Combined with the Ilizarov circular external fixator and the Chasles theorem of six-axis motion15, 16, the hexapod external fixator has played an important role in orthopedic and reconstructive surgery due to the advantages of simultaneous correction of multiplanar spatial deformities without frame modification4, 7, 8, 10, 12, 17–19. Initially developed for gradual deformity correction, the hexapod external fixator expanded to conduct the management of fracture and bone nonunion11, 12, 19–21.
The high theoretical accuracies of 1/1000000 inch and 1/10000 degrees are extreme for clinical practice, but with approximate correction accuracies of 1mm and 1° using the hexapod external fixator15, 22. Accurate radiographic analysis of deformity and mounting parameters are crucial for the success of hexapod external fixation treatment. Although lots of satisfactory clinical outcomes have been manifested in the HEF treatment, no technique is perfect in fact, as most parameter measurement techniques are subjective and heavily depend on human evaluators. Malcorrection, insufficient correction, or unexpected translation-angulation can be presented by subtle errors in the parameter definition. Lots of previously published methods have been described to improve parameter accuracy, including CT scans, intraoperative fluoroscopy, postoperative radiography, and determination of the radiographs’ orthogonality15, 23–29.
Gantsoudes et al.15 utilized equipment that already available in a TSF treatment would be used to obtain intraoperative orthogonal images, they thought their technique was quick, cheap, and easily reproducible. Ahrend et al.29 taken postoperative radiographs with the help of a rotation rod, the results manifested that the variability of rotation on radiographs was lower with the rotation rod, and more reproducibly comparable radiographs can be obtained. Kanellopoulos et al.25 described a noninvasive technique using a specifically designed radiolucent frame to determine the reference ring perfectly orthogonal in single exposures for each radiographic view. Deakin DE et al.26 acquired perfectly aligned radiographs with the help of a frame-mounted spirit level. Wright et al.28 described a silhouette technique to produce adequate orthogonal imaging. Kucukkaya et al.24 introduced a technique for determining the mounting parameters using computed tomography, and it is especially advantageous for cases with rotational deformity. Liu et al.27 precisely measured the deformity and mounting parameters with the help of the elliptic registration technique and three-dimensional reconstruction. Gessmann et al.30 declared that the mounting parameters can be accurately measured with radiographic techniques when using calibration markers and a software calibration tool.
The aforementioned techniques all concentrated on precise parameter calculation, none of them focused on the influence of the bony ends’ movement trajectory on the correction effectiveness during the fracture reduction process. At present, the mainstream hexapod external fixation system all performed one-step reduction trajectory, the inherent limitation is that the collision and interference between the irregular bony end in the process of fracture reduction often result in an incomplete reduction or failed reduction. In those complex cases, this drawback always results in repeated radiographs which expose the patient to further radiation exposure and make the reduction procedure time-consuming.
In the present study, the three key points reduction trajectory of “extension-rotation-reduction” was used to resolve this problem. In the whole process, the crucial step is “extension” to provide sufficient space for the relative movement of the two bony ends. Notably, accurate parameter measurements are equally important. In this series of 57 patients with tibial shaft fractures treated by the HEF, there were no statistically significant differences between the two groups in the final clinical outcomes. Although both groups achieved satisfactory outcomes, the average number of repeated radiographs after the first postoperative radiograph and mean duration of deformity correction in Group Ⅱ were all less than those in Group Ⅰ. Repeated reduction may aggravate the internal soft tissue damage which is not beneficial for fracture healing, and the long duration of fracture reduction will make patients uncomfortable. Therefore, according to our experience, the three key point trajectory of “extension-rotation-reduction” is recommended due to the shorter reduction duration with lower potential radiation exposure, especially for those complex fractures with irregular bony ends.
The present study had several limitations. First of all, considering the small sample size in a single-center, a conservative attitude should be adopted regarding the interpretations of our results. Besides, compared with the conventional one-step reduction trajectory, this three key point trajectory is relatively tedious but the results manifest the superiority.