The 3-kilogram weight-lifting flexion radiograph: a new diagnostic method for segmental sagittal lumbar instability: a cross-sectional study

Article information

Asian Spine J. 2025;19(5):717-727

Publication date (electronic) : 2025 June 24

doi : https://doi.org/10.31616/asj.2025.0065

Koopong Siribumrungwong¹, Warunyoo Suttikadsanee², Waroot Pholsawatchai¹, Sorrawich Singhatanadgige², Thongchai Suntharapa²

¹Department of Orthopaedics, Chulabhorn International College of Medicine, Thammasat University, Pathum Thani, Thailand

²Department of Orthopaedics, Faculty of Medicine, Thammasat University, Pathum Thani, Thailand

Corresponding author: Waroot Pholsawatchai, Department of Orthopaedics, Chulabhorn International College of Medicine, Thammasat University, 99 Moo 18, Paholyothin Road, Klong Nueng, Klong Luang, Pathum Thani 12120, Thailand, Tel: +66-2-5644440-9, Fax: +66-2-5644118, E-mail: warootmountaindew@gmail.com

Received 2025 February 6; Revised 2025 March 8; Accepted 2025 March 9.

Abstract

GRAPHICAL ABSTRACT

Introduction

Chronic low back pain is a common condition among patients attending orthopedic outpatient departments, with a prevalence ranging from 50% to 70% among orthopedic patients [1]. The etiology of low back pain is multifactorial, with lumbar spinal instability being a significant contributor to chronic back pain [2,3]. Lumbar spinal instability affects 13%–30% of individuals with mechanical low back pain, with the L4/L5 segment being the most commonly involved level. Early detection of spinal instability is crucial, as it significantly impacts the treatment plan and clinical outcomes [4].

Numerous diagnostic tools have been proposed for identifying spinal instability, each with varying degrees of accuracy and reliability. Among these, flexion-extension radiographs are commonly used due to their simplicity, non-invasiveness, and cost-effectiveness. However, their utility is often limited by the lack of standardized protocols, resulting in inconsistencies in measurement techniques across different practitioners and institutions. Moreover, flexion-extension radiographs exhibit a low sensitivity for detecting dynamic instability, particularly in its early stages or in cases with subtle abnormalities [4–8].

Previous studies have established the diagnostic value of facet joint fluid on magnetic resonance imaging (MRI) in detecting lumbar instability. Cho et al. [4] identified the facet fluid sign (fluid >1 mm) as a reliable indicator, with 87.30% sensitivity, 87.10% specificity, and 87.23% accuracy [9,10]. Rihn et al. [9] found a strong correlation between facet fluid on MRI and sagittal instability on flexion-extension radiographs, reporting a positive predictive value of 82%. Furthermore, in the study by Hung et al. [11], facet fluid >1.45 mm had an area under the curve of 0.77, with sensitivity of 73.9% and specificity of 67.2%, reinforcing the role of MRI in diagnosing lumbar instability. The presence of facet fluid should raise suspicion for instability and prompt further evaluation with stress radiographs, such as standing lateral flexion-extension radiographs.

Ameet et al. explored the potential of weight-lifting radiographs as a diagnostic tool for spinal instability. In their study, applying a 4.55-kg weight during radiographic imaging resulted in an average anteroposterior (AP) translation of 5±0.3 mm in healthy individuals. While this method shows promise in identifying spinal instability, its sensitivity is limited by the overlap between the degree of AP translation observed in normal physiological conditions and pathological cases, increasing the risk of false-positive diagnoses. Nonetheless, the findings suggest that weight-lifting radiographs may enhance diagnostic sensitivity by amplifying instability in affected spinal segments and revealing subtle abnormalities [12,13].

Recognizing the limitations of a 4.55-kg weight, our study employed a 3-kg weight for radiographic evaluation. This decision was informed by ergonomic studies and guidelines, which recommend that acceptable lifting weights should not exceed 10% of an individual’s body weight [14–17]. The suitability of this weight was further corroborated using the ergonomic National Institute for Occupational Safety and Health (NIOSH) equations, confirming the safety and feasibility of lifting a 3-kg weight for most individuals [18].

This study aimed to investigate the diagnostic utility of 3-kg weight-lifting flexion radiographs in detecting segmental spinal instability, with a specific focus on the L4/L5 segment. The effectiveness and clinical applicability of this novel approach was evaluated in comparison to conventional radiographs and the MRI-based assessment of the “facet fluid” sign.

