
==== Front
Chin J Traumatol
Chin J Traumatol
Chinese Journal of Traumatology
1008-1275
1008-1275
Elsevier

S1008-1275(24)00078-6
10.1016/j.cjtee.2024.06.002
Original Article
An experimental study of a novel external fixator based on universal joints for the rapid initial stabilization of mandibular comminuted fractures
Hu Zhi-Qiang ab1
Dai Tai-Qiang c1
Zhao Lu a
Li Xun a
Gong Xu d
Jia Hao a
Kong Hai-Tao a
Cai Bo-Lei a
Chang Shi-Ping spchang4578@163.com
c⁎⁎
Tian Lei tianleison@163.com
a⁎
a State Key Laboratory of Oral and Maxillofacial Reconstruction and Regeneration, National Clinical Research Center for Oral Diseases, Shaanxi Clinical Research Center for Oral Diseases, Department of Oral and Maxillofacial, School of Stomatology, The Fourth Military Medical University, Xi'an, 710032, China
b The 80th Group Army Hospital of PLA, Weifang, 261000, Shandong province, China
c The Third Affiliated Hospital, The Air Force Military Medical University, Xi'an, 710038, China
d State Key Laboratory of Oral and Maxillofacial Reconstruction and Regeneration, National Clinical Research Center for Oral Diseases, Shaanxi Clinical Research Center for Oral Diseases, Department of Dental Materials, School of Stomatology, The Fourth Military Medical University, Xi'an, 710032, China
⁎ Corresponding author. tianleison@163.com
⁎⁎ Corresponding author. spchang4578@163.com
1 These authors contributed equally to this work.

09 7 2024
9 2024
09 7 2024
27 5 254262
3 3 2024
9 5 2024
3 6 2024
© 2024 Chinese Medical Association. Production and hosting by Elsevier B.V.
2024
Chinese Medical Association
https://creativecommons.org/licenses/by-nc-nd/4.0/ This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Purpose

Shaping and assembling contemporary external fixators rapidly for the severe mandibular fractures remains a challenge, especially in emergency circumstance. We designed a novel external fixator that incorporates universal joints to provide the stabilization for mandibular comminuted fractures. This study aims to confirm the efficacy of this novel external fixator through biomechanical tests in vitro and animal experiments.

Methods

In vitro biomechanical tests were conducted using 6 fresh canine with mandibular defect to simulate critical comminuted fractures. Three mandibles were stabilized by the novel external fixator and other mandibles were fixed by 2.5 mm reconstruction plates. All fixed mandibles were subjected to loads of 350 N on the anterior regions of teeth and 550 N on the first molar of the unaffected side. The stability was evaluated based on the maximum displacement and the slope of the load-displacement curve. In animal experiments, 9 beagles with comminuted mandibular fractures were divided into 3 groups, which were treated with the novel external fixation, reconstruction plate, and dental arch bar, respectively. The general observation, the changes in animals’ weight, and the surgical duration were recorded and compared among 3 groups. The CT scans were performed at various intervals of 0 day (immediately after the surgery), 3 days, 7 days, 14 days, 21 days, and 28 days to analyze the displacement of feature points on the canine mandible and situation of fracture healing at 28 days. The statistical significance was assessed by the two-way analysis of variance test followed by the Bonferroni test, enabling multiple comparisons for all tests using GraphPad Prism10.1.0 (GraphPad Inc, USA).

Results

The outcomes of the biomechanical tests indicated that no statistically significant differences were found in terms of the maximum displacement (p = 0.496, 0.079) and the slope of load displacement curves (p = 0.374, 0.349) under 2 load modes between the external and internal fixation groups. The animal experiment data showed that there were minor displacements of feature points between the external and internal fixation groups without statistic difference, while the arch bar group demonstrated inferior stability. The CT analysis revealed that the best fracture healing happened in the internal fixation group, followed by the external fixation and arch baring at 28 days after fixation. The external fixation group had the shortest fixation duration (25.67 ± 3.79) min compared to internal fixation ((70.67 ± 4.51) min, p < 0.001) and arch baring ((42.00 ± 3.00) min, p = 0.046).

