Tendon Anatomy and Tendon Disorders of the Wrist

Abstract

Background

Wrist pain is common and can be attributed to tendon pathologies.

Methods

This review aims to give a structured review of tendon anatomy, discuss anatomical pitfalls, and provide an overview of typical tendon pathologies of the wrist based on the current literature.

Results

Typical tendon pathologies of the wrist include de Quervain tenosynovitis, proximal and distal intersection syndrome, extensor and flexor pollicis longus tendon ruptures, and extensor carpi ulnaris subsheath injury. Typical pitfalls are multiple bundles of the abductor pollicis longus tendon or the centrally increased signal of the extensor carpi ulnaris tendon.

Conclusion

Both ultrasound and MRI are appropriate modalities for assessing the tendons of the wrist. Knowledge of normal anatomy, variants, pathologies, as well as appropriate imaging is crucial to determine the diagnosis.

Key Points

• Multiple tendon bundles of the abductor pollicis longus are a common anatomical feature and should not be mistaken for tendon splitting.

• An anatomical pitfall resembles the frequently found centrally increased signal of the extensor carpi ulnaris tendon caused by fibrovascular tissue.

• In order to visualize the diagnosis of a proximal intersection syndrome, the MR scan field needs to include the area approximately 4 to 8 cm proximal to Lister’s tubercle.

• The tendons of the thumb, i.e., extensor and flexor pollicis longus, are most commonly torn after distal radial fracture (EPL) and osseous hardware fixation (FPL).

Citation Format

• Marth T, Grob NA, Jacobson JA et al. Tendon Anatomy and Tendon Disorders of the Wrist. Rofo 2025; DOI 10.1055/a-2499-5875

Zusammenfassung

Hintergrund

Handgelenkschmerzen sind häufig und können auf Pathologien der Sehnen beruhen.

Methoden

Dieser Review soll eine strukturierte Übersicht der Sehnenanatomie geben, Fallstricke diskutieren und einen Überblick über typische Sehnenpathologien des Handgelenks verschaffen, basierend auf der aktuellen Literatur.

Ergebnisse

Typische Sehnenpathologien des Handgelenks sind eine de Quervain Tendovaginitis, ein proximales und distales Intersektionssyndrom, Rupturen der extensor und flexor pollicis longus-Sehnen und Verletzungen der Subscheide der extensor carpi ulnaris-Sehne. Typische Fallstricke sind multiple Sehnenbündel der abductor pollicis longus-Sehne oder ein zentral erhöhtes Signal der extensor carpi ulnaris-Sehne.

Schlussfolgerung

Ultraschall und MRT sind beides geeignete bildgebende Modalitäten zur Untersuchung der Handgelenksehnen. Kenntnis der normalen Anatomie, Varianten, Pathologien sowie der angebrachten Bildgebung ist entscheidend, um die Diagnose zu stellen.

Kernaussagen

• Multiple Sehnenbündel der abductor pollicis longus-Sehne sind ein häufiges anatomisches Merkmal und sollten nicht als Aufspaltung der Sehne fehlinterpretiert werden.

• Ein anatomischer Fallstrick ist das häufig erhöhte zentrale Signal der extensor carpi ulnaris-Sehne, welches durch fibrovaskuläres Gewebe hervorgerufen wird.

• Um ein proximales Intersektionssyndrom darzustellen, muss der MR-Scanbereich 4 bis 8 cm proximal des Lister Tuberkels mit einbeziehen.

• Daumensehnen sind häufig rupturiert nach distaler Radiusfraktur (extensor pollicis longus) und osteosynthetischer Versorgung (flexor pollicis longus).

Keywords

wrist - tendons - MR-imaging - ultrasound - intersection syndrome - De Quervain tenosynovitis

