AOA Critical Issues Symposium: So, You Want to Be a Department Leader: Essentials for Success

ABSTRACT

Most health systems are vertically integrated, and the leaders of orthopaedic surgery departments or service lines must have a comprehensive understanding of their role in the strategic plan of the health system. Orthopaedic surgery departments must be profitable while supporting the tripartite mission of excellence in clinical care, research, and education. This symposium had 4 specific objectives: to discuss how to (1) create synergy between the department or service line and the health system, (2) develop a strategy to enhance financial stability and revenue growth, (3) develop a comprehensive plan to enhance recruitment and retention of a diverse faculty, and (4) consider alternative strategies to foster education and research, even when the health system may be more focused on revenue generation.

Spine fusion by gene therapy

Over 250 000 patients each year undergo a spine fusion procedure in the US. This constitutes 50% of all bone graft procedures. Despite best efforts, a large percentage of spine fusions (up to 35%) fail to form a solid bony arthrodesis. This is a significant clinical problem and has led to research in bone formation biology to augment spine fusion rates. Both recombinant and purified osteoinductive cytokines have been studied in pilot and pivotal studies in humans. At this point, recombinant human bone morphogenetic protein-2 has received FDA approved for lumbar interbody application with titanium cages. Despite these successes, limitations of directly applying osteoinductive proteins related to cost and carriers remain to be overcome. Gene therapy for spine fusion and other bone healing applications are being pursued as an alternative strategy. This article will review the state of the art of local gene therapy for bone formation and to highlight specific issues, which must be addressed when pursuing such a program. A critical step in using gene therapy for bone formation is choosing an appropriate osteoinductive gene. In choosing the gene, one must consider the differences in efficacy of the gene as well as the gene availability due to proprietary constraints. The choice of delivery vector is important. Factors such as the potency of the gene and the specific application intended play a role in this decision. Next, the effective dose, transduction time, and gene transfer method must be established. The choice of carrier material to form the scaffold for the new bone formation is another critical step that must be optimized for successful bone formation. Finally, a strategy for in vitro and in vivo testing must be developed to maximize the chances of success in human trials.

Do the intervertebral disc cells respond to different levels of hydrostatic pressure?

OBJECTIVE

To test the hypothesis that hydrostatic pressure directly affects the synthesis of collagen and proteoglycan by intervertebral disc cells.

DESIGN

By the use of pressure vessels, hydrostatic pressure was applied to intervertebral disc cells cultured in alginate.

BACKGROUND

The influence of compression (both hydrostatic and axial) on chondrocyte metabolism was examined in a number of earlier studies. However, in most of these studies, articular cartilage, not intervertebral disc was used, and in none of these was hydrostatic pressure applied to intervertebral disc cells cultured in alginate.

METHODS

Fresh cells were harvested from the lumbar intervertebral discs of dogs. Before their suspension in an alginate gel system, the cells were plated and expanded until they reached confluence. Then, by use of the alginate gel system, the cells were exposed (for up to 9 days) to specific values of hydrostatic pressure inside two stainless steel pressure vessels. One vessel was kept at 0.35 MPa and the other at atmospheric pressure (approximately 0.1 MPa). The effects of 0.35 MPa were compared against atmospheric pressure by measuring the incorporation of [3H]-proline and [35S]-sulfate into collagen and proteoglycans, respectively, for the anulus cells and nucleus cells separately, and by determining whether this incorporation was reflected by changes in the levels of mRNA for aggrecan and Types I and II collagen.

RESULTS

Proteoglycan synthesis was inhibited at 0.35 MPa as compared to atmospheric pressure for both the nucleus and anulus cells, whereas collagen synthesis was stimulated in the nucleus cells, but inhibited in the anulus cells. The mRNA levels of collagen 1A and collagen 2A decreased in the anulus but showed a differential response in the nucleus (collagen 1A increased, while collagen 2A decreased). The mRNA levels for aggrecan core protein decreased in the anulus and increased in the nucleus.

CONCLUSIONS

Hydrostatic pressure directly affects the synthesis of collagen and proteoglycan by the intervertebral disc cells.

RELEVANCE

This in vitro study reveals the direct effect of hydrostatic pressure on disc cells, in the absence of other factors. However, circumspection must be applied when comparisons between these results, from in vitro experiments on dog disc cells, are extrapolated and applied to the whole discs of humans.

Anterior lumbar interbody fusion using a barbell-shaped cage: a biomechanical comparison

There are drawbacks to using threaded cylindrical cages (e.g., limited area for bone ingrowth and metal precluding radiographic visualization of bone healing). To somewhat offset these drawbacks, a barbell-shaped cage has been designed. The central core of the barbell can be wrapped with collagen sheets infiltrated with bone morphogenetic protein. The obvious theoretical advantages of a barbell cage have to be weighed against potential biomechanical disadvantages. Our purpose was to compare the biomechanical properties of an anterior lumbar interbody reconstruction using 18-mm-diameter threaded cylindrical cages, with a reconstruction using barbell cages (18-mm diameter and 6 mm wide at both cylindrical ends, with a round 4-mm-diameter bar joining the two ends). Twelve cadaveric lumbar motion segments were tested. Three L5-S1 segments received two threaded cylindrical cages, and three L5-S1 segments received two barbell cages. Three L3-L4 segments received one threaded cylindrical cage, and three L3-L4 segments received one barbell cage. A series of biomechanical loading sequences were carried out on each motion segment, and stiffness curves were obtained. After the biomechanical testing, an axial compressive load was applied to the motion segments until failure. They were then radiographed and bisected through the disc, and the subsidence (or penetration) of the cage(s) in the cancellous bone of the vertebral bodies was measured. There was no difference in terms of stiffness between the motion segments with the threaded cylindrical cage(s) inserted and those with the barbell cage(s) inserted (p > 0.15). The average values of subsidence was 0.96 mm for the threaded cylindrical cage group and 0.80 mm for the barbell cage group (difference not significant: p = 0.38). The results suggest that a reconstruction using barbell cages is a biomechanically acceptable alternative to one using threaded cylindrical cages.

Demineralized bone matrix, bone morphogenetic proteins, and animal models of spine fusion: an overview

Preclinical investigations on the use of bone morphogenetic proteins (BMP) in the spine have yielded promising results. This has led to the preliminary introduction of these growth factors in controlled clinical trials. Initial data made available suggest that these differentiating factors will play a major role in the treatment of spinal disorders in the future. This article reviews key preclinical studies and their results that formed the basis for introduction into clinical trials. Non-primate and non-human primate models of spine fusion with BMP are reviewed objectively, and important issues regarding carrier, dose, and site of implantation are discussed. Finally, exciting new gene therapy research is discussed, with comments made on its applicability for the future.

The value of magnetic resonance imaging of the lumbar spine to predict low-back pain in asymptomatic subjects : a seven-year follow-up study

BACKGROUND

In 1989, a group of sixty-seven asymptomatic individuals with no history of back pain underwent magnetic resonance imaging of the lumbar spine. Twenty-one subjects (31%) had an identifiable abnormality of a disc or of the spinal canal. In the current study, we investigated whether the findings on the scans of the lumbar spine that had been made in 1989 predicted the development of low-back pain in these asymptomatic subjects.

