606 - Ganglion Cyst Formation Following Posterior Tibialis Tendon Transfer in Acquired Flatfoot Deformity

Left dorsal foot ganglion cyst in the setting of prior posterior tibialis tendon transfer, associated with mechanical overuse and acquired flatfoot deformity.

Case Description:

A 40-year-old female with a history of bilateral ACL reconstructions presented in March 2025 with a left dorsal foot ganglion cyst. Her left knee surgery in 2002 was complicated by an iatrogenic common peroneal nerve injury, resulting in immediate foot drop. In 2005, she underwent posterior tibialis tendon transfer, regaining functional dorsiflexion within two months. Over the next two decades, she developed unilateral left arch collapse and a dropped navicular. Following increased activity and gardening, she noted a dorsal foot cyst associated with mild pain and dorsiflexion fatigue. She denied trauma, redness, ankle swelling, or new numbness. Exam revealed a palpable, mildly tender cyst without neurovascular compromise; MRI confirmed a 16 mm well-circumscribed lesion near the posterior tibialis insertion. Conservative treatment was initiated with supportive footwear, compression, icing, and temporary off-the-shelf inserts while awaiting custom orthotics. At follow-up, ultrasound showed cyst reduction with corresponding pain relief, and improved gait mechanics.

Discussions:

Ganglion cyst formation and acquired flatfoot deformity are both rare complications of posterior tibialis transfer, with only a handful of cases reported. By altering biomechanics, flatfoot deformity increases tendon and joint stress, potentially leading to chronic inflammation and cyst formation. In this case, conservative management, including compression, icing, supportive footwear, and inserts was employed as first-line therapy. Ultrasound was valuable for monitoring cyst progression, demonstrating size reduction and symptom improvement. While custom orthotics are standard postoperatively following tendon transfer, they may also help prevent cyst recurrence and slow progression of flatfoot deformity in this patient population.