Unicompartmental Knee Arthroplasty

Unicompartmental knee arthroplasty (UKA) has had varying degrees of acceptance since its introduction in the 1950s.[1, 2, 3, 4, 5, 6]  Frequent and early failures were cited in initial studies. Consequently, by the late 1980s, the popularity of UKA had waned. However, after recognizing the need to avoid overcorrection of the mechanical axis, surgeons ultimately were able to reduce the risk that pain in the "normal" compartment would cause early failure.

With the publication of several studies in the late 1980s and 1990s reporting 10-year survivorship rates exceeding 93%, enthusiasm for UKA again increased, survivorship being comparable to that for total knee arthroplasty (TKA). Although in 1994, the Swedish Joint Replacement Registry reported a high percentage of poor results for UKA, these outcomes mainly reflected the fact that the surgery was performed on patients with chronic inflammatory arthritis.[7]

Interest in UKA was also stimulated by the introduction of the mobile-bearing form of the procedure. Goodfellow and O'Connor, along with others, published excellent long-term survivorship rates associated with this technique.[8]  Their rationale for the procedure's success was clearly stated: a mobile bearing (also called a meniscal bearing) provides the unique combination of complete congruency of the articular surface (to minimize wear and creep) and total freedom of movement (to accommodate the preferred motion pattern of the retained natural compartment).  However, mobile-bearing procedures have higher failure rates due to bearing dislocations.[9]

Subsequently, there was a heightened interest in UKA, particularly because of the introduction of the minimally invasive parapatellar technique. This form of the procedure potentially can reduce morbidity, complications, and length of hospital stay. Many view UKA as preferable to high tibial osteotomy (HTO) in relatively young patients with medial compartment arthritis.

Advances in the use of robotics have been reported to reduce technical problems related to bone resection and implant positioning and lower revision rates.[10]

Theoretical advantages of the procedure include preservation of uninvolved tissue and bone, reduced operating time, better range of motion (ROM), improved gait, and increased patient satisfaction. Failure and revision are more common during the early (< 5 years) postoperative period and arehave been associated with incorrect indications and/or technical errors during surgery.[11] With appropriate patient selection, careful surgical technique, and proper implant design, UKA can now be viewed as a procedure with reliable medium- to long-term success.[12, 13]

Because UKA is typically performed for medial osteoarthritis, this article focuses primarily on that condition.

For patient education information, see the Arthritis Health Center, as well as Knee Pain Health Center.

Careful patient selection is critical for UKA if reliable results are to be achieved.[14] The arthritis should be predominantly confined to a single compartment. (Medial compartment osteoarthritis is usually on the anteromedial aspect of the tibial plateau, and lateral compartment osteoarthritis is typically on the femoral side.) No significant degenerative changes in the other (medial, lateral, or patellofemoral) compartments should be present, and both cruciate ligaments should be intact.

Absence of the anterior cruciate ligament (ACL) is a contraindication; this ligament makes the combined rolling and sliding at the meniscal femoral and meniscal tibial interfaces possible, which may yield near-normal joint kinematics and mechanics.

The operation is also indicated in patients with osteonecrosis of the femoral condyle. Not all of the unicompartmental replacements are suitable for the lateral side, because the ligaments of the lateral compartment are more elastic than those of the medial side.

Malalignment of the limb should be passively correctable to neutral and not beyond. This usually is possible in patients with a varus deformity less than 15° or a valgus deformity less than 20°.

The deformity of the knee should be only mild; therefore, a flexion contracture should be less than 15°. UKA with excision of osteophytes in the notch cannot correct moderately severe flexion contractures.

Ideally, it should be possible to flex the knee to 110°. This is important for the preparation of the femoral condyle during the operation.

