---
title: "Reliability of ultrasound measurement of medial knee joint space width during valgus stress"
id: "plos-one-4-relative-and-absolute-reliability-of-medial-knee-joint-space-width-measurement"
canonical_url: "https://medichelpline.com/clinical-feed/plos-one-4-relative-and-absolute-reliability-of-medial-knee-joint-space-width-measurement"
content_type: "clinical_feed_article"
specialty: "Radiology"
source_name: "PLOS ONE (Medicine)"
source_url: "https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0358761"
published_at: "2026-09-18T14:00:00.000Z"
evidence_level: "Journal Feed"
license: "CC-BY-NC-4.0 / Informational Use"
---
# Reliability of ultrasound measurement of medial knee joint space width during valgus stress
## Provenance & Clinical Metadata
- **Canonical URL:** https://medichelpline.com/clinical-feed/plos-one-4-relative-and-absolute-reliability-of-medial-knee-joint-space-width-measurement
- **Specialty:** [Radiology](https://medichelpline.com/clinical-feed/radiology.md)
- **Primary Source:** PLOS ONE (Medicine)
- **Source URL:** [Original Journal Publication](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0358761)
- **Published At:** 2026-09-18T14:00:00.000Z
- **Evidence Rating:** Journal Feed
## Executive GIST (TL;DR)
- This study assessed both relative and absolute reliability of ultrasound-based measurement of changes in **medial knee joint space width** during a standardized **valgus stress** test in healthy adults. - Seventeen healthy male participants (34 knees) were measured under unloaded and valgus-loaded conditions (10 Nm). - Images were obtained with an 8–15 MHz linear transducer and analyzed with ImageJ; the joint space width was defined between the distal medial femoral edge and the most proximal medial tibial plateau. - Two physical therapists with different experience levels performed imaging; a third experienced therapist applied the valgus load. - Measurements were acquired on two occasions (Day 1 and Day 2, one week apart) to evaluate intra-rater (same examiner across days and within-day repeats) and inter-rater reliability. - A significant increase in medial joint space width under valgus loading (mean increase 1.33–1.41 mm) was observed (p < 0.001). - Relative reliability: intra-rater ICC(3,1) across two days = 0.88 (95% CI 0.77–0.94); inter-rater ICC(3,1) = 0.84 (95% CI 0.70–0.92). Within-day intra-rater ICCs ranged 0.85–0.93. - Absolute reliability: Bland-Altman showed only random error (no fixed or proportional bias). Intra-rater SEM = 0.19 mm (MDC 95 = 0.51 mm); inter-rater SEM = 0.23 mm (MDC 95 = 0.65 mm). - The authors conclude ultrasound provides sufficient reliability to quantify changes in medial joint space width during valgus stress in healthy individuals; changes above the MDC thresholds likely exceed measurement error. - The study notes limitations: small sample of healthy young males only, measurements performed with specific protocol and personnel; authors recommend larger studies and evaluation in patients with ligamentous injury to confirm clinical applicability.
