The major risk factor for fracture healing is poor reduction. Fractures that healed with an angular deformity greater than 5°in any anatomical plane or a translational deformity greater than 10 mm are regarded as malunion9. Even minor degrees of alteration in the mechanical axis has been demonstrated to significantly affect the progression to arthritis in both knee and ankle20, 21. It is generally accepted that anatomic reduction is a crucial step for most fracture treatments.
To improve reduction accuracy, many computer-assisted techniques have been developed. Koo et al.22, 23 performed an algorithm for closed fracture reduction using a unilateral external fixator, whereas there lacked a device which could facilitate its implementation in clinical practice. Hofstetter et al.24, 25 and Weil et al.26 developed fluoroscopy-based navigation techniques in which the bone fragments are displayed through superimposed line graphics to provide the real-time spatial relationship between the bone fragments, and navigation systems based on CT were also developed for the accurate reduction27, 28.
The hexapod external fixation offers the ability of simultaneous correction of the multidirectional deformities without frame modification, and was widely used for limb lengthening, deformity correction, and fracture reduction in recent years3, 4, 11–13, 16, 29, 30. In contrast, there are still some inherent limitations that can affect effectiveness. The postoperative adjustments are based on the measurements of standard orthogonal AP and lateral radiographic31, whereas they are often conducted subjectively by radiologists and position the limbs for taking the orthogonal radiographs especially in patients with severe deformities is challenging. Besides, lots of published data demonstrated that radiographs performed with malrotation of the limb will lead to wrong measurements of the mechanical axis, the mechanical femorotibial axis (mFTA), the femoral anatomic mechanical angle (AMA), the mechanical lateral distal femoral angle (mLDFA) or the mechanical medial proximal tibial angle (mMPTA) 18, 19, 32, 33. As described, malrotation on radiographs can lead to wrong anatomical measurements, however the assessment of this malrotation is difficult. It can be assumed that rotation-related changes in the measured anatomic alignment can influence the degree of deformity correction as well as the treatment success.
Moreover, the reference ring must be perpendicular to the reference bony fragment in AP and lateral view, which is a challenge for surgeons during the surgical installation. For the mounting parameters, the whole reference ring must be visible on the radiographs, while the lateral border of the reference ring was incomplete or obscured in most radiographs in our retrospective analysis. The spatial information, especially for the rotational deformities, can’t be obtained from the radiographs which are shown in the 2D planes, and they were calculated by clinical examination in traditional usage which is not accurate17. Additionally, there are 13 parameters for surgeons to measure and subjective errors will occur inevitably, the majority of residual deformities have been reported due to inaccurate mounting parameters34. The aforementioned situations may result in a time-consuming process, which is often followed by repeated radiographs including further radiation exposure for the patient.
To prevent the miscalculation of the parameters needed by the hexapod external fixation system, several techniques have been proposed31, 34–36. Kucukkaya et al.34 calculated the mounting parameters using the tomographic images in CT, they demonstrated its advantages, especially in deformities with a rotational deformity. Gantsoudes et al.36 declared it is easily reproducible in the operating room and allows for accurate measurement of the mounting parameters when conducted intraoperative measurement. Ahrend et al.37 used rotation rod to control limb rotation before taking radiographs and enhance the standard radiographic procedure, they concluded that the variability of rotation on radiographs was lower with the rotation rod, and more reproducible and better comparable radiographs can be conducted. However, these techniques are all dependent on the measurements in 2D planes.
The presented new method which uses a postoperative CT scans to establish the relationship between the bone and ring, and replaced the spatial changes in bony fragments with that in the two rings. In our study, there was no need to conduct orthogonal AP and lateral X-rays to measure so many parameters for surgeons, and this way may be a potential benefit to prevent wrong measurements. Besides, surgeons conducted virtual reduction under the direct vision by Mimics, and the three-point trajectory of “extend-rotate-reduct” can easily avoid the collision and interference between the irregular bony end in the process of fracture reduction. In this method, the most important one for surgeons is just keeping the stable fixation in bony fragment and its corresponding ring, and there was no need to ensure that the reference ring was perpendicular to the reference bony fragment.
The present study had several limitations. Firstly, the new method here may be limited by the two software, Mimics and SolidWorks, which required the surgeons to have a basic knowledge of these software. The procedures are tedious and time-consuming in inexperienced hands, especially under the influence of metal artifacts for 3D reconstruction. A practical software which contains the aforementioned comprehensive function need to be developed for these limitations. Secondly, the control group in which conduct parameters measurement on 2D radiographs is needed to compare the accuracy of the present method, whereas the clinical outcomes have demonstrated the effectiveness of our new method. Thirdly, considering the higher radiologic exposure in CT than X-rays, our method is suggested to apply in severe open fractures or those unusually complex cases with severe deformities, especially in those with extensive soft tissue damage and internal fixation is impossible or inadvisable.