Aim: A study was undertaken to determine the status of Hoffa's infrapatellar 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 characteristics of the fat pad was performed for each individual case.
Results: Hoffa's fat pad was abnormal in all cases. Shear injury from the inferior pole of the patella was present in 16 cases. Intrasubstance disruption with fluid filled clefts were noted in 12 cases. In 17 cases diffuse oedema of Hoffa's fat pad had occurred. In nine cases the damaged fat pad mimicked a loose body, while in five cases an intra-articular post-traumatic loose body was identified.
Conclusion: Post-traumatic change in Hoffa's fat pad is a constant secondary 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 fat pad from loose osteo-chondral bodies can be difficult, requiring further correlative imaging.