Metal-backedversusall-polyethylene unicompartmental knee arthroplasty

Abstract

ObjectivesUp 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).Materials and MethodsA 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.ResultsExperimental 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.ConclusionAP 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

Ten-year survival and patient-reported outcomes of a medial unicompartmental knee arthroplasty incorporating an all-polyethylene tibial component

“…Concerns have thus been raised regarding the incidence of tibial loosening/subsidence in all-polyethylene tibial components [ 4 , 24 ], and the role of implant stiffness in unexplained pain [ 12 , 19 , 25 ]. Such variable survival rates from 56% at 7 years (32–75 95% CI) [ 4 ] to 87.6% at 10 years [ 9 ] in all-polyethylene designs suggest that not all all-polyethylene designs are equal, and this may reflect component thickness [ 13 ]. Components of 6 mm thickness have been significantly associated with early clinical failure [ 26 ], increased wear and osteolysis [ 27 ].…”

“…Pathological cancellous bone overload and tibial subsidence may affect thinner implants more so than thicker implants. Finite element analysis has shown that increasing all-polyethylene thickness to 10 mm still under-performs in terms of proximal tibial strain compared to metal-backed implants [ 13 ] with the added cost of increased tibial resection and associated reduction in cancellous bone strength [ 28 ].…”

“…Elevated proximal tibial strain with repetitive microfracture and remodelling may contribute to this pain [ 11 ]. Biomechanical studies have demonstrated greater microdamage in composite bone models implanted with all-polyethylene UKA tibial components compared to metal-backed implants [ 12 ] and finite element analysis has shown elevated strain to be dependent upon all-polyethylene implant thickness [ 13 ]. Elevated proximal tibial strain under all-polyethylene UKA tibial components may cause changes in local cancellous bone architecture and predispose to early failure by tibial subsidence, aseptic loosening or ongoing pain.…”

New insights into the biomechanics of Legg-Calvé-Perthes’ disease

“…FE modelling has being increasingly used for implant biomechanical optimization, 17 , 18 custom-implant development, 19 , 20 and tissue engineering. 21 We have previously developed an FE model of a healthy 7.9-year-old male hip from CT data.…”

Optimal interference of the tibial component of the cementless Oxford Unicompartmental Knee Replacement

“…In contrast, excessive interference increases the assembly load required to implant the component which in turn generates stress in the bone, trabecular damage and increases the risk of fractures. 2 , 3 …”