Bone graft materials in fixation of orthopaedic implants in sheep

Bone graft is widely used within orthopaedic surgery especially in revision joint arthroplasty and spine fusion. The early implant fixation in the revision situation of loose joint prostheses is important for the long-term survival. Bone autograft has been considered as gold standard in many orthopaedic procedures, whereas allograft is the gold standard by replacement of extensive bone loss. However, the use of autograft is associated with donor site morbidity, especially chronic pain. In addition, the limited supply is a significant clinical challenge. Limitations in the use of allograft include the risk of bacterial contamination and disease transmission as well as non-union and poor bone quality. Other bone graft and substitutes have been considered as alternative in order to improve implant fixation. Hydroxyapatite and collagen type I composite (HA/Collagen) have the potential in mimicking skeletal bones. The osteoconductive properties of the composite might be improved by adding bone marrow aspirate (BMA), which can be harvested during surgery. Other alternatives to bone graft are demineralised bone matrix (DBM) and human cancellous bone (CB). DBM is prepared by acid extraction of human bone and includes bone collagen, morphogenetic proteins and growth factors. The combination of DBM with CB and with allograft might improve the healing potential of these grafts around non-cemented orthopaedic implants and thereby the implant fixation. Study I investigates the effect of HA/Collagen composite alone and in combination with BMA on the early fixation of porous coated titanium implants. In addition, the study compares also the effect of autograft with the gold standard allograft. By using a sheep model, the implants were inserted in the trabecular bone of femoral condyles. The test biomaterials were placed in a well defined peri-implant gap. After the observation period, the bone-implant specimens were harvested and evaluated mechanically by a destructive push-out test and analyzed histologically qualitatively and quantitatively. Study II investigates the effect of DBM alone and in combination with CB or allograft. The control group in study I and II was allograft. Study III is a methodological study and investigates the potential systematic bias by applying the traditional sampling method, which includes evaluating the mechanical fixation by using the superficial part and the histological analysis by using the profound part of the implant. The implants in this study were inserted in the proximal humerus and only allograft was used in the peri-implant gap. In study I, the mechanical testing showed failure by the preloading in the composite group with and without BMA. There were no bone ongrowth and sparely bone formation in the gap by the composite group. Adding BMA to the composite has no beneficial effect on implant fixation. No significant difference between autograft and allograft on mechanical fixation, bone ongrowth and bone formation. In study II the combination of DBM with CB or allograft showed no significant differences on the mechanical testing and histological analysis to the control group, whereas DBM alone showed significant low mechanical fixation, low bone ongrowth and low bone formation. Study III showed no significant difference between the sampling methods. In conclusion, HA/Collagen composite alone or in combination with BMA has no effect on the early fixation of porous coated titanium implants. Autograft has comparable effect on allograft with regard to early implant fixation. The combination of DBM with CB may represent an alternative to allograft. In study III, mechanical testing and histological analysis can be applied either from the superficial or the profound part of the implant. By applying the histological analysis from the superficial part and the mechanical testing from the profound part, an extra section is required.