METHODS

A questionnaire concerning the development and duration of low-back pain over a seven-year period was sent to the sixty-seven asymptomatic individuals from the 1989 study. A total of fifty subjects completed and returned the questionnaire. A repeat magnetic resonance scan was made for thirty-one of these subjects. Two neuroradiologists and one orthopaedic spine surgeon interpreted the original and repeat scans in a blinded fashion, independent of clinical information. At each disc level, any radiographic abnormality, including bulging or degeneration of the disc, was identified. Radiographic progression was defined as increasing severity of an abnormality at a specific disc level or the involvement of additional levels.

RESULTS

Of the fifty subjects who returned the questionnaire, twenty-nine (58%) had no back pain. Low-back pain developed in twenty-one subjects during the seven-year study period. The 1989 scans of these subjects demonstrated normal findings in twelve, a herniated disc in five, stenosis in three, and moderate disc degeneration in one. Eight individuals had radiating leg pain; four of them had had normal findings on the original scans, two had had spinal stenosis, one had had a disc protrusion, and one had had a disc extrusion. In general, repeat magnetic resonance imaging scans revealed a greater frequency of disc herniation, bulging, degeneration, and spinal stenosis than did the original scans.

CONCLUSIONS

The findings on magnetic resonance scans were not predictive of the development or duration of low-back pain. Individuals with the longest duration of low-back pain did not have the greatest degree of anatomical abnormality on the original, 1989 scans. Clinical correlation is essential to determine the importance of abnormalities on magnetic resonance images.

Adenoviral delivery of LIM mineralization protein-1 induces new-bone formation in vitro and in vivo

BACKGROUND

The LIM mineralization protein-1 (LMP-1) gene encodes for an intracellular protein that induces the expression of several bone growth factors. The purpose of the present study was to determine the feasibility and the optimal dose of adenoviral delivery of the LMP-1 cDNA to promote spinal fusion.

METHODS

A replication-deficient human recombinant adenovirus was constructed with the LMP-1 cDNA driven by a cytomegalovirus promoter. In phase 1, an in vitro dose-response experiment was performed to determine the optimal adenovirus-LMP-1 (AdLMP-1) concentration and infection time. In phase 2, nine rabbits had a single-level posterolateral arthrodesis of the lumbar spine with implantation of a carrier matrix loaded with bone-marrow-derived buffy-coat cells that had been infected for ten minutes with adenovirus containing the cDNA for LMP-1 (AdLMP-1) or beta-galactosidase (AdBgal). In phase 3, posterolateral arthrodesis of the spine was performed with implantation of cells infected with AdLMP-1 (ten rabbits) or cells infected with an empty adenovirus that did not contain LMP-1 cDNA (ten rabbits) and the results were compared. In this phase, peripheral-blood-derived buffy-coat cells were used instead of bone-marrow-derived cells and a collagen-ceramic-composite sponge was used as the carrier.

RESULTS

In phase 1, the in vitro dose-response experiment showed that a multiplicity of infection of 0.25 plaque-forming units per cell was the most efficient dose. In phase 2, the implants that had received cells infected with AdLMP-1 induced a solid, continuous spinal fusion mass at five weeks. In contrast, the implants that had received cells infected with AdBgal or a lower dose of AdLMP-1 induced little or no bone formation. In phase 3, a solid spinal fusion was observed at four weeks in all ten rabbits that had received cells infected with AdLMP-1 and in none of the ten rabbits that had received cells infected with the empty adenovirus. Biomechanical and histological testing of the AdLMP-1-treated specimens revealed findings that were consistent with a high-quality spinal fusion.

CONCLUSIONS

Adenoviral delivery of LMP-1 cDNA promotes spinal fusion in immune-competent rabbits.

The effect of nicotine on gene expression during spine fusion

STUDY DESIGN

A rabbit model of posterolateral spine fusion was used to investigate the effect of nicotine on cytokine expression during spine fusion.

OBJECTIVES

To determine the effects of nicotine on the known gene expression pattern of bone morphogens and related proteins expressed during spine fusion.

SUMMARY OF BACKGROUND DATA

The mechanism by which nicotine increases the pseudarthrosis rate of spine fusion is unknown. Recently, a distinct temporal and spatial pattern of cytokine expression during bone formation has been described. The authors hypothesized that nicotine would alter this known pattern, thereby revealing the mechanism by which nicotine exerts its effect.

METHODS

Twenty-eight New Zealand White rabbits underwent posterolateral spine fusion with autogenous bone graft. Fourteen rabbits received systemic nicotine by a miniosmotic pump. Fusions were harvested at 0, 2, 5, and 7 days and 2, 3, and 4 weeks after arthrodesis. Specimens were divided into the outer zones adjacent to the transverse processes and the central zones between the transverse processes. Gene expression of type I and II collagen, bone morphogenic protein-2, -4, and -6 and basic fibroblast growth factor (bFGF) and vascular endothelial growth factor (VEGF) was then measured at each time point in each of the two zones.

RESULTS

Nicotine inhibited expression of all cytokines measured, mainly in the central zone. However, the previously described temporal and spatial patterns of expression were preserved.

CONCLUSIONS

Nicotine inhibits expression of a wide range of cytokines, including those associated with neovascularization and osteoblast differentiation. Therefore, the effects of nicotine appear to involve more than just local vasoconstriction.

Gene therapy for spine fusion

Spine fusion is a commonly performed yet often unsuccessful surgical procedure. As many as 40% of patients undergoing spine fusion may have a nonunion or failure to form a continuous bone bridge. This clinical challenge has focused much of the attention of osteoinductive bone growth factors toward spine applications. Clinical pilot and pivotal trials will show the feasibility of recombinant and purified bone growth factors to promote spine fusion in humans. Despite this, strategies of gene therapy for spine fusion and other bone healing applications are being pursued. This article reviews the state of the art of local gene therapy and highlights specific issues that must be addressed when pursuing a gene therapy program. Perhaps the most critical step in gene therapy for bone formation is choosing an appropriate osteoinductive gene. Such choices may be limited by differences in efficacy of the chosen gene and availability because of proprietary constraints. The choice of delivery vector is crucial and depends on the potency of the gene and the specific application intended. Establishing the effective dose, transduction time, and gene transfer method are important decisions. The choice of carrier material to form the scaffold for the new bone formation is paramount to successful bone formation. Finally, a strategy for in vitro and in vivo testing must be developed to maximize the chances of success in human trials.

Biology of lumbar spine fusion and use of bone graft substitutes: present, future, and next generation

Posterolateral lumbar spine arthrodesis is a commonly performed procedure, yet the biology of healing is poorly understood. Nonunion, or failure to achieve a solid bony fusion, occurs in up to 40% of patients. We first developed and validated a rabbit model to characterize the healing process by measuring macroscopic parameters, microscopic parameters, and gene expression. We found that presently available osteoconductive and weakly osteoinductive materials were insufficient to replace autografts, but could in some cases serve as bone graft extenders. In contrast, two osteoinductive growth factors currently in development could replace autograft in non-human primates and in humans, but may be limited by the high dose required, carrier variability, and high cost. We identified, cloned, and sequenced a novel complementary DNA (cDNA) encoding for an intracellular protein LMP-1, which is expressed during the first few hours of osteoblast differentiation. LMP-1 expression is able to induce many BMPs, their receptors, and other bone growth factors. Local implantation of bone marrow cells transfected with LMP-1 cDNA induced spine fusion in 100% of sites tested; no bone formed at the control sites without LMP-1. This strategy of local gene therapy may provide a basis for the next generation of bone graft substitutes.

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

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 shown rhBMP-2 to be an effective substitute for autogenous bone graft, but there are no studies to date documenting such efficacy for human spine fusion.

