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Chapter 6124femur, which implies subjects were over 16.6 years of age.14 The exact age and sex distribution among our subjects could not be retrieved due to a strict anonymization protocol . The presence of open epiphysis can influence the choice of treatment in ACL injury.12 Whether the age of the subjects influences an observers performance to determine the femoral footprint of the ACL is unknown. Our findings are comparable to the findings from Rachmat et al who demonstrated a mean intraobserver accuracy of 4.30 mm when identifying the femoral footprint of the ACL on MRI.15 It has to be noted however that Rachmat et al used one cadaveric specimen with an intact ACL. Our study is thus more representative of the clinically relevant situation. Adding MRI’s of subjects with intact ACL’s to our database could have introduced a learning effect with the observers. The effect of background and experience may then have been biased. Therefore our study only included MRI with confirmed rupture of the ACL. In our study orthopedic surgeons were able to determine the same point (femoral footprint) with a mean difference of 2.97 mm between two assessments of 20 scans. A high diversity in the size and shape of the femoral footprint has been reported9, and this footprint appears to be ribbon shaped with a length of 16mm and a width of 8mm.9,18 In this light, a mean 3D difference of 2-5 mm can be regarded acceptable. The orthopedic surgeons showed the highest agreement within their group compared to the other two groups, followed by the orthopedic residents. These findings seem to indicate a lack of “in-field” experience of radiologists compared to the orthopedic surgeons and orthopedic residents. Possibly, witnessing or performing an ACL reconstruction (or knee surgery in general) repeatedly, leads to more consistency in defining the location of the ACL footprint. As residents in orthopedic surgery, not specialized in ACL reconstruction, attained comparable group interobserver reliability compared to the orthopedic surgeons, the effect of general surgical experience seems to be more relevant than experience in ACL reconstruction specifically. This emphasizes that femoral tunnel Mark Zee.indd 124 03-01-2024 08:56