title: The 2014 Frank Stinchfield Award: The 'landing zone' for wear and stability in total hip arthroplasty is smaller than we thought: a computational analysis.
abstract: BACKGROUND
  Positioning of total hip bearings involves tradeoffs, because cup orientations most favorable in terms of stability are not necessarily ideal in terms of reduction of contact stress and wear potential. Previous studies and models have not addressed these potentially competing considerations for optimal total hip arthroplasty (THA) function.
  
  
  OBJECTIVE
  We therefore asked if component positioning in total hips could be addressed in terms of balancing bearing surface wear and stability. Specifically, we sought to identify acetabular component inclination and anteversion orientation, which simultaneously resulted in minimal wear while maximizing construct stability, for several permutations of femoral head diameter and femoral stem anteversion.
  
  
  METHODS
  A validated metal-on-metal THA finite element (FE) model was used in this investigation. Five dislocation-prone motions as well as gait were considered as were permutations of femoral anteversion (0°-30°), femoral head diameter (32-48 mm), cup inclination (25°-75°), and cup anteversion (0°-50°), resulting in 4320 distinct FE simulations. A novel metric was developed to identify a range of favorable cup orientations (so-called "landing zone") by considering both surface wear and component stability.
  
  
  RESULTS
  When considering both wear and stability with equal weight, ideal cup position was more restrictive than the historically defined safe zone and was substantially more sensitive to cup anteversion than to inclination. Ideal acetabular positioning varied with both femoral head diameter and femoral version. In general, ideal cup inclination decreased with increased head diameter (approximately 0.5° per millimeter increase in head diameter). Additionally, ideal inclination increased with increased values of femoral anteversion (approximately 0.3° per degree increase in stem anteversion). Conversely, ideal cup anteversion increased with increased femoral head diameter (0.3° per millimeter increase) and decreased with increased femoral stem anteversion (approximately 0.3° per degree increase). Regressions demonstrated strong correlations between optimal cup inclination versus head diameter (Pearson's r=-0.88), between optimal cup inclination versus femoral anteversion (r=0.96), between optimal cup anteversion versus head diameter (r=0.99), and between optimal cup anteversion and femoral anteversion (r=-0.98). For a 36-mm cup with a 20° anteverted stem, the ideal cup orientation was 46°±12° inclination and 15°±4° anteversion.
  
  
  CONCLUSIONS
  The range of cup orientations that maximized stability and minimized wear (so-called "landing zone") was substantially smaller than historical guidelines and specifically did not increase with increased head size, challenging the presumption that larger heads are more forgiving. In particular, when the cup is oriented to improve not only stability, but also wear in the model, there was little or no added stability achieved by the use of larger femoral heads. Additionally, ideal cup positioning was more sensitive to cup anteversion than to inclination.
  
