BONING UP

At the human body shop, bioceramics or biopolymers combined with bone cells and growth factors are likely to lead the parts list

A woman struck by a car in Florida loses 3 inches of her crushed lower leg and could be left to live with a drastic disability for the rest of her life. A Detroit teenager with a marble-sized divot in his thigh from the removal of a benign tumor could spend the rest of his life being overly cautious.

But the woman is now back to ballroom dancing, and the teenager is swimming again and plans to go skiing.

These dramatic tales of people who against great odds recovered from accidents or diseases to live reasonably normal lives are examples of the benefits reaped from research that began in the 1980s to characterize with increasing accuracy the content and structure of human bone. The success of this research now is resulting in new drugs and treatments for bone diseases, a host of new synthetic materials to repair bone fractures or to serve as bone replacements, and tissue engineering techniques to induce bone growth from scratch using a patient's own bone cells.

"There is more activity now in this field than there ever has been before, which I find gratifying," says Ralph E. Holmes, professor of surgery and head of the division of plastic surgery at the University of California, San Diego, Medical Center. Holmes, who invented a scanning electron microscope backscatter imaging technique to quantitatively measure ingrowth of bone in implants, has been a leader during the past 20 years in evaluating many synthetic materials. "I think the current activity is a sign of the maturity of all the sciences involved and a recognition that not just one thing can make bone heal," Holmes observes.

Although bone seems lifeless, it actually is made up of a very alive, porous framework that is constantly rebuilding itself. Bone is a composite material made up of collagen protein fibers threading through hydroxyapatite, Ca₅(PO₄)₃OH. Hydroxyapatite makes up about 70% of bone structure and essentially all of the enamel in teeth. The collagen fibers, a bundled array of cross-linked helical polypeptide strands, provide extra strength, allowing bones to flex under stress.

BENEATH BONE'S HARD EXTERIOR...

Bone tissue replaces itself through the action of cells called osteoclasts that produce acids to dissolve (resorb) hydroxyapatite and enzymes to break down collagen. The resulting release of calcium and proteins prompts other cells called osteoblasts to lay down new matrix that mineralizes and forms hydroxyapatite and collagen. Some growth factors, such as bone morphogenetic proteins, are manufactured by bone cells themselves to either increase or decrease bone remodeling.

Blood vessels snake through the bone framework, too, carrying many different compounds that orchestrate bone remodeling, such as calcitrol (a form of vitamin D-3), calcitonin (a thyroid hormone that prevents bone resorption), parathyroid hormone (which works with calcitrol to regulate calcium and phosphate metabolism), and prostaglandins (fatty acids that perform hormonelike functions). Beneath all of that is the bone marrow that creates the osteoblasts and osteoclasts as well as the red and white blood cells.

Skeletal deficiencies from trauma, tumors and bone diseases, or abnormal development frequently require surgical procedures to attempt to restore normal bone function. Although most of these treatments are successful, they all have associated problems and limitations.

For a minor fracture, usually a few weeks in a cast are all that is needed for the bone to repair itself. For a more severe fracture or one in a particularly tricky position, a bone cement or filler may be used to help strengthen the fractured bone so it heals faster. These procedures may or may not require metal hardware such as plates, pins, or screws for extra support.

For severe fractures, a bone graft may be needed. In a typical bone graft, natural bone or a synthetic material is shaped by the surgeon to fit the affected area and held in place with hardware. Over time, the natural bone growth process takes over and at least partially resorbs the grafted bone. Key to the level of resorption is the porosity of the implant material that allows ingrowth of vascularized tissue.

Surgeons have been performing bone grafts for years. The preferred method is a procedure called an autograft, where bone fragments are taken from a patient's own body-usually a hip (iliac crest), the pelvis, or ribs-and affixed to healthy bone. An alternative method, called an allograft, uses bone donated from a cadaver and works nearly as well.

Bone grafts are painful, complicated procedures that generally involve a long recovery period. Drawbacks to autografts are the two surgical procedures needed, meaning longer hospitalization and recovery time, and higher cost. Besides, the body doesn't carry much spare bone.

