Introduction: Incorporation of Silicon into the HA structure enhances the bioactivity of Hydroxyapatite (HA). Silicon substituted calcium phosphate (SiCaP/SiHA) has been introduced as an osteoconductive material for bone formation. However, the osseoinductive capacity of this biomaterial has not been assessed. A previous study by Hing et al shows that bioactivity of stoichiometric hydroxyapatite bone substitute materials is enhanced by increasing the level of porosity within the implant struts [1].

The aim of this study was to test the hypothesis that SiCaP bone graft results in superior osseoinduction compared to stoichiometric HA and osseoinduction enhancement using high microporosity materials.

Methods: Implantation of 32 bone graft plugs (16 granular and 16 blocks) with 3 different strut porosities: 20% SiHA, 35% SiHA, 10% SiHA and 20% HA, all with matched 80% total porosity supplied by ApaTech Ltd into the paraspinalis muscle of 4 sheep for 12 weeks. HA and %SiHA locations were randomized at implant sites.

Following euthanasia at 12 weeks histomorphometry was carried out to calculate Percentage of bone, soft tissue and implant area and Percentage of the amount of bone in contact with the calcium phosphate surface (% Bone attachment). Further evaluation of Calcium, Phosphate and Silicon levels within the implants and surrounding bone was carried out by Scanning Electron Microscopy (SEM) and EDAX.

Results: Bone formation was observed within the pores of both granules and blocks of SiCaP and HA implants. Greater bone formation and attachment was detected in scaffolds with higher strut porosity (SiHA35) compared to implants of the same chemical composition but lower strut porosity (SiHA10, SiHA20. More bone formation and contact was observed in SiHA implants (SiHA20) compared to matched porosity HA implants where the amount of bone formed was minimal. Uniform distribution of Silicon (Si) was visible within the SiHA scaffold struts according to EDAX results. Greater quantities of Si existed in newly formed bone as compared to soft tissue adjacent to the SiHA implants. Silicon was not detected in either soft or hard tissues adjacent to HA implants.

Conclusion: Both microporous HA and SiCaP promote bone ingrowth, as ectopic bone formation was observed in all four groups of synthetic materials. Matched porosity SiCaP is more osseoinductive than HA. Increasing strut porosity results in promotion of osseoinductivity. High strut porosity (> 10%) block environment contributes to greater osseoinductive behaviour. In conclusion we report that presence of silicon and the strut porosity influence the osseoinductive capacity of calcium phosphate bone substitute biomaterials.

We previously demonstrated that cartilaginous tissue was induced on a reamed acetabular articulation in an ovine hemiarthroplasty model with three different femoral head sizes. At maximum loading during stance phase, the acetabular peak stresses immediately after reaming could reach approximately 80 MPa under direct implant-bone contact with in-vitro measurements.

We aimed to establish finite element (FE) models of the ovine hip hemiarthroplasty which examine stress distribution on the reamed acetabula by three head sizes. We hypothesized that the stress distribution did not differ between different sizes when the joint is congruent and that the peak stresses in the acetabulum immediately after reaming occurred in the dorsal acetabulum.

Three two-dimensional FE models of ovine hip hemi-arthroplasty were built; each comprised a head component, 25, 28, and 32 mm in diameter, and an acetabular component. The acetabular geometry was acquired from an ovine acetabular histological section. The head was moved to partly intersect with the acetabulum representing the reaming procedure and a congruent contact was confirmed. Cortical bone and cancellous bone were modelled as linear elastic, with moduli of 20 and 1.2 GPa, respectively. Variable moduli were also assessed. The finest mesh for each model consisted of over 100,000 four-node quadrilateral elements. Loading conditions were chosen to represent peak hip joint force developed during the stance phase. Stress distribution in the acetabular area in contact with the head was plotted against the articulating arc length.

The results confirmed that the stress distribution between different prosthetic head sizes in a reamed hemiarthroplasty model did not change when the joint was congruent. The peak compressive stresses occurred in the dorsal acetabulum with the 32 mm model being the highest at approximately 69 MPa, the 28 mm model at 63 MPa, and the 25 mm model at 54 MPa. An increase in the cancellous modulus and a decrease in the cortical modulus increased the peak stresses in the dorsal acetabulum.

This presents an indicative study into the effect of prosthetic femoral head sizes on the stress distribution in the acetabulum. The idealized 2-D models showed reasonable agreement when compared quantitatively with the in vitro study.

Introduction: The aim of this study is to develop a novel approach to tissue engineering in vivo, in which the adaptive response of skeletal tissues to the imposed mechanical environment will be utilised to induce a cartilaginous resurfacing of the acetabular articulation in a hemi-arthroplasty model of hip replacement. Our hypothesis was that a cartilaginous resurfacing of subchondral bone can be induced by applying stresses of 0 to 3 MPa to the articular surface of the acetabulum. We used an ovine hemiarthroplasty model where the stresses on the acetabulum were engineered by using different femoral head sizes.

Methods: Three groups of six sheep received unilateral hip hemi-arthroplasties and were sacrificed 24 weeks post-operatively to harvest the acetabula. At operation, acetabular cartilage was removed completely and the subchondral bone was reamed down and left bleeding. Three femoral head sizes, 25, 28, and 32-mm, were used to induce different contact stress levels. Vertical ground reaction force (GRF) data were measured and normalised by body weight for both limbs pre-operatively and every 4 weeks post-operatively. Five specimens from each group and eight unoperated controls were processed and stained with Safranin O and Sirius Red. Cartilage proteoglycans in the regenerated tissues from four specimens in the 25-mm group were detected by immunoblotting using specific monoclonal antibodies.

Results: The operated limbs were subjected to an average of 80 to 90% pre-operative GRF after the eighth post-operative week and maintained till the end of the study. No significant difference was noted during the period between the three groups. A layer of regenerated tissue was noted on all specimens processed and was Sirius positive. Four operated specimens processed in the 25-mm group and three in the 28-mm group were Safranin O positive. The presence of cartilage aggrecan, cartilage link proteins, biglycan, and decorin was confirmed by immunoblotting.

Discussion and Conclusion: We conclude that a cartilaginous resurfacing of acetabulum can be induced in vivo under the mechanical environment imposed by our hemi-arthroplasty model. This approach may be advantageous in clinical practice as a regenerated acetabular cartilaginous surface would avoid the problems associated with wear of the plastic acetabular cup and replacement of the acetabulum.