There has been increasing recognition of the “chain of correlations” extending from the pelvic alignment to the spine [5], and the PI is a standard measurement for the surgical treatment of ASD patients. Various formulas related to the PI have been proposed for the surgical treatment of ASD [20, 21], and Schwab et al. [22] suggested a simplistic formula (LL=PI+9 [±9]) to estimate the mean lumbar lordosis from the mean PI. Accurate PI measurement is thus a prerequisite for spine surgeons in the treatment of patients with ASD.
Pelvic Incidence and Pelvic Rotation
The PI is generally measured as the angle between the perpendicular line from the sacral plate and the line connecting the midpoint of the sacral plate to the midpoint of the bicoxofemoral axis in 2D sagittal radiographs of standing whole-spine lateral radiographs [7]. However, an image of the 3D pelvis on 2D radiographs can be influenced by the pelvic position and orientation [12], for which there is a difficulty in precisely identifying the sacral endplate and the bicoxofemoral axis [23]. In addition, radiological measurements, including those of the PI, may be influenced by the surgeon’s knowledge and consequent experience of the anatomical landmarks [6].
In clinical practice, malposition or malorientation of the pelvis is commonly observed in standing whole-spine lateral radiographs because of factors such as the patient’s incorrect standing position, pelvic obliquity due to leg length discrepancy, and divergent X-ray beam, which could cause an error in the measurement of spinopelvic parameters [24, 25]. Thus, in 1998, Jackson et al. [24] highlighted the need for an accurate imaging technique for the pelvis to achieve more precise radiological measurements, including those of the PI, and subsequently proposed the geometrical rules to show that all of the radiographs presented 15° or less vertical pelvic rotation with simultaneous 20º or less tilt in the horizontal plane.
Tyrakowski et al. [12], using a single radiological phantom, defined 0º rotation as the complete overlapping of the femoral heads in the anteroposterior direction on lateral radiographs and produced radiographs through rotation at 5º intervals up to 45º along the vertical axis. As a result, the PI was shown to vary according to the pelvic position on the axial plane. The proper maximal angle of rotation of the pelvis for a reliable PI measurement on lateral radiographs was reported to be 30°. They also reported 2 years later in a study on PI measurements based on horizontal pelvic rotation that the PI may be influenced by the pelvic rotation on the coronal plane upon radiography and that a substantial error of PI measurements may occur upon 20º or more horizontal rotation [25]. In our study, similar results were obtained that the PI of an acceptable error of 6º on radiographs [12, 19] was 35º in the horizontal pelvic rotation and 30º in the vertical pelvic rotation.
This study agrees with the two previous studies by Tyrakowski et al. [12, 25] in that the changes in the PI according to the horizontal and vertical rotation of the pelvis were analyzed. However, the key difference lies in the PI measurement method. Through the conventional measurements based on simple radiological images, as in the studies by Tyrakowski et al. [12, 25], the measured values cannot be accurately reproduced by repeated measurements with a constant probability of both intra- and inter-rater errors. In this study, on the contrary, a higher reliability of result values could be achieved by using CT scans and conducting 3D measurements using a 3D model based on several specialized programs, including AutoCAD. Another notable difference from the studies by Tyrakowski et al. [12, 25], where a single radiological phantom was used, is that the measurements in this study were taken from 30 actual patients. Through such highly reliable data from analyzing actual patients, we revealed that the measurement of the PI could be influenced by the horizontal and vertical rotation (0º, 5º, 10º, 15º, 20º, 25º, 30º, 35º, and 40º, respectively) of the pelvis while acquiring the radiograph.
Optimal Pelvic Incidence Evaluation
For an ideal assessment of pelvic parameters, including the PI, it is crucial to acquire radiographs that allow precise identification of the sacral endplate in a straight line with two overlapping femoral heads [6]. Despite this, spinopelvic parameters are usually measured on 36-inch-long cassette lateral radiographs of the spine, and the projection of whole-spine radiographs is centered on the 12th vertebra [23]. Therefore, obtaining the perfect superposition of the two femoral heads and precisely identifying the sacral endplate are usually impossible using whole-spine radiographs. In particular, the sacral endplate could show an overlap of the lumbar spine and the pelvic bony structures on whole-spine radiographs. At the same time, the presence of a buttock or ilium shadow could interfere with the precise evaluation of the sacral endplate. The rotation of the pelvis could also deform the shape of the sacral endplate to an oval on the radiograph [25].
