Background
The subacromial space (SAS) is comprised of the humeral head inferiorly, and the anteroinferior surface of the anterior third of the acromion, the coracoacromial ligament and acromioclavicular joint superiorly [
1]. Reduction of the SAS can lead to subacromial impingement syndrome (SAIS), whereby the tissues occupying the space, notably the supraspinatus tendon, become compressed and subsequently damaged [
2]. SAIS is one of the most common shoulder disorders, accounting for 48% of all clinical diagnoses [
3]. Narrowing of the SAS can result from musculoskeletal exposures that modify healthy joint kinematics [
2,
4‐
6] or innate morphological parameters [
7‐
9]. Both mechanisms pose risk of SAIS and/or rotator cuff tears and subsequent pain, functional limitations, and further tissue injury [
10‐
12]. Thus, accurate measurement of the SAS width is beneficial for risk identification and prevention.
A variety of imaging-based methods have been used to quantify the SAS. Radiography has been used to capture the acromiohumeral interval (minimum SAS width), demonstrating high reliability [
8,
13‐
16]. However, changes in arm position and X-ray beam orientation have been shown to overestimate SAS measurements [
14,
17]. While radiography captures a single static image, biplane fluoroscopy enables investigators to view the structures dynamically across simultaneous images [
18]. This method is particularly advantageous as it offers the same benefits as conventional radiography, while allowing for three-dimensional measurement of the SAS in both static and dynamic states [
18,
19]. Researchers investigating the SAS have also used computed tomography (CT) scans, which yields greater precision compared to conventional radiography [
20,
21]. Bey and colleagues [
19,
22‐
24] developed a model-based motion tracking technique that uses CT scans alongside biplane radiographs to track in vivo glenohumeral kinematics with a high degree of accuracy compared to dynamic radiostereometric analysis (RMS errors < 0.385 mm for the scapula and < 0.374 mm for the humerus) [
24]. This method has been subsequently adopted by other research groups and deemed the gold standard for quantifying the SAS [
25]. However, while deemed noninvasive, these techniques still expose participants to radiation, albeit the effective dosage is reportedly low [
25]. Magnetic resonance imaging (MRI) has been regarded as the most appropriate imaging technique for complex anatomical structures [
26‐
28]. Research using MRI to capture the SAS width reports high precision, reliability, and accuracy of the anatomical models reconstructed from MRI images [
27,
29]. While these methods have been deemed effective for measuring the SAS, they are not readily accessible and can be expensive. Ultrasound (US) is another imaging modality that has been used to measure the SAS. US is efficient, noninvasive, relatively easy to administer, and inexpensive. However, US may overestimate the minimum SAS width, as the lateral positioning of the probe may capture the distance between the acromion and lateral humerus rather than the minimum distance between the coracoacromial arch and the superior-most point on the humeral head.
Several studies have investigated the inter- and intra-rater reliability of SAS measures using US. Among studies where the agreement in SAS measures between experienced clinicians was quantified, excellent intra- (ICC: ≥ 0.92) and inter- (ICC: ≥ 0.88) rater reliability was reported [
30‐
32]. In these studies, participants included asymptomatic individuals, and patients with SAIS and/or rotator cuff tendinopathy. Further, excellent within-day (ICC ≥ 0.98) and day-to-day (ICC ≥ 0.96) intra-rater reliability has been reported for expert raters [
33]. Other studies have compared the agreement of SAS measurements captured by both novice and expert raters. Novice raters included physical therapists, physiotherapy students, and orthopedic residents who received practice and/or training prior to commencing the study [
34‐
37]. These studies revealed that the novice raters demonstrated good to excellent inter-rater reliability among one another (ICC: 0.79) [
35] and compared to an expert (0.70–0.77) [
34,
36,
37]. Intra-rater reliability for the novice rater was reported to be excellent (ICC ≥ 0.84) [
35,
37]. SAS measurement agreement between expert clinicians and non-clinician novice raters, as well as the agreement between their US measures and that quantified from MRI, has not been studied.
In an effort to improve reliability of US-based measures, researchers have implemented various tools and methodologies. Bulbrook et al. [
38] introduced a 3D-printed novel transducer attachment to improve the reliability of capturing muscle architecture parameters in the thigh. In the shoulder, an inclinometer has been attached to the US probe in effort to assist in maintaining a consistent probe angle between repeated images [
39]. However, researchers have not studied whether SAS measures captured by two raters are in better agreement when the same transducer angle is adopted. Although measurements performed by expert raters have previously demonstrated excellent reliability, minimal research exists that examines the validity of SAS compared to other imaging modalities and the sensitivity to transducer tilt variation. It is also important to determine whether a novice non-clinician rater with minimal US training can reliably and accurately measure the SAS. Confirmed reliability of a novice researcher, as compared to an expert clinician, and validity, as compared to MRI, would enable more feasible and accessible laboratory-based assessments of SAIS risk.
The primary purposes of this study were to: (1) evaluate the agreement of SAS measures captured using US between a novice with introductory training and an expert and (2) evaluate the agreement between US and MRI measures of the SAS. We hypothesized that the SAS measurements of novice and expert raters would have moderate to excellent agreement. We also expected the US measures to overestimate the minimum SAS captured from MRI due to the imaging capabilities of US. The secondary purpose of this research was to determine whether inter-rater reliability was improved when the transducer tilt angle was consistent between raters. We hypothesized that inter-rater reliability would be improved when a consistent transducer tilt angle was adopted.
Limitations
Certain limitations of this research should be considered. First, participant positioning during US imaging was standardized across all participants and raters. As participants may have had to change positions (seated, prone) between raters, it is possible that minor differences in posture existed between the two raters, which may have affected the SAS width. As well, with measurements captured from real-time freeze-framed images, it is uncertain whether differences between raters were due to differences in the US image or the measurement protocol. Further, intra-rater reliability of US imaging was assessed from the three repeated measurements in each position. The three measures were captured consecutively as the entire US imaging protocol was blocked randomized. This may have affected the reliability. Lastly, post hoc power calculations for ICC measures revealed a high power between ultrasound measures of novice and expert raters for both seated (p2 = 0.97) and supine (p2 = 0.86). However, power was lower for ICCs between ultrasound and MRI (p2 = 0.14–0.60). Thus, future investigations of image modality agreement should consider a larger sample size.
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