Reconstruction of large mandibular defects is a challenge for oral and maxillofacial surgeons. Untreated critical-sized defects and severe fractures may result in functional deficiencies and disfigurement.1–5 Mandibular internal fixation by means of locking reconstruction plating may be used for mandibular reconstruction after resection surgery or for fracture fixation.6
Fixation failure is an important complication that can occur when bone quality is poor or mechanical load circumstances are unfavorable.7 Screw loosening, plate exposure through oral mucosal ulceration, and collateral damage to important adjacent anatomic structures are potential complications when a bone plate is positioned near the alveolar margins and sustains excessive stress.4,8–10 The plate-to-bone fixation strength depends on a multitude of factors, including plate positioning, configuration, and strength; screw geometry; bone quality; and load.7 Patient outcome is optimized by selection of a plate system with mechanical and biological advantages relative to the fixation goals and fracture configuration circumstances.7
Miniplate systems for mandibular reconstruction designed for use in humans3,11–13 may be applied on the alveolar margin of the mandible to counter mandibular bone stresses in accordance with the tension band principle.3,14 This is attributable to the mandibular geometry and loading conditions during prehension and mastication that are consistent with maximum tensile stresses on the alveolar margin and maximum compressive stresses on the ventral border of the mandible.3,14 Application of the tension band principle takes advantage of the fact that anatomic reconstructions are strongest when fixation devices are loaded in tension and placement of a small plate along the line of tensile stress (Champy lines) would impart sufficient strength to neutralize applied functional forces after fixation.3,14,15 However, the tension band principle also relies on interfragmentary compression, which is not present in critical-sized defects. Furthermore, experiments that yielded this information were obtained from studies13–15 performed on humans, and parallels between humans and dogs with regard to biomechanical principles have not been verified.
Locking reconstruction plates and screws16–18 were developed to provide additional mechanical advantage over the use of conventional plates. Novel integration of a rigid interface between the plate and screws enhances the stability of repaired bone constructs and allows more flexibility in plate position and application. The system is characterized by the presence of threads on the screws and plates that allows screws to be locked to a plate, which enhances both primary and secondary stability.18
Descriptions in the veterinary literature in which miniplates were used along the alveolar margin in conjunction with a locking or conventional plate have resulted in anecdotal reports of exposure of the miniplate through the alveolar and gingival mucosa.8,19 Conversely, clinical experiences of the authors during the past several years with the use of locking reconstruction plates for repair of critical-sized defects and fracture management has revealed that within the population of treated dogs, a single LP placed in a buccal position (just ventral to the tooth roots and dorsal to the mandibular canal) did not result in clinical or radiographic evidence of collateral damage to blood vessels, nerves, or tooth roots or plate exposure through the mucosa and provided solid and reliable internal fixation.6,20
However, a fundamental question remains with regard to stabilization of midbody mandibular critical-sized defects. It would be important to know if there is a biomechanical need for a 2-plate system or whether a single LP placed strategically without damaging important anatomic structures (tooth roots and the mandibular canal) would provide sufficient stabilization for uncomplicated healing. Therefore, the objective of the study reported here was to compare the biomechanical strength of 2 fixation methods for an unstable critical-sized defect and compare the strength for those constructs to that of an intact mandible. We hypothesized that the biomechanical strength of a single mandibular reconstruction plate positioned at a midmandibular position would not differ from that for a 2-plate construct with plates positioned on dorsal and ventral portions of the mandibular ramus. We further hypothesized that placement of a miniplate and screws on the alveolar margin (ie, a 2-plate construct) would result in significant differences in the radiographic evidence of tooth root injury, compared with results for a single LP.
Materials and Methods
Samples
Mandibles (n = 24) were obtained from 12 fresh-frozen adult canine cadavers (body weight, 30 to 35 kg). Mandibles were excised and debrided of muscle tissue and then separated at the symphysis to yield 2 mandibles/cadaver.
Study design
Biomechanical properties of mandibles with a critical-sized defect stabilized with 1 of 2 fixation methods were evaluated. A blocked study design was used to allow for statistical comparisons among all treatments within dogs, thus minimizing the effects of variation for individual dogs on the desired study objectives. A third of the mandibles (n = 8) were allowed to remain intact. A critical-sized defect was made in the remaining 16 mandibles; a single LP (8) or an LMP (8) was used for fixation of these mandibles.
