Introduction to Patellofemoral Instability
Recurrent dislocation of the patella represents a formidable challenge in orthopedic surgery, demanding a nuanced understanding of the extensor mechanism's complex biomechanics. While instability can occasionally follow a violent initial traumatic dislocation in a structurally normal knee, it occurs far more frequently in knees harboring one or more underlying anatomical abnormalities. These structural aberrations predispose the patellofemoral joint to subluxation or frank dislocation under minimal physiological loads.
In these predisposed knees, minor trauma or even simple pivoting maneuvers are sufficient to precipitate an initial dislocation. The underlying pathological condition invariably causes an abnormal excursion of the extensor mechanism over the femoral condyles. Consequently, successful management—whether conservative or operative—mandates a comprehensive evaluation of the anatomical factors dictating patellar tracking.
Anatomy and Biomechanics of the Extensor Mechanism
The stability of the patellofemoral joint relies on an intricate interplay between dynamic muscular forces and static ligamentous and osseous restraints.
Dynamic Stabilizers: The Quadriceps Vector
Anatomical studies have meticulously delineated the contributions of the various portions of the quadriceps muscle to knee extension and patellar tracking. The muscular vectors are not uniform; rather, they exert multidirectional forces that must remain in equilibrium:
* Vastus Lateralis (VL): Pulls laterally relative to the frontal plane of the femur at an angle of 7 to 10 degrees.
* Vastus Medialis Longus (VML): Represents the proximal portion of the vastus medialis, with its muscle fibers pulling at a 15 to 18-degree medial angle.
* Vastus Medialis Obliquus (VMO): Represents the distal, more horizontal fibers of the vastus medialis, pulling at a relatively horizontal 50 to 55 degrees medially.
Clinical Pearl: The primary function of the vastus medialis obliquus (VMO) is not knee extension, but rather to stabilize the patella against the lateral pull of the vastus lateralis. This makes the VMO the critical dynamic stabilizer of the patella. VMO dysplasia or atrophy is a ubiquitous finding in chronic patellar instability.
Static Stabilizers: Osseous and Ligamentous Restraints
Static factors serve as the primary stabilizers of the patella, particularly in the early degrees of knee flexion before the patella fully engages the trochlear groove. These include:
1. Osseous Morphology: The shape of the patella (Wiberg classification) and the depth of the femoral sulcus (trochlea).
2. Patellar Height: A patellar tendon of appropriate length ensures timely engagement of the patella into the trochlea during flexion.
3. Medial Capsuloligamentous Complex: A normally tensioned medial capsule reinforced by the patellofemoral and patellotibial ligaments.
The Medial Patellofemoral Ligament (MPFL)
The main factor that results in recurrent patellar dislocation is the incompetence of the medial patellofemoral ligament (MPFL). Biomechanical studies demonstrate that the MPFL provides 50% to 60% of the restraining force against lateral patellar displacement from 0 to 30 degrees of knee flexion.
The MPFL is an extrasynovial ligament, situated in layer 2 of the medial knee, much like the superficial medial collateral ligament (sMCL). Injury to the MPFL during a dislocation event may result in:
* Minimal long-term damage with a return to full function.
* Mild-to-moderate plastic deformation resulting in chronic laxity.
* Complete incompetence from avulsion (femoral or patellar) or interligamentous mid-substance failure, leading to recurrent instability.
Pathoanatomy and Biomechanical Vectors
Static and dynamic forces inherently tend to displace the patella laterally. Understanding the vectors that exacerbate this lateral force is paramount for surgical decision-making.
The Q-Angle (Quadriceps Angle)
First described by Brattström, the Q-angle is formed by the line of pull of the quadriceps mechanism and that of the patellar tendon as they intersect at the center of the patella.
