Biomaterial developments for bone tissue engineering

•: Growth factors (bone morphogenetic protein, transforming growth factor-beta, fibroblast growth factor, platelet-derived growth factor, and insulin-like growth factor) are proteins secreted by cells that act on the appropriate target cell or cells to carry out a specific action.

•: Because growth factors are expressed during different phases of fracture-healing, it has been thought that they may serve as potential therapeutic agents to enhance bone repair.

•: The selection of an appropriate carrier or delivery system for a particular growth factor is essential in order to induce a specific biologic effect.

•: There are a number of potential clinical applications for growth factors in the enhancement of bone repair, including acceleration of fracture-healing, treatment of established nonunions, enhancement of primary spinal fusion or treatment of established pseudarthrosis of the spine, and as one element of a comprehensive tissue-engineering strategy that could include gene therapy to treat large bone-loss problems.

Growth factors are proteins that serve as signaling agents for cells. They function as part of a vast cellular communications network that influences such critical functions as cell division, matrix synthesis, and tissue differentiation. The results of experimental studies have established that growth factors play an important role in bone and cartilage formation, fracture-healing, and the repair of other musculoskeletal tissues. Recently, with the advent of recombinant proteins, there has been considerable interest in the use of growth factors as therapeutic agents in the treatment of skeletal injuries. As growth factors become available as therapeutic agents, it is essential that orthopaedic surgeons understand their biological characteristics and clinical potential. The purpose of this review is to define the mechanisms of action, functions, and potential clinical applications of a variety of growth factors that may be used clinically to treat problems associated with the repair of bone.

Growth factors are proteins secreted …

The role of growth factors in the repair of bone. Biology and clinical applications.

Angiogenesis and bone repair.

Study Design. A prospective randomized clinical study was conducted. Objective. To determine whether the dose and carrier that were successful in rhesus monkeys could induce consistent radiographic spine fusion in humans. of Background Data. Preclinical studies have demonstrated that recombinant human bone morphogenetic protein-2 (rhBMP-2), an osteoinductive bone morphogenetic protein, is successful at generating spine fusion in rabbits and rhesus monkeys. Methods. For this study, 25 patients undergoing lumbar arthrodesis were randomized (1:2:2 ratio) based on the arthrodesis technique: autograft/Texas Scottish Rite Hospital (TSRH) pedicle screw instrumentation (n = 5), rhBMP-2/TSRH (n = 11), and rhBMP-2 only without internal fixation (n = 9). On each side, 20 mg of rhBMP-2 were delivered on a carrier consisting of 60% hydroxyapatite and 40% tricalcium phosphate granules (10 cm 3 /side). The patients had single-level disc degeneration, Grade 1 or less spondylolisthesis, mechanical low back pain with or without leg pain, and at least 6 months failure of non-operative treatment. Results. All 25 patients were available for follow-up evaluation (mean, 17 months; range 12-27 months). The radiographic fusion rate was 40% (2/5) in the autograftl TSRH group and 100% (20/20) with rhBMP-2 group with or without TSRH internal fixation (P = 0.004). A statistically significant improvement in Oswestry score was seen at 6 weeks in the rhBMP-2 only group (-17.6; P = 0.009), and at 3 months in the rhBMP-2/TSRH group (-17.0; P = 0.003), but not until 6 months in the autograft/TSRH group (-17.3; P = 0.041). At the final follow-up assessment, Oswestry improvement was greatest in the rhBMP-2 only group (-28.7, P< 0.001). The SF-36 Pain Index and PCS subscales showed similar changes. Discussion. This pilot study is the first with at least 1 year of follow-up evaluation to demonstrate successful posterolateral spine fusion using a BMP-based bone graft substitute, with radiographs and CT scans as the determinant. Consistently, rhBMP-2 was able to induce bone in the posterolateral lumbar spine when delivered at a dose of 20 mg per side with or without the use of internal fixation. Patients with spondylolisthesis classified higher than Meyerding Grade 1 or with more than 5 mm of translational motion may still require internal fixation. Some patients did smoke during the postoperative period, and all in the rhBMP-2 groups still obtained solid fusions. Conclusions. Consistently, rhBMP-2 with the biphasic calcium phosphate granules induced radiographic posterolateral lumbar spine fusion with or without internal fixation in patients whose spondylolisthesis did not exceed Grade 1. Statistically greater and quicker improvement in patient-derived clinical outcome was measured in the rhBMP-2 groups.

