A kinematic description of dynamic midfoot break in children using a multi-segment foot model

Abstract

Midfoot break (MFB) is a foot deformity that occurs most commonly in children with cerebral palsy (CP), but may also affect children with other developmental disorders. Dynamic MFB develops because the muscles that cross the ankle joint are hypertonic, resulting in a breakdown and dysfunction of the bones within the foot. In turn, this creates excessive motion at the midfoot. With the resulting inefficient lever arm, the foot is then unable to push off the ground effectively, resulting in an inadequate and painful gait pattern. Currently, there is no standard quantitative method for detecting early stages of MFB, which would allow early intervention before further breakdown occurs. The first step in developing an objective tool for early MFB diagnosis is to examine the difference in dynamic function between a foot with MFB and a typical foot. Therefore, the main purpose of this study was to compare the differences in foot motion between children with MFB and children with typical feet (Controls) using a multi-segment kinematic foot model. We found that children with MFB had a significant decrease in peak ankle dorsiflexion compared to Controls (1.3 ± 6.4° versus 8.6 ± 3.4°) and a significant increase in peak midfoot dorsiflexion compared to Controls (15.2 ± 4.9° versus 6.4 ± 1.9°). This study may help clinicians track the progression of MFB and help standardize treatment recommendations for children with this type of foot deformity.

Introduction

Midfoot break (MFB) is a severe foot deformity that can cause chronic pain and reduced ankle power, which can make walking difficult. This type of foot deformity occurs most commonly in children with cerebral palsy (CP) [1]. CP is defined as a group of permanent disorders of the development of movement and posture causing activity limitation that are attributed to non-progressive disturbances that occurred in the developing foetal or infant brain [2]. MFB can also affect children with other developmental disorders, such as spina bifida. At the Shriners Gait Lab (SGL) at Sunny Hill Health Centre for Children in Vancouver, BC, Canada, at least 10% of all children diagnosed with CP present to the lab with a MFB. In addition several children with diagnoses other than CP present to the lab with MFB every year.

Classically, as the severity of MFB worsens, surgical management becomes necessary. Prolonged weight-bearing in the face of a MFB may lead to the development of painful calluses over prominent osseous anatomy (e.g. navicular or plantarflexed talus). This may make the wearing of any potential corrective or supportive orthoses difficult or even impossible. The existing surgical treatment modalities aim to establish a plantigrade foot whereby both the hindfoot and forefoot are on the ground together during standing [3]. The long term morbidity following surgical treatment of MFB can be serious, thus early diagnosis and preventative treatments for MFB are critical.

Clinical: MFB is a 3-dimensional foot deformity that can be described in terms of forefoot and hindfoot positioning in the coronal, sagittal, and transverse planes. MFB can be fixed or dynamic (flexible). In a foot with MFB, increased tightness in the gastroc-soleus complex pulls the hindfoot and talus into equinus. These increased muscle forces overpower the tibialis posterior and spring ligament, causing a collapse of the longitudinal arch. This results in a forefoot position that is dorsiflexed, abducted and supinated relative to the hindfoot. In addition, the navicular becomes subluxated dorsally and laterally on the talus [1].

Diagnosing a dynamic MFB is done on clinical exam by stabilizing the sub-talar joint, and observing whether or not dorsiflexion occurs through the midfoot. In a typical foot, there is no dynamic motion at the midfoot; therefore, midfoot motion observed during this exam indicates the presence of midfoot break. Clinical diagnosis of MFB can be confirmed with a radiograph of the standing foot measuring the talo-navicular angle on both the AP and the lateral views [4]. However, radiographs provide no information about the dynamic motion between the hindfoot and forefoot. Confirmation of dynamic MFB relies on both radiographic results and the experience of the physician. The lack of objectivity in this type of clinical assessment forms the basis for this study.

Gait analysis: Three-dimensional gait analysis has been used to assess joint motion for over 20 years [5]. Gait analysis is routinely performed on children with CP and other developmental disorders to provide a dynamic assessment of walking, but there is currently no information in the literature describing MFB in relation to gait parameters. Conventional gait analysis methods model the foot as a single segment based on the assumption that the foot is rigid [5]. In this paper, we refer to this as the conventional foot model, (CFM). However, the assumption of foot rigidity is not always true for individuals with foot deformities. Many multi-segment foot models have been developed and tested in recent years to address this issue [6], [7], [8], [9], [10]. Despite the vast number of foot models emerging in the literature, few of these models are practical for routine clinical use, particularly in children.

The two multi-segment foot models considered for this study were developed to address the paediatric population directly. The oxford foot model (OFM) is a validated model that has been used clinically for several years and has been adapted for use in children [11]. The OFM defines the foot segments based on underlying bony anatomy, and thus relies heavily upon accurate marker placement by clinicians for repeatable segment orientation. Repeatability tests in children have shown minimal day-to-day variability in the sagittal plane, but much greater variability in the transverse plane [11].

The Shriners Hospital for Children Greenville Foot Model (SHCG model) was developed to address the problem of day-to-day variability caused by inconsistency in marker placement [12]. The model minimizes the dependence on marker placement by utilizing a “Smart Surface” (either the floor or a wedge) to help define the plantar surface of the foot. Only a few markers require specific anatomical placement, while others are simply tracking markers. The SHCG model was recently modified to provide different options for the plantar surface references (mSHCG model) [8]. The plantar surface of the forefoot and hindfoot can be defined by the Smart Surface (Option 1), using radiographs (Option 2), or using anatomically placed markers (Option 3). The developers of the mSHCG conducted repeatability tests using Option 3 and showed that, similar to the OFM, the mSHCG foot model was more repeatable in the sagittal plane compared to the transverse and coronal planes. Their results also showed that the mSHCG model (Option 3) was equally or more repeatable than the OFM.

Early identification of MFB is critical for intervention and prevention of progression of this condition. There is currently no standard clinical description for MFB, and thus early indications of MFB can be easily overlooked. The purpose of this study was twofold: (1) to compare the use of a single-segment foot model versus a multi-segment foot model to describe foot and ankle kinematics in children with MFB, and (2) to characterize MFB using a multi-segment foot model. Selection of the preferred multi-segment foot model involved comparing the repeatability of the OFM to the mSHCG model to determine the most appropriate multi-segment model for MFB assessment.

This study focused on sagittal plane kinematics as a first step in describing MFB. By characterizing the kinematic profiles of midfoot break using gait analysis, we aim to provide clinicians with an objective tool for identifying the onset of MFB with the goal of prevention and standardization of treatment recommendations.