Dynamic Radiography Analysis of Functional Safe Zone During Sit-to-Stand Motion After Total Hip Arthroplasty and Its Association with Preoperative Factors

Abstract

Background:

Functional safe zones for implant positioning in total hip arthroplasty (THA) have primarily been evaluated in static postures. However, no studies have examined sit-to-stand movements, which carry a high risk of posterior dislocation. In this study, we recorded sit-to-stand motion post-THA using a dynamic x-ray system to analyze pelvic tilt and hip flexion angles during sitting and standing, including sex differences and the influence of sitting posture, and identify preoperative factors associated with standing alignment.

Methods:

We retrospectively evaluated 118 patients 6 months post-THA. Sequential lateral radiographs were used to capture maximal anterior pelvic tilt during standing. The sacral slope (SS) and pelvic–femoral angle (PFA) were measured; changes from the sitting position were calculated. Sex differences and correlations with sitting posture were assessed, and regression analyses identified predictors of standing SS and PFA.

Results:

The mean ΔSS was 29.6° ± 8.7°, and the mean ΔPFA was 24.8° ± 8.7°. ΔSS showed no sex difference (p = 0.331), whereas ΔPFA was larger in women (25.8° ± 8.6°) than in men (20.5° ± 8.3°, p = 0.011). ΔSS correlated negatively with sitting SS (r = −0.550, p < 0.001), and ΔPFA correlated positively with sitting PFA (r = 0.478, p < 0.001). Age, preoperative hip flexion, and pelvic incidence–lumbar lordosis (PI-LL) moderately predicted standing PFA.

Conclusion:

During sit-to-stand motions post-THA, the pelvis tilted anteriorly by approximately 30°, and the hip flexed by approximately 25°, influenced by sex and sitting posture. Standing hip flexion was affected by age, preoperative hip flexion angle, and PI-LL.

Level of Evidence:

Diagnostic Level III. See Instructions for Authors for a complete description of levels of evidence.

Introduction

Total hip arthroplasty (THA) provides excellent pain relief and functional restoration; however, postoperative dislocation remains problematic. Conventional safe ranges for cup orientation, such as the Lewinnek safe zone¹ and formula-based positions to minimize impingement, have been proposed^2,3. However, these approaches disregard individual postural and motion differences, and dislocations can occur even at “optimal” angles⁴. Recently, attention has shifted toward developing functional safe zones that reflect spinopelvic and hip motion^5-8.

Evaluation of spinal alignment and pelvic tilt in standing and sitting positions showed that limited lumbosacral motion increases the demand on the hip joint, thereby elevating impingement risk⁹. Vigdorchik et al.¹⁰ proposed determining cup angles based on the spinopelvic alignment and the degree of pelvic tilt change from the standing to sitting position. Most studies rely on static assessments of standing and sitting postures. Meanwhile, dislocation typically occurs during transitions, and posterior dislocation frequently occurs during sit-to-stand¹¹ because, when rising from sitting, the pelvis tilts further anteriorly and the hip flexes more^12,13. Therefore, evaluating dynamic motion from sitting to standing may help identify safer implant positioning. However, few studies have quantitatively assessed coordinated pelvic–hip motion during sit-to-stand using imaging, and the influence of sex or sitting posture on these postural changes remains unclear.

Dynamic Digital Radiography (DDR) is a new high-frequency sequential x-ray imaging technology enabling low-dose assessment of joint motion during activity^14,15. This system enables imaging-based evaluation of sit-to-stand dynamics. Because preoperative hip deformity may affect sit-to-stand motion, post-THA is suitable. Clarifying the relationship between the standing-up posture and preoperative factors may support preoperative planning.

We used DDR to evaluate pelvic and hip dynamics from sitting to standing post-THA. We aimed to quantify pelvic tilt and hip flexion angles and their changes, including sex and sitting posture effects, and determine the associations between pelvic tilt and hip flexion angles during standing up and preoperative factors. We hypothesized that sit-to-stand kinematics differ according to sex and sitting posture and are related to preoperative parameters.

