No augmentation of morselized and impacted bone graft by OP-1 in a weight-bearing model

Revision pedicle screws with impaction bone grafting: a case series

BACKGROUND

Pedicle screw fixation in spinal constructs can be subject to failure requiring revision surgery. In cases of aseptic loosening various salvage techniques have been described. Revision screws augmented with cement have become popular but are not without risks. Larger diameter screws are often used but result in reducing bone stock or expanding the pedicles. We present a novel technique of pedicle screw revision by impaction bone allografting and a case series.

METHODS

The failed screws are removed. The screw track is probed to check its integrity. Milled bone allograft is funneled into the screw hole and sequentially impacted, before insertion of a replacement screw. We report a case series and describe a single case where this method has been used. Information was gathered from the electronic patient record in our hospital.

RESULTS

Ten screws were revised in 7 patients. Mean age at first surgery was 60.86 (48-76) years. Average time between first surgery and revision was 12.6 (4.7-49.9) months. Average follow-up was 26.2 (5.7-62.2) months and no screws showed any signs of loosening.

CONCLUSIONS

Impaction grafting with bone allograft is a technique for pedicle screw salvage that can be used safely and effectively as an alternative to cemented screws, when pedicle screws have failed by aseptic loosening. It avoids the risks associated with cemented screws and in our series was successful.

Biodegradable Screws Containing Bone Morphogenetic Protein-2 in an Osteoporotic Rat Model

OBJECTIVE

The aim of this study was to evaluate the effect for biodegradable screws containing bone morphogenetic protein-2 (BMP-2) in an osteoporotic rat model.

METHODS

Twenty-four female Wistar rat (250-300 g, 12 weeks of age) were randomized into four groups. Three groups underwent bilateral ovariectomy (OVX). Biodegradable screws with or without BMP-2 were inserted in the proximal tibia in two implantation groups. The extracted proximal metaphysis of the tibiae were scanned by exo-vivo micro-computed tomography. Evaluated parameters included bone mineral density (BMD), trabecular bone volume (BV/TV), trabecular number, trabecular thickness, and trabecular separation (Tb.Sp). The tibia samples were pathologically evaluated by staining with by Hematoxylin and Eosin, and trichrome.

RESULTS

Trabecular formation near screw insertion site was evident only in rats receiving BMP-2 screws. BMD and BV/TV significantly differed between controls and the OVX and OVX with screw groups. However, there were no significant differences between control and OVX with screw BMP groups. Tb.Sp significantly differed between control and OVX and OVX with screw groups (p<0.05), and between the OVX and OVX with screw BMP group (p<0.05), with no statistically significant difference between control and OVX with screw BMP groups. Over the 12 weeks after surgery, bone lamellae in direct contact with the screw developed more extensive and thicker trabecular bone around the implant in the OVX with screw BMP group compared to the OVX with screw group.

CONCLUSION

Biodegradable screws containing BMP-2 improve nearby bone conditions and enhance ostoeintegration between the implant and the osteoporotic bone.

Topical bisphosphonate augments fixation of bone-grafted hydroxyapatite coated implants, BMP-2 causes resorption-based decrease in bone

