Introduction

When bone reconstruction of the posterior atrophic maxilla is needed, different surgical procedures can be used. Following the loss of teeth, alveolar ridge resorption leads to a combined vertical and horizontal reduction of the bony support, at the same time increasing maxillary sinus pneumatization. Such a condition makes implant placement impossible, either because of insufficient vertical bone volume or an alteration of the intermaxillary relationship which is not compatible with prosthetically guided implantology. As for other areas, bone augmentation procedures can be performed prior to placement of the implant, concurrent with implant placement, or subsequent to it.

Up to 10 years ago, autogenous bone was considered to be the gold standard in the reconstruction of atrophic areas of the jaws, due to its osteoconductive, osteoinductive and regenerative properties; it should still be chosen as the most suitable material in severe atrophies (class V and VI according to Cawood and Howell). In the treatment of smaller defects (class III and IV), bone substitutes of synthetic and xenogenic origin have been playing an important role in implant surgery. All those materials, generally called “biomaterials”, are able to favor the adherence of cells and tissue regeneration thanks to a variable degree of osteoconductive activity; after being tested for several years through randomized prospective or retrospective studies, they can be considered as a reliable way to rebuild bone.

Something fundamental is to remember that the difficulty in rebuilding a bone defect is more related to its extent than its depth. In other words, a very deep but localized defect is easier to treat than a superficial but extensive one. Careful preoperative analysis of the site to be regenerated is important instead of deciding at the time of surgery.

Biomaterials

The term biomaterial generally indicates any substance used to create a medical device destined for diagnosis, prevention, control, mitigation or therapy of a human disease, on condition that it persists in the body for at least 30 days after implantation. First of all, cytotoxicity, genotoxicity, and hemocompatibility of a biomaterial have to be evaluated. After that, attention has to be paid to its macrostructure and microstructure, by evaluating the isotropy. Finally, its mechanical, physical, and chemical properties should be taken into consideration. Which characteristics should a biomaterial have to be considered for implantation in the human body? They can be summarized as follows:

• non-carcinogenic;
• non-antigenic;
• hydrophilic;
• radiopaque;
• easy handling;
• versatile (usable in several clinical fields).

Biomaterials can be obtained from the patient (autogenous), from beings belonging to the same species (homologous), from beings belonging to different species (xenogenic) or from minerals (alloplasts). Apart from autogenous bone, which has osteoconductive, osteoinductive, and osteoproliferative properties, and homologous bone, whose properties are mainly osteoconductive and slightly inductive, all the biomaterials used for bone regeneration are only osteoconductive scaffolds. Bone substitutes were created in order to promote bone regeneration, avoiding the necessity of harvesting bone from the patient. The first materials on the market were represented by ceramic hydroxyapatite of different macrostructures (coral, bioglass, ceramic hydroxyapatite) and the osteoconductive potential, and the resorbability, were not excellent with regard to the implant field. A few years later, demineralized freeze dried bone allograft (DFDBA) from human donors was introduced in the US; osteoinductive properties were claimed for this, because the demineralization process was able to expose bone morphogenetic proteins (BMP). In addition, some publications confirmed an osteoconductive property for this material. Unfortunately, the properties of DFDBA were not confirmed by later histologic and clinical studies in sinus elevation and guided bone regeneration (GBR) procedures.

At the same time, xenogenic anorganic bone was obtained from cattle, followed by similar materials from equine or porcine sources. These mineral scaffolds, resulting from a treatment to eliminate any trace of organic material, promote colonization of bone tissue via osteoconduction. They are slowly replaced by newly formed bone; both the quality and the quantity of lamellar bone is well documented and, at the moment, they are considered a first-choice material in bony defect repair in implantology, with the exception of class V and VI atrophies (Cawood and Howell), where the use of autogenous bone, alone or in association with xenogenic materials, is mandatory to rebuild the bony architecture prior to implant placement.

