The Institutional Review Board (IRB) of Veterans Health Service Medical Center approval was obtained prior to this study (IRB 2024-09-020-001). We retrospectively collected data from asymptomatic Korean adults between March 2007 and September 2010. The study was limited to male populations, as the participants were drawn from those visiting the Veterans Health Service Medical Center. Regarding inclusion criteria, participants had to be over 50 years old, have no spinal pathology, and have no history of spinal trauma, surgery, or disorders affecting the area below the hip. Additionally, individuals with a history of neck, back, or lower limb pain were excluded. A total of 183 male populations were initially screened. However, 33 were subsequently excluded for various reasons : seven due to scoliosis greater than 10° in the coronal plane, one due to pelvic obliquity from leg length discrepancy, eight due to the presence of wedging vertebrae, nine due to lumbar disc space narrowing, four because of bilateral isthmic defects in the lumbosacral region, and four due to the presence of metallic foreign bodies (from traditional acupuncture) in the back muscles. Ultimately, 150 male populations were included in the study.

Standing plain lateral radiographs of the entire spine, including the pelvis, were analyzed in asymptomatic adult male volunteers. The protocol for acquiring these standing plain radiographs followed the methodology described by Horton et al. [6]. The radiographs were stored in a picture archiving and communication system (PACS; Maroview; Marotech, Seoul, Korea) in the Digital Imaging and Communications in Medicine format. Radiographic measurements were performed using the ruler and protractor functions in the Maroview PACS. The Sagittal vertical axis (SVA) was measured using a plumb line from the center of the C7 vertebra (C7PL) to the posterosuperior corner of the S1 upper endplate (UEP). Thoracic kyphosis (TK) was measured between the UEP of T5 and the lower endplate (LEP) of T12, while LL was measured between the T12 LEP and the S1 UEP. LL was also divided into two components : ULL, between the T12 LEP and the L4 UEP; and LLL, between the L4 UEP and the S1 UEP. SS was defined as the angle between the horizontal line and the line along the sacral endplate, while PI represented the angle formed by a line connecting the center of the sacral endplate to the femoral head axis. PT was measured as the angle between the line from the center of the sacral endplate to the bicoxofemoral axis and the C7PL. These sagittal distance and angular parameters were measured by an orthopedic spine surgeon and a spinal neurosurgeon.

The upper lumbar distribution index (ULDI) was calculated as the ratio of ULL to total LL multiplied by 100, while the LLL distribution index, or LDI, was computed as the ratio of LLL to LL multiplied by 100. These indices quantify the proportion of lordosis in the upper and lower arcs, respectively, relative to the entire lumbar curve. The theoretical Roussouly type was determined using methodology previously described by Pizones et al. [20]. Based on their PI values, participants were classified into four theoretical Roussouly types : type 1, PI less than 45° with the apex of the LL below the L4 vertebral body; type 2, PI less than 45° with the apex of the LL above the L4-L5 disc space; type 3, 45°≤ PI <60°; and type 4, PI ≥60°.

Intraclass correlation coefficients (ICCs) were calculated to assess the reliability of the measurements, with the following benchmarks : perfect reliability at 1.0, excellent reliability between 0.8 and 0.99, substantial reliability between 0.6 and 0.79, and poor reliability at less than 0.59. The correlations among various sagittal parameters were examined using Pearson correlation coefficients. Pearson correlation and simple linear regression analyses were employed to explore the relationships between these parameters. The mean value for each parameter was calculated according to the PI subgroup (PI <40°, 40°≤ PI <50°, 50°≤ PI <60°, and 60°≤ PI) and the theoretical Roussouly type. A p-value of less than 0.05 was considered to indicate statistical significance. Statistical analyses were performed using SPSS version 22.0 (IBM Corp., Armonk, NY, USA).

The final cohort for analysis comprised 150 adult male populations, with an average age of 64.1±6.4 years. The participants had a mean height of 167.0±5.5 cm, a weight of 67.3±9.8 kg, and a body mass index of 24.1±3.1 kg/m² (Table 1). These baseline characteristics did not differ significantly among the subgroups categorized by PI and Roussouly type. All ICCs exceeded 0.8 according to the method of Shrout and Fleiss, demonstrating excellent reliability between measurements.