Abbreviations

ACP: acute calcific periarthritis

APL: abductor pollicis longus

DECT: dual-energy computed tomography

ECRB: extensor carpi radialis brevis

ECRL: extensor carpi radialis longus

ECU: extensor carpi ulnaris

EDC: extensor digitorum communis

EDM: extensor digiti minimi

EI: extensor indicis

EPB: extensor pollicis brevis

EPL: extensor pollicis longus

FCR: flexor carpi radialis

FCRB: flexor carpi radialis brevis

FCU: flexor carpi ulnaris

FPL: flexor pollicis longus

MCP: metacarpophalangeal joint

MRI: magnetic resonance imaging

PL: palmaris longus

RA: rheumatoid arthritis

STT: scapho-trapezium-trapezoid

UTE: ultrashort echo time

Introduction

The anatomy of the wrist tendons can be divided into the flexor and extensor tendons based on their function and location. Furthermore, the extensor tendons can be assigned to different compartments defined by the extensor retinaculum. Anatomic variants of the tendons and muscles are common and must be distinguished from pathological findings. Pathologies of the wrist tendons are common and are especially reported among individuals involved in activities with repetitive stress on their wrists, e.g., de Quervain tenosynovitis, which is more common in mothers. Imaging techniques such as ultrasound and magnetic resonance are suitable for depicting the different pathologic conditions. However, advanced imaging techniques like dual-energy computed tomography will also be briefly discussed. In this article, we will review the normal anatomy and anatomical variants of the wrist tendons and provide an overview of common pathologies and their presentation on imaging. This will be done by discussing common pathologies and imaging modalities especially in context with the current literature. However, this article does not represent a meta-analysis but rather an overview on these topics, and thus might be considered an unstructured literature review with a focus on clinical relevance. Additionally, postoperative findings will be not discussed in this article. Nevertheless, we believe that we are providing a structured review of a clinically relevant topic that might be of value for interested readers.

Imaging techniques

Ultrasound has gained popularity in the daily clinical practice of hand surgeons and has become a routine diagnostic tool. The lack of ionizing radiation and the low cost, portability, dynamic real-time assessment, and comparison with the unaffected hand are some of the advantages [1]. An additional benefit of ultrasound is that it can be more readily available than other cross-sectional imaging. The ancillary use of duplex Doppler and power Doppler can further reveal blood flow on the synovial membranes and is recommended for the detection of tenosynovitis [2] [3]. The superficial location of the wrist tendons allows the use of high-frequency ultrasound probes with high spatial resolution. Tendons at the level of the wrist can be examined with a linear ultrasound transducer (10–18 MHz) or even a high-frequency ultrasound probe (14–33 MHz) such as a “hockey stick” transducer with a smaller field-of-view [2]. In the setting of distal radius fractures, diagnostic ultrasound is an accurate tool with a sensitivity and specificity of 88% and 99%, respectively, for identifying a specific tendon as ruptured and 88% and 87%, respectively, for tendon abnormalities in general [4]. While ultrasound is dependent on the experience of the examiner, MRI is a noninvasive imaging technique that can produce cross-sectional images on any plane, allowing simultaneous examination of soft tissues, synovium, tendons, articular cartilage, and bone. It delivers the information obtained by ultrasound more objectively and with higher contrast resolution and is considered as the reference standard [5]. In the setting of early inflammatory arthritis, MRI appears to be more sensitive than ultrasound for detecting tenosynovitis [6] [7] and its use may be helpful for detecting ongoing disease in rheumatoid arthritis [8]. Bone marrow edema that cannot be depicted on ultrasound can be visualized via MRI, which is especially of interest in the setting of rheumatoid arthritis, where it is thought to be a strong predictor for erosive progression [9] [10]. When comparing 1.5T systems with 3T systems, the latter should be favored, because it provides a nearly twofold increase in signal-to-noise ratio, that can be used to increase the speed of imaging, achieve a better contrast-to-noise ratio, and improve spatial resolution [11]. The use of 7.0T to image the musculoskeletal system is still in the early, primarily research stages. In tendon imaging there is a special focus on ultrashort echo time (UTE) magnetic resonance imaging, which makes it possible to image short T2 tissues, such as tendons, with a high signal. However, most of the UTE-MRI techniques are still in the validation phase [12].

Computed tomography is in general not used for tendon imaging. However, dual-energy computed tomography is used in the diagnosis of gout arthropathy, which can involve the wrist tendons as well, with a reported sensitivity and specificity of 89% and 91%, respectively [13].

Specific tendon pathologies are going to be addressed in this article while providing an overview of parameters on different imaging modalities.

Anatomy of the extensor tendons of the wrist

At the level of the distal forearm, the extensor tendons are guided through six dorsal extensor sheets ([Fig. 1]), formed by the extensor retinaculum, a strong, fibrous, oblique running band, consisting of a strengthened part of the distal forearm fascia. The extensor retinaculum runs from the anterior border of the radius to the triquetral and pisiform bones. Beginning at the radius and moving toward the ulna, the first dorsal compartment contains the abductor pollicis longus (APL) and the extensor pollicis brevis (EPB) tendons [14] [15]. In 40% of cases, there may be a complete or partial septation between the two tendons, which splits the compartment into two sub-compartments [16]. The APL tendon frequently consists of multiple bundles, with even up to nine bundles described in the literature [17] [18]. Thus, the diagnosis of splitting of the APL tendon should be handled with caution.

