Treating large mandibular defects (Proceedings)

Bridging reconstruction plates were the first implant used with success in the human mandible. Despite their greater ease of use and application in this location as compared to conventional plates, the low perioperative morbidity was unfortunately followed by high long-term morbidity. It was believed that the pressure between the bone/implant interface interfered with circulation, leading to bone resorption in this area. This resorption led to loss of contact between the plate and bone, and subsequent instability of the entire construct. The uncontrolled forces of mastication resulted in the originally well-fixed screws/fixation to become overloaded, leading to screw loosening, resulting in further bone resorption and loss of the fixation.

One of the earliest uses of the locking plate technology was with the treatment of large mandibular defects in humans using the THORP® (Titanium-coated Hollow Screw and Reconstruction Plate) system, which was introduced in 1984 (Synthes®; Davos, SW) or repair of mandibular fractures, especially as bridging plates for large defects, and was generally replaced by the UniLOCK® system after it was introduced in 1999 (Synthes®; Davos, SW). The primary advantages of the latter are a thinner profile plate and a simpler (2-piece) locking screw/plate design.

The basic premise for this fixation type was to change from the mixed-mode of load transfer of standard plating systems to a screw-only mode of load transfer. In the former, load transfer occurs directly between the screw and plate and the screw dependent friction generated between the plate and the bone; in all cases contact, or compression, between plate and bone is required. With the locking plate systems, there is direct load transfer between the screw and the plate so that contact, or compression, between plate and bone is not required.

Conventional plate/screw systems require precise adaptation of the plate to the underlying bone. Without this intimate contact, tightening of the screws will draw the bone segments toward the plate, resulting in alterations in the position of the osseous segments (loss of primary reduction) and the occlusal relationship. With locking plate/screw systems it becomes unnecessary for the plate to intimately contact the underlying bone in all areas. As the screws are tightened they "lock" to the plate, thus stabilizing the segments without the need to compress the bone to the plate, which makes it impossible for the screw insertion to alter the reduction.

Another potential advantage in locking plate/screw systems is that they do not disrupt the underlying cortical bone perfusion as much as conventional plates, which compress the undersurface of the plate to the cortical bone.

A third advantage to the use of locking plate/screw systems is that the screws are unlikely to loosen from the plate (loss of secondary reduction). Conventional screws are loaded in bending, but the locking screw – since it is locked in the plate – is in a neutral position without creating any unnecessary stresses in the bone. This difference results in an increase of the construct yield strength with the locking screws. Finally, because the screws are locked within the plate, implant loosening will not occur as a result of micromotion. Micromotion can result in screw loosening with a standard plate and screw, which then causes a loss of the frictional interface between the plate and bone, the consequence of which is total implant failure. Locking plate/screw systems have been shown to provide more stable fixation than conventional non-locking plate/screw systems subjected to the unique environment of the high loads of mastication in a cantilevered fashion.

The 2.4 UniLOCK® system is a locking reconstruction plate of a low profile design. Standard 2.4-mm screws (1.8-mm drill bit) or locking screws, either 2.4-mm or 3.0-mm (1.8-mm or 2.4-mm drill bit, respectively) can be used with these plates. The larger screw is designed as a "rescue" screw if either of the 2.4-mm screws strip their bone threads. The locking screws have a special double-lead thread beneath the screw head that engages and locks into the threaded holes of the plate. Plates can be cut to the appropriate lengths, and because of the reconstruction plate configuration, may be contoured 3-dimensionally.

A 2.0-mm Locking Mandible System was introduced in 2000 (Synthes®; Davos, SW). In this system, 4 plate sizes are available: 1.0 mm thickness X 4.8 mm width (mini), 1.3 mm thickness X 5.0 mm width (intermediate), 1.5 mm thickness X 6.5 mm width (large), and 2.0 mm thickness X 6.5 mm width (extra). A number of different lengths are available depending on the plate size: 20-hole mini, 12-hole intermediate, 12- and 20-hole large and extra. These plates can be cut to the desired length. The plates will accommodate both standard and locking 2.0-mm screws. The locking screws have conical double-lead locking threads.

It is important to recognize that because of the capability for 3-dimensional bending that in all of these systems the in-plane bending must be performed first, followed by the out-of-plane bending, then finally any twisting. Specialized instrumentation is available in order to protect the screw-holes from deformation – thus protecting the locking mechanism (screw inserts only are available with the 2.4-mm reconstruction plate). These plating systems have been used in veterinary surgery identical to their usage in humans for mandibular fracture fixation. In addition, they have also been successfully utilized in veterinary patients for spanning large defects. In the latter, the addition of a variety of bone grafts has been used so as to restore bony continuity.

