Correspondence: Stephen Eric Metzinger, MD, Aesthetic Surgical Associates, 3601 Houma Blvd, Suite 300, Metairie, LA 70006 (email@example.com).
Objective To determine the usefulness of resorbable plating systems in load-bearing applications of the mandible and the location of critical failure.
Methods An osteotomy was created in 24 fresh cadaveric mandibles at the angle and fixated by the Champy technique with similar resorbable craniofacial plating systems from 4 manufacturers. Each mandible was held rigid as a material test system applied a downward force anteriorly. The critical tolerance was measured and the type of failure was noted.
Results Critical failure occurred at forces from 34.6 to 137.8 N. We found a statistically significant difference between the plating groups (P<.001 for all comparisons). The point of failure was almost uniformly at the plate.
Conclusions Critical failure was overwhelmingly due to rupture of the plate rather than to stripping or shearing of the screws as had been strongly expected. We found differences in plate strengths for this particular application and did not evaluate their respective long-term resorptive properties. We do not advocate that single resorbable plate fixation be the sole means of mandible angle fracture fixation, regardless of the plating system used.
Advances in pediatric craniofacial and orthognathic surgical fields have led to wider use of resorbable plating systems in the rigid fixation of bone trauma. This trend is supported by the desire to limit the amount of permanent hardware in patients. Although metal plates have been reliable with low complication rates, they have drawbacks.1 These include the undesirable effects of palpability, visibility, temperature sensation, and interference with radiological studies.2 Metallic plates can also migrate, become exposed (possibly requiring subsequent removal), interfere with facial growth, and create stress shielding, in which overly rigid fixation causes bone weakening.3 Titanium originating from the plates has been found to shed into adjacent tissues and can be traced to regional lymph nodes and distant organs. The ultimate effect of titanium deposition within adjacent soft tissue and regional lymph nodes has yet to be fully elucidated.4-7
Use of resorbable plating systems in osteosynthesis of the mandible is on the rise, particularly in children. This is partially driven by concerns for restriction of normal mandibular growth that occurs with fixation using metallic plating systems.8 This complication may be limited by use of hardware that resorbs over time.
We undertook this study to compare 4 different plating systems currently available and to evaluate their integrity and resistance to physiologic forces. We used fresh human cadaveric mandibles to reproduce the application and fixation of the plates as realistically as possible, creating a mechanical model with human bone. We also wanted to compare the different properties of the bone to determine whether those properties affect the results. We modified the illustration by Blez and Kahn9 to create our study.
Twenty-four fresh cadaveric mandibles were obtained (12 dentulous and 12 edentulous). Sixteen of the mandibles were from females and 8 were from males. These were divided into 2 test groups based on dentition. Four plating systems were used (Table 1).
To prepare the osteotomy/plate construct, a 1.8-mm hole was drilled on each side of the inferior portion of the osteotomy, 6 mm from each other and 5 mm from the angle of the mandible. The osteotomies were created 45° from horizontal on each mandibular angle and 2.5 cm anterior to the lingula. A 24-gauge steel wire was placed through the 1.8-mm holes to prevent the osteotomy from sliding during force testing; the wire also served as a hinge. The resorbable plates were trimmed to 7 holes, if needed, allowing for 3 holes on each side of the osteotomy. The plates were then contoured and fixed with the standard Champy technique along the osteosynthesis line.10-11 The plates were then placed in a heat bath and contoured to the shape of the site to be fixed. The plates were placed 3 mm inferior to the superior border of the angle of the mandible. The mandible was drilled and tapped, and the plates were fixed with their respective screws as presented in Table 2.
To test each side of the mandible independently, each mandibular condyle was encased in plaster of paris before testing and placed on the testing apparatus. The opposite condyle was left free (Figure 1). When force was applied to the first side, the second side was not being stressed. Therefore, our mechanical construct allowed each angle osteotomy/plate construct to be stressed, independent of the opposite side. After each mandible was prepared and placed in the test apparatus, a test system (Bionix Material Test System; MTS System Corp, Eden Prairie, Minn) delivered a downward perpendicular force on the anterior mandible via 2 lateral points on the steel rod (Figure 1). The lingula of each side of the mandible was the reference point from which measurements were made. Transosseous steel rods were placed 5.0 cm anterior to the lingula through each side of the mandibular parasymphysis exiting the contralateral side, equidistant from the superior and inferior edges. This served as the lever to which uniform force is applied. Our setup ensured symmetric loading of the mandible and simulated bilateral physiologic loads.
A dentulous mandible specimen is shown on the experimental apparatus. The Bionix Material Test System (MTS System Corp, Eden Prairie, Minn) was used to deliver a downward perpendicular force on the anterior mandible via 2 lateral points on the steel rod. This setup ensured symmetric loading of the mandible and simulated bilateral physiologic loads. The steel wire, which prevents the osteotomy from sliding during force testing and also serves as a hinge, is visible on the left, just below the mandibular plate.
