Dental implant therapy is a well-documented and well-supported procedure with high predictability,1 particularly if hard tissue is optimal. But in cases with insufficient bone volume, a bone augmentation procedure is required. Many techniques have proved to be effective for horizontal and vertical hard tissue augmentation, such as Guided Bone Regeneration (GBR) or autogenous bone block graft.2–4
Reducing flap tension
Independent from the bone augmentation itself, primary wound healing appears crucial for a positive outcome. Unfortunately, complications associated with these procedures are not irrelevant. Studies report complication rates of 45% with vertical GBR3 or 29.8% with vertical block grafts.4 Flap tension for primary wound closure of mucoperiostal flaps seems to the most important factor.5 Minimal flap tension (lower than 5 gr) is associated with 100% primary wound closure, while increasing the flap tension increases the incidence of wound dehiscence. This means that soft tissue release is a crucial step in bone augmentation surgery. Clinically, three types of flaps can be released by periostal incisions and coronal advancement – these are the lingual and vestibular flaps in the mandible, and the vestibular flap in the maxilla.
Three types of released and advanced flaps
The mandibular lingual flap is released by interrupting the thin periosteum layer with an elevator or dissector (Fig. 1A-C). The use of a blade may be avoided or limited to the mesial area of the mandible, where the periosteum is thicker.6 In the distal area the more superficial fibers of the mylohyoid muscle may be denuded simply by elevating a full thickness flap, since the mylohyoid line is in a more cranial position. In some cases, detaching muscle fibers from the internal face of the flap may increase the coronal advancement of the flap.7
The mandibular vestibular flap is released with a scalpel blade (15c). The tip of the blade should contact the superficial inner face of the flap starting from the vertical releasing incision and moving distally or mesially (Fig. 2A-D). The blade works with the cutting face upside down and the non-cutting area facing the flap. Once the periosteum is interrupted, the flap can be elongated with an elevator or dissector, avoiding vascular damage. It is important to work where the flap reaches the fornix, so as not to weaken the flap itself. Consider the position of the mental foramina and the mental nerve, maintaining a safe distance of six mm from these anatomical structures.
The maxillary vestibular flap is released in a manner similar to the mandibular vestibular flap, except that usually, once the periosteum is interrupted, the flap is elongated with a blade (instead of elevator or dissector) because the density of elastic muscle fibers inhibits coronal advancement.
Avoiding the killer loop effect
The surgeon should keep in mind that a released and elongated flap does not necessarily result in a correctly released flap. When the flaps are not optimally released, the suture lines, which are usually composed of horizontal mattress sutures and single sutures, may make the marginal part of the wound ischemic, which can result in necrosis. This “killer loop effect” of horizontal mattress sutures is amplified by increasing the residual flap tension at the end of the surgery. The application of a breaking force suture seems to reduce the marginal flap tension prior to the horizontal mattress.8 Two suturing technique employ a breaking force suture – one in the maxilla and one in the mandible.
The maxillary suture involves the entire thickness of the palatal flap and just the coronal periosteum layer of the vestibular flap. The needle engages the palatal flap seven to ten mm apical to the flap margin, then moves to the vestibular side and engages the periosteum on the coronal margin of the releasing periosteum incision. It then moves palatally again through the entire palatal flap. After the suture is tied, the vestibular flap slips coronally (Fig. 3A-D). Then mattress sutures and single sutures are applied with no or minimal residual tension.
The mandibular suture involves the inner part of the flaps. The needle engages the periosteum on the coronal margin on the lingual flap, then moves to the vestibular flap, engaging the coronal margin of the released periosteum in the same manner. After the suture is tied, both flaps move coronally and will usually come in contact with one another. Then mattress sutures and single sutures are applied with no or minimal Tension (Fig. 4A-D).
The breaking force suture involves just the periosteal layers without creating any killer loop effect, while leaving the vascularity intact. This suture can reduce the residual flap tension prior to wound closure by about 87%.8
An alternative: The tunnel approach
Flap design can also be an opportunity to improve or simplify the primary wound closure and reduce the risk of dehiscence. The tunnel approach proposed by Khoury is one of these opportunities.9 The technique avoids any crestal incision in the area of the graft. A single vertical incision is made at the distal aspect of the mesial tooth closest to the defect. The flap is then elevated as a full thickness flap, tunneling around the defect. In this way there is optimal graft protection, and wound dehiscence is avoided by the continuity of the soft tissue over the graft. Visualization of the surgical field is obviously limited, and this must be taken into consideration. The anatomy of the defect must be analyzed preoperatively, and the bone defect should be quite smooth and regular without deep bone septa, which could interfere with the correct elevation of a full thickness flap.10 Nevertheless, many publications report limited or no soft tissue complications.9–11
Soft tissue adheres more tenaciously to bone than dentin
Considering flap stability as a factor influencing flap tension, Werfully et al. (2002)12 demonstrated differences in flap adhesion to different substrates. Their study in dogs revealed that, at all healing times, the flap tensile strength at the flap-bone interface was at least twice that of the flap-dentin interface (after seven days: 5.08 N versus 1.82 N, respectively). This means that soft tissue is able to adhere more strongly and quickly to bone than to dentin. The difference is the favorable relationship between the underlying coagulum of the flap with bone, whereas the coagulum-dentin interface is more tenuous. Extrapolating these findings to augmentation, a key factor for stress reduction of the flap margins in early healing could be the capacity of the coagulum and the overlying flap to adhere to the graft material. It is reasonable to assume that the stability of the complex coagulum-flap interface is not optimal when a graft material is the interface (non-resorbable idrophobic PTFE membrane), which might explain the high complication rate associated with augmentation procedures. But for overall flap stability, the suture line is the key to maintaining primary wound closure.