Fixation method
Both the single LP and LMP fixation methods involved use of a 24-hole plate cut in the middle to yield two 12-hole, 2.4/3.0-mm titanium LPs.a Plates were contoured, as needed, in 3 planes to adapt to the buccal (abaxial) surface of the mandible just dorsal to the mandibular canal and ventral to the tooth roots. Each plate was secured on each side of the defect with four 3-mm-diameter, 16-mm-long, bicortical locking screwsb (Figure 1). Placement of plates and screws was determined on the basis of results of radiography and subjective assessment of spatial location of teeth roots, the mandibular canal, and the ventral border of the mandible. In addition, the LMP fixation method involved use of a 20-hole (cut to the appropriate size), 1-mm-thick, nonlocking adaptation miniplateb secured with 7 to 9 (depending on the amount of bone available for screw insertion) 2-mm-diameter, 14-mm-long, bicortical nonlocking screws inserted at the alveolar margin.
Photographs of a critical-sized defect created by segmental mandibulectomy that excised the mandibular first molar and was stabilized by use of a single LP secured to the buccal surface halfway between the dorsal and ventral borders of the mandible (A) or an LMP construct secured to the buccal surface near the alveolar and ventral margins (B).
Citation: American Journal of Veterinary Research 77, 5; 10.2460/ajvr.77.5.445
After the plate or plates and screws were applied for the LP and LMP groups, a critical-sized defect was created to standardize the gap defect and to avoid an inadvertent change in the mandibular dimension such as increasing or decreasing the gap size and position. A segmental mandibulectomy of the segment that included the mandibular first molar was performed (Figure 1). Care was taken during creation of the defect to avoid damage to the plates and screws.
Radiography
A lateral radiographic view of all specimens was obtained by use of a digital radiography system (46 kVp and 6.0 mAs with a 1-m generator-to-detector distancec,d) before and after plate stabilization. Radiographs were evaluated separately by 2 investigators (BA and FJMV) for radiographic evidence of superimposition or proximity of screws to tooth roots or mandibular canal structures to reflect potential damage, as was described elsewhere.21–23 The number of screws that penetrated a tooth root or the mandibular canal was quantified.
Mechanical testing
Intact mandibles and mandibular-defect constructs were loaded in cantilever bending by use of a servohydraulic mechanical testing systeme (Figure 2). The caudal end of the ramus of the mandible that contained the sites for insertion of the muscles of mastication was fixed in PMMAf to hold it rigidly in place in the mechanical testing system. The rostral aspect of the mandible was loaded to simulate prehension and mastication. Load was applied in a direction perpendicular to the occlusal surface of the canine teeth, which were embedded in PMMA to distribute the load over the rostral end of the mandible. Custom loading fixtures were used to accommodate rotation of the rostral aspect of the mandible as the structure deformed about the ramus during loading. Mandibular-defect constructs were loaded in a single-load-to-failure test under displacement control at 1 mm/s, and simultaneously, load and axial displacement were recorded at 102 Hz. Tests were automatically stopped at 70% peak load after a peak was detected.
Illustration of cantilever bending of a mandibular construct conducted by affixing the ramus with PMMA to hold it rigidly in place in the mechanical testing system and applying a load to the canine teeth in a direction perpendicular to the body of the mandible.
Citation: American Journal of Veterinary Research 77, 5; 10.2460/ajvr.77.5.445
Mode of failure
The mode of failure (bone fracture, plate failure, or plate bending) was recorded. Photographs were obtained before and after tests to aid in determination of mode of failure.
Data analysis
Load-displacement data were plotted for each test. Yield for each construct was determined by detecting a deviation from linearity with a running least squares mean regression line and 0.08% displacement offset criteria. Stiffness of constructs prior to yield was calculated as the slope of the middle third of the data between the start of the loading curve and construct yield. Construct failure was determined as the point at maximum load. Stiffness after yield was calculated as the slope of the middle third of the data between construct yield and construct failure. Yield and failure loads and displacements were the respective values at the yield and failure points. Yield and failure energies were calculated as the integrals under the loaddeformation curve to the yield point and to the failure point.
Statistical analysis
Effect of treatment group (LP, LMP, or intact) on mandibular stiffness, yield strength, and failure strength was assessed by use of Wilcoxon tests.g Nonparametric methods were selected because of the small sample size and large differences in variance among treatment groups (with regard to the means of the treatment groups) and because transformation of the data did not result in normality of the residuals in an ANOVA. Student t tests were used to assess differences between LP and LMP constructs for screw penetration of teeth and the mandibular canal. For all tests, values of P < 0.05 were considered significant.