Force was applied and measured until a critical failure occurred in the integrity of the fixation (Figure 2). The amount of the force and the site of failure were recorded. Three hemimandibles served as a control group (Table 2). The osteotomy/plate construct in the control group was created with the techniques outlined but used a curved 6-hole titanium mandible fracture plate (Stryker Craniomaxillofacial, Freiburg, Germany) that was fixed with six 2.0 × 6-mm screws in the same fashion as for the resorbable systems (Figure 3).
An example of critical failure at the plate: the plate has broken at screw hole 3 from the right side. The steel rod, on which the force is applied, is visible on the right.
One of the control hemimandibles, which used a curved 6-hole titanium mandibular fracture plate (Stryker Craniomaxillofacial, Freiburg, Germany) fixed with six 2.0 × 6-mm screws.
The force required for critical failure (force to failure) was analyzed via multivariable and 2-way analysis of variance (ANOVA), with plate system, sex, and dentition as grouping variables and force to failure as the dependent variable. The Fisher P–least significant difference (PLSD) test was used to perform post hoc group comparisons. Significance for all tests was set at P<.05.
The individual testing results for the 4 groups are shown in Table 3 and Table 4. The mean ± SD force to failure in each group of plating systems is shown in the following tabulation.Article
In our control group, the mean force to failure was 340.67 ± 24.19 N.
The analysis of variance revealed a significant effect of the plating system (P<.001). Post hoc results showed that groups C and D were the strongest and statistically similar. Groups A and B were the weakest and essentially the same. The differences between groups A and C and groups A and D were significant (P<.001), as were the differences between groups B and C and groups B and D (P<.001).
The individual focus of failure was also recorded. In 29 hemimandibles, the point of failure was plate breakage at 1 of the holes. In 15 hemimandibles, stretching of the plate caused failure. Only 4 hemimandibles had a different type of failure. In 1 case, the plate broke with 1 screw pulled out; in 2, the plate disengaged through the screw heads while the screw heads remained intact; and in 1, the plate disengaged through 1 screw head and sheared off the head of 2 other screws. In 2 of our controls, the mandible cracked at 322 and 332 N; in the third control, the plate stretched and then failed at 368 N (data not shown).
The mean force to failure required in dentulous and edentulous mandibles is compared in Figure 4. Two-way ANOVA, analyzing plating systems and dentition status, showed no effect of teeth on the results and no effect of teeth on the plating systems (P = .28 for dentulous vs edentulous groups, Fisher PLSD test).
The mean ± SD force required to cause critical failure in the dentulous and edentulous mandibles tested by plating system group.
The mean force to failure in each group by sex is compared in Figure 5. When we accounted for plate and sex, we found only a marginal effect of sex on the plating sytem used (P = .10, ANOVA). However, in the post hoc analysis, sex was significant (P = .007, Fisher PLSD test).
The mean ± SD force required to cause critical failure in each group by sex and plating system group.
We measured the distances between the steel rod at the anterior mandible (K), the steel wire at the angle (H), and the center of the plate over the osteotomy site (P) (Table 4). We found a statistically significant difference for the mean ± SD H-P distances by sex (females, 14.16 ± 3.00 mm; males, 16.62 ± 4.84 mm; P = .03). In contrast, the mean ± SD K-H distances by sex were similar (females, 42.75 ± 4.13 mm; males, 42.12 ± 5.45 mm; P = .66).
The safety and efficacy of resorbable material in animal studies began more then 30 years ago.12-13 However, the literature regarding the biomechanical properties of the resorbable systems is limited. This is particularly so in the case of mandibular fractures and osteotomies. The focus has been largely on clinical efficacy and biological properties.14 Studies have evaluated resorbable hardware in metacarpal fractures, resorbable screws in condylar osteotomies in sheep, and resorbable plates in mandibular osteotomies in sheep.15-18
The safety and efficacy of resorbable plates in craniomaxillofacial surgery have also been established in the pediatric population.19-21 In addition, intraoral monocortical miniplate fixation has been established as an effective method for rigid fixation of mandibular fractures.22 One study had good results in 2 pediatric mandibular fractures, using a 6-hole resorbable plate in each case.23
The most significant finding from our study was the almost uniform site of failure—the plate—during the stress on the plated osteotomy. We had expected the failure to be at the screw head, where it would be sheared off of the body. We did not expect screws to be pulled out because of the location of the plates and the direction of the force. Failure was largely of 2 types: almost two thirds of the failures were due to actual cracks or breaks in the plate itself; about one third were due to stretching of the plate, and more than half of these occurred in plates from group D. We presume that the polymer used in the group D plates had more ductile properties. The low number of cases of screw failure demonstrates the greater integrity of the screws compared with the plates against forces perpendicular to the length of the screw when 6-screw fixation is used (3 screws on each side of the osteotomy). This was true for both 2.0-mm and 2.2-mm screws that were 6, 7, and 8 mm long.
