title: Metal-backed versus all-polyethylene unicompartmental knee arthroplasty: Proximal tibial strain in an experimentally validated finite element model.
abstract: OBJECTIVE
  Up to 40% of unicompartmental knee arthroplasty (UKA) revisions are performed for unexplained pain which may be caused by elevated proximal tibial bone strain. This study investigates the effect of tibial component metal backing and polyethylene thickness on bone strain in a cemented fixed-bearing medial UKA using a finite element model (FEM) validated experimentally by digital image correlation (DIC) and acoustic emission (AE).
  
  
  METHODS
  A total of ten composite tibias implanted with all-polyethylene (AP) and metal-backed (MB) tibial components were loaded to 2500 N. Cortical strain was measured using DIC and cancellous microdamage using AE. FEMs were created and validated and polyethylene thickness varied from 6 mm to 10 mm. The volume of cancellous bone exposed to < -3000 µε (pathological loading) and < -7000 µε (yield point) minimum principal (compressive) microstrain and > 3000 µε and > 7000 µε maximum principal (tensile) microstrain was computed.
  
  
  RESULTS
  Experimental AE data and the FEM volume of cancellous bone with compressive strain < -3000 µε correlated strongly: R = 0.947, R2= 0.847, percentage error 12.5% (p < 0.001). DIC and FEM data correlated: R = 0.838, R2= 0.702, percentage error 4.5% (p < 0.001). FEM strain patterns included MB lateral edge concentrations; AP concentrations at keel, peg and at the region of load application. Cancellous strains were higher in AP implants at all loads: 2.2- (10 mm) to 3.2-times (6 mm) the volume of cancellous bone compressively strained < -7000 µε.
  
  
  CONCLUSIONS
  AP tibial components display greater volumes of pathologically overstrained cancellous bone than MB implants of the same geometry. Increasing AP thickness does not overcome these pathological forces and comes at the cost of greater bone resection.Cite this article: C. E. H. Scott, M. J. Eaton, R. W. Nutton, F. A. Wade, S. L. Evans, P. Pankaj. Metal-backed versus all-polyethylene unicompartmental knee arthroplasty: Proximal tibial strain in an experimentally validated finite element model. Bone Joint Res 2017;6:22-30. DOI:10.1302/2046-3758.61.BJR-2016-0142.R1.
authors.affiliation: School of Engineering, University of Edinburgh, Alexander Graham Bell Building, Mayfield Road, Edinburgh EH9 3JL, UK chloe.scott@nhslothian.scot.nhs.uk.
authors.forename: C E H
authors.initials: CE
authors.lastname: Scott
authors.affiliation: Cardiff School of Engineering, Cardiff University Institute of Mechanical and Manufacturing Engineering, Queen's Buildings, The Parade, Cardiff CF24 3AA, UK.
authors.forename: M J
authors.initials: MJ
authors.lastname: Eaton
authors.affiliation: Department of Orthopaedics, Royal Infirmary of Edinburgh, 51 Little France Crescent, Old Dalkeith Road, Edinburgh EH16 4SA, UK.
authors.forename: R W
authors.initials: RW
authors.lastname: Nutton
authors.affiliation: Department of Orthopaedics, Royal Infirmary of Edinburgh, 51 Little France Crescent, Old Dalkeith Road, Edinburgh EH16 4SA, UK.
authors.forename: F A
authors.initials: FA
authors.lastname: Wade
authors.affiliation: Cardiff School of Engineering, Cardiff University Institute of Mechanical and Manufacturing Engineering, Queen's Buildings, The Parade, Cardiff CF24 3AA, UK.
authors.forename: S L
authors.initials: SL
authors.lastname: Evans
authors.affiliation: School of Engineering, University of Edinburgh, Alexander Graham Bell Building, Mayfield Road, Edinburgh EH9 3JL, UK.
authors.forename: P
authors.initials: P
authors.lastname: Pankaj
keywords: Bone strain; Finite element analysis; Unicompartmental knee arthroplasty