Results
Samples
Mandibles were of a similar size among all groups. Mean ± SD length of the mandibles as measured from the head of the mandible to the most rostral aspect of the mandible was 15.3 ± 0.4 cm. Mean mandibular height as measured from the alveolar margin to the ventral border at the mesial aspect of the first molar was 2.5 ± 0.09 cm.
Mechanical variables
Median moment arm lengths of mandibles in the LP and LMP groups were not significantly different from the median moment arm length of mandibles in the intact group (Table 1). Intact mandibles were significantly stronger and stiffer and required more energy to yield during cantilever bending, compared with results for LP and LMP mandibular constructs. The LMP constructs had stiffness, strength, and energy to yield values that were < 30% of values for intact mandibles. The LP constructs had stiffness, strength, and energy to yield values that were 15% of values for intact mandibles. Displacement at yield and energy to fail did not differ significantly between intact mandibles and the LP and LMP mandibles. The LP and LMP constructs had a larger plastic displacement to failure after yield than did intact mandibles (Figure 3).
Force-displacement curves that indicate the yield (circles) and failure points (triangles) for a representative intact mandible (gray line) and representative mandibles with a critical-sized defect stabilized with a single LP (solid black line) or an LMP (dashed black line) construct.
Citation: American Journal of Veterinary Research 77, 5; 10.2460/ajvr.77.5.445
Median (interquartile range) values for mechanical test variables from a single-load-to-failure test of intact mandibles (n = 8 canine cadavers) and mandibles with a critical-sized defect stabilized with a single LP (8) or a 2-plate LMP (8) construct.
| Group | Wilcoxon P values* | |||||
|---|---|---|---|---|---|---|
| Variable | Single LP | LMP | Intact | Intact vs LMP | Intact vs single LP | Single LP vs LMP |
| Prior to yield | ||||||
| Stiffness (N/mm) | 3.9 (3.8–4.6) | 7.9 (6.5–8.8) | 25.9 (18.0–32.8) | 0.005 | 0.005 | 0.005 |
| Yield | ||||||
| Displacement (mm) | 9.6 (9.1–11.1) | 11.3 (8.0–15.5) | 13.4 (10.4–15.7) | 0.718 | 0.104 | 0.505 |
| Load (N) | 40.6 (38.4–47.3) | 88.1 (63.4–123.2) | 283.5 (226.6–367.1) | 0.021 | 0.005 | 0.005 |
| Energy (N•mm) | 232 (206–307) | 554 (295–1,092) | 2,154 (1,476–2,866) | 0.039 | 0.026 | 0.058 |
| After yield | ||||||
| Stiffness (N/mm) | 0.4 (0.3–0.5) | 1.2 (1.1–2.5) | 5.1 (3.6–11.5) | 0.017 | 0.005 | 0.005 |
| Failure | ||||||
| Displacement (mm) | 111.1 (94.3–122.0) | 66.5 (42.6–76.0) | 29.7 (22.3–35.2) | 0.086 | 0.014 | 0.021 |
| Load (N) | 124.8 (80.8–251.2) | 178 (155–226) | 425 (375–527) | 0.007 | 0.032 | 0.209 |
| Energy (N•mm) | 9,042 (6,803–11,254) | 8,524 (4,855–13,151) | 8,771 (6,118–13,992) | 0.796 | 0.796 | 0.959 |
| Moment arm (mm) | 93.0 (91.0–94.0) | 93.0 (92.0–95.0) | 93.0 (92.5–96.5) | 0.790 | 0.344 | 0.557 |
Values were considered significantly different at P < 0.05.
The LMP construct was twice as stiff and 2.2 times as strong and required 2.9 times the energy to yield, compared with values for the LP construct, but there was no significant difference in displacement to yield between the LMP and LP constructs. Data for after yield indicated that the LMP construct was 4.8 times as stiff as the LP construct. The LMP construct did not have greater strength or energy to fail than did the LP construct.
Mode of failure
Intact mandibles failed as a bone fracture adjacent to PMMA fixture (rostrally in 3 mandibles and caudally in 5 mandibles; Figure 4). The LMP mandibles failed as a bone fracture at caudal screw holes of the LP in 2 constructs and as stretching or breakage of the alveolar miniplate in 6 constructs. The LP mandibles failed as a bone fracture at the caudal screw holes of the LP in 3 constructs and plate bending with no obvious bone or plate fracture in 5 constructs.
