Transient Local Bone Remodeling Effects of rhBMP-2 in an Ovine Interbody Spine Fusion Model.

BACKGROUND: Recombinant human bone morphogenetic protein-2 (rhBMP-2) is a powerful osteoinductive morphogen capable of stimulating the migration of mesenchymal stem cells (MSCs) to the site of implantation and inducing the proliferation and differentiation of these MSCs into osteoblasts. Vertebral end-plate and vertebral body resorption has been reported after interbody fusion with high doses of rhBMP-2. In this study, we investigated the effects of 2 rhBMP-2 doses on peri-implant bone resorption and bone remodeling at 7 time points in an end-plate-sparing ovine interbody fusion model. METHODS: Twenty-one female sheep underwent an end-plate-sparing discectomy followed by interbody fusion at L2-L3 and L4-L5 using a custom polyetheretherketone (PEEK) interbody fusion device. The PEEK interbody device was filled with 1 of 2 different doses of rhBMP-2 on an absorbable collagen sponge (ACS): 0.13 mg (1×) or 0.90 mg (7×). Bone remodeling and interbody fusion were assessed via high-resolution radiography and histological analyses at 1, 2, 3, 4, 8, 12, and 20 weeks postoperatively. RESULTS: Peri-implant bone resorption peaked between 3 and 8 weeks in both the 1× and the 7× rhBMP-2/ACS-dose group. Osteoclastic activity and corresponding peri-implant bone resorption was dose-dependent, with moderate-to-marked resorption at the 7×-dose level and less resorption at the 1×-dose level. Both dose (p < 0.0007) and time (p < 0.0025) affected bone resorption significantly. Transient bone-resorption areas were fully healed by 12 weeks. Osseous bridging was seen at all but 1 spinal level at 12 and at 20 weeks. CONCLUSIONS: In the ovine end-plate-sparing interbody fusion model, rhBMP-2 dose-dependent osteoclastic resorption is a transient phenomenon that peaks at 4 weeks postoperatively. CLINICAL RELEVANCE: Using the U.S. Food and Drug Administration (FDA)-approved rhBMP-2 concentration and matching the volume of rhBMP-2/ACS with the volume of desired bone formation within the interbody construct may limit the occurrence of transient bone resorption.

Radiographic, biomechanical, and histological evaluation of rhBMP-2 in a 3-level intertransverse process spine fusion: an ovine study.

OBJECTIVE The objective of this study was to evaluate bone grafts consisting of rhBMP-2 on an absorbable collagen sponge with a ceramic composite bulking agent, rhBMP-2, directly on a ceramic-collagen sponge carrier or iliac crest bone graft (ICBG) in combination with local bone graft to effect fusion in a multisegmental instrumented ovine lumbar intertransverse process fusion model. METHODS Thirty-six sheep had a single treatment at 3 spinal levels in both the right and left intertransverse process spaces. Group 1 sheep were treated with 7.5 cm3 of autograft consisting of ICBG plus local bone for each intertransverse process space. For Groups 2-4, 4 cm3 of local bone was placed within the intertransverse process space followed by 4.5-5 cm3 of the rhBMP-2 graft material. Group 2 animals received 1.5 mg/cm3 rhBMP-2 on an absorbable collagen sponge with a commercial bone void filler consisting of Type I lyophilized collagen with a biphasic hydroxyapatite/ß-tricalcium phosphate ceramic with local bone. Group 3 animals received 0.75 mg/m cm3 of rhBMP-2 on a collagen ceramic sponge carrier with local bone. Group 4 animals received 1.35 mg/cm3 of rhBMP-2 on the same collagen ceramic sponge carrier with local bone. Sheep were euthanized 6 months postoperatively. Manual palpation, biomechanical testing, CT, radiography, and undecalcified histology were performed to assess the presence of fusion associated with the treatments. RESULTS All animals in Groups 2-4 that received grafts containing rhBMP-2 achieved radiographic and CT fusion at all 3 levels. In Group 1 (bone autograft alone), only 19% of the levels demonstrated radiographic fusion, 14% resulted in possible radiographic fusion, and 67% of the levels demonstrated radiographic nonfusion. Biomechanical testing showed that Groups 2-4 demonstrated similar stiffness of the L2-5 segment in all 6 loading directions, with each of the 3 groups having significantly greater stiffness than the autograft-only group. In Group 1, only 2 of 18 levels were rated as achieving bilateral histological fusion, with an additional 3 levels showing a unilateral fusion. The majority of the treated levels (13/18) in Group 1 were scored as histological nonfusions. There were no histological nonfusions in Groups 2 through 4. All 18 levels in Group 2 were rated as bilateral histological fusions. A majority (34/36) of the levels in Group 3 were rated as bilateral histological fusions, with 2 levels showing a unilateral fusion. A majority (35/36) of the levels in Group 4 were rated as bilateral histological fusions, with 1 level showing a unilateral fusion. CONCLUSIONS In the ovine multilevel instrumented intertransverse process fusion model, rhBMP-2 was able to consistently achieve CT, radiographic, biomechanical, and histological fusion. Compared with ICBG, the gold standard for bone grafting, rhBMP-2 was statistically superior at achieving radiographic and histological fusion.