METHODS

Fourteen patients with single-level lumbar degenerative disc disease refractory to nonoperative management were randomized to receive lumbar interbody arthrodesis with a tapered cylindrical threaded fusion cage filled with rhBMP-2/collagen sponge or autogenous iliac crest bone. Patients were evaluated with radiographs, sagittally reformatted computed tomography scans, and Short Form-36 and Oswestry outcome questionnaires.

RESULTS

All 11 patients who received rhBMP-2 were judged by three independent radiologists to have solid fusions (at 6, 12, and 24 months postimplantation), whereas only 2 of the 3 control patients, who received the standard treatment of autogenous iliac crest bone, were deemed to be fused. The Oswestry Disability Questionnaire scores of the rhBMP-2 group improved sooner (after 3 months) than those of the autograft group, with both groups demonstrating similar improvement at 6 months. Short Form 36 scores continued to improve up to 24 months.

CONCLUSION

The arthrodesis was found to occur more reliably in patients treated with rhBMP-2-filled fusion cages than in controls treated with autogenous bone graft, although the sample size was limited. There were no adverse events related to the rhBMP-2 treatment. This study is one of the first to show consistent and unequivocal osteoinduction by a recombinant growth factor in-humans.

Osteoinductive bone graft substitutes

This review will summarize the major efforts currently underway to develop osteoinductive bone graft substitutes for minimally invasive spine fusions. The primary categories of substitute include purified bone growth factors, recombinant bone growth factors, and growth factors delivered by gene therapy approaches. Clinical trials are underway for the purified and recombinant bone growth factors and pre-clinical studies have yielded promising results for a variety of gene therapy techniques for generating bone.

Recombinant human bone morphogenetic protein-2 overcomes the inhibitory effect of ketorolac, a nonsteroidal anti-inflammatory drug (NSAID), on posterolateral lumbar intertransverse process spine fusion

STUDY DESIGN

An animal model of posterolateral intertransverse process spine fusion healing.

OBJECTIVE

To evaluate the effect of systemic ketorolac, alone and in combination with locally applied recombinant human bone morphogenetic protein-2, on spine fusion healing.

SUMMARY OF BACKGROUND DATA

The effect of nonsteroidal anti-inflammatory drugs on bone graft healing in animals remains controversial. However, most studies point to the inhibition of fracture repair, especially during the early healing period.

METHODS

Forty-nine adult New Zealand white rabbits underwent single-level lumbar fusion with autologous iliac bone graft. Two mini-osmotic pumps were implanted subcutaneously and filled with saline as a control or ketorolac. Rabbits were divided into three groups: 1) control (saline in pump); 2) nonsteroidal anti-inflammatory drug (ketorolac in pump); 3) nonsteroidal anti-inflammatory drug (ketorolac in pump) and bone morphogenetic protein (bone graft soaked in a 3.0 mg solution of recombinant human bone morphogenetic protein-2. All rabbits were killed after 6 weeks.

RESULTS

In the control group, 75% (12 in 16) of the surviving rabbits were judged to have solidly fused lumbar spines as compared with only 35% (6 in 17) of the animals that received ketorolacachieved fusion (P = 0.037). Of the animals that received ketorolac and recombinant bone morphogenetic protein-2, 100% (9 in 9) fused.

CONCLUSIONS

The results of this study confirm the detrimental effect of a commonly used nonsteroidal anti-inflammatory drug on spinal fusion during the immediate postoperation period in a established rabbit model of posterolateral lumbar spine fusion. The addition of recombinant bone morphogenetic protein-2 to the autograft bone was able to compensate for the inhibitory effect of ketorolac on bone formation. On the basis of these data, caution is urged in the routine use of nonsteroidal anti-inflammatory drugs for postoperation analgesia in patients undergoing spine arthrodesis.

Bioactive factors for bone tissue engineering

Orthopaedic surgery is currently in the midst of a transformation from bone grafting and the use of bone graft substitutes to bone tissue engineering. Bioactive bone growth factors likely will play a particularly important role in this emerging field. This article will review the three leading strategies for using bioactive factors for bone tissue engineering: extraction and partial purification of growth factors, recombinant protein synthesis, and gene therapy. Preclinical and early clinical trial results with bone morphogenetic protein-2, bone morphogenetic protein-7 (osteogenic protein-1), and NeOsteo bovine bone protein extract will be reviewed. In addition, the current obstacles to clinical implementation of bone tissue engineering will be reviewed.

Osteoinductive bone graft substitutes for spinal fusion: a basic science summary

An understanding of precise biologic mechanisms at work during spinal fusion healing is just beginning to evolve. Current molecular biology research has shown this process to be multifactorial and extremely complex. With the advent of improved animal models to study the biology of spinal fusion, essential information regarding the basic science behind arthrodesis has advanced knowledge of this process. Moreover, with advances in local gene therapy as well as osteoinductive proteins and osteoinductive carrier matrices, the orthopedic surgeon will soon enter into a new era of biologic manipulation for fusion.

Natural history of rheumatoid arthritis of the cervical spine

This article reviews the natural history of rheumatoid arthritis involving the cervical spine with special attention given to predictors of paralysis. Understanding the natural history of rheumatoid arthritis of the cervical spine is necessary to determine the benefit of various interventions. The primary treatment goal for cervical instability is prevention of irreversible neurologic injury. The natural history of rheumatoid arthritis for a period of 10 years or more is one of significant disease progression. The natural history of cervical instability in patients with rheumatoid arthritis is more variable, with only some patients having a neurologic deficit develop. Recent studies support prophylactic stabilization of the rheumatoid cervical spine to prevent paralysis in high risk patients. However, proponents for prophylactic arthrodesis must acknowledge that not all cervical instability in rheumatoid arthritis progresses to neurologic deficit, and surgical intervention in patients with rheumatoid arthritis incurs added morbidity and mortality. Identifying the risk factors for progression of cervical instability is the first step in eliminating morbidity and mortality from spinal cord and brain stem compression. Surgical stabilization is indicated not only for those patients with paralysis, but for the subgroups of patients with cervical rheumatoid disease who are at risk for spinal cord and brain stem compression. The posterior atlantodental interval is the most reliable screening tool and predictor of progressive neurologic deficit.

The effect of hydrostatic pressure on intervertebral disc metabolism

STUDY DESIGN

By the use of pressure vessels, hydrostatic pressure was applied to intervertebral disc cells cultured in an alginate.

OBJECTIVE

To test the hypothesis that hydrostatic pressure directly affects the synthesis of collagen and proteoglycan by the intervertebral disc cells.

SUMMARY OF BACKGROUND DATA

The influence of compression (both hydrostatic and mechanical) on chondrocyte metabolism was examined in a number of earlier studies. However, in most of these studies, articular cartilage, not intervertebral disc, was used, and in none of these was hydrostatic pressure applied to intervertebral disc cells cultured in alginate.

METHODS

Fresh cells were harvested from the lumbar intervertebral discs of dogs. Before their suspension in an alginate gel system, the cells were plated and expanded until they reached confluence. Then, by use of the alginate gel system, the cells were exposed (for up to 9 days) to specific values of hydrostatic pressure inside two stainless steel pressure vessels. One vessel was kept at 1 MPa and the other at atmospheric pressure. The effects of 1 MPa were compared against atmospheric pressure by measuring the incorporation of [3H]-proline and [35S]-sulfate into collagen and proteoglycans, respectively, for the anulus cells and nucleus cells separately, and by determining whether this incorporation was reflected by changes in the levels of mRNA for aggrecan and Types I and II collagen.

