Irradiation has no effect on the incorporation of impacted morselized bone: a bone chamber study in goats

Background Gamma irradiation has been widely used for sterilization of bone allografts. However, gamma irradiation alters proteins. This is favorable when it reduces immunogenicity, but is undesirable when osteoinductive proteins are damaged. Although the effect of gamma irradiation on BMPs has been studied, the effect of irradiation on the process of incorporation of morselized bone chips remains unclear. We studied the effects of sterilization by gamma irradiation on the incorporation of impacted morselized allografts.

Methods Bone chambers with impacted allografts, rinsed impacted allografts, allografts that were rinsed and subsequently irradiated, and an empty control were implanted in proximal medial tibiae of goats. Incorporation was evaluated using histology and histomorphometry.

Results Histology revealed evidence of bone graft incorporation, which proceeded in a similar way in unprocessed, rinsed, and both rinsed and irradiated bone grafts. After 12 weeks, no difference in bone and tissue ingrowth was found between the unprocessed, the rinsed, and the rinsed and subsequently irradiated allografts. The amount of unresorbed graft remnant was highest in the unprocessed bone grafts.

Interpretation We conclude that sterilization with gamma irradiation does not influence the incorporation of impacted rinsed bone allografts.     

Incomplete incorporation of morselized and impacted autologous bone graft: A histological study in 4 intracorporally grafted lumbar fractures

Morselized and impacted bone allografts are used successfully in hip and knee revisions, but experiments using bone chambers indicate that impaction actually can delay ingrowth of new bone into a graft. To understand the remodeling and incorporation process of morselized and impacted grafts, we studied the incorporation of morselized impacted autografts in lumbar fractures histologically.

4 patients were operated on for Th XII-LI fractures. The fractures were stabilized by VSP plates and transpedicular screws in the vertebrae above and below the fractured one. Autologous bone graft was packed into the fractured vertebral body through one of the pedicles. After 18-20 months, the plates were removed and biopsies were obtained from various locations in the fractured vertebra. All fractures were at this time clinically and radiographically healed. Histologically, in all cases, large areas of the autograft in the vertebral body were unvascularized and partially or entirely necrotic. As with morselized bone in hip revisions, evaluation of graft incorporation requires histological examination. Full osseous incorporation of a graft is not always necessary for a good clinical result.     

Incomplete incorporation of morselized and impacted autologous bone graft: a histological study in 4 intracorporally grafted lumbar fractures

Morselized and impacted bone allografts are used successfully in hip and knee revisions, but experiments using bone chambers indicate that impaction actually can delay ingrowth of new bone into a graft. To understand the remodeling and incorporation process of morselized and impacted grafts, we studied the incorporation of morselized impacted autografts in lumbar fractures histologically. 4 patients were operated on for Th XII-LI fractures. The fractures were stabilized by VSP plates and transpedicular screws in the vertebrae above and below the fractured one. Autologous bone graft was packed into the fractured vertebral body through one of the pedicles. After 18-20 months, the plates were removed and biopsies were obtained from various locations in the fractured vertebra. All fractures were at this time clinically and radiographically healed. Histologically, in all cases, large areas of the autograft in the vertebral body were unvascularized and partially or entirely necrotic. As with morselized bone in hip revisions, evaluation of graft incorporation requires histological examination. Full osseous incorporation of a graft is not always necessary for a good clinical result.     

No effect of ketoprofen and meloxicam on bone graft ingrowth: A bone chamber study in goats

Background and purpose?There is increasing awareness that non‐steroidal anti‐inflammatory drugs (NSAIDs), and especially the cyclooxygenase‐2 (COX‐2) selective ones, may retard bone healing. We have used NSAIDs (indomethacin for at least 7 days) to prevent heterotopic ossification after acetabular reconstructions using impacted bone grafts. The long‐term clinical results have been satisfying, making it difficult to believe that there is an important negative effect of NSAIDs on graft incorporation. We studied the effect of two different NSAIDs on bone and tissue ingrowth in a bone chamber model in goats, using autograft, rinsed allograft, and allograft that had been rinsed and subsequently irradiated.

Methods?9 goats received no NSAIDs, 9 received ketoprofen, and 9 received meloxicam—all for 6 weeks. In each goat 6 bone chambers were implanted: 2 filled with autograft, 2 with rinsed allograft, and 2 with allograft that had been rinsed and irradiated. The amount of bone ingrowth and total tissue ingrowth was compared between the groups.

