Anterior-posterior trochlear measurements of normal and dysplastic trochlea by axial magnetic resonance imaging.

Different surgical techniques have been described to correct trochlear dysplasia, without clear descriptions of the various types of trochlear dysplasia. In describing trochlear dysplasia, there exist no clear criteria to distinguish between decreased trochlear depth (heightened trochlea floor) and flattened lateral and/or medial condylar height. The current study aims to build a database of axial MRI measurements of normal and abnormal trochlear shape to create a foundation for the selection of the necessary surgical correction to more normal trochlear anatomy. We prospectively examined 152 subjects: 30 patients with patellar instability due to trochlea dysplasia and 122 subjects without any symptoms or objective findings related to the patellofemoral joint. MRI was performed in both groups. The height of the medial and lateral condyle, and the center of the trochlea was measured on axial MR images. The height of these different locations was compared to the total width of the femoral condyle and expressed in percentages. The statistical analysis was conducted with the Student's t test at SPSS software. For intraobserver reliability 20 randomly taken MRI were evaluated twice. The intraobserver reliability was determined by calculating the kappa values investigated parameter. In normal subjects, the height of the lateral condyle was 81% of the width of the femoral condyle (100 units),the trochlear central height was 73%, the medial condylar height was 76%. In patients with patellar instability, the lateral condylar height was 82% and showed no significant difference compared to the normal group (P = 0.082). The trochlear central (77%) and medial condylar height (79%) were significantly different (P < 0.001) compared to the normal subjects. The location of pathology in patients with patellar instability was decreased lateral condylar height in five cases (16.6%) and decreased central/medial height in 25 cases (83.4%). A height of the lateral condyle <77% was documented to be pathologic. There was also a significant difference (P < 0.001) between males and females comparing the different heights of the trochlea to the total width of the femoral condyle. The resultant percentages of all three height measurements, the lateral, central, and medial heights, were greater in males than in females. The intraobserver reliability was perfect for all investigated parameters. In conclusion, (1) the presented measurement scheme on axial MRI is a reliable method to calculate the height of the trochlea in different locations, (2) a more objective assessment of the trochlear pathology is possible, (3) in five of six cases the pathology is located in the center and/or medial trochlea, and (4) in our series of patellofemoral instability patients, most would benefit from a deepening trochleaplasty as the surgical procedure of choice to correct dysplasia.