Overview
Patellofemoral instability occurs when the patella repeatedly moves out of its normal position, almost always toward the outside of the knee. Some patients are predisposed to instability because of underlying anatomy, including generalized ligamentous laxity, trochlear dysplasia, patella alta, or valgus alignment of the knee.
Other patients have relatively normal anatomy but develop instability after a traumatic injury. This can occur with a twisting or pivoting injury when the foot is planted and the knee turns inward, or from a direct blow to the inside of the patella. In some patients, both underlying anatomy and an injury contribute to the problem.
Initial Treatment
A first-time patellar dislocation can produce a significant knee effusion, which is often bloody. When the swelling is substantial, I will often aspirate the knee to remove the fluid. Most first-time dislocations are initially treated without surgery. Physical therapy focuses on restoring motion and strength and improving control of the knee. Many patients will never experience another dislocation. Patients with underlying ligamentous laxity, trochlear dysplasia, patella alta, abnormal alignment, or a significant injury to the medial patellofemoral ligament (MPFL) are at greater risk for recurrent instability.
When Is Surgery Considered?
Once the patella has dislocated repeatedly, particularly after more than a couple of episodes, surgical stabilization should be considered. An important part of this decision is determining why the patella is unstable. An isolated MPFL reconstruction can work extremely well in appropriately selected patients, but it may not be enough when substantial bony abnormalities, such as severe trochlear dysplasia or other significant alignment problems, are driving the instability. Those situations may require additional procedures. I also prefer to address recurrent instability before repeated dislocations produce progressive cartilage damage. Each dislocation can injure the articular cartilage on the back of the patella and within the patellofemoral joint. Significant cartilage injury can sometimes occur with even a single dislocation.
MPFL Reconstruction
When an isolated soft-tissue stabilization is appropriate, I perform an MPFL reconstruction using an allograft hamstring tendon. The MPFL functions essentially as a leash that helps prevent the patella from translating too far toward the outside of the knee. During reconstruction, the ligament is recreated from the medial side of the patella to its attachment on the medial distal femur. The graft travels from the patella to the femur and back to the patella in a V-shaped configuration. The reconstruction is secured with dissolvable anchors in the patella and a screw in the femur, recreating this restraint and stabilizing the patella.
Patellofemoral Stabilization
I begin with knee arthroscopy to evaluate the joint, assess the articular cartilage, and identify and remove any loose bodies when present. The reconstruction is then performed through two relatively small open incisions: one along the medial side of the patella and another along the medial distal femur. The allograft tendon is positioned and secured to reconstruct the MPFL and restore stability to the patella.
Recovery
Patients initially use a knee brace locked in extension while walking. Full weight-bearing is allowed, with crutches as needed for comfort and stability. Range of motion begins early. Physical therapy begins within the first two weeks and focuses initially on range of motion and quadriceps activation. As quadriceps control returns, the brace is progressively unlocked and is typically discontinued within the first 4-6 weeks. Physical therapy then progresses toward strengthening and increasingly functional activities. Most patients can progress back toward sports over approximately 3-4 months, depending on strength, function, and the demands of their particular sport.
Expected Results
MPFL reconstruction has a high success rate, with success primarily measured by restoration of patellar stability and prevention of recurrent dislocation. The procedure cannot reverse cartilage damage that occurred before surgery. Stabilizing the patella helps prevent the additional damage caused by repeated dislocations, but existing patellofemoral cartilage damage can still progress over time and may continue to produce symptoms.
Risks and Complications
Complications are uncommon but can include infection, recurrent instability, stiffness, and DVT or pulmonary embolism. For blood-clot prevention, I typically use baby aspirin twice daily for three weeks. Patients with a previous history of DVT or other significant risk factors may require stronger anticoagulation, such as Xarelto.