Normal Scapholunate and Lunotriquetral Ligaments.

The scapholunate ligament and the lunotriquetral ligament are the two intrinsic carpal ligaments of greatest clinical significance; disruption of these ligaments may cause instability and pain. Both of these ligaments can be evaluated best on gradient echo coronal images using thin sections. These ligaments are horseshoe-shaped, bandlike, or triangle-shaped structures on the proximal aspects of the carpal bones to which they attach ( Fig. 12-1 ).

Normal intrinsic carpal ligaments. Gradient echo coronal image of the wrist. The scapholunate ( arrow ) and lunotriquetral ( arrowhead ) ligaments are located on the proximal aspects of the carpal bones to which they attach, best depicted on coronal images.

The scapholunate ligament has volar, middle, and dorsal portions ( Fig. 12-2 ). Studies have shown that only the volar and, especially, the dorsal portions are important for wrist stability. The volar portion is looser so that it can accommodate the various articulating curvatures of the scaphoid and lunate. Perforations (or communicating defects) in the middle portion are common and do not seem to relate to symptoms when they are an isolated finding.

Normal scapholunate ligament: three portions. A , Diagram showing the different appearances of the volar, middle, and dorsal portions of the scapholunate ligament, which correspond to the MRI appearances. B , Gradient echo coronal image of the wrist. The volar portion of the ligament ( arrowhead ) is trapezoidal and intermediate signal. C , Gradient echo coronal image of the wrist. The middle portion of the scapholunate ligament ( arrowhead ) is lower signal than the volar portion and triangular in configuration. D , Gradient echo coronal image of the wrist. The dorsal portion ( arrow ) is a band of even lower signal, and the strongest portion of the ligament.

The volar portion of the scapholunate ligament is trapezoidal in configuration, with intermediate signal intensity on gradient echo images. The middle portion of the scapholunate ligament is triangular in shape, with lower signal intensity than the volar component, and both portions are routinely heterogeneous in signal intensity. The dorsal portion, the most important for carpal stability, is a low signal intensity, homogeneous band. Generally, the volar portion of the scapholunate ligament attaches to cortical bone, whereas the middle and dorsal portions attach to hyaline cartilage or a combination of cartilage and cortical bone. The reason for the higher signal intensity of the volar and middle portions of the scapholunate ligament compared with the dorsal portion is related to the lower density of collagen fibers and the higher proportion of loose connective tissue and vascular tissue in these regions.

The lunotriquetral ligament is smaller and more taut than the scapholunate ligament, but has a similar shape and frequently displays a heterogeneous low signal intensity on coronal gradient echo images. The lunotriquetral ligament may attach to hyaline articular cartilage or cortical bone. Its stronger and thicker volar component blends with the triangular fibrocartilage (TFC).

Intermediate signal intensity may partially or completely traverse the substance of the lunotriquetral ligament and scapholunate ligament in asymptomatic individuals ( Fig. 12-3 ). It should be considered an abnormal finding (torn ligament) only if this signal intensity is as high in signal as that of fluid on whatever type of T2 sequence is being used. Similarly, high signal intensity between articular cartilage and the ligament should indicate an avulsed ligament only if the signal intensity is as bright as fluid.

Scapholunate ligament: normal variation. Coronal fatsuppressed T2W image of the wrist. Intermediate signal traversing the intercarpal ligaments is a normal finding ( arrowhead ) because it does not become as high signal as fluid.

Abnormal Scapholunate and Lunotriquetral Ligaments.

Scapholunate instability is the most common carpal instability. Clinically, patients complain of pain and weakness on the dorsal radial aspect of the wrist. Abnormalities on MRI that indicate a scapholunate ligament abnormality are listed:

• 1.
  

  Discontinuity of the ligament, with or without an increased space between the scaphoid and lunate bones

  
  
• 2.
  

  Complete absence of the scapholunate ligament

  
  
• 3.
  

  Distorted morphology with fraying, thinning, and irregularity

  
  
• 4.
  

  Elongated ligament with an increased intercarpal space ( Fig. 12-4 )

  
  

  
  

  
  

  

  
  

  
  

  Scapholunate ligament abnormality. A , Coronal fat-suppressed T2W image shows fluid signal between the scaphoid and lunate ( arrow ). Scapholunate distance is wide. B , Fast T2 with fat saturation coronal image of the wrist (different patient than in A ). There is distortion of the morphology of the scapholunate ligament, which is frayed and with a vertical high signal tear through it ( arrows ). C , Gradient echo coronal image of the wrist (different patient than in A and B ). The space between the scaphoid and lunate bones is increased, and the ligament ( arrow ) is stretched, although it remains intact. This was the result of inflammation associated with rheumatoid arthritis. Multiple bone erosions, subchondral cysts, and a triangular fibrocartilage tear also are present.

