Comparison of Strengths of Mandibular Angle Fractures Following Different Plate Designs: A Human Cadaver Study

Abstract

Study Design: This institutional cross-sectional study using cadaveric mandibles aimed to measure and compare the strengths of three plating designs utilized in osteosynthesis of mandibular angle fractures. Objective: There have been prior studies on angle fracture fixation though few biomechanical studies on human cadaveric specimen. This study aims to directly compare the biomechanical strength of different plating designs to the mandibular angle fracture using a human cadaveric specimen substrate. Methods: After receiving an angle osteotomy and either single plate, two plate, or 3D plate fixation, the specimens underwent biomechanical testing using the Instron 5565 mechanical testing unit. The primary outcomes measured were peak load at which permanent deformation started, displacement value at peak load, and load necessary for a specific amount of displacement at 1, 3, 5, and 7 mm. Results: There were 15 hemi-mandibles in each group. Based on data analysis of all the specimens, there were no significant differences in the mandibular height, ramus width, mandibular thickness, angle height, and gonial angle between the hemimandibles. This study demonstrated a statistically significant increased strength performance of the 3D plate over the single plate fixation and the 2-plate over the single plate fixation. The results between 2-plate and 3D plate were in similar values. Conclusions: In terms of biomechanical strength, the 3D plate and two plate designs outperform the single plate design to mandibular angle fractures. There are various anatomical and patient specific situations that can aid in selection between them. In the absence of the favorable angle fracture and patient, biomechanical strength to the method of fixation selection needs to be considered.

Introduction

The mandible is one of the most frequently affected bones in facial fractures, often with fractures occurring at the mandibular angle (MAF). The most common causes of MAF are assault, motor vehicle accidents, and falls with differences in frequency based on age and gender.[1,2,3] Therefore, these routine traumatic injuries necessitate the selection of optimal treatment, often involving open reduction combined with internal fixation. The Champy et al technique has been successfully utilized by surgeons for internal fixation of maxillofacial fractures, employing the usage of miniplates positioned along the ideal lines of osteosynthesis.[4] Even with proper technique, postoperative complications when treating mandibular fractures continue to occur at a high rate.[5] Mandibular angles can be difficult in treatment due to lack of toothborne stabilization ability, unfavorable fracture patterns due to muscular pull, presence of third molars, and avoidance of neurovascular injury in the inferior alveolar canal.[6] This can be compounded with patient specific factors such as bruxism, dystonia, poor patient compliance, and social circumstances. Recently, the usage of three-dimensional (3D) plates has become increasingly more common in the treatment of MAFs.[7]

In addition to the 3D plate’s quadrilateral shape conferring a geometrically stable configuration,[8] the 3D plate system allows for greater malleability, greater torsional stability, and a slimmer profile compared to traditional miniplates for internal fixation of MAFs.[7,8] Based on the meta-analysis conducted by Al-Moraissi et al comparing the clinical outcomes of standard and 3D miniplate fixation in MAFs, 3D miniplate fixation reduced the risk of postoperative complications, including trismus and hardware failure, by 58% and generated greater interfragmentary stability compared to standard miniplates.[7] A portion of this difference in interfragmentary stability likely has to do with the comparative peak load capacities of the two methods. Prior biomechanical studies have compared fixation techniques in mandibular angles. Studies have been conducted on single plate vs 3D plate for MAF fixation but were predominantly completed on polyurethane synthetic or animal mandibular specimens.[9,10,11,12,13,14,15,16,17] Studies completed in vivo examined outcomes such as rates of hardware failure, infection, etc.[7,8,18,19] There have also been recent systematic reviews and meta-analyses demonstrating that the single plate system has reduced postoperative complication rates compared to the two plate system,[20,21] while a 3D plate exhibits reduced postoperative complication rates and markedly decreased hardware failure incidence when compared to a single plate fixation in the management MAFs.[7,20,22,23] All synthetic mandible models developed to date have reported that the two plate technique provides much more stability than a single plate,[9,10,24] and sheep models have demonstrated that a single plate fixation has the lowest resistance to compression load.[15] The biomechanics of the 3D plate for the treatment of MAFs are still in dispute. Some studies have shown the 3D plate to have lower resistance values than a single or two plate fixation in synthetic models,[9,11] but in a sheep model study the 3D plate had more favorable biomechanical behavior than a single plate and no significant differences with the two plate technique.[16] Similarly, synthetic models showed no differences between a 3D curved with 8-hole plate and dual plating for bending and torsion resistance.[14] Munante-Cardenas et al[9] showed two parallel plates had statistically greater resistance to compression load than 3D fixation systems; however, the authors used a thinner 3D plate. In contrast, Haug et al[12] demonstrated 3D plates had higher yield load and displacement than two plate systems.

