Sensitivity of Neck Model Estimates to Variation in Vertebral Kinematics

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Abstract

Measurement of loads and displacements within the head and neck is very challenging, and for this reason musculoskeletal (MS) models are useful tools for investigating biomechanical phenomena. These models rely on a wide range of model parameters, such as tissue mechanical properties and intervertebral kinematics. Although intervertebral kinematics (IVK) exhibit considerable variation among subjects, the sensitivity of MS models to variation in IVK parameters has not yet been quantified. The goal of this study was to quantify variation in model estimates of extension strength, flexion strength and fatigability (the ratio of gravitational load to muscular capacity) due to variation in IVK.

In the first specific aim, we used radiographic measures of IVK over the range of head motion to quantify variation in the intervertebral motion distribution (IVMD), a subset of IVK parameters relating the amount of rotation at an individual motion segment to the magnitude of head rotation. By developing subject-specific MS models we were able to quantify the sensitivity of model estimates to variation in the IVMD parameters. We found estimates of flexion strength and fatigability to be substantially sensitive to variation in the IVMD.

Next, we developed MS models with entirely subject-specific IVK, including the location of intervertebral centers of rotation (IVCR) and neutral posture, in addition to IVM parameters. We observed similar trends compared to part one, and the variation in fatigability estimates increased substantially when all IVK parameters varied compared to just the IVMD.

In the third specific aim of this work examined the influence of subject-specific IVK on an ergonomic investigation of forward head postures while subjects used a tablet PC in multiple configurations. Radiographs were used to develop subject-specific MS models; photographs were used to inform a generic head and neck model. We compared subject-specific and generic model fatigability estimates, and found similar trends between model types across subjects, but different magnitudes in fatigability increase from the neutral posture.

Overall, this work shows that MS models of the head and neck are sensitive to variation in IVK parameters specifically, and inter-subject variation is an important consideration during the development and application of MS generally.

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