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Patellar Instability and Patellar Dislocations: A Comprehensive Guide

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1. ANATOMY AND BIOMECHANICS

• The femoral trochlear groove guides the patella through a cam-like mechanism
• The lateral trochlear facet is normally more prominent than the medial, resisting lateral patellar displacement
• The patella begins engaging the trochlea at approximately 20-30 degrees of knee flexion - below this arc, soft tissues dominate

• Medial Patellofemoral Ligament (MPFL): The primary medial restraint (50-60% of resistance to lateral translation). Runs from the superomedial patella to the medial femoral epicondyle (Schottle's point)
• Vastus Medialis Obliquus (VMO): Active dynamic medial stabilizer
• Lateral retinaculum: Normally thicker/stronger - imbalance predisposes to lateral dislocation
• Patellotibial and patellomeniscal ligaments: Secondary restraints

• Q angle: Angle between ASIS-patella line and patella-tibial tubercle line; normal <15° men, <20° women
• TT-TG distance (tibial tubercle to trochlear groove): Normal 9-13 mm; >20 mm = high risk for instability
• Patella alta: Caton-Deschamps index >1.2 or Insall-Salvati ratio >1.2
• Trochlear sulcus angle: Normally <140° on axial views

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2. CLASSIFICATION OF PATELLAR INSTABILITY

A. Functional Classification (Lyon/Dejour School)

• Patellar instability: current approach, PMC 2026

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B. Types of Patellar Dislocation by Direction

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C. Trochlear Dysplasia: Dejour Classification

• Type 0 (no dysplasia): Sulcus angle <157°; lateral trochlear inclination (LTI) >14°
• Type 1 (low-grade): Sulcus angle >157°; LTI <14°; central bump <5 mm
• Type 2 (moderate): Sulcus angle/LTI unmeasurable; central bump <5 mm
• Type 3 (high-grade): Sulcus angle >157° or unmeasurable; LTI <14° or unmeasurable; central bump >5 mm

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3. RISK FACTORS AND PREDISPOSING ANATOMY

• Miller's Review of Orthopaedics 9th Ed, p. 365
• Campbell's Operative Orthopaedics 15th Ed, p. 1861

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4. SIGNS AND SYMPTOMS

History

• Mechanism: Pivoting/twisting injury with foot planted (non-contact); external tibial rotation with knee in slight flexion; or direct blow to the medial aspect of the patella
• Subjective dislocation event: Patient may report seeing or feeling the kneecap "slip out," followed by spontaneous reduction as the knee extends
• Pop: Often heard/felt - can mimic ACL tear; in patients aged 9-14 years, this mechanism is more likely a patellar instability event than ACL injury
• Previous episodes: Recurrence rate 15-60% after first dislocation; younger patients and those with dysplasia at higher risk

Symptoms

Physical Examination Findings

• Knee held in flexion (guarding)
• Visible lateral displacement of the patella (if not spontaneously reduced)
• Swelling/effusion
• Bruising (ecchymosis) developing within hours, particularly medially (from medial retinaculum/MPFL tear)

• Medial tenderness: Tenderness along the medial retinaculum, medial patellar border, and MPFL origin at the medial femoral condyle - highly specific for MPFL rupture
• Joint effusion: Ballottement sign; wave sign
• Hemarthrosis: Suggests osteochondral fracture
• Defect: Palpable defect at medial retinaculum

• Miller's Review of Orthopaedics 9th Ed, p. 365
• Rosen's Emergency Medicine, p. 721

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5. INVESTIGATIONS AND EVALUATION

Radiographs (First-Line)

• Empty trochlear groove (if not yet reduced)
• Displaced patella (lateral)
• Joint effusion (radiopaque soft tissue swelling)
• Osteochondral fracture fragment - most commonly at the medial inferior patella edge or lateral femoral condyle
• Lipohemarthrosis (on horizontal beam lateral) - suggests intra-articular fracture

• Crossing sign: Trochlear groove line crosses the anterior femoral condyle outline = dysplasia present
• Supratrochlear spur: Bony bump proximal to trochlea = Type B dysplasia
• Double contour sign: Hypoplastic medial condyle creates second contour = Type C/D dysplasia

CT Scan

• Gold standard for measuring TT-TG distance (most accurate)
  • Normal: 9-13 mm
  • 15-20 mm: borderline

  • 20 mm: significantly associated with instability

• Quantifies trochlear morphology in cross-section
• Detects osseous loose bodies
• Identifies torsional deformities (femoral anteversion, tibial torsion)
• Dynamic CT can assess patellar tracking in different flexion angles

MRI (Essential in Acute Dislocation with Effusion)

The International Patellofemoral Study Group recommends MRI for patients with large hemarthrosis to detect osteochondral fractures.

