A MECHANISM FOR ANATOMICAL ASSESSMENT OF ACL STRENGTH IN CADAVER KNEES: A STANDARDISED TEST FOR EVALUATION OF ACL CONSTRUCT IN BI-CRUCIATE RETAINING IMPLANTS

INTRODUCTION In native knees the anterior cruciate ligament (ACL) plays a major role in joint stability and kinematics. Sacrificing the ACL in contemporary total knee arthroplasty (TKA) is known to cause abnormal knee motion, and reduced function. Hence, there is growing interest in the development of ACL retaining TKA implants. Accommodation of ACL insertion around the tibial eminence is a challenge with these designs. Therefore, a reproducible and practical test setup is necessary to characterize the strength of the ACL/bone construct in ACL retaining implants. Seminal work showed importance of loading the ACL along its anatomical orientation. However, prior setups designed for this purpose are complex and difficult to incorporate into a standardized test for wide adoption. The goal of this study was to develop a standardized and anatomically relevant test setup for repeatable strength assessment of ACL construct using basic force-displacement testing equipment. METHODS Cadaver knees were positioned with the ACL oriented along the loading axis and being the only connection between femur and tibia. 15° knee flexion was selected based on highest ACL tensions reported in literature. Therefore, the fixtures were adjusted accordingly to retain 15° knee flexion when the ACL was tensioned. The test protocol included 10 cycles of preconditioning between 6N and 60N at 1mm/s, followed by continuous distraction at 1mm/s until failure ( Fig. 1 ). Eleven cadaveric knees (4 male, 7 female; 70.9 yrs +/−13.9 yrs) were tested using this setup to characterize a baseline ACL pullout strength (peak load to failure) in native knees. RESULTS The average ACL pullout strength was 935.6N +/−327.5N with the extremes ranging from a minimum of 346N to a maximum of 1425N. There were five failure modes observed: [1] ACL avulsion from the femur with bony attachment (one knee), [2] ACL pull-off from the femur w/o bony attachment (two knees), [3] ACL tear (three knees), [4] ACL pull-off from the tibia w/o bony attachment (one knee), [5] ACL avulsion from the tibia with bony attachment (three knees). One knee showed a combined failure mode of 2 & 4, meaning part of the ACL was pulled off the femur and part pulled off the tibia. CONCLUSION There was a large variation in failure load between specimens. The knee with the minimum failure load had severe arthritis, osteophytes and signs of ACL deficiency. The average failure load (935.6N +/−327.5N) is in line with those published in literature for a comparable age group. This indicates that failure loads and modes obtained with more complex setups could be reproduced by using standard uniaxial load frames and simple fixtures. The failure modes in our experiment were evenly spread between mid-substance, and insertions (either femur or tibia). This test could be used as a standardized method to investigate the strength of the ACL complex following procedures such as ACL reconstruction, partial- and total knee arthroplasty. In particular, this setup provides a reliable mechanism for evaluation of the ACL-bone construct in bi-cruciate retaining (BCR) TKA, which is likely required for regulatory pathways.