Materials and Methods

Patient Selection

This retrospective, single-center study was approved by the institutional review board (Approval Number: 6422). Given its retrospective design, informed consent was obtained through an opt-out form on the website.

Between November 2019 and May 2024, Japanese patients who underwent unilateral THA and DDR of the sit-to-stand motion at 6 months postoperatively were included. At our institution, THA is routinely performed using a supine anterolateral approach. Cases requiring bone grafting or with severe motion limitation (hip flexion ≤ 50°) were treated using other approaches, but only supine anterolateral cases were included to eliminate the influence of approach-related soft tissue differences. Exclusion criteria were prior contralateral THA, contralateral hip osteoarthritis (Kellgren–Lawrence grade ≥ 3), advanced lower extremity degenerative disease or surgery, scoliosis (Cobb angle ≥ 10°), or lumbar fusion history. Of 305 THAs performed during the study period, 187 were excluded, leaving 118 hips for analysis (21 men and 97 women, mean age: 67.9 ± 10.1 years), with diagnoses of osteoarthritis (n = 109) and osteonecrosis (n = 9) (Supplementary Figure 1, Supplementary Table 1).

Surgical Procedure and Rehabilitation

All surgeries were performed under general anesthesia using a supine anterolateral approach and a uniform cementless system (Pinnacle cup and Tri-Lock stem; DePuy Synthes), and full weight-bearing walking was initiated on postoperative day 1. Surgical procedure and rehabilitation details are presented in Supplementary Methods.

Preoperative and Postoperative Evaluations

Preoperatively, the hip flexion range of motion was measured in the supine position, and lumbar lordosis (LL) and pelvic incidence (PI) were measured on lateral standing spinopelvic radiographs (Supplementary Figure 2A). Spinopelvic alignment was expressed as PI-LL. Lumbar mobility was defined as the change in LL between maximal extension and flexion (Supplementary Figure 2B). At 6 months after THA, sit-to-stand motion was recorded using DDR (Fig. 1). From sequential DDR images, we selected the image just before rising (sitting) and the image showing the maximal anterior pelvic tilt (standing up) to measure the sacral slope (SS) and pelvis–femoral angle (PFA) (Fig. 2). Detailed preoperative and postoperative evaluations are provided in Supplementary Methods.

Fig. 1.

Fig. 1-A Lateral imaging of sit-to-stand using a dynamic digital radiography system. Fig. 1-B Sequential lateral pelvic radiographs during sit-to-stand from a chair.

Fig. 2.

SS and PFA in the standing-up when the pelvis is at maximum anterior tilt and the hip joint is at maximum flexion. PFA, pelvic–femoral angle, and SS = sacral slope.

Two orthopedic surgeons (H.S. and D.K.) independently measured preoperative spinopelvic and standing-up radiographic parameters twice, 2 weeks apart, to assess intraobserver and interobserver reliability using intraclass correlation coefficients. Preoperative spinopelvic alignment showed almost perfect reliability, whereas SS and PFA during standing demonstrated substantial to almost perfect reliability (Supplementary Table 2).

Statistical Analysis

Sex differences in demographics, preoperative range of motion, spinopelvic alignment, and SS/PFA parameters were analyzed using independent t-tests. Pearson’s correlations were used to assess the relationships between changes in SS, PFA, and sitting values. To identify preoperative factors affecting standing-up posture, univariable regression analyses were performed with SS and PFA at standing-up as dependent variables and 7 independent factors: age, sex, body mass index (BMI), diagnosis, preoperative hip flexion, PI-LL, and lumbar mobility. Variables with p < 0.30 were entered into multivariable models, with this relatively liberal threshold chosen to avoid excluding potentially relevant predictors at the screening stage. Sex and diagnosis are coded as dummy variables. Sample size was estimated using G*Power 3.1.0 under a multiple regression model (7 predictors, f² = 0.15, α = 0.05, power = 0.80) for the primary analyses, requiring at least 103 cases. The analyses of sex differences in SS and PFA were therefore considered exploratory within this clinically representative cohort. Statistical analyses were conducted using SPSS v29.0 (IBM), with p < 0.05 considered significant.

