The new method allowed relatively easy measurement of changes in pelvic tilt among the examined patients while changing from a standing to a lying position. Furthermore, the measurements could be performed in a consulting room, without specialized equipment or additional personnel, making the whole procedure easy and inexpensive. Our findings confirm that changes in the pelvic setting described by radiological parameters (SS, PT, PI) vary between individual patients. Some surgeons agree that the placement of the cup of the hip joint during THA should also take into account functional changes in the position of the pelvis [8, 18].
While most patients in the present study had an anterior slope in supine position that was reduced while standing, different patterns were also noticed. This could be explained by the fact that pelvic tilt results from a dynamic balance of forces exerted by the lumbar spine, both hip joints, muscles and ligaments, and is related to individual patent factors, such as joint laxity. The balance is susceptible to both external conditions, such as reorientation of the body concerning gravitational force [3], and internal conditions, such as spine deformities, spine fusion, and dysfunction between the lumbar spine and hip [10, 19, 20]. The internal conditions that affect the pelvic tilt also include secondary effects caused by hip flexion contractures. In case of THA, the pelvic mobility of each individual, and the associated change in alignment of the implanted acetabulum, could result in a loss of the desired safe Range of Motion (ROM).
Many researchers report the need to identify patients in whom such changes can have a significant impact on the short- and long-term outcomes of THA [8, 17, 18]. However, this is difficult to achieve on a larger scale due to the limitations of radiological measurement, e.g. with the X-ray device in the horizontal position. The Smart Navigated Ultrasound system used in the present study is a noninvasive cost-effective system based on generally-available components, which can be used as an alternative approach for measuring the pelvic tilt in the APP in standing and supine positions. The system can also be used as a dynamic tool in different body positions.
So far, PT has been measured mostly using planar X-ray imaging [21] or CT [22]. However, as these measurements can be performed only in the supine position, it is difficult to compare our results with prior data. Nevertheless, previous studies performed in both laboratory and clinical settings confirm that navigated ultrasonography demonstrated good intra- and inter-rater repeatability [13, 23]. An experimental study by Martin et al. found that an ultrasound-based measuring system for pelvic tilt yields maximum deviations of ± 1 degree and ± 1.4 degrees based on different measurement scenarios [23]. Another study on navigated smart device-based ultrasound system by Margues et al. reports intra-rater reliability ranging from good to excellent for the supine position and moderate to excellent for the upright position [13].
The degree of PT in the APP affects the functional orientation of the cup in both the supine and standing positions, and consequently, the safe range of motion [8, 17, 18]. The absolute angular value of the functional change in the pelvic tilt, i.e. the difference in angular value between the standing and supine positions, is equal to the absolute value of the PT change in APP [15]. It is essential to identify cases where a change of pelvic position occurs with a change of body position, as a serious tilt change can significantly affect the possible results of surgical procedures involving implantation of a hip joint, particularly with regard to the Lewinnek Safe Zone [4, 10]. In cases where the real pelvic tilt is unknown, implantation of the cup in a supine position may lead to incorrect placement (e.g., cases 6, 9 and 13 from our study). When moving from a supine to a standing position, the cup, being in an anteverted position, is more “open”, and anteversion was found to increase in the majority of participants. Thus, incorrect cup placement can lead to posterior impingement, anterior edge-loading or anterior dislocation [9]. This complication would be possible in cases 6, 11, 13, 15 and 16, where the cup position could change more than 10°. It is worth noting that in three cases, the pelvic tilt change was greater than 20°, which can substantially influence hip stability or cause impingement. Knowledge about the difference between the real pelvic positions in the standing or supine positions may avoid future problems associated with loss of ROM or hip instability.
Eftekhary et al. argue that less anteversion is recommended for patients with spinal deformity, in whom the pelvis tilts back from a supine position to a standing AP pelvic X-ray, to prevent anterior dislocation in a standing position. This is because posterior PT increases functional anteversion, with each 1° change in posterior PT increasing anteversion by 0.8°. These patients must be identified prior to surgery to minimize the risk of back impact, front edge loading and dislocation. For patients with anterior standing PT, it should be determined whether this risk is due to flexion contracture of the hip or true spinal deformity. In the case of flexion contracture, it may resolve to some extent after THA, and therefore anatomical positioning is acceptable. If the spine is deformed, it is suggested to increase the anteversion to account for the flexion of the femur while sitting [9].
Strengths and limitations of the study. Although the method described is useful in assessing pelvic tilt and might be useful for fulfilling the goals described by Eftekhary et al. [9], our work has two main limitations. The first is that the group included only a relatively small number of patients, and hence generalized conclusions should be drawn with caution. The second is that it does not make any comparison between ultrasound measurements and CT findings; however, previous studies performed in both laboratory and clinical settings have found navigated ultrasonography to have good intra- and inter-rater repeatability [13, 23]. Neither limitation affects the main goal of this work, which was to evaluate the potential of the SNU system to identify cases where implantation of the acetabular component could be problematic and would require a broader scope of diagnostic tests, e.g. radiological tests. As this goal has been met, future studies and patient examinations are planned to determine the level of agreement between SNU and other methods.