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Patellar Instability

From WikiSM

Other Names

  • Unstable Patella
  • Lateral Patellar Instability
  • Patellofemoral Instability
  • Recurrent Patellar Dislocation
  • Patellar Dislocation
  • Patellar Subluxation
  • Chronic Patellar Instability
  • Recurrent Patellar Subluxation

Background

History

  • 1950s–1970s: Trillat & Lyon School established patellar instability as a biomechanical malalignment problem and introduced early realignment procedures such as tibial tubercle transfer[1]
  • Mid–Late 20th Century the use of lateral release, medial reefing, and other nonanatomic procedures with inconsistent outcomes highlighted the need for better understanding of patellofemoral mechanics [2]
  • In 1980 Dejour identified key anatomic risk factors (trochlear dysplasia, patella alta, increased TT–TG distance)[3]
  • Biomechanical studies established the medial patellofemoral ligament as the primary restraint to lateral patellar translation[4]

Epidemiology

  • Incidence
    • 29/100,000 per capita risk of dislocation among adolescents (need citation)
  • Prevalence
    • 2-3% of presentations involving the knee joint include a history of patellar dislocation (need citation)
    • 50-60% of first time lateral patellar dislocations occur secondary to a sport related injury (need citation)
    • Up to 40% of skeletally immature patients may develop recurrent instability[5]
    • Up to 10% of patients may develop contralateral patellar instability[6]

Introduction

Normal patellofemoral joint, patellar instability and patellar dislocation
Lateral patellar instability in flexion. The patella dislocates laterally beyond 40º of knee flexion[7]

General

  • Characterized by abnormal lateral displacement of the patella, ranging from subluxation to recurrent dislocation of the patellofemoral joint
  • Common causes include traumatic injury, trochlear dysplasia, patella alta, increased TT–TG distance, and ligamentous laxity
  • Patients typically report anterior knee pain, a sensation of the knee “giving way,” recurrent dislocations, and apprehension with knee flexion or activity
  • Diagnosis is made clinically, imaging can assess alignment and structural abnormalities
  • Treatment from nonoperative management to surgical intervention depending on severity

Etiology

  • General
  • Acute traumatic
  • Episodic/ recurrent instability/ chronic patholaxity
    • Most common type, which occurs after an initial dislocation
    • Seen in adolescent athletes, more commonly in women
  • Alternative patterns described by Chotel in pediatrics[8]
    • Congenital dislocation
    • Permanent dislocation
    • Habitual dislocation in knee flexion
    • Habitual dislocation in knee extension
  • Syndromic instability

Associated Conditions

Anatomy of the Patellofemoral Joint

Anatomy of the Medial Patellofemoral Ligament
  • Patella
  • Medial Patellofemoral Ligament
    • Primary static restraint to lateral instability during first 30° of flexion
    • Prevents excessive lateral movement of the patella
    • Part of the medial patellar retinaculum
  • Medial Patellar Retinaculum
    • Important stabilizer of the patella
    • Resists lateral patellar displacement /dislocation
  • Vastus Medialis Obliquus (VMO)
    • Most distal portion of the medial quadriceps muscle
    • Exerts a medially directed force that helps keep the patella in position
  • Trochlear Groove of the Femur
    • Variants in trochlear morphology can predispose the patella to maltracking[9]
    • Gross subluxation/dislocation
    • Can influence recurrent patellar instability
  • Tibial Tubercle
    • Arises from the lateral aspect of the proximal tibia
    • Excessive lateralization increases tibial external rotation, severe genu valgum, or even increased femoral anteversion[10]
  • All effect patellar tracking

