Study on the correlation between shear wave elastography and MRI grading of meniscal degeneration

Abstract

Background

Shear Wave Elastography (SWE) offers quantitative insights into the hardness and elasticity characteristics of tissues. The objective of this study is to investigate the correlation between SWE of the menisci and MRI-assessed degenerative changes in the menisci, with the aim of providing novel reference source for improving non-invasive evaluation of meniscal degenerative alterations.

Methods

The participants in this study were selected from patients who underwent knee joint MRI scans at our hospital from February 2023 to February 2024. The anterior horns of both the medial and lateral menisci were evaluated using SWE technique. The differences in elastic values of meniscus among different MRI grades were compared. The correlation between elastic values and MRI grades, as well as various parameters, was analyzed. Using MRI Grade 3 as the gold standard, the optimal cutoff value for meniscal tear was determined. The intraclass correlation coefficient (ICC) was employed to evaluate the reliability of repeated measurements performed by the same observer.

Results

A total of 104 female participants were enrolled in this study, with 152 lateral menisci (LM) and 144 medial menisci (MM) assessed. For the male group, 83 individuals were included, with 147 LM and 145 MM evaluated. The results demonstrated statistically significant differences in the elasticity values of the menisci at the same anatomical sites across different MRI grades (P < 0.001). Within the same grade, the MM had higher elasticity values than the LM, showing a statistically significant difference (P < 0.001). The elasticity values of the menisci were higher in males compared to females. There were statistically significant positive correlations between the elasticity values of the menisci and age, BMI, and MRI grade. The ICC for repeated measurements within the observer demonstrated good reliability (> 0.79).

Conclusions

The meniscal elasticity values measured by SWE exhibit a significant positive correlation with the grades of degeneration assessed by MRI. Furthermore, the elasticity values of the meniscus are found to increase with advancing age and elevated BMI.

Background

The meniscus is essential for the proper function of the knee joint, as it is tasked with the critical responsibilities of pressure bearing and distribution, shock dampening, and the preservation of joint integrity throughout a range of motions [1]. In the context of an aging demographic coupled with the prevalence of sports-related activities, the incidence of meniscal degeneration and trauma has risen, posing as prevalent osteoarticular afflictions. These conditions may precipitate an accelerated decline in the condition of the knee’s articular cartilage, potentially culminating in the development of osteoarthritis of the knee [2]. Consequently, the timely and precise identification of meniscal pathology alongside the implementation of proactive therapeutic measures are paramount for clinical practice.

Arthroscopy is widely recognized as the gold standard in diagnosing meniscal lesions; nonetheless, its invasive nature typically reserves it for therapeutic interventions subsequent to the verification of meniscal impairment. Magnetic resonance imaging (MRI) offers excellent resolution in visualizing meniscal structures, making it the favored non-invasive diagnostic modality. MRI boasts an impressive sensitivity of 88% and a high specificity of 94% in the detection of meniscal injuries [3]. Despite these advantages, MRI incurs significant costs, is prone to motion-related artifacts, and is not recommended for patients with metallic implants or those suffering from claustrophobia. High-definition ultrasonography emerges as an economically viable, non-invasive, and expeditious alternative; yet, the debate surrounding its diagnostic accuracy persists [4].

Shear wave elastography (SWE) is an ultrasound imaging technique based on the principles of quantitative measurement of tissue elasticity. It measures transient displacements in the tissue by applying external stress, thus providing quantitative information about tissue hardness and elastic characteristics. The assessment of tissue resilience is quantified through measurements in kilopascals (kPa) or as a speed in meters per second (m/s), correlating linearly with the degree of tissue rigidity [5]. SWE serves as a valuable diagnostic tool for examining diverse injuries and pathological states within the musculoskeletal framework. Notably, in scenarios of muscle strains, atrophy, and tendinosis, the quantified elasticity tends to be reduced, in contrast to the elevated values observed in instances of muscle spasticity and ligamentous thickening [6].

The objective of this study is to investigate the correlation between SWE and MRI in assessing degenerative changes of the meniscus, aiming to provide novel reference source for clinical diagnosis and treatment.

