Within Patient Radiological Comparative Analysis of the Performance of Two Bone Graft Extenders Utilized in Posterolateral Lumbar Fusion: A Retrospective Case Series

Abstract

Two bone graft extenders differing in chemical composition were implanted contralaterally in 27 consecutive patients undergoing instrumented posterolateral lumbar fusion as standard-of-care. Bone marrow aspirate and autogenous bone graft were equally combined either with β-tricalcium phosphate (β-TCP) or a hybrid biomaterial [containing hyaluronic acid (HyA) but lacking a calcium salt] and implanted between the transverse processes. Fusion status on each side of the vertebrae was retrospectively graded (1-5 scale) on AP planar X-ray at multiple visits as available, through approximately 12 months. Additionally, consolidation or resorption since prior visit for each treatment was recorded. Sides receiving β-TCP extender showed marked resorption prior to bone consolidation during the first 6 months. By contrast, sides receiving the hybrid biomaterial containing integrated HyA showed rapid bone consolidation by week 6-8, with maintenance of initial bone volume through 12 months. Fusion grade was superior for the hybrid biomaterial, differing significantly from β-TCP at day 109 and beyond. Fusion success at >12 months was 92.9 vs. 67.9% for the hybrid biomaterial and β-TCP-treated sides, respectively. The hybrid biomaterial extender demonstrated a shortened time-to-fusion compared to the calcium-based graft. Mode of action has been demonstrated in the literature to differ between these compositions. Therefore, choice of synthetic biomaterial composition may significantly influence the mode of action of cellular events regulating appositional bone growth.

Keywords: biomaterial scaffold; bone graft extender; bone marrow aspirate; endochondral ossification; hyaluronic acid; spine fusion.

Figure 1

Examples of fusion grade definition.

Figure 2

Predicted mean differences with 95% confidence intervals. From day 109 forward, the predicted mean fusion grade (grade 4 probably fused/grade 5 definitely fused) for segment receiving InQu extender was found to be statistically significantly (p < 0.05) better than the predicted mean for those sides receiving β-TCP extender.

Figure 3

Kaplan–Meier analysis of time-to-fusion. The log-rank test of the homogeneity of the Kaplan–Meier curves showed that the time-to-fusion for the InQu-treated side at 206 days was significantly (p < 0.0309) shorter than the β-TCP curve at 318 days.

Figure 4

(A) InQu resorption/consolidation assessment by subject over time. Each line lies in the upper half of the graph demonstrating that there is no significant initial resorptive phase, only intervals of increasing consolidation over time. (B) β-TCP resorption/consolidation assessment by subject over time. A flat line demonstrates for most of the β-TCP-treated fusion segments between days 91–135 no change in radiographic appearance from surgery. Once the mineral content has been cleared, an increase in the height of the plotted lines (consolidation) was observed for one or more time intervals in 21 of 28 fusion segments. However, from day 135 and on, 7 of 28 fusion segments (25%) showed a steady decline in line height toward the negative portion of the graph, indicating that more resorption had occurred than consolidation.

Figure 5

Comparative radiography depicting resorption and consolidation of two bone graft extenders. A 46-year-old female with history of active tobacco use and diabetes underwent PL fusion of the lumbar spine at L2-3 and L5-S1. AP radiographs at L2-3 are shown over time. Left side received bone autograft, BMA, and β-TCP putty. Right side received equal mix of bone autograft, BMA, and InQu putty. Sequential assessment over 24 weeks illustrates marked resorption of ceramic bone graft, whereas the opposite side receiving hybrid extender showed consolidation of applied graft at 6 weeks, with sustained bone volume at 24 weeks.