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Reliability on determining the footprint of the torn ACL on MRI1256positioning remains a surgical decision, although it may not always has to be taken in the operating theatre. The excellent ICC values mainly show that the observers are consistent with locating the same point. It may seem tempting to compare the ACL insertion points as determined by the observers to a predefined point measured from the posterior cortex, for instance as defined by Piefer.13This would not be in accordance with the patient specific (instrumentation) concept and would lead to a generalized approach for each patient. No gold standard, such as confirmation by arthrotomy, was used in this study to prove that this point is actually the femoral insertion of the ACL. This is due to the fact that scans of patients with torn ACL’s were used and not cadaver samples. The down side of using cadavers is the intactness of the ACL. The ultimate goal of identifying the femoral insertion of the ACL is to give the surgeon the optimal information about where the femoral tunnel should be located. This is, obviously, only necessary in case of a torn ACL. Therefore for clinical purposes, this study was set up to use scans of a cohort of patients resembling the relevant population. As a consequence, we included subjects who have undergone routing 2D MRI scans of the knee for clinical purposes. It has been shown that 3D MRI improves overall image quality and quantitative contrast ratio4, although it has not been more accurate in diagnosing ligamentous injuries compared to 2D MRI.7 It has been demonstrated that there is no advantage in localizing the ACL attachment centers when using 3D MRI over 2D MRI.20In our study manual segmentation of the MRI scans was performed to create a 3D model of the distal femur. Automatic or semi-automatic segmentation techniques have been described in the literature.1-3 Although the correctness of the 3D model was not evaluated in this study, evaluation of the segmentation technique was done prior to this study. Unpublished data showed an excellent surface comparison when comparing 3D models derived from 2D MRI, 3D MRI and CT. The fact that orthopedic surgeons reach a high group interobserver agreement may be the effect of a monocenter study. There may be a Mark Zee.indd 125 03-01-2024 08:56