gms | German Medical Science

The purpose of this study was to evaluate the mid-term osteointegration of two bioabsorbable cages. We compared the interbody fusion of an autologous tricortical iliac crest bone graft with two bioabsorbable cages in a sheep cervical spine fusion model. This study was designed to determine whether there are differences between the three interbody fusion techniques in (1) the ability to preserve postoperative distraction, (2) the biomechanical stability, and (3) the histological characteristics of intervertebral bone matrix formation.

48 sheeps underwent C3/4 discectomy and fusion: Group 1: autologous tricortical iliac crest bone graft (n = 16); Group 2: bioabsorbable cage made of PLDLLA filled with autologous cancellous bone grafts (n = 16); Group 3: bioabsorbable cage made of Resorbon (Biomet Merck) filled with autologous cancellous bone grafts (n = 16). Radiographic scans were performed pre- and postoperatively,after 1, 2, 4, 8, 12 weeks and then monthly, respectively. At the same time points disc space height (DSH) was measured. After 12 weeks 8 animals of each group were killed, the other 8 animals of each group were killed after 1 year. Post mortem fusion sites were evaluated using functional radiographic views

in flexion and extension. Quantitative computed tomographic scans (QCT) were performed to assess bone mineral density (BMD), bone mineral content (BMC) and bony callus volume (BCV). Biomechanical testing was performed in flexion, extension, axial rotation and lateral bending. Stiffness, range of motion (ROM), neutral (NZ) and elastic zone (EZ) were determined. Histomorphological and histomorphometrical analysis were performed. Foreign body reactions of the implants were graded histologically.

During the 1 year follow-up the Resorbon-cage group showed significantly higher values for DSH compared to the bone graft group and the PLDLLA-cage group. Additionally, the Resorbon-cage group demonstrated a significantly higher stiffness and lower ROM, NZ, EZ in axial rotation and lateral bending than any other group. There was no difference for BMD and BMC between all groups. However, the histomorphometrical parameters demonstrated highest bone volume in the Resorbon-cage group. Histological examination revealed that 2/8 of Resorboncage specimens obtained interbody fusion within 12 weeks and 7/8 within 1 year. PLDLLA-cages showed no interbody fusion during this period. Further, the PLDLLA-cages showed grade II-III foreign body reactions (fibrotic tissue, osteolysis) in all fusion areas after 12 weeks and 1 year. In contrast, the Resorbon-cage group demonstrated only two animals with grade I foreign body reaction after 12 weeks and 2 grade I and 1 grade II foreign body reactions after 1 year.

After 1 year there was no significant difference between the bioabsorbable PLDLLA-cage and the tricortical bone graft. In comparison to the tricortical bone graft the bioabsorbable Resorbon-cage showed significantly better distractive properties, a significantly higher biomechanical stiffness and an advanced interbody fusion. There was no difference between all groups in the amount of new formed bone matrix after 12 weeks. After 1 year the Resorbon-cage group showed an advanced bone matrix formation and in most cases a solid interbody fusion. Although the fate of the foreign body reactions is currently unclear for both bioabsorbable cages, the appearance of osteolysis associated with use of the PLDLLA-cage allows scepticism regarding the value of this bioabsorbable implant.

Submitted for Oral Presentation:

- NASS 2003, San Diego, October 21-25, 2003

- Cervical Spine Research Society 2003, Barcelona, 19.-20. Juni 2003

- Eurospine , Prag, October 1-4, 2003

- Biotechnology AO/Synthes , Freiburg, March 28, 2003

- DGW, Bochum, May 30-31, 2003

- WorldSpine II, Chicago, August 10-13, 2003