INTRODUCTION TO PATELLAR INSTABILITY AND DISLOCATION

Patellar dislocation is a debilitating orthopedic condition that predominantly affects young, active individuals. The vast majority of these dislocations occur in a lateral direction, driven by a combination of traumatic forces and underlying anatomical predispositions such as trochlear dysplasia, patella alta, or an excessive tibial tubercle-trochlear groove (TT-TG) distance.

Historically, nonoperative management was the gold standard for first-time dislocators. However, contemporary orthopedic literature demonstrates a high rate of recurrent instability, cartilage damage, and progressive patellofemoral osteoarthritis in patients with disrupted medial-sided restraints. Consequently, open reduction and anatomical repair—or reconstruction—of the medial patellofemoral ligament (MPFL) and the vastus medialis obliquus (VMO) complex have become paramount in the surgical armamentarium of the orthopedic surgeon.

This masterclass delineates the precise surgical techniques, biomechanical considerations, and postoperative rehabilitation protocols required to successfully manage patellar dislocations, ensuring the restoration of native patellofemoral kinematics.

SURGICAL ANATOMY AND BIOMECHANICS

A profound understanding of the medial soft-tissue restraints is non-negotiable for successful surgical intervention. The medial patellofemoral complex is the primary restraint to lateral patellar translation, contributing approximately 50% to 60% of the total restraining force from 0 to 30 degrees of knee flexion.

The Vastus Medialis Obliquus (VMO)

The VMO is a dynamic stabilizer of the patella. Surgeons must pay careful attention to that portion of the vastus medialis that originates in the region of the femoral adductor tubercle.

Surgical Warning: If the VMO origin at the adductor tubercle has been disrupted and has retracted proximally, the angle of insertion of the vastus medialis muscle fibers into the superomedial pole of the patella is significantly altered. These fibers are vital to the prevention of recurrent lateral dislocation of the patella. Failure to restore this precise vector will result in persistent lateral tracking and surgical failure.

The Medial Patellofemoral Ligament (MPFL)

The MPFL is a distinct condensation of capsular fibers located in layer 2 of the medial knee.
* Femoral Origin: Located in a saddle-shaped depression between the adductor tubercle and the medial epicondyle (often radiographically identified via Schöttle’s point).
* Patellar Insertion: Attaches to the superomedial two-thirds of the patella, blending seamlessly with the undersurface of the VMO.
* Biomechanics: The MPFL is most taut in full extension and early flexion (0° to 30°). As the knee flexes beyond 30°, the patella engages the bony architecture of the trochlea, and the MPFL becomes lax.

INDICATIONS FOR SURGERY

Surgical intervention is tailored to the chronicity of the dislocation, the patient's anatomical risk factors, and the presence of concomitant intra-articular pathology.

• Acute Primary Repair: Indicated for first-time dislocators with a clearly identifiable, avulsed medial restraint (e.g., a bony avulsion from the patella or femur), or those with concomitant osteochondral fractures requiring excision or fixation.
• MPFL Reconstruction: The gold standard for recurrent patellar instability, patients with generalized ligamentous laxity, or cases where the native medial tissues are too attenuated for a robust primary repair.
• Concomitant Procedures: A lateral release may be performed if indicated (e.g., severe lateral retinacular tightness with a negative passive patellar tilt). However, routine lateral release is discouraged as it may exacerbate medial instability if not carefully considered.

PREOPERATIVE PLANNING AND POSITIONING

Imaging

Standard radiographic evaluation must include weight-bearing anteroposterior (AP), lateral, and axial (Merchant or Sunrise) views. The lateral radiograph is scrutinized for patella alta (Caton-Deschamps index) and trochlear dysplasia (crossing sign, supratrochlear spur). Magnetic Resonance Imaging (MRI) is mandatory to assess the location of the MPFL tear (femoral, mid-substance, or patellar), evaluate the VMO, and identify osteochondral lesions. Computed Tomography (CT) is utilized to measure the TT-TG distance; a distance greater than 20 mm may necessitate a concomitant tibial tubercle osteotomy.

Patient Positioning

1. The patient is placed supine on a standard radiolucent operating table.
2. A high-thigh tourniquet is applied.
3. The operative extremity is prepped and draped free to allow full, unhindered range of motion (ROM) from 0 to 120 degrees of flexion.
4. A lateral post or sandbag may be utilized to maintain the leg in a neutral position.
5. Fluoroscopy must be available and positioned on the contralateral side of the table.

SURGICAL TECHNIQUE: OPEN REDUCTION AND PRIMARY REPAIR

Primary repair is highly effective when the pathology involves a discrete avulsion of the MPFL/VMO complex, particularly in acute settings.