Materials and Methods

This was a cross-sectional study involving patients with chronic low back pain, recruited from Thammasat University Hospital between January 2020 and December 2021. The inclusion criteria comprised patients aged 35–75 years, with a body mass index (BMI) ranging from 18.5 to 25 kg/m², exhibiting symptoms suggestive of lumbar instability. Furthermore, all participants met the criteria for MRI evaluation and exhibited a positive facet fluid sign in at least one lumbar level.

Exclusion criteria were applied to ensure participant safety and result accuracy. Patients were excluded if they had a history of previous lumbar spine surgery, presented with red flag signs of low back pain, progressive neurological deficits, cauda equina syndrome, or conus medullaris syndrome. Additional exclusion criteria included muscle weakness in the arms or legs that would prevent weight-lifting tasks or standing for radiographic imaging, cognitive impairments hindering the ability to follow study procedures, or a history of recent back injury. Patients with conditions unrelated to orthopedic causes, such as urinary tract stones, gynecological disorders, or aneurysms, were also excluded.

This study was conducted in compliance with the principles enshrined in the Declaration of Helsinki. The study protocol was approved by the Research Ethics Committee of Thammasat University (registration number: MTU-EC-OT-6-099/63). Written informed consent was obtained from all participants.

Radiographic imaging, MRI imaging, and measurement techniques

Participants underwent lumbosacral (LS) spine radiographs in the lateral view for three positions: lateral flexion, lateral flexion with a 3-kg weight on their back, and lateral extension. All radiographs were obtained by verified radiologic technicians under the supervision of the author. The X-ray beam was positioned 1.5 meters from the participant. During lateral flexion radiographs (with and without weight), participants flexed to an angle close to 90° and held the position for 5 seconds before the radiograph was obtained. A 3-kg weight was placed at the T12 level during weighted flexion radiographs. For lateral extension radiographs, participants extended their back as much as possible (Fig. 1).

Fig. 1

Flexion position (A), flexion with 3-kg weight on the back position (B), and extension position (C).

All patients underwent LS spine MRI in the supine position. To facilitate the accumulation of fluid in the facet joints, potentially indicative of facet joint laxity, MRI technicians waited for 5 minutes before initiating the scan. A musculoskeletal radiologist performed the LS spine MRI assessments, identifying the facet fluid sign by assessing areas of high signal intensity within the facet joints. Facet fluid thickness was measured, and a threshold of ≥1 mm was considered indicative of lumbar spinal instability, consistent with established criteria [4].

Radiographs were independently reviewed by two certified orthopedic spine surgeons. Measurement techniques for translation and angular rotation were adapted from the study by Rathod et al. [3], while diagnostic criteria for segmental sagittal lumbar instability were based on the study by Cho et al. [4]. Radiographic assessments were conducted at the following five spinal levels: L1/2, L2/3, L3/4, L4/5, and L5/S1. Changes in translation and angular rotation in conventional flexion radiographs and 3-kg weight-lifting flexion radiographs were compared with those of conventional extension radiographs (Figs. 2, 3). Translation changes of ≥5 mm and angular rotation changes of ≥10° were considered indicative of segmental sagittal lumbar instability.

Fig. 2

Translation; flexion radiograph (A), 3-kg weight-lifting flexion radiograph (B), and extension radiograph (C). No translation occurred at any levels except anterior translation of L4 on L5 on flexion (6.92 mm) and on extension (5.96 mm). Hence, the sagittal translation is 6.92–5.96=0.96 mm. But with 3-kg weight, the anterior translation on flexion was 12.19 mm, so the sagittal translation is 12.19–5.96=6.23 mm.

Fig. 3

Angular rotation; flexion radiograph (A), 3-kg weight-lifting flexion radiograph (B), and extension radiograph (C). At L3/4 level, conventional radiography has angular rotation of −3°–(−15°)=12° but the 3-kg weight-lifting radiograph has angular rotation of 2°–(−15°)=17° and at L4/5 level, conventional radiography has angular rotation of −1°–(−5°)=4° but 3-kg weight-lifting radiograph has angular rotation of 3°–(−5°)=8°.

