Yu-Long Sun, PhD1; Chunfeng Zhao, MD1; Gregory D. Jay, MD, PhD2

Thomas M. Schmid, PhD3; Kai-Nan An, PhD1; Peter C. Amadio, MD1

Abstract
Background:  
Lubricin facilitates boundary lubrication of cartilage. The synthesis of lubricin in cartilage is regulated by mechanical stimuli, especially shear force. Lubricin is also found in flexor tendons. However, little is known about the effect of mechanical loading on lubricin synthesis in tendons or about the function of lubricin in flexor tendons. The purpose of this study was to investigate the relationship of mechanical loading to lubricin expression and gliding resistance of flexor tendons.

Methods:  
Flexor tendons were harvested from canine forepaws that had been suspended without weight-bearing for twenty-one days and from the contralateral forepaws that had been allowed free motion. Lubricin expression in each flexor tendon was investigated with real-time RT-PCR (reverse transcription polymerase chain reaction) and immunohistochemistry. Lubricin in the flexor tendon was extracted and quantified with ELISA (enzyme-linked immunosorbent assay). The friction between the flexor tendon and the proximal pulley was measured.

Results:  
The non-weight-bearing flexor tendons had a 40% reduction of lubricin expression (p < 0.01) and content (p < 0.01) compared with the flexor tendons in the contralateral limb. However, the gliding resistance of the tendons in the non-weight-bearing limb was the same as that of the tendons on the contralateral, weight-bearing side.

Conclusions:  
Mechanical loading affected lubricin expression in flexor tendons, resulting in a 40% reduction of lubricin content, but these changes did not affect the gliding resistance of the flexor tendons.

Clinical Relevance:  
The gliding resistance of flexor tendons was not affected after a period of limited motion. This suggests that physical activity after a short period of limited motion does not lead to wear of intact tendons and their surrounding tissue.