The bone morphogenetic proteins antagonist Noggin inhibits membranous ossification

Bone morphogenetic proteins (BMPs) are expressed and secreted during fracture repair. Although they are likely to be required for this process, little is known about their physiological role in bone regeneration. Noggin is a protein that specifically binds and inactivates several BMPs. It plays fundamental roles during early embryonal development and limb morphogenesis by this BMP-inactivating activity. This study shows that Noggin can modify bone formation in vivo in the adult animal and, thus, indirectly, that BMP signaling is indispensable in this process. A noggin mutein (hNgdeltaB2-Fc) engineered so as to display increased bioavailability was used. Bilateral titanium bone chambers were inserted in 70 rats, and side comparisons for bone formation in the chambers were done. The hNgdeltaB2-Fc had no effect on total amount of tissue formed in the chamber but decreased the amount of bone compared with both buffer controls and a control made up of an Fc-tagged IL-6Ralpha protein, which had no effects of its own. Also, wild-type noggin inhibited bone formation. Thus, endogenous BMP signaling is necessary for normal bone regeneration.

Bone graft incorporation. Effects of osteogenic protein-1 and impaction

Impaction of cancellous bone grafts in a bone chamber in rats in a previous study led to decreased ingrowth of new bone after 6 weeks compared with unimpacted grafts. The current study analyzes whether this decrease represented a final loss of ingrowth or just a delay, if the decrease was influenced by immunologic factors, and if it was possible to influence the inhibitory effect by adding a bone morphogenetic protein. Bone chambers with impacted or unimpacted bone grafts were implanted bilaterally in rat tibias. The mean bone ingrowth distance into the graft was measured on histologic sections. Three experiments were done: (1) the bone ingrowth into impacted and unimpacted grafts was studied at 6 and 12 weeks; (2) the immunologic influence was studied by comparing isogeneic grafts with allogeneic grafts; and (3) the authors tried to influence the decrease in bone ingrowth in impacted grafts by adding osteogenic protein-1. Bone ingrowth into the impacted graft was decreased at 6 weeks but not at 12 weeks. No difference was found between isografts and allografts at 6 weeks. With the addition of osteogenic protein-1, the impacted grafts showed dramatically increased bone ingrowth. Impacted bone grafts are incorporated at a slower rate than were structural grafts. The delay can be reversed by adding osteogenic protein-1, making ingrowth faster than in structural bone.

OP-1 for cervical spine fusion: bridging bone in only 1 of 4 rheumatoid patients but prednisolone did not inhibit bone induction in rats

We used OP-1 (also called BMP-7) on a collagen type-1 carrier in atlanto-axial posterior fusions to promote bony healing after wire fixation. 4 patients who had instability between the atlas and axis due to rheumatoid disease received the implants. The patients were examined with conventional radiography postoperatively at 2, 6 and 10 months. In 3 patients, no new bone formation was detectable. In 1 patient, new bone bridged the fusion site at 6 months. 3 patients were on chronic steroid treatment, including the patient in whom bone formation was detected. To determine whether steroid treatment could be responsible for the low rate of bone induction, 24 rats each received OP-1 implants in an abdominal muscle pouch. They were divided into 3 groups receiving saline, 0.1 or 1.0 mg/kg BW of prednisolone daily until they were killed 3 weeks postoperatively. Specimens were decalcified for histology and the amount of calcium in the decalcifying solution was measured. All groups showed ossicles induced by OP-1, and no effect of prednisolone was detected. Thus the failures in the patients may have causes other than prednisolone treatment.

Intraosseous BMP implants in rabbits. Inhibitory effect on bone formation

The bone harvest chamber is a model for rapid spontaneous bone healing in rabbits. We have previously shown inhibition of bone formation by using BMP-2 on a collagen carrier in this intraosseous model, despite bone formation when depositing BMP-2 on a similar carrier subfascially in the same animals. The doses were 12 and 0.6 micrograms/5 mm3 chamber volume. As these findings conflicted with most other experiments dealing with the skeletal response to BMP-2, we repeated the previous experiments with variations. We studied: 1) a lower BMP-2 dose, 2) a different type of BMP (BMP-7/OP-1), 3) a different carrier (hydroxyapatite), 4) a different chamber construction allowing contact with extraskeletal tissue and 5) BMP-2 on the original collagen carrier in an acutely inserted chamber in rats. We also studied the border between the BMP-2 implant and the preexisting bone to see whether BMP-2 caused premature differentiation of the callus so that proliferation was stopped and a bone cyst formed. The low dose of BMP-2 reduced tissue ingrowth and tended to reduce bone formation. BMP-7 showed the same inhibitory effects as BMP-2. BMP-2 on a hydroxyapatite carrier also inhibited bone formation in the chamber. In the chamber that allowed contact with extraskeletal tissue, we observed no effects of BMP-2. The border between the BMP-2 implant and the preexisting bone did not look like a cyst wall. BMP-2, from the same batch, on a similar collagen carrier, regularly increased bone formation in the acutely inserted bone chamber in rats, thereby excluding major defects in the BMP-2 implants. The inhibition in this specific model is a consistent finding and not due to an overdose, a specific BMP-type, a specific carrier or premature callus differentiation.