Risk Factors

Bony / Anatomic / Biomechanical Factors

  • Trochlear Dysplasia (OR: 18.1)[11]
    • Abnormal geometry of the trochlear groove that reduces the bony constraint for patellar tracking
    • Present in 68.3% of patients with recurrent dislocation[10]
  • Patella Alta
    • High riding patella delays patellar engagement in the trochlear groove
    • Caton-Deschamps >1.2 or Insall-Salvati >1.2
    • Found in 60% of recurrent dislocation patients[12]
  • Increased TT–TG distance (OR: 2.1)
    • Lateral positioning of the tibial tubercle increases lateralizing forces on the patell
    • Present in 42% of recurrent cases[13]
  • Patellar tilt (Reflects maltracking and lateral soft tissue imbalance)
  • Lateral femoral condyle hypoplasia (decreased lateral bony restraint)
  • Genu Valgum: Increased Q-angle contributing to lateral tracking
  • Miserable Malalignment Syndrome: femoral anteversion, genu valgum, external tibial torsion

Soft Tissue Factors

Patient-Specific Factors

  • Adolescent age (OR 2.61-2.72 for recurrence)[14]
    • Particularly those under 18 with open physes
    • Highest risk group for primary and recurrent instability
  • History of patellar dislocation
    • Strong predictor of recurrence
  • Female sex
  • Race
    • Reported associations in African American and Caucasian populations
  • Family history

Systemic / Connective Tissue Disorders

Differential Diagnosis

Clinical Features

Clinical example of laterally dislocated patella[7]
Patellar Apprehension Test

History

  • Important to review history of previous patellar dislocations, episodes of patellar instability
    • About 1/3 of patients will have recurrent dislocation, often within first 2 years[14]
    • Contralateral dislocations occur in 5.4% of patients
  • Patient will complain of anterior knee pain
  • Must review risk factors (anatomic, biomechanical, etc)

Physical Exam: Physical Exam Knee

  • Begin with observation of gait and standing alignment
    • May reveal genu valgum, pes planus, or an increased Q-angle
  • In acute dislocation, deformity, effusion/ hemarthrosis may be present
  • Evalaute range of motion, which should be intact
  • Tenderness
    • Commonly along medial patellar facets, inferior pole of the patella, medial parapatellar structures
    • Indicates MPFL injury

Special Tests

Evaluation

AP Knee xray demonstrating laterally dislocated patella in a 12 year old
AP Knee xray with osteochondral defect following patella dislocation

Radiographs

  • Standard Radiographs Knee
    • Ideally, standard AP and lateral weight bearing views, as well as sunrise view
    • May not be possible in setting of acute dislocation
    • Help identify fractures of the patella, avulsion fractures, loose bodies and sometimes large cartilage defects
    • PA radiographs at 45 degrees flexion may aid in assessment of the coronal alignment of the tibiofemoral joint
  • Lateral views and Sunrise or Merchant views
    • Provide information to trochlear morphology, patellar height and patellar tilt
  • Lateral patellar Tilt ((Laurin’s angle))
    • Assessed by the lateral patellofemoral angle on sunrise or merchant view
    • Angle is measured between a line along the subchondral bone of the lateral trochlear facet and posterior femoral condyles
    • Normal: angle greater than 11° that opens laterally
    • Abnormal angles: parallel or open medially
  • Patellar height
    • Can be measured by both direct and indirect methods
    • The Insall-Salvati Ratio: ratio measuring the length of the patella ligament, patellar length
      • A normal ratio is 1.0; a ratio of 1.2 suggests patella alta and 0.8 patella baja
    • Caton-Deschamps Index (CDI): distance between the distal point of the patellar articular surface and the anterior superior margin of the tibia, divided by the patellar articular surface length
      • A normal ratio is 1.0; a ratio of less than 0.6 suggests patella baja and a ratio of 1.3 suggests patella alta
    • Blackburne-Peel method (BP): ratio of the height of the lower pole of the articular surface above a tibial plateau line to the articular surface length of the patella
      • Normal between 0.54- 1.06; A ratio of less than 0.54 is considered to be patella alta
    • Technique described by Blumensaat uses the roof of the intercondylar notch as a reference line and is one of the most commonly used direct methods for the assessment of patellar height
  • True lateral radiographs and sunrise views can help identify other risk factors
    • The trochlear findings were elucidated by Dejour and Le Coultre and were subsequently revised to create the trochlear dysplasia classification system [18]
    • Crossing sign: occurs when the trochlear groove lies in the same plane as the anterior border of the lateral condyle, which represents a flattened trochlear groove
    • Double contour sign: occurs when the anterior border of the lateral condyle lies anterior to the anterior border of the medial condyle, which represents a convex trochlear groove or hypoplastic medial condyle
    • Supratrochlear spur can arise from the proximal aspect of the trochlea and can also indicate a risk factor

