Hypermobility Spectrum disorders (HSD) are connective tissue disorders characterized by symptomatic synovial joint hypermobility etiologically related to genetic and pathologic factors (1–5). HSD is a new terminology introduced in 2017 for hypermobility-related disorders to highlight the wide heterogeneities within joint hypermobility-related conditions, and to replace the terms “Joint Hypermobility Syndrome” (JHS) and “Ehlers-Danlos Syndrome Hypermobility Type” (EDS-HT) (6). HSD diagnosis is likely to capture the majority of patients previously diagnosed as JHS/EDS-HT (7). The subdivisions of HSD are localized, peripheral, generalized, and historical HSD (6). HSD is the most common type of connective tissue disorder, where its prevalence was 55% of women patients attending physiotherapy with musculoskeletal symptoms (8). The prominent symptomatic features of HSD include chronic symptomatic joint hypermobility, recurrent joint dislocation, and muscle weakness (2, 9–14). It is a multi-systemic disorder, affecting the musculoskeletal, cardiovascular, digestive, and autonomic nervous systems, as it alters the mechanical properties of these systems’ connective tissues (2).

Hypermobility spectrum disorders lacks a consistent and fixed constitutional pattern as it involves huge spectra of symptomatic features, severity levels, and systemic involvement, which complicate its identification. The complexity of HSD identification and diagnosis is highly related to the subjectivity of the approaches used, as they rely on past medical history and clinical examination using the historical Brighton or Villefranche criteria or the 2017 HSD classification framework (2, 6, 7, 15, 16). The current approaches are highly dependent on the experience of the examiner, and the memory of the patient. The predominant clinical feature of joint hypermobility, assessed using the Beighton score, is used to guide the identification process, but could be affected by the regression of the condition, and replaced with joint stiffness, which further complicates the diagnostic process. A recent review related the diagnostic delay of hEDS/HSD to the lack of a confirmatory diagnostic approach (17). Ultimately, there is a need for a more objective diagnostic procedure to improve the subjectivity of existing diagnostic approaches.

Advances in radiology medicine could offer a more objective approach for the diagnosis of HSD. Compression sonoelastography (SEG) is an ultrasound-based system which is designed to examine the biomechanical properties of structures through color-coded images and strain ratio (18, 19). The strain ratio is an objective elasticity measure provided in real-time (18, 19). Introducing SEG to assist in the diagnosis of HSD could be valuable as a more objective indicator. SEG may be able to detect alterations in the biomechanical properties of the musculotendinous structures in HSD theoretically caused by several factors. Firstly, the biomechanical rigidity of musculotendinous structures in HSD could be reduced due to genetic mutations in the gene encoding collagen, which is essential to confer tensile strength and rigidity (4, 10, 20). Biomechanical properties could also be altered as a result of the primary features of HSD, including joint hypermobility and muscular weakness (2, 4, 10). Such genetic and symptomatic factors strongly suggest changes in the biomechanical properties of the musculotendinous structures in HSD. Consequently, SEG could help in HSD identification, diagnosis, and monitoring.

The value of SEG for the examination of people with HSD was previously explored by our research group in two studies (21, 22). The first study supported the feasibility of SEG in terms of training requirements, patient tolerance, and examination of the GM muscle in JHS (21). The second study found significant reductions in musculoskeletal elasticity in people with HSD when compared to a control group in various anatomical structures (22). That study provided initial evaluations for the deltoid muscle, biceps brachii muscles, brachioradialis muscle, rectus femoris muscle, patellar tendon, and the gastrocnemius-Achilles tendon (GM-AT) complex, but none were examined intensively (22). However, it was clear that the GM-AT complex showed the highest differences between groups and the least error rate in comparison to the other structures examined (22). Alsiri et al. (22) strongly suggested intensively exploring the GM-AT complex in people with HSD relative to a control group as a promising indicator for identifying, diagnosing, and monitoring changes in biomechanical properties in response to the progression of the condition, or to interventions (22).

