Impact on spino-pelvic alignment by total knee arthroplasty to correct knee flexion deformity in osteoarthritis patients: a retrospective cohort study
Introduction
Knee osteoarthritis (KOA) is a common degenerative joint disorder. Its clinical characteristics include knee pain and restricted mobility, with advanced stages presenting as knee flexion and varus or valgus deformities, as well as associated spinal issues such as lumbar and back pain, leading to various long-term outcomes, including physical limitation, functional disability, psychological stress, and impacting the quality of life (1). The diagnosis of KOA depends on the clinical physical examination and imaging studies, with necessary laboratory tests to rule out other causes (such as rheumatoid factor to exclude rheumatoid arthritis). Recently, Karpinski reported that a novel approach, vibroacoustic processes, could provide a non-invasive method to facilitate the diagnosis of KOA (2). Pathologically, KOA is characterized by the progressive breakdown of articular cartilage, changes in subchondral bone, and synovial inflammation. While treatments to directly repair articular cartilage are still under development, the total knee arthroplasty (TKA), whether it is cemented or cementless, is the current widely accepted surgical intervention for KOA, aiming to alleviate knee pain, restore the knee’s physiological morphology and joint function, and improve the patient’s quality of life (3,4). Although most patients experience symptomatic improvement following TKA, some could have knee flexion deformity recurrence or worsening spinal problems, indicating an interrelationship between the knee joint and the spine and pelvis (5-7). The human body can maintain an upright posture and walk with minimal energy expenditure due to a delicate balance and interplay between knee joint flexion and extension, as well as appropriate spino-pelvic alignment. TKA can alter knee flexion contraction, which in turn leads to corresponding changes in spino-pelvic sagittal alignment. Unfortunately, physicians often focus on the knee joint and rarely consider its impact on spino-pelvic alignment.
Proper alignment of the spine, pelvis, and lower limbs is vital for maintaining a stable standing posture and gait. Abnormalities in spino-pelvic alignment can lead to knee flexion deformity and vice versa (5-9). For example, valgus knees can result in pelvic drop and contralateral spinal curve to maintain head balance (10). Varus knees may induce hip abduction and pelvic tilting (PT), which can influence lumbar scoliosis (11). Knee flexion contractures can cause anterior PT and lumbar lordosis (LL) (12). The biomechanical study found that postoperative knee varus or valgus deformity can cause changes in stress distribution within the knee joint, potentially contributing to functional disability (13). Offierski et al. first proposed the concept of “hip-spine syndrome”, suggesting a correlation between hip disorders and lumbar pain (14). Tsuji et al. and Murata et al. introduced the concept of “knee-spine syndrome”, suggesting that degenerative changes in the knee could influence the lumbar spine (15,16). Research focusing on the impact of TKA on spino-pelvic alignment, particularly in correcting knee flexion deformity, is scant, with existing studies primarily concentrating on sagittal rather than coronal alignment (5,6,17). Puthiyapura et al. observed no significant changes in spino-pelvic parameters after TKA and that the degree of knee flexion angle (KFA) correction did not significantly affect spino-pelvic parameters (5). Lee et al. found that sacral slope (SS) increased in patients with KFA correction ≥10°, with no significant changes in other spino-pelvic parameters after TKA (17). Innocenti et al. studied the effects of different types of spino-pelvic alignments on knee flexion contracture and found that type II B spino-pelvic imbalance was an independent risk factor for knee flexion contracture after TKA (18).
The low-dose biplanar X-ray device (EOS) whole-body X-ray imaging system, a pioneering technology in scanning, evaluates overall body alignment with significantly lower radiation doses than conventional X-ray systems. This technology captures weight-bearing, undistorted, full-body images in a single shot, eliminating image stitching. It provides comprehensive data on the alignment of the spine, pelvis, and lower limbs and is increasingly used in clinical settings (6,7,19). In patients undergoing TKA, the EOS imaging system can be applied for preoperative planning and postoperative evaluations (6). However, a detailed and comprehensive EOS analysis of the impact of TKA on spino-pelvic alignments following TKA is still desired.