  
  CONCLUSIONS
  Positioning THA bearings involves tradeoffs regarding stability and long-term bearing wear. Cup positions most favorable to minimization of wear such as low inclination and elevated anteversion were detrimental in terms of construct stability. Orientations were identified that best balanced the competing considerations of wear and stability.
authors.affiliation: Department of Orthopaedics and Rehabilitation, University of Iowa Hospitals and Clinics, Iowa City, IA, USA, jacob-elkins@uiowa.edu.
authors.forename: Jacob M
authors.initials: JM
authors.lastname: Elkins
authors.forename: John J
authors.initials: JJ
authors.lastname: Callaghan
authors.forename: Thomas D
authors.initials: TD
authors.lastname: Brown
mesh_terms: D019644:Arthroplasty, Replacement, Hip; D001363:Awards and Prizes; D020342:Finite Element Analysis; D006622:Hip Prosthesis; D006801:Humans; D061825:Metal-on-Metal Joint Prostheses; D019736:Prostheses and Implants; D011474:Prosthesis Design; D017755:Prosthesis Fitting
references.citation: J Bone Joint Surg Am. 1994 May;76(5):677-88
references.pmid: 8175815
references.citation: J Bone Joint Surg Br. 2011 Aug;93(8):1011-6
references.pmid: 21768621
references.citation: J Arthroplasty. 1998 Sep;13(6):621-6
references.pmid: 9741436
references.citation: J Biomech. 2003 Apr;36(4):577-91
references.pmid: 12600348
references.citation: J Orthop Res. 2012 Feb;30(2):169-77
references.pmid: 21812025
references.citation: Clin Orthop Relat Res. 2010 Sep;468(9):2328-32
references.pmid: 20480405
references.citation: J Bone Joint Surg Am. 2013 Oct 2;95(19):1760-8
references.pmid: 24088968
references.citation: J Bone Joint Surg Am. 2009 Nov;91(11):2598-604
references.pmid: 19884433
references.citation: J Biomed Mater Res B Appl Biomater. 2007 May;81(2):508-15
references.pmid: 17041924
references.citation: J Arthroplasty. 1994 Apr;9(2):119-25
references.pmid: 8014641
references.citation: J Arthroplasty. 1998 Aug;13(5):530-4
references.pmid: 9726318
references.citation: Iowa Orthop J. 2012;32:17-21
references.pmid: 23576916
references.citation: J Bone Joint Surg Am. 2009 Jan;91(1):128-33
references.pmid: 19122087
references.citation: J Bone Joint Surg Am. 1992 Jul;74(6):849-63
references.pmid: 1634575
references.citation: J Orthop Res. 2004 Jul;22(4):815-21
references.pmid: 15183439
references.citation: J Arthroplasty. 2006 Aug;21(5):696-701
references.pmid: 16877155
references.citation: J Biomech. 2006;39(7):1315-23
references.pmid: 15894324
references.citation: J Bone Joint Surg Br. 2008 Sep;90(9):1143-51
references.pmid: 18757952
references.citation: Clin Orthop Relat Res. 2006 May;446:22-8
references.pmid: 16672867
references.citation: J Biomech. 2001 Jul;34(7):873-81
references.pmid: 11410171
references.citation: J Bone Joint Surg Br. 1986 May;68(3):409-13
references.pmid: 3733807
references.citation: J Bone Joint Surg Br. 2007 Sep;89(9):1158-64
references.pmid: 17905951
references.citation: Clin Orthop Relat Res. 2011 Feb;469(2):454-63
references.pmid: 20953853
references.citation: Orthop Clin North Am. 2001 Oct;32(4):569-77, vii
references.pmid: 11689370
references.citation: Clin Orthop Relat Res. 1998 Oct;(355):152-62
references.pmid: 9917600
references.citation: J Bone Joint Surg Br. 1993 Mar;75(2):228-32
references.pmid: 8444942
references.citation: Med Eng Phys. 2007 May;29(4):465-71
references.pmid: 16901743
references.citation: Clin Orthop Relat Res. 2011 Jun;469(6):1642-50
references.pmid: 20963530
references.citation: J Arthroplasty. 2002 Apr;17(3):282-8
references.pmid: 11938502
references.citation: J Bone Joint Surg Br. 2005 Jun;87(6):762-9
references.pmid: 15911655
references.citation: J Bone Joint Surg Br. 2009 Oct;91(10):1287-95
references.pmid: 19794161
references.citation: J Bone Joint Surg Br. 1990 Nov;72(6):971-9
references.pmid: 2246300
references.citation: J Bone Joint Surg Am. 2012 Sep 19;94(18):1655-61
references.pmid: 23289127
references.citation: Clin Orthop Relat Res. 2014 Feb;472(2):529-42
references.pmid: 24218160
references.citation: J Bone Joint Surg Br. 1994 Jan;76(1):45-8
references.pmid: 8300680
references.citation: J Arthroplasty. 2004 Dec;19(8 Suppl 3):66-70
references.pmid: 15578556
references.citation: Clin Orthop Relat Res. 2010 Sep;468(9):2313-20
references.pmid: 20559767
references.citation: J Bone Joint Surg Am. 1978 Mar;60(2):217-20
references.pmid: 641088
references.citation: Clin Orthop Relat Res. 1990 Dec;(261):159-70
references.pmid: 2245542
references.citation: Orthop Clin North Am. 2011 Apr;42(2):241-50, ix
references.pmid: 21435498
references.citation: Clin Orthop Relat Res. 1983 Mar;(173):151-8
references.pmid: 6825326
references.citation: J Bone Joint Surg Br. 2008 Oct;90(10):1291-7
references.pmid: 18827237
references.citation: J Orthop Res. 2011 Nov;29(11):1642-8
references.pmid: 21495065