Patellar Dislocation and Instability
Keith A. Stuessi
Brent R. Becker
BasicsDescription
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Patellar instability is defined as hypermobility of the patella in either the medial or lateral direction.
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Medial instability is extremely rare.
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Complete dislocation and subluxation represent variations in severity of instability.
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Acute dislocation typically occurs with a twisting injury and strong contraction of the quadriceps; rarely it is due to direct trauma to the medial aspect of the patella.
Risk Factors
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Prior history of subluxed or dislocated patella
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Recurrence rate 15–50% after initial dislocation
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Adolescent females
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Patella alta (“high-riding patella”)
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Excessive genu valgum
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Weak vastus medialis
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Excessive tibial torsion
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Family history of patellar instability
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Trochlear dysplasia
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Lateralized tibial tuberosity
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Risk factors associated with developmental dysplasia (1st-born girl, high birth weight, deliver by C-section, breech delivery)
Commonly Associated Conditions
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Avulsion fracture of the superior medial pole of the patella
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Osteochondral fractures of the lateral femoral condyle or posterior patellar articular surface
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Tear of the medial patellofemoral ligament
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Concomitant major ligamentous or meniscal injury
DiagnosisPre Hospital
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Patient has severe pain and may have heard a pop at time of dislocation.
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Knee is usually held in 20–30 degrees of flexion, and patella is palpable laterally.
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Acutely swollen knee
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Hemarthrosis
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Tenderness to palpation over the medial edge of patella
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Tenderness just proximal to medial femoral epicondyle
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Consider subluxation if:
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History is consistent of a dislocation but pain and examination findings have resolved
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Look for patella alta (high-riding patella) or laterally displaced patella.
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History
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Initial or recurrent?
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History of previous knee injury or patellofemoral pain syndrome?
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Does your patella feel like it is slipping or moving laterally on certain movements?
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Do you have swelling?
Physical Exam
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Immediately after dislocation, may show patella dislocated laterally and prominence medially due to uncovered medial femoral condyle
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Obvious effusion
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Tenderness most apparent over the medial retinaculum and vastus medialis
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Limited range of motion with knee in extended position
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Fear of redislocation when knee is flexed
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Positive apprehension sign with movement of patella laterally
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Check ACL and meniscus, as up to 12% of patellar dislocations have associated major ligamentous or meniscal injury.
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“J” sign: Seated patient straightens the knee; the patella moves outward instead of straight upward.
Diagnostic Tests & Interpretation
Imaging
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Standard anteroposterior, lateral, and patellar views (“sunrise” or “tunnel” view)
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Sunrise view mandatory. Rule out presence of associated osteochondral fractures. Avulsion fracture or calcification along the medial edge of the patella is considered pathognomonic for patellar dislocation
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CT more sensitive than plain films for identifying patellar malalignment
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MRI more informative than CT, as it can evaluate articular cartilage
Differential Diagnosis
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Subluxation vs dislocation
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Although history of patellar dislocation is fairly classic, consider other entities that cause early effusions, eg, anterior cruciate ligament (ACL) tear, meniscal tear, and tibial plateau fractures.
P.443
Treatment-
If not reduced spontaneously, may require conscious sedation for pain control and muscle relaxation
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Alternatively, arthrocentesis performed with instillation of 10–15 mL of lidocaine and/or bupivacaine
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Extension of the leg with hip flexed (reduces tension of quadriceps tendon)
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Gentle pressure on patella directed lateral to medial
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Postreduction radiographs to confirm reduction and rule out fractures
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Examine anterior ACL and medial and lateral menisci to rule out accompanying tears.
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After reduction, rest, ice, focal compression, and elevation are indicated for the 1st 24–48 hr.
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Knee immobilization is maintained for ∼2–3 wks, although early passive range of motion in terminal extension is allowed to minimize disuse atrophy.
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Hinged brace may be substituted for knee immobilizer as early as 1 wk.
Medication
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Trial of NSAIDs drugs
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Tylenol
Additional Treatment
Referral
Orthopedic referral indicated if:
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Osteochondral fracture noted on either plain radiographs or MRI
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Recurrent patellar dislocations despite adequate rehabilitation, especially in younger patients (<14 yrs old), in whom recurrence rates can reach 60%
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Evidence of joint locking
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High-risk athlete participates in activities involving pivoting and is at increased risk of recurrent patellar dislocation
Additional Therapies
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A significant number of patients develop large hemarthrosis. Aspiration may be considered after 48–72 hr, both to relieve pain and to check for fat globules, which might help diagnose an occult osteochondral fracture.
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Isometric quadriceps exercises are begun as soon as possible, although it is often difficult and painful for the athlete to produce a contraction that involves the vastus medialis.
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Active range of motion exercises (closed chain) are started at 1 wk and physical therapy consultation given for medial quadriceps strengthening (vastus medialis oblique).
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Knee immobilizer or hinged brace is used for ambulation until 100 degrees of painless flexion is present, there is no effusion, and a normal heel-to-toe gait is possible.
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Immobilizer or hinged brace ultimately is replaced with a neoprene sleeve with a lateral buttress until normal, painless activities of daily living are possible.
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Rehabilitation alone appears to be effective as operative treatment that is followed by rehabilitation in children under 16 yrs with patellofemoral instability (1)[A].
Surgery/Other Procedures
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Typically patients who fail a conservative trial of ∼6 mos become surgical candidates.
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Surgical management includes lateral retinacular release alone, proximal extensor mechanism realignment alone, or combined proximal and distal realignment.
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Minimally invasive patellar realignment includes lateral retinacular release and plication of the medial retinaculum.
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Minimally invasive patellar realignment improves morbidity and recovery when compared with open realignments (2)[C].
References
1. Palmu S, Kallio PE, Donell ST, et al. Acute patellar dislocation in children and adolescents: a randomized clinical trial. J Bone Joint Surg Am. 2008;90:463–470.
2. Andrish J. The management of recurrent patellar dislocation. Orthop Clin North Am. 2008;39:313–327, vi.
Additional Reading
Drez D, Delee JC, eds. Orthopaedic sports medicine: principles and practices. Philadelphia: WB Saunders, 1994.
Garth WP, Pomphry M Jr, Merrill K. Functional treatment of patellar dislocation in an athletic population. Am J Sports Med. 1996;24:785–791.
Iobst CA, Stanitski CL. Acute knee injuries. Clin Sports Med. 2000;19:621–635, vi.
Kilgore KP. The knee: patellar dislocation. In: Ruiz E, Cicero JJ, eds. Emergency management of skeletal injuries, 1st ed. St. Louis: Mosby-Year Book, 1995.
Roberts DM, Stallard TC. Emergency department evaluation and treatment of knee and leg injuries. Emerg Med Clin North Am. 2000;18:67–84, v–vi.
CodesICD9
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718.86 Other joint derangement, not elsewhere classified, involving lower leg
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836.3 Dislocation of patella, closed
Clinical Pearls
Return to sports: Evidence of adequate healing (absence of sensations of instability, lack of effusion, and absence of pain on patellofemoral compression) and adequate function (able to perform rotational movements such as pivoting, cutting, and twisting without evidence of instability). Athlete may need McConnell taping or patellar stabilizing braces to accomplish this.