Incidence and risk factors of acute traumatic primary patellar dislocation.

PURPOSE: The purpose of this study was to investigate incidence, nature, and risk factors of primary traumatic patellar dislocations.

METHODS: We identified acute first-time traumatic patellar dislocations from a national hospital discharge register. Patients with previous patellar dislocations, subluxations, or knee traumas were excluded. The sample consisted of 128,714 Finnish male conscripts (median age 20). Background risk factor data were obtained from a Finnish conscript service database. The dislocators were observed during their service period for a short-term outcome.

RESULTS: From the 128,714 male conscripts, 278 had patellar dislocations, and 72 were classified as having sustained first-time traumatic patellar dislocations. The 128,436 nondislocators served as a control group. The incidence of acute traumatic primary patellar dislocations among male conscripts was 77.4 (95% CI: 61.1-96.8) per 100,000 persons per year. The male patients with traumatic primary patellar dislocations were taller (P = 0.03) and weighed more (P = 0.02) than the controls. Hemarthrosis was present in all patients, and when MRI or open surgery was performed, medial retinacular disruption and medial patellofemoral ligament (MPFL) injury were identified. Patients' return to military service was unrelated to the choice of treatment.

CONCLUSION: Primary patellar dislocation is not a negligible cause of morbidity among young male adults. It can be concluded that hemarthrosis and MPFL rupture are the definite signs of an acute traumatic primary patellar dislocation. Height and weight were significant risk factors, whereas poor physical performance was not associated with primary patellar dislocation.