• Rockwood and Green's Fractures in Adults 10th Ed, p. 3020

Arthroscopy (Selective Use)

• Not routine; reserved for:
  • Large hemarthrosis with suspected loose osteochondral fragments
  • Failure of conservative management
  • Pre-surgical assessment
  • Concurrent intra-articular pathology

Patient-Reported Outcome Measures

• Kujala Score (AKPS): Anterior knee pain scale; gold standard for patellofemoral outcomes
• IKDC (International Knee Documentation Committee)
• Lysholm score
• KOOS (Knee Injury and Osteoarthritis Outcome Score)

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6. EVALUATION FRAMEWORK (SYSTEMATIC APPROACH)

STEP 1: Establish dislocation vs subluxation vs alternative diagnosis
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STEP 2: Assess for urgent reduction (if still dislocated)
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STEP 3: Post-reduction imaging (X-ray: AP + lateral + sunrise)
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STEP 4: Identify osteochondral fracture / loose bodies
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STEP 5: MRI if hemarthrosis or suspected osteochondral injury
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STEP 6: Risk-stratify for recurrence (age, sex, trochlear dysplasia, 
         TT-TG distance, patella alta, prior episodes)
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STEP 7: If surgical candidate: CT for TT-TG; full morphological assessment
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STEP 8: Outcome measures + physiotherapy assessment

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7. TREATMENT

A. Emergency / Acute Management: Closed Reduction

1. Provide adequate analgesia (oral analgesics usually sufficient; rarely needs sedation)
2. Patient supine, hip flexed to relax hamstrings
3. Slowly and gently extend the knee (passively)
4. Apply inferomedial pressure to the lateral patella as the knee extends
5. A second provider may assist with gentle downward pressure on the lateral patella to "open" the medial facet
6. Confirm reduction - a palpable/audible "click" as patella seats into trochlea

• Neurovascular examination
• Knee immobilizer in full extension
• Post-reduction radiographs: AP + lateral + sunrise (to confirm centering + exclude fracture)
• Weight bear as tolerated with crutches
• Orthopedic follow-up within 1-2 weeks

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B. Conservative (Non-Operative) Treatment

• Knee immobilizer in extension
• Cryotherapy + NSAIDs for pain and swelling
• Isometric quadriceps exercises

• Patella-stabilizing brace
• Progressive range of motion
• Closed-chain quadriceps exercises (especially VMO emphasis)
• Hip abductor/external rotator strengthening
• Proprioception training

• Return when full passive ROM regained, no effusion, quadriceps strength ≥80% of contralateral side
• Functional sport-specific training

• Recurrence rates 15-44% (conservative) to 15-60% in some series
• Recurrence risk highest in: age <15, trochlear dysplasia, patella alta, elevated TT-TG, female sex
• Surgically and conservatively managed patients have similar long-term functional outcomes in first-time dislocators without major structural injury

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C. Surgical Treatment

• Osteochondral fracture with free fragment (fix or remove)
• Large complete MPFL avulsion/midsubstance tear (selected cases)
• Failure of conservative management / recurrent instability
• First-time dislocation in high-risk patients (trochlear dysplasia grade B-D, TT-TG >20 mm, high-level athletes)
• Medial iatrogenic instability (if from prior surgery)

1. MPFL Reconstruction (Proximal Realignment)

• Autograft: gracilis or semitendinosus tendon
• Graft routed from patella (patellar tunnels or suture anchors) to anatomic femoral origin
• Schottle's point = anatomic femoral attachment of MPFL, identified on lateral radiograph:
  • 1 mm anterior to posterior cortex extension line
  • 2.5 mm distal to posterior origin of medial femoral condyle
  • Proximal to posterior point of Blumensaat line

