Spinal Instability and Spinal Fusion Surgery Treatment & Management

Updated: Jan 14, 2016
  • Author: Peyman Pakzaban, MD; Chief Editor: Brian H Kopell, MD  more...
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Treatment

Medical Therapy

In acute overt instability, stabilization of the spine is required in all cases. In this context, medical treatment refers to the use of external bracing for spine stabilization. If instability is due to an osseous fracture, if the fracture fragments can be reduced to near-anatomic alignment, and if there is no significant neural compression after reduction, the patient may be treated nonsurgically with a brace until the fracture heals.

In anticipated instability (eg, extensive discitis and osteomyelitis treated with debridement, decompression and antibiotics), bracing may be used as a temporary means of stabilization, before fusion is undertaken or until spontaneous fusion occurs.

Many forms of external orthoses and braces are available. In the cervical spine, a halo offers the greatest amount of stabilization. Rigid cervical collars (eg, Philadelphia collar, Miami collar) and various cervicothoracic orthoses provide intermediate amounts of stabilization, while soft collars provide little stabilizing benefit. In the case of thoracic and lumbar spine, the only brace that provides significant stabilizing benefit is a rigid TLSO (thoraco-lumbo-sacral orthosis) brace. Rigid lumbar braces that do not extend to the chest and soft braces/corsets provide little stabilizing benefit.

In chronic overt instability and covert instability, medical treatment plays a more prominent role. If not at risk for imminent neurological deterioration, the patients with these forms of instability generally undergo conservative (nonsurgical) treatment first. Fusion is reserved for those in whom conservative treatment fails (see Indications).

Conservative treatment may include some or all of the items below:

  • Medications: Analgesics, anti-inflammatories, muscle relaxants, tricyclic antidepressants, anti-epileptics
  • Physical therapy
  • Behavior modification (smoking cessation, weight loss)
  • Injection therapy (eg, epidural or facet steroid injections)
  • Transcutaneous electrical nerve stimulation (TENS)
  • Psychological treatment (especially for depression)
  • Alternative treatments (eg, acupuncture, biofeedback)
Next:

Surgical Therapy

Once the decision has been made to fuse a particular spine segment, there may be several surgical methods to accomplish this task. After a particular method is selected, the etiology of instability is no longer relevant, as the technical steps would be the same. The following is a discussion of the most commonly employed fusion techniques in various regions of the spine.

Atlantoaxial (C1-2) instability

Atlantoaxial instability may be caused by a variety of conditions, including odontoid fractures, rupture of the transverse ligament, ligamentous incompetence due to rheumatoid arthritis, and congenital instability associated with os odontoideum.

C1-2 fusion with cable fixation

Before the advent of screw fixation techniques, wiring of C1-2 posterior elements with an interposed bone graft was the only method of fusion of the atlantoaxial segment. The monofilament wires used in the past are gradually being abandoned in favor of multistranded braided cables, which offer greater flexibility, strength, and fatigue resistance.

Three techniques are available. In the Gallie technique, a cable loop is passed under C1 posterior arch from below, folded over the C1, and hooked under the base of C2 spinous process. The free ends of the cable are then brought together in the midline to secure a unicortical onlay bone graft against the decorticated surfaces of C1 and C2 laminae. Although relatively safe and easy to perform, this technique provides little rotational stability and has a higher pseudarthrosis rate due to the onlay nature of the graft.

C1-2 fusion with cable fixation (Gallie technique) C1-2 fusion with cable fixation (Gallie technique). In this case, the fusion is supplemented with transarticular screws.

In the Brooks technique, one (central) or two (lateral) bicortical bone grafts are wedged between C1 posterior arch and C2 lamina. A tension band is then constructed by passing two separate cables under both C1 and C2 laminae and attaching their free ends posterior to the graft or grafts. The cancellous surfaces of the graft are in good contact with the decorticated undersurface of C1 arch and top rim of C2 lamina, placing the graft(s) under compression, thus enhancing fusion rates. In addition, the Brooks method provides greater rotational stability due to bilateral engagement of the C2 lamina. The problem with this technique is that sublaminar passage of the cable under both C1 and C2 substantially increases the technical difficulty and risk of spinal cord injury during the procedure, particularly if the canal diameter is already compromised by the underlying pathology.

C1-2 fusion and cable fixation (Brooks technique). C1-2 fusion and cable fixation (Brooks technique). Image courtesy of Synthes, Inc.

The newer Sonntag technique combines the ease and safety of the Gallie technique with the superior biomechanical features of the Brooks technique. Here, the cable loop is passed only under the C1 lamina and hooked under C2 spinous process base, as in the Gallie technique. The difference is that the bicortical bone graft is wedged between C1 and C2 posterior elements (similar to Brooks technique) and the free ends of the cables are attached under C2 spinous process base. Since (unlike Brooks) there is no C2 sublaminar wire to protect against anteropulsion of the graft, a notch is made in the inferior portion of the graft and it is wedged over the superior surface of the C2 spinous process.

C1-2 fusion with cable fixation (Sonntag technique C1-2 fusion with cable fixation (Sonntag technique): coronal (left) and sagittal (right) CT reconstructions.

Regardless of the technique used for C1-2 cable fixation, a halo is generally applied until fusion occurs (usually 3-6 mo), creating a major drawback from the standpoint of patient comfort and rehabilitation.

C1-2 transarticular screw fixation

Screw fixation of the atlantoaxial segment provides immediate rigid fixation of the joint and eliminates the need for a halo. The main consideration is the risk of injury to the vertebral artery. In about 18% of the cases, the vertebral artery rides high after emerging from the C2 transverse foramen, positioning itself in the path of the screw. Before surgery, it is imperative to perform a high-resolution CT scan with sagittal reconstructions to detect this variant anatomy and avoid screw insertion on that side.

The technique is as follows: The patient is placed in prone position and the head is carefully flexed under fluoroscopic guidance and fixed in a Mayfield head holder. An incision is made from the skull base to C7. In addition, small stab incisions may be necessary more inferiorly and laterally to for placement of the drill in the correct trajectory. C1, C2, and C3 are exposed to the lateral margin of the lateral masses.

The ligamentum flavum above C2 is removed to expose the C2 nerve root (greater occipital nerve), which runs posterior to the facet joint, unlike any other location in the spine. The nerve and its surrounding venous plexus are retracted superiorly to expose the facet joint. A 2.5-mm drill is used to establish the crew trajectory, starting at the C2-3 facet edge about 2-3 millimeters lateral to the medial aspect of the lateral mass. Drilling is performed in 2 mm increments pointing 10 degrees medially in a cephalad trajectory aimed at the posterior cortex of the anterior arch of C1 under continuous lateral fluoroscopic imaging. As the drill crosses the C1-2 articulation, decreased mobility of C1 is often immediately palpable. To correct any subluxation, C1 may be pushed or pulled in anterior/posterior directions before the drill crosses the facet joint.

The drill hole is then screwed with a self-tapping screw of the appropriate length (alternatively, it can be tapped and then screwed). The procedure is then repeated on the opposite side. The posterior surfaces of the C1 and C2 lateral masses and the posterior aspect of the facet joint are decorticated with a drill and packed with cancellous bone graft.

C1-2 transarticular screw. Notice the proximity of C1-2 transarticular screw. Notice the proximity of vertebral artery to the typical screw trajectory.

