A Novel External Fixator to Facilitate Accurate and Ecient Fracture Treatment

Background: The rapid popularization and application of bone external xation technology has led to external xators emerging as remarkable methods to treat various complex fractures. Because of their instability, assembly complexity and poor controllability, however, new external xators are challenging to design in terms of facilitating accurate and ecient fracture treatment. Methods: We designed a novel external xator that could feasibly treat clinical complex fractures. A prospective study was conducted on patients with various complex fractures from September 2017 to September 2019. During this period, we treated 24 patients with a universal joint external xator and then evaluated its therapeutic effects. In this study, adult cadaver femur fracture xations were modelled by using universal joint (UJ) and AO external xators, and the mechanical stability was assessed by applying a biomechanical testing device. Results: The designed device not only ts the repair site but could also be conveniently installed and implanted. All patients achieved good xation and fracture healing with the universal joint fracture external xators. In in vitro biomechanical testing, constructs were loaded under axial compression, lateral compression and torsion. Overall axial stiffness showed no signicant differences between the two groups. The lateral stiffness was 1.554 ± 0.017 mm for the UJ group and 1.342 ± 0.020 mm for the AO group (p<0.001) under a 600 N load, while the torsional stiffness was 15.727 ± 0.141° for the UJ group and 14.472 ± 0.292° for the AO group under a 40 Nm load. Conclusion: In this small and preliminary study, the universal joint external xator, which achieved a stable mechanical structure, was capable of intensive and ecient treatment for comprehensive fractures. If these ndings can be conrmed in a larger study, this external xator may be widely used in certain situations where traditional external xators are not a convenient option.


Introduction
External xation systems are widely used for treating fractures. There are many commonly accepted indications for the application of an external xator, such as open fractures that require continuous debridement, soft tissue coverage and eventual stable internal xation [1]. Furthermore, there are many types of external bone xators applied to the fracture healing process despite their numerous disadvantages, which include potential nail infection, instability, assembly complexity and poor controllability [2,3]. At the scene of a disaster, there may be many casualties with complicated conditions, and it is necessary to quickly x any fractures and effectively reduce bleeding to reduce patients' pain and create conditions for further treatment. The unilateral Bastiani external xator is widely used but suffers from complex operation, the need for image-assisted equipment and instability [4]. Hence, it is not suitable for application in emergency treatment, such as scenes of mass accidents. For open fractures with extensive soft tissue injury, the "reduction-rst" type of external xator is often di cult to apply due to the lack of safe nail channels [5]. Meanwhile, the existing "nail-priority" external xator has a complicated combination structure and requires many components that need to be connected to achieve a stable structure, and there are no speci c rules for the combination and consumption of the parts [6,7].
The use of external xators thus places high requirements on the distribution of the product and the technical level of the surgeon, and conventional external xators are di cult to apply at injury scenes, for patient transfers and in conditions where no image assistance is possible.
Accordingly, designing a targeted external xator has become a critical issue. For patients with severe trauma fractures, an external xator is used for temporary xation to treat the fracture. Not only is the operation simple, convenient and fast, but it can also give the patient a speci c recovery time, allowing the surgeon to choose the appropriate date to perform the planned operation and repair the non-fatal wound. In the present study, we designed a novel external xator for facilitating fracture treatment that is more applicable to various environments. Treating fracture patients, we assessed the clinical e cacy and validated the mechanical stability of our universal joint external xator.

External Fixator Designs
Application To overcome the shortcomings of existing external xators, which cannot be well adapted to controlled xation of accident victims' injury sites, we invented a novel external xator that not only conveniently performs damage-controlled xation of various complex fractures but also applies the original external xator to secondary restoration, performing the nail channel's non-invasive changes when the patients' general condition and medical conditions permit.
Technical Scheme a) The external xator is designed as a single-arm external xator with few components, which is convenient for distribution, disinfection and transportation. b) The xation method is designed to be a multiplanar xation that prioritizes nail penetration and increases xation strength. c) Between both ends of the fracture, a connection is designed that can be greatly deformed in all directions and is su ciently exible to allow nail penetration and noninvasive secondary reduction and xation. Instrument Structure We used Mimics 19.0 software to design a universal joint fracture external xator based on a three-dimensional model and nailing plan ( Figure 1). This novel device is composed of brackets, nail lockers and xed nails. The brackets are composed of a distal bracket, a proximal bracket and a coupler. The distal bracket and the proximal bracket are the main frames of the external xator and are used as the attachment brackets for the nails and nail lockers for the distal and proximal fractures, respectively. The coupler, which is a fastening universal bonder, connects the distal and proximal brackets together. When the coupler is loose, the distal and proximal brackets can be relatively closed, shifted, angled and twisted. When the coupler is tightened, the device becomes a rigid xator.

