Aim

Photodynamic therapy (PDT) requires a photosensitiser, a light source of an appropriate wavelength, and the presence of molecular oxygen. Once stimulated to its excited phase by the light, the photosensitiser reacts with oxygen to form free radicals of ‘singlet oxygen’ which is cytotoxic to microorganisms.

We aim to demonstrate the effectiveness of PDT as an in-vitro antimicrobial technique against Staphylococcus aureus, Methicillin resistant Staphylococcus aureus, Staphylococcus epidermidis, Pseudomonas aeruginosa and Acinetobacter bauminii. This will form the scientific basis for further animal and human studies assessing PDT for treatment of periprosthetic infections, septic arthritis, and open fractures.

Long-term survival of massive prostheses used to treat bone cancers is associated with extra-cortical bone growth and osteointegration into a grooved hydroxyapatite coated collar positioned adjacent to the transection site on the implant shaft [1]. The survivorship at 10 years reduces from 98% to 75% where osteointegration of the shaft does not occur. Although current finite element (FE) methods successfully model bone adaption, optimisation of adventitious new bone growth and osteointegration is difficult to predict. There is thus a need to improve existing FE models by including biological processes of osteoconduction and osteoinduction.

The principal bone adaptation criteria is based on the standard strain-energy remodeling algorithm, where the rate of remodeling is controlled by the difference in the stimulus against the reference value [3]. The additional concept of bone connectivity was introduced, to limit bone growth to neighbouring elements (cells) adjoining existing bone elements. The algorithm was developed on a cylindrical model before it was used on an ovine model.

The geometry and material properties from two ovine tibiae were obtained from computed tomography (CT) scans and used to develop FE models of the tibiae implanted with a grooved collar. The bones were assigned inhomogeneous material properties based on the CT grey values and typical ovine walking load conditions were applied. The FE results show a region of bone tissue growth below the implanted collar and a small amount of osteointegration with the implant, which is in good agreement to clinical results. Some histological results suggest that further bone growth is possible and potential improvements to the model will be discussed. In summary, by including an algorithm that describes osteoconduction, adventitious bone growth can be predicted.

Introduction Bone graft supply for impaction grafting can be problematic due to the supply of graft, sterilisation, which alters the biological properties of the graft, and the immunogencity of the graft which may lead to graft rejection. Reducing the amount of graft can be accomplished by using increased amounts of synthetic materials such as hydroxyapatite (HA). This study evaluated the effect of using mixtures of porous HA (Apapore™) with allograft for cemented impaction allografting of the femoral stem in an ovine model. The aim was to test the hypothesis that increased quantities of Apapore™ will be stable and induce similar bone remodelling to that where a 50:50 mixture with allograft was used.

Method Twelve hemi-arthroplasty femoral components were inserted into the right hip of skeletally mature female commercially cross-bred sheep weighing between 65 and 80kg. Femoral components were manufactured from Cobalt Chromium alloy and cemented in place following impaction of the femoral canal. Animals were randomly placed into one of two groups according to the allograft-apapore mixture used. Group 1: Apapore:allograft mixed 50:50. Group 2: Apapore: allograft mixed 90:10. Six animals were investigated in each group. Implants remained in vivo for 6 months. In order to quantify bone formation rates, oxytetracycline injections were given 2 months post-surgery and 3 weeks later, followed by a third administration in the fifth month post-surgery and 3 weeks later. Animals were walked over a force plate pre-operatively and at 8, 16 and 24 weeks post-operatively. Twelve readings of maximum force (Fmax, N/m2) were taken and average values of right over left were calculated as a percentage (%AR/AL) and represented how well the animal used its operated leg where 100% represents full weight-bearing. Thin sections (~70μm thick) were prepared through four regions of the femur. The proximal, mid and tip of the femoral component region and one distal to the implant tip were analysed where bone area, Apapore™ area, Apapore™-bone contact and cement mantle thickness were quantified and compared using image analysis techniques.

Results In both groups, the use of graft resulted in the formation of a cancellous network of bone on the endosteal surface which incorporated the Apapore™ granules. When all regions were compared, femoral bone turnover results demonstrated significantly increased rates in group 1 (0.0021mm day-1) when compared with group 2 (0.0015mm day-1) (p< 0.05). No significant differences were identified when the proximal, mid and tip regions in the two groups were compared however, significantly increased turnover was identified in the distal region in group 1 (0.0027 mm day-1) when compared with group 2 (0.0013mm day-1) (p< 0.05). In both groups increased turnover was observed in the proximal, tip and distal regions with least in the mid region of the stem. Ground Reaction Force (GRF) results demonstrated no significant differences between the two experimental groups at 8, 16 and 24 weeks postoperatively. In both groups, a significant decline in function was demonstrated 8 weeks post-op when compared with pre-operative values and in both groups function gradually increased over time. Results for new bone area demonstrated significantly increased new bone in the proximal and distal regions in both groups (proximal =7.94mm2 and 7.13mm2; distal =7.03mm2 and 8.17mm2, group 1 and 2 respectively) with least new bone in the mid region of the stem (4.53mm2 and 4.79mm2). No significant differences in any of the regions were demonstrated when group 1 and 2 were compared. In both groups, significantly increased amounts of Apapore™ was observed in the proximal and distal regions of the femoral stem with least in the mid and tip region. No significant difference in cement mantle thickness was identified between the two groups.

Discussion Results demonstrated that hips maintained functional stability when a higher amount of Apapore™ mixture was used. Results for bone turnover rates and the amount of new bone formation in the 90:10 mixture demonstrated Apapore™ to be a comparable and suitable alternative to replace allograft in impaction grafting of a femoral component.