Mechanical changes in the Achilles tendon due to insertional Achilles tendinopathy

Abstract

Insertional Achilles tendinopathy (IAT) is a painful and debilitating condition that responds poorly to non-surgical interventions. It is thought that this disease may originate from compression of the Achilles tendon due to calcaneal impingement. Thus, compressive mechanical changes associated with IAT may elucidate its etiology and offer clues to guide effective treatment. However, the mechanical properties of tissue have not been characterized. Therefore, the objective of this study was to measure the mechanical properties of excised IAT tissue and compare with healthy cadaveric control tissue. Based on the known compositional changes in the Achilles tendon caused by IAT, we hypothesized that the compressive modulus of IAT tissue would be significantly higher than in healthy control tissue and would correlate with symptom severity. We further hypothesized that IAT tendons would exhibit an altered distribution of compressive mechanical properties across the Achilles tendon insertion. Tissue from the Achilles tendon insertion was acquired from healthy donors and from patients undergoing debridement surgery for IAT. Several (~15) tissue specimens from each donor were then mechanically tested under cyclic unconfined compression at two different loading rates and the acquired data was analyzed to determine the distribution of mechanical properties (secant and tangent compressive moduli and transition strain) for each donor. While the median mechanical properties of tissue excised from IAT tendons were not significantly different than healthy tissue, the distribution of mechanical properties within each donor was dramatically altered. In particular, healthy tendons contained more low modulus (compliant) and high transition strain specimens than IAT tendons, as evidenced by a significantly lower 25th percentile secant modulus and higher 75th percentile transition strain. Furthermore, these parameters were significantly correlated with symptom severity as measured with the VISA-A scale. Finally, it was found that preconditioning and slow loading both reduced the secant modulus of healthy and IAT specimens, suggesting that slow, controlled ankle dorsiflexion prior to activity may help IAT patients manage disease-associated pain.

Insertional achilles tendinopathy associated with altered transverse compressive and axial tensile strain during ankle dorsiflexion

“…8 Tendinopathy is also associated with changes in tendon mechanics, with IAT tendons demonstrating increased compressive modulus compared with healthy controls, indicating an intrinsically more rigid tissue. 9 These histological changes in IAT tendon are nearly identical to tendon alterations caused by high levels of transverse compression. [10][11][12] Specifically, in vivo studies indicate that regions of tendon adjacent to bone that experience repeated compression exhibit increased type II collagen and aggrecan content, decreased collagen alignment and increased compressive modulus characteristics generally associated with cartilage.…”

“…At the tendon insertion of patients with IAT there are increased cartilage matrix proteins (type II collagen and aggrecan) and rounder tendon cells, which suggest metaplasia to cartilage cells (chondrocytes) . Tendinopathy is also associated with changes in tendon mechanics, with IAT tendons demonstrating increased compressive modulus compared with healthy controls, indicating an intrinsically more rigid tissue . These histological changes in IAT tendon are nearly identical to tendon alterations caused by high levels of transverse compression .…”

Achilles tendon compositional and structural properties are altered after unloading by botox

“…Also in humans, it has been shown that static training leads to increased tendon stiffness whereas dynamic training does not 44 . Another study in human tendons has also reported that healthy tendons are softer and more compliant compared to tendons with tendiopathy 45 .…”

Aging does not alter tendon mechanical properties during homeostasis, but does impair flexor tendon healing

“…To analyze acquired load-extension (i.e., force-displacement) curves and determine the non-linear mechanical properties of tested tendons, the ramp to failure portion of each mechanical test was fit to a piecewise bilinear function as described previously. 15,16 The slope of the first portion of the bilinear fit was taken to be the toe stiffness of the specimens, a measure of its resistance to deformation at small extensions. The slope of the second portion of the bilinear fit was taken to be the linear stiffness of the specimen, a measure of its resistance to deformation at large extensions.…”