Managing mandibular fractures in the dog (Proceedings)

An understanding of the functional anatomy is the prerequisite for successful application of the fracture fixation devices in the unfamiliar location of the mandible. These biomechanical principals must account not only for the very large forces generated, but also the position of the teeth that can – and often do – interfere with implant application. Bending forces are the primary distracting forces acting on the mandible that must be neutralized (Fig. 1). A continuum of tensile to compressive stresses exists from one side of the bone to the other during bending stress. Maximal tensile stresses are present at the oral (alveolar) surface, and maximal compressive stresses are present at the aboral surface (Fig. 1a); therefore, distraction is created at the oral margin (Fig. 1b). These bending moments increase from caudal to cranial; furthermore, shear forces are maximal at the ramus, while rotational forces are most prominent rostral to the canine teeth - and maximal at the mandibular symphysis. Bending moments, however, remain as the most significant force that must be neutralized due to the anatomic configuration of a long lever arm with absence of supplemental support.

Fig. 1 A) Line drawing of an the mandible demonstrating bending moments, i.e., the continuum of tensile to compressive stresses from oral to aboral bone surface with closure of the jaw during normal (chewing, biting) function (medium arrows). Large arrows indicate pull of the major muscles of mastication (T, temporalis m; M, masseter m; D, digastricus m; P, pterygoideus m). B) With a fracture, distraction occurs at the oral (alveolar) margin; compression occurs only at the point of bone fragment contact (C).

Application of the fixation must consider the tension and compression surfaces of the bone. All fixation devices are strongest in tension (stresses parallel to the longitudinal axis of the implant); therefore, they should be placed along the lines of tensile stress, or on the tension surface of the bone. In cases of mandibular fractures, this location is along the alveolar border; however, the presence of the teeth will interfere with this most optimal biomechanical location.

The objectives of treatment are to provide early rigid skeletal fixation, with restoration of dental occlusion, thereby achieving an early return to function (eating and drinking – without bypassing the oral cavity via alternate feeding methods, e.g., esophagostomy/gastrostomy tube placement). Techniques and devices most often described for this purpose include: intraosseous wire, used alone or in combination with other skeletal fixation devices, and bone plates and screws.

Intraosseous wires

Intraosseous wire fixation is a simple technique that is designed to be used on the basis of the tension-band principle. Due to the wire's small size, a great deal of versatility is possible with the wire location. This permits the tooth roots to be easily avoided when placing wires on the biomechanically advantageous alveolar surface of the bone. Intraosseous wire fixation is used to provide direct support for distracting (due to bending) and shearing forces at the fracture site. A single heavy wire suture (e.g., 1.5-mm) applied to thicker bones, as in the mandibular body or ridges of the vertical mandibular ramus, sufficiently neutralizes bending forces by locating the wire nearer to the alveolar/oral margin rather than to the ventral, or aboral, margin of the bone. This location effectively prevents bending and separation at the tension side of the mandible during normal function (chewing, biting). If anatomic reduction can be maintained, lateral bending, shear and rotational forces on the mandible are minimal. Single intraosseous wires, although most useful in neutralizing bending forces, still may not effectively neutralize shear or rotational forces despite the latter's generally lower magnitude. The addition of a second, or divergent, wire greatly enhances neutralization of these forces. When bony defects are present, either due to trauma or tooth loss this form of fixation is not effective, as it cannot maintain distraction because it cannot function as a buttress device. The limitation of orthopedic wire, therefore, is that it must be used with an understanding of the functional anatomy (placed along the lines of tensile stress) and with an anatomic reconstruction of all bone fragments (used as a rigid suture to compress the fractured bone fragments together).

Wire placement

Orthopedic wire should be flexible enough to permit manipulation through drill holes in the bone and to allow proper tightening. The use of large diameter wire (1.0-1.5-mm) provides the best rigidity for fixation, but also is more difficult to position and properly tighten. Wire diameter must be at least 1.0-1.5-mm, wire smaller than 1.0-mm is unlikely to be of sufficient strength to maintain fracture reduction even in the smallest of animals. The direction of the drill-holes through the bone also is important in facilitating insertion of the wire and proper tightening. Aiming the drill hole toward the fracture site results in a sloping hole that facilitates passage of the orthopedic wire (Fig. 2). Generally, in mandibular fractures the bone fragments are not very mobile despite the surgical exposure obtained, and the orthopedic wire must be passed through the drill-hole with the bone fragments in situ. The wire is most easily passed through the drill-hole in one fragment from outside to inside (lateral to medial), retrieved, made into a large loop and then passed through the drill-hole in the second bone fragment from inside to outside.