Conclusion

The conclusion of this study highlighted the efficacy and reliability of this novel external fixator in managing mandibular fractures rapidly, offering a viable option for the initial stabilization of comminuted mandibular fractures in the setting of emergency rescue.

Keywords

External fixation
Mandibular fracture
Battle injuries
Damage control
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pmc1 Introduction

It has been reported that the occurrence and severity of craniomaxillofacial battle injuries (CMFBIs) both increased greatly in modern military confrontations. Data related to the Russia-Ukraine conflict indicates that the incidence of CMFBIs has increased to 34.87%, which is the top on the list of trauma registry.1 Mandibular fractures are common seen in CMFBIs, accounting for 43% of all craniomaxillofacial (CMF) fractures.2 Mandibular fracture can result in critical airway complications and severe bleeding, which may threaten the patient's life and need to be treated in emergency. Improper management may also lead to chewing and swallowing difficulties, as well as facial deformities.3 Therefore, timely and proper stabilization of comminuted mandibular fractures is crucial in emergency environments, such as military conflict or serious natural calamity.4,5

The initial approach to treating CMFBIs involves damage control and rapid evacuation4, which include wound debridement, temporary external fixation, and delayed primary closure following damage control principles6. Presently, the primary methods of external fixation for mandible fractures predominantly encompass maxillomandibular fixation with arch bars and external fixator fixation. Nevertheless, teeth loss and comminuted fractures in severe facial trauma frequently pose challenges in executing arch bar fixation, even impossible sometimes. The external fixators available in the market encounter several shortcomings during their application including difficulties in shaping the bars due to the fixation positions of multiple pins and limited portability caused by various bulky auxiliary instruments. Consequently, both arch bars and the present mandibular external fixator are not suitable for the swift implementation of damage control in military conflict settings.

To expedite the initial stabilization and management of comminuted mandibular fractures resulting from CMFBIs, we developed a novel mandibular external fixator based on universal joints. In vitro biomechanical tests and animal experiments were carried out to assess the stability and efficacy of this novel external fixator for treating mandible comminuted fractures.

2 Methods

2.1 Structural design of the external fixator

The novel external fixator is composed of 3 main components: titanium ball pins, preformed bars, and universal joints. The ball pin contains 2 parts which are bone fixation part and skin penetration part. The former part is designed to be a depth-limiting screw with a maximum length of 12 mm. The latter part exhibits a spherical shape with a roughened texture, whose friction coefficient is maintained within the range of 0.2 – 0.4.

The universal joint comprises a composite structure by linking 2 sets of clamps (Fig. 1A). Each set of clamps is interconnected using standard M4 bolts and nuts, while the 2 sets of clamps are joined by a dumbbell-shaped bar to enable universal adjustment (Fig. 1B). The preformed bar, made by titanium (6 mm in diameter, TA1) and preshaped to resemble the mandible, serves to connect different joints (Fig. 1C). The weight of the whole external fixator, with 6 universal joints secured, is approximately 136 g.Fig. 1 The design diagram and image of the novel external fixator: (A) a design diagram depicting the structure of the ball pin and universal joint, (B) various directions and forms of the universal joint, and (C) an image of the assembled external fixator intended for animal experimental.

Fig. 1

Before utilization, the number of pins and the estimated fixation site are predetermined based on the specific condition of the fracture or bone defect. The pins are inserted into the cortical bone of the mandible according to the fundamental principles of external fixation, while the spherical ends extend beyond the skin and are inserted into the socket of the universal joint through the specified notches of the joint clamps. Subsequently, each set of universal joints is linked to the preformed bar. Finally, the positions of the bars and joints are adjusted, and the bolts are tightened with a torque of 5 N to complete the fixation process.