Bone allograft is used in total joint arthroplasties in order to enhance implant fixation. BMPs are known to stimulate new bone formation within allograft, but also known to accelerate graft resorption. Bisphosphonates are strong inhibitor of bone resorption. The aim of this study was to investigate whether the bisphosphonate zoledronate was able to counteract the accelerated graft resorption without interfering with the BMP induced bone formation. In the present study the two drugs alone and in combination were studied in our canine model of impaction bone grafting. We included 10 dogs in this study. Cancellous allograft bone grafts were soaked in either saline or zoledronate solution (0.005mg/mL) and then vehicle or BMP2 (0.15mg rhBMP2) was added. This produced four treatment groups: A) control, B) BMP2, C) zoledronate and D) BMP2+zoledronate. The allograft treated with A, B, C or D was impacted into a circumferential defect of 2.5mm around HA-coated porous Ti implants. Each dog received all four treatment groups with two implants in the distal part of each femur. The group with allograft soaked in zoledronate (C) showed better biomechanical fixation than all other groups (p<0.05). It had less allograft resorption compared to all other groups (p<0.005) without any statistically significant change in new bone formation. The addition of BMP2 to the allograft did not increase new bone formation significantly, but did accelerate allograft resorption. This was also the case where the allograft was treated with BMP2 and zoledronate in combination (D). This caused a decrease in mechanical implant fixation in both these groups compared to the control group, however only statistically significant for the BMP2 group compared to control. The study shows that topical zoledronate can be a valuable tool for augmenting bone grafts when administered optimally. The use of BMP2 in bone grafting procedures seems associated with a high risk of bone resorption and mechanical weakening.

In vitro and in vivo evaluation of bone formation using solid freeform fabrication-based bone morphogenic protein-2 releasing PCL/PLGA scaffolds

The aim of this study was to develop novel polycaprolactone/poly(lactic-co-glycolic acid) (PCL/PLGA) scaffolds with a heparin-dopamine (Hep-DOPA) conjugate for controlled release of bone morphogenic protein-2 (BMP-2) to enhance osteoblast activity in vitro and also bone formation in vivo. PCL/PLGA scaffolds were prepared by a solid freeform fabrication method. The PCL/PLGA scaffolds were functionalized with Hep-DOPA and then BMP-2 was sequentially coated onto the Hep-DOPA/PCL/PLGA scaffolds. The characterization and surface elemental composition of all scaffolds were evaluated by scanning electron microscope and x-ray photoelectron spectroscopy. The osteoblast activities on all scaffolds were assessed by cell proliferation, alkaline phosphatase (ALP) activity and calcium deposition in vitro. To demonstrate bone formation in vivo, plain radiograph, micro-computed tomography (micro-CT) evaluation and histological studies were performed after the implantation of all scaffolds on a rat femur defect. Hep-DOPA/PCL/PLGA had more controlled release of BMP-2, which was quantified by enzyme-linked immunosorbent assay, compared with Hep/PCL/PLGA. The in vitro results showed that osteoblast-like cells (MG-63 cells) grown on BMP-2/Hep-DOPA/PCL/PLGA had significantly enhanced ALP activity and calcium deposition compared with those on BMP-2/Hep/PCL/PLGA and PCL/PLGA. In addition, the plain radiograph, micro-CT evaluation and histological studies demonstrated that the implanted BMP-2/Hep-DOPA/PCL/PLGA on rat femur had more bone formation than BMP-2/Hep/PCL/PLGA and PCL/PLGA in vivo.

rhBMP-2/calcium phosphate matrix induces bone formation while limiting transient bone resorption in a nonhuman primate core defect model

BACKGROUND

Transient bone resorption limits the use of recombinant human bone morphogenetic protein-2 (rhBMP-2)/absorbable collagen sponge in metaphyseal bone. The purpose of the present study was to evaluate the efficacy of rhBMP-2/calcium phosphate matrix (CPM) to induce bone formation while limiting transient bone resorption in nonhuman primate core defects.

METHODS

Metaphyseal core defects were created in eighteen cynomolgus monkeys. rhBMP-2 retention was evaluated in the distal part of the radius. Bone formation was evaluated at eight weeks following treatment with 1.5 or 4.5-mg/mL rhBMP-2/CPM, CPM alone, or no treatment in the distal part of the radius, the proximal part of the tibia, and the proximal part of the femur; at twenty-four weeks following treatment with 1.5-mg/mL rhBMP-2/CPM or CPM alone in the proximal part of the tibia; and at one, two, and four weeks following treatment with 1.5-mg/mL rhBMP-2/CPM or no treatment in the distal part of the radius. Bone resorption was evaluated at four weeks following treatment with 1.5, 2.0, 3.0, and 4.5-mg/mL rhBMP-2/CPM or CPM alone in the distal part of the femur. Evaluations were performed with use of scintigraphy, radiographs, histological analysis, and computed tomography.

