Local Tumor Necrosis Factor Alpha Exposure Inhibits Posterolateral Fusion Rates in a Rat Model

STUDY DESIGN: Randomized, preclinical animal study. OBJECTIVE: To determine whether exposure to TNF-α levels affects Mesenchymal stem cell (MSC) osteogenic differentiation and if brief, localized TNF-α at the fusion site affects posterolateral arthrodesis in a rat model. SUMMARY OF BACKGROUND DATA: Elevated tumor necrosis factor-α (TNF-α) has been shown in degenerative spine pathology, pseudarthrosis, and systemic inflammation. Yet the effect of local localized TNF-α exposure on subsequent arthrodesis remains incompletely understood. METHODS: MSCs were examined in vitro with TNF-α exposure. Bilateral posterolateral fusions at L4-L5 were performed on 35 Wistar Kyoto male rats. Demineralized bone matrix (DBM) was used for fusion in both groups, but the treatment group also received a low dose of TNF-α (20 uL of 50 ng/mL) on an absorbable collagen sponge. Animals sacrificed at day 2 and day 4 had the local fusion mass harvested and processed for cytokine analysis with ELISA. Serum was also collected by cardiac puncture. Animals sacrificed at 4 weeks were assessed by manual palpation and microCT for fusion, as well as serum cytokine levels with ELISA. RESULTS: TNF-α suppressed MSC osteogenesis in a dose-dependent manner, starting at 0.1 ng/mL. TNF-α protein rose within the fusion mass at postoperative day (POD) 2 in the treatment group compared to controls (P <0.05), while IL-1β levels remained unchanged. At 4 weeks, manual palpation demonstrated fusion in 4/5 control animals, and no fusion in any TNF-α-treated animal (P=0.048). Micro-CT and histological analysis revealed bilateral fusion in 4/5 controls compared with 1/5 TNF-α animals, with the remainder showing unilateral or no fusion (P = 0.286). CONCLUSIONS: A brief, localized increase in TNF-α levels after surgery significantly impairs posterolateral fusion in rats. Limiting early TNF-α activity at the fusion site may improve arthrodesis outcomes, particularly in patients with elevated inflammatory profiles.