Fig. 1 Normal anatomy of the wrist at the level of the extensor retinaculum on an axial T2-weighted MR image. The colored arrows point to the extensor tendons, with the color-matched roman numerals indicating their equivalent extensor compartment. I (blue arrows) = first extensor compartment containing the abductor pollicis longus (APL) and the extensor pollicis brevis (EPB) tendons. II (orange arrows) = second extensor compartment containing the extensor carpi radialis longus (ECRL) and the extensor carpi radialis brevis (ECRB) tendons. III (green arrow) = third extensor compartment containing the extensor pollicis longus tendon (EPL). IV (red arrows) = fourth extensor compartment containing the extensor digitorum (ED) and extensor indicis (EI) tendons. V (grey arrow) = fifth extensor compartment containing the extensor digiti minimi tendon (EDM). VI (yellow arrow) = sixth extensor compartment containing the extensor carpi ulnaris tendon (ECU). The white arrow points to Lister’s tubercle (LT), separating the II from the III compartment. The mnemonic at the bottom of the image makes it easier to remember the sequence of -longus and -brevis in the first three extensor compartments.

Approximately four to eight centimeters proximal to the dorsal tubercle of the radius (Lister’s tubercle) lies the “proximal intersection” (proximal crossing), where the tendons from the first compartment run across over tendons of the second extensor compartment [14]. The latter is located between the first compartment and Lister’s tubercle and contains the extensor carpi radialis longus (ECRL) and extensor carpi radialis brevis (ECRB) tendons with communicating tendon sheets. They also communicate through a normal foramen with the extensor pollicis longus (EPL) tendon at the distal intersection point [19]. Thus, if fluid is present in the tendon sheath at this distal location it tends to surround all three tendons (ECRL, ECRB, EPL). The ECRL inserts at the dorsum of the second metacarpal bone. The ECRB runs on the ulnar side of the ECRL and inserts at the base of the third metacarpal bone and the adjacent parts of the second, and sometimes forth metacarpal bone [15]. Additional muscle and tendon segments are common in the second extensor compartment [14]. The EPL tendon is located in the third dorsal extensor compartment. The EPL tendon passes on the ulnar side of the Lister’s tubercle, crosses obliquely above the radial wrist extensor tendons, and forms the “anatomical snuffbox” at the base of the thumb with the EPB tendon. The EPL lies immediately adjacent to the EPB tendon at the dorsolateral aspect of the first metacarpophalangeal joint (MCP) and inserts at the base of the distal phalanx of the thumb [15]. Of note: To make it easier to remember the names of the tendons of the first three extensor compartments, it is helpful to know that they end alternating: “Longus, Brevis, Longus, Brevis, Longus” (APL, EPB, ECRL, ECRB, EPL) ([Fig. 1]) [20].

The fourth dorsal compartment of the wrist contains the extensor indicis (EI) and the extensor digitorum communis (EDC) tendons. The fifth dorsal compartment of the wrist contains the extensor digiti minimi (EDM) tendon, which commonly has two slips. The sixth dorsal compartment contains the extensor carpi ulnaris (ECU) tendon [14] [15] [21]. The tendon is formed by spiral fibers originating from two muscle bellies with fibrovascular tissue at the center of the tendons at the level of the distal radioulnar joint, explaining a frequently found centrally increased signal within the tendon [22]. At the level of the distal ulna, the ECU tendon is covered by a subsheath. The tendon’s position in the wrist changes with forearm pronation and supination undergoing a physiologic movement and can be seen outside the ulnar groove on axial MR imaging and ultrasound in asymptomatic subjects in supination [23] [24] [25]. The ECU tendon inserts at the base of the 5^th metacarpal on the dorsal surface [15].