This study aimed to employ the EOS X-ray imaging system to assess the effects of TKA on both sagittal and coronal spino-pelvic alignments in patients with KOA undergoing correction for knee flexion deformity. We hypothesized that specific sagittal and coronal alignments of the spine and pelvis would change following correction of knee flexion deformity, and that the correction of knee flexion deformity could have a significant influence on the sagittal spino-pelvic alignments. We present this article in accordance with the STROBE reporting checklist (available at https://qims.amegroups.com/article/view/10.21037/qims-2025-898/rc).
Methods
Study design and participant selection
A retrospective cohort study was conducted on patients who underwent TKA for KOA from April 2023 to May 2024. This retrospective study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments and was approved by the Ethics Committee of Baoding No. 1 Central Hospital (No. 2024-201). The written informed consent was waived due to the retrospective study design.
Inclusion criteria were patients with (I) TKA due to varus-type KOA; (II) complete records for 6-month postoperative follow-up outcomes; and (III) whole-body EOS imaging captured within 1 week before surgery and at 6 months postoperatively. Exclusion criteria were patients with (I) valgus-type KOA; (II) spinal, pelvic, or other lower limb surgery before or after TKA; (III) a history of traumatic fractures of the spine, pelvis, or lower limbs; or (IV) spinal infections or spondylolisthesis.
Patient demographics, including age, sex, and BMI, were recorded.
Image acquisition
All patients underwent whole-body EOS imaging within 1 week before surgery and at the 6-month postoperative follow-up. The imaging was performed using the EOS dual-plane system (EOS Imaging, Paris, France), which captured weight-bearing, full-body anteroposterior and lateral X-ray images. Patients were positioned at the center of the floor inside the EOS imaging booth during the imaging process. They stood with feet together, slightly staggered (typically with the right foot slightly forward), toes and patellae oriented directly forward, and knees extended as much as possible. The upper limbs were flexed at approximately 90 degrees, with fists clenched at the level of the collarbones or cheeks. The scanning process, covering from head to feet, took 15–20 s with a fixed distance of 130 cm between the detector and the X-ray beam source, employing a tube voltage of 90 kV and a tube current of 250 mA. Both anteroposterior and lateral images were acquired simultaneously. These images were then transferred to the SterEOS workstation (V.1.6.6.8201, EOS Imaging, Paris, France), where two physicians, proficient in SterEOS operations, performed 3D modeling and measurements.
Radiological parameter measurement
Measurement of spino-pelvic sagittal parameters
In EOS lateral images, multiple spino-pelvic sagittal parameters were measured, including pelvic incidence (PI), PT, SS, thoracic kyphosis (TK), LL, sagittal vertical axis (SVA), spino-sacral angle (SSA), T1 spinopelvic inclination (T1SPI), and T9 spinopelvic inclination (T9SPI) before and after TKA (Figure 1A-1D). PI was defined as the angle between the line from the center of the femoral heads to the midpoint of the upper endplate of S1 and the perpendicular line from the midpoint of the S1 upper endplate (normal =45±15°). If the femoral heads were not overlapping, the midpoint of the line connecting the centers of the femoral heads was taken (6). SS was the angle between the upper endplate of S1 and the horizontal line (normal =37.7°±9.7°) (Figure 1A) (6). PT was the angle between the line from the center of the femoral heads to the midpoint of the upper endplate of S1 and the vertical axis through the center of the femoral heads (normal =12.6°±4.5°). If the femoral heads were not overlapping, the midpoint of the line connecting the centers of the femoral heads was taken (Figure 1A) (6). TK is measured as the angle between the upper endplate of T1 and the lower endplate of T12, with normal values ranging from 20 to 40 degrees. LL is defined by the angle between the upper endplate of L1 and the upper endplate of S1, typically averaging 54.6 degrees with a standard deviation of ±10 degrees (6). The SVA represents the horizontal distance between the plumb line passing through the center of C7 and the posterior superior corner of the S1 upper endplate. If the plumb line is anterior to the posterior edge of S1, it is recorded as a positive value (+), with a normal value being less than 5 cm (6,7).
