Changes in ADC caused by tensile loading of rabbit Achilles tendon: Evidence for water transport

Water diffusion measurements were performed on rabbit Achilles tendons duri ng static tensile loading and tendons in an unloaded state. The apparent di ffusion coefficient (ADC) was measured along two directions: parallel and p erpendicular to the long axis of the tendon. Tendons were studied after bei ng prepared in two ways: (a) after being stored frozen in phosphate-buffere d saline (PBS) and (b) freshly isolated. Statistically significant directio nal anisotropy was observed in the ADC in all tendons. The ADC was signific antly greater in the direction parallel to the long axis of the tendon than in the perpendicular direction. The anisotropy is attributed to the greate r restrictions seen by the water molecules in the perpendicular direction a nd is consistent with the known geometry of the tendon. Storage in PBS caus ed tendons to swell. This increased the ADC measured along both directions and reduced the anisotropy. The existence of anisotropy in the ADC was not related to the orientation of the specimen in the magnet. The ADC increased along both directions following the application of a 5-N tensile load; the increase was greatest along the perpendicular axis of the tendon. In order to determine whether load-related changes in the ADC reflected changes in interfibrilar spacing, we used electron microscopy to measure load-related changes in fibril spacing. Load-related changes in fiber spacing could not account for the observed changes in the ADC. The increase in ADC caused by loading was attributed to the extrusion of tendon water into a bulk phase a long the outside surface of the tendon. In PBS-stored samples, enough fluid was extruded that it could be visualized. The transient response of the AD C to a 5-N tensile load was also studied. The absolute ADC in both directio ns increased with loading and recovered to baseline upon unloading. The tra nsient changes in ADC, for both loading and unloading, had a mean time cons tant of approximately 15 min. The magnitude of the load-induced transient A DC changes was comparable to that seen in the static-loading experiments. ( C) 2000 Academic Press.