If vertebral artery injury is encountered on one side, the screw is left in place and screw placement on the opposite side is avoided in order to prevent bilateral injury. A postoperative vertebral angiogram is performed to rule out pseudoaneurysm formation.

C1-2 transarticular screw fixation is best supplemented with C1-2 cable fixation in order to provide better bone substrate for fusion than what can be packed in the facet joints. The patient is placed in a Philadelphia collar postoperatively.

C1-2 lateral mass/isthmus fixation

When preoperative CT imaging reveals that a high-riding vertebral artery would be in the trajectory of a C1-2 screw, an alternative technique can be employed. A screw is inserted in C1 lateral mass. A second screw is inserted into the C2 isthmus. The two screws are then connected with a rod or a plate. The procedure is repeated on the opposite side. The technique for exposure is identical to C1-2 transarticular screw placement. After the ligamentum flavum is resected, the medial wall of the C2 isthmus is exposed and palpated with a Penfield 4 instrument.

Although the isthmus is sometimes called the C2 pedicle, this is not strictly correct. The isthmus is a tubular structure that courses medially and superiorly, connecting each C2 lateral mass to the body, and is more correctly identified as the pars equivalent. The drill and screw are directed along the visualized trajectory of the isthmus. The entry point is at the center of the lateral mass and the trajectory is angled 25 degrees medially and 25 degrees cephalad. The more medial trajectory of the screw in this technique helps avoid vertebral artery injury. Palpation of the medial wall of the isthmus during screw insertion helps avoid breach of the spinal canal.

Odontoid screw fixation

This technique is reserved to certain type-2 odontoid fractures. Its main advantage is that it directly repairs the odontoid fracture, thus avoiding a C1-2 fusion and maintaining range of motion. Its shortcoming is the limited circumstances in which it can be employed.

Odontoid fractures are categorized according to the following scheme (Table 4) and treated according to the algorithm in Table 1.

Table 4. Odontoid Fracture Classification (Open Table in a new window)

Type Fracture Anatomy
1 Fracture through the odontoid tip (rare)
2 Fracture across the base of the odontoid process (most common)
2A As in type 2, except with comminution of fracture line, reducing the possibility of healing of fracture in halo or with odontoid screw
3 Fracture extension into C2 vertebral body; because of larger bone contact area, fracture usually heals well in a halo

Odontoid screw fixation applies only to those type 2 odontoid fractures that are reducible to less than 3 mm of fracture fragment displacement and are not associated with rupture of transverse ligament or a tumor in C2. In addition, type 2A fractures, fractures in patients older than 65 years, slanted fracture lines, and old nonhealing fractures may be better treated by C1-2 fusion, as odontoid screw fixation is less likely to yield a satisfactory outcome in these circumstances.

The technique is as follows: the procedure is performed from an anterior approach under biplanar fluoroscopy. The neck is slightly extended. A transverse incision is created on the right side of the neck, overlying the C5-6 interspace. Dissection is carried down to the spine as in anterior cervical fusion (see below). A plane is developed cephalad along the anterior aspect of the spine to the C2-3 disc space.

An Apfelbaum retractor specifically designed for this technique is deployed. A Kirschner wire (K-wire) is placed at the anterior-inferior margin of C2 and a shallow pilot hole is drilled under fluoroscopy toward the odontoid tip. The entry point is along the midline if one screw is desired and lateral to the midline if two screws are intended. A reamer is placed over the K-wire to create a tangential slot along the anterior margin of C3 and C2-3 disc space. The reamer is removed and replaced with a drill guide. The spikes of the drill guide are tapped into the anterior body of C3 to stabilize it. The K-wire is removed and replaced with a drill through the drill guide.

Drilling is performed toward odontoid tip. The screw hole is tapped and screwed with a lag screw of appropriate length to engage the cortex of the odontoid tip. The lag screw will help compress the fracture fragment against the C2 body. If a second screw is inserted, it does not have to be a lag screw. A Philadelphia collar is applied postoperatively for 3 months.

Subaxial cervical instability

Anterior cervical fusion

Anterior cervical fusion is one of the most commonly used fusion techniques in spine surgery. The anterior approach is used increasingly in preference to the posterior approach to the cervical spine, [8] as it provides distinct advantages with regard to decompression, fusion, and instrumentation. Most pathological processes in the cervical spine, especially degenerative and neoplastic disorders, affect the structures anterior to the spinal cord. An anterior approach to the cervical spine permits thorough decompression of the spinal canal without manipulating the spinal cord. Furthermore, an anterior approach permits placement of the bone graft in an interbody position under compression, which significantly enhances the success of fusion. Finally, the relatively large surface areas and volumes of the vertebral bodies compared to the posterior cervical elements render them ideal substrates for instrumentation.

Anterior cervical plate, applied in this case afte Anterior cervical plate, applied in this case after 2-level anterior cervical discectomy and fusion. Image courtesy of Synthes, Inc.

The most commonly employed indications for anterior cervical fusion are in treatment of degenerative disorders. Large central disc herniations with cord compression and chronic disc-osteophyte complexes at one or more levels cannot be safely removed from a posterior approach. Cervical spinal stenosis associated with kyphosis is best treated through an anterior approach since a multilevel posterior decompression by laminectomy does not relieve the stretching of spinal cord over disc-osteophytes and may exacerbate the kyphosis in the long run. Anterior cervical discectomy is effective not only for treatment of neural compression (myelopathy and radiculopathy), but also for treatment chronic axial pain associated with disc degeneration and correction of spinal deformity (kyphosis).

Large central disc herniations (A and B) and cervi Large central disc herniations (A and B) and cervical spondylotic myelopathy with kyphosis (C) are two common indications for anterior cervical discectomy and fusion.

Anterior cervical corpectomy (removal of vertebral body) is employed when the pathology extends behind the vertebral body anterior to the cord (eg, large osteophytes or ossification of posterior longitudinal ligament) or involves the vertebral body itself (eg, tumor or burst fracture). Traumatic cervical dislocations associated with disc herniation should be treated through an anterior approach.

C5-6 bilateral jumped facets associated with disc C5-6 bilateral jumped facets associated with disc herniation (left) was treated with C6 anterior cervical decompression and fusion (right).

The technique is as follows: The patient is placed on the operating table in supine position. Rolls are placed under the shoulders and under the hip from which iliac crest graft is to be harvested. The arms are padded and tucked by the patient’s sides. The shoulders are taped to the foot of the bed to permit unencumbered visualization of the spine by fluoroscopy. The fluoroscopic C-arm is positioned in cross-table lateral orientation and draped.

The skin incision is typically made on the right side of the neck for a right-handed surgeon and the left side of the neck for a left-handed surgeon, as this significantly facilitates access. If C7-T1 disc is the target, some surgeons prefer a left-sided approach to minimize the risk of recurrent laryngeal nerve (RLN) palsy, although this puts the thoracic duct at risk. If unilateral RLN palsy is present preoperatively, surgery must be performed from the side of the palsy to avoid the risk of bilateral RLN palsy. A transverse skin incision over a skin crease centered along the anterior border of sternocleidomastoid muscle provides the best cosmetic result for 1- and 2-level fusions. With practice, a 3-level fusion can also be performed through a transverse incision. An oblique vertical incision along the anterior border of the sternocleidomastoid muscle is used if greater rostrocaudal exposure is required.

