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

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Background/Purpose: Children demonstrating equinus gait, characterized as \<10° of 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 (i.e., 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 intra-operative observations of improved ankle dorsiflexion with plantaris lengthening in addition to GS complex lengthening, a plantaris tenotomy or excision is sometimes considered 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). If this surgical trial finds that plantaris significantly contributes to equinus contracture, it may support using exclusively open surgical techniques (rather than percutaneous), so the plantaris can be identified and addressed. In addition, if the effect of plantaris is significant, it is possible that early isolated release of the plantaris tendon may be an option while preserving the gastrocsoleus complex and thereby reducing post-operative weakness. This approach may also reduce the long-term requirement for Achilles tendon lengthening with improved preservation of plantarflexion strength by reducing the feedback mechanism from the plantaris to the gastrocsoleus complex. Primary objective: 1) Compare the effect of plantaris tenotomy in the setting of equinus contracture on passive ankle dorsiflexion relative to that of TA lengthening or GN recession. Secondary objectives: 1. Quantify the proportion of participants with an identifiable plantaris 2. Describe the location of the insertion of the plantaris tendon in those with an identified plantaris Hypothesis 1. Plantaris significantly contributes to equinus contracture in children: plantaris tenotomy significantly improves passive ankle dorsiflexion, independent of the GS mechanism Treatment groups: Participants scheduled for surgical lengthening of the GN or TA Group 1: TA lengthening or GN recession, then Plantaris tenotomy Group 2: Plantaris tenotomy, then TA lengthening or GN recession A computer-generated uneven block randomization sequence will be used. Population: Inclusion Criteria (All of the following criteria must be met): * Ability to provided informed consent/assent in English. * Pediatric patients (4-17 years) who have consented for surgery for the management of equinus contracture \* (either TA lengthening or GN recession) at the Stollery Children's Hospital * Known underlying diagnosis of any of the following: idiopathic toe walking, cerebral palsy, hereditary spastic paraparesis, traumatic brain injury, spinal cord injury/tethering, hereditary sensory-motor neuropathy, stroke * Ability to maintain hindfoot and midfoot neutral during assessment * Passive plantarflexion on affected side greater than 20° and greater than degree of equinus contracture. * Note: may be isolated or in conjunction with other orthopaedic procedures; in bilateral ankle equinus procedures, data will be collected bilaterally, but included as a single participant (i.e., single randomization). Exclusion Criteria (Any one or more of the following): * Unable to provide informed consent/assent in English. * Previous surgery for equinus * Limb deficiency on affected side * Knee flexion contracture of greater than 5° * Surgical intervention of the lower extremities below the affected knee in the last twelve months * BoNTA injections below the affected knee within the last six months * Known or suspected arthrofibrosis. Screening and enrollment: A member of the patient's care team (i.e. surgeon, nurse, or resident) will identify potential participants, completing a Consent to Contact from when patients are consented or scheduled for the index procedure. A Research Associate will contact the potential participant or their legal guardian and explain the study. If they wish to take part, they will provide consent via e-consent on REDCap. Once consent is obtained, notification will be sent to the treating surgeons' office, along with the participant's randomization allocation. 42 participants (21 per group, 20 for power and one in case a patient doesn't have a plantaris or whose physical examination changes between consent and surgery) will be included. Pre- and post-intervention assessments will be performed intra-operatively. Maximum passive ankle dorsiflexion will be measured by the surgeon with a sterile goniometer at four different time points, with the knee maximally extended. The time points for measurement are: 1. Prior to skin incision. 2. Prior to division of first tendon/aponeurosis 3. After division of first tendon/aponeurosis 4. After division of second tendon/aponeurosis To minimize confounding by any co-existing intrinsic foot deformities, ankle dorsiflexion will be measured with the long axis of the leg and long axis of the heel pad as the points of reference. After both structures (i.e., plantaris tendon and GN aponeurosis or Achilles tendon) have been identified, but prior to measurement (2) above, the plantaris location will be documented. In the event that the plantaris is not identifiable intra-operatively, only measurement (4) above will be excluded as only one tendon/aponeurosis will be divided. In GN recession cases where there is inadequate dorsiflexion after both procedures, the surgeon may complete a partial lengthening of the soleus, with a fifth set of measurements documented after completion. Outcomes assessment will all be completed intra-operatively so there will be no loss to follow-up. Sample Size Justification: Due to the paucity of literature, estimate of required sample size is based on a minimal clinically important difference (MCID) derived from the experience of the study surgeons. There were 27 and 28 eligible patients in 2020 and 2021 respectively. Data analysis: Descriptive statistics will characterize baseline demographics and any systematic differences among groups will be risk-adjusted. Maximum passive ankle dorsiflexion during surgery will be examined using ANOVA or corresponding Kruskal-Wallis model. Further Justification: Surgery is routinely done for patients with equinus contracture involving lengthening of the tendoachilles or gastrocnemius and then plantaris to achieve the desired ankle range of motion is obtained. In this study, we want to understand the contribution of plantaris to equinus. If plantaris can be cut first and the desired range of motion is obtained, then the tendoachilles and gastrocnemius need not be cut. This will reduce post-operative recovery time. Group 1 is standard of care. Group 2 provides the possibility of a lesser surgical intervention. Thus, the study may provide the possibility of a less invasive surgery for the study patient with quicker recovery.