Step 1: Approach and Exposure

• Make a longitudinal or gently curved incision over the medial aspect of the knee, centered over the palpable defect or the medial border of the patella.
• Dissect through the subcutaneous tissues to expose the extensor retinaculum.
• Carefully evacuate the hemarthrosis. Inspect the joint for osteochondral fragments. If an osteochondral fracture of the lateral femoral condyle or medial patellar facet is present, it must be addressed via fixation (using headless compression screws or bioabsorbable pins) or excision.

Step 2: Identification of Pathology

• Identify the torn edges of the MPFL and the VMO.
• Assess the femoral origin near the adductor tubercle. If the VMO origin is disrupted and retracted proximally, it must be mobilized.
• Assess the patellar insertion. Mid-substance tears are less amenable to primary repair and may require augmentation or reconstruction.

Step 3: Anatomical Repair

• Patellar Avulsions: Debride the medial edge of the patella to bleeding bone. Place two or three double-loaded suture anchors into the superomedial border of the patella. Pass the sutures through the avulsed edge of the MPFL and VMO using a horizontal mattress or Krackow configuration.
• Femoral Avulsions: Identify the anatomical footprint distal to the adductor tubercle. Prepare the bone bed and utilize a suture anchor to reattach the ligamentous complex.
• VMO Advancement: Ensure the VMO is advanced distally and laterally to restore its native insertion angle. This step is critical to re-establish the dynamic medial vector required to prevent recurrent lateral dislocation.

Step 4: Lateral Release (If Indicated)

• Assess patellar tracking and lateral tension.
• A lateral release may be performed if indicated by excessive lateral tightness that prevents centralization of the patella without undue tension on the medial repair.
• Perform the release from the inside out, carefully preserving the superior lateral geniculate artery to prevent patellar avascular necrosis.

Step 5: Closure

• Cycle the knee through a full range of motion to assess the integrity of the repair and ensure the patella tracks centrally within the trochlear groove.
• Close the wound in layers. Repair the retinaculum meticulously.
• Close the subcutaneous tissue and skin.
• Apply a sterile dressing and a knee immobilizer.

SURGICAL TECHNIQUE 60-2: MPFL RECONSTRUCTION

For recurrent instability or attenuated tissues, an anatomical MPFL reconstruction using an autograft or allograft is the procedure of choice.

Step 1: Graft Preparation

• Prepare a semitendinosus autologous graft or utilize a suitable allograft (e.g., semitendinosus or gracilis).
• If harvesting an autograft, make a small oblique incision over the pes anserinus, identify the semitendinosus tendon, and harvest it using an open-ended tendon stripper.
• Whipstitch both ends of the graft using high-strength, non-absorbable sutures. Size the graft to determine the appropriate tunnel diameters.

Step 2: Incision and Retinacular Exposure

• Center the skin incision between the medial edge of the patella and the adductor tubercle. Alternatively, two smaller incisions can be used to minimize morbidity.
• Identify the extensor retinaculum.
• Make a small incision at the medial edge of the patella (exposing the superomedial two-thirds) and another small incision just distal to the adductor tubercle to expose the femoral footprint.

Step 3: Graft Passage

• Create a soft-tissue tunnel in layer 2 of the medial knee.
• Using a large hemostat or a specialized tendon passer, pass the graft through the tunnel between the capsule and the extensor retinaculum.
• Ensure the graft remains extra-articular and does not tether the overlying skin or underlying capsule.

Step 4: Femoral Fixation and Isometry Testing

• Identify the anatomical femoral footprint (Schöttle’s point) using fluoroscopy: anterior to the posterior cortical line, distal to the posterior intersecting line, and proximal to the level of the posterior medial condyle.
• Drill a guide pin across the femur, directed slightly anteriorly and proximally to avoid the intercondylar notch. Over-drill with a reamer matching the graft size.
• Secure the graft to the femur using a high-strength suture, an interference screw, or a suture anchor.
• Cycling the Knee: Before final patellar fixation, pass the free ends of the graft over the patella. Apply tension to the graft and cycle the knee from 0 to 90 degrees of flexion to ensure correct isometry. The graft should be taut in full extension and become slightly lax as the knee flexes beyond 30 degrees.

Clinical Pearl: Over-tensioning the MPFL graft is the most common cause of surgical failure, leading to medial patellofemoral cartilage overload, severe pain, and arthrofibrosis. The graft should act as a check-rein, not a tight medial tether. Tension the graft at 30 degrees of knee flexion, ensuring the lateral patellar facet is flush with the lateral trochlea.

