McCahill, Jennifer L.; (2023) Clinical Meaningfulness of the Oxford Foot Model to Assess Foot Deformity during Gait. Doctoral thesis (Ph.D), VU University, Amsterdam.

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Abstract

The prevalence of foot deformity is globally high affecting populations across the lifespan. Foot deformity can be present from birth (such as clubfoot), emerge during growth/ present over-time (flat foot, hallux valgus), or occur following an injury or neurological event (cerebral palsy, stroke). A person with a foot deformity has altered foot structure and potentially foot function. This in turn may limit their activities in daily life due to long-lasting pain and diminished walking capacity. Despite the common clinical presentation of foot deformity altering an individual’s function, the correlation between the amount of structural deformity and its effects on quality of life remains unclear. The challenge for health professionals is to identify a clinically meaningful level of deformity that warrants intervention to maximise an individual’s participation in society. The available assessments of foot deformity in the literature are largely static measures despite previous studies showing significant differences between static structure and dynamic foot function. Optimal assessment of abnormal foot structure could be achieved through assessment in three dimensions during gait and function. Three-dimensional gait analysis is an assessment tool which measures dynamic deformity in the lower limbs. More recently, three-dimensional multi-segment foot models have been developed to improve our understanding of foot motion during gait, such as the Oxford Foot Model (OFM). The OFM was developed to measure tibia, hindfoot, forefoot and hallux motion in a clinical setting. As a relatively recent development in the assessment of dynamic foot motion, rigorous clinimetric testing of the OFM is still lacking, limiting its full potential for clinical applications and research utility. Therefore the general aim of this thesis was to establish the clinical role of the Oxford Foot Model to assess foot function during gait in the presence of deformity. Chapter I is the introduction of the thesis detailing the prevalence of foot deformity and the lack of dynamic foot assessments available in the literature. Gait, gait analysis and foot kinematics are discussed, leading to the clear gap in the literature providing the basis of the research aim. Chapter II is a repeatability study to justify the use of the OFM in populations with known foot deformity. Previously the repeatability of the OFM had been assessed in adults and children healthy populations. The OFM was designed to be adaptable in its application to measure different types of foot deformity therefore, it is important to know its repeatability in pathological conditions. This study assessed the intra and inter-rater repeatability of marker placement in children with clubfoot and in children with hemiplegic cerebral palsy compared to a typically developing population. The results of this study show that the OFM provides repeatable results in healthy children, as well as in children with either congenital or acquired foot deformity. Chapter III builds on previous research completed in Oxford evaluating the repeatability of the hindfoot marker in the OFM suggesting that the axes of the hindfoot are most sensitive to marker placement on the posterior aspect of the heel. Since other multi-segment foot models also use a similar marker, it is important to find methods to place this as accurately as possible. The aim of this pilot study was to test two different ‘jigs’ (anatomical alignment devices) against the eyeball marker placement method to improve reliability of heel marker placement and calculation of hindfoot angles. Two gait analysts (one beginner and one experienced with the foot model) completed this repeatability study on 10 healthy adult subjects using a ratio caliper and heel mould, both designed by three-dimensional printing, against eyeball marker placement. The intra-tester and inter-tester repeatability of hindfoot marker placement were assessed for 5 clinically relevant variables of the OFM. Overall the results showed there was low intra-tester and inter-tester variability suggesting good sensitivity of the OFM to detect meaningful clinical differences. The use of the ratio caliper may improve intra-tester variability, but did not seem superior to the eyeball method of marker placement for inter-tester variability. The use of a heel mould was discouraged. Chapter IV addresses the lack of available dynamic assessment tools of foot function in the literature. To summarise the quality of foot motion over the gait cycle, the Foot Profile Score (FPS) was defined as a single score based on the OFM kinematics expressing the overall deviation of foot function relative to the norm. The aim of this study was to define and validate the FPS by studying its properties and design, and analyse it against a clinical assessment of foot deformity. Concurrent validity was established for the FPS analysing the relationship with Clinical Foot Deformity Score (CFDS) in 60 subjects with a condition affecting the lower limbs. Content validity was established for the six Foot Variable Scores (FVS) that make up the FPS using a multiple regression of the CFDS on the 6 FVS in the 60 subjects. Predictive validity was established analysing the relationship of the FPS and GPS comparing 60 lower limb involvement subjects with 60 subjects with isolated foot deformity. The FPS has become the first validated score of dynamic foot motion. Chapter V analyses the responsiveness of the FPS in a clinical population. The FPS enables clinicians and researchers to quantify deviations of foot motion during gait, to monitor change in foot/ankle motion over time, and to measure the outcome of intervention. With the creation of a new outcome measure, it is important to test its responsiveness to intervention in a clinical population. Firstly, we defined the minimal clinically important difference (MCID) for the FPS based on the regression of the FPS on the Clinical Foot Deformity Scale (CFDS) presented in Chapter IV. Using the MCID, we applied it to a clinical population of 37 children with cerebral palsy, spastic hemiplegia, comparing their FPS before and after isolated foot and ankle surgery. A regression analysis looked at potential relationships between the change in FPS and their pre-operative FPS, age at surgery, and time since surgery. An MCID of 2.4 degrees for the FPS indicated a clinically meaningful improvement in foot function, which was evident in 76% of children with hemiplegia post isolated foot/ankle surgery. Moreover, the FPS responded with larger improvements for more deformed feet. These findings suggest the FPS is sufficiently responsive in a clinical population and should be considered when indicating and evaluating foot surgery. Chapter VI investigates if older symptomatic children with clubfoot deformity differ in perceived disability and foot function during gait, depending on initial treatment with Ponseti or surgery, compared to a control group. The second aim was to investigate correlations between foot function during gait and perceived disability in this population. Foot function was assessed by the OFM kinematics and plantar pressure and correlated with parent-reported outcome measures including the Oxford Ankle Foot Questionnaire, the Disease Specific Index for clubfoot and the Pediatric Quality of Life Inventory 4.0. Our findings suggest that symptomatic children with clubfoot deformity present with similar degrees of gait deviations and perceived disability regardless of whether they had previously been treated with the Ponseti Method or surgery. The presence of sagittal and coronal plane hindfoot deformity and coronal plane forefoot deformity were associated with higher levels of perceived disability, regardless of their initial treatment. This was the first study to compare outcomes between Ponseti and surgery in a symptomatic older clubfoot population seeking further treatment. In addition, it was the first paper to correlate foot function during gait and perceived disability to establish a link between deformity and subjective outcomes. In Chapter VII, the main findings of the presented studies were critically discussed, leading to clinical implications and ideas for future research. To summarise, this thesis was able to establish the Oxford Foot Model (OFM) and its summary score, the Foot Profile Score (FPS), provide clinically meaningful information for treatment indication and evaluation of dynamic foot deformity during gait in the presence of foot deformity.

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