Clinically, this angle is represented by the intersection of two lines:
1. A line drawn from the Anterior Superior Iliac Spine (ASIS) to the center of the patella.
2. A second line drawn from the center of the tibial tuberosity to the center of the patella.
Surgical Warning: For the Q-angle measurement to be accurate and reproducible, the patella must be centered on the trochlea. This is achieved by flexing the knee to exactly 30 degrees. Measuring the Q-angle in full extension can yield falsely normal results if the patella is laterally subluxated.
Normative Values:
* Males: 8 to 10 degrees.
* Females: 15 degrees ± 5 degrees (due to a naturally wider pelvis).
This inherent valgus angle imparts a lateral force vector to the patellofemoral joint as the knee is extended.
Drivers of an Increased Q-Angle
Pathological elevation of the Q-angle exponentially increases the lateral subluxation force on the patella. Factors that can increase this angle include:
* Genu Valgum: Increases the valgus vector of the extensor mechanism.
* Increased Femoral Anteversion: Internally rotates the femur beneath the extensor mechanism.
* External Tibial Torsion: Lateralizes the distal attachment of the patellar tendon.
* Laterally Positioned Tibial Tuberosity: Directly increases the lateral vector (quantified by the TT-TG distance).
The combination of femoral anteversion, genu valgum, and external tibial torsion is classically referred to as "Miserable Malalignment Syndrome."
Clinical Evaluation and Diagnostic Imaging
Physical Examination
A meticulous physical examination is required to assess both dynamic tracking and static laxity.
* J-Sign: As the knee extends from a flexed position, the patella abruptly deviates laterally in the final 10 to 15 degrees of extension, indicating loss of trochlear constraint and MPFL laxity.
* Patellar Apprehension Test: A laterally directed force applied to the patella at 20-30 degrees of flexion elicits guarding and apprehension.
* Lateral Glide Test: Displacement of the patella laterally by more than 2.5 quadrants indicates gross medial restraint incompetence.
Imaging Protocols
Magnetic Resonance Imaging (MRI) is the gold standard to evaluate the site and extent of MPFL injury and is strictly indicated for instability. However, a complete radiographic workup is mandatory:
* Standard Radiographs: AP, True Lateral (at 30 degrees flexion to assess patella alta via the Insall-Salvati ratio and trochlear dysplasia via the crossing sign), and Merchant/Sunrise views.
* CT Scan: Essential for calculating the Tibial Tubercle-Trochlear Groove (TT-TG) distance. A TT-TG distance > 20 mm is highly pathological and often necessitates a tibial tubercle osteotomy.
* MRI: Evaluates the MPFL (femoral avulsion vs. patellar avulsion), assesses for osteochondral fractures (frequently seen on the medial patellar facet or lateral femoral condyle), and evaluates cartilage integrity.
Surgical Management of Recurrent Patellar Dislocation
Operative intervention is indicated for recurrent instability that has failed conservative rehabilitation, or in the acute setting if there is a displaced osteochondral fracture or massive structural avulsion.
Medial Patellofemoral Ligament (MPFL) Reconstruction
Because MPFL incompetence is the primary driver of recurrent dislocation, isolated lateral release is historically obsolete and contraindicated for episodic instability. MPFL reconstruction is the workhorse procedure for restoring medial restraint.
1. Graft Selection
Autografts (gracilis or semitendinosus) are most commonly utilized. Allografts may be considered in revision settings or to minimize donor-site morbidity, though they carry a slightly higher risk of elongation.
2. Femoral Tunnel Placement (Schöttle's Point)
The most critical step in MPFL reconstruction is identifying the isometric femoral attachment. Non-anatomic femoral placement is the leading cause of graft failure and postoperative stiffness.
* Fluoroscopic Landmarks: On a strict lateral radiograph, Schöttle's point is located:
1. 1 mm anterior to the posterior cortical line of the femoral diaphysis.
2. 2.5 mm distal to the posterior origin of the medial femoral condyle.
3. Proximal to the level of the posterior articular margin (Blumensaat’s line).
3. Patellar Fixation
The native MPFL inserts onto the proximal half of the medial patellar border. The graft is typically secured using two suture anchors or transosseous tunnels in the proximal and middle thirds of the medial patella.
