Hyperilexion injuries of the human cervical spine commonly occur i n vehicular crashes, contact sports, diving, and fal ls. Flexion related injuries constitute a h igh percentage of al l cervical spine injuries. The objective of this study was to determine the mechan isms and tolerance of the human cervical spine under hyperilexion loading conditions through in vitro biomechanical experimentation. The impact load was delivered by an electrohydraulic piston to the cranium of a cadaver head-neck complex in a preflexed configuration. Measured biomechanical quantities inc luded head impact forces, neck forces and moments, kinematics, spinal cord pressures, and pre-test al ignment parameters. A total of thirteen specimens were tested. Minor inju ries included mainly disruption of the lower cervical sp ine posterior l i gament complex at one level. Major injuries included extensive l igamentous injury usual ly with vertebral fractures and/or complete dis locations. The anterior eccentricity of the occipital condyle with respect to T1 body was the most critical variable which influenced the loading condition and injury outcome. The spinal cord pressures were consistent with the severity of joint d isruption and load magnitude. The average moment magnitudes for minor injuries were 52 Nm, and 97 Nm for specimens with major trauma. Using logistic regression techniques, the 25 % probability of major neck injury was determined to occur at 1 850 N of axial neck force and 62 N m of injury bending moment. H U MAN C ERVICAL SPINE hyperflexion injuries commonly occur in vehicular crashes, contact sports, and falls. Hyperilexion of the cervical spine is a forced forward bending exceeding the normal physiological range. The result ing trauma may include those classified in the literature as hyperflex ion, flexion, compressive flexion, and distractive flexion injuries (Allen et al., 1 982; Yoganandan et al., 1 990; Harris and M irvis 1 996) . Epidemiological studies have indicated that the flexion mechanism is the most common cause of neck trauma and accounts for 48-70% of al l cervical spine injuries (Allen et al., 1 982; Yoganandan et al., 1 990). JRCOBI Conference Göteborg, September 1998 249 Hyperflexion injuries are often associated with localized l igamentous disruption of the cervical spine (Harris and Mirvis 1 996) . They may be characterized by local ized kyphotic angulation at the level of injury, anterior dislocation or rotation of the subluxed vertebra, anterior narrowing and posterior widening of the disc space, dislocation of the articulating facets, or widening of the space between spinous processes ("fanning") (Green et al., 1 98 1 ; Harris and Mirvis 1 996) . In the absence of dislocation, flexion injuries are difficult to detect by radiographic means since there are often no bony fractures. The injured cervical spine may appear normal in radiographic fi lms when the spine is in neutral or extended positions. The radiographic signs of injury are only evident in flexion. Such injuries may lead to late dislocation or late instabi l ity of the vertebral column with potential ly disastrous consequences (Green et al., 1 98 1 ) . These injuries are difficult to stabilize with non-operative treatment (Savini et al., 1 987) . Results from our previous experimental studies have demonstrated that the pre alignment condition of the head-neck complex significantly influences the injury mechanism and biomechanical response variables (Pintar et al., 1 990a; Pintar et al., 1 995a; Pintar et al., 1 995b). Determining the correlation between the al ignment condition, biomechanical responses, and injury mechanism may assist in the evaluation of injury risks and preventive measures. The present in vitro experimental study was designed to biomechanically determine the mechanism of injury and human tolerance for hyperflexion injury of the cervical spine with head impact.