Changes in dorsal horn neuronal responses in an experimental wrist contracture model.

Joint contracture, a major complication after casting, usually makes the therapeutic outcome worse by causing a limited range of motion and related pain. We developed rat models of wrist contracture with fracture of the radius (group A) and wrist contracture without fracture (group B), and investigated whether contracture and fracture changed the characteristics of cervical dorsal horn neuronal responses and the behavior of the animals. After 4 weeks of immobilization, both groups showed wrist contracture and disuse tendencies in the treated forelimb. In an electrophysiological study, the responses of 403 cervical dorsal horn neurons to mechanical stimuli were examined. In normal (control) animals, the neurons had the following distribution: 63% were low-threshold (LT); 15% were high-threshold (HT); and 22% were wide-dynamic-range (WDR). In group A, the distribution of the neurons changed to 51% LT, 16% HT, and 33% WDR. Similar changes were observed for group B. Responses during wrist movement were also examined. Forty-one percent of cells in the control group were responsive to the movements, whereas the number of neurons responding to motion stimulus in both groups A and B was increased, to 77%. The changed population of WDR and LT neurons responding to wrist movement suggests that the characteristics of dorsal horn neurons may undergo plastic changes after contracture.