## Clinical Analysis & Structured Key Points
[ Skip to main content ](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0358761#main-content) Advertisement * [plos.org](https://plos.org/) * [Create account](https://community.plos.org/registration/new) * [Sign in](https://journals.plos.org/user/secure/login?page=%2Fplosone%2Farticle%3Fid%3D10.1371%2Fjournal.pone.0358761) * * About * Browse * Publish * [](https://journals.plos.org/plosone/ "PLOS One") * Search [advanced search](https://journals.plos.org/plosone/search) * [Browse Topics](https://journals.plos.org/plosone/subjectAreaBrowse) Browse Subject Areas ? Click through the PLOS taxonomy to find articles in your field. For more information about PLOS Subject Areas, click [here](https://github.com/PLOS/plos-thesaurus/blob/master/README.md "Link opens in new window"). [](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0358761) [](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0358761) * 0 [Save](https://journals.plos.org/plosone/article/metrics?id=10.1371/journal.pone.0358761#savedHeader) [Total Mendeley and Citeulike bookmarks.](https://journals.plos.org/plosone/article/metrics?id=10.1371/journal.pone.0358761#savedHeader) * 0 [Citation](https://journals.plos.org/plosone/article/metrics?id=10.1371/journal.pone.0358761#citedHeader) [Paper's citation count computed by Dimensions.](https://journals.plos.org/plosone/article/metrics?id=10.1371/journal.pone.0358761#citedHeader) * 0 [View](https://journals.plos.org/plosone/article/metrics?id=10.1371/journal.pone.0358761#viewedHeader) [PLOS views and downloads.](https://journals.plos.org/plosone/article/metrics?id=10.1371/journal.pone.0358761#viewedHeader) * 0 [Share](https://journals.plos.org/plosone/article/metrics?id=10.1371/journal.pone.0358761#discussedHeader) [Sum of Facebook, Twitter, Reddit and Wikipedia activity.](https://journals.plos.org/plosone/article/metrics?id=10.1371/journal.pone.0358761#discussedHeader) Open Access Peer-reviewed Research Article # Relative and absolute reliability of medial knee joint space width measurement using ultrasound imaging during valgus stress testing * Ryoga Hayashi, Roles Conceptualization, Formal analysis, Investigation, Methodology, Visualization, Writing – original draft Affiliation Nantaru Orthopedic Clinic, Otaru, Hokkaido, Japan ⨯ * Hayato Kawaji, Roles Investigation, Resources, Writing – review & editing Affiliation Soseigawadori Orthopedic Clinic, Sapporo, Hokkaido, Japan ⨯ * Satoru Kojima Roles Conceptualization, Investigation, Methodology, Project administration, Supervision, Writing – review & editing * E-mail: skojima@hoku-iryo-u.ac.jp Affiliation School of Rehabilitation Sciences, Health Sciences University of Hokkaido, Kanazawa, Tobetsu-cho, Japan [ ![ORCID logo](https://journals.plos.org/resource/img/orcid_16x16.png) https://orcid.org/0009-0000-7677-3817 ](https://orcid.org/0009-0000-7677-3817 "ORCID Registry") ⨯ # Relative and absolute reliability of medial knee joint space width measurement using ultrasound imaging during valgus stress testing * Ryoga Hayashi, * Hayato Kawaji, * Satoru Kojima ![PLOS](https://journals.plos.org/resource/img/logo-plos-full-color.svg) x * Published: September 18, 2026 * * [Article](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0358761) * [Authors](https://journals.plos.org/plosone/article/authors?id=10.1371/journal.pone.0358761) * [Metrics](https://journals.plos.org/plosone/article/metrics?id=10.1371/journal.pone.0358761) * [Comments](https://journals.plos.org/plosone/article/comments?id=10.1371/journal.pone.0358761) * [Media Coverage](http://plos.altmetric.com/details/doi/10.1371/journal.pone.0358761) * [Abstract](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0358761#abstract0) * [Introduction](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0358761#sec001) * [Materials and methods](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0358761#sec002) * [Results](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0358761#sec006) * [Discussion](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0358761#sec007) * [Conclusions](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0358761#sec008) * [Supporting information](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0358761#sec009) * [References](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0358761#references) * [Reader Comments](https://journals.plos.org/plosone/article/comments?id=10.1371/journal.pone.0358761) * [Figures](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0358761) ## Abstract This study aimed to evaluate both the relative and absolute reliability of ultrasound-based measurements of changes in medial knee joint space width under valgus stress in healthy individuals. Seventeen healthy adult males (34 knees) underwent ultrasound