2.2 In vitro biomechanical tests on the canine mandible

2.2.1 Construction of an in vitro canine mandible defect model

Six fresh canine mandibles were selected as experimental specimens with all soft tissues stripped off and were divided into 2 groups evenly. The ascending ramus of the mandibles was embedded in self-curing resin (OSTRON P500, acrylic tray, and baseplate resin) to facilitate the following manipulations. Since it is difficult to establish a communited manibular fracture model in vitro, the segmental bone defects in the mandibular body of the same stress patterns were made to mimic comminuted fractures. The mandible osteotomy extended from the mesial aspect of the left first molar to the distal aspect of the first premolar (Fig. 2).Fig. 2 Biomechanical tests were conducted in vitro using a canine mandibular bone defect model. (A) Anterior tooth load with external fixation, which involved bilateral fixation with three pins on each side of the bone defect. (B) Applying molar load with external fixation, which also included bilateral fixation with three pins on each side of the bone defect. (C) Applying anterior tooth load in the control group, a 2.5 mm reconstruction plate and 6 locking screws were used to fix the mandible. (D) Applying molar load in the control group.

Fig. 2

2.2.2 Methods of biomechanical tests on the canine mandible

Two prevalent occlusal loading patterns, which are located in the anterior region of teeth and molars, were employed for biomechanical tests. According to the principles of external fixation, increased pin quantity and distance between pins will enhance stability. Moreover, bilateral fixation is noted to offer greater stability compared to unilateral fixation.7 Therefore, the novel external fixator used in the experimental group was bilateral fixed with 3 pins evenly implanted at the anterior end of the bone defect and unaffected side (Fig. 3). Additionally, 3 pins were implanted at the posterior side of the bone defect (Fig. 3).8 All 6 pins were positioned at least 5 mm above the mandibular border, maintaining a distance of 10 mm between the fractured bone end and the neighboring pins. A 2.5 mm reconstruction plate with 6 locking screws (Matrixachusetts, DePuy Synthes, USA) was used to fixthe defect mandible in the control group (Fig. 3). Then all mandible specimens were put on the universal material testing machine (Instron 34SC-1, USA).Fig. 3 Establishment of the canine comminuted mandibular fracture model and the treatment with a novel external fixator, reconstruction plate, and arch bar. (A) Diagram of an osteotomy model. The red dotted line represents the osteotomy line, while the yellow line represents the mandibular neurovascular bundle. (B) A photograph depicting the actual osteotomy procedure. (C) Photograph showing the assembled external fixator. (D) Photograph showing the internal fixation with a 2.5 mm reconstruction plate. (E) Photograph showing the arch bar fixation.

Fig. 3

The anterior tooth load was applied at a maximum of 350 N with the rate of 1 mm/min on the anterior tooth region of the canine mandibular to simulate the maximum occlusal force of the canine anterior teeth. The molar load was applied at a maximum of 550 N with the rate of 1 mm/min on the first molar of the unaffected side to simulate the maximum occlusal force of the canine molar. All measurements were performed by a single investigator and validated by another individual to document the load-displacement curves at both mandibular load points.

2.3 Design of animal experiments

2.3.1 Establishment of the canine comminuted mandibular fracture model

All animal procedures and experiments were reviewed and approved by the Institutional Animal Care and Use Committee at the Air Force Military Medical University (Xi'an, China; ethical review number: 2023 (kq-043)) and strictly followed the international and national animal testing regulations.

