Patellar dislocation

1. High-Yield Summary

Acute patellar dislocation represents a traumatic disruption of the patellofemoral extensor mechanism characterized by lateral displacement of the patella out of the femoral trochlear groove. The condition predominantly affects active adolescents and young adults, with peak incidence occurring between 10 and 19 years of age. The injury is typically sustained through a non-contact dynamic knee valgus vector coupled with knee flexion and external tibial rotation while the foot is planted.

• Primary Static Restraint: The medial patellofemoral ligament (MPFL) provides 50% to 60% of the total medial restraining force resisting lateral patellar translation from 0° to 30° of knee flexion; beyond 30°, patellofemoral stability is primarily governed by osseous engagement within the concave femoral trochlear groove.
• Nonoperative Management for Primary Injury: Acute first-time dislocators without displaced osteochondral shear fractures or mechanical locking are managed nonoperatively with short-term (2 to 3 weeks) extension bracing followed by progressive active rehabilitation targeting vastus medialis obliquus (VMO), hip abductor, and core musculature.
• Indications for Acute Operative Intervention: Displaced osteochondral fracture (typically originating from the inferomedial patellar facet or lateral femoral condyle impaction ridge, >= 1.0 cm²), irreducible dislocation, or macroscopic complete MPFL disruption with displaced bony avulsion warrant prompt surgical fixation or reconstruction.
• Gold Standard Reconstructive Stabilization: Anatomical MPFL reconstruction anchored at Schöttle’s point represents the gold standard surgical intervention for recurrent instability, restoring native kinematics and reducing redislocation rates from over 40% down to 2% to 5%.
• A la Carte Osseous Realignment: Management of recurrent instability requires objective evaluation of predisposing skeletal anomalies; concurrent bony realignment is indicated for tibial tubercle-trochlear groove (TT-TG) distance >= 20 mm (tibial tubercle osteotomy medialization), severe patella alta with Caton-Deschamps index > 1.2 (tibial tubercle distalization), and severe high-grade trochlear dysplasia (Dejour Type B or D with trochlear bump > 3-5 mm, indicating deepening trochleoplasty).

2. Anatomy

The patellofemoral joint represents an intricate articulation between the triangular sesamoid patella and the femoral trochlea, dependent upon a delicate balance of static soft-tissue stabilizers, dynamic muscular vectors, and three-dimensional osseous architecture.

Medial Patellofemoral Ligament and Medial Stabilizers

The medial patellofemoral ligament (MPFL) constitutes the major passive soft-tissue stabilizer resisting lateral displacement of the patella. Residing within layer two of the medial knee (deep to the crural fascia and superficial to the joint capsule), the MPFL measures 55 to 65 mm in length and 10 to 12 mm in width, exhibiting an average ultimate tensile failure load of approximately 208 N.

The femoral origin of the MPFL is localized to a precise bony depression situated between the medial femoral epicondyle and the adductor tubercle, historically and radiographically defined as Schöttle’s point. On a true lateral fluoroscopic radiograph with complete superposition of the posterior femoral condyles, Schöttle’s point lies 1.3 mm anterior to the tangent of the posterior femoral cortex, 2.5 mm distal to the perpendicular line drawn from the posterior origin of the medial femoral condyle, and proximal to the posterior extension of Blumensaat’s line.

Distally, the MPFL fans out in a triangular configuration to insert broadly along the proximal two-thirds (upper 50% to 60%) of the medial patellar margin and into the deep undersurface of the vastus medialis obliquus (VMO) tendon. Secondary medial stabilizers include the medial patellotibial ligament (MPTL) and medial patellomeniscal ligament (MPML), which run obliquely from the inferomedial patellar border to the anteromedial tibia and medial meniscus anterior horn, providing secondary restraint against lateral translation and patellar tilt at knee flexion angles exceeding 30°.

The vastus medialis obliquus (VMO) muscle provides dynamic medial stabilization. Its muscle fibers originate from the adductor magnus tendon and distal medial intermuscular septum, orienting obliquely at 50° to 55° relative to the longitudinal femoral shaft axis before inserting into the superomedial patellar border and superficial MPFL fibers. The VMO does not extend the knee independently; rather, it dynamically counters the lateralizing pull of the vastus lateralis during terminal knee extension.

Lateral Retinacular Complex

The lateral retinaculum is organized into two anatomical layers. The superficial layer consists of longitudinal fibers derived from the fascia lata and the aponeurosis of the vastus lateralis. The deep layer comprises the transverse epicondylopatellar band (running from the lateral epicondyle to the lateral patellar border), the deep patellotibial band, and patellomeniscal bands. Pathological contracture of the deep transverse retinaculum restricts medial patellar translation, increases lateral patellofemoral contact pressures, and generates an adverse tilting moment arm.

