Abstract

A brief review is provided of characterization of the spine and of the various biomechanics models for the head/neck/spine system. This work was motivated by several biomechanics problems currently under consideration. First effort was to assess and discuss status of the leading intervertebral disc implants, and secondly to formulate numerous suitable dynamic models of biomechanical response, for example: for the “whiplash” problem, for a seated and helmeted crew member during a crash scenario, analysis of the High Altitude Low Opening Parachute (HALO) jump, and the effects of artillery fire on crews of combat vehicles . Therefore both quasi-static response and dynamic behavior were of interest. Characterization of the mechanical properties of the human body remains a challenge to biomedical engineers. The spine is a segmented structure containing \iscoelastic and kinematic elements. Efforts to characterize this complex structure have included in-vivo range of motion, head/neck response to forward, backward and lateral impact “jerks”, intervertebral disc pressure measurements, cadaveric whole spine tests, experiments on vertebrae and discs as well as isolated spinal ligament studies. With regard to crash survivability, a great deal of biodynamics research has been conducted and some general guidelines have been determined. But much more research is needed to provide accurate, proven figures. Whole body survival criteria have been derived based on test subjects seated with “correct” upright posture, and for single peak impacts. The magnitude, direction and duration of applied accelerative force have definite effects on human tolerance, as shown in a widely used data summary. For instance a spineward chest-back accelerative force of 45 G has been tolerated voluntarily when pulse duration is less than 0.044 seconds. However when the pulse duration increases to 0.2 seconds the tolerable force magnitude is 25 G. This paper reviews the readily available literature and concludes with parametric data for dynamic analysis of occupant response. Consideration of the available data indicates a wide range in mechanical properties of spinal ligaments, vertebrae and muscle groups. Coefficients of variance, cv, (standard deviation divided by mean value) were found to range as follows: Spinal ligament failure load cv’s from 30 to 85%, maximum deformation-35 to 72%, and stiffness from 33 to 84%; Vertebrae failure stresses from 44 to 64%, failure strains from 30 to 41%; Intervertebral discs with normal disc failure loads with c.v. of 10 % compared to 6% for degenerated discs, while cadaveric Spine failure loads variances were from 27 to 72%, and equivalent bilinear spine stiffness K1 had 40% and K2 62% variance. K1 = 101(40)[.40]**, K2 = 153(94)[.62] Newtons/mm. Range of motion and reflex times and neck strengths are observed to be influenced by sex and age. Both range of motion and neck strength decreased with aging. Coefficients of variance of male reflex times and strengths were found to be less variable than those of females, in the young and middle age groups. In general, male neck strengths were 1 1/2 to 2 times that of females. A recent study in Quebec of 5000 whiplash cases found that women are 50 % more likely than men to suffer whiplash injuries. The annual incidence of whiplash was found to be 86 per 100,000 for females compared with 54 for men. Mass moments of inertia, masses and associated centers of gravity and segment lengths are provided for a 50th percentile U.S. male aviator and for the “SOMLA” occupant man model. Regarding artificial intervertebral disc implants, review of the state-of-the-art indicates that no existing implants duplicate the full range of capabilities of the human disc or spine. As for crash survivable aircraft seats, civil aircraft, US Army helicopters and even NASA’s space shuttle crew seats could be significantly improved use of advanced materials, and properly design impact attenuation systems. The wide variation of spine strengths indicates the probabilistic design and analysis techniques should be applied. Finally it is evident that application of advanced composites and so-called smart materials must be based on an in-depth understanding of biomechanics and likely failure modes of the human body.

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