Measurements were obtained using the Picture Archiving and Communication System imaging system (SYNAPSE; Fujifilm Medical Systems, Lexington, MA, USA), with digital calibration to ensure accuracy.

Results

The study enrolled a total of 46 participants (37% male and 63% female). The mean age of participants was 54.9 years (range, 35–71 years) and the mean BMI was 22.5 kg/m² (range, 19.2–24.5 kg/m²) (Table 1). The prevalence of lumbar instability among the participants, including its distribution by spinal level and sex, is summarized in Table 2.

Table 1

Demographic data (N=46)

Table 2

Prevalence of lumbar instability of participants in this study with subdivision of disease prevalence by sex

Diagnostic performance and reliability at the L4/L5 level

At the L4/L5 level, the diagnostic performance of conventional flexion radiographs and 3-kg weight-lifting flexion radiographs was compared to the gold standard MRI. The conventional method yielded a sensitivity of 44.44%, specificity of 80.00%, PPV of 88.89%, NPV of 28.57%, and an overall accuracy of 52.17%. In contrast, the 3-kg method demonstrated significantly higher sensitivity (83.33%) while maintaining a specificity of 70.00%. Additionally, the 3-kg method achieved a PPV of 90.91%, an NPV of 53.85%, and an overall accuracy of 80.43%. These findings suggest that the 3-kg method is more effective in detecting true positives while maintaining acceptable specificity (Table 3).

Table 3

Comparison of diagnostic values between the conventional flexion radiographs and the 3-kg weight-lifting flexion radiographs

The McNemar test revealed significant differences between conventional method and MRI (p=0.00) as well as between conventional and 3-kg methods (p=0.00), highlighting the limited performance of conventional method compared to the other methods. However, no significant difference was observed between the 3-kg method and MRI (p=0.51), suggesting that the diagnostic performance of the 3-kg method closely aligns with that of MRI at this level (Table 4).

Table 4

Comparison between three tests (divided into three subgroups) using McNemar test

Reliability analysis using ICCs revealed excellent interobserver and intraobserver reliability for both conventional and 3-kg radiographs. Specifically, translation measurements yielded ICC values of 0.99 for both interobserver and intraobserver reliability in both methods. Similarly, angular rotation measurements also demonstrated ICC values of 0.99 for conventional and 3-kg methods, confirming high consistency and reproducibility in radiographic measurements (Table 5).

Table 5

Intraclass correlation coefficient showing interobserver and intraobserver reliability

Discussion

Mechanical low back pain caused by lumbar spinal instability is commonly encountered in orthopedic practice. Early diagnosis through appropriate screening tests can help physicians confidently diagnose lumbar spinal instability and make appropriate preoperative plans. Current diagnostic tools for lumbar spinal instability include dynamic lateral LS spine radiographs and facet fluid signs on MRI [5]. While lateral LS spine radiographs can diagnose spondylolisthesis, they have limited ability in detecting dynamic instability. Dynamic radiographs, such as lateral flexion-extension radiographs, can help identify dynamic instability, whereas LS spine MRI enables the diagnosis of lumbar instability by identifying facet fluid signs. In a study by Cho et al. [4], facet fluid sign exhibited a sensitivity of 87.30%, specificity of 87.10%, PPV of 93.22%, NPV of 77.14%, and accuracy of 87.23%.

The present study utilized facet fluid sign as the gold standard to evaluate and compare the diagnostic values of conventional and 3-kg methods. Given that lumbar spinal instability is a critical cause of mechanical low back pain, accurate diagnosis is vital for effective management and surgical planning. This study demonstrates that the 3-kg weight-lifting flexion radiograph offers enhanced sensitivity and accuracy in detecting lumbar spinal instability, particularly at the L3/4 and L4/5 levels.

This study enrolled 46 patients with chronic low back pain and a diagnosis of lumbar instability. The prevalence of lumbar instability was highest at L4/5 (76.09%), followed by L3/4 (54.35%) and L5/S1 (41.3%), consistent with previous findings [4,20]. Males and females showed similar prevalence at L2/3 and L3/4, but females had a higher prevalence at L4/5 and L5/S1, likely due to anatomical and biomechanical differences. These findings emphasize the need for targeted diagnostic approaches based on the level and patient demographics.