Contraindications for UKA include the following:

• Inflammatory arthropathy
• Previous HTO with overcorrection
• Sepsis
• Cruciate ligament lesion
• Medial or lateral subluxation (usually associated with a torn ACL)
• Tibial or femoral shaft deformity
• Flexion contracture greater than 15°
• Varus deformity greater than 15° (medial UKA)
• Valgus deformity greater than 20° (lateral UKA)
• Flexion less than 110°

UKA is controversial in the presence of patellofemoral joint arthritis, youth and high activity level, obesity, chondrocalcinosis, and crystalline arthropathy.

Patellofemoral joint arthritis

Progression of osteoarthritis in the patellofemoral joint after UKA is rare, according to some studies. In the Swedish Registry, no UKAs required revision for patellofemoral problems.[7]

Murray et al reported that residual postoperative pain was independent of the state of the patellofemoral joint, and no knee surgery was revised because of patellofemoral problems.[15, 16, 17]  UKA improves the mechanical axis and patellar tracking and allows more normal kinematics and rapid quadriceps rehabilitation. For these reasons, osteoarthritis of the patellofemoral joint may not be considered an absolute contraindication.

However, other investigators and surgeons have reached the opposite conclusion; thus, many consider patellofemoral disease to be an absolute contraindication for UKA. For more information, see Patellofemoral Joint Arthritis.

Youth and high activity level

Because younger patients tend to be more active than older individuals, they would also seem likely to have a higher revision rate for UKA than older individuals would. However, although some studies have found such differences in revision rates, others have not. Minimally invasive techniques may reduce morbidity, complications, and length of hospital stay, which may favor the use of UKA in younger patients.[18]

Obesity

Technical difficulties and increased risk of complications are associated with obesity. However, obesity is not considered a contraindication particularly for the UKA mobile-bearing design. This is because a correlation has not been found between obesity and wear.

Chondrocalcinosis

It may be necessary to differentiate between patients with a generalized chondrocalcinosis with synovitis and, effectively, an inflammatory condition in the knee (which is a contraindication for UKA) and patients with calcification in the meniscus without generalized evidence of inflammation.

Crystalline arthropathy

Many patients with osteoarthritis have calcium pyrophosphate crystal deposition in their articular surfaces and suffer from crystalline inflammatory arthropathy. Although Brumby et al indicated that crystalline arthropathy is a contraindication for unicompartmental replacement,[19] Murray et al disagreed.[15]

Complication prevention

A history of trauma should alert the surgeon to the possibility of a remote fracture or articular or ligamentous damage. A history of pain in multiple joints should draw attention to the possibility of inflammatory arthritis.

The physical examination is crucial for appropriate patient selection for unicompartmental knee arthroplasty (see Indications and Contraindications). The integrity of the ACL, the mechanical alignment and range of motion of the knee, and collateral stability must be assessed.

When nonoperative and, possibly, arthroscopic procedures fail, the surgeon may consider HTO, UKA, or TKA. UKA appears to result in better function, greater pain relief, less morbidity, and higher patient satisfaction than do HTO and TKA.

The long-term survival rate for UKA[20]  is higher than that for HTO and comparable to that for TKA. With strict indications, newer prostheses, and attention to surgical technique, UKA has become a valuable treatment for unicompartmental knee arthritis.

In a prospective study, Berger et al found that in a 3-month period following knee arthroplasty, no patients in the investigation who had undergone UKA required hospital readmission, compared with 9% of patients who received TKA.[21] In the study, the authors looked at 111 patients who had undergone primary knee arthroplasty, of whom 25 underwent UKA and 86 underwent TKA. Of the 111 patients, 104 (24 with UKA and 80 with TKA) met discharge criteria and were discharged directly to home.

Saenz et al evaluated the clinical and radiographic outcomes of the EIUS unicompartmental prosthesis and found it to be associated with higher revision rates than were metal-backed implants.[22] The implant survival rate was 89%, with 16 knees either revised or scheduled for revision. The reasons for revision included aseptic loosening of the tibial component, progressive symptomatic patellofemoral disease, and tibial component subsidence.

Unicompartmental knee arthroplasty vs high tibial osteotomy

Resurfacing methods are gaining popularity. Results comparing HTO with UKA favor the latter.

