Intraoperative tools, such as CT-based navigation systems and robot-assisted surgery, are used for accurate cup placement, and precise cup placement accuracy has been reported. However, their use requires registration to obtain accurate location information, and individual preoperative planning is time-consuming and expensive. In recent years, simpler portable navigation systems have been developed, and their accuracy has been reported.
The accuracy of cup placement by various portable navigation systems in the supine position is 2.6°–3.7° for RI and 2.8°–3.8° for RA [10,11,12]. Okamoto et al. reported that in DAA-THA, the cup placement rate within ± 5° was 56.6% in the alignment guide group and 72.2% in the portable navigation (Hip Align®) group [13]. The accuracy of cup placement using various portable navigation systems in the lateral decubitus position is 2.5°–4.6° for RI and 2.3°–6.5° for RA [4, 14].
Supine approaches, such as DAA and ALS, have the advantage of easy fluoroscopy access. Fluoroscopy is available at all institutions and is easy to perform without additional costs. In this study using intraoperative fluoroscopy, Lewinnek's safe zone achievement rate was 98.1% (151/154 joints), and the accuracy of individual RI/RA was within ± 5° at > 80%. This result is comparable to those of studies using portable navigation, indicating the usefulness of intraoperative fluoroscopy. There have been reports of the usefulness of using fluoroscopy intraoperatively, as in this study, and it is said to contribute to improving the accuracy of cup placement [15]. On the other hand, the use of fluoroscopy does expose the patient and surgical staff to radiation. Fluoroscopy is often used in orthopedic surgery, even for common fracture surgery, and radiation exposure is an unavoidable problem for orthopedic surgeons and staff. Jinnai et al. reported significantly shorter fluoroscopy time and lower radiation exposure level for DAA-THA compared to proximal femur fractures [16]. In a recent systematic review, it was also reported that radiation exposure level during anterior THA is low and does not affect the surgeon or patient [17]. Therefore, we consider THA using fluoroscopy to be a safe procedure.
We targeted RI 40° and RA 15°, following Lewinnek's safe zone concept. However, in recent years, there have been many reports of dislocations, even in cases within the safe zone [18]. Concepts such as “functional safe zones,” in which cups are seated considering individual spinopelvic mobility, have been proposed [19]. Spinal parameters and pelvic motion have large personal deviations, and there are still many unknown and no established goals. However, we believe it is important to reduce outliers from the target range, as systematic reviews have shown that the risk of postoperative dislocation increases with deviation from the target range [20]. In our institution, preoperative AP radiographs of the bilateral hips in the supine, standing, and sitting positions were obtained to assess pelvic motion during postural changes. In general, the posterior pelvic tilt from supine to standing position was 5° and from standing to sitting was 20° [21]. However, some cases of pelvic positional changes differ between these motions. A large posterior pelvic tilt in the supine to standing position increased the risk of anterior dislocation in the hip extension position. A small posterior pelvic tilt from the supine to sitting position is associated with a high risk of posterior dislocation due to anterior impingement in the hip flexion position. The target RA value was adjusted to account for these pelvic motions in postural change, and there were no cases of dislocation or other complications in the short term, averaging 1.5 years postoperatively. However, further studies are required to determine these target angles.
Our study had several limitations. First, this was a retrospective study, not a randomized study. Long-term dislocation rates and clinical outcomes have not yet been evaluated. The angle of the cup position should be investigated further, as it affects clinical outcomes and complications. In addition, the surgeons were not identical, and each of the four surgeons had different years of experience. Differences in cup placement accuracy with years of experience have also been reported but were not examined among surgeons in this study [22]. However, we believe inter-operator error is minimized because all four fixed surgeons participated in the surgery in all cases and evaluated the angle of the cup position. Most patients in this study had few obesity and Crowe types I or II, with only mild deformities. There are reports that obese patients and patients with severe deformities are less accurate in navigation, so it is possible that the results would have been different if more obese or deformed patients had been studied [3]. Finally, as described above, there is the issue of radiation exposure. Our technique uses fluoroscopy, as appropriate, during cup placement. The more time fluoroscopy is used, the more exposed physicians, nurses, and others are. Although not done in this study, we believe that it is necessary to evaluate the time spent using fluoroscopy and make efforts to reduce the use of fluoroscopy.
