Abstract
The role of the anterior neck muscles in cervical dysfunction has become an area of interest in the physiotherapy literature, resulting in the development of new methods for assessing and treating dysfunction of these muscles. However, these methods are based primarily on electromyographic (EMG) and various imaging studies, and lack a detailed anatomical or biomechanical foundation. The purpose of this work was to examine the morphology and basic biomechanics of the sternocleidomastoid (SCM); scalenus anterior, medius, and posterior; longus capitis and colli; rectus capitis anterior and lateralis muscles with a view to better understanding the capabilities of these muscles. This will contribute to our understanding of the mechanisms of cervical disorders and inform more evidence based approaches to treatment.
This research was completed in three stages: dissection of the fascicular anatomy of the anterior neck muscles in embalmed cadavers; magnetic resonance imaging (MRI) of these muscles in young volunteers; and biomechanical modelling. Descriptive and morphological data from both the dissection and MRI studies were recorded, and cross-referenced for input into the biomechanical model. The biomechanical study involved calculating the peak force capabilities of each fascicle, and deriving the orientation and distribution of these forces across the cervical motion segments using CT scans. The result was a detailed breakdown of the peak torque, compression and shear forces generated by the anterior neck muscles at a fascicular level, calculated with reference to each cervical motion segment.
The dissection study revealed several interesting findings regarding the structure of these muscles, adding considerable detail to anatomical textbook descriptions. Findings are described for each individual muscle. The MRI study found substantially larger muscle volumes than found in the dissection study, due to changes both with age and embalming. Biomechanical modelling demonstrated that in the neutral position the anterior neck muscles are not capable of exerting large forces, and do not act equally on all the cervical motion segments. Moment generating capacity into flexion was dominated by the SCM, and increased at lower levels in the cervical spine. All muscles were capable of producing compression, and total compression capacity remained relatively even at different cervical levels. Shear capacity was minimal, and was only potentially produced by the SCM in the lower cervical spine.
The anterior neck muscles are complex and interesting muscles for which textbook descriptions tend to be simplified. These muscles act closely on the cervical motion segments, producing largely compressive forces. The more deeply placed longus and scalene muscles demonstrated a limited capacity for producing flexion moments, especially compared to the SCM. At C2-3 the SCM produced a net extension moment, suggesting that at this level (and above) the longus capitis and colli may proportionally play a greater role in cervical flexion. However, the force capabilities of these muscles remain very small and may be insufficient to produce actions attributed to these muscles in the clinical literature. This research presents data fundamental to understanding the function of these muscles, and which has the potential to contribute towards many different biomechanical applications in future research.