RESULTS

Comparisons with atmospheric pressure yielded the following findings: 1) In the incorporation studies, the nucleus and anulus cells exhibited a differential response to a hydrostatic pressure of 1 MPa. Collagen and proteoglycan syntheses were stimulated in the nucleus cells and inhibited in the anulus cells. 2) There was no significant increase in cell proliferation, as measured by DNA content, at 1 MPa for either the anulus or nucleus cells. 3) The mRNA levels of collagen (Col 1A1 and Col 2A1) and aggrecan increased at 1 MPa in both the nucleus and anulus cells.

CONCLUSIONS

Hydrostatic pressure directly affects the synthesis of collagen and proteoglycan by the intervertebral disc cells.

Posterolateral lumbar intertransverse process spine arthrodesis with recombinant human bone morphogenetic protein 2/hydroxyapatite-tricalcium phosphate after laminectomy in the nonhuman primate

STUDY DESIGN

A nonhuman primate lumbar intertransverse process arthrodesis model was used to evaluate recombinant human bone morphogenetic protein 2 (rhBMP-2) in a hydroxyapatite-tricalcium phosphate (HA-TCP) carrier as a complete bone graft substitute.

OBJECTIVES

To assess the ability of a ceramic material to serve as a carrier for various doses of rhBMP-2 as a bone graft substitute in a primate model of posterolateral intertransverse process spinal fusion after laminectomy.

SUMMARY OF BACKGROUND DATA

The reported non-union rates for posterolateral lumbar spine fusion with autogenous iliac crest bone range from 5-35%. Recombinant human bone morphogenetic protein 2 has shown potential to serve as a bone graft substitute for posterolateral intertransverse process spine fusion. Although a resorbable collagen sponge was a suitable carrier in rabbits and dogs, it was too compressible for the paraspinal muscles in rhesus monkeys. This failure of the collagen carrier has prompted evaluation of the feasibility of an alternative carrier material and the required dose of rhBMP-2.

METHODS

Twenty-one adult rhesus monkeys underwent a laminectomy at L4-L5 followed by bilateral intertransverse process arthrodesis via the same midline incision (n = 16) or a minimally invasive video-assisted posterolateral approach (n = 5). Bone graft implants on each side consisted of either 5 cm3 of autogenous iliac crest bone or 60:40 HA-TCP blocks (1.2 x 0.5 x 3.7 cm) loaded with a solution containing 0, 6, 9, or 12 mg of rhBMP-2 per side. The monkeys were killed 24 weeks after surgery. Inspection, manual palpation, radiography, and histology were used to assess fusion and to detect any bony growth into the laminectomy defect.

RESULTS

Fusion was not achieved in any of the monkeys treated with autogenous iliac crest bone graft. Both of the monkeys treated with the HA-TCP blocks with 0 mg rhBMP-2 achieved fusion. All 15 monkeys treated with the HA-TCP blocks and either of the three doses of rhBMP-2 achieved solid fusion. Two animals had extension of the fusion on one side because of malpositioned ceramic block. The results in animals fused via the minimally invasive video-assisted technique were the same as inthose fused with the open technique. Histologic analysis showed some ingrowth of bone into the ends but not-through the ceramic block in the absence of rhBMP-2. When the ceramic blocks were loaded with rhBMP-2 there was a dose-dependent increase in the amount and quality of bone throughout the ceramic carrier based on qualitative assessment. No significant bone encroachment on the exposed thecal sac through the laminectomy defect was observed in any of the monkeys.

CONCLUSION

Hydroxyapatite-tricalcium phosphate proved to be a suitable carrier for rhBMP-2 in the posterolateral spine fusion model in rhesus monkeys. Even in the presence of a laminectomy defect, there was no evidence of bone induction outside the confines of the ceramic carrier.

Posterolateral intertransverse process spinal arthrodesis with rhBMP-2 in a nonhuman primate: important lessons learned regarding dose, carrier, and safety

Recombinant osteoinductive proteins have been used successfully in canine and rabbit models of posterolateral intertransverse process arthrodesis, but little is known about the ability of these compounds to achieve fusion in nonhuman primates. The goals of this investigation were to compare different combinations of recombinant human bone morphogenetic protein-2 (rhBMP-2) dosages and carriers in a nonhuman primate model of posterolateral intertransverse process spinal fusion and to determine the feasibility of using rhBMP-2 in the presence of exposed dura in a laminectomy model. Posterolateral intertransverse process arthrodeses were performed at L4-5 in 29 rhesus monkeys. The most striking findings were as follows: rhBMP-2 could induce bone in a nonhuman primate spine; the presence of a laminectomy defect with exposed dura did not preclude the safe use of rhBMP-2 for posterolateral fusion; soft tissue compression of the collagen sponge carrier prevented bone induction at standard BMP doses, presumably due to squeezing of the protein out of the sponge; and longer rhBMP-2 loading time into the collagen carrier and mechanical protection from the soft tissue compression both allowed more bone induction at a lower dose of rhBMP-2.

Graft healing in laryngotracheal reconstruction: an experimental rabbit model

The actual sequence of events in graft healing in laryngotracheal reconstruction (LTR) is not well understood. To investigate sequential changes in graft healing, we submitted 21 rabbits to LTR with an anterior auricular cartilage graft. They were immediately extubated after surgery, and no stent was used. Rabbits were painlessly sacrificed at 7 different time periods (0, 1, 2, 3, 4, 6, and 10 weeks after surgery). Cross sections of the larynx and graft were cut and stained with hematoxylin-eosin and trichrome. Epithelialization progressed very rapidly and was complete by 21 days. During the 10 weeks, there was a progressive necrosis and resorption of the original graft cartilage. However, neochondrification progressed very rapidly and resulted in excellent structural support of the graft in the airway. Problems, such as infection and granulation tissue formation, were relatively minor and infrequent.

Molecular biology and spinal disorders. A survey for the clinician

Over the past 10 years, advances in molecular biology techniques have extended the potential for understanding spinal disorders from the microscopic (histologic) level down to the molecular level of gene expression within individual cells. These advances are initiating new avenues of research and, ultimately, novel clinical treatments. The intent of this update is to provide the spine clinician with a basic understanding of molecular biology, the type of information that may be learned from its application, and the potential for gene therapy in spine disorders.

New formulations of demineralized bone matrix as a more effective graft alternative in experimental posterolateral lumbar spine arthrodesis

STUDY DESIGN

A rabbit model of posterolateral intertransverse process spine arthrodesis was used.

OBJECTIVE

To determine the efficacy of two new formulations of demineralized bone matrix.

SUMMARY OF BACKGROUND DATA

The flowable gel form of Grafton (Osteotech, Eatontown, NJ) demineralized bone matrix has been shown to have osteoinductive properties in various models and currently is used clinically as bone graft material in posterolateral lumbar spine arthrodesis. Two new formulations of Grafton, one made of flexible sheets (Flex) and the other made in a malleable consistency (Putty), have improved handling characteristics compared with the gel form.

METHODS

In this study, 108 New Zealand white rabbits underwent bilateral posterolateral intertransverse spine arthrodesis at L5-L6 using autogenous iliac crest bone graft alone (control), one of the new forms of demineralized bone matrix (DBM; made from rabbit bone) alone or in combination with autogenous iliac crest bone. Rabbits were killed 6 weeks after surgery. The lumbar spines were excised, and fusion success or failure was determined by manual palpation and radiography. Specimens also were processed for undecalcified histologic analysis.

