Dose-dependent effects of topical vancomycin and teicoplanin on fusion mass formation in a rat posterolateral lumbar arthrodesis model

Purpose Topical glycopeptide antibiotics are widely used during spinal fusion procedures to reduce the risk of surgical site infection; however, their dose-dependent effects on fusion biology remain unclear. This experimental study aimed to compare the effects of locally applied vancomycin and teicoplanin at different doses on fusion mass formation in a rat posterolateral lumbar arthrodesis model. Methods Forty-five female Wistar rats underwent unilateral L4–L5 posterolateral fusion and were randomized into five groups (n = 9 each): control (no topical antibiotic), low-dose vancomycin (14.3 mg/kg), high-dose vancomycin (71.5 mg/kg), low-dose teicoplanin (15 mg/kg), and high-dose teicoplanin (50 mg/kg). Autologous iliac crest bone graft was combined with the assigned antibiotic solution and placed onto the decorticated fusion bed. Fusion was evaluated at 8 weeks using micro-computed tomography (fusion mass volume), manual palpation (fusion score), and qualitative histological assessment (hematoxylin–eosin and Masson’s trichrome staining). Results Micro-CT fusion scores did not differ significantly among groups (p = 0.106). In contrast, fusion mass volume showed a significant intergroup difference (p = 0.018). The high-dose vancomycin group demonstrated the lowest fusion mass volume (median 7.3 mm³ [4.4–14.9]), which was significantly reduced compared with the control group (13.6 mm³ [10.3–19.7]) and both teicoplanin groups (median range 13.1–13.7 mm³; post hoc p ≤ 0.007). Manual palpation scores showed a non-significant trend toward lower values in the high-dose vancomycin group (p = 0.073). Histologically, fusion masses in the high-dose vancomycin group appeared less organized, whereas both teicoplanin groups demonstrated bone morphology comparable to controls. Conclusion In this rat posterolateral fusion model, high-dose topical vancomycin was associated with a reduction in fusion mass volume, despite similar fusion scores. In contrast, topical teicoplanin at both low and high doses did not adversely affect fusion mass formation. These findings highlight the importance of dose selection when using topical vancomycin and indicate that teicoplanin may be considered a bone-compatible alternative for local application during spinal fusion.