Allografts eliminate the need for a second surgery, but after sterilization, the donated bone loses much of its strength. In addition, when bone grafts are taken from cadavers, there is the risk of rejection or of transmitted diseases such as hepatitis B or AIDS caused by infection with the human immunodeficiency virus. Allograft procedures have been on the decline since late 1993 when the Food & Drug Administration issued regulations governing bone and tissue banks because of the risk of transmitting infectious diseases.

A third option, which today is becoming more viable, is use of a synthetic material to replace lost bone. Synthetic materials have the advantage of eliminating the need for surgery to claim bone for the graft procedure, and eliminating the chance for rejection or transmission of infectious disease. An additional benefit is a significant reduction in medical cost and, in general, a faster recovery time.

The ideal bone substitute, according to Holmes, would approximate the autograft, requiring minimally that it be biocompatible and osteoinductive so that the body's natural bone-making process eventually would replace the implanted material. Most synthetic materials, however, are weakly resorbed or do not resorb at all. But low resorption is not necessarily a drawback, he says.

In some applications, such as a long bone fracture, one might hope for complete resorption of a synthetic material. But in other cases, such as reconstruction of the chin, resorption may not be desirable at all. Thus, a combination of autograft and synthetic material with hardware often turns out to be the best solution, Holmes notes.

The driving force to develop new treatments for bone diseases, the fractures associated with them, and fractures from trauma is that they make up an important segment of the health care industry. Americans suffer some 5.6 million fractures, and surgeons perform about 3.1 million orthopedic procedures in the U.S. each year, according to the American Academy of Orthopaedic Surgeons. Industry experts expect these numbers to increase steadily as the general population grows older.

In 1995, the latest year for which statistics are available, there were some 426,000 bone-graft procedures in the U.S., according to Medical Data International (MDI), a market research firm based in Irvine, Calif. That makes bone second only to blood transfusions on the list of transplanted materials.

The worldwide bone-graft market was estimated to be about $800 million annually, about half in the U.S., MDI reports. In 1995, about 58% of bone-graft procedures were autografts, compared with 34% allografts, and 8% synthetic materials, and nearly half of the procedures involved the spine. The percentage of synthetic procedures is expected to rise substantially as new biomaterials continue to come onto the market.

Treating bone diseases
When osteoclasts dissolve old bone faster than osteoblasts can replace it, the result is osteoporosis. Osteoporosis is characterized by loss of bone density that can lead to debilitating fractures of the hip and spine. The disease usually strikes postmenopausal women (age 50 to 70) as a result of decreased estrogen and progestin levels. The disease also can occur later in life (after age 75) in women and, as a result of testosterone imbalance, in men.

According to the National Osteoporosis Foundation, an estimated 20 million women in the U.S. have osteoporosis. That number is projected to grow to more than 35 million by 2015. Furthermore, the financial impact of osteoporosis in the U.S. has been estimated to be almost $14 billion per year in medical costs and lost productivity. These sobering statistics are the driving force for research into drugs to prevent or treat osteoporosis and other bone diseases. Industry analysts expect the global osteoporosis market for pharmaceuticals to reach $5 billion by 2005.

Osteoporosis can be detected by radiological imaging techniques used to measure loss of bone density. Besides prevention through diet, a number of therapies are being used to treat or prevent the disease. Estrogen replacement therapy, approved in 1988 by FDA, is common. Combined estrogen-progestin therapy as well as calcitonin therapy also are being used.

Diphosphonates are a class of compounds that are now becoming available in the U.S. as nonhormonal drugs for prevention of osteoporosis and as a treatment to help prevent bone fractures in patients who already have osteoporosis. The drugs are also proving successful for treatment of Paget's disease of bone (a chronic dissolving of normal bone followed by disorganized, enlarged, and weakened bone formation) and heterotopic ossification (abnormal overgrowth of bone, usually at postoperative sites such as the hip). Although the exact mechanism of action of diphosphonates is unclear, they accumulate on bone surfaces and inhibit osteoclast resorption, allowing the bone-formation process to dominate.

