Augmentation improves human cadaveric vertebral body compression mechanics for lumbar total disc replacement

Abstract

Study design: Cadaveric biomechanical study.

Objective: To quantify the effects of vertebral body augmentation on biomechanics under axial compression by a total disc replacement (TDR) implant.

Summary of background data: TDR is a surgical alternative to lumbar spinal fusion to treat degenerative disc disease. Osteoporosis in the adjacent vertebrae to the interposed TDR may lead to implant subsidence or vertebral body fracture. Vertebral augmentation is used to treat osteoporotic compression fracture. This study sought to evaluate whether vertebral augmentation improves biomechanics under TDR axial loading.

Methods: Forty-five L1-L5 lumbar vertebral body segments with intact posterior elements were used. Peripheral quantitative computed tomography scans were performed to determine bone density, and specimens were block-randomized by bone density into augmentation and control groups. A semiconstrained keeled lumbar disc replacement device was implanted, providing 50% endplate coverage. Vertebral augmentation of 17.6% +/- 0.9% vertebral volume fill with Cortoss was performed on the augmentation group. All segments underwent axial compression at a rate of 0.2 mm/s to 6 mm.

Results: The load-displacement response for all specimens was nonlinear. Subfailure mechanical properties with augmentation were significantly different from control; in all cases, the augmented group was 2 times higher than control. At failure, the maximum load and stiffness with augmentation was not significantly different from control. The maximum apparent stress and modulus with augmentation were 2 times and 1.3 times greater than control, respectively. The subfailure stress and apparent modulus with augmentation were moderately correlated with bone density whereas the control subfailure properties were not. The augmented maximum stress was not correlated with bone density, whereas the control was weakly correlated. The maximum apparent modulus was moderately correlated with bone density for both the augmented and the control groups.

Conclusion: Augmentation improved the mechanical properties of the lumbar vertebral body for compression by a TDR implant.

Figure 1

Representative peripheral quantitative computed tomography (pQCT) scan of lumbar vertebral specimen. Trabecular bone density was evaluated in a 10 mm diameter region of interest (white circle).

Figure 2

Fluoroscopic images A) pre-augmentation (Arrow indicates point of Cortoss injection), B) post-augmentation, C) post-testing

Figure 3

Representative load-displacement response from three specimens demonstrating variability in maximum load. Most samples exhibited the “typical” response of uniform load to failure. Some samples exhibited a small break in the curve (square) or an early drop followed by a continued increase to failure. The maximum load determined for each sample is indicated by #.

Figure 4

Average (standard deviation) sub-failure and maximum properties for control (white) and augmented (black) specimens. A) load, B) stiffness, C) apparent stress, and D) apparent modulus, *p≤0.05.

Figure 5

Correlation with bone density for apparent material properties, normalized by geometry. A) sub-failure stress, (B) sub-failure modulus, (C) maximum stress, (D) maximum modulus, *p≤0.05.