Fig. 2 A) Orthopedic wire passed through the drill-hole in one bone fragment from outside (O) to inside (I), retrieved, looped, and then passed through the drill-hole in the adjoining bone fragment. B) The wire is pulled through the second bone fragment (reducing the size of the loop) while simultaneously untwisting the loop - thus avoiding a major kink in the wire. C) The wire then is pulled through the bone fragments a sufficient distance to remove the kinked area from between both bone fragments.

Wire tightening

Two methods for tightening orthopedic wire are used: either a twist knot or tension loop. Both methods have been advocated in long bone fractures, but the twist method is more easily applied and allows for better control of tightening with mandibular fracture fixation. Tension loop wire easily is over-tightened when used in the repair of these fragile bones, i.e., flat bones with thin cortices, and may result in the wire pulling through the bone and/or creation of additional fractures. The hand twist technique results in a better "feel" for fracture reduction and wire tightening in repair of these fractures. It is emphasized that whichever technique is used, it is essential that the wire be applied tightly in order to prevent any fragment distraction.

Tightening the twist knot is most easily accomplished with wire twisting forceps (similar in appearance to a needle holder). When twisting wire, the tension applied should be greatest early during the twist when the wire is being bent around the obtusely angled drill-holes (Fig. 3). The application of sufficient wire tension, when large diameter wires are used, is more easily achieved when the majority of the drill-holes have obtuse rather than acute angles on the side opposite the twist as wire slides more readily when pulled around obtusely vs. acutely angled corners. To ensure that sufficient tension has been applied to the buried side of the wire loop (side opposite the twist), and the fracture has been firmly compressed, a second instrument may be used to temporarily lever under the loop before the twisting is complete. The wire should be bent over perpendicular to the loop and against the bone during completion of the final twist, and then cut to maintain at least three twists. The twist should be bent over away from the gingival margin. Twisting with uneven tension on the two strands of wire, continued twisting after adequate tension is achieved, bending the twist over after twisting is completed, and bending the twist back parallel with the loop all result in loss of tension or metal fatigue which may result in premature loosening or breakage of the wire.

Fig. 3 A) Angled drill-holes (toward the fracture line) result in obtusely angled corners and wire orientation enabling the wire to slide (small arrows) early in the wire tightening process [large arrow: tension applied to the twist]. B) A single angled drill-hole results in one acutely angled corner (open arrow) and the remainder of the wire obtusely oriented early in the wire tightening process [large arrow: tension applied to the twist]. The wire slides around the obtusely angled corners (small arrows), but "locks" - and does not slide - at the acutely angled corner (open arrow). C) If neither drill-hole is angled, both areas of acutely angled corners will "lock" (open arrows) resulting in a loose wire since the wire cannot tighten sufficiently on the opposite side of the bone (curved arrow). D) As the wire is tightened - as in (A) - the wire begins to traverse acutely angled corners on the side of the bone adjacent to the twist, and can no longer slide; therefore, the tension on the twist (large arrow) must be diminished. The wire on the opposite bone surface should be tight at this time; this can be further ensured by levering a second instrument under the loop before proceeding with final wire tightening.

Mastering the technique of tightening orthopedic wires is critical to the successful performance of this fracture fixation technique.

Wire patterns

A simple interrupted pattern is the ideal goal for wire application (a single wire placed along the lines of tensile stress, re: a tension-band wire), which neutralizes the bending forces. This single wire is most often supplemented by a second wire, usually placed parallel to the first wire, and at the aboral mandibular surface. This second "stabilization" wire acts to neutralize the rotational and shear forces. Two wires, placed a short distance apart from each other, also allow their support to be distributed over a greater area of the fracture.

Fig. 4 Single wire spanning a cranial mandibular fracture. The cranial drill-hole must be angled sufficiently to exit just caudal to the mandibular symphysis (this portion of the wire must be passed first (outside-O to inside-I) to facilitate wire placement through both bone fragments.

Wire placement is planned such that all wires are applied perpendicular to the fracture line whenever possible. All drill-holes are angled towards the fracture line so as to allow the wire to slide more easily, and thereby tighten without restriction. All holes are drilled, and wires placed, before any wires are tightened (this is especially important in bilateral body or ramus fractures when the mandibular symphysis is intact). Wires are tightened caudal to cranial, working toward the symphysis. Wires are tightened carefully in order to avoid loosening all previously tightened wires. Symphyseal fractures are wired last. A single 1.0-1.5-mm (20- or 18-gauge) tension-band wire usually is adequate to achieve stable fixation (Fig. 4); however, a second wire (Figs. 5), a stabilization wire, usually is placed to obtain enhanced fracture repair stability (especially in bilateral fractures). A general rule is to place as few wires as necessary to achieve stable fixation. Wires are twisted evenly and firmly and bent perpendicular to the wire strand and away from the gingival margins. The wire twist is cut after it has been bent. All wires are pressed firmly against the bone in order to avoid subsequent penetration of the oral mucosa.

