Total Ankle Arthroplasty: Is Ankle Plantarflexion Strength Important?

Abstract

Category: Ankle Arthritis Introduction/Purpose: Ankle push off power, which requires gastroc-soleus muscle strength, is a critical aspect of healthy gait and increases as gait speed increases. It is therefore surprising that one-to-two-year outcomes following total ankle arthroplasty (TAA) include improved gait speed but deficient ankle power. One possible explanation for low ankle power following TAA is ankle plantarflexion weakness. Information on plantarflexion strength is extremely limited in people before or after TAA. Evaluating plantarflexion strength may inform postoperative expectations and guide rehabilitation programs. The purpose of this study was to evaluate the change in ankle plantarflexion strength, ankle power during gait, and gait speed before and after TAA in people with end-stage ankle arthritis, and in comparison to a healthy matched control group. Methods: Twenty-five participants were included in this prospective case-control study. TAA group participants (n = 13) [mean (SD): Age 60.9 (15.3) years; BMI 30.53 (5.5) Kg/m2; 85% male] with end-stage ankle arthritis who received a TAA were evaluated preoperatively and six months postoperatively. Performance of adjunct soft tissue procedures were patient specific (5/13 participants received tendo-achilles lengthening). All patients received formal physical therapy. Healthy control participants (n=12) were matched to the TAA group on age, gender and BMI. Ankle peak isokinetic plantarflexion strength (torque at 60 and 120 degrees/second; Nm/kg) was measured with an instrumented dynamometer. Peak ankle power (joint torque x segmental velocity; W/kg) was calculated via three-dimensional multi-segment foot motion analysis while participants walked barefoot over a force plate at controlled speeds. Gait speed (m/s) was measured with the Six-Minute Walk Test. Appropriate nonparametric comparisons were made to evaluate differences across time, between limbs, and between groups. Results: Compared to preoperative values, involved limb ankle strength was preserved at 60 and 120 degrees/second following TAA (both p > 0.59). Postoperative involved limb ankle strength at both speeds were 37-56% lower than the non-involved limb and control group (all p < 0.05) (Figure 1). Similarly, involved limb ankle power was preserved following TAA (p = 0.43), but remained at least 38% lower than the non-involved limb or control group (both p < .01). A subset analysis revealed that TAA participants with tendo-achilles lengthening had 25-33% less involved limb postoperative ankle power and strength than TAA participants without tendo-achilles lengthening. Interestingly, gait speed increased following TAA (p = 0.01) and was similar to control group speeds [TAA 1.5 vs. Control 1.6 m/s; p = 0.59]. Conclusion: Robust improvements in gait speed were observed following TAA. These values approached normative gait speed in spite of diminished ankle strength and power. Ankle plantarflexion weakness reduces the capacity to generate ankle power during gait, regardless of possible contributing factors (i.e. preexisting atrophy/weakness, tendo-achilles lengthening). Accordingly, improvements in gait speed were likely linked to proximal joint compensations (i.e. hip, knee). The long term consequences of plantarflexion weakness may negatively affect implant loading. Study findings provide new information and point to the importance of targeting strength during postoperative TAA rehabilitation, potentially adjusting strategies for patients receiving soft tissue lengthening procedures.