T1SPI/T9SPI is the angle between the line from the center of T1 or T9 to the center of the femoral heads and the vertical axis through the center of the femoral heads. If the femoral heads do not overlap, the midpoint of the line connecting their centers is used. If the line from the center of T1 or T9 to the center of the femoral head is anterior to the vertical axis, the angle is recorded as a negative value (−) (20).
SSA is the angle formed by the line from the center of C7 to the midpoint of the upper endplate of S1 and the S1 upper endplate, with normal values around 133±8 degrees (9). The differences between preoperative and postoperative values of these parameters are calculated and represented as delta (Δ), for example, ΔSSA = pre-SSA − post-SSA.
Measurement of spino-pelvic coronal parameters
Two spino-pelvic coronal parameters were measured in EOS anteroposterior images, including PO and C7-central sacral line (C7-CSL) (Figure 2A,2B). PO was the angle between the line connecting the topmost points of the iliac crests and the horizontal line. If the left iliac crest was higher than the right, the angle was recorded as a positive value (+) (6). C7-CSL was the horizontal distance between the plumb line through the center of the C7 vertebra and the midline of the sacrum. If the plumb line was to the right of the sacral midline, it was recorded as a positive value (+) (6). The difference between preoperative and postoperative values of these parameters was represented as delta (Δ).
HKA
HKA was measured on EOS anteroposterior images. HKA was defined as the acute angle formed by the mechanical axes of the femur and tibia in the coronal plane, with varus angles recorded as positive values (+) (Figure 2) (6). ΔHKA was the difference between preoperative and postoperative values.
KFA
KFA was measured using lateral images from the EOS system. KFA is defined as the acute angle formed by the mechanical axes of the femur and tibia in the sagittal plane, with flexion angles noted as positive values (+) (Figure 3A) (7). ΔKFA was the difference between the preoperative and postoperative values.
Building on prior research, a ΔKFA >10° could affect other spino-pelvic measurements (5,17). Consequently, we selected a cutpoint for ΔKFA at 10° to categorize patients into two groups: Group A, with a ΔKFA of less than 10°, and Group B, with a ΔKFA of 10° or greater.
Two physicians re-measured the above radiological parameters after two months from the initial measurements to assess the inter-observer and intra-observer reliability. The mean values measured by the two physicians were used for subsequent statistical analysis.
Statistical analysis
Using the formula N=(Zα+Zβ)2×σ2/δ2 from a previous study, a sample size (N) of 34 was calculated with a paired difference (δ) of 4, difference standard deviation (σ) of 8, α of 0.05, and β of 0.10 (17). Statistical analyses were performed using SPSS Statistics 27.0 software (IBM Corp, Armonk, NY, USA). The Kolmogorov-Smirnov test was employed to evaluate the normality of distributions. Normally distributed metric data were expressed as mean ± standard deviation, and non-normally distributed data were presented as median (interquartile range). Missing data were handled using multiple imputation. Changes in parameters pre- and post-surgery were analyzed using paired t-tests or Wilcoxon signed-rank tests. Demographic parameters between subgroups were compared using independent sample t-tests or χ2 tests. The false discovery rate (FDR) was utilized to adjust for multiple comparisons. Correlations between ΔKFA and Δ spino-pelvic parameters were assessed using Pearson or Spearman correlation and multivariate linear regression analysis. A strong correlation was indicated by 0.5≤ | correlation coefficient (CC) | <1.0, a moderate correlation by 0.3≤ | CC | <0.5, and a weak correlation by | CC | <0.3 (21). A P<0.05 was considered statistically significant.
Inter-observer and intra-observer reliability were calculated using the intraclass correlation coefficient (ICC), assuming a two-way mixed-effect model and consistency type. Reliability was graded according to Fleiss, with values above 0.75 representing excellent reliability, 0.40–0.75 as moderate-to-good reliability, and below 0.40 as poor reliability (22).
