The hexapod external fixator provides advantages of simultaneous correction of multiplanar spatial deformities without frame modification, playing a vital role in orthopedic and reconstructive surgery6–12, 23. Accurate radiographic analysis of bony deformities and mounting parameters are crucial for the success of hexapod external fixation treatment. Postoperative adjustments require precise radiographic imaging of the frames and fracture site in both the standard AP and lateral views. These radiographs must be taken in the orthogonal plane to generate accurate prescriptions based on a computer program13, 14, 24. However, it may be difficult to achieve in the common clinical practice. Many radiographs are usually obtained subjectively by radiographers, and they are not absolutely orthogonal for the postoperative deformities measurement. Inaccurate radiographic imaging can lead to wrong parameter measurements for hexapod external fixator, resulting in incorrect prescriptions as well as insufficient deformity correction18–20, 25.
Lots of previously published methods have been described to obtain the standard orthogonal radiographs for postoperative correction planning of hexapod external fixator. Deakin DE et al.14 used a frame-mounted spirit level to help the radiographer produce perfectly aligned radiographs. Ahrend et al.15 conducted postoperative radiographs with the help of a rotation rod, concluding that the variability of rotation on radiographs was lower with the rotation rod and more reproducible and better comparable radiographs can be obtained. Kanellopoulos et al.13 developed a noninvasive guiding frame to conduct reproducible and consistent x-rays oriented orthogonally to the reference ring at different points in the correction. Although satisfactory results of reducing repeated radiographs have been determined by the aforementioned techniques, it seems time-consuming to work in inexperienced hands.
Gantsoudes et al.26 obtained intraoperative orthogonal images with the help of a rod marker, while these images were usually inadequate that just covered a small visual field, and the radiographic process might add anesthesia time. Besides, Sokucu et al. 27 declared that there is no difference between measurements taken during perioperative fluoroscopy and postoperative radiograph. Wright et al.24 introduced a silhouette technique to obtain adequate orthogonal imaging, resulting in an improvement in the adequacy of planning imaging and a reduction of repeated radiographs requirement. Subsequently, Al-Uzri et al.28 also designed and described a guideline to improve the quality of postoperative radiographs significantly. Compared to two-dimensional radiographs, the computed tomography remains the gold standard for accurate parameter measurement with the additional advantage of rotational deformities calculation29; However, there is a drawback of significantly higher radiation exposure.
In the present study, a noninvasive and simple device was used to improve postoperative radiographs for the correction planning of hexapod external fixator. Basic principles of imaging via orthogonal views were used in this technique. In fact, even if the anteroposterior X-rays is not taken in the neutral position of the limb, as long as the anteroposterior and lateral X-rays are taken when the two perpendicular sides of the foot rings are parallel to the horizontal line, the two radiographs taken at this time are perpendicular to each other. In the two consecutive groups, there were no statistically significant differences in the demographic data, original postoperative deformities, residual deformities after final correction, external fixation time, and the final clinical outcomes. However, compared to patients without additional foot ring used, there were fewer repeated radiographs after the first postoperative radiographs and less mean time to the satisfactory reduction achieved in patients with additional foot ring used. Our results manifested this device may ensure orthogonal radiographs for the parameter measurement, resulting in less radiation exposure and correction duration.
The additional foot ring is a user-friendly and cost-efficient device. It is easy for both the patient and radiographer to control the limb rotation and determine the radiographs' orthogonality during the radiographic process, just making the two perpendicular sides of the foot ring parallel to the horizontal plane respectively. Notably, the foot ring can be reused without increasing the cost burden on patients. Furthermore, radiation exposure, duration of deformity correction, and cost for patients might be reduced due to the less repeated radiographs with the wrong position.
According to our experience, this device has demonstrated an improvement in the orthogonality of postoperative radiographs for hexapod external fixator and a reduction in repeated imaging requirements. The radiographers involved also conclude that this way can easily obtain a good orthogonal view. Although we do not accurately define the radiation exposure for repeat imaging, radiation exposure reduction can be extrapolated due to the fewer repeated images.
The present study had several limitations. First of all, considering the small sample size, a conservative attitude should be adopted regarding the interpretations of our results. Besides, the patient has to be turned in an inconvenient position, especially for those with polytrauma or severe limb deformity, and it may be considered one limitation of this study. Moreover, during the anteroposterior view, adjusting the mounting holes on the distal hexapod ring and the foot ring to ensure that the lower leg was neutral may also be a time-consuming process, and an installation-friendly device is therefore needed to resolve this problem. Finally, if there was any rotational correction, it will change the position of ankle and distal bony end, and repeated mounting parameters measurement is needed.