Primary Investigator University of Rochester Rochester, United States
Introduction: : Tendons are long cords of strong, flexible material that connect muscle to bone and are necessary for movement of the body[1]. The tissue is also porous[2], allowing fluid to flow through pores along its length. The permeability of tendon is a property that describes how easily fluid is squeezed from the tissue. Tendons are often found with structures known as bursae[3]: closed, fluid-filled sacs located between tendons and bones, which are lined with a thin synovial membrane and filled with synovial fluid. The most abundant molecule in synovial fluid is albumin[4], a macromolecule that is also the most abundant protein in blood. Previous studies from our lab show that accumulation of albumin (or other macromolecules of equivalent size) is detrimental to the health of articular cartilage (AC) because these macromolecules can infiltrate and expand cartilage pores, increasing tissue permeability and reducing its resistance to deformation under mechanical loading. To build on these findings, the objective of this study is to define how the accumulation of macromolecules within the pores of tendon, which are similar in size to those found in AC, influence its response to mechanical loading. The results of this study could on day impact care for painful tendon conditions such as Achilles tendinopathy or rotator cuff disease by defining how the molecular composition of synovial fluid – which can be clinically modified – affects tendon health. We hypothesize that the accumulation of a critical concentration of soluble macromolecules is detrimental to tendon health because it increases tissue permeability.
Materials and
Methods: : Porcine flexor tendons were dissected and frozen laying flat. Then, 3 mm biopsy punches were taken from the frozen tendon through the minor axis of the tendon down its length. The punches were then assigned to three experimental groups: a fresh group without incubation (n=10), a group incubated in Phosphate-Buffered Saline (PBS) solution for 2 days at 37℃ (n=21), and a group incubated in a solution of 30 mg/ml Bovine Serum Albumin (BSA) for 2 days at 37℃ (n=21). Once the samples had been incubated for their respective times, a confined compression stress-relaxation test was performed using a custom designed and fabricated device that interfaces with a uniaxial tensile test machine (Fig. 1). The samples were inserted into the top of a 3 mm diameter hole in the confined compression chamber of the device, which features thin slits down its length to allow for fluid exudation (Fig. 1). After preloading, a stress relaxation test was performed. Specifically, specimens were rapidly compressed to a strain of 0.5 and held at this fixed strain level for 5 minutes while the load was recorded. The stress was computed as the force divided by the cross-sectional area, and the acquired data were assumed to fit an equation of the form σ=σ_0 e^(-t/τ)+ε_0 H_A,[5] where ε_0 is the applied compressive strain (0.5). Based on this model, H_A and the relaxation time τ was calculated and the permeability (k) was determined according to k=d^2/(τH_A ), where d is the initial specimen thickness.
Results, Conclusions, and Discussions:: Results and
Discussion: Contrary to our hypothesis, an accumulation of albumin in porcine flexor tendon did not affect the permeability of the tendon compared to control specimens incubated in PBS (Fig. 2). This result contrasts with our previous findings in murine cartilage, where albumin infiltration increased permeability due to the generation of osmotic pressure gradients within the tissue’s heterogeneous pores. These contrasting findings may reflect differences in pore structure between cartilage and tendon, or species differences between murine and porcine models. In this study, untreated specimens exhibited lower permeability and higher aggregate modulus than both treatment groups, likely due to swelling or protease activity during culture (Fig. 2, 3). However, since both albumin- and PBS-incubated specimens were maintained under similar conditions, comparisons between these groups remain valid. A limitation of this study is that we did not confirm infiltration of albumin throughout the tendon over the incubation time period. Future studies will use immunohistochemistry to verify albumin penetration into the tendon core.
Conclusion: Our finding that tendon permeability was unchanged after albumin incubation suggest differential effects of albumin infiltration on tendon and cartilage and may indicate that increases in synovial albumin levels (e.g., in individuals with joint diseases) may not adversely impact tendon function.
Acknowledgements and/or References (Optional): : [1] Wang et al., 2012; [2] Connizzo et al., 2017; [3] Benjamin et al., 2014; [4] Bennike et al., 2014; [5] Chin et al., 2011
