PIGMENTED VILLONODULAR SYNOVITIS IN THE ANKLE OF A PEDIATRIC PATIENT: A CASE REPORT.

BACKGROUND: Pigmented villonodular synovitis (PVNS), also known as diffuse tenosynovial giant cell tumor, is a rare, benign but aggressive synovial proliferative disorder. It most commonly affects the knee; involvement of the foot and ankle is uncommon (incidence ~1.8 per million), and pediatric cases are especially rare. This rarity often leads to delayed diagnosis, as early symptoms can mimic benign causes (e.g. sprains or "growing pains") and initial radiographs may be unremarkable.

CASE PRESENTATION: We report a case of a 12-year-old girl with a four-year history of chronic ankle pain and swelling. She had been misdiagnosed with growth-related pain or recurrent ankle sprains after undergoing multiple X-ray examinations that showed no abnormalities. Over time, a progressive ankle joint swelling and pain with restricted motion developed. On examination, she had tenderness over the ankle (medially and laterally) with limited range of motion; neurovascular status was intact. Laboratory work-up showed a normal white blood cell count and an elevated erythrocyte sedimentation rate (ESR of 30 mm/hour, normal <20), consistent with mild inflammation. Plain radiographs remained normal, but MRI revealed diffuse synovial thickening with confluent nodular masses in the anterior and posterior ankle joint recesses, with heterogeneous signals and minor bone erosion, suggestive of PVNS. Arthroscopic biopsy confirmed tenosynovial giant cell tumor (PVNS). The patient underwent an open synovectomy via combined anterior and posterolateral incisions (double incision) to achieve maximal synovial resection. Approximately six weeks post-operatively, a single-session adjuvant radiosynoviorthesis (RSO) was performed - an intra-articular injection of Yttrium-90 colloid (approximately 5 mCi) - to irradiate any remaining synovial tissue and reduce recurrence risk. Postoperatively, her pain improved with no immediate complications. At 12-month follow-up, the patient had no evidence of recurrence and demonstrated marked improvement: her ankle pain resolved (VAS pain score improved from 6/10 preoperatively to 0-1/10 post-treatment) and range of motion normalized (dorsiflexion 20° and plantarflexion 45°, restored from a preoperative ~50% limitation). She returned to full, pain-free weight-bearing ambulation without assistive devices.

CONCLUSION: PVNS of the ankle is a challenging diagnosis in children due to its rarity and nonspecific presentation. This case highlights that persistent pediatric ankle pain with swelling, even with normal initial X-rays, should raise suspicion for uncommon pathologies like PVNS. Early MRI is warranted for definitive diagnosis. Complete surgical synovectomy remains the primary treatment for diffuse PVNS; however, adjuvant intra-articular RSO (radioisotope synovectomy) can be safely utilized to minimize the high recurrence risk associated with diffuse disease. Our patient's excellent outcome at one year - with restored function and no recurrence - underscores the effectiveness of an aggressive combined approach in managing diffuse PVNS of the ankle in a pediatric patient.