Working Diagnosis:
Ganglion cyst of the left knee.

Treatment:
Eight mL of viscous, honey-colored fluid was aspirated from the cyst and then 20 mg of triamcinolone and 3 mL of bupivacaine were injected under ultrasound guidance.

At four-week follow-up, the patient reported mild recurrence of swelling without pain. Bedside ultrasound redemonstrated the cyst, reduced in size and without septations. Six mL of thin, yellow-brown fluid was aspirated from the cyst and triamcinolone/bupivacaine injection was repeated.

Eight weeks after his first visit, he returned reporting continued mild swelling of the medial left knee. Bedside ultrasound revealed the cyst measuring 12.6 cm^3. Honey-colored, viscous fluid was aspirated from the cyst. The area was then injected with 10 mL (100 mg) of doxycycline as a sclerosing agent. One hour later, the cyst was again aspirated.

He returned for follow-up fourteen weeks after initial presentation. He had complete resolution for four weeks following the doxycycline procedure, then had mild return of the swelling after increasing his activity level. Ultrasound revealed recurrence of the cyst, now less than 10 cm^3. 6 mL of yellow fluid was aspirated from the cyst. Then, 10 mL of doxycycline (100 mg) was injected. After one hour, the cyst was drained Case Photo #2 .

Outcome:
With each procedure, patient noted subjective improvement along with decrease in size of the cyst. Following the second doxycycline injection, the patient reported no significant swelling and denied any noteworthy pain. Three months following his second doxycycline injection he had returned to all of his desired activities without limitations or a return of his symptoms. He did not require any surgical intervention to address the ganglion cyst.

Author's Comments:
The differential diagnosis for cystic lesions about the knee includes popliteal, meniscal, and ganglion cysts. Diagnostic consideration should be made for potential mimickers including vascular aneurysms, epidermal cysts, and tumors. Doxycycline has been used as an injectable sclerosing agent in aneurysmal bone cysts and soft tissue cavities such as Morel-Lavallee lesions. In this case, injection of doxycycline led to clinical resolution of the patient's ganglion cyst.

Editor's Comments:
Bedside ultrasound is an important diagnostic tool in identifying and directing therapy at multi-septated cysts. Multiple chemicals that have been used to ganglion cysts with mixed results include corticosteroids, sclerosants (tetracycline, doxycycline, phenols, hypertonic saline), and hyaluronidase. Corticosteroids decrease inflammation leading to less fluid within the ganglion cyst. Sclerosants enhance adhesions of the ganglion cyst walls with resolution of the cyst. Hyaluronidase breaks down hyaluronic acid which is makes up the majority of fluid within the ganglion cyst and leads to resolution of the cyst.

References:
1. Stein D, et al. Cysts about the knee: evaluation and management. J Am Acad Orthopedics Surg 21 (2013) 469-479
2. Wang CJ. Multiple ganglion cysts of the knee. Arthroscopy 18:8 (2002) E42
3. Not JH, et al. Intraarticular epidermal cyst of the knee. J Orthop Sci 18 (2013) 340-342
4. Hardy T, et al. Metastatic peripheral primitive neuroectodermal tumor/Ewing's sarcoma of the knee mimicking a popliteal cyst in an adult patient. J Clin Rheum 15:3 (2009) 150
5. Doyal A, et al. Recurrent aneurysmal bone cyst of the cervical spine in childhood treated with doxycycline injection. Skeletal Radiol 44 (2015) 609-612
6. Shields WE, et al. Percutaneous doxycyline treatment of juxtaphyseal aneurysmal bone cysts. J Pediatr Orthop 36 (2016) 205-212
7. Bansal A, et al. Doxycyline sclerodesis as a treatment option for persistent Morel-Lavallee lesions. Injury, Int J Care Injured 44 (2013) 66-69

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