PREDICTORS OF PATELLOFEMORAL SUBLUXATION

Abstract

Introduction: Patellofemoral joint subluxation is associated with pain and dysfunction. The causes of patel-lofemoral subluxation are poorly understood and multi-factorial, arising from abnormalities of both bone and soft tissues. This study aims to identify which anatomical variables assessed on Magnetic Resonance (MR) images are most relevant to patellofemoral subluxation.

Method: A retrospective analysis of MR studies of 60 patients with suspected patellofemoral subluxation was performed. All patients were graded for the severity/ magnitude of radiological subluxation using a dynamic MR scan (Grade 0 [nil] to Grade 3 [subluxed]. The patient scans were assessed using a range of anatomical variables, these included:

• Patella alta,

• Patella type (Wiberg classification),

• Trochlea sulcus angles for bone and cartilage,

• The shortest horizontal distance between the most distal part of the vastus medialis obliquis (VMO) muscle to the supra-medial aspect of the patella,

• Trochlea and patella cartilage thickness (maximum depth),

• The horizontal distance between the tibial tubercle and the midpoint of the femoral trochlea (TTD),

• Patella Engagement – represented as the percentage of the patella height that is captured in the trochlea groove when the knee is in full extension,

• A Discriminant Analysis test for multi-variant analysis was applied to establish the relationship between each bony/soft tissue anatomical variable and the severity/magnitude of patellofemoral subluxation.

Results: The distance of the VMO from the patella (p < 0.001), TTD (p < 0.001) and Patella Engagement (p < 0.001) showed highly significant relationships with patellofemoral subluxation.

Conclusions: The following three anatomical variables are associated with patellofemoral subluxation: the distance of the VMO muscle from the patella, TTD and Patella Engagement.

This is the first study to establish that patella engagement is related to PFJ subluxation showing that the lower the percentage engagement of the patella in the trochlea, the greater the severity/magnitude of patellofemoral subluxation. The finding provides greater insight into the aetiology and understanding of the mechanism of symptomatic PFJ subluxation.