Nine adult male beagles, each weighing approximately 15 kg, were divided into 3 groups: external fixation group, internal fixation group, and arch bar group, with 3 beagles allocated randomly to each group. All beagles were anesthetized by intramuscular injection of xylazine hydrochloride (0.1 mL/kg, Beite Biotechnology Research Institute Co., Ltd, Changsha, China) and 3% pentobarbital (0.3 mL/kg, Sinopharm Chemical reagent Co., LTD, Shanghai, China). Additionally, lidocaine with epinephrine was administered at the surgical sites for local anesthesia. A submandibular approach was performed to expose the mandible, the skin layers, subcutaneous tissue, muscle, and periosteum were dissected to reveal the underlying bone surface. A reciprocating saw was utilized to generate 3 vertical and 2 horizontal osteotomy lines at the mandibular body with meticulous consideration given to protect the neurovascular bundle. The incision was then closed in layers in the external fixation and arch bar groups, while the incision was closed after the internal fixation with the reconstruction plate was completed in the internal fixation group.

2.3.2 Fracture fixation methods

The occlusion and loose bone fragments in each group underwent manual reduction before stabilization. In the external fixation group, after the positions of pin fixation were confirmed according to the fracture site, 5 mm skin incision was performed. Then pin tracks were prepared by drilling the cortical bone and the pins were implanted into the mandible along the pin track. The method of assembling the external fixator in beagles was as same as the method used in vitro biomechanical tests, which had 3 pins fixed on each side of the fracture and the pins were connected rigidly with a preformed titanium arch by a universal joint. In the internal fixation group, a 2.5 mm reconstruction plate with 3 locking screws (8 mm) on each side of the fracture site was used to fix the fractures following the AO principle. In the arch bar group, the arch bar was ligature on the teeth with a 0.3 mm wire, followed by stabilization of self-curing resin around the arch bar.

Postoperatively, all beagles were administered daily intramuscular injections of ceftiofur sodium (5 mg/kg; obtained from Henan Huaxu Biotechnology Co., Ltd.) for 3 days, and a soft diet for 2 weeks. Each beagle received a CT scan (uCT760, Shanghai United Imaging Medical Technology Co, Ltd. China) at 6 different time intervals following the surgical procedure (0, 3, 7, 14, 21, and 28 days). On the 28th day, all beagles were weighed to compare the difference between pre- and post-surgery.

2.4 Main measurement and observation indicators

2.4.1 Measurement and observation indicators for in vitro biomechanical tests

The maximum displacement refers to the extent of movement on the anterior teeth and molars under the greatest applied load. The slopes of the linear segments of the load-displacement curve of the 2 occlusal modes (anterior teeth and molars) were computed to compare the differences between the experimental and control groups.

2.4.2 Measurement and observation indicators for animal experiments

The general observation was conducted to evaluate the post-surgical infection status, and then the changes in animals' weights between pre-surgery and the 28th day after surgery were recorded and compared. The second indicator involved the measurement of displacement of feature points (Δd). The raw CT scan data of animals underwent processing by MIMICS 21.0 software (Materialise NV, BEL) to extract the hemi-mandibular mask and generate a 3-dimensional (3D) object based on pixels with designated gray values (HU value: 226 – 3071). Feature points were selected and marked in the 3D objects of the mandible (Fig. 4). The changes of these points’ position were measured at different time intervals to assess the stability of different treatments. The 3D mandibular objects on the 3rd, 7th, 14th, 21st, and 28th days were aligned to the mandible object at the first time point (baseline data, 0 day after surgery) respectively through points registration in MIMICS, so that each feature point was endowed a unique coordinate value (x, y, z). The disparities in coordinate values of those feature points between different time intervals and baseline data, which were indicated by the displacement of feature points (Δd) were computed using the formula provided below. Here, the letter ‘n' represents the ordinal number of the feature point, and the letter ‘i' represents the ordinal number of the time point. This calculation was performed to determine the displacement of the feature points within each group.Fig. 4 The distribution of feature points in the canine's mandible: (1) the most prominent point on the lateral side of the condyle; (2) the most prominent point on the medial side of the condyle; (3) the mandibular angle point; (4) the most prominent cusp of the first molar; (5) the lowest point of the mandible at the proximal end of the fracture; (6) the lowest point of the mandible at the distal end of the fracture; (7) the most anterior point of the mental foramen; (8) the cusp of the second premolar; and (9) the cusp of the canine.

