Table of Contents
- 01. Rotator Cuff
- 1. Anatomy and Function
- 2. Pathogenesis
- 3. Terminology
- 4. Natural History
- 5. Ultrasound Technique
- 6. Spectrum of Findings
- 7. Ultrasound Pitfalls
- 8. References
- 02. Postoperative Rotator Cuff
- 03. Long Head of Biceps Brachii Tendon
- 04. Bursae
- 05. Joint Spaces
- 1. Acromioclavicular Joint
- 2. Glenohumeral Joint
- 3. Sternoclavicular Joint
- 4. Scapulothoracic
- 5. References
- 06. Fractures
- 07. Os acromiale
- 08. Adhesive Capsulitis
- 09. Deltoid
- 10. Pectoralis Major
- 11. Web Exclusive: Sternocostal Joints
- 12. Web Exclusive: Musculoskeletal Ultrasound Educational Videos
- a. Best Original Manoeuver
- b. Snapping Film Direction
- c. Actor in a Leading Role
- d. Actress in a Leading Role
- e. Actor in a Supporting Role
- f. Actress in a Supporting Role
- g. Best Correlation Between Clinical and Sonographic Findings
- h. Joint Instability Short Film Documentary
- i. Unedited Film
- j. Real Time Impingement Short Film
- k. Nerve Compression Short Film
- l. Doppler Feature Special Award for Lifetime Achievements
- m. Interventional Feature Special Award for Distinguished Body of Work
- n. Honorary Award
1.2.2. Acromioclavicular Joint Ultrasound: Ganglion
The acromioclavicular joint (ACJ) ganglion is a common benign pseudocystic lesion filled with gelatinous material that communicates with the joint space.23-25 The wall of these lesions is composed of multidirectional strata of collagen and has no synovial membrane.
According to a review article published in 2010, forty-one cases of ACJ ganglion have been described, and most occurred in patients with an associated rotator cuff tear.26 However, available data from case reports is limited and most studies lack histopathological correlation or include overdistended ACJ in the definition of ganglion. In the following paragraphs, we only consider ganglion as a synonymous with lesions eccentrically located in relation to the joint space.
The pathogenesis of ACJ ganglion is enigmatic, and three proposed mechanisms derive from the observation of lesions located elsewhere in the body. In the first proposed mechanism, joint abnormalities lead to altered biomechanics, weaken the joint capsule, and eventually cause leakage of intra-articular fluid into the periarticular tissue. Intra- and extra-articular fluids communicate via a pedicle, which contains a one-way valve mechanism. Such a one-way valve is thought to result from a number of microcysts that communicate with the primary ganglion and are present in the tissue surrounding the tortuous pedicle lumen.27 Subsequent reaction between extra-articular fluid and the periarticular tissue encapsulates fluid and forms the wall of the ganglion.28 In the second proposed mechanism, the ganglion results from an extra-articular degenerative process and the gelatinous material represents the final end-product of a myxoid change in collagen or connective tissue that subsequently forms a pedicle and communicates to the joint.28 Lastly, in the third proposed mechanism, joint stress stimulate hyaluronan secretion by the mesenchymal cells located adjacent to the joint capsule. These cells are widely distributed in adult connective tissues and are also responsible for the formation of joint capsules and bursae.29 The accumulation of fluid eventually induces the formation of a pseudocapsule lined by collagen from compression of surrounding tissues. As none of these mechanisms individually explain all of the known features of ACJ ganglia, their origin is thought to be multifactorial and defined on a case-by-case analysis.
ACJ ganglion typically presents as a painless enlarging mass depicted as anechoic lesion adjacent to the ACJ (figure 5-7). Atypical findings such as thick walls and septations are not uncommon. Internal low-level echoes represent artifact, hemorrhage, or the relative proportion of the constituents of mucin that may change with the age of the ganglion.30,31 Acoustic enhancement is described but particularly difficult to demonstrate because most lesions are small and located close to the bone.
Figure 5-7. ACJ ganglion.
The differential diagnosis of a palpable lump located over the ACJ includes osteophytes (figure 5-8), synovial cysts, and supra-acromial bursitis. Synovial cysts are a simple herniation of the joint capsule and distinguished from ganglia on the basis of the presence of a synovial membrane, but these terms are loosely applied because imaging findings are similar and secondary changes in the capsule may make histologic differentiation difficult, if not impossible.32 Both ganglia and synovial cysts may communicate with the ACJ and this finding does not help in differentiating between these two lesions. However, this information should always be reported to the referring physician because failure to resect such a communication invariably leads to recurrence.33-35 On daily practice, we liberally use the term ganglion as a descriptive, not histopathological diagnosis. It should be emphasized that since ganglia has no epithelial lining, these lesions are actually pseudocysts, not true cysts.
Studies on the natural history of untreated ACJ ganglia are lacking. Anecdotal experience suggest that changes in volume are common and at least one case in the literature refers spontaneous resolution.36 Needle aspiration or surgical removal of these ganglia may be tricky and results in unpredictable outcomes without addressing the underlying pathological process.