This article discusses the normal anatomy and pathologic appearances of the intrinsic and extrinsic wrist ligaments using MR Imaging. Technological advances in surface coil design and higher magnetic field strengths have improved radiologists’ ability to consistently visualize these small ligaments in their entirety. Wrist ligament anatomy, in the context of proper physiologic function, is emphasized, including common normal variants, and their appearances on MR imaging. The spectrum of disorders, incorporating overlapping appearances of senescent degenerative changes, and destabilizing ligament tears, is outlined. The diagnostic performance of MR imaging to date for various ligament abnormalities is discussed, along with significant limitations.
Key points
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Ligamentous injury to the wrist is a common cause of chronic wrist pain and carpal instability.
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MR imaging, coupled with knowledge of normal anatomy, allows consistent visualization of all major intrinsic and extrinsic wrist ligaments.
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Ligament tears are diagnosed by the presence of abnormal signal hyperintensity on fluid-sensitive sequences, ligament discontinuity, or altered morphology.
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MR imaging has excellent specificity, good sensitivity, and substantial interobserver agreement for diagnosis of partial and complete scapholunate and lunotriquetral ligament tears. Sensitivity is enhanced with higher field strengths and use of magnetic resonance arthrography.
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Appropriate clinical management is guided by wrist MR imaging interpretation.
Discussion of the problem
There is a high prevalence of ligamentous injury to the wrist in the setting of trauma, particularly in the presence of bone abnormalities. A recent study found that 60% of patients presenting for wrist MR imaging following trauma had intrinsic ligament injury and 75% had extrinsic ligament injury. Such traumatic ligament injuries have delayed functional consequences such as progressive carpal instability with secondary deterioration of the wrist joint and chronic wrist pain. However, a history of trauma is not always elicited, and this has led to the theory that, in some cases, degenerative changes in ligaments alone may cause pain in the stable wrist, and may extend over time, possibly via increased local motion, to complete tears and resultant instability. Regardless of cause, appropriate clinical management is predicated on accurate and timely diagnosis.
MR imaging is a proven, established technology for the detection, evaluation, and follow-up of disorders of the wrist, including ligamentous disorders. Wrist MR imaging frequently alters planned clinical management, including the decision to perform surgery. All of the major intrinsic and extrinsic wrist ligaments are easily identified on 1.5 T and 3 T MR imaging with a range of accuracies for tear detection depending on the particular ligament and disorder type. MR imaging is considered appropriate in the work-up of both radial-sided and ulnar-sided wrist pain with normal or nonspecific radiographs, according to the American College of Radiology Appropriateness Criteria.
Discussion of the problem
There is a high prevalence of ligamentous injury to the wrist in the setting of trauma, particularly in the presence of bone abnormalities. A recent study found that 60% of patients presenting for wrist MR imaging following trauma had intrinsic ligament injury and 75% had extrinsic ligament injury. Such traumatic ligament injuries have delayed functional consequences such as progressive carpal instability with secondary deterioration of the wrist joint and chronic wrist pain. However, a history of trauma is not always elicited, and this has led to the theory that, in some cases, degenerative changes in ligaments alone may cause pain in the stable wrist, and may extend over time, possibly via increased local motion, to complete tears and resultant instability. Regardless of cause, appropriate clinical management is predicated on accurate and timely diagnosis.
MR imaging is a proven, established technology for the detection, evaluation, and follow-up of disorders of the wrist, including ligamentous disorders. Wrist MR imaging frequently alters planned clinical management, including the decision to perform surgery. All of the major intrinsic and extrinsic wrist ligaments are easily identified on 1.5 T and 3 T MR imaging with a range of accuracies for tear detection depending on the particular ligament and disorder type. MR imaging is considered appropriate in the work-up of both radial-sided and ulnar-sided wrist pain with normal or nonspecific radiographs, according to the American College of Radiology Appropriateness Criteria.