Fig. 4

The third indicator was the surgical fixation operation time. The treatment time consuming was recorded to evaluate the convenience of the manipulation in different groups. For the external fixation group and the arch bar group, only the fixation time was recorded. For the internal fixation group, both the time of exposure of the bone surface and the time of incision closure were included in addition to the fixation time. The final indicator was the evaluation of radiographic images. The fracture healing status in CT images of 3 groups at different time intervals were analyzed.Δdni=(xni−xn0)2+(yni−yn0)2+(zni−zn0)2

2.4.3 Statistical analysis

Two-way analysis of variance was used to analyze the difference in the displacement of feature points, while one-way analysis of variance was selected for the statistical analysis of other measurement indicators. The statistical analysis software utilized in this study was GraphPad Prism version 10.1.0 (316).

3 Results

3.1 Evaluation of in vitro biomechanical tests

No collapse or deformation of the external fixator was detected in any group in vitro biomechanical tests. There was no statistically significant difference observed when assessing maximum displacements at the anterior tooth region between the experimental group and the control group (p = 0.496), although the average displacement of the experimental group surpassed that of the control group, as indicated in Fig. 5A. There was also no significant difference observed under the molar loading conditions between the 2 groups. However, the average displacement in the experimental group was found to be lower than that in the control group (Fig. 5B, p = 0.079).Fig. 5 The results of in vitro biomechanical test. (A) The comparison of maximum displacements under anterior tooth loading (with a maximum load of 350 N). (B) The comparison of maximum displacements under molar loading (with a maximum load of 350 N). (C) The comparison of the slopes of the load-displacement curves under anterior tooth loading. (D) The comparison of the slopes of the load-displacement curves under molar loading.

Ex. Fix: groups fixed with an external fixator; 2.5 mm RP: group fixed with internal fixation using a 2.5 mm reconstruction plate; ns: p > 0.05.

Fig. 5

The comparison of load-displacement curve slopes under both anterior teeth and molars loading conditions revealed no significant difference between the experimental and control groups. The average slope of the experimental group was found to be lower compared to that of the control group, as illustrated in Fig. 5C (p = 0.374) and Fig. 5D (p = 0.349). The findings suggest that the stability of the novel external fixator based on the universal joint is similar to that of the 2.5 mm reconstruction plates in the treatment of segmental mandibular defect.

3.2 Results of animal experiments

3.2.1 General observations from the animal experiments

In the group treated with arch bars, 1 beagle developed a severe infection at the fracture site, while the remaining 2 beagles displayed suboptimal oral hygiene, thereby heightening their vulnerability to infections. Beagles in the external fixation group exhibited mild erythema around the surgical incision and pins at the 1st week after surgery. This symptom was relieved by topical disinfection with povidone-iodine swabs for 4 days, 3 times a day. Beagles in the internal fixation group exhibited mild redness and swelling at the incision site, which subsequently resolved within a week. Beagles in the arch bar group exhibited reduced appetite and a notable weight loss (Fig. 6), with an average of 1.93 kg, indicating a statistically significant difference compared to other groups. Conversely, no abnormal behavior was observed in either the external or internal fixation groups. The average weight gain was 1.53 kg and 1.30 kg, respectively, indicating no significant difference in body weight between the 2 groups.Fig. 6 Variations in the weight of the experimental beagles were recorded for each group both pre-surgery and 28 days post-surgery. On the 28th day, a significant decrease was observed in the average body weight of beagles in the arch bar group, showing statistical differences compared to both the external fixation group and internal fixation group.

Ex. Fix: group fixed with an external fixator; 2.5 mm RP: group fixed with internal fixation using a 2.5 mm reconstruction plate; Arch bar: group fixed with an arch bar; ns: p > 0.05, ∗∗∗: p < 0.001.