RESULTS

Manual palpation of the harvested lumbar spines revealed that the fusion rates of the Flex-DBM/Auto group (9/9, 100%) and Putty-DBM/Auto group (10/10, 100%) were superior (P < 0.01) to those of the Auto/control group (3/9, 33%). As a stand-alone graft substitute, Flex-DBM performed superiorly with a fusion rate of 11/11 (100%) compared with that of Putty-DBM (10/12, 83%) and Gel-DBM (7/12, 58%). The devitalized version of Flex-DBM had a fusion rate of 4/11 (36%), which was comparable with the devitalized Putty-DBM rate of 4/12 (33%). Both were superior (P < 0.05) to the devitalized Gel-DBM rate of 0/12 (0%). More mature fusions with greater amounts of trabecular bone were present radiographically and histologically in rabbits that received all forms of demineralized bone matrix than in those in which autograft was used.

CONCLUSIONS

The new flexible sheet and malleable putty forms of demineralized bone matrix were effective as graft extender and graft enhancer in a model of posterolateral lumbar spine fusion. These newer formulations of Grafton appear to have a greater capacity to form bone than the gel form or autogenous bone graft alone in this model.

The use of coralline hydroxyapatite with bone marrow, autogenous bone graft, or osteoinductive bone protein extract for posterolateral lumbar spine fusion

STUDY DESIGN

A posterolateral lumbar arthrodesis animal model using coralline hydroxyapatite as a bone graft substitute.

OBJECTIVE

To determine the effectiveness of coralline hydroxyapatite as a bone graft substitute for lumbar spine fusion when used with bone marrow, autogenous bone graft, or an osteoinductive bone protein extract.

SUMMARY OF BACKGROUND DATA

Coralline hydroxyapatite is commonly used as a bone graft substitute in metaphysial defects but its use in a more challenging healing environment such as the posterolateral spine remains controversial. There are no published animal studies in which the use of coralline hydroxyapatite has been evaluated in a posterolateral lumbar arthrodesis model.

METHODS

Single-level posterolateral lumbar arthrodesis was performed at L5-L6 in 48 adult New Zealand White rabbits. Rabbits were assigned to one of three groups based on the graft material they received: 3.0 mL coralline hydroxyapatite 1.5 mL plus bone marrow; 1.5 mL coralline hydroxyapatite plus 1.5 mL autogenous iliac crest bone; and, 3.0 mL coralline hydroxyapatite plus 500 micrograms bovine-derived osteoinductive bone protein extract on each side. Rabbits were killed after 2, 5, or 10 weeks, and the spines were excised and evaluated by manual palpation, radiographs, tensile biomechanical testing, and nondecalcified histology.

RESULTS

Fusions were assessed by manual palpation at 5 weeks for comparisons among the three groups of graft materials. The coralline hydroxyapatite used with bone marrow produced no solid fusions (0/14). When combined with an equal amount of autogenous iliac crest bone, coralline hydroxyapatite resulted in solid fusion in 50% (7/14) of the rabbits (P < 0.05). When combined with the osteoinductive growth factor extract, the coralline hydroxyapatite resulted in solid fusion in 100% (11/11) of the rabbits (P < 0.05). The fusion masses in the growth factor group were significantly stronger (1.8 +/- 0.2 vs. 1.3 +/- 0.1; P = 0.02) and stiffer (1.5 +/- 0.2 vs. 1.2 +/- 0.1, P = 0.04) based on tensile testing to failure when normalized to the adjacent unfused level.

CONCLUSION

These data indicate that coralline hydroxyapatite with bone marrow was not an acceptable bone graft substitute for posterolateral spine fusion. When combined with autogenous iliac crest bone graft-coralline hydroxyapatite served as a graft extender yielding results comparable to those obtained with autograft alone. Coralline hydroxyapatite served as an excellent carrier for the bovine osteoinductive bone protein extract yielding superior results to those obtained with autograft or bone marrow.

Inflammatory disorders of the cervical spine

The most common inflammatory disorders affecting the cervical spine include adult and juvenile rheumatoid arthritis, ankylosing spondylitis, Reiter's syndrome, and psoriatic arthritis. These disorders are characterized by typical deformities and instabilities of the cervical spine that result from the destruction caused by synovitis in bony and ligamentous structures in the neck. The treatment of these inflammatory lesions differs from the treatment of similar lesions found in the posttraumatic or degenerative spine. This article attempts to outline the epidemiology, clinical manifestations, and natural history of these conditions. Various radiographic parameters for evaluating disease progression have been used over the years, and their usefulness is reviewed in the context of recent studies better defining the radiographic natural history of these lesions. An algorithm for the use of the various imaging methods including magnetic resonance scanning is provided, and recent progress in delineating the proper timing of surgical intervention and the predictors of neurologic recovery is presented. The current surgical procedures available to treat these conditions are discussed with emphasis on distinguishing those cases in which stabilization alone is required from those in which a decompression procedure is also necessary.

LMP-1, a LIM-domain protein, mediates BMP-6 effects on bone formation

Glucocorticoids can promote osteoblast differentiation from fetal calvarial cells and bone marrow stromal cells. We recently reported that glucocorticoid specifically induced bone morphogenetic protein-6 (BMP-6), a glycoprotein signaling molecule that is a multifunctional regulator of vertebrate development. In the present study, we used fetal rat secondary calvarial cultures to determine genes induced during early osteoblast differentiation as initiated by glucocorticoid treatment. Glucocorticoid, and subsequently BMP-6, was found to induce a novel rat intracellular protein, LIM mineralization protein-1 (LMP-1), that in turn resulted in synthesis of one or more soluble factors that could induce de novo bone formation. Blocking expression of LMP-1 using antisense oligonucleotide prevented osteoblast differentiation in vitro. Overexpression of LMP-1 using a mammalian expression vector was sufficient to initiate de novo bone nodule formation in vitro and in sc implants in vivo. These data demonstrate that LMP-1 is an essential positive regulator of the osteoblast differentiation program as well as an important intermediate step in the BMP-6 signaling pathway.

Lumbar spine fusion by local gene therapy with a cDNA encoding a novel osteoinductive protein (LMP-1)

STUDY DESIGN

A posterior arthrodesis animal model using local expression of a newly discovered osteoinductive protein delivered in bone marrow cells.

OBJECTIVE

To introduce the concept of local gene therapy and determine its feasibility for achieving lumbar spine fusion using a gene for a novel osteoinductive protein: LIM Mineralization Protein-1 (LMP-1).

SUMMARY OF BACKGROUND DATA

Extensive work is currently underway to improve the healing success and morbidity associated with the gold standard bone-grafting material of autogenous iliac crest. As a result, alternative osteoinductive proteins and new delivery methods warrant investigation. The authors' laboratory recently identified a novel gene that had osteoinductive capacity in vitro and is therefore a candidate for a new in vivo osteoinductive agent.

METHODS

Single-level posterior lumbar and thoracic arthrodesis was attempted in 14 athymic rats. The graft material, which consisted of a devitalized bone matrix (no osteoinductive activity) soaked with 0.75 to 1.5 million bone marrow cells, was inserted with the dorsal spine exposed. In each rat, one site received marrow cells transfected with the cDNA encoding a novel osteoinductive protein. At the other site for a control, the marrow cells were transfected with the reverse copy of the cDNA that did not express any protein. Transfection of marrow cells for 2 hours was accomplished using the mammalian expression vector pCMV2. Rats were killed after 4 weeks, and the spines were evaluated by manual palpation, radiographs, and nondecalcified histology.