Trochlear Morphology and Osseous Congruency

The femoral trochlea is an asymmetric, cartilaginous saddle-shaped groove. In the normal knee, the lateral trochlear facet is higher, steeper, and extends further proximally than the medial facet, providing a mechanical bony buttress against lateral patellar translation once the patella engages at 20° to 30° of knee flexion. Normal trochlear parameters include a sulcus angle of 138° +/- 6° (measured on axial projections) and a lateral trochlear inclination angle exceeding 11°. Trochlear depth normally exceeds 4 mm. Dysplastic loss of this concave geometry deprives the extensor mechanism of its native bony constraint.

Internervous Planes and Surgical Corridors

Surgical exposure of the medial patellofemoral restraints utilizes the medial parapatellar interval. This plane is developed between the femoral nerve-innervated quadriceps extensor apparatus (vastus medialis and rectus femoris) proximally and the sensory terminal distribution of the saphenous nerve distally. Exposure of the tibial tuberosity for realignment osteotomy proceeds through a longitudinal anteromedial incision, developing a subperiosteal interval along the anterior tibial cortex superficial to the anterior compartment fascia, preserving the tibialis anterior muscle innervated by the deep peroneal nerve.

Neurovascular Danger Zones

Three critical neurovascular structures are at risk during patellofemoral stabilization procedures:

• Infrapatellar Branch of the Saphenous Nerve (IPBSN): Arising from the saphenous nerve in the distal thigh or adductor canal, the IPBSN courses obliquely across the anteromedial joint line, passing distal to the medial epicondyle and crossing the patellar tendon. It is highly susceptible to traction, transection, or entrapment during medial retinacular incisions, hamstring autograft harvesting, and anterior tibial tubercle exposures, predisposing patients to painful neuroma formation or disabling anteromedial knee hypoesthesia.
• Descending Genicular Artery: Emerging from the femoral artery just proximal to the adductor hiatus within Hunter’s canal, the descending genicular artery bifurcates into a saphenous branch and articular (musculoarticular) branches that descend along the anterior border of the adductor magnus tendon toward the medial epicondylar region. Aggressive dissection near the adductor tubercle during femoral tunnel preparation risks arterial laceration and troublesome intraoperative hemorrhage.
• Popliteal Neurovascular Bundle: Located directly posterior to the posterior femoral condyles and posterior cortex. Bicortical over-drilling or unguided reamer advancement during femoral tunnel preparation carries catastrophic risk of popliteal artery or tibial nerve laceration. Safe instrumentation mandates strict fluoroscopic guidance and bicortical tactile control.

3. Pathogenesis

Patellar dislocation is fundamentally a failure of static and dynamic extensor restraints under severe biomechanical stress, compounded in many cases by pre-existing anatomical dysplasia.

Vector Forces and Mechanism of Injury

The classic mechanism of primary patellar dislocation is a non-contact athletic trauma occurring in early knee flexion (20° to 40°). The foot is planted firmly on the playing surface while the athlete decelerates or cuts, generating an internal femoral rotation and dynamic knee valgus vector against external tibial rotation. This composite kinetic chain vector creates a profound laterally directed force vector acting on the patella.

At 0° to 30° of knee flexion, the patella is entering the proximal entrance of the trochlea and has not yet achieved deep bony engagement within the trochlear sulcus. The entire burden of stabilizing the patella against lateral translation falls upon the medial soft-tissue restraints, specifically the MPFL. Direct contact injury, accounting for less than 10% of presentations, results from a blunt force applied directly to the medial patellar aspect with the knee flexed, forcibly driving the patella over the lateral trochlear ridge.

Disruption Mechanics of the Extensor Restraints

When the lateral displacement force exceeds the structural threshold of the MPFL (yield tensile load approximately 208 N), macroscopic structural disruption occurs. The anatomical zone of MPFL failure varies across patient cohorts:

• Femoral Origin Disruption: Represents the most frequent site of failure, observed in 60% to 70% of cases, occurring as an avulsion or interstitial tear at or immediately adjacent to Schöttle’s point.
• Patellar Insertion Avulsion: Occurs in 15% to 20% of acute injuries, frequently carrying a microscopic or macroscopic sleeve of periosteum or subchondral bone.
• Midsubstance Tearing: Occurs in 15% to 20% of cases through interstitial attenuation and longitudinal fiber disruption.
• Multifocal / Bipolar Tearing: Observed in 10% to 15% of high-energy pediatric and adolescent injuries, characterized by concurrent disruption at both the femoral and patellar attachments.

Osteochondral Shearing Mechanics and Kiss Contusions

As the patella displaces laterally over the prominent lateral femoral condyle, its inferomedial articular facet is violently compressed against the acute anterolateral edge of the lateral condyle. Following lateral dislocation, spontaneous reduction typically occurs as the patient extends the knee or contracts the quadriceps, driving the medial patellar facet across the lateral condylar ridge a second time.

This dual impaction mechanism produces a pathognomonic reciprocal bone contusion pattern, termed the “kiss contusion”, characterized on magnetic resonance imaging by intense trabecular microfracture and marrow edema localized to the anterolateral femoral condyle and the inferomedial patellar facet. In 30% to 40% of acute dislocations, the shear stresses exceed the shear strength of uncalcified articular cartilage and subchondral bone, resulting in displaced osteochondral fractures of the medial patellar facet or lateral condyle, producing macroscopic intra-articular loose bodies.

