Measuring Neck Morphologies Using Upright Magnetic Resonance Imaging

Because the head supported mass worn by military pilots adds loading to the head-neck complex, morphometrical characteristics of neck muscles are expected to be different from those that do not wear the added mass. To accurately determine segmental neck loads using computational whole-body human or head-neck models, it is important to include accurate morphometrical properties of cervical muscles from the population of interest, in this case military pilots. Routine radiographic imaging (x-rays and computed tomography scans) is less optimal, while magnetic resonance imaging (MRI) is appropriate. Although MRI scanners are generally available in clinical settings, recumbent scanning does not include the axial load acting on the head-neck complex. An upright scanner is efficacious to quantify cervical muscle morphometrical properties from pilot populations. The objective of the study is to determine the morphological properties of cervical muscles in a group of military pilots using upright magnetic resonance images (MRI).

Military pilots from the 115th Fighter Wing of the Wisconsin Air National Guard were recruited after obtaining U.S. government approvals. T1- and T2-weighted MRIs were obtained with subjects in the neutral sitting position maintaining their head-necks with the Frankfort plane horizontal. Two observers measured the cross-sectional areas of the sternocleidomastoid and trapezius muscles from C2 to C7 levels, and data were processed for the intra- and interclass correlation analysis. The radius was measured as the linear distance between the centroid of the muscle and the respective vertebral body at its centroidal point. The centroidal angulation was measured between the line joining the centroid of the body and muscle and the line joining the centroid of the vertebral body and tip of the spinous process.

The mean age, stature, weight, and body mass indices of seven male pilots were 38 years, 1.8 m, 86 kg, and 27 kg/m2, respectively. For both muscles cross-sectional areas were the least at the C2-C3 level, and the area was the greatest at the C6-C7 level for the sternocleidomastoid muscle and C7-T1 level for the trapezius muscle. While the radius data were relatively invariant along subaxial spinal levels, for the trapezius and sternocleidomastoid muscles, radii were level dependent. Angulations increased consistently along the caudal direction for both muscles.

Variations in muscle morphologies have implications in spinal load-sharing patterns under different types of loadings: pure sagittal, pure coronal, and combined/oblique loading modes, and high-g forces. These data can be used as a first step in computational modeling to quantify segmental forces, moments, and local stress-strain patterns in spinal components (discs, facet joints, and ligaments) and are applicable to military subjects and military loading scenarios.