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Man with posterior knee mass

Issue: August 2019

ByGeorge F. Hatch III, MD

ByHarrison F. Kay, MD

ByMoé R. Takeda, MD

ByJihoon T. Choi, MD

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A 36-year-old man was referred for the management of ongoing posterior right knee pain and discomfort following a complicated surgical history for his knee. He sustained a partial tear of his right quadriceps tendon in 2003 and underwent surgical repair. In 2004, he sustained a complete retear of the same quadriceps tendon and underwent revision surgery. In 2017, he sustained another knee injury at which time he underwent arthroscopic partial medial and lateral meniscectomy.

He received a corticosteroid knee injection due to ongoing pain 2 months after surgery. Shortly thereafter, he developed knee effusion and erythema.

What is the next step in the workup of postoperative knee pain, erythema and swelling after corticosteroid injection?
See answer on on next page.

A large popliteal mass extending down posterior aspect of the thigh

The differential diagnosis for erythema and effusion after corticosteroid injection included hematoma and infection. The patient underwent aspiration. It revealed negative gram stain and 80,000 white blood cells indicative of septic arthritis. He underwent arthroscopic irrigation and debridement. Cultures from a specimen grew methicillin-sensitive Staphylococcus aureus. He was treated with IV antibiotics.

Two months later, the patient was found to have a large popliteal mass extending down the posterior aspect of the thigh. He denied any signs of acute infection. His physical exam was remarkable for a large palpable area of fluctuance over the posterior knee, as well as areas of fluid tracking down the posterior aspect of his leg and ankle.

Differential diagnosis now included abscess and synovial (Baker’s), meniscal and ganglion cysts.

Source: Harrison F. Kay, MD

Radiographs of the right knee demonstrated deformity of the patella and heterotopic ossification (HO) of the quadriceps tendon consistent with previous surgery, as well as a moderate to large knee effusion (Figure 1). MRI of the right lower extremity demonstrated a knee joint effusion with extensive synovitis, a large Baker’s cyst measuring up to 7.1 cm with a posterior medial joint capsule defect, and numerous lobulated collections that contained synovitis and debris extending along the posteromedial calf about 40 cm down to the level of the ankle (Figure 2). Ultrasound-guided aspiration and synovial biopsy yielded cloudy serosanguineous fluid with debris. Four soft tissue core biopsy samples were obtained and sent for culture and gram stain. All cultures were negative. The patient elected for excisional biopsy of the cystic mass.

Source: Moé R. Takeda, MD

Surgical excision was performed with an extensile, posteromedial approach (Figure 3). The cystic mass was found to originate between the interval of the medial head of the gastrocnemius and the semimembranosus, consistent with a Baker’s cyst. A pedunculated stalk involving the posteromedial capsule was noted. The cystic mass traversed the length of the leg, narrowing proximally and broadening distally to involve the posteromedial ankle (Figure 4). The mass was removed and measured more than 35 cm in length. Caseous granular debris was noted throughout. Intraoperative frozen sections were negative for infection with no acute inflammation. All intraoperative cultures were negative. Surgical pathology of the cyst demonstrated chronic inflammation and hemosiderin-laden macrophages consistent with Baker’s cyst (Figure 5).

The patient was followed at 1-week intervals. He remained neurovascularly intact with full knee and ankle range of motion (ROM). His posterior leg did not show evidence of recurrent fluid collection. While using compressive dressings, the patient was mobilized in physical therapy with full ROM of the ipsilateral knee and ankle with the goal of retaining motion and strength while avoiding fluid re-accumulation. By 3 months, the patient’s leg had returned to baseline with no persistent swelling (Figure 6). The patient could return to full activity.

Source: Harrison F. Kay, MD

To the authors’ knowledge, this is the first and largest reported case of a massive Baker’s cyst not associated with arthroplasty.

It presented in the setting of prior meniscectomy and knee septic arthritis. The diagnosis was complicated by infection and concerns of an abscess related to septic arthritis. A capsular defect led to massive extra-articular synovial fluid collection via a communication between the medial head of the gastrocnemius and semimembranosus. These cysts, even when large, can be treated successfully with surgical excision.

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• For more information:
• Jihoon T. Choi, MD; George F. Hatch III, MD; Harrison F. Kay, MD; and Moé R. Takeda, MD, can be reached in the department of orthopedic surgery, Keck School of Medicine of the University of Southern California, 1520 San Pablo St., Suite 2000, Los Angeles, CA 90033. Choi’s email: jihoon.choi@med.usc.edu. Hatch’s email: ghatch@med.usc.edu. Kay’s email: harrison.kay@med.usc.edu. Takeda’s email: mtakeda@dhs.lacounty.gov.
• Edited by Joshua D. Johnson, MD, and Nicholas A. Trasolini, MD. Johnson is a chief resident in the department of orthopedic surgery at Mayo Clinic in Rochester, Minnesota. He will be a musculoskeletal oncology fellow at MD Anderson following residency. Trasolini is the administrative chief resident in the department of orthopedic surgery at Keck Medical Center of the University of Southern California and will be a sports medicine fellow at Rush University Medical Center following residency.

Disclosures: Choi, Hatch, Kay and Takeda report no relevant financial disclosures.