4. Natural History

Patellar dislocation displays a distinct epidemiological distribution and variable natural history governed by skeletal maturity and underlying anatomical morphology.

Epidemiology

The annual incidence of acute primary patellar dislocation in the general population ranges from 5.8 to 29 per 100,000 person-years. The demographic distribution peaks dramatically among adolescents aged 10 to 19 years, where the incidence surges to 110 to 147 per 100,000 person-years. While overall gender incidence across the general lifespan is roughly equivalent, adolescent females exhibit a higher relative incidence, attributable to increased ligamentous laxity, wider pelvic geometry with an elevated quadriceps angle (Q-angle), dynamic knee valgus, and higher rates of underlying trochlear dysplasia.

Recurrence Rates and Risk Factors

The natural history following nonoperative management of a first-time acute patellar dislocation varies significantly depending upon baseline anatomical risk factors:

• Baseline Recurrence Rate: In patients with normal skeletal alignment and normal trochlear morphology, the rate of secondary dislocation following conservative care ranges from 15% to 44%.
• High-Risk Pediatric / Dysplastic Cohort: In pediatric and adolescent patients with open physes and predisposing anatomical dysplasia, recurrent dislocation occurs in 70% to 80% of conservatively managed knees.
• Persistent Functional Impairment: More than 50% of first-time dislocators report chronic knee dysfunction, including persistent anterior knee pain, subjective instability, apprehension during cutting maneuvers, or reduced athletic performance at 5 to 10 years follow-up.
• Validated Recurrence Risk Factors: Primary anatomical determinants of recurrence include high-grade trochlear dysplasia (Dejour Types B, C, and D), patella alta (Caton-Deschamps index > 1.2), elevated tibial tubercle-trochlear groove distance (TT-TG >= 20 mm), skeletal immaturity (age = 4/9), and a history of contralateral patellar dislocation.

Long-Term Degenerative Trajectory

Recurrent instability and mechanical shear injuries inflict cumulative structural damage on patellofemoral articular cartilage. Long-term natural history investigations demonstrate that traumatic patellar dislocation confers a more than three-fold increase in the odds of radiographic patellofemoral osteoarthritis compared to uninjured controls (odds ratio 3.3, 95% CI 2.1 to 5.2). The presence of an osteochondral fracture or full-thickness chondral defect at the index injury substantially accelerates degenerative joint progression.

5. Clinical Presentation

Thorough evaluation integrates a detailed traumatic history with targeted provocative physical examination maneuvers to identify structural instability, retinacular pathology, and anatomical malalignment.

History

Patients typically describe an acute, non-contact twisting event during sports or pivoting, accompanied by an audible or palpable “pop”, sudden knee collapse (“giving way”), and immediate severe pain. A key historical clue is whether the patient observed the patella displaced laterally onto the outer aspect of the knee. Spontaneous reduction frequently occurs when the patient attempts to straighten the knee or upon initial handling by on-field personnel.

A tense, painful knee hemarthrosis typically evolves rapidly within 1 to 2 hours of injury, caused by tearing of the vascularized MPFL, medial retinaculum, or subchondral bone bed. Mechanical symptoms, such as true catching, locking, or inability to achieve full terminal extension, strongly suggest the presence of a displaced osteochondral loose body within the joint. Clinicians should systematically document prior episodes of subluxation, contralateral knee instability, generalized joint hypermobility, and family history of patellofemoral disorders.

Physical Exam

Physical examination begins with visual inspection of limb alignment, joint effusion, and extensor lag. In the acute setting, examination is limited by pain and hemarthrosis; in subacute or chronic evaluations, provocative testing reliably identifies patellar instability and retinacular pathology:

• Patellar Apprehension Test (Fairbank’s Sign): With the patient supine and the knee relaxed at 20° to 30° of flexion, the examiner applies a gentle, laterally directed translation force to the medial patellar margin while monitoring the patient’s facial expression and muscular response -> Involuntary quadriceps contraction, marked anxiety, verbal apprehension, or active resistance against further lateral translation [Sensitivity 88%, Specificity 92%].
• Moving Patellar Apprehension Test (Sage Test): The examiner passively flexes and extends the knee through a continuous arc from 0° to 90° of flexion and back while applying a continuous lateral displacement force to the patella; the maneuver is immediately repeated while applying a medially directed force -> Apprehension, pain, and voluntary quadriceps guarding during lateral displacement that are completely abolished when repeated with medial displacement [Sensitivity 94%, Specificity 88%].
• Patellar Glide Test: With the knee supported in 30° of flexion and the quadriceps fully relaxed, the examiner divides the patellar transverse width into four quadrants and gently translates the patella medially and laterally -> Lateral translation exceeding 2 quadrants (> 50% of patellar width) indicates medial patellofemoral ligament incompetence; medial glide of less than 1 quadrant denotes lateral retinacular tightness, whereas medial glide exceeding 3 quadrants indicates generalized ligamentous hyperlaxity [Sensitivity 82%, Specificity 84%].
• Bassett’s Sign (Focal MPFL Tenderness): Direct digital palpation of the soft-tissue depression between the medial femoral epicondyle and adductor tubercle -> Focal, exquisite point tenderness at the femoral insertion of the medial patellofemoral ligament indicative of acute rupture [Sensitivity 84%, Specificity 76%].
• Patellar Tilt Test: With the knee in full extension and quadriceps relaxed, the examiner attempts to elevate the lateral patellar border anteriorly above the horizontal plane while stabilizing the medial border -> Inability to elevate the lateral patellar facet to or above the neutral horizontal plane (0° tilt) denotes severe lateral retinacular contracture [Sensitivity 75%, Specificity 89%].
• J-Sign Assessment: The examiner observes patellar tracking as the patient actively or passively extends the knee from 90° of flexion to full extension -> Abrupt, sudden lateral deviation of the patella as it exits the bony constraints of the trochlear groove during terminal 20° to 0° of extension, tracing an inverted “J” configuration, indicative of severe maltracking, patella alta, or trochlear dysplasia [Sensitivity 70%, Specificity 85%].

