title: Low femoral antetorsion as a risk factor for bony impingement after bipolar hemiarthroplasty.
abstract: BACKGROUND
  Reports of dislocation after bipolar hemiarthroplasty (BHA) abound in literature, and several studies have mentioned the factors that are associated with an increased risk of dislocation. However, there is no report detailing the pattern of impingement in BHA and how femoral antetorsion can affect the range of motion (ROM) after BHA.
  
  
  OBJECTIVE
  The purpose of this study was to evaluate the pattern of impingement in BHA and whether femoral antetorsion affects the ROM after BHA using three-dimensional (3D) dynamic motion analysis.
  
  
  METHODS
  Using the computed tomography (CT) data of 60 patients (60 hips), including 31 men and 29 women who underwent BHA for the treatment of idiopathic osteonecrosis (ION) of the femoral head, we calculated the antetorsion of the femoral neck, ROM of flexion (Flex), internal rotation (Int-R), and external rotation (Ext-R) using a CT-based 3D simulation software. We evaluated the pattern of impingement and the relationship between femoral antetorsion and ROM in BHA. As for the implant position in the 3D simulation software, the anteversion of the femoral implant was set to be the same as the natural antetorsion of the femoral neck and neck length was set to be the standard neck in all cases.
  
  
  RESULTS
  This study revealed the mechanism of impingement in BHA: (1) bone to bone impingement and (2) implant to bone impingement. We found a significant decrease in the ROM of Flex and Int-R inversely proportional to the femoral antetorsion. In patients with lower femoral antetorsion, the ROM of Flex and Int-R decreased due to bony impingement (the anterior great trochanteric region of the femur impinges on the anteroinferior edge of the anteroinferior iliac spine). Whereas, high anteversion of the femoral implant may decrease the ROM of Ext-R; however, our results also showed that even the lowest ROM of Ext-R with 10° hip extension was over 40°.
  
  
  CONCLUSIONS
  We demonstrated that lower femoral antetorsion substantially affects the ROM of Flex and Int-R due to bony impingement. For these patients, there should be consideration given to retaining femoral "anterior offset" in BHA.
authors.affiliation: Department of Orthopaedic Surgery, Graduate School of Biomedical Sciences, Hiroshima University, 1-2-3 Kasumi, Minami-ku, Hiroshima, 734-8551, Japan. readymade_jpn@yahoo.co.jp.
authors.forename: Takeshi
authors.initials: T
authors.lastname: Shoji
authors.affiliation: Department of Orthopaedic Surgery, Hiroshima Prefectural Rehabilitation Center, 295-3, Taguchi, Saijou-town, East Hiroshima, Hiroshima Prefecture, Japan. yasuyuji@hiroshima-wsc.jp.
authors.forename: Yuji
authors.initials: Y
authors.lastname: Yasunaga
authors.affiliation: Department of Orthopaedic Surgery, Graduate School of Biomedical Sciences, Hiroshima University, 1-2-3 Kasumi, Minami-ku, Hiroshima, 734-8551, Japan. tacyama@hiroshima-u.ac.jp.
authors.forename: Takuma
authors.initials: T
authors.lastname: Yamasaki
authors.affiliation: Department of Orthopaedic Surgery, Graduate School of Biomedical Sciences, Hiroshima University, 1-2-3 Kasumi, Minami-ku, Hiroshima, 734-8551, Japan. rousai12330472@yahoo.co.jp.
authors.forename: Soutarou
authors.initials: S
authors.lastname: Izumi
authors.affiliation: Department of Orthopaedic Surgery, Graduate School of Biomedical Sciences, Hiroshima University, 1-2-3 Kasumi, Minami-ku, Hiroshima, 734-8551, Japan. s_hachi5581@yahoo.co.jp.
authors.forename: Susumu
authors.initials: S
authors.lastname: Hachisuka
authors.affiliation: Department of Orthopaedic Surgery, Graduate School of Biomedical Sciences, Hiroshima University, 1-2-3 Kasumi, Minami-ku, Hiroshima, 734-8551, Japan. ochim@hiroshima-u.ac.jp.
authors.forename: Mitsuo
authors.initials: M
authors.lastname: Ochi
mesh_terms: D000328:Adult; D000368:Aged; D019644:Arthroplasty, Replacement, Hip / Q000009:adverse effects / Q000639:trends*; D060750:Bone Anteversion / Q000150:complications / Q000000981:diagnostic imaging*; D005260:Female; D057925:Femoracetabular Impingement / Q000000981:diagnostic imaging* / Q000209:etiology; D062785:Hemiarthroplasty / Q000009:adverse effects / Q000639:trends*; D006801:Humans; D008297:Male; D008875:Middle Aged; D011183:Postoperative Complications / Q000000981:diagnostic imaging* / Q000209:etiology; D011859:Radiography; D016059:Range of Motion, Articular / Q000502:physiology; D012307:Risk Factors