The effect of nucleotomy on lumbar spine mechanics in compression and shear loading

Study Design. An in vitro biomechanical investigation on human cadaveric sp ecimens was conducted before and after nucleotomy. Endplate and vertebral b ody deformation patterns were measured under compression and shear loading, in addition to kinematics and disc pressure. Objective. The working hypotheses of this study were that in compression, n ucleotomy results in an altered deformation pattern of the endplate and tha t in shear, nucleotomy does not result in an altered endplate deformation p attern or disc pressure. Summary of Background Data. The pressure distributions within the intervert ebral disc have been studied in compression loading but not in shear loadin g. Severe degeneration and surgical nucleotomy result in small nuclear pres sure and altered loading distribution in compression. The effect of these c hanges on the vertebral endplate and the response under shear loads are not well understood. Methods. Five L3-L4 and two L4-L5 functional spinal units were tested under compression and shear loading, intact and after nucleotomy. Vertebral body deformations, intradiscal pressure, and intervertebral kinematics were mea sured. A series of compression-type (maximum 1000 N) and shear-type (maximu m 500 N) loads were applied. Results. With nucleotomy, the disc pressure and the endplate strains decrea sed under compression, but the vertebral rim strains did not change. In she ar, the vertebral rim and endplate strains did not change with nucleotomy. Disc pressure was lower in shear than in compression. Conclusion. Nucleotomy resulted in decreased disc pressure, decreased endpl ate deformation, and modified loading patterns onto the inferior vertebra i n compression loading. However, nucleotomy did not appreciably affect the b ehavior of the disc in shear loading.