  
  

The accuracy of MRI has been reported as 90% compared with arthrography and 95% compared with surgery (arthroscopy and arthrotomy).

Scapholunate instability occurs when the scapholunate ligament is completely torn or stretched, allowing the scaphoid and lunate bones to dissociate. The scaphoid tilts in a volar direction (rotatory subluxation), whereas the lunate tilts in a dorsal direction (dorsal intercalated segmental instability). The relationship of the osseous structures can be detected on sagittal MRIs ( Fig. 12-5 ). Rotatory subluxation is the most common instability pattern in the wrist. The dorsal intercalated segmental instability pattern of carpal instability also can occur with an unstable fracture of the scaphoid, even though the scapholunate ligament is intact. Partial tears more commonly affect the weaker volar ligamentous attachment.

Dorsal intercalated segmental instability. Sagittal fatsuppressed T2W image of the wrist. The lunate (L) is tipped in a dorsal direction relative to the capitate (C) and radius (R) because of rotatory subluxation of the scaphoid that occurred from disruption of the scapholunate ligament.

Scapholunate ligament disruption and chronic rotatory instability of the scaphoid also may lead to the capitate migrating proximally and may cause scapholunate advanced collapse (SLAC) wrist. The SLAC wrist consists of scapholunate ligament disruption, degenerative changes between the scaphoid and the distal radius, and proximal migration of the capitate between the scaphoid and lunate bones ( Fig. 12-6 ).

Scapholunate advanced collapse wrist. Coronal fatsuppressed T2W image of the wrist. The scapholunate ligament is absent, and the space between the scaphoid (S) and lunate (L) is increased. The capitate (C) is migrating proximally between the two bones ( arrow ). There is loss of cartilage and a decreased space between the scaphoid and radius from degenerative joint disease.

Disruption of the lunotriquetral ligament is not as easy to diagnose as disruption of the scapholunate ligament because of its smaller size. Similar abnormalities to those seen in an abnormal scapholunate ligament are present in an abnormal lunotriquetral ligament ( Fig. 12-7 ). Tears of the lunotriquetral ligament are the second most common cause of carpal instability and result in the lunate tilting in a volar direction (volar intercalated segmental instability) secondary to the disruption of the triquetral attachment. There is a strong association between tears of the TFC and lunotriquetral ligament tears. The alignment of the carpal bones on sagittal images depends on wrist position. The lunate tends to volar flex and dorsiflex relative to the radius when the wrist is placed in radial and ulnar deviation. Proper positioning of the hand and wrist in the magnet is important to prevent this pitfall. Normally, the distal radius, lunate, and capitate all align colinearly, or nearly so, just as they do on a lateral wrist radiograph.

Lunotriquetral tear and consequent volar intercalated segmental instability. A , Gradient echo coronal image of the wrist. There is high signal through a disruption of the lunotriquetral ligament with fragments of the ligament ( arrow ) seen on either side of the tear. B , T1 sagittal image of the wrist. The lunate (L) is tipped in a volar direction relative to the capitate (C) and radius (R) because of the lunotriquetral ligament tear, resulting in carpal instability (volar intercalated segmental instability).

Volar and Dorsal Ligaments.

The extrinsic carpal ligaments can be seen best on gradient echo coronal images; they are seen in cross section on sagittal images ( Fig. 12-8 ). These ligaments course between the carpal bones and the radius on the volar and the dorsal sides of the wrist. The extrinsic ligaments run obliquely and are best imaged on oblique sagittal images. Because imaging usually is not done in this plane, it generally requires several adjacent coronal images to see an entire ligament. The volar ligaments are stronger and thicker than the dorsal ligaments and are major stabilizers of wrist motion. Extrinsic ligaments are thickenings of the joint capsule and are intracapsular and extrasynovial. Extrinsic ligaments appear as striated fascicular structures with alternating bands of low and intermediate signal intensity on coronal MRIs.

Extrinsic ligaments. A , Gradient echo coronal image of the wrist. Portions of the two major volar carpal extrinsic ligaments are shown coursing obliquely as striated low signal structures. rlt, radiolunotriquetral ligaments; rsc, radioscaphocapitate. B , T1 sagittal image of the wrist. Volar and dorsal extrinsic ligaments are seen as round, low signal structures ( arrowheads ) in cross section in this plane of imaging.

