title: Longitudinal evaluation of time related femoral neck narrowing after metal-on-metal hip resurfacing.
abstract: OBJECTIVE
  To track the short-term neck narrowing changes in Birmingham metal-on-metal hip resurfacing (MOMHR) patients.
  
  
  METHODS
  Since 2001, the Center for Hip and Knee Replacement started a registry to prospectively collect data on hip and knee replacement patients. From June 2006 to October 2008, 139 MOMHR were performed at our center by two participate surgeons using Birmingham MOMHR prosthesis (Smith Nephew, United States). It is standard of care for patients to obtain low, anteriorposterior (LAP) pelvis radiographs immediately after MOMHR procedure and then at 3 mo, 1 year and 2 year follow up office visits. Inclusion criteria for the present study included patients who came back for follow up office visit at above mentioned time points and got LAP radiographs. Exclusion criteria include patients who missed more than two follow up time points and those with poor-quality X-rays. Two orthopaedic residency trained research fellows reviewed the X-rays independently at 4 time points, i.e., immediate after surgery, 3 mo, 1 year and 2 year. Neck-to-prosthesis ratio (NPR) was used as main outcome measure. Twenty cases were used as subjects to identify the reliability between two observers. An intraclass correlation coefficient at 0.8 was considered as satisfied. A paired t-test was used to evaluate the significant difference between different time points with P < 0.05 considered to be statistically significant.
  
  
  RESULTS
  The mean NPRs were 0.852 ± 0.056, 0.839 ± 0.052, 0.835 ± 0.051, 0.83 ± 0.04 immediately, 3 mo, 1 year and 2 years post-operatively respectively. At 3 mo, NPR was significantly different from immediate postoperative X-ray (P < 0.001). There was no difference between 3 mo and 1 year (P = 0.14) and 2 years (P = 0.53). Femoral neck narrowing (FNN) exceeding 10% of the diameter of the neck was observed in only 4 patients (5.6%) at two years follow up. None of these patients developed a femoral neck fracture (FNF).
  
  
  CONCLUSIONS
  Femoral neck narrowing after MOMHR occurred as early as 3 mo postoperatively, and stabilized thereafter. Excessive FNN was not common in patients within the first two years of surgery and was not correlated with risk of FNF.
authors.affiliation: Wenbao Wang, Jeffrey A Geller, Rohit Hasija, Jung Keun Choi, David A Patrick Jr., William Macaulay, Center for Hip and Knee Replacement, Department of Orthopaedic Surgery, NewYork-Presbyterian Hospital at Columbia University Medical Center, New York, NY 10032, United States.
authors.forename: Wenbao
authors.initials: W
authors.lastname: Wang
authors.forename: Jeffrey A
authors.initials: JA
authors.lastname: Geller
authors.forename: Rohit
authors.initials: R
authors.lastname: Hasija
authors.forename: Jung Keun
authors.initials: JK
authors.lastname: Choi
authors.forename: David A
authors.initials: DA
authors.lastname: Patrick
authors.forename: William
authors.initials: W
authors.lastname: Macaulay
keywords: Arthroplasty; Complications; Femoral neck narrowing; Hip joint; Hip resurfacing