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Biologic Options in Interbody Fusion
Introduction
Interbody fusion devices aim to provide anterior column support as bony fusion between adjacent vertebral bodies progresses. Irrespective of the material utilized for structural support, depending on exact surgical technique, supplemental graft material should be placed within and/or around the allograft or cages in order to achieve a solid fusion. Achieving a solid arthrodesis can be directly correlated to long-term clinical outcomes and the durability of the procedure. Spine biologics can aid in facilitating arthrodesis by altering the existing environment by enhancing specific cellular and molecular activity. As a result, the field of biologics has expanded rapidly over recent years to include not only autogenous bone graft but also allograft, demineralized bone matrix, ceramic carriers, recombinant growth factors, and tissue engineering therapies.
The ideal bone graft substitute possesses three distinct properties: osteogenesis, osteoconduction, and osteoinduction. Osteogenic grafts contain osteoprogenitor or osteogenic precursor cells capable of directly forming bone. Osteoinduction is the mechanism whereby these precursor cells are stimulated to differentiate into mature osteoblasts, whereas osteoconductive materials provide a biocompatible physical structure or scaffold that supports the formation of new bone ( Table 17.1 ). One additional feature of bone grafts, which is more commonly discussed in maxillofacial surgery, is that of osseointegration. This refers to an implant’s ability to bind to bone without any intervening tissue. Surgeons should assess the biologic requirements of the respective fusion site and select a bone graft strategy based on these properties.
TABLE 17.1
Review of the Osteoinductive, Osteoconductive, and Osteogenic Properties of Various Bone Graft Substitutes and Extenders
| Bone Graft | Osteoinductive | Osteoconductive | Osteogenic |
|---|---|---|---|
| Autograft | √ | √ | √ |
| Allograft | √ | ||
| DBM | √ | √ | |
| Ceramic | √ | ||
| rhBMP | √ | ||
| Platelet | √ | ||
| MSCs | √ |
DBM , demineralized bone matrix; MSCs, mesenchymal stem cells; rhBMP, recombinant human bone morphogenetic protein.
Autograft
Iliac crest autograft contains all three properties for bone formation and remains the gold standard for fusion procedures as it has a number of advantages. Depending on the procedure, it can be obtained anteriorly or posteriorly as well as through the same incision versus a separate incision. It is cost effective, readily available, and is biocompatible without risk of antigenicity. The main drawback of its use is related to donor site morbidity, which can include pain, paresthesias, hematoma, and infection with an incidence rate as high as 50% in some series. In one multicenter prospective study, Sasso et al. found that 31% of patients reported pain even at 2 years postoperatively, suggesting that the duration is not merely transient in many patients. Of note, there was no significant difference in pain scores when comparing posterior versus anterior harvest sites.
One alternative to iliac crest bone graft (ICBG) and its potential harvest-related complications is local autograft. This can be harvested from the spinous processes, lamina, and facets during both open and minimally invasive procedures. A systematic review of clinical studies demonstrated similar fusion rates when comparing local bone autograft with ICBG, 79% and 89%, respectively. One primary limitation of local autograft is the potential for volume constraints, particularly in single-level fusions. Sengupta et al. compared ICBG with local autograft in a retrospective review and found similar healing rates in one-level fusions; however, local bone autograft had a significantly lower fusion rate compared with ICBG in multilevel fusions, 20% vs. 66%, respectively ( P = .029). As a result, a number of bone graft extenders have subsequently been developed to remedy this volume-related limitation.
Allograft
Allograft, bone obtained from human donors, serves as an osteoconductive agent, providing a scaffold for bone formation. Allograft is processed and preserved through freeze-drying or freezing. The osteogenic potential of the graft is sacrificed as bone cells are eliminated during its processing which decreases the risk of disease transmission, antigenicity, and infection. Therefore, it is recommended that allograft should always be applied in conjunction with autograft or another osteoinductive agent in the lumbar spine. Nonetheless, allograft is incredibly versatile as it is available in multiple forms including powder, strips, bone chips, and cage-type formulations. Femoral ring allograft has historically been one of the most common materials utilized in anterior lumbar interbody fusions. Thalgott et al. compared freeze-dried with frozen allografts from a single manufacturer in a prospective, randomized study with a minimum follow-up of 24 months. Frozen grafts were cooled and stored at -70°C and dehydrated via lyophilization, whereas freeze-dried grafts were stored at room temperature. They found freeze-dried allografts more likely to break intraoperatively and were also more likely to require reoperation for pseudarthrosis ( P = .026). Over 85% of the revision surgeries were performed in the freeze-dried group; however, most of these patients were noted to be smokers. Despite this disparity, there was no difference in Oswestry disability index (ODI), Short Form-36 (SF-36), and pain scale scores.
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