Anatomical description and biomechanics of the anterior cruciate ligament in the goat knee

Aim: The aim of this study was to perform a detailed qualitatively and quantitatively assessment of the goat ACL, its bundles, its insertion sites and to describe the biomechanical function of the different bundles in order to evaluate its use for a double bundle ACL reconstruction model. Materials and Methods: Ten non-paired adult goat knees were used. We measured the insertion site area of each bundle. We analyzed the ratio between the femoral and tibial insertion site areas and the midsubstance cross-sectional area of the ACL. Microscribe 3D and 3D-laser camera were used to record the data. A CASPAR Stäubli RX90 robot with a six degree-of-freedom load cell was used for measurement of anterior tibial translation (ATT) [mm] and in situ forces [N] at 30° (full extension), 60°, 90° as well as rotational testing at 30° in 14 paired goat knees before and after each bundle was cut. Results: Three bundles could be clearly identified in each ACL: anteromedial (AM), intermediate (IM) and posterolateral (PL) bundle. On the tibial side, the insertion of the IM and PL bundles could not be identified separately. On the femur, the area of insertion site, represented as a percentage of the entire footprint, was 54.3 ± 7.8% for AM, 9.9 ± 3.8% for IM and 35.8 ± 4.4% for PL bundle. The area of tibial insertion was 68.6 ± 4.7% for AM and 31.4 ± 4.7% for IM/PL bundle. The differences between the entire femoral (51.9 ± 4.6 mm2) and tibial (81.1 ± 11.9 mm2) footprint areas and between each bundle were statistical significant (p < 0.05). All insertions had significantly larger areas than the ligament midsubstance cross-sectional area (21.76 ± 7.26 mm2) (p < 0.05). When the AM-bundle was cut, the ATT increased significantly at 60° and 90° of flexion (p<0.05). When the PL-bundle was cut, the ATT increased only at 30°. However, most load was transferred through the big AM-bundle while the PL-bundle shared significant load only at 30°, with only minimal contribution from the IM-bundle at all flexion degrees. Conclusion: The precise knowledge of the ACL anatomy in the goat knee is necessary when a goat model is planned. Though anatomically discernible, the IM-bundle plays only an inferior role in ATT and might be neglected as a separate bundle during reconstruction. The goat ACL shows some differences to the human ACL, whereas the main functions of the ACL bundles are similar.