Anatomy
The ligaments of the wrist guide and constrain the complex motion of the carpus relative to the forearm and metacarpals, and facilitate transmission of force between carpal bones. They are commonly divided into intrinsic and extrinsic groups. Intrinsic ligaments arise and insert entirely within the carpus onto carpal bones, whereas extrinsic ligaments arise in the forearm, or extend onto metacarpals, and have additional attachments to retinacula and/or tendon sheaths. The appropriate nomenclature is to name the ligaments for the bones from which they originate and onto which they insert, proximal to distal, radial to ulnar. This article describes some of the most important ligaments organized by anatomic location.
Interosseous Ligaments
Interosseous ligaments are intrinsic intercarpal ligaments that unite carpal bones either within a carpal row or between carpal rows. The most important and well-studied are those that separate the radiocarpal and midcarpal compartments, providing the flexible linkage of the proximal carpal row, the scapholunate ligament (SLL) and lunotriquetral ligament (LTT). In contradistinction, the 3 distal carpal row interosseous ligaments that unite the trapezium with the trapezoid, the trapezoid with the capitate, and the capitate with the hamate, allow normal communication between the midcarpal and common carpometacarpal compartments, as they do not extend from volar wrist joint capsule to dorsal capsule.
Scapholunate ligament
The SLL is a C-shaped structure, approximately 18 mm in length, and 2 to 3 mm in thickness, connecting the mutually articulating surfaces of the ulnar scaphoid and radial lunate ( Fig. 1 ). It has 3 histologically and functionally distinct segments. Knowledge of segmental anatomy is crucial because the site and extent of the ligament disruption may be used to differentiate between a traumatic tear and a degenerative perforation, the latter of which sometimes represents asymptomatic senescent change.
Dorsal scapholunate ligament
The dorsal segment of the SLL is a true histologic articular ligament, with normally taut transversely oriented collagen fascicles averaging 3 to 5 mm in proximal to distal length ( Fig. 1 ). It has a trapezoidal shape in the axial plane, with shorter volar fibers, and is intimately associated with the dorsal joint capsule. Distally it merges with the dorsal intercarpal ligament (DIL).
Volar scapholunate ligament
The volar segment of the SLL is a much thinner ligament (no more than 1 mm thick), with slightly obliquely oriented collagen fibers from proximal-ulnar to distal-radial. The lunate attachment is just dorsal to the long radiolunate ligament (LRL) attachment and immediately proximal to the cartilage surface. It is not normally possible to directly visualize the volar SLL with an arthroscope, because thin laminae of collagen fibers extending from the radioscapholunate neurovascular bundle isolate it from the radiocarpal joint.
Proximal (membranous) scapholunate ligament
Unlike the dorsal and volar segments of the SLL, the proximal or membranous segment is grossly anisotropic fibrocartilage rather than a true ligament. It has a pliable consistency and its attachments blend with the articular cartilage of the scaphoid and lunate. Usually there is a meniscus-like extension that protrudes into the scapholunate joint space, with wedge-shaped or triangular cross-sectional geometry, usually nicely shown in the coronal pane with MR imaging ( Fig. 1 ). This segment is the weakest part of the SLL, prone to degenerative perforations, and the only area of the SLL that can be readily seen by arthroscopy in the absence of scapholunate dissocation.
Lunotriquetral ligament
The lunotriquetral ligament (LTL) is slightly longer, measuring approximately 20 mm in length, and more V shaped than the SLL; however, it also has 3 histologically and functionally distinct segments. The LTL segments could be viewed as the reverse of the SLL segments, in that the volar LTL is thicker and potentially more important than the dorsal LTL, whereas the dorsal SLL is thicker than the volar SLL.
Dorsal lunotriquetral ligament
The dorsal segment of the LTL is a true histologic ligament with transverse collagenous fibers, measuring up to only 1 to 1.5 mm in thickness, making it challenging to identify on MR imaging. It is covered by the dorsal radiocarpal ligament (DRL).