Methods

Study subjects

This prospective study was approved by the Medical Ethics Committee at Shenzhen Hospital (Futian) of Guangzhou University of Chinese Medicine (No.GZYLL(KY)-2022-039). Written informed consent was obtained from all study participants prior to their inclusion. The study population consisted of individuals who underwent knee MRI scans at Shenzhen Hospital (Futian) of Guangzhou University of Chinese Medicine between February 2023 and February 2024. Previous research [7] has demonstrated gender variations in the elastic modulus of the meniscus. As a result, the research subjects were categorized into male and female groups based on the following inclusion criteria:

• 1. Chinese citizens aged between 40 and 69 years old.

• 2. Individuals who have not previously taken part in other clinical trial studies.

• 3. All participants must be willing to adhere to the experimental protocol and sign an informed consent form.

Exclusion criteria were as follows:

• 1. Patients with a history of knee joint surgery;

• 2. Pregnant or lactating women;

• 3. Individuals with heart disease, respiratory system diseases, or other serious illnesses that prevent them from undergoing ultrasound evaluation;

• 4. Individuals with skin damage or infection that makes them unsuitable for ultrasound examination;

• 5. Poor quality of SWE (The image must meet all three of the following criteria concurrently: (I) The motion stability index (M-STB) exhibits a rating of 5 stars; (II) The quality control chart consistently displays green; (III) The reliability index (RLB index) exceeds 98%.).

Instruments and methods

The MRI examinations were conducted using the Philips Healthcare Ingenia 1.5T MRI scanner equipped with an 8-channel phased-array knee coil as the receiving coil. In the context of our investigation, the following MRI protocols were utilized for scanning:

The T1WI_TSE_SAG sequence was configured with a TR of 492 ms, a TE of 8 ms, a FOV measuring 160 mm by 160 mm by 79 mm, and a voxel size of 0.5 mm by 0.59 mm by 3.0 mm, with the entire scanning process taking 1 min and 52 s.

For the T2WI_SPAIR_COR sequence, the parameters included a TR of 2865 ms, a TE of 70 ms, a FOV of 160 mm by 160 mm by 80 mm, with voxel dimensions of 0.58 mm by 0.69 mm by 3.5 mm, and the scan duration was 2 min and 52 s.

The T2WI_SPAIR_TRA sequence was set with a TR of 3000 ms, a TE of 60 ms, covering a FOV of 160 mm by 160 mm by 92 mm, with voxel dimensions of 0.55 mm by 0.65 mm by 3.5 mm, and the scanning time was 2 min and 36 s.

Lastly, the PDW_SPAIR_SAG sequence employed a TR of 2776 ms, a TE of 30 ms, a FOV of 160 mm by 160 mm by 79 mm, and a voxel size of 0.62 mm by 0.62 mm by 3.0 mm, with the scan time being 2 min and 52 s.

Patients were situated in a supine position, ensuring both knees were securely immobilized and aligned with an external rotation of 15 degrees to optimize stability. The imaging procedure was conducted in a craniocaudal approach, commencing at the level of the feet and progressing towards the head.

All SWE assessments were performed utilizing the Mindray Company’s Resona 7T ultrasound diagnostic system. This system is furnished with an L14-5 high-frequency linear array transducer, which boasts a frequency spectrum spanning from 5 to 14 MHz, and is seamlessly integrated with SWE capabilities.

Initially, a senior radiologist, who is also a co-author of this study, conducted an MRI analysis of the anterior horns of the menisci, using the classification system suggested by Lotysch et al. [8]. This system categorizes the meniscal status as follows: Grade 0 denotes a meniscus without any abnormalities. Grade 1 is indicative of small foci or punctate areas of elevated signal intensity within the substance of the meniscus, which are not in continuity with the joint surface. Grade 2 refers to the presence of linear or horizontal regions of increased signal intensity within the meniscus that do not reach or contact the joint surface. Grade 3 describes a meniscus with regions of increased signal intensity that extend to and involve at least one of the articular surfaces.

