Injury Threshold of Rectus Capitis Muscles at the Atlanto-occipital Joint.

Objective: The objective of this study was to collect muscle stiffness data from the 4 rectus capitis (RC) muscles to better understand their role in stabilizing the atlanto-occipital joint. The passive load displacement properties of these muscles have not been previously reported.

Methods: Rectus capitis muscles were removed from 3 unembalmed head and neck specimens. Passive length-force (stiffness) data were collected by using a servo-controlled hydraulic test machine. Multivariate analysis of variance with Bonferroni correction was used to assess the significance of the differences among passive stiffness within the elastic region of each muscle and the load and strain at the yield points.

Results: Rectus capitis lateralis (RCL) muscles failed at significantly higher levels of load and strain compared with the other 3 pairs of muscles. Passive stiffness of both RCL and RC anterior muscles was significantly higher than the other 2 pairs of muscles.

Conclusion: The anatomic location of the RCL muscles, along with their high levels of passive stiffness, would be expected to facilitate the maintenance of atlanto-occipital joint congruence during normal daily activities. The level at which the RC posterior minor muscles failed could put them at risk of a strain injury during a rear end motor vehicle accident. Diagnostic and treatment protocols that apply forces to the upper cervical spine should be tailored to consider the patient's age, gender, and history of previous injuries to avoid overstretching RC muscles.