A computed tomography (CT) based morphometric study of superior pubic ramus anatomy among Arabs to determine safe intramedullary pubic rami screw insertion

To assess the morphometric variables of the superior pubic ramus in an Arab/ Middle Eastern population to establish a safe pubic screw fixation technique. Cross-sectional retrospective analysis of computed tomography (CT) pelvic images. Morphometric data were extracted including; on pubic ramus length, insertion angles, potential danger zones and ramus diameters. The correlation between pubic rami diameter and patient demographics was also analyzed. A total of 231 participants were included (45% female). The mean pubic ramus length was 104 mm in females and 127 mm in males. The narrowest canal diameters at the para-symphyseal area were; 7.35 mm (males) and 4.75 mm (females). The mediolateral insertion angle was 49.4° in females and 41.8° in males. The cephalic-caudal angle was 49.9° in males and 42.1° in females. The mean distance from the lateral ilium entry point to the joint articular surface was 23.5 mm in males and 19.9 mm in females. The symphysis pubis to tubercle exit point was higher in females than males (24.2 mm vs 16.6 mm, respectively). There was a significant positive correlation between age and pubic ramus diameters in all age groups. The results from this study suggest that percutaneous pubic rami screw fixation using the standard 6.5 or 7.3 mm cannulated screw system may potentially be unsafe in female Arab patients. This subset of patients may require alternative non-cannulated screws (3.5–4.5 mm) or plate options. Further, female patients may have a higher risk of acetabular joint penetration, while males have a potentially higher risk of pudendal nerve injury.


Introduction
Pelvis morphology variability between individual and different ethnic groups has been described and highlighted especially in the literature relating to obstetrics and gynecology [2]. The morphological and morphometric variation of the anterior osseous pathway of the pelvic ring often affects the treatment decision and implant selection in pelvic and acetabular fractures. In the last decade, percutaneous acetabulum fixation had gained widespread popularity. Routt et al. [9], described percutaneous retrograde pubic ramus screw insertion. The biomechanical cadaveric comparison between percutaneous retrograde pubic ramus screws fixation and plating showed comparable results, further solidifying its use [6,10]. Report of percutaneous fixation in isolated pubic ramus fracture in the geriatric population showed improved pain levels [14]. However, percutaneous fixation of the pelvic ring requires an adequate understanding of the 1 3 radiological anatomy of the bony pelvis and its relation to the adjacent vital structures [5]. Safe insertion of the percutaneous screw depends on pubic rami diameter, insertion trajectory and the degree of the curvature of the pubic rami [1,3]. A recent study assessed participants in Japan and found that insertion of 6.5 mm retrograde cannulated screw was safe in their studied population based on the average of the canal diameter at the para-symphyseal level in males (13.5 mm) and females (10.7 mm) [11]. Whereas a study on an Indian population concluded that percutaneous retrograde pubic ramus screw insertion with a similar diameter was only applicable in 54% of the studied population [13]. Thus highlighting the need to delineate these geographical variations to establish safe pubic screw insertion techniques in this specific population. This study represents the first to analyze an Arab/Middle Eastern population.

Study design
The study design was a cross-sectional retrospective analysis. The study protocol was approved by the Institutional Ethical Review Board (UID 1652/2021).

Sample group
Computed tomography pelvic images of 231 consecutive patients (103 females, 128 males) were obtained from the radiology department system during the period of 2018 to 2021. The inclusion criteria were as follow: Arab nationality or Middle Eastern origin, skeletally mature and normal CT osseous findings. Exclusion criteria were the presence of pelvic or acetabular deformity, any history of the acetabulum or pelvis ring fractures, and osteoarthritic changes in symphysis pubis or sacroiliac joints. Patients with metabolic disorders involving the bones such as Paget's disease were excluded. Patients with primary or metastatic osseous lesions were excluded. To reduce heterogeneity, Arab nationalities were defined as any ethnic groups that originate from Middle Eastern countries. All included patients were of Arabic ethnic backgrounds, share the same language, descend from two parents of Middle Eastern origin and possess a Middle Eastern nationality.