Results

Up to the 6-month evaluation, no patient experienced postoperative hip dislocation or clinically apparent instability. Clinical/radiographic evaluations are summarized in Table I. PI was significantly higher in women than in men. Post-THA, the mean changes during sit-to-stand were 29.6° ± 8.7° (mean ± SD) for SS and 24.8° ± 8.7° for PFA. There was no sex difference in ΔSS; however, ΔPFA was significantly greater in women than in men. PFA values were also greater in females at sitting and standing positions. ΔSS correlated negatively with sitting SS (r = −0.550, p < 0.001), and ΔPFA correlated positively with sitting PFA (r = 0.478, p < 0.001) (Supplementary Figure 3).

TABLE I.

Patient Clinical/Radiographic Evaluation

Variable	Value
	Total (n = 118)	Female (n = 97)	Male (n = 21)	p
Preoperative hip flexion (range, angle)	85.1 ± 12.6 (60 to 120)	84.6 ± 13.4 (60 to 120)	87.1 ± 7.9 (70 to 100)	0.422
ΔLL (range, angle)	26.0 ± 14.0 (−2 to 76)	25.1 ± 14.5 (−2 to 76)	30.1 ± 11.2 (12 to 50)	0.141
PI (range, angle)	49.0 ± 10.5 (21 to 83)	50.3 ± 10.6 (21 to 83)	43.0 ± 8.0 (24 to 53)	0.003
LL (range, angle)	40.9 ± 14.7 (−15 to 75)	41.0 ± 15.5 (−15 to 75)	40.6 ± 10.7 (25 to 58)	0.903
PI-LL (range, angle)	8.0 ± 16.5 (−43 to 74)	9.2 ± 17.3 (−43 to 74)	2.4 ± 10.6 (−20 to 21)	0.085
Before standing up SS (range, angle)	16.2 ± 11.4 (−20 to 48)	16.2 ± 11.5 (−20 to 48)	16.2 ± 11.2 (−3 to 42)	0.997
Standing up SS (range, angle)	45.9 ± 11.2 (5 to 83)	46.2 ± 11.4 (5 to 83)	44.2 ± 9.9 (26 to 66)	0.452
ΔSS (range, angle)	29.6 ± 8.7 (10 to 56)	30.0 ± 8.6 (11 to 56)	27.9 ± 9.0 (10 to 44)	0.331
Before standing up PFA (range, angle)	136.6 ± 17.3 (90 to 187)	139.4 ± 16.8 (90 to 187)	123.9 ± 13.5 (91 to 150)	<0.001
Standing up PFA (range, angle)	111.7 ± 15.2 (72 to 166)	113.6 ± 15.5 (72 to 166)	103.4 ± 10.3 (84 to 122)	0.004
ΔPFA (range, angle)	24.8 ± 8.6 (6 to 51)	25.8 ± 8.6 (6 to 51)	20.5 ± 8.3 (7 to 36)	0.011

LL = lumber lordosis, PFA = pelvic-femoral angle, PI = pelvic incidence, and SS = sacral slope.

In regression analyses (Supplementary Table 3), BMI, lumbar mobility (ΔLL), and PI-LL had p < 0.30 for standing SS. All preoperative parameters had p < 0.30 for PFA. In multivariable analyses, PI-LL was an independent predictor of SS with low explanatory power, whereas age, preoperative hip flexion, and PI-LL were independent predictors of PFA with moderate determination (Table II).

TABLE II.

Multiple Linear Regression Analysis of Related Factors for Standing Up SS and Standing Up PFA

Factor	B	β	95.0% CI	p
Standing up SS
Intercept	47.8	—	45.662 to 50.018	<0.001
PI-LL, degrees	−0.251	−0.365	−0.371 to −0.131	<0.001
BMI	0.155	0.055	−0.507 to 0.818	0.641
ΔLL, degrees	−0.037	−0.046	−0.226 to 0.152	0.697
Standing up PFA
Intercept	111.7	—	91.746 to 109.445	<0.001
Age, years	0.255	0.164	0.033 to 0.478	0.025
Preoperative hip flexion, degrees	-0.257	−0.209	-0.429 to −0.085	0.004
PI-LL, degrees	0.568	0.613	0.436 to 0.700	<0.001
Sex (female = 0, male = 1)	−2.498	−0.063	−10.931 to 5.936	0.556
BMI	−0.209	−0.048	−0.961 to 0.543	0.581
Diagnosis (OA = 0, ION = 1)	0.848	0.015	−11.776 to 13.471	0.894
ΔLL, degrees	0.109	0.088	−0.101 to 0.318	0.304

