[An improved vertebral body replacement for the thoracolumbar spine. A biomechanical in vitro test on human lumbar vertebral bodies].

BACKGROUND: In recent years, the use of expandable titanium cages for vertebral body replacement in the thoracolumbar spine has been well established for the treatment of tumors, unstable traumatic lesions, or posttraumatic deformity. Collapse of the implant into the vertebral body remains a point of concern. A biomechanical compression test was designed to assess implant subsidence for a newly developed prototype for vertebral body replacement in the thoracolumbar spine using human cadaveric lumbar vertebrae. The objective of this study was to compare the compressive performance of a new expandable cage with modified end-plate design with three commonly available expandable cages for vertebral body replacement.

MATERIALS AND METHODS: The compressive strengths at the implant-vertebral body interface were measured via axial loading of the new prototype (Synex II) in comparison with three different expandable titanium cages: Synex I (Synthes), Obelisc (Ulrich Medical) and X-Tenz (DePuy Spine). Twenty-four intact, fresh frozen human lumbar vertebrae (L1-L4) were distributed into four identical groups according to BMD (determined by quantitative computed tomography) and the vertebral level. Specimens were loaded in the craniocaudal direction with a material testing machine at a constant speed of 5 mm/min. Load displacement curves were continuously recorded for each specimen until failure (diminishment of compressive force (F)/obvious implant migration through the vertebral body endplate). One-way analysis of variance and post-hoc tests (Bonferroni) were applied to detect differences at 1, 2, 3, 4 mm displacement (F1-4 mm), and Fmax between implant groups.

RESULTS: The four expandable cages did not show statistically significant biomechanical differences in terms of maximum compression force (Fmax) until failure: Synex II (1,782 N/4.7 mm); Synex I (1,645 N/4.7 mm); Obelisc (1,314 N/4.2 mm); X-Tenz (1,470 N/6.9 mm). However, the mean compression force until 4 mm displacement (F1-4 mm: 300-1,600 N) was highest for Synex II. The difference at 2 mm displacement was significant (p=0.028) between Synex II (F2 mm=879 N) and X-Tenz (F2 mm=339 N).

CONCLUSION: The modified endplate design of the new prototype was found to improve its compressive performance under constrained uniaxial loading conditions at the implant-bone interface. The improved compressive behaviour of the new implant might help to reduce the risk of implant subsidence and collapse into the vertebral body in vivo.