Results?There were no statistically significant differences in bone ingrowth between the different groups. Also, no differences in bone ingrowth were found with respect to the type of graft used. Furthermore, there was no statistically significant difference in the total amount of ingrowth of fibrous tissue between the treatment groups.

Interpretation?No differences in bone ingrowth in titanium bone chambers could be detected with both ketoprofen and meloxicam compared to untreated control animals. This confirms our hypothesis that the effect of NSAIDs on the incorporation and ingrowth of bone graft is limited.     

Rinsing of allograft bone does not improve implant fixation

Background and purpose Impacted morselized allograft bone is a well-established method for reconstructing bone defects at revision surgery. However, the incorporation of bone graft is not always complete, and a substantial volume of fibrous tissue has been found around grafted implants. We hypothesized that rinsing the bone graft may improve graft incorporation by removing the majority of immunogenic factors present in blood, marrow, and fat.

Methods We implanted a cylindrical (10- × 6-mm) porous-coated Ti implant into each proximal tibia of 12 dogs. The implants were surrounded by a 2.5-mm gap into which morselized fresh frozen allograft bone was impacted. The bone graft was either (1) untreated or (2) rinsed in 37°C saline for 3 × 1 min. After 4 weeks, the animals were killed and implant fixation was evaluated by mechanical push-out and histomorphometry.

Results The groups (rinsed vs. control) were similar regarding mechanical implant fixation (mean (SD)): shear strength (MPa) 2.7 (1.0) vs. 2.9 (1.2), stiffness (MPa/mm) 15 (6.7) vs. 15 (5.6), and energy absorption (kJ/m²) 0.5 (0.2) vs. 0.6 (0.4), The same was evident for the new bone formation on the implant surface and around the implant: ongrowth (%) 6 vs. 7 and ingrowth (%) 9 vs. 9. Although not statistically significant, a 61% reduction in fibrous tissue ongrowth and 50% reduction in ingrowth were found in the rinsed group.

Interpretation Within the limits of this experimental model, we did not detect any benefits of rinsing morselized allograft bone prior to impaction grafting.     

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.     

No positive effects of OP-1 device on the incorporation of impacted graft materials after 8 weeks

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.     

Selection of bone grafts for revision total hip arthroplasty

The selection of bone grafts to reconstruct deficient bone for revision hip replacement requires an understanding of specific bone graft functions and the critical steps of the biologic incorporation of the graft into the host. Bone grafts provide functions of osteogenesis, either graft derived or by osteoinduction, osteoconduction, or both, and mechanical support. Autologous cancellous bone provides excellent osteogenesis and osteoconduction without structural support. Nonvascularized cortical autografts provide mechanical support and are somewhat osteogenic. Allogeneic cancellous bone is osteoconductive and minimally osteoinductive, whereas cortical allografts provide structural support, if not freeze-dried, and are somewhat osteoconductive. Allogeneic demineralization bone matrix is highly osteoinductive. The selection of the appropriate bone graft depends on the classification of the bone deficiency. Cavitary (contained) defects can be reconstructed with cancellous morselized autograft, frozen or freeze-dried allograft, or allogeneic demineralized bone matrix. Segmental defects require bulk corticocancellous and/or cortical autografts or allografts. The ultimate incorporation of the bone graft depends on the interaction of the graft and the host's mechanical and biologic environment, and host-bone graft contact and stability. Optimum bone graft selection will enhance the clinical outcomes in revision total hip arthroplasty.     

Wire mesh allows more revascularization than a strut in impaction bone grafting: an animal study in goats

Segmental defects can be reconstructed with a cortical strut or a metal wire mesh when using bone impaction grafting in the femur. We hypothesized that structural grafts would negatively influence revascularization of the underlying impacted grafts compared with an open wire mesh. A standardized large medial wall defect was reconstructed with a strut or a mesh in six goats per group. In all femurs impaction grafting was done in combination with a cemented collarless double-tapered highly polished Exeter stem. After 6 weeks the femurs were harvested. A high rate of periprosthetic fractures was observed (three of seven and two of six for the strut and mesh groups, respectively). Histologic analysis showed different revascularization and tissue ingrowth patterns for both reconstruction techniques. In the strut group, fibrous tissue ingrowth was limited to the edges of the defect. Medially behind the strut no or limited fibrous tissue ingrowth was found. In the mesh group, fibrous tissue and blood vessels penetrated the mesh and a superficial zone of revascularized grafts was observed. Although revascularization, concomitant graft resorption and bone incorporation may compromise the short-term stability of the stem after surgery, the long-term stability of the stem probably is best guaranteed by graft incorporation.     