Transient soft-tissue edema associated with implantation of increasing doses of rhBMP-2 on an absorbable collagen sponge in an ectopic rat model.

BACKGROUND: Recombinant human bone morphogenetic protein-2 (rhBMP-2) is an osteoinductive protein. However, soft-tissue edema adjacent to the site of rhBMP-2 implantation has been reported. This animal study was designed to examine soft-tissue edema associated with increasing rhBMP-2 doses with implantation on an absorbable collagen sponge (ACS) and with injection directly into muscle. METHODS: Thirty-six Lewis rats received intramuscular implantation of rhBMP-2 on an ACS (Part I) or intramuscular injection of rhBMP-2 solution (Part II). Part-I sites received rhBMP-2/ACS at doses of 0 µg, 30 µg (normal), 129 µg (mid), or 450 µg (high). Part-II sites received rhBMP-2/ACS or rhBMP-2 intramuscular injection at doses of 10 µg (normal) or 150 µg (mid). A previous rat model showed 10 µg to be 100% effective at inducing osseous spinal fusion. In our study, T2-weighted magnetic resonance imaging (MRI) was performed at two and seven days to assess edema volume, and statistical comparisons were carried out with analysis of variance (ANOVA). Cellular response, vascularity, and ossification were examined histologically. RESULTS: Quantitative MRI demonstrated similar peri-implant edema volumes in the control (buffer on an ACS) and normal-dose rhBMP-2 groups. Higher doses resulted in increased edema volume. Edema decreased significantly from two to seven days. Similar capillary densities were observed in all rhBMP-2 groups at two days, and there was dose-dependent increased ossification at seven days. Compared with the rhBMP-2 injection, implantation of the rhBMP-2/ACS resulted in increased edema. This edematous response was transient in all groups. Minimal or no ossification occurred after the rhBMP-2 injections. CONCLUSIONS: Transient peri-implant soft-tissue edema occurred in a dose-dependent manner following implantation of rhBMP-2/ACS in this rat model. The normal dose of rhBMP-2/ACS produced edema similar to that in the controls. Finally, rhBMP-2 solutions injected directly into muscle resulted in minimal soft-tissue edema.

Short-term osteoclastic activity induced by locally high concentrations of recombinant human bone morphogenetic protein-2 in a cancellous bone environment.

STUDY DESIGN: An experimental study investigating osteoclastic activity induced by rhBMP-2 in sheep. OBJECTIVE: To examine the effects of increasing local rhBMP-2 concentration on osteoclastic response and peri-implant bone resorption. SUMMARY OF BACKGROUND DATA: Level I clinical studies have established the safe and effective volume and concentration of rhBMP-2 delivered on an absorbable collagen sponge. However, peri-implant bone resorption appearing as decreased mineral density has been observed radiographically in rare instances after implantation of rhBMP-2 on an absorbable collagen sponge (rhBMP-2/ACS). METHODS: Bilateral corticocancellous defects were created in the distal femora of 30 adult sheep. Combinations of rhBMP-2/ACS implant volume (V) (1V = normal fill or 2V = overfilled) and rhBMP-2 solution concentration (x) (1x = normal concentration or 3.5x = hyperconcentrated) resulted in local rhBMP-2 concentrations of 0x, 1x, 2x, 3.5x, and 7x the estimated effective concentration for this model. Faxitron radiography, quantitative CT, histology, and quantitative histomorphometry were conducted in a blinded fashion to analyze the effect of the treatments. RESULTS: At 1 week, the normal fill-normal concentration implants (1x) produced the least transient osteoclastic activity resulting in limited peri-implant resorption. Overfilled-hyperconcentrated implants (2x, 3.5x) demonstrated moderate resorption zones. Overfilled-hyperconcentrated implants (7x) demonstrated extensive osteoclastic activity and marked resorption. Results at 4 and 8 weeks revealed dense osteoid and bone in the voids with progressive bony healing. Control defects showed no osteoclastic activity with little to no bony healing. CONCLUSION: Increasing the local rhBMP-2 concentration by overfilling the defect with rhBMP-2/ACS or hyperconcentrating the rhBMP-2 solution on the absorbable collagen sponge led to a concentration-dependent osteoclastic resorption of peri-implant bone. The osteoclastic effect was transient, and progressive healing took place over the 8-week survival period.

