Is the Etiology of Pretibial Cyst Formation After Absorbable Interference Screw Use Related to a Foreign Body Reaction?

Arthroscopically assisted anterior cruciate ligament reconstruction using a bioabsorbable tibial fixation screw is occasionally complicated by pretibial cyst formation. The few case reports describing pretibial cyst formation noted several graft types and fixation techniques, making it difficult to establish one etiology. Some literature suggests cysts form from communication between the joint and pretibial area leading to extravasation of joint fluid, maturing into a cyst. We propose the development of cysts after PLLA screw use may be related to a foreign body reaction. We propose this foreign body reaction (1) relates to the biochemical breakdown of bioabsorbable materials; and (2) differs from cystic formations resulting from joint communication. We retrospectively reviewed seven patients who developed pretibial cysts at least 2 years after original primary ACL reconstruction surgery. MRI was used to visualize the extent of cystic formation. Cysts were treated by d,bridement with specimens sent for histologic analysis. Cyst appearance had a 3-year incidence of 5%. No cyst had an infectious etiology. In all cases, the tibial screw outline was present on MRI, although intraoperatively, the screw was substantially decomposed. Grafts were well incorporated and none of the knees demonstrated anterior laxity. Histologically, cyst material contained fragments of PLLA surrounded by foamy histiocytes, suggesting a foreign body reaction. No cysts recurred. Tibial cysts occur in a subset of patients undergoing ACL reconstruction using a bioabsorbable PLLA interference screw. We suspect they arise from a foreign body response to the screw breakdown. Removal is well tolerated. Level IV, therapeutic study. See Guidelines for Authors for a complete description of levels of evidence.