The platysma is divided in line with the skin incision. A subplatysmal dissection is carried out. A large external jugular vein is best mobilized and retracted to the side, whereas a smaller one can be ligated and divided. An avascular plane is developed medial to the sternocleidomastoid muscle and carried medial to the carotid sheath to reach the anterior border of the cervical spine. Gentle medial retraction of the midline structures significantly facilitates this task. The omohyoid muscle courses obliquely in this region from an inferior-lateral to a superior-medial location. For C3-4, C4-5, and C5-6 discs, the plane of dissection is superior to the omohyoid muscle, while for C6-7 and C7-T1 disc, it is below the omohyoid muscle.

Occasionally, for an extensive procedure that spans several segments above and below C6, the omohyoid muscle is divided and later reapproximated. Small tributaries to the internal jugular vein running transversely across the field of exposure can be coagulated and divided. If the common facial vein is hindering the exposure of the upper cervical spine, it can be ligated and divided. The carotid sheath is never entered. If the ansa cervicalis is encountered, it can be mobilized either medially or laterally. In very high exposures, care is taken to prevent injury to the hypoglossal nerve deep to the digastric muscle. In very low exposures, care is taken to spare any neural structures that might correspond to a variant crossing of the recurrent laryngeal nerve.

The prevertebral fascia is opened and the esophagus and pharynx are retracted toward the contralateral side. A transverse cervical artery, often accompanied by a vein, is usually identified over the C7 vertebral body in the superior extension of the mediastinal fat pad. This artery and the fat pad can usually be swept inferiorly and preserved. If exposed, this artery should be carefully inspected at the end of the procedure to make sure that it is intact. If damaged by stretching, it must be ligated to avoid the risk of a catastrophic postoperative neck hematoma.

The attachment of longus coli muscles to the anterolateral aspects of the vertebral bodies above and below the target site are divided. Bleeding from muscle edges and bone is controlled with electrocautery and bone wax. Excessive monopolar electrocautery is avoided to prevent the risk of thermal damage to the nearby sympathetic chain with resultant Horner syndrome. If large anterior osteophytes are present, they are resected flush with the anterior surface of the vertebral bodies. An anterior cervical self-retaining retractor is inserted and its lips are secured under the mobilized edges of the longus coli muscles. If sufficient dissection of the longus coli muscles is not carried out, the retractors will not remain in place, creating a significant nuisance during the remainder of the case. Many contemporary retractors have blades that distract not only medial-laterally but also superior-inferiorly, providing an excellent exposure.

Caspar posts are inserted in the midportion of the vertebral bodies above and below the target site under fluoroscopic guidance. The Caspar distractor is then used to distract the disc space(s). The anterior longitudinal ligament and the anterior annulus of the disc are resected. Under the operating microscope, the contents of the disc(s) are thoroughly evacuated to expose the converging posterior lips of the superior and inferior endplates and the intervening posterior annulus of the disc.

The posterior annulus is temporarily left in place as a protective shield while the posterior lips of the endplates and the underlying osteophytes are meticulously drilled with a bur under the operating microscope until they are reduced to thin shells of cortical bone. The posterior annulus of the disc is the completely resected. The herniated disc material is removed. The residual osteophyte shells are elevated away from the dura with a small hook or a small up-angled curette and removed. The posterior longitudinal ligament is completely resected from side to side to fully expose and decompress the dura.

In rare instances, an ossified or thickened posterior longitudinal ligament is fused to the dura and cannot be completely resected without risking dural laceration. The medial aspects of the uncinate processes are resected to ensure decompression of the symptomatic nerve roots. A nerve hooks is passed laterally into the neural foramina to ensure their patency. Bleeding from the lateral epidural veins is readily controlled with Gelfoam. Care is taken during lateral dissection to avoid injury to the vertebral arteries, which are located lateral and anterior to the uncovertebral joints. A careful review of the preoperative CT scan or MRI axial images will provide useful information about the proximity of the vertebral artery to the neural foramina and any aberrant looping of that vessel into the normal course of the exposure.

If a corpectomy is to be performed, the discs above and below the vertebral body are first resected, as described above. The anterior aspect of the vertebral body is readily resected with a large rongeur. The posterior half of the vertebral body is carefully drilled until the posterior wall is thinned down to a shell of bone. Venous bleeding from the lateral walls of the resected vertebral body is controlled with bone wax. The posterior cortex and posterior longitudinal ligament are carefully elevated away from the dura and resected.

After satisfactory decompression is obtained, the endplates are decorticated with a drill, in preparation for fusion. The cartilaginous endplate is removed but excessive removal of the bony endplate is avoided in order to minimize settling of the graft. Posterior ledges in the endplates are left behind to prevent retropulsion of the graft.

The height of the interspace is measured. An appropriately sized tricortical bone graft is harvested from the anterior iliac crest or fashioned out of cadaveric allograft bone. The graft is inserted into the interspace and tapped in place under fluoroscopic guidance. The distraction is then released and the distraction posts are removed. An appropriately sized anterior cervical plate is affixed to the vertebral bodies above and below the fused segment(s) with cancellous screws under optional fluoroscopic guidance. The screws are locked utilizing the locking mechanism specific to the plate.

The retractors are removed. Hemostasis is secured. The wound is irrigated. The platysma is closed with absorbable sutures. The skin is closed in subcuticular fashion.

Anterior cervical discectomy and fusion: A. Disc r Anterior cervical discectomy and fusion: A. Disc removed and interspace prepared to receive graft; B. Iliac crest bone graft harvested; C. Bone graft; D. Graft inserted into disc space; E. Plate screwed to anterior surface of vertebral bodies.

Some surgeons do not perform a fusion after an anterior cervical discectomy. This is only appropriate if the operation consists of a limited central single level discectomy for a soft central disc herniation in absence of spondylosis, wherein comprehensive removal of the posterior longitudinal ligament and uncovertebral joints has not been performed. Even if spontaneous fusion occurs as desired in these cases, the resultant reduction in foraminal height may predispose the patient to future nerve root compression.

It is acceptable to perform a single level discectomy and fusion without plating, relying on the tension band provided by the middle and posterior columns to promote stability and fusion. A plate is employed for all multilevel discectomies and corpectomies.

No collar, soft collar, or a rigid cervical collar may be applied for 1-6 weeks postoperatively, depending on the extent of the procedure. After multilevel corpectomies, a halo is considered.

Lateral mass screws

Excellent stabilization of the subaxial cervical spine can be achieved from a posterior approach by using lateral mass screws. Here screws are inserted into the lateral masses at trajectories designed to avoid the vertebral artery and cervical nerve roots. The adjacent screws are linked by bilateral plates or rods. The technique can be employed whether or not a previous laminectomy has been performed.

Two techniques are available. In the Magerl technique, the entry point is 2 mm medial and 2 mm superior to the center of lateral mass and the screw trajectory is 20-25 degrees lateral and parallel to facet surface in the sagittal plane. In the Roy-Camille technique, the entry point is just above the center of the lateral mass and the trajectory is directed only 10 degrees laterally and perpendicular to the lateral mass surface in the sagittal plane. Although, the Magerl technique leaves a greater safety zone between the screw tip and the nerve root, it is not always easy to achieve the cranial angulation that the technique requires during surgery. Prior to placement of rods or plates, the facet joints and lateral mass surfaces are decorticated and packed/covered with cancellous bone. If laminae are present, corticocancellous strips of bone graft can be placed over the decorticated laminae and wired in place.

