# Dynamic Deformation Behavior of the Porcine Anterior Cruciate Ligament Enthesis Under Anterior Tibial Loading

## TL;DR

This study examines how the structure of the ACL enthesis in pigs changes under load, revealing differences in fiber angles and cartilage presence that affect load transmission.

## Contribution

The study reveals regional differences in fiber orientation angles and fibrocartilage presence in the ACL enthesis under loading, linking these to load-transduction mechanisms.

## Key findings

- Fiber orientation angles in the femoral enthesis region were significantly higher than in the ligament region under loading.
- The tibial enthesis region showed no significant difference in fiber orientation angles between enthesis and ligament regions.
- Uncalcified fibrocartilage was present in the femoral enthesis but absent in the tibial enthesis of the ACL.

## Abstract

This study determined the insertion angle at the porcine anterior cruciate ligament (ACL) enthesis under joint loading to provide information on the structure and mechanical function of the enthesis. Ten intact porcine knee joints were harvested, and an anterior tibial load was applied using a robotic testing system. After dissecting a portion of the ACL enthesis along ligament fibers, the remaining enthesis was imaged using a digital microscope while reproducing the three-dimensional intact knee motion. Fiber orientation angles (FOAs) in the enthesis region (0–300 µm from the ligament-bone boundary) and the ligament region (500–2000 µm from the ligament-bone boundary) were analyzed in the femoral and tibial entheses of the anteromedial bundle (AMB) of the ACL under loading. On the femoral side, the FOA in the enthesis region was significantly higher than that in the ligament region by approximately 10 degrees under loading (n = 5, p < 0.05 in paired t-test). In contrast, the FOAs in the enthesis and ligament regions on the tibial side were nearly equal under loading, with no significant difference (n = 5, p > 0.15 in paired t-test). Histological examination indicated that uncalcified fibrocartilage (UF) was abundant in the enthesis region of the AMB femoral enthesis while the UF was not observed in the enthesis region of the AMB tibial enthesis. Thus, the current data suggest that the regional dependence and independence in FOA are caused by the presence or absence of UF and contributes to a moderate and subtle load-transduction in the ACL enthesis.

The online version contains supplementary material available at 10.1007/s10439-024-03654-2.