RESULTS

Seventy-eight percent, 64%, 50%, 35%, and 12% of the rhBMP-2 was retained in the distal part of the radius at one, seven, fourteen, twenty-one, and forty-nine days after surgery. rhBMP-2/CPM increased bone formation within core defects and surrounding trabeculae compared with CPM alone or no treatment at all anatomic locations at eight weeks, and bone formation was ongoing in the rhBMP-2/CPM-treated proximal tibial sites at twenty-four weeks. Bone formation began in the trabeculae surrounding the core defects at one week and was observed adjacent to the resorbing CPM within the core defects and in the surrounding trabecular bone at two and four weeks in the rhBMP-2/CPM-treated distal radial sites. Bone formation was confined to the region immediately surrounding the core defects in the untreated distal radial sites at all time points. Transient bone resorption was only observed in the distal femoral sites treated with 4.5 mg/mL of rhBMP-2/CPM at two weeks.

CONCLUSIONS

Treatment of nonhuman primate metaphyseal core defects with 1.5 to 3.0-mg/mL rhBMP-2/CPM resulted in bone formation without transient bone resorption.

CLINICAL RELEVANCE

rhBMP-2/CPM may be useful to accelerate healing of metaphyseal bone defects in humans.

Local treatment of cancellous bone grafts with BMP-7 and zoledronate increases both the bone formation rate and bone density: a bone chamber study in rats

BACKGROUND AND PURPOSE

The remodeling of morselized bone grafts in revision surgery can be enhanced by an anabolic substance such as a bone morphogenetic protein (BMP). On the other hand, BMPs boost catabolism and might cause a premature resorption, both of the graft and of the new-formed bone. Bisphosphonates inactivate osteoclasts and can be used to control the resorption. We studied a combination of both drugs as a local admix to a cancellous allograft.

METHODS

Cancellous bone allografts were harvested and freeze-dried. Either saline, BMP-7, the bisphosphonate zoledronate, or a combination of BMP-7 and zoledronate were added in solution. The grafts were placed in bone conduction chambers and implanted in the proximal tibia of 34 rats. The grafts were harvested after 6 weeks and evaluated by histomorphometry.

RESULTS

Bone volume/total volume (BV/TV) was 50% in the grafts treated with the combination of BMP-7 and zoledronate and 16% in the saline controls (p < 0.001). In the zoledronate group BV/TV was 56%, and in the BMP group it was 14%. The ingrowth distance of new bone into the graft was 3.5 mm for the combination of BMP-7 and zoledronate and 2.6 mm in the saline control (p = 0.002). The net amount of retained remodeled bone was more than 4 times higher when BMP-7 and zoledronate were combined than in the controls.

INTERPRETATION

An anabolic drug like BMP-7 can be combined with an anti-catabolic bisphosphonate as local bone graft adjunct, and the combination increases the amount of remaining bone after remodeling is complete.

rhBMP-2 induces transient bone resorption followed by bone formation in a nonhuman primate core-defect model

BACKGROUND

Bone resorption preceding bone formation has been reported following the administration of recombinant human bone morphogenetic protein-2 (rhBMP-2) delivered in an absorbable collagen sponge (ACS) in metaphyseal bone. This study characterizes treatment with rhBMP-2/ACS in metaphyseal bone with use of a nonhuman primate core-defect model.