Common pathologies of the extensor tendons of the wrist

De Quervain tenosynovitis

De Quervain tendinopathy is defined as tenosynovitis of the first dorsal extensor compartment tendons (APL and EPB), as they pass through the fibro-osseous sheath at the wrist joint. Commonly seen as an overuse injury in activities involving repeated movements of the hand such as radioulnar deviation or repetitive gripping with the thumb, it has been described during pregnancy and in the immediate postpartum period (thought to be a result of endocrine influences and fluid retention) in new mothers (prolonged carrying of babies with the wrist held in flexion and ulnar deviation and with the thumb in extension) and in athletes participating in golf or racquet sports. Risk factors include female gender and increased age, repeated or sustained wrist bending activities, and twisting motions [26] [27] [28] [29] [30] [31]. The diagnosis is usually made by physical examination, termed the Finkelstein test [32]. The patient presents with wrist pain localized to the dorsoradial side of the wrist, with gradual onset of pain not associated with trauma, with limitation of thumb motion, and with swelling [26] [33]. MRI and ultrasound might be able to identify various changes associated with de Quervain tenosynovitis, such as sheath or retinaculum thickening, increase in tendon thickness, and presence of sheath effusion often with surrounding reactive soft tissue ([Fig. 2]) [34]. The appearance of tendinopathy can range from mild tendon swelling to longitudinal splitting of the tendon in severe cases [35]. Based on ultrasound-guided studies, the retinacular sheath is nearly three times thicker in patients with de Quervain tenosynovitis [36].

Fig. 2 (A and B) De Quervain’s tenosynovitis in a 59-year-old man. Longitudinal (A) and transverse (B) ultrasound images showing the first extensor compartment. The white arrows show thickening of the overlying retinaculum and the synovial sheath. (C) Ultrasound image of De Quervain’s tenosynovitis in a 64-year-old woman shows that the pathology is additionally accompanied by peritendinous subcutaneous (arrowhead) and synovial hyperemia (arrow) on Doppler imaging at the first extensor compartment. Rad = Radius; Scaph = Scaphoid; Trap = Trapezium; EPB = extensor pollicis longus; APL = abductor pollicis longus.

Nonsurgical treatment includes rest and splinting and can be combined later on with corticosteroid injections into the first extensor compartment. If conservative treatment fails, surgical release may be necessary [26] [37].

Patients with de Quervain disease are more likely to have a septum dividing the compartment (62.2%) in comparison to normal cadaver wrists (43.7%), which has implications for both conservative and surgical treatment, because the presence of a septum may limit the reach of the injected steroid to only a portion of the compartment and surgical treatment may result in inadequate decompression of all compartments [38] [39]. Ultrasound is a useful tool to depict such types of anatomic variations in the first extensor compartment [39].

Proximal intersection syndrome

Proximal intersection syndrome refers to noninfectious peritendinous inflammation due to overuse at the site where the tendons of the first dorsal extensor compartment (APL and EPB) cross the tendons of the second dorsal compartment (ECRL and ECRB), approximately 4 to 8 cm proximal to Lister’s tubercle. The most commonly proposed etiology is due to friction [26] [40], while others have postulated stenosis of the second extensor compartment [41]. Reported activities involve repetitive flexion and extension of the wrist, frequently seen in association with athletic activities, including rowing [42], skiing [43], and tennis [44]. The syndrome is also seen in patients working with agricultural or digging tools as well as in carpenters, office workers [45], and supermarket checkout clerks [46]. Patients present with pain proximal to the wrist, located dorsally, often with localized swelling at the intersection of the first and second dorsal compartments, and there is often audible crepitus or “squeaking” with wrist motion [47] [48]. The proximal location should be taken into account when planning the MR examination, as most standard wrist MRI protocols will not include this area [49]. On ultrasound, edematous changes in the APL and EPB (typically at the myotendinous junctions), the loss of a hyperechoic plane dividing the two different compartments, sheath effusion and swelling, typically of the tendons in the second extensor compartment can be noted [2]. MRI findings include peritendinous edema and fluid around all four tendons at the point of the intersection, commonly adjacent subcutaneous edema, and proximal and distal extension of the peritendinous edema and fluid ([Fig. 3]) [50] [51].

Fig. 3 T2-weighted fat-saturated axial MR image in an 18-year-old female at the level of the proximal intersection in the distal forearm. Shown is the crossing of the first extensor compartment tendons (blue arrows; APL= abductor pollicis longus, EPB = extensor pollicis brevis), over the second extensor compartment tendons (orange arrows; ECRL = extensor carpi radialis longus, ECRB = extensor carpi radialis brevis). Fluid around all four tendons can be noted in this patient with proximal intersection syndrome (white arrow).