Posterior cervical wiring

Monofilament wires, double-stranded twisted wires, Drummond wires (wire loops passing through a button), and braided multistranded cables can be used for wiring of posterior cervical elements.

Spinous process wiring is the simplest technique but provides no limitation of extension and little rotational stability. In this technique, a wire is passed through the center of one spinous process around the base of a lower spinous process. Multiple wire loops can be used to connect multiple spinous processes in an interlocking chain array. More complicated spinous process wiring patterns (eg, Bohlman triple-wire technique) allow firm attachment of bone graft strips to the sides of the spinous processes.

Sublaminar wiring technique involves the use of a loop of wire that is carefully passed under the lamina and the cut in half. Each wire is then brought laterally to each side and tightened around a rod or bone graft. The technique is repeated for two or more laminae.

When a laminectomy has been performed, the only available structure for wiring is the facet joint. Wires passed through the midportion of the inferior articulating facet are tightened around a rod or bone graft strip.

In general, wiring techniques are inferior to anterior or posterior screw/plate/rod fixation methods.

Lumbar spinal instability

Posterior and posterolateral noninstrumented lumbar fusion

Noninstrumented posterior or posterolateral fusion of the spine is fairly simple to perform and is an acceptable treatment for certain instances of degenerative instability, when the patient is not believed to be a candidate for pedicle screw insertion. Because of its greater susceptibility to pseudarthrosis, it is not recommended for situations in which overt instability is present. The patient is kept in a TLSO brace until the fusion solidifies.

The technique is as follows: The lumbar spine is exposed in standard fashion through a posterior midline incision. Bilateral exposure of the laminae is extended further laterally to completely expose of the facet joints and transverse processes of the vertebrae to be fused. Usually, a decompressive laminectomy is carried out to treat neural compression. In this process, medial facetectomies may be carried out to fully decompress the lateral recesses, if necessary. The transverse processes, lateral aspect of the facet joints and the synovial facet surfaces are decorticated with a high-speed drill.

Corticocancellous strips of bone are harvested from posterior iliac crest and placed in the "lateral gutters" over the lateral aspect of facet joints and transverse processes. The space between the articulating surfaces of the facet joints is packed with cancellous bone graft. The fusion mass may be supplemented with cortical bone obtained from the laminectomy.

If the laminar surfaces have not been completely removed (eg, only a laminotomy has been performed), bone graft can be applied to the decorticated laminae and spinous processes to produce a true posterior fusion. The posterolateral fusion may be supplemented by a posterior lumbar interbody fusion (PLIF).

Instrumented lumbar fusion with pedicle screws

Pedicle screw fixation is the most commonly used approach for internal stabilization of the lumbar spine. Screws are inserted into the pedicles of the vertebrae to be fused and connected to each other with bilateral rods or plates.

The technique is as follows: The spine is exposed (and decompressed if necessary) as for a noninstrumented fusion. Pedicle screws are inserted into the pedicles above and below the motion segment to be fused. The main concern during pedicle screw insertion is to avoid breach of the pedicle wall and injury to the exiting nerve root. If a laminectomy or upper laminotomy has been carried out, it is possible to visualize or palpate the medial and inferior surfaces of the pedicle, which are in contact with the nerve root. In this case, only lateral fluoroscopy is necessary to guide the entry and trajectory of the screw in the sagittal plane. If the pedicle has not been exposed by laminectomy/laminotomy, AP and lateral fluoroscopy are usually used. The inferior-lateral aspect of the pedicle can also be exposed by subperiosteal dissection from a lateral approach along the base of the transverse process.

Intraoperative fluoroscopy for pedicle screw inser Intraoperative fluoroscopy for pedicle screw insertion.

The entry point to the pedicle is located at the junction of lines bisecting the transverse process and superior articulating facet. A starting hole is created at the entry point with an awl or a drill. A pedicle probe is then used to establish the path of the screw under fluoroscopic guidance. The probe may be stimulated with a nerve stimulator to elicit EMG activity from the lower extremities. If EMG is elicited at low stimulation currents, contact between the probe and the nerve root is suspected and the probe is removed and reinserted differently.

The trajectory in the sagittal plane is parallel to the superior endplate for –L1-L5 and toward the sacral promontory for S1. In the axial plane, the probe is directed slightly medially into the vertebral body. As one descends the lumbar spine, the medial angulation of the pedicles increases. After the probe is removed, the screw path is tapped and screwed with the appropriate size screw. In order to avoid penetration of the anterior cortex of the vertebral body, the screw should not extend beyond 80% of the diameter of the vertebral body on lateral fluoroscopy.

Once all of the screws have been inserted, they are usually linked with rods, although notched plates also exist for this purpose. Depending on the instrumentation system used, the screw/rod interface may be fixed, requiring extensive contouring of the rod to fit the screw heads, or variable, requiring minimal contouring. Before the screw/rod interface is tightened and locked, the pedicle screws can be used to distract or compress the vertebral bodies. If reduction of spondylolisthesis is desired, the inferior screw is locked to the rod and the superior screw is pulled back toward the rod, the latter acting as a cantilever beam. If adequate decompression of the spinal canal and nerve roots has been carried out by laminectomy and partial facetectomy, it may not be necessary to reduce the spondylolisthesis, in which case the fusion is said to have been performed in situ. Some surgeons prefer to attempt to reduce all spondylolisthesis in order to restore sagittal balance.

Pedicle screw fixation of lumbar spine. Image cour Pedicle screw fixation of lumbar spine. Image courtesy of Synthes, Inc.

After pedicle screw insertion, a posterolateral bony fusion is performed as previously described. Instrumented posterolateral fusion can be further supplemented by interbody fusion (ie, PLIF, TLIF, or ALIF), thus producing a global fusion.

Lumbar interbody fusion

Lumbar interbody fusion refers to replacement of disc space with bone. Because of the substantial surface area of the vertebral endplates and the fact that the bone graft is under compression, interbody fusion enjoys a favorable fusion environment. It is ideally supplemented with pedicle screw instrumentation to provide internal fixation and may or may not be further supplemented with posterolateral fusion.

Combined interbody and posterolateral lumbar fusio Combined interbody and posterolateral lumbar fusion with pedicle screws: coronal (left) and sagittal (right) CT reconstructions.

Interbody fusion can be performed through a posterior approach (posterior lumbar interbody fusion [PLIF]), a posterolateral approach (transforaminal lumbar interbody fusion [TLIF]), or an anterior approach (anterior lumbar interbody fusion [ALIF]). Recently, a far lateral approach (extreme lateral interbody fusion [XLIF]) has also been described.

The original PLIF technique was performed through a routine posterior exposure for lumbar discectomy. After the disc space was thoroughly evacuated, the endplates were decorticated with large angled curettes and bone rasps. The disc space was then packed with autologous cancellous bone. Nowadays, PLIF, TLIF, and ALIF are usually performed with the aid of interbody implants. The implants, which are made of a PEEK polymer and machined cortical allograft bone, or metal cages are filled with cancellous bone before insertion into the disc space. The remaining disc space around the implant is also packed with cancellous bone. The 3 techniques differ only in the method of insertion of the interbody implant.

