Hearing and Seeing Nerve/Tendon Snapping: A Systematic Review on Dynamic Ultrasound Examination

Nerve/tendon snapping can occur due to their sudden displacement during the movement of an adjacent joint, and the clinical condition can really be painful. It can actually be challenging to determine the specific anatomic structure causing the snapping in various body regions. In this sense, ultrasound examination, with all its advantages (especially providing dynamic imaging), appears to be quite promising. To date, there are no comprehensive reviews reporting on the use of dynamic ultrasound examination in the diagnosis of nerve/tendon snapping. Accordingly, this article aims to provide a substantial discussion as to how US examination would contribute to ‘seeing’ and ‘hearing’ these pathologies’ different maneuvers/movements.


Introduction
Snapping commonly occurs as a result of the sudden displacement of an anatomical or pathological structure during the movement of an adjacent joint [1]. Apart from causing curiosity, the clinical scenario can often be accompanied by discomfort or pain, limiting daily professional/sporting activities [1]. Snapping is usually audible and palpable, but rarely visible; therefore, imaging and detecting the actual/responsible structure is crucial, but difficult as well [2][3][4]. Although radiographs, computed tomography and magnetic resonance imaging (MRI) are used for assessing several anatomic structures in this aspect, ultrasound (US) examination appears to be superior and able to contribute more [3,4]. Apart from its high resolution as regards nerve/tendon imaging, the dynamic evaluation of the structures in a patient-and physician-friendly approach is paramount for better understanding 'snapping' [2][3][4]. US examination provides a precise (real-time) correlation between the symptoms and the movement of the suspected structure [1,5,6]. Depending on the suspected nerve/tendon, snapping can be triggered/tested during any position, also conveniently reassuring the patient [5]. Prompt dynamic US examination requires excellent knowledge on US physics, and consequently regarding various artifacts and interactions with different tissues, for better interpretation of the US images/videos [2][3][4].
Although the use of US examination for snapping is well-known, there is no comprehensive review present in the pertinent literature describing how sonographic 'seeing' or 'hearing' can be performed. As such, the purpose of this article was to report the significance/utility of US examination in the diagnosis of snapping tendons and nerves.

Materials and Methods
The present review was performed according to the Preferred Reporting Items for Systematic reviews and metanalysis (PRISMA). The literature research was carried out using databases like PubMed, Scopus and Web of Science. The following keyword combinations were run: "snapping" OR "popping" OR "dislocation" OR "subluxation" AND "ultrasound imaging" AND/OR "ultrasonography" AND "tendons" OR "nerves". No publication date or language restrictions were imposed. The initial search yielded 220 papers for nerves and 99 papers for tendons. Thereafter, 120 articles for nerves and 20 for tendons were removed before screening ( Figure 1). The retrieved studies (100 papers for nerves and 79 for tendons) were then reviewed. Papers focusing on treatment, surgery or treatment that did not discuss how to perform US examination, or those not published in English, were excluded. A total of 72 papers for nerves and 65 for tendons were further reviewed for their titles and abstracts. Finally, 60 papers for nerves and 62 for tendons were identified for full-text reading, whereby 40 papers for nerves and 48 for tendons were included in this systematic review.

Results
Papers selected as regards the US imaging of nerve/tendon snapping either in patients or healthy subjects were analyzed. The agreement between the authors for including the articles was perfect (Cohen's k = 0.87). The main characteristics of the studies (published between 1983 and 2023) are summarized in Table 1 for nerves  and in Table  2 for tendons [1,.

Results
Papers selected as regards the US imaging of nerve/tendon snapping either in patients or healthy subjects were analyzed. The agreement between the authors for including the articles was perfect (Cohen's k = 0.87). The main characteristics of the studies (published between 1983 and 2023) are summarized in Table 1 for nerves  and in Table 2 for tendons [1,.       The 40 papers reviewed for nerves comprised 16 original articles, 6 reviews, 14 case reports, 3 retrospective studies and 2 letters to the editor . Similarly, 49 papers reviewed for tendons comprised 7 original articles, 17 reviews, 23 case reports/series and 1 retrospective study . For nerves, 1156 males (63.8%) and 657 females (36.2%) with an average age of 29.8 ± 15 years had been studied. For tendons, 455 males (29.9%) and 1066 females (70.1%) with an average age of 24.3 ± 14 years had been studied. Usually, the snapping was assessed using B-mode imaging, either with linear or curvilinear probes. The most commonly involved structures were the ulnar nerve (87.5%) and iliopsoas tendon (37.5%).