Broughton et al demonstrated good results in 76% of patients in a replacement group and in 43% of patients in an osteotomy group.[23]  ROM, speed of rehabilitation, and perioperative morbidity were significantly better for UKA, and no signs of late deterioration were present. Weale et al, after a 12- to 17-year follow-up period, also reported better function and longer survival in the unicompartmental group.[24]

Other publications have similarly shown more favorable results with arthroplasty.[25, 26]  The functional advantages of UKA over HTO have also been demonstrated by using gait analysis, with patients displaying a more normal gait and better stair-climbing ability after UKA than they did after HTO.

If a revision to TKA becomes necessary, the results are now believed to be generally better if the revision occurs after a failed UKA than they are following a failed HTO. (Previous HTO is a contraindication for UKA.)

Unicompartmental knee arthroplasty vs total knee arthroplasty

In the late 1980s, UKA waned in popularity, largely because of problems with patient selection, operative technique, and polyethylene wear. Later, as understanding of the procedure and the associated prostheses improved, the long-term results of UKA became comparable to those of TKA.[27, 28]

Functional outcome with UKA is superior to that with TKA, with the former providing better ROM and ambulatory function. Laurencin et al found that UKA also results in less pain, more stability, and better stair-climbing ability than does TKA.[29]  In addition, the cost of UKA is about 57% of the cost of TKA.

Dalury et al found little or no difference in outcome between patients who received TKA and those who received UKA, except for slightly better ROM with UKA than with TKA (123º ± 9º and 119.8º ± 7º, respectively). Of the 23 patients in the study, 11 expressed no preference for either knee and 12 preferred the unicompartmental knee; no patient preferred the total knee.[27]

Advances in instrumentation for unicompartmental knee arthroplasty (UKA) have increased the surgeon’s ability to implant the components in proper alignment and should theoretically increase the longevity of the arthroplasty. Instrumentation is crucial for proper alignment and orientation with some types of UKA systems, such as the Oxford Phase III prosthetic.[30, 31, 32]  Other prosthetic systems, such as the Repicci II, rely more on anatomic landmarks to ensure proper component placement.

The patient is placed in a supine position, with the draped leg set on a thigh support or in a legholder. In positioning the patient, it is necessary to ensure that the knee is free to flex to at least 110-120°.

A sandbag or other bump is affixed to the table to help maintain flexion of the knee. A strap placed around the thigh and leg may serve the same function. The leg is draped free, and it is helpful to place a mark or ball of tape over the anterior superior iliac spine or the femoral head.

Mobilization of the knee and patient can start on postoperative day 1. Recovery of knee function is usually rapid, with considerably less pain than that which occurs with a total knee arthroplasty (TKA). Early mobilization is encouraged, and passive assistance by the physical therapist is advised. The patient can frequently be discharged after 2-3 days. In some centers, the procedure is now performed in day care.

Mechanical prophylaxis against deep vein thrombosis is recommended for the duration of the hospitalization. Postdischarge prophylaxis is at the discretion of the surgeon.

The concept behind unicompartmental knee arthroplasty (UKA) is the replacement of only the damaged part of the knee and the preservation of as much normal tissue and bone as possible, to allow the restoration of normal kinematics.

In the past, UKA was performed through a standard anterior approach with dislocation of the patella, as was used for total knee arthroplasty (TKA). However, the minimally invasive form of unicompartmental arthroplasty has gained popularity. The smaller incision and arthrotomy of this technique cause less damage to the extensor mechanism, because the patella is not dislocated and the suprapatellar synovial pouch remains intact. (See the images below.)

Follow-up research demonstrated decreased morbidity, faster rehabilitation, and reduced length of hospital stay among patients who undergo the less invasive medial UKA. With appropriate pain control, the surgery can safely be done as an outpatient procedure, providing substantial cost savings, and patients have reported feeling less pain postoperatively than they did preoperatively. Nevertheless, surgeons utilizing the minimally invasive approach must contend with a restricted visual field, which makes UKA a demanding procedure.

