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

STUDY DESIGN: An in vitro biomechanical investigation on human cadaveric specimens was conducted before and after nucleotomy. Endplate and vertebral body 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, nucleotomy results in an altered deformation pattern of the endplate and that in shear, nucleotomy does not result in an altered endplate deformation pattern or disc pressure. SUMMARY OF BACKGROUND DATA: The pressure distributions within the intervertebral disc have been studied in compression loading but not in shear loading. Severe degeneration and surgical nucleotomy result in small nuclear pressure and altered loading distribution in compression. The effect of these changes 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 measured. A series of compression-type (maximum 1000 N) and shear-type (maximum 500 N) loads were applied. RESULTS: With nucleotomy, the disc pressure and the endplate strains decreased under compression, but the vertebral rim strains did not change. In shear, 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 endplate deformation, and modified loading patterns onto the inferior vertebra in compression loading. However, nucleotomy did not appreciably affect the behavior of the disc in shear loading.

A comparative biomechanical investigation of anterior lumbar interbody cages: central and bilateral approaches.

BACKGROUND: Some biomechanical studies have been performed to evaluate the stabilization provided by interbody cages, but there are virtually no comparative data for the different designs. Furthermore, most investigators have used animal models, which may have led to different results due to morphological variation in the end plates and articular facets. The objectives of the current study were to evaluate whether two different anterior cage designs (BAK and SynCage) performed differently with respect to immediate stabilization of the spine, whether the cages stabilized the spine significantly compared with its intact condition, and whether the addition of supplementary translaminar screw fixation further stabilized the spine. Stabilization was defined as a reduction in motion after insertion of an implant. METHODS: Twelve lumbar functional spinal units from human cadavera were tested under pure moments of flexion, extension, bilateral axial rotation, and bilateral lateral bending to a maximum of ten newton-meters. The relative intervertebral motions were measured, with use of an optoelectronic camera system, under three test conditions: with the spine intact, after insertion of anterior interbody cages, and after insertion of anterior interbody cages supplemented with translaminar screw fixation. Six specimens were tested for each type of cage: a bilateral, porous, threaded cylinder (BAK) and a central, porous, contoured implant with end-plate fit (SynCage). RESULTS: The cages performed in a similar manner in all directions of loading, with no significant differences between the two designs. The cages significantly stabilized the spine compared with its intact condition in flexion, axial rotation, and lateral bending (the median value for motion was 40, 48, and 29 percent of the value for the intact condition, respectively; p = 0.002 for all three directions). Compared with the cages alone, translaminar screw fixation provided no additional stabilizing effect in these directions but it significantly increased the stability of the spine in extension (the median value for motion was 34 percent of the value with the cages alone; p = 0.013). CONCLUSIONS: There were no differences in the stabilization provided by the two different cage designs. Use of the cages alone stabilized the spine in all directions except extension, and use of supplementary translaminar screw fixation provided additional stabilization only in extension. CLINICAL RELEVANCE: This study demonstrated that interbody cages do not stabilize the lumbar spine in extension, and this observation was not altered by the use of substantially different designs. If the lack of stabilization in extension is a clinical problem, possible solutions include the avoidance of extension postoperatively or the use of supplementary fixation.

The role of supplemental translaminar screws in anterior lumbar interbody fixation: a biomechanical study.

The immediate stabilization provided by anterior interbody cage fixation is often questioned. Therefore, the role of supplementary posterior fixation, particularly minimally invasive techniques such as translaminar screws, is relevant. The purpose of this biomechanical study was to determine the immediate three-dimensional flexibility of the lumbar spine, using six human cadaveric functional spinal units, in four different conditions: (1) intact, (2) fixed with translaminar screws (TLS), (3) instrumented with anterior interbody cage insertion with the BAK system and (4) instrumented with BAK cage with additional TLS fixation. Flexibility was determined in each testing condition by measuring the vertebral motions under applied pure moments (i.e. flexion-extension, bilateral axial rotation, bilateral lateral bending) in an unconstrained manner. Anterior fixation with the BAK alone provided significant stability in flexion and lateral bending. Additional posterior TLS significantly reduced the motion in extension and axial rotation. TLS fixation alone resulted in smaller rotations than BAK fixation in all loading directions. Based on these results, it seems that interbody cage fixation with the BAK system stabilizes the spine in some, but not all, loading directions. The problematic loading directions of extension and axial rotation can be substantially stabilized by using translaminar screw fixation. However, one should emphasize that the degree of stability needed to achieve solid fusion is not known.