To investigate the impact of weight positioning, the 3-kg weight was placed farther from the L4/5 level compared to other lumbar levels, except for L5/S1, resulting in greater moment arm effects. This set-up simulates real-life scenarios, where the load exerted on the body during flexion or lifting creates stress on the L4/5 level more than that on the other segments due to the moment arm. Additionally, L4/5 is the most mobile segment of the lumbar spine, contributing to the higher prevalence of lumbar instability observed at this level in the study. Although the L5/S1 level exhibited the longest moment arm, the prevalence of instability at this level was lower than that at L4/5. This can be attributed to the anatomical position of the L5 vertebral body below the inter-crestal line and the stabilizing effects of spinopelvic ligaments. These biomechanical and anatomical factors help explain the distribution of lumbar instability across levels observed in this study.

A comparative analysis of diagnostic trends at the L3/4 and L4/5 levels revealed improved diagnostic values for the 3-kg weight-lifting flexion radiograph, particularly in terms of sensitivity, NPV, and accuracy. At the L4/5 level, the 3-kg weight-lifting flexion radiograph showed a 38.89% higher sensitivity, a 10% lower specificity, and a 28.6% higher accuracy than the conventional flexion radiograph. Similarly, at the L3/4 level, it showed 52% higher sensitivity, 14.29% lower specificity, and 21.74% higher accuracy. There were no significant differences (p>0.05) between the 3-kg weight-lifting flexion radiograph and MRI for these levels, suggesting that the 3-kg weight-lifting flexion radiograph may not be inferior to MRI for diagnosing lumbar spinal instability. Moreover, at these levels, the 3-kg weight-lifting flexion radiograph demonstrated a strong screening capability, with an NLR in the range of 0.1–0.2. However, its utility as a confirmation test was limited, with a PLR between 2 and 5.

At the L1/2 and L2/3 levels, the 3-kg weight-lifting flexion radiograph demonstrated higher sensitivity, albeit with a slightly reduced specificity, compared to conventional flexion radiographs. Subgroup analysis revealed no significant differences between conventional radiographs and MRI or between 3-kg weight-lifting flexion radiographs and MRI in this respect. However, the clinical applicability of these findings is limited due to the relatively low prevalence of lumbar instability at these levels. At the L1/2 level, the 3-kg weight-lifting flexion radiograph showed a PLR of >10, indicating a robust confirmation test, but its NLR of >0.2 classified it as a poor screening test. At the L2/3 level, both the 3-kg weight-lifting and conventional flexion radiographs showed PLRs of 2–5 and NLRs of >0.2, indicated their limited effectiveness as either screening or confirmation tests.

At the L5/S1 level, the 3-kg weight-lifting flexion radiograph showed a 50% sensitivity compared to the 25% sensitivity of the conventional flexion radiograph; however, the diagnostic performance of the former remained significantly different from that of MRI. Subgroup analysis revealed no significant differences between conventional flexion radiographs and 3-kg weight-lifting flexion radiographs (p>0.05). Both methods exhibited NLRs of >0.2, indicating limited effectiveness as screening tests. One possible explanation for these findings is the fixed weight of 3 kg used in this study, which may not account for individual variability in body weight. While a higher weight could potentially enhance diagnostic accuracy, it would also increase risk of adverse effects, such as exacerbated back pain. Both radiographic methods exhibited high specificity (92.31%). The 3-kg weight-lifting flexion radiograph also showed a PLR in the range of 5–10, suggesting its potential as an effective confirmation test.

Importantly, none of the patients reported worsening of back pain after undergoing the 3-kg weight-lifting flexion radiograph, indicating the safety of the procedure. This finding is consistent with those of prior studies suggesting the safety of using weights under 10% of body weight [14–17]. The ergonomic validation using the NIOSH equation further supported the appropriateness of using a 3-kg weight for clinical settings.

Previous studies have highlighted the variability in dynamic radiograph techniques and measurement methods [10]. Esmailiejah et al. [21] reported overall diagnostic values for dynamic radiographs, including a sensitivity of 84.8%, specificity of 100%, PPV of 100%, NPV of 79.1%, and accuracy of 90.4%. However, these results were not stratified by specific lumbar levels and relied on subjective assessments. Many previous studies have compared new diagnostic tools to conventional flexion and extension radiographs, which are often used as reference standards despite their limitations [8]. This study incorporated measurement techniques and diagnostic criteria established by Cho et al. [4], while introducing novel radiographic techniques and calculations to determine diagnostic values across all lumbar spine levels.

