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DOI:10.2214/AJR.05.0764
AJR 2007; 188:198-202
© American Roentgen Ray Society

Clinical Observations

Extensor Retinaculum of the Wrist: Sonographic Characterization and Pseudotenosynovitis Appearance

Brian L. Robertson1, David A. Jamadar1, Jon A. Jacobson1, Monica Kalume-Brigido1, Elaine M. Caoili1, Zvi Margaliot2 and Michel O. De Maeseneer1,3

1 Department of Radiology, University of Michigan Hospitals, 1500 E Medical Center Dr., TC2910, Ann Arbor, MI 48109.
2 Division of Plastic Surgery, Department of Surgery, Trillium Health Centre, Mississauga, ON, Canada.
3 Present address: Division of Radiologic Sciences, Wake Forest University, Winston-Salem, NC 27157-1088.

Received May 4, 2005; accepted after revision July 25, 2005.

 
Address correspondence to D. A. Jamadar.


Abstract
Top
 Abstract
Introduction
 Subjects and Methods
 Results
 Discussion
 References
 
OBJECTIVE. We have found in our practice that the normal extensor retinaculum of the wrist may appear hypoechoic on sonography and, because it is closely applied to the extensor tendons, may simulate tenosynovitis. This study prospectively evaluates the extensor retinaculum in 50 healthy adult volunteers, characterizing its sonographic appearance.

CONCLUSION. The extensor retinaculum has a characteristic appearance on sonography. A hypoechoic appearance from anisotropy should not be confused with tenosynovitis.

Keywords: anatomy • hand • sonography • tenosynovitis • wrist


Introduction
Top
 Abstract
 Introduction
Subjects and Methods
 Results
 Discussion
 References
 
Sonography of the hand and wrist has been used successfully to evaluate a number of musculoskeletal disorders. Several examples include the diagnosis of carpal tunnel syndrome, ganglionic cyst, and tendon abnormalities, such as a tendon tear and tenosynovitis [1]. Particularly in the distal extremities, where there is less soft-tissue attenuation, high-frequency probes may be used to produce exquisite detail of anatomy and any abnormality.

At the dorsum of the wrist, the extensor retinaculum is a fibrous band that holds the extensor tendons in place and prevents bow-stringing (Figs. 1 and 2). By extending fascial attachments to the underlying bones and periosteum, the retinaculum forms six compartments over the dorsal wrist, each of which contains various tendons [2, 3].


Figure 1
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  		Fig. 1 —Illustration of dorsal wrist shows extensor retinaculum (blue) extending obliquely across dorsal wrist. Note extensor digitorum tendons (red) and tendon sheaths (yellow).

 

Figure 2
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  		Fig. 2 —Photograph of dissected right dorsal wrist of elderly male cadaver shows extensor retinaculum (arrowheads) superficial to extensor tendons (T). Left is ulnar, and right is radial.

 
In our clinical practice, we have observed a variable appearance of the normal extensor retinaculum on sonography that, because of its close proximity to the extensor tendons, may be misinterpreted as complex fluid in the tendon sheath or in the adjacent soft tissues and thus prompt the diagnosis of tenosynovitis. This study characterizes the sonographic appearance of the extensor retinaculum at the wrist in a population of healthy adult volunteers.


Subjects and Methods
Top
 Abstract
 Introduction
 Subjects and Methods
Results
 Discussion
 References
 
Institutional review board approval was obtained before commencing this study. The initial in vivo study population consisted of 51 healthy volunteers. Exclusion criteria consisted of a history of wrist trauma or of a systemic disease, such as rheumatoid arthritis or psoriasis, or a history of tenosynovitis at the wrist. The final study population consisted of 50 subjects, with one subject with psoriasis excluded.

One hundred wrists of 50 healthy subjects (15 men, 35 women) were examined. The subjects had a mean age of 33.4 years (age range, 20-58 years). Six of the 50 subjects were left hand-dominant, 43 were right hand-dominant, and one was ambidextrous.