6. Imaging and Diagnostic Studies

Diagnostic imaging progresses systematically from standard plain radiography to multiplanar magnetic resonance imaging and computed tomography to comprehensively characterize osseous morphology, soft-tissue tears, and chondral integrity.

Radiographic Evaluation

Plain radiography represents the mandatory initial diagnostic modality. A complete patellofemoral series includes anteroposterior (AP), true lateral (at 30° flexion), and axial (Merchant or Laurin) views.

The true lateral radiograph is the cornerstone for assessing trochlear dysplasia and patellar height. Strict projectional criteria demand complete superposition of the posterior femoral condyles, with less than 2 mm of rotational discrepancy. Three classic radiographic signs characterize trochlear dysplasia on the true lateral view:

• Crossing Sign: The line of the trochlear groove crosses the anterior contour of the lateral femoral condyle, indicating that the groove has flattened completely and become flush with the condyle.
• Supratrochlear Spur: A bony prominence projecting > 3 mm anterior to the continuation of the anterior femoral cortex line at the superior terminus of the trochlea.
• Trochlear Bump: Anterior protrusion of the trochlear floor > 3 mm relative to the line of the anterior femoral cortex.
• Double Contour Sign: A distinct sclerotic line projecting posterior to the anterior lateral condyle contour, reflecting a hypoplastic, convex medial facet.

Patellar height is quantitatively evaluated on the true lateral projection at 30° of knee flexion utilizing validated indices:

• Caton-Deschamps Index: Calculated as the ratio of the distance from the inferior edge of the patellar articular cartilage to the anterior-superior margin of the tibial plateau divided by the length of the patellar articular surface. Normal range is 0.8 to 1.2. Patella alta is defined as > 1.2 (severe alta > 1.3); patella baja is defined as < 0.6. This index is advantageous due to its reproducibility and reliability following tibial tubercle osteotomy.
• Insall-Salvati Ratio: The length of the patellar tendon (from inferior patellar pole to tibial tubercle) divided by the greatest diagonal length of the patella. Normal range is 0.8 to 1.2. Values > 1.2 indicate patella alta; values < 0.8 indicate patella baja. It is susceptible to distortion in patellae with elongated non-articular inferior poles.
• Blackburne-Peele Ratio: The perpendicular distance from the inferior articular margin of the patella to a line projected along the tibial plateau divided by the length of the patellar articular surface. Normal range is 0.5 to 1.0; values > 1.0 denote patella alta.

Axial views (Merchant view at 45° flexion or Laurin view at 20° to 30° flexion) assess patellofemoral tracking and facet geometry:

• Sulcus Angle: Formed by the lines connecting the deepest point of the trochlear groove to the highest peaks of the medial and lateral condyles. Normal is 138° +/- 6°; an angle > 145° signifies a flattened, dysplastic trochlea.
• Congruence Angle: The angle between the zero-reference bisector of the sulcus angle and a line connecting the sulcus apex to the lowest point of the patellar median ridge. Normal is medial (-6° +/- 11°); lateral deviation > +16° confirms pathological lateral patellar subluxation.
• Lateral Patellofemoral Angle (Laurin): Formed between the intercondylar line and the lateral patellar facet line; it normally opens laterally. An angle that is parallel or opens medially signifies abnormal lateral patellar tilt.

Computed Tomography

Computed tomography (CT) with axial superimposed slices is the gold standard for quantifying extensor mechanism lateralization and lower-extremity rotational profile:

• Tibial Tubercle-Trochlear Groove (TT-TG) Distance: Measured by overlaying the axial slice displaying the deepest point of the trochlear groove onto the axial slice passing through the center of the tibial tuberosity, referenced perpendicular to the posterior femoral condylar line. Normal is = 20 mm, serving as a primary indication for medializing tibial tubercle osteotomy.
• Tibial Tubercle-Posterior Cruciate Ligament (TT-PCL) Distance: Measures the transverse distance between the center of the tibial tuberosity and the medial border of the posterior cruciate ligament insertion. Normal is < 24 mm. Because it is independent of the trochlear groove, it distinguishes true anatomical lateralization of the tibial tubercle from apparent lateralization caused by severe trochlear rotation or medial trochlear hypoplasia.
• Rotational Profile: Axial rotational CT quantifies femoral anteversion (normal 10° to 15°; excessive > 25° to 30°) and external tibial torsion (normal 20° to 30°; excessive > 40°), which represent critical proximal and distal malalignment drivers of recurrent instability.

