We proposed a method which could automatically identify the spatial configuration of the frame by adding markers to TSF to reduce the measurement error and improve the reduction accuracy. According to Table 2, the residual deformities were significantly smaller in experimental group than control group (P < 0.05). These indicated that the marker-3D measurement method could further improve the accuracy and reduce the residual deformities comparing to traditional X-ray measurement method.
Compared with the traditional measurement that uses X-Ray imaging for planning, fractures reconstructed with the marker-3D measurement method showed better reduction accuracy, which could come from reduced measurement error and the reduction of axial rotational deformity. Traditional measurement method requires multiple measurements and adjustments after surgery to achieve satisfactory reduction[15]. In most of the patients with 3D measurement method, the satisfactory reductions could be achieved by the initial adjustment plan.
Other orthopedic surgeons had done a lot of research in these areas. Simpson et al used CT images for 3D reconstruction to perform virtual surgery [14]. He introduced a tracking stylus to digitize the connection holes as reference points on the TSF ring, or used the information of the bone surface for registration. This method avoided the measurement of parameters to reduce measurement errors, but the result was greatly interfered by the choice of connecting holes. The positions of these connecting holes may be affected by the installation of Kirschner wires and struts, resulting in the failure to find a suitable connection hole as a reference point, and metal artifacts would also affect the imaging.
Similar to our study, Tang et al designed a hexapod automatic fracture reduction device, similar to the Stewart platform, and then tested in animal models[23]. With the help of three-dimensional reconstruction from CT scan, 12 marker balls were used to replace 12 screw bolts, and the hinger’s length was directly identified by software, then the electric hinges automatically reduced the fracture. However, this automatic reduction may not be able to consider the soft tissue, the blood supply, and the shape of fracture during the process of reduction, the bone segments may even get stuck during the process of reduction.
Du H et al improved the above device for clinical usage and redesigned the device as a combination of a positioning unit, a reduction unit, and a control center [26]. Four non-special marking points of the positioning unit were used for registration to obtain the length of the struts. They introduced a series-parallel configuration to convert the 6-DOF movements of the hexapod mechanism into relevant movements of two holders, which prevented the device from jamming during resetting and improved the portability of the device. However, there were several disadvantages in this design. First, the positioning unit was composed of four parts which were complicated to disassemble and could lose precision during the process. Second, the various customized rings and devices may limit the clinical application of the device. The Last but not the least, this method only provided a temporary reduction method during surgery, it still needs effective external fixation or internal fixation after the reduction, and the device also needed to be customized, which was not suitable for clinical promotion.
In this study, to improve the ease of use, we used standard TSF, designed a marker, and developed a software. The markers could be installed freely, and the software was able to automatically identify the position of the marker balls. With the help of the markers, the processes of measurement could be finished automatically, which helped avoiding manual measurement errors. In addition, a set of computer-assisted TSF reduction software based on CT images was introduced. The position and posture of bone segments were automatically recognized by the markers[24]. The adjustment plan of external fixation for fracture reduction was obtained through the final relative displacement transformation matrix.
With the help of CT data, the proposed marker-3D measurement method could accurately obtain the axial information of the injured limb and generate adjusting plan, compared with the traditional measuring method based on X-ray. In order to ensure the safety of the reduction and to avoid the soft tissue damage around the fracture, the self-developed software was able to examine the path of the fracture reduction process. Two basic principles were used in the reduction process: (1) necessary bone segment traction and rotation need to be applied to avoid the collision of the fracture segments; (2) the bone segments are aligned with minimal movement while avoid overstretching of soft tissue. As a result, the marker-3D measurement method illustrated high reduction accuracy.
Previous studies have proved that the lower limb malalignment will increase the risk of knee OA and medial meniscus lesions[16–19]. The marker-3D measurement method had high reduction accuracy and will be able to effectively improve the alignment comparing to traditional X-ray method. The better the alignment is, the better the patient's long-term prognosis will be. The marker method makes the treatment process intuitive and convenient; it has a wide prospect of application.
The use of markers to achieve automatic measurement also had the following shortcomings: (1) During CT scanning, the metal parts of TSF could produce metal artifacts interfering with the morphology of the 3D reconstruction of the bone, thereby affecting the accuracy of the reduction; (2) The sample size of this study was relatively small, and larger sample size could help to verify the effectiveness of this study; (3) The surgeons need computer skills and are able to master the use of 3D reconstruction and other software;(4) The radiation load of CT is larger than X-ray.
In the next step, the imaging technology needs to be improved to minimize the metal artifacts in the reconstruction process for improving the accuracy of reduction. Our registration remained as a manual point-to-point registration in this study, how to achieve automatic registration will be one of our next research objectives. In addition, we are planning to use optical trackers and markers to evaluate fractures and fracture reduction more comprehensively[27]. Furthermore, the automatic reduction robot system will be used as our next research direction.
In conclusion, we introduced a marker-3D measurement method that introduced the marker onto the current mainstream TSF for easy installation and simple operating. The marker is simple to install and disassemble, and is fully compatible with the current mainstream external fixation instruments. The reconstructed 3D bone model provided axial information of patients and could also help surgeons better understand the mechanism of injury. The marker-3D measurement method is able to improve the accuracy of fracture reduction and avoid manual measurement error in the clinical application of TSF. This method in clinical application is conducive to the patient’s rehabilitation and bone healing.