Biologic Options in Interbody Fusion

17

Biologic Options in Interbody Fusion MARCO C. MENDOZA, BRETT D. ROSENTHAL, AND WELLINGTON K. HSU

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.1 Surgeons should assess the biologic requirements of the respective fusion site and select a bone graft strategy based on these properties.

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.2 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.3

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.4 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).5 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 TABLE Review of the Osteoinductive, Osteoconductive, 17.1 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.

145

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SE C T I O N 4    Adjunct instrumentation in Lumbar Interbody Fusion

spine. Nonetheless, allograft is incredibly versatile as it is available in multiple forms including powder, strips, bone chips, and cagetype 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.6 

Demineralized Bone Matrix Demineralized bone matrix (DBM) is a bone graft extender developed from human cadaveric bone by means of acid extraction. This process results in a matrix containing a type I collagen framework in addition to a number of growth factors such as bone morphogenetic proteins, transforming growth factor-β (TGF-β), insulin-like growth factor, and fibroblast growth factor. These components make DBM both osteoinductive and osteoconductive. However, DBM comes in a variety of formulations with over 50 commercial DBM products available for use in the lumbar spine. Bae et al.7 evaluated the quantity of bone morphogenetic protein (BMP) in different products in addition to the variability of BMP in varying lots from the same manufacturer. Utilizing enzyme-linked immunosorbent assay, they identified BMP-2, BMP-4, and BMP-7. Both BMP-2 and BMP-7 were found in all DBM products in low concentrations (∼20 to 200 ng/g); however, BMP-4 was not detected in all samples. Additionally, the variability of BMP concentrations among different lots of the same DBM formulation was higher than the variability of concentrations among different DBM formulations. Despite the questionable reliability of providing consistent osteoinduction, evidence in the literature supports the use of DBM as a bone graft extender in posterolateral lumbar spine fusion surgery.8 However, it should be noted that DBM lacks the structural stability needed for lumbar interbody fusion procedures and, in this setting, should be used in combination with a structural spacer. Thalgott et  al. reported a 96% fusion rate in their series of 50 patients who underwent anterior lumbar interbody fusion with the use of titanium mesh cages, coralline hydroxyapatite, and DBM as part of a circumferential fusion.9 

Ceramics Ceramic-based bone grafts are a type of synthetic graft with osteoconductive properties, supporting new bone ingrowth but lack any osteoinductive potential. They convey numerous advantages including nearly unlimited supply, easy sterilization, and lack immunogenicity as they are biologically inert and generally do not induce an inflammatory response. Drawbacks include their brittle structure and low tensile strength obviating the need for protection from excessive force until a solid fusion has occurred. Calcium sulfate is resorbed in only a few weeks after implantation and therefore should not be used as a scaffold in lumbar fusion surgery. Ideally, the scaffold should facilitate bony ingrowth and then resorb as the fusion develops. As a result,

more commonly used compounds include β-tricalcium phosphate and hydroxyapatite. β-tricalcium phosphate is resorbed over a period of months, making it more suitable for use in lumbar spine fusions, whereas hydroxyapatite is resorbed over the course of years.10 In a recent systematic review with a collective population of 1332 patients, ceramics demonstrated an overall fusion rate in the lumbar spine of 86.4%. All interbody fusion studies reviewed included posterior instrumentation and were subsequently grouped together as circumferential fusions, which included anterior, posterior, and transforaminal techniques. The overall fusion rate for interbody fusions was not statistically different from the posterolateral technique, 88.8% versus 85.6%, respectively (P = .64).11 This review suggests that ceramic-based scaffolds are an effective bone graft extender in each of these techniques; however, variability in assessing fusion status and the number of included patients may have led to the lack of difference between the two groups. 

