Study Probes Effect of Tibial Component Positioning in Ankle Arthroplasty

Acadaveric study of the role of tibial component position in altering kinematics following total ankle arthroplasty (TAA) with a modern fixed-bearing device found normal joint action in the coronal and sagittal planes. Increased internal rotation in the transverse planes was associated with medial positioning of the tibial component.

The study’s findings, presented during the American Orthopaedic Foot & Ankle Society Annual Meeting by Constantine Demetracopoulos, MD, of Hospital for Special Surgery in New York, may provide insight into the influence of TAA design and implant position on ankle mechanics. The study indicated the need for further research to determine how implant position and altered kinematics affect load transfer at the bone-implant interface.

“Implant position in TAA has been associated with abnormal joint contact mechanics and ligament engagement patterns, possibly leading to component wear and early failure,” the authors wrote. However, no validated method to reliably determine landmarks to reference the ideal position of the implants has been described. According to the authors, this study, which received the International Federation of Foot & Ankle Societies Award of Excellence, is the first to directly correlate the medial position of the tibial implant with changes in ankle kinematics. Previous studies, they noted, have analyzed the influence of the position of the implants on functional outcomes, but the medial-lateral position of the tibial component has not been specifically evaluated.

“During operative procedures, the tibial component is usually positioned in order to preserve bone stock and protect the medial and lateral malleolus,” the authors wrote. “However, the medial-lateral position of the tibial implant in relation to the center of the tibial axis is often not regarded. This finding could have clinical implications for techniques implemented during operative procedures and for the development of new instrumentation systems.”

Quest for physiologic motion

TAA is still considered a novel procedure compared to hip and knee arthroplasty, the authors noted. “Even though significant advancements in implant design and operative techniques have been achieved in the past decades, ankle mechanics following TAA are still poorly understood, and it is debatable whether current TAA implants are capable of replicating physiologic ankle motion,” they wrote. “Furthermore, the effect of implant position on ankle kinematics is still unclear.”

Ankle kinematics in the setting of TAA is contingent upon implant geometry. Current TAAs offer various shapes and degrees of constraint. The differences arise from a lack of consensus with respect to the ideal implant design, the authors wrote. “Some modern TAAs have an asymmetrical design that mimics the native talar geometry, in an attempt to provide motion in three planes and reproduce physiologic ankle kinematics.”

The tibial implant position effect

With this study, the investigators sought to compare the three-dimensional kinematics pre- and post-TAA with the use of a modern fixed-bearing implant, as well as correlate variations in ankle kinematics with the position of the tibial implant, using cadaveric gait simulation for the analysis of ankle kinematics during the gait cycle. They hypothesized that ankle kinematics would be altered following TAA and that the changes would be correlated with variations in tibial implant position.

Eight mid-tibia cadaveric specimens were used. The specimens had no history of trauma or previous surgery and did not present with any foot deformities. Gait parameter inputs were obtained from healthy individuals from a previous in vivo study. Ankle joint kinematics were recorded as outcome measures. Researchers simulated the stance phase of gait both pre- and post-TAA in each specimen using a 6-degree-of-freedom robotic platform. The tibia was potted and secured to a static mounting fixture around the platform.

Each of the nine extrinsic tendons of the foot were freed from muscle attachments and connected to a cable instrumented with a load cell. Linear motor-applied physiologic forces were obtained from previous studies. A robotic platform reproduced tibial-ground kinematics by moving a forceplate in relation to the specimen (Fig. 1). Ankle kinematics were recorded and calculated by a three-dimensional motion-capture system used to track reflective markers fixed to the tibia and talus with intracortical pins.

An iterative learning control algorithm was used to optimize the forceplate trajectory, according to tibial-ground motion and ground reaction forces. After the trajectory was optimized, ankle kinematics were recorded and calculated for each specimen from a series of three simulations in the coronal, transverse, and sagittal planes.

Surgical procedures were performed by a single fellowship-trained foot and ankle orthopaedic surgeon. A modern fixed-bearing TAA (Salto Talaris, Integra LifeSciences) was used according to the manufacturer’s recommended protocol. Using reconstructed CT data, the researchers characterized three-dimensional tibial component position relative to a standard ankle joint reference. The reference planes for the ankle joint were obtained from three perpendicular lines formed by the anatomic axis of the tibia, the transmalleolar axis, and a line perpendicular to the first two lines.

Differences in ankle joint kinematics were identified only in the transverse plane, where internal talar rotation was significantly increased following TAA compared to the native condition (Fig. 2). The medial position of TAA tibial component was found to be positively associated with increased internal talar rotation (b = 1.861 degrees/mm, R2 = 0.72, P = 0.008). No other measurements of tibial component position (anterior-posterior/inferior-superior position, sagittal/frontal/transverse plane angle) were found to be significantly associated with altered ankle kinematics following TAA (All b < 0.1 and P > 0.05).

Fig. 1 Cadaveric gait simulator
Courtesy of Hospital for Special Surgery

Fig. 2 Using 95 percent confidence intervals to calculate joint rotations during stance phase, the researchers noted differences in ankle kinematics pre- and post-total ankle arthroplasty (TAA) only in the transverse plane, where an increased internal talar rotation in relation to the tibia was observed following TAA.

Larger image (PDF)

Abnormal kinematics

In this study, ankle kinematics pre- and post-TAA were comparable in the coronal and sagittal planes. “However, an increased internal talar rotation was observed in the transverse plane, which was associated with increasing medial positioning of the tibial implant,” the authors noted. “We believe that the asymmetrical design of the implant when associated with a medial positioning of the tibial component allows excessive internal rotation. Since the Salto Talaris is a fixed-bearing implant, a medial translation of the tibial component is followed by the talar component. This leads either to an incongruent articular contact pattern or to a translated talus in relation to its original position, resulting in abnormal ankle kinematics.”

The authors wrote that gait studies with patients have been conducted to evaluate kinematics following TAA but that cadaveric gait simulation “allows the comparison of ankle kinematics pre- and post-TAA with controlled parameters, avoiding bias caused by differences in anatomy, gait patterns, or muscle activation. In addition, it allows measurement of implant position based on post-TAA CT scans that would not be possible in living subjects without exposing them to unnecessary radiation.”

They concluded, “We believe this investigation provides future insight to the influence of TAA design and implant position on ankle mechanics. Furthermore, this study brings to light the effect of the medial-lateral position of the tibial component on ankle kinematics. Further studies are warranted to determine the mechanisms by which TAA design and position affect ankle kinematics.”

The abstract for this presentation may be found online at Foot & Ankle Orthopaedics: Saito G, Sturnick D, Deland J, Ellis S, Demetracopoulos C. Influence of Tibial Component Position on Altered Kinematics Following Total Ankle Arthroplasty During Simulated Gait. Foot Ankle Ortho 2018;3(3) doi: 10.1177/2473011418S00105. https://doi.org/10.1177/2473011418S00105.

Terry Stanton is the senior science writer for AAOS Now. He can be reached at tstanton@aaos.org.

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