METHODS

Unilateral proximal femoral core defects were treated with 360 microg of rhBMP-2/ACS or ACS alone or were left untreated in seven, five, and five adult male cynomolgus monkeys, respectively. Distal femoral core defects in seven of the above animals were treated with 360 microg of rhBMP-2/ACS in one limb and ACS alone in the contralateral limb. Retention of rhBMP-2 in the proximal part of the femora was determined with use of tracer amounts of (125)I-rhBMP-2 imaged with a gamma camera. The distal part of the femora was evaluated with in vivo computed tomography. Computed tomography and histological evaluation were performed on harvested segments in all animals at twenty-four weeks. The histological response in the proximal and distal parts of the femora containing core defects treated with 360 microg of rhBMP-2/ACS in one limb and ACS alone in the contralateral limb was evaluated at one, two, and four weeks in three animals per time point.

RESULTS

Approximately 39.9%, 24.2%, 3.4%, and 0.5% of the rhBMP-2 was retained in the proximal part of the femora at one, seven, fourteen, and twenty-one days, respectively. The mineral density and trabecular volume fraction of the core defects treated with rhBMP-2/ACS, those treated with ACS alone, and untreated core defects in the proximal part of the femora were 81%, 54%, and 20%, respectively, and 94%, 36%, and 31%, respectively, of the corresponding region in the contralateral limbs at twenty-four weeks. The mineral density and trabecular volume fraction of the region surrounding the core defects treated with rhBMP-2/ACS, those treated with ACS alone, and untreated core defects were 112%, 105%, and 104%, respectively, and 117%, 108%, and 107%, respectively, of the corresponding region in the contralateral limbs. Treatment with rhBMP-2/ACS increased the size of the proximal and distal core defects compared with treatment with ACS alone. Histological evaluation of the rhBMP-2/ACS-treated limbs demonstrated that bone resorption was initiated at one week in association with osteoclasts and receptor activator of nuclear factor-kappaB ligand-positive stained spindle-shaped cells and peaked at two weeks. Bone formation was observed at two weeks and was ongoing at twenty-four weeks.

CONCLUSIONS

Treatment of metaphyseal core defects with rhBMP-2/ACS resulted in bone resorption followed by bone formation in nonhuman primates.

Impaction bone grafting in revision hip surgery: past, present and future

Joint replacement surgery can have excellent clinical results. However, as the number of patients undergoing surgery increases, the number of failed joint replacements is set to rise. One of the greatest challenges for the revision surgeon is the restoration of bone stock. This article focuses upon revision hip surgery, with particular reference to the scope of the problem; historical and current solutions to bone loss in the femur and acetabulum; the clinical results following revision surgery; and the basic science behind impaction bone grafting, before ending with possible future directions for improving the restoration of bone stock.

The management of bone loss in revision total knee replacement

The management of bone loss in revision replacement of the knee remains a challenge despite an array of options available to the surgeon. Bone loss may occur as a result of the original disease, the design of the prosthesis, the mechanism of failure or technical error at initial surgery. The aim of revision surgery is to relieve pain and improve function while addressing the mechanism of failure in order to reconstruct a stable platform with transfer of load to the host bone. Methods of reconstruction include the use of cement, modular metal augmentation of prostheses, custom-made, tumour-type or hinged implants and bone grafting. The published results of the surgical techniques are summarised and a guide for the management of bone defects in revision surgery of the knee is presented.

Strain imparted during impaction grafting may contribute to bony incorporation: an in vitro study of the release of bmp-7 from allograft

In order to investigate the osteoinductive properties of allograft used in impaction grafting and the effect of strain during impaction on these properties, we designed an in vitro experiment to measure strain-related release of bone morphogenetic protein-7 (BMP-7) from fresh-frozen femoral head allograft. A total of 40 10 mm cubes of cancellous bone were cut from ten samples of fresh-frozen femoral head. The marrow was removed from the cubes and the baseline concentrations of BMP-7 were measured. Specimens from each femoral head were allocated to four groups and subjected to different compressive strains with a material testing machine, after which BMP-7 activity was reassessed. It was present in all groups. There was a linear increase of 102.1 pg/g (95% confidence interval 68.6 to 135.6) BMP-7 for each 10% increase in strain. At 80% strain the mean concentration of BMP-7 released (830.3 pg/g bone) was approximately four times that released at 20% strain. Activity of BMP-7 in fresh-frozen allograft has not previously been demonstrated. This study shows that the freezing and storage of femoral heads allows some maintenance of biological activity, and that impaction grafting provides a source of osteoinductive bone for remodelling. We have shown that BMP-7 is released from fresh-frozen femoral head cancellous bone in proportion to the strain applied to the bone. This suggests that the impaction process itself may contribute to the biological process of remodelling and bony incorporation.