The most important volar ligaments are the radioscaphocapitate and radiolunotriquetral ligaments. The radioscaphocapitate ligament originates on the volar surface of the radial styloid process and courses obliquely across the waist of the scaphoid without attaching to it (it acts as a seatbelt to maintain the position of the scaphoid), to insert on the center of the capitate. The radiolunotriquetral ligament is the largest ligament of the wrist. It arises adjacent to (on the ulnar side of) the radioscaphocapitate ligament on the radial styloid. It runs obliquely to attach to the volar surfaces of the lunate and triquetrum.

The dorsal extrinsic ligaments of the wrist run obliquely between the distal radius and to each of the carpal bones of the proximal carpal row (radioscaphoid, radiolunate, and radiotriquetral ligaments). There are other small extrinsic carpal ligaments that are not discussed here.

The good news about all of these extrinsic ligaments is that no one knows with certainty what importance they have clinically. The MRI appearance of normal extrinsic ligaments has been extensively described, but the ability of MRI to detect abnormalities is unknown. The only value in radiologists knowing about them is so that they do not cause confusion during interpretation of MRIs of the wrist. We spend little to no time analyzing these ligaments.

Normal Triangular Fibrocartilage.

The TFC is a fibrocartilaginous biconcave disk with an asymmetric bow-tie shape, similar to the temporomandibular joint disk ( Fig. 12-10 ). The TFC is positioned in the ulnocarpal space with attachments on the medial side to the ulnar styloid process by two thin bands of TFC tissue. At its radial attachment, there is hyaline cartilage interposed between the TFC and the radius that must not be confused with a detached or torn TFC. The TFC attaches directly to the cartilage, which is the articular surface of the distal radioulnar joint. The thickness of the TFC is inversely proportional to the degree of ulnar variance. In other words, the TFC is thinner in patients with positive ulnar variance, which may predispose it to tear, and thicker in patients with negative ulnar variance. The TFC is depicted best on coronal MRIs. It may be diffusely low signal intensity regardless of pulse sequence, or have intermediate signal intensity in its substance from asymptomatic myxoid degeneration.

Normal triangular fibrocartilage. A , Coronal gradient echo image of the wrist. The triangular fibrocartilage ( small arrowhead ; TFC) is a biconcave structure attaching to the intermediate signal cartilage on the radius ( white arrow ). This image also shows the ulnar collateral ligament well ( large arrowhead ). B , Gradient echo coronal image of the wrist. The ulnar attachment of the triangular fibrocartilage consists of two thin bands of tissue ( arrowheads ).

Abnormal Triangular Fibrocartilage.

Any structure of the TFCC can be abnormal, but the TFC is the main component to show abnormalities. Clinically, patients complain of ulnar-sided wrist pain and tenderness. An audible click with pain may be elicited by rotation of the forearm. On MRI, the TFC can be evaluated similarly to the meniscus in the knee. High signal intensity within the substance of the TFC has no clinical significance, whereas high signal intensity extending through either the proximal or the distal surface of the TFC indicates a tear ( Fig. 12-11 ). TFC tears may be partial or full thickness, extending partially or completely through the substance of the TFC. Partial tears are noted more frequently at the proximal articulating surface with the distal radioulnar joint. Fluid in the distal radioulnar joint was previously believed to be a secondary sign of a TFC tear, but a small amount of fluid may be present in this joint in most individuals.

Triangular fibrocartilage tears. A , Coronal gradient echo image of the wrist. A gap in the radial aspect of the triangular fibrocartilage ( arrowhead ) from a tear. B , Gradient echo coronal image of the wrist (different patient than in A ). A perforation at the radial attachment of the triangular fibrocartilage is evident as a high signal line ( arrowhead ) traversing the triangular fibrocartilage. Note bone marrow edema in the lunate as a result of ulnolunate impaction. C , Gradient echo coronal image of the wrist (different patient than A and B ). There is a high signal irregularity ( arrowhead ) on the distal surface of the triangular fibrocartilage from a partial tear.

The location of a tear has therapeutic implications because of the vascular supply of the TFC. The peripheral 20% of the TFC on the ulnar margin is well vascularized, and tears may heal with nonoperative therapy if properly immobilized or with primary repair. The remainder of the TFC is essentially avascular, and perforations or tears in the central and radial portions of the TFC usually are débrided. Many individuals have high signal intensity within the substance of the TFC and perforations, but have no symptoms. The intrasubstance signal is probably from myxoid degeneration. The asymptomatic perforations are probably degenerative in nature because most traumatic tears are symptomatic. As with all imaging, the presence of abnormal findings does not ensure that symptoms are a consequence of the abnormalities, and correlation of the MRI and clinical findings is mandatory in planning proper management of patients. The TFC may be traumatically detached from its ulnar attachment and may become interposed between the radius and ulna, preventing proper reduction of the distal radioulnar joint ( Fig. 12-12 ).