Distal intersection syndrome

Distal intersection syndrome is located distal to Lister’s tubercle, where the third extensor compartment tendon (EPL) crosses over the second extensor compartment tendons (ECRL and ECRB). It is less common than the classic forearm intersection syndrome. Patients sometimes present after a traumatic event and symptoms can mimic those of osteoarthritis or a ganglion cyst. Due to mechanical friction, Lister’s tubercle can irritate the EPL, particularly if there has been a previous fracture [52] [53] [54]. The inflammation can spread to the communicating second and third extensor compartments at the crossing point [19]. Imaging shows peritendinous edema, fluid, and distention of the second and third extensor compartment at their crossing point, and additional findings like tendinosis with thickening and/or increased signal on fluid-sensitive sequences on MRI in the EPL, ECRB, and ECRL ([Fig. 4]), partial tendon tears, and reactive marrow edema of Lister’s tubercle [52] [54]. Most patients respond to conservative treatment, but some patients may progress to a complete tear of the extensor pollicis longus needing synovectomy and tendon repair [52] [54].

Fig. 4 T2-weighted fat-saturated MR image of a 33-year-old female with distal intersection syndrome. The extensor pollicis longus tendon (EPL; green arrow) crosses over the second extensor compartment tendons (orange arrows; ECRL = extensor carpi radialis longus tendon, ECRB = extensor carpi radialis brevis tendon). Fluid around all three tendons can be noted (white arrows).

Extensor pollicis longus tendon rupture at the level of Lister’s tubercle

Rupture of the EPL tendon is a well described complication of distal radius fractures, with a reported incidence of 0.2–5%, occurring on average 6–8 weeks after fracture [55]. On imaging, a tendon tear is seen as discontinuity of the tendon, stump retraction, and fluid in the tendon gap, which appears hypoechoic on ultrasound ([Fig. 5]) and hyperintense on fluid-sensitive MR sequences [56]. The mechanical theory hypothesizes that there are dorsal cortical irregularities from fractures that can injure the tendon. The vascular theory outlines a watershed area of the EPL tendon at the level of Lister’s tubercle in which the tendon sheath is poorly vascularized, whereby almost all nutrition is provided by the synovial fluid. Swelling, edema, and hematomas narrow the space and result in increased pressure on the EPL tendon in the watershed area, thereby decreasing vascularity and nutrition and eventually causing ischemia and rupture [55]. The incidence is higher in nondisplaced fractures versus displaced fractures, suggesting that an intact extensor retinaculum, which most likely is the case in nondisplaced distal radius fractures, creates a rigid space in which the EPL tendon becomes compressed by the aforementioned swelling, edema, hematoma and eventually in a later stage by fracture callus [55]. Extensor tendon ruptures were also noted after volar plating, including damage secondary to screw penetration of the dorsal cortex, with the EPL tendon being the most ruptured extensor tendon [57]. Spontaneous EPL tendon ruptures can occur with underlying chronic inflammation such as rheumatoid arthritis or use of local or systemic steroids [58].

Fig. 5 (A and B) 46-year-old man after a fall on ice. Lateral radiograph of the wrist (A) shows a slightly dislocated distal radius fracture with a dorsal cortical step off (arrow). The combined longitudinal ultrasound images (B) depict the full thickness tear of the extensor pollicis longus tendon (EPL) with retraction of the EPL stumps (arrows). The dehiscence of the tendon stumps is visualized by the green markers. The radius fracture is located between the retracted stumps (arrowhead). Rad = Radius.

Extensor carpi ulnaris subsheath injury

The ECU tendon in the sixth dorsal extensor compartment is additionally held in place at the level of the ulnar head by the ECU subsheath to prevent subluxation with wrist rotation [14] [26]. A rupture of this subsheath can result in a luxation of the ECU tendon especially in supination ([Fig. 6]). Most patients recall a traumatic episode, and many of these episodes involve a sporting activity like tennis or golf, where supination, ulnar deviation, and wrist flexion increase the angulation of the ECU tendon relative to the ulna, resulting in maximal force upon the ECU subsheath [59] [60]. Three types of disruption of the fibro-osseous sheath are reported ([Fig. 7]) [61]. The fibro-osseous sheath can rupture ulnar, and the torn sheath will then lay superficial to the tendon. When the fibro-osseous sheath ruptures radially, the torn sheath lies in the ulnar groove beneath the tendon. And lastly, a detachment of the periosteum from the ulnar side of the ulna in continuity with the fibro-osseous sheath can occur, forming a false pouch into which the tendon can dislocate [61]. Ultrasound is the modality of choice to check for ECU subsheath injury, due to the possibility of dynamic evaluation of instability ([Fig. 8]). In supination, the unstable ECU tendon subluxates/luxates volarly while relocating into the groove with pronation. Additionally, it gives the opportunity to examine the contralateral side to make sure the subluxation is not physiologic [24] [62]. However, this diagnosis might be harder on MRI if the exam is only performed in pronation. Other MRI signs of subsheath injuries include tendinopathy, tenosynovitis, and marrow edema in the ulnar head, while acute injuries of the subsheath will be associated with edema and hemorrhage surrounding the tendon [60] [63]. The clinical significance of the tendon subluxation or dislocation is questionable, as it can be found in asymptomatic volunteers [23] [24].

