The Effect of Targeting the Plantaris Muscle-tendon Unit in Surgical Correction of Ankle Equinus in Children

Children demonstrating equinus gait, characterized as <10° of ankle dorsiflexion relative to the leg, are commonly referred for specialist assessment. Equinus gait or deformity can occur in isolation (e.g. idiopathic toe-walking), with other foot deformities (e.g. clubfoot), or with underlying neuromotor conditions (e.g. cerebral palsy). A 2019 study found 83.3% of patients surveyed with cerebral palsy had equinus.

In children with underlying spasticity and no significant contracture, chemodenervation via Botulinum toxin (BoNTA) injections of the gastroc-soleus (GS) complex is often used to maintain functional range and optimize gait. Typically, injections become less effective over time, often leading to equinus contracture, functional impairment, and the need for orthoses or surgery. Once surgical intervention (i.e. tendoachilles (TA) lengthening or gastrocnemius (GN) recession) is required, contracture of the plantaris muscle has also been noted, usually requiring concomitant or later lengthening via tenotomy. Surgical GS lengthenings to treat ankle equinus can result in prolonged and sometimes incomplete recovery of ankle plantarflexion strength despite intensive physiotherapy, adversely affecting gait and lower limb function.

The presence and course of plantaris are believed to be variable. Recent evidence indicates that it is present in 98-100% of the population while historical estimates ranged from 80-93%. Historically considered a vestigial structure with no significant functional impact, more recently, plantaris contraction is considered to impact knee flexion and ankle plantarflexion as well as possibly contribute to external rotation of the lower leg and hindfoot inversion, similar to the GN. Given these potential functional similarities and empiric intra-operative observations of improved ankle dorsiflexion with plantaris lengthening in addition to GS complex lengthening, a plantaris tenotomy is often performed during surgery for equinus contracture. However, clinical recognition of the plantaris contribution to equinus gait and/or contracture remains inconsistent, and the effect of treatments targeting the plantaris are not well described in the literature.

The overall goal of this proposed study is to examine the contribution of the plantaris to ankle equinus by assessing the short-term impact of surgical treatment targeting the plantaris muscle-tendon unit to manage children with equinus gait and/or contracture.

The trial will compare changes to passive ankle dorsiflexion after plantaris tenotomy performed either a) before TA lengthening or GN recession or b) after TA lengthening or GN recession (in a single surgical encounter).