Volar lunotriquetral ligament
The volar region is the thickest segment, composed of transversely oriented collagen fascicles that interweave with the ulnocapitate ligament (UCL), measuring up to 2.5 mm in thickness. It transmits the extension moment of the triquetrum.
Proximal (membranous lunotriquetral ligament)
Similar to the proximal SLL, the proximal LTL is composed of fibrocartilage and often shows a meniscal projection into the lunotriquetral joint. However, it is much thinner than the proximal SLL, often no more than 1 to 1.5 mm, which has made it unreliable to visualize on MR imaging in the past.
Scaphotrapeziotrapezoid ligament
The scaphotrapeziotrapezoid ligament (STL) is composed of collagenous fibers that span the volar and dorsal aspects of the distal pole of the scaphoid and proximal trapezium with some fibers extending to the trapezoid. Some fibers from the radioscaphocapitate ligament (RSL) blend with the volar STL, which is not visible arthroscopically.
Volar Capsular Ligaments
Volar radiocarpal ligaments
These extrinsic extrasynovial capsular ligaments span the width of the palmar rim of the distal radius and attach distally to carpal bones ( Fig. 2 ). They play roles in preventing ulnar translation of the carpus, and secondary stabilization of the scaphoid and lunate. The osseous attachments cannot be directly visualized with multiportal wrist arthroscopy.
Radial collateral ligament
The radial collateral ligament (RCL), or radioscaphoid ligament, runs from the radial styloid process to the scaphoid tuberosity and the flexor carpi radialis tendon ( Fig. 2 ). It is sometimes considered part of the RSL rather than a separate structure.
Radioscaphocapitate ligament
The RSL arises from the tip of the radial styloid process through the middle of the scaphoid fossa, supports the waist of the scaphoid, attaches to the proximal cortex of the distal scaphoid pole, and distally interdigitates with fibers from the UCL and palmar scaphotriquetral ligament, with only about 10% of fibers ultimately inserting on the capitate ( Fig. 2 ). This ligament plays a role in scaphoid stability, acting as a sling or “seat belt” at the scaphoid waist. In general, it is easily seen on arthroscopy, except for the radial styloid attachment and where it crosses the scaphoid waist, making tears in these areas difficult to diagnose.
Long radiolunate ligament
The LRL, also known as the radiolunotriquetral ligament, arises from the palmar rim of the remaining scaphoid fossa, passes anterior to the proximal scaphoid pole, and attaches to the radial volar lunate cortex ( Fig. 2 ). In some cases it extends in 1 continuous sheet to the triquetrum or it may exist as 2 separate radiolunate and lunotriquetral portions, with superficial fibers blending with the LTL. It serves as a volar sling for the lunate.
Radioscapholunate bundle
At one time there was a structure described as the radioscapholunate ligament, ulnar to the LRL, and radial to the short radiolunate ligament (SRL), which is now known to represent a neurovascular-containing mesocapsule, providing no mechanical strength. It merges with the proximal SLL, dividing the proximal from volar segments ( Fig. 3 ). The term radioscapholunate bundle, rather than ligament, may be preferred.
Short radiolunate ligament
The SRL arises from the entire width of the volar rim of the lunate fossa of the radius and attaches distally onto the radial half of the lunate volar cortex ( Fig. 2 ). The lunate is therefore strongly anchored to the radius via both the LRL and SRL, which may explain why it tends to remain in place despite the traumatic force of a complete perilunate dislocation.
Volar midcarpal ligaments
The most important of the intrinsic capsular midcarpal ligaments include the arcuate and deltoid ligament complexes. Both form a V, flanking the proximal normal zone of volar capsular deficiency, in the carpal tunnel floor, known as the space of Poirier, through which volar lunate dislocation occurs. Although technically separate structures, particularly proximally, the arcuate and deltoid ligaments blend imperceptibly with each other as they approach their common attachment on the capitate, making them practically indistinguishable distally. In the literature, the arcuate ligament is sometimes referred to as the deltoid ligament and the deltoid ligament referred to as the arcuate ligament.