Results
There were 70 patients, 26 males and 44 females, with an average age of 68.7±5.1 years (54–79 years). There were 35 cases involving the left knee and 35 involving the right knee. The average BMI was 27.5±3.7 kg/m2 (range, 22.2–37.5 kg/m2). The KFA values (7.0°±3.2°) after TKA decreased compared to those (15.4°±6.8°) before TKA (Figure 3B). Groups A and B had 39 and 31 patients, respectively (Figure 4).
No significant differences were present in age, sex, and BMI, but a significant difference in ΔKFA existed between Groups A and B (Table 1). The inter-observer and intra-observer ICCs for all radiological measurements demonstrated high reliability, with values of 0.860 [95% confidence interval (CI): 0.778–0.924] and 0.920 (95% CI: 0.845–0.958), respectively.
Table 1
| Characteristics | Group A (n=39) | Group B (n=31) | P |
|---|---|---|---|
| Age (years) | 67.8±4.6 | 69.9±5.7 | 0.171† |
| Sex (female) | 27 (69.0) | 21 (67.7) | 0.863‡ |
| BMI (kg/m2) | 26.6±2.8 | 28.9±4.5 | 0.056† |
| KFA (°) | |||
| Before TKA | 11.2±3.1 | 21.5±5.8 | <0.001† |
| After TKA | 5.9±3.0 | 8.4±2.7 | 0.004† |
| ΔKFA | 5.1±2.3 | 13.2±4.3 | <0.001† |
†, the statistical value from the independent sample t-test, with values presented as mean ± standard deviation; ‡, the statistical value from the χ2 test, with values presented as number (%). Δ, the difference between preoperative and postoperative values. Group A: ΔKFA <10°; Group B: ΔKFA ≥10°. BMI, body mass index; KFA, knee flexion angle; TKA, total knee arthroplasty.
Changes in spino-pelvic parameters post-TKA in all patients
Table 2 shows that sagittal parameter measurements, PT, SS, LL, SVA, SSA, T1SPI, and T9SPI, showed significant changes after TKA. Both measurements in the coronal parameters (PO and C7-CSL) did not show statistically significant changes
Table 2
| Parameter | Pre-TKA | Post-TKA | Δ | P |
|---|---|---|---|---|
| HKA (°) | 12.7±4.9 | 3.5±1.9 | 9.2±3.8 | <0.001† |
| PT (°) | 17.2±8.6 | 10.0±6.1 | 7.2±4.4 | <0.001† |
| PI (°) | 47.2±9.2 | 47.1±8.9 | 0.1±0.7 | 0.636† |
| SS (°) | 30.1±8.9 | 37.2±8.4 | −6.9±4.3 | <0.001† |
| TK (°) | 40.2±7.6 | 39.3±8.3 | 1.2±2.5 | 0.617† |
| LL (°) | 44.7±6.7 | 46.6±8.1 | −1.9±3.4 | <0.001† |
| SSA (°) | 122.1±6.4 | 119.4±7.2 | 2.7±3.5 | <0.001† |
| SVA (mm) | 15.4 (25.7) | 38.8 (21.6) | −27.2 (26.8) | <0.001‡ |
| T1SPI (°) | 3.2±4.8 | −2.6±4.4 | 5.8±2.1 | <0.001† |
| T9SPI (°) | 9.5±4.7 | 3.5±4.3 | 6.0±2.7 | <0.001† |
| PO (°) | −0.4 (2.7) | −0.2 (2.1) | −0.2 (2.3) | 0.526‡ |
| C7-CSL (mm) | 8.6 (18.5) | 5.7 (11.9) | 0.9 (4.5) | 0.063‡ |
†, the statistical value from the paired t-tests, with values presented as mean ± standard deviation; ‡, the statistical value from the Wilcoxon signed-rank tests, with values presented as median (interquartile range). Δ, the difference between preoperative and postoperative values. CSL, central sacral line; HKA, hip-knee-ankle; LL, lumbar lordosis; PI, pelvic incidence; PO, pelvic obliquity; PT, pelvic tilt; SS, sacral slope; SSA, spino sacral angle; SVA, sagittal vertical axis; T1SPI, T1 spinopelvic inclination; T9SPI, T9 spinopelvic inclination; TK, thoracic kyphosis; TKA, total knee arthroplasty.
