Normal and slightly degenerated lumbar intervertebral disc speciinem (L1–L4) taken at autopsy from 16 individuals between 20 and 57 years of age have been studied to evaluate the effect of forward, hackward and sideward tilt on the intradiscal pressure.
In a vertically loaded and tilted disc the hydrostatic properties of the nucleus pulposus are not affected.
In disc-preparations containing one disc and parts of adjoining vertebral bodies it has been found that compared to the intradiscal pressure in the non-tilted case, the pressure in healthy or slightly degenerated specimens shoxs an increase on tilted loading. No difference was observed betwecn L1, L2, L3 and L4 in this respect. The recorded increase in pressure in absolute measure at a tilt of 5° is about 0.7 kp./cm.² independently of the applied load in the interval 2-10 kp./cm.².
The percentage increase produced by a tilt of 5° will be for an external load of 2 kp./cm.2: 22 %, for 10 kp./cm.²: 5 %. The increase in pressure and the relative diminution of this increase with higher loads is explained by the fact that the lateral bulging of the annulus fibrosus on tilted loading follows a nonlinear pattern, so that the nucleus pulposus has relatively less volume at its disposal with vertical loading on a tilted than on a non-tilted disc. With increasing loads this effect will, expressed as a percentage, become less and less pronounced.
The relation between tilt and increase in intradiscal pressure has been estimated and it can on the basis of the experimental findings of this examination be expected that a tilt of morc than 5° map lead to an appreciably larger increase in intradiscal pressure. The increase of backward tilt for example from 5° to 8° resulted in a more than double incrcase of pressure from 0.7 kp./cm.² to 1.5 kp./cm.² compared with the pressure in the non-tilted disc.
It can thus be demonstrated that a backward tilt not only will subject the dorsal part of the annulus fibrosus in a normal lumbar disc to an increased vertical stress, but also to a tangential stress which per unit of area may be 6 or 7 times the external load. This fact may form the mechanical background to the production of dorsal annulus ruptures in the lumbar intervertebral discs.
In lumbar disc specimens with arches and facets still attached there exist a small "intrinsic" pressure of 0.7 kp./cm.², probably exerted by the capsules and ligaments surrounding the posterior bony elements. When these elements are removed no intradiscal pressure can be recorded in the unloaded discs.
The changes in intradiscal pressure caused by 5° forward, backward or sideward tilt in these specimens when subjected to vertical loading up till 220 kp. are not influenced by the posterior bony or ligamentous elements. Thus arches and facets do not have any significant effect on the tensile strains in the annulus fibrosus resulting from a forward, backward or sideward tilt of 5°.
The results that are presented clearly demonstrate the mechanism by which the lumbar discs manage to combine their function of withstanding a heavy vertical stress with the ability to tilt in various directions.