All ultrasonographic examinations were conducted by another author, an ultrasonographer with over five years of experience in musculoskeletal ultrasound diagnostics. The outcomes of the MRI were disclosed to the investigative team and the study participants solely post the conclusion of the SWE evaluations on the subjects. Prior to this disclosure, the radiologist maintained the confidentiality of the MRI results for all parties involved.

Before the commencement of the SWE procedure, participants were instructed to avoid any form of strenuous exercise affecting the lower limbs, including but not limited to extended periods of running, cycling, and squat exercises, for a two-hour period preceding the examination. They were also asked to rest in a horizontal position for a duration of five minutes before the examination. The individuals were positioned in a supine manner on the diagnostic table, with a circular pillow placed at the popliteal fossa to maintain a slight bend in the knees, typically between 15 and 25 degrees. The examiner initiated a preliminary two-dimensional grayscale scan in muscle and bone mode to capture an optimal coronal view of the anterior horns of both the medial and lateral menisci. Subsequently, the examination protocol transitioned to the “STE elasticity” mode, which was configured to measure within a range of 0-200 kPa, without the application of additional pressure. The visual representation of tissue elasticity was depicted through a color spectrum, with deep blue indicative of the least rigid tissues and deep red signifying the most rigid. A Motion Stability Index (M-STB) rating of 5 stars suggested that the participant’s respiratory and positional movements exerted minimal influence on the elasticity measurements, thereby denoting superior motion stability. Moreover, a green quality control chart and a Reliability Index (RLB index) exceeding 98% confirmed the acquisition of high-fidelity shear wave elastography images, which were then suitable for capture and subsequent detailed analysis.

Data collection

We positioned a circular sampling frame with a 2 mm diameter at the center of the meniscus as the region of interest (ROI). The numerical values for the following parameters were obtained: the average value (EMean), maximum value (EMax), minimum value (EMin), and standard deviation (ESD) of Young’s modulus. In order to minimize measurement errors, we conducted three repeated scans and measurements on each section, recorded the EMean values for the three measurements, and then calculated the average as the final measurement result.

In addition, we also recorded some basic information of each research subject, including gender, age, height, weight, and body mass index (BMI)(Figs. 1 and 2) .

Fig. 1

A 65-year-old male patient presented with a tear in the anterior horn of the medial meniscus (MM) of the left knee joint. Image A shows a proton density-weighted sequence MRI, revealing a torn signal in the coronal plane of the MM anterior horn (indicated by the red circle). Image C presents a 2D grayscale ultrasound image, indicating a torn hypoechoic area in the coronal plane of the MM anterior horn (indicated by the red triangle). Image B displays a shear wave elastography (SWE) confidence map of the meniscus, appearing uniformly green. Image D presents a color-coded SWE map, with deep blue indicating extreme softness and deep red indicating extreme hardness. The torn meniscus exhibits a region with light green coding, indicating increased hardness within the torn area. A region of interest (ROI) with a diameter of 2 mm is placed at the central position of the meniscus for measurement. The reliability index (RLB) is 100%. (F, femur; MM, medial meniscus; T, tibia)

Fig. 2

The patient’s knee arthroscopy image

Statistical methods

Statistical analysis was conducted using IBM SPSS 25.0 software. The continuous variables (age, height, weight, BMI, and elasticity values) were reported as mean ± SD. Comparisons between the elastic values of menisci in the same site but with different MRI grades were assessed using one-way analysis of variance (ANOVA) with the least significant difference (LSD) method for post-hoc pairwise comparisons. Independent samples t-tests were used to compare elastic values between different sites with the same MRI grade. Correlation analyses between different parameters were performed using either Pearson’s or Spearman’s correlation analysis, depending on the nature of the data. The optimal cut-off value for diagnosing meniscal tear was determined by selecting the highest Youden’s index, using MRI Grade 3 as the gold standard. Sensitivity, specificity, area under the curve (AUC), and 95% confidence interval (CI) were then calculated. Finally, the intraclass correlation coefficient (ICC) was utilized to assess the reliability of repeated measurements conducted by the same observer. A p-value of less than 0.050 was considered statistically significant.

