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Dislocated kneecap

Dislocated kneecap

Ryan Shulman explains why the trend is moving towards surgical management of this painful injury

The stereotypical presentation of a patient with dislocation of the patella (kneecap) is a young, overweight and inactive female. Recent research has shown, however, that athletes of both genders are at increased risk of sustaining this injury.

Thanks to some large prospective trials, we now have a better understanding of which populations are at greatest risk, what percentage of each population is at risk of ongoing symptoms of pain, instability and recurrence, and which methods of treatment are more likely to improve outcomes. Some trials have reported rates of redislocation of up to 44%, and symptoms of recurrent instability at rates greater than 50% with non-operative treatment(1). As a result surgical management is becoming increasingly common.

Fithian et al(2)have performed one of the largest prospective cohort studies of patellar dislocation. Their data showed that in the high-risk age group (between 10 and 17 years old), girls were approximately 33% more vulnerable than boys for first-time dislocation. Within the same age range, the risk for patellar dislocation among girls with a previous history of patellofemoral complaints was three times greater than that for boys with prior subluxation/dislocation.

The same study also found that 61% of primary dislocations occurred during sporting activity and 9% occurred during dancing activities. In a similar but smaller study conducted by Nietosvaaraet al (3), 55% of the sample was injured in sporting activities. Neither study details a breakdown of injury by type of sporting activity. In a recent comprehensive analysis of knee injuries sustained by English professional rugby players, Dallalana et al(4)reported only one patello-femoral dislocation in a total cohort of 211 injured players, representing 0.5% of reported knee injuries. This population is of course not representative of the high risk population reported by Fithianet al.

Dislocation of the patella occurs when the kneecap is forced laterally (sideways) out of its groove (the trochlea) over the lateral condyle (outer surface) of the femur (see Figure 1, overleaf). In just over 90% of cases the patella will relocate spontaneously, but occasionally strong analgesia and sedation may be required to ease the patella manually back into the trochlear groove.

Most patients will be able to describe a specific incident of knee trauma which caused the dislocation, though details are often sketchy. Occasionally the cause may be a fall onto the patella, or a direct blow, eg by a hockey stick, but more often the incident occurs during a non-contact movement, such as a twisting or cutting move during sporting or dancing manoeuvres. Patients report a sensation of the knee giving way or ‘popping’, and there is almost immediate swelling. The main differential diagnosis at this stage is rupture of the anterior cruciate ligament (ACL), with or without significant meniscal injury.


The clinician should take a careful history, in particular family history and any previous dislocations of either knee – the risk of a subsequent dislocation on either side increases six-fold after a first dislocation(2). General hypermobility is also a recognised predisposing factor.

Patellar dislocation can cause a range of associated damage to other knee structures, so an examination of the affected knee needs to take account of this. Support for the patella laterally comes from fascial connections between the iliotibial band, biceps femoris and the lateral patellar retinaculum. Tightness in these lateral structures may cause tilting or displacement of the patella and may contribute to lateral dislocation.

The trochlear groove acts to centralise the patella and conducts compressive forces from the patella to the femur during flexion and extension of the knee. A shallow trochlear groove will significantly limit the inherent bony stability of the patello-femoral articulation. If a patient with recurrent patella dislocation does have a groove which is abnormal (ie too shallow), reconstructive surgery to deepen the groove is sometimes offered.

All primary acute dislocations of the patella should have an x-ray assessment made. The clinician should make a general assessment of knee ligaments and patella tracking and tilt, to rule out concomitant injury. Moderate to severe swelling will usually represent an intra-articular bleed (haemarthosis); if swelling is significant, a recent systematic review(5) suggests needle aspiration will improve patient comfort and assist with diagnosis.

Severe intra-articular bleeding represents significant damage to medial structures and usually high force, and is also highly suggestive of associated osteochondral damage, a significant risk factor for repeated dislocation and ongoing symptoms.

Osteochondral lesions, which are poorly diagnosed on plain x-ray, have been reported to occur in up to 95% of first time dislocations(6). If suspected, MRI of the knee should be requested.

Other signs of a poor prognosis for nonoperative treatment are:

* grossly dislocatable patella

* palpable gaps in the VMO (vastus medialis obliquus muscle), adductor mechanism or MPFL (medial patello-femoral ligament, the primary ligament restraining lateral patellar movement).

Those who repeatedly dislocate a knee should be evaluated for lower limb alignment, including assessment of:

* Q angle

* tibial and femoral torsion

* excessive subtalar pronation

* general joint laxity, and

* patellar tracking through passive range of motion and in functional activities (walking, jogging, cutting and jumping).