The biomechanical behavior of mandibles has been investigated by different approaches, including computer models (e.g., finite element analysis) and physical models such as synthetic materials, animal bone, and cadaveric bone. Physical models allow testing on a gross level to give fatigue performance and fracture strength.[25] In regards to physical properties of non-human mandibles compared to human cadaveric mandibles, previous studies reported the synthetic mandible replicas failed in peak load value between 350 and 675 N;[12,26] however, the cadaveric mandibles failed between 579.46 and 1783.57 N.[27] In order to offer a more accurate representation of in vivo performance of these fixation methods, a human mandible would be preferable.

This study was performed to determine strength differences between single plated, two plated, and 3D plated fixation of mandibular angle fractures in human mandibles. The study aimed to compare peak load and terminal vertical displacement of cadaveric angle-fractured hemimandibles with a single plate, two plate, and 3D ladder plate fixated fractures in human cadaveric specimens. The authors hypothesized that utilizing the 3D plate and the two plate method would have significantly greater peak loads compared to the single plate and exhibit no difference in peak loads between the two plate and 3D plate fixations of a mandibular angle fracture in human mandibles. This information could be useful to the surgeon in fixation selection individualized to the patient.

Materials and Methods

Study Design

A cross-sectional cadaveric study of 15 human cadavers were provided by the Anatomical Donor Program, School of Medicine, University of Alabama at Birmingham. All donors underwent the embalming procedure and protocol at the School of Medicine Anatomy Laboratory. Inclusion criteria were male or female who presented with an intact edentulous, partially edentulous, or dentate mandible. Exclusion criteria of the donors included presence of osteoporosis, metastatic bone cancer, metabolic bone disorders, or mandibular tumors at time of death. Mandibles with third molars were also excluded from the study. The personal identities of all donors were kept confidential and ethnicity was not identified in this study. All cadaveric skulls and mandibles were previously hemisected in the midsagittal plane. Mandibles were dissected by skilled oral maxillofacial surgery residents, fellows, and faculty. The principles of the Declaration of Helsinki were followed for cadaveric specimens.

Variables

The primary predictive variables were age of donor, gender, mandibular height (millimeters, mm), ramus width (mm), height and thickness of angle of the mandible (mm), gonial angle (degree), and dental status of the plated mandibles with single plate or 3D plate. The primary outcomes were peak load in newtons (N) defined as the load at which permanent deformation started, displacement value at peak load (mm), and load (N) necessary for a specific amount of displacement at 1, 3, 5, and 7 mm between the different plates.

Data Collection Methods

Thirty human hemimandibles were cleaned and divided in three groups by the remaining dental status:[27,28] edentulous (EM), partially edentulous (PE), and dentate mandible (DM). Each hemimandible was marked in the area of confluence of the alveolar crest and external oblique ridge (point A) to the inferior border of the mandibular body perpendicularly (point B).[27,28] The third point (C) was obtained on the inferior border of the mandible 10 mm posterior from point B.[9,29,30] The mandibular body height was measured from point A to B. The height of angle of the mandible was detected from point A to C, then the thickness of angle of the mandible was done at the midpoint (point D) of this line. After that the ramus width was measured from the coronoid notch (point E) of anterior ramus to posterior ramus (point F) parallel to occlusal plane (Figure 1). Finally, the gonial angle was determined at the points connecting the articulare, the gonion, and the menton.[31] The measurements were measured twice and averaged.