Bone Morphogenetic Proteins Bone morphogenetic proteins belong to the TGF-β superfamily of growth factors. They act through serine-threonine kinase receptors and transduce their signal via the SMAD pathway12 (Fig. 17.1). This subsequently results in the induction of bone formation through the differentiation, maturation, and proliferation of mesenchymal precursor cells into osteogenic cells. More than 20 types are described; however, only 2 commercial forms are available for clinical use: recombinant human bone morphogenetic protein-2 (rhBMP-2; Infuse) and rhBMP-7 (OP-1). The family of proteins were first discovered by Dr. Marshall Urist in 1965, but not until 2002 did the US Food and Drug Administration (FDA) approve their utilization clinically. Specifically, the use of rhBMP-2 is currently approved as a component of a titanium cage for anterior lumbar interbody fusion. Despite this single FDA-approved indication, rhBMP-2 is frequently used for a number of off-label applications, including posterolateral spine fusions, posterior lumbar interbody fusions, transforaminal lumber interbody fusions, and cervical spine procedures. Only two commercial forms of recombinant BMP are available for clinical use: rhBMP-2 (Infuse) (Medtronic, Memphis, TN) and rhBMP-7 (OP-1) (Stryker, Kalamazoo, MI). These proteins are water-soluble and are rapidly diffused from the surgical site when used independently. As a result, matrix carriers or scaffolds are required to decrease diffusion away from the desired site of application. The most common carrier currently utilized is a type-1 absorbable collagen sponge. It is deformable and can be easily inserted into a cage for interbody fusions. In 2002, Burkus et al. reported their results in an FDA-regulated, multicenter prospective randomized study of 279 patients who underwent anterior lumbar interbody fusion using two tapered titanium threaded fusion cages.13 The rhBMP-2 group showed a statistically significant improvement in clinical outcomes, including back and leg pain scores as well as the ODI. Additionally, 32% of patients in the ICBG group reported graft site discomfort at 2-year follow-up. In a systematic review, Galimberti et al. also echoed favorable results for rhBMP-2 in anterior lumbar interbody fusion procedures, showing a significant improvement in fusion rates. However, they did not find any statistically significant improvement in fusion rates in posterior lumbar interbody fusion and transforaminal lumbar interbody fusion procedures. It should be noted that these conclusions are limited by the heterogeneity of rhBMP-2 dosing and varying levels of evidence.14

CHAPTER 17  Biologic Options in Interbody Fusion

BMP-2/4 Group BMP-2 BMP-4

BMPR-I Group BMPR-IA BMPR-IB

OP-1 Group BMP-5 BMP-7 BMP-6 BMP-8A BMP-8B

ALK-1 Group ALK-2

BMPR-II

Smad1 Smad5

ActR-II

Smad4

Smad8

ALK-1 Group

ActR-IIB

BMP-9 Group BMP-9 BMP-10

ALK-1 ALK-2 BMPR-I Group

GDF-5 Group GDF-7 GDF-6 GDF-5

BMPR-IB

Ligands

Type II receptor

A BMP ligands

147

Type I receptor

R-Smad

co-Smad

CD44

RGM

Endofin GS domain kinase domain

P

R-Smad Type I Type II receptor receptor

P

P P

co-Smad

coactivators

DNA binding proteins

B • Fig. 17.1 Signal Transduction by BMP receptors and Smads12. A. Relationships between serine-threonine kinase receptors (type I and type II) and Smad proteins in signal transduction. B. Signaling from bone morphogenetic protein (BMP) receptors at the plasma membrane to the nucleus by Smads. ActR, activin receptor; ALK, activin receptor-like kinase; BMPR, bone morphogenetic protein receptor; RGM, receptor guidance molecule.

Early evidence demonstrated encouraging results, with increased arthrodesis rates and decreased reoperation rates when using rhBMP2. This initial enthusiasm was subsequently tempered by a growing body of evidence elucidating potential adverse effects, including prevertebral swelling, hematoma formation, and increased cancer risk in patient exposed to rhBMP-2. Other described complications included radiculitis, heterotopic ossification, osteolysis, seroma, and retrograde ejaculation.15 This ultimately led to an industry-sponsored clinical trial (the Yale Open Data Access project), which was led by two research groups at the University of York and Oregon Health and Science University.16 Both groups concluded that rBMP-2 did result in high fusion rates in lumbar spine cases but demonstrated no significant difference in back pain and leg scores when compared to ICBG.

