Category: Basic Sciences/Biologics; Ankle; Hindfoot Introduction/Purpose: The tall Controlled Ankle Motion (CAM) boot and the short CAM boot are commonly used devices to immobilize the foot and ankle. However, the effect of these devices on joint contact pressures is unknown. The objective of this study is to assess the effect of the tall CAM boot and short CAM boot on contact pressures of the ankle, subtalar, talonavicular, and calcaneocuboid joints. We hypothesize that both the tall CAM boot and short CAM boot will reduce contact pressures of the ankle and hindfoot joints, with the tall CAM boot having the greatest effect. Methods: Eight lower extremity cadaver specimens were mounted on a servohydraulic test frame. The specimens were loaded to 700 N at a cyclical frequency of 1 Hz with the posterior tibial, peroneus longus, peroneus brevis, flexor hallucis longus, flexor digitorum longus, and Achilles tendon physiologically tensioned. TekScan (TekScan, Boston, MA) pressure sensors were placed in the ankle, subtalar, talonavicular, and calcaneocuboid joints. In the sagittal plane, the specimens were loaded on a neutral surface, followed by 20o of dorsiflexion. Each specimen served as its own control, with contact pressures measured with no immobilization (control), followed by placement in a short CAM boot and tall CAM boot. In addition, contact pressures in the immobilized limbs were measured at muscle loads both equal to and half of the load applied to the control in order to account for decreased muscle activation during immobilization. Results: There was no difference in the average and peak contact pressures of the ankle, subtalar, talonavicular and calcaneocuboid joints when comparing the short CAM boot to no immobilization at equal tendon loads. The tall CAM boot significantly decreased average and peak contact pressures of the ankle, subtalar, and talonavicular joints when compared to no immobilization. The tall CAM decreased the contact pressures of the talonavicular and subtalar joint to a greater degree than the ankle joint. The reduction in contact pressures was accentuated when the load applied to the tendons was decreased in accordance with diminished muscle activation during immobilization. Neither immobilization device decreased the contact pressures of the calcaneocuboid joint at equal tendon loads. Neither CAM boot changed the center of pressure of any joint. Conclusion: Immobilization in a tall CAM boot decreases contact pressures of the ankle and hindfoot in both a neutral position and in dorsiflexion. A tall CAM boot should be used clinically if the goal of its use is to maximally reduce contact pressures of the ankle and hindfoot. The tall CAM boot is better at reducing the contact pressures of the subtalar and talonavicular joint than the ankle joint.