Use of the Zebris Ultrasonic Three-Dimensional Motion Analysis System to Evaluate Range of Motion of the Cervical Spine in 61 Healthy Adults

Introduction

The cervical section is the most mobile part of the spinal column in the human body. Complex movements in all the planes are possible because of the different structure of the upper segments of the cervical spine (C1 and C2) [1].

Many people experience problems related to reduced cervical spine mobility due to degeneration, traumatic pain, compression of nerve roots and many other conditions [2,3]. Cervical deformities and disorders cause significant functional limitations and are a serious health problem because they can lead to severe disability and consequently adversely affect the patient’s quality of life. Objective measurements of the range of motion make it possible to design effective therapy programs or to assess the effects of therapy, and they provide feedback during the evaluation of therapeutic interventions administered [4–10].

The range of motion, which is a functional measure of the cervical spine, can be examined by means of several easy-to-use clinical tools. There are several devises which can be applied for this purpose in the clinical practice; these include a centimetre tape, a goniometer, an inclinometer or special apps downloaded to a phone [11–17].

Many therapists need objective tools to measure the cervical spine range of motion quickly and easily. Specialized tests facilitate the process of designing the therapy and enable monitoring of the effects of the therapeutic techniques applied. Moreover, such assessments may provide motivation for patients to continue the therapy. Both the diagnostic tests and the devices used for such evaluations must be accurate and effective, and the measurements performed using these must be reproducible, which is why such research is needed for future users of MCS Zebris [6,7].

Devices used in clinical practice must be tested for reliability, reflected by the consistency of measurements performed over time, by investigators or subjects, as well as their validity corresponding to reproducibility of the measurements [4–17]. Authors of many studies have investigated the reliability of devices such as goniometers and inclinometers, as well as advanced assessment methods enabled by mobile phone technology [12–20].

The Zebris CMS 20s device is an advanced cervical spine mobility assessment tool that uses ultrasound technology to precisely track movement in the 3 anatomical planes. The device consists of a measuring unit emitting ultrasound signals and a reference frame mounted on the head of the subject, which enables accurate and objective measurement of the range of movement in real time [12,15–20].

Although this system has already been validated and is widely used to measure cervical range of motion [12,15–20], there is a need for further research to optimize the measurement protocols and assess inter-rater variability in specific populations, such as the Polish population. The present study not only adds new data to the existing literature, but also provides the first evidence for the Polish population, which may have important implications for future clinical trials and diagnostic practice in Poland. Based on the available literature, we can identify several key research gaps that this study aims to fill.

First, previous studies of the Zebris 3D Ultrasonic system considered limited groups of participants, without the use of appropriate sampling methods and adequate statistical estimation of data, or were performed in specific laboratory conditions [12,16,18,19]. The present study involved a larger sample of healthy adults, which was determined using a sample size calculator to ensure it was adequately powered for the feasibility study and to make it possible to update the normative values for cervical range of motion. Furthermore, many earlier studies focused exclusively on reproducibility of the results, and did not always assess the differences between the independent investigators performing the measurements (inter-rater reliability). The current research project engaged 2 independent observers with more than 15 years of experience in motion analysis, as well as inter- and intra-rater reliability studies; consequently, it was possible to assess potential differences resulting from manual placement of the sensors and the way the measurement was conducted. Studies by Cagnie et al [15] and Quinlan et al [18] focused on assessing one-time measurements or short-term reproducibility, but it appears no studies have assessed the stability of results over longer periods of time. In the current study, the measurements were repeated after 2 weeks, so we were able to assess whether the Zebris system produces consistent results, not only in the short term, but also in the longer term, which is crucial for practical application of the tool in clinical and research settings. Finally, previous studies have used a variety of approaches, ranging from in vitro analyses [19] to limited in vivo studies that often focused on selected aspects of mobility (eg, Luedtke et al [20] performed only cervical flexion rotation testing). The present study used a holistic approach, enabling assessment of all movements and planes of cervical motion, and providing a complete picture of the functionality of this spinal segment.

Therefore, this study aimed to assess the inter-rater variability and test-retest reliability of the Zebris ultrasonic three-dimensional motion analysis system in measuring cervical range of motion in 61 healthy young Polish adults.

Material and Methods

ETHICS STATEMENT:

The study was approved by the Research Ethics Committee for Physiotherapists at the Polish Physiotherapy Association on July 4, 2024 (Approval No. 1/07/2024). All procedures were conducted in accordance with the principles of the Declaration of Helsinki and the relevant guidelines and regulations. Prior to participation, all individuals were provided with detailed information about the purpose, procedures, potential risks, and benefits of the study. Written informed consent was obtained from all participants before their inclusion in the study. Participation was voluntary, and participants were informed of their right to withdraw from the study at any time without any consequences.