Hemimandibles were mounted in self-cure acrylic resin in 9.5 × 3.5 × 3 centimeters molds made of silicone. The acrylic resin was placed from the coronoid process and sigmoid notch to the ramus of the mandible encompassing the condyle and posterior border of the ramus to mimic the position of the condyle in the temporomandibular joint and to eliminate the possibility of fracture at this site (Figure 2). After the acrylic had set, blocks with mounted hemimandibles were removed from their molds and labeled with the donor number. Then, the hemimandibles were uniformly sectioned on the mandibular angle region from point A to C using a microreciprocating saw, with a .4 mm thick blade.

This was to control the location of the fracture for standardization.

Proximal and distal segments of the fractured hemimandibles were reduced in anatomic position and fixated with three different osteosynthesis techniques (Figure 2):

(1)

Left hemimandibles (n = 15): one titanium 2.0 mm, 4-hole noncompression plate, 4 monocortical titanium fixation screws with 7.0 mm in length (KLS Martin, Freiburg, Germany) along lateral aspect of the mandible near the external oblique ridge. Following testing, these specimens plate was removed and the 3D plate (as below) was adapted.

(2)

Left hemimandibles (n = 15): a 3D curved titanium 2.0 mm, 10-hole noncompression plate, 8 monocortical titanium fixation screws with 7.0 mm in length (KLS Martin, Freiburg, Germany) along the lateral aspect of the mandible. The distance between upper and lower bar of 3D plate was 7.5 mm.

(3)

Right hemimandibles (n = 15): two titanium 2.0 mm, 4-hole noncompression plate, 8 monocortical titanium fixation screws with 7.0 mm in length (KLS Martin, Freiburg, Germany) along the upper and lower border of the lateral aspect of the mandible. The distance between midpoint of upper and lower plates were measured twice and averaged.

The custom-fabricated jig with a hemimandible was fixated and tested using an Instron 5565 mechanical testing unit (Instron Corp, Norwood, MA) (Figure 3). The testing unit was performed with a static load cell that was set to produce linear displacement with compression mode at a rate of 1 mm per minute. The vertical compression load was applied to the first molar region and progression continued until permanent distortion of the plate or screws occurred (Figure 4). During the test, data were acquired digitally, and load-displacement graphs were drawn by Bluehill 2 software (Instron Corp). The peak load, displacement at peak load and load application at displacement values of 1, 3, 5, and 7 mm were recorded.

Statistical Analysis

Descriptive statistics were calculated for all variables: n (%) for categorical variables, mean (SD) for continuous variables. Mandible measurements for left and right were highly correlated (all Pearson’s r > .86) therefore average measurement for mandibular height and thickness, ramus width, angle height, and gonial angle were used. The difference between plate techniques for each outcome measures was determined as first plate technique—second plate technique (e.g., 3D plate-Single plate) and the percentage difference calculated as [(first-second)/second] *100 (e.g., [(3D plate-Single plate)/Single plate*100]). Unadjusted paired t-test was used to compare the mean of donor characteristics and strength between plate types. Likelihood ratio chi-square test was used for gender comparison. Due to the high correlation between jaw measurements, individual covariate adjusted linear models were used to assess the difference between the plating systems and outcomes, also adjusted for age and sex of donor. Due to the high level of correlation of outcome measures, no adjustments were made for multiple testing. Normality was visually assess using Normal Quantile Plots. Sample size was based upon donor availability, no a priori power or sample size was conducted. Edentulous status was not used as a potential covariate given that only 1 donor had dentate type, and only 3 were partial. P values of <.05 were considered significant. All statistics were performed using JMP Pro 14 or SAS 9.4 (SAS Institute, Cary, NC, USA).

Results

The donors comprised of 7 males (46.7%) and 8 females (53.3%), with mean of donor age at death 84.1 (SD 8.7) years (range, 59 to 93 years), and the majority (11, 73.3%) were edentulous (

Table 1. Summary of Measures.

