Incorporation of morsellized bone graft under controlled loading conditions. A new animal model in the goat.

The aim of this study was to develop a new animal model in which we could assess the in vivo effects of mechanical stimuli in the incorporation process of impacted morsellized bone grafts. The subcutaneous pressure implant SPI was developed for use in the goat. This device can generate controlled loading conditions onto a fixed amount of bone graft in the distal femur. Twenty goats were divided into three groups: non-loaded, 2 or 4 MPa loads (1 Hz, 1 h/day). The goats were sacrificed after 3, 6 or 12 weeks. The results were documented by clinical observations, quantitative bone density from QCT-scanning and histomorphometry. Nine post-mortem knee specimens were prepared in a similar manner to the experimental knees to determine the reproducibility and mechanical stability of the grafting method. Three goats were lost due to complications, the others functioned clinically well. Histology showed invasion of the bone graft by a front of vascular fibrous tissue after which osteoclasts resorbed the dead bone graft, followed by woven bone apposition on the graft remnants. At 12 weeks the loaded grafts had transformed into a vital trabecular structure. QCT bone density measurements revealed persistently high densities in the 12-weeks 4 MPa specimens, but reduced densities in the 2 MPa and non-loaded specimens. Morphometrically, the mineralising surface was larger in the 4 MPa group (P = 0.02) and the incorporation and remodelling processes had advanced more rapidly in the 2 MPa specimens (P = 0.04). Although the numbers investigated in this study in each group were low, statistical differences were found in the amount of graft left after incorporation and in the apposition rate of the new bone. In the future this model will be used to study the incorporation potential of different types of bone graft and bone graft substitutes.

Role of growth factors in the incorporation of unloaded bone allografts in the goat.

Revision of a failed total joint replacement often demands bone grafting methods to restore deficient bone stock. However, impaired allograft incorporation can be the result of inadequate host or graft properties. The stimulation of bone healing with growth factors could provide a new approach to deal with this problem. The repeated sampling bone chamber was used in the goat to investigate the properties of bone allografts enriched with transforming growth factor-beta, recombinant human bone morphogenetic protein-2, and basic fibroblastic growth factor under unfavorable vascular and nonloaded conditions. Ten goats each had three bone chambers implanted in the medial proximal tibia. Different carrier allograft bone preparations were used for each growth factor based on convention and previously reported results. The period between implantation and chamber harvest was 8 weeks. The concentrations of the growth factors used was 0, 1, or 10 micrograms of transforming growth factor-beta 2, 0, 1, or 5 micrograms of bone morphogenetic protein-2, and 0, 40, 200 ng of basic fibroblastic growth factor. The specimens were analyzed histomorphometrically for the amount of soft tissue ingrowth, bone ingrowth, and the number of osteoclasts. In all specimens, a resorption front grew into the graft followed by fibrovascular tissue and, in some instances, bone. In the 5 micrograms of bone morphogenetic protein-2 specimens, larger amounts of soft tissue and woven bone were present, whereas in the specimens that received 10 micrograms of transforming growth factor-beta 2, there was a decrease in the amount of tissue and bone ingrowth. Two hundred nanograms of basic fibroblastic growth factor had a negative effect on soft tissue formation but increased the amount of vascular elements containing erythrocytes. The number of osteoclasts was higher in the 5-microgram bone recombinant human morphogenetic protein-2 specimens. In the clinical arena with absence of good perigraft vascularization and loading, bone morphogenetic protein-2 may have a strong stimulatory effect on bone graft incorporation.

Mechanical characteristics of impacted morsellised bone grafts used in revision of total hip arthroplasty.

The use of impacted, morsellised bone grafts has become popular in revision total hip arthroplasty (THA). The initial stability of the reconstruction and the effectiveness of any subsequent process of revitalisation and incorporation will depend on the mechanical integrity of the graft. Our aim in this study was to document the time-dependent mechanical properties of the morsellised graft. This information is useful in clinical application of the graft, in studies of migration of the implant and in the design of the joint. We used 16 specimens of impacted, morsellised cancellous bone from the sternum of goats to assess the mechanical properties by confined compression creep tests. Consideration of the graft material as a porous, permeable solid, filled with fluid, allowed determination of the compressive modulus of the matrix, and its permeability to fluid flow. In all specimens the compression tests showed large, irreversible deformations, caused by flow-independent creep behaviour as a result of rolling and sliding of the bone chips. The mean permeability was 8.82 *10(-12) m4/Ns (SD 43%), and the compressive modulus was 38.7 MPa (SD 34%). No correlation was found between the apparent density and the permeability or between the apparent density and the compressive modulus. The irreversible deformations in the graft could be captured by a creep law, for which the parameters were quantified. We conclude that in clinical use the graft is bound to be subject to permanent deformation after operation. The permeability of the material is relatively high compared with, for example, human cartilage. The confined compression modulus is relatively low compared with cancellous bone of the same apparent density. Designs of prostheses used in revision surgery must accommodate the viscoelastic and permanent deformations in the graft without causing loosening at the interface.

Viability of the acetabular bone bed at revision surgery following cemented primary arthroplasty.

Loosening of total hip replacements is often associated with severe loss of periprosthetic bone. The notion exists that the remaining bone is sclerotic, avascular, and displays little osteogenic activity, and that it therefore potentially compromises the revitalization of bone grafts used to restore bony defects. To verify this opinion we studied the bone characteristics in acetabular bone biopsies taken at primary total hip arthroplasty (PTH) and revision total hip arthroplasty (RTH) for a cemented PTH. In 6 PTH patients and in 10 RTH patients, acetabular bone biopsies were taken from the roof, the center, and the lower rim of each acetabulum. Specimens were evaluated by light microscopy and histomorphometrically measured for specimen size, bone area, perimeter, active osteoid perimeter, number of vessels, and osteoclasts. The vascularity and vitality appeared to be comparable in the RTH and PTH bone biopsies. However, the trabecular organization of the RTH bone differed from that of the PTH biopsies. In the PTH biopsies, the trabeculae were running perpendicular to the subchondral bone layer, whereas in the RTH biopsies the layers of bone were oriented parallel to the implant surface. There was abundant remodeling activity in the RTH bone, with large quantities of active osteoid and osteoclasts. These histologic parameters differed, but not statistically significant, from the PTH biopsies. In conclusion, we found that at revision, the acetabular bone was viable with sufficient vascularity and remodeling activity to provide an acceptable recipient host bone bed for revision surgery combined with bone grafting.