General Session: Innovative Technologies I - Hall F

Presented by: G.M. Williams

Author(s):

W.R. Walsh(1), R.A. Oliver(1), M.H. Pelletier(1), T. Wang(1), E.R. Walsh(1), J.M. Page(2), G.M. Williams(2)

(1) University of New South Wales, Prince of Wales Hospital, Surgical and Orthopaedic Research Laboratories, Sydney, NSW, Australia
(2) NuVasive, San Diego, CA, United States

Abstract

Introduction: Optimizing the microstructure of calcium phosphate (CaP) has emerged as a key strategy to enhance and control the osteogenic cellular response and resulting bone formation. This study evaluated the in vivo performance of a novel microstructured CaP material, characterized by micropores < 1µm, bound within an absorbent type I collagen matrix as both an autograft bone substitute and extender in an established model of posterolateral lumbar fusion (PLF) in rabbits. This new graft material (AttraX® Scaffold) was compared to a positive control of autologous iliac crest bone graft and two commercially available bone graft materials (AttraX Putty and Formagraft®) to assess their relative fusion performance as influenced by differences in binder composition and CaP microstructure.

Methods: 90 skeletally mature female NZ White rabbits were assigned to 5 groups for single level Boden PLF model following ethical approval. Groups included: 1) Autograft (2cc per side); 2) AttraX Scaffold + Autograft (1cc + 1cc); 3) AttraX Putty + Autograft (1cc + 1cc); 4) AttraX Scaffold (2cc); and 5) Formagraft (2cc). Endpoints included mechanical assessment of fusion (manual palpation and robotic range of motion testing), radiographic assessment (plain films, Faxitron, and microCT), histology and histomorphometry at 6, 9, and 12 weeks. Fusion rates were analyzed by Kruskal-Wallis tests, while ROM and histomorphometry were analyzed by ANOVA and post-hoc tests when appropriate.

Results: By radiography and histology, all groups demonstrated progressive new bone formation, remodeling, and partial resorption of graft materials within the fusion masses from 6 to 12 weeks. Bilateral fusion rates progressed with time and were equivalent among all groups at 6 and 12 weeks by palpation, radiographs, and microCT (Table 1). Range of motion was reduced in all groups from time zero supporting other fusion assessments. Notably, AttraX Scaffold (Groups 2 & 4) had significantly higher fusion rates by microCT at 9 weeks (p< 0.05) and greater bone tissue by histomorphometry at 12 weeks (p< 0.01) compared to Formagraft. No adverse effects were noted with any of the graft materials.

Discussion: The microstructured CaP grafts performed in a similar manner regardless of the polymeric binder composition (collagen versus alkylene oxide copolymer) and were equivalent to the autograft positive control by measures of radiographic and mechanical fusion. Fusion outcomes were similar for AttraX Scaffold when used as either an autograft substitute or extender. Moreover, compared to non-microstructured CaP-collagen (Formagraft), the microstructured CaP-collagen (AttraX Scaffold) showed evidence of earlier fusion and faster bone formation as indicated by microCT and histomorphometry. This study provides comparative performance data of microstructured and non-microstructured CaP biomaterials in a translational model of posterolateral spine fusion. The results offer evidence that optimized microstructure is an important driver of the biological response of fusion.

Table 1: Bilateral fusion rates at 6, 9, and 12 wk