The PLIF technique is performed as follows: Laminectomy (or bilateral hemilaminectomy), bilateral medial facetectomy, and bilateral discectomy are carried out at the target segment. The traversing and exiting nerve roots are identified bilaterally and the intervening epidural veins are bipolar coagulated and divided. The endplates are thoroughly decorticated. The traversing nerve roots and the dural sac are retracted medially as the interbody implants are inserted bilaterally under fluoroscopic guidance. Depending on the type of the implant, specific instruments are used for preparation of the disc space and insertion of the implant.

The TLIF technique is performed as follows: This technique is usually performed unilaterally and does not require an extensive laminectomy. It lends itself to open or minimally-invasive approaches. A partial facetectomy is performed to unroof the neural foramen and identify the exiting and traversing nerve roots. A unilateral discectomy is performed and the endplates are thoroughly decorticated using long angled curettes and bone rasps. A banana-shaped interbody implant packed with bone is inserted into the disc space via a transforaminal approach and tapped in place under fluoroscopic guidance. Because of its shape, as the implant is inserted and tapped, it gradually assumes a transverse orientation within the disc space. The disc space posterior to the implant is packed with cancellous bone.

The ALIF technique is performed as follows: The appropriate lumbar or lumbosacral segment is reached through an anterior transperitoneal or extra-peritoneal approach (see below). This can be accomplished by an open or laparoscopic method. The L5-S1 disc is always approached between the iliac vessels, often requiring mobilization and lateral retraction of the left iliac vein. For the L4-5 disc space, the level of aortic bifurcation and size of the left iliac vein determine the direction of vessel retraction. The iliolumbar vein is divided for L4-5 disc access. The anterior longitudinal ligament and the anterior annulus of the disc are incised and the disc contents are evacuated. The endplates are prepared and the interbody implants are inserted utilizing instruments and methods specific to the type of implant used.

ALIF is sometimes combined with pedicle screw instrumentation, necessitating an anterior-posterior approach, which avoids opening the spinal canal. In conditions that require decompression of the spinal canal, a PLIF or TLIF can be performed in conjunction with pedicle screw instrumentation, thus performing a global fusion through a single approach without opening the abdomen.

Lumbar corpectomy

A lumbar corpectomy is generally performed for neoplastic disease affecting the vertebral body but may be performed for other indications, such as burst fractures with substantial retropulsion that cannot be reduced through a posterior approach or extensive vertebral osteomyelitis with pathological fracture that cannot be adequately debrided and decompressed from a posterior approach.

The technique is as follows: The L2-L5 segments are approached through a left retroperitoneal approach with the patient is lateral decubitus position. The retroperitoneal approach is usually provided by a general surgeon. The peritoneum is mobilized forward until the psoas muscle is visualized. The kidney is mobilized forward. The ureter is found over the psoas muscle and mobilized forward with its surrounding fat. The sympathetic chain and the genitofemoral nerve are identified over the psoas muscle and preserved. The psoas muscle attachments to the lateral aspects of the vertebral bodies are mobilized posteriorly with a Cobb periosteal elevator to expose the lateral aspect of the pedicles. The segmental vessels are ligated and divided over midportion of the vertebral bodies.

The vertebral body bone is removed from a left to right approach. The posterior margin the vertebral body is identified at the level of pedicle and followed inferiorly. Retropulsed bone fragments or ventral epidural tumor are removed. The discs above and below the level of corpectomy are removed and the endplates decorticated.

Reconstruction across the gap produced by corpectomy requires a large interbody implant. This could be a piece of tibial or femoral allograft, cored out and filled with autologous bone. Alternatively, a Steinman pin wedged between the adjacent vertebral bodies, surrounded by methylmethacrylate can be used in the case of malignant disease, when bony fusion is not realistically expected. Most commonly, metal cages of fixed height (Harms cage) filled with bone or expandable metal cages are used to reconstruct the vertebral body defect. A plate or plate-rod system is screwed to the lateral aspect of the vertebral bodies above and below the level of corpectomy, the latter providing the advantage of compression across the cage.

Anterolateral lumbar corpectomy followed by recons Anterolateral lumbar corpectomy followed by reconstruction with a fixed-height cage and a dynamic rod system that allows compression across the cage. Image courtesy of Synthes, Inc.

Thoracic and thoracolumbar instability

Posterior thoracic and thoracolumbar instrumentation

Since the advent of Harrington rods for posterior thoracolumbar stabilization and deformity correction, there has been a significant evolution in posterior instrumentation constructs. Harrington rods were nonsegmental systems attached to the spine via hooks at proximal and distal ends of long rods and relied primarily on distraction for deformity correction. Modern posterior thoracolumbar instrumentation constructs differ from Harrington rods in several key aspects:

  • They are segmental; that is, they are attached to the spine not only at proximal and distal ends of rods but at multiple intervening segments.
  • They are modular and hybrid, allowing simultaneous use of hooks (laminar hooks, transverse process hooks, pedicle hooks), wires, and pedicle screws.
  • They rely on 3-point-bending biomechanical principles more than distraction for deformity correction and stabilization.

These instrumentation systems can be used for treatment of acute overt instability due to trauma, neoplasms, or infections, in addition to deformity correction. Hooks can be applied above a lamina, facing down, or below a lamina, facing up. When 2 hooks are placed on the same lamina or adjacent laminae, facing each other, a "claw" construct is formed. Claw constructs help secure the ends of the rods against the spine. When placement of laminar hooks is not possible (because of laminectomy) or desirable, transverse process hooks can be used. Hooks can also be placed under pedicles (inserted through the thoracic facet joint and screwed to the undersurface of the pedicle).

When the construct relies primarily on hooks, a long segment of the spine is instrumented (generally 3 segments above and 2 segments below) to prevent failure of the construct. The segments receiving bone graft would be shorter than the instrumented segments, known as the "rod long, fuse short" practice. Increasingly, pedicle screws are playing a prominent role in segmental modular constructs in the thoracic and thoracolumbar regions. The increased stability conferred by the screw-based systems allows construction of shorter constructs spanning the unstable motion segment.

A modular posterior thoracolumbar instrumentation A modular posterior thoracolumbar instrumentation system, which is attached to the spine by a combination of screws and hooks, in turn attached to long rods. In this case, it is used for correction of scoliosis, using 3-point bending biomechanical principles. Image courtesy of Synthes, Inc.

Posterior systems also allow reduction of anterior (vertebral body) fractures using segmental distraction (see the image below). Above all, use of posterior instrumentation for thoracic and thoracolumbar spinal instability is clearly augmented by the fact that exposure of this portion of the spine is far easier from a posterior versus an anterior approach.

Example of application of biomechanical principles Example of application of biomechanical principles to spine surgery. Insertion of special pedicle screws (Schanz screws) pivoting on a rod transfers the instantaneous axis of rotation (IAR) to the screw/rod interface. Compression of the proximal end of the screws produces distraction-reduction of the vertebral burst fracture. If the posterior longitudinal ligament is intact, retropulsion is corrected by ligament taxis. Image courtesy of Synthes, Inc.

Anterior thoracic and thoracolumbar instrumentation

There are instances in which thoracic and thoracolumbar instability cannot be adequately addressed from a posterior approach. These include pathological fractures of the vertebral bodies due to tumor or infection, producing cord compression, and some traumatic burst fractures (associated with retropulsion and rupture of posterior longitudinal ligament) that cannot be reduced by posterior distraction. In these cases of anterior pathology, an anterior approach is preferred.