Manipulating the anabolic and catabolic response in bone graft remodeling: synergism by a combination of local BMP-7 and a single systemic dosis of zoledronate

Remodeling of a bone graft can be influenced both by anabolic substances, such as a bone morphogenic protein (BMP) and by anticatabolic substances, such as the bisphosphonates. BMPs are potent bone anabolic substances, but also boost catabolism and cause resorption. Bisphosphonates inhibit osteoclast function and can be used to postpone resorption. In the present study a combination of both drugs was explored in a rat bone chamber model. Cancellous bone grafts were treated with either BMP-7 or saline and placed in a bone chamber implanted in the proximal tibia. After 2 weeks, an injection of either zoledronate 0.1 mg/kg or saline was given subcutaneously. The rats were killed after 6 weeks, and bone ingrowth distance into the graft and graft resorption were measured by histomorphometry. BMP-7 significantly (p = 0.007) increased new bone ingrowth distance into the graft from 2.0 mm (SD = 0.98 mm) in the controls to 3.1 mm (SD = 0.93 mm). If bisphosphonate was not given, most of the newly formed and old graft bone was resorbed. A single injection of zoledronate significantly (p < 0.001) increased the trabecular volume/total volume to 40% (SD = 9%) compared to 14% (SD = 10%) in the nonbisphosphonate treated. In total, the net amount of bone increased by 400% when BMP-7 and zoledronate combined was compared to saline. A bone graft can be treated with BMP-7 to increase new bone formation and at the same time be protected against premature catabolism by a single dose of a bisphosphonate. This combination might be useful in various conditions in orthopedic reconstruction.

In vitro and in vivo optimization of impaction allografting by demineralization and addition of rh-OP-1

Impaction allografting is a bone tissue engineering technique currently used in lower limb reconstruction orthopedic surgery. Our hypothesis was that biological optimization can be achieved by demineralization and addition of osteogenic protein-1 (OP-1) to the allograft. The objective of our in vitro study was to evaluate human mesenchymal stem cell (MSC) proliferation (Alamar Blue assay, titrated thymidine assay, total DNA Hoechst 33258, and scanning electron microscopy) and osteogenic differentiation (alkaline phosphatase assay) in two types of impacted carrier, namely, demineralized bone matrix (DBM) and insoluble collagenous bone matrix (ICBM), with or without OP-1. The objective in vivo was to compare the osteogenic potential of impacted DBM with or without OP-1, with that of impacted fresh frozen allograft (FFA), again with or without OP-1. DBM + OP-1 optimized osteoinduction and significantly improved (p < 0.05) proliferation and differentiation in comparison to the majority of all other graft preparation in vitro. In addition, DBM + OP-1 was significantly superior, with regard to osteogenesis, compared to the impacted FFA alone (p < 0.001), FFA + OP-1 (p = 0.01) and DBM alone (p = 0.02) in vivo. We propose that partial demineralization and addition of OP-1 provides a good method for improving the osteoinductive properties of fresh allograft currently used in the impaction grafting technique.

