Changes in bone architecture during spinal fusion: three years follow-up and the role of cage stiffness.

STUDY
DESIGN: A morphometrical analysis of microcomputed tomography-based reconstructions of bone from the fusion zone in spinal cages.
OBJECTIVE: To describe the architectural changes of the fusing bone tissue in time and to study the effect of cage stiffness on the development of the bone architecture within the cage. SUMMARY OF BACKGROUND DATA: Interbody fusion within spinal cages is routinely evaluated as either successful or not successful. The quality of the fusion, however, strongly depends on the architecture (i.e., structure and density) of the bone tissue within the cage. Bone architecture obviously changes during the fusion process, and cage stiffness is known to play a pivotal role.
METHODS: Nine bone samples were available from a long-term in vivo study on resorbable spinal cages in Dutch milk goats described elsewhere. Follow-up periods of 3, 6, 12, 24, 30, and 36 months were considered for cages made of titanium or Poly L-Lactic Acid. The specimens were scanned with a resolution of 13 microm in a microcomputed tomography system. From the resulting reconstructions, the bone density; trabecular thickness, spacing, and number; connectivity density; and structure model index were assessed.
RESULTS: We found a homogenization of all bone structure indexes along the spinal axis with time, and remarkably faster in the Poly L-Lactic Acid cages than in the titanium cages. After longer follow-up periods, a coarser bone structure with larger trabecular thickness and intertrabecular spacing was found. The structure model index appeared to be sensitive for nonunions.
CONCLUSIONS: More "mature" spinal fusions showed a coarser and more homogeneous bone structure. High cage stiffness had a deteriorating effect on the fusion rate. The structure model index appears to be an interesting parameter for quantifying the quality of bone in the fusion zone.