Magnetic Resonance Imaging

Magnetic resonance imaging (MRI) is essential in the acute setting to define soft-tissue pathology and articular damage. Key MRI sequences include proton-density fat-suppressed (PDFS) and T2-weighted turbo spin-echo views:

• MPFL Disruption Characterization: Identifies the exact anatomical location (femoral origin at Schöttle’s point vs. patellar insertion vs. midsubstance) and severity of the tear, guiding surgical repair versus reconstruction.
• Bone Contusion Mapping: Visualizes the diagnostic reciprocal “kiss contusion” edema pattern at the anterolateral femoral condyle and inferomedial patellar facet.
• Chondral and Osteochondral Injury: Detects occult chondral shear injuries, subchondral fracture lines, cartilage delamination, and intra-articular loose bodies located in the suprapatellar pouch, gutters, or intercondylar notch.
• Lateral Trochlear Inclination (LTI): Measured on the most proximal axial image depicting cartilaginous trochlea as the angle between the subchondral bone of the lateral trochlear facet and the posterior condylar axis. An LTI < 11° demonstrates high sensitivity and specificity for trochlear dysplasia.

7. Classification

The validated classification system governing clinical evaluation and operative decision-making in patellar instability is the Dejour Classification of Trochlear Dysplasia. Based on true lateral plain radiographs and axial cross-sectional imaging, it stratifies anatomical dysplasia into four distinct types that dictate surgical planning.

8. Treatment

Management of patellar dislocation spans conservative rehabilitation protocols for acute primary injuries to comprehensive, multi-component anatomical reconstruction for recurrent instability.

Medical / Nonoperative

Nonoperative management is the primary standard of care for acute, first-time patellar dislocations in the absence of mechanical entrapment or displaced osteochondral fractures:

• Immobilization Parameters: The injured limb is placed in a knee immobilizer or hinged knee brace locked in extension for 2 to 3 weeks during ambulation to protect the healing MPFL. Prolonged rigid casting (> 4 weeks) is strictly contraindicated, as it produces joint arthrofibrosis, chondral degeneration, and severe quadriceps inhibition without lowering redislocation rates.
• Protected Weight-Bearing: Weight-bearing as tolerated with crutches is encouraged immediately in full extension. Early protected weight-bearing promotes articular cartilage nutrition and limits bone mineral loss.
• Range of Motion Progression: Unlocked passive and active-assisted range of motion from 0° to 90° is initiated at 2 weeks, progressing toward full flexion by 6 weeks post-injury.
• Structured Active Rehabilitation:
  • Quadriceps Activation: Isometric quadriceps sets, straight leg raises without extensor lag, and neuromuscular electrical stimulation (NMES) to overcome arthrogenic muscle inhibition.
  • VMO Conditioning: Closed kinetic chain strengthening, including terminal knee extensions (0° to 30°) and wall slides (0° to 45°), optimizing dynamic medial patellar vector.
  • Kinetic Chain & Core Stabilization: Progressive strengthening of hip abductors (gluteus medius) and external rotators to counter dynamic knee valgus and internal femoral rotation. Lumbo-pelvic core stabilization is integrated to control trunk lean.
  • External Patellar Support: Dynamic patellar stabilizing braces (e.g., J-brace or patellar tracking orthosis) or McConnell taping during the transition to sport-specific activities.

Operative

The overarching goals of surgical stabilization are to restore the static medial checkrein against lateral translation in early flexion, optimize patellofemoral tracking without excessive joint constraint, correct underlying skeletal malalignment, and restore articular congruency.