• Isolated medial retinacular plication and VMO advancement have largely fallen out of favour due to higher failure rates

2. Tibial Tubercle Osteotomy (Distal Realignment)

• Anteromedialization (Fulkerson osteotomy): Combined anteromedial transfer of the tibial tubercle
  • Indicated when: TT-TG >20 mm, associated distal patellar chondromalacia (anteriorization offloads the patella)
  • Contraindication: Proximal patellar arthrosis of the medial facet
• Medial transfer alone (Elmslie-Trillat): For isolated TT-TG elevation without chondral disease
• Fixed with screws; partial weight-bearing post-op

3. Trochleoplasty

• Surgical deepening/reshaping of the trochlear groove
• Indicated for: High-grade trochlear dysplasia (Dejour B/D with supratrochlear bump) when bony containment is insufficient
• Technically demanding; usually combined with MPFL reconstruction
• Sulcus-deepening trochleoplasty (Bereiter technique) most common

4. Lateral Release / Lateral Retinacular Lengthening

• Isolated lateral release should NOT be performed for patellar instability (causes medial iatrogenic instability)
• Can be added as an adjunct to address excessive lateral tightness alongside other procedures

5. Proximal Femoral Osteotomy

• For patients with significant femoral anteversion (>25-30°) contributing to instability
• Derotational femoral osteotomy corrects excessive anteversion

6. Loose Body Removal / Osteochondral Repair

• Arthroscopic removal of free intra-articular fragments
• OATS (osteochondral autograft transfer) or autologous chondrocyte implantation for large cartilage defects

Recurrent instability / first-time with risk factors
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Assess TT-TG distance (CT)
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TT-TG <20 mm                    TT-TG >20 mm
     ↓                                ↓
MPFL reconstruction alone       MPFL + tibial tubercle 
                                 anteromedialisation
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Assess trochlear dysplasia (Dejour)
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Grade A/C (no bump)            Grade B/D (bump present)
MPFL +/- TTO               MPFL + trochleoplasty +/- TTO

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8. SPECIAL CONSIDERATIONS

Pediatric Patients (Open Physes)

• First-time dislocation remains most common cause of traumatic hemarthrosis in children
• Bony procedures at tibial tubercle risk physeal damage until skeletal maturity
• MPFL reconstruction can be performed physeal-sparing
• Higher recurrence risk in children <15 years

Superior Patellar Dislocation

• Rare (fewer than 50 cases reported)
• Presents with: high-riding patella, acute knee pain, inability to perform straight leg raise, skin dimple deformity
• Often in patients with large osteophytes (knee osteoarthritis) - the osteophyte entraps the patella in hyperextension
• Requires manual reduction; may need surgical osteophyte removal if recurs

Iatrogenic Medial Dislocation

• Occurs after overzealous lateral release or distal realignment over-correction
• Presents with medial apprehension, locking in flexion
• Requires revision realignment (medial retinacular release + lateral reconstruction)

Habitual Dislocation

• Occurs predictably at a consistent flexion angle during normal walking
• Usually congenital/developmental basis
• Often requires surgical correction combining MPFL reconstruction + trochleoplasty

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9. COMPLICATIONS

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Summary Table

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• Miller's Review of Orthopaedics, 9th Ed - pp. 364-366 (patellar instability, Dejour classification, MPFL)
• Campbell's Operative Orthopaedics, 15th Ed 2026 - p. 1861 (patellar dislocations, treatment)
• Rockwood and Green's Fractures in Adults, 10th Ed 2025 - pp. 3019-3021 (acute patellar dislocations)
• Rosen's Emergency Medicine - pp. 720-721 (acute management)
• Tintinalli's Emergency Medicine - p. 960 (patellar dislocations in children)
• Patellar instability: current approach (PMC, 2026) - Dejour V3.0 classification, updated surgical algorithms

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Patellar Instability and Patellar Dislocations: A Complete Clinical Guide

