Characterization of a developing lumbar arthrodesis in a sheep model with quantitative instability.
BACKGROUND
Mechanical forces have been considered responsible for stress shielding an arthrodesis, but the biology of a developing lumbar fusion has not been well characterized.


OBJECTIVE
A large animal model was used to test the hypothesis that mechanical forces modify the biological processes involved in a developing bony fusion.


METHODS
Lumbar fusion was performed in an ovine model using custom instrumentation that permitted a controlled degree of anterior-posterior translation after surgery. Fusion sites were evaluated by radiography, microradiography, histology and histomorphometry at time points that corresponded with predicted early and later stages of bone healing.


METHODS
Fourteen skeletally mature ewes underwent lumbar spinal fusion under general anesthesia. In the control (stable) group, the spine was rigidly fixed with a cage anteriorly and pedicle screws posteriorly. In the experimental (unstable) group, the spine was destabilized by an annulectomy (with no anterior implant) and custom pedicle screws that allowed 2 mm of anterior-posterior translation. Animals were euthanized 6 and 12 weeks after surgery.


RESULTS
Radiographs confirmed that the fusion mass had not fully consolidated at either time point. Microradiographs revealed a trend toward increased bone formation at 6 weeks in the stable case as compared with the unstable, but by 12 weeks, this trend had reversed (p=.03). Intramembranous bone formation was the primary mechanism of healing near the transverse process in animals with both stable and unstable fixation. In the area between the two transverse processes, new bone formation occurred primarily through endochondral ossification. At 12 weeks, the stable case had significantly more cartilage formed (p=.023) but less newly formed bone (p=.07) as compared with the quantitatively unstable.


CONCLUSIONS
This clinically realistic animal model allowed characterization of the biology of the developing arthrodesis before fusion. Under stable or unstable conditions, endochondral ossification was the predominant mechanism of new bone formation within the intertransverse process region. This finding, which contrasts with previous reports from small animal models of spine fusion, may reflect a difference in biology that results from the increased size of the intertransverse space in sheep as compared with small animals. Interestingly, mechanical instability increased the formation of new bone within this region, but not at the transverse process. Endochondral ossification therefore appears to respond to mechanical factors in the fusion site. The ovine model shows promise as an alternative to the rabbit model and may provide a more stringent test for potential new surgical and nonsurgical strategies for spine fusion.
Cherry Way Orthopaedics, P.C., 425 First Avenue, Pittsburgh, PA 15219, USA. CHERRYWAY@POL.NET
Mark R
MR
Foster
Matthew J
MJ
Allen
Joanne E
JE
Schoonmaker
Hansen A
HA
Yuan
Atsunori
A
Kanazawa
Soo-An
SA
Park
Baowei
B
Liu
D000818:Animals; D001174:Arthrodesis / Q000379:methods; D001696:Biomechanical Phenomena; D001863:Bone Screws; D002356:Cartilage / Q000502:physiology; D005260:Female; D007103:Immobilization; D007593:Joint Instability / Q000000981:diagnostic imaging; D008159:Lumbar Vertebrae / Q000000981:diagnostic imaging / Q000502:physiology*; D023421:Models, Animal; D010012:Osteogenesis; D011859:Radiography; D012756:Sheep; D013997:Time Factors