We acknowledge that lumbar instability can occur even without the presence of a facet fluid sign. Rihn et al. [9] found that while facet fluid on MRI correlated with sagittal instability on flexion-extension radiographs, approximately 18% of patients with radiographic instability did not exhibit facet fluid. This indicates that this radiological marker, though valuable, is not universally present in all cases of instability. Similarly, Cho et al. [4] reported high diagnostic performance of the facet fluid sign, with sensitivity and specificity exceeding 87%. However, the absence of facet fluid did not definitively rule out instability. Our study aimed to assess the effectiveness of a novel radiographic technique using the facet fluid sign as a diagnostic reference, given its established association with instability. Recognizing that instability can still be present without detectable facet fluid, future studies should also assess this technique in patients lacking this MRI marker.

Our study utilized the facet fluid sign as the reference standard for detecting lumbar instability, given its established association with degenerative spondylolisthesis, but not lytic spondylolisthesis. To maintain a homogeneous study population and ensure validity, we excluded patients with lytic spondylolisthesis from this study. The role of flexion-extension radiography in assessing instability was first reported by Knutsson [22], who demonstrated that disk degeneration leads to vertebral displacement and tilting movements. Subsequent studies, including Jeong et al. [23], confirmed the effectiveness of dynamic radiographs in evaluating instability in both lytic and degenerative spondylolisthesis, using criteria such as angular motion greater than 10° and translational movement exceeding 3 mm. These findings suggest that our weight-lifting flexion radiograph technique may also apply to lytic spondylolisthesis. However, further research is required to validate its efficacy in this patient population.

To the best of our knowledge, this is the first study to provide a comprehensive diagnostic assessment of all lumbar spine levels using the 3-kg weight-lifting flexion radiograph. Notably, it provides level-specific diagnostic values rather than relying solely on significant translations or angular rotations, which can lead to false-positive results. The study’s inclusion criteria ensured the selection of participants with both clinical and radiographic evidence of lumbar instability, yielding valid and reliable findings. Furthermore, excellent Inter- and intraobserver reliability were demonstrated using intraclass correlation coefficients, underscoring the robustness of the study.

Despite these strengths, some limitations of this study must be acknowledged. The use of facet fluid signs on MRI as the reference standard has inherent limitations, including variability in fluid presence due to factors such as muscle spasms or gas in the facet joints [10]. Additionally, the study’s generalizability may be limited, as it was conducted on a normal-weight population according to the World Health Organization’s classification. Participants with extreme weight conditions might yield less accurate results. A high BMI implies increased body mass, which can result in a thicker body composition. This may affect spinal alignment and lead to discrepancies in spine positioning and misalignment in imaging results. Further studies should explore the use of adjustable weights and include larger, more diverse populations to refine the diagnostic utility of this technique.

The weight-lifting or stress radiograph technique may pose challenges for patients with severe back symptoms, raising concerns regarding patient safety. The stress radiograph can potentially introduce further injury or exacerbate pain. To ensure patient safety and prevent discomfort, we excluded such patients from our study. For patients with severe back symptoms, alternative diagnostic methods, such as dynamic radiographs or MRI, may be more appropriate.

Furthermore, although all participants exhibited clinical signs suggestive of lumbar instability, we did not quantify symptom severity. Future research could incorporate standardized clinical scoring systems, such as the Visual Analog Scale or Oswestry Disability Index, to further explore these correlations and enhance the clinical relevance of the findings.

Lastly, our study primarily focused on degenerative spondylolisthesis. To expand the generalizability of our findings, future research should evaluate the applicability and accuracy of this technique in patients with other types of spondylolisthesis, especially lytic spondylolisthesis. This would help determine its clinical utility across various spondylolisthesis subtypes.

Conclusions

This study demonstrates that the 3-kg weight-lifting flexion radiograph can significantly enhance diagnostic sensitivity and accuracy for lumbar instability, particularly at the L3/4 and L4/5 levels, making it a reliable screening tool. Additionally, it may serve as a useful confirmation test for lumbar instability at the L1/2 and L5/S1 levels. The technique is safe, effective, and exhibits excellent diagnostic alignment with MRI, offering a valuable alternative for patients who cannot undergo MRI. These findings support the broader clinical application of the 3-kg weight-lifting flexion radiograph in the diagnosis of lumbar instability.

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