assessments under unloaded and valgus-loaded (10 Nm) conditions. The medial knee joint space width was measured using a standardized ultrasound protocol, and changes were calculated as the difference between loaded and unloaded conditions. Intra- and inter-rater reliability between two physical therapists with different experience levels were evaluated using intraclass correlation coefficients, and absolute reliability was assessed via Bland-Altman analysis, standard error of measurement, and minimal detectable change at the 95% confidence level. A significant increase in medial knee joint space width (ranging from 1.33 to 1.41 mm) was observed under valgus stress compared to the unloaded condition (_p_ < 0.001). The intraclass correlation coefficient for intra-rater reliability across two separate measurement days was 0.88 (95% confidence interval: 0.77–0.94), whereas that for inter-rater reliability was 0.84 (95% confidence interval: 0.70–0.92). Bland-Altman analysis revealed only random measurement error, with minimal detectable change at the 95% confidence level values below 1.0 mm (0.51 mm intra-rater; 0.65 mm inter-rater). Ultrasound imaging could provide sufficient reliability to quantify changes in medial knee joint space width during valgus stress testing in healthy individuals. These values suggest that changes exceeding these thresholds may reflect changes beyond measurement error. However, further studies with larger sample sizes and involving patients with ligamentous injuries are needed to confirm the clinical applicability of this method. ## Figures ![Table 2](https://journals.plos.org/plosone/article/figure/image?size=inline&id=10.1371/journal.pone.0358761.t002) ![Table 3](https://journals.plos.org/plosone/article/figure/image?size=inline&id=10.1371/journal.pone.0358761.t003) ![Fig 3](https://journals.plos.org/plosone/article/figure/image?size=inline&id=10.1371/journal.pone.0358761.g003) ![Fig 1](https://journals.plos.org/plosone/article/figure/image?size=inline&id=10.1371/journal.pone.0358761.g001) ![Fig 2](https://journals.plos.org/plosone/article/figure/image?size=inline&id=10.1371/journal.pone.0358761.g002) ![Table 1](https://journals.plos.org/plosone/article/figure/image?size=inline&id=10.1371/journal.pone.0358761.t001) ![Table 2](https://journals.plos.org/plosone/article/figure/image?size=inline&id=10.1371/journal.pone.0358761.t002) ![Table 3](https://journals.plos.org/plosone/article/figure/image?size=inline&id=10.1371/journal.pone.0358761.t003) ![Fig 3](https://journals.plos.org/plosone/article/figure/image?size=inline&id=10.1371/journal.pone.0358761.g003) ![Fig 1](https://journals.plos.org/plosone/article/figure/image?size=inline&id=10.1371/journal.pone.0358761.g001) ![Fig 2](https://journals.plos.org/plosone/article/figure/image?size=inline&id=10.1371/journal.pone.0358761.g002) ![Table 1](https://journals.plos.org/plosone/article/figure/image?size=inline&id=10.1371/journal.pone.0358761.t001) **Citation:** Hayashi R, Kawaji H, Kojima S (2026) Relative and absolute reliability of medial knee joint space width measurement using ultrasound imaging during valgus stress testing. PLoS One 21(9): e0358761. https://doi.org/10.1371/journal.pone.0358761 **Editor:** Emad A. Aboelnasr, Cairo University Faculty of Physical Therapy, EGYPT **Received:** March 18, 2026; **Accepted:** September 5, 2026; **Published:** September 18, 2026 **Copyright:** © 2026 Hayashi et al. This is an open access article distributed under the terms of the [Creative Commons Attribution License](http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. **Data Availability:** All relevant data are within the manuscript and its [Supporting Information](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0358761#sec009) files. **Funding:** The author(s) received no specific funding for this work. **Competing interests:** The authors have declared that no competing interests exist. ## Introduction Injuries to the medial structures of the knee, particularly the medial collateral ligament (MCL), are common and may result in medial knee instability [[1](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0358761#pone.0358761.ref001)–[5](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0358761#pone.0358761.ref005)]. The valgus stress test is widely used to evaluate the integrity of these structures [[3](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0358761#pone.0358761.ref003)–[6](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0358761#pone.0358761.ref006)]. However, the test relies largely on the examiner’s subjective assessment of the medial joint opening, which may reduce its objectivity and reproducibility [[7](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0358761#pone.0358761.ref007)]. To overcome this limitation, quantitative