Results

A total of 104 female participants were included in the study, with 44 individuals undergoing bilateral knee joint meniscal scans and 60 individuals undergoing unilateral knee joint meniscal scans. Altogether, 152 lateral menisci (LM) and 144 medial menisci (MM) were included for statistical analysis. The male group consisted of 83 individuals, with 63 individuals undergoing bilateral knee joint meniscal scans and 20 individuals undergoing unilateral knee joint meniscal scans. A total of 147 LM and 145 MM were included for statistical analysis. Basic demographic data are presented in Table 1. The distribution of menisci based on the MRI degeneration grade is shown in Table 2 for both gender groups.

Table 1 Baseline characteristics

Table 2 Meniscus degeneration grades distribution for lateral and medial Meniscus

Table 3 presents a comparison of meniscal elasticity values at different MRI grades within the same gender group, for both LM and MM. Using the LSD method for pairwise comparisons, all p-values were found to be less than 0.001, indicating statistical significance. Within the same grade, MM exhibited significantly higher meniscal elasticity values than LM. Furthermore, when comparing among different gender groups, males demonstrated significantly higher meniscal elasticity values than females, for the same location and grade.

Table 3 Meniscal elasticity values according to Meniscal Degeneration Grading Via MRI

Regarding correlation, as shown in Table 4, the elasticity values of the LM and MM groups in both gender categories demonstrate statistically significant positive associations with age, BMI, and MRI-Grade. No correlations have been observed with height and weight.

Table 4 Correlation between Meniscal elasticity values and age, BMI, and MRI degeneration Grade

Using MRI Grade 3 as the reference standard, we determined the optimal cutoff value for diagnosing meniscal tears to be the threshold with the highest Youden’s index. This cutoff value was then used to calculate the corresponding sensitivity, specificity, AUC, and 95%CI, as presented in Table 5.

Table 5 Sensitivity, specificity, Youden’s index, AUC and 95% CI of the elasticity values for Torn Menisci

Regarding the reliability of repeated measurements by the same internal observer, as presented in Table 6, the ICC and 95% CI for the elasticity values of the LM and MM are 0.793 (0.747–0.833) and 0.803 (0.759–0.842), respectively.

Table 6 Intraclass Correlation Coefficient for Intraobserver Measurement of Lateral and medial Meniscus elasticity values

Discussion

The meniscus is a highly hydrated fibrocartilaginous tissue consisting of water, extracellular matrix (ECM), and cells. Collagen is the predominant component in the ECM, followed by proteoglycans (PGs) [9].

Battistelli et al. [10] conducted a study on histopathological differences between healthy and torn menisci. In healthy meniscal samples, collagen fibers and PGs were evenly distributed in the internal region, with minimal calcium deposits. Additionally, the collagen fibers exhibited regular periodic orientation, indicating structural integrity and favorable tissue engineering characteristics. In contrast, torn meniscal samples showed disrupted fibrocartilage, decreased and disorganized collagen fibers, a significant increase in PGs with localized accumulation, as well as extensive areas of calcification. Torn meniscal samples typically maintained an intact surface but exhibited evident degradation in the internal tissue. These findings suggest a degenerative process in the meniscus that likely initiates internally and gradually spreads to the surface over time, ultimately resulting in complete destruction or loss of meniscal tissue. These findings support the MRI grading system for meniscal degeneration proposed by Lotysch et al. [8], indicating that histopathological changes can be graded and diagnosed through MRI evaluation.

These variations also provide histological foundations for the application of SWE in the meniscus. In our study, the elastic values of the meniscus significantly increased with the degeneration grade of MRI, demonstrating a strong positive correlation. This finding is consistent with the results of Gurun [11] and Park et al. [12].

Multiple previous studies [7, 11, 12] have confirmed that the hardness values of the MM are higher than those of the LM. Our research findings similarly demonstrate that the elasticity values of the meniscus are consistently higher in the MM compared to the LM at the same MRI grade, with statistical significance. A study [13] has found that, in comparison to the anterior horn of the LM, the anterior horn of the MM exhibits significantly higher content of collagen fibers and PGs throughout the tissue depth, suggesting a denser internal tissue structure in the MM. On the other hand, due to anatomical differences between the MM and LM [14], the MM is firmly attached to the capsule and synovium, making it less mobile. Consequently, the MM is more susceptible to degeneration and tears. This explains why the elasticity values of the MM are consistently higher than those of the LM.

