Comparative biomechanical compression testing with a new implant for vertebral body replacement

The authors present a new titanium implant for replacement of the vertebral body (Synex(TM)). Possible indications would be fractures or dislocations with destruction of the anterior column, posttraumatic kyphosis as well as tumors in the throracolumbar spine. The construction has to be completed by a stabilizing implant. For best fit and contact to adjacent end-plates Synex(TM) is distractable in situ. The possibility of secondary dislocation or loss of correction should thereby be minimised. Objectives. We performed comparative compression tests with Synex(TM) and MOSS(TM) ("Harms 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 end-plate in uniaxial loading via both implants to exclude a caving of Synex(TM) in vivo. Methods. 12 human cadaveric specimens of intact vertebrae (L1) were divided in 2 similar groups (matched pairs) according to bone mineral density (BMD), determined using DE-QCT. The specimens were loaded with axial compression force at a constant speed of 5 mm/min to failure and the displacement was recorded with a continuous load-displacement curve. Results. The mean ultimate compression force (F-max) showed a tendency towards a higher result testing Synex(TM) with 3396 N versus 2719 N (non significant). The displacement until F-max was 2.9 mm in group 5 (Synex(TM)), which was half as long as in group M (5.8 mm). The difference was significant (p<0.001). The compression force was twice as high and significantly (p<0.05) higher with Synex(TM) at a displacement 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. Conclusions. Synex(TM) was found to be at least comparable to MOSS(TM) for suspensory replacement of the vertebral body at the thoracolumbar spine. A possible consequence of the significantly higher mean compression forces between 1 and 2 mm displacement might be a decreased segmental deformation or loss of correction.