The current intensive study of the GM-AT complex was therefore conducted to provide valuable clinical indications and recommendations for clinical examination, for example specifying the exact anatomical region and muscle contraction status to be used. To explore the clinical value of SEG examinations, the predominant symptomatic features of HSD were explored and correlated with HSD findings, including joint hypermobility and pain. We hypothesize that HSD has a significant impact in reducing the stiffness of the GM-AT complex. The study aimed to explore the biomechanical properties of the GM-AT complex in people with HSD through comparison with a control group using compression SEG at different anatomical regions and during rest and maximal isometric contraction. Secondary aims were to correlate SEG findings with the major symptomatic features of HSD including joint hypermobility and pain to determine the clinical value of SEG results, and to undertake exploratory Receiver Operating Characteristics (ROC) curve analysis to explore potential diagnostic performance.

The HSD group included 28 patients, 26 women and two men, aged 28.7 ± 8.4 years (mean ± SD). The control group included 28 participants, 26 women and two men, aged 31.5 ± 8.7 years. No statistically significant differences were found between the two groups for age, height, or weight (all p > 0.05), which indicate that the two groups are homogenous at baseline (Table 1). The Beighton score was significantly higher in the HSD group (p = 0.001) (Table 1). The HSD group included 20 patients with generalized HSD, seven patients with localized HSD, and one patient with historical HSD. Three patients from the HSD group, and one participant from the control group were left side dominant. In terms of ethnicity, all participants in both groups were Asian. Nationalities of women in the HSD group included Kuwait (n = 22), Saudi Arabia (n = 1), Philippines (n = 1), India (n = 1), and Lebanon (n = 1). Men in the HSD group were from Kuwait (n = 1) and Egypt (n = 1). Women in the control group were from Kuwait (n = 15), India (n = 9), Egypt (n = 1), and Syria (n = 1). Men in the control group were from Kuwait (n = 1) and Saudi Arabia (n = 1).

During rest, statistically significantly greater elasticity was identified in the HSD group relative to the control group at the GM-AT musculotendinous junction (strain ratio 2.05 ± 1.31 vs. 1.48 ± 0.49, respectively), mid AT (3.60 ± 1.97 vs. 2.66 ± 1.00), and distal AT (4.57 ± 2.69 vs. 3.22 ± 1.94) (all p < 0.05) (Table 2). During maximal isometric plantar flexion contraction, the descriptive statistics showed higher elasticity in the HSD group compared to the control group in GM-AT musculotendinous junction (3.40 ± 2.16 vs. 2.62 ± 1.07), and mid AT (10.75 ± 5.29 vs. 8.49 ± 3.53). However, the observed differences were not statistically significant (all p > 0.05) (Table 2). The descriptive statistics pointed toward higher elasticity of the GM in HSD relative to controls during rest (4.05 ± 1.43 vs. 3.62 ± 0.78), but not during contraction (4.23 ± 1.29 vs. 4.19 ± 1.31) and neither were statistically significant (both p > 0.05) (Table 2). No significant differences were found between the two groups in the cross-sectional areas (all p > 0.05) (Table 2).

The intensity of joint pain was significantly greater among the HSD group at the hip, knee, and ankle joints, bilaterally (all p < 0.001) (Table 3). During rest, weak but statistically significant correlations were found between the intensity of hip joint pain and the strain ratio of the GM; r = 0.362 (p = 0.008), and between the intensity of knee joint pain and the strain ratio of the GM-AT musculotendinous junction; r = 0.360 (p = 0.008) (Table 4). During maximal contraction, weak but statistically significant correlations were determined between the Beighton score and the strain ratio of the GM-AT musculotendinous junction and mid AT; r = 0.318 (p = 0.017) and r = 0.267 (p = 0.047), respectively (Table 4). The Beighton score showed significant correlations with pain intensity at the hip, knee and ankle joints (all p < 0.05) (Table 4).

Receiver operating characteristics curves for the strain ratio at the proximal, mid, and distal AT at rest are presented in Figure 2. These suggested that the strain ratio may not, on its own, distinguish between those with and without HSD. This was confirmed by analysis of the area under the curve (AUC), yielding a maximum point estimate of 0.635 for the distal AT (Figure 2). None of the AUC estimates were statistically significant (all p > 0.05). Linear regression equations were constructed to predict the strain index, but the R square values were very low (Appendix).

