Patella Fractures 

In the 1800s, treatment of patella fractures was quite controversial. Owing to lack of adequate fixation techniques, surgical experience, and imaging, treatment most often was extension splinting. This led to poor results because of intra-articular incongruity, nonunion, and poor motion. Initial attempts at operative stabilization of these fractures yielded rates of satisfactory repair of less than 50%. Treatment methods then progressed to excision of fractured patellar fragments or of the entire patella. Despite encouraging early results, long-term follow-up revealed degenerative changes in the femoral condyles and decreased quadriceps power.^{[1, 2, 3, 4, 5, 6, 7]}

With the introduction of the AO group (Arbeitsgemeinschaft für Osteosynthese or Association for the Study of Osteosynthesis) in the 1950s, a new technique for fixation of these fractures was developed. The concept of the anterior tension band technique was introduced. This allowed for stable fixation, early motion, and improved rates of bony union.

Patella fractures become problematic if the extensor mechanism of the knee is nonfunctional, articular congruity is lost, or stiffness of the knee joint ensues. To avoid these problems, the surgeon must achieve anatomic restoration of the joint and must allow early motion. See radiographs below.

Recent studies

Rabalais et al compared 2 tension-band techniques, with stainless-steel wire and ultra-high-molecular-weight polyethylene cable, in transverse patella fractures in 8 cadaver knees. Fixation consisted of figure-of-8 and parallel wire configurations along with Kirschner wires and polyethylene cable. The parallel wire configuration showed better results than the figure-of-8 construct in monotonic and cyclic loading models, and the ultra-high-molecular-weight polyethylene cable and the 18-gauge steel wire performed similarly.^[8]

El-Sayed and Ragab reported on arthroscopic-assisted reduction and fixation of displaced transverse fractures of the patella in 14 patients with displaced transverse fractures of the patella. Patients were treated by arthroscopic-assisted closed reduction of the fracture with percutaneous screw fixation. Patella fractures were found to unite in a mean of 7 weeks. All 14 patients regained full extension of the knee, and full range of motion was achieved in 12 patients, with 2 patients experiencing a 10º loss of flexion.^[9]

Wright et al performed materials testing on No. 5 FiberWire suture and 18-gauge stainless-steel wire tension by using them both for band fixation on a transverse patellar fracture model and performing 3-point bending tests. The investigators found that FiberWire maintained its initial stiffness until failure and that during 3-point-bend testing, double-strand FiberWire had a higher failure load than stainless-steel wire when the suture was tied and locked under tension created by a modified Wagoner hitch.^[10]

Dargel et al compared interfragmentary compression with tension band wiring. They concluded that fragment fixation is an acceptable technique for treating osteosynthesis of a transverse patella fracture.^[11]

Muzaffar et al successfully treated minimally displaced two-part patellar fractures with percutaneous K-wire fixation and compression that is subsequently augmented with a cast.^[12]

Frequency

Patella fractures account for approximately 1% of all skeletal injuries.

The subcutaneous location of the patella makes it prone to injury. Fractures occur as a result of a compressive force such as a direct blow, a sudden tensile force as occurs with hyperflexion of the knee, or from a combination of these. A variety of fracture patterns result, depending on the mechanism of injury. The most common patterns are often described as stellate or transverse. Less common patterns include vertical, marginal, osteochondral, or sleeve fractures. Sleeve fractures are seen exclusively in the pediatric population.^[13] On radiographs, sleeve fractures are represented by a small bony avulsion fracture. However, they are actually larger than they appear on radiographs because they are surrounded by a significant portion of articular cartilage.

A direct blow to the patella most often results in a stellate fracture pattern. The compressive forces applied to the patella result in a comminuted pattern. The energy of the blow is absorbed by the fracture and may cause damage to the articular cartilage of both the patella and the femoral condyles. Free osteochondral lesions, therefore, must be excluded. Approximately 65% of these fractures do not involve the extensor retinaculum. If the extensor mechanism has not been disrupted and if intra-articular step-off is less than 2 mm, the fracture may be treated with a nonoperative modality.

Another mechanism of injury to the patella is a tensile force, as is sustained with hyperflexion of the knee with an eccentric contraction of the quadriceps. Approximately 35% of these are nondisplaced fractures with an intact retinaculum. This type of fracture, with less than 2 mm of intra-articular step-off, can be treated with a nonoperative modality.

A combination of these 2 mechanisms can lead to a variety of other fracture patterns. A displaced transverse fracture can have comminution if a blow to the knee occurs after the tensile force. For instance, a hyperflexion moment to the knee resulting in a transverse fracture pattern can be followed by a fall onto the knee, which causes comminution.

The patient usually presents with pain in the affected knee. The history reveals a direct blow to the knee, a fall, or a combination of both. Overlying abrasions, ecchymosis over the anterior aspect of the knee, or both may be present. Any lacerations must be assumed to communicate with the joint until disproved by the saline load test. Because the retinaculum may have a large tear, it may be necessary to inject a significant amount of saline (up to 100 mL) to exclude an open joint. An accompanying intra-articular effusion may be present, which, if aspirated, will reveal fat globules. If the fracture is displaced, a defect is palpable at the fracture site.

The extensor mechanism must always be evaluated. As a result of the pain associated with the injury and hemarthrosis, the patient may be unable to perform a straight leg raise. Aspiration of the hemarthrosis under sterile conditions and the instillation of lidocaine may relieve the pain enough to perform a reliable examination. Disruption of the extensor mechanism results in the inability to extend the knee against gravity and usually implies that a tear is present in the medial and lateral quadriceps expansion.

Indications for operative treatment include disruption of the extensor mechanism, articular incongruity with more than 2 mm of step-off, or more than 3 mm of separation between primary fracture fragments.

The patella is the largest sesamoid bone in the body. It is contained within the extensor mechanism, consisting of the quadriceps tendon proximally and the patellar ligament distally. At this location, it serves to increase the extensor moment by nearly 30%. The patella is covered at its proximal aspect by thick cartilage. Owing to its relatively small size, the patellofemoral joint is exposed to the highest contact stress of any weightbearing joint. Therefore, any aberration in its anatomy or alignment may lead to symptomatic degenerative changes.

Relative contraindications to closed treatment of patella fractures include open fractures and intra-articular displacement with disruption of the extensor mechanism.^{[14, 15, 16]} Contraindications to operative repair of patella fractures include a preexisting lack of active extensor function, septic arthritis, and fixed flexion contractures of the knee.