After written informed consent was obtained from the study subjects, sonographic examinations were performed using a 7- to 15-MHz compact linear array transducer (model HDI 5000, Philips-Advanced Technology Laboratories). All scanning was performed by a board-certified sonographer (8 years of experience) who had been specifically trained in musculoskeletal sonography (2.5 years of musculoskeletal sonography experience). For the sonography examinations, each subject sat with the arm and hand extended and placed in a prone position on the examination table. The table was positioned to level the arm comfortably. Liberal transmission gel was used in place of a standoff pad. Sonographic scanning was performed on the prone dorsal wrist where the retinaculum was identified by its expected anatomic location and compact fibrillar sonographic texture.

Images were obtained along the long axis (longitudinal) and along the short axis (transverse or cross section) of the retinaculum of each wrist. A minimum of five images of each wrist were obtained. Along the short axis of the retinaculum, sonographic images were obtained with the sound beam perpendicular to the retinaculum and in obliquity to determine the effect of transducer positioning on echogenicity. The sonographer also evaluated the extensor retinaculum with power Doppler sonography (color power angiography, 78%; Wall filter, medium; pulse repetition frequency, 700). Hand dominance was recorded; both right and left wrists were examined in all subjects.


Figure 3
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  		Fig. 3A —37-year-old woman with normal extensor retinaculum. Sonograms along long axis of extensor retinaculum (A) and along short axis of extensor retinaculum (B) show normal subtle compact fibrillar extensor retinaculum (arrows) to be relatively hypoechoic compared with adjacent extensor digitorum tendons (ED). Note radius (R) in B, lunate (L) in A and B, capitate (C) in B, and scaphoid (S) in A; distal is to right in B.

 


Figure 4
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  		Fig. 3B —37-year-old woman with normal extensor retinaculum. Sonograms along long axis of extensor retinaculum (A) and along short axis of extensor retinaculum (B) show normal subtle compact fibrillar extensor retinaculum (arrows) to be relatively hypoechoic compared with adjacent extensor digitorum tendons (ED). Note radius (R) in B, lunate (L) in A and B, capitate (C) in B, and scaphoid (S) in A; distal is to right in B.

 
The sonographic images were retrospectively analyzed by a fellowship-trained musculoskeletal radiologist (8 years of experience in musculoskeletal sonography) and by the same sonographer who had obtained the images. The time interval between image acquisition and retrospective image analysis was 7-10 days. Sonographic images obtained along the short axis of the extensor retinaculum were used for measurements. On these static images, the extensor retinaculum was identified by its characteristic location and compact fibrillar echotexture. Retinaculum thickness and the short-axis width were measured by the radiologist using measurement calipers at an image workstation (Kinet Dx Workstation 3000, Siemens Medical Solutions).


Figure 5
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  		Fig. 4 —50-year-old woman with normal extensor retinaculum. Sonogram along short axis of extensor retinaculum shows its characteristic fusiform thickening and location (arrows). Note hypoechoic appearance relative to extensor digitorum tendons (ED), although internal echoes are still visible. Note radius (R), lunate (L), and capitate (C) bones; distal is to right.

 


Figure 6
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  		Fig. 5A —37-year-old man with normal extensor retinaculum. Sonograms show effect of angling transducer between relatively perpendicular (A) to retinaculum (arrows, A) and at angle (B) to retinaculum (arrows, B). Retinaculum appears more hypoechoic when beam is at angle. Note extensor digitorum tendons (ED) and radius (R), lunate (L), and capitate (C) bones; distal is to right.

 


Figure 7
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  		Fig. 5B —37-year-old man with normal extensor retinaculum. Sonograms show effect of angling transducer between relatively perpendicular (A) to retinaculum (arrows, A) and at angle (B) to retinaculum (arrows, B). Retinaculum appears more hypoechoic when beam is at angle. Note extensor digitorum tendons (ED) and radius (R), lunate (L), and capitate (C) bones; distal is to right.

 


Figure 8
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  		Fig. 6 —48-year-old man with 19-year history of psoriasis and asymptomatic dorsal wrist. Sonogram shows retinaculum retains fusiform shape (arrows), but flow adjacent to retinaculum is increased and flow around extensor digitorum tendons (ED) is increased. Note radius (R). Distal is to right.

