Current Concepts in Patellar Instability

By Marc R. Safran, MD

Recent interest in patellar instability has focused on the role of the medial patellofemoral ligament (MPFL), a well-defined structure in more than 90% of cadaver knees. Biomechanical studies suggest it is a prime static stabilizer to prevent patellar dislocation, providing 41-80% of the restraint to lateral displacement, with the greatest contribution in the first 15° of knee flexion.1 The MPFL is in layer II of the medial knee soft tissues and runs from its femoral attachment just distal to the adductor tubercle and postero-superior to the medial epicondyle, to the upper two-thirds of the medial patellar margin. In some series, the MPFL attaches to the underside of the VMO instead of the patella. The MPFL is 5-6 cm in length, becoming narrower and thinner as it approaches its femoral attachment.

Until recently, the MPFL has been overlooked in the pathophysiology of patellar instability. It has been commonly thought that lateral patellar dislocation was associated with tears of the medial retinaculum and/or the VMO; however, recent research reveals these structures are not injured, and the MPFL is the essential lesion with patellar dislocation.2 The injury to the MPFL can occur anywhere along its course—midsubstance, off its femoral attachment, or from its patellar attachment—and can be injured in more than one location with a single patellar dislocation.

Many other factors are felt to play a role in recurrent patellar instability, including trochlear dysplasia, increased Q-angle, and excessive genu valgum. The Q-angle gives a static determination of one component of the vector of pull of the extensor mechanism and is best measured with the knee in 90° of flexion. Those with recurrent patellar dislocation and an increased Q-angle may be considered for an operation medializing the tibial tubercle to decrease this displacing vector of lateral pull. The Hauser procedure (simple medialization of the tibial tubercle) has an obligatory posteriorization of the tubercle due to the triangular shape of the tibia. This posteriorization increases the forces on the patellofemoral joint and usually results in arthritis. Alternatively, Fulkerson developed a tibial tubercle osteotomy that provides for anteromedialization.3 This oblique osteotomy goes from anteromedial to posterolateral so that the tubercle is slid anteriorly and medially (AMZ Tracker, Mitek Worldwide, Westwood, Mass). The osteotomy is usually 7-10 cm long to allow a large contact surface to reduce the risk of non-union. The osteotomy is internally fixed by 2 low profile screws to reduce postoperative symptoms related to their subcutaneous position.

The recurrence of acute patellar dislocations has been reported to be 15-44%. Maenpaa and Lehto reported a prospective randomized trial of treatment of patellar dislocation and found the recurrence rate was approximately 3 times greater in the group treated with early range of motion.4 A way to consistently predict who will have a recurrence is important, as is a way to reduce the likelihood of recurrence. In a prospective natural history study of 125 first-time patellar dislocators, Fithian showed that 17% had a recurrence of dislocation or subluxation within 2-5 years, while 9% had a dislocation of the contralateral patella (unpublished data). The predictors for recurrence included younger age at the time of initial dislocation, while the risk of dislocating the contralateral patella was increased in those with a family history of patellar problems and hip dysplasia.

A few authors in the 1990s recommended primary repair of the MPFL after initial dislocation for young, athletic individuals who some authors feel are at higher risk of recurrent dislocation. Several small, noncontrolled studies have shown MPFL repair to be successful in preventing recurrent patellar instability. However, 2 recent prospective, controlled studies were unable to find a reduction in recurrence of patellar instability for first-time patellar dislocation when comparing those treated surgically (including MPFL repair) and those treated nonoperatively.

At this point, surgery can only be recommended for acute, first-time patellar dislocations if there is an associated osteochondral fracture that requires removal or internal fixation; repairing the MPFL at its site of injury should also be considered at that time. Acute repair of an avulsion from the patella may be managed with suture anchors or bony tunnels, while repair of an avulsion from the femoral attachment is usually accomplished using a low profile screw with a spiked soft tissue washer or suture anchors. Midsubstance injuries may be repaired primarily; however, the strength of the repair is of question and many would recommend augmentation of the MPFL. It is critical when performing an MPFL repair or reconstruction not to overconstrain the patellofemoral joint thereby limiting motion.

For recurrent dislocations, addressing the cause of recurrence is critical. A lateral release does not address the cause of recurrence and is generally not recommended in the management of patellar instability. As the essential lesion in patellar instability, the MPFL generally warrants repair. Although at this time repair of the MPFL for the first-time dislocation is not indicated, exceptions may be repair of an MPFL tear when operating for an acute osteochondral lesion. However, more study is needed to further define who may benefit from early surgery. 

Dr. Safran, Co-Director, Sports Medicine, Associate Professor, UCSF Department of Orthopaedic Surgery, San Francisco, CA, is Associate Editor of Sports Medicine Reports.

References

1. Tuxoe JI, et al. Knee Surg Sports Traumatol Arthrosc. 2002;10:138-140.

2. Nomura E, et al. Knee. 2002;9:139-143.

3. Fulkerson JP, et al. Am J Sports Med. 1990;18: 490-496.

4. Maenpaa H, Lehto MU. Am J Sports Med. 1997;25: 213-217.