External fixation characterized by rapid, simple, effective, minimally invasive and fixated reliably, which can preserve the biomechanical microenvironment needed for fracture healing has a clear role in the management of unstable fractures, especially for high-energy injuries with extensive soft tissue damage15,16. The Ilizarov method of deformity correction was the most important contribution in the field of deformity correction, whereas years of training and frame changing were needed when used the Ilizarov frame for multiplanar deformities. The hexapod system such as TSF, based on the same principle as the Ilizarov system, comprised of two rings and six struts, uses a six-axis deformity analysis incorporated within a computer-based software to establish a virtual hinge in 3D space, around which multiple deformities including translation, angulation, rotation and shortening are corrected simultaneously by adjusting the strut lengths17,18.
Precise parameters measurement are required when applying the TSF,the accuracy of the frame is depend upon the input of precise parameters. For the six deformity parameters, the translation and angulation of the coronal plane and sagittal plane respectively and the axial plane translation can be obtained from the AP and lateral X-rays. However, we can’t acquire the axial plane angulation (rotation) which was theoretically calculated by the spatial information through the 2D X-rays. The axial plane angulation is determined by clinical examination in traditional measurement which is not accurate19,20. We used the CT scans data obtained from the affected tibia to reconstruct a 3D model, and calculated the actual axial plane angulation by virtual reduction assisted by Mimics.
The TSF system required orthogonal AP and later X-rays when define the deformity parameters21–24, which was conducted subjectively by radiologists and it is impossible to determine the orthogonality of the X-rays in clinical practice. Additionally, X-rays may need to be taken repeatedly because of the manual measurement errors. Considering above mentioned possible error, foot ring was used
and determined that the two vertical edges are parallel to the reference platform when conducted the AP and lateral X-rays (Fig. 7). This method contributed to reduce the possible error caused by different postures of the affected tibia during radiological examination.
The mounting parameters define the location of the origin relative to the reference ring, which is essential to describe the relationship between the frame and bone. One third of the patients suffered residual deformity after the initial correction10,17,25−27. The majority of residual deformities consist of insufficient correction or unexpected translation-angulation with the correction of TSF have been reported due to inaccurate mounting parameters19.Many methods have been proposed to define the mounting parameters, including intraoperative fluoroscopy, postoperative radiography and CT scans19,21,28,29.
Gantsoudes et al.28 declared it is easily reproducible in the operating room and allows for accurate measurement of the mounting parameters when conducted intraoperative measurement, whereas Sami et al.30 thought that there is no difference between perioperative and postoperative measurements. These methods may not be very accurate when the images of the reference ring are incomplete or the reference marks are overlapped and obscured. Kucukkaya et al.19 calculated the mounting parameters using the tomographic images in CT, they demonstrated its advantages, especially in deformities with a rotational deformity. But we thought the biggest drawback of this method for the rotary frame angle was that they only calculate the projection of the true angle onto the horizontal plane, not the true angle in spatial position if the reference ring is not parallel to the horizontal plane. The projection is less than the true spatial angle as the generally accepted theory.
The methods of elliptic registration and the intersection of the two diagonals belonged to the rectangle were used in our study to helped us accurately find the center of the reference ring, especially in conditions that the images of the reference ring is incomplete or overlapped and obscured, it greatly prevents the measurement errors caused by incorrect center of the reference ring. The rotary frame angle is also determined by clinical examination in traditional measurement which is not accurate20. We rotated the 3D model to ensure that the reference ring was parallel to the horizontal plane in AP view by Mimics, the measurement of the spatial angle is transformed into the measurement of the planar angle in this way. In addition, the rotary frame angle can be measured easily with the use of the visualized master lab.
The present study had several limitations. Firstly, we used two types of software Mimics and CorelDRAW which required the surgeon to have a basic knowledge of these software, the procedures are tedious and time-consuming, especially under the influence of metal artifacts for 3D reconstruction. Secondly, we conducted elliptical registration and fracture reduction by visual observation, there will be inevitable subjective error.