Validating the Pig ACL as a Model for Pre-Clinical Testing of Ligament Repair Techniques

Purpose: Advancements in surgical repair and reconstruction of the anterior cruciate ligament (ACL) necessitate an appropriate animal model for pre-clinical testing. Furthermore, pre-clinical testing using cadaveric tissue is less readily available and comes at a greater cost as compared to animal tissue. Although the literature suggests the use of a pig model as a good biomechanical alternative for knee joint studies, only a limited number of studies have investigated similarities in knee joint anatomy. This study specifically aims to establish a method for comparing the geometric length, width and thickness of the ACL between humans and pigs in an effort to provide a measurement of proportionality between the two. Although the pig ACL may be smaller in overall size, we hypothesize that the dimensions will be proportional. Developing a repeatable measurement method and understanding geometric differences can help in study design (e.g., selecting suture size for ligament repair), control for experimental variation, and adds to our understanding of the mechanical differences that may be observed during testing. Although this is a preliminary study, future studies expanding this work will help determine if the pig is an appropriate alternative. Projecting ahead, as these examinations advance into in vivo studies, a validated pig model can serve as a way to follow long-term outcomes. Methods: Multi-planar digital images were acquired of a sectioned anterior cruciate ligament (ACL) from both a juvenile Yorkshire pig and a 78 year old male cadaver. Each image was imported and calibrated using imageJ (1.49v, National Institutes of Health, USA). Measurements were taken by two independent observers to determine ACL length, width, and thickness for comparison between human and pig ACL dimensions. A paired t-test was performed to establish inter-observer image measurement reliability (alpha=0.05). Results: No significant difference between readers was found in the ACL dimensions measured (p=0.23). The pig ACL had a length of 25.9-mm, a width of 10.6-mm, and a thickness of 4.1-mm. The length and thickness of the pig ACL were both 24% smaller than the human ACL. The width however did not match the scaling factor found in the other two dimensions resulting in only a 14% decrease as compared to the human ACL. Conclusions: This preliminary study establishes a reliable method for measurement of ligament dimensions. Although limited in sample size, the comparison between the ACL of a Yorkshire pig and a human was found to be proportional in two of the three dimensions studied. Moreover, visual comparison showed the ligaments to have very similar planar geometries. The study is currently ongoing and more samples are being collected and analyzed. Such geometric information will help add to our understanding of the ACL as a structure and add to the biomechanical tissue data available in the literature. Moving forward, we hope to use the pig as a model to study two novel ACL repair techniques.