assessment methods have been developed. Although stress radiography is considered a reference method for evaluating medial knee laxity [[8](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0358761#pone.0358761.ref008),[9](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0358761#pone.0358761.ref009)], its clinical use is limited by radiation exposure and equipment requirements. Ultrasound imaging has emerged as an attractive alternative because it is non-invasive, radiation-free, portable, and capable of providing real-time dynamic assessment of medial knee structures [[10](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0358761#pone.0358761.ref010)–[13](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0358761#pone.0358761.ref013)]. Previous studies have demonstrated that ultrasound measurements of changes in medial knee joint space width under valgus stress correlate well with stress radiography [[10](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0358761#pone.0358761.ref010),[12](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0358761#pone.0358761.ref012)] and exhibit high intra- and inter-rater reliability [[10](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0358761#pone.0358761.ref010),[11](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0358761#pone.0358761.ref011)]. These findings suggest that ultrasound may be useful for quantifying medial knee laxity. However, previous studies have primarily focused on relative reliability, which reflects measurement consistency but does not indicate the type or magnitude of measurement error. For clinical decision-making, absolute reliability is equally important because it enables determination of whether an observed change exceeds measurement error and likely represents a change beyond measurement error. Parameters such as the standard error of measurement (SEM) and minimal detectable change at the 95% confidence level (MDC95) provide this information. Although absolute reliability of ultrasound measurements has been reported for several knee laxity-related assessments [[14](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0358761#pone.0358761.ref014),[15](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0358761#pone.0358761.ref015)], no study has investigated the absolute reliability of ultrasound-based measurements of changes in medial knee joint space width under valgus stress. Consequently, the threshold for distinguishing changes beyond measurement error in medial joint space width remains unknown. Therefore, the purpose of this study was to evaluate both the relative and absolute reliability of ultrasound-based measurements of changes in medial knee joint space width under valgus stress in healthy individuals. Establishing SEM and MDC95 values would provide useful thresholds for distinguishing changes beyond measurement error and may facilitate the clinical application of ultrasound-based assessment of medial knee laxity. We hypothesized that: (a) the measurement of changes in medial knee joint space width under valgus stress would demonstrate high relative reliability both within and between examiners, and (b) when assessing absolute reliability, the errors would be primarily random, with the magnitude of the errors being less than 1.0 mm, both within and between examiners. ## Materials and methods ### Participants The sample size was originally determined to detect significant differences in medial knee joint space width between unloaded and loaded conditions. Sample size estimation was performed using G*power software (Version 3.1.9.7; Heinrich Heine University, Düsseldorf, Germany) based on a paired _t_ test with a two-tailed analysis, assuming an effect size of 0.50, an alpha level of 0.05, and statistical power (1 – _β_) of 0.80. The results indicated that a sample size of 34 was required. Given the influence of hormonal fluctuations on joint laxity in females [[16](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0358761#pone.0358761.ref016)–[18](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0358761#pone.0358761.ref018)], only male participants were included in this study to eliminate potential confounding effects. Participants were recruited through convenience sampling from the students at our university. Exclusion criteria included a history of knee surgery or trauma, subjective knee pain, severe leg malalignment, inflammatory diseases affecting the knee joint, and BMI ≥ 30. Consequently, 17 healthy adult males (34 knees) were enrolled in this study. Participants were prospectively recruited between May 24, 2022, and September 6, 2022. Written informed consent was obtained from all participants. This study was approved by the Ethics Committee of Health Sciences University of Hokkaido (Approval No. 