Tuber-to-Anterior Process Angle (TAPA): A Cadaveric Study and Surgical Technique for Placing Axial Calcaneal Screws

Abstract

Category: Ankle, Hindfoot Introduction/Purpose: Operative management of displaced intra-articular calcaneus fractures frequently involves placement of P: A screws in order to maintain calcaneal length and axial alignment. While plate fixation may still be more commonly utilized, screw-only fixation for the treatment of these fractures has been demonstrated by several authors to be safe and effective. Directing fixation from the tuber into the anterior process can be challenging. Understanding the calcaneal long-axis as it relates to the long-axis of the foot, a measurement we call the Tuber-to-Anterior Process Angle (TAPA), can facilitate the ease and accuracy of guide-wire and cannulated screw placement. The goals of this study are to define the TAPA, and to report a surgical technique which uses this angle to facilitate axial screw placement. Methods: Eight cadaver feet were utilized. A longitudinal pin was placed from the center of the posterior calcaneal tuber to the 2nd metatarsal heads. Next, the calcaneocuboid joint was exposed, and a wedge of cuboid was removed. A calcaneal pin was then placed, exiting at the center of the anterior process. The tips of two small K-wires were impacted into the medial and lateral-most aspects of the articular surface. A true axial view of each specimen was obtained. On these images, a digital line was drawn from the posterior tuber starting point and the central calcaneal pin, representing the calcaneal long-axis. The angle subtended by this line and the long-axis of the foot represents the TAPA. Digital lines were drawn between the posterior tuber starting point and the medial and lateral K-wires, and the range between their subtending angles represents the axial plane tolerance for screw placement within the anterior process. Results: The average TAPA measured 10.9 ± 1.3 degrees (range: 8.4-13.0). The average angle, as measured to the medial extent of the anterior calcaneus, measured 2.8 ± 1.3 degrees (range: 0.4-4.3). The average angle, as measured to the lateral extent of the anterior calcaneus, measured 19.0 ± 2.7 degrees (range: 15.7-22.7). Conclusion: Knowledge of the Tuber-to-Anterior Process Angle (TAPA), found to be 10.9 degrees (± 1.8 degrees) laterally deviated from the long-axis of the foot, simplifies placement of posterior-to-anterior screws in the calcaneus. Understanding this relationship reduces reliance on intraoperative axial fluoroscopy and increases operative efficiency. Furthermore, this knowledge can be applied during other procedures in which axial screw placement is performed, such as with corrective calcaneal osteotomy.