The Evolution of the Socket Graft: Technique Considerations

Once the dental surgeon has decided to delay implant placement and place bone grafting material into the extraction socket, a host of factors must then be considered before proceeding. Primarily, the type of defect present must be identified. Then, the clinician can decide how best to proceed utilizing a combination of flap techniques, bone grafting materials, biologic materials, and barrier membranes. The overall goal in bone grafting the extraction defect is to allow for adequate bone to be present when the time comes to place the dental implant. Now, more than ever, clinicians have a vast array of materials and options to use, ranging from new products to biologic materials and surgical techniques that have been refined over the past several decades.

When evaluating the extraction socket the clinician must categorize the defect by the number of walls remaining. Five-wall defects are the most predictable sockets to treat, as all the walls of the extraction socket are present: buccal, lingual/palatal, mesial, distal, and apical floor. If the walls of the defect are relatively thick and have good interseptal bone present, as in the molar region, then grafting is not necessarily required. However, grafting is needed when the five-wall defect is present in the anterior region and the labial bone is very thin.

A four-wall defect has one of the extraction walls missing, usually the labial plate or a portion of the labial plate. A three-wall defect is present when two of the walls of the extraction socket are missing. Typically, this defect has the labial and lingual/palatal walls missing and is termed a "through and through defect." Two-wall defects are those that have only two walls remaining, and one-wall defects are typically referred to as a "knife-edge ridge."¹ These last two defects are the most difficult to regenerate adequate bone for the placement of implants and are usually found in long-span healed extraction sites.

After the defect has been identified the surgeon has many options regarding preserving or rebuilding the alveolar ridge. There is no "one" method to do this. Bone regenerative materials fall into three main categories: osteogenic, osteoinductive, and osteoconductive.

An osteogenic graft transfers viable bone cells and is capable of forming bone on its own. The only bone grafting material that is osteogenic is autogenous bone. The drawback of this type of graft is the need for a second-site surgery with its associated co-morbidities.

Osteoinductive materials are able to induce bone formation and regeneration through the addition of growth factors into the recipient site. Currently, platelet-rich fibrin (PRF), a second-generation platelet concentrate²; bone morphogenic proteins (BMPs); members of the transforming growth factor-β superfamily; and placental tissues have the ability to induce bone formation, are anti-inflammatory and angiogenic, and eliminate morbidity associated with autogenous grafting.^3,4 The high concentration of platelets in PRF provide several key growth factors, such as platelet-derived growth factor (PDGF), vascular endothelial growth factor (VEGF), and transforming growth factor β1 and β2 (TGF-β), which act to stimulate cell proliferation and enhance angiogenesis.⁵ BMP has been proven to induce the differentiation of mesenchymal cells into osteoblasts.⁶ It is packaged with a collagen carrier that does not provide any osteoconductive properties; therefore, mineralized bone is often added to the BMP and covered with a rigid titanium mesh framework to inhibit the forces of muscles and tissue.

Osteoconductive materials consist of allografts (human-derived), alloplasts (synthetic), and xenografts (animal-derived) that come in the form of particles and blocks. These products provide a scaffold for new bone to form from existing bone but do not form bone on their own. Allograft cadaver bone is one of the most widely used bone grafting materials. It is available in a demineralized freeze-dried product, a mineralized freeze-dried product, or a combination of the two. The particles are also available in cancellous, cortical, and a cortical and cancellous mix. Demineralized bone is a weak osteoinductive material that requires a carrier and is available as a bone paste. These bone pastes may also include cancellous bone, which adds osteoconductive properties to their osteoinductive benefits.

Grafting the extraction socket is crucial. As much as 50% of the horizontal width of bone can resorb in the first 6 months after extraction.⁷ Therefore, it is advisable to graft the fresh extraction socket to preserve the alveolar ridge as opposed to grafting the healed extraction socket to reconstruct the alveolar ridge. Evidence shows that the coronal one-third of the extraction socket does not heal as well as the apical one-third due to factors such as surgical trauma, early clot retraction, contamination with saliva, bacteria, and oral debris, and epithelial migration into the void.⁸ Ideally, implants should be placed with at least 2 mm of buccal bone to achieve less long-term bone loss; studies have shown that this also helps maintain thicker labial gingiva.⁹

The following techniques, as suggested by the author, are the culmination and modification of the works of many surgeons (see Acknowledgment). It is assumed that all extractions are performed as atraumatically as possible and sockets have been debrided free of granulation tissue and irrigated with sterile saline.