). There were no differences in age of donor (P = .72), gender (P = .91), or edentulous status (P > .99) by plate type. There were no significant differences in the mandibular height, ramus width, mandibular thickness, angle height, and gonial angle between the hemimandibles. There were significant differences using paired t-test for peak load (N), displacement at peak load (mm), and force applied (N) for plate displacement of 1, 3, 5, and 7 mm.

Table 1,

Table 2. Mean Difference Between Plate Types.

and

Table 3. Mean Percentage Difference and P-value Between Plate Types.

is a summary of the data compiled together and split for comparison between plating types as well as difference and percentage difference when comparing either group.

3D Plate vs Single Plate

In individual covariate adjusted linear models, there were no associations between gender, age, ramus width, or mandibular thickness for any of the outcomes. There were no significant adjusted models for displacement at peak load, or for force at 1 and 3 mm. For peak load, the difference between plate type remained when adjusted for average mandibular height (P = .0045). From the linear model, an average height of 21.7 mm would result in no difference in peak load. Peak load difference is higher for single plates compared to 3D above that height and below 21.7 mm, 3D peak load was higher than single plate.

At force applied for 5 mm displacement, age (P = .0244), average angle height (P = .0172), and average gonial angle (P = .0159) were associated with a difference between 3D and single plate. As age increases, the difference between 3D and single plate decreases. Lower average angle heights have larger differences in 3D and single plate, meaning as the angle height increases the difference between 3D and single plate decreases.

At force applied for 7 mm displacement, average angle height (P = .0102) was again associated with difference between 3D and single plate. Lower average angle heights have larger differences in 3D and single plate. As the angle height increases, the difference between 3D and single plate decreases. The percentage difference between plating modalities was also significant at each measurement. The 3D plate had 146.8% difference at peak load, 235.8% difference at force 1 mm, 190.1% difference at force 3 mm, 156.4% difference at force 5 mm, and 123.0% difference at force 7 mm. The mandible displacement at peak load difference was 70.9% for 3D compared to single plate. This means greater mandible displacement at peak load with a single plate compared to 3D plate.

Single Plate vs Two Plate

A summary of all covariate and primary outcome measurements is again shown in Table 1, Table 2 and Table 3. Again, there were no significant differences in the mandibular height, ramus width, mandibular thickness, angle height, and gonial angle between the right and left side hemimandibles, which correlate to the two plate and single plate hemimandibles, respectively. Significant differences were found in all outcome variables including peak load, displacement at peak load, and force at all displacement values between the two plate and single plate fixation methods.

All outcome variable models were adjusted for age and sex of donor and were iteratively checked for average mandible height, average ramus width, average mandibular thickness, average angle height, and average gonial angle. Since there were no significant differences in the height measures by jaw side, the average height was used as a potential predictor of outcome measures. No difference in peak load, displacement at peak load, or force at 5 mm of displacement was found when adjusted for age, sex, average mandible height, average ramus width, average thickness, average angle height, or average gonial angle. For force at 1 mm of displacement, only average ramus width was found to correlate with the difference in force required between two plates and single plate. As the average ramus width increased, the force required for displacement to 1 mm for two plates increased more than with a single plate with a P value of .028.

For force at 3 mm of displacement, sex was associated with a difference in force required for displacement between the two plate and single plating techniques. The average force in males was 38.9 N and in females was 11.5 N at 3 cm of displacement. For force at 7 mm of displacement a statistically significant difference was found for average mandibular thickness (P = .05), average angle height (P = .01), and average gonial angle (P = .05). As average mandibular thickness increased, the force required for displacement of two plates was higher than for a single plate. As average angle height increased, the difference between two plate and single plate techniques decreases, similarly for average gonial angle. At smaller angles, two plates require more force for displacement to 7 mm than single plate technique, but at higher angles, the difference is such that the single plate technique had a similar or higher force for displacement to 7 mm than the two plate technique.