Patient Inclusion Criteria
After receiving approval from Jiangxi Provincial People's Hospital A liated to Nanchang University, patients who suffered from various traumas complicated with fracture were considered as the research objects in a prospective trial, which took place in the hospital from September 2017 to September 2019.
Based on the willingness of the patients and their guardians to be treated with UJ external xators, 24 patients were included in this study. We evaluated the clinical e cacy of the UJ external xators in terms of the practicality of disease and related complications. The radiographs were analysed pre-xation, postxation and at the nal follow-up.

Biomechanical Testing
After receiving approval from Jiangxi Provincial People's Hospital A liated to Nanchang University, we generated 10 approximate adult cadaver femur fracture models ( Figure S1), which were divided into two groups according to external xators: the UJ group (n=5) and the AO group (n=5). The two groups were then tested non-destructively by using a CTM2200 microcomputer-controlled electronic universal testing machine, a Zwick (B22.5/TSIS, Germany) material testing machine and a 100 Nm microcomputercontrolled torsion testing machine to determine axial, lateral and torsional stiffness ( Figure 2). For axial and lateral compression, a maximum load of 600 N was applied at respective loading speeds of 20 N/s and 50 N/s with a preload of 200 N, and then we measured the displacement of the specimens. For torsion, a maximum load of 40 Nm was applied at a rotational speed of 0.3°/s, and then we measured the rotation angle.

Statistical Analysis
Data are presented as the mean ± standard deviation using IBM SPSS Statistics v.21.0 (IBM Corp., Armonk, N.Y., USA). If the collected data were distributed normally and with homogeneity of variance, independent sample t tests were used to determine any signi cant differences between the groups. If not, the Kruskal-Wallis rank sum-test was used. A p-value < 0.05 was considered statistically signi cant.

Advantages of the UJ External Fixator
First, the UJ external xator is a versatile device that prioritized nail penetration and is capable of singlearm multi-dimensional xation ( Figure 1A). It can avoid the wound surface and connect the bone nails in multiple dimensions to form a strong multi-planar xation. A single-arm xator can x nails around approximately 2/3 the diameter of the bone. Second, it noninvasively corrects reduction by utilizing UJ external xators (Figure 1 B and C). The number of components did not increase nor did the position of the nails change when we needed to correct fracture reductions. Third, compared with traditional external xators, the UJ external xator consists of fewer components, which are easier to carry and assemble, and delivers more power. Given that it can be exibly adjusted, the UJ external xator could be used for pelvic fractures, limb fractures and ankle fractures, among others.

Clinical application
Twenty-four patients with different fracture types were nally included in this study. All patients were followed up for an average of 23.7 months (21 36 months). There were 17 males and 7 females, and the age ranged from 7 to 60 years old. Patients who were treated in trauma scenes or in the hospital were evaluated at the time of placing the UJ external xator to assess the effect of damage-controlled xation, fracture xation and complications. Through radiographic observation, we assessed the patients' fracture xation status as well as the condition of the fracture healing. During the treatment, the UJ external xator was xed rmly without nail loosening and displacement. The fracture patients healed well during follow-up. There were no intraoperative or postoperative complications, including infection, delayed union or breakage of UJ external xators. A clinical case analysis of the use of an UJ external xator for a pelvic fracture is shown in Figure 3.

In Vitro Testing
Given the experimental reliability, there were no between-group difference in terms of nail distance or nail insertion angle in the sagittal, coronal and horizontal planes (Tables S1-S4). For axial compression, Table   I shows that no signi cant differences were detected between the two groups for loads of 100 N, 200 N, 400 N and 600 N. For lateral compression, Table II shows that there were no signi cant differences for loads of 100 N, 200 N and 400 N. However, the UJ group exhibited poorer stiffness with a load of 600 N (equivalent to the weight of an average adult) than the AO group (p<0.001). For torsion, Table III demonstrates that the UJ external xator was likely to twist when the load exceeded 40 Nm.