22R172175). ### Procedures The medial knee joint space width was measured using a diagnostic ultrasound system (Noblus; Hitachi Medical Corp., Tokyo, Japan) equipped with an 8–15 MHz linear transducer. Two licensed physical therapists performed the measurements. Examiner 1 was in their second year of clinical practice, and Examiner 2 had over 10 years of clinical experience. Two examiners (Examiners 1 and 2) underwent prior training until they achieved intra-rater reliability (ICC ≥ 0.80), as demonstrated in previous studies [[10](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0358761#pone.0358761.ref010),[11](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0358761#pone.0358761.ref011)]. Subsequently, the formal ultrasound measurements were performed. A third examiner (Examiner 3), a licensed physical therapist with over 10 years of experience in musculoskeletal rehabilitation, applied valgus stress during the measurement. Participants were positioned supine with the knees fixed at 20° flexion and the ankles in neutral dorsiflexion using a custom-made fixation device ([Figs 1a](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0358761#pone-0358761-g001) and [1b](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0358761#pone-0358761-g001)). Examiners identified the MCL attachment sites on the femur and tibia via ultrasound imaging. These sites were marked with a pen and connected with a straight line to guide probe placement. The ultrasound probe was aligned along this line to visualize the long axis of the MCL. The probe was then positioned parallel to the anterior edge of the MCL and adjusted to achieve clear visualization of the bone boundary. Images were acquired repeatedly until three successful trials were obtained. [![thumbnail](https://journals.plos.org/plosone/article/figure/image?size=inline&id=10.1371/journal.pone.0358761.g001)](https://journals.plos.org/plosone/article/figure/image?size=medium&id=10.1371/journal.pone.0358761.g001 "Click for larger image") Download: * [PNG larger image](https://journals.plos.org/plosone/article/figure/image?download&size=large&id=10.1371/journal.pone.0358761.g001) * [TIFF original image](https://journals.plos.org/plosone/article/figure/image?download&size=original&id=10.1371/journal.pone.0358761.g001) Fig 1. (a) A custom-made fixation device. **(b)** Knee fixed at 20° of flexion and ankle positioned in neutral dorsiflexion using the device. **(c)** Measurement of medial knee joint space width using ultrasound imaging. [ https://doi.org/10.1371/journal.pone.0358761.g001](https://doi.org/10.1371/journal.pone.0358761.g001) Ultrasound images were acquired under both unloaded and loaded conditions. For loaded condition measurements, a valgus load of 10 Nm was applied to the knee [[8](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0358761#pone.0358761.ref008),[10](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0358761#pone.0358761.ref010),[19](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0358761#pone.0358761.ref019)]. Examiner 3 stabilized the lateral side of the knee with one hand and applied valgus force to the medial malleolus using a handheld dynamometer (µTas F1; Anima Corp., Tokyo, Japan) with the other hand ([Fig 1c](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0358761#pone-0358761-g001)). The valgus force was calculated by dividing 10 Nm by the length of the lower leg for each subject. The applied force was continuously monitored to ensure consistency throughout the procedure. Ultrasound images were captured on two separate occasions: Day 1 and Day 2 (one week later). Imaging on Day 1 was performed by Examiner 1, while imaging on Day 2 was conducted by both Examiner 1 and 2 to assess intra- and inter-rater reliability. For the reliability assessments across the two separate measurement days, the skin markings were removed and re-identified to ensure independence. However, for same measurement day inter-rater assessments, the markings were retained. Ultrasound images were imported into a PC and analyzed using ImageJ software (Version 1.53; National Institutes of Health, USA). The medial knee joint space width was defined as the distance between the distal end of the medial femoral edge and the most proximal medial surface of the tibia plateau along the extension of the medial femoral edge [[20](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0358761#pone.0358761.ref020)] ([Fig 2](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0358761#pone-0358761-g002)). Image scale calibration was performed, and measurements were quantified using the distance measurement tool in ImageJ. Changes in the medial knee joint space width were calculated by subtracting the values under unloaded condition
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