Step 5: Patellar Fixation

• Create a bony trough or drill two transverse tunnels in the superomedial half of the patella.
• Attach the graft to the patella with an interference screw, suture anchors, or by passing the graft through the transverse tunnels and suturing it to itself.
• Ensure the fixation is robust and the patella is centralized.

Step 6: Retinacular Repair and Closure

• Repair the medial retinaculum over the graft with figure-of-eight sutures to restore the local anatomy and provide additional soft-tissue stabilization.
• Close the wound in layers.
• Apply a controlled-motion knee brace locked in extension.

POSTOPERATIVE CARE AND REHABILITATION

Rehabilitation is as critical as the surgical execution. The protocol varies slightly depending on whether a primary repair or a reconstruction was performed, but the underlying principles of protecting the healing tissue while preventing arthrofibrosis remain constant.

Protocol Following Primary Repair

• Immobilization: The limb is immobilized in a knee immobilizer for 10 to 14 days to protect the acute repair.
• Early Motion: Early range of motion is begun to prevent arthrofibrosis and to promote the formation of strong collagen along the lines of stress. This is typically initiated under the strict guidance of a physical therapist.
• Bracing: A Palumbo-type brace (a dynamic patellar stabilization brace) is added at 2 weeks to provide ongoing medial support during functional activities.
• Weight Bearing: Walking on crutches with weight bearing to tolerance is begun during the first week. The crutches are discontinued when full weight bearing can be tolerated, control of the limb is regained, and limping is no longer a problem.
• Strengthening: Appropriate rehabilitation of the musculature of the extremity is essential. Stiff-legged resistance exercises with weights, followed by short-arc knee extension exercises, are recommended.
• Precautions: A full range of motion of the knee against resistance during early rehabilitation places excessive forces on the patellofemoral joint and should be strictly avoided.
• Return to Play: Quadriceps strengthening is continued for 3 to 4 months, and strength can be documented objectively using isokinetic testing. We allow a return to full activity (sports) when quadriceps strength reaches 90% of the uninvolved side (Limb Symmetry Index > 90%).

Protocol Following MPFL Reconstruction (Technique 60-2)

• Immobilization: The limb is immobilized in a controlled-motion knee brace set at 30 degrees of flexion for the first 2 weeks. This slight flexion prevents the patella from engaging the proximal trochlea too aggressively while the graft incorporates.
• Ambulation: The brace is locked in full extension for ambulation for the first 6 weeks to protect the graft from sudden eccentric quadriceps loads that could compromise fixation.
• Motion Progression: Motion is gradually increased under supervision. The goal is to achieve 90 degrees of flexion by week 4, and full ROM by week 6.
• Advanced Rehabilitation: Beyond 6 weeks, the rehabilitation protocol mirrors that of the primary repair. Emphasis is placed on core stability, gluteal strengthening, and closed-kinetic-chain quadriceps exercises. Open-kinetic-chain exercises are restricted to short arcs (90° to 45°) to minimize patellofemoral joint reaction forces.

COMPLICATIONS AND PITFALLS

Orthopedic surgeons must be vigilant regarding potential complications associated with patellar stabilization procedures:

1. Recurrent Instability: Often the result of failing to address underlying bony abnormalities (e.g., severe trochlear dysplasia or elevated TT-TG distance) or non-anatomical placement of the femoral tunnel during reconstruction. If the femoral tunnel is placed too proximal, the graft will tighten excessively in flexion, potentially stretching out or failing.
2. Arthrofibrosis: A direct consequence of prolonged immobilization or over-tensioning of the MPFL graft. Early, controlled ROM is the primary preventative measure.
3. Patellar Fracture: Drilling tunnels that are too large or too close together in the patella creates a stress riser. Utilizing suture anchors or limiting tunnel diameter to 3.2 mm or 4.0 mm mitigates this risk.
4. Medial Overload: Caused by over-tensioning the graft, leading to iatrogenic medial patellofemoral arthritis. The graft must be tensioned as a check-rein, allowing 1 to 2 quadrants of lateral patellar glide.

CONCLUSION

The open reduction and repair of patellar dislocations, alongside modern MPFL reconstruction techniques, represent highly successful interventions for restoring patellofemoral stability. By meticulously adhering to anatomical principles—specifically the restoration of the VMO insertion angle and the precise isometric placement of the MPFL—surgeons can reliably eliminate recurrent instability. Coupled with a structured, evidence-based postoperative rehabilitation protocol, patients can expect a predictable return to high-level functional and athletic activities.