Discussion
Since DBM for clinical use is very expensive compared to autologous bone grafting, effectiveness of such products is mandatory. While DBM gel as an autograft extender showed improved fusion in dogs, heterologous DBM provided equal fusion capacity as autologous bone graft in rats. Comparable results were also obtained when DBM was used as partial substitute of autologous bone graft in a postero-lateral fusion model in rabbits. In another rabbit model, DBM showed osteoinductive ability, while allogenic deep-frozen cortical bone did not. Radiographic investigations in humans showed equal fusion rates of postero-lateral arthrodesis performed in patients with local autologous bone graft augmented with DBM and in those with autologous bone graft from the iliac crest alone. Radiographic progression of the fusion with time was also shown when DBM was used as bone graft extender to decrease the need of autologous bone graft. Intra-individual comparison of DBM augmented autograft at one side and autograft without adjunct at the contralateral side in patients who underwent postero-lateral fusion showed equal results in radiographic assessment. An ex vivo histological analysis of different types of DBM products used for sinus lift procedures in oral surgery showed superiority of DBX® in terms of new bone formation and low residual demineralised matrix compared to other products. In a recently published case report of a 7-year old male child suffering from idiopathic scoliosis who underwent posterior spinal fusion, DBM was used as sole graft source. Eleven month later on the basis of a routine surgical exploration, osseous biopsies were taken from the fusion site and histological analysis was performed. In this case, investigators found only mature bone with no residual DBM graft and concluded therefore DBM to be fully incorporated in the fusion mass.
Our results tend to support these overall findings of reliable remodelling of DBM to new bone. The histological analysis showed significant new bone formation and decreasing residual DBM material in all of our patients depending on the time span DBM was in situ. Additionally, cortical bone chips converted as well in the same manner. The verifiable process of "creeping substitution" allowed confirming active incorporation and reorganisation of the demineralised matrix.
Further analysis should be made to determine whether additional cortical bone chips are necessary to enhance new bone formation or if DBM putty alone provides best performance in postero-lateral fusion of the spine. Comparative study designs will help to analyse the bone remodelling patterns of different treatment courses such as the use of DBM versus autologous bone grafting or spontaneous fusion based on posttraumatic hematoma.
Limitations of our study are the small number of patients included in the collective and the lack of quantitative morphometric analysis based on standardized bone samples, which of course was not applicable in the clinical setting. Furthermore the results of our investigation are not applicable to postero-lateral fusion in general since our study population is relatively youg (mean age 44 years) and the reason for a fusion was trauma in eight of nine cases. Additionally there may be a bias with regard to the harvesting site of the bony samples, which were all taken around the rods during implant removal and therefore may not be representative for all fusion areas.