Arcuate ligament
The arcuate ligament is composed of the scaphocapitate ligament (SCL) or radial limb and triquetrohamocapitate ligament (THL) or ulnar limb, neither of which are visible by arthroscopy ( Fig. 2 ). It has also been described in the radiology literature as the distal band of the palmar scaphotriquetral ligament. The palmar scaphotriquetral ligament is another intrinsic capsular ligament extending from scaphoid to triquetrum, at the level of the midcarpal joint, superficial to the SCL and THL.
Deltoid ligament
The distal RSL and UCL constitute radial and ulnar limbs, respectively, of the deltoid ligament. Distally they form transversely oriented interdigitating fibers that support the head of the capitate in the midcarpal joint, like a labrum. Fibers from the deltoid ligament also interdigitate with the palmar scaphotriquetral ligament.
Volar ulnocarpal ligaments
Ulnolunate and ulnotriquetral ligaments
The ulnolunate ligament (ULL) and ulnotriquetral ligament (UTL) both arise proximally from the volar radioulnar ligament of the triangular fibrocartilage complex (TFCC), and form the anterior and ulnar aspects of the ulnocarpal joint capsule. They exist as a continuous sheet, also including the SRL, without any demarcation between them, distinguished only by their distal attachments ( Fig. 2 ). The ULL attaches distally to the volar ulnar cortex of the lunate, and the UTL attaches to the proximal and ulnar surfaces of the triquetrum, sometimes with a few fibers attaching to the ulnar styloid process as well.
The UTL often has 2 normal perforations: a proximal opening to the prestyloid recess, and a distal pisotriquetral joint orifice. The distal triquetral attachment of the UTL, where tears are sometimes seen on MR imaging, is difficult to see with arthroscopy. The TFCC is discussed in MRI of the Triangular Fibrocartilage Complex by ME Cody, DT Nakamura, KM Small and H Yoshioka.
Ulnocapitate ligament
The UCL is superficial to the other 2 ulnocarpal ligaments, and is the only one to attach directly to the ulnar head fovea. It reinforces the volar LTL and then passes radially and distally into the midcarpal joint to contribute to the deltoid ligament, with only about 10% of fibers directly attaching to the capitate ( Fig. 2 ).
Dorsal Capsular Ligaments
Early ligament sectioning studies focused on evaluation of intrinsic and volar extrinsic ligaments via dorsal approaches to elevate soft tissue flaps; however, this disrupted the dorsal capsular ligaments in the process. There is now mounting evidence that the dorsal capsular ligaments of the wrist play as important a role in carpal stability as volar capsular and interosseous ligaments. The DRL and DIL form a V with the apex pointed at their common attachment on the triquetrum ( Fig. 3 ). This angle of intersection at the triquetrum goes from acute in wrist extension to nearly orthogonal in palmar flexion. It is thought that this linkage system may provide stability between the radius and scaphoid indirectly without compromising motion. The osseous attachments cannot be directly visualized with multiportal wrist arthroscopy.
Dorsal radiocarpal ligament
Also known as the dorsal radiotriquetral ligament, in most cases the DRL originates from the dorsal rim of the distal radius, spanning the Lister tubercle to the sigmoid notch. It then passes obliquely distally and ulnarly to attach to the dorsal ulnar horn of the lunate, dorsal LTL, and fourth and fifth extensor compartment septa, and terminates just proximal to the radial dorsal ridge of the triquetrum ( Fig. 4 ). Three less common variants have been described, which involve either separate bands or additional radial fibers that may cover the dorsal proximal scaphoid.
