Validation of the Neck Injury Criterion (n Ic) Us Ing Kinematic and Clinical Resul Ts from Human Su Bjects in Rear-en D Collisions

Neck lnjury C riterion (N IC) values were calculated using human subject kinematic data and compared to their clin ical results. Twenty-nine percent (29%) and 38% of the subjects exhibited whiplash-associated disorders (WAD) at rear-end speed changes of 4 and 8 km/h, respectively. None of the subjects' N I C values exceeded 1 5 m2/s2, which had been proposed as a tolerance level for AIS-1 cervical injury. N I C was not able to predict the presence of symptoms in our test population. This may be due to differences between our subjects' anatomical source of pain and the nature and type of injury predicted by N IC . A NECK INJURY CRITERION (NIC) to mathematical ly model and predict neck injuries in low-speed rear-end automobile collisions has been proposed based on the relative acceleration and velocity between the top and the bottom of the cervical spine (Boström, et a l . , 1 996). This criterion was based on a theory first presented by Aldman ( 1 986) and a pig model developed by Svensson, et al . ( 1 993), which produced histopathologic findings especially to the dorsal root gangl ion in the lower cervical spine after rapid induced head motion in the sagittal plane (Örtengren, et al . 1 996). Using Evans Blue dye conjugated to Albumin (EBA) örtengren, et al . ( 1 996) found leakage indicative of cell membrane dysfunction i n the cervical spinal ganglia, which Svensson, et a l . ( 1 993) correlated with measured pressure gradients in the intervertebral canal of the cervical spine. Svensson (1 993) proposed that the ganglia pathologies could have been caused by mechanical stresses and strains due to relative motion between vertebral segments but that a lack of observed trauma to the cervical vertebrae, discs, and ligaments supported Aldman's ( 1 986) theory that the ganglia pathology was caused by hydro-dynamic effects from change in the inner volume of the spinal canal during the rapid extension/flexion motions of the neck. Svensson, et al . ( 1 993) theorized that this pressure gradient injury IRCOBJ Confere11ce Göteborg, September 1998 335 336 mechanism and resultant pathology could be responsible for AIS-1 symptoms experienced by occupants in rear-end collisions. The work of Penning (1 992) focused on rearward translation (retraction) of the head relative to the torso as a potential cervical injury mechanism and the recent work of Svensson, et al. ( 1 993) and Boström, et al. ( 1 996, 1 997) addressed retraction as a potential injury mechanism during low-speed rear-end impacts. These authors postulated that as the head and neck passed through phases during the rearward motion the cervical spine changed shape between a straight and an S-shape, which probably caused the pressure transient along the spinal canal. They also suggested that the passage of maximal retraction was the critical phase in the head/neck kinematics. Consequently, Boström, et a l . ( 1 996) proposed a Neck lnjury Criterion (NIC) with the equation NIC = arel * L + Vrei2, where are1 and Vrel were the relative horizontal acceleration and velocity between the bottom (T 1 ) and top ( C 1 ) of the cervical spine (T 1 -C 1 ) . The term L represented the length of the cervical spine, which was set at 0.2 m for the pig model and assumed by these researchers to be a similar value in humans (Boström, et al. 1 996). Svensson, et al . ( 1 993) acknowledged that the pig model could not provide quantitative data regarding the injury threshold for human beings; however Boström, et al . ( 1 996) froposed a preliminary estimate of human tolerance level of NIC < 1 5 m2/s . The authors recommended that this proposed threshold be validated, falsified or modified using accident and volunteer test data. Boström, et al . ( 1 997) suggested that the N IC value should be calculated at the instant of maximal retraction but that due to insufficient human whiplash kinematic data that this point needed to be assumed. Therefore, it was proposed that the NIC value be calculated at 50mm of relative T1 -C1 displacement, whereby N IC-50 equals NIC at the time when the double integral of are1 equals 50 mm. Boström, et al . ( 1 997) used kinematic data from volunteer sied tests conducted at 8 .5 and 1 0.5 km/h sied velocity with an average deceleration of 2 .5 g to evaluate the N IC-50 (Eichberger, et a l . , 1 996). These N I C-50 values were calculated from ehest acceleration data rather than T1 acceleration data and estimated head center of gravity (CG) acceleration rather than C 1 acceleration data. The averaged N IC-50 values reported from sixteen volunteer tests across five different car seats were al l below the 1 5 m2/s2 threshold. E ichberger, et al . ( 1 996) reported that three volunteers suffered from neck compla ints on the day following the tests, which lasted approximately twenty-four hours and one volunteer complained of cervical distortion symptoms for about two weeks. No statistical analyses were reported by Boström, et al . ( 1 997) regarding correlation between those volunteers with and without symptoms and N IC-50 values and it is not known if any of those individual NIC50 values exceeded the 1 5 m2/s2 threshold. S iegmund, et al. ( 1 997) recently reported detailed head and neck kinematic data from human subjects exposed to a 4 and 8 km/h speed change. Brault, et a l . ( 1 998a) recently published the clinical results of these human subject tests including the presence, severity and duration of cervical symptoms. Symptoms consistent with whiplash-associated disorders (WAD) including cervical symptoms of AIS-1 level were recorded. The availabil ity of IRCOBJ Confere11ce Gii teborg, September 1998 these combined kinematic and clinical data presents a unique opportunity to test the proposed Neck lnjury Criterion. The primary purpose of this paper was to correlate NIC values calculated from human subject kinematic data to the subjects' presence of symptoms. The detailed human head and neck kinematic data were also used to investigate the differences between using data at the head CG and data at the atlanto-occipital (A/O) joint to represent C1 in the NIC equation and determination of retraction values.