The aim of this study was to test for side-to-side variability, to determine the effect of patient positioning on the NSA using both anteroposterior supine and upright pelvic radiographs, and to provide reference NSA values, as well as studying differences in terms of sex. According to our findings, the mean NSA on the upright radiographs was found to be 131.21° ± 4.72°, and on the supine position the mean NSA was 133.06° ± 5.71°. Our results showed no significant side-to-side variability between the left and right femur on upright X-ray; however, a significant difference was found with regards to the supine position. Our study also showed a significant difference in the mean values of the NSA between females and males only in the upright positions. Additionally, a significant difference was found in the NSA between the upright and supine position. Overall, the study had an intra-rater ICC of 0.898 and 0.858 for the two reviewers.
Femoral neck angle is known to vary during growth, between geographical locations, and across temporal periods. The cause of these differences between distinct regions and populations is hypothesized to be either as a result of differences in activity levels or a consequence of climate-induced body proportions [17]. For our sample, the mean NSA was found to be 131.21° ± 4.72° in the upright position and 133.06° ± 5.71° for supine radiographs. In a systematic review including 26 publications reporting the measurement of the NSA on conventional radiograph, the mean NSA of healthy adults (5,089 hips) was 128.8° [9]. In the systematic review, the NSA ranged from 123° to 137.3°, showing significant variation between the studies. The variability could be the result of the geographical differences as well as the different methods used to measure the NSA. Similar studies done on NSA using other radiological modalities, such as computed tomography and dual-energy X-ray absorptiometry (DEXA) scans, have described a similar range for NSA values [18,19,20]. In our sample, the NSA value was found to be higher than the mean of most studies found in the literature. Anatomical studies done on femoral neck angle measurement have shown a significant increase in mean neck-shaft angles in populations with a sedentary lifestyle [21], which may contribute to the higher NSA as numerous studies have described many population to have a public health problem of sedentary lifestyle and higher obesity rates [22, 23]. The pelvic bony geometry is complex and known differences exist between both sexes. Our study also showed a significant difference in the mean values of the NSA between females and males in upright positions but not on the supine position. This is similar to the finding of Chiu et al. that amongst the Malaysian population, females had a significantly higher NSA than males [24]. Nevertheless, various studies have shown no differences in NSA between males and females [25, 26].
Femoral neck angle measurement has various clinical implications in adults, one potential implication is to aid in determining NSA after an injury. This is of importance in the fixation of unstable hip fractures as having a nail angle less than the native NSA leads to more varus reductions and fracture displacement [27]. Some studies have hypothesized that due to the dominance of one leg over another, bone lengths and angles may be affected [28]. Our results showed no significant side-to-side variability between the left and right femur on upright X-rays; however, a significant difference was observed in the supine position. In a study done by Rogers et al. on 203 patients to check for side-to-side variability of the NSA using upright anteroposterior pelvis radiographs, no significant variability between the two angles was found [29]. Similarly, a study done in India on 110 patients using supine anteroposterior pelvis radiographs concluded that the NSA angle of the contralateral femur can be used as a template during repair [25]. Future randomized control trials comparing the outcome of using the NSA of the contralateral femur versus other methods during surgery would provide more conclusive evidence.
Our study had an intra-rater ICC of 0.898 (95% CI 0.840–0.934) and 0.858 (95% CI 0.736–0.918) of the two reviewers and an overall inter-rater reliability of 0.688 (CI 0.128–0.851). Mast et al. in their study on 20 radiological images, found the intra-rater reliability of 0.94 for the first observer and 0.95 for the second observer and inter-rater reliability of NSA to be 0.58 [30], while Nelitz et al. reported an intra-rater reliability range between 0.76 to 0.90 and inter-rater reliability range between 0.72 to 0.89 [31]. Bouttier et al. has reported the same high intra-rater reliability of 0.90 for both observers and inter-rater reliability of 0.83 [32]. Nevertheless, other studies have shown higher ICC on computed tomography, such that one study done by Boese et al. reported an a intra-rater reliability of 0.995 and inter-rater reliability of 0.914 [33]. The authors attribute this higher intra-rater and inter-rater reliability to the negation of the femoral rotation effect in CT scans and clearly defined measurement protocol. This suggests that the ICC for the NSA has moderate to good reliability according to the cut-off values reported by Koo and Li, and the reliability can be increased when using a CT scan compared to the X-ray images [16].
To date, there is no established method for measuring the NSA. While some studies have described methods that utilize pelvic radiographs [2, 34, 35], other studies have described methods that require imaging the whole femur [36, 37]. Furthermore, some studies have found a difference between non-corrected and rotation-corrected measurements of the NSA, such that in a systematic review done by Boese et al., with positional correction the NSA was found to be 128.8° versus 131.6° in the subgroup without correction [9]. To add to this, our study compared NSA in an upright versus supine position, such that a significant difference was found between the two X-ray radiographic patient positioning. According to our search, no other study has compared the NSA between the two positions. Taking pelvic radiographs in the upright position may allow for a more accurate assessment of the NSA when the pelvis is influenced by weight-bearing. Also, the upright position may limit the patient from unknowingly internally or externally rotating the femur during the imaging process. However, taking an upright image may be impossible when a patient has a hip fracture. Various studies have also suggested other imaging modalities to measure NSA [18,19,20, 38].
In a study that compared radiographic measurements on standing and supine pelvic radiographs, it was found that standing pelvic radiographs resulted in lower lateral center edge angle and acetabular depth measurements, a lower likelihood of a positive crossover sign or ischial spine sign, and a higher acetabular inclination. Accordingly, they suggested using standing anteroposterior pelvic radiographs to obtain the most precise pelvic radiographic parameters [39]. Another study compared supine and standing pelvis radiographs in the evaluation of pincer-type femoroacetabular impingement, found that in the standing position there was a decrease in the incidence and amount of the ischial spine sign and crossover sign and a small increase in inclination; therefore, they recommended the use of standing pelvis radiographs in nonarthritic hip pain [40]. Trying to explain our results in supine radiographs, pelvis X-rays taken in a supine position might not have been absolutely symmetrical, allowing for a significant difference in NSA measurements. This could be the case because, in a standing posture, it is much simpler to manage the little variations in rotation in both hips to create symmetrical hips; however, in the supine position, it is harder to control the symmetry between the right and left femur as well as the rotation. We suggest that radiographs to be taking in standing position whenever possible since it is more clinically relevant because the pelvis is in a more functional position. Another reason might be the effect of lower limb dominance mentioned earlier, which is usually the right side [28]. However, lower limb dominance was not addressed in this study, so this remains a plausible but hypothetical explanation.
This study's main strength is that it presents a comprehensive assessment of the NSA measurement on radiographic images using a well-defined measurement method that enables the reproducibility of the study for future comparison. In addition, this is the first study to directly compare supine and standing plain pelvic radiographs with regards to femoral neck-shaft angle. The study’s limitations are mainly a result of the retrospective design, which caused us to rely solely on the patients’ records to exclude any deformities or previous trauma that may have affected the NSA. Secondly, the study sample was limited due to numerous radiographical images being excluded as a result of poor image quality and asymmetry.