Dorsal intercarpal ligament
This intrinsic capsular ligament, also known as the dorsal scaphotriquetral ligament, inserts distal to the dorsal ridge of triquetrum on its radial aspect. It then passes radially, where it attaches to the dorsal distal lunate and dorsal groove of scaphoid, with additional fibers attaching distally to the dorsal proximal rim of the trapezium in most wrists ( Fig. 4 ). In one cadaveric study, there were additional fibers from the DIL that attached to the trapezoid in 42% and the capitate in 7%. The 2 most commonly described variants include a single band that branches radially to attach to the scaphoid and trapezium, and 2 distinct fascicles.
Mr imaging
Protocol
The salient considerations of optimal wrist MR imaging are well summarized in a practice parameter developed collaboratively by the American College of Radiology, the Society of Computed Body Tomography and Magnetic Resonance, the Society for Pediatric Radiology, and the Society of Skeletal Radiology.
Depending on the clinician’s needs and choice of coil, the patient can be positioned in the bore of the scanner either supine or prone. The supine position with the wrist imaged alongside the patient may be more comfortable and decrease the risk of claustrophobia. However, because of decreased magnetic field homogeneity peripherally, image quality may suffer from decreased signal/noise ratio (SNR) or incomplete chemically selective fat saturation. Scanning the patient prone with the upper extremity overhead, the so-called superman position, allows the wrist to be imaged in the homogenous center of the magnetic field, at the increased risk of motion artifact from potential discomfort.
Thin high-resolution images are required to evaluate the small ligaments of the wrist, which often take a double-oblique course in three-dimensional (3D) space. In order to achieve the requisite spatial resolution, a high SNR is required, and is achieved primarily through the use of an appropriate local receiver coil. There are many different acceptable surface coil design options. Without the proper coil, MR imaging cannot reliably diagnose wrist ligamentous disorders. The smallest surface coils, known as microscopy coils, are less than 5 cm in diameter, show even higher SNR and contrast/noise ratio than the standard small surface coil, but have a limited field of view (FOV), precluding imaging of the entire wrist at once.
A static magnetic field strength of at least 1 T is recommended. Higher field strengths provide greater SNR, which facilitates improved visualization of wrist ligaments. Although all of the major wrist ligaments are routinely identified at 1.5 T, imaging at 3 T has additional benefits, including increased ligament conspicuity. The additional SNR obtained from greater than 1.5 T imaging can be exploited to increase spatial resolution, obtain a larger FOV without losing spatial resolution, or scan in a shorter time period while maintaining adequate spatial resolution.
Wrist ligaments are optimally visualized with an FOV of 6 to 8 cm, assuming there is adequate SNR available, otherwise up to 12 cm may still allow consistent visualization of wrist ligaments and TFCC. A rectangular FOV can save imaging time without sacrificing spatial resolution. Higher imaging matrices are necessary for adequate spatial resolution, but are limited by available intravoxel SNR and imaging time. At least 256 steps in the frequency direction and 192 steps in the phase direction for two-dimensional (2D) imaging is recommended, and slice thickness should be no greater than 3 mm to minimize partial volume effects. Two-dimensional fast spin echo (FSE) slices 2 mm thick are routinely obtained with 3 T magnets.
The thinnest slice thicknesses obtainable, typically 0.6 to 1.2 mm, are acquired through volumetric 3D pulse sequences. Initially these volumetric acquisitions were of the gradient echo (GRE) variety, resulting in T2*-weighted images, and thought to be most useful for assessing the interosseous ligaments because of the thin sections obtainable. However, more recently, isotropic 3D FSE sequences have become available, which have superior SNR, in addition to the usual isotropic 3D acquisition benefits of ability to reconstruct data into any arbitrary plane with thin slice thickness. However, at this time, blurring in reconstructed planes remains a problem because of T2 decay during the required long echo trains. Whether obtained via 3D reconstructions or primary 2D acquisitions, oblique planes may add significant value to assessment of intrinsic and extrinsic ligaments, which also run in oblique planes.