Currently, the cost-effectiveness and clinical indications for which BMP is most applicable are controversial. Identification of the proper carrier for various clinical scenarios is essential to reduce expenditures and potentially complications of recombinant proteins. Future investigations are currently underway in an effort to reduce the concentration of growth factors required for arthrodesis by improving carrier properties. Peptide amphiphile (PA) molecules, one promising alternative, are composed of nanofiber structures that mimic extracellular filaments and display biologic cues for cellular regeneration on its surface. Lee et al. developed a novel approach whereby a PA system was able to bind both endogenous and exogenous BMP-2. Using this technology in a rat model, PA with a binding affinity specific for BMP-2 led

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SE C T I O N 4    Adjunct instrumentation in Lumbar Interbody Fusion

to successful fusion rates at a dose 10-fold lower than the required therapeutic dose. Moreover, a fusion rate of 42% was achieved even without the use of exogenous BMP-2.17 

Platelet Concentrates The release of growth factors, including platelet-derived growth factor (PDGF) and TGF-β, promote the differentiation and proliferation of mesenchymal stem cells, which can in turn enhance bone healing. As a result, methods have been developed to increase the concentrations of these growth factors from a patient’s blood into autogenous growth factor (AGF) concentrates. AGFs are typically combined with thrombin and either ICBG, local autograft, or allograft. The osteoconductive capacity and lack of immunogenicity of AGFs make them an attractive option; however, harvesting does require a preoperative blood draw followed by processing, which potentially increases operative time. Hee et  al. performed a prospective study evaluating the efficacy of AGF in instrumented transforaminal lumber interbody fusions. They found no significant difference in pseudarthrosis rates between AGF/ICBG versus ICBG alone, 4% and 6%, respectively. However, they did note that AGF may facilitate faster fusions because 96% of patients in the AGF group demonstrated bony consolidation on radiographs at 6 months compared with 64% of patients in the control group (P <.05).18 Based on the current evidence, the addition of platelet concentrates to autograft for posterolateral and interbody spine fusion does not appear to add any benefit to fusion rates. Furthermore, there are limited data regarding the necessary concentration needed for platelet gels to have an osteogenic effect.19 

Mesenchymal Stem Cells Mesenchymal stem cells (MSCs) are pluripotent stem cells that have the capacity to differentiate into multiple cell lines. In the proper environment, MSCs can differentiate into osteoblasts and promote lumbar spine fusion. MSCs can be found in bone marrow, periosteum, muscle, and adipose tissue. Autologous bone marrow aspirates (BMAs) can be harvested from the vertebral body or iliac crest. BMA lacks structural support, obviating the need for it to be combined with an appropriate carrier such as ceramics or allograft. Neen et al. evaluated the efficacy of bone marrow aspirate on a collagen/hyaluronic acid (HA) matrix compared with autograft in anterior and posterolateral spine fusions. They found equivalent fusion rates when the posterolateral approach was used, but noted BMA to be inferior to autograft for anterior interbody fusions.20 Overall, as described by Khashan et al. in a systematic review, limited evidence exists to support the use of BMA or MSCs with synthetic or allograft materials as a substitute or as a supplement to autologous bone graft.21 In contrast to autogenous stem cells, data regarding the use of allogeneic mesenchymal precursor cells (MPCs) appear promising. Despite techniques that attempt to concentrate the cell component of bone marrow, producing adequate quantities of progenitor cells remains problematic. Recent advances in immunoselection have led to the isolation and purification of MPCs from donor bone marrow. In a recent ovine study,