Osteogenic protein-1 increases the fixation of implants grafted with morcellised bone allograft and ProOsteon bone substitute: an experimental study in dogs

Impacted bone allograft is often used in revision joint replacement. Hydroxyapatite granules have been suggested as a substitute or to enhance morcellised bone allograft. We hypothesised that adding osteogenic protein-1 to a composite of bone allograft and non-resorbable hydroxyapatite granules (ProOsteon) would improve the incorporation of bone and implant fixation. We also compared the response to using ProOsteon alone against bone allograft used in isolation. We implanted two non-weight-bearing hydroxyapatite-coated implants into each proximal humerus of six dogs, with each implant surrounded by a concentric 3 mm gap. These gaps were randomly allocated to four different procedures in each dog: 1) bone allograft used on its own; 2) ProOsteon used on its own; 3) allograft and ProOsteon used together; or 4) allograft and ProOsteon with the addition of osteogenic protein-1. After three weeks osteogenic protein-1 increased bone formation and the energy absorption of implants grafted with allograft and ProOsteon. A composite of allograft, ProOsteon and osteogenic protein-1 was comparable, but not superior to, allograft used on its own. ProOsteon alone cannot be recommended as a substitute for allograft around non-cemented implants, but should be used to extend the volume of the graft, preferably with the addition of a growth factor.

No positive effects of OP-1 device on the incorporation of impacted graft materials after 8 weeks: a bone chamber study in goats

BACKGROUND

Bone morphogenetic proteins (BMPs) have the potential to improve clinical outcome after hip revision surgery by improving graft incorporation and implant fixation. However, impaction of cancellous bone grafts and TCP/HA bone substitute mixed with OP-1 device in a bone chamber in goats in a previous study led to reduced fibrous tissue ingrowth after 4 weeks. New bone formation was not promoted by OP-1. In the current study we examined whether this reduction represented a final loss of ingrowth or was just a delay, and whether the reduction can be overcome and ultimately results in a better late ingrowth.

METHODS

Bone chambers with impacted allografts and impacted TCP/HA granules mixed with 2 doses of OP-1 device were implanted in proximal medial goat tibias. Impacted allografts and TCP/HA not treated with OP-1 served as controls. After 8 weeks, the incorporation was evaluated using histology and histomorphometry.

RESULTS

Histology revealed evidence of bone graft incorporation, which proceeded in a similar way in both allografts and TCP/HA, with and without the addition of OP-1. After 8 weeks, no difference in bone ingrowth was found between the OP-1 groups and their controls. It was only in the allografts that the addition of OP-1 resulted in more fibrous tissue ingrowth.

INTERPRETATION

We conclude that the previously observed delay in fibrous tissue ingrowth can be only partially overcome.

Stability of fused versus nonfused THA femoral impaction grafts

Impaction grafting for THA involves compacting morselized cancellous bone (MCB) into a cavitary defect to build up bone stock. Ideally, the MCB subsequently remodels into a new contiguous cancellous lattice. A recent laboratory model of MCB fusion allows simulating an impaction graft construct in this ideal eventual clinical state. The purpose of the present study was to determine the relative stability of femoral impaction graft constructs in which the MCB has fused versus that for MCB in the freshly impacted nonfused condition. Cemented femoral impaction graft constructs were created in composite femurs. For fused constructs, the MCB was mixed with an amine epoxy that causes the MCB to set up into a contiguous structure biomechanically comparable to intact cancellous bone in compression. The constructs were loaded with 500,000 physiologic gait cycles. Three-dimensional motion was measured between the femur and the stem. The fused femoral impaction grafts were much more stable than the nonfused grafts at the proximal stem location, but MCB fusion had only a modest effect on distal stem stability. These results indicate that most of the opportunity to reduce femoral stem micromotion and migration lies proximal, and that steps to enhance impaction graft remodeling and fusion are most effectively focused proximally.

