Tensile mechanical changes in the Achilles tendon due to Insertional Achilles tendinopathy

Insertional Achilles tendinopathy (IAT) is a painful condition that is challenging to treat non-operatively. Although previous studies have characterized the gross histological features, in vivo strain patterns and transverse compressive mechanical properties of tissue affected by IAT, it is not known how IAT impacts the tensile mechanical properties of the Achilles tendon insertion along the axial/longitudinal direction (i.e., along the predominant direction of loading). To address this knowledge gap, the objectives of this study were to 1) apply ex vivo mechanical testing, nonlinear elastic analysis and quasilinear viscoelastic (QLV) analysis to compare the axial tensile mechanical properties of the Achilles tendon insertion in individuals with and without IAT; and 2) use biochemical analysis and second harmonic generation (SHG) imaging to assess structural and compositional changes induced by IAT in order to help explain IAT-associated tensile mechanical changes. Tissue from the Achilles tendon insertion was acquired from healthy donors and from patients undergoing debridement surgery for IAT. Tissue specimens were mechanically tested using a uniaxial tensile (stress relaxation) test applied in the axial direction. A subset of the donor specimens was used for SHG imaging and biochemical analysis. Linear and non-linear elastic analyses of the stress relaxation tests showed no significant tensile mechanical changes in IAT specimens compared to healthy controls. However, SHG analysis showed that fibrillar collagen was significantly more disorganized in IAT tissue as compared with healthy controls, and biochemical analysis showed that sulfated glycosaminoglycan (sGAG) content and water content were higher in IAT specimens. Collectively, these findings suggest that conservative interventions for IAT should target restoration of ultrastructural organization, reduced GAG content, and reduced resistance to transverse compressive strain.