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Patellar Dislocation
From WikiSM
Contents
Other Names
- Kneecap Dislocation
- Dislocated Kneecap
- Patellar Subluxation
Background
- This page describes subluxations and dislocations of the Patella
- Dislocation: defined as patella disengages completely from the trochlear or femoral groove
- Subluxation: defined as partial disengagement and spontaneous reduction
History
Epidemiology
- Represents about 2-3% of knee injuries
- Overall incidence is around 8 to 43 per 100,000[1][2]
- Highest among adolescents aged 14 to 18 years
- Males and females seem to be affected equally
- Recurrence rate following first time dislocation is 15-60%[3]
Pathophysiology
- General
- It is important to take thorough history to determine if occurrence was first time dislocation
- Patients may describe a popping sensation without obvious dislocation suggesting subluxation or dislocation with spontaneous reduction
- Lateral Dislocation
- Most common by far
- Occurs with contact during contact or collision sport
- Without contact during aggressive cutting or pivoting maneuver
- Other Directions
- Medial dislocations, intra-articular and superior dislocations are rare
- Associated with congenital conditions, quadriceps atrophy or iatrogenic
Etiology
- Typically traumatic from:
- Non-contact twisting injury to the knee
- Direct blow to the medial aspect of the knee
- Tibia is often externally rotated with foot planted, knee slightly flexed with a valgus position
- Generalized ligamentous laxity
- These patients may have more benign etiology
- Often subluxation events rather than frank dislocation
Pathoanatomy
- Vastus Medialis Obliquus (VMO)
- Most distal portion of the medial quadriceps muscle
- Exerts a medially directed force that helps keep the patella in position
- Medial retinaculum
- Medial Patellofemoral Ligament (MPFL)
- Primary static restraint to lateral instability during first 30° of flexion
- Prevents excessive lateral movement of the patella
- Almost universally torn during dislocations
Associated Conditions
- Miserable Malalignment Syndrome
- Osteochondral Defect Knee
- Patellar Instability
- Patellofemoral Pain Syndrome
Risk Factors
- General
- Female > male[4]
- Younger age
- Muscle related factors
- Weakness or atrophy of Vastus Medialis Obliquus
- Excessive tone of the Vastus Lateralis or Iliotibial band
- Anatomic factors
- Increased Q Angle
- Patella Alta or high riding patella
- Trochlear dysplasia
- Excessive lateral patellar tilt
- Lateral femoral condyle hypoplasia
- Laterally located tibial tubercle
- Generalized ligamentous laxity
- Connective Tissue Disorder
- Marfan Syndrome
- Ehlers Danlos Syndrome
- Down Syndrome
- Due to congenitally small patella, hypoplastic condyle
Differential Diagnosis
- Fractures
- Dislocations & Subluxations
- Patellar Dislocation (and subluxation)
- Knee Dislocation
- Proximal Tibiofibular Joint Dislocation
- Muscle and Tendon Injuries
- Ligament Pathology
- Arthropathies
- Bursopathies
- Patellofemoral Pain Syndrome (PFPS)/ Anterior Knee Pain)
- Neuropathies
- Other
- Bakers Cyst (Popliteal Cyst)
- Patellar Contusion
- Pediatric Considerations
- Patellar Apophysitis (Sinding-Larsen-Johnansson Disease)
- Patellar Pole Avulsion Fracture
- Tibial Tubercle Avulsion Fracture
- Tibial Tuberosity Apophysitis (Osgood Schalatters Disease)
- Proximal Tibial Metaphyseal Fracture
- Proximal Tibial Physeal Injury
Clinical Features

Clinical example of laterally dislocated patella[5]
- History
- Most describe pain and potentially a noticeable deformity of the knee
- Usually preceded by direct trauma to the knee or a sudden change in direction
- Many describe a sense of giving way or instability when the dislocation occurs
- Feel a pop or multiple pops and there may be generalized pain in the anteromedial knee
- Exam: Physical Exam Knee
- Acutely dislocated patella is typically easy to see
- May have joint effusion or hemarthrosis (#2 cause in pediatrics behind ACL[6])
- Carefully identify the patellar poles, medial and lateral joint lines and retinaculum
- Palpable tenderness along the medial retinaculum or MPFL may be appreciable
- Patellar laxity, looseness, or increased motion may be noted
- Important to stress other structural ligaments if possible
- Range of motion is very limited in dislocation, may be intact in subluxation
- Strength testing can be limpted in knee extension, hip abduction
- Normal patellar glide medially and laterally is between 25-50% the width of the patella
- Other findings include femoral anteversion, patella alta, tibial torsion, genu recurvatum, genu valgum or varum, pes planus, and general ligamentous laxity
- Special tests
- Special tests may need to be deferred in the acute setting depending on how symptomatic patient is
- Consider evaluating for ligamentous laxity using the Beighton Score[7]
- Patellar Apprehension Test: Apply medial and lateral pressure to the patella testing for apprehension
- Patellar J Sign: Evaluate path of patella during flexion and extension
Evaluation
Knee MRI axial view with marrow edema of the medial patellar facet and two small osteochondral fragments[8]
Knee MRI coronal view with marrow edema of the lateral femoral condyle[9]
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
- Plain radiography
- 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
- 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: 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 [10]
- 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
- Computed tomographic (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
- Indicated if any evidence of loose body or Osteochondral Lesion on radiographs
- Common Findings[11]
- Bruising pattern of lateral femoral condyle, medial patella
