Using DBMs in clinical orthopedics

Demineralized bone matrix has a variety of uses but no industry standard exists for its processing.

Issue: October 2005

ByDouglas W. Jackson, MD

Dr. Tom Einhorn is a frequent contributor to both articles and commentary in Orthopedics Today. In addition, he is a well-respected mentor, teacher, respected researcher and contributor to orthopedic literature. I turned to Tom to share with our readers some succinct answers to key questions on the current status of demineralized bone in orthopedics.

Douglas W. Jackson, MD: What is the definition of “demineralized bone matrix?”

Thomas A. Einhorn, MD [photo]
Thomas A. Einhorn

Thomas A. Einhorn, MD: Demineralized bone matrix (DBM) is a product of cortical bone that has been taken through a demineralization process, usually involving exposure to 0.6N hydrochloric acid. It is used clinically as a bone graft substitute based on its complement of osteoinductive growth factors (such as bone morphogenetic protein, or BMP) and osteoconductive proteins. Indeed, all commercially available DBMs come from cortical bone as studies have shown that cancellous bone is not particularly osteoinductive. However, there is no industry standard for DBM processing, and many DBM products exist in varying particle sizes and are prepared with different methods. A variety of solvents can be used, including water, detergent, alcohol and in some cases, oxidizing agents such as hydrogen peroxide. Some tissue banks sterilize with gamma irradiation and others with electron beam radiation. Few, if any, tissue banks have systematically analyzed their processes and formulations to determine which are most advantageous.

A fair amount of research has been done on DBM preparations and these have dispelled many logical assumptions. For example, while the growth factor activity of DBM preparation may vary, this variation is not due to inherent differences in growth factor complement in the bone source but rather how the DBM is processed. Moreover, while all tissue banks and DBM suppliers in the United States draw from the same donor pool, it has been shown that age and gender are not relevant factors in the biological activity of the finished product. Again, this strongly suggests that how DBM is processed and formulated have the biggest effects on its potential efficacy.

DBMs can be formulated with a variety of carriers. Water is an important component of the carrier mix but various commercial DBM preparations include other components such as glycerol, starch, hyaluronic acid, calcium sulfate, human collagen, porcine collagen and lecithin. These components, in addition to the various methods of sterilization and water content, potentially have large effects on the finished product and, consequently, its performance as a bone graft substitute. In particular, water-based formulations have a limited shelf-life — usually no more than one year. How DBM is milled is also important; all formulated DBM products today are particles which the exception of Grafton (Osteotech, Eatontown, N.J.) which is milled as fibers. This may be advantageous to its osteoconductive activity.

Jackson: Are there good preclinical data that DBM enhances osteoinduction and/or osteoconduction?

Einhorn: Marshall Urist’s seminal discovery that DBM induces bone formation when implanted into an extra skeletal site comprised the first set of pre-clinical data to show that DBM has an inductive effect on cells. Since that observation was made in 1965, more than a thousand articles have appeared in the literature demonstrating bone induction in orthotopic, heterotopic and skeletal defect sites in animals. Indeed, careful studies were performed documenting the biochemical responses to DBM in rodent tissues. These reports have shown a highly reproducible and consistent expression pattern of specific osteogenic genes. Subsequent studies elucidated the specific BMPs and other members of the transforming growth beta molecule superfamily, which are present in DBM.

“How DBM is processed and formulated have the biggest effects on its potential efficacy.”
— Thomas A. Einhorn

However, the most clinically relevant analysis to be conducted in an animal study is a so-called bioassay, in which the bone forming effect of DBM is assessed in vivo. Using an immunologically privileged athymic rat, Peterson et al performed spine fusions with human DBMs and compared the radiologic, histologic and manual mechanical testing results among Grafton putty, DBX putty, (Musculoskeletal Transplant Foundation, Edison, N.J.), and Allomatrix injectable putty (Wright, Arlington, Tenn.) in comparison to control. When results were analyzed at four and eight weeks, Grafton was found more effective than DBX, and Allomatrix was found no better than control. In a subsequent study intended to determine if DBM is indeed osteoinductive, Martin et al used a rat spine fusion model to determine the healing potential of Grafton DBM after treatment with a guanidine extraction procedure known to obliterate any osteoinductive activity. The results showed excellent fusion in untreated implants but no healing in guanidine extracted implants confirming the osteoinductive DBM activity.