Two Plate vs 3D Plate

A summary of measurements is found Table 1, Table 2 and Table 3. There were no associations between predictive variables, except with the average mandibular height and differences in peak load between the plating techniques. The average mandibular height was significantly associated with peak load differences between the two plate and 3D plate (P = .032). Below about 19 mm in mandibular height, the peak load of 3D plate was higher than the two plate, but at higher average mandibular heights the two plate had a higher peak load than the 3D plate. From this data the 3D plate may be interpreted to be more suitable for a smaller jaw, whereas the two plate would be better suited for a larger jaw than the 3D plate fixation. The difference of displacement at peak load of the plating systems was not associated with all predictive variables.

Differences of force at displacement values at 1, 3, and 5 mm had association with age [P = .022 for 1 mm, P = .058 for 3 mm, and P = .011 for 5 mm] and sex (P = .004 for 1 mm, P = .011 for 3 mm, and P = .008 for 5 mm). As age increased, the difference in force between 3D and two plate decreased. For females, the average difference was higher for the 3D than the two plate. On the contrary, two plate had a greater average difference than the 3D plate for males. In addition, the average gonial angle was also associated with the difference in force at displacement value 5 mm (P = .025). The two plate system withstood a greater force than the 3D plate system in hemimandibles with an average gonial angle above 123°, but the 3D plate withstood a higher force than the two plate when the average gonial angle was below 123°.

There was an association between the difference in force at displacement value at 7 mm and the average mandibular thickness only (P = .005). When the average mandibular thickness was below 7.5 mm, the force for displacement of the 3D plate was higher than the two plate, but the opposite was seen when the average mandibular thickness was higher than 7.5 mm.

The distance between the upper and lower plates in two plate fixation was 7.6 + 1.5 mm. There was a positive correlation between the distance and force at 1 mm (R = .52, SE = 4.49, P = .047), 3 mm (R = .59, SE = 8.13, P = .020), 5 mm (R = .54, SE = 9.77, P = .037), and 7 mm (R = .59, SE = 9.49, P = .020).

Discussion

A 3D, grid, or matrix plate can be considered a two plate osteosynthesis, with two plates joined by an interconnecting crossbar.[14] Because of its interconnected design and box configuration, a broad platform is created that increases the fracture stability in comparison to a single plate. The 3D plate allows for almost no movements at the superior and inferior borders from torsional and bending forces, providing greater resistance against gap opening at the inferior border with biting forces. In addition, it could be a timesaving alternative to conventional two plate fixation by placing only 1 unit instead of 2 plates. Furthermore, it is simple to apply because of its low profile while maintaining strength and malleability.[15,18,19] Studies of plated hemimandibles with a single plate found no association between biomechanical behaviors and age, gender, dental status, the side of plate application (right/left), mandibular height, height or width of fracture line.[28,32]

For native hemimandibles, previous studies have reported that males had significantly higher mandibular height and peak load than females and there were significant correlations between mandibular height and either gender or peak load. In contrast, there was no significant difference in peak load or displacement in relation to dental status.[27] Another factor affecting the strength of mandibular angle area is the gonial angle. There were reports that patients with a high gonial angle were more susceptible to angle fractures because the height of the mandible at the angle and body regions were decreased and the cortical bone was thinner when compared to patients with low or normal gonial angle.[31,33] Compromised screw anchorage and greater displacement of fracture segments occurs in those patients due to the decreased cortical bone thickness; an additional plate and screws are recommended.[31] As our results show and dicussed further below, these factors of mandibular height and gonial angle played a role in performance of the plating systems.

3D Plate vs Single Plate

Our study demonstrates significant difference in strength performance of the 3D plate over the single plate fixation with regards to force tolerated at peak load, mandible displacement, and force applied to displace the fracture at 1, 3, 5, and 7 mm. When strengths are compared by a percentage, there was a 146.8% increase in strength of peak load of 3D plate over the single plate and 235.8% at 1 mm displacement. There was a notable association with mandibular angle height. Greater mandibular angle heights showed significantly less of a difference between the two fixation methods. In hemimandibles with increased mandibular angle height (>21.7 mm), the difference between 3D and single plate is not as pronounced as compared to the mandibles with less height.

Single Plate vs Two Plate

Significant differences were found in all outcome variables including peak load, displacement at peak load, and force at all displacement values between the two plate and single plate fixation methods. As the average ramus width increased, the force required for displacement to 1 mm for two plates increased more than with a single plate.

