Abstract
BackgroundThe challenging biological and mechanical environment of posterolateral fusion (PLF) requires a carrier that spans the transverse processes and resists the compressive forces of the posterior musculature. The less traumatic posterolateral approach enabled by minimally invasive surgical techniques has prompted investigations into alternative rhBMP-2 carriers that are injectable, settable, and compression-resistant. In this pilot study, we investigated injectable low-viscosity (LV) polymer/composite bone grafts as compression-resistant carriers for rhBMP-2 in a single-level rabbit PLF model.MethodsLV grafts were augmented with ceramic microparticles: (1) hydrolytically degradable bioactive glass (BG), or (2) cell-degradable 85% β-tricalcium phosphate/15% hydroxyapatite (CM). Material properties, such as pore size, viscosity, working time, and bulk modulus upon curing, were measured for each LV polymer/ceramic material. An in vivo model of posterolateral fusion in a rabbit was used to assess the grafts’ capability to encourage spinal fusion.ResultsThese materials maintained a working time between 9.6 and 10.3 min, with a final bulk modulus between 1.2 and 3.1 MPa. The LV polymer/composite bone grafts released 55% of their rhBMP-2 over a 14-day period. As assessed by manual palpation in vivo, fusion was achieved in all (n = 3) animals treated with LV/BG or LV/CM carriers incorporating 430 μg rhBMP-2/ml. Images of μCT and histological sections revealed evidence of bone fusion near the transverse processes.ConclusionThis study highlights the potential of LV grafts as injectable and compression-resistant rhBMP-2 carriers for posterolateral spinal fusion.
Highlights
The challenging biological and mechanical environment of posterolateral fusion (PLF) requires a carrier that spans the transverse processes and resists the compressive forces of the posterior musculature
Material preparation and characterization The Low-viscosity poly(ester urethane) (LV-PUR)/ceramic composite grafts were prepared in a multistep process: (1) preparation of a polyester triol which is the foundation of the LV-PUR, (2) surface modification of the ceramic particles, and (3) combination of the LV-PUR, the ceramic component, and a polymerization catalyst to form a composite graft
While the absorbable collagen sponge (ACS) carrier retains recombinant human bone morphogenetic protein-2 (rhBMP-2) at the site of placement for 2 weeks [28], the ACS carrier is ineffective for PLF at clinically relevant doses due to soft tissue compression of the collagen [29]
Summary
The challenging biological and mechanical environment of posterolateral fusion (PLF) requires a carrier that spans the transverse processes and resists the compressive forces of the posterior musculature. The less traumatic posterolateral approach enabled by minimally invasive surgical techniques has prompted investigations into alternative rhBMP-2 carriers that are injectable, settable, and compression-resistant. In this pilot study, we investigated injectable low-viscosity (LV) polymer/composite bone grafts as compression-resistant carriers for rhBMP-2 in a single-level rabbit PLF model. Use of rhBMP-2-loaded ACS in a posterolateral fusion procedure is more challenging than interbody fusion due to the less favorable biological and mechanical conditions for bone healing [5]. Recent studies involving combining the rhBMP-2/ACS with a bulking agent, such as calcium phosphate granules, show improved posterolateral fusion rates in a non-human primate model compared to rhBMP-2/ACS alone, revealing the utility of a compression-resistant rhBMP2 carrier [5,6,7,8,9,10]
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