Dimple of Venus Is Associated with the Increased Pelvic Incidence Angle and More Sagittally Oriented Facet Joint

Spinopelvic balance and spinopelvic junction anatomy have been increasingly investigated recently. Studies have generally focused on measurements of the radiological images of the spinopelvic junction anatomy. Pelvic incidence angle (PIA) and facet joint angle are the two studied radiological measurements at the spinopelvic junction. These radiological studies investigated the relationship between diseases at the level of L5-S1 such as facet joint degeneration, facet joint tropism, intervertebral disc degeneration, intervertebral disc hernia and the PIA, and facet joint orientation [1‒4]. Increased PIA has been shown to correlate with more sagittal facet joint orientation [1]; more sagittally oriented facet joint angle has been reported to be associated with diseases such as increased facet joint degeneration [2]. Similarly, it is known that there is a relationship between increased PIA and increased facet joint degeneration at the level of L5-S1 [3]. Imagama et al. [4] described the relationship between low PIA and intervertebral disc degeneration. A recent study has revealed the relationship between the dimple of Venus and increased PIA [5].

Anatomical structures of the spinopelvic junction including the PIA, sacral slope, facet joint angle individually vary from person to person. However, possible reasons for these differences are not known and there is little information in the literature regarding potential anthropometric landmarks specialized in estimating the spinopelvic junction values such as the PIA, sacral slope, and facet joint angle.

Inspection is an important step during the examination of a patient. Some anatomical landmarks have been previously shown to be important in the development of certain diseases. For example, the association between posterior shoulder dislocation and shoulder dimple has been previously reported [6]. Sacral dimple is an important landmark at inspection of patients with occult spinal dysraphisms [7].

The dimple of Venus, the subject of the present study, is in the lower back at the level of the spinopelvic junction. To the best of our knowledge, the effect of the dimple of Venus on spinopelvic junction anatomy/pathoanatomy has not been previously reported. Individual structures of the sacral morphology, PIA, facet joint angle and genetic inheritance of the dimple of Venus suggest that these parameters may be related to one another [1, 8, 9]. We aimed to examine possible relationships between the dimple of Venus and the spinopelvic junction anatomy/pathoanatomy including the PIA, sacral slope, facet joint angle/tropism/degeneration, and intervertebral disc degeneration/hernia at the level of L5-S1. The relationship between the dimple of Venus and low back pain was also explored in patients who attended the orthopedics and traumatology outpatient clinic for any reason other than acquired or congenital diseases of the pelvic girdle and vertebral column. We examined lumbar MRI to evaluate the facet joint angle, facet joint degeneration and intervertebral disc degeneration, and lateral lumbosacral X-ray to assess the PIA of the patients.

This retrospective study was conducted at the University of Giresun-Health Ministry Giresun Training Hospital after obtaining the Ethics Committee approval (31/05/2017-03-05, KAEK-18). Patients who attended the Orthopedics and Traumatology outpatient clinic between July 2017 and March 2020 and met the inclusion and exclusion criteria were included in the study. The inclusion criteria were (1) being older than 18 years of age, (2) attending the outpatient clinic with lower extremity complaints such as knee/leg/ankle pain other than low back pain and undergoing lumbar MRI for differential diagnoses, (3) having undergone a lumbar MRI examination and lumbosacral X-ray in the last 1 year, (4) the whole vertebral colon and pelvic girdle can be radiologically evaluated by lumbar/hip/pelvic X-ray. Overweight patients (body mass index ≥30 kg/m²) and those with a surgery/fracture of the hip/pelvis bone/vertebral column, any congenital/acquired disease such as Legg-Calve-Perthes disease, slipped capital femoris, developmental hip dysplasia, spina bifida, scoliosis, block/hemiblock/butterfly vertebra or any known rheumatological diseases were excluded from the study. Demographic and clinical characteristics such as age, gender, body mass index, and low back pain felt in the lumbar region were recorded.

Radiological evaluations were performed by two observers at different times. In order to ensure that the methods used in the study are standardized, the two observers made and evaluated the radiological measurements of the first 10 patients together; these patients were subsequently excluded from the study. The evaluations were conducted through the Picture Archiving and Communication System of the hospital. The facet joint angle was considered as the angulation between the facet joint and the planum medianum of the vertebrae, as previously [9]. Facet tropism was determined as the difference between the right and left facet joints as proposed previously [10]. The facet joint and the presence of intervertebral disc degeneration were assessed as suggested earlier [11]. The sagittal and coronal diameter of the vertebral canal was measured on MRI as described by Hughes et al. [12]. Intervertebral disc herniation, if present, was assessed on sagittal/transverse lumbar MRI. The PIA, pelvic tilt, and sacral slope were identified on lateral lumbar X-ray images [13].

