Fracture of the medial margin of the patella, a finding diagnostic of lateral dislocation.

Abstract

Afracture or cortical irregularity of the medial articular margin of the patella was noted in a number of patients admitted to The Hospital for Special Surgery, New York, with the diagnosis of recurrent lateral dislocation of the patella. We regard this roentgen finding as pathognomonic evidence of previous lateral dislocation of the patella. In a few instances, we have been able to diagnose this previous dislocation from the x-ray examination without knowledge of the patient's history or clinical findings. The purpose of this presentation is to discuss and illustrate the roentgen abnormalities which follow lateral dislocation of the patella and to point out the importance of the tangential view of the patella in the examination of the knee. Presentation of Material Records of 150 patients admitted to the hospital with a diagnosis of lateral dislocation of the patella were obtained, and their roentgen examinations and charts reviewed. In 16 cases, 12 females and 4 males, the fracture of the medial articular margin of the patella was demonstrated. Not all the 150 patients had a complete roentgen examination, including tangential views, and therefore the exact statistical occurrence of the marginal fracture could not be established. A concurrent finding also previously described (1–4), the compression fracture or irregularity of the lateral margin of the lateral femoral condyle, was seen in only a few cases. It appears to be less common and more difficult to evaluate than the patellar rim fracture. In none of the 16 cases was any congenital abnormality of the patella such as hypoplasia demonstrated. A history of previous trauma was obtained in 12. Mechanism of Injury The laterally dislocated patella locks itself over the lateral margin of the lateral femoral condyle; then, upon contraction of the quadriceps, the patella is pulled back into its normal position. The impingement of the medial margin of the patella upon the lateral condyle causes a compression defect in the condyle and a fracture of the medial margin of the patella either by compression or by shearing as the patella slides back, pulled by the contracting quadriceps. This process produces a predominantly chondral fracture at the margin of the articular surface, which becomes radio-graphically visible only when a sufficient portion of subchondral bone is involved. Discussion Of the 16 patients in whom the medial marginal fracture of the patella was demonstrated, the youngest was twelve years of age. We assume that the fracture also occurs in younger children but is not radio-graphically detectable because the articular cartilage is thick and the ossification center proportionally small. Here the fracture is purely chondral in nature and therefore invisible. A dislocating patella in a patient eight years of age is demonstrated in Figure 1. No marginal fracture is visible.