Five-wall defects with thin walls are treated by the placement of cortical cancellous mineralized bone into the defect. The wound is covered with a placental membrane and then an absorbable collagen sponge. These are held in place with a figure-of-eight suture using resorbable suture material. Healing time is 3 to 4 months.

Four-wall defects, which are typically missing the buccal wall of bone, require the use of some type of barrier membrane, such as a high-density polytetrafluoroethylene (PTFE) membrane. Minimal flap elevation is preferable, but depending on the degree of missing bone a flap may need to be elevated. When raising a flap, the clinician should not extend too far beyond the area to be grafted, as this will help keep the graft contained to the site. Graft containment is also aided by using a mix of cortical cancellous particles with a demineralized bone paste that has cancellous bone within it. This mixture forms a firm, moldable graft material that contains osteoconductive and some osteoinductive properties. This is then covered with two membranes in a layered fashion. First, the amniotic membrane is placed directly on the bone, and a second layer of PTFE, which is exposed to the oral cavity, is placed over it. The PTFE should not contact the adjacent teeth and should overlap the palatal ridge of bone and cover the grafted defect. A nonresorbable suture, such as 4-0 PTFE, is left in place for preferably 4 to 6 weeks. The PTFE membrane is also removed at that time. Healing time is 4 to 6 months.

Three-wall defects are treated by the addition of a platelet concentrate such as PRF. PRF is generated by drawing blood from the patient and spinning it down to separate out the red blood cells from the platelets, fibrin, leukocytes, and growth factors. During the centrifuge process one or two of the tubes can be removed early before clotting and mixed with cortical cancellous mineralized autograft. Once the mixture coagulates the clinician has a firm mixture of bone similar to a bone paste, but containing the patient's PRF instead of reverse phase medium. This is currently referred to as "sticky bone." Once the centrifuge has completed spinning, the remaining tubes will contain a coagulated mixture of platelets, fibrin, leukocytes, and growth factor. The red blood cells are easily removed from the "fibrin clot," and the clot can be compressed and used as a membrane over the graft or cut into pieces and mixed with the bone graft material. The bone graft material and PRF should then be covered with either a titanium-reinforced PTFE membrane or a collagen barrier membrane that will not resorb for 4 to 6 months. This decision of which membrane covering to use is based on the size of the defect and whether or not the clinician believes that a large amount of wound contraction may occur. The titanium-reinforced PTFE membrane will help to counteract wound contraction. These wounds are closed primarily with 4-0 PTFE sutures that are removed in 4 to 6 weeks. The PTFE membrane is removed at the time of implant placement.

The treatment of two- and one-wall defects requires the clinician to maximize the amount of osteoinduction by utilizing BMP mixed with cortical cancellous allograft bone particles. For smaller grafts, a titanium-reinforced PTFE is used, but larger grafts require titanium mesh containment. The mesh is most easily utilized by having a 3D stereolithic model made of the dental arch in question and pre-bending the plate prior to surgery. The mesh is covered with a drapable, non-cross-linked porcine resorbable membrane. The wound is closed primarily with 4-0 PTFE sutures that are removed at 4 to 6 weeks. The mesh is removed at the time of implant placement. Healing typically requires 8 months.

In general, the larger the defect, the more biologic materials are needed and the longer the membrane should last. Twenty years ago patients were referred for a consultation to evaluate if they were candidates for implant therapy. Today, it is rare that a patient is not a candidate for bone grafting and eventual implant therapy. Bone grafting materials and tissue engineering continue to evolve and are an exciting adjunct to the field of implant dentistry.

The author acknowledges Drs. Michael Block, Daniel Cullum, Bach Le, Robert Marx, Craig Misch, Anthony Sclar, and the late Philip Boyne for having input into the grafting techniques discussed.