Limb-immobilization produced chronic pain behavior in rats

Abstract

Clinically, it is known that chronic pain develops following soft tissue injuries without a clear nerve lesion. In this study, we tried to evaluate effects of cast limb-immobilization on the sensory and musculoskeletal system of the rat. Adult male Sprague-Dawley rats, weighting 300-400g, were used. Left hindlimb was immobilized by plaster casts for two weeks: the hip, knee and ankle being flexed in the standing position. We measured the following items bilaterally: 1. von Frey tests (VFTs) at the plantar surface and at the medial surface of the calf, 2. pressure pain threshold (PPT) of the calf using a push-pull gauge, 3. the maximum thickness of the calf to evaluate muscle atrophies, 4. the hindpaw thickness and circumference to evaluate the edema, 5. range of motion (ROM) of the hip, knee and ankle, 6. skin temperature of the hindpaws. Pain behaviors evaluated by VFTs and PPT increased bilaterally (ipsilaterally predominantly) and lasted for more than 8 weeks after cast-off. In addition, a bilateral decrease of the calf thickness and ipsilateral reduction of ROMs and increases in the circumference, thickness and skin temperature of the hindpaws were observed in the immobilization side up to 4 weeks after cast-off.It is noteworthy that a simple limb-immobilization caused pain behaviors, which lasted long after recovery various signs of peripheral tissue dysfunction. The animal model developed in this study is valuable for further investigation of chronic pain. [Jpn J Physiol 55 Suppl:S168 (2005)]