title: How Does Wear Rate Compare in Well-functioning Total Hip and Knee Replacements? A Postmortem Polyethylene Liner Study.
abstract: BACKGROUND
  The longevity of total hip (THR) and knee replacements (TKR) that used historical bearing materials of gamma-in-air sterilized UHMWPE was affected more by osteolysis in THRs than in TKRs, although osteolysis remains a concern in TKRs. Therefore, the study of polyethylene wear is still of interest for the knee, particularly because few studies have investigated volumetric material loss in tibial knee inserts. For this study, a unique collection of autopsy-retrieved TKR and THR components that were well-functioning at the time of retrieval was used to compare volumetric wear differences between hip and knee polyethylene components made from identical material.
  
  
  OBJECTIVE
  The following questions were addressed: (1) How much did the hip liners wear and what wear patterns did they exhibit? (2) How much did the knee inserts wear and what wear patterns did they exhibit? (3) What is the ratio between TKR and THR wear after controlling for implantation time and patient age?
  
  
  METHODS
  We compared 23 THR components (Harris-Galante [HG] and HG II) and 20 TKR components (Miller-Galante [MG II]) that were retrieved postmortem. The components were made from the same polyethylene formulation and with similar manufacturing and sterilization (gamma-in-air) processes. Twenty-one patients (12 males, nine females) had THRs and 16 (four males, 12 females) had TKRs. Patients who had TKRs had an older (p = 0.001) average age than patients who had THRs (age, 75 years; SD, 10, versus 66 years; SD, 12, respectively). Only well-functioning components were included in this study. Therefore, implants retrieved postmortem from physically active patients and implanted for at least 2 years were considered. In addition, only normally wearing TKR components were considered, ie, those with fatigue wear (delamination) were excluded. The wear volume of each component was measured using metrology. For the tibial inserts an autonomous mathematic reconstruction method was used for quantification.
  
  
  RESULTS
  The acetabular liners of the THR group had a wear rate of 38 mm(3) per year (95% CI, 29-47 mm(3)/year). Excluding patients with low-activity, the wear rate was 47 mm(3) per year (95% CI, 37-56 mm(3)/year). The wear rate of normally wearing tibial inserts was 17 mm(3) per year (95% CI, -6 to 40 mm(3)/year). After controlling for the relevant confounding variable of age, we found a TKR/THR wear rate ratio of 0.5 (95% CI, 0.29-0.77) at 70 years of age with a slightly increasing difference with increasing age.
  
  
  CONCLUSIONS
  Excluding delamination, TKRs exhibited lower articular wear rates than THRs for historical polyethylene in these two unique cohorts of postmortem retrievals.
  
  
  CONCLUSIONS
  The lower TKR wear rate is in line with the lower incidence of osteolysis in TKRs compared with THRs.
authors.affiliation: Department of Orthopedic Surgery, Rush University Medical Center, Room 204b, Orthopedic Building, 1611 W Harrison Street, Chicago, IL, 60612, USA. Robin_pourzal@rush.edu.
authors.forename: Robin
authors.initials: R
authors.lastname: Pourzal
authors.affiliation: Department of Orthopedic Surgery, Rush University Medical Center, Room 204b, Orthopedic Building, 1611 W Harrison Street, Chicago, IL, 60612, USA.|Department of Bioengineering, University of Illinois at Chicago, Chicago, IL, USA.
authors.forename: Christopher B
authors.initials: CB
authors.lastname: Knowlton
authors.affiliation: Department of Orthopedic Surgery, Rush University Medical Center, Room 204b, Orthopedic Building, 1611 W Harrison Street, Chicago, IL, 60612, USA.
authors.forename: Deborah J
authors.initials: DJ
authors.lastname: Hall
authors.affiliation: Department of Orthopedic Surgery, Rush University Medical Center, Room 204b, Orthopedic Building, 1611 W Harrison Street, Chicago, IL, 60612, USA.
authors.forename: Michel P
authors.initials: MP
authors.lastname: Laurent
authors.affiliation: Department of Orthopedic Surgery, Rush University Medical Center, Room 204b, Orthopedic Building, 1611 W Harrison Street, Chicago, IL, 60612, USA.
authors.forename: Robert M
authors.initials: RM
authors.lastname: Urban
authors.affiliation: Department of Orthopedic Surgery, Rush University Medical Center, Room 204b, Orthopedic Building, 1611 W Harrison Street, Chicago, IL, 60612, USA.
authors.forename: Markus A
authors.initials: MA
authors.lastname: Wimmer
mesh_terms: D000367:Age Factors; D000368:Aged; D000369:Aged, 80 and over; D019644:Arthroplasty, Replacement, Hip / Q000009:adverse effects / Q000295:instrumentation*; D019645:Arthroplasty, Replacement, Knee / Q000009:adverse effects / Q000295:instrumentation*; D001344:Autopsy; D001696:Biomechanical Phenomena; D020878:Device Removal; D005260:Female; D006621:Hip Joint / Q000503:physiopathology / Q000601:surgery*; D006801:Humans; D007719:Knee Joint / Q000503:physiopathology / Q000601:surgery*; D008297:Male; D008875:Middle Aged; D010014:Osteolysis / Q000209:etiology; D020959:Polyethylene / Q000737:chemistry*; D011474:Prosthesis Design; D011475:Prosthesis Failure; D012307:Risk Factors; D013314:Stress, Mechanical; D013997:Time Factors; D016896:Treatment Outcome
chemical_list: D020959:Polyethylene