Changes in spino-pelvic parameters post-TKA in Groups A and B
Table 3 details the spino-pelvic parameter changes after TKA for patients in Groups A and B. Both groups exhibited similar changes post-TKA, except for LL, which showed a significant increase in Group B but not in Group A. Table S1 shows that male and female patients had similar changes in all the spino-pelvic parameter measurements after TKA.
Table 3
| Parameter | Group A (n=39) | Group B (n=31) | |||||||
|---|---|---|---|---|---|---|---|---|---|
| Pre-TKA | Post-TKA | Δ | P | Pre-TKA | Post-TKA | Δ | P | ||
| HKA (°) | 10.6±4.1 | 3.0±1.9 | 7.6±2.5 | <0.001† | 15.5±4.6 | 4.1±1.7 | 11.4±3.6 | <0.001† | |
| PT (°) | 17.4±9.3 | 10.4±6.1 | 7.0±4.2 | <0.001† | 16.8±7.7 | 9.5±6.1 | 7.2±4.1 | <0.001† | |
| PI (°) | 47.9±9.9 | 47.7±9.6 | 0.2±0.6 | 0.068† | 46.1±8.3 | 46.3±8.0 | −0.2±0.7 | 0.175† | |
| SS (°) | 30.6±9.2 | 37.2±8.6 | −6.6±4.5 | <0.001† | 29.4±8.6 | 36.8±8.2 | −7.4±4.1 | <0.001† | |
| TK (°) | 40.4±6.5 | 39.4±7.5 | 1.0±2.4 | 0.087† | 39.7±9.1 | 38.6±9.3 | 1.2±2.0 | 0.077† | |
| LL (°) | 43.7±6.5 | 44.2±8.6 | −1.5±1.7 | 0.571† | 45.4±6.8 | 48.3±7.3 | −3.0±2.9 | <0.001† | |
| SSA (°) | 122.6±5.8 | 120.7±5.5 | 1.9±2.4 | 0.004† | 122.2±6.1 | 119.1±5.2 | 3.1±2.6 | 0.001† | |
| SVA (mm) | 14.6 (28.9) | 37.0 (18.5) | −30.4 (30.5) | <0.001‡ | 20.0 (33.1) | 40.7 (31.2) | −25.5 (23.9) | <0.001‡ | |
| T1SPI (°) | 4.3±4.5 | −1.0±4.3 | 5.3±2.0 | <0.001† | 3.7±4.5 | −1.4±4.4 | 5.1±2.1 | <0.001† | |
| T9SPI (°) | 10.8±4.5 | 3.8±4.1 | 7.0±2.7 | <0.001† | 7.6±4.4 | 3.1±4.5 | 4.5±2.0 | <0.001† | |
| PO (°) | −0.3 (3.0) | −0.1 (2.2) | −0.2 (2.1) | 0.662‡ | −0.8 (2.2) | −0.1 (2.0) | −0.7 (2.5) | 0.197‡ | |
| C7-CSL (mm) | 4.5 (10.2) | 3.7 (8.9) | 0.8 (3.2) | 0.078‡ | 4.7 (12.2) | 3.5 (9.3) | 1.2 (6.1) | 0.068‡ | |
†, the statistical value from the paired t-tests, with values presented as mean ± standard deviation; ‡, the statistical value from Wilcoxon signed-rank tests, with values presented as median (interquartile range). Δ, the difference between preoperative and postoperative values. Group A: Δ knee flexion angle <10°; Group B: Δ knee flexion angle ≥10°. CSL, central sacral line; HKA, hip-knee-ankle; LL, lumbar lordosis; PI, pelvic incidence; PO, pelvic obliquity; PT, pelvic tilt; SS, sacral slope; SSA, spino sacral angle; SVA, sagittal vertical axis; T1SPI, T1 spinopelvic inclination; T9SPI, T9 spinopelvic inclination; TK, thoracic kyphosis; TKA, total knee arthroplasty.