Fig. 6

3.2.2 Analysis of the feature points

The superimposition of 3D mandibular objects of different groups at different time points (Fig. 7) roughly reflects the stability of the 3 fracture fixation methods. It is apparent that the area of colour overlapped in the external fixation group and the internal fixation group was wider than that in the arch bar group at every time point, indicating that there was a higher degree of bone fragment displacement in the arch bar group compared with the other 2 groups.Fig. 7 The outcomes of the superimposition of the 3-dimensional (3D) objects in the external fixation group, the internal fixation group, and the arch bar group. The 3D mandibular object obtained immediately after the surgery (0 day) served as the baseline data and was marked in red. The 3D mandibular objects at different time intervals were shown by distinct colors. After registration and alignment with the baseline data at 0 day, the matched parts were displayed in red, whereas the non-matched parts were depicted in various colors, which were yellow at 3 days, cyan at 7 days, purple at 14 days, green at 21 days, and white at 28 days. The variance in the arch bar group was observed obviously at the anterior region of the fracture, whereas in the other 2 groups, the variance was evenly distributed, which meant the deformation of the mandible was not obvious compared with the baseline data.

Ex. Fix: group fixed with an external fixator; 2.5 mm RP: group fixed with internal fixation using a 2.5 mm reconstruction plate; Arch bar: group fixed with an arch bar; d: day.

Fig. 7

Concerning the displacements of the feature points, no significant differences were observed at points 7 to 9 between the groups treated with external fixation and internal fixation at all time intervals. Nevertheless, significant differences were observed in the displacement at points 8 and 9 in the external fixation and internal fixation group compared with the arch bar group at the 3rd day. However, the displacement at point 7 was only noted difference between the internal fixation and arch bar groups. Subsequently, after 7 days, there was a significant difference compared with in the arch bar group and the external fixation or internal fixation groups in the displacements at points 7, 8, and 9, as depicted in Fig. 8. The results indicate that the fixation efficacy of external and internal fixation methods shows no difference, whereas the arch bar method demonstrated less effectiveness. Interestingly, the displacement of feature at point 6 showed no significant difference in any groups at every time point. The data for pairwise comparisons can be found in Appendix 1.Fig. 8 The difference in displacement of feature points among different groups. Statistically significant differences exist in the displacement at feature points 7, 8, and 9. The displacement of the dental arch bar group exhibits the greatest magnitude, showing statistically significant differences when compared to both the external fixation and internal fixation groups.

Ex. Fix: group fixed with an external fixator; 2.5 mm RP: group fixed with internal fixation using a 2.5 mm reconstruction plate; Arch bar: group fixed with an arch bar; Δd: displacement of feature points; d: day.

Fig. 8

Analysis of the displacement curves at feature points indicated that both the external fixation and internal fixation groups exhibited minimal variability and significant overlap (red and blue lines in Fig. 9). These findings indicate the consistent effectiveness of these methods over time. In contrast, the group treated with arch bars exhibited a significant increase in displacements at points 7 to 9 over a 14-day period, which was subsequently followed by a stabilization of the curves (green lines in Fig. 9). This indicates a slightly lower initial stability of the arch bars.Fig. 9 Trends of the displacements of feature points in each group at different time points. Trends of the displacements at feature points 6 to 9 in the 3 groups over time. In comparison to the displacement at feature points 7, 8, and 9 observed in the external fixation and internal fixation groups, the displacement within the arch clamp group exhibited a sharp increase over 14 days, followed by stabilization. The magnitude of the increase escalated from point 6 to point 9.

Ex. Fix: group fixed with an external fixator; 2.5 mm RP: group fixed with internal fixation using a 2.5 mm reconstruction plate; Arch bar: group fixed with an arch bar; Δd: displacement of feature points; d: day.