Biomaterials Currently Used in Osseointegration

Anorganic Bovine Bone

This is an osteoconductive and slow-resorbing material composed of an anorganic mineral matrix deprived of the organic scaffold in order to leave intercrystalline microtunnels and microcapillaries between the bovine apatite crystals. The high osteoconductivity is due to the natural microstrucure of the material, which demonstrates a large inner surface area and a system of intracrystalline spaces and microtunnels available for ingrowth of blood vessels and osteoblast migration. Long-term stability has been proved by many clinical studies. There must be no direct contact of the material with the implant surface for good implant osseointegration. The only early contact is between the clot and the matrix particles, and the angiogenesis and osteoblasts deposition occur from there. Integration is due to replacement of the bone substitute with newly formed bone. Histomorphometric analysis demonstrated that anorganic bovine bone increases the mineral portion in regenerated areas as compared to host bone areas. Some of the material remains in the bone tissue and is slowly embedded in lamellar bone, resulting in denser bone, and this could explain the high survival rate of implants placed in areas augmented with it.

Calcium Phosphate

Depending on the Ca/P ratio, the presence of water and impurities, this material crystallizes into two different shapes:

• calcium hydroxyphosphate or hydroxyapatite (HA);
• beta calciumtriphosphate (beta TCP).

Synthetic HA has the same chemical composition as human HA, but a slightly different structure. Its rate of metabolism by the human body depends on its structure, chemical composition, and interface surface area.

Beta TCP is also resorbable and is slowly converted to HA inside the human body; it supports early bone apposition (woven bone), although beta TCP’s degradation products may provoke an inflammatory response that impairs and reverses bone apposition in the defect site. Histologic evaluations showed a solid host–bone connection at 9 months, but newly formed bone is confined to the periphery of the graft, the center being partially filled with connective tissue. This material can only be said to be basically resorbable, and no conversion process to trabecular, functional spongiosa occurs, since remodeling cannot take place. A recent report on sinus elevation showed no difference between a test site treated with beta TCP and a control site treated with autogenous bone in terms of quantity and rate of ossification in a significant sample of patients.

Calcium Sulfate

This material has been used as a synthetic grafting material. Once the sulfate is embedded with water, an exothermic reaction results in crystallization and hardening of the granules. The host bone forms in concentric layers around the resorbing sulfate, which is different to the situation with other ceramics. Dense calcium sulfate does not seem to stimulate early bone apposition, but bone repair is more advanced at 12 weeks as compared with those treated with beta TCP, despite beta TCP’s support for direct bone apposition at 1 week. Calcium sulfate appears to provide a more stable osteoconductive scaffold and seems to be a good material in sinus elevation. The osteoconductivity of calcium sulfate is documented, but the clinical experience remains limited.

Calcium Carbonate

This material is derived from calcified coral polyp skeletons with an aragonite crystal structure and mineral trace elements. Histologic examinations showed absence of direct bony ingrowth of the carbonate in the bone, while connective tissue appeared to encapsulate the implant, isolating it from the autogenous bone and preventing osteoconduction. Bone remodeling looks incomplete due to an insufficient amount of newly formed bone.

Bioactive Glass

This is a resorbable amorphous material composed of silicon dioxide, calcium oxide, and sodium oxide. The osteoconductive property is documented, but the most reliable clinical studies in sinus elevation were carried out using a mix of bioactive glass and autogenous bone in single-arm studies or comparing a mix with a control test represented by autogenous bone alone. Studies evaluating bone repair using bioglass alone are necessary to verify the properties of such a material.

Demineralized Freeze-Dried Bone Allograft

This material, from human donor bone, has been extensively used in the treatment of periodontal and periapical osseous defects. The material provides a source of type I collagen, which is the only organic component of bone. The processed bone results in lyophilized particles demineralized with hydrochloric acid. The particles are then recovered by centrifugation, frozen and freeze-dried again. Many reports confirm its ability to induce new bone formation thanks to the exposure of BMP, whose inductive properties are well known. Despite this property, DFDBA’s osteoinductive action has never been demonstrated and it seems that the regenerated bone is insufficient either in quality or in quantity to allow predictable implant placement, particularly following sinus elevation procedures. Other clinical studies confirm the reliability of DFDBA in sinus elevations, but always in conjunction with other materials. Similar results are reported relating to the use of DFDBA in association with expanded polytetrafluoroethylene (e-PTFE) membranes.

Surgical Techniques

The choice of procedure for reconstructing the posterior areas of the maxilla depends both on the depth and the extent of the defect. Starting from the postextraction defects up to the most severe atrophies, the classification according to Cawood and Howell clarifies, in a simple way, what kind of surgical technique is the most suitable for each specific bony defect. In accordance with that classification, the defects can be listed as follows:

• postextraction sites;
• horizontal defects (including dehiscences and fenestrations around implants);
• vertical defects;
• combine/>
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