[Introduction of a new intervertebral spacer for cervical fusion: results of a controlled animal study].

Aim: This study investigated the fusion properties of a new intervertebral spacer that is directly fixed to the adjacent vertebrae by screws. The new spacer was compared with an identical spacer without direct fixation but with an additional plate and with an autologous bone graft.
Methods: A single-level, cervical fusion was performed on 18 mature Merino sheep. Three treatment groups of six sheep each were used. Group A used an autogenous bone graft and an additional anterior plate. Group B used a titanium solid spacer with a titanium plasma-sprayed (TPS) surface and an additional anterior plate. Group C used a similar spacer as group B that was directly fixed with the adjacent vertebrae with 4 screws. The standing period was 12 weeks. The substrates were evaluated radiographically and histologically.
Results: Dislocation of the spacer or the bone graft occurred in one of the 6 cases in each group A and B, while 5 of the 6 spacers were dislocated in group C. Successful fusion was found in 4 of the 6 cases in groups A and C and in 5 of the 6 cases in group B. The histomorphometry revealed a direct bone apposition at the interface of the bone graft (group A) at 11.6% of the surface. The surfaces of the spacers were osseointegrated in 1.1% in group B and 1.0% in group C.
Discussion: As the dislocation rate shows, the fixation of the new spacer (group C) seems to be less stable than the fixation of the bone graft or the spacer with the additional plate. Although the intervertebral fusion is comparable for all groups, the extent of the osseointegration is much higher for the bone graft than for the spacers. In conclusion, the bone graft may offer a much better long-term stability than do the spacers.