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A Soft-Tissue Mass About the Knee Following Intra-Articular Injection of Hylan-G1
Histopathologic slides of tissue obtained with an excisional biopsy of a soft-tissue mass were sent to the musculoskeletal pathologist at our institution from a community hospital. The accompanying clinical history documented an expanding painful mass, of two months' duration, in the posterolateral aspect of the left knee of a thirty-six-year-old woman with adult-onset diabetes and hypertension. The accompanying report on the magnetic resonance imaging findings noted a nonspecific soft-tissue mass (Figs. 1-A and 1-B). The pathologist at the community hospital, who sent the slides, suspected a diagnosis of myxoid sarcoma but requested a second opinion.
Fig. 1-A |
Fig. 1-A Coronal gradient-echo magnetic resonance image (repetition time, 450 msec; echo time, 9.4 msec) of the left knee (lateral is to the right, and medial is to the left), demonstrating a nonspecific hyperintense multilobulated mass lateral to the iliotibial band in the subcutaneous tissues. The black arrow demonstrates high signal intensity from diffuse edema, and the white arrow indicates some of the hyperintense lobules of the mass.
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Fig. 1-B |
Fig. 1-B Coronal fat-suppressed fast-spin-echo T2-weighted image (repetition time, 4566 msec; echo time, 49.5 msec), demonstrating the hyperintense mass throughout the subcutaneous tissues lateral to the iliotibial band (narrow arrows) as well as a small knee effusion (wide arrow).
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Histologically, the tissue showed a cellular infiltrate within a myxoid ground substance that appeared to dissect through surrounding fibrous connective tissue (Figs. 2-A, 2-B, and 2-C). This pattern was thought to represent aggressive growth. The cells showed mild to moderate degrees of nuclear atypia. Occasional nuclei with an open chromatin pattern and small but readily discernible nucleoli were also present. Rare mitotic figures were identified. The interpretation by our pathologist, communicated to the community hospital, was a low-grade fibromyxoid sarcoma. The key element in this interpretation was the dissection of myxomatous material between collagen bundles.
Fig. 2-A |
Fig. 2-B |
Fig. 2-C |
Figs. 2-A, 2-B, and 2-C Photomicrographs of tissue obtained with an excisional biopsy of a mass lateral to the left knee, demonstrating myxoid tissue dissecting between collagen bundles (arrow in Fig. 2-A), mild-to-moderate nuclear atypia including variably sized pyknotic nuclei (arrows in Fig. 2-B), and increased mitotic figure frequency with three or four noted per ten high-power fields (arrow in Fig. 2-C) (hematoxylin and eosin, ×40 for Fig. 2-A and ×1000 for Figs. 2-B and 2-C).
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The patient was referred to our clinic for further evaluation, where additional historical information was ascertained. Approximately six weeks prior to the clinical presentation with the enlarging mass, the patient had completed a course of three weekly injections of Hylan G-F 20 (nine and seven days apart) for the treatment of osteoarthritis of the left knee. The Hylan G-F 20 was administered with a medial parapatellar injection with the knee flexed to 45°. Generalized knee swelling, warmth, and mild erythema developed between the first two injections. The knee inflammation persisted, and erythema and swelling in the leg, swelling in the foot, paresthesias of the dorsum of the foot, and a small posterolateral knee lump that subsequently began to enlarge developed between the second and third injections. These symptoms eventually prompted the evaluation that led to the excisional biopsy.
On physical examination, the excisional biopsy wound was found to have dehisced and was draining serous fluid. There was no palpable mass, but there was diffuse swelling lateral to the left knee.
A new magnetic resonance imaging scan, acquired nine weeks after the first scan, demonstrated extensive fluid-filled cysts with surrounding inflammation encompassing the posterior and lateral aspects of the knee. Contrast-medium-enhanced computed tomography of the chest, abdomen, and pelvis showed no metastatic disease.
We proposed exploration of the still-open wound for repeat biopsy and secondary wound closure. At surgery, myxomatous material was identified deep to the previous surgical field in multiple cyst-like spaces. No discrete mass was found. The peroneal nerve was surrounded by fibrous tissue. The cystic material was resected en bloc, and the surrounding tissues were extensively biopsied.
Histologic examination of these specimens revealed fibro-inflammatory tissue with little myxoid material (Figs. 3-A and 3-B). The bulk of the tissue exhibited fibrosis, chronic nongranulomatous inflammation, and small reactive vascular channels.
Fig. 3-A |
Fig. 3-B |
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Figs. 3-A and 3-B Photomicrographs of tissue obtained with the second excisional biopsy, demonstrating bland inflammatory fibrovascular tissue, including typical granulation tissue, with numerous small vascular channels (arrow in Fig. 3-A) and areas of normal subcutaneous adipose tissue ("a" in Fig. 3-B) surrounded by mature scar-like collagenous matrix ("c" in Fig. 3-B) (hematoxylin and eosin, ×40 for Fig. 3-A and ×250 for Fig. 3-B).
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