Background

Radiological and clinical results of total shoulder arthroplasty are dependent upon ability to accurately measure and correct glenoid version. There are a variety of imaging modalities and computer-assisted reconstruction programmes that are employed with varying degrees of success. We have compared three freely available modalities: unformatted 2D CT; formatted 2D CT; and 3D CT reconstructions.

Introduction: Periprosthetic bone loss, brought about by wear particle induced osteolysis, presents a major challenge and compromises outcome in revision Total Hip Replacement. Poor bone stock at revision hip replacement is the main indication for impaction allografting. There are well documented limitations in the use of bone graft. Autogenous bone graft is osseoinductive, though donor site morbidity and the limited amount available restrict its use. An alternative is allogenic bone graft from cadaveric femoral heads. The drawbacks of using allograft are a limited supply and the risk of disease transmission. An alternative may be the use of bone substitute materials. Usually these are used in conjunction with allograft and therefore a number of drawbacks still apply. This study investigates the use of impaction grafting without bone graft. In this study we tested Apopore, 60% porosity, 2–5 mm hydroxyappatite (HA) granules (ApaTech Ltd) in an animal impaction model with allograft as control. Hypothesis Impaction using porous granular HA induces a similar volume of new bone compared with impaction using allograft.

Methods and Materials: Cylindrical defects of 15mm diameter were created in the medial femoral condyles of 12 sheep (6 sheep in each group) and filled with 3.5 grams of either morselised ovine allograft, washed and defatted according to North London Tissue Bank protocols, or porous HA granules impacted with a specially designed impactor, 20 times with a force of 3 KN. This force was similar to that measured during impaction grafting in clinical cases. After 6 weeks the sheep were euthanized, samples embedded in resin and the amount of bone formation measured by histomorphometric analysis.

Results: Under the impaction forces used the HA graft was more impacted than allograft. In the impacted HA graft the average pore size was smaller than for impacted allograft. After 6 weeks more new bone formation was observed at the host implant interface than the middle of the implant in both groups. At the implant host interface there was 26.64% (± 2.13%) new bone formation in the allograft and 21.13% (± 4.51%) new bone formation in the HA implant. In the middle of the implants allograft produced 11.01% (± 2.07%) new bone whilst the HA produced 7.23% (± 4.05%) new bone. Two tailed t-test showed no significance in either region, p=0.28 at the interface and p=0.40 in the middle. Allograft underwent resorption, from 39.37% at time zero to 5.66% (± 2.04%) at 6 weeks, a total reduction of 85%, where as the volume of HA granules remained the same and was 49% at time zero and 48.59% (± 1.69%) at 6 weeks. Two tailed t-test showed a significant difference (p< 0.0001) between allograft and HA at 6 weeks.

Conclusions: This study shows that granular porous HA induced a similar level of bone formation as compared with allograft. Resorption of allograft in this model allowed greater ingrowth of fibrous tissue. This makes the structural scaffold much more porous, compromising stability of the construct. The HA was not resorbed after 6 weeks and hence may be more stable. HA also has the advantage of being readily available. This study demonstrates that a bone substitute material does not need to be mixed with allograft.

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.