Photographs of failure modes among representative intact mandibles and mandibles with a critical-sized defect stabilized with single LP and LMP constructs. A—Rostral failure at PMMA fixation site on an intact mandible. B—Elongation of the alveolar miniplate and bending of the LP in an LMP construct. C—Breakage of the alveolar miniplate and bending of the LP in an LMP construct. D—Bending deformation of the LP in an LP construct. E—Deformation of the LP with bone fracture contiguous with the caudal screws in an LP construct. F—Bone fracture contiguous with the caudal screws in an LMP construct.
Citation: American Journal of Veterinary Research 77, 5; 10.2460/ajvr.77.5.445
Tooth root and mandibular canal damage attributable to plating methods
Injury to tooth roots detected radiographically before mechanical loading was significantly (P < 0.001) greater for LMP-reconstructed mandibles (mean ± SD, 5.4 ± 0.5 root injuries/mandible) than for LP-reconstructed mandibles (mean, 0.6 ± 0.7 root injuries/mandible). Similarly, the mean number of screws that had approximation or superimposition on the mandibular canal was significantly (P < 0.001) greater for LMP-reconstructed mandibles (3.8 ± 0.7 screws/canal) than for LP-reconstructed mandibles (1.5 ± 0.8 screws/canal; Figure 5).
Lateral radiographic views of 2 mandibles obtained before mechanical loading. Notice that there is more frequent screw penetration of tooth roots and the mandibular canal in the LMP construct (A) than in the LP construct (B).
Citation: American Journal of Veterinary Research 77, 5; 10.2460/ajvr.77.5.445
Discussion
To the authors’ knowledge, the present study was the first in which the biomechanical aspects of reconstruction of segmental mandibular defects of dogs by means of internal fixation have been described. The study was designed to provide evidence-based information about the optimal plate location and number of plates that should be used in mandibular reconstruction to reduce the incidence of complications. There were 3 cardinal findings. First, in comparison with intact mandibles, reconstructed mandibles remained significantly weaker, regardless of plate number and location of the construct. Second, there were no consistent biomechanical differences at failure between a single-plate construct at the middle of the mandibular height versus a 2-plate construct, but the 2-plate construct had greater strength and stiffness prior to yield. Third, radiographic evidence of screws and tooth root and mandibular canal approximation was detected significantly more often for the 2-plate construct than for the single-plate construct.
The 2-plate construct outperformed the single-plate construct for the in vitro test conditions of the present study. The 2-plate construct had approximately twice the stiffness and yield strength prior to yield, compared with results for the single-plate construct. This finding was not surprising because the large gap in the mandible precluded load sharing between bone fragments. Thus, the implants sustained the entire load. Therefore, the addition of a second adaptation miniplate bridging the defect gap and sharing the load with the LP helped to resist bending. However, both single- and 2-plate constructs are likely to sustain perioperative loads in patients. Yield loads for both fixation methods were much lower than that of the intact mandibles in the present study; however, this in vitro study simulated a worst-case scenario in which we tested only a single mandible without supporting musculature and loads applied solely to the canine teeth. Canine teeth have the greatest moment arm from the temporomandibular joint and cause the entire load to contribute to a large bending moment at the defect site. In vivo, loads are distributed over the canine teeth as well as over the premolars and molars. The bending moment acting on the critical-sized defect would be much smaller in vivo because the load is distributed over smaller moment arms to individual teeth and the portion of the load acting on the teeth caudal to the defect would not act to deform the construct. Bite force also generally is greater at the carnassial teeth than at the canine teeth24; the canine teeth were loaded in the present study. Furthermore, yield loads (forces at which the structure permanently deforms) detected from single- and 2-plate constructs of the mandible in the present study are similar to the typical bilateral mandibular bite force values of live dogs of similar size.25 Live dogs have a contralateral mandible and robust masticatory muscles that act as substantial supporting structures. Considering these factors, it is likely that either fixation method would provide sufficient stability for defect healing. The clinical experience of the authors would support this conclusion.
