Association Between Tibial External Rotation and Hindfoot Varus in Patients With Neurologic Disorders: A 3D Modeling Analysis Using Weight-Bearing CT

Research Type: Level 4 – Case series Introduction/Purpose: Hindfoot varus is a deformity characterized by an inverted heel, leading to abnormal foot and ankle alignment. It can result from congenital conditions, neurologic disorders, or diseases like Müller-Weiss Disease (MWD), contributing to lateral ankle instability, pain, and gait abnormalities. External tibial torsion, a rotational deformity of the tibia, is common in adults and often linked to cavus deformity, and varus ankle osteoarthritis. This misalignment alters foot positioning, disrupts gait mechanics, and increases the risk of lower limb injuries. This study uses 3D modeling with weight-bearing CT to examine the relationship between external tibial torsion and hindfoot alignment. Additionally, we present two clinical cases of tibial rotational osteotomy, evaluating postoperative outcomes and providing insights for surgical planning in hindfoot varus correction. Methods: Hindfoot alignment measurements, particularly those using the calcaneal axis, are highly sensitive to foot rotation, requiring precise imaging. We evaluated two patients with neurological disorders presenting with increased external tibial torsion and hindfoot varus. Tibial torsion was assessed using EOS and 3D modeling with weight-bearing CT (WBCT, PedCAT™, CurveBeam LLC, USA). WBCT images were obtained with patients standing comfortably. Hindfoot alignment was measured using Foot Ankle Offset (FAO), a fully 3D axial plane parameter per Lintz et al. FAO was calculated as the ratio of the talar dome projection distance from the midfoot line, divided by foot length. A perfectly aligned foot and ankle yield FAO = 0%. Lintz developed a linear model correlating tibial torsion and FAO using WBCT. Results: The first patient, a 32-year-old male with cerebral palsy, had tibial torsion of 44°, and FAO of -13% preoperatively. He underwent right tibial 10° derotation osteotomy with IM nail fixation, calcaneal valgising osteotomy, tendo Achilles and plantar fascia release, tibialis posterior tendon transfer, and flexor tendon tenotomy. The second patient, a 61-year-old male with post-polio syndrome, had tibial torsion of 50° and FAO of -10%. He underwent PSI-assisted femoral valgising osteotomy with allograft, right tibial 10° internal rotation varising osteotomy with IM nail fixation, calcaneal valgising osteotomy, midfoot dorsolateral closing wedge osteotomy, and tarsal tunnel release. Hindfoot alignment, estimated based on tibial torsion, was significantly varus in both patients. However, a 10° tibial derotation effectively improved alignment, though additional calcaneal osteotomy was required. Conclusion: This study uses 3D modeling with weight-bearing CT to assess the association between tibial torsion and hindfoot alignment, aiding surgical planning for hindfoot varus. Tibial de-rotation may be effective for severe external tibial torsion with varus hindfoot alignment. The relationship in neurologic disorder patients may differ from non-pathologic populations in previous study, possibly following a different slope and a non-linear pattern. Further analysis with a larger sample using 3D modeling is needed to validate these findings and clarify the biomechanical relationship between tibial torsion and hindfoot alignment. 3D Modeling for Surgical Planning of Tibial De-Rotation and Association Between Tibial Torsion and Hindfoot Alignment: Estimated Relationship Tibial torsion was corrected using tibial osteotomy with an intramedullary nail. Preoperative tibial torsion and the estimated change in hindfoot varus after surgery were plotted along with the estimated association line.