TABLE 17.2 Bone Graft Advantages/Disadvantages

Bone Graft

Advantages

Disadvantages

Autograft

Gold standard, biocompatible

Donor site morbidity, volume limitations

Allograft

Abundant supply, decreased antigenicity, multiple forms

Decreased osteogenic activity

DBM

Multiple forms, osteoconductive

Variable composition

rhBMP

Increased fusion rates, availability, osteoinductive

Cost, seroma, radiculitis, heterotopic ossification, osteolysis

Ceramics

Availability, lack of immunogenicity, compressive resistance

Potential brittle handling properties with low tensile strength, varying resorption

MSC

Osteogenic, easily accessible

No structural support, variability in bone marrow cellularity

Platelets

lack of immunogenicity, easily accessible

Preoperative blood draw, unclear dosing response for osteogenesis

DBM, demineralized bone matrix; MSCs, mesenchymal stem cells; rhBMP, recombinant human bone morphogenetic protein.

  

Wheeler et  al. evaluated the fusion capacity of allogeneic MPCs delivered on an osteoconductive scaffold.22 Results from manual palpation, functional radiographs, x-ray, computed tomography, and fusion site histologic analysis indicated that fusion success using MPCs did not differ from that achieved using iliac crest autograft. Additionally, regardless of dose, there was no evidence of adverse systemic or local responses. Future studies may prove this to be a viable option in spinal arthrodesis. 

Conclusion A successful spine fusion is a multifactorial process dependent on the type of bone graft selected in addition to a number of local and systemic factors that affect the healing response. Although autograft iliac crest is considered the gold standard, potential complications led to the development of a number of alternatives, each with their own advantages and disadvantages (Table 17.2). The efficacy of these materials varies widely in the literature and should be critically evaluated prior to implantation. Despite the potential added cost23 and side effects (Table 17.3), bone graft substitutes may have a role in facilitating bony union, particularly in more stringent biologic environments such as smokers and osteoporotics. It is critical for surgeons to assess the host biologic environment and ensure that all necessary elements are present to promote bone healing.

CHAPTER 17  Biologic Options in Interbody Fusion

TABLE 17.3 Costs and Side Effects of Various Bone Grafts

Biologic

Cost

Allograft

Adverse Effects Infection, disease transmission, host rejection

Cancellous allograft (chips/freezedried)

$376/30 mL

Cortical allograft (femoral shaft)

$530–$1,681/3– 20 cm

DBM (Grafton/ Allomatrix)

$726–$1,225/10 mL

Ceramics

Host rejection, variable composition Varying resorption, potential brittle handling, low tensile strength, fracture

Calcium Sulfate (Osteoset)

$655/10 mL

Calcium Phosphate (CopiOs)

$1520/10 mL

Tricalcium Phosphate (Vitoss, Orthovita)

$875/10 mL

rhBMPs (Infuse)

$3500–$4900, small-large

Ectopic bone formation, allergic reaction, prevertebral swelling, possible oncogenicity, retrograde ejaculation, seroma, osteolysis

DBM, Demineralized bone matrix; rhBMPs, recombinant human bone morphogenetic protein.

  

References 1. Branemark PI. Osseointegration and its experimental background. J Prosthet Dent. 1983;50:399–410. 2. Silber JS, Anderson DG, Daffner SD, et  al. Donor site morbidity after anterior iliac crest bone harvest for single-level anterior cervical discectomy and fusion. Spine (Phila Pa 1976). 2003;28:134–139. 3. Sasso RC, LeHuec JC, Shaffrey C, et  al. Iliac crest bone graft donor site pain after anterior lumbar interbody fusion: a prospective patient satisfaction outcome assessment. J Spinal Disord Tech. 2005;18(suppl):S77–S81. 4. Hsu WK, Nickoli MS, Wang JC, et al. Improving the clinical evidence of bone graft substitute technology in lumbar spine surgery. Global Spine J. 2012;2:239–248. 5.  Sengupta DK, Truumees E, Patel CK, et  al. Outcome of local bone versus autogenous iliac crest bone graft in the instrumented posterolateral fusion of the lumbar spine. Spine (Phila Pa 1976). 2006;31:985–991.

