Functional data for the diagnosis of patellofemoral laxity obtained by MRI during quadriceps isometric contraction.

Patellofemoral instability is related to anatomy. Magnetic resonance imaging (MRI) provides anatomic detail, but spoiled gradient echo (SPGR) imaging during isometric quadriceps contraction provides objective functional data for diagnosing patellofemoral laxity. Knee MRI studies and medical charts of 398 patients were retrospectively reviewed. Two independent blinded observers evaluated the knee MRI studies for patellofemoral morphology and patellar position on axial SPGR images during relaxation and isometric quadriceps contraction for lateral patellar migration. Charts were reviewed for history of patellofemoral instability or dislocation. Patients were divided into 2 groups: group 1 comprised patients with 2.5 mm or more of lateral patellar migration on axial SPGR images, and group 2 comprised patients with less than 2.5 mm of patellar subluxation. Logistic regression models were used to determine relationships between patellofemoral subluxation of 2.5 mm or more and (1) history of dislocation or clinical patellofemoral instability, (2) grade 4 chondromalacia on MRI, (3) corrected central trochlear height, and (4) differential trochlear height (corrected for lateral condylar height). Statistically significant associations were found between patellar subluxation and each of the above 4 clinical/morphologic measures. Lateral patellar migration of 2.5 mm or more on SPGR obtained during quadriceps contraction had statistically significant associations with the above 4 measures. Evaluation of SPGR of quadriceps contraction provides objective functional information about patellofemoral instability for management decisions.