Lubricin in human achilles tendon: The evidence of intratendinous sliding motion and shear force in achilles tendon

Abstract

Achilles tendon is one of the most commonly injured tendons. Mechanical force is regarded as a major causative factor. However, the biomechanics of Achilles tendon and mechanical mechanism of the injuries are unclear. Lubricin expresses at regions exposed to sliding motion and shear force in a number of tissues. This study investigated the distribution and concentration of lubricin in human Achilles tendons for better understanding the biomechanics of Achilles tendon. Achilles tendons were harvested from nine cadavers. Lubricin was extracted from various locations proximal to the calcaneal insertion and quantified with ELISA. The distribution of lubricin was investigated with immunohistochemistry. Lubricin was mainly identified at the interfaces of tendon fascicles, especially in the mid-portion of the tendon. The concentration of lubricin in Achilles tendons varied by individual and the distance from its calcaneal insertion. The distal portion of the tendon had a higher concentration of lubricin than the proximal regions of the tendon. This study suggests the presence of intratendinous sliding motion of fascicles and shear force at interfaces of fascicles in human Achilles tendon. Shear force could be an important mechanical factor for the development of Achilles tendinopathy and rupture.

Three-dimensional morphology and strain of the Achilles free tendon immediately following eccentric heel drop exercise

“…MVIC, we cannot discount small changes in volume (minimal detectable change for 3DUS volume ∼0.2 ml; Obst et al, 2014b) or redistribution of fluid along the tendon length (Sun et al, 2015). Regardless, our findings demonstrate that acute changes in tendon transverse morphology after exercise may be more pronounced when measured under tensile load, and that such changes may largely be dependent on changes in tendon length due to mechanical creep.…”

The “other” 15–40%: The Role of Non‐Collagenous Extracellular Matrix Proteins and Minor Collagens in Tendon

“…In addition, elastic fibers were prominent in the IFM in the superficial digital flexor tendon in horse, the equivalent of the Achilles tendon, suggesting a role for elastic fibers in mediating recoil of energy storing tendons . Lubricin is a glycoprotein that was identified in the IFM where it may facilitate the gliding between tendon fascicles due to its anti‐adhesive properties . This role was supported in pullout tests of lubricin knockout mouse‐tail tendon fascicles, which showed increased gliding resistance .…”

Experimental evaluation of multiscale tendon mechanics

“…A unique mucin‐glycoprotein known as lubricin or proteoglycan 4, has also localized in soft connective tissues, such as cartilages, ligaments, menisci, intervertebral discs, and tendons . It has been identified to be an essential part for cartilage lubrication and further cytoprotection, including preventing synovial fluid proteins from depositing onto cartilage and reducing chondrocyte apoptosis .…”

“…A unique mucin‐glycoprotein known as lubricin or proteoglycan 4, has also localized in soft connective tissues, such as cartilages, ligaments, menisci, intervertebral discs, and tendons . It has been identified to be an essential part for cartilage lubrication and further cytoprotection, including preventing synovial fluid proteins from depositing onto cartilage and reducing chondrocyte apoptosis . Particularly for tendons (i.e., flexor tendon, Achilles tendon) at anatomical locations where inter‐or intra‐tendinous shear forces occur, lubricin has been suggested to facilitate tendon gliding against surrounding tissues on the tissue scale and assist the relative movement between tendon fascicles on the microscale .…”

“…It has been identified to be an essential part for cartilage lubrication and further cytoprotection, including preventing synovial fluid proteins from depositing onto cartilage and reducing chondrocyte apoptosis . Particularly for tendons (i.e., flexor tendon, Achilles tendon) at anatomical locations where inter‐or intra‐tendinous shear forces occur, lubricin has been suggested to facilitate tendon gliding against surrounding tissues on the tissue scale and assist the relative movement between tendon fascicles on the microscale . Flexor tendons treated with lubricin showed lower gliding resistance and decreased adhesions than untreated tendons .…”