Neither fixation method provided complete stability to a mandible with a critical-sized defect. Intact mandibles were stronger and stiffer and required more energy to reach yield and maximum load. This was not an unexpected outcome because although a plate or plates restored anatomic alignment of remaining bone fragments, mandibular architecture was not restored in the defect site, nor was load transfer through the mandible body possible. Consequently, optimization of tissue regeneration and bone healing is critical to regain full function during the initial period of high interfragmentary strain.26,27 Therefore, the ideal treatment strategy would combine surgical and regenerative approaches. One approach would be use of a compression-resistant matrix infused with recombinant human-bone morphogenetic protein-2, which has reproducibly achieved timely regeneration of critical-sized bone defects.6,20,28 The matrix may share some minimal load bearing in the immediate postoperative period, and acceleration of regeneration by use of a compression-resistant matrix and recombinant human–bone morphogenetic protein-2 at a defect site likely enhances load sharing as the defect heals. The plate can act as a parallel load-bearing element that shields the mandible after defect healing.27
In the present study, we found that alveolar plates lost the mechanical advantage in gap situations when plates functioned as buttress or bridging plates. Plate application in an alveolar position is potentially desirable when anatomic reduction is possible, but plate exposure through the mucosa is a common complication. Alveolar plates may prove advantageous as tension band plates when anatomic reduction allows for load sharing across bone fragments under compression. Alveolar plates appear to be associated with a high complication rate. In both veterinary and human medicine, plate exposure and extrusion are the most commonly reported complications.3,28–32 Although plate exposure through the gingiva in dogs has been reported in experimental10 and clinical conditions,3,28 the actual incidence is unknown. However, an incidence of plate exposure of 9.9% to 10.5% has been reported for humans.30,33 In humans, the incidence of additional surgeries and hardware removal is significantly higher for the 2-plate combination of dorsal and ventral plates, compared with results for a single-plate construct.32
Furthermore, placement of a nonlocking miniplate along the alveolar margin (ie, alveolar plate) resulted in the radiographic evidence of substantial tooth root injury (defined as superimposition or proximity of the screw to a tooth root). Although apparent root contact can be a result of parallax error (false-positive result), apparent noncontact when there is in fact contact (false-negative result) is unlikely.22,23 On the basis of results of previous studies21–23 conducted to evaluate dental root damage caused by screws, radiographic assessment of root injury is fairly consistent, but the actual extent of the damage can only be reliably ascertained histologically. The teeth and the mandibular canal structures occupy most of the volume of the mandibles.3 This complex anatomic structure makes it challenging to apply a plate with fixed screw positions without invading tooth roots or vascular structures. Investigators of a previous study10 reported that placement of a plate at a dorsal location on the buccal aspect of the mandible (ie, dorsal to the canal and ventral to the alveolar margin) resulted in damage to 61% of the roots, with the damage involving the periodontal ligament, dentine, cementum, pulp, and periapical tissues. Dental damage may result in pain, infection, tooth death, periapical lesions, and ultimately failure of fixation because of persistent infection and inflammation at the fixation site.3,10 However, under favorable conditions (no inflammatory infiltrate or pulpal invasion), healing of the tooth root can occur if root damage inflicted by a screw is limited to the cementum or the dentin.21,23 Therefore, it is prudent to identify a fixation method that will provide acceptable rigid biomechanical stabilization yet avoid dental damage. Because of the substantial trauma to dental structures with the alveolar plate in the LMP method, we recommend use of a single plate positioned dorsal to the mandibular canal and just ventral to the dental roots. Dental radiography and CT are useful during staging and surgical planning to minimize disruption of teeth and vascular structures.6,34
In the study reported here, a 2-plate construct was stronger prior to yield than was a single-plate construct for stabilization of critical-sized defects, and both plate constructs were weaker than intact mandibles. However, considering the rigorous nature of the in vitro tests and reported bite loads, both plate configurations would be likely to sustain postoperative loads. Because application of an alveolar plate resulted in radiographic concerns of invasion of tooth and vascular structures and because there is a historical prevalence of plate extrusion, we recommend that only 1 LP be used for mandibular reconstruction of critical-sized defects. We also recommend that plate stabilization be augmented by regenerative treatments to optimize healing.
Acknowledgments
Supported by the Companion Animal Memorial Fund, administered by the Center for Companion Animal Health at the University of California-Davis, and by DePuy Synthes Vet, a division of DePuy Orthopaedics Inc.
The authors declare that there were no conflicts of interest.
The authors thank Maria Maroulis for facilitating donation of materials used in the study.
ABBREVIATIONS
| LMP | Locking plate combined with an alveolar miniplate |
| LP | Locking plate |
| PMMA | Polymethylmethacrylate |
Footnotes
VPT4120.24, provided by DePuy Synthes Vet, a division of DePuy Orthopaedics Inc, Paoli, Pa.
449.020, provided by DePuy Synthes Vet, a division of DePuy Orthopaedics Inc, Paoli, Pa.
Mark III, Sound-Eklin DR, Carlsbad, Calif.
Generator HF100/30þ, MinXray Inc, Northbrook, Ill.
MTS Systems Corp, Eden Prairie, Minn.
Coe tray plastic, GC America Inc, Alsip, Ill.
SAS, version 9.4, SAS Institute Inc, Cary, NC.
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