This is the first study to intensively explore the GM-AT complex in people with HSD using SEG technology. The findings from this study have implications for presenting an objective procedure to assist in the clinical examination of patients with HSD in terms of the diagnosis, identification process and monitoring of progression. Statistically significant reductions in the strain ratio were identified in the HSD group relative to the control group during rest at the musculotendinous junction, mid AT, and distal AT. Strain ratio was also found to be reduced during maximal isometric plantar flexion contraction, yet the reductions observed did not reach statistical significance. The objective procedure introduced shows that using SEG might be clinically valuable as it demonstrated weak but significant correlations with the major symptomatic features of HSD, including joint hypermobility and pain intensity at the lower limb joints. Although there were statistically significant between-group differences at the proximal, mid and distal AT strain ratio at rest, ROC curve analysis suggested that the strain ratio may not have adequate sensitivity and specificity when used alone as a diagnostic criterion. SEG might need to be combined with clinical examination to enhance its diagnostic capability. However, it should be noted that the study was not specifically designed to explore sensitivity and specificity and is likely to have been under-powered for such analysis (some p-values were approaching statistical significance). Further specific investigation of strain ratio as a potential diagnostic criterion is warranted, in both HSD and hEDS.

Various elements related to etiological and symptomatic features could explain the reduction in strain ratio observed the GM-AT complex in HSDs. The biomechanical properties of the musculotendinous structures are highly reliant on the connective tissues. Specifically, the connective tissues bind the underlying structures and form a framework of support to maintain the strength of the musculoskeletal system. Indeed, HSDs are connective tissue disorders, and collagen is the main type of fibers found in the connective tissues (36). Collagen gives the musculoskeletal structures their mechanical strength, and forms 65–80% of tendon composition (36). However, various studies have found mutations, and molecular defects in the genes encoding collagen, and in the enzymes responsible for collagen modification in people with JHS/EDS-HT (1, 4). However, the genetic etiology of HSD is not yet conclusive and is sometimes contradictory. The etiology has also been related to abnormalities in an extracellular glycoprotein (tenascin-X), which bridges between collagen fibers and contributes toward collagen formation, and in fibroblast mechanobiology of cell adhesion and cytoskeleton organization (20, 37). The abnormalities of vital components of the connective tissues in HSD could explain the significant reduction in the strain ratio of the GM-AT complex identified by the current study. Moreover, muscle weakness in HSD could explain the reduced strain ratio. Strength was found to be significantly reduced in HSD including the muscles of the shoulder, hand, hip, knee, and foot (15, 26, 38). Muscle strength was associated with activity limitation in HSD, which could lead to a vicious cycle of further weakness due to the muscular disuse effect (15). Muscle strength could be further reduced due to participation restrictions due to joint pain in HSD, increasing the impact of the symptomatic features on reducing the strain ratio of musculotendinous structures (21).

Two studies were performed previously using SEG to explore the GM-AT complex in HSDs during rest (21, 22), but contraction status was not explored previously. The midpoint of the AT was explored at rest in HSD by one previous study (22). The mean difference between the two groups of the mid-AT was relatively similar at 1.32 in the previous study (22), and 0.94 in the current study. However, no significant difference in the strain ratio was identified by the previous study, while the current study found increased elasticity of the AT at the mid-point. The current study was designed to be more powerful in terms of sample size for AT examination, which could explain the ability of the current study to detect between-group differences. The previous study included 21 patients with HSD, and so the sample size increased by 33.3% in the current study, reaching 28 patients in the HSD group (22). The GM muscle was examined in both previous studies, supporting the current identification of the insignificant impact of HSD on the GM biomechanical properties (21, 22). The first was a small feasibility study comparing ten patients with JHS relative to ten healthy controls (21). However, the findings cannot be directly compared because of using different analysis approaches, where software was used to count the color pixels of the colored SEG images in the previous study (21). Even so, no significant differences were found in the GM muscle elasticity in the proportion of red (soft tissues), green (medium tissue), and blue pixels (hard tissue) (all p > 0.05) (21). The second study used the strain ratio and found no significant difference in GM elasticity in the HSD group compared to the control group (p > 0.05), in line with the findings of the current study (22). Although not statistically significant, descriptive statistics suggested a higher proportion of soft tissues in GM muscle for JHS participants by Alsiri (21), and a higher strain ratio of the GM muscle in HSD participants in the current study (21). Notably, the mean difference in GM elasticity between the two groups was similar at 0.43 in the current study and 0.47 in the previous study (22). A type II error could explain the insignificant differences found in the current and previous studies.