• Medial Patellofemoral Ligament (MPFL) Reconstruction:
  • Graft Selection: Autograft semitendinosus or gracilis, or soft-tissue allografts (semitendinosus, gracilis, or anterior tibialis). Autografts provide high structural tensile properties; allografts avoid donor-site morbidity and hamstring weakness.
  • Patellar Fixation: Two parallel 3.5 to 4.5 mm drill tunnels placed in the proximal half of the medial patellar margin (spaced >= 15 mm apart), or dual suture anchors/short aperture sockets along the proximal two-thirds of the medial patellar border. Drilling must avoid penetrating the anterior patellar cortex or the articular cartilage.
  • Femoral Tunnel Placement: Identified under true lateral fluoroscopic guidance at Schöttle’s point. A guide pin is drilled through Schöttle’s point, directed anteriorly and proximally (30° anterior, 15° proximal) to avoid the intercondylar notch, posterior cortex, and distal femoral physis. The femoral tunnel is reamed to graft diameter (typically 6 to 7 mm).
  • Graft Tensioning & Fixation: The graft is secured using an interference screw (bioabsorbable or PEEK) with the knee positioned in 20° to 30° of flexion. Critical surgical pearl: The graft must be secured with minimal manual tension while ensuring the patella is seated centrally within the trochlear groove. The patella must retain 1 to 2 quadrants of passive lateral translation after fixation to prevent joint over-constraint.
• Tibial Tubercle Osteotomy (TTO):
  • Fulkerson Anteromedialization (AMZ): Indicated for elevated TT-TG (>= 20 mm) combined with distal or lateral patellar chondromalacia. An oblique planar osteotomy (45° to 60°) allows 10 to 15 mm of medial translation and 5 to 10 mm of anteriorization, unloading the lateral patellofemoral facet.
  • Straight Medialization (Elmslie-Trillat): Indicated for isolated elevated TT-TG (>= 20 mm) with preserved patellofemoral articular cartilage.
  • Distalization: Indicated for severe patella alta (Caton-Deschamps index > 1.2 to 1.3). The osteotomized tubercle shingle is advanced distally (1 mm distalization per 0.1 reduction in Caton-Deschamps index) to facilitate early trochlear engagement. Rigid fixation is achieved using two or three 4.5 mm bicortical lag screws.
• Trochleoplasty: Indicated in skeletally mature patients with severe high-grade trochlear dysplasia (Dejour Type B or D) exhibiting a prominent supratrochlear bump (> 3-5 mm) and persistent instability. Techniques include the Bereiter thin-flap osteoplasty (elevating a flexible chondral flap, deepening the cancellous subchondral floor, and re-securing the flap with resorbable tapes) or Dejour groove-deepening trochleoplasty. Trochleoplasty is always combined with MPFL reconstruction.
• Lateral Retinacular Lengthening: Indicated strictly as an adjunct when the lateral retinaculum remains contracted following medial stabilization, evidenced by a negative patellar tilt test (< 0° or inability to invert horizontal). Performed via Z-plasty or coronal step-cut lengthening. Isolated lateral retinacular release is strictly contraindicated for patellar instability.

Indications & Contraindications

Surgical decision-making relies upon rigorous objective thresholds:

• Indications for Acute Operative Stabilization:
  • Displaced osteochondral fracture >= 1.0 cm² requiring open or arthroscopic reduction and internal fixation (using headless compression screws or bioabsorbable pins) or fragment excision.
  • Macroscopic, complete MPFL disruption with a large, displaced bony avulsion fragment.
  • Irreducible acute patellar dislocation or soft-tissue entrapment.
• Indications for Delayed / Reconstructive Stabilization:
  • Recurrent patellar dislocation (>= 2 verified dislocation episodes).
  • Persistent objective patellar instability following failure of >= 3 to 6 months of supervised, compliant active physical therapy.
  • Tibial tubercle-trochlear groove (TT-TG) distance >= 20 mm (indication for concurrent TTO medialization; relative indication for 15 to 19 mm).
  • Severe patella alta with Caton-Deschamps index > 1.2 (indication for concurrent TTO distalization).
  • Severe trochlear dysplasia (Dejour Type B or D with trochlear bump > 3-5 mm) refractory to soft-tissue stabilization.
• Surgical Contraindications:
  • Skeletal Immaturity: Open proximal tibial and distal femoral physes represent an absolute contraindication to standard tibial tubercle osteotomy and standard transphyseal femoral tunnel drilling. Requires physis-sparing MPFL reconstruction (adductor magnus sling or subphyseal suture anchor fixation) and distal soft-tissue realignment (e.g., modified Roux-Goldthwait).
  • Severe Generalized Patellofemoral Osteoarthritis: Kellgren-Lawrence grade 3 to 4 or diffuse Outerbridge grade IV articular loss across both patella and trochlea represents a contraindication to isolated soft-tissue stabilization; managed via patellofemoral arthroplasty or total knee arthroplasty.
  • Habitual Dislocation in Deep Flexion: Dislocation driven by severe quadriceps contracture or abnormal iliotibial band tethers represents a contraindication to isolated MPFL reconstruction; requires extensor mechanism lengthening or quadricepsplasty.
  • Isolated Lateral Retinacular Release: Strictly contraindicated as a stand-alone procedure for patellar instability due to high rates of recurrent instability and iatrogenic medial subluxation.

9. Outcome

Clinical outcomes following patellar stabilization are evaluated using validated patient-reported outcome measures (PROMs), objective redislocation rates, and return-to-sport metrics.

Patient-Reported Outcome Measures

The primary validated tools utilized in patellofemoral clinical trials include:

• Kujala Anterior Knee Pain Scale: A 13-item, 100-point disease-specific questionnaire evaluating subjective pain, limp, weight-bearing, walking, stair climbing, running, and jumping. In contemporary surgical MPFL reconstruction cohorts, mean baseline scores (45 to 60 points) improve significantly to 85 to 94 points at intermediate and long-term follow-up.
• International Knee Documentation Committee (IKDC) Subjective Score: Demonstrates marked post-operative improvement, with cohort means increasing from 48-55 points preoperatively to 82-90 points postoperatively.
• Banff Patella Instability Instrument (BPII): A 23-item patellar instability-specific PROM with high construct validity and responsiveness, quantifying functional quality of life, sport readiness, and psychological apprehension.

