MRI Web Clinic — April 2004

ACL Ganglion Cysts
Michael E. Stadnick, M.D.

Clinical History: 37 year-old male with no known injury, complaining of diffuse knee pain and stiffness, worsening over the last 3 months. Sagittal T1-weighted (1a) and fat-suppressed proton density-weighted (1b) images are provided. What are the findings? What is your diagnosis?

1a
1b
Figure 1:

Sagittal T1-weighted (1a) and fat-suppressed proton density-weighted (1b) images

 

 

Findings

 

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Figure 2:

(2a) T1-weighted sagittal image demonstrates an enlarged, club-shaped anterior cruciate ligament (arrows).

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Figure 3:

(3a) A proton density-weighted sagittal image at the same level demonstrates increased signal within the enlarged anterior cruciate ligament (arrows) with faintly identified ACL fibers. Intraosseous increased signal is present within the tibial spine deep to the posterior extent of the ACL insertion (arrowhead).

 

 

Diagnosis

Anterior cruciate ligament ganglion cyst with intraosseous extension.

Discussion

Anterior and posterior cruciate ligament ganglion cysts are reported in the literature as being rare with an incidence of approximately 0.5% to 1%.1,2 However, in our experience, this entity is encountered more commonly, most often within the anterior cruciate ligament. The etiology of cruciate ligament ganglion cysts is uncertain. Remote ligamentous trauma as well as mucinous degeneration associated with aging likely play a role in the development of cruciate cysts.

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Figure 4:

(4a) 3D rendering of an ACL ganglion viewed from the medial side with the medial femoral condyle removed.

Two characteristic MR appearances have been described for ganglion cysts of the ACL. The first resembles ganglia in other periarticular locations, presenting as a discrete fluid signal cystic structure within the substance or on the surface of the ACL(5a).3 The second form has been likened to a celery stalk in appearance.4 The ACL demonstrates diffuse thickening containing ill-defined intraligamentous T2-hyperintensity, which splays the intact ACL fibers (6a).

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Figure 5:

(5a) Cystic form of ACL ganglion cyst. T2-weighted sagittal view demonstrates localized fluid signal within the ACL (arrows).

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Figure 6:

(6a) "Celery stalk" form of ACL ganglion cyst. Proton density-weighted sagittal demonstrates a diffusely increased size of the ACL with ill-defined increased signal and interspersed intact ACL fibers.

 

The soft-tissue thickening and increased signal intensity along the course of the ligament with either form of cyst may be confused with the appearance of an ACL tear. Several critical distinctions allow a confident diagnosis of an ACL ganglion cyst. With cruciate ligament ganglia, intact ACL fibers can often be visualized coursing through or around the cyst. On physical exam, the Lachman test for ACL instability is negative with an ACL ganglion. The patient’s clinical history and symptoms do not support an ACL injury. Furthermore, the ancillary MR findings of an ACL tear, such as bone bruises and the MR anterior drawer sign, are absent.

When a cruciate ligament ganglion is discovered incidentally, no intervention is necessary. However, in some patients, ACL ganglion cysts cause significant clinical symptoms. These patients may complain of pain and stiffness, and large ganglia may cause limitation of extension. For such patients, decompression of the ganglion at arthroscopy or via CT guidance is indicated and provides prompt relief.3,5,6

An interesting feature of cruciate ganglia, which has not been widely reported, is their tendency to extend into adjacent intra-articular spaces and osseous structures. Extraligamentous extension into the joint and intercondylar notch produces a higher incidence of mechanical symptoms (7a). It is postulated that intraosseous extension may be a finding more often associated with pain (8a). This association has not yet been proven.

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Figure 7:

(7a) Intra-articular extension of a cruciate ligament ganglion. An anteriorly protruding ganglion (arrow) arising from an ACL ganglion cyst (arrowheads) is well seen on a proton density-weighted sagittal image.

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Figure 8:

(8a) Intraosseous extension of an ACL ganglion. A proton density-weighted coronal view from the same patient as seen in (A) and (B) reveals intraosseous ganglia (arrows) communicating with the ACL ganglion cyst (short arrow). Reactive marrow edema (arrowheads) surrounds the intraosseous ganglia.

 

 

Conclusion

Anterior cruciate ganglion cysts are probably more prevalent than stated in the literature. These cysts have a variable MR appearance and are not always confined to the substance of the ligament. In most cases, consideration of the clinical history, careful evaluation of the MR appearance, including the lack of ancillary findings of an ACL tear, and physical examination allow confident differentiation of a ganglion cyst from a cruciate ligament tear.

References

1 Brown MF, Dandy DJ: Intra-articular ganglia in the knee. J Arthosc 6:322, 1990.

2 Kaplan PA, Helms CA, Dussault R, Anderson MW, Major NM: Musculoskeletal MRI. 1st ed. 2001.

3 Do-Dai DD, Youngberg RA, Lanchbury FD, et al: Intraligamentous ganglion cysts of the anterior cruciate ligament: MR findings with clinical and arthroscopic correlations. J Comput Assist Tomogr 1996 Jan-Feb; 20(1): 80-4.

4 McIntyre J, Moelleken S, Tirman P: Mucoid degeneration of the anterior cruciate ligament mistaken for ligamentous tears. Skeletal Radiol 2001 Jun; 30(6): 312-5.

5 Stoller DW: Magnetic Resonance Imaging in Orthopaedics and Sports Medicine . 2nd ed. Lippincott-Raven; 1997.

6 Antonacci VP, Foster T, Fenlon H, Harper K, Eustace S: Technical report: CT-guided aspiration of anterior cruciate ligament ganglion cysts. Clin Radiol. 1998 Oct;53(10):771-3.

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