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Patient Specific Instrumentation in ACL reconstruction1397planned drill trajectory. Both the pre- and post-procedural scans were segmented as described before. The post-procedural drill trajectories were easily identified and segmented as cylinders on all post-procedural scans (See Figure 4). The position of these cylinders was compared to the pre-procedural planned drill trajectories. Measurements were performed in Mimics. Distances from cylinder edge to cylinder edge were recorded in mm using a digital ruler. Because of the oblique drilling trajectory, the center of the cylinder was hard to determine, therefore we chose for edge-to-edge measurements and added the diameter of the RetroDrill (3.5mm) to this measurement. All measurements were performed by one trained observer. The measurements were repeated by the same observer more than 2 weeks later to determine the intra-observer reliability of the measurements. The intraclass correlation coefficient (ICC 2-way random, absolute agreement) was calculated between the first and second assessment. A value less than 0.5 was considered to be indicative of poor reliability, value between 0.5 and 0.75 indicates moderate reliability, a value between 0.75 and 0.9 indicates good reliability, and a value greater than 0.90 indicates excellent reliability. ResultsThe introduced hooks provided a very good fit in the intercondylar notch as shown in table 1. The two orthopedic surgeons reported similar results as shown in table 1. Table 1. Overview of the fitting properties of the patient specific guides as judged by the two orthopedic surgeons. Fitting properties were rated on a 5 point Likert scale (1= very poor, 2= poor, 3= moderate, 4= good, 5 = very good)Observer Cadaver 1 Cadaver 2 Cadaver 3 Cadaver 4Orthopedic Surgeon 1 Good Very Good Very Good Very GoodOrthopedic Surgeon 2 Good Very Good Very Good Very GoodUsing the 3D printed guide hooks resulted in a mean difference of 5.0 mm (SD 1.0 mm range 3.8-6.7mm) between the planned and actual drilled trajectory. For an example, see Figure 4. In Figure 4, the planned drill trajectory is displayed as the dark-gray cylinder. The actual drilled tunnel is displayed in red.Mark Zee.indd 139 03-01-2024 08:56