title: Subspine impingement: 2 case reports of a previously unreported cause of instability in total hip arthroplasty.
abstract: BACKGROUND
  Instability is a common cause of revision hip arthroplasty and is frequently due to improper component placement and subsequent component impingement. Impingement of the greater trochanter upon the anterior inferior iliac spine (AIIS) has been described as a cause of symptomatic femoroacetabular impingement (FAI), but has never been described as a cause of instability following total hip arthroplasty (THA).
  
  
  METHODS
  We present 2 cases of patients undergoing THA. Each patient was evaluated preoperatively and found to have a prominent AIIS, which was concerning due to it overhanging the anterolateral acetabular lip. Both patients had intraoperative posterior instability of their THA, the cause of which was determined to be impingement of the greater trochanter upon a prominent AIIS. Open resection of the AIIS was performed with subsequent resolution of impingement.
  
  
  METHODS
  AIIS impingement has been reported as a cause of symptomatic FAI. In these case reports, open or arthroscopic resection of the AIIS resulted in resolution of symptoms. Morphologically distinct subtypes of the AIIS have been previously described based upon computed tomography, and some subtypes are associated with a high risk of impingement in the native hip. No previous studies have described this phenomenon in the setting of THA.
  
  
  CONCLUSIONS
  Instability is a common cause of revision THA. Impingement of the greater trochanter upon a prominent AIIS is a previously unreported cause of THA instability which can be addressed with intraoperative resection of the AIIS with good result.
authors.affiliation: Department of Orthopaedic Surgery, NYU Hospital for Joint Diseases, New York - USA.
authors.forename: Roy I
authors.initials: RI
authors.lastname: Davidovitch
authors.affiliation: Department of Orthopaedic Surgery, NYU Hospital for Joint Diseases, New York - USA.
authors.forename: Edward M
authors.initials: EM
authors.lastname: DelSole
authors.affiliation: Department of Orthopaedic Surgery, NYU Hospital for Joint Diseases, New York - USA.
authors.forename: Jonathan M
authors.initials: JM
authors.lastname: Vigdorchik
mesh_terms: D019644:Arthroplasty, Replacement, Hip / Q000009:adverse effects*; D001182:Arthroscopy / Q000379:methods*; D019299:Decompression, Surgical / Q000379:methods*; D003937:Diagnosis, Differential; D005260:Female; D057925:Femoracetabular Impingement / Q000175:diagnosis / Q000209:etiology* / Q000601:surgery; D005271:Femur Head Necrosis / Q000601:surgery; D005471:Fluoroscopy; D006621:Hip Joint / Q000000981:diagnostic imaging / Q000503:physiopathology / Q000601:surgery; D006801:Humans; D008297:Male; D008875:Middle Aged; D015207:Osteoarthritis, Hip / Q000601:surgery; D016059:Range of Motion, Articular / Q000502:physiology; D012086:Reoperation; D014057:Tomography, X-Ray Computed