Discussion
The external skeletal xation technique is a minimally invasive operation in which nails are percutaneously implanted and connected with external frames. The stress transmitted between nails stimulates the regeneration and reconstruction of bone tissue [8]. Previous studies have justi ed the bene ts of an external xator in the treatment of fractures, as it can (1) maintain fracture stability by resisting rotation, shearing and varus-valgus forces [9,10]; (2) alleviate articular surface stress for articular fractures, thus protecting the articular surface [11]; and (3) supply a suitable environment for soft tissue and muscle recovery [12]. At present, external xators are rarely used in trauma scenes, especially in eld medical treatment. Hybrid external xators have relatively more functions, but a large number of components must be used to meet the needs of different types of fractures [6,13]. In addition, when an external xator is attached to a joint, it will affect joint movement to some extent. Joint stiffness is a welldocumented complication during treatment external xators, even resulting in heterotopic ossi cation [13][14][15][16].
In this study, we designed a novel external xator that aims to overcome the shortcomings of existing external xators that cannot be well adapted to control the xation of an accident victim's injury site. The new external xator is not only convenient for performing damage-controlled xation of various complex fractures but can also be exibly adjusted. The UJ external xator is an intensive device that has a number of advantages over traditional external xators. Several of these are listed as follows.
1. Convenient delivery. The new xator is small and requires no extra components.
2. Easy learning. The only tools required are an electric drill, a wrench and a nailer; junior physicians can manipulate the external xator quickly.
3. Broad versatility. When applied to many kinds of fractures, the UJ external xator can continue noninvasive controlled xation to the nal xation.
4. No Sterilization. External xators and tools can carry bacteria, but only the nails need to be sterilized, which makes the new xator suitable for reuse in battle elds and disaster areas.
In our study, we observed a good xation effect in which fracture patients healed well during follow-up according to radiographic observation. Because the UJ external xator can be adjusted exibly, joint deformity and stiffness can be avoided, and cross-joint xation does not require pre-nail implantation after xation. Therefore, the dynamic external xator is a reliable and effective device for treating fractures, allowing early postoperative functional rehabilitation and restoring joint function. During the treatment, the UJ external xator was xed rmly without nail loosening and displacement. Moreover, there were no intraoperative or postoperative complications, including infection, delayed union or breakage of the UJ external xators. Given that it supplies a suitable condition for fracture healing, the UJ external xator achieved an acceptable treatment result.
Previous studies have con rmed that xator stiffness is closely related to stability and fracture healing [17][18][19]. The overall stiffness of the fracture xation device directly impacts the axial, torsional, and shear interfragmentary movement at the fracture site [20][21][22]. During axial and lateral biomechanical testing, we used a maximum load of 600 N, which is similar to the weight of an average adult, to estimate the mechanical performance of the UJ external xator. Our data suggest that the UJ external xator exhibited biomechanical stability similar to that of AO external xators, which indicates that the UJ external xator conformed to clinical application standards. We found no signi cant differences in the stiffness of the rotational planes with a load below 40 Nm. This demonstrates that the UJ external xator had a normal anti-torsion effect. Therefore, the UJ external xator, with its good biomechanical stability, will lead to shear interfragmentary movement and promote fracture healing.
Some limitations should not be ignored. First, because the UJ external xation technology was new to them, the patients and their guardians were fully informed of the purpose and the potential advantages and disadvantages of the device before surgery, as that was when we were able to communicate with them. Second, the relatively small sample size resulted in insu cient power for identifying signi cant differences, and a larger study will be performed to assess the mechanical stability.

Conclusions
In this small, preliminary study, we show that the UJ external xator prioritizes nail penetration and possesses a exible spatial structure to produce an intensive and e cient treatment for patient fractures. It can be noninvasively adjusted to ensure reduction of the fracture and can improve the operation skills of surgeons. In addition, the UJ external xator has a stable mechanical structure. If these ndings can be con rmed in a larger study, this external xator may be widely used in certain situations where a traditional external xator is not a convenient option.

Declarations
Ethics approval and consent to participate This prospective study was approved by the Ethics Committee of Jiangxi Provincial People's Hospital A liated to Nanchang University. All participants included in this study were informed to use this novel external xator based on their willingness and provided written informed consent. All methods were carried out in accordance with relevant guidelines and regulations.

Consent to publication
Not applicable.

Availability of data and material
This article is a clinical retrospective study, and the available data were collected from 24 patients. The biomechanical data of the external xator specimens were provided by the Biomechanical Laboratory of Shanghai University. If someone wants to request the data from this study, he or she can contact Corresponding author (Xieping Dong, xpdongdoctor@163.com) at any time.
MKS, HPL, and XPD contributed to conception and design. MKS, ZZA and JNL contributed to acquisition and analysis of data. MKS and XPD involved in drafting the manuscript. HPL and XPD revised this study critically for important intellectual content. All authors gave nal approval of the version to be published. Corresponding author is XPD and HPL.    A 31-year-old female was brought to our emergency department by ambulance after sustaining trauma in a road tra c accident with poor vital signs (BP, 57/22 mmHg). On examination, the pelvic compression test was positive. Plain radiograph showed that the displacement fracture of the pubic symphysis was not reset with the waist or girdle (A and B). In the supine position, bilateral iliac xations were performed followed by application of the universal joint external xator without imaging assistance (C and D).