BACKGROUND The progression of subtalar and midfoot arthritis is well documented following ankle arthrodesis, and gait analysis has suggested that a functional gait pattern following arthrodesis may be due to tarsal hypermobility compensating for lost tibiotalar motion. We present a prospective radiographic study comparing the pre- and postoperative range of motion of the foot following ankle arthrodesis utilizing reliable anatomic landmarks to measure the sagittal range of motion. MATERIALS AND METHODS Between 2002 and 2006, we performed 154 arthrodesis procedures of the ankle. Patients were suitable for inclusion in this study if an isolated arthrodesis of the ankle was performed for post traumatic arthritis only with a minimum of 1-year followup and who had not undergone any prior nor subsequent hindfoot surgery. Standardized pre- and postoperative passive plantar- and dorsiflexion radiographs were obtained, and reproducible anatomic landmarks were then used to measure and compare tibiotalar, subtalar and medial column (talonavicular, naviculo-middle cuneiform and middle cuneiform-first metatarsal) motion. These measurements were repeated at 6, 12, and a mean of 33 months at final followup evaluation. The SF-36, calcaneal pitch, and the presence of radiographic and symptomatic hindfoot arthritis were noted for each patient pre and postoperatively. RESULTS There were 48 patients who met the inclusion criteria. The mean preoperative sagittal motion (tibiotalar, medial column and subtalar combined) was 37.2 degrees, of which 17.8 degrees was tibiotalar motion. This decreased to a mean of 22.6 degrees postoperatively. The postoperative mean subtalar range of motion increased by 4.1 degrees (from 5.2 degrees to 9.3 degrees) (p < 0.0001), and the medial column motion increased by 2.1 degrees postoperatively (from 14.3 degrees to 16.4 degrees) (p < 0.003). Both of these results were statistically significant. There was a compensatory increase of the combined subtalar and medial column motion after arthrodesis of 10.8%. Using regression analysis, there was a significant correlation between the preoperative tibiotalar motion and the final difference in the subtalar range of motion (p = 0.03) and the combined motion of the medial column and the subtalar joint (p < 0.0001). Quality of life was positively associated with increased compensatory motion of the hindfoot and midfoot after ankle arthrodesis. There was an inverse association between the calcaneal pitch angle and the range of motion postoperatively. CONCLUSION This prospective study demonstrated a statistically significant relative hypermobility of the subtalar and medial column joints following ankle arthrodesis, and may account for the functional gait which can be achieved following ankle arthrodesis. The significantly increased subtalar range of movement appeared to cause impingement of the posterior part of the posterior facet of the subtalar joint which may account for the increased incidence of subtalar arthritis following arthrodesis. Preoperative arch height can be used to predict both residual motion and function after ankle arthrodesis.