The sagittal parameters in this study were as follows. The mean C7 SVA was -0.2±2.8, with 133 instances of C7 SVA ≤4 cm and 17 instances where 4< SVA ≤9 cm. Pelvic parameters included PI of 48.6°±8.6°, SS of 37.3°±6.8°, and PT of 11.3°±6.4°. The sample included 139 participants with PT <20° and 11 participants with 20°< PT ≤30°. TK was 30.0°±8.8°, and LL was 57.5°±9.0°. Within LL, ULL and LLL were 17.9°±8.0° and 39.7°±6.8°, respectively. The ULDI and LDI were 30.4%±11.7% and 69.7%±11.7%, respectively. The differences between PI and LL (PI-LL), PI and LLL (PI-LLL), and PI and ULL (PI-ULL) were -8.9°±8.0°, 9.0°±9.3°, and 30.4°±8.9°, respectively. We observed 148 instances of PI-LL ≤10° and remaining two individuals in which 10°< PI-LL ≤20°. The results of the correlation analysis are presented in Table 2. PI was positively correlated with PT, LL, ULL, LLL, PI-LL, PI-ULL, PI-LLL, and ULDI and negatively correlated with LDI. PT was not significantly related to LL. Furthermore, PT tended to increase with ULL, PI-LL, PI-ULL, PI-LLL, and ULDI and decrease with LLL and LDI. The results of simple linear regression analyses between PT and several parameters are shown in Table 3. All variables except LL were linearly correlated with PT. Among the variables, PI-LL had the highest regression coefficient (β). However, the coefficient of determination (R²) for PI-LLL was as high as 0.554, indicating the accuracy of this method for predicting PT.

As indicated in Table 4, participants were divided into four subgroups based on PI : PI <40°, 40°≤ PI <50°, 50°≤ PI <60°, and 60°≤ PI. We observed changes in various parameters as PI increased. Specifically, as PI rose from low (PI <40°) to high (60°≤PI), the values of PT, LL, ULL, PI-LL, PI-ULL, and PI-LLL increased. Additionally, with an increase in PI, ULDI significantly increased from 25.9%±12.7% to 38.9%±7.1%, while LDI decreased from 74.1%±12.7% to 61.1%±7.1% (Fig. 1). However, mean LLL did not increase beyond 41.7°±6.9° (ranged from 23° to 58°), regardless of PI.

Similarly, as the Roussouly type progressed from type 1 (PI <45°) to type 4 (PI ≥60°), increases were observed in PI, PT, LL, ULL, PI-LLL, and ULDI. The LDIs corresponding to Roussouly type were 81.0%±10.7%, 65.3%±11.0%, 68.0%±10.6%, and 62.6%±8.2%, respectively (Fig. 2). Additionally, as Roussouly type increased, ULDI rose significantly from 19.0%±10.5% to 37.4%±7.7%, while LDI decreased from 81.0%±10.7% to 62.6%±8.2% (Table 5). The LDIs of Roussouly types 1 and 4 were significantly higher and lower, respectively, than those of types 2 and 3 (p<0.001). However, regardless of Roussouly type, mean LLL did not exceed 41.9°±6.6° (ranged from 23° to 58°).

Several studies have identified LL as a key factor in spinal sagittal balance. Initially, research focused on the relationship between LL and PI or SS. Subsequently, studies have explored morphological classification models of LL [4,10,11,16-18,21,23]. Despite this, detailed research on the components of LL—specifically ULL and LLL—remains scarce, although Yilgor and colleagues [26,27] have discussed LLL and used relative LL and LDI to predict postoperative construct failure. In this study, we further investigated parameters potentially associated with ULL or LLL, the constituents of LL. We found that as PI and Roussouly type increase, the contribution of ULL to LL also increases, up to 38.9%. Conversely, LLL substantially impacts LL in patients with a low PI and those classified as Roussouly type 1.

