MAGNETIC-RESONANCE-IMAGING OF PATELLOFEMORAL RELATIONSHIPS

Patellofemoral relationships were analyzed in 11 patients (13 knees) w ith patellar dislocation and 15 asymptomatic subjects (15 knees) at 0- degrees and 20-degrees of flexion. The measurements were made from fiv e consecutive axial images through the patellofemoral joint. The six i ndices measured were lateral patellar tilt (LPT), lateral patellofemor al angle (LPA), lateral patellar displacement (LPD), patella-lateral c ondyle index (L/PW), congruence angle (CA), and sulcus angle (SA). The reproducibility of the method was evaluated. The difference between t he two study groups was more evident at 0-degrees than at 20-degrees o f knee flexion. Significant differences were noted between measurement s made at different levels of the joint, particularly in the controls. Isometric contraction of the quadriceps muscle lateralized and tilted the patella slightly in both groups. L/PW with and without quadriceps muscle contraction, and LPA with reference to the anterior condyles d ifferentiated between the two study groups most clearly. LPT and LPA w ith reference to the anterior condyles differentiated the study groups better than LPT and LPA with reference to the posterior condyles. The reproducibility was good except for interobserver comparison of CA an d SA. The use of an imaging plane selected at the midpoint of the pate llar articular cartilage increases the sensitivity of the measurements , since it takes into account both the height of the patella and the t endency towards lateralization. These results indicate that patellar t ilt is best measured with the LPA index and patellar lateralization wi th the L/PW index at 0-degrees knee flexion. This study should always include isometric contraction of the quadriceps muscle.