Does the Subtalar Joint Compensate for Ankle Malalignment in End-stage Ankle Arthritis?

BACKGROUND: Patients with ankle arthritis often present with concomitant hindfoot deformity, which may involve the tibiotalar and subtalar joints. However, the possible compensatory mechanisms of these two mechanically linked joints are not well known. QUESTIONS/PURPOSES: In this study we sought to (1) compare ankle and hindfoot alignment of our study cohort with end-stage ankle arthritis with that of a control group; (2) explore the frequency of compensated malalignment between the tibiotalar and subtalar joints in our study cohort; and (3) assess the intraobserver and interobserver reliability of classification methods of hindfoot alignment used in this study. METHODS: Between March 2006 and September 2013, we performed 419 ankle arthrodesis and ankle replacements (380 patients). In this study, we evaluated radiographs for 233 (56%) ankles (226 patients) which met the following inclusion criteria: (1) no prior subtalar arthrodesis; (2) no previously failed total ankle replacement or ankle arthrodesis; (3) with complete conventional radiographs (all three ankle views were required: mortise, lateral, and hindfoot alignment view). Ankle and hindfoot alignment was assessed by measurement of the medial distal tibial angle, tibial talar surface angle, talar tilting angle, tibiocalcaneal axis angle, and moment arm of calcaneus. The obtained values were compared with those observed in the control group of 60 ankles from 60 people. Only those without obvious degenerative changes of the tibiotalar and subtalar joints and without previous surgeries of the ankle or hindfoot were included in the control group. Demographic data for the patients with arthritis and the control group were comparable (sex, p=0.321; age, p=0.087). The frequency of compensated malalignment between the tibiotalar and subtalar joints, defined as tibiocalcaneal angle or moment arm of the calcaneus being greater or smaller than the same 95% CI statistical cutoffs from the control group, was tallied. All ankle radiographs were independently measured by two observers to determine the interobserver reliability. One of the observers evaluated all images twice to determine the intraobserver reliability. RESULTS: There were differences in medial distal tibial surface angle (86.6°±7.3° 95% CI, 66.3°-123.7°) versus 89.1°±2.9° [95% CI, 83.0°-96.3°, p<0.001), tibiotalar surface angle (84.9°±14.4° 95% CI, 45.3°-122.7° versus 89.1°±2.9° 95% CI, 83.0°-96.3°, p<0.001), talar tilting angle (-1.7°±12.5° 95% CI, -41.3°-30.3°) versus 0.0°±0.0° [95% CI, 0.0°-0.0°, p=0.003), and tibiocalcaneal axis angle (-7.2°±13.1° 95% CI, -57°-33°) versus -2.7°±5.2° [95% CI, -13.3°-9.0°, p<0.001) between patients with ankle arthritis and the control group. Using the classification system based on the tibiocalcaneal angle, there were 62 (53%) and 22 (39%) compensated ankles in the varus and valgus groups, respectively. Using the classification system based on the moment arm of the calcaneus, there were 68 (58%) and 20 (35%) compensated ankles in the varus and valgus groups, respectively. For all conditions or methods of measurement, patients with no or mild degenerative change of the subtalar joint have a greater likelihood of compensating coronal plane deformity of the ankle with arthritis (p<0.001-p=0.032). The interobserver and intraobserver reliability for all radiographic measurements was good to excellent (the correlation coefficients range from 0.820 to 0.943). CONCLUSIONS: Substantial ankle malalignment, mostly varus deformity, is common in ankles with end-stage osteoarthritis. The subtalar joint often compensates for the malaligned ankle in static weightbearing. LEVEL OF EVIDENCE: Level III, diagnostic study.