MR imaging findings in anterior cruciate ligament reconstruction: evaluation of notchplasty.

OBJECTIVE Notchplasty, resection of bone from the roof and lateral side of the inter-condylar notch, is frequently performed during anterior cruciate ligament (ACL) reconstruction to avoid abrasion or deflection of the ACL graft by the femur (graft impingement). Graft impingement can develop months after the initial reconstruction despite adequate notchplasty. Such impingement has recently been attributed to fibrocartilage overgrowth of the notchplasty site. The objectives of this study were to evaluate the MR appearance of the notchplasty site immediately after surgery and 6 months later and to investigate whether cartilage overgrowth of the notchplasty site can be detected with MR imaging. SUBJECTS AND METHODS Thirty-three patients who underwent bone-tendon-bone autograft ACL reconstruction had MR scans of the intercondylar notch 6 months after surgery. Twenty-five of these patients also had MR scans in the immediate postoperative period. The appearance of the notchplasty site and evidence for cartilage overgrowth of the notchplasty site were evaluated. Arthroscopic correlation was available for six patients. RESULTS On MR images, the margins of the notchplasty site were identified in all patients. Contour of the notchplasty site along the lateral notch wall was frequently concave toward the notch, a finding not commonly seen in an unoperated notch. Six months later, the shape of the notchplasty site had not changed, but evidence for recortication of the notchplasty site (seen as a layer of signal void 0.5-1.5 mm in thickness over the previously exposed cancellous bone) was seen in 94% of the patients. Additionally, a second layer of signal void, 1-5 mm in thickness with signal intensity identical to that of hyaline cartilage, was seen overlying the notchplasty site in 64% of patients. Arthroscopic correlation available in six patients suggested that this second layer represented fibrocartilage overgrowth of the notchplasty site. CONCLUSION Characteristic changes of the intercondylar notch after notchplasty can be seen on MR images. This preliminary study also suggests that a thin layer of cortical bone forms over the notchplasty site in most patients within 6 months of surgery. Perhaps more significantly, overgrowth of the notchplasty site by fibrocartilage may also be detected in some patients. Further experience is needed to determine whether MR imaging is a useful method for identifying cartilage overgrowth after notchplasty.