The quantitative data obtained in the study were presented as mean and standard deviation, or median, quartile 1 and quartile 3, while the qualitative data were summarized by count and percentage. The Shapiro-Wilks test was used to evaluate whether the distributions of continuous variables were normal. According to whether continuous variables had a normal distribution or not, the two independent sample t and Mann-Whitney U tests were employed to compare continuous variables between patients with and without the dimple of Venus. Between-group comparisons were performed with the χ² test. The correlation coefficient between quantitative variables was detected by the Intraclass Correlation Coefficient (ICC) test. In order to determine after which cutoff value the likelihood of the presence of the dimple of Venus increases, receiver operating characteristics analysis was used for performance measures regarding the PIA, SS, right, and left facet joint angle values. Multiple logistic regression analysis was performed to determine the relationship between the dimple of Venus and the selected variables. For this aim, we used the scale of Hosmer and Lemeshow. The purposeful selection process begins by a univariate analysis of each variable. Variables with a significance value of less than 0.10 were included in the logistic model. A p value lower than 0.05 was regarded as significant for all analyses. The SPSS software package (version 23) was employed in all statistical analyses.

We studied 370 patients with the mean age of 48.9 ± 13.87 years. There were 134 males with the mean age of 47.86 ± 14.50 years and 236 females with the mean age of 48.49 ± 13.49 years. Table 1 shows the distribution of the qualitative variables with respect to the presence of the dimple of Venus. The dimple of Venus was detected in 41.9% of the patients. The mean PIA value was 52.79 ± 9.97° in females and 51.75 ± 8.93° in males. Among the patients with dimple of Venus, the PIA, coronal canal diameter, and sacral slope value were higher (p < 0.001, p = 0.027, and p = 0.001, respectively) (p < 0.001); the right and left facet joint angles were lower (p = 0.017, p = 0.001, respectively) (Table 2) (Fig. 1). The PIA was observed to be higher in males than in females, but the difference was not statistically significant (p = 0.318). The mean coronal canal diameter was 17.58 ± 4.43 mm in females and 20.3 ± 4.15 mm in males (p < 0.001).

Table 3 shows the comparisons of the quantitative variables. There was a significant weak and positive correlation between the PIA and coronal canal diameter (p = 0.023 r = 0.188), between the sacral slope and tropism (p = 049 r = 0.158), and between the sacral slope and coronal canal diameter (p = 0.013 r = 0.206).

In male patients with the dimple Venus, weak, positive and significant correlations were detected between the intervertebral disc degeneration and left facet joint degeneration (p = 0.03, r = 0.25), right facet joint degeneration (p = 0.04, r = 0.25), and intervertebral disc herniation at the level of L5-S1 (p = 0.01 r = 0.31), whereas there was a moderate, positive, and significant correlation between the left facet joint degeneration and right facet joint degeneration (p < 0.01, r = 0.73). In females with the dimple Venus, a weak, positive, and significant correlation was detected between intervertebral disc herniation at the level of L5-S1 and intervertebral disc degeneration (p = 0.01, r = 0.35), while the correlation between the left facet joint degeneration and right facet joint degeneration was moderate, positive and significant (p < 0.01, r = 0.66). In patients without the dimple Venus, it was determined that in males, the correlations between intervertebral disc herniation at the level of L5-S1 and intervertebral disc degeneration and between the left facet joint degeneration and right facet joint degeneration were moderate, positive, and significant (p < 0.01, r = 0.55; p < 0.01, r = 0.60, respectively); in females, on the other hand, there existed a weak, positive, and significant correlation between the intervertebral disc degeneration and left facet joint degeneration (p = 0.04, r = 0.16), right facet joint degeneration (p = 0.02, r = 0.25) and intervertebral disc herniation at the level of L5-S1 (p = 0.01, r = 0.27), a moderate positive and significant correlation between the left facet joint degeneration and right facet joint degeneration (p < 0.01 r = 0.72).

The results of univariate and multivariate logistic regression analyses regarding the presence of the dimple are presented in Table 4. According to the univariate analysis results, gender, PIA, sacral slope, right and left facet joint angles and coronal canal diameter were significant risk factors for the likelihood of the presence of the dimple of Venus. The results obtained by multivariate logistic regression analysis, on the other hand, revealed that a one-unit increase in the PIA, increased the likelihood of the presence of the dimple of Venus by 2.17 times (95% CI: 1.719–2.758; p < 0.001). On the contrary, a negative relationship was found between the likelihood of the presence of the dimple of Venus and the right facet joint angle (hazard ratio: 0.933; 95% CI: 0.873–0.998; p = 0.038) and left facet joint angle (hazard ratio: 0.831; 95% CI: 0.734–0.941; p = 0.004). Receiver operating characteristics analysis demonstrated that the cut-off value was ≥54.1° with a sensitivity of 97% and a specificity of 94% (p < 0.01) for the PIA, ≥40° with a sensitivity of 81% and a specificity of 81% (p < 0.001) for the SS, ≤35.3° with a sensitivity of 40% and a specificity of 74% (p < 0.025) for the right facet joint angle, and ≤34.6° with a sensitivity of 41% and a specificity of 81% (p < 0.001) for the left facet joint angle (Fig. 2a, b). The interobserver agreement between the two researchers who performed the measurements was found to be strong for the right facet joint angle (r = 0.89) and tropism (r = 0.89), and very strong for the left facet joint angle (r = 0.97), sagittal canal diameter (r = 0.97) and coronal canal diameter (r = 0.94).