RESULTS

In the pivotal experiment, successful spine fusion was obtained in 9/9 (100%) of the sites receiving marrow cells transfected with the active LMP-1 cDNA and in 0/9 (0%) of the sites receiving marrow cells transfected with the reverse (inactive) LMP-1 cDNA. Radiographs and histology confirmed the manual palpation results, demonstrating controlled new bone formation in the carrier and marrow transfected with the active LMP-1 cDNA and essentially no bone induction in the sites treated with marrow cells that did not express the protein.

CONCLUSIONS

These data confirm that local delivery of the novel LMP-1 cDNA using bone marrow cells is feasible in vivo. Furthermore, these results demonstrate that posterior thoracic or lumbar spine fusion can be achieved in rats by local delivery of the LMP-1 cDNA.

The biology of posterolateral lumbar spinal fusion

This article reviews the existing knowledge base concerning the biology of spinal fusion, with the understanding that the focus is weighted toward posterolateral lumbar spinal fusion because of a relative paucity of biologic information on healing of other types of fusions. The discussion focuses first on the basic science of spinal fusion healing from the standpoint of animal modeling. Next, the discussion centers on the multitude of local factors that can affect fusion healing. Finally, the numerous systemic factors known to affect fusion healing are discussed.

Outcome assessment after spinal fusion: why and how?

Outcomes of medical interventions today must reflect a more complete array of measurements, including health status, patient satisfaction, medical costs, and quality of life. This article discusses the rationale and methodology involved with measurement of health status, particularly as it applies to spinal disorders and spinal fusion. Generic and disease-specific instruments are reviewed, and their limitations are also discussed.

Bone repair and enhancement clinical trial design. Spine applications

Spine fusion represents over half of all current bone grafting procedures. It is an important clinical area for bone augmentation because use even of the most successful graft material, autogenous iliac crest bone, is fraught with an unacceptably high failure rate. Various new options that biologically will control bone induction currently are under development. To prevent unnecessary clinical failures, it is important that surgeons understand the principles of clinical trial design, whether they actively are using the reported results or simply reviewing them in an effort to decide if they will use the new technology in their own practice. This review highlights the important issues relevant to designing a clinical trial for studying the efficacy of a bone augmentation product in a spine application. Some of the design challenges are generic, but others are unique to the spine because of difficulty in defining a clear diagnostic group, difficulty in noninvasively assessing the technical success of surgery, and frequent dissociation of the clinical and radiographic results.

Biologic enhancement of spinal fusion

Scientific advances in the past decade have generated considerable clinical interest in developing biologic tools that may ultimately enhance spinal fusion. This article reviews the current understanding of each of these and other fusion-enhancing tools with particular attention to the results of in vivo animal experimentation and, where available, objective clinical data.

The Marshall R. Urist Young Investigator Award. Gene expression during autograft lumbar spine fusion and the effect of bone morphogenetic protein 2

A prospective animal study of posterolateral lumbar spine arthrodesis was performed to determine the temporal and spatial pattern of gene expression and to determine the effect of recombinant human bone morphogenetic protein 2 on the gene expression pattern of a healing spine fusion mass. In Group 1, 20 adult New Zealand rabbits underwent L4-L5 posterolateral intertransverse process arthrodesis using autograft alone. Two rabbits were euthanized at each of the following points: 0, 2, and 4 days, and 1, 2, 3, 4, 5, 6, and 10 weeks after surgery. The same surgical technique was used for 16 rabbits in Group II, except that the autograft first was soaked in a solution of recombinant human bone morphogenetic protein 2 before implantation. Ribonucleic acid was extracted from different regions of the fusion mass at each point and analyzed for expression of bone and cartilage related genes using reverse transcription polymerase chain reaction. A reproducible temporal sequence and spatial pattern of gene expression was found in healing spine fusions. In the central portion of the fusion mass a temporal lag in gene expression was observed that parallels the lag in healing within the central zone previously observed in histologic studies. Treatment of bone graft with recombinant human bone morphogenetic protein 2 resulted in an increase in the early expression of bone morphogenetic protein 6 which was associated with expression of higher levels of Type I collagen, osteocalcin, and other bone related genes. These findings suggest that central nonunion may be associated with delayed expression of osteoblast related genes in the central region of the forming fusion mass. The growth factor, recombinant human bone morphogenetic protein 2, increased the level of bone related gene expression throughout the fusion mass, eliminated the delay in healing within the central zone, and may decrease the likelihood of a nonunion.

Biomechanical study of lumbar pedicle screws: does convergence affect axial pullout strength?

We tested the hypothesis that two pedicle screws placed in convergence offer more resistance to axial pullout than do two pedicle screws placed in parallel. Eight fresh cadaveric lumbar spines, L2-L5, were harvested. Individual vertebra were then paired for testing. Into each L2 and L3 vertebra, a pair of pedicle screws were inserted. The screws were placed parallel (i.e., at 0 degrees convergence) in L2 and at 30 degrees convergence in L3, in the first pair. In the second pair of L2 and L3, this order was reversed. Alternating the convergence angle was carried out on each successive pair of L2 and L3. A section of standard longitudinal rod was attached to each pedicle screw by using top-loading three-point shear clamp fixation with tangential lock screws. Transverse connectors were attached superiorly and inferiorly to the longitudinal rods. As in L2 and L3, each L4 and L5 received a pair of screws placed at either 0 or 30 degrees convergence, with the angle of convergence alternated on each successive pair. Each pair of screws in each vertebra was tested in axial pullout at 1 mm/s. Fourteen pairs of vertebra were tested and two findings emerged: paired pedicle screws at 30 degrees of convergence offered more resistance to axial pullout (28.6% on average) and sustained higher loads at the clinical threshold of loosening (101% on average) than paired pedicle screws placed in parallel. Thus there seems to be an advantage to screws placed in 30 degrees of convergence as compared with screws in parallel.

Vascularization of the fusion mass in a posterolateral intertransverse process fusion

STUDY DESIGN

A previously characterized rabbit model was used to study vascularization of the fusion mass in a posterolateral intertransverse process fusion.

OBJECTIVES

To determine the interosseus origin of the new blood vessels in a posterolateral intertransverse process fusion mass and to test the hypothesis that bone incorporation and the extent of vascularization are closely related.

SUMMARY OF BACKGROUND DATA

It has been reported that vascularization is essential for bone graft incorporation. There are, however, few reports dealing with vascularization of the spinal arthrodesis.

METHODS

Thirty-one adult New Zealand White rabbits underwent bilateral intertransverse process fusion, using autogenous iliac crest bone graft. The rabbits were killed at 3 weeks (n = 6) and 6 weeks (n = 25) after surgery, and colored silicone was injected to fix the vasculature. A semiautomated image analysis system was used to assess the percentage of the area of vascularization in the fusion mass and the transverse processes.

RESULTS

The major interosseus blood supply for vascularization of the autogenous bone graft came from upper and lower transverse processes. There were three types of fusion mass observed at 6 weeks after surgery: solid type, solid type with cartilaginous cleft, and nonunion type. There was significantly less vascularization of the fusion mass and of the transverse processes in the nonunion type compared with that in the solid type and with that in the cartilaginous cleft type.

CONCLUSIONS

There is a close correlation between bone incorporation and the extent of vascularization in a posterolateral intertransverse process fusion.