DIPHOSPHONATES HELP BOOST BONE DENSITY

The use of diphosphonates to treat bone disease was first advanced by emeritus research chemist M. David Francis and coworkers at Procter & Gamble in Cincinnati. Francis' other research accomplishments include discovering the benefits of adding fluoride salts to toothpaste to prevent tooth decay and adding pyrophosphate (P₄O₇^4-) for tartar control (C&EN, March 11, 1996, page 34).

Francis led the development of etidronate (disodium 1-hydroxyethane-1,1-diphosphonate), a compound that Procter & Gamble originally had looked at as a detergent additive to chelate calcium and other hard-water ions. Etidronate, now marketed by Procter & Gamble as Didronel, is used in the U.S. and abroad to treat Paget's disease and heterotopic ossification. Procter & Gamble also markets Didronel in 17 countries for osteoporosis.

However, in April of this year, Merck announced that it had become the first company to receive final U.S. approval for a diphosphonate to help prevent or treat osteoporosis. Alendronate (sodium 4-amino-1-hydroxybutylidene-1,1-diphosphonate) is now marketed in 49 countries as Fosamax.

FDA's decision to clear the drug, according to the company, was based primarily on two-year results of an ongoing six-year study of more than 1,600 women ages 45 to 59. The drug is working as well as hormone replacement therapy to increase bone mass at the hip and spine, while a placebo group is showing a gradual loss of bone mass.

Procter & Gamble has another diphosphonate compound, risedronate (sodium 2-(3-pyridinyl)-1-hydroxyethylidene-1,1-diphosphonate), that is in late-stage Phase III clinical trials for treatment of osteoporosis and Paget's disease. In March, the company filed a new-drug application with FDA for treatment of Paget's disease, and plans to file another application for treatment of osteoporosis in 1998. In May, Procter & Gamble announced it was forming a global alliance with Hoechst Marion Roussel-the pharmaceutical company of Germany's Hoechst-to commercialize the new drug under the name Actonel.

Mending and making bones
Poly(methyl methacrylate), an old standard, has been used for decades as a synthetic filler to repair skeletal defects and affix metal implants to bone. Usually methyl methacrylate is polymerized in situ at the site where additional bone is needed. The polymer hardens to become stronger than bone and is generally a good substitute.

One of the advantages of poly(methyl methacrylate) as well as similar types of bone fillers and cements is that they can be injected through the skin. But there are a couple of problems critical to the use of these materials. One is that they usually must cure in situ and in doing so can generate heat that could damage surrounding soft tissues. Another, and perhaps more important issue, is that most of the materials are minimally degradable or don't degrade at all, nor can they support ingrowth of new bone tissue.

The concept of developing materials that are biocompatible and can at least be partially resorbed in the natural bone-growth process grew out of the ground-breaking research in the 1960s by Marshall R. Urist, a professor of orthopedic surgery at the University of California, Los Angeles.

Urist was the first researcher to conclusively demonstrate the phenomenon of osteoinduction, or the natural process of bone desorption and formation. For more than a century, surgeons had recognized the ability of demineralized bone to aid bone healing. But it was Urist's work on implanting demineralized bone segments into animals and being able to induce new bone growth within the implants that led him to conclude in 1965 that cells in the bone matrix stimulated cells at an implant site to differentiate into osteoblasts and osteoclasts. He later showed that bone morphogenetic proteins are the sole inducers of bone cell differentiation. His original paper reporting those findings was recently reprinted as a landmark paper in the Journal of NIH Research [9, 43 (1997)].

Urist's findings coupled with better diagnostic techniques subsequently gave rise to a greater understanding of bone structure and the bone-formation process. This understanding, in turn, led to the development of a host of calcium-based synthetic bone products designed to mimic natural bone and to actually be resorbed by the body. It has been only recently, after many years of animal trials, that these materials have advanced to the point that they are acceptable for use in humans and are starting to gain FDA approval.

One company well ahead in the synthetic bone implant market is Interpore International of Irvine, Calif. In November 1992, Interpore became the first company to receive FDA approval for a synthetic bone-void filler.