Fig. 9

3.2.3 Interpretation of the CT images

The CT scans indicated the changes in fracture line for all 3 fixation methods. The obscure fracture lines in the mandible implied successful bone healing happened in the internal fixation and external fixation groups, as indicated by the red arrow in Fig. 10. The fracture lines in the arch bar group were still obvious at 28 days after surgery, which indicated the status of poor bone healing.Fig. 10 CT scans were conducted to assess the fracture area in 3 groups: external fixation, internal fixation, and arch bar, at various time intervals (0 day, 3 days, 7 days, 14 days, 21 days, 28 days). CT imaging revealed that the internal fixation group exhibited the best fracture union on the 28th day, followed by the external fixation group and the arch bar group. The bone healing phenomenon can be observed in EX group and RP group, while the bone healing could hardly be found in the arch bar group. The red arrow indicates the location where the bone fractures have healed obviously.

Ex. Fix: group fixed with an external fixator; 2.5 mm RP: group fixed with internal fixation using a 2.5 mm reconstruction plate; Arch bar: group fixed with an arch bar; d: day

Fig. 10

3.2.4 Analysis of the fixation time

Significant differences were observed in the duration of the fixation process among the 3 groups. The external fixation group had the shortest time consumption, with an average duration of (25.67 ± 3.79) min, followed by the arch bar group, which had an average duration of (42.00 ± 3.00) min (p = 0.046). In contrast, the group treated with internal fixation exhibited the longest fixation time, with a duration of (70.67 ± 4.51) min (p < 0.001), as depicted in Fig. 11.Fig. 11 Comparison of fixation time among 3 groups. The average fixation time was found to be the shortest in the external fixation group (25.67 ± 3.80) min, followed by the arch bar group (42.00 ± 3.00) min, and the longest in the internal fixation group (70.67 ± 4.51) min.

2.5 mm RP: group fixed with internal fixation using a 2.5 mm reconstruction plate; Arch bar: group fixed with an arch bar; Ex. Fix: group fixed with an external fixator ∗∗: p < 0.01; ∗∗∗: p < 0.001; ∗∗∗∗: p < 0.0001.

Fig. 11

4 Discussion

Comminuted mandibular fractures and composite tissue defects are frequently observed in CMFBIs and will cause damage of the airway, occlusion, and facial aesthetics. Although open reduction and rigid fixation (ORIF) is regarded as the golden standard for treating mandible fractures and bone defects, it is not the case in emergency settings, especially in the battlefield environment. According to the principle of damage control, timely debridement, stabilization of fractures, and subsequent precise surgical intervention are crucial in the initial management of CMFBIs. This approach aims to stabilize the fractured bone fragments as soon as possible while minimizing periosteum stripping and vascular disruption, thereby enhancing tissue healing and infection resistance.9,10 Nevertheless, the ORIF requires specialized personnel, specific equipments, and more time, which may be impossible to carry out in emergency settings.11

Maxillomandibular fixation with arch bars is commonly conducted to fix jaw fractures and restore normal occlusal as a main sloe method or adjuvant method of ORIF.8 However, severe mandibular fractures are frequently associated with teeth loss or looseness, and significant occlusal discrepancies, making arch baring unfeasible. Consequently, external fixation appears to be a better option for the initial treatment of severe mandibular injuries in military medical support.

Currently, the commonly utilized external fixators in maxillofacial surgery is manufactured by the Synthes Company.12 Nevertheless, its complex assembling process and the requirement for precise bar shaping prior to fixation present certain limitations. The adjustments of the angles and positions of the pins is a hard and time-consuming work, and the general portability of the Synthes external fixation system is not suitable for rescuing patients in the battlefield. To overcome these limitations and enhance the efficiency of fixation for prompt treatment, our research team has devised a novel external fixator, which utilizes a ball-and-socket structure in the universal joint to address the problems associated with bar shaping caused by varying pin implantation angles and the irregular anatomy of the mandible. This design streamlines the assembling process, decreases the time consuming for fixation, and enables further adjustments after fixation to optimize bone fragment alignment. Additionally, the fixator is constructed from lightweight titanium alloy, which improves portability and facilitates transportation.

