Читать книгу Bone and Soft Tissue Augmentation in Implantology - Группа авторов - Страница 21

For small augmentations, the bone core technique is recommended. A bone core is harvested using trephine burs of different diameters, but on average a 3.5-mm external and a 2.5-mm internal diameter (see Chapter 4). The bone cores are used together with bone chips to augment the bone immediately after implant placement. This trabecular bone core can be used analogous to the cortical bone plate, providing a small bony sheet for the bone particles, again requiring stabilization with micro screws. After 3 months of healing, the implants and the grafted bone are exposed, and the width of the grafted area measured. Bone cores grafted completely inside the bony contours demonstrated no resorption 3 months postoperatively, while in most cases bone cores grafted partially outside the bony contours showed partial resorption of the bone outside the bony contours.⁶⁷ Similar to the bony lid technique, at the re-entry 3 months later the average width of the area reconstructed with the trabecular bone core only lost 0.3 mm, which is around 13% of the original dimension, again suggesting good volume stability. What we can learn from this approach is that, in a clinical scenario, the resorption of cortical bone plates as well as trabecular bone cores is low, and that bone chips are well integrated into the newly formed bone after 3 months.^67-69 Taken together, autografts in this particular indication allow and may even support the occurrence of natural bone formation originating from the host bone and maybe also from the transplanted autografts. In addition, and interestingly, the augmented volume remains rather stable, with around 7% to 13% resorption after 3 months.

Bone resorption also occurs after tooth extraction, as reported in canine models⁵ and clinical cases,¹⁰⁴ and when the facial bony wall is thin it even disappears, probably due to the lack of vascular supply.²⁶ The questions are as exciting as they are important: 1) Why are transplanted autographs resorbed? 2) Why does this resorption occur partially but not completely, depending on the size and anatomy of the autograft? 3) Why is it difficult to predict the extent of resorption?

There are certainly many reasons for bone resorption – some are known (e.g. the influence of muscle activity), while others are as yet unknown. In case of human sinus augmentation with pure autogenous grafts, around 40% of bone volume is lost within 6 months, probably through the respiratory pressure on the sinus mucosa covering the grafted and non-mechanical resistant trabecular bone,^31,47,101 similar to a canine model.¹⁰² In alveolar cleft patients, transplanted iliac bone showed comparable bone resorption rates of less than 40% within 6 months.^39,136 On a cellular level, the resorption of autologous bone chips by osteoclasts within 1 week is particularly obvious in the pig mandibular defect mentioned above.¹⁰⁰ It seems that the resorption of transplanted bone that undergoes necrosis is prone to resorption – similar to local bone areas, with microcracks that undergo fatigue damage and are replaced by remodeling.¹⁰⁷