The company's Pro Osteon hydroxyapatite is made from coral through a thermochemical process developed in the 1970s. Currently, it is the only synthetic product on the market that has a porous infrastructure similar to natural bone. The interconnected structure of the coral remains intact throughout processing, providing a matrix through which blood vessels and new bone tissue can grow.

INTERPORE'S SYNTHETIC BONE GRAFT...

Interpore acquires between 2 and 4 tons of coral each year from atolls in the Pacific and Indian Oceans to make its product, less than 1% of the total annual amount of coral imported into the U.S., the company says. The amount of coral harvested for import is controlled by the Convention on International Trade of Endangered Species of Wild Fauna & Flora (CITES) and is generally agreed upon by ecologists to present little threat to fragile reefs as long as the coral is harvested in ways that sustain the reefs as living structures. One coral "head" weighing 150 to 200 lb provides enough material for several hundred bone grafts.

The synthetic material is prepared by heating the coral-which is essentially calcium carbonate-with ammonium phosphate at more than 200 °C for 24 to 60 hours to obtain about 95% hydroxyapatite. The material is processed into block or granular form and sterilized by gamma radiation.

Used similarly to natural bone in autograft procedures, the synthetic material has about the same length of healing time. A surgeon can shape a block of the material, for example, to fit into a fracture crevice or a carved out portion of a long bone. The graft area is then stabilized with a metal plate and screws, which later can be removed.

The natural porosity of the material does have the drawback of reducing its strength, notes David C. Mercer, Interpore's president and chief executive officer. But the porous structure provides room for bone tissue to immediately grow into the pores of the implant. However, the material is only partially resorbed and replaced by natural bone. The company is now evaluating in preclinical studies a related new product that has a higher resorption rate.

Pro Osteon is currently approved for nonweight-bearing treatment of fractures at the wide end of long bones and for jaw and reconstructive facial surgery, according to Mercer. However, the material has been used in many cases to replace a short section of a long bone, he says. In such cases, the limb must be immobilized for a long period-perhaps several years-to ensure new bone growth has become strong enough to support weight.

Sales of Pro Osteon have increased steadily since 1992, reaching $11.7 million in 1996, up 47% from the previous year. Sales in the first half of 1997 continue to be strong, increasing 13% from last year's first half to $6.3 million. Interpore also has approval to sell its bone substitute in 41 countries and began international marketing in 1995.

Although synthetic biomaterials on the market or under development work well for their intended functions, none has yet proven to be sufficiently strong or able to be processed into a large enough piece to stand in as a complete replacement for long bones.

Research by University of Texas, Austin, chemistry professor Richard J. Lagow, however, appears to have come the closest to that goal. Lagow makes a high-purity hydroxyapatite from scratch by reacting calcium metal, calcium hydroxide, and phosphoric acid at 700 to 850 °C. Lagow also has come up with analogous high molecular weight linear calcium polyphosphates by reacting hydroxyapatite with phosphoric acid. The success of his research has led to the creation of a small Austin-based company, called OsteoMedica Inc., to develop the synthetic materials as potential complete bone substitutes.

Key to developing these compounds for implant materials, Lagow says, is their high purity, which does not retard bone growth. Also important, he says, is holding the reaction temperature below the 1,200 °C temperature at which calcium phosphate fuses into a ceramic.

Besides controlling bulk size and shape, during his proprietary synthesis and processing method, Lagow can moderate the interconnecting porosity of the synthetic material-in the range of 150- to 400-µm pores-to match the density of different types of bone. "Interconnecting means that the body can vascularize it quickly and bone can then grow into the material much faster because there is a greater surface area for the osteoblasts and osteoclasts to work," Lagow notes. "Otherwise, the osteoclasts must tunnel through the bone matrix to resorb the synthetic material, which is a much slower process."

Lagow has collaborated with UT Austin chemical engineering professor Joel W. Barlow and others to develop a selective laser sintering technology that they have patented to fabricate complex bone shapes from the calcium phosphate materials.

Barlow and his research group developed a technique whereby they use a spray drier to coat hydroxyapatite or calcium phosphate powders with a poly(methyl methacrylate) that acts as a binder. The materials readily fuse in the sintering process but maintain their interconnecting pore structure. And, unlike other bioceramics being investigated as bone substitutes, they retain their high strength during processing, Lagow says.