Use of recombinant human bone morphogenetic protein-2 to achieve posterolateral lumbar spine fusion in humans: a prospective, randomized clinical pilot trial: 2002 Volvo Award in clinical studies.

Study Design.A prospective randomized controlled human clinical pilot trial.Objectives.To determine the feasibility of using rhBMP-2/collagen as a substitute for autogenous bone graft inside interbody fusion cages to achieve arthrodesis in humans.Summary of Background Data.Preclinical studies have s

The use of rhBMP-2 in interbody fusion cages. Definitive evidence of osteoinduction in humans: a preliminary report.

STUDY DESIGN Nineteen dogs underwent L4-L5 intertransverse process fusions with either 58 micrograms, 115 micrograms, 230 micrograms, 460 micrograms, or 920 micrograms of recombinant human bone morphogenetic protein-2 carried by a polylactic acid polymer. A previous study (12 dogs) compared 2300 micrograms of recombinant human bone morphogenetic protein-2, autogenous iliac bone, and carrier alone in this model. All fusions subsequently were compared. OBJECTIVES To characterize the dose-response relationship of recombinant human bone morphogenetic protein-2 in a spinal fusion model. SUMMARY OF BACKGROUND DATA Recombinant osteoinductive morphogens, such as recombinant human bone morphogenetic protein-2, are effective in vertebrate diaphyseal defect and spinal fusion models. It is hypothesized that the quality of spinal fusion produced with recombinant human bone morphogenetic protein-2, above a threshold dose, does not change with increasing amounts of inductive protein. METHODS After decortication of the posterior elements, the designated implants were placed along the intertransverse process space bilaterally. The fusion sites were evaluated after 3 months by computed tomography imaging, high-resolution radiography, manual testing, mechanical testing, and histologic analysis. RESULTS As in the study using 2300 micrograms of recombinant human bone morphogenetic protein-2, implantation of 58-920 micrograms of recombinant human bone morphogenetic protein-2 successfully resulted in intertransverse process fusion in the dog by 3 months. This had not occurred in animals containing autograft or carrier alone. The cross-sectional area of the fusion mass and mechanical stiffness of the L4-L5 intersegment were not dose-dependent. Histologic findings varied but were not related to rhBMP-2 dose. Inflammatory reaction to the composite implant was proportional inversely to the volume of the fusion mass. CONCLUSIONS No mechanical, radiographic, or histologic differences in the quality of intertransverse process fusion resulted from a 40-fold variation in dose of recombinant human bone morphogenetic protein-2.

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Effective doses of recombinant human bone morphogenetic protein-2 in experimental spinal fusion.