Fig. 6 Axial T2-weighted MR image of a 33-year-old man. Posttraumatic rupture of the extensor carpi ulnaris subsheath (arrow) with luxation of the extensor carpi ulnaris tendon (arrowhead).

Fig. 7 The three patterns of disruption of the extensor carpi ulnaris subsheath (green) are shown in this schematic illustration. The disruption of the subsheath results in transient dislocation of the extensor carpi ulnaris tendon (orange) in supination (left side), followed by relocation in pronation (right side). In A the fibro-osseous sheath is ruptured on the ulnar side, resulting in transient dislocation of the tendon in supination, followed by relocation in pronation, with the tendon returning to a position beneath the subsheath. When the fibro-osseous sheath ruptures radially (B), the tendon will lie on top of the torn subsheath after relocation in pronation. In C, a detachment of the periosteum occurred on the ulnar side of the ulna in continuity with the fibro-osseous sheath, forming a false pouch into which the extensor carpi ulnaris tendon can dislocate in supination.

Fig. 8 A dynamic ultrasound examination of a 20-year-old female with ulnar-sided wrist pain reveals luxation (arrow) of the extensor carpi ulnaris tendon (asterisk) over the styloid process of the ulna (S) in supination, compared to neutral position which shows the tendon more centered in the ulnar groove (G). Of note: R marks the distal radius.

Anatomy of the flexor tendons of the wrist

Most flexor tendons of the wrist are located deep with respect to the flexor retinaculum (transverse carpal ligament) ([Fig. 9]). The flexor retinaculum is a fascial band located at the volar aspect of the hand, near the wrist, forming the carpal tunnel [15]. It attaches radial to the scaphoid tubercle and the trapezium and ulnar to the pisiform and hook of the hamate [14]. From superficial to deep, the tunnel contains the flexor digitorum superficialis tendons and the flexor digitorum profundus tendons on the ulnar side, and the median nerve and flexor pollicis longus tendon (FPL) on the radial side. Three flexor tendons run outside the carpal tunnel: the flexor carpi ulnaris tendon (FCU), which attaches to the pisiform bone, the flexor carpi radialis tendon (FCR), and the palmaris longus tendon (PL) [15] [26]. The FCR runs radial to the carpal tunnel in its own fibro-osseous tunnel separated from the carpal tunnel by a vertical retinacular septum [64]. The palmaris longus lies subcutaneous above the flexor retinaculum and is present in at least 75–85% of the population [15] [26].

Fig. 9 Axial T2-weighted MR images of the wrist showing the flexor tendons: (A) at the level of the distal radius, (B) at the level of the proximal carpal row showing the insertion of the flexor carpi ulnaris tendon (FCU; yellow arrow) to the pisiform bone (asterisk), (C) at the level of the flexor retinaculum. The tendons of the flexor pollicis longus (FPL; orange arrow), flexor digitorum superficialis (FDS, grey arrows) and flexor digitorum profundus (FDP; red arrows) course inside the carpal tunnel, while the tendon of the flexor carpi radialis (FCR; blue arrow) courses in its own fibro-osseous sheath. PL (green arrow) = palmaris longus tendon.

Common pathologies of the flexor tendons of the wrist

Flexor tendon ruptures

Flexor tendon ruptures can occur accidentally, e.g., due to an injury with a knife. Additionally, flexor tendon ruptures can occur after distal radius volar plating. Approximately 33% of surgeons reported in a time period over one year after plating at least one flexor tendon injury, with the flexor pollicis longus being the most commonly reported tendon injury [57]. On imaging, the tendon tear is seen – as described earlier in the setting of EPL tendon ruptures – as a discontinuity of the tendon, stump retraction, and fluid in the tendon gap that is hypoechoic on ultrasound and hyperintense on fluid-sensitive MR sequences.

Flexor carpi radialis tendinopathy

At the level of the wrist, the FCR tendon runs in a close anatomical relation to the subjacent volar surfaces of the scaphoid and trapezium, in contact with the volar aspect of the capsule of the scapho–trapezium–trapezoid (STT) articulation or triscaphe joint. This close relationship to the STT bones can make the tendon vulnerable to injury by cortical bone irregularities as have been described in case reports in scaphoid fracture [65], fracture malunion [66], and joint degeneration [67]. The coexistence of FCR tendinopathy and STT arthritis can be demonstrated by MR imaging. It can lead to a partial-thickness tear or complete discontinuity of the FCR tendon ([Fig. 10]) [67].

