Implant-related infection model in rat spine

The rate of postoperative infections is approximately 1% in spine surgery. However, when metal implants are used, postoperative infection rates significantly increase and were reported between 2.1 and 8.5%. This study aim to set up an infection model in the rat spine with a metal implant.

Forty white male Sprague Dawley rats were randomly divided in four groups. In all rats, under operation microscope, a 3 mm titanium microscrew was implanted in the thoracolumbar area (T10-L1) after laminar decortication. In Group I (control group), sterile isotonic solution and in other three groups, different concentrations of Staphylococcus aureus [Group II: (10²), Group III: (10³), Group IV: (10⁶)] were squirted on the decorticated lamina site. All animals were sacrificed after 2 weeks, and then blood cultures and cultures from fascia, muscle and bone were obtained. Bacterial number in each tissue was measured as colony-forming unit per gram tissue. Titanium microscrews were placed in 0.5 ml tryptic soy broth and vortexed than plated on trypticase soy agar to determine bacterial growth. Two animals from each group were subjected to histological examination.

Blood cultures obtained by intra-atrial puncture after 2 weeks were negative in all groups indicating no systemical infection developed. Bacterial cultures were negative in all specimens of Group I (control group). A significant osseous infection was confirmed in Groups II, III and IV. Comparison of bacterial counts in bone cultures showed no significant difference between Group III (10³ CFU/10 μl) and Group IV (10⁶ CFU/10 μl) (P > 0.05), while both groups had significantly higher counts than Group II (10² CFU/10 μl) (P > 0.05). Microscopic findings of supurrative inflammation were present only in Group IV (10⁶ CFU/10 μl).

This study shows that inoculation of S. aureus in 10⁶ CFU/10 μl concentration at the decorticated lamina after implantation of a titanium screw in rat spine is a reproducible model for spinal infection and can be used for the animal model of prophylaxis and treatment and of postoperative infection.