SPINE Volume 20, Number 24, pp 2669-2682 @1995, Lippincott—Raven Publishers Evaluation of rhBMP-2 With an OPLA Carrier in a Canine Posterolateral (Transverse Process) Spinal Fusion Model Harvinder S. Sandhu, MD,* Linda E. A. Kanim, MA,* J. Michael Kabo, PhD,* Jeffrey M. Toth, PhD,i Erik N. Zeegen, BA,* David Liu,* Leanne L. Seeger, MD,T and Edgar G. Dawson, MD* Study Design. Posterolaterai l.4—L5 transverse pro- cess fusions were done on 14 adult beagles. Six were implanted with recombinant human bone morphoge- netic protein-2 carried by open-cell polylactic acid poly- mer delivery vehicle. Six received autogenous iliac bone graft. Two received carrier alone. Eleven were killed 3 months after implantation. One in each group was maintained for 8 months. Objectives. To compare recombinant human bone morphogenetic protein-2 and open-cell polvlactlc acid polymer with autogenous iliac bone for inducing trans- verse process fusion in the canine by 3 months and to determine whether transverse process decortication and implantation of carrier alone causes spontaneous transverse process fusion in the canine. Summary of Background Data. Recombinant human bone morphogenetic proteins have healed segmental long bone defects in several models. They have in- duced interlaminar and facet fusions in canines. Inter- Iaminar and facet fusions have occurred after sham decortications in canines. Recombinant human bone morphogcnetic protein-2 has not been evaluated for transverse process fusion in canines. Transverse pro- cess fusion is a preferred clinical method for achieving posterior lumbar fusion. Methods. Fusion sites were evaluated by serial com- puted tomography scans. After the dogs were killed, explanted spines were subjected to manual testing, me- chanical testing, high resolution radiography, and histo- Iogic analysis. Results. One hundred percent of recombinant hu- man bone morphogenetic protein-2-implanted sites had solid transverse process fusion oy 3 months according From the “Department of Orthopaedic Surgery and the &#39;|&#39;Department of Radiology, UCLA School of Medicine, Los Angelcs, California, and the :tDepartment of Orthopaedic Surgery, The Medical College of Wisconsin, Milwaukee, Wisconsin. Funding provided by Sofamor Danek USA, Memphis, Tennessee, and Genetics Institute, Andover, Massachusetts. Presented in part at the Scoliosis Research Society, Portland, Oregon, September 1994. Accepted for publication May 15, 1995. Device status category: 7. to all measures. No autografted sites were fused at this interval. Ossoous bridging of posteroiateral gutters oc- curred in the recombinant human bone morphogenetic protein-2-implanted sites after 2 months, the earliest radiographic measure. None of the carrier-only sites showed bone formation. conclusions. Recombinant bone morphogcnetlc pro- tein-2 carried by open-cell polylactic acid polymer is superior to autogenous iliac bone for producing radio- graphically and mechanically solid transverse process fusions in canines by 3 months. Spontaneous trans- verse process fusion does not occur in canines after decortication and open-cell polylactic acid polymer im- plantation. [Key words: bone induction. bone morpho- genetic protein. canine. polylactic acid polymer, pos- tcrolateral fusion. recombinant human bone morphogenetic protein-2| Spine 1995:20:2669-2682 Posterolateral transverse process fusion (TPF)is a fre- quently used method for achieving primary lumbar intersegmental arthrodesis.” Autogenous corticocan- cellous bone chips from the iliac crest, combining osteo- genic, osteoconductive, and osteoinductive properties, are currently the most successful grafting material.15’18 The incidence of pseudarthrosis with use of autogenous bone has been reported as high as 40% for multilevel fusions.3 Additional disadvantages with iliac crest graft relate to the adjuvant procedure required to procure the graft. Younger and Chapman noted minor complications such as harvest site pain, hypersensitivity, and buttocks an- esthesia in 20.6% of patients and major complications such as deep infections and vascular injuries in 8.6% of patients.7&#39;”&#39;39 Fresh, frozen, lyophilized, and demineralized bone matrix from allogeneic sources are used as alternatives or supplements to autogenous bone.21’22&#39;28 These prep- arations are impaired by lower osteogenic capacity, higher resorption rate, larger host immunologic re- sponse, and reduced revascularization.” There are con- 2669

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Evaluation of rhBMP-2 with an OPLA carrier in a canine posterolateral (transverse process) spinal fusion model.