Vrtovec et al. [11] reported that, for PI measurements, 2D radiographic images showed approximately 5° overestimation compared to 3D CT images and that the manual measurements through 2D cross-section could not reflect the precise center and inclination of the 3D anatomical structure. In addition, Yamada et al. [26] analyzed the reliability of measuring spinopelvic parameters, including the PI, on standing whole-spine lateral radiographs and standing lateral pelvis radiographs, and reported that PI also tends to be larger approximately 5º due to a large projection angle to the sacral endplate in standing whole-spine lateral radiographs compared with standing lateral pelvic radiographs. Chen et al. [27] also reported that, as the vertical projection point is positioned higher than the spinopelvic area in the whole-spine radiograph, the femoral heads failed to form an alignment and the sacral endplate could not be sharply defined. However, in the pelvic radiograph, the vertical projection point of the radiograph tube falls in the spinopelvic area so that the femoral heads are aligned and an accurate identification of the sacral endplate is possible, and the optimized radiographic intensity in the pelvic area contributes to a more precise visualization of the femoral heads and the sacral endplate through increased signals in the pelvic area. Thus, compared to whole-spine radiographs, standing pelvis radiographs would be more effective in analyzing spinopelvic parameters, including the PI.
In treating ASD patients, the measurement of the spine Cobb’s angle using whole-spine radiographs should be performed, however, a greater emphasis is placed on standing lateral pelvic radiographs than whole-spine radiographs in evaluating spinopelvic parameters, including the PI. To minimize measurement errors according to the horizontal and vertical rotation of the pelvis, the following methods are suggested for PI measurements: In producing the standing pelvic lateral radiographs, the pelvis should first be adjusted horizontally by placing the feet above a block in the case of pelvic obliquity on the whole-spine radiographs. After checking the greater trochanter (GT) of the femur through palpation, the center points of the X-ray tube and cassette should be positioned approximately 3 cm above the GT at 150º to produce maximum overlapping of the two femoral heads so that they are positioned at the center of the produced images (true pelvis lateral radiograph, Fig. 7-A). And even in the case of complete overlap of the two femoral heads, the first sacral endplate boundary may be unclear. In such cases, the proximal and distal boundaries of the upper endplate should be precisely identified in reference to the sagittal cut on CT or MRI of the sacral endplate, and the drawings can be made on the standing pelvic lateral radiographs (Fig. 7-B). The subsequent PI measurement is anticipated to be more accurate based on the angle between the key line from the center of the sacral endplate and the line connecting the identified center of the sacral endplate and the center of the two femoral heads in maximum overlap (Fig. 7-C).
Limitations
This study has some limitations. First, as the study was conducted retrospectively, several confounding variables may exist. Second, the PI measurements were taken using only the 3D model of special mechanical programs, including AutoCAD, to prevent direct comparison with 2D radiographs. Nevertheless, more precise PI measurements using the 3D model are thought to differentiate this study from previous studies. A comparative analysis between 3D and 2D radiographs was conducted in a follow-up study. Third, with the recent advancement of novel imaging techniques, including EOS imaging (Biospace Med, France), far more accurate angle measurements have become possible. However, considering that most clinics have not yet acquired the EOS, the method based on true pelvic lateral radiographs suggested in this study is anticipated to serve as a useful guideline for spine surgeons planning surgical treatment for ASD. Fourth, the level of radiation exposure may increase owing to the additional radiological imaging to obtain standing pelvic lateral radiographs together with whole-spine radiographs. However, as the PI is a critical parameter in treating ASD patients and one that sets the standard in the surgical treatment, the potential increase in additional radiation exposure for more accurate PI measurements is a risk outweighed by therapeutic benefits for ASD patients. This coincides with the recommendations of the International Commission on Radiological Protection (ICRP) [28]: In conditions where the source of exposure is subject to control, it is desirable and reasonable to set specific dose limitations so that the associated risk is judged to be appropriately small in relation to the benefits resulting from the practice.