Symptomatic middle facet talocalcaneal coalition is frequently associated with rearfoot arthrosis that is often managed surgically with rearfoot fusion. However, no objective method for classifying the extent of subtalar joint arthrosis exists. No study has clearly identified the extent of posterior facet arthrosis present in a large cohort treated surgically for talocalcaneal coalition through preoperative computerized axial tomography. The authors conducted a retrospective review of 21 patients (35 feet) with coalition who were surgically treated over a 12-year period for coalition on at least 1 foot. Using a predefined original staging system, the extent of the arthrosis was categorized into normal or mild (Stage I), moderate (Stage II), and severe (Stage III) arthrosis. The association of stage and age is statistically significant. All of the feet with Stage III arthrosis had fibrous coalitions. No foot with osseous coalition had Stage III arthrosis. The distribution of arthrosis staging differs between fibrous and osseous coalitions. Only fibrous coalitions had the most advanced arthrosis (Stage III), whereas osseous coalitions did not. This suggests that osseous coalitions may have a protective effect in the prevention of severe degeneration of the subtalar joint. Concomitant subtalar joint arthrosis severity progresses with age; surgeons may want to consider earlier surgical intervention to prevent arthrosis progression in patients with symptomatic middle facet talocalcaneal coalition.