The minimally invasive technique can also be used for lateral UKA, although some authors prefer the standard total knee approach. With the less invasive technique, the exposure is limited, and thus, positioning the prosthetic components is more difficult.

Optimal postoperative alignment

The optimal tibiofemoral alignment after UKA has yet to be determined. Extremes of overcorrection and undercorrection are undesirable. Overcorrection may result in medial or lateral subluxation and increased loading of the unreplaced compartment (medial or lateral). Undercorrection causes varus or valgus alignment of the leg and can potentially overload the implant.[33, 34]  Most surgeons advocate undercorrection of the mechanical axis by 2-3° to avoid overloading the normal compartment.

In a restrospective study of two matched groups of patients, Herry et al concluded that restitution of joint-line height in resurfacing UKA can be improved with robotic-assisted surgery.[35]

In general, medial and lateral soft-tissue releases are contraindicated, because knees requiring these have too much preoperative deformity for UKA. Angular correction is usually obtained with removal of peripheral osteophytes that tent the capsule and the collateral ligaments.

Computer-assisted surgery can improve the postoperative alignment of medial UKA over that of conventional techniques. Real-time information about the leg axis at each step of the operation should diminish the danger of overcorrection or undercorrection.

The navigation system may also be helpful in achieving a more precise component orientation. However, all investigations into the navigation system's use have consisted either of bone-saw or of cadaver studies. As a result of the limited exposure and restricted visual field in vivo associated with the minimally invasive procedure, it is difficult to apply the studies' results to the optimal positioning of components. Further development of the navigation technique will determine whether it is more accurate than the conventional method.[36, 37, 38]

Converting a UKA to a TKA (see the images below) is somewhat more difficult than performing a primary TKA. The results are acceptable but arguably not as good as they are with primary TKA. Despite the benefit of the conservative bone cuts used for UKA, stemmed components and augments may be needed for bone loss associated with component removal or osteolysis.

Bone defects, if present, usually can be treated with a local autograft. The cumulative revision rate at 10 years is more than three times higher for patients in whom a failed UKA was revised to a further UKA than for those in whom it was revised to a TKA.[26]

Early

Infection, superficial and deep, is possible. With lateral UKA, palsy of the common peroneal nerve may occur, though this is more common with TKA in patients with severe flexion and valgus deformity.

A tibial plateau fracture (see the image below) may result if too much stress is applied with cementation of the tibial component. The vertical tibial cut may act as a stress riser. In general, forceful impaction of the component is minimal, and this complication should be unusual.

Ligamentous instability is rarely a problem in properly selected patients with an intact anterior cruciate ligament (ACL). Soft-tissue releases should be minimal. Knee joint stiffness may occur.

Late

Late infection may occur, usually from hematogenous seeding. In addition, prosthesis failure or loosening is possible.

The bearing of a mobile-bearing unicompartmental knee prosthesis may become dislocated, especially with lateral UKA (see the images below). A high proxima, varus tibial angle, along with damage to or overdistraction of the lateral soft-tissue structures, is thought to contribute to this problem. Polyethylene wear can occur, but it may be less with a mobile-bearing design.

A systematic review by Ko et al found that the overall incidence of complications for mobile-bearing designs was not significantly different from that for fixed-bearing designs; however, mobile-bearing designs were more susceptible to reoperations in scenarios involving aseptic loosening, progression of arthritis, or implant dislocation.[14]

Undercorrection or overcorrection of the deformity and malpositioning of the components may cause late complications. In the case of overcorrection, excessive load on the opposite compartment might accelerate degenerative changes. Undercorrection places excessive load on the prosthesis, and loosening and failure may result. Improper placement of the components can cause subluxation of the tibia on the femur or impingement of the patella on the femoral component.

Most patients develop a radiolucent line in the tibial bone-cement interface. If these lines are less than 1 mm, they usually do not progress. The appearance of a similar line under the femoral component is expected but is not easily demonstrated because of the nonplanar form of the femoral bone-cement interface.