Correlation between ΔKFA and Δ spino-pelvic parameters
Pearson analysis indicated a negative correlation between ΔKFA and ΔT1SPI [P=0.019, Pearson correlation coefficient (PCC): −0.330] and ΔKFA and ΔT9SPI (P=0.004; PCC: −0.404). However, other parameters were not significantly correlated with ΔKFA (Table 4). Multivariate linear regression analysis revealed that ΔKFA was significantly associated with ΔT1SPI (β=−0.341, 95% CI: −0.741 to 0.059, P=0.037) and ΔT9SPI (β=−0.437, 95% CI: −0.962 to 0.089, P=0.011), indicating that TKA could correct the knee flexion deformity, with improved spino-pelvic sagittal alignment.
Table 4
| Parameter | CC | 95% CI | P |
|---|---|---|---|
| ΔPT (°) | 0.104 | −0.187 to 0.412 | 0.471 |
| ΔPI (°) | 0.197 | −0.459 to 0.118 | 0.170 |
| ΔSS (°) | −0.137 | −0.429 to 0.164 | 0.343 |
| ΔTK (°) | −0.081 | −0.365 to 0.214 | 0.578 |
| ΔLL (°) | 0.245 | 0.029 to 0.545 | 0.086 |
| ΔSSA (°) | −0.112 | −0.377 to 0.146 | 0.441 |
| ΔSVA (mm)† | 0.177 | −0.101 to 0.443 | 0.218 |
| ΔT1SPI (°) | −0.330 | −0.512 to 0.054 | 0.019 |
| ΔT9SPI (°) | −0.404 | −0.630 to −0.139 | 0.004 |
| ΔPO (°)† | −0.131 | −0.501 to 0.232 | 0.365 |
| ΔC7-CSL (mm)† | 0.156 | −0.139 to 0.432 | 0.278 |
†, the statistical value from Spearman analysis and the rest value from Pearson analysis. Δ, the difference between preoperative and postoperative values. CC, correlation coefficient; CI, confidence interval; CSL, central sacral line; KFA, knee flexion angle; LL, lumbar lordosis; PI, pelvic incidence; PT, pelvic tilt; PO, pelvic obliquity; SS, sacral slope; SSA, spino sacral angle; SVA, sagittal vertical axis; T1SPI, T1 spinopelvic inclination; T9SPI, T9 spinopelvic inclination; TK, thoracic kyphosis.
Changes in HKA post-TKA
Significant decreases in the HKA alignment were observed following TKA, as outlined in Table 2. The average change in HKA was 9.2° across all patients, with specific changes of 7.6° in Group A and 11.4° in Group B (P<0.001, Table 3). Similar trends were noted among both male and female patients, as detailed in Table S1.
Discussion
This study applied the EOS whole-body X-ray imaging system to investigate the impact of knee flexion deformity correction by TKA on spino-pelvic sagittal and coronal alignment in patients with KOA. We found that TKA significantly improved knee flexion and varus deformities, with a subsequent reduction in PT, SSA, T1SPI, and T9SPI, and an increase in SS, LL, and SVA postoperatively, with no significant changes observed in TK, PI, PO, and C7-CSL. Similar results were observed in Groups A and B, except that the postoperative LL increase was more prominent in Group B. The change in KFA showed a significant correlation with the changes in T1SPI and T9SPI, but there was no sufficient evidence of correlation with other parameters. This study demonstrated that, following TKA, knee flexion contracture in KOA patients is corrected, resulting in compensatory changes in the sagittal alignment of the spine and pelvis, which alleviates the forward-leaning posture of the spine and pelvis. These changes could be beneficial for KOA patients, allowing them to maintain the stability of their body’s center of gravity with minimal energy expenditure when standing after TKA.