- Disruption of the MPFL (at the medial femoral epicondyle insertion)
- Articular cartilage injuries if present
Classification
Dejour Classification of Trochlear Dysplasia
- Type A: flatter than normal with a sulcus angle greater than 145°
- Type B: which is flat
- Type C: which is convex
- Type D: which is convex with a supratrochlear spur
Management
Patellar Dislocation Reduction
- Acute dislocations require reduction
- Consideration should be made to transfer the patient to the emergency department depending on the clinical context
- Patient will require some degree of analgesia, possibly procedural sedation
- Procedure
- Dislocation should be obvious
- Patient should have knee in a resting position of slight flexion
- With distal hand gently extend knee
- Simultaneously, proximal hand applies gentle medial pressure to the patella
- As the knee extends, the patella should slide over the lateral femoral condyle and pop into the groove
- Patients typically feel much better after reduction is complete
Nonoperative
- Indications
- First time dislocation
- No evidence of osteochondral defect, intra-articular damage
- Relative rest from offending activities
- NSAIDS, Ice Therapy
- Patellar J Brace
- Often helpful initially to provide additional stability
- Kinesiology Tape can be considered
- Especially McConnell technique
- Physical Therapy
- Begin around 2-4 weeks
Operative
- Indications
- Presence of osteochondral defect or loose body
- Subluxation of patella on sunrise view
- Failure to improve with conservative management
- Anatomic factors predisposing to dislocation
- Recurrent Dislocation
- Technique
- Arthroscopy with or without open debridement
- MPFL repair (re-attachment) or reconstruction (proximal realignment)
- Lateral release (distal realignment)
- Tibial Osteotomy (distal realignment)
- Trochleoplasty
Rehab and Return to Play
Rehabilitation
- Goals
- Core stability
- Strengthen extensor mechanism, especially VMO
- Proprioception
- Four phases of rehabilitation before full return to activity:
- Protective phase (up to 6 weeks post-operation)
- Moderate protection phase (weeks 7-12)
- Minimum protection phase (weeks 13-16)
- Return to activity phase (weeks 17-20+)
Return to Play
- After first dislocation, majority of athletes can return to play at about 3-4 weeks[12]
Complications and Prognosis
Prognosis
- Symptoms
- Risk of redislocation
- 20-40%, higher if second dislocation[15]
- After second dislocation, subsequent risk is 50% or greater (need citation)
- Risk increases if presence of trochlear dysplasia, patella alta, laterally placed tibial tubercle
- Sillanpaa et al studied early stabilization surgery for primary dislocation[16]
- Redislocation rate was significantly lower in surgical group compared to nonsurgical group
- No benefit from surgery was seen at long-term follow up
Complications
- Osteochondral Defect
- Patellar Instability or recurrent dislocations
- Patellofemoral Osteoarthritis
- Patellofemoral Pain Syndrome
- Chronic pain
- Inability to return to sport
See Also
- Internal
- External
- Patella Dislocation: Introduction and Diagnosis
- Patella Dislocation: Management and Treatment
- Sports Medicine Review Knee Pain: https://www.sportsmedreview.com/by-joint/knee/
References
- ↑ Jain NP, Khan N, Fithian DC. A treatment algorithm for primary patellar dislocations. Sports Health. 2011 Mar;3(2):170-4.
- ↑ Ries, Zachary, and Matthew Bollier. "Patellofemoral instability in active adolescents." The journal of knee surgery 28.04 (2015): 265-278.
- ↑ Fithian DC, Paxton EW, Stone ML, Silva P, Davis DK, Elias DA, White LM. Epidemiology and natural history of acute patellar dislocation. Am J Sports Med. 2004 Jul-Aug;32(5):1114-21.
- ↑ DeFroda, Steven F., et al. "Diagnosis and management of traumatic patellar instability in the pediatric patient." Orthopedics 40.5 (2017): e749-e757.
- ↑ Sanchis-Alfonso, Vicente, et al. "Failed medial patellofemoral ligament reconstruction: causes and surgical strategies." World journal of orthopedics 8.2 (2017): 115.
- ↑ Pedowitz, Jason M., et al. "Recurrence of patellar instability in adolescents undergoing surgery for osteochondral defects without concomitant ligament reconstruction." The American journal of sports medicine 47.1 (2019): 66-70.
- ↑ Beighton P, Solomon L, Soskolne CL. Articular mobility in an African population. Ann Rheum Dis. 1973 Sep;32(5):413-8
- ↑ https://radiopaedia.org/cases/25852
- ↑ https://radiopaedia.org/cases/25852
- ↑ Dejour D, Le Coultre B. Osteotomies in patello-femoral instabilities. Sports Med Arthrosc. 2007 Mar;15(1):39-46
- ↑ 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.
- ↑ Hergenroeder, Albert C., and Richard G. Bachur. "Approach to acute knee pain and injury in children and skeletally immature adolescents."
- ↑ Atkin DM, Fithian DC, Marangi KS, Stone ML, Dobson BE, Mendelsohn C. Characteristics of patients with primary acute lateral patellar dislocation and their recovery within the first 6 months of injury. Am J Sports Med. 2000 Jul-Aug;28(4):472-9.
- ↑ Mäenpää H, Lehto MU. Patellar dislocation. The long-term results of nonoperative management in 100 patients. Am J Sports Med. 1997 Mar-Apr;25(2):213-7
- ↑ Mäenpää H, Huhtala H, Lehto MU. Recurrence after patellar dislocation. Redislocation in 37/75 patients followed for 6-24 years. Acta Orthop Scand. 1997 Oct;68(5):424-6.
- ↑ Sillanpää PJ, Mattila VM, Mäenpää H, Kiuru M, Visuri T, Pihlajamäki H. Treatment with and without initial stabilizing surgery for primary traumatic patellar dislocation. A prospective randomized study. J Bone Joint Surg Am. 2009 Feb;91(2):263-73.
Created by:
John Kiel on 25 June 2019 19:03:38
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Last edited:
4 October 2022 18:51:42
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