The elastic modulus of the meniscal tissue was found to be higher in the male group, consistent with previous studies [7] and the findings of Pereira et al. [15]. This difference may be attributed to factors such as gender-related mechanical loading and physiological hormones. Men generally have a higher body mass and muscle strength, which increases mechanical loading and promotes collagen synthesis. Collagen is an important component of the meniscus, and its content is positively correlated with the hardness of the meniscal tissue [16]. On the other hand, compared to men, women tend to have a higher body fat percentage, which can lead to a decrease in the content of type I and type III collagen [17]. Studies have shown that estrogen receptors are present in the meniscus of the knee joint [18]. The lack of estrogen can result in subchondral bone resorption, articular cartilage degeneration, and meniscal cell apoptosis [19]. These findings suggest that estrogen plays an important role in maintaining the health of the meniscus.

As individuals age, the composition of the meniscus undergoes a series of complex changes, including chondrocyte senescence, accumulation of advanced glycation end-products (AGEs), and calcium deposition [20]. Aging of meniscus chondrocytes leads to damaged meniscal epithelial cells and disturbed tissue homeostasis. The accumulated AGEs and calcium deposition render the tissue stiff and fragile, ultimately causing degenerative changes or tears in the meniscus. These alterations result in decreased elasticity, increased brittleness, and diminished compressive ability of the meniscus, consequently leading to an increase in hardness values. Our study found a positive correlation between the hardness value of the meniscus and age, consistent with previous research findings [7, 11].

Research [9] has shown that obesity is a significant risk factor for knee injuries. The incidence and severity of knee joint pathology are positively correlated with BMI. When the knee joint is under load, the body weight exerts pressure on the meniscus. A higher BMI implies additional load and pressure, which may lead to meniscal damage and tears. Compared to individuals with normal weight, obese individuals have a nearly four times higher probability of experiencing a meniscal tear. The findings of this study suggest a positive correlation between the elastic modulus of the meniscus and BMI, which aligns with the results of Dag et al. [5].

Limitations

Although Table 3 shows statistical differences in the elastic values of menisci among different MRI grades, it is also apparent that the differences between adjacent grades are not substantial, particularly between Grade 0 and Grade 1. This suggests that specific elastic data may have limited clinical significance in practice. However, these data do indicate a certain association between elastic values and MRI grades. Additionally, in Table 4, we found a very strong correlation (r > 0.9), indicating that meniscal SWE can reflect the degeneration level seen on MRI.

We identified the optimal cut-off value, at which the Youden’s index was highest, for diagnosing meniscal tears using MRI Grade 3 as the reference standard. This approach yielded excellent sensitivity, specificity, AUC, and 95%CI. However, it is important to note that these findings may not be universally applicable due to the limited age range (40–69 years) of our study population and potential variations in ultrasound devices. Therefore, caution is advised when generalizing these results.

Due to the narrow space between the femur and tibia, we only assessed the coronal plane of the anterior horn of the meniscus, which can be easily visualized using ultrasonography. However, the most severe degeneration of the meniscus is typically observed in the posterior horn [21]. Therefore, the clinical applicability of this study is significantly limited.

In this study, the SWE assessment of the meniscus was conducted by the same ultrasonographer, and no inter-operator repeatability studies were carried out. While the ICC of intra-observer repeated measurements demonstrated good levels, the variability between operators, inherent in ultrasound examination, could be a cause for concern. Thus, the reliability of the results still requires further validation.

Conclusions

In conclusion, our research findings demonstrate a significant positive correlation between the elastic values of the meniscus measured by SWE and the degeneration grades evaluated by MRI, and indicate the positive influence of age and BMI on meniscus elasticity. However, in view of the limitations of this study, these findings require further empirical research in more diverse groups for confirmation.