For force at 7 mm of displacement, a statistically significant difference was found for average mandibular thickness, average angle height, and average gonial angle. As average mandibular thickness increased, the force required for displacement of two plates was higher than for a single plate. As average angle height or average gonial angle increases, the difference between two plate and single plate techniques decreases. At smaller angles the two plate technique required more force for displacement to 7 mm than the single plate technique, but at higher angles the difference is such that the single plate technique had a similar or higher force for displacement to 7 mm than the two plate technique.

Two Plate vs 3D Plate

Our biomechanical study found that the two plate and 3D plate fixation techniques had similar stability with compression loading. For clinical implications, if biomechanical stability is the only factor considered, surgeons can use both osteosynthesis systems for MAF management depending on preference; however, other factors should be considered.

The hemimandibles of females with a smaller mandibular height and higher gonial angles fixated with a 3D plate were stronger than those with the two plate. Conversely, the hemimandibles of males with greater mandibular heights and lower gonial angles fixated with the two plate system had increased stability. Additionally, the distance between the upper and lower plate had a positive correlation with the force for displacement of plated hemimandibles in two plate fixation. These results could be applied clinical scenarios. First, 3D plate osteosynthesis should be selected when treating MAF that occurs in a female patient with a small mandible with a high gonial angle, such as in atrophic edentulous mandibles. In contrast, male patients with a tall mandible and low or normal gonial angle with MAF should be treated with the two plate osteosynthesis. Secondly, the lower plate should be placed as low as possible when using the two plate technique for MAF management to act as a compression band at the inferior border of the mandible. The additional lower plate in two plate osteosynthesis provides better stability, avoids opening of the inferior fracture gap, and lateral displacement of the inferior border of the mandible.[34]

Final

This study does find significant differences when comparing osteosynthesis between 3D, two plate, and single plate fixation in mandibular angle fractures. The Champy et al technique has been successfully utilized by surgeons for internal fixation of maxillofacial fractures, employing the usage of miniplates positioned along the ideal lines of osteosynthesis.[4] Even with proper technique, postoperative complications when treating mandibular fractures, especially mandibular angle fractures, continue to occur at a high rate. MAFs can be difficult in treatment due to lack of toothborne stabilization ability, unfavorable fracture patterns due to muscular pull, presence of third molars, and avoidance of neurovascular injury in the inferior alveolar canal.[6] This can be compounded with patient specific factors such as bruxism, dystonia, poor patient compliance, and social circumstances. These factors necessitate the need for alternative methods of fixation in an absence of the ideal patient and situation. The information presented in this in vitro study can further aid the surgeon in fixation method selection from a biomechanical stability standpoint. Figure 5 shows an algorithm that could be considered for selection of fixation technique to the mandibular angle fractures.

In the ideal fracture pattern and ideal patient (lack of patient specific factors to influence healing such as bruxism, dystonia, poor compliance, and social circumstances), a single plate should be considered for adequate fixation. In the absence of these factors then additional fixation could be considered by means of two plate or 3D plate. As depicted above, if the mandibular body height in the region of the angle is >19 mm, then two plate fixation could be considered the ideal method to fixate. If the height is less than 19 mm then 3D plate should be considered. Additionally, the data supports that when the angle height is greater than 22 mm, then single plate could be considered again over 3D plate. The difference in fixation strength is less when >22 mm, but the 3D plate remains superior. Lastly, the surgeon may consider application of a 3D plate in all these situations because of its adaptability and convenience.

The human cadaveric mandible would be the most appropriate model as a replica for the clinical situation; however, there are some drawbacks of these models.[35] Standardization of a cadaveric study is complicated due to variable size and bone density, representing an important disadvantage. This study used the same donor for comparison between fixation systems, so this limitation was eliminated. Other limitations include the age of donor, lack of randomization for grouping, and limited availability to obtain larger sample sizes to characterize the associations. Other obstacles could be for study design, ethical, practical, and financial issues. A randomized cadaveric study should be further investigated.