MRI appearances of the infrapatellar fat pad in occult traumatic patellar dislocation

AIM: A study was undertaken to determine the status of Hoffa's infrapatella r fat pad in instances of acutely dislocated patellae. MATERIALS AND METHODS: The study consisted of MR examinations performed on 18 consecutive patients with acutely dislocated and relocated patellae with a mean interval between injury and MR examination date of 14.8 days (range 1-60 days). An analysis of the attachments and intrinsic signal characteri stics of the fat pad was performed for each individual case. RESULTS: Hoffa's fat pad was abnormal in all cases. Shear injury from the i nferior pole of the patella was present in 16 cases, Intrasubstance disrupt ion with fluid filled clefts were noted in 12 cases. In 17 cases diffuse oe dema of Hoffa's fat pad had occurred. In nine cases the damaged fat pad mim icked a loose body, while in five cases an intra-articular post-traumatic l oose body was identified. CONCLUSION: Post-traumatic change in Hoffa's fat pad is a constant secondar y MR feature not previously reported, that can be added to the spectrum of indirect findings in cases of occult patellar dislocation, In addition, the MRI distinction between post-traumatic changes in Hoffa's infrapatellar fa t pad from loose osteo-chondral bodies can be difficult, requiring further correlative imaging. Apostalaki, E. et al. Clinical Radiology (1999) 54, 74 3-747. (C) 1999 The Royal College of Radiologists.