Enhancement of bone formation in the setting of repeated tissue deformation

The purpose of this study was to investigate whether the osteoinductive recombinant human bone morphogenetic protein 2, combined with a collagen carrier, could enhance bone formation when exposed to controlled amounts of tissue deformation. Chambers that allow for the multiple harvestings of tissue specimen were used. The devices were implanted in the tibial metaphyses of skeletally mature New Zealand White rabbits. A tissue ingrowth canal in each device either was left empty or filled with a collagen carrier with or without 0.6 microgram of recombinant human bone morphogenetic protein 2. The tissue ingrowth canal was deformed cyclically during a period of 2 minutes daily, according to a previously described deformation protocol. The tissue that developed in the chambers was harvested every 3 weeks. Undecalcified histologic sections of each tissue sample were stained with trichrome and von Kossa stains and subjected to histomorphometric analysis. It was found that deformation decreased the area occupied by bone trabeculae in the empty chambers and carrier controls. The amount of bone formed in the chambers treated with bone morphogenetic protein 2 was significantly greater than that in the chambers subjected to micromotion and left empty or implanted with the collagen carrier. The amount of bone in chambers with motion and bone morphogenetic protein 2 was equal to that in chambers left empty and not subjected to micromotion. Qualitative histologic analysis of the bone formed with bone morphogenetic protein 2 revealed normal bone trabeculae. These findings indicate that bone morphogenetic protein 2 may be useful in augmenting bone formation in conditions that otherwise would favor the formation of fibrous tissue.

BMP-2 can inhibit bone healing. Bone-chamber study in rabbits

We came across a bone growing situation in which BMP-2 inhibits bone formation. Collagen sponges with BMP-2 were inserted in titanium bone chambers in rabbits. In this model, bone formation occurs spontaneously. With 2.4 and 0.12 micrograms BMP-2 per mm3 of the chamber volume, bone ingrowth was reduced to 16% and 42% of collagen controls, respectively. Sponges with 2.4 micrograms BMP-2 per mm3 that were placed in a similar titanium chamber in an intramuscular subfascial site induced bone formation in 4 cases of 5. The BMP-2 concentrations used are considered optimal for extraskeletal bone induction, and intramuscular sponges in rats all induced bone. The inhibitory effect is most likely model-related, but indicates unknown effects of BMP-2.

Transforming growth factor beta and bone morphogenetic protein 2 for bone ingrowth: a comparison using bone chambers in rats

We compared the ability of transforming growth factor beta 1 (TGF beta 1) and bone morphogenetic protein 2 (BMP-2) to increase the penetration distances for fibrous tissue and bone into porous coralline hydroxyapatite. Forty-four rats received pairs of titanium bone chambers implanted bilaterally in their proximal tibiae. These chambers allow tissue ingrowth from one end of a long cylinder, so that ingrowth distances along the cylinder axis can be measured. Since bone never grows as far as fibrous or undifferentiated mesenchymal tissue, we measured both total tissue ingrowth and bone ingrowth distances. All chambers were implanted with cylinders of porous hydroxyapatite (Interpore 200). The hydroxyapatite cylinders were treated with either 1, 33, or 1000 ng of TGF beta 1 or BMP-2 prior to implantation, and compared with untreated contralateral control implants at 6 weeks. The bone ingrowth distance into the porous hydroxyapatite showed a trend toward inhibition with TGF beta 1 as compared to untreated controls (p < 0.08), and there was a negative correlation between TGF beta 1 dose and ingrowth distances for both bone (p < 0.01) and total tissue (p < 0.01). BMP-2 greatly increased bone (p < 0.001) and total tissue (p < 0.001) ingrowth distances.