Influence of TAL-TATS surgery on energy production of Triceps Surea – a musculoskeletal modeling evaluation

Abstract

Introduction Cerebral palsy is the most common paediatric motor disability in Europe [1]. It is known to cause structural and functional gait deformities, such as equinus foot and toewalking. A common surgical option to rehabilitate toewalking is Tendo Achilles Lengthening (TAL) surgery. However, this procedure can lead to plantar flexor weakening, which can develop into a drop-foot or even crouch gait post-surgery [2]. To prevent these complications, a previous study recommended adding Tibialis Anterior Tendon Shortening (TATS) to the surgical procedure [3]. The combined TAL-TATS procedure has shown improvement towards typically developing gait patterns after surgery during early and midterm evaluations regarding clinical and overall gait scores [3,4], but it remains unknown how the procedure affects muscle function of the triceps surae. As conventional gait analysis does not provide detailed information on single muscles, this study proposes a musculoskeletal modelling approach, which will allow more in-depth investigation of muscle function before and after TAL-TATS intervention. Research Question Does the TAL-TATS procedure lead to weakening of the triceps surae (as shown by a reduction in energy production) during gait? Methods 3D gait analysis outcomes of 10 patients who underwent a combined TAL-TATS procedure (5 years 6 months ± 1 year 1 month post-surgery [mean±SD]) were analysed using the Twente Lower Extremity Model v1.1 from the AnyBody repository (5 segments; 5 degrees of freedom; 159 muscle fibers in 37 muscle groups; simple muscles; polynomial recruitment solver power 3). Resulting joint moments of every muscle fiber with respect to a certain joint were determined in order to compute the power of a muscle. This enabled computation of muscle moments of biarticular muscles at both joints separately. Muscle power was computed by multiplying these moments with the corresponding joint velocity. Integration of this power over the time of interest revealed the produced or absorbed energy of a muscle per joint [5]. Pre-operative muscle power over the ankle join of the gastrocnemius and soleus were compared against postoperative findings, using paired t-tests and statistical parametric mapping. Results Outcomes showed no significant reduction in produced energy during push off over the ankle joint for both Gastrocnemius (p=0.81) and Soleus (p=0.32), while there was a significant reduction in absorbed energy by the triceps surae during the load-response phase (p<0.001), when comparing pre- to postoperative gait (Figure 1). Discussion In conclusion, no indicators for reduced energy production of the triceps surae during gait after TAL-TATS procedure were found. The common fear of weakening plantar flexors after lengthening is not supported by the current findings. This outcome supports previous statements that the combined TAL-TATS procedure could prevent plantar flexor weakening after surgery. Further validation of the proposed method and further investigation of the TAL-TATS procedure are needed to establish added value of TAL-TATS to TAL.