Characterization of a developing lumbar arthrodesis in a sheep model with quantitative instability

Area of Science:

• Spinal fusion biology
• Orthopedic research
• Biomechanical analysis

Background:

• Mechanical forces are implicated in stress shielding of arthrodesis.
• The biological processes of developing lumbar fusion remain incompletely understood.

Purpose of the Study:

• To investigate the hypothesis that mechanical forces influence the biological processes of developing bony fusion.
• To characterize the biology of lumbar fusion in a large animal model.

Main Methods:

• A lumbar fusion ovine model was utilized with custom instrumentation allowing controlled anterior-posterior translation.
• Fusion sites were assessed using radiography, microradiography, histology, and histomorphometry at 6 and 12 weeks post-surgery.
• Control (stable) and experimental (unstable) groups were compared, with instability introduced via annulectomy and translation-allowing pedicle screws.

Main Results:

• Radiographs showed incomplete fusion consolidation at both time points.
• Microradiography indicated a trend reversal in bone formation between stable and unstable groups by 12 weeks (p=.03).
• Endochondral ossification was the primary healing mechanism in the intertransverse region, while intramembranous bone formation dominated near the transverse process.

Conclusions:

• Endochondral ossification is the predominant mechanism for new bone formation in the intertransverse lumbar fusion site, irrespective of stability.
• Mechanical instability was found to increase new bone formation in the intertransverse region, but not at the transverse process.
• The ovine model provides a clinically relevant platform for studying spinal fusion biology and evaluating novel fusion strategies.