LAGOW'S METHOD TO SYNTHESIZE...

The laser can be guided to form intricate bone shapes by computer, using data sources such as magnetic resonance imaging or computed tomography. The molded ceramic is then heated to remove the polymer and further processed. Thus far, the researchers have been able to generate a wide range of bone sizes and shapes. The technology has since been licensed to BioMedical Enterprises Inc., San Antonio, which is pursuing biocompatability studies of fabricated calcium phosphate implants.

OsteoMedica's goal is eventually to provide surgeons with synthetic molded bone "blanks" that can be custom shaped to fit a patient's needs. The company's bioceramic, called Megagraft 1000, so far has been successful in tests replacing tibia sections in rabbits and other animals.

And in a very successful study on dogs, sections of the radius were replaced with the synthetic bone. After 11 months, support plates and screws were removed, and the dogs eventually regained full use of their legs. One of the most promising aspects of the study was that, some three years after the time of the implant, the synthetic material was completely resorbed- results that haven't been reported for other synthetic materials. OsteoMedica is preparing to begin clinical trials in humans for spinal fusions in the U.K. and Australia, where regulatory requirements aren't as stringent as in the U.S.

"Being able to synthesize novel hydroxyapatite material to make it accessible to higher bone ingrowth and remodeling rate is a critical step," says UC San Diego's Holmes, who has evaluated both Interpore's and OsteoMedica's products. While he finds both materials work well for their intended use, a product such as OsteoMedica's hydroxyapatite being available in different pore sizes would be particularly useful for a wide range of applications to control the level of resorption, he notes. "With a range of porosities and resorption rates, surgeons could learn to choose the synthetic material for a particular application much in the way they choose a suture material."

Glen O'Sullivan, an assistant professor of orthopedics at Stanford University Medical Center, also has used Interpore's product in his patients and has worked on a clinical study of spinal fusions in sheep using OsteoMedica's material. He, too, finds both materials perform well for their designed use.

"One advantage of OsteoMedica's product is it is one of the hardest materials," O'Sullivan notes. "For example, Interpore's material can be crumbled between the fingers, while OsteoMedica's is strong enough to drill holes in it. This makes OsteoMedica's a good candidate for use in the spinal column, whereas Interpore's would not be-and it isn't approved by FDA for that."

Holmes and O'Sullivan agree that there isn't one material that is going to be suitable for all applications. "With any of these new products," O'Sullivan says, "one likes to be optimistic, but the hopes and expectations may not always pan out for all applications." It makes sense, he adds, that FDA only approves what might appear to be broadly applicable bioceramic materials for a narrow range of applications.

There are perhaps dozens of calcium-based synthetic materials in addition to Interpore's and OsteoMedica's that have received approval for use in the past couple of years or are anticipating approval soon. In 1996, for example, FDA approved OsteoSet, a calcium sulfate (plaster of paris) bone-void filler developed by Wright Medical Technology, Arlington, Tenn., that is reasonably resorbed by the body in as little as eight weeks. Another product cleared in 1996 by FDA for repair of cranial defects is a hydroxyapatite bone cement developed by American Dental Association Health Foundation researchers called BoneSource. The material will be manufactured by OsteoGenics Inc. and distributed by Howmedica Leibinger, a division of Pfizer.

An injectable bone cement developed by Norian Corp., Cupertino, Calif., is described by the company as a biocompatible, moldable compound made by mixing calcium phosphate, tricalcium phosphate, and calcium carbonate with sodium phosphate solution into a toothpastelike substance. Norian currently has approval to market its Skeletal Repair System in Europe and Canada, and is working toward regulatory approval in Japan and the U.S., mainly for treatment of wrist and hip fractures.

New directions
In 1993, FDA approved one of the first bone graft substitutes that capitalized on a new concept for products to facilitate bone repair. Collagraft, marketed by Bristol-Myers Squibb subsidiary Zimmer Inc., Warsaw, Ind., is a hydroxyapatite/tricalcium phosphate and bovine collagen that must be mixed with a patient's own bone marrow.