Discussion
To the best of our knowledge, this review article is a unique summary of 89 publications on US examination for nerve/tendon snapping. In particular, having also summarized the relevant maneuvers/movements, we have demonstrated the utility of dynamic US imaging as a gold standard diagnostic method for snapping. Of note, this type of assessment not only ascertains the snapping structure, but also the possible abnormalities in relation to the clinical condition   (Table 3). The etiology of snapping is linked to a wide range of functional factors [1,9,10,34], especially in biomechanical disorders in which the underlying mechanism is complex. Repetitive movements, overuse, muscle and fascial imbalances or structural abnormalities can be reasons for snapping. In some cases, snapping may be painless, while in some others it can be accompanied by significant discomfort/pain. Additionally, patients affected by snapping/popping phenomena are more susceptible to developing chronic pain and limited joint movement . The majority of the literature reviewed in this study highlighted the role of US examination to unravel difficulties in diagnosing and unveiling the exact biomechanical alterations associated with snapping/popping due to ambiguous symptoms and signs [1][2][3][4][5]. In this regard, performing a simple US examination following an inconclusive physical examination can undoubtedly be contributory .
Snapping phenomena varied between genders; while the prevalence values were 63.8% (nerves) and 29.9% (tendons) in males, they were, respectively, 36.2% and 70.1% in females. All this could be determined by a different tissue composition in the different sexes. We believe that this 'almost complete' opposition needs further investigation.

Nerve Snapping
Concerning nerve snapping (Video S1), the most common regions were reported to be the elbow and the ulnar nerve (87.5%) (Figure 2, Video S2), followed by the medial antebrachial cutaneous nerve (at the elbow), the median nerve (at the wrist) and the sciatic nerve (in the thigh) (Video S3). For the ulnar nerve (snapping over the medial epicondyle), dynamic and short-axis imaging at the cubital tunnel level have been used during various positions of elbow flexion/extension. Additionally, isometric triceps contraction has also been used in certain cases [42].

Tendon Snapping
Regarding tendon snapping, several factors such as a conflict with bony structures, other tendons (intersection), retinacula and thickened pulleys, or instability caused by the rupture of retinacula, have been reported [1]. The iliopsoas tendon was the most In this context, considering the benefits of immediate/dynamic visualization of the complete movement, along with the nerve structure along its entire trajectory, dynamic imaging appears to be the most advantageous imaging modality [42,44,47]. Moreover, Shen et al. [44] reported an instability of the ulnar nerve in children, possibly in relation to the flexible retinaculum of the cubital tunnel. Schertz et al. [19] demonstrated that the morphological compression and dislocation of the ulnar nerve correlated with symptomatology. They postulated that patients with anatomic and/or dynamic variation of the ulnar nerve and its surrounding structures were more prone to developing ulnar-nerve-related complaints [19]. Similarly, the snapping of the medial antebrachial cutaneous nerve over the medial epicondyle was assessed during elbow flexion [23]. Median nerve snapping over the palmaris longus tendon [46], the sciatic nerve at the ischiofemoral space (during hip rotations) [28], the proper digital nerve of the fifth toe (during flexion/extension) [29] and the superficial radial nerve during thumb flexion/extension [34] are some other scenarios reported in the literature. Dynamic US examination can be readily performed from a technical standpoint. However, it is imperative for the sonographer to possess detailed knowledge of the local anatomy in order to precisely identify the possible anatomical variations. While such variations can be evaluated (generally statically) by using computed tomography and/or magnetic resonance imaging, US is a far more accessible and affordable diagnostic modality . It is noteworthy that a visualization of the snapping can also be coupled with the sensation or sound of snapping during real-life instances.