B = unstandardized coefficient, β = standard coefficient, PFA = pelvic-femoral angle, SS = sacral slope, LL = lumber lordosis, PI = pelvic incidence, OA = osteoarthritis, and ION =0 idiopathic osteonecrosis.

Standing up SS: R² = 0.133, adjusted R² = 0.125.

Standing up PFA: R² = 0.461, adjusted R² = 0.446.

Discussion

This study demonstrated that during sit-to-stand motion post-THA, the pelvis tilted anteriorly by approximately 30°, and the hip flexed by approximately 25°. Women showed greater changes in hip flexion angle than men, with smaller hip flexion angles in sitting and standing positions; however, these findings should be interpreted cautiously because of the unbalanced sex distribution and structural differences between men and women (Table I). Sitting posture correlated with motion changes; patients with less anterior pelvic tilt and smaller hip flexion in the sitting position had larger increases in pelvic tilt and hip flexion during rising (Supplementary Figure 3). Age, preoperative hip flexion angle, and PI-LL independently predicted PFA during standing up, with moderate model fit (Table II).

Recent studies on THA stability have mainly assessed standing and sitting positions^16-20. Patients with a large PI-LL (flatback deformity) showed limited posterior pelvic tilt during sitting, predisposing them to anterior impingement²¹. Similarly, higher flatback prevalence and smaller changes were observed in cup anteversion in dislocation cases²². However, these studies focused only on the pelvic tilt and did not directly measure hip flexion or confirm impingement. In a study evaluating pelvic tilt and PFA, when the sum of cup anteversion and PFA in the sitting position was <151°, the functional safe zone was exceeded, increasing posterior dislocation risk²³. Innmann et al.²⁴ further emphasized assessing the deep-flexed seated position for patients with greater hip flexion. Using motion capture and finite element modeling, it was shown that standing up from a chair carries the highest dislocation risk among daily activities¹¹. Thus, assessing motion while standing up is crucial but difficult with conventional imaging. DDR offers higher frame rates and lower radiation doses, allowing clear dynamic imaging that is comparable to static views. Here, we captured clear sit-to-stand images, achieving acceptable reliability for the SS and PFA measurements.

Millington et al.²⁵ stated that the trunk must lean forward to shift its center of gravity before rising. Our study showed that this motion involved approximately 30° of anterior pelvic tilt and 25° of hip flexion (Table I). Compared with the previously observed 13° pelvic tilt and 12° hip flexion from sitting to deep flexion²⁴, our findings indicate that assessing motion during standing up is essential for evaluating anterior impingement post-THA. Although pelvic tilt did not differ by sex, women showed smaller hip flexion angles in sitting and standing but greater changes during rising (Table I). This may be influenced by the higher PI in women. For the same pelvic tilt in sitting, a higher PI shifts the hip center anteriorly, reducing the sitting hip flexion angle (Fig. 3) and thus increasing the change during rising (Supplementary Figure 3). Moon et al.²⁶ also reported a greater PI in women. These suggest that women rely more on hip movement while standing up, which is an important consideration for implant positioning.

Fig. 3.

Relationship between PI and PFA with the same sacral slope, a smaller PI results in a smaller PFA (Fig. 3-A), whereas a larger PI results in a larger PFA (Fig. 3-B). PFA = pelvic–femoral angle, and PI = pelvic incidence.