In late 1996, Interpore signed a license and development agreement with Quantic Biomedical, San Rafael, Calif., for a technology to use a gellike material containing bone growth factors from a patient's own blood that can be combined with its coral-based Pro Osteon to provide accelerated bone growth. Preclinical feasibility trials are expected to begin by the end of this year.

These examples illustrate the impact that the "delayed discovery" of bone morphogenetic proteins (BMPs) has had in opening the research community to a new direction: the concept that BMPs, bone cells, and various hormones could form the basis of an engineered system for bone repair that includes bioceramics or biopolymers. In essence, the world of bioceramics is being wed to the world of tissue engineering.

Although Urist's work on osteoinductivity was definitive, most researchers weren't convinced until BMPs actually began to be cloned by recombinant DNA methods some 20 years after his landmark research paper.

In 1988, the first group to clone a BMP was that of senior director John M. Wozney at biotechnology company Genetics Institute, Andover, Mass. [Science, 242, 1528 (1988)]. Today, one of Genetics Institute's molecules, BMP2, is in clinical trials for fracture repair, spinal fusion, and other possible applications. Several other biotechnology and pharmaceutical companies are testing the more than 30 BMPs cloned thus far for potential use in bone- and tooth-mending applications. "BMPs are destined to bring osteogenesis under the control of surgeons before the turn of the century," Urist noted in a commentary on his landmark paper.

"Despite the great advances in the synthetic materials, one still needs the bone cells," O'Sullivan states. "Bone cells are needed in the implant material and you want a means of stimulating bone cell activity. This is where research with BMPs is really going to take off." O'Sullivan notes that even during autograft procedures, surgeons attempt to aspirate bone cells from adjacent bone to incorporate into the implanted bone, and sometimes use material extracted from presurgical blood donation by the patient to help induce implant bone growth and resorption.

O'Sullivan points out that a critical step remaining for BMPs is to find the optimum carrier for implantation. Although bioceramics likely will work well, he says, they do have the problem of slow resorption. Thus many researchers believe that biodegradable polymers will work best as delivery devices for human growth factors, he says.

One of the leaders in the biodegradable polymer area is Antonios G. Mikos, an associate professor in the department of chemical engineering and the Institute of Biosciences & Bioengineering at Rice University, Houston. "The advantage of using polymers is that one can very accurately engineer their mechanical properties and degradation characteristics," he explains. The size and shape of the scaffold can be made to order as well, depending on which bone a potential patient may need.

Mikos and his research group are working on strategies to naturally grow bone from scratch either in vitro or in vivo by seeding natural or synthetic polymer scaffolds with bone cells or to use the scaffolds as conduits to induce new bone growth from surrounding tissues. Several substrate materials are being investigated by a number of researchers, Mikos notes, including poly(-hydroxy esters), polyanhydrides, polyimides, polyphosphazenes, and collagen.

The success of such strategies is dependent on the scaffold material's being biocompatible, osteoconductive, and quickly degradable into products that can be metabolized or excreted, he explains. For example, poly(lactic-co-glycolic acid) breaks down to lactic acid and glycolic acid, which are metabolized in the body and excreted as carbon dioxide and water.

Osteoblast transplantation onto a polymer scaffold would eliminate the problem of donor scarcity, immune rejection, and pathogen transfer by taking the needed cells from a patient's own body, Mikos points out. Although osteoblasts may be obtained by a variety of methods, including bone chips from an injury site or enzymatic digestion of harvested bone, the most desirable method would be to obtain the cells from the patient's own bone marrow. Osteoblasts obtained from bone marrow, for example, can also be expanded in tissue culture in a lab and seeded onto a polymer scaffold for implantation.

Poly(lactic-co-glycolic acid) has been extensively investigated as a material for tissue-engineering scaffolds because it already has been approved by FDA for use in surgical sutures, can be made with controlled pore size, and degrades well. The first such scaffolds were designed by biomedical and chemical engineering professor Robert S. Langer at Massachusetts Institute of Technology and Joseph P. Vacanti of Harvard Medical School in the late 1980s to create an in vitro environment that enables cells to organize themselves to form functioning tissues. Langer and Vacanti prepared crude scaffolds by bonding together poly(lactic-co-glycolic acid) fibers into a two-dimensional network.