The upper thoracic region (T1-3) is approached anteriorly by extending an anterior cervical approach inferiorly to the mediastinum via a partial or complete median sternotomy. The fusion and instrumentation methods here are identical to anterior cervical procedures.

The mid-thoracic (T4-T11) region is generally approached by a transthoracic approach from the left side. This usually involves a thoracotomy with opening of the pleura and deflation of the ipsilateral lung, which provides excellent exposure of the entire T4-T11 region from an anterolateral perspective. Alternatively, an extracavitary (extrapleural) approach can be used, which provides a more limited rostrocaudal and anterior exposure.

The thoracolumbar region (T12-L2) is approached by an anterior thoracolumbar approach, which combines a thoracotomy with a retroperitoneal approach to the upper lumber spine and requires division and mobilization of the diaphragm.

When a thoracic or thoracolumbar corpectomy is performed, the corpectomy gap has to be reconstructed in the same fashion as a lumbar corpectomy (see above). For this purpose, the most recent additions to the surgeon's armamentarium are expandable cages, which are placed in the corpectomy defect and then expanded to engage and distract the adjacent vertebral bodies. These are then filled with bone and supplemented with a plate. The plate is applied to the lateral surface of the vertebral bodies and serves as a tension band construct.

Anterolateral thoracic corpectomy followed by reco Anterolateral thoracic corpectomy followed by reconstruction with an expandable cage and a fixed plate/screw system. Image courtesy of Synthes, Inc.

In addition to permitting more thorough decompression of anterior pathology, anterior thoracic and thoracolumbar reconstructions allow one to limit the instrumented fusion to the pathological motion segment, sparing the adjacent segments.

Anteroposterior and lateral radiographs of anterio Anteroposterior and lateral radiographs of anterior thoracic corpectomy and reconstruction for pathological fracture due to vertebral osteomyelitis.
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Preoperative Details

Routine preoperative tests usually consist of complete blood count, electrolytes, BUN, creatinine, glucose, prothrombin time (PT), activated partial thromboplastin time (aPTT), international normalized ratio (INR), chest radiograph, and electrocardiogram. Blood is typed and screened. If extensive blood loss is anticipated, one or two units of packed red blood cells are cross-matched or a cell saver is used. Alternatively, if the procedure is scheduled electively, the patient may donate autologous blood several weeks before the surgery. Thigh-high compression stockings (TED hose) and sequential compression devices are applied for DVT prophylaxis preoperatively and are not removed until the patient is mobilized postoperatively.

In patients who are at particular risks for deep venous thrombosis and pulmonary embolism (eg, paraplegic, quadriplegic, or bed-bound prior to surgery), subcutaneous injections of low-molecular-weight heparin may begin before the surgery, weighing the individual patient's risk of postoperative epidural hematoma against the risk of pulmonary embolism. Meticulous attention to hemostasis, liberal use of closed wound drainage, and careful postoperative neurological evaluation are indispensable when heparin is used. An antibiotic with antistaphylococcal activity, usually a first generation cephalosporin, is given within one hour prior to the skin incision and continued for 3 doses postoperatively.

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Intraoperative Details

Intraoperative details specific to each fusion technique have been summarized above. The following are general concepts pertaining to intraoperative management of all fusion procedures.

Positioning the patient for surgery is of utmost importance for safe and effective conduct of the procedure. For posterior cervical procedures, a prone position is preferred. Although some surgeons use a sitting position to minimize bleeding from epidural veins, this position puts the patient at risk for intraoperative hypotension and venous air embolism. Meticulous surgical technique, judicious use of bipolar coagulation, and use of an operating microscope when necessary permit all posterior cervical procedures to be performed safely in prone position. The patient's head may be immobilized by 3-point skeletal fixation in a Mayfield head holder, which also permits precise control of cervical contour. If the head is positioned over a foam or horseshoe head holder instead, special attention should be given to avoiding compression of the eyes, which could result in raised intra-ocular pressure and retinal ischemia.

For posterior lumbar and thoracolumbar fusions, the patient is positioned prone over a frame or table that permits the abdomen to hang free. Otherwise, the increased intra-abdominal pressure would interfere with venous return and would increase intraoperative bleeding. The Wilson frame fulfills this requirement and provides the fastest and least cumbersome means for positioning the patient. Certain other spine frames and tables (eg, Andrews table) allow the patient to be positioned in knee-to-chest position.

The resultant lumbar flexion facilitates access to the spinal canal and disc spaces by increasing the interlaminar and posterior interbody distances. However, if the patient is fused in this position, the natural lumbar lordosis is lost, resulting in "flat back" syndrome. Other spine tables, such as Jackson table, which allows the patient to be flipped from supine to prone position and vice versa, are useful for combined anterior-posterior procedures. Regardless of the position or frame used, all pressure points must be carefully padded to avoid compression neuropathy.

Intraoperative fluoroscopy is essential for safe and accurate instrumentation of the spine. A radiolucent frame should be used when applicable to allow for lateral and AP fluoroscopy. The C-arm should be draped in sterile fashion and positioned so that it can be readily moved in and out of imaging position. Alternatively, fluoroscopy-based stereotactic navigation can be used, wherein a computer with sophisticated stereotactic software permits virtual fluoroscopic navigation throughout the surgery. In this case, AP and lateral fluoroscopic images are taken after the spine is exposed and a bone-mounted stereotactic frame is attached. The C-arm is then removed and the procedure is performed based on computer-assisted navigation of the original fluoroscopic images.

Intraoperative neurophysiological monitoring for spine procedures consists of recording somatosensory evoked potentials (SSEP), motor evoked potentials (MEP), or electromyography (EMG) in order to detect and correct factors that lead to neurological compromise during the surgery. During pedicle screw placement, stimulation of the pedicle probe with a nerve stimulator at subthreshold currents would result in EMG activity in the lower extremities if the probe is in contact with the nerve root, prompting its repositioning.

Microdissection under an operating microscope allows safer decompression of neural elements, particularly where visualization is limited (eg, anterior cervical discectomy and osteophyte resection). The microscope can be moved out of position when the more delicate decompression phase of the procedure is completed before proceeding with fusion and instrumentation.

Anesthetic considerations during fusion surgery include maintenance of adequate blood pressure and fine-tuning of muscle relaxation during the surgery while maintaining the depth of anesthesia. Muscle relaxation facilitates the initial exposure of the spine, but must be avoided or reversed if intraoperative MEP or EMG is to be used. When muscle relaxation is not present, it is important to maintain deep anesthesia to prevent patient movement during critical parts of the procedure, such as decompression of the spinal cord. Adequate blood pressure must be maintained at all timed to avoid neural ischemia, which could exacerbate existing neural injury. In patients with labile blood pressure or cardiopulmonary risk factors, an arterial line may be inserted for continuous monitoring. If adequate peripheral venous access is not available, a central line is inserted. If the procedure is expected to last more than two hours, a bladder catheter is inserted.

Modern operating room setup for spine surgery with Modern operating room setup for spine surgery with fluoroscopy unit, neurophysiological monitoring equipment, operating microscope, and digital radiology monitors.
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Postoperative Details

Pain is controlled aggressively after fusion surgery with parenteral opiates for the first 12-36 hours, after which the patient is converted to oral analgesics. Muscle relaxants and benzodiazepine anxiolytics can help reduce the requirement for opiates. If significant postoperative pain is anticipated, intravenous patient-controlled analgesia (PCA) with a continuous basal rate is employed.

