title: Is the etiology of pretibial cyst formation after absorbable interference screw use related to a foreign body reaction?
abstract: BACKGROUND
  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.
  
  
  OBJECTIVE
  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.
  
  
  METHODS
  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 débridement with specimens sent for histologic analysis. Cyst appearance had a 3-year incidence of 5%.
  
  
  RESULTS
  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.
  
  
  CONCLUSIONS
  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.
  
  
  METHODS
  Level IV, therapeutic study. See Guidelines for Authors for a complete description of levels of evidence.
authors.affiliation: Center for Shoulder, Elbow and Sports Medicine, 622 W 168th Street, PH-1117, New York, NY 10032, USA.
authors.forename: Guillem
authors.initials: G
authors.lastname: Gonzalez-Lomas
authors.forename: Ryan T
authors.initials: RT
authors.lastname: Cassilly
authors.forename: Fabrizio
authors.initials: F
authors.lastname: Remotti
authors.forename: William N
authors.initials: WN
authors.lastname: Levine
mesh_terms: D000328:Adult; D016118:Anterior Cruciate Ligament / Q000000981:diagnostic imaging / Q000601:surgery*; D000070598:Anterior Cruciate Ligament Injuries; D001182:Arthroscopy / Q000009:adverse effects* / Q000295:instrumentation; D001672:Biocompatible Materials; D001863:Bone Screws / Q000009:adverse effects*; D003646:Debridement; D005548:Foreign-Body Migration / Q000209:etiology* / Q000473:pathology / Q000601:surgery; D006801:Humans; D007718:Knee Injuries / Q000000981:diagnostic imaging / Q000601:surgery*; D019344:Lactic Acid; D008279:Magnetic Resonance Imaging; D008875:Middle Aged; D009519:New York City; D011091:Polyesters; D011108:Polymers; D011474:Prosthesis Design; D011859:Radiography; D012086:Reoperation; D012189:Retrospective Studies; D013581:Synovial Cyst / Q000209:etiology* / Q000473:pathology / Q000601:surgery; D013710:Tendons / Q000637:transplantation*; D013977:Tibia / Q000000981:diagnostic imaging / Q000473:pathology / Q000601:surgery*; D013997:Time Factors; D014182:Transplantation, Autologous; D014184:Transplantation, Homologous; D016896:Treatment Outcome; D055815:Young Adult
chemical_list: D001672:Biocompatible Materials; D011091:Polyesters; D011108:Polymers; D019344:Lactic Acid; C033616:poly(lactide)