Cellular Bone Matrices Reconsidered

Cellular bone matrices (CBMs) are a biologic admixture of minimally processed allogeneic cellular constituents (usually mesenchymal stem cells [MSCs]) with processed allogeneic bone (often demineralized bone matrix [DBM] and lyophilized cancellous bone)1. In commercial form, CBMs include, and can also be referred to as, cellular bone allografts (CBAs). CBMs constitute about 21% of all bone graft materials utilized in spine surgery and account for about $300 million of the $1.4 billion in spine osteobiologic sales in the U.S. alone in recent years2,3. Advocates for the use of CBMs contend that this biologic admixture provides (qualitatively) all of the bone-forming properties found in autologous cancellous bone graft (i.e., osteoconduction, osteoinduction, and osteogenesis). Given the heterogeneity of various study parameters and the absence of a high-quality study with sufficient power to draw conclusions regarding the relative effectiveness of various bone graft materials, we are left with inferring the relative cost-effectiveness or value of various bone graft materials on the basis of limited evidence. CBMs are associated with substantially higher costs compared with some acellular bone-graft products4. The estimated cost of CBM per cubic centimeter is ∼$525 compared with ∼$17 for cancellous bone chips and ∼$151 for DBM1. CBMs are processed aseptically, as they cannot be sterilized because of the presence of living cells contained within. The cellular element is separated from other marrow cellular elements and then cryopreserved. CBMs have been marketed under a variety of trade names, including SygnaCel Cellular Bone Matrix (Medtronic), Osteocel Plus (Globus Medical/NuVasive biologics), Osteocel Pro (Globus Medical/NuVasive biologics), Osteocel Max (Globus Medical/NuVasive biologics), OsteoVive Plus (Xtant Medical), Trinity Evolution (Orthofix), Trinity Elite (Orthofix), Via Graft (VIVEX Biologics), ViviGen (DePuy Synthes), ViviGen Formable (DePuy Synthes), BIO4 Viable Bone Matrix (Stryker), PrimaGen Advanced Allograft (Highridge Medical), map3 Cellular Allogeneic Bone Graft (RTI Surgical), V92 and V92-FC (Paragon 28/Vivex Biologics), SCYLLA and SCYLLA-F (OrthoAxis), and Magnus Viable Cellular Allograft (Royal Biologics)5. Harvesting, segregating, preserving, and testing the live cells (MSCs or lineage-committed bone cells) incurs additional production expenses over acellular allograft alternatives, and, as a result, CBMs are priced at a premium5. Promoters and advocates of CBMs often contend that these allogeneic grafts are immunologically privileged. But what does the literature indicate? Schu et al. reviewed the immunogenicity of allogeneic mesenchymal stem cells. They stated that there was evidence of immune blunting with use of MSCs in vitro, but, in vivo, MSCs activate T-cell-mediated binding in the presence of inflammatory cytokines. Further, in both non-human primates and rats, allogeneic MSCs induced antibody formation6. In vivo, allogeneic MSCs can feature both anti-inflammatory and proinflammatory phenotypes, contingent on the microenvironment. Experimental evidence demonstrates that the presence of inflammatory cytokines (certain concentrations of interferon-gamma [IFN -γ] and tumor necrosis factors [TNF-α and TNF-β]) elicit the MSC’s proinflammatory phenotype, enhancing immunogenicity and diminishing immunosuppression. Allogeneic MSCs elicit both humoral and cellular immunity in an immunocompetent host7,8. Additionally, activated natural killer cells are known to specifically target cells with low or absent major histocompatibility complex (MHC) class-I T-cell epitopes, such as MSCs, and kill them8. The potential for “anti-donor memory reaction” with subsequent exposure to human leukocyte antigen (HLA) mismatch is a theoretical risk7,9. Substantiating this concern are the findings of a systematic review of 8 prospective cohort studies on immune sensitization following allogeneic bone grafting in humans: analysis of bone-soluble protein demonstrated sensitization in 55% of the patients who had received allograft bone9. Although the preparation of CBMs reduces the natural immunogenicity of constituent allogeneic bone and its cellular elements, these findings are of sufficient relevance to raise an additional concern regarding CBM use, particularly in a repeated manner in the same recipient. Second, we know that MSCs do not directly produce bone. Bone is derived from osteoblasts differentiated from MSCs, after further differentiation of the MSCs into committed preosteoblasts. The known interactions between the body’s immune cells, inflammatory cytokines, MSCs, and osteoblasts and osteoclasts are sufficiently complex to defy simple characterization through in vitro testing alone. B lymphocytes promote osteoclast differentiation and maturation, whereas osteoclasts exert inhibitory feedback on B-cell activity. Different subsets of T lymphocytes regulate osteoclasts in distinct ways. T cells exert bidirectional regulatory effects on osteoblasts, whereas B cells have relatively limited influence on osteoblast