Morphometric parameters assessment
The properties of the Computed tomography (CT) (General Electric Co. 2009) scanner included a resolution Gemstone Spectral image (GSI) with acquisition parameters of 465 mA, 120 kVp, 0.984 pitch, 39.3 mm/rotation speed, 0.8 (s) rotation time and slice thickness of 3.6 mm with helical reconstruction cuts of 0.625 mm.
The morphometric parameters assessment technique is similar to that used by Suzuki et al. [11] in their assessment of a Japanese population. The axis was drawn by connecting 3 anatomical landmark points which included; the center of the pubic tubercle, the mid portion of the pubic ramus shaft and a cephalic portion of the acetabulum. After that, reconstructed cross-sectional images with 1 mm cuts were created perpendicular to this defined axis. The length of the axis was measured to represent the length of the superior pubic ramus. The pubic ramus diameter was measured at 3 areas that correspond to the narrowest canal diameter: the para symphyseal area, base of the ramus, and mid superior acetabular area. The diameter was calculated in these areas by drawing perfect circles that come into contact with the two bone cortices (Fig. 1).
The antegrade insertion angles were measured in correspondence to the vertical reference line in 2 views: the reconstructed obturator oblique and the inlet views. The obturator oblique reconstructed view was used to assess the insertion angle in cephalic-caudal orientation and to measure the distance from the entry point to the joint surface ( Fig. 2: Left). Whereas, the inlet reconstructed view was used to assess the insertion angle in medial to lateral orientation and to measure the distance from the exit point to the symphysis pubis ( Fig. 2: Right). The entry and exit points were defined as the junction of the axis of the ramus to the outer ilium cortex. In, antegrade screws, the exit point was the junction between the drawn axis of the ramus and the anterior cortex of the pubic tubercle ( Fig. 2, Left). The opposite is true for retrograde screws. All the morphometric measurements were performed by an independent radiologist who was blinded to the study design.

Statistical analysis
Statistical analysis was performed using R v 3.6.3. Counts and percentages were used to summarize categorical variables and the mean ± standard deviation was used for continuous variables. Unpaired t-test was used to compare the diameter between males and females. Pearson's correlation was used to assess the association between age and the diameter of the pubic ramus in the para symphyseal, supra-acetabular areas and the base of the ramus. Hypothesis testing was performed at 5% level of significance.

Results
The mean age for males was 42 years (range 20-76 years) which was statistically comparable to the mean age of the female group (44 years, range 21-84 years) ( Table 1). The 1 3 narrowest canal diameter in males was 7.35 mm at the para-symphyseal region, whereas, the widest diameter in male was 8.67 mm at the base of the ramus. In females, all canal diameters at the three measured regions were significantly smaller (P < 0.001) than male's diameters. The smallest pubic rami canal diameter in females was found to be 4.75 mm at para symphyseal region while the widest was 6.83 mm at the base of the ramus. The length of the ramus was significantly higher in males than females using the obturator reconstruction view (127 mm ± 12.5 vs. 122 mm ± 7.25, respectively, P < 0.001) and the inlet reconstruction view (114 mm ± 17.3 vs. 104 mm ± 15.1, respectively, P < 0.001).
Using the inlet reconstruction view, the medio-lateral insertion angle was significantly higher in females than males (49.4° vs. 41.8°, respectively, P < 0.001), whereas the cephalic-caudal angle was significantly higher in males than females (49.9° vs. 42.1°, respectively, P < 0.001). The mean distance from the lateral ilium entry point to joint articular surface was significantly higher in males (23.5 mm) than females (19.9 mm) (P < 0.001). The symphysis pubis to tubercle exit point was also measured and found to be significantly higher in females than males (24.2 mm vs 16.6 mm, respectively, P < 0.001).
The pubic rami diameter result analysis was repeated after stratification of data according to the age group since aging can change pubic ramus diameter. An analysis of the pubic diameter data in 3 age groups (18-39 years, 40-59 years and above 60 years) found that the average diameter was significantly higher in males than females in three areas of the pubic ramus and the results were consistent across all age groups (Table 2).
Further, Pearson's correlation analysis revealed a statistically significant positive correlation was observed between age and the pubic ramus diameter at the para symphyseal area (r = 0.127, P = 0.054), base of ramus (r = 0.153, P < 0.05), and supracetabular area (r = 0.158, P < 0.05) indicating that higher age is associated with higher pubic ramus diameter in the three areas (Fig. 3).