CT

  • Can more accurately characterize the morphology of the trochlea
  • Assess femoral and tibial torsion
  • Tibial tubercle to trochlear groove (TT-TG) distance
    • Assesses relative rotation of femur to tibia
    • The TT-GG distance is between two perpendicular lines; one from the posterior cortex to the tibial tubercle and one from the posterior cortex to the trochlear groove
    • Average 8-10 mm in pediatric and adult patients; a TT-TG distance of greater than 20 is highly associated with patellar instability

MRI

  • Common Findings[19]
    • Bruising pattern of lateral femoral condyle, medial patella
    • Disruption of the MPFL (at the medial femoral epicondyle insertion)
    • Articular cartilage injuries if present

Classification

Classification of patella instability and maltracking[20]

Parikh Classification of Patellar Instability

  • Type I: first patellofemoral dislocation with (A) or without (B) osteochondral fracture[21]
  • Type II: recurrent subluxation (A) or dislocation (B)
  • Type III: dislocatable patella by the examiner or patient which is either passive (A) or habitual in flexion/extension (B).
  • Type IV: dislocated patella that is either reducible (A) or irreducible (B).

Management

Patellar J Brace


Nonoperative

  • Indications
    • Majority of cases
    • Absence of loose bodies or osteochondral fragments, other soft tissue injuries
  • NSAIDS
  • Physical Therapy
    • Goals: restoring neuromuscular control and patellar stability[22]
    • Emphasis: Quadriceps/ VMO Strengthening gluteal, core and hip strengthening
    • Flexibility and stretching
  • Kinesiology Taping
  • Knee Immobilizer
    • May be indicated early in acute setting
  • Patellar Brace
    • Patellar stabilizing sleeve or "J" brace
    • Often used for 6-8 weeks of recovery with the goal of return to full motion
  • Activity modification

Operative

  • Indications
    • Failure of conservative management
    • Associated osteochondral fragment (≥ 5 mm)
    • Associated osseous avulsion of the MPFL
    • Associated meniscus tear
  • Techniques
    • Medial patellofemoral ligament reconstruction
    • MPFL reconstruction with autograft vs allograft
    • Trochleoplasty
    • Tibial tubercle osteotomy
    • Arthroscopic debridement (removal of loose body) vs Repair
    • Fulkerson-type osteotomy (anterior and medial tibial tubercle transfer)
    • Tibial tubercle distalization
    • Lateral release

Rehab and Return to Play

General patellar instability exercises
Patellar instability exercises

General Principles of Rehabilitation

  • Primary rehab goals: reduce pain/swelling, restore full ROM (0–135°), improve patellar tracking, and regain neuromuscular control[23]
  • Strength focus: VMO, quadriceps, glutes, hip girdle, and core to optimize patellofemoral stability and alignment
  • Mobility work: stretch tight lateral structures (hamstrings, quads, calf, IT band, lateral retinaculum) to reduce lateral patellar tracking
  • Neuromuscular training + adjuncts: proprioception, balance, functional progression, with bracing, taping, orthotics, and activity modification as needed
  • Progression & return-to-sport: gradual, criteria-based loading; return when ≥90% limb symmetry, functional confidence, and psychological readiness are achieved

Phase 1: Acute Phase (Weeks 1-2)

  • Goals: Pain and edema control, protect healing tissues, initiate early motion
  • Consider Motion-restricting knee brace: 0-20-40° ROM
  • Weight-bearing: Partial weight-bearing (30-60% bodyweight) with crutches
  • Range of motion: Gentle passive and active-assisted ROM within brace limits
  • Modalities: Ice, compression, elevation for edema management
  • Muscle activation: Quadriceps sets, gluteal sets, ankle pumps
  • Patellar mobilization: Gentle medial glides (if not contraindicated post-surgery)