Triangular fibrocartilage detachment. Coronal fat-suppressed T2W image of the wrist. The ulnar attachment of the triangular fibrocartilage has been traumatically detached with discontinuity and a gap of the triangular fibrocartilage ( arrow ) through the two thin bands that attach it to the ulnar styloid process.

Traumatic tears of the TFC often are associated with injuries of adjacent structures, such as the extensor carpi ulnaris tendon sheath and lunotriquetral ligament, which can be shown readily by MRI. MRI is very accurate for diagnosing TFC tears; compared with arthrography and surgery, it has an accuracy rate of 95%. Tears in the central and radial portions are best shown, whereas tears near the ulnar attachment are less accurately diagnosed because synovitis or synovial proliferation in the prestyloid recess may mimic a tear.

Normal Radioulnar Ligaments.

The volar and dorsal radioulnar ligaments are broad, striated bands that originate on the volar and dorsal cortex of the sigmoid notch of the distal radius. The ligaments pass on the volar and dorsal surfaces of the TFC and blend with it. The radioulnar ligaments attach to the ulnar styloid process medially, and to the distal radius laterally. The dorsal and volar radioulnar ligaments can be distinguished from the TFC proper because they tend to have flat superior and inferior margins, rather than being biconcave, and they attach directly to bone, rather than to cartilage on the radius ( Fig. 12-13 ). These structures are low signal intensity on all pulse sequences and are best depicted on coronal images.

Normal radioulnar ligament. Coronal fat-suppressed T2W image of the wrist. The volar radioulnar ligament is seen as a low signal structure ( arrowheads ) attaching to the bones of the radius and ulna, with straight proximal and distal margins. These all are features distinguishing it from the adjacent triangular fibrocartilage proper. The dorsal radioulnar ligament has an identical appearance to the volar ligament.

Abnormal Radioulnar Ligaments.

Disruption of the volar or dorsal radioulnar ligaments is associated with instability of the distal radioulnar joint. Disrupted ligaments can be seen on MRI, and subluxation or dislocation of the distal radioulnar joint can be shown easily on axial MRIs ( Fig. 12-14 ). Distal radioulnar joint instability is diagnosed when the ulna does not articulate properly with the sigmoid notch of the distal radius and is displaced in either a dorsal or volar direction from this notch.

Distal radioulnar joint: normal and abnormal. A , Axial fat-suppressed T2W image of the wrist. The normal concentric relationship of the radius and ulna is shown ( arrows ) in the distal radioulnar joint of a patient with intact radioulnar ligaments. B , Axial T1W image of the wrist (different patient than in A ). Disruption of the dorsal radioulnar ligament resulted in dorsal subluxation of the ulna relative to the radius with loss of the concentric relationship of the two bones in the distal radioulnar joint ( arrows ).

Normal Extensor Carpi Ulnaris.

The extensor carpi ulnaris tendon and its sheath, which is a component of the TFCC, can be seen on coronal MRIs, but is best depicted in the axial plane, as is the case with many other tendons ( Fig. 12-16 ). The tendon sheath is not evident on MRI, unless there is fluid in it. The extensor carpi ulnaris tendon is located in the groove on the dorsum of the ulna in neutral and pronation positioning of the wrist, whereas subluxation can be seen if imaged in supination.

Normal extensor carpi ulnaris tendon. A , Coronal fatsuppressed T2W image of the wrist. The extensor carpi ulnaris tendon is seen on the ulnar and dorsal side of the wrist ( arrowheads ). B , T1 axial image of the wrist. The normal position of the extensor carpi ulnaris is evident in the groove on the dorsum of the ulna ( arrow ).

Abnormal Extensor Carpi Ulnaris Sheath.

Traumatic disruption of the extensor carpi ulnaris tendon sheath can result in subluxation or dislocation of the extensor carpi ulnaris tendon at the level of the distal ulna, out of its normal groove, in a medial direction. Associated tenosynovitis is common. Subluxation and tenosynovitis are best shown on axial images ( Fig. 12-17 ).

Dislocated extensor carpi ulnaris tendon. Axial T1W image of the wrist. The extensor carpi ulnaris tendon ( arrow ) is dislocated in an ulnar direction from its normal position in the groove ( arrowhead ) on the dorsum of the ulna, indicating disruption of the extensor carpi ulnaris tendon sheath, which is a component of the triangular fibrocartilage complex.