Summary

This study validates the adolescent porcine stifle joint as a model for studying ACL biomechanics under axial and torsional loads, crucial for understanding ACL injuries in adolescents. The findings highlight the comparable damage caused by torsional and axial stresses, emphasizing the importance of considering growth plate strength in injury prevention and treatment strategies for young athletes.

Abstract

Introduction

Anterior Cruciate Ligament (ACL) injuries are increasingly common in children aged 10-13, with a significant proportion resulting from non-contact mechanisms, particularly involving torsional loads on the knee. Understanding the biomechanics of ACL injuries in adolescents is challenging due to the limited availability of cadaveric specimens. This study aims to validate the adolescent porcine stifle joint as a suitable model for studying ACL biomechanics, specifically under axial and torsional loads. The research focuses on assessing the ACL's deformation rate (strain), stiffness, and load-to-failure, providing critical data to validate the porcine model for translational research.

Methods

Thirty fresh porcine stifle joints from Yorkshire pigs, aged 2 to 4 months and weighing approximately 90 lbs, were used. The joints were stored at -22°C and thawed for 24 hours at room temperature before testing. The samples were randomly divided into three groups: a control group subjected only to a load-to-failure test, and two experimental groups subjected to 100 cycles of loading. The loading conditions applied were axial loads of 500N and torsional loads inducing a 30-degree twist, simulating the mechanical conditions likely to cause ACL injuries. The femur was externally rotated, and the knee was internally rotated to achieve the angular displacement. The stifle joints were dissected, leaving the ACL, meniscus, and stabilizing ligaments intact. The samples were then mounted on a servo-hydraulic material testing machine, aligned at a 20-degree angle between the femur and tibia, and subjected to unidirectional tensile loading at 1 mm/sec until rupture. Load and displacement data were recorded at 100 Hz.

Results

Significant differences in load-to-failure were observed among the different loading conditions. The one-way ANOVA showed statistically significant differences in maximum failure force between the control and the 500N axial load group (p = 0.014) and between the control and the 500N torsional load group (p = 0.003). However, no significant difference was found between the 500N axial and torsional load groups (p = 0.2645). The ruptured ACLs closely resembled adolescent ACL injuries in their detachment patterns. Additionally, stiffness measurements post-cyclic loading indicated that the torsional loads induced comparable, if not greater, damage to the ACL as axial loads. Interestingly, two samples exhibited failure at the distal femur growth plates, suggesting that the growth plates in adolescent porcines are weaker than the ligaments.

Discussion

The findings indicate that torsional loads are as damaging to the ACL as axial loads of equivalent magnitude, reinforcing the importance of considering both types of stress in understanding ACL injury mechanisms. The study also highlights the vulnerability of growth plates in younger populations, which may be a critical factor in ACL injuries.

Clinical Relevance: This research validates the adolescent porcine model as a reliable surrogate for studying ACL biomechanics, particularly in the context of axial and torsional loads. The findings emphasize the need to consider the impact of growth plate strength in ACL injury prevention and treatment strategies, especially in pediatric populations. The study's insights can guide the development of more effective interventions and protective measures in young athletes.