Fluid-sensitive pulse sequences, such as T2-weighted, T2*-weighted (GRE), short-tau inversion recovery (STIR), intermediate-weighted (long recovery time; echo time, 30–60 milliseconds), and proton density–weighted sequences, are most helpful for evaluating the wrist ligaments. Fat suppression, whether accomplished via spectrally selective radiofrequency pulses; a phase-dependent method (Dixon technique); or, if necessary, STIR, enhances diagnostic yield ( Box 1 ). Gadolinium-enhanced magnetic resonance arthrography (MRA) may improve diagnostic performance for ligamentous disorders ; this is discussed in a separate article MR arthrography of the wrist and elbow by Bancroft and colleagues. In addition, a type of fast GRE pulse sequence known as balanced steady-state free precession is able to generate images rapidly enough (<0.6 seconds per image) to allow the moving wrist to be imaged. This kinematic or active MR imaging pulse sequence could potentially be used to investigate dynamic carpal instability by supplementing an otherwise high-spatial-resolution examination with high-temporal-resolution images, although this has yet to be proved.
| Minimal Requirements | Ideal | |
|---|---|---|
| Field Strength (T) | 1 | 3 |
| Field of View (cm) | ≤12 | 6–8 |
| Frequency Matrix | 256 | ≥384 |
| Phase Matrix | 192 | ≥256 |
| Slice Thickness (mm) | ≤3 | 1–2 |
| Pulse Sequences | Fat-saturated fluid-sensitive sequences (30-60 milliscecond echo time) | |
Normal MR Imaging Appearance
Interosseous ligaments
Ligaments are optimally evaluated with slices oriented with respect to their long axis, or orthogonal to potential tears. Therefore, although the proximal segments of SLL and LTL are best seen in the coronal plane, the dorsal and volar segments are better appreciated on the axial images. Oblique axial images through the long axes of these ligaments further improve visualization and sensitivity for tears, particularly of LTL, which is usually more deviated from the anatomic axial plane than SLL ( Fig. 5 ). Imaging the wrist in radial and ulnar deviation may have the same effect, with the added benefit of changes in radiocarpal joint width ; however, normal interosseous ligament morphology and signal may be altered with wrist position.
Dorsal and volar segments
Scapholunate ligament and lunotriquetral ligament
The appearance of the interosseous ligaments of the proximal carpal row vary depending on the slice thickness, spatial resolution, pulse sequence, coil, and wrist position. Usually the dorsal and volar components of both SLL and LTL are bandlike on axial images, with predominantly low signal intensity on all pulse sequences, because of the homogeneous transversely oriented collagen fascicles ( Fig. 5 ; Fig. 6 ). However, the volar SLL often shows striated heterogenous increased signal intensity, particularly on GRE sequences, probably related to associated loose vascular connective tissue. Occasionally, the volar or dorsal SLL appears triangular on axial images, similar to the proximal SLL on coronal images, suggesting continuation of the intra-articular meniscoid component ( Fig. 6 ).
Proximal segments
Scapholunate ligament
Most of the fibrocartilaginous proximal SLL has a triangular shape on coronal images and shows heterogenous, predominantly low, signal intensity. It normally attaches to intermediate-signal-intensity hyaline cartilage on both the lunate and the scaphoid. Coronal slices approaching the volar SLL instead usually show a trapezoid-shaped heterogenous intermediate-signal-intensity ligament, attaching directly to cortex. Coronal images approaching the dorsal SLL often show a bandlike low-signal-intensity structure variably attaching to cortex and articular cartilage ( Fig. 7 ).
