Hoffa's fat pad injuries and their relationship with anterior cruciate ligament tears: new observations based on MR imaging in patients and MR imaging and anatomic correlation in cadavers.

PURPOSE: To determine the normal anatomic relationships of Hoffa's fat pad with the anterior cruciate ligament (ACL) and with the frequency of Hoffa's fat pad abnormalities in ACL-deficient knees.
DESIGN: Retrospective clinical study on patients and observational anatomic study on cadavers. The study was approved by the Institutional Review Board. PATIENTS: MR imaging studies of the knees of 100 patients (21-48 years old) with or without arthroscopically proven tears of the ACL, performed at a single institution, were reviewed by two readers for abnormalities of Hoffa's fat pad. Ten cadaveric knee specimens were studied with MR imaging and Faxitron radiographs, and by inspection of sections and histology.
RESULTS: Alterations in Hoffa's fat pad on MR imaging were present in 64% (32/50) of patients with torn ACLs, and in 24% (12/50) of patients without a tear of the ACL (P < 0.05). Hoffa's fat pad inserted into the intercondylar notch in 50% (5/10) of cadaveric specimens, four in conjunction with the ligamentum mucosum and in one in an isolated fashion. Histological study demonstrated the composition of the ligamentum mucosum and Hoffa's fat pad and their course and insertion sites in the intercondylar notch.
CONCLUSION: Abnormalities of Hoffa's fat pad, such as focal and diffuse edema, tears, scars and synovial proliferation, are more common in knees with torn ACLs than in knees with intact ACLs.