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University of California San Francisco San Francisco, CA 94117-1349
Jacobson et al. [1] reported
on the sonographic characteristics of the dorsal aspect of the scapholunate
ligament. Although my colleagues and I strongly support the use of sonography
in musculoskeletal imaging, we have concerns regarding its suitability for
making the diagnosis of carpal
instability.
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Carpal stability depends on a complex structure of extrinsic and intrinsic ligaments (Figs. 1A, 1B). Ventral extrinsic ligaments include the radioscaphocapitate, radiolunotriquetral, and radiolunate ligaments. The dorsal extrinsic radiocarpal ligament consists of the radioscaphoid, radiolunate, and radiotriquetral portions. Important intrinsic ligaments are the lunotriquetral ligament and the scapholunate ligament, which consist of ventral, central, and dorsal portions. Although most radiologists agree that the dorsal (the strongest) and ventral parts of the scapholunate ligament are important stabilizers, controversy exists regarding the clinical implications of lesion location and size [2–5].
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Scapholunate instability remains a diagnostic problem clinically and radiologically. Therapeutic options, including temporary immobilization and arthrodesis, vary considerably depending on the time elapsed since the injury, the degree of stability, and the age and personal living circumstances of the patient. This instability has a poor prognosis if insufficiently diagnosed and treated; thus, a comprehensive assessment of all stabilizers and other relevant intraarticular and periarticular structures is essential for appropriate clinical management [2, 4, 6]. Sonographic identification of the presence or absence of abnormality of the dorsal part of the scapholunate ligament neither definitely proves nor disproves instability.
The insufficiency of sonography for making this diagnosis is partially supported by the findings of Jacobsen et al. [1] in eight cadaveric wrists and of Griffith et al. [5] in 100 healthy individuals, although both research groups regard this modality as a valuable tool. Griffith et al. found that the dorsal part of the scapholunate ligament was completely visible in only 48% of the wrists and partially visible in 30%; they found that the ventral part of the scapholunate ligament was completely visible in only 7% of the wrists, partially visible in 9%, barely visible in 8%, and not seen in 76%. Jacobson et al. were not able to classify 50% of the subjects according to the standards and concluded that nonvisualization of the ligament indicates an abnormality. This determination contradicts that of Griffith et al., who concluded that nonvisualization of the dorsal part of the scapholunate ligament does not automatically indicate an abnormality because this ligament was either barely visible (8%) or not visible (15%) in 23% of healthy subjects in their study.
Although Jacobson et al. [1] mentioned the small sample size as a study limitation, the fact that the findings for a sonographically normal scapholunate ligament are based on a single wrist may also present a problem. Another concern is that the T1-weighted MR imaging sequence used in the Jacobson et al. study (slice thickness, 3 mm; gap, 0.3 mm; matrix, 256 x 192) seems to be less suitable than a T1-weighted three-dimensional gradient-echo sequence (effective slice thickness, 0.6–1.5 mm; matrix, 256 x 256), which offers higher resolution and multiplanar capability [2, 3, 6].
In summary, my concern is with the real value of sonography for this complex abnormality. If such diversity exists in the appearance (detectable or nondetectable) and the gradation (normal or abnormal) in these recent studies in the assessment of only one third of a single ligament of the large complex of carpal stabilizers, what would be the diagnostic or therapeutic consequence of an abnormal or a normal sonographic finding for the scapholunate ligament? Would there not be too many open questions, implying the need for further diagnostic procedures if relevant carpal instability is suspected clinically or on unenhanced radiographs? Most likely, the surgeon would favor arthroscopy, which could simultaneously provide therapy, whereas the radiologist would suggest MR imaging [3, 4].
References
University of Michigan Ann Arbor, MI 48109-0326
Valley Radiologists, Ltd. Glendale, AZ 85301
University of Missouri Columbia, MO 65212
We thank Mohr et al. for their comments regarding our article, "Sonography of the Scapholunate Ligament in Four Cadaveric Wrists: Correlation with MR Arthrography and Anatomy" [1].
With improved resolution of sonographic transducers, it is possible to identify both intrinsic and extrinsic ligaments of the wrist. The first step in determining the effectiveness of sonography in evaluating carpal ligaments is to understand their normal sonographic appearance. We chose the dorsal aspect of the scapholunate ligament for this initial step because it is easily identified and is one of the important stabilizers of the wrist. The use of healthy volunteers in this process has limitations in that ligament disruption may be seen in asymptomatic wrists, and sonographic evaluation would be biased with knowledge that healthy volunteers were imaged. For this reason, we used cadaveric specimens in our study. The cadaveric wrists were imaged with sonography in a blinded fashion before other imaging and anatomic sectioning were performed. To provide a strong gold standard, we used a combination of arthrography, MR arthrography, and cadaveric sectioning to determine whether the scapholunate ligament was normal or abnormal. Because of this strict gold standard, only four of the initial eight specimens could be unequivocally classified as normal or abnormal using arthrography and MR imaging. A 3-mm slice thickness was used for MR imaging to match the slice thickness of anatomic sectioning, thus providing a direct correlation.
As stated in our abstract, the goal of our study was to characterize the sonographic appearance of the dorsal aspect of the scapholunate ligament [1]. We have shown that a normal scapholunate ligament has a hyperechoic fibrillar echotexture, and absence of this appearance suggests abnormality. We agree that the diagnosis of carpal instability is complex, and it is unclear what role sonography will eventually have in establishing this diagnosis. Further studies are needed to confirm these initial observations of the scapholunate ligament and to provide additional information about other carpal ligaments. Dynamic imaging, which may offer additional information, is one advantage of sonography that was not assessed in our study.
References
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