Future Perspectives Assessing Pros/Cons of Dynamic US Examination in the Evaluation of Nerve/Tendon Snapping
To date, the concept of dynamic US imaging has been widely accepted [2][3][4]. However, several of the dynamic US assessments obtained can contribute to the exact diagnosis and monitoring of the snapping condition if they are properly interpreted in a clinical/surgical/rehabilitative context. In terms of therapeutic approach, different publications reported that the precise detection of the cause and its severity played an important role. Accordingly, conservative vs. surgical treatment alternatives can be promptly applied, as well as followed, thereafter. Needless to say, the former group includes proper posture maintenance, excessive movement avoidance, regular stretching and strengthening, all of which aim to help muscle/fascial balance and flexibility [95].
However, while the pros are that dynamic US imaging enables real-time and multidirectional US observation, providing a more accurate, precise and objective approach to assessing the nerves and tendons movement, the cons would be that in some cases, during dynamic US imaging, it can be difficult to identify which anatomic structure is snapping. Bjerre et al. [39] reported that an accessory snapping triceps tendon can clinically be confused with the snapping of the ulnar nerve [39], as the two structures are closely located at the medial epicondyle. Moreover, a careful evaluation of nearby anatomical structures is mandatory, with particular attention on the various movement directions and degrees during the maneuvers. For example, Asopa et al. [68] demonstrated that the pes anserinus snapping can be secondary to a meniscal cyst, and only by dynamic US imaging was it possible to underline the snapping cause, avoiding incorrect surgery. The MRI revealed a lobulated parameniscal cyst, but it was unable to provide a definitive explanation for the snapping sensation. In contrast, dynamic US imaging permits the successful identification of both meniscal cysts and tears, permitting the observation that the sartorius was anterior to the cyst in the neutral position, while the gracilis tendon was located posteriorly. Inevitably, during knee flexion, the sartorius tendon snapped over the cyst and moved to a posterior position at the front edge of the gracilis tendon. When extending the knee back to a neutral position with active quadriceps muscle contraction, the sartorius tendon swiftly moved forward, traversing over the cyst, resulting in a distressing snapping sensation [68].
Due to the superior sensitivity of dynamic US examination in comparison with static US examination and MRI, it has the potential to be an initial modality or to reduce the numbers of imaging examinations. While interest in this US examination is increasing, there are several issues to be considered and solved. First, more methodologically rigorous studies are still needed. The issues in conducting clinical studies include the choice of reference standards for the final diagnosis, the competency of examiners and the standardization of findings. Second, there were few pieces of evidence on the utility of dynamic US examination to differentiate particular nerve/tendon snapping based on the standardization of dynamic US maneuvers. Third, knowledge of anatomical variations is crucial to better highlight the correct anatomical structure snaps and the reasons that determine it.
The utility of dynamic US examination in nerve/tendon snapping has been shown mainly in the fields of physical and rehabilitative medicine, radiology, orthopedics and neurology. Collaboration between these specialties is indispensable for the further development of this assessment modality.
The limitations of this review would be the small number of patients included in different studies and the heterogeneity of the article types. Also, taking into account the possible variations as regards the expertise of sonographers and the device settings, it was not reasonable or conclusive to carry out further statistical analysis.

Conclusions
In closing, this review shows that dynamic US examination can be efficiently incorporated as an extension of physical examination for the evaluation of nerve/tendon snapping in daily clinical practice. It is noteworthy that such an assessment would not only unmask the actual cause/structure responsible for snapping, but would also guide the treatment as well as the close follow up during management. To this end, simultaneously 'seeing' and 'hearing' the snapping under US examination is invaluable for musculoskeletal physicians.
Supplementary Materials: The following supporting information can be downloaded at https:// www.mdpi.com/article/10.3390/s23156732/s1 , Video S1. Snapping of the radial nerve. Video S2. Snapping of the ulnar nerve. Video S3. Snapping of the sciatic nerve. Video S4. Snapping of the peroneal tendons.
Funding: This research received no external funding.

Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author. The data are not publicly available due to privacy.