PASSIVE MOTION - THE DOSE EFFECTS ON JOINT STIFFNESS, MUSCLE MASS, BONE-DENSITY, AND REGIONAL SWELLING

We studied the effects of passive motion on joint stiffness, muscle ma ss, bone density, and regional swelling after an intra-articular injur y. Instrumentation was applied to the hindlimbs of thirty adolescent N ew Zealand White rabbits to allow either passive motion or immobilizat ion of the ankle. The knee was immobilized by the locking together of Steinmann pins that had been placed within the medullary canals of the tibia and femur. An intra-articular injury was produced by drilling o f the tibial pin through the ankle joint into the talus and subsequent withdrawal of the pin from the ankle joint. The rabbits were divided into five groups, and they received four, eight, twelve, sixteen, or t wenty-four hours of passive motion each day during the three-week peri od of study. One ankle of each rabbit was moved through an arc of 90 t o 170 degrees of dorsiflexion at one cycle per minute, while the contr alateral ankle was immobilized in 100 degrees of dorsiflexion with an aluminum splint, which was fixed to the aluminum block that was used t o stabilize the knee joint. We found that sixteen and twenty-four hour s of passive motion prevented stiffness of the joint. Passive motion f or shorter periods was ineffective, even harmful, and resulted in stif fness ratios that were as much as four times higher than those of the control limbs (those treated with immobilization). Swelling of the lim b decreased only in the group that received twenty-four hours of passi ve motion. Muscle mass increased by an average of 13 per cent (range, 4 to 34 per cent), in comparison with that of the immobilized limbs in every group that was treated with passive motion. Bone density was ma intained only in the limbs in which the ankle became stiff (ankles tha t had been treated with passive motion for twelve hours or less). An i nverse relationship was noted between the duration of passive motion a nd the radiographic density of the distal tibial metaphysis; this rela tionship was statistically significant (p < 0.01). The limbs treated f or twelve, eight, or four hours each day showed progressively greater bone density in comparison with those treated with immobilization or w ith sixteen or twenty-four hours of passive motion.