No effect of bone morphogenetic protein-7 (OP-1) on the incorporation of impacted bone grafts in a realistic acetabular model

Bone morphogenetic proteins (BMPs) accelerate bone repair in experimental and clinical conditions. Impacted Morsellized Cancellous Bone grafts (MCB) are successfully used to reconstruct bone defects after failed hip implants. The main question in this study was if BMP-7 (OP-1) mixed with MCB could accelerate the incorporation of MCB and prevents the formation of a soft tissue interface after remodeling of the MCB. A large loaded defect in the acetabulum of goats was reconstructed with a wire mesh and with MCB or MCB mixed with OP-1. After 6 weeks, no differences were found in the revascularization process, in the number of osteoclasts resorbing the MCB, and in the thickness and appearance of the fibrous interface between MCB with or without OP-1. After 6 weeks, enchondral bone had formed in the bone graft layer and on the periosteal anterior and superior rim in the OP-1 group only. More periosteal bone and more bone in the holes of the mesh had been formed in most OP-1 goats. Most MCB was replaced by new lamellar bone after 15 weeks in both groups. We speculate that during or directly after impaction most of the OP-1 is released from the carrier inducing an early effect outside the reconstructive layer at the periosteal side of the acetabulum. Probably most OP-1 has left the reconstruction by the time new vessels and progenitors reached the bone graft. These results do not support the use of OP-1 in impaction bone grafting in patients.

High doses of OP-1 inhibit fibrous tissue ingrowth in impaction grafting

A major concern in using growth factors in impaction grafting is the potential stimulation of the osteoclastic lineage. A solution would be using an osteoconductive material resistant to resorption and providing initial stability after reconstruction. Growth factors may promote bone formation in combination with such graft materials. We determined whether OP-1 would promote the incorporation of impacted morselized allografts and tricalcium phosphate/hydroxyapatite (TCP/HA) into host bone, whether bone formation would be preceded by an initial process of accelerated resorption, and whether the response to OP-1 remodeling/incorporation would be dose-related. We performed two bone chamber studies in goats to ascertain the early effects of OP-1 dose on resorption and incorporation of impacted morselized allografts and TCP/HA. After 4 weeks, the incorporation process of impacted morselized allografts and TCP/HA was not promoted by OP-1. We observed no signs of accelerated resorption preceding bone formation. An increase in OP-1 dose resulted in an inhibition of fibrous tissue formation but OP-1 did not promote bone formation. Early failures in impaction grafting, using mixes with OP-1, might be explained by the lack of fibrous tissue ingrowth and not by increased resorption and remodeling.

Osteointegration of hydroxyapatite-titanium implants coated with nonglycosylated recombinant human bone morphogenetic protein-2 (BMP-2) in aged sheep

Osteointegration of metal implants into aged organisms can be severely compromised due to reduced healing capacity of bone, lack of precursor cells for new bone formation, or osteoporosis. Here, we report on successful implant healing in a novel model of aged sheep in the presence of nonglycosylated bone morphogenetic protein 2 (BMP-2). Ewes of 8 to 12 years with significant radiologic and histologic signs of osteoporosis and adipocytic bone marrow received a cylindrical hydroxyapatite-titanium implant of 12 x 10 mm. BMP-2 has been produced as a bacterial recombinant fusion protein with maltose-binding protein and in vitro generation of mature BMP-2 by renaturation and proteolytic cleavage. A BMP-2 inhibition ELISA was developed to measure the in vitro release kinetics of bioactive human BMP-2 from immersed solid implant materials by using Escherichia coli expressed and biotinylated recombinant human BMP-2 receptor IA extracellular domain (ALK-3 ECD). The implants were placed laterally below both tibial plateaus, with the left leg implant carrying 380 microg BMP-2. Both implant types became integrated within the following 20 weeks. The control implant only integrated at the cortical bone, and little new bone formation was found within the pre-existing trabecular bone or the marrow cavity. Marrow fat tissue was partially replaced by unspecific connective tissue. In contrast, BMP-2-coated implants initiated significant new bone formation, initially in trabecular arrangements to be replaced by cortical-like bone after 20 weeks. The new bone was oriented towards the cylinder. Highly viable bone marrow appeared and filled the lacunar structures of the new bone. In mechanical tests, the BMP-2-coated implants displayed in average 50% higher stability. This animal model provided first evidence that application of nonglycosylated BMP-2 coated on solid implants may foster bone healing and regeneration even in aged-compromised individuals.

