Patellar dislocation is a rather common injury and the cause of knee haemarthrosis in adolescents and young adults.
Treatment of primary (the first) dislocation is often conservative.
Mechanism of injury
Valgus bend and external rotation of the lower leg while the knee is flexed causes the patella to dislocate over the lateral crest of femoral sulcus.
Also a direct blow to the knee or falling over may cause dislocation.
Symptoms and findings
A typical patient is a teenager or a young adult (< 30-35 years) with haemarthrosis of the knee (at first dislocation; habitual dislocations are not associated with haemarthrosis).
There is often merely the patient's own account of the kneecap being dislocated (patella slipping out of place and back into place noticed by the patient him-/herself)
The energy leading to the dislocation may vary
Family history
Difficulty in weight bearing
Pain and palpable tenderness on the medial side of the patella, on the medial epicondyle of the femur and on the upper crest of the lateral condyle of the femur. Sometimes the dislocation is visible.
The patella can be displaced laterally more on the injured side compared with the intact side. This procedure is painful (the apprehension sign).
X-ray (including the patellar projection i.e. Laurin projection) is necessary to find any loose bodies. Patellar axial projection may reveal tilting and lateralization of the patella (pictures 12).
In obvious haemarthrosis of the knee, performing MRI is recommended in order to detect typical injuries and for differential diagnosis (rupture of the anterior cruciate ligament, meniscal tear).
Treatment
Reduction
The knee is extended and the patella is pressed in medial direction.
Effective pain medication is required for the reduction to be successful, for example by a combination of alfentanil and midazolam, both intravenously.
Evacuation of the haemarthrosis
Haemarthrosis is aspirated. Especially in the case of major haemarthrosis, it usually alleviates the pain significantly.
Presence of fat droplets suggests the possibility of an osteochondral fracture.
Indications for operative treatment
Loose body on x-ray needing fixation
Operative treatment of recurrent dislocations is considered depending on the age of the patient and the frequency of dislocations. The key question is: Do you trust your knee?
Conservative treatment
Primary dislocation with no clear loose bodies in the joint
Acute phase of recurrent dislocation. An operation is performed later if necessary.
Thigh muscle exercises are started and a patellar stabilizing orthosis is used when the oedema has diminished (in 2-3 days).
Short immobilization may be used to relieve pain.
References
Yao LW, Zhang C, Liu Y et al. Comparison operative and conservative management for primary patellar dislocation: an up-to-date meta-analysis. Eur J Orthop Surg Traumatol 2015;25(4):783-8. [PubMed]
Salonen EE, Magga T, Sillanpää PJ, et al. Traumatic Patellar Dislocation and Cartilage Injury: A Follow-up Study of Long-Term Cartilage Deterioration. Am J Sports Med 2017;45(6):1376-1382. [PubMed]
Smith TO, Gaukroger A, Metcalfe A et al. Surgical versus non-surgical interventions for treating patellar dislocation. Cochrane Database Syst Rev 2023;1(1):CD008106. [PubMed]