Recurrence Rates and Joint Stability

Comparative evidence confirms the superior mechanical stabilization achieved by anatomical operative reconstruction over nonoperative care in recurrent cohorts:

• Operative vs. Nonoperative Recurrence: In the landmark Cochrane systematic review of randomized trials (Smith et al. 2023), surgical intervention reduced the relative risk of recurrent dislocation by 43% compared to non-surgical treatment (RR 0.57, 95% CI 0.41 to 0.80).
• Anatomical MPFL Reconstruction Survivorship: Isolated or combined anatomical MPFL reconstruction yields clinical redislocation rates of 2% to 5% at 5 to 10 years of follow-up, contrasting with nonoperative recurrence rates exceeding 40% in high-risk populations.

Return-to-Play Timelines

Overall return-to-sport rates following anatomical MPFL reconstruction range from 80% to 90%, with 65% to 75% of competitive athletes successfully returning to their pre-injury level of competition. Light straight-line jogging and functional running drills are initiated at 3 to 4 months postoperatively. Full unrestricted return to pivoting and contact athletics occurs between 6 and 9 months, contingent upon passing objective functional testing criteria (limb symmetry index >= 90% on single-leg hop testing, full active range of motion, and complete absence of subjective apprehension).

10. Complications

Complications following patellar dislocation and reconstructive surgery range from intraoperative technical pitfalls to late patellofemoral degenerative arthritis, categorized systematically across perioperative timelines.

11. Pearls and Pitfalls

Mastery of patellofemoral stabilization mandates adherence to strict technical pearls while avoiding common diagnostic and procedural pitfalls.

Technical Pearls

• Fluoroscopic Verification of Schöttle’s Point: Never drill the femoral guidewire without establishing a perfect true lateral fluoroscopic radiograph. Both posterior femoral condyles must overlap completely (< 2 mm discrepancy). An oblique projection of as little as 5° shifts the apparent anatomical landmark by several millimeters, predisposing to non-isometric tunnel placement and catastrophic graft overtensioning in flexion.
• The Checkrein Tensioning Principle: The MPFL functions natively as a stabilizing checkrein against lateral translation, not a rigid static tenodesis. Secure the graft at 20° to 30° of knee flexion with the patella resting naturally in the trochlear groove, applying only minimal manual tension. Once secured, the patella must retain 1 to 2 quadrants (8 to 10 mm) of passive lateral translation and tilt symmetrically. If the patella cannot be displaced laterally by at least one quadrant, the graft is overtensioned and must be immediately reset.
• Physis-Preserving Architecture in Skeletally Immature Knees: In pediatric patients with open physes, standard transphyseal femoral drilling risks angular deformity or leg length discrepancy. Position the femoral guide pin strictly distal to the distal femoral physis under fluoroscopic guidance, angled distally and anteriorly, or utilize an adductor magnus tendon sling technique to avoid the physis altogether.
• Patellar Tunnel Safety Margins: Maintain a minimum 15 mm bony bridge between parallel patellar drill tunnels. Drill in the anterior half of the patellar thickness parallel to the articular cartilage under direct tactile and visual control to completely eliminate the risk of articular surface violation or iatrogenic patellar fracture.

Surgical Pitfalls

• The Anterior/Proximal Femoral Trap: Placing the femoral tunnel even 5 mm anterior or proximal to Schöttle’s point causes the reconstructed ligament to lengthen significantly during knee flexion. This generates massive patellofemoral contact pressures, causes severe flexion block, accelerates medial facet chondromalacia, and predisposes to graft rupture.
• The Isolated Lateral Retinacular Release Fallacy: Performing an isolated lateral release for patellar instability fails to correct the incompetent medial restraints and destabilizes the extensor mechanism, frequently precipitating debilitating iatrogenic medial patellar subluxation. Lateral release or lengthening should only be performed as an adjunct to MPFL reconstruction in the setting of documented lateral retinacular contracture with a negative tilt test.
• Neglecting Structural Osseous Malalignment: Performing isolated soft-tissue MPFL reconstruction in the presence of an unaddressed TT-TG distance >= 20 mm, severe patella alta (Caton-Deschamps > 1.3), or femoral anteversion > 30° subjects the soft-tissue graft to pathological lateral vectors, leading to stretching, graft failure, and recurrent dislocation.
• Missing Occult Osteochondral Shear Fractures: A tense hemarthrosis following acute patellar dislocation contains an osteochondral fragment in up to 30% to 40% of cases. Failure to obtain thorough MRI or perform diagnostic arthroscopic joint exploration risks missing free-floating intra-articular fragments that cause irreversible cartilage destruction and mechanical locking.

12. Controversy

Substantial debate persists regarding surgical timing, realignment thresholds, and the role of bony trochlear reshaping in complex patellar instability.