Improved healing efficacy in canine ulnar segmental defects with increasing recombinant human bone morphogenetic protein-2/allograft ratios.

OBJECTIVES: A validated canine critical-sized segmental defect model was used to compare efficacy of volumetric ratios of recombinant human bone morphogenetic protein-2 (rhBMP-2)/absorbable collagen sponge (ACS) with cancellous allograft bone or biphasic calcium phosphate ceramic granules using radiographic, biomechanical, and histologic analyses. METHODS: Eighteen mixed-breed hounds received bilateral critical-sized ulnar segmental defects. The six treatment groups included 1:5, 1:2, and 1:1 volumetric ratios of rhBMP-2/ACS to cancellous allograft chips; a 1:1 volumetric ratio of rhBMP-2/ACS to biphasic calcium phosphate ceramic granules; iliac crest autograft alone; and cancellous allograft chips alone (n = 6 ulna per group). Animals were euthanized at 12 weeks and analyzed by radiography, torsion testing, and histology. RESULTS: rhBMP-2/ACS groups demonstrated higher radiographic union than autograft or allograft at 12 weeks. Both treatment groups receiving a 1:1 ratio demonstrated union rates of 100% (12/12 ulna) compared with 17% (1/6) and 0% (0/6) for autograft and allograft, respectively. Torsion testing to failure demonstrated significant increases in maximum torque for all rhBMP-2 groups compared with autograft, but no differences between the rhBMP-2 groups. Ulnae treated with the 1:1 volume ratio of rhBMP-2/ACS to ceramic had the highest histologic union grades followed by 1:1 and 1:2 volume ratios of rhBMP-2/ACS to allograft. CONCLUSIONS: In this study, efficacy of the treatment correlated with the ratio of rhBMP-2/ACS to allograft. As volume of rhBMP-2/ACS decreased below a 1:1 volume ratio, the overall efficacy decreased, thus indicating a potential limit to extending rhBMP-2/ACS past a 1:1 volumetric ratio in large segmental defects. Furthermore, ceramic was found to be a successful replacement for cancellous allograft bone at a 1:1 volumetric ratio. Clinical application using graft expanders or bone void fillers with rhBMP-2 should be used carefully and requires further investigation.

A comprehensive clinical review of recombinant human bone morphogenetic protein-2 (INFUSE Bone Graft).

The combination of recombinant human bone morphogenetic protein-2 (rhBMP-2) on an absorbable collagen sponge (ACS) carrier has been shown to induce bone formation in a number of preclinical and clinical investigations. In 2002, rhBMP-2/ACS at a 1.5-mg/cc concentration (INFUSE Bone Graft, Medtronic Spinal and Biologics, Memphis, TN) was FDA-approved as an autograft replacement for certain interbody spinal fusion procedures. In 2004, INFUSE Bone Graft was approved for open tibial fractures with an intermedullary (IM) nail fixation. Most recently, in March 2007, INFUSE Bone Graft was approved as an alternative to autogenous bone grafts for sinus augmentations, and for localised alveolar ridge augmentations for defects associated with extraction sockets. The culmination of extensive preclinical and clinical research and three FDA approvals makes rhBMP-2 one of the most studied, published and significant advances in orthopaedics. This review article summarises a number of clinical findings of rhBMP-2/ACS, including the FDA-approved investigational device exemption (IDE) studies used in gaining the aforementioned approvals.