Adjacent Joint Kinematics after Ankle Arthrodesis in Cadaveric Gait Simulation

Abstract

INTRODUCTION: Ankle arthrodesis has remained an effective treatment for minimizing pain and increasing function for patients suffering end-stage ankle arthritis. Unfortunately, approximately one-half patients treated with ankle arthrodesis develop adjacent joint arthritis [1]. The etiology of this clinical issue not fully understood due to the difficulty of investigating these joints in vivo. Cadaveric simulation provides a unique capability of studying intrinsic foot and ankle joint mechanics, while controlling for variabilities present in vivo as a function of degenerative disease and subsequent compensations [2]. The objective of this study was to establish the effect of ankle arthrodesis on adjacent joint kinematics using cadaveric gait simulation. We hypothesized that adjacent joint motion would be increased after immobilization of the ankle joint. METHOD(S): Four mid-tibia cadaveric specimen (male, average age at death: 34) with no foot and ankle malalignment or history of lower extremity trauma or surgery were used in this study. Each specimen was prepared for testing using previously described techniques [2]. In short, each tibia was potted and secured to a static mounting fixture about a six-degree of freedom robotic platform. The nine extrinsic ankle tendons were isolated and connected to linear actuators instrumented with load cells in series. During simulations, while motors actuate the foot, a force plate was moved relative to the stationary specimen through an inverse tibial kinematic path calculated from the previously published in vivo data of healthy gate [3]. Three-dimensional ankle kinematics were captured using a motion capture system which tracked reflective marker clusters that were secured to the tibia, talus, calcaneus, and navicular using intracortical bone-pins (Figure 1 A). For each specimen, an optimized force plate trajectory was first generated with the native condition of the foot and ankle. An iterative control system was used to make minor changes to both the force plate trajectory and muscle force target in order to minimize the error between the target and measured ground reaction forces. A series of simulations were then recorded with the specimen in its intact condition. The robotic platform was then used to position the foot in a neutral sagittal and slightly everted orientation while compressing the ankle joint. One cortical screw was passed medially to immobilize the ankle in the standardized position. Following, under fluoroscopic guidance, two additional cortical screws were passes in the anteroposterior plane to increase resistance to large sagittal moments. After ankle arthrodesis, foot and ankle kinematics were again recorded using the same muscle force and kinematic inputs as the intact condition. Motion of the tibia and talus were monitored to confirm sufficient ankle joint immobilization. To assess the effect of ankle arthrodesis during simulated walking on adjacent joint kinematics, pre- and post-arthrodesis kinematics of the subtalar and talonavicular joint were directly compared along the stance phase and differences were assessed using two-tailed, paired Student's t-tests with an alpha value set at p = 0.05. RESULT(S): Subtalar and talonavicular joint plantarflexion was greater during early stance phase in the presence of ankle arthrodesis (Figure IB and 1C). The talonavicular joint also dorsiflexed more during late stance following ankle arthrodesis (Figure 1C). Unlike in the sagittal plane, ankle arthrodesis had no detectable effect on transverse or frontal plane motions. DISCUSSION: This study revealed that the sagittal plane motions of both subtalar and talonavicular joints sustain altered motion. Alternatively, both joints retain normal motion in the transverse and frontal planes. These sagittal plane differences in adjacent joint motion findings are a logical result of ankle immobilization, which has its primary motion in the sagittal plane. However, these findings are difficult to detect, and previous reports have focused on motions during static and non-weight bearing tests. The results of this study provide further insight into how the joint motions are redistributed to adjacent joints after arthrodesis during an active and weight bearing task such as walking. Further studies are needed to identify changes in contact mechanics as a result of altered adjacent joint kinematics. However, the findings of this study expand the on previously reported effects of ankle arthrodesis, which can be further investigated using these cadaveric simulation techniques. It may be that intrinsic characteristics in some patients may be better suited for these effects, and those patients may be targeted for such treatment. SIGNIFICANCE: An improved understanding on how ankle arthrodesis affects adjacent joint mechanics may provide clinicians with better insight when providing treatment options for end stage ankle arthritis.