Gender- and region-dependent local facet joint kinematics in rear impact: implications in whiplash injury.

Study Design: Localized facet joint kinematics resulting from whiplash acceleration were analyzed in the dynamic domain during the time of cervical S-curvature using intact head and neck specimens and a pendulum mini-sled loading apparatus.

Objectives: To determine the effects of gender, impact severity, cervical level, and anatomic joint region on shear and distraction motion of lower cervical facet joints.

Summary Of Background Data: Clinical and experimental studies identify cervical facet joints to be a likely location of whiplash injury. Epidemiologic studies report that female occupants sustain a greater percentage of whiplash injuries. Previous experimental studies have not analyzed facet joint motion as a function of variables such as gender.

Methods: Intact head and neck complexes were subjected to whiplash acceleration using a pendulum mini-sled apparatus at four impact severities. Facet joint kinematics were analyzed using digital high-resolution video at 1000 frames per second during the time of maximum cervical S-curvature. Shear and distraction motions were analyzed in the ventral and dorsal joint regions from C4-C5 to C6-C7 levels. Analysis of variance techniques were used to analyze biomechanical data.

Results: Intact head and neck complexes sustained cervical S-curvature during whiplash loading. Lower cervical facet joints demonstrated dorsally directed shear motion with distraction in the ventral and compression in the dorsal regions of the joint. Magnitudes of distraction and compression were significantly lower than shear motion (P < 0.05). Facet joint shear and distraction motion increased with impact severity. Lower cervical facet joint shear and distraction motions in female specimens were greater than in male specimens. This difference reached statistical significance at C4-C5 (P < 0.05).

Conclusions: Secondary to whiplash loading, lower cervical facet joints responded with a shear plus distraction mechanism in the anatomic ventral and shear plus compression mechanisms in the dorsal region. Injury to the ventral region stems from tensile failure of the joint capsule. Injury to the dorsal region stems from pinching of the joint capsule or synovial fold and contact between subchondral bone of superior and inferior facet processes. Because excess spinal motion is biomechanically related to abnormalities and because lower cervical facet joints sustain greater motion in female specimens, this population is more likely to be injured under whiplash loading. Potential contributors for the susceptibility of females to injury, including genotypic (apolipoprotein APOE-epsilon4), hormonal, structural, and tolerance factors, are discussed.