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SECTION 1 — ANATOMY AND STABILIZERS OF THE PATELLOFEMORAL JOINT

Bony Stabilizers

• The lateral trochlear facet is normally taller and more prominent than the medial, creating a "wall" that resists lateral patellar displacement
• The patella engages the trochlea at approximately 20-30° of knee flexion - below this arc, soft tissues provide primary restraint
• Any trochlear shallowness (dysplasia) eliminates this bony containment

Soft Tissue Stabilizers

Key Biomechanical Measurements

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SECTION 2 — CLASSIFICATION OF PATELLAR INSTABILITY

A. By Functional Pattern (Lyon/Dejour Classification)

B. By Direction of Dislocation

C. Dejour Classification of Trochlear Dysplasia

• Type 0 (no dysplasia): Sulcus angle <157°; LTI >14°
• Type 1 (low-grade): Sulcus angle >157°; LTI <14°; central bump <5 mm
• Type 2 (moderate): Sulcus angle/LTI unmeasurable; central bump <5 mm
• Type 3 (high-grade): Bump >5 mm; most severe - requires trochleoplasty

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SECTION 3 — RISK FACTORS AND PREDISPOSING ANATOMY

• Campbell's Operative Orthopaedics 15th Ed, p. 1861
• Miller's Review of Orthopaedics 9th Ed, p. 365

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SECTION 4 — SIGNS AND SYMPTOMS

History

• Mechanism: Pivoting/twisting on a planted foot (most common - non-contact); external tibial rotation with knee flexed; or direct blow to the medial patella
• Age 9-14: A pop + swelling after twisting is more likely patellar instability than ACL tear
• Prior episodes: Recurrence rate is 15-75% after first dislocation
• Spontaneous reduction: Patella often self-reduces as the knee extends; patient describes seeing or feeling the kneecap "pop out and back in"

Symptoms

Physical Examination

• Knee held in partial flexion (guarding)
• Visible lateral displacement if still dislocated
• Diffuse swelling and ecchymosis (especially medially - from medial retinaculum/MPFL tear)
• Quadriceps wasting in chronic cases

• Medial retinacular tenderness - the cardinal sign; along medial patellar border and at MPFL attachment on medial femoral condyle
• Hemarthrosis / joint effusion - ballottement, fluctuance
• Palpable defect in the medial retinaculum (acute)
• Diffuse patellar tenderness

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SECTION 5 — INVESTIGATIONS AND EVALUATION

Step-by-Step Investigation Framework

ACUTE PRESENTATION
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1. Clinical examination → Confirm dislocation or spontaneous reduction
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2. Plain radiographs (AP + lateral ± Merchant/sunrise)
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3. Reduce if still dislocated → Post-reduction films
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4. Is there hemarthrosis or suspected osteochondral fracture?
   YES → MRI
   NO → Conservative management
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5. Recurrent/chronic instability: CT for TT-TG measurement
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6. Risk stratification → Guide surgical planning

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Radiography (First-Line in All Cases)

• Empty or shallow trochlear groove (AP/axial)
• Laterally displaced patella
• Soft tissue swelling/effusion
• Osteochondral fracture fragment at medial inferior patellar margin (pathognomonic avulsion) or lateral femoral condyle
• Lipohemarthrosis on horizontal-beam lateral = intra-articular fracture

• Crossing sign: Floor of trochlear groove crosses the anterior femoral condyle line = dysplasia present
• Supratrochlear spur: Bony prominence proximal to trochlea = Type B/D
• Double contour: Hypoplastic medial condyle line below crossing sign = Type C/D

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MRI (Recommended for All Acute Dislocations with Effusion)

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CT Scan (For Surgical Planning - TT-TG Measurement)

• Precise trochlear depth and sulcus angle measurement
• Torsional profile (femoral anteversion, tibial torsion)
• Detect osseous loose bodies
• Dynamic CT to assess patellar tracking during flexion

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Arthroscopy (Selective)

• Large hemarthrosis with suspected loose osteochondral fragments
• Concurrent intra-articular pathology
• Pre-surgical assessment of cartilage
• Removal of free fragments

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Patient-Reported Outcomes

• Kujala score (AKPS) - gold standard patellofemoral outcome tool
• IKDC - International Knee Documentation Committee
• KOOS - Knee Injury and Osteoarthritis Outcome Score
• Lysholm score