Phase 2: Early Strengthening (Weeks 3-4)

  • Goals: Progress ROM, restore full weight-bearing, initiate quadriceps recruitment
  • Knee brace: 0-10-60° ROM
  • Weight-bearing: Progress to full weight-bearing as tolerated
  • Range of motion: Active ROM exercises, goal of 0-90° by week 4
  • Strengthening exercises:
    • Quadriceps recruitment with emphasis on vastus medialis obliquus (VMO)
    • Straight leg raises (4 planes)
    • Double leg squats with isometric hip adduction (0-45°)
    • Closed-chain terminal knee extension
    • Hip abduction and external rotation strengthening (side-lying clamshells, standing hip abduction)
  • Flexibility: Hamstring, quadriceps, gastrocnemius, iliotibial band, tensor fascia lata stretching

Phase 3: Progressive Strengthening (Weeks 5-8)

  • Goals: Restore full ROM, advance strengthening, improve neuromuscular control
  • Knee brace: 0-0-90° ROM, wean as tolerated
  • Range of motion: Full ROM (0-135°)
  • Strengthening exercises:
    • Lunges (forward, lateral, reverse)
    • Single-leg squats (0-60°)
    • Leg press with neutral foot position
    • Step-ups and step-downs (4-6 inch height)
    • Core stability exercises (planks, side planks, dead bugs)
    • Hip strengthening progression (resistance band work, single-leg bridges)
  • Neuromuscular training:
    • Balance exercises (single-leg stance, foam pad progressions)
    • Sensorimotor training for leg axis stabilization
    • Proprioceptive exercises

Phase 4: Advanced Strengthening and Sport Specific Training (Weeks 9-16)

  • Goals: Maximize strength and power, restore dynamic stability, prepare for return to sport
  • Strengthening exercises:
    • Progressive resistance training (squats, deadlifts, leg press at maximal intended velocity)
    • Plyometric exercises (box jumps, lateral hops, depth jumps)
    • Agility drills (cutting, pivoting, deceleration)
    • Sport-specific movement patterns
  • Lateral trunk muscle training
  • Continue hip and core strengthening
  • Gradual increase in training volume and intensity based on sport demands
Return to play infographic for Patellar Instability

Return to Play

  • Functional Testing Requirements:
    • Quadriceps strength: ≥90% limb symmetry index (LSI) on isokinetic testing
    • Single-limb hop testing: ≥90% LSI on all hop tests (single hop, triple hop, crossover hop, timed hop)
    • Lateral step-down test: Proper form without knee valgus or pain
    • Side hop test: ≥90% LSI
    • Lateral leap and catch test: Successful completion with proper landing mechanics
    • Y-balance test: ≥90% LSI in all directions
    • Depth jump: Proper landing mechanics with minimal ground contact time
  • Return to Sport Progression:
    • Phase 1: Non-contact sport-specific drills (weeks 12-16)
    • Phase 2: Controlled contact drills with gradual intensity increase (weeks 16-20)
    • Phase 3: Full practice participation (weeks 20-24)
    • Phase 4: Return to competition (typically 4-6 months post-injury or post-surgery)
  • Timeline Considerations:
    • Nonoperative management: 3-4 months minimum
    • Post-MPFL reconstruction: 4-6 months minimum
    • Post-tibial tubercle osteotomy or trochleoplasty: 6-9 months minimum
  • Return to sport following Menetrey guidelines[24]
    • Full recovery of knee motion
    • Recovery of strength
    • Absence of a knee effusion, pain
    • Competence with sport specific exercises
  • Strongly consider