Fig. 5 Transverse mandibular fractures; two wires are placed across each fracture line to enhance the stability of the repair by further distributing the points of fixation. The wires closest to the oral margin (arrows) are of greatest importance base upon biomechanical considerations, i.e., the tension-band wire, neutralizing bending forces; the aboral wires, so-called "stabilization" wires, act to neutralize the rotational and shear forces.

Intraosseous wiring techniques rely on the static forces generated by the tension of the wire and by the frictional forces generated between each corresponding bone fragment. The prerequisite, is accurate anatomic reduction so that the wire can obtain sufficient neutralization across two broad, opposing bone fragments, thereby providing adequate stability for healing. This degree of stability is sufficient only in relatively stable fractures, i.e., those that can be anatomically reconstructed.

Significant comminution or bone loss precludes the ability to obtain such precise anatomic apposition of the bone fragments. Since intraosseous wires are passed through drill-holes of slightly greater diameter than the wire, the wire/bone contact is limited to a single, small area; therefore, limited interfragmentary compression is generated by the wire, and it is not possible to achieve continuous interfragmentary compression across each and every bone fragment in comminuted fractures. Intraosseous wires also provide only two-dimensional stability, as rotation still may occur around the wire. In unstable fractures with comminution, or multiple areas of involvement, the end result is that motion occurs despite the placement of multiple intraosseous wires. The disadvantage of the intraosseous wiring technique is that only simple adaptation (neutralization) can be achieved; therefore, in highly comminuted fractures stable fixation is not possible and eventual bony collapse and soft tissue shrinkage will occur. Restoration of normal occlusion and stabilization of the fractures with severe comminution or gaps must be accomplished using alternate fixation techniques, e.g., plate fixation used to span the fracture gap as a buttress device.

Screws and plates

Standard plate fixation of mandibular fractures is useful in managing mid-body and some ramus fractures, especially in cases in which there is severe comminution, tooth or bone loss. Screw and plate fixation offers the advantages of inherent 3-D stability. Compared to intraosseous wire fixation, there is much greater implant/bone contact, which is due to the greater circumferential bone contact with the screw; therefore, much greater interfragmentary compression can be generated, both between the screw and bone and between the opposing bony fragments.

Compression plates (have been recommended as the ideal method of fixation of mandibular fractures. Reconstruction plates also are recommended, and have the further advantage of allowing their three-dimensional bending/contouring, in comparison to standard plates, permitting the implant to be contoured more closely to the shape of the mandible; as such, they are considered the ideal fixation for long segments of comminuted mandibular fractures. Plate fixation, however, is on the "wrong" side of the mandible due to the anatomic restrictions related to the tooth roots, and must be secured away from the alveolar margin, or on the compression side of the mandible. This biomechanically unfavorable position is overcome by using this larger implant to counter this disadvantage. Furthermore, interdental wires are added to provide supplemental fixation on the tension-band surface of the mandible.

Accurate contouring of these larger implants may be difficult in some areas of the mandible due to anatomic irregularities of the bone surface, especially at the junction of the mandible and ramus. Failure to carefully match the shape of the plate with the bone will result in a step at the fracture site when the plate is secured (and the screws tightened), resulting in malocclusion. The result is abnormally high forces on the bone and implant loosening. A further disadvantage of standard plate application is the difficulty of obtaining adequate purchase in the thin bone of the ramus due to the large size of the screws. An alternative to the standard plate fixation is the miniplate design. The miniplate design is based upon application of the tension-band principle, placing the fixation devices along the lines of tensile stress. By placing such a small device in this manner, it remains of sufficient size and strength to neutralize the applied functional forces after fixation - identical to the principles applied to intraosseous wire fixation. The small size of these implants allows their placement in similar locations as intraosseous wire fixation: the plates may be placed close to the alveolar border and the screws angled so as to avoid impingement on the tooth roots.

Additional stability also can be obtained to counter shear and rotational forces, similar to intraosseous wire fixation, by using a "balanced" approach to implant application. Two points of fixation may be obtained in order to more effectively neutralize all of the distractive forces: tension-band fixation on the traction side of the bone (alveolar border) and stabilization fixation on the compression side of the bone (aboral margin); the torsional and shear forces are neutralized by applying a second fixation device parallel to the first, again similar to that described with intraosseous wire fixation.

References

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