Fig. 10 (A) sagittal T2-weighted fat-saturated MR image of a normal flexor carpi radialis tendon (arrowhead) inserting at the base of the second metacarpal bone (arrow). (B) sagittal image of the wrist of a 69-year-old female. The patient reported radiopalmar pain without history of trauma. The open arrowheads delineate the retracted stumps of the torn flexor carpi radialis tendon. II = Second metacarpal bone; Td = Trapezoid; S = Scaphoid; R = Radius.

Anatomical variants

In most cases the lumbrical muscles arise from the flexor digitorum profundus tendons just distal to the carpal tunnel [15] [68]. In about 22% of individuals, the most proximal portions of the lumbrical muscles can be found between the deep flexor tendons within the carpal tunnel ([Fig. 11]) [68] and can be a cause of carpal tunnel syndrome [69].

Fig. 11 T2-weighted axial MR image of a 16-year-old male. As an anatomical variant, a lumbrical muscle (arrowhead) can be found between the deep flexor tendons within the carpal tunnel. This finding was asymptomatic. Normal signal intensity of the median nerve (arrow). The study was conducted for suspected TFCC lesion.

A rare accessory muscle, the flexor carpi radialis brevis (FCRB), was described to be associated with pain by causing an atypical intersection syndrome of the wrist. This intersection occurs between the tendon of the FCRB muscle and the tendon of the FCR muscle, and was reported with an associated longitudinal split tear of the FCR tendon [70].

Inflammatory diseases

In general, tendons can be damaged by inflammatory processes, such as chronic inflammatory underlying conditions, in particular rheumatoid arthritis (RA) [71]. The most common tendon pathology due to inflammatory disease is tenosynovitis. However, in severe cases, a tendon rupture can also occur [71].

In RA, tenosynovitis occurs frequently, as it targets synovial tissue, with reported prevalence of about 50% to 80% [72], as an early inflammatory feature [73] [74]. Tenosynovitis may manifest before development of clinical arthritis [75] and be associated with subsequent bone erosion [76]. An important complication of RA in the wrist is tendon rupture, which seems to be related to synovial invasion of the tendon [77] [78] and mechanical attrition. In particular, these tendon ruptures were commonly reported at predilection sites of bone erosions, which can lead to sharp spurs. Especially the extensor tendons of the ulnar-sided wrist, the EDM, and the EDC of the fifth digit are at risk. This is because the eroded ulnar head or ulnar styloid process, in combination with a volar subluxation of the radius, causes the dorsal riding ulna to rub against the overlying tendons [26] [79]. Rupture of the EPL tendon is common [80], while on the flexor side, rupture of the FPL at the palmar scaphoid referred to as a Mannerfelt lesion, and rupture of the FDP of the index finger have been reported [79]. MRI and ultrasound play an important role in the initial diagnosis, staging, treatment monitoring, and assessment of remission in patients with RA [81].

Tenosynovitis is included in the RA MRI scoring system and is defined as peritendinous effusion and/or tenosynovial postcontrast enhancement, seen on axial sequences over ≥ 3 consecutive slices [82]. A small amount of fluid can be seen in a normal tendon sheath, but a fluid thickness of more than 1 mm is suggested as abnormal. The tenosynovium may be thickened and revealed as a bright signal surrounding a tendon on T1-weighted postcontrast and T2-weighted sequences ([Fig. 12]). The tendon itself may show morphological and signal changes, best identified on axial images as an increased signal on T1 with a low signal on T2 or an increased signal on both T1 and T2 sequences, and may show postcontrast enhancement [83] [84].

Fig. 12 44-year-old female with diagnosed seropositive rheumatoid arthritis. In the T1-weighted image (A), a thickened tendon sheath (white arrowhead) of the sixth dorsal extensor compartment can be appreciated with the extensor carpi ulnaris tendon marked with an asterisk. After gadolinium administration (B), contrast enhancement of the tenosynovium (white arrowhead) signaling tenosynovitis of the sixth dorsal extensor compartment is seen.

On ultrasound, tenosynovitis may appear as hypoechoic or anechoic thickened tissue, and may or may not be accompanied by fluid within the tendon sheath. This can be assessed on two perpendicular planes and may be associated with a Doppler signal [85].

