Biomechanical comparison of the efficacy of Cfr-PEEK and titanium systems in the fixation following sagittal split advancement osteotomy: a biomechanical study
Sagittal split ramus osteotomy (SSRO) is one of the most preferred methods for managing developmental and congenital deformities in the mandible . Fixation stability is a prerequisite for the success of this procedure . However, stability problems and skeletal recurrence still continue to be a challenge after SSRO . Although there are many studies in this field, there still needs to be more consensus regarding the fixation system that will provide ideal stability, especially for large advancements . In orthognathic surgery, titanium plate systems have been regarded as the “gold standard” for fixation . However, they do have several disadvantages, including local irritability, growth disturbance, migration, infection, dysesthesia, interference with radiological examination, accumulation of metallic debris in regional lymphatic nodes, and potential stress shielding that leads to plate removal in more than 10% of cases . Polyetheretherketone (PEEK), a member of the polyaryletherketone (PAEK) family, is a polymer that has been used in spine, orthopedic, and craniomaxillofacial surgery since the 1980s . The low Young’s modulus of PEEK (3 4 GPa), which is closer to that of cortical bone (12-120GPa), is the main property of this material over its metallic counterparts (110GPa) . PEEK is easily adaptable by adding other components, including carbon fibers . PEEK, reinforced with carbon fibers, is known as Cfr-PEEK, a composite material . The elastic modulus of Cfr-PEEK (18GPa) is similar to the cortical bone; therefore, compared to titanium, it could have less stress shielding effect . Other important benefits of this material are its compatibility with imaging modalities, chemical stability and resistance, and sterilization by standard methods such as steam and gamma . This study aims to evaluate the efficacy of Cfr-PEEK in fixation following SSRO by comparing it with titanium in vitro.   Study groups This biomechanical experimental study conducted on sheep mandibles in Gaziosmanpaşa University research laboratory by following the recommendations of the Checklist for Reporting In-Vitro Studies (CRIS) . Fresh sheep mandibles at comparable developmental stages were purchased from a local butcher and used in this study. Soft tissues were removed from 14 sheep mandibles and split into two in the midline to create 28 hemi-mandibles. The samples were kept at -15̊C for 2 days until all testing was complete. The samples were thawed at room temperature before experiments. Mandibles were randomly divided into four groups with sealed envelopes. Only the researcher who performed the statistical analysis was blinded to the groups. Group 1. 5 mm advancement-A 4-hole titanium mini plate and screw ( n = 7). Group 2. 5 mm advancement-A 4-hole Cfr-PEEK mini plate and titanium screw ( n = 7). Group 3. 10 mm advancement-A 4-hole titanium mini plate and screw ( n = 7). Group 4. 10 mm advancement-A 4-hole Cfr-PEEK mini plate and titanium screw ( n = 7). Osteotomy Vertical and horizontal osteotomies were performed by marking anatomical landmarks with a stent using a round diamond disc under water cooling. At the anterior margin of the ramus, an oblique osteotomy was performed, and from the edge of the horizontal osteotomy to the basal portion perpendicularly, a monocortical osteotomy was performed. Then, it was connected with the vertical osteotomy. Then, splitting was performed using an osteotome. The segments were repositioned to obtain 5-mm and 10-mm advancement. Fixation materials were applied with reference points marked on all hemi-mandibles to ensure standardization. Titanium and Cfr-PEEK systems were used for fixation after advancement. Cfr-PEEK plates were produced by Abay Health Services (Bursa/Türkiye) with a granule composition of 30% carbon and 70% peek with injection molding technology. There is a 3 mm long carbon fiber in the granule. For the fixation of the models, a 1 mm profile thickness plate and 7.0 mm long screws titanium (Trimed ® , Turkey) and custom-made 2 mm profile thickness Cfr-PEEK plates and 7.0 mm long titanium screws (Trimed ® , Turkey) were used. All fixations were done by one investigator (EBS). Biomechanical testing A custom-made fixation appliance was used to fix the hemi-mandibles to the test device (Autograph AGS X; Shimadzu Co, Japan). Trapezium X software was used for data acquisition and analysis. This software provides real-time displacement monitoring. The software automatically compensates for machine compliance and environmental variations during testing, ensuring consistent and repeatable results. Before testing, the system underwent routine calibration by using the software. The displacement resolution of the AGS-X testing system is 0.01 μm, as specified by Shimadzu Co. According to the manufacturer’s specifications, the measurement accuracy for displacement is ± 0.5% of the measured value. Each model was fixed in a similar fashion to the testing machine, and the occlusal plane was positioned parallel to the ground plane and stabilized in the condylar and coronoid areas. The compression load that simulated the masticatory loads was then applied from the molar region. After resetting the device, the experiment started. A load of 50 N was applied to the hemi-mandibles in the vertical direction to simulate the bite forces, and the displacement data under linear loads were measured . Because a bad split occurred, two were excluded. Therefore, seven repetitions were performed in groups 2 and 4, and six repetitions in groups 1 and 3. The data was recorded digitally on the software (Autograph AGS-X, Shimadzu, Kyoto, Japan), showing the load and displacement results. Sample size Sample size calculated with G*Power version 3.1. To detect a significant difference in the stability variable between different fixation techniques with an estimated effect size of 2.41, the minimum sample size was obtained as 5 per group at a significance level of 0.05 and a power of 0.90 . Statistical analysis The statistician made a blind assessment of the groups. The IBM SPSS Statistics for Windows software (version 20.0, IBM Corp, Armonk, NY) was used for the statistical analysis. The normality of the data was measured by the Shapiro-Wilks test. The comparison between groups was analyzed using the One-Way ANOVA test with pairwise comparisons. A p -value < 0.05 is considered as significant.   