Management of low back pain. Current assessment and formulation of a blueprint for the health care delivery system of the future

This article outlines the authors' recommendations for future health care delivery to patients with low back pain and presents one potential blueprint for a modified delivery system. A group of multidisciplinary physicians and support staff that functions as a unified team with common goals is best positioned to overcome the challenges of providing quality care in a managed care environment. The burden on those who manage patients with spine disorders is to define and prove that high quality medical care results in genuine cost savings and better value for the health care customer.

An assessment of the early management of spine problems and appropriateness of diagnostic imaging utilization

Low back pain may affect up to 80% of all adults and is the second leading reason for physician visits in ambulatory medicine. Estimates for the annual direct medical costs for treating patients with back pain approach $25 million, despite the fact that it is a self-limited condition in at least 90% of patients with recovery occurring within 6 to 12 weeks. Recent scrutiny of health care delivery has produced numerous observations documenting a high variability in use of resources for medical conditions including low back pain, but few studies have attempted to examine the medical appropriateness of diagnostic and therapeutic decisions.

Reversing the inhibitory effect of nicotine on spinal fusion using an osteoinductive protein extract

STUDY DESIGN

The effect on spinal fusion of an osteoinductive bone protein extract in the presence of a known inhibitor of spinal fusion (systemic nicotine) was studied prospectively in an animal model of posterolateral lumbar fusion.

OBJECTIVES

To evaluate the ability of a bovine-derived osteoinductive bone protein extract to overcome the inhibitory effect of nicotine in a rabbit spine fusion model.

SUMMARY OF BACKGROUND DATA

Multiple studies have demonstrated the ability of a variety of osteoinductive growth factors to serve as a bone graft substitute for lumbar spinal fusion under "normal" healing conditions.

METHODS

Forty-eight adult female New Zealand white rabbits underwent spine arthrodesis at L5-L6 while receiving systemic nicotine through a subcutaneous miniosmotic pump. Arthrodesis was performed using one of the following three graft materials: 1) autogenous iliac crest, 2) osteoinductive bone protein delivered in an allogeneic demineralized bone matrix/ collagen carrier, or 3) osteoinductive bone protein delivered with autogenous iliac crest. Fusions were assessed by blinded manual palpation, radiography, and biomechanical testing.

RESULTS

Of the 44 rabbits manually tested by blinded observers, all 14 in the osteoinductive bone protein plus autogenous iliac crest bone group had solid fusions (14 of 14), whereas the fusion rate was less in the osteoinductive bone protein plus demineralized bone matrix group (nine of 14, 64%; P = 0.02), and there were no fusions in the autogenous iliac crest only group (0 of 16, 0%; P = 0.000001). The use of osteoinductive bone protein with autogenous bone produced stronger and stiffer fusions compared with those using autogenous bone alone or osteoinductive bone protein with allograft bone.

CONCLUSIONS

Cigarette smoking and nicotine are inhibitory factors in the healing of fractures and spine fusions. This study shows that the inhibitory effect of nicotine can be overcome with an osteoinductive bone growth factor in an animal model.

Experimental posterolateral lumbar spinal fusion with a demineralized bone matrix gel

STUDY DESIGN

A controlled rabbit model of lumbar posterolateral intertransverse process arthrodesis was used to evaluate a bone graft substitute.

OBJECTIVE

To determine the efficacy of demineralized bone matrix gel as an autograft extender, using different ratios of demineralized bone matrix to autograft and to determine the efficacy of demineralized bone matrix as an autogenous bone graft enhancer by adding it to the usual quantity of autograft.

SUMMARY OF BACKGROUND DATA

Autogenous bone is considered the most effective bone graft material for posterolateral lumbar arthrodesis, yet nonunions occur in up to 30% of patients. In addition, donor site complications may occur in 25-30% of patients. This has prompted the search for and investigation of bone graft extenders, enhancers, and substitutes. Commercially available demineralized bone matrix gel is one possible graft extender and enhancer, which, unlike mineralized allografts, has osteoinductive properties. Although the gel is in common use, the efficacy of demineralized bone matrix when used for posterolateral spine arthrodesis has not been examined in prospective clinical studies. Furthermore, no known animal studies have tested demineralized bone matrix gel in a posterolateral arthrodesis model.

METHODS

Forty-seven New Zealand white rabbits underwent bilateral posterolateral spine arthrodesis at L5-L6 using autogenous iliac crest bone graft alone or in combination with demineralized bone matrix. Four groups were formed on the basis of the ratio of autograft to demineralized bone matrix: autograft alone (3 mL), 100:0 group; autograft (3 mL) and demineralized bone matrix (1.5 mL), 100:50 group; autograft (1.5 mL) and demineralized bone matrix (1.5 mL), 50:50 group; and autograft (0.75 mL) and demineralized bone matrix (2.25 mL), 25:75 group. Rabbits were killed 6 weeks after surgery. Inspection, manual palpation, radiographic film, and histologic evaluation were used to assess fusion.

RESULTS

All groups had similar fusion rates (66-73%) based on manual palpation. Rabbits implanted with demineralized bone matrix had more mature fusion masses, evidenced by the greater trabecular bone formation seen on radiographic film and histologic study.

CONCLUSIONS

Demineralized bone matrix was effective as a graft extender when used in up to a 3:1 ratio with autograft in a rabbit posterolateral spine fusion model. When less than the standard volume of autograft was used, the addition of demineralized bone matrix gel lead to fusion success rates comparable to those of the standard amount of autograft alone. However, demineralized bone matrix did not increase the frequency of successful fusion when added to the standard amount of autograft.

Glucocorticoid-induced differentiation of fetal rat calvarial osteoblasts is mediated by bone morphogenetic protein-6

Glucocorticoids (GCs) at physiological concentrations promote osteoblast differentiation from fetal calvarial cells, calvarial organ cultures, and bone marrow stromal cells; however, the cellular pathways involved are not known. Bone morphogenetic proteins (BMPs) are recognized as important mediators of osteoblast differentiation. Specific roles for individual BMPs during postembryonic membranous bone formation have yet to be determined. We recently reported that GC potentiated the osteoblast differentiation effects of BMP-2 and BMP-4, but not of BMP-6, which, by itself, was the most potent of the three. In the present study, we used fetal rat secondary calvarial cultures to study the role of BMP-6 during early osteoblast differentiation. Treatment with the GC triamcinolone (10(-9) M) resulted in a 5- to 8-fold increase in BMP-6 steady-state messenger RNA levels, peaking at 12 h. In contrast, BMPs -2, -4, -5, -7, and transforming growth factor (TGF)-beta1 messenger RNA levels increased by less than 2-fold, after GC treatment, compared with untreated control cultures at 24 h. BMP-6 protein secretion increased 6- to 7-fold by 12 h and 12-fold (from 7.5 to 90 ng/ml) by 24 h, as measured by quantitative Western analysis. Treatment of cells with oligodeoxynucleotides antisense to BMP-6 diminished secretion of BMP-6 protein and significantly inhibited the GC-induced differentiation, as determined by a 10-fold decrease in the number of mineralized bone nodules, compared with controls that were treated with sense oligonucleotides or no oligonucleotides (ANOVA, P < 0.05). The antisense oligonucleotide inhibition of differentiation was rescued by treatment with exogenous recombinant human BMP-6. We conclude that GC-induced differentiation of osteoblasts from the pluripotent precursors is mediated, in part, by BMP-6. These results suggest that BMP-6 has an important and unique role during early osteoblast differentiation.