PARTICIPANTS AND SETTING:

The study was conducted in the Spine Kinesiology Laboratory at the University of Rzeszów. The participants were recruited from among students at the University of Rzeszów by means of advertisements posted on notice boards. The study group included 61 healthy individuals, 40 women and 21 men, reporting no pain in the cervical spine, aged 23–27 years, with mean age of 23 years. All participants provided voluntary consent to be involved in the study. Those eligible to participate were healthy young adults without problems in the cervical spine. The exclusion criteria were: age below 18 and above 35 years; presence of cervical spine disorders and injuries, pain, or coexisting neurological, orthopedic or rheumatic diseases potentially affecting spinal mobility; as well as lack of consent to participate in the study.

STUDY PROTOCOL:

In line with the study protocol, the assessment of cervical range of motion (ROM) was performed using the Zebris CMS 20s device (Zebris Medical GmbH, Germany), which enables ultrasound-based three-dimensional motion analysis. The device consists of a measurement unit with ultrasonic sensors and a head-mounted reference frame, allowing precise tracking of movement in all anatomical planes.

Participants were seated in an upright position on an adjustable chair, with their back straight, gaze forward, feet flat on the ground, and hands resting on their knees. Before they performed each movement, participants received verbal instructions and observed a demonstration of the movement. Movements were performed actively without external assistance, and each measurement was repeated 3 times to obtain an average value. The measurements were performed with an accuracy of 1°. Additionally, certain control measures were implemented to ensure data reliability. Before the start of the trial, a calibration procedure was used to verify the measurement accuracy of the Zebris CMS 20s. To minimize variability, the raters received training and followed a standardized measurement protocol. All measurements were performed during the same time of day to avoid potential diurnal variations in cervical mobility.

PROCEDURES:

The measurements were carried out using the Zebris CMS 20s (Zebris Medical GmbH, Germany), which enables ultrasonic testing of the cervical range of motion (ROM) (Figure 1). This device is widely used in research [12,15–20], and it provides objective, repeatable, and highly accurate measurements. It consists of a measuring unit with ultrasonic sensors and a reference frame attached to the subject’s head, to enable precise tracking of movement in all anatomical planes.

The study assessed inter-observer variability (Rater 1 versus Rater 2) as well as repeatability of measurements over time (test-retest). For this purpose, the Intraclass Correlation Coefficient (ICC) was used to determine the degree of agreement between measurements taken by 2 independent raters and between 2 measurement sessions conducted 2 weeks apart. Statistical analysis showed that the level of inter-rater variability was acceptable, which reflects the consistency of the measurements.

Cervical mobility was assessed in the 3 anatomical planes, by measuring the range of flexion-extension, right and left side bend and right and left rotation. The measurements were performed by 2 independent raters, each with 15 years of experience in motion analysis. The measurements were recorded simultaneously under standardized conditions in a quiet, well-lit room with a constant ambient temperature. To ensure the reliability and repeatability of the measurements, the same participants were reassessed 2 weeks later under identical conditions by the same raters.

During the trial, participants sat upright in an adjustable chair, with their back straight, eyes looking forward, feet positioned flat on the floor, and hands resting on their knees. Before performing each movement, participants received verbal instruction and watched a demonstration of the movement. The movements were performed actively, without external assistance, and each measurement was repeated 3 times to obtain an average value. Measurements were performed with an accuracy of 1°.

To minimize bias, the investigators (Rater 1 and Rater 2) performed their measurements independently, without communicating with each other. During both evaluation sessions, the same protocol was strictly followed to ensure consist results.

DATA ANALYSIS:

The statistical analyses of the collected data were performed using Statistica 13.3 software (StatSoft). Parametric tests were used for the analyses if their basic assumptions were fulfilled with regard to the conformity of the distributions of the analysed variables to normal distribution, and the homogeneity of variance. The conformity of the distributions with the normal distribution was assessed using the Shapiro-Wilk W test, while the homogeneity of variance was assessed using the Levene’s test. Descriptive statistics (mean, median, minimum, and maximum values as well as standard deviations) were calculated for all variables. The t test for independent variables was used to assess differences in the average level of a numerical characteristic in the 2 populations. The t test for dependent variables was applied to assess within-group variability in the 2 groups. The correlation of 2 variables with a normal distribution was determined using Pearson’s linear correlation coefficient. To assess the reliability of the measurements and inter-rater agreement, intraclass correlation coefficients (ICC) were calculated. The ICC analysis made it possible to assess the consistency of the measurements and to determine whether they were independent or systematically inter-related.