Non-operative treatment

Most patients with patellofemoral instability should be treated initially with aggressive, well monitored, non-operative rehabilitation(7). After dislocation, initial healing and pain relief can be helped if the knee is immobilised in a brace. The total immobilisation period is three to six weeks, and there is no good evidence to suggest that one or other form or brace should be preferred: a full brace, posterior support or patella tracking orthoses are all options.

To reduce joint stiffness, after about three weeks the therapist should begin gentle passive mobilisation, avoiding the original injurious movement, within the range of mid-flexion to full extension. This movement is unlikely to impede MPFL healing and joint movement is essential to safeguard the health of articular cartilage.

The athlete can begin isometric quadriceps strengthening exercises and straight leg raises as soon as tolerated, progressing to inner range (reduced range) knee extension exercises as tolerated. The patella transmits progressively less force as the knee comes into extension; particularly at the first 20-30 degrees of flexion. Thus gentle open-chain knee exercises in this range are well tolerated and should be encouraged to minimise the loss of quadriceps strength. After about six weeks, the athlete should be able to tolerate a progression to jogging and straight line sprints, and the therapist can introduce figure of eight and stepping/cutting manoeuvres.

Operative management

The literature is still rather tentative on whether conservative or operative management is best for initial dislocations. Two studies(8,9), have shown no significant differences between the two approaches.

Relative indications for early surgical treatment include:

* concurrent osteochondral injury

* palpable disruption of the MPFL-VMOadductor mechanism

* MRI findings of a large complete avulsion or midsubstance rupture of the MPFL

* an obviously subluxed patella on x-ray compared to the other knee

* patients who fail to improve with non-operative management(2).

Where an adequate repair can be undertaken on a large osteochondral defect, this should be done. While there is little evidence to show that recurrence or patellofemoral pain is reduced, most surgeons would argue that unstable areas of chondral damage should be trimmed arthroscopically.

Athletes who wish to return to sport early and whose functional demands are high, may require early surgical management. MPFL reconstruction using hamstring grafts and repair of VMO and retinacular structures can have most athletes back to training at four to six months, assuming no osteochondral damage.

As chondral lesions are so common, associated loose bodies can be the cause of further symptoms of pain and locking further down the non-operative rehabilitation pathway. Arthroscopic removal is the suggested treatment at this stage(10).

In the case of chronic repeat dislocations, the main purpose of surgery is to address the underlying cause of symptoms. Trochlear dysplasia (malformation) can be treated by reconstruction.

Patients with functionally increased Q angle may benefit from a procedure in which the tibial tubercle is relocated closer in to the body’s midline. This procedure can be combined with MPFL repair. Results from these procedures are variable and are affected by a multitude of factors. In particular, the procedure can increase PFJ joint reaction forces and thus lead to ongoing patellofemoral joint pain.


1.Cofield, RH, Bryan, RS (1977) ‘Acute dislocation of the patella: results of conservative treatment’. J Trauma.1977;17:526-531.

2.Fithian, DC, Paxton, EW, et al (2004) ‘Epidemiology and natural history of acute patellar dislocation’. Am J Sports Med. 2004; 32:1114-1121.

3. Nietosvaara ,Y, Aalto, K, Kallio, PE (1994) ‘Acute patellar dislocation in children: incidence and associated osteochondral fractures’. J Pediatr Orthop. 14:513-515.

4. Dallalana, RJ, Brooks ,JHM, et al (2007) The ‘Epidemiology of knee injuries in English professional Rugby Union.’ Am J Sports Med. 35; 818-830.

5. Stefancin, JJ, Parker, RD (2007) ‘First-time traumatic patellar dislocation: a systematic review’. Clin Orthop Relat Res. Feb;455:93-101.

6. Nomura, E, Inoue, M, Kurimura, M (2003) ‘Chondral and osteochondral injuries associated with acute patellar dislocation’. Arthroscopy. 19:717-721.

7.Hinton, RY, Sharma, KN (2003) ‘Acute and recurrent patellar instability in the young athlete’. Orthop Clin N Am 34:385-396.

8.Nikku, R, Nietosvaara ,Y et al (2005) ‘Operative treatment of primary patellar dislocation does not improve medium-term outcome: a seven-year follow-up report and risk analysis of 127 randomized patients’.Acta Orthop.76:699-704.

9.Andrade, A, Thomas, N (2002) ‘Randomized comparison of operative vs nonoperative treatment following first time patellar dislocation’. Presented at: The European Society of Sports Traumatology, Knee Surgery and Arthroscopy; Rome, Italy.

10. Mehta, VM, Inoue, V et al (2007) An Algorithm Guiding the Evaluation and Treatment of Acute Primary Patellar Dislocations (Review article). Sports Med Arthrosc Rev .5:78-81


Dislocated kneecap