Lunotriquetral ligament
The proximal fibrocartilaginous LTL is also triangular on most coronal images, although it may appear more linear as the volar LTL is approached ( Fig. 7 ). Although initially described as having mostly homogeneous low signal intensity, subsequent evaluation with higher-resolution techniques using a microscopy coil have described 3 different major types of presumed normal signal intensity. The most common (45.5%) consisted of linear intermediate or high signal intensity traversing the distal surface of the proximal LTL (type 2), followed by homogeneous low signal intensity (type 1, 33.8%), followed by linear intermediate or high signal intensity traversing both proximal and distal surfaces (type 3, 20.8%). An amorphous shape is more commonly seen in older individuals, possibly representing degenerative change.
Scaphotrapeziotrapezoid ligament
When seen, the STL usually shows mildly heterogenous low signal intensity, with striations, on fluid-sensitive sequences. The dorsal component is reportedly more easily seen; however, there is a paucity of imaging literature regarding this ligament at this time.
Volar capsular ligaments
Capsular ligaments generally appear on MR imaging as linear hypointense structures, often with alternating bands of intermediate signal intensity leading to a striated appearance. In general, they are optimally evaluated in the axial and sagittal planes, with the exception of the RCL, which is better evaluated in the axial and coronal planes.
Radial collateral ligament
The RCL is a thin, low-signal-intensity structure that arises from the tip of the radial styloid process, radial to the common origin of RSL and LRL, and inserts on the radial aspect of the scaphoid waist ( Fig. 8 ). It is more easily identified in the presence of a radiocarpal joint effusion, or with MRA, and measures approximately 5.1 mm by 1.2 mm on average in the axial plane.
Radioscaphocapitate and long radiolunate ligaments
The radial and palmar origins of the RSL and LRL on the radial styloid process are intimately associated, and often cannot be distinguished on MR imaging. Proximally they run nearly parallel and immediately adjacent to each other, the LRL proximal and ulnar to the RSL, with a fluid-signal-intensity interligamentous sulcus, sometimes visible on MR imaging ( Fig. 9 ). Both ligaments take a distal and ulnar oblique course, traversing a groove in the scaphoid waist, most easily shown in the sagittal plane. As previously noted, the RSL attaches to the palmar aspect of the distal scaphoid pole and the central palmar capitate, whereas the LRL inserts widely on the palmar lunate ( Fig. 10 ). The RSL measures approximately 7.0 mm by 2.8 mm and the LRL 8.0 mm by 3.1 mm in the sagittal plane.
Short radiolunate ligament
Most commonly, the SRL appears as a homogeneously low-signal-intensity focal thickening of the joint capsule between the volar lip of the lunate facet and the volar lunate cortex ( Fig. 11 ). A study using MRA in cadaveric wrists more than a decade ago reported that the SRL was not well seen on MR imaging.
Arcuate ligament
The SCL, or radial limb of the arcuate ligament, and THL, or ulnar limb, appear as low-signal-intensity bands, often with intermediate signal striations, on all magnetic resonance sequences ( Fig. 12 ). The SCL arises from the scaphoid tuberosity, in common with the fibrous band of the RSL, and courses in a distal ulnar oblique fashion, 60° to the horizontal, deep to the RSL, to attach to the volar capitate body ( Fig. 13 ). The THL is seen to originate from the volar triquetrum and courses 30° from the horizontal in a distal-radial oblique fashion to attach to the volar capitate. A minority of patients in one series had secondary fibers from the THL attaching to the hamate as well. These oblique ligaments may be best evaluated using oblique planes prescribed or reformatted along the long axes of these structures. The SCL measures approximately 7.3 mm by 2.6 mm, and the THL 6.3 mm by 2.4 mm in the sagittal plane.
Ulnocarpal ligaments
Both the UTL and ULL are short, thick, low-signal-intensity structures on MR imaging, arising from the volar radioulnar ligament ( Fig. 14 ). The UTL has a visible broad insertion on the volar aspect of the triquetrum, and the ULL inserts on the volar aspect of the lunate, in common with the LRL. The UTL measures approximately 5.8 mm by 3.0 mm, and the ULL 5.4 mm by 2.6 mm in the axial plane. The MR imaging appearance of the UCL has yet to be reported.
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