PubMed · 4648029
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R Contini. 1972. Body segment parameters. II.. https://pubmed.ncbi.nlm.nih.gov/4648029/
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The study of natural interfaces may provide information necessary to engineer functionally graded biomaterials for bioengineering applications. In this study, the mechanical, structural, and chemical composition variations associated with a region between cementum and dentin were studied with the use of nanoindentation, microindentation, optical microscopy, and Raman microspectroscopy techniques. Three-millimeter-thick transverse sections (N = 5) were obtained from the apical one-third of the roots of sterilized human molars. The samples were ultrasectioned at room temperature with the use of a diamond knife and an ultramicrotome. Longitudinal ground sections of 100 microm thickness were prepared and stained with von Kossa stain to determine the mineralized regions within the molar roots. Raman microspectroscopy was used to determine the relative inorganic content, mainly apatite (PO4(3-)nu1 mode at 960 cm(-1)) and organic content, mainly collagen (C--H stretch at 2940 cm(-1)) between cementum and dentin bulk tissues. The microindentation and nanoindentation results indicated a gradual transition in hardness from cementum to dentin over a width ranging from 100 to 200 microm. However, the variation in hardness data for cementum and dentin by nanoindentation was larger (0.62 +/- 0.21, 0.77 +/- 0.14 GPa) than from microindentation (0.49 +/- 0.03, 0.69 +/- 0.07 GPa). Within the 100 to 200 microm region there was a 10 to 50 microm fibrillar hydrophilic cementum-dentin junction (CDJ) with mechanical properties significantly lower than either the cementum or the dentin side of CDJ. Light microscopy revealed a 100 to 200 microm translucent region between cementum and dentin. Raman microspectroscopy results showed a variation in organic and inorganic composition 80 to 140 microm wide. It was concluded that a morphologically and biomechanically different CDJ lies within a wider cementum-dentin interphase. Hence, cementum, dentin, and the interphase can be classified as a functionally graded dental tissue within the root of a tooth.
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.
STUDY DESIGN: The biomechanical responses resulting from laminectomy with graded unilateral and bilateral facetectomy were quantified using a three-dimensional nonlinear finite element model of the C2-C7 motion segments. OBJECTIVE: To study the influence of laminectomy with graded unilateral and bilateral facetectomy on the cervical spinal biomechanics. SUMMARY OF BACKGROUND DATA: Cervical spinal stenosis is a condition that is caused by the narrowing of the spinal canal. Laminectomy and facetectomy are commonly used surgical procedures for decompressing cervical spinal stenosis. Resection of the posterior structures causes instability and affects the internal stresses of the cervical spinal components. However, the influence of these surgical procedures on the biomechanical responses of the cervical spine has not been studied. METHODS: A nonlinear finite element model of the intact C2-C7 was constructed and validated. Ten surgically altered models were created from the intact model and were tested under physiologic loading. Because of the inclusion of five motion segments, it was possible to determine the intersegmental responses and internal cortical shell and disc stresses in the adjacent altered and unaltered spinal components. RESULTS: Under combined flexion and extension, intersegmental motions at C4-C5 and C5-C6 increased significantly after C5 laminectomy. Subsequent facetectomy performed at C5 and C6 on the laminectomized model only affected the responses at the C5-C6 segment. Overall, slight intersegmental responses of up to 5% were observed at the adjacent levels of C3-C4 and C6-C7. Laminectomy did not cause any significant increase in the intersegmental motions under lateral bending and axial rotation. Extending the surgical procedures to unilateral and bilateral facetectomy only increased the intersegmental motions slightly. Similar increases in the intervertebral disc and the cortical shell stresses were observed. These findings may partially explain the clinical observations of enhanced osteophytes formation. CONCLUSIONS: This study provides a better understanding of the surgically altered cervical spinal biomechanics and may help formulate treatment strategies such as spinal implants.