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Patient Specific Instrumentation in ACL reconstruction1357surrounding structures on these images. Voxel size of 0.4x0.4x3.0mm was selected with a field of view of 160mm, a flip angle of 1500, a repetition time of 3530 ms. and an echo time of 41 ms. The scanning protocol used in this study was similar to the routine clinical protocol for diagnosing ACL injury. This avoids the need for additional scans when this concept is used for clinical purposes in the future. Files were saved for further processing in 16-bit Digital Imaging and Communications in Medicine (DICOM) file formats. Segmentation Procedure.Using the Mimics Innovation Suite Software version 21.0 (Materialise, Leuven, Belgium) the images were segmented to obtain accurate 3D models of the knee. The MRI images were semi-automatically segmented with the use of the livewire technique as previously described.29 Using this technique, the software is able to semi-automatically distinguish different gray scales in order to select a region of interest. The region of interest consisted of the femur including the overlying cartilage. Intra-observer reliability for the segmentation method was evaluated using repeated segmentations. The total absolute mean distance between models was 0.20 mm. 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 Unpublished data showed an excellent surface comparison when comparing 3D models derived from 2D MRI compared to CT.The center of the femoral origin of the ACL was determined on the MRI images and marked by a circle of 2 mm in diameter. This point was referred to as “ACL origin”, see figure 1. Previous research has shown that the identification of the femoral insertion using this method has a high intra- and interobserver reliability, even in the presence of ACL injury.29 Intra-observer reliability for this method has been shown to be excellent with an ICC of > 0.98 and excellent interobserver reliability with an ICC of > 0.96. Mark Zee.indd 135 03-01-2024 08:56