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SECTION 6 — TREATMENT

A. Emergency Management: Closed Reduction

1. Analgesia (oral/IV; conscious sedation usually not needed)
2. Patient supine, hip flexed to relax hamstrings
3. Gently and slowly extend the knee while simultaneously applying inferomedially-directed pressure to the patella
4. If difficult: gentle downward pressure on the lateral patellar border to "open" the medial facet
5. A "click" is felt/heard as the patella seats into the trochlea
6. Neurovascular check immediately after reduction
7. Post-reduction radiographs (AP + lateral + sunrise) to confirm centring and exclude osteochondral fracture

• Knee immobilizer in full extension
• Weight bear as tolerated with crutches
• Orthopedic follow-up within 1-2 weeks
• Ice + elevation + NSAIDs for pain/swelling

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B. Conservative (Non-Operative) Management

• Recurrence rate: 15-44% (Rockwood & Green) to 15-75% (Campbell's)
• No long-term functional difference vs surgery in first-time dislocators without osteochondral injury

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C. Surgical Treatment

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Surgical Procedure 1: MPFL Reconstruction (Proximal Realignment)

• Graft: gracilis or semitendinosus tendon (autograft or allograft)
• Patellar attachment: suture anchors or bone tunnels at superomedial patellar border
• Femoral attachment: Schottle's point (anatomically critical)

• Miller's Review of Orthopaedics 9th Ed, p. 366

• Isolated lateral release (contraindicated for instability - causes medial iatrogenic dislocation)
• VMO advancement (Green procedure)
• Medial retinacular plication alone

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Surgical Procedure 2: Tibial Tubercle Osteotomy (Distal Realignment)

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Surgical Procedure 3: Trochleoplasty

• Indicated for: High-grade trochlear dysplasia with supratrochlear bump (Dejour Type B, D, and V3.0 Type 3) where the bony deformity is too severe for soft tissue procedures alone
• Technique: Bereiter sulcus-deepening trochleoplasty - the supratrochlear bump is resected/levelled by elevating a thin osteochondral flap, deepening the underlying groove, then reseating the cartilage flap
• Usually combined with MPFL reconstruction
• Technically demanding; requires significant surgical expertise
• Indicated when: central bump >5 mm on MRI (V3.0 Type 3)

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Current Treatment Algorithm (2024-2026)

ACUTE FIRST-TIME DISLOCATION
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Osteochondral fracture present?
   YES → Surgery (fix or remove fragment)
   NO → Conservative management
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RECURRENT INSTABILITY / SURGICAL CANDIDATE
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Step 1: ALWAYS include MPFL reconstruction (ruptured >90% of cases)
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Step 2: Evaluate TT-TG distance (CT)
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      TT-TG <20 mm              TT-TG >20 mm
      MPFL alone                 MPFL + tibial tubercle
                                  osteotomy (medialization)
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Step 3: Evaluate trochlear dysplasia (Dejour)
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    Type A/C (no bump)        Type B/D or V3.0 Type 3 (bump)
    No trochleoplasty          Add trochleoplasty
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Step 4: Evaluate patella height
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    Patella alta (CDI >1.2)    Target: PHI <1.16 post-op
    Consider tubercle          SPE >38%, TT-TG 8-14 mm post-op
    distalization

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D. Specific Management of Rare Dislocation Types

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SECTION 7 — COMPLICATIONS

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SECTION 8 — SUMMARY TABLE

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• Campbell's Operative Orthopaedics, 15th Ed 2026, p. 1861 - patellar dislocations, treatment
• Miller's Review of Orthopaedics, 9th Ed, pp. 364-366 - patellar instability, Dejour classification, MPFL, surgical techniques
• Rockwood and Green's Fractures in Adults, 10th Ed 2025, pp. 3019-3021 - acute patellar dislocations, preferred treatment
• Rosen's Emergency Medicine, pp. 720-721 - clinical features, closed reduction technique
• Tintinalli's Emergency Medicine - pediatric patellar dislocations
• Patellar instability: current approach (PMC 2026) - Dejour V3.0 classification, 2024 updated treatment algorithm