Lateral patellar dislocation is a common cause of acute traumatic haemarthrosis in young active patients, usually occurring during sporting activites. Patients can often be unaware it has occurred. Often magnetic resonance imaging offers the first diagnosis. Most patellar dislocations are treated conservatively with an emphasis on early return to movement.

We report on a series of 80 consecutive patients who were diagnosed as having had a transient patellar dislocation by MRI from November 2001 to April 2008 as evidenced by the characteristic countercoup pattern of bone bruising seen on the lateral femoral condyle. In addition to the patellar findings, the images were reviewed with specific reference to the medial collateral ligament, a heretofore undescribed concomitant injury.

During the study period, 80 patients (66 males, 14 females) were diagnosed on MRI as having had transient patellar dislocation. The mean age (mean +/− standard deviation) of the cohort was 23.9+/−7.5 years (range:11–60 years). In all but two cases, normal anatomical alignment had been restored. In addition to multiple patellar chondral findings, the condition of the MCL was commented upon in 77 cases (96.3%). Of these, 40 (51.9%) had documented damage to the MCL. These injuries were classified as grade 1 (n=20), grade 1/2 (n=2), grade 2 (n=13), grade 2/3 (n=2) and grade 3 (n=3). Male patients were more likely to have had MCL damage 54.5% vs. 28.6% (p=0.07, Chi-Square).

These results serve to highlight the co-existence of MCL injuries with patellar dislocation to a relatively high incidence. This injury should be suspected and examined for in the case of prolonged symptoms after dislocation especially in male patients.

Lateral patellar dislocation is a common cause of acute traumatic haemarthrosis in young active patients, usually occurring during sporting activites. However, patellar dislocation is usually transient with patients often unaware it has occurred. Often magnetic resonance imaging (MRI) offers the first diagnosis. Most patellar dislocations are treated conservatively with an emphasis on early return to movement.

We report on a series of 30 patients who were diagnosed as having had a transient patellar dislocation by MRI from December 2001 to October 2007 as evidenced by the characteristic countercoup pattern of bone bruising seen on the lateral femoral condyle. In addition to the patellar findings, the images were reviewed with specific reference to the medial collateral ligament, a heretofore undescribed concomitant injury.

During the study period, 30 patients (26 males, 4 females) were diagnosed on MRI as having had transient patellar dislocation. The mean age (mean +/− standard deviation) of the cohort was 23.1+/−6.1 years (range:14 – 36 years). In all but one case, normal anatomical alignment had been restored. In addition to multiple patellar chondral findings, the condition of the MCL was commented upon in 29 cases (97%). Of these, 12 (41%) had documented damage to the MCL. These injuries were classified as grade 1 (n=7), grade 2 (n=3) and grade 2/3 as defined by incomplete detachment of the MCL from the medial femoral condyle (n=2).

These results serve to highlight the co-existence of MCL injuries with patellar dislocation to a relatively high incidence. This injury should be suspected and examined for in the case of prolonged symptoms after dislocation. In addition, the current vogue for early rehabilitation needs to be regarded with some circumspection.