In 1991, working with Langer and Vacanti, Mikos (then at MIT) further developed the polymer scaffolds by incorporating sodium chloride crystals into the copolymer matrix by adding crystals to a solution of the dissolved polymer. The salt crystals were later leached out, leaving behind a porous polymer matrix. Mikos was able to control porosity and pore size by varying the concentration and size of the crystals.

In another technique, Mikos extruded polymer fibers and aligned them in the shape of the desired scaffold. He embedded the arrangement in a polymer with a higher melting point and bound the scaffold together by heating. After cooling, he selectively dissolved the embedding medium, leaving behind an interconnected, highly porous structure.

Mikos, graduate student Susan L. Ishaug-Riley, and coworkers have recently conducted feasibility studies to show that bone formation in vitro and in vivo is possible by culturing rat osteoblasts in three-dimensional poly(lactic-co-glycolic acid) foams of different pore sizes (shown on the cover of this issue).

In one study, the polymer foams supported the proliferation of the seeded rat osteoblasts in vitro to form a calcified bonelike tissue after two months [J. Biomed. Mater. Res., 36, (1997)]. The goal of the study was to gain a better understanding of the important parameters in the design of an osteoblast foam-culture system before attempting osteoblast transplantation in vivo.

In a subsequent in vivo study, rat bone marrow osteoblasts were seeded onto polymer foams and implanted into the rat mesentery (the membrane of the abdominal cavity) [J. Biomed. Mater. Res., 36, 1 (1997)]. Growth of islands of mineralized bonelike tissue in the foam surrounded by fibrovascular tissue was observed within one week and had significant penetration of bone tissue into the scaffold after seven weeks.

The findings were encouraging, the researchers note, because they indicate that the regenerative potential of the seeded polymer scaffolds for new bone growth with transplanted cells and secreted bone growth factors may further induce bone growth from adjacent bone. Ishaug-Riley received a student outstanding research award from the Society for Biomaterials for the in vivo study at the society's 23rd annual meeting in New Orleans in May.

Osteoblast transplantation is not a straightforward approach, Mikos says, noting that because bone is highly vascularized, it is not possible to engineer and grow a complete bone or bone fragment in vitro and transplant it. "The maximum thickness of new bone one can create in vitro is a few hundred micrometers, which is not significant for clinical applications. However, the goal is to form new bone tissue in vivo and not in vitro. Then, vascularization becomes equally important to bone formation and necessary for regeneration."

A critical issue for cell transplantation is which phenotype of transplanted cell should be used. "It is not clear if one should transplant osteoblasts or preosteoblasts or progenitor cells," he says.

Mikos believes it will be possible in the next decade for tissue-engineered implants to be used for the reconstruction of skeletal deformities resulting from trauma, tumors, or abnormal development. "I hope that new cell-based therapies will be developed for the treatment of osteoarthritis and osteoporosis based on combinations of degradable biomaterials, growth factors, and cells," he says. "Yet, the main drawback with new polymers is the time and effort needed to get FDA approval for their use."

O'Sullivan is optimistic about the prospects of tissue engineering in bone repair, but also cautious about a couple of potentially critical problems. FDA currently is not certain about how to regulate tissue-engineered products, he says. (A problem with tissue cultures is potential contamination with a fungus, bacteria, or mold.) "And it will be interesting to see if the new technology will become available in an affordable manner, given that the couple of companies working on BMPs have spent a tremendous amount of money during the past few years to develop a research infrastructure." He thinks the BMP companies will end up controlling the technology development path for the synthetic implant companies.

"The next step is going to be a fine balance between cost and whether the outcome is going to be worth it."

Interpore's bone-void filler is made from coral that is converted to hydroxyapatite and processed into block or granular form. X-ray images of an ankle show a fracture (indicated by red arrows) in which the bone fragment was removed and replaced by the synthetic material. Metal plates and screws observed in the postoperative X-ray hold the implant in place and support the ankle until it heals.