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6. Thalgott JS, Fogarty ME, Giuffre JM, et al. A prospective, randomized, blinded, single-site study to evaluate the clinical and radiographic differences between frozen and freeze-dried allograft when used as part of a circumferential anterior lumbar interbody fusion procedure. Spine (Phila Pa 1976). 2009;34:1251–1256. 7. Bae HW, Zhao L, Kanim LE, et  al. Intervariability and intravariability of bone morphogenetic proteins in commercially available demineralized bone matrix products. Spine (Phila Pa 1976). 2006;31:1299–1306; discussion 1307–1308. 8. Kang J, An H, Hilibrand A, et al. Grafton and local bone have comparable outcomes to iliac crest bone in instrumented single-level lumbar fusions. Spine (Phila Pa 1976). 2012;37:1083–1091. 9. Thalgott JS, Giuffre JM, Klezl Z, et al. Anterior lumbar interbody fusion with titanium mesh cages, coralline hydroxyapatite, and demineralized bone matrix as part of a circumferential fusion. Spine J. 2002;2:63–69. 10. Jamali A, Hilpert A, Debes J, et al. Hydroxyapatite/calcium carbonate (HA/CC) vs. plaster of Paris: a histomorphometric and radiographic study in a rabbit tibial defect model. Calcif Tissue Int. 2002;71: 172–178. 11. Nickoli MS, Hsu WK. Ceramic-based bone grafts as a bone grafts extender for lumbar spine arthrodesis: a systematic review. Global Spine J. 2014;4:211–216. 12. Miyazono K, Kamiya Y, Morikawa M. Bone morphogenetic protein receptors and signal transduction. J Biochem. 2010;147:35–51. 13. Burkus JK, Gornet MF, Dickman CA, et al. Anterior lumbar interbody fusion using rhBMP-2 with tapered interbody cages. J Spinal Disord Tech. 2002;15:337–349. 14. Galimberti F, Lubelski D, Healy AT, et al. A systematic review of lumbar fusion rates with and without the use of rhBMP-2. Spine (Phila Pa 1976). 2015;40:1132–1139. 15. Tannoury CA, An HS. Complications with the use of bone morphogenetic protein 2 (BMP-2) in spine surgery. Spine J. 2014;14: 552–559. 16. Fu R, Selph S, McDonagh M, et  al. Effectiveness and harms of recombinant human bone morphogenetic protein-2 in spine fusion: a systematic review and meta-analysis. Ann Intern Med. 2013;158: 890–902. 17. Lee SS, Hsu EL, Mendoza M, et al. Gel scaffolds of BMP-2-binding peptide amphiphile nanofibers for spinal arthrodesis. Adv Healthc Mater. 2015;4:131–141. 18. Hee HT, Majd ME, Holt RT, et  al. Do autologous growth factors enhance transforaminal lumbar interbody fusion? Eur Spine J. 2003;12:400–407. 19. Padilla S, Orive G, Sanchez M, et al. Platelet-rich plasma in orthopaedic applications: evidence-based recommendations for treatment. J Am Acad Orthop Surg. 2014;22:469–470. 20. Neen D, Noyes D, Shaw M, et al. Healos and bone marrow aspirate used for lumbar spine fusion: a case controlled study comparing healos with autograft. Spine (Phila Pa 1976). 2006;31:E636–E640. 21. Khashan M, Inoue S, Berven SH. Cell based therapies as compared to autologous bone grafts for spinal arthrodesis. Spine (Phila Pa 1976). 2013;38:1885–1891. 22. Wheeler DL, Fredericks DC, Dryer RF, et al. Allogeneic mesenchymal precursor cells (MPCs) combined with an osteoconductive scaffold to promote lumbar interbody spine fusion in an ovine model. Spine J. 2016;16:389–399. 23. Roberts TT, Rosenbaum AJ. Bone grafts, bone substitutes and orthobiologics: the bridge between basic science and clinical advancements in fracture healing. Organogenesis. 2012;8:114–124.