To identify the most sensitive clinical examination for the biomechanical properties of HSD, the present study employed two muscular statuses: rest and maximal contraction, of different anatomical regions of the GM-AT complex. Examination at rest seems more sensitive, as it succeeded in detecting between-group differences; therefore, it is recommended for clinical examination. GM-AT musculotendinous junction, mid AT and distal AT, are recommended as anatomical regions for clinical examination. Yet, it seems that mid and distal AT are more sensitive as they showed the highest mean difference of 0.94 and 1.34 for strain ratio, respectively. Clinically, the mid AT is more applicable and standardizable being between the two malleoli. The distal AT was more difficult to examine because it is against the calcaneus. Accordingly, the current study presents three clinical examinations to determine the biomechanical properties of the GM-AT complex in HSD for patient identification, as a supplementary diagnostic tool, and for monitoring purposes. The superior test could be considered the mid-AT, followed by the distal AT, then the GM-AT musculotendinous junction, all during rest. The examination under maximum voluntary contraction was introduced to potentially increase the sensitivity of the test. It was reported previously that changing the contraction status of the AT from rest to contraction changed the strain ratio, and examination at rest showed large variabilities in strain ratio compared to contraction examination (21, 39). However, these observations were reported as a response for healthy AT (21, 39). The current study shows that for people with HSD, the examination at rest is more sensitive for detecting differences compared to contraction examinations. For contraction examination, the AT was hard to deform with the mild compressions recommended for SEG examination, therefore, a greater amount of compression was required to cause the required tissue deformation to generate elastograms. Moreover, the descriptive statistics identified by the current study could be clinically utilized to determine the expected range of strain index for patients with HSD while correlating the strain index with the color-coded images of the SEG (patients with HSD would manifest an increase in red areas of the examined structures). More importantly, recording the strain index for each patient could be potentially used as a benchmark to determine the effectiveness of management as well as the progression of the condition.

The study could be limited due to several factors. Firstly, the operator was not blinded, which could cause expectation bias. However, the SEG is equipped with a visual quality indicator to standardize the applied compressions, and the operator showed high reliability for examining the GM-AT complex (22). Patients with recent injury within the last 3 months were excluded from the study as it was considered that such structures would not accurately represent the biomechanical properties of patients with HSD due to ongoing tissue healing. However, an alternative perspective is that people with HSD may experience recurrent injuries and dislocations (11, 21), and the exclusion of such patients could have affected the generalizability of the findings be only including patients who were less severely affected by HSD. The study sample was exclusively Asian and, although ethnic differences in strain ratio values have not yet been established, care should be taken in extrapolating the findings to other populations. Shear-wave type of SEG is recommended for future research, which produces shear waves for strain measurement to remove the requirement for manual compressions applied by the examiner in compression SEG (40). Two examination statuses were used in the present study–rest and contraction–but comparing the data from these two tests is difficult because mild compressions were used at rest, and harder compressions during contraction. The current study also had a number of strengths, including a robust methodology of highly standardized procedures, focused anatomical areas, justified sample size, and in-depth statistical analysis approaches.

In conclusion, people with HSD demonstrated greater elasticity of the GM-AT complex at rest than age and gender-matched control participants. Future research is required to establish the diagnostic, clinical, and research utility of strain ratio measurement in HSD and in hEDS.

The original contributions presented in this study are included in this article/supplementary material, further inquiries can be directed to the corresponding author.

The studies involving human participants were reviewed and approved by Kuwait Ministry of Health Ethics Committee. The patients/participants provided their written informed consent to participate in this study.

NA designed the study, performed the data collection and statistical analysis, and wrote the manuscript. SP designed the study, statistically analyzed part of the data, and wrote the manuscript. Both authors contributed to the article and approved the submitted version.