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

Study Design. Using human cadaver spines, the authors investigated mechanic al properties of the interface between titanium mesh cage and vertebra in r espect to vertebral bone mineral density. Objectives. The objective of this study is to examine the effects of the si ze of the mesh cage and an internal end ring system on interface mechanical properties in reference to vertebral bone mineral density. Summary of Background Data. A titanium mesh cage has recently been develope d for anterior spinal reconstruction. The cage provides immediate postopera tive stability and facilitates bony union with cancellous bone packed in th e cage itself. Mechanical properties of the interface between the cage and vertebra, however, a re yet to be clarified in osteoporotic spine. Methods. Twenty-five lumbar vertebrae harvested from embalmed human cadaver s (n = 20) were used. The vertebrae were divided into four experimental gro ups according to the applied cage conditions: phi 25 mm cage without intern al end ring (L-), phi 19 mm cage without internal end ring (S-), phi 25 mm cage with internal end ring (L+), and phi 19 mm cage with internal end ring (S+). Bone mineral density of whole vertebral body was measured by dual en ergy radiograph absorptiometer (DXA). Peripheral quantitative computed tomo graphy was used to determine local bone mineral density of subchondral canc ellous bone of vertebral body. Each cage was compressed on vertebral endpla te 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 loadin g with a loading rate of 0.5 mm/min. Relationships between the mechanical p roperties and vertebral bone mineral densities were evaluated. In 11 specim ens acoustic emission during compression loading was measured and simultane ously recorded in load-deformation data. After the mechanical test microrad iograms of midsagittal sections of the vertebrae were taken to observe fail ure patterns of endplate or trabecular bone. Results. Vertebrae compressed with large cages (group: L- or L+) showed gre ater maximum load than those compressed with small cages (group: S- or St). The internal end ring contributed to higher maximum-load. The size of the cage or the internal end ring, however, did not have any effect on stiffnes s. Maximum load and stiffness were positively correlated with whole vertebr al bone mineral density measured by dual energy radiograph absorptiometer o r local cancellous bone mineral density of subchondral bone measured by per ipheral quantitative computed tomography, Correlation coefficient and P val ue were more significant in the association of the mechanical properties an d subchondral bone mineral density measured by peripheral quantitative comp uted tomography than in the association of the parameters and whole vertebr al bone mineral density measured by DXA. A load-deformation curve with an a coustic emission event count rate showed that significant acoustic emission signals were generated around maximum load. On microradiographic study mos t vertebrae compressed with the cage showed encroachment of the cage spikes into the endplate or trabecular structure, preserving structures of the mo st central portion of the vertebrae. Conclusion. A titanium mesh cage with larger diameter and/or augmentation o f internal end ring produces a significant increase of the interface streng th between the cage and the vertebra. A positive correlation between the in terface strength and vertebral bone mineral density suggests that vertebral bone mineral density is an important parameter for successful spinal recon struction, and also implies that in severe osteoporotic spine the stability of the cage is declined, and other instrumentation should be combined.