In vivo evaluation of a resorbable osteoinductive composite as a graft substitute for lumbar spinal fusion

The purpose of this prospective animal study was to evaluate the efficacy of a resorbable coral particulate to serve as a carrier with several doses of a bovine-derived osteoinductive bone protein mixture. A previously validated rabbit model for posterolateral intertransverse process lumbar spinal fusion was used. Posterolateral intertransverse process arthrodeses were performed at L5-L6 in 64 adult New Zealand white rabbits. The bone graft substitute evaluated consisted of a Biocoral/collagen composite with one of four doses (0, 100, 300, or 1,000 micrograms) of a bovine-derived osteoinductive bone protein extract (BP). Fusions were assessed at 5 weeks by manual palpation, radiography, histology, and biomechanical testing. Use of the Biocoral/collagen carrier without BP resulted in no solid fusions. Biocoral/collagen with 100 micrograms BP resulted in solid fusions in 31% (4 of 13) of the rabbits. Biocoral with 300 micrograms or 1,000 micrograms BP resulted in solid spinal fusion in all rabbits (27 of 27). There were no neurologic complications related to the growth factor or carrier. Small subcutaneous collections of serous fluids were noted in 26% of the animals in the 1st postoperative week, but resolved without problems by the 5th week. Such seromas may limit the clinical utility of this growth factor-carrier combination. Biocoral/collagen combined with the appropriate dose of bovine-derived osteoinductive bone protein was efficacious as a bone graft substitute for achieving posterolateral lumbar spinal fusion in the rabbit model. A dose of 300 micrograms BP was determined as the threshold to reliability produce solid spinal fusions.

Analysis of chondroitin sulfate in lumbar intervertebral discs at two different stages of degeneration as assessed by discogram

Previous studies have presented evidence that an underlying cause of intervertebral disc degeneration is related to changes in the sulfation of the proteoglycans. The sulfation of the chondroitin in cadaveric lumbar intervertebral discs, at two different stages of degeneration as assessed by discogram, were analyzed. Fourteen of 28 lumbar discs were graded 2 and the other 14 were graded 4 (i.e., more degenerated). From each disc, six regional segments were carefully isolated. Proteoglycans were solubilized from the disc tissue with 4 M GuHCl. Chondroitin sulfate chains were analyzed by diethylaminoethyl (DEAE)-Sephacel and high-performance liquid chromatography (HPLC) anion exchange chromatography. The major differences in sulfation of the chondroitin between grade 2 and grade 4 discs only occurred in the posterior central annulus and nucleus segments. The chondroitin in the posterior central nucleus segments of the grade 2 and grade 4 intervertebral discs were undersulfated as compared with the other segments, and the differences between these segments and the others were more accentuated in the grade 4 discs than in grade 2 discs.

Video-assisted lateral intertransverse process arthrodesis. Validation of a new minimally invasive lumbar spinal fusion technique in the rabbit and nonhuman primate (rhesus) models

STUDY DESIGN

Cadaveric anatomic and in vivo survival animal studies were performed to develop a new arthrodesis technique for the lumbar spine.

OBJECTIVES

To examine the feasibility, efficacy, and safety of a minimally invasive lumbar intertransverse process arthrodesis technique using an osteoinductive growth factor (rhBMP-2) delivered in a collagen sponge carrier. The technique was first developed using a rabbit model and modified for the nonhuman primate (rhesus monkey), a larger animal with the most similar bone biology to humans.

SUMMARY OF BACKGROUND DATA

The morbidity of conventional posterolateral lumbar intertransverse process arthrodesis includes graft donor site morbidity; paraspinal muscle devascularization, denervation, and scarring and nonunion in up to 36% of patients. Minimally invasive anterior lumbar interbody arthrodesis techniques have been developed, but these often require a metal implant and carry risks to major vessels and development of retrograde ejaculation. A minimally invasive technique for posterolateral intertransverse process arthrodesis has not been described previously.

METHODS

In Part 1, we examined seven New Zealand white rabbits and five rhesus monkeys at necropsy and during nonsurvival surgeries to determine the best access routes and to develop special instruments for video-assisted lateral intertransverse process arthrodesis. In Part 2, 38 New Zealand white rabbits underwent L4-L5 intertransverse process arthrodesis: the "OPEN" group (n = 16) underwent a standard open muscle-splitting approach using rhBMP-2 (bone morphogenetic protein) and collagen as a bone graft substitute; the "video-assisted control" group (n = 6) underwent video-assisted lateral intertransverse process arthrodesis with the collagen implant only (no growth factor); and the "video-assisted-BMP" group (n = 16) underwent video-assisted lateral intertransverse process arthrodesis with rhBMP-2 and collagen as the graft material. In Part 3, rhesus monkeys (n = 4) underwent video-assisted lateral intertransverse process arthrodesis using rhBMP-2 and collagen after laminectomy of L4-L5.

RESULTS

In Part 1, we identified expedient, minimally invasive routes to the intertransverse process interval appropriate for each species: an intermuscular approach for the rabbit and a lateral, extramuscular approach for the rhesus monkey. In Part 2, all rabbits in the OPEN and video-assisted-BMP groups achieved solid intertransverse process lumbar fusions when assessed at 10 weeks. There were no neurologic impairments nor any difference between the two groups in the frequency of postoperative infection or other complications. None of the animals in the video-assisted control group showed evidence of fusion. In Part 3, exposure, decortication and grafting with rh-BMP-2 and collagen was accomplished successfully in all four monkeys through the video-assisted minimally invasive approach without complications.

CONCLUSION

Video-assisted lateral intertransverse process arthrodesis is a feasible, effective, and safe method of lumbar spinal fusion in the rabbit and rhesus monkey. Use of this arthrodesis procedure will minimize the morbidity of paraspinal muscle denervation and devascularization seen with open intertransverse process fusion techniques, and the use of an osteoinductive growth factor will eliminate the problem of graft donor site morbidity and possibly increase the chances for successful fusion.

Noninvasive markers of bone metabolism in the rhesus monkey: normal effects of age and gender

Measurement of bone turnover in conditions such as osteoporosis has been limited by the need for invasive iliac bone biopsy to reliably determine parameters of bone metabolism. Recent advances in the area of serum and urinary markers of bone metabolism have raised the possibility for noninvasive measurements; however, little nonhuman primate data exist for these parameters. The purpose of this experiment was to define the normal range and variability of several of the newer noninvasive bone markers which are currently under investigation in humans. The primary intent was to determine age and gender variability, as well as provide some normative data for future experiments in nonhuman primates. Twenty-four rhesus macaques were divided into equal groups of male and female according to the following age groupings: 3 years, 5-10 years, 15-20 years, and > 25 years. Urine was collected three times daily for a four-day period and measured for several markers of bone turnoverm including pyridinoline (PYD), deoxypyrodinoline (DPD), hydroxyproline, and creatinine. Bone mineral density measurements of the lumbar spine were performed at the beginning and end of the study period. Serum was also obtained at the time of bone densitometry for measurement of osteocalcin levels by radioimmunoassay. There were no significant differences in bone mineral density, urine PYD, or urine DPD based on gender. Bone density was lowest in the youngest animals, peaked in the 15-20-year group, but again decreased in the oldest animals. The osteocalcin, PYD, and DPD levels followed an inversely related pattern to bone density. The most important result was the relative age insensitivity of the ratio of PYD:DPD in monkeys up to age 20 years. Since bone density changes take months or years to become measurable and iliac biopsies are invasive, the PYD/DPD marker ratio may have important implications for rapid noninvasive measurement of the effects of potential treatments for osteoporosis in the non-human primate model.