Immediate Surgical Stabilization vs. Conservative Care in First-Time Dislocators

The traditional treatment paradigm reserves operative intervention for recurrent dislocators or those with displaced osteochondral fractures. However, recent Level I systematic reviews (Smith et al. Cochrane 2023) demonstrate that immediate surgical stabilization significantly reduces the relative risk of recurrent dislocation by 43% compared to nonoperative management (RR 0.57, 95% CI 0.41 to 0.80). Despite this reduction in recurrence, intermediate-term functional PROMs and adverse event profiles show no statistically significant difference between cohorts. Consequently, widespread consensus favors a selective approach: first-time dislocators with normal osseous anatomy are managed conservatively, whereas patients presenting with multiple high-risk anatomical factors (skeletal immaturity, high-grade trochlear dysplasia, patella alta, elevated TT-TG) are considered for early primary stabilization.

Management of Borderline TT-TG Distance (15 to 20 mm)

While a TT-TG distance >= 20 mm is an established threshold for tibial tubercle osteotomy (TTO) medialization, management of the borderline zone (15 to 19 mm) remains controversial. One school of thought advocates isolated MPFL reconstruction, arguing that anatomical soft-tissue reconstruction provides sufficient mechanical restraint, avoiding the morbidity, delayed union, and hardware complications associated with TTO. The opposing biomechanical view maintains that an elevated TT-TG exerts an uncorrected lateral vector on the graft, leading to progressive graft attenuation, persistent anterior knee pain, and late recurrent instability. Increasingly, surgeons utilize the TT-PCL distance (threshold >= 24 mm) and patient-specific cartilage status to guide decision-making in this intermediate cohort.

Deepening Trochleoplasty: Geographic and Technical Divergence

Deepening trochleoplasty directly corrects the primary anatomical deformity in high-grade trochlear dysplasia (Dejour Types B and D with a prominent bump). Although widely performed in specialized European patellofemoral centers with excellent long-term joint stability and patient satisfaction, the procedure has historically faced reluctance among North American surgeons. Concerns include a steep learning curve, risks of osteochondral flap necrosis, severe postoperative arthrofibrosis requiring secondary manipulation, and long-term patellofemoral degenerative arthritis. As thin-flap techniques and specialized instrumentation have matured, selective utilization of trochleoplasty in severe recurrent instability is gradually expanding in specialized tertiary centers worldwide.

13. Evidence-Based Synthesis

Modern clinical decision-making is grounded in high-quality randomized controlled trials and prospective registry analyses, corroborated by seminal biomechanical and anatomical descriptions.

14. References

• Smith TO, Gaukroger A, Metcalfe A et al. Surgical versus non-surgical interventions for treating patellar dislocation. Cochrane Database of Systematic Reviews. 2023. doi:10.1002/14651858.CD008106.pub4 [Level I: Systematic Review & Meta-Analysis of RCTs]
• Flores GW, de Oliveira DF, Ramos APS et al. Conservative management following patellar dislocation: a level I systematic review. Journal of Orthopaedic Surgery and Research. 2023. doi:10.1186/s13018-023-03867-6 [Level I: Systematic Review of Level I Studies]
• Whittaker JL, Losciale JM, Juhl CB et al. Risk factors for knee osteoarthritis after traumatic knee injury: a systematic review and meta-analysis of randomised controlled trials and cohort studies for the OPTIKNEE Consensus. British Journal of Sports Medicine. 2022. doi:10.1136/bjsports-2022-105496 [Level I: Systematic Review & Meta-Analysis (OPTIKNEE Consensus)]
• Dejour H, Walch G, Nove‐Josserand L, Guier C. Factors of patellar instability: An anatomic radiographic study. Knee Surgery, Sports Traumatology, Arthroscopy. 1994. doi:10.1007/bf01552649 [Level IV: Comparative Anatomic & Radiographic Series]
• Kujala UM, Jaakkola LH, Koskinen SK et al. Scoring of patellofemoral disorders. Arthroscopy. 1993. doi:10.1016/s0749-8063(05)80366-4 [Level IV: Clinical Metric Validation Study]

15. Master Class Pro-Tip

During medial patellofemoral ligament (MPFL) reconstruction, isometric femoral fixation is the single most critical determinant of clinical success. Always establish the true lateral fluoroscopic projection by ensuring absolute superposition of the posterior femoral condyles with zero rotational obliquity. Under fluoroscopy, place your guide pin at Schöttle’s point: 1.3 mm anterior to the posterior femoral cortex line and 2.5 mm distal to the posterior origin of the medial femoral condyle tangent. Before reaming the femoral socket, wrap the graft around the guide pin and cycle the knee through a full arc from 0° to 110°. A truly isometric graft will exhibit less than 1 to 2 mm of excursion. Fix the graft in 20° to 30° of flexion with the patella centered in the trochlear groove, applying only minimal manual tension. The reconstructed patella must retain 1 to 2 quadrants of passive lateral translation; over-tensioning the graft converts a stabilizing checkrein into a destructive tether that accelerates medial patellofemoral chondrosis and causes persistent flexion loss.

Master Class Surgical Pearl: Anatomical MPFL Isometric Fixation