One of the most interesting uses of DBM is as a bone graft extender or composite graft with autologous bone. Studies in rabbits have shown that the amount of autologous bone needed to achieve spinal fusion can be reduced by 75% when the missing volume is made up with DBM gel. While it is unclear if this is a combined osteoinductive and osteoconductive effect or an osteoconductive effect alone, this concept may hold significant clinical promise. In my opinion, the most impressive study to date is by Cammissa et al who performed a multicenter equivalency trial in 120 patients who underwent posterolateral spinal fusion with pedicle screw fixation. Each patient served as their own control and underwent iliac crest bone grafting on one side of the spine and treatment with a Grafton DBM/autologous bone composite (2-to-1 ratio) on the contralateral side. At 24 months’ follow-up, there was an equivalent fusion rate using these two types of graft materials.

Jackson: Are there clinical settings in which DBM bone graft substitutes can eliminate the need for harvesting bone from the patient?

Einhorn: This question has been asked many times and although several authors have tried to answer it, none have succeeded in doing so with what would be considered level I evidence (ie, from a high quality randomized controlled trial or a systematic analysis of randomized controlled trials). Instead, only case reports, case series and case control studies have appeared in the literature. Some of these reports show that, in certain cases, so-called local bone can be used (presumably autologous bone taken from skeletal structures at surgical site intervention but not as part of a purposeful iliac crest bone harvesting procedure). Other reports are contaminated with cases in which allogeneic bone chips were used in addition to the DBM. To my knowledge, only one study involved a prospective randomized design in which no additional material was implanted and no autologous bone was harvested. Geesink et al, in an investigation intended to test the ability of OP-1 implant (Stryker Biotech, Hopkinton, Mass.) to heal a critical-sized, 15 mm fibular osteotomy defect, randomized patients to four experimental groups: no treatment, treatment with collagen alone, treatment with 2 ccs Grafton DBM, and treatment with 2.5 mgs. OP-1 implant in a 3 cc volume. After 12 months, none of the unfilled or collagen-treated defects healed, five of six of the OP-1 treated defects healed, and all of the Grafton DBM defects healed. There were no significant differences between Grafton DBM and OP-1 and both Grafton and OP-1 were significantly better than either the untreated or the collagen treated defects.

Jackson: Are there good clinical data to support the use of DBM as a delivery system for growth factors or as part of a composite graft involving autologous bone marrow?

Einhorn: The only growth factors currently available for use in orthopedic patients are BMP-2 and OP-1 (BMP-7). As the FDA approval process for each of these requires a specific formulation involving a collagen product, the ability to use either of these with DBM would be considered an off-label use of the BMP. Despite this, I am aware of several surgeons who have combined the above-mentioned BMP products with DBM in clinical settings. Unfortunately, from these anecdotal experiences, there is no way of knowing what role DBM played in the clinical outcome.

The use of autologous bone marrow continues to stimulate great interest by clinicians and scientists and I, for one, support this enthusiasm. Although the number and concentration of osteoprogenitor cells in a normal human bone marrow aspirate may be too small for clinical efficacy, procedures to rapidly and reliably increase these numbers and concentrations will very likely have an important impact on the efficacy of autologous bone marrow as a bone graft material. A few studies have tested the use of DBM as a delivery vehicle for autologous bone marrow and suggest some efficacy in that application. Further research is needed before this can be widely recommended.

Dr. Einhorn or his department have been a paid consultant to Stryker Biotech, Genetics Institute (now Wyeth Research), Osteotech and Aastrom Biosciences Inc.

For more information:

Peterson B, Whang PG, Iglesias R, Wang JC, et al. Osteoinductivity of commercially available demineralized bone matrix. Preparations in a spine fusion model. J Bone Joint Surg Am. 2004;86:2243-2250.

Martin GJ, Boden SD, Titus L, Scarborough NL. New formulations of demineralized bone matrix as a more effective graft alternative in experimental posterolateral lumbar spine arthrodesis. Spine. 1999;24:637-645.

Cammissa FP Jr, Lowery G, Garfin SR, et al. Two-year fusion rate equivalency between Grafton DBM gel and autograft in posterolateral spine fusion: a prospective controlled trial employing a side-by-side comparison in the same patient. Spine. 2004;29:660-666.

Geesink RG, Hoefnagels NH, Bulstra S. Osteogenic activity of OP-1 bone morphogenetic protein (BMP-7) in a human fibular defect. J Bone Joint Surg Br. 1999;81:710-800.