Our experiment tested the plates by simulating incisor force through the anterior steel rod. The mean force to failure in each different plating system group ranged from 49.5 to 88.9 N. In one study,24 average incisor bite force ranged from 62.7 N in the first 6 weeks after fixation to 120.5 N after that, with 150.9 N being the average for controls. Another study25 showed a mean incisor force of approximately 150 N and a maximum incisor bite force of approximately 200 N; these values were not significantly different between males and females.
It is not known how much the physical properties are affected by contouring the plates with heat; therefore, it cannot be determined whether heating the plates affects the results.26 Degradation of the plating systems occurs through hydrolysis, with subsequent metabolism of the molecules and elimination of the end products by respiration.3 This was not evaluated and, likewise, long-term strength comparisons cannot be extrapolated from this study.
Groups A and B had similar results at lower forces. They also shared the same polymer material in the same composition—polylactic acid—but were designed differently. Groups C and D had similar results at higher forces. Groups C and D also were composed of polylactic acid but had different components of polyglycolic acid. Combining copolymers, which can give the plates desired qualities,3, 21 probably accounts for part of the difference in strength. The other part was likely the size of the plate, with plates from groups C and D being physically larger, particularly those from group D, a 2.2-mm plate (which is the company's equivalent for the 2.0-mm plate). In the literature, larger or multiple plates have been used in mandibular fixation studies, for the most part because of their reduced strength compared with metal plates.27-28 However, studies reviewed by Tams et al29 have also shown that larger polylactic acid plates and screws had long degradation periods, with increased complications.
Landes et al30 cited the disadvantage of screw breakage during fixation when using resorbable systems. We did not find this to be a problem. Like those authors, we agree that contouring the resorbable plate was easier than contouring metallic plates. In general, we found the various resorbable systems to be fairly straightforward to use.
Dentition had no effect on the force to failure for each plating system. This makes sense because failure occurred entirely within the hardware systems and no bone fractures were created. Insufficient force was generated to cause a bone failure before a hardware failure because of the generally low forces at which failure occurred.
Although ANOVA indicated that the effect of sex on the results was marginal, there was a statistically significant difference between the results for male and female mandibles on the basis of the post hoc Fisher PLSD test. This can be accounted for biomechanically by the results of the measurements between the steel rod serving as the bite force, the steel wire serving as a hinge, and the plate (Table 3). We found the K-H distance to be essentially equal, but found the H-P distance to be greater in male mandibles. Therefore, the force multiplying factor (K-H/H-P) will be greater for women. A similar masticatory force will result in greater tension across the plate owing to the smaller moment arm. Anatomically, this increased tension is the result of the smaller size of the mandible angle in females.
This study has 3 limitations. We did not evaluate plating system strength in the long term and therefore did not observe the effect of resorption on the strength of the different plating systems. Also, we did not include resorbable mandibular plating systems, and our results should not be extrapolated to them. A secondary limitation was the number of controls used. Although few in number, our controls demonstrated consistency in strength and gave a general idea of the difference between metal-system and resorbable-system strengths. The third limitation was the use of osteotomy, which behaves differently than fractures.
In this series, failure at the mandible angle fixation was due to the resorbable plates and not the screws and occurred at small to moderate forces. There was a significant difference between systems in this particular application of force that was likely accounted for by material and size. Our results do not take into consideration the long-term resorption qualities of each system. The dentition did not affect the results. The difference between the results for male and female mandibles was owing to the size of the mandible angle.
We suggest that resorbable plates hold promise in many aspects of craniofacial surgery and that improvements in their properties may, in the future, make them the gold standard in rigid fixation. It is largely recognized that the strength of the resorbable material is the limiting factor to its use.31-32 In mandible angle fracture or osteotomy, we do not advocate using a single craniofacial plate alone but may consider additional plates, with or without mandibulomaxillary fixation, or stronger resorbable plates capable of withstanding greater load-bearing forces. This capability is important, considering that mandible angle fractures have the highest rate of postsurgical complications.29 We do not recommend using metal screws with the resorbable plates because the resorbable screws work well.
Accepted for Publication: August 2, 2006.
Financial Disclosure: Dr Metzinger is a craniomaxillofacial faculty member for AO North America.
Funding/Support: This study was supported in part by a financial grant from AO North America. The plating systems were provided by Synthes, OsteoMed, W. Lorenz Surgical, Inc, and Stryker Craniomaxillofacial.
Disclaimer: The use of resorbable craniofacial plates in rigid fixation of the mandible is not recommended by Synthes, OsteoMed, W. Lorenz Surgical, Inc, or Stryker Craniomaxillofacial.
Previous Presentation: This study was presented in part as a poster at the annual Clinical Congress of the American College of Surgeons; October 10-14, 2004; New Orleans, La.
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