An experimental study on the interface strength between titanium mesh cage and vertebra in reference to vertebral bone mineral density.

Abstract

Using human cadaver spines, the authors investigated mechanical properties of the interface between titanium mesh cage and vertebra in respect to vertebral bone mineral density. The objective of this study is to examine the effects of the size of the mesh cage and an internal end ring system on interface mechanical properties in reference to vertebral bone mineral density. A titanium mesh cage has recently been developed for anterior spinal reconstruction. The cage provides immediate postoperative stability and facilitates bony union with cancellous bone packed in the cage itself. Mechanical properties of the interface between the cage and vertebra, however, are yet to be clarified in osteoporotic spine. Twenty-five lumbar vertebrae harvested from embalmed human cadavers (n = 20) were used. The vertebrae were divided into four experimental groups according to the applied cage conditions: Phi25 mm cage without internal end ring (L-), Phi19 mm cage without internal end ring (S-), Phi25 mm cage with internal end ring (L+), and Phi19 mm cage with internal end ring (S+). Bone mineral density of whole vertebral body was measured by dual energy radiograph absorptiometer (DXA). Peripheral quantitative computed tomography was used to determine local bone mineral density of subchondral cancellous bone of vertebral body. Each cage was compressed on vertebral endplate via a specially designed device connected to a material testing machine. Maximum load and stiffness of the interface between the cage and vertebra were measured from load-deformation data in quasi-static compression loading with a loading rate of 0.5 mm/min. Relationships between the mechanical properties and vertebral bone mineral densities were evaluated. In 11 specimens acoustic emission during compression loading was measured and simultaneously recorded in load-deformation data. After the mechanical test microradiograms of midsagittal sections of the vertebrae were taken to observe failure patterns of endplate or trabecular bone. Vertebrae compressed with large cages (group: L- or L+) showed greater maximum load than those compressed with small cages (group: S- or S+). The internal end ring contributed to higher maximumload. The size of the cage or the internal end ring, however, did not have any effect on stiffness. Maximum load and stiffness were positively correlated with whole vertebral bone mineral density measured by dual energy radiograph absorptiometer or local cancellous bone mineral density of subchondral bone measured by peripheral quantitative computed tomography. Correlation coefficient and P value were more significant in the association of the mechanical properties and subchondral bone mineral density measured by peripheral quantitative computed tomography than in the association of the parameters and whole vertebral bone mineral density measured by DXA. A load-deformation curve with an acoustic emission event count rate showed that significant acoustic emission signals were generated around maximum load. On microradiographic study most vertebrae compressed with the cage showed encroachment of the cage spikes into the endplate or trabecular structure, preserving structures of the most central portion of the vertebrae. A titanium mesh cage with larger diameter and/or augmentation of internal end ring produces a significant increase of the interface strength between the cage and the vertebra. A positive correlation between the interface strength and vertebral bone mineral density suggests that vertebral bone mineral density is an important parameter for successful spinal reconstruction, and also implies that in severe osteoporotic spine the stability of the cage is declined, and other instrumentation should be combined.