Fusion in degenerative spondylolisthesis: comparison of osteoconductive and osteoinductive bone graft substitutes

The emergent widespread options of bone graft substitutes for spinal fusion procedures vary in their osteobiologic activity. A majority of current literature focuses on the comparison of osteoinductive (OI) or osteoconductive (OC) bone graft substitutes individually against ICBG. These studies have demonstrated the legitimacy of bone graft substitutes, but despite the widespread use in spinal fusion procedures there is a dearth in the current literature in the direct comparison of OC and OI substitutes. This retrospective comparative analysis compares the efficacy of OI vs. solely OC agents in producing radiographic fusion on patients with DS.

Patients, who underwent a lumbar fusion for DS with at least 6 months post-op radiographs, were divided based on whether they received an OC or OI bone graft substitute. The OC groups included allografts, calcium phosphate, ceramics and hydroxyapatite products. The OI group included bone morphogenic protein, demineralized bone matrix, and stem cell-based products.

Using a conservative hierarchical approach to determine fusion, fusion criteria included stringent use of multiple measurement methods including flexion/extension x-rays, Lenke and Brantigan CT fusion measurement criteria, and history of revision surgery due to pseudoarthrosis.

A total of 126 patients (78 OI, 48 OC) met the studies inclusion criteria for the assessment for fusion. The mean time for flexion-extension radiographic evaluation was 13.1 months for the OI group and 15 months for the OC group. The mean time for CT scan evaluation was 18 months for the OI group and 15.9 months for the OC group. Using the stated hierarchical criteria for fusion stated above, the fusion rate for the OI group was 87.18%, and the fusion rate for the OC group was 93.75%. The difference in OI and OC groups was not statistically significant (P = 0.367). Based on the demographic data collected, there were no statistically significant factors determining fusion.

With the vastly growing market for OI and OC materials commonly used in lumbar spinal fusions, the options for surgical treatment for degenerative spondylolisthesis are ever expanding. No significant difference was found when comparing fusion rates between the two types of materials in this retrospective analysis. Interestingly, TLIF procedures provided lower fusion rates than posterolateral fusion procedures. This may be due to a small sample size but the association with a minimally invasive technique warrants investigation. Due to the substantial difference in price between the OI and OC materials and the lack of evidence supporting higher fusion rates with more expensive OI agents, it is incumbent on the spine community to consider and reassess the products that are routinely used.