title: Increase in physical activity after resurfacing hip arthroplasty is associated with calcar and acetabular bone mineral density changes.
abstract: OBJECTIVE
  Bone preservation is an important advantage of the resurfacing hip arthroplasty (RHA) concept. We hypothesised that patients' increase in physical activity level after RHA would positively relate with periprosthetic bone mineral density (BMD) changes and thus facilitate bone preservation.
  
  
  METHODS
  BMD-changes were prospectively recorded in 38 patients after RHA. Dual-energy absorptiometry was used to quantify BMD-changes in 6 periprosthetic regions of interest preoperatively, at 6 months, 1, 2 and 3 years postoperative. The effect estimates of patients' physical activity, according to their Harris Hip Score (HHS) and University of California at Los Angeles (UCLA) Activity Score, on these BMD changes were assessed in linear mixed models.
  
  
  RESULTS
  The UCLA (coefficient = 0.02 (95% CI, 0.010-0.034); p<0.001) and HHS (coefficient = 0.002 (95% CI, 0.001-0.003); p<0.001) were associated with the BMD in the calcar region. As for BMD changes in the femoral neck only the HHS was associated (coefficient = 0.0006 (95% CI, <0.0001-0.001); p = 0.04). Both the UCLA and the HHS were inversely associated with BMD in the medial acetabular region (UCLA: coefficient = -0.02 (95% CI, -0.038 to -0.007); p = 0.005, HHS: coefficient = -0.002 (95% CI, -0.003 to -0.001), the same accounted for the HHS to BMD-change cranial to the acetabulum (-0.001 [95% CI, -0.0018 to -0.0001]; p = 0.03). For the caudal acetabular and femoral subtrochanteric region no relation with BMD-changes was found.
  
  
  CONCLUSIONS
  The increase in activity scores after RHA was indeed associated with an increase in BMD in the calcar region, however unexpectedly also with a BMD decrease on the acetabular side. Stress shielding from the implant is the most likely causative factor.
authors.affiliation:  Department of Orthopaedics, Rijnstate Hospital, Arnhem - The Netherlands.
authors.forename: Davey M J M
authors.initials: DM
authors.lastname: Gerhardt
authors.affiliation:  Department of Orthopaedics, Radboud University Medical Centre, Nijmegen - The Netherlands.
authors.forename: Gerjon
authors.initials: G
authors.lastname: Hannink
authors.affiliation:  Department of Radiology and Nuclear Medicine, Rijnstate Hospital, Arnhem - The Netherlands.
authors.forename: Ton
authors.initials: T
authors.lastname: Rijnders
authors.affiliation:  Department of Orthopaedics, Rijnstate Hospital, Arnhem - The Netherlands.
authors.forename: Job L C
authors.initials: JL
authors.lastname: van Susante
mesh_terms: D015502:Absorptiometry, Photon / Q000379:methods; D000046:Academic Medical Centers; D000077:Acetabulum / Q000000981:diagnostic imaging / Q000503:physiopathology; D000328:Adult; D000368:Aged; D019644:Arthroplasty, Replacement, Hip / Q000009:adverse effects / Q000379:methods*; D015519:Bone Density / Q000502:physiology*; D016723:Bone Remodeling; D015331:Cohort Studies; D016001:Confidence Intervals; D015444:Exercise / Q000502:physiology*; D005260:Female; D005270:Femur Head / Q000000981:diagnostic imaging / Q000503:physiopathology; D005500:Follow-Up Studies; D006622:Hip Prosthesis; D006801:Humans; D016014:Linear Models; D008297:Male; D008875:Middle Aged; D011182:Postoperative Care / Q000379:methods; D011446:Prospective Studies; D018570:Risk Assessment; D016896:Treatment Outcome