Extrinsic and intrinsic innervation of the guinea pig knee joint: toward an animal model for manipulative therapy.

OBJECTIVES: Besides spinal conditions, knee joint problems are one of the most common ailments of the musculoskeletal system. Problems with the knee can be classified into (1) poor mechanics, (2) traumatic injury, and (3) arthritic changes. All these problems can produce pain. Conservative treatment such as chiropractic manipulative therapy can be helpful in alleviating some of the pain. The present study was carried out to investigate the extrinsic and intrinsic innervation of the knee joint of the guinea pig in the hope of shedding light on future study of the underlying mechanism of chiropractic manipulative therapy on knee joint pain conditions. METHODS: A total of 16 guinea pigs were used in the present study. Microdissection was performed to reveal the gross anatomy of nerve innervation. Histology and immunohistochemistry were also performed to identify nerve endings and immunoreactive fibers in different articular tissues. RESULTS: Gross dissection revealed the general pattern of guinea pig knee joint innervation. By using gold chloride preparations, various kinds of nerves and terminals or endings were identified in these tissues. Immunohistochemistry also revealed immunoreactivities in nerve fibers in different joint tissues. CONCLUSIONS: The distribution of the different nerve endings showed a characteristic pattern in different articular tissues. They were considered to be mechanoreceptors (types I, II, and III) and pain receptors (type IV). The structural characteristics and distribution patterns of the different types of nerve endings suggest that the roles of the different nerve endings vary in different parts of the articular tissues.

Preliminary morphological evidence that vertebral hypomobility induces synaptic plasticity in the spinal cord.

OBJECTIVE: A widely accepted theoretical model suggests that vertebral hypomobility can cause pain and abnormal spinal mechanics because of changes in sensory input from spinal and paraspinal tissues. The purpose of this pilot study was 3-fold: (1) to make a preliminary determination if chronic vertebral hypomobility at L4 through L6 in the rat would affect synaptic density and/or morphology in the superficial dorsal horn of the L2 spinal cord level, (2) to identify relevant outcome variables for future studies, and (3) to obtain preliminary data that would permit estimating an appropriate sample size for future studies. METHODS: Using an established rat model, we fixed 3 contiguous lumbar segments (L4-L6) for 8 weeks with a specially engineered vertebral fixation device. Electron micrographs were obtained from 2 animals from the experimental (fixed) group and each of 3 control groups (no surgery, surgery but no devices implanted, and devices implanted but not fixed). Synapses were randomly selected using a stereological approach and were analyzed for symmetry, curvature, type of postsynaptic profile, and perforations. The synaptic density was also estimated. RESULTS: There was increased synaptic density and percentage of positively curved synapses in the dorsal horn of experimental animals as compared with controls. Experimental animals had a lower percentage of axospinous synapses, with a concomitant increase in the percentage of synapses on dendritic shafts. CONCLUSIONS: These preliminary data suggest for the first time that chronic vertebral hypomobility at L4 through L6 in the rat affects synaptic density and morphology in the superficial dorsal horn of the L2 spinal cord level. More definitive studies are warranted, and the biologic significance of these finding should be investigated.