Macroscale and microscale mechanical and biochemical properties of the equine superficial digital flexor tendon are regionally dependent.

Objective: To examine regional mechanical and biochemical differences across the length of the equine superficial digital flexor tendon (SDFT). Methods: Equine SDFTs were collected postmortem from 9 donors and divided into proximal, middle, and distal regions based on the location of the musculotendinous and osteotendinous junctions. Tensile testing was performed on samples to obtain macroscale mechanical properties, and atomic force microscopy was done to obtain microscale mechanical properties. Biochemical assays and histological staining were done to assess glycosaminoglycan (GAG), collagen, and DNA content in each region. Results: Regional differences were present in macroscale mechanical properties and biochemical properties. Differences were primarily seen between the middle and distal region, with the distal region having a 3-fold increased GAG content (Δ14.15 µg GAG/mg sample; 95% CI, -0.32 to 28.62) and a 1.67-fold decreased macroscopic tensile modulus (Δ-68.2 MPa; 95% CI, -107.9 to -28.5). A more disorganized collagen fibril matrix was seen in the distal region through histological staining. Conclusions: Mechanical, biochemical, and histological regional differences exist with the equine SDFT. In particular, the distal region showed a disorganized matrix with higher GAG content and decreased modulus in comparison to the proximal and middle regions. Clinical Relevance: Our data provide information that helps better understand normal SDFT function and presentation of tendinopathy by providing regional structural and biochemical and mechanical information within the SDFT.