This biomechanical experimental study conducted on sheep mandibles in Gaziosmanpaşa University research laboratory by following the recommendations of the Checklist for Reporting In-Vitro Studies (CRIS) . Fresh sheep mandibles at comparable developmental stages were purchased from a local butcher and used in this study. Soft tissues were removed from 14 sheep mandibles and split into two in the midline to create 28 hemi-mandibles. The samples were kept at -15̊C for 2 days until all testing was complete. The samples were thawed at room temperature before experiments. Mandibles were randomly divided into four groups with sealed envelopes. Only the researcher who performed the statistical analysis was blinded to the groups. Group 1. 5 mm advancement-A 4-hole titanium mini plate and screw ( n = 7). Group 2. 5 mm advancement-A 4-hole Cfr-PEEK mini plate and titanium screw ( n = 7). Group 3. 10 mm advancement-A 4-hole titanium mini plate and screw ( n = 7). Group 4. 10 mm advancement-A 4-hole Cfr-PEEK mini plate and titanium screw ( n = 7).   Vertical and horizontal osteotomies were performed by marking anatomical landmarks with a stent using a round diamond disc under water cooling. At the anterior margin of the ramus, an oblique osteotomy was performed, and from the edge of the horizontal osteotomy to the basal portion perpendicularly, a monocortical osteotomy was performed. Then, it was connected with the vertical osteotomy. Then, splitting was performed using an osteotome. The segments were repositioned to obtain 5-mm and 10-mm advancement. Fixation materials were applied with reference points marked on all hemi-mandibles to ensure standardization. Titanium and Cfr-PEEK systems were used for fixation after advancement. Cfr-PEEK plates were produced by Abay Health Services (Bursa/Türkiye) with a granule composition of 30% carbon and 70% peek with injection molding technology. There is a 3 mm long carbon fiber in the granule. For the fixation of the models, a 1 mm profile thickness plate and 7.0 mm long screws titanium (Trimed ® , Turkey) and custom-made 2 mm profile thickness Cfr-PEEK plates and 7.0 mm long titanium screws (Trimed ® , Turkey) were used. All fixations were done by one investigator (EBS).   A custom-made fixation appliance was used to fix the hemi-mandibles to the test device (Autograph AGS X; Shimadzu Co, Japan). Trapezium X software was used for data acquisition and analysis. This software provides real-time displacement monitoring. The software automatically compensates for machine compliance and environmental variations during testing, ensuring consistent and repeatable results. Before testing, the system underwent routine calibration by using the software. The displacement resolution of the AGS-X testing system is 0.01 μm, as specified by Shimadzu Co. According to the manufacturer’s specifications, the measurement accuracy for displacement is ± 0.5% of the measured value. Each model was fixed in a similar fashion to the testing machine, and the occlusal plane was positioned parallel to the ground plane and stabilized in the condylar and coronoid areas. The compression load that simulated the masticatory loads was then applied from the molar region. After resetting the device, the experiment started. A load of 50 N was applied to the hemi-mandibles in the vertical direction to simulate the bite forces, and the displacement data under linear loads were measured . Because a bad split occurred, two were excluded. Therefore, seven repetitions were performed in groups 2 and 4, and six repetitions in groups 1 and 3. The data was recorded digitally on the software (Autograph AGS-X, Shimadzu, Kyoto, Japan), showing the load and displacement results.   Sample size calculated with G*Power version 3.1. To detect a significant difference in the stability variable between different fixation techniques with an estimated effect size of 2.41, the minimum sample size was obtained as 5 per group at a significance level of 0.05 and a power of 0.90 .   The statistician made a blind assessment of the groups. The IBM SPSS Statistics for Windows software (version 20.0, IBM Corp, Armonk, NY) was used for the statistical analysis. The normality of the data was measured by the Shapiro-Wilks test. The comparison between groups was analyzed using the One-Way ANOVA test with pairwise comparisons. A p -value < 0.05 is considered as significant.   The displacement values among the groups were presented as mean ± SD and shown in Table . The results of this study show that there was a significant difference among the displacement values of four groups at 50 N of loading ( p < 0.05) (Fig. ). When the difference was evaluated, the pairwise displacement value of group 1 did not show significant differences compared to groups 2, 3, and 4 ( p = 0.507, p = 0.191, p = 0.756). Group 2 showed a significant difference compared to group 3 ( p = 0.009), while showed no significant difference compared to group 4 ( p = 0.088). Group 3 did not show a significant difference compared to group 4 ( p = 0.654). The highest displacement values were observed in group 3 and the lowest in group 2. The Cfr-PEEK plates fixed groups showed lower displacement values than their titanium counterparts in 5- and 10-mm advancements.   