The load applied in the biomechanical tests is 350 N and 550 N, which closely approximates the average maximum bite force of a live beagle in anterior teeth and molars respectively.13 The bite force of humans is notably lower than that of beagles, which means the load range utilized in this study covers the physiological range of motion load that the human occlusal system experiences. The evaluation of biomechanical tests in vitro indicated no significant difference in maximum displacement or load-displacement curve slope between the external fixation and internal fixation groups across the 2 loading conditions. This implies that the static stability of the external fixator is similar to that of the 2.5 mm reconstruction plate.

In the animal experiments, 1 beagle in the arch bar group developed a bone infection as a result of difficulties in maintaining oral hygiene and unstable bone fracture fixation. The beagles in the external fixation and internal fixation groups exhibited mild redness and swelling at the incision site, which subsequently resolved. A comparison of feature points on the canine mandible revealed no significant differences between the external fixation and internal fixation groups, suggesting the comparable efficacy in the management of comminuted fractures. The external fixator group demonstrated superior fixation compared to the arch bar group at feature points 7 to 9 (Fig. 8).

The results indicated that both external fixators and reconstructive plates offered stable fixation within 28 days, while arch bars exhibited lower stability within 14 days. CT imaging demonstrated the highest rate of successful fracture union in the internal fixation group, followed by the external fixation group, with the arch bar group showing the least successful union. Moreover, the shortest fixation time was taken in the external fixator group. Overall, the results of the stability and effectiveness demonstrated the efficacy of fixation in both the external fixation and internal fixation groups. Nevertheless, fixation with arch bars demonstrated slightly lower effectiveness.

The experimental results confirm the stability and efficacy of the novel external fixator; however, there are still some limitations. Firstly, the sample size for each group is small. Further experiments are necessary to comprehensively verify the efficacy of this external fixator. Secondly, to standardize the data in the present study, a fracture model induced by high-energy projectiles is excluded. As a result, pertinent data regarding the management of comminuted fractures arising from gunshot wounds and blast injuries is not acquired. Furthermore, the external fixator is only utilized on animal subjects and its effectiveness in clinical applications still needs to be further studied.

We invented a novel titanium external fixator based on universal joints to manage mandibular fractures rapidly and proved its efficacy and reliability through in vitro biomechanical tests and animal experiments. The advantages of easily assembling, lightweight structure, and compact size offer a viable option for the initial stabilization of comminuted mandibular fractures in the setting of emergency, especially in the aspect of military rescue.

Funding

This work was supported by Research projects of State Key Laboratory of Oral & Maxillofacial Reconstruction and Regeneration [grant number:2020ZB03 and LX2021-101 ], and Key research and development program of Shaanxi Province [2024SF-YBXM-264] .

Ethical statement

All procedures in this study were performed in full compliance with the institutional and protection laws and approved by the animal welfare and ethical committee (School of Stomatology, The Fourth Military Medical University). The obtained ethical review number is 2023 (kq-043).

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Author contributions

Zhi-Qiang Hu wrote and revised the manuscript, conducting the experiments and overseeing data management.

Tai-Qiang Dai performed experimental operations, and contributed to the statistical analysis and the creation of figures.

Lu Zhao performed experimental operations and contributed to the preparation of experimental materials.

Xun Li performed experimental operations and data collection.

Xu Gong helped with the design of the study and some data collection.

Hao Jia performed experimental operations.

Hai-Tao Kong contributed to the data collection.

Bo-Lei Cai contributed to the supervision of the experimental process.

Shi-Ping Chang was responsible for the design of experiments, and the supervision of the experimental process.

Lei Tian conceived the study and guided the study design, and reviewed of the manuscript finally.

Appendix A Supplementary data

The following is/are the supplementary data to this article.Multimedia component 1

Multimedia component 1

Peer review under responsibility of Chinese Medical Association.

Appendix A Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjtee.2024.06.002.
==== Refs
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