Involvement of the Medial Column Ligaments in Biomechanically Simulated Adult Acquired Flatfoot Deformity

Author: Arjun Sivakumar

Sivakumar, Arjun, 2018 Involvement of the Medial Column Ligaments in Biomechanically Simulated Adult Acquired Flatfoot Deformity, Flinders University, College of Science and Engineering

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Background: Adult acquired flatfoot deformity (AAFD) is one of the most common etiologies, estimated to affect approximately 8% of adults (aged 21 and over) in the U.S. (NPD Group for the Institute for Preventive Foot Health, National Foot Health Assessment 2012). This corresponds to approximately 19 million people (U.S. Census Bureau, 2017). Classification systems describing the symptoms of AAFD are widely used in the clinical assessment and treatment of the condition (‘Stage I – IV Flatfoot’) and recognize the spring ligament as the most contributing stabilizing structure of the medial ankle during all stages of the condition, with the deltoid ligament described to only experience substantial deterioration at the end stage of the condition (Stage IV flatfoot). In spite of this, the biomechanical involvement of these ligaments as the physical deformity progresses has not been established. Objectives: The aim of this study was to better understand the biomechanical contribution of the primary static stabilizing ligaments of the medial column (spring ligament, deltoid ligament, talocalcaneal interosseous, cuneonavicular ligaments) during the physical progression of AAFD. Methods: 9 unmatched Fresh-frozen cadaveric feet specimens were used. The age of the specimens ranged from 44 to 84 years and the cause of death was not related to the foot. Except for the posterior tibial tendon (dynamic stabilizer), all other structures were kept intact. Small tantalum beads were carefully implanted at the attachment sites of the deltoid, talocalcaneal, navicular cuneiform and spring ligaments (superomedial calcaneonavicular and inferior calcaneonavicular). A characteristic physical deformity anatomically similar to AAFD was progressively produced in the cadaveric feet using a novel six degree of freedom (6DOF) Hexapod Robot. This was produced under load control through applying 10 degrees of dorsiflexion, followed by a series of 5 incremental loads (100N, 230N, 460N, 690N, 920N) with the foot fully unconstrained. Left and right stereo-radiographs were taken of the foot in its unloaded state, as well as at each of the loadsteps. Radiostereometric Analysis (RSA) was then used to, digitize and track the displacement of the tantalum markers in 3D space and calculate the strain within each of the ligaments at each progressive loadstep. Results: The strains for each of the 11 ligaments at 5 progressive loads were calculated, from which the sequence of recruitment of the ligaments could be observed at each stage through the interacting elongations and subsequent contractions of the stabilizing ligaments at progressive loads. The tibiocalcaneal portion of the deltoid ligament exhibited the highest strain at each loading condition. Conclusion: Under the tested conditions, the biomechanical contribution of the static stabilizing of the medial column as the physical deformity progresses was evaluated. The deltoid ligament was found to experience higher strains than the other ligaments at each stage of the condition. An understanding of the contribution of the ligaments during the onset of AAFD is essential for optimizing the management of symptomatic AAFD. From this study, it is suggested that more attention be given to the deltoid ligament at all stages of AAFD.

Keywords: Adult Acquired Flatfoot Deformity, Medial Column Ligaments, Biomechanics, Contribution, Strain, Radiostereometric Analysis
Subject: Engineering thesis

Thesis type: Masters
Completed: 2018
School: College of Science and Engineering
Supervisor: Associate Professor John Costi