Standard fixation in orthognathic surgery has still been accepted as titanium osteosynthesis . However, the disadvantages originated from the titanium material, leading to research for new materials. Materials that have an elastic modulus similar to cortical bone, such as PEEK and their composites like Cfr-PEEK, have been gaining popularity in recent decades . To the best of our knowledge, there is no study that explores the use of Cfr-PEEK in SSRO. Therefore, the main aim of this study was to investigate the mechanical properties of Cfr-PEEK following 5- and 10-mm mandibular advancement in SSRO by comparing it with titanium using a biomechanical model. The results of this study showed that with the least displacement values, Cfr-PEEK plates are more stable than their titanium counterparts. To investigate the biomechanical behavior of fixation materials, biomechanical tests are efficient tools before their usage in clinical practice . To create an ideal in vitro test environment, the test model that is structurally and morphologically closest to the human mandible should be chosen. To date, for this purpose, cadaveric mandible, porcine hemimandible, sheep hemimandible, and rib of animal origin have been used with different models . Although the cadaver mandible is the biomechanically most acceptable test model, it has disadvantages such as being expensive, physical properties can be changed due to formalin fixation, challenging to obtain, and has a risk of cross infection . The sheep mandible is widely preferred in biomechanical test models since it has close physical features with the human jawbone . Also, sheep mandible is easy to obtain, cost-effective, and resistant to change after freezing . Thus, in this study, sheep hemimandibles were used to evaluate the displacement and 50 N force applied to the models based on the study by Harada et al.  stated that bite forces varied between 29 N and 70 N in the postoperative two weeks in SSRO. In bone healing, biomechanical parameters can determine the outcome. The mechanical environment provided by fracture fixation regulates the biological environment in the healing period . In this sense, rigid plates reported causing bone resorption by stress shielding effect . However, by sharing larger stresses with the bone, Cfr-PEEK materials are considered to reduce the stress shielding effect and hence prevent bone resorption and failure . Also, Cfr-PEEK is considered to increase the contact area between plate and bone because of its high Young’s modulus . Studies reported different results in terms of the biomechanical properties of Cfr-PEEK material. Altiparmak et al.  compared Cfr-PEEK and titanium subperiosteal implants using the finite element method. They reported that the stress values in titanium are nearly two times that of the PEEK plate. They also observed two-fold higher stress in titanium screws compared to Cfr-PEEK screws. In another study, Padolino et al.  for proximal humeral fractures compared Cfr-PEEK and titanium locking plates and observed more bone remodeling under Cfr-PEEK and more bone resorption under titanium plates were observed. However, for distal radius fractures, Mugnai et al.  compare stainless steel, titanium, and Cfr-PEEK volar locking plates and stated that Cfr-PEEK shows high plastic deformation, increased risk of breakage, and higher failure under lower loads. In maxillofacial surgery, a limited number of studies have been conducted in the field of trauma  and no studies in the field of orthognathic surgery have been conducted in terms of the usage of Cfr-PEEK as a fixation material. In their study evaluating the feasibility of Cfr-PEEK plates for managing atrophic mandibular fracture with the finite element method, Diker and Bayram  found that stress values for the Cfr-PEEK system were lower than titanium. They concluded that Cfr-PEEK materials are usable for managing atrophic mandibular fractures. In another finite element study, Avci et al.  compare the stability of Cfr-PEEK, titanium, and resorbable plates for fractures of mandibular angulus. Researchers reported that Cfr-PEEK plate/screws have lower stresses compared to titanium, and 2 mm thick Cfr-PEEK plates may be used as an alternative to titanium plates with 1 mm thickness. The present study is unique in terms of using custom-made Cfr-PEEK plates manufactured and evaluated in an SSRO model in vitro. The present study compared the biomechanical behavior of Cfr-PEEK and titanium plates on the in vitro SSRO model. Both 5 mm and 10 mm advancement models fixated with Cfr-PEEK showed lower displacement values. These results may originate from the fact that the Cfr-PEEK composite was manufactured with 30% carbon and 70% PEEK granules, and this adds a comparable mechanical property to the Cfr-PEEK with titanium. By improving the manufacturing process of Cfr-PEEK, more optimum material can be obtained. This study has some limitations. First, artificial in vitro models cannot fully reproduce the clinical environment. Also, although a sheep mandible is an accepted model used, there are still differences between behavior of different species. Regarding the experimental setup, only vertical loads from the molar region were applied, and no dynamic testing was conducted. Besides these limitations, the main aim of this study was to test the mechanical behavior of different materials subjected to the same loads. In this study, we presented an in vitro set-up for the biomechanical evaluation of Cfr-PEEK as an osteosynthesis material in SSRO. According to our study, Cfr-PEEK provides better results than titanium in terms of displacement. However, future experimental and clinical studies that include a larger sample comparing different fracture scenarios need to be done.   