An imbalance in spinal sagittal alignment could lead to increased pelvic retroversion and knee flexion, whereas increased knee flexion could result in anterior tilting of the spine and pelvis. This compensatory mechanism allows the human body to maintain optimal balance with minimal energy expenditure (8,19,20,23). Few studies have reported the changes in spino-pelvic alignment after TKA, and most of them mainly focused on sagittal balance, whereas coronal balance was often missed (5,7,17). These studies utilized simple radiographic measurements of spino-pelvic sagittal parameters to evaluate the interrelation between the spine, pelvis, and knee joint, yielding divergent conclusions. Although plain radiographs offer the advantages of being convenient, quick, and cost-effective, localized radiographic assessments cannot simultaneously reflect compensatory responses from other body parts, which is insufficient for the research development of the knee-spine syndrome. The advent of the biplanar low-dose EOS whole-body X-ray imaging system has opened new research perspectives. The EOS imaging system can obtain complete, weight-bearing, frontal, and lateral images of the entire body in a single scan without stitching or amplification distortion, reducing radiation dose without compromising image quality. The system has SterEOS 2D and 3D workstations to post-process the acquired images (24,25). Several previous studies used EOS imaging to evaluate the relationship between spinal alignment and lower limb alignment. Zhang et al. assessed the correlation between coronal imbalance of degenerative scoliosis and lower limb parameters using EOS imaging (26). They found that the knee and ankle joints had more severe degeneration on the main curved side of patients. Fu et al. used EOS to analyze the relationship between spinal sagittal imbalance and KOA, reporting that the progression of KOA and low back pain contribute to the severity of sagittal spinal imbalance, with pelvic retroversion serving as its compensatory mechanism (19). However, to date, there have been few studies utilizing EOS whole-body X-ray images to assess the impact of TKA on spinal and pelvic alignment. Kim et al. found that the change of HKA was significantly correlated with ΔPO, and the change of KFA was significantly correlated with ΔPT and ΔSS (6). However, the study of Kim et al. had certain limitations. Their study only analyzed the effects of ΔHKA on the coronal alignment of the spine pelvis and ΔKFA on the sagittal alignment of the spine pelvis. However, it did not analyze the effects of ΔHKA on the sagittal alignment of the spine pelvis or ΔKFA on the coronal alignment of the spine pelvis.
Our study utilized EOS whole-body X-ray imaging to evaluate the impact of knee flexion deformity correction by TKA on spino-pelvic sagittal alignment in patients with KOA. The findings revealed that PT decreased and SS increased, whereas PI remained unchanged after TKA, aligning with the results of Kitagawa et al. (9). PI describes the anatomical configuration of pelvic sagittal alignment, which typically increases during growth, stabilizes in adulthood, and is not influenced by changes in imaging position or posture (27). In contrast, SS and PT are sensitive to variations in body position and posture (28). Correcting knee flexion deformities likely involves the interaction of the hamstring, quadriceps, trunk, and pelvic muscles, promoting hip extension and leading to increased SS and decreased PT (17). Unlike the findings of Kitagawa et al. (9) and Kim et al. (7), our study observed an increase in LL post-TKA, particularly in patients with ΔKFA ≥10°, whereas changes were less pronounced in patients with ΔKFA <10°. This variation could be attributed to the lumbar spine’s greater range of motion and its capacity for more pronounced compensatory adjustments in response to alterations in KFAs. The observed differences might also be related to the limited sample size of our study, suggesting the need for further research with larger cohorts to validate these findings. These results showed that the change in lower limb alignment had a significant influence on adjacent movable segments, such as the pelvis and lumbar vertebrae (17).
