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Some mechanical tests on the lumbosacral spine with particular reference to the intervertebral discs; a preliminary report.

1957; National Institutes of Health; Volume: 39-A; Issue: 5 Linguagem: Inglês

Thornton Brown, Robert J. Hansen, Alvin J. Yorra,

Musculoskeletal pain and rehabilitation

The investigations reported here represent an effort to explore the possibilities of obtaining quantitative data on the mechanical properties of the lumbosacral spine by applying to fresh autopsy specimens of the spine testing techniques used in civil or mechanical engineering. Few tests have been made and the data are insufficient. However, the findings to date are of interest and encourage us to continue these investigations. With the exception of the markedly osteoporotic spines which failed at relatively low axial loads, the ultimate axial compressive load for the lumbar discs teste ranged from 1,000 to 1,300 pounds. Stiffness values for these discs ranged from 470 to 8,250 pounds per inch initially but increased to 12,000 to 20,000 pounds per inch after the applied load reached 200 to 400 pounds. Failure under axial compression took place in the vertebral end-plates even when well developed ruptures of the annulus were present. Failure of the plate ranged from imperceptible cracks to more or less complete collapse of the endplate depending on the condition of the bone (presence or absence of osteoporosis) and on the size of the load applied. Under the experimental conditions employed here, which involved a gradually increasing load over periods ranging from ten to thirty minutes, the volumes of five discs decreased under axial compression by from 1.0 to 2.5 cubic centimeters before failure occurred due to fracture of one of the vertebral end-plates and collapse of the underlying vertebral body. This decrease in volume is thought to be due both to collapse of the fissures and spaces ordinarily present within all adult discs and to the passage of fluid across the vertebral end-plates into the medullary spaces of the adjacent vertebral bodies. Tests of the tensile strength of various portions of two intervertebral discs revealed that the tensile strength ranged from 0 to 700 pounds per square inch. In general, the weakest areas in both discs were the central and lateral portions. The tensile strength of three specimens of ligamentum flavum ranged from 226 to 373 pounds per square inch. Two discs (the fourth lumbar) were subjected to combined axial loading and bending while the expansion and contraction of the discs were recorded at various points about the circumference of the annulus. During bending the discs expanded on the concave side and contracted on the convex side of the curve, while at points located at 90 degrees to the plane of motion little or no expansion or contraction occurred. The only exceptions to this general pattern of behavior were in the posterolateral portions of the discs, where annular ruptures are most frequently found. The greatest expansion and contraction occurred On the anterior aspect of these discs during straight flexion and extension. One fatigue test was performed in which cyclic bending and compression stresses were imposed on a single disc. For technical reasons the test was not satisfactory. It is of interest, however, that rapid failure of the annulus occurred under the conditions of this experiment characterized by a horizontal tear of all but the most peripheral fibers of the annulus. The resulting picture was not unlike that seen in some extensively degenerated dises. The findings of particular interest in the work presented here are as follows: 1. Under axial compressive stress, failure of the disc complex invariably took place in the cartilaginous plate. In the few specimens tested the type of failure varied according to the condition of the bone rather than according to the condition of the disc. In the spines from younger individuals small cracks occurred, while in the old spines with osteoporosis, there was more or less total collapse of the plate. The similarity of the small cracks produced in these specimens to those found in the autopsy material studied by Beadle and by Coventry, Ghormley, and Kernohan was quite striking. 13, 27 have been inclined to assume that these defects were of little clinical significance due to their small size. However, the findings presented here suggest that such an assumption may not be valid. Failures of the end-plate may play a role in the causation of pain in some back injuries when the roentgenograms show no abnormality. In fact, Hirsch 23 reported one case in which failure of the plate was demonstrated in a man in whom sudden low-back pain developed after a strenuous effort to right an overturned car. However, the special technique of tomography was required to visualize the defect. The relief of back pain resulting from the intradiscal injection of a local anaesthetic or the anti-inflammatory hormone, hydrocortisone, might be explained by this mechanism also. 2. In these tests failure of the annulus fibrosus occurred only as the result of extremely rapid cyclic bending combined with mild axial compression. However, no protrusion of disc material was produced in this short period but rather a linear horizontal tear through all but the most peripheral fibers which still remained intact. In the one specimen in which failure was induced by flexion, the inferior posterior margin of the superior vertebra was avulsed, but the annulus remained intact. 3. It is commonly stated that the nucleus pulposus is the structure by which stresses are distributed uniformly to the annulus fibrosus and cartilaginous plates 40. It is furthermore stated in the literature that the nucleus pulposus moves toward the convex side of the curve when the spine bends to either side or forward and backward 36. The behavior of the two specimens subjected to combined axial loading and bending in this investigation did not seem consistent with this concept. The annulus invariably bulged on the concave side, apparently as the result of compression between the opposing vertebral surfaces. The thickest and strongest portion of the annulus, that is its anterior third, bulged the most. This would be more consistent with direct compression of the annulus than with bulging caused by deformation of the nucleus pulposus or displacement of the nucleus toward the opposite side. If there were a significant amount of displacement of the nucleus toward the convex side, it would not be anticipated that the annulus would retract on this side, as was observed in these specimens. Therefore during bending the annulus in these rather aged discs would appear to be subjected primarily to direct compression rather than to hoop tension as would be the case if its only function were to resist deformation of the nucleus pulposus. Friberg, as the result of his studies of fresh specimens of the spine, also concluded that the annulus had a weight-bearing function. This behavior of the disc may well vary with age. The more fluid nucleus of the child may displace more during bending than the relatively fibrotic and fragmented nucleus of the adult. 4. Under axial compressive loads the volume losses of the intervertebral discs before failure in the four specimens tested were 1.0, 1.5, 2.5, and 2.5 cubic centimeters, respectively, during time intervals ranging from ten to thirty minutes. Previous clinical observations on the disappearance of water-soluble radiopaque solutions after injection into the disc space11 have suggested that fluid transfer across the cartilaginous plates occurs quite rapidly during life even in the recumbent position when compression forces are minimal. It remains to be determined how such factors as age and degeneration of the cartilaginous plates effect. the rate and amount of this transfer. Degenerative changes which occur within the disc with advancing age may be related in some way to alterations in this fluid transfer. The experimental methods reported here are thought to show sufficient promise to warrant a more extensive testing program on specimens from different age groups and in sufficient numbers to obtain data of statistical significance. Furthermore, other tests would be desirable. For instance, impact tests might furnish quantitative data on the mechanism of injuries associated with falls and other sudden movements. The effect of other types of stress including shear and axial torsion would also be of interest.