Prognosis and Complications

Severe Patellofemoral OA

Prognosis

  • Recurrence Rates
    • More than 33.6% of first time dislocations will re-dislocate following conservative management[14]
    • That risk approaches 36% over 20 years
    • Greatest risk is in the first 2 years
  • Risk factors for recurrence
    • Younger age (OR 2.61)
    • Open physes (OR 2.72)
    • Trochlear dysplasia (OR 4.15)
    • Elevated tibial tubercle-trochlear groove distance (OR 2.87)
    • Patella alta (OR 2.38)
  • Surgical outcomes[22]
    • Lower recurrence rates compared to conservative management
    • Surgery reduces the risk of recurrence with an odds ratio of 0.45 compared to nonoperative treatment (low quality evidence)
  • Return to sport[25]
    • Only 2/3 of patients return to their pre-injury sport level after patellar dislocation
    • Return to sport rates range from 64-82% depending on the study and treatment approach

Complications

  • Osteochondral Defect[26]
    • Occur in 70% of first time dislocations on MRI
    • Primarily affect medial patellar facet, lateral femoral condyle
  • Patellofemoral Osteoarthritis
    • 7.8 fold increased risk following patellofemoral dislocation[27]
    • Cumulative incidence: 5 years (1.2%), 10 years (2.7%), 15 years (8.15%), 20 years (14.8%), 25 years (48.9%)
    • Up to 20% of cases at 20 years following initial dislocation[6]
  • Patella Fracture
    • Can occur as a surgical complication
  • Recurrent instability
  • Failure of surgical fixation
  • Inability to return to sport
  • Quadriceps weakness
  • Pediatric specific complications
    • Physeal arrest
  • Surgical complications
    • MPFL Reconstruction complications range from 0-32.3%[28]
    • Tibial tuberocle osteotomy includes nonunion, malunion, compartment syndrome, fracture
    • Trochloeplasty include arthrofibrosis, persistent pain, cartilage damage, and progression of arthritis