A combination of bisphosphonate and BMP additives in impacted bone allografts

OP-1 increases bone ingrowth distance of new bone into allografts (Tägil et al. 2000), but the bone density after incorporation may be reduced by an increase in resorption (Höstner et al. 2000). Bisphosphonates inactivate osteoclasts and can be used to increase allograft bone density after incorporation (Aspenberg and Åstrand 2002). A combination of locally-applied bisphosphonate and OP-1 in the graft could therefore be expected to increase both new bone ingrowth and density. We tested this by using a rat bone chamber model. OP-1 alone increased the ingrowth distance of bone. Clodronate increased final bone density greatly, but reduced the ingrowth distance of new bone into grafts that were extremely impacted. This reduction was improved by adding OP-1. Regardless of graft density, combinations of OP-1 and clodronate included a high final bone density, but the ingrowth distances were shorter than with OP-1 alone. These data indicate that new bone and tissue ingrowth into a compacted graft depends on resorption and that resorption is a prerequisite for the stimulating effect of OP-1 in this experimental set-up. Although the problems associated with the use of OP-1 in impaction grafting may be solved by adding a bisphosphonate, some of the benefits of OP-1 can be lost.     

A combination of bisphosphonate and BMP additives in impacted bone allografts

OP-1 increases bone ingrowth distance of new bone into allografts (T?gil et al. 2000), but the bone density after incorporation may be reduced by an increase in resorption (H?istner et al. 2000). Bisphosphonates inactivate osteoclasts and can be used to increase allograft bone density after incorporation (Aspenberg and Astrand 2002). A combination of locally-applied bisphosphonate and OP-1 in the graft could therefore be expected to increase both new bone ingrowth and density. We tested this by using a rat bone chamber model. OP-1 alone increased the ingrowth distance of bone. Clodronate increased final bone density greatly, but reduced the ingrowth distance of new bone into grafts that were extremely impacted. This reduction was improved by adding OP-1. Regardless of graft density, combinations of OP-1 and clodronate included a high final bone density, but the ingrowth distances were shorter than with OP-1 alone. These data indicate that new bone and tissue ingrowth into a compacted graft depends on resorption and that resorption is a prerequisite for the stimulating effect of OP-1 in this experimental set-up. Although the problems associated with the use of OP-1 in impaction grafting may be solved by adding a bisphosphonate, some of the benefits of OP-1 can be lost.     

Fibrous tissue armoring increases the mechanical strength of an impacted bone graft

Impacted, morselized bone allografts are used with good clinical results in revision of hip prostheses with loosening and osteolysis. The impacted bone graft appears radiographically to remodel, but histological analyses have shown a heterogeneous picture with a mixture of living and dead bone. Thus, complete remodeling of the graft may be neither a prerequisite nor a cause of the good clinical results. The present study concerns the mechanical effect of the mere armoring of the bone graft by ingrowing fibrous tissue. We compared the compression strength of freshly-impacted grafts to grafts that had been inserted into a bone chamber and thus were penetrated by fibrous tissue growing in between the graft trabeculae. The compressive strength was doubled after 4 weeks of fibrous ingrowth. We conclude that the mechanical properties of an impacted graft are enhanced by armoring with ingrowing fibrous tissue. Strengthening of the parts of the impacted grafts which have not yet remodeled, would be clinically relevant for the outcome of the operation, since these parts are at high stress during the whole remodeling period. Complete osseous remodeling may not be necessary to obtain a good clinical result with a morselized impacted graft.     