Discussions
The utilization of navigation technology to aid in percutaneous acetabular and pelvis fracture fixation assists in the safe insertion of percutaneous screws [7,12]. However in reality, operative navigation has not been implemented worldwide due to its cost. Thus emphasizing preoperative planning using CT imaging. Appropriate preoperative assessment will allow the surgeon to estimate the length and delineate the safe osseous pathway. Previous literature has highlighted the need to examine geographical morphological variations as general sizing guides may not be as accurate in different populations [11,13].
The average length of the pubic ramus in our studied population was approximately between 104 and 127 mm, which was similar to previous literature [3,7,11]. In a recent study performed in India, the mean length of safe screw trajectory was shorter (96.6 mm in males and 83.95 mm in females) than our reported results [13]. Our research revealed that the narrowest pubic canal diameter in males was 7.35 mm in all age groups, whereas the narrowest pubic canal diameter in females was 4.75 mm. This finding yields important clinical data since the most commonly used cannulated screws in the pelvis and acetabulum surgeries have diameters of 6.5 mm, 7.0 mm and 7.3 mm. Therefore, the use of cannulated screws of size 6.5 or 7.3 mm may potentially be unsafe in a subset of the female subgroup. If percutaneous pubic rami screws are performed in female patients, a solid 3.5 (mm) or 4.5  This conforms with other studies similarly reporting that safe screw diameter in males is larger than females [3,8,13]. A European pelvis CT-based morphometric study found that the safe screw diameter is 9.2 mm in males and 7.0 mm in females [7]. Whereas in Japan, the authors reported in their study that percutaneous pubic rami screw insertion can be safely used in both males and females with safe screw diameters of 13.5 mm and 10.7 mm, respectively [11]. Our study highlighted that the pubic rami diameter increase with age and this could be explained by cancellous bone resorption and cortical wall thinning in the older age group. In geriatric patients, percutaneous pubic rami screws can be considered as a safe treatment option without sizing restrictions [14].
The two most commonly used intraoperative fluoroscopic views for either antegrade or retrograde pubic rami screw insertion are the obturator oblique view and the inlet view. In the obturator reconstructed view, our cephaliccaudal insertion angle in males was 49.9° compared to 42.1° in females. The steepness of the angle in males compared to females can be explained by their androidshaped pelvis. In addition, we found in the obturator reconstructed view that the distance between the lateral ilium entry point and the acetabular articular surface is less than 20 mm in females, while in males was 23 mm. This may put females at risk of hip joint violation due to their smaller pubic rami diameter. In the inlet reconstruction view, the medial-lateral insertion angle in males was 41.8° compared to 49.4° in females. This difference can be explained by the gynecoid or platypelloid-shaped female pelvis. Suzuki et al. [11], concluded that the cephalic insertion angle of the guide wire in males and females should be 66° and 67° respectively, while the lateral insertion angle shoulder be 54.1° and 55.9° degrees, respectively. Our findings showed that the exit point to symphysis pubis distance is smaller in male patients (16 mm) than females (24.1 mm), which is similar to previous reports [3,7,13]. This finding makes pudendal nerve injury more likely in males than females since it exits 0.8 mm (range 0-2 mm) lateral to the pubic tubercle [4]. This is an important consideration to avoid potential sexual dysfunction in a male patient. The main limitation of our study is that we did not use the 3D virtual software that can determine the cortical screw perforation point for the selected screw diameter and the safe containment of screw trajectory in multiple dimensions. Another limitation of this study is that the clinical data including; height and weight were not assessed.

Conclusion
Pelvic morphology varies among patients. Results from this study conclude that the standard 6.5-7.3 mm pubic rami screws may potentially be unsafe in the female subset. Thus, females are more at risk of cortical violation due to the smaller diameter of the pubic rami. Whereas males are more at risk of pudendal nerve injury due to the smaller symphysis pubis exit point distance. Providers should exercise caution when selecting screw sizes in the female subgroup and should consider smaller-sized screws (3.5-4.5 mm) if possible or plate fixation as an alternative.

Code of availability
Not applicable.