The in vivo behaviour of a sol–gel glass and a glass-ceramic during critical diaphyseal bone defects healing

The in vivo evaluation, in New Zealand rabbits, of a sol-gel glass 70% CaO-30% SiO2 (in mol%) and a glass-ceramic obtained from thermal treatment of the glass, both bioactive in Kokubo's simulated body fluid (SBF), is presented. Femoral bone diaphyseal critical defects were filled with: (i) sol-gel glass cylinders, (ii) glass-ceramic cylinders, or (iii) no material (control group). Osteosynthesis was done by means of anterior screwed plates with an associate intramedullar Kirschner wire. Each group included 10 mature rabbits, 9 months old. Follow-up was 6 months. After sacrifice, macroscopic study showed healing of bone defects, with bone coating over the cylinders, but without evidence of satisfactory repair in control group. Radiographic study showed good implant stability and periosteal growth and bone remodelling around and over the filled bone defect. The morphometric study showed minimum evidences of degradation or resorption in glass-ceramic cylinders, maintaining its original shape, but sol-gel glass cylinders showed abundant fragmentation and surface resorption. An intimate union of the new-formed bone to both materials was observed. Mechanical study showed the higher results in the glass-ceramic group, whereas sol-gel glass and control group showed no differences. The minimum degradation of glass-ceramic cylinders suggests their application in critical bone defects locations of transmission forces or load bearing. The performance of sol-gel glass cylinders suggests their usefulness in locations where a quick resorption should be preferable, considering the possibility of serving as drug or cells vehicle for both of them.

Delivery of bone morphogenetic proteins for orthopedic tissue regeneration

Carriers for bone morphogenetic proteins (BMPs) are used to increase retention of these factors at orthopedic treatment sites for a sufficient period of time to allow regenerative tissue forming cells to migrate to the area of injury and to proliferate and differentiate. Carriers can also serve as a matrix for cell infiltration while maintaining the volume in which repair tissue can form. Carriers have to be biocompatible and are often required to be bioresorbable. Carriers also have to be easily, and cost-effectively, manufactured for large-scale production, conveniently sterilized and have appropriate storage requirements and stability. All of these processes have to be approvable by regulatory agencies. The four major categories of BMP carrier materials include natural polymers, inorganic materials, synthetic polymers, composites of these materials. Autograft or allograft carriers have also used. Carrier configurations range from simple depot delivery systems to more complex systems mimicking the extracellular matrix structure and function. Bone regenerative carriers include depot delivery systems for fracture repair, three-dimensional polymer or ceramic composites for segmental repairs and spine fusion and metal or metal/ceramic composites for augmenting implant integration. Tendon/ligament regenerative carriers range from depot delivery systems to three-dimensional carriers that are either randomly oriented or linearly oriented to improve regenerative tissue alignment. Cartilage regenerative systems generally require three-dimensional matrices and often incorporate cells in addition to factors to augment the repair. Alternative BMP delivery systems include viral vectors, genetically altered cells, conjugated factors and small molecules.

Impaction bone-grafting in revision joint replacement surgery

The standard graft material for impaction bone-grafting is fresh-frozen femoral head allograft morselized to a particle size as large as is practical to ensure stability and allow new bone formation. The graft must be sufficiently compacted to provide immediate mechanical stability; this requires containment of the graft and substantial impaction energy. Diaphyseal bone fracture and excessive implant migration are the most common complications of the operation. Impaction bone-grafting in revision total hip replacement has produced good medium-term results on both the acetabular and the femoral side. The use of compacted morselized bone graft is a relatively new technique in revision knee surgery and requires longer-term follow-up with larger numbers of patients to assess its value.