The current study revealed the effect of the dimple of Venus on the anatomy of the spinopelvic junction. The prominent anatomical findings related to the presence of the dimple of Venus are increased PIA and more sagittally oriented facet joint at the L5-S1 level. In terms of pathoanatomical findings, the intervertebral disc degeneration, intervertebral disc herniation, facet joint degeneration at the level of L5-S1 were not related to the presence or absence of the dimple of Venus. Moreover, low back pain was not related to the presence or absence of the dimple of Venus. As a surface anatomical finding, the dimple of Venus may not have a relationship with low back pain. The dimple of Venus, an anthropometric marker on the body surface, can be safely used to estimate the PIA and facet joint angle without radiological examination.

The clinical significance of the current study can be interpreted mainly in terms of biomechanics. Spinopelvic parameters, including the pelvic incidence and sagittal facet joint orientation, play a crucial role in spinal biomechanics, spinal pathologies, hip pathologies [1‒4, 14, 15]. The anatomical compatibility of PIA and facet joint orientation with each other is a well-known issue [1, 3]. The relationship between PIA or facet joint orientation with spinal diseases was described previously [1, 2, 4, 9, 14, 16‒18]. The PIA and stiffness of the lumbar vertebrae are gaining more importance in hip surgery. Understanding the spinopelvic relationship is of importance to minimize postoperative instability of total hip arthroplasty [14]. Stepinski et al. [14] emphasized the significance of preoperative planning of hip arthroplasty, as lower PIA is associated with posterior dislocation, while increased PIA is associated with anterior dislocation [14, 15]. Moreover, the risk of periprosthetic hip dislocation increases after lumbar spinal fusion, which is mostly performed to treat facet joint degeneration [7]. Acetabulum anteversion increases with increasing pelvic inclination. The PIA is related not only to the facet joint angle but also to anatomical acetabular anteversion [19]. Additionally, decreased PIA is associated with femoroacetabular impingement, one of the deformities of the hip joint [20].

The original Schober test and modified Schober index use the dimple of Venus as a clinical landmark to evaluate lumbar mobility [21]. On the study on whether the dimple of Venus can be utilized to evaluate and predict pelvic torsion showed that the dimple did not significantly predict pelvic torsion [22]. Balta et al. [5] did not find a statistically significant relationship between the dimple of Venus and sacral anatomy in terms of sacral screw corridor. Despite being widely used, the PIA at the spinopelvic junction cannot be estimated from any anatomical landmark and physical or anthropometric examination findings [23]. A simple clinical examination finding such as the Dimple of Venus may be more effective than radiological imaging methods. Yamada et al. [23] found no significant correlation between eight different anatomical landmarks and the PIA.

We report here that subjects with the dimple of Venus had a significantly higher coronal canal diameter. This finding may be attributed to gender difference, as the prevalence of the dimple of Venus was higher in males in our study group considering that the diameter of the vertebral canal is known to be wider in males than in females [24].

Our study may lead to further research on the Venus dimple in relation to diseases of the spinopelvic junction and hip. Another issue that merits research is the relationship between the dimple of Venus and personalized preventive rehabilitation programs. The limitations of our study are the retrospective nature of our study design, and the fact we did not consider other factors that may affect spinopelvic parameters, such as physical activity levels, and not including any disease control groups. The relationship between the dimple of Venus and the sacral/lumbar pedicle screw trajectory is another issue lacking in the present study due to not using computerized tomography of the spinopelvic junction.

Our study provides new insights into the possible effects of the dimple of Venus on the anatomy of the spinopelvic junction anatomy. Our findings suggest that the presence of the dimple of Venus is associated with increased pelvic incidence and a more sagittally oriented facet joint, which may increase the risk of developing spinal pathologies. Further, longitudinal studies are needed to investigate the dynamic changes over time in spinopelvic parameters and the dimple of Venus.

Approval was obtained from the Clinical Research Ethics Committee; Decision Date and Number: 31/05/2017-03-05, KAEK-18. The research was conducted ethically in accordance with the World Medical Association Declaration of Helsinki.

The authors have no conflicts of interest to declare.

This study received no financial support.

Authors Kürşad Aytekin and Orhan Balta participated in the design, execution, and analysis of the paper and approved the final version.

Data supporting the findings of this study are available on request from the corresponding author.