 
For each wrist, the echogenicity of the extensor retinaculum was characterized by consensus agreement as hypoechoic, isoechoic, or hyperechoic relative to the adjacent tendon tissue. If a retinaculum had several echogenicities on different images from the same wrist, each was recorded. The shape of the retinaculum was also characterized on short-axis images of the retinaculum. The static power Doppler images were also reviewed by consensus and were characterized as showing positive or negative flow within the retinaculum. The power Doppler settings were configured for the first subject at a threshold just short of aliasing to optimize for visualization of low-velocity flow and were then reproduced for the remaining subjects.

Approval for use of a cadaveric specimen was obtained from the anatomic donations department. The skin and subcutaneous tissues were dissected, by one of the authors, off the dorsum of the right wrist of a single formalin-fixed adult male cadaver to expose the superficial aspect of the wrist (Fig. 2).


Results
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 Abstract
 Introduction
 Subjects and Methods
 Results
Discussion
 References
 
Sonography revealed the extensor retinaculum in all wrists located superficial to the extensor tendons at the level of the radiocarpal joint (Figs. 3A and 3B). Along the long axis of the extensor retinaculum (Fig. 3A), the continuous homogeneous compact fibrillar echotexture was identified superficial to the extensor tendons and deep to the subcutaneous fat. Along the short axis of the extensor retinaculum, a fusiform appearance was noted with thickening of the central portion at the level of the radiocarpal joint, tapering both distally and proximally (Fig. 3B). This configuration was observed in all 100 retinacula, with variation only in the long-axis dimension and thickness.

Sonography along the short axis of the extensor retinaculum showed a maximum thickness of 0.6-1.7 mm in men (mean ± SD, 1.16 ± 0.24 mm) and 0.5-1.6 mm in women (mean, 1.06 ± 0.22 mm). The short axis of the retinaculum varied from 8.2 to 23 mm in men (mean, 15.6 ± 2.73 mm) and from 8.0 to 19.8 mm in women (mean, 12.9 ± 2.40 mm).

Using the two-tailed Student's t test, we found a statistically significant difference in the short-axis dimension of the retinaculum between men and women (p < 0.001). Significant differences between men and women were also noted in the retinacular short-axis dimensions in both right (p = 0.003) and left (p = 0.001) hands. A trend was noted when assessing the differences in thickness of the extensor retinaculum between men and women (p = 0.6): No significant differences in thickness (men: p = 0.718; women: p = 0.873) or short-axis dimension (men: p = 0.718; women: p = 0.603) were noted when comparing the effects of handedness, or hand dominance.

In all 100 extensor retinacula, when imaged perpendicular to the retinaculum, a subtle compact fibrillar echotexture was seen slightly hypoechoic relative to tendon (Figs. 3A and 3B). When imaged obliquely, all retinacula became more hypoechoic, although internal echoes were still identified (Fig. 4). None of the retinacula were found to be hyperechoic relative to tendon. In all wrists, there was no flow with power Doppler imaging.

The cadaveric part of the study was performed on an elderly male whose age at the time of death and cause of death were unknown. The dissection revealed a fibrous band in the expected location of the extensor retinaculum extending obliquely across the dorsal wrist. A photograph was obtained to illustrate gross anatomy (Fig. 2).


Discussion
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
References
 
The extensor retinaculum is a fibrous thickening of the investing fascia, extending around the wrist to become continuous with the volar carpal ligament. Proximally the extensor retinaculum arises from the antebrachial fascia of the forearm and distally it tapers, becoming indistinguishable from the loose fascia over the metacarpals, which is part of the superficial fascia of the dorsum of the hand [2, 3]. At the level of the radiocarpal joint, the extensor retinaculum courses obliquely across the dorsal carpus and is thickened centrally along its long axis, tapering proximally and distally. The function of the extensor retinaculum is to prevent bowstringing by keeping the various extensor tendons in close proximity to the underlying radius and ulna but separate from each other at the wrist [2, 3]. Portions of the extensor retinaculum may be used as a graft in the reconstruction of interosseous ligaments of the wrist [4] or in the reconstruction of pulleys in the fingers [5].

In our clinical practice, we have noted a variable sonographic appearance of the extensor retinaculum. We found that the fusiform shape and possible hypoechogenicity may cause the normal extensor retinaculum to simulate tenosynovitis of the extensor tendons. This study characterizes the extensor retinaculum in healthy volunteers with regard to its shape, size, and echogenicity.

