Biomechanical compression tests with a new implant for thoracolumbar vertebral body replacement

The authors present an investigation into the biomechanical functioning of a new titanium implant for vertebral body replacement (Synex). Possible ind ications are fractures and/or dislocations with damage of the anterior colu mn, posttraumatic kyphosis and tumors of the thoracolumbar spine. The const ruction must be supplemented by a stabilizing posterior or anterior implant . For best fit and contact with adjacent end-plates, Synex is distractable in situ. We performed comparative compression tests with Synex and MOSS ("H arms mesh cage") on human cadaveric specimens of intact vertebrae (L1). The aim of the study was to measure the compressive strength of the vertebral body endplate in uniaxial loading via both implants to exclude collapse of Synex in vivo. Twelve human cadaveric specimens of intact vertebrae (L1) we re divided into two identical groups (matched pairs) according to bone mine ral density (BMD), determined using dual-energy quantitative computed tomog raphy (DE-QCT). The specimens were loaded with an axial compression force a t a constant speed of 5 mm/min to failure, and the displacement was recorde d with a continuous load-displacement curve. The mean ultimate compression force (F-max) showed a tendency towards a higher reading for Synex: 3396 N versus 2719 N (non-significant). The displacement until F-max was 2.9 mm in the Synex group, which was half as far as in the MOSS group (5.8 mm). The difference was significant (P <0.001). The compression force was twice as h igh, and significantly (P <0.05) higher with Synex at displacements of 1 mm , 1.5 mm and 2 mm. A significant (P <0.001) correlation (R=0.89) between F- max and BMD was found. Synex was found to be at least comparable to MOSS co ncerning the compressive performance at the vertebral end-plate. A possible consequence of the significantly higher mean compression forces between an d 2 mm displacement might be decreased collapse of the implant into the ver tebral body in vivo.