function. Conversely, osteoblasts promote the maturation and differentiation of both T cells and B cells10. Osteogenic differentiation of MSCs results in increased expression of MHC class-II molecules and the costimulatory molecules, as well as decreased expression of the coinhibitory molecules. Monocytes and allogeneic T cell and B cell-mediated immune responses resulted at the cellular and systemic levels8. Although specific in vitro testing (e.g., leukocyte transformation testing) of a specific CBM having lineage-committed precursor bone cells (sometimes referred to as a viable bone allograft) has shown a relative absence of a cellular immune response11, these allogeneic cells are not known to be immunologically privileged in vivo. Osteoblasts naturally express MHC-I molecules, and, in the presence of inflammatory cytokines, they can both upregulate MHC class-I and induce the expression of MHC class-II molecules12,13. Therefore, because of immune responses, one would not reasonably expect MSCs or lineage-committed precursor bone cells to be able to directly or indirectly osteoproliferate in vivo in an immunocompetent host. Given the concerns regarding escalating health-care expenditures and the gatekeepers that scrutinize surgical product introduction and procurement (e.g., value analysis committees), one would imagine that there is reasonable scientific and clinical evidence to support the use of CBMs. One would also expect the surgeons themselves to have investigated the claims associated with these products when deciding to use them over other bone-graft materials (e.g., autograft, DBM, allogeneic bone, bone morphogenetic protein [BMP]-2, etc.). However, there is little or no published experimental evidence supporting the notion that allogeneic MSCs provide enhanced bone healing beyond that associated with carrier DBM and/or lyophilized allogeneic cancellous bone. Cuomo et al. utilized a critical bone defect in an athymic rat model to test the value of adding MSCs to DBM. The authors found no reliable healing effect in critical-sized bone defect with use of bone marrow aspirate or mesenchymal stem cell-enriched bone marrow aspirate mixed with demineralized bone matrix14. Similarly, Abedi et al., in another athymic rat test model, found no difference in inter-transverse process fusion rates when 2 different commercially available CBM preparations were utilized in the “vital” and “devitalized” states15. McAnany et al. found a lower rate of fusion at 1 year in 1- and 2-level cervical fusions when comparing patients fused with CBMs (87.7%) versus bone allograft alone (94.7%)2. In several systematic reviews that examined the efficacy of MSCs and CBMs with synthetic or allograft extenders, there was a lack of enhanced benefit (over allogeneic bone-graft carriers)1,16–18. Lastly, disease transmission must be considered when evaluating the merits of utilizing a nonsterile allogeneic cell or tissue graft. Two separate tuberculosis (TB) transmission clusters resulting from the use of MSCs in spine surgery occurred in the U.S., in 2021 and 2023. One involved 113 patients and the other, 36 patients. As of December 20, 2023, laboratory-confirmed tuberculosis had been identified in 5 of 36 recipients of surgical bone allografts, and 2 affected patients died from the infection. Whole-genome sequencing showed that positive Mycobacterium tuberculosis infection isolates from these patients were found to be genetically consistent with organisms isolated from common graft lot material19. The global bone graft and bone-graft substitute market is predicted by some analysts to reach $5.3 billion by 203220. Health-care costs are outpacing general inflation, and health insurance premiums have increased by 52% in the last 10 years21. With all of the emphasis on value-based surgical care and the imperative for clinicians to practice evidence-based medicine, it would be incumbent on surgeon utilizers to reconsider the use of CBMs. Given the experimental and clinical evidence available to date, the premium costs, and the potential for disease transmission, it is challenging to justify the use of CBMs as a bone-graft material. Although the intent of the present article is to shed light on the value proposition of CBMs, it is my opinion that current experimental and clinical evidence does not support the use of CBMs over acellular DBMs with lyophilized cadaveric-sourced cancellous bone (the primary bone carrier constituents of CBMs). If donor-graft osteogenesis is desired, autogenous cancellous bone remains the “gold standard.” There are a variety of minimally invasive surgery autologous graft-harvest tools and methods that avoid or mitigate historical open harvest-related complications. In addition, equating “local autograft” bone—usually an admixture of cortical and cancellous bone, often with fibrous tissue impurities—with pure cancellous autograft is not substantiated. Experimental evidence in athymic rats comparing human-sourced “local bone” to iliac crest-derived cancellous bone demonstrated significant differences in their respective capacity for osteoproliferation22.