As for gout, deposition of uric acid crystals may involve tendons in the wrist, most commonly enveloping the tendons (45%) [86], leading to tenosynovitis and tendon ruptures [87] [88].

On ultrasound, tophaceous deposits are hyperechoic with an anechoic rim. Additionally, they have a nodular or infiltrative appearance and may exhibit posterior acoustic shadowing due to sound-beam attenuation and possible calcification [89]. MRI cannot specifically identify uric acid crystals, while features of gout may vary. Gouty tophi have an intermediate or low signal intensity on T1-weighted images and heterogeneous signal intensity on T2-weighted sequences, possibly due to varying amounts of calcium, and can express uniform enhancement or a non-enhancing center [89].

Dual-energy computed tomography (DECT) has the advantage of diagnosing urate crystals by exploiting the photon energy–dependent attenuation of different materials, displaying them in 2D or 3D color-coded maps ([Fig. 13]) [90].

Fig. 13 54-year-old male with gout crystal arthropathy. T1-weighted axial MR image at the level of the ulnar head (A) shows peritendinous soft-tissue inflammation around the extensor carpi ulnaris tendon (white arrowhead) with strong contrast enhancement in the post gadolinium, T1-weighted fat-saturated image (B) and contrast enhancement of the little erosion in the ulnar head (white arrow). Urate crystals could be confirmed with dual-energy computed tomography, where crystals are marked in green in the color-coded 3D image (white open arrowhead) (C). The erosion can be depicted by computed tomography, as shown in the coronal plane (D) at the base of the ulnar styloid process (white arrow). Small tophi calcifications are depicted as well (white open arrowhead).

In psoriatic arthritis, the tendons are commonly affected by inflammatory changes as well, with no difference in frequency compared to RA. However, in RA, extensor tendons might be more commonly affected than flexor tendons, whereas in psoriatic arthritis the opposite can be observed [91]. Inflammatory changes in flexor tendons of the digits are most frequently observed, followed by the extensor carpi ulnaris and flexor carpi radialis tendons of the wrist [91] [92].

In general, soft-tissue calcifications can lead to inflammatory reactions in the surrounding soft tissue, which can affect tendons [93] [94]. This mechanism is particularly known in the shoulder as calcific tendinitis. In rare cases, such calcifications can also affect the wrist tendons, notably the flexor carpi ulnaris at the pisiform ([Fig. 14]). Rupture of the calcium deposits is thought to result in an acute inflammatory response, where clinical symptoms can be easily misdiagnosed as acute infection [93]. In these cases, imaging is crucial to depict the calcifications, in order to avoid unnecessary invasive interventions. On ultrasound, calcium hydroxyapatite depositions can be seen as echogenic areas with shadowing within the tendon or around the tendon sheath [95]. On radiographs, they demonstrate a characteristic amorphous, “cloud-like” appearance, with no cortex or internal trabecular pattern [93]. On MR imaging, calcium deposits are hypointense on T1- and T2-weighted images and can be surrounded by an inflammatory, edema-like reaction [96].

Fig. 14 (A, B, C) 38-year-old male with peritendinitis calcarea and partial tear of the flexor carpi ulnaris (FCU) tendon. AP radiograph of the wrist (A) shows a cloud-like calcification (arrow) distal to the ulnar styloid process. Coronal T2-weighted fat-saturated MR image (B) shows the calcification as a hypointense structure (arrow) with surrounding edema along the FCU tendon (arrowhead). On the axial T2-weighted fat-saturated image (C), the hypointense calcification (arrow) and the partial tear of the FCU (arrowhead) can be seen as hyperintense signal with fiber disruption within the tendon.

Conclusion

This article gives an overview over the anatomy and the pathologies of the wrist tendons. Knowledge of this anatomy and its anatomical variants is key to avoid misdiagnosis, and to understand certain pathologies. Anatomical crossings of tendons, for example, facilitates intersection syndromes. The tendons of the thumb, i.e. extensor and flexor pollicis longus, are most commonly torn after distal radial fracture (EPL) and osseous hardware fixation (FPL). Both ultrasound and MR imaging are appropriate modalities to assess the tendons of the wrist.

Conflict of Interest

Balgrist University Hospital and Balgrist Campus each have an academic research collaboration with Siemens Healthineers. Balgrist University Hospital also has an academic research collaboration with Bayer.

Acknowledgement

The authors like to thank Sara Erostrato for her effort in preparing the graphics for figure 7 of this article.

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Publication History

Received: 17 July 2024

Accepted after revision: 27 November 2024

Article published online:
11 February 2025