In addition to ensuring general medicine shear conditions when planning dental rehabilitation with implants, of course, the respective regional bone situation is of great importance. Without a suitable local bone bearing implants can not be inserted sufficient statistic in the sense of "backward planning". If the space in bone stock is inadequate, the optimization is the first task.
Possibilities of optimization of the bone bed are given by using the body's own materials, through the use of bone substitute materials (BS) and membranes or in conjunction with all the above options, as well as by distraction osteogenesis.
Cause of a primarily non-performance implantation bone stock can be:
- genetic defect,
- after tumor resection,
- and the general atrophy of the jaws after loss of the dentition.
After tooth loss outweighs the reduction of the width of the maxillary alveolar ridge (sagittal resorption typ), in the lower jaw of the height reduction of the alveolar ridge (vertical resorption typ). Resorptions are less pronounced, after the initialyear of tooth loss, i.e. after incorporation of a dental prosthesis. In the maxilla, the alveolar ridge can dwindle down to a few millimeters thick bone lamella. For the repair of bone defects autogenous bone is still considered to be the ideal graft material.
Guidelines for bone grafting
- The recipient region must have an adequate blood supply. This is extremely important for the preservation of all living cells of the transplant surface ensured.
- Between graft and host bone, a direct bone contact area must be existing. This facilitates bone resorption and bone formation by "creeping substitution".
- The recipient region must have an adequate blood supply. This is important for the preservation of all living cells is ensured on the graft surface.
The repair of autologous graftBone repair is based on three basic mechanisms:
Osteogenesis: the ability to form in the graft of the surviving bone osteoblasts.
Osteoconduction: The introduced graft serves as a lead structure for the sprouting of vessels from the adjacent bone stock. The newly formed bone deposits on the transplanted tissue.
Osteoinduction: Osteoinduction effect of bone proteins (growth inducing factors for example = bone morphogenetic protein, BMP) to reach the pluripotent mesenchymal cells, with the ingrowth end vessels in the bone graft. Under the action of these proteins wachstumsinduzierenden the pluripotent Mesenchymzellem differentiate into cartilage, or bone-forming cells. The biological transplant healing depends on the storage tissue and the mechanical rest.
The healing of the graft always runs in multiple phases, used in the first two weeks initially resorptive process and only occurs consecutively with the capillary ingrowth from the surrounding tissue bearing the beginning of the repair. Graft osteoblasts, which are supplied with blood by diffuse, survival in this first phase and begin osteoneogenesis.
By the third week, the second phase begins. Osteoclastic bone cells penetrate the ingrowth end of the storage vessels into the graft tissue and resorb the bone graft. Below osteoblasts build new bone on (woven bone). In the subsequent period, the entire free transplanted bone is resorbed and replaced successively.
In the following, it is in the third phase from the 4th Week under functional stress to the remodeling of the bone into lamellar bone braid. The raised mounting operations lead to a loss of volume, so that always a certain over-contouring of the bone graft is necessary.
We know that not functionally loaded bone is resorbed over a relatively short period of time. It is also known that the last phase of bone healing, so the trabecular pattern in clarifying functional integration of the graft is favored over a load introduction by implants. These processes are not or only insufficiently to develop, if the quality of the transplant bed is restricted hinschtlich the vascularization of the adjacent bearing bone and the surrounding soft tissues.
This can for example by a previous radiotherapy, massive inflammation or multiple previous surgeries, which led the case for extensive scarring. The decision whether an intraoral bone harvesting is sufficient or extraoral donor site must be sought should already take place within the implant planning.
Similarly, the question of the time of implant placement should be taken when osteoplastic replacement in individual cases. The synchronous implant placement is possible if sufficient stability of the implant is given into the local residual bone.
If this condition, it is time to act two. The implantation is carried out in this case after an installation phase of the bone graft of four to six months. Advantage of simultaneous implant placement during the implantation of bone graft is that even after the required four to six months of healing of the bone graft and the implants can be a functional load of the built-bone on the kaufunktionell loaded implants, thus early for adequate functional loading of the augmented mandible leads. The absorption rate in bony integrated implants from the second year after autologous bone grafting is indicated by 0.2 mm per year.
Choice of the augmentation
Clinically it should be considered already in the planning phase which präopertativen bone replacement material is suitable for the individual case. Here, the autologous bone is by definition also be seen as a bone substitute material such other materials. Once the basic mechanisms of bone healing were explained with the use of an autologous graft in the preceding paragraph will be presented briefly below, the specific characteristics and indication of the most common bone substitute materials.
Bone substitute materials (= BS) must be characterized by biocompatibility. Long-term safety also includes the degradation products arising from the degradation. Bone substitute materials general posses osteoconductive and rare -inductive properties.
All on the market at the time BS are characterized by two specific properties:
- With respect to their biological behavior, which is to be assessed as Osteoconductive.
- Respect to their formulation, which is always in particulate form.
From a materials science point of view BS must be divided into defect fillers of the long term stability such as for example hydroxylapatites, the resorbable BS´s, which are substituted by de novo bone formation, so the onlyserve as temporary defect filler. These include phase-pure, unsintered hydroxyapatite, and the alpha-and beta-tricalcium phosphates, which may release large amounts of calcium ions and / or implant particles due to their solubility and hence have high demand of phagozytosis.
In allogeneic transplants donor and recipient are not genetically identical but belong to the same species (man-man). There is a possible risk of infection by pathogens transfer from the donor to the recipient. In addition, the risk of infection which reason we use at the site of implantation up to 20% no quantifiable allografts.
Xenogenic bone grafts derived from another species (eg, cattle, pigs) and are genetically different. They are used today only after appropriate treatment, eg Pyrolisation to turn immunological risks. For the usefulness of these materials, the course prescribed large interconnector animal cavities play an important role, entering the bearing bone ingrowth from forming osteocytes on the track vessels and all the material can be penetrated by de novo bone formation. The cavities are filled with new formed bone, the autogenous bone connected to the BS like a wallpaper.
Intraoral donor sites for autogenous bone grafts avascularAs oral donor sites in the mandible are suitable:
- angle of the jaw,
- mandibular border,
- retromolar region.
Extra-oral donor sites for autogenous bone grafts avascularCommon donor sites of avascular bone grafts are:
- anterior and posterior iliac crest
Clinical augmentationDue to a relatively thin cortical layer and an additional loose cancellous bone of the upper jaw has primarily an more unfavorable implant bed than the lower jaw. For the atrophic maxilla different augmentation procedures for widening and internal and external ridge structure were developed. To widen the alveolar ridge one can laterally apply an autogenous corticocancellous bone graft. The stabilization of the bone graft is done by osteosynthesis screws. An implant insertion is made secondary to bony incorporation of the bone graft. When it comes to vertical alveolar ridge augmentation one has to take into consideration that is resolves in a reduction of the vertical distance between the upper and lower jaw, which has to be considered with regard to the prosthetic restoration. This dimensional changes can be done either through a so-called vertical augmentation or by a interpositional osteoplasty. The stabilization of the bone grafts can be performed in the horizontal augmentation by simultaneously implant placement or using osteosynthesis crews fixed to the residual. In these two-stage procedures three to four months after the augmentation the implant is placed.