Increased subcortical patellar signal intensity at T2-weighted MR imaging: a subacute finding after knee injury

PURPOSE: To investigate the temporal prevalence of a rim of increased signal intensity in the subcortical part of the patella at T2-weighted magnetic resonance (MR) imaging after arthroscopic knee surgery or knee injury. MATERIALS AND METHODS: The prospective and retrospective components of the study were performed after receiving approval from the institutional research ethics board. Written informed consent was obtained for the prospective component of the study. The need for informed consent was waived for the retrospective component of the study. The authors performed a prospective evaluation of serial MR images of the knee of four patients (three men and one woman; age range, 28-53 years; mean age, 36.8 years) after arthroscopic anterior cruciate ligament (ACL) reconstruction. They also performed a retrospective review of MR images of the knee from 90 consecutive patients (59 male and 31 female patients; age range, 13-66 years; mean age, 34.6 years) referred for the evaluation of an ACL injury. All imaging studies were evaluated in consensus by two musculoskeletal radiologists for the presence of a rim of increased signal intensity in the subcortical part of the patella at T2-weighted imaging. The presence of this signal intensity pattern was correlated with the time to imaging from surgery or joint injury. Logistic regression modeling was performed with increased subcortical signal intensity as the outcome and a second-degree polynomial for elapsed time as the predictor. RESULTS: All four patients who underwent imaging prospectively after ACL surgery showed increased T2-weighted signal intensity in the subcortical part of the patella 12 weeks after surgery. None of the patients showed these signal intensity changes at 6 or 24 weeks. A subcortical rim of increased T2-weighted signal intensity was seen in the patella in 24 (27%) of the 90 consecutive patients who underwent imaging for the assessment of possible ACL injury. Significant independent differences were seen in the prevalence of increased T2-weighted signal intensity in the subcortical part of the patella when related to time between injury and MR imaging (P = .002), with an increase in prevalence as time since injury increased and a decrease in prevalence in patients who underwent MR imaging more than 20 weeks after injury (P = .013). CONCLUSION: Increased T2-weighted signal intensity in the subcortical part of the patella appears to be a subacute transient MR finding seen after knee surgery or injury.