Viscoelastic characteristics of the canine cranial cruciate ligament complex at slow strain rates

Proteoglycans play a role in the viscoelastic behaviour of the canine cranial cruciate ligament

Proteoglycans (PGs) are minor extracellular matrix proteins, and their contributions to the mechanobiology of complex ligaments such as the cranial cruciate ligament (CCL) have not been determined to date. The CCLs are highly susceptible to injuries, and their extracellular matrix comprises higher PGs content than the other major knee ligaments. Hence these characteristics make CCLs an ideal specimen to use as a model in this study. This study addressed the hypothesis that PGs play a vital role in CCL mechanobiology by determining the biomechanical behaviour at low strain rates before and after altering PGs content. For the first time, this study qualitatively investigated the contribution of PGs to key viscoelastic characteristics, including strain rate dependency, hysteresis, creep and stress relaxation, in canine CCLs. Femur-CCL-tibia specimens (n = 6 pairs) were harvested from canine knee joints and categorised into a control group, where PGs were not depleted, and a treated group, where PGs were depleted. Specimens were preconditioned and cyclically loaded to 9.9 N at 0.1, 1 and 10%/min strain rates, followed by creep and stress relaxation tests. Low tensile loads were applied to focus on the toe-region of the stress-strain curves where the non-collagenous extracellular matrix components take significant effect. Biochemical assays were performed on the CCLs to determine PGs and water content. The PG content was ∼19% less in the treated group than in the control group. The qualitative study showed that the stress-strain curves in the treated group were strain rate dependent, similar to the control group. The CCLs in the treated group showed stiffer characteristics than the control group. Hysteresis, creep characteristics (creep strain, creep rate and creep compliance), and stress relaxation values were reduced in the treated group compared to the control group. This study suggests that altering PGs content changes the microstructural organisation of the CCLs, including water molecule contents which can lead to changes in CCL viscoelasticity. The change in mechanical properties of the CCLs may predispose to injury and lead to knee joint osteoarthritis. Future studies should focus on quantitatively identifying the effect of PG on the mechanics of intact knee ligaments across broader demography.

The anterior cruciate ligament in murine post-traumatic osteoarthritis: markers and mechanics

Background

Knee joint injuries, common in athletes, have a high risk of developing post-traumatic osteoarthritis (PTOA). Ligaments, matrix-rich connective tissues, play important mechanical functions stabilising the knee joint, and yet their role post-trauma is not understood. Recent studies have shown that ligament extracellular matrix structure is compromised in the early stages of spontaneous osteoarthritis (OA) and PTOA, but it remains unclear how ligament matrix pathology affects ligament mechanical function. In this study, we aim to investigate both structural and mechanical changes in the anterior cruciate ligament (ACL) in a mouse model of knee trauma.

Methods

Knee joints were analysed following non-invasive mechanical loading in male C57BL/6 J mice (10-week-old). Knee joints were analysed for joint space mineralisation to evaluate OA progression, and the ACLs were assessed with histology and mechanical testing.

Results

Joints with PTOA had a 33–46% increase in joint space mineralisation, indicating OA progression. Post-trauma ACLs exhibited extracellular matrix modifications, including COL2 and proteoglycan deposition. Additional changes included cells expressing chondrogenic markers (SOX9 and RUNX2) expanding from the ACL tibial enthesis to the mid-substance. Viscoelastic and mechanical changes in the ACLs from post-trauma knee joints included a 20–21% decrease in tangent modulus at 2 MPa of stress, a decrease in strain rate sensitivity at higher strain rates and an increase in relaxation during stress-relaxation, but no changes to hysteresis and ultimate load to failure were observed.

Conclusions

These results demonstrate that ACL pathology and viscoelastic function are compromised in the post-trauma knee joint and reveal an important role of viscoelastic mechanical properties for ligament and potentially knee joint health.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13075-022-02798-7.

Ligament mechanics of ageing and osteoarthritic human knees

Knee joint ligaments provide stability to the joint by preventing excessive movement. There has been no systematic effort to study the effect of OA and ageing on the mechanical properties of the four major human knee ligaments. This study aims to collate data on the material properties of the anterior (ACL) and posterior (PCL) cruciate ligaments, medial (MCL) and lateral (LCL) collateral ligaments. Bone-ligament-bone specimens from twelve cadaveric human knee joints were extracted for this study. The cadaveric knee joints were previously collected to study ageing and OA on bone and cartilage material properties; therefore, combining our previous bone and cartilage data with the new ligament data from this study will facilitate subject-specific whole-joint modelling studies. The bone-ligament-bone specimens were tested under tensile loading to failure, determining material parameters including yield and ultimate (failure) stress and strain, secant modulus, tangent modulus, and stiffness. There were significant negative correlations between age and ACL yield stress (p = 0.03), ACL failure stress (p = 0.02), PCL secant (p = 0.02) and tangent (p = 0.02) modulus, and LCL stiffness (p = 0.046). Significant negative correlations were also found between OA grades and ACL yield stress (p = 0.02) and strain (p = 0.03), and LCL failure stress (p = 0.048). However, changes in age or OA grade did not show a statistically significant correlation with the MCL tensile parameters. Due to the small sample size, the combined effect of age and the presence of OA could not be statistically derived. This research is the first to report tensile properties of the four major human knee ligaments from a diverse demographic. When combined with our previous findings on bone and cartilage for the same twelve knee cadavers, the current ligament study supports the conceptualisation of OA as a whole-joint disease that impairs the integrity of many peri-articular tissues within the knee. The subject-specific data pool consisting of the material properties of the four major knee ligaments, subchondral and trabecular bones and articular cartilage will advance knee joint finite element models.