Prophylactic antibiotics, started before the surgery, are continued for 3 doses (24 h) after surgery. Further antibiotic administration after this point in absence of infection has not been shown to be of benefit and may lead to emergence of antibiotic-resistant pathogens.

If dexamethasone is used preoperatively and intraoperatively for neuroprotection, it is discontinued after the surgery as quickly as the patient’s neurological condition permits. Prolonged postoperative use of corticosteroids may increase the risks of wound infection and dehiscence.

Intravenous fluids are used until the patient can tolerate food and drink by mouth. In anterior thoracolumbar procedures, nasogastric drainage may be required if paralytic ileus occurs. Routine orders for anti-emetics, antacids, and stool softeners are written.

If closed wound drainage is employed, the drain is removed when its output diminishes (usually on postoperative day 1). In transthoracic procedures, the chest tube is removed when the lungs have fully re-expanded and pneumothorax has resolved. If significant blood loss has occurred during or after the surgery (through the drain or chest tube), hemoglobin levels are checked. If symptomatic anemia exists, blood transfusion is considered.

Wound hematoma is of particular significance after anterior cervical surgery. Small neck hematomas may cause dysphagia, odynophagia, hoarseness, or sore throat, and are treated conservatively. However, a large neck hematoma can result in upper airway compromise, which constitutes a life-threatening emergency and requires immediate surgical evacuation. Although rare, such hematomas develop within the first 24 hours after the surgery; hence, the general practice of keeping the patients in hospital overnight after such procedures. Neurological deterioration within the first 24-48 hours after surgery should raise suspicion of epidural hematoma, prompting immediate imaging studies or surgical re-exploration.

Early mobilization of the patient after fusion surgery not only expedites rehabilitation but also prevents certain complications such as DVT, atelectasis, and pneumonia. If fusion is performed without instrumentation, an external orthosis is employed until the fusion has matured. After instrumented fusion, an external orthosis may still be applied to supplement the internal instrumentation, depending on the type and extensiveness of the procedure and the risk of instrumentation failure.

Fever is not uncommon after fusion surgery. A low-grade fever on the first or second postoperative day is usually due to atelectasis and is treated with incentive spirometry and early patient mobilization. High-grade or protracted fever should be worked up to exclude pneumonia, urinary tract infection, wound infection, and bacteremia. The incision should be examined daily (by the healthcare staff during the hospitalization and by the patient and family after discharge). Occasionally, fever occurs in absence of any evidence of infection after an operation that required significant muscle retraction and manipulation, in which case it may be due to pyrogenic substances released as a result of muscle necrosis.

The patient is usually discharged from the hospital within 24-48 hours after an elective fusion operation. Debilitated and elderly patients and those with neurological or systemic injuries may require longer hospitalization.

Although initiation of exercise therapy is often delayed until more than 3 months after lumbar fusion, Abbott et al showed that rehabilitation can be conducted during the first 3 postoperative months. All patients in the study received a home program focusing on pain-contingent training of functional strength and endurance of back, abdominal, and leg muscles; stretching; and cardiovascular fitness. One group also received 3 outpatient sessions focusing on modifying maladaptive pain cognitions, behaviors, and motor control; the patients who also received psychomotor therapy demonstrated significantly improved functional disability, self-efficacy, outcome expectancy, and fear of movement/(re)injury. [16]

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Follow-up

Patient follow-up is geared toward assessment of functional recovery (pain and neurological function), radiographic assessment of fusion, and detection of delayed postoperative complications.

The first follow-up visit is scheduled about 7-10 days after surgery to assess the condition of the wound, remove staples and sutures, and address the patient’s questions and concerns. The second and third followup visits are usually scheduled at 6 weeks and 3 months after surgery, although considerable variation exists in practice patterns. The focus of these visits is to ensure that the wound has healed properly, the fusion has progressed well, the patient’s neurological function has improved as expected, the patient’s preoperative pain syndrome has resolved or diminished, the pain medications are tapered off, the brace is discontinued, and rehabilitation measures are instituted when appropriate.

If all has progressed well, additional follow-up can be performed by telephone, mailed questionnaires, or online, as dictated by specific practice patterns. Routine radiographic studies are performed at predefined intervals (eg, 6 wk, 6 mo, and 1 y postoperatively) until the fusion is deemed to be solid. Routine CT scan or MRI is not required after fusion surgery, unless there is concern for a specific problem that requires such imaging.

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Complications

Specific complications of fusion surgery include injury to nearby structures specific to the particular operation/approach (eg, recurrent laryngeal nerve palsy after anterior cervical surgery).

General complications of fusion surgery include the following:

  • Infection of soft tissues, epidural space, bone, disc space, fusion mass, or hardware, which may require removal of instrumentation and/or prolonged IV antibiotic treatment
  • Bleeding, which may require transfusion
  • Wound hematoma, which may require surgical evacuation, particularly if it causes airway compromise after anterior cervical surgery
  • Epidural hematoma, necessitating surgical evacuation
  • Nerve root or spinal cord injury
  • Cerebrospinal fluid leakage or pseudomeningocele formation, requiring repair
  • Vascular injury (eg, vertebral or carotid artery injury in the cervical spine, iliac vessel injury in the lumbar spine)
  • Organ injury (eg, esophagus, pharynx, rectum)
  • Nerve injury (eg, recurrent laryngeal nerve, hypoglossal nerve)
  • Pseudarthrosis, requiring redo fusion
  • Excessive subsidence, displacement, or breakage of a structural graft
  • Hardware failure (loosening, pullout, breakage), requiring reoperation
  • Discomfort associated with the hardware
  • Wound dehiscence

Complications associated with iliac crest bone graft harvest include the following:

  • Infection
  • Bleeding, hematoma formation, bruising
  • Pelvic fracture
  • Pain of musculoskeletal or neuralgic origin
  • Numbness around or related to the incision
  • Sacroiliac joint dysfunction and pain

Systemic complications of fusion surgery include but are not limited to the following:

  • Deep venous thrombosis, pulmonary embolism
  • Myocardial infarction, congestive heart failure
  • Atelectasis, pneumonia
  • Urinary tract infection
  • Peripheral nerve injury related to patient positioning
  • Blindness related to intraoperative retinal ischemia

A potential long-term complication of fusion in the cervical or lumbar spine is adjacent segment degeneration, also known as transition level syndrome. The intended loss of motion across the fused motion segment or segments increases the biomechanical stress on the adjacent motion segments. Over time, this may result in disc herniation, accelerated disc or facet degeneration, or spinal stenosis at the adjacent segments above or below the level of fusion. Although some instances of adjacent segment disease are undoubtedly due to the natural history of the underlying degenerative disease, and others are due to unintended injury to the adjacent segment elements (eg, facet joints) during the original fusion operation, the rest are thought to be caused by the biomechanical mechanism described above.

Transition level syndrome: C6-7 disc herniation de Transition level syndrome: C6-7 disc herniation developed 6 years after C4-5 and C5-6 anterior cervical discectomy and fusion.