Statistical significance corresponded to P<0.05. The sample selection calculator was used to calculate the minimum sample size. A fraction size of 0.8 was applied, with a maximum error of 6%.

Results

ASSESSMENT OF INTER-RATER AGREEMENT:

Inter-rater agreement was assessed, and for this purpose the analysis was conducted to compare results recorded by Rater 1 and Rater 2 in the first measurement (A) and in the second measurement (B), with all measurements conducted in the same direction of movement and during the same period of time by Rater 1 and Rater 2. The results were highly consistent (P<0.001). The strength of the relationship was 0.94–0.98 (Table 1). The best results were identified for flexion r=0.98 and side bend to the right r=0.98

POTENTIAL SOURCES OF VARIABILITY:

Although the results demonstrated high inter-rater reliability, minor variability may have arisen from slight differences in the manual placement of sensors by the raters, despite standardized protocols.

The analyses also compared the mean results recorded in the measurements A and B in each direction of movement by Rater 1 and Rater 2. The mean results of the measurements did not significantly differ between the raters (P>0.05) (Table 2).

ASSESSMENT OF INTRA-RATER AGREEMENT:

Intra-rater agreement was assessed by comparing the results recorded by the same investigator (first Rater 1 and then Rater 2) at 2 timepoints, 2 weeks apart. The results recorded by Rater 1 in measurements A and B did not differ significantly (P>0.05) (Table3).

Likewise, the results obtained by Rater 2 in Measurement A and B did not differ significantly (P>0.05) (Table 4).

The strength of reliability, or the degree of the agreement between the measurements, was also assessed using ICC. The analyses tested the correspondence between measurements performed by the 2 raters (A vs B) and consistency of measurements performed by each Rater at 2 timepoints (1 vs 2). The correlation coefficient rICC assumes the same values as the Pearson’s linear correlation coefficient, from −1 to 1. The present findings show strong inter-rater reliability and intra-rater reliability reflected by rICC values close to 1 (Table 5).

Discussion

PRACTICAL IMPLICATIONS:

The study shows that measurements of cervical spine range of motion performed with the Zebris ultrasound system are highly consistent and reproducible. This suggests the device can be useful in clinical practice, both in diagnosing problems and in monitoring therapeutic progress.

LIMITATIONS:

The present study enrolled only young adults, limiting generalization of results to a more diverse population. It is therefore important to continue related research in more diverse demographic groups. The current findings suggest that there is a need for further research using a broader spectrum of methods to more accurately determine the feasibility of the device in assessing spinal range of motion. Further comparative studies are also needed to enable assessment of mobility measurements performed in different positions, which may contribute to a better understanding of using the device in practical clinical applications. At the first stage we assessed intra- and inter-rater reliability but we are also planning further research to investigate concurrent validity of the Zebris ultrasonic three-dimensional motion analysis system compared to a cervical range of motion goniometer (CROM) and virtual reality (VR) goggles. The present study focused on evaluating the Zebris system as a stand-alone measurement tool, which was an intentional objective of the project. Furthermore, by conducting this study in a group of healthy individuals, we were able to create a database for normal cervical range of movement for the Polish population. However, we understand that a comparison with other methods could provide additional information on the accuracy and validation of the Zebris system. We are planning to include such comparisons in future studies. Our study was conducted in a group of healthy volunteers to assess normative cervical mobility parameters in the Polish population and inter-observer variability under controlled conditions. The inclusion of patients with conditions affecting cervical mobility (eg, degenerative disease, trauma) would be a valuable addition. However, it was our objective to first perform the assessment of healthy Polish participants, and after the reliability of the device has been shown in healthy individuals, we intend to assess the reliability and validity of the tool in patients with cervical spine discopathy. We are planning to carry out a study which will involve such groups of patients. In the present study we assessed inter-observer variability by comparing measurements of all 6 movements in 3 planes, performed twice, 2 weeks apart, by 2 independent raters with 15 years of experience in research focusing on inter- and intra-rater reliability. Although this approach is consistent with the methodology used in previously published studies where reliability assessment was performed based on measurements performed by 1 or 2 raters, we plan to increase the number of investigators and the number of repeated measurements to obtain more representative data.

Conclusions

The study demonstrates that the Zebris ultrasonic three-dimensional motion analysis system presents excellent inter-rater and intra-rater agreement in measurements of cervical spine range of motion in healthy young Polish adults. These findings support use of the device in clinical practice for diagnosing cervical spine mobility disorders, planning therapeutic interventions, and monitoring treatment outcomes. Future studies need to verify the reliability of the system in more varied populations, including patients with different spinal conditions.