Fibrous tissue armoring increases the mechanical strength of an impacted bone graft

Impacted, morselized bone allografts are used with good clinical results in revision of hip prostheses with loosening and osteolysis. The impacted bone graft appears radiographically to remodel, but histological analyses have shown a heterogeneous picture with a mixture of living and dead bone. Thus, complete remodeling of the graft may be neither a prerequisite nor a cause of the good clinical results. The present study concerns the mechanical effect of the mere armoring of the bone graft by ingrowing fibrous tissue. We compared the compression strength of freshly-impacted grafts to grafts that had been inserted into a bone chamber and thus were penetrated by fibrous tissue growing in between the graft trabeculae. The compressive strength was doubled after 4 weeks of fibrous ingrowth. We conclude that the mechanical properties of an impacted graft are enhanced by armoring with ingrowing fibrous tissue. Strengthening of the parts of the impacted grafts which have not yet remodeled, would be clinically relevant for the outcome of the operation, since these parts are at high stress during the whole remodeling period. Complete osseous remodeling may not be necessary to obtain a good clinical result with a morselized impacted graft.     

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

Introduction: Impacted morselized bone allograft is thought to be remodeled to a great extent. We have previously shown that load-bearing increases the remodeling of impacted morselized bone allografts in a rabbit knee prosthesis model. Bone Morphogenic Proteins (BMPs) also stimulate bone formation and bone allograft remodeling. In this study, our aim was to determine whether it was possible to increase further the remodeling of impacted morselized bone allografts by combining load-bearing with a BMP. Experiment: A solution of Osteogenic Protein-1 (OP-1, also called BMP-7) was added to freeze-dried bone allograft chips before impaction grafting in our rabbit knee prosthesis model. 23 skeletally mature rabbits received an uncemented tibial knee prosthetic component inserted into a bed of impacted morselized bone allograft. 12 rabbits were given OP-1-treated allograft (50 µg OP-1 per gram allograft), and 11 rabbits vehicle-treated allograft. Each rabbit received mean 0.53 g graft. The rabbits were killed after 3 or 6 weeks and the grafted region was examined by histomorphometric assessment of the volume fraction of newly formed bone and remaining graft. Results and interpretation: We found that OP-1 did not increase the bone density (graft plus new bone) to any substantial extent. However, we can not exclude that this might be due to a carrier problem, since the OP-1 was added as a solution directly to the dry graft.     

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

INTRODUCTION: Impacted morselized bone allograft is thought to be remodeled to a great extent. We have previously shown that load-bearing increases the remodeling of impacted morselized bone allografts in a rabbit knee prosthesis model. Bone Morphogenic Proteins (BMPs) also stimulate bone formation and bone allograft remodeling. In this study, our aim was to determine whether it was possible to increase further the remodeling of impacted morselized bone allografts by combining load-bearing with a BMP. EXPERIMENT: A solution of Osteogenic Protein-1 (OP-1, also called BMP-7) was added to freeze-dried bone allograft chips before impaction grafting in our rabbit knee prosthesis model. 23 skeletally mature rabbits received an uncemented tibial knee prosthetic component inserted into a bed of impacted morselized bone allograft. 12 rabbits were given OP-1-treated allograft (50 microg OP-1 per gram allograft), and 11 rabbits vehicle-treated allograft. Each rabbit received mean 0.53 g graft. The rabbits were killed after 3 or 6 weeks and the grafted region was examined by histomorphometric assessment of the volume fraction of newly formed bone and remaining graft. RESULTS AND INTERPRETATION: We found that OP-1 did not increase the bone density (graft plus new bone) to any substantial extent. However, we can not exclude that this might be due to a carrier problem, since the OP-1 was added as a solution directly to the dry graft.     

Fibrous tissue armoring increases the mechanical strength of an impacted bone graft

Impacted, morselized bone allografts are used with good clinical results in revision of hip prostheses with loosening and osteolysis. The impacted bone graft appears radiographically to remodel, but histological analyses have shown a heterogeneous picture with a mixture of living and dead bone. Thus, complete remodeling of the graft may be neither a prerequisite nor a cause of the good clinical results. The present study concerns the mechanical effect of the mere armoring of the bone graft by ingrowing fibrous tissue. We compared the compression strength of freshly-impacted grafts to grafts that had been inserted into a bone chamber and thus were penetrated by fibrous tissue growing in between the graft trabeculae. The compressive strength was doubled after 4 weeks of fibrous ingrowth. We conclude that the mechanical properties of an impacted graft are enhanced by armoring with ingrowing fibrous tissue. Strengthening of the parts of the impacted grafts which have not yet remodeled, would be clinically relevant for the outcome of the operation, since these parts are at high stress during the whole remodeling period. Complete osseous remodeling may not be necessary to obtain a good clinical result with a morselized impacted graft.     

Is an Impacted Morselized Graft in a Cage an Alternative for Reconstructing Segmental Diaphyseal Defects?