STUDY DESIGN L4-L5 intertransverse process fusions were produced with 58 micrograms, 230 micrograms, or 920 micrograms of recombinant human bone morphogenetic protein-2 in 20 dogs. Eleven had traditional decortication of posterior elements before insertion of the implant. Nine were left undecorticated. All animals were evaluated 3 months after surgery. OBJECTIVES To determine whether decortication is a prerequisite for successful fusion in the presence of osteoinductive proteins such as bone morphogenetic protein-2. SUMMARY OF BACKGROUND DATA Recombinant osteoinductive proteins can induce de novo bone in ectopic soft-tissue sites in the absence of bone marrow elements. Traditional methods for achieving spinal fusion rely on exposure of bone marrow through decortication to facilitate osteogenesis. It is hypothesized that the presence of an implanted osteoinductive protein obviates the need for exposure and release of host inductive factors. METHODS Recombinant human bone morphogenetic protein-2-induced intertransverse process fusions were performed with and without decortication. Fusion sites were evaluated by computed tomography imaging, high-resolution radiography, manual testing, mechanical testing, and histologic analysis. RESULTS One hundred percent of decorticated spines and 89% of undecorticated spines were clinically fused by 3 months. Ninety-one percent of decorticated spines and 78% of undecorticated specimens exhibited bilateral transverse process osseous bridging. The only spines that failed to achieve solid bilateral arthrodesis were in the lowest dose group. With the higher two doses, there was histologic evidence of osseous continuity between the fusion mass and undecorticated transverse processes. CONCLUSIONS There were no statistical differences in clinical and radiographic fusion rates between decorticated and undecorticated sites. With higher doses of recombinant human bone morphoganetic protein-2, there was little histologic distinction between fusions in decorticated versus undecorticated spines.

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Experimental spinal fusion with recombinant human bone morphogenetic protein-2 without decortication of osseous elements.

Study Design. Twenty sheep underwent anterior lumbar interbody fusions with either a threaded titanium interbody fusion device (cage, n = 8), autogenous iliac crest dowel graft (autograft, n = 6), or interbody decortication only (sham, n = 6). Two sheep had misplaced cages and were excluded. Sheep were killed after 6 months. Objectives. To determine whether this model is useful for examining the distractive and fixation properties of interbody fusion cages. Summary of Background Data. Interbody fusion cages are used in anterior lumbar interbody fusion procedures to provide immediate intersegmental fixation and to distract and preserve interbody height. The process of physiologic anchorage by bone ingrowth into such devices is under investigation. Methods. Sheep were radiographed immediately after surgery and 2, 4, and 6 months after surgery. Interbody distraction and angulation were measured with a digital photo image analyzer at each time point. After the sheep were killed, stiffness to flexion, extension, and lateral bending moments were measured. Twelve untreated cadaver spines were also tested for comparison. Results. After surgery, interbody distraction successfully occurred in cage and autograft-implanted sites. Loss of interbody height ensued, however, in all groups during the first 2 months. Percentage loss of height was lowest in cage sites. By 6 months, only cage sites remained distracted beyond normal. Fusions in all groups were stiffer than untreated spines. Autograft sites were stiffer than cage sites to lateral bending. Sham sites were stiffer than cage and autograft sites to flexion, but this likely resulted from complete intervertebral collapse. Conclusion. Despite early subsidence, interbody fusion cages successfully distracted and preserved interbody spaces. This model is useful for investigating methods of improving distraction and stabilization.

Distractive properties of a threaded interbody fusion device. An in vivo model.

In this study, truncal deformity in patients with scoliosis was evaluated by circumferential scanning using an optoelectronic device (Optronic Torsograph, Anima Corp., Tokyo, Japan). This device generates cross-sectional skin surface topography at 10 axial levels to provide a three-dimensional representation of truncal shape. Ninety-three patients with suspected idiopathic scoliosis were evaluated with conventional Cobb measurements, and their parameters were computed using the device's measurements. The geometric cross-sectional indexes correlated poorly with the Cobb angle and were highly variable during repeated measurements. The optoelectronic device's angle, a measure of centroid curvature, correlated highly with the Cobb angle for primary spinal curves. The highest correlation was observed in thoracolumbar curves. This optoelectronic device has clinical relevance as a method for further scoliosis screening and monitoring the progression of spinal curvature in a patient.

J. M. Kabo

Papers

Effective doses of recombinant human bone morphogenetic protein-2 in experimental spinal fusion.

Evaluation of rhBMP-2 with an OPLA carrier in a canine posterolateral (transverse process) spinal fusion model.

Experimental spinal fusion with recombinant human bone morphogenetic protein-2 without decortication of osseous elements.

Distractive properties of a threaded interbody fusion device. An in vivo model.

Optoelectronic evaluation of trunk deformity in scoliosis.