PT is a measure of the extent to which the pelvis tilts backward. Essentially, as LL decreases, PT increases to prevent inclination of the trunk [9]. Furthermore, retroversion of the pelvis leads to hip extension as a compensatory action to balance the body’s center of gravity. When this compensatory capacity is maximized, knee flexion serves as an additional compensatory mechanism [1,3,14,15]. Consequently, if pelvic retroversion occurs, the energy expenditure during gait increases, while the gait distance is shortened.

Few studies have attempted to clarify the relationship between PT and other parameters. While some researches suggest that PT is directly related to PI and that health-related quality of life decreases as PT increases in pathological situations [1,2,8,13,16,17]. Moreover, a previous report demonstrated that changes in PT were associated with changes in LL, indicating that alterations in LL following spine surgery contribute to determining the value of PT [18]. However, the present study found no such relationship between LL and PT. Nevertheless, the relationship between PI-LLL and PT displayed the highest coefficient of determination in a simple linear regression analysis, suggesting that PT is significantly related to LLL according to PI (Table 3). The existence of this relationship in normal asymptomatic populations suggests that greater emphasis should be placed on the correction of LLL when performing lower lumbar fusion under pathological conditions.

In the present study, we established normative values for ULL, LLL, ULDI, and LDI based on PI and Roussouly type. We observed that as PI increased, the mean ULDI rose from 25.9%±12.7% to 38.9%±7.1%, while the mean LDI decreased from 74.1%±12.7% to 61.1%±7.1%. In spinal realignment surgery, surgeons must consider restoring the normative LDI for each patient according to their PI and Roussouly type. Nonetheless, our findings indicate that mean value of LLL did not exceed around 42° (ranged from 23° to 58°), regardless of these individual characteristics. It means that the LLL does not increase unlimitedly even in patients with very high PI or Roussouly type 4. Therefore, an appropriate degree of ULL should be added for patients with a high PI or those classified as Roussouly types 2, 3, or 4. Additionally, we found that LDI represents a minimum of 61.1% of the total LL value. When performing spinal fusion, it is thus important to ensure that LDI continues to account for above 61% of LL. Based on notable previous research regarding GAP score, the range for LDI corresponding to alignment is between 50% and 80% [12,13]. The authors also suggested that an LDI exceeding 80% represents a risk factor for mechanical failure following spinal realignment surgery. However, our data revealed that the mean LDI was 81.0%±10.7% in asymptomatic individuals classified as Roussouly type 1. Moreover, we have frequently observed excellent clinical- and radiographic outcomes without any mechanical failure in Roussouly type 1 patients having exceeding 80% LDI after surgery (Fig. 3). Consequently, we propose that the optimal LDI and ULL should be customized based on the individual’s PI and Roussouly type.

This study had several limitations. At first, the study sample was restricted to men due to the demographics of the public veterans’ hospital where the research was conducted. Consequently, the findings may not be directly applicable to females or to younger individuals in terms of sex and lifestyle. Given that spinopelvic alignment and pelvic morphology can differ between males and females, this should be noted as a potential constraint on generalizability. Secondly, the study’s cross-sectional design can restrict its ability to assess changes in spinal alignment or lordosis distribution over time. Longitudinal data would be valuable in understanding how these parameters evolve with aging or how they might relate to the onset of degenerative spinal conditions. In addition, the participants lacked age diversity, with most being of old age. While this reflects the typical patient demographic for spinal disease and is thus relevant for surgical applications and research in elderly patients, we recommend investigating the distributions of ULL, LLL, and LDI values across a broader age range. By exclusively focusing on asymptomatic individuals, the study may overlook subtle or early biomechanical alterations present in those with mild or emerging symptoms. These transitional changes between normal aging and pathological states could be crucial in understanding risk factors for spinal imbalance or surgical failure. Including mildly symptomatic subjects in the future research might help bridge the gap between normative anatomy and clinically relevant spinal deformities. Lastly, this study was fundamentally an “observational” study designed to estimate the average spinopelvic parameters in asymptomatic individuals. Thus, the present data cannot determine the optimal target angle for surgery. Future study by surgical series can elucidate the optimal target angle more clearly.