Large diaphyseal bone defects often are reconstructed with large structural allografts but these are prone to major complications. We therefore asked whether impacted morselized bone graft could be an alternative for a massive structural graft in reconstructing large diaphyseal bone defects. Defects in the femora of goats were reconstructed using a cage filled with firmly impacted morselized allograft or with a structural cortical autograft (n = 6 in both groups). All reconstructions were stabilized with an intramedullary nail. The goats were allowed full weightbearing. In all reconstructions, the grafts united radiographically. Mechanical torsion strength of the femur with the cage and structural cortical graft reconstructions were 66.6% and 60.3%, respectively, as compared with the contralateral femurs after 6 months. Histologically, the impacted morselized graft was replaced completely by new viable bone. In the structural graft group, a mixture of new and necrotic bone was present. Incorporation of the impacted graft into new viable bone suggests this type of reconstruction may be safer than reconstruction with a structural graft in which creeping substitution results in a mixture of viable and necrotic bone that can fracture. The data suggest that a cage filled with a loaded morselized graft could be an alternative for the massive cortical graft in reconstruction of large diaphyseal defects in an animal model. Each author certifies that he or she has no commercial associations (eg, consultancies, stock ownership, equity interest, patent/licensing arrangements, etc) that might pose a conflict of interest in connection with the submitted article.     

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.     

The effect of impaction and a bioceramic coating on bone ingrowth in porous titanium particles

Background and purpose Porous titanium (Ti) particles can be impacted like cancellous allograft bone particles, and may therefore be used as bone substitute in impaction grafting. We evaluated the effect of impaction and of a thin silicated biphasic calcium phosphate coating on osteoconduction by Ti particles.

Methods The bone conduction chamber of Aspenberg was used in goats and filled with various groups of coated or uncoated small Ti particles (diameter 1.0–1.4 mm). Impacted allograft bone particles and empty chambers were used in control groups. Fluorochromes were administered at 4, 8, and 12 weeks. Maximum bone ingrowth distance was evaluated by histomorphometric analysis.

Results Histology of Ti particle graft cylinders showed a dense matrix with narrow inter-particle and intra-particle pores (< 100 μm), occluding the lumen of the bone chamber. Bone ingrowth distances gradually increased with time in all groups. Maximum bone ingrowth distance was higher in originally empty chambers than those with allograft bone particles (p = 0.01) and Ti particles (p < 0.001). Maximum bone ingrowth in allograft bone particles was higher than in all Ti groups (p ≤ 0.001). Impaction reduced osteoconduction and the coating partially compensated for the negative effect of impaction, but these differences were not statistically significant. No osteolytic reactions were found.

Interpretation Osteoconduction in the bone conduction chamber was reduced more by the insertion of small Ti particles than by insertion of small allograft bone particles. The osteoconductive potential of porous Ti particles should be studied further with larger-sized particles, which may allow bone ingrowth after impaction through larger inter-particle pores.     

Acetabular reconstruction with impacted morselized cancellous allografts in cemented hip arthroplasty: A histological and biomechanical study on the goat

Bone defects in total hip arthroplasty revision surgery can be restored with different types of bone graft. The use of impacted morselized allograft chips in combination with cement is the treatment of our choice. To establish the incorporation capacity of the grafts and mechanical stability of the implant, an animal model in the goat was developed. An acetabular defect was created and restored with morselized grafts and a cemented cup. Postoperative performance of the reconstruction was followed both histologically and biomechanically. Histology showed that consolidation of the graft with the host bone bed had occurred within 3 weeks. In the following period a front of vascular sprouts infiltrated the graft. Graft resorption, woven bone deposition, and subsequent remodeling resulted in a new trabecular structure. This structure contained only scarce remnants of the original dead graft material. At the graft-cement interface, graft resorption and new bone formation had resulted in areas of direct vital bone-cement contact. Locally, a soft tissue interface was present. After longer follow-up periods, progressive interface formation and loosening of the cups were found in most animals. Mechanical testing showed that the stability of the reconstruction increased during the first 12 postoperative weeks. Thereafter, the stability decreased, probably by soft-tissue interface formation at the graft cement-interface. We conclude that cemented morselized allografts have a high capacity to incorporate. Initial cup stability is adequate to provoke graft incorporation with decreasing stability after the incorporation process has been completed.