Image Quiz

A Soft-Tissue Mass About the Knee Following Intra-Articular Injection of Hylan-G (continued)

Answer: Viscosupplementation pseudotumor.

On the basis of the operative findings and a review of the first biopsy slides in light of the newly discovered history of viscosupplementation injections, the musculoskeletal pathologists interpreted the lesion as being reactive, inflammatory tissue rather than a malignant tumor.

One year after the final resection of the cysts, the patient had a healed wound but advanced symptomatic osteoarthritis of the knee. No recurrence was palpable. Repeat magnetic resonance imaging demonstrated resolution of the cystic inflammation without a recurrent mass.

Discussion

Use of intra-articular viscosupplementation has increased dramatically in recent years. Associated complications have been reported. Complications associated specifically with Hylan G-F 20, commercially available as Synvisc (Wyeth-Ayerst Pharmaceuticals, Philadelphia, Pennsylvania), have included both transient and, rarely, prolonged inflammatory reactions.

Because of the temporal sequence, we suspect that the mass in our patient was an atypical inflammatory response to a course of viscosupplementation therapy. The patient had generalized knee warmth and erythema during the course of the injections, beginning after the first injection. Such reactions to viscosupplementation are common. The reported prevalence of acute inflammatory reactions to a first course of treatment with Hylan G-F 20 has ranged from 2% to 27%. Most reactions involve mild pain at the injection site or generalized knee inflammation that typically resolves spontaneously over days or weeks. Nonetheless, persistent inflammatory reactions lasting many months have been reported. Some documented reactions have even included chronic granulomatous inflammation. We are not aware of any reports of a mass associated with viscosupplementation. The anatomical explanation of how an anteromedial intra-articular injection can lead to a posterolateral cystic mass is not clear.

The histologic features of the cystic mass are also not easily explained. Most troubling initially was the combination of mitotic figures and nuclear atypia with dissection of myxomatous material between collagen bundles. While it is well recognized that the histologic appearance of an inflammatory, reactive process can mimic that of a malignant tumor, such interpretations usually depend on specific exclusion of an infiltrative growth pattern. The initial diagnosis of a low-grade malignant tumor was based on the appearance of dissecting myxomatous material infiltrating through collagen bundles, but this interpretation was subsequently questioned when the pathologists learned of the temporally related injections.

Among the interpretations that were offered as the magnetic resonance images were reviewed by multiple musculoskeletal imaging experts was that the cystic mass may have arisen from a ruptured popliteal cyst in this patient with advanced osteoarthritis and a complex medial meniscal tear (Figs. 4-A and 4-B). Lateral popliteal cysts, involving the popliteus bursa, are less common than enlargements of the medial semimembranosus bursa originally described by Baker in 1877. However, both dissection and rupture of such lateral popliteal cysts have been reported. Lateral cysts have been found to rupture into the subcutaneous tissues around the fibular head, cause peroneal nerve dysfunction, and even extend further into the anterior compartment. Medial and directly posterior popliteal cyst ruptures are frequently associated with swelling and erythema of the leg and foot. This condition has been named the "pseudo-thrombophlebitic syndrome." Such a pattern is consistent with the swelling of the foot and leg that our patient recalled from the time that she noticed the mass. Ruptured popliteal cysts can temporarily maintain an intra-articular communication, exposing the periarticular tissue to additional insult from any generalized intra-articular inflammation.


Fig. 4-A
Fig. 4-B		  
Figs. 4-A and 4-B Axial gradient-echo magnetic resonance images (repetition time, 450 msec; echo time, 9.4 msec) of the left knee (lateral is to the right, and medial is to the left), made before the first excisional biopsy. These images are at the level of the joint line (Fig. 4-A) and a few millimeters distal to it (Fig. 4-B), and they demonstrate a small volume of residual hyperintense signal in the region of the popliteal bursa (arrows with white dot), with the multilobulated mass and subcutaneous edema lateral to the lateral head of the gastrocnemius muscle and tracking anteriorly (multiple arrows). The point adjacent to the anterolateral margin of the lateral head of the gastrocnemius appears to provide a potential conduit for dissection to the posterolateral subcutaneous tissues, from a ruptured lateral popliteal cyst.

For larger view, click on image

We therefore concluded that a lateral popliteal cyst was a part of a generalized intra-articular inflammatory reaction to viscosupplementation; this cyst ruptured, exposing the posterolateral periarticular tissues to dissecting volumes of inflammatory synovial fluid, resulting in an inflammatory mass.

In summary, we report a large cystic mass with sarcoma-like pathologic features in temporal association with a course of intra-articular viscosupplementation with Hylan G-F 20. As with any intervention, the risks and benefits of intra-articular viscosupplementation must be weighed carefully. Perhaps more important is awareness of this possible complication to avoid the potential for drastic mismanagement.

Reference

1. Jones KB, Patel PP, DeYoung BR, Buckwalter JA. Viscosupplementation pseudotumor. A case report. J Bone Joint Surg Am. 2005;87:1113-9.