We enjoyed the article “Alar, Transverse, and Apical Ligament Strain Due to Head-Turned Rear Impact” by Maak et al (Spine 2006;31:632–8) describing the results of their cadaveric spinal injury experiments with a focus on upper cervical ligament loading. Their study elegantly showed the relative differences in loads that occur in the alar, transverse, and apical ligaments of the upper cervical spine in rear impacts when the head is rotated. However, the authors did make a rather significant error in concluding that “the alar, transverse, and apical ligaments are not at risk for injury caused by head-turned impacts up to 8g.” The level of force required to cause injury in real-world collisions is determined by observational study of such collisions and retrospective reconstruction of the forces at which the injuries occurred. An experimental study of the characteristics of ligament proxies in 6 cadaveric spines with an average age of 80 years and head rotation of less than 30° is not a valid basis from which to extrapolate a lower injury threshold for the general population exposed to real-world crashes.
Maak et al evaluated the effect of varying a single predictor of injury: impact load severity. Even when looking at experimentally induced fractures, an injury with a relatively narrow variance in comparison with ligament injury, researchers have found that experimental data cannot accurately predict a lower threshold for injury in the real world using only impact load severity.1 Biomechanical study of injury mechanisms under experimental conditions is an excellent method for elucidating how such injuries occur, not dictating when they can or cannot occur. This is accomplished with epidemiologic study of the real thing.