A biomechanical evaluation of the design and fit of cervical collars

Name: Laurence Jeppe Russell

Abstract

Background: The fit of cervical collars is an important aspect of their effective use. A contributing factor to the fit of collars is the tension by which straps and adjustable components are tightened. High strap tension may increase the risk of secondary issues such as pressure ulcers, or high intracranial pressure (Brannigan et al., 2022). Whereas, loose-fitting collars will not effectively immobilise the cervical spine.

Aim: This study aimed to assess the biomechanical fit of two commercial collars, using a custom displacement rig and physical phantom model.

Methods: This study investigated the Miami J collar (Ossur, Sweden), which is formed of separate front and back panels attached with hook and loop straps. The collar was applied to a physical phantom headform (i-bodi, UK) manufactured from a stiff polymer and silicone skin layer, with its geometry based on the NIOSH medium headform. During periods of collar application, interface pressure sensors (operating range: 10-258 mmHg) were strategically placed to estimate the contact conditions (occiput, shoulders, and neck). The collars were applied in three different tightnesses: low, medium, and high. The tightness was increased and decreased by displacing the straps 5 mm each way from medium tightness. Then the front piece of the collar was replaced with a displacement-driven load cell attached to both straps. The tension measurements were taken 60 seconds after each of the displacement steps to minimise the damping effects in the system. The strap tension and interface pressures were measured across a range of displacements. Comparisons between strap load, interface pressure and collar design were then made.

Results: The strap tensions associated with each of the tightnesses were 12.9, 14.2, and 28.0 N for low, normal, and high tightness respectively. The corresponding interface pressure, at the selected locations, revealed interface pressures of 26-45 mmHg across the three tightnesses. The mean interface pressure for low, medium and high tightness was 35, 36, and 43 mmHg respectively, demonstrating a clear correlation between tension and the interface pressures. After each displacement step, the measured load would peak and then follow a pattern of exponential decay. This was indicative of a stress relaxation behaviour. A strong correlation was found between the measured tension in the load cell and the interface pressures. This allowed for the quantification of strap tension for each collar tightness.

Conclusions: This study presents a physical platform which can be employed to evaluate the interface pressure and strap tension of a commercial cervical collar in a lab setting. The strap tension is an important factor in the fit and function of cervical collars. This study has demonstrated that physical models can be used to estimate the biomechanical interactions between collar tension and interface pressure values at key sites. This approach could be complemented by computational modelling, where different head geometries and collar types could be evaluated efficiently.

References: Brannigan et al. (2022) Adverse Events Relating to Prolonged Hard Collar Immobilisation: A Systematic Review and Meta-Analysis. Global Spine Journal