Pitfall: Avoid placing patellar anchors too anteriorly or drilling tunnels too deeply, as this creates a stress riser that can lead to a catastrophic transverse patellar fracture.
4. Biomechanics of Graft Tensioning
The graft must act as a check-rein, not a medializing force.
* Tensioning should be performed at 30 degrees of knee flexion.
* The patella should be able to be displaced laterally by 1 to 2 quadrants with a firm endpoint.
* Over-tensioning will lead to medial patellofemoral cartilage overload, severe pain, and loss of knee flexion.
Tibial Tubercle Osteotomy (TTO)
In patients with a pathological Q-angle driven by a lateralized tibial tubercle (TT-TG > 20 mm) or severe patella alta (Caton-Deschamps index > 1.2), an isolated MPFL reconstruction will fail due to excessive lateral biomechanical forces. A TTO is required to normalize the extensor vector.
Fulkerson Anteromedialization
The Fulkerson osteotomy is the gold standard for concurrent lateralization and patellofemoral chondrosis.
1. Approach: A longitudinal incision is made lateral to the tibial crest. The anterior compartment musculature is elevated.
2. Osteotomy: An oblique osteotomy is performed from anteromedial to posterolateral. The angle of the cut dictates the ratio of anteriorization (which unloads the patellofemoral joint) to medialization (which corrects the Q-angle).
3. Translation: The tubercle is translated medially (typically 10-15 mm) to normalize the TT-TG distance to approximately 10-12 mm.
4. Fixation: The osteotomy is rigidly fixed using two or three 4.5 mm cortical lag screws placed from anterior to posterior.
Trochleoplasty
In cases of severe Dejour Type B, C, or D trochlear dysplasia (characterized by a flat or convex trochlear vault), soft tissue and distal bony procedures may be insufficient. A sulcus-deepening trochleoplasty involves elevating the articular cartilage of the trochlea, burring a new V-shaped sulcus in the subchondral bone, and securing the cartilage down into the new groove. This is a highly specialized procedure reserved for refractory cases.
Postoperative Rehabilitation Protocol
Successful outcomes depend heavily on a structured, phased rehabilitation program to protect the surgical reconstruction while restoring function.
- Phase 1: Protection (Weeks 0-2)
- Weight-bearing as tolerated (WBAT) with the knee locked in extension in a hinged brace.
- Focus on edema control, quadriceps activation (isometric sets), and patellar mobilization to prevent arthrofibrosis.
- Phase 2: Early ROM (Weeks 2-6)
- Unlock brace for passive and active-assisted Range of Motion (ROM).
- Goal: 0 to 90 degrees of flexion by week 4; 120 degrees by week 6.
- Initiate closed-chain kinetic exercises (e.g., mini-squats) within the pain-free arc.
- Phase 3: Strengthening (Weeks 6-12)
- Discontinue brace once normal gait mechanics and straight-leg raise without extension lag are achieved.
- Progressive resistance training focusing on VMO hypertrophy, gluteal strengthening, and core stability.
- Phase 4: Return to Play (Months 4-6)
- Initiate plyometrics, agility drills, and sport-specific training.
- Clearance for sports requires >90% limb symmetry index on functional hop testing and isokinetic strength testing.
Conclusion
Recurrent patellar dislocation is a multifactorial pathology that demands a rigorous, individualized approach. By thoroughly evaluating the dynamic stabilizers (VMO vs. VL vectors), static restraints (MPFL integrity), and osseous alignment (Q-angle, TT-TG distance, and trochlear morphology), the orthopedic surgeon can tailor the operative intervention. Whether utilizing an isolated MPFL reconstruction or combining it with a tibial tubercle osteotomy, restoring the precise biomechanical equilibrium of the extensor mechanism is the cornerstone of achieving long-term patellofemoral stability.
