Measurement of functional heel pad behaviour in-shoe during gait using orthotic embedded ultrasonography
Introduction
The bony prominences at the plantar surface of the foot are covered by anatomical structures which protect against the high loads generated during activities of daily living and recreational pursuits. These structures are made up of small chambers of fibrous connective tissue containing columns of fat which vary in both size and orientation depending on their location relative to the bone [1]. A number of previous reports have suggested changes to the mechanical properties of these cushioning structures may be associated with conditions such as plantar heel pain [2], [3], [4], type 2 diabetes [5], [6], and with ageing in general [7].
In this article we focus on the bulk properties of the soft tissue at the plantar surface of the calcaneus, commonly referred to as the heel pad [8]. Previous in vivo studies have incorporated radiographic methods to image the heel pad during gait [4], [9], [10], however exposure of the subject to ionising radiation may limit the clinical and research utility of this approach. Ultrasonography (US) has been suggested as an alternative imaging modality, and has been demonstrated to be reliable and to give comparable measurements of heel pad thickness to those obtained using radiographic imaging during static loaded and unloaded conditions [11]. To estimate the mechanical properties of plantar tissues, investigators have used US transducers combined with load cells to simultaneously measure tissue deformation and the force applied, either as indentors [6], [7], [12] or embedded in the floor [13]. These quasi static approaches have the limitation that they may not reflect the dynamic properties of the foot during gait [14] or are limited to a single, initial step. Neither approach can be used in combination with footwear.
Orthotic devices such as heel cups or foot orthoses are regularly prescribed as conservative interventions when plantar heel pain is present [15] although it should be noted that evidence for their efficacy is limited [16]. The mode-of action of these devices is unclear, and proposed mechanisms include redistribution of pressure to reduce peak forces or prevention of lateral displacement of the tissue around the bony prominence to reduce compression and provide greater protection under load. State of the art computer aided design and manufacturing technologies now present the opportunity to embed sensors within orthotic structures to monitor physiological and biomechanical properties of the foot during gait [17].
In this study we present a novel method which utilises orthotic heel inserts with an embedded US transducer to allow measurement of the functional behaviour of the heel pad in-shoe during gait. Reliability of the method is tested and the biomechanical response of the heel pad to a contoured heel cup versus a flat insert are compared, with our primary hypothesis being that the contoured heel cup will reduce overall compression of the heel pad during stance phase of gait.
Section snippets
Participants
Sixteen participants (8♀) were recruited from student and staff bodies at Glasgow Caledonian University. Participants were eligible for inclusion if they had no history of heel pain, and no current or significant history of a diagnosable musculoskeletal, orthopaedic or neurological disorder affecting the lower limb or foot. The group mean age was 39.2 years (SD 8.1), height 1.73 m (SD 0.08), and weight 75.2 kg (SD 9.6). Shoe size ranged from UK 4 (female) to UK 11 (male). Ethical review of this
Results
Inter-session reliability for compression was found to be excellent with ICC2,1 values of 0.95, 95% CI [0.7, 0.99] for the inexperienced rater and values of 0.94, 95% CI [0.63, 0.99] for the experienced rater. Inter-rater reliability was also excellent with an ICC2,1 of 0.89, 95% CI [0.67, 0.97].
Data for three feet were not included in the main analysis due to the US videos from at least one test condition being of insufficient quality to allow accurate identification of the calcaneal border.
Discussion
In this study we have demonstrated a method which utilises a high resolution US transducer embedded in an orthotic insert to measure in vivo functional heel pad behaviour during gait. The technique has excellent reliability and US experience is no barrier to use. To the best of our knowledge this is the first study to directly measure the effect of an orthotic intervention on the heel pad.
We found evidence that the use of a contoured heel cup reduces the loading and deformation of the central
Acknowledgements
This work was supported by an equipment grant from Tenovus Scotland (grant number S12/9). DET is funded through an Arthritis UK fellowship (grant number 17832). The funders had no input into the study design, collection, analysis and interpretation of data; the writing of the manuscript; or the decision to submit for publication.
Conflict of interest statement
The authors declare that they have no conflict of interest relating to the material presented in this article.
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