Our results show that the normal extensor retinaculum has a characteristic appearance superficial to the extensor tendons, tapering proximally and distally with a subtle compact fibrillar echotexture, at the level of the radiocarpal joint. The maximal thickness of the retinaculum ranged from 0.5 to 1.7 mm, with no effect related to handedness. The difference in the short-axis dimension of the retinaculum between men and women may be in part explained by the overall size differences between these two groups, although this was not directly assessed in this study.

With regard to echogenicity, the normal extensor retinaculum showed a subtle fibrillar echotexture when imaged perpendicular to the sound beam and was relatively hypoechoic compared with adjacent tendon. The extensor retinaculum appeared even more hypoechoic when imaged obliquely. This can be explained by anisotropy, which may be seen when scanning other structures in the musculoskeletal system, such as tendons and ligaments.

The sonographic appearance of compact linear fibrillar structures as hyperechoic, hypoechoic, or even anechoic is due to the sonographic phenomenon of anisotropy [6-8]. Anisotropy is a sonographic property of linearly organized tissues, such as tendons, ligaments, and nerves, where sonographic appearance is determined in part by the angle of insonation of the ultrasound beam. When the beam is perpendicular to a structure such as the extensor retinaculum, that structure will appear reflective and the internal stacked parallel linear structure will be visible. When the beam is angled away from perpendicular, these linear reflective structures appear less hyperechoic. Even an angle of as little as 2° off the perpendicular can produce this phenomenon [6]. Angling the transducer back and forth during real-time scanning to optimize reflectivity (Figs. 5A and 5B) will help to distinguish anisotropy from hypoechoic abnormalities such as tenosynovitis [8].

Tenosynovitis is the result of a disorder of the synovium surrounding the tendon that causes synovial proliferation and the production of fluid that accumulates around the involved tendon [9]. The etiology of tenosynovitis includes infectious, inflammatory, and posttraumatic causes. Sonographically, although simple fluid surrounding a tendon typically appears anechoic, both the synovium and complex fluid may appear hypoechoic [10]. The presence of flow on color or power Doppler imaging may help differentiate complex fluid from synovitis, with the latter possibly showing increased flow, helping to differentiate inflamed synovium from a normal extensor retinaculum. Knowledge of the anatomy of the wrist, the location of the synovial tendon sheaths in relation to the tendons and retinaculum, and patient symptoms also help to differentiate between the two. In the subject with psoriasis, one of our exclusion criteria, sonography showed flow adjacent to the extensor retinaculum (Fig. 6).

We found that the normal extensor retinaculum may appear as abnormal hypoechoic tissue surrounding the extensor tendons because of anisotropy; however, the characteristic location of the extensor retinaculum at the level of the radiocarpal joint and its typical shape, width, and thickness should indicate that the tissue is the normal extensor retinaculum. By angling the transducer during real-time imaging to obtain a perpendicular orientation between the sound beam and the extensor retinaculum, the subtle compact fibrillar echotexture of the normal extensor retinaculum can be detected. Comparison with the contralateral side, another advantage of sonography, may also be helpful because symmetry is often found in normal structures.

One potential limitation of this study is the absence of a gold standard. However, the known anatomic location of the extensor retinaculum and typical compact fibrillar appearance of ligaments and ligamentlike structures made identification and evaluation possible. In addition, the consistent identification of this structure in all asymptomatic wrists is also evidence that a normal anatomic structure was identified. Last, interobserver variability for extensor retinaculum identification and measurements was not tested; however, characterization and measurements were made with agreement by consensus.

In summary, the normal extensor retinaculum of the wrist has a characteristic location and sonographic appearance that will allow identification and prevent misinterpretation as tenosynovitis.


References
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
 References
 

  1. Middleton WD, Teefey SA, Boyer MI. Hand and wrist sonography. Ultrasound Q2001; 17:21 -26[Medline]
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D. A. Jamadar, B. L. Robertson, J. A. Jacobson, G. Girish, B. J. Sabb, Y. Jiang, and Y. Morag
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Am. J. Roentgenol., January 1, 2010; 194(1): 216 - 225.
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