Our study results also found that SVA increased, whereas T1SPI, T9SPI, and SSA decreased, following the TKA. The change in knee flexion deformity was significantly correlated with changes in T1SPI and T9SPI, but not with other parameters. SVA, T1SPI, T9SPI, and SSA are radiographic parameters used to assess the overall sagittal balance of the spino-pelvic alignment (9,29). SVA was correlated with balance function in patients with lower back pain (24), sagittal balance in patients with ankylosing spondylitis with thoracolumbar kyphosis (30), quality of life in patients with adult spinal deformity (31), postoperative outcomes in patients with degenerative lumbar spondylolisthesis (32), and postoperative functional assessment in patients with lumbar spinal stenosis (33). However, SVA’s accuracy can be compromised by patient positioning and compensatory adjustments in the lower limbs, making it less reliable for patients with severe spinal deformities (34). In contrast, T1SPI and T9SPI can provide greater precision in evaluating overall sagittal balance (35). The SSA, which integrates the C7 and SS, reflects the overall kyphosis of the spine (36). Our results showed that correcting knee flexion deformity improved the spino-pelvic sagittal alignment, which could benefit patients’ posture and gait after surgery. To our knowledge, this is the first study to demonstrate the relationship between KFA, TISPI, and T9SPI in patients with KOA undergoing TKA, providing a valuable reference for future research.
Our study also evaluated the impact of knee flexion deformity changes on the overall coronal alignment of the spino-pelvic structure. C7-CSL is a parameter indicating the relationship between the cervical spine and the sacrum and is widely used to assess the spino-pelvic coronal balance (24). Our study showed no statistically significant changes in C7-CSL and PO after TKA. We speculated that the corrected knee flexion deformity by TKA might have no significant effect on the overall coronal alignment of the spino-pelvis, or it might be related to the short postoperative follow-up time. We will continue to observe in the follow-up studies.
There are several limitations in this study. First, it only included patients with unilateral varus-type KOA undergoing TKA, introducing a selection bias. Because KOA typically involves both knee joints, we plan to expand our research to include patients with bilateral TKA in future studies. Second, the postoperative follow-up period was limited to 6 months, which may not fully capture the effects of TKA on spino-pelvic alignment. We intend to conduct longer-term follow-ups of 1 year or more to comprehensively assess changes in spino-pelvic alignments. Third, although the study standardized the imaging posture for patients, some may have experienced difficulty in maintaining the standard position due to lumbar or knee pain, potentially affecting the measurement results. Fourth, the small sample size in the current study may lack sufficient statistical power to detect specific potential changes or correlations in these radiographic parameters. The study analyzed imaging findings in patients from a local hospital. However, it lacked detailed information on comorbidities, activity level, and KOA duration, which could limit the generalizability of the research findings. Finally, we only analyzed the imaging changes following TKA, without addressing the clinical impact of these changes, such as quality of life, pain intensity, and functional improvements. Future studies should be performed to correlate the imaging changes with clinically meaningful improvements.
Conclusions
The knee flexion deformity correction by TKA in KOA patients may lead to improved spino-pelvic sagittal alignment. Following TKA, LL increased in KOA patients who had a knee flexion deformity correction of ≥10°. Furthermore, the KFA change was correlated with the changes in T1SPI and T9SPI in KOA patients after TKA. The forward-leaning posture of the spine and pelvis was alleviated, which was beneficial for KOA patients in maintaining the stability of their body’s center of gravity with minimal energy expenditure when standing after TKA. Therefore, clinicians should be aware of potential changes in spino-pelvic alignment when correcting lower limb alignment. A comprehensive preoperative evaluation may enhance surgical outcomes and prognosis.
Acknowledgments
None.
Footnote
Reporting Checklist: The authors have completed the STROBE reporting checklist. Available at https://qims.amegroups.com/article/view/10.21037/qims-2025-898/rc
Data Sharing Statement: Available at https://qims.amegroups.com/article/view/10.21037/qims-2025-898/dss
Funding: None.
Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://qims.amegroups.com/article/view/10.21037/qims-2025-898/coif). The authors have no conflicts of interest to declare.
Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. This study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. The study was approved by the Ethics Committee of Baoding No. 1 Central Hospital (No. 2024-201). The written informed consent was waived due to the retrospective study design.
Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.
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