Symptomatic adjacent segment disease is more likely to develop if the adjacent segment is already diseased, albeit asymptomatically, at the time of the original fusion operation. In order to avoid reoperation in this situation, it is common practice to fuse the adjacent degenerated motion segment at the same time as fusion of the symptomatic motion segment. If the degenerated adjacent segment is felt to be contributing to the patient’s pain syndrome, its fusion is further justified. By the same token, if a patient presents with significant multilevel degenerative disease, fusion should be avoided if at all possible, unless sufficient indication exists for fusion of all of the affected motion segments (eg, multilevel cervical spondylosis with myelopathy).

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Outcome and Prognosis

Outcome after fusion surgery is measured in terms of the 3 cardinal clinical manifestation of spinal instability: neurological function, pain, and disabling deformity. With overt instability (eg, trauma, tumor, infection), neurological function after surgery is directly related to preoperative neurological status and cannot be used as a measure of success of fusion surgery. For instance, after a thoracolumbar fracture-dislocation with cord laceration and paraplegia, the success of fusion surgery should not be measured in terms of recovery of neurological function, but in terms of addressing disabling deformity and pain.

In conditions with overt instability, deformity and pain outcome measures correlate closely with radiographic success of fusion. Since modern fusion and instrumentation techniques ensure radiographic success in most of these cases, the outcome of fusion surgery for treatment of overt instability is generally good, and the necessity of lumbar fusion for overt instability is not questioned.

The situation is entirely different in the case of covert instability as it applies to degenerative disease, where there is not a strong correlation between successful radiographic fusion and clinical improvement, and the former cannot be used as a surrogate marker for the latter. Consequently, there is now a great deal of interest in direct assessment of clinical outcome after fusion surgery for degenerative spine disease.

In a multicenter randomized controlled trial, the Swedish Lumbar Spine Study Group provided one of the rare instance of class I scientific evidence in spine literature. [17] Of the 294 patients with disabling back pain due to 1- or 2-level degenerative disease without spinal stenosis or spondylolisthesis (covert instability: dysfunctional motion segment), the lumbar fusion group did significantly better than the conservatively treated group in terms of pain relief, degree of disability, return to work status, and degree of satisfaction. In contrast to this study, a smaller Norwegian study of degenerative back pain [18] failed to show a statistically significant difference between lumbar fusion and a very aggressive regimen of physical and cognitive treatment (25 h of physical therapy per wk for 8 wk, followed by a comprehensive home exercise program, individual counseling, lessons, group therapy sessions, and peer group discussions).

Importantly, both groups experienced significant improvements over baseline with a trend toward greater improvement in the surgical group. This study has been criticized for its small number of patients and large confidence intervals in the data, suggesting that it lacked sufficient power to detect a statistical difference. [14] Furthermore, it is unclear whether large-scale implementation of such a vigorous physical and cognitive program is realistic in the everyday clinical setting.

Such discrepancies in the spine literature are the norm, not the exception, often dealing with studies of far less scientific rigor than those mentioned above. In an effort to produce evidence-based treatment recommendations, the American Association of Neurological Surgeons/Congress of Neurological Surgeons (AANS/CNS) Joint Section on Disorders of the Spine and Peripheral Nerves took up the monumental task of analyzing all of the available literature on lumbar fusion for degenerative disease. [14] The ensuing recommendations were ranked according to the strength of the supporting evidence as follows:

  • Standards - Highest level of clinical confidence; based on one or more well-designed comparative studies, or less well-designed randomized controlled trials (class I evidence)
  • Guidelines - Intermediate level of confidence; based on one or more well-designed comparative studies, or less well-designed randomized controlled studies (class II evidence)
  • Options - Lowest level of confidence; based on case series, comparative studies with historical controls, expert opinion, and flawed randomized controlled trials (class III evidence)

Most studies were believed to provide class III evidence; therefore, most recommendations were provided at the lowest options level. Some of the more salient recommendations are described below, along with their rankings:

  • Lumbar fusion is recommended as a treatment for carefully selected patients with disabling low back pain due to one- or two-level degenerative disease without stenosis or spondylolisthesis (standards).
  • Lumbar fusion is not recommended as routine treatment following primary or recurrent disc excision, unless there is evidence of preoperative lumbar deformity, instability, or chronic low back pain (options).
  • Posterolateral lumbar fusion is recommended for patients with lumbar stenosis and associated spondylolisthesis who require decompression (guidelines).
  • Posterolateral lumbar fusion is not recommended as a treatment option in patients with lumbar stenosis without preexisting spinal instability (spondylolisthesis) or likely iatrogenic instability (options).
  • In the context of single-level stand-alone ALIF or ALIF with posterior instrumentation, addition of posterolateral fusion is not recommended as it increases operating time and blood loss without influencing the likelihood of fusion or functional outcome (guidelines).
  • In the workup of discogenic pain, the following is recommended:
    • MR imaging should be used as the initial diagnostic test instead of discography.
    • MR imaging-documented disc spaces that appear normal should not be fused.
    • Lumbar discography should not be used as a stand-alone test for surgical decision-making.
    • If discography is performed, both a concordant pain response and morphological abnormalities be present prior to considering fusion (guidelines).
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Future and Controversies

A great deal of controversy remains regarding the application of fusion surgery in the treatment of degenerative spine disease without overt instability. In the future, these controversies will be addressed by a 2-pronged approach. First, rigorous randomized controlled trials are needed to better assess the efficacy of existing methods of fusion. Second, novel treatment strategies are needed to replace fusion surgery.

Disc arthroplasty and posterior dynamic stabilization devices are 2 strategies that are currently being investigated. Some brands of artificial disc for treatment of symptomatic lumbar degenerative disc disease have received FDA approval. Short-term studies reveal equivalent results for disc arthroplasty and lumbar fusion. [19]

Artificial lumbar disc. Image courtesy of Synthes, Artificial lumbar disc. Image courtesy of Synthes, Inc.

A recent prospective, randomized, controlled multicenter study designed to show the "noninferiority" of cervical total disc replacement (TDR) revealed that this technology was at least equivalent to anterior cervical discectomy and fusion with regard to outcome at 24 months. [20]

Although most primary outcome measures such as pain scores and neurological success were equivalent between both groups, the disc replacement group showed a lower requirement for analgesics and lower reoperation rate compared with the fusion group at 24 months. Although these results show promise for total disc replacement, it should be noted that the study was limited to patients with single level disc disease with radiculopathy and can not be generalized to patients with multilevel disc herniations, spondylosis, spondylolisthesis, and degenerative disc disease. In addition, long-term followup studies are needed to determine whether these benefits last, whether motion preservation with artificial discs persists over the long term, and whether the frequency of transition-level syndrome is decreased.

Posterior dynamic stabilization devices come in several varieties. The most promising of these are pedicle screw-based system, where the screws are linked by flexible members instead of rigid rods. The theoretical goal is to limit movement to a zone where neutral or near-neutral loading of spine occurs, or conversely prevent movement into a zone where abnormal loading occurs. Again, the few clinical trials that have been conducted have produced clinical outcomes comparable with fusion. [21]

The challenges facing artificial disks and posterior dynamic stabilization devices are two-fold. First, they have to improve upon lumbar fusion outcomes. Second, these mechanical devices have to continue to function indefinitely, as opposed to current spine implants, which are shielded from biomechanical stress once bony fusion is achieved.

In the long-term future, biological rather than mechanical treatment strategies directed at repairing and maintaining the degenerated spine elements are more likely to provide a satisfactory solution to the problem of degenerative spine disease.

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