Considering the standing-up posture, where anterior impingement is most likely, determining the cup and stem angles may help reduce dislocation risk. In this study, age, preoperative hip flexion, and PI-LL were predictors of PFA during standing; older age, smaller hip flexion, and larger PI-LL were associated with greater PFA, suggesting a lower impingement risk (Table II). Older patients and those with limited hip flexion may have reduced soft tissue flexibility, which restricts postoperative hip motion. A large PI-LL, an indicator of spinopelvic sagittal balance on standing²⁷, reflects flat back alignment with posterior pelvic tilt, which can persist while sitting and standing, thereby increasing PFA. PI-LL affects pelvic and hip alignment during standing and sitting^28,29, suggesting that it also influences dynamic motion during sit-to-stand. ΔLL was significantly associated with the PFA during standing up in the univariable analysis but not in the multivariable model. Therefore, although lumbosacral motion may influence hip kinematics, it may not play a major independent role once overall spinopelvic alignment and hip factors are considered.

With CT-based 3D simulation enabling assessment of implant position and impingement, our regression model could help predict postoperative hip flexion during standing up and integrate it into preoperative planning. If the simulated impingement angle is smaller than the predicted flexion angle, impingement may occur postoperatively, warranting adjustment of the cup, stem anteversion, or offset. Incorporating these predicted values in future studies may help verify and prevent impingement while standing up and contribute to a more precise understanding of dynamic functional safe zones.

Limitations

First, surgeries were performed by multiple surgeons, although all were experienced and adhered to a standardized protocol to minimize variability in postoperative soft-tissue tension and stability. Second, some patients were excluded because dynamic imaging was limited to selected surgeons during the initial phase of system implementation. As it was not intentionally selective and the sample size was sufficient, selection bias was unlikely. Third, only a single dynamic recording was obtained per patient. However, as patients practiced beforehand and multiple recordings would have increased radiation exposure, a single low-dose recording was considered adequate. Fourth, dynamic assessments were performed only at 6 months, when recovery is generally stable in our practice; however, this clinically chosen time point may not reflect the full time course of THA mobility and stability, and potential longitudinal changes in spinopelvic parameters and hip motion could not be evaluated, warranting longer-term, multi-time-point follow-up studies. Fifth, although DDR enabled high-frame-rate dynamic imaging, it provided only lateral 2D views, precluding assessment of multiplanar motion such as axial rotation. Nevertheless, the main parameters are routinely assessed on lateral radiographs and are directly relevant to impingement and dislocation during sit-to-stand; therefore, a 2D lateral approach remains appropriate for our research aim. Sixth, the cohort mainly comprised patients with primary osteoarthritis and without severe spinal deformity, and detailed radiographic grading of spinal or hip degeneration was not performed. Furthermore, we did not systematically evaluate systemic comorbidities or implant-related factors. Future studies should incorporate these variables to provide a more comprehensive assessment of factors affecting postoperative kinematics. Finally, all participants were Japanese, and pelvic morphology, PI, and spinopelvic alignment may differ in other ethnic populations. In addition, the observed sex imbalance and baseline differences may have influenced the observed sex-related patterns in hip motion. Larger, more balanced, and ethnically diverse cohorts with adjusted analyses are required to confirm our exploratory sex-related and ethnicity-related findings. Taken together, these limitations substantially restrict causal inference and preclude a definitive description of functional, motion-based safe zones from this cohort alone. Therefore, our findings should be interpreted as hypothesis-generating and as a basis for future confirmatory studies.

Conclusion

During the sit-to-stand motion post-THA, the pelvis tilted anteriorly by approximately 30°, and the hip flexed by approximately 25°. Women showed greater hip motion changes than men, correlating with sitting posture. Hip flexion during standing was associated with age, preoperative hip flexion, and PI-LL. Incorporating these parameters into predictive models may enable individualized preoperative planning that anticipates postoperative standing-up posture. Dynamic imaging and analysis methods may help establish functional, motion-based safe zones for implant positioning post-THA; however, further validation in larger cohorts is warranted.

Appendix

Supporting material provided by the author is posted with the online version of this article as a data supplement at jbjs.org (http://links.lww.com/JBJSOA/B214, http://links.lww.com/JBJSOA/B215, http://links.lww.com/JBJSOA/B216, http://links.lww.com/JBJSOA/B217, http://links.lww.com/JBJSOA/B218, http://links.lww.com/JBJSOA/B219, http://links.lww.com/JBJSOA/B220). This content has not been copyedited or verified.