See Also

Internal

External

References

  1. ↑ Arendt, Elizabeth A. “Building Bridges Across the Ocean: A Historic Review of Patellar Dislocation.” Knee Surgery, Sports Traumatology, Arthroscopy, vol. 21, no. 2, 2013, pp. 279–293.
  2. ↑ Eliasberg, Christopher D., et al. “Failure of Patellofemoral Joint Preservation.” Operative Techniques in Sports Medicine, vol. 28, no. 2, 2020.
  3. ↑ Dejour, Henri, et al. “Factors of Patellar Instability: An Anatomic Radiographic Study.” Knee Surgery, Sports Traumatology, Arthroscopy, 1987.
  4. ↑ Surendran, S., et al. “Patellar Instability – Changing Beliefs and Current Trends.” Journal of Clinical Orthopaedics and Trauma, vol. 5, no. 3, 2014, pp. 150–156.
  5. ↑ Lewallen LW, McIntosh AL, Dahm DL. Predictors of recurrent instability after acute patellofemoral dislocation in pediatric and adolescent patients. Am J Sports Med. 2013;41(3):575–81.
  6. ↑ 6.0 6.1 Sanders TL, Pareek A, Hewett TE, Stuart MJ, Dahm DL, Krych AJ. High rate of recurrent patellar dislocation in skeletally immature patients: a long-term population-based study. Knee Surg Sports Traumatol Arthrosc. 2017
  7. ↑ 7.0 7.1 Sanchis-Alfonso, Vicente, et al. "Failed medial patellofemoral ligament reconstruction: causes and surgical strategies." World journal of orthopedics 8.2 (2017): 115.
  8. ↑ Chotel F, Berard J, Raux S. Patellar instability in children and adolescents. Orthop Traumatol Surg Res. 2014;100(1 Suppl):S125–37.
  9. ↑ Weber-Spickschen TS, Spang J, Kohn L, Imhoff AB, Schottle PB. The relationship between trochlear dysplasia and medial patellofemoral ligament rupture location after patellar dislocation: An MRI evaluation. Knee. 2011;18:185–8
  10. ↑ 10.0 10.1 Steensen RN, Bentley JC, Trinh TQ, Backes JR, Wiltfong RE. The prevalence and combined prevalences of anatomic factors associated with recurrent patellar dislocation: A magnetic resonance imaging study. Am J Sports Med. 2015;43:921–7.
  11. ↑ Christensen TC, Sanders TL, Pareek A, Mohan R, Dahm DL, Krych AJ. Risk factors and time to recurrent ipsilateral and contralateral patellar dislocations. Am J Sports Med. 2017;45(9):2105–10.
  12. ↑ Only, Arthur J., Elizabeth A. Arendt, and Betina B. Hinckel. "Anatomic risk factors for lateral patellar instability." Arthroscopy 40.11 (2024): 2642-2644.
  13. ↑ Steensen, Robert N., et al. "The prevalence and combined prevalences of anatomic factors associated with recurrent patellar dislocation: a magnetic resonance imaging study." The American journal of sports medicine 43.4 (2015): 921-927.
  14. ↑ 14.0 14.1 14.2 Huntington, Lachlan S., et al. "Factors associated with an increased risk of recurrence after a first-time patellar dislocation: a systematic review and meta-analysis." The American journal of sports medicine 48.10 (2020): 2552-2562.
  15. ↑ Beighton P, Solomon L, Soskolne CL. Articular mobility in an African population. Ann Rheum Dis. 1973 Sep;32(5):413-8
  16. ↑ Ahmad, Christopher S., et al. "The moving patellar apprehension test for lateral patellar instability." The American Journal of Sports Medicine 37.4 (2009): 791-796.
  17. ↑ Zimmermann, Felix, Michael C. Liebensteiner, and Peter Balcarek. "The reversed dynamic patellar apprehension test mimics anatomical complexity in lateral patellar instability." Knee Surgery, Sports Traumatology, Arthroscopy 27.2 (2019): 604-610.
  18. ↑ Dejour D, Le Coultre B. Osteotomies in patello-femoral instabilities. Sports Med Arthrosc. 2007 Mar;15(1):39-46
  19. ↑ Elias DA, White LM, Fithian DC. Acute lateral patellar dislocation at MR imaging: injury patterns of medial patellar soft-tissue restraints and osteochondral injuries of the inferomedial patella. Radiology. 2002 Dec;225(3):736-43.
  20. ↑ Frosch, K-H., and A. Schmeling. "A new classification system of patellar instability and patellar maltracking." Archives of Orthopaedic and Trauma Surgery 136.4 (2016): 485-497.
  21. ↑ Parikh SN, Lykissas MG. Classification of lateral patellar instability in children and adolescents. Orthop Clin North Am. 2016;47(1):145–52.
  22. ↑ 22.0 22.1 Hing, Caroline B., et al. "Surgical versus non‐surgical interventions for treating patellar dislocation." Cochrane Database of Systematic Reviews 11 (2011).
  23. ↑ Watson, Richard, et al. "Lateral patellar dislocation: a critical review and update of evidence-based rehabilitation practice guidelines and expected outcomes." JBJS reviews 10.5 (2022): e21.
  24. ↑ Menetrey J, Putman S, Gard S. Return to sport after patellar dislocation or following surgery for patellofemoral instability. Knee Surg Sports Traumatol Arthrosc. 2014;22(10):2320–6.
  25. ↑ Ménétrey, Jacques, Sophie Putman, and Suzanne Gard. "Return to sport after patellar dislocation or following surgery for patellofemoral instability." Knee Surgery, Sports Traumatology, Arthroscopy 22.10 (2014): 2320-2326.
  26. ↑ Vollnberg, Bernd, et al. "Prevalence of cartilage lesions and early osteoarthritis in patients with patellar dislocation." European radiology 22.11 (2012): 2347-2356.
  27. ↑ Sanders, Thomas L., et al. "Patellofemoral arthritis after lateral patellar dislocation: a matched population-based analysis." The American journal of sports medicine 45.5 (2017): 1012-1017.
  28. ↑ Jackson, Garrett R., et al. "Complication rates after medial patellofemoral ligament reconstruction range from 0% to 32% with 0% to 11% recurrent instability: a systematic review." Arthroscopy: The Journal of Arthroscopic & Related Surgery 39.5 (2023): 1345-1356.
Created by:
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Last edited:
14 April 2026 19:49:37
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