With large mandibular defects in humans, various grafting techniques have been used in order to re-obtain bony continuity of the bone. The use of vascular grafts has greatly improved the results of these procedures and also reduced the number of complications. Most commonly, these have been the result of tumor ablation or severe trauma. The largest issue for such reconstructions is the unfavorable environment – due to the oral contamination present – and the requirements of sufficient structural integrity/stability to counter the forces of mastication. Bone union may be difficult to attain across large defects, especially with the requirement of continued jaw mobility.

Both vascular and avascular (free) grafts have been used with success in mandibular reconstruction. The main difference is the live cells that vascularized bone maintains, which are capable of regeneration and thus providing more rapid healing and immediate structural support with early callus formation – especially in instances where there may be a tenuous fixation. In addition, the vascularized grafts have been shown to survive in a radiated bed with evidence of callus formation and a fully viable bone marrow. The vascularized graft first used was a rib transposition. Others now commonly used include: the iliac crest, fibula, or scapula. The success of vascularized bone grafting in clinical studies has exceeded 90%, demonstrating the safety and reliability of mandibular reconstruction with vascularized bone. At present, in the vast majority of mandibular reconstructions, the iliac crest, fibula, or scapula is used.

Despite the obvious advantages of vascularized bone grafts for mandibular reconstruction in humans, this is not a practical technique in veterinary surgery. The obvious limitations include equipment (an operating microscope) and a specialized set of surgical skills (that requires specialized training and constant honing). Furthermore, there is an additional size constraint with small animal practice, as our patients are considerably smaller than the human counterpart, adding to the difficulty of the technical demands of microantastomic vascular techniques. Therefore, use of bone to restore immediate structural continuity in small animals has been confined to the use of avascular grafts, using primarily either rib or ulna. The basic prerequisites of a free avascular bone graft include: a good (vascular) soft-tissue bed in which the graft has close contact, and absolute mechanical stability for the graft. Autogenous cancellous grafting is recommended at the host-graft interface to promote union by providing greater osteogenic and osteoinductive potential at these critical sites. Despite fairly rapid healing and incorporation at the graft-host interface (~8-12 wks), complete graft incorporation does not occur. The major drawback of these grafts is that large areas of bone become nonviable, and the graft must be replaced by host bone, which occurs via Haversian remodeling. This process takes much time (years) and never is 100% complete. Another potential disadvantage is the presence of nonviable bone in a possibly infected environment. Furthermore, the degree of stability that must be provided by the fixation cannot be compromised. There is an expected loss of ~25% of the original graft size due to the remodeling; therefore, size overcompensation must be considered at the time of the original procedure, although this may not always be possible due to the confines of the local environment. Avascular grafts generally are limited to ~5-cm in length, as longer grafts result in necrosis at the central portion of the graft, with subsequent structural failure.

Autogenous cancellous bone grafting can be used to span large gaps, and remains the "gold standard"; however, without structural support the fixation remains at risk for early failure due to premature implant failure, although the use of the locking systems helps to address this issue. Once again, however, there is a limited source of autogenous bone graft available, and alternate techniques must be applied when very large defects are present. Banked allograft may be used to expand the size of the graft, but one drawback is the possibility of disease transmission; this is a considerable concern in humans. In small animals, commercially available banked bone is available in a number of forms (Veterinary Transplant Services, Inc.; Kent, WA): cancellous bone, coritcocancellous chips, and demineralized bone matrix powder, or a combination of these substances.

Alternative options have been suggested for the reconstruction of large mandibular defects in humans due to the high morbidity, inconvenience and cost associated with the reconstruction procedures, this despite the current popularity of the microvascular transfer techniques. These involve the bone morphogenetic proteins (BMPs) that have been shown to be the most effective osteoinductive growth factors both in vitro and in vivo; BMP-2 and BMP-7 (OP-1) have been identified as the most potent of these, and have been reproduced using recombinant DNA technology (recombinant human BMP's – rhBMPs). The major limitation for their clinical use has been the delivery system, or carrier. The carriers most often used at this time include: ceramics such as tricalcium phosphate (TCP), and non-ceramics such as calcium phosphate-based cements (CPCs), natural polymers [different forms of collagen such as: gelatin, demineralized bone matrix (DBM)], and composites [varying combinations of synthetic and natural polymers (HA impregnated PLA sponges, collagen-PLG-alginate, and PLGA-gelatin), or mineral components with natural polymers (collagen-TCP)].

Some of the first experimental evaluations of the rhBMPs in the mandible were performed in an experimental canine mandibular defect model (2001). Subsequent studies at that time in nonhuman primates also were shown to be effective at quickly and effectively inducing osseous regeneration of the mandible. A few case report series have been published (off-label use) showing the success of this approach in a number of human patients.

Similar reports also have been published in veterinary surgery, again demonstrating their utility for treatment of large mandibular defects and their reconstruction with minimal complications. The rapid bone healing observed in these cases, along with the absence of serious complications, mimics those found under more controlled experimental conditions. These cases demonstrate the tremendous promise for the treatment of massive bony mandibular defects in the future.

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