Metal-backed versus all-polyethylene unicompartmental knee arthroplasty: Proximal tibial strain in an experimentally validated finite element model

Abstract

Objectives: 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).

Materials and 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.

Conclusion: 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.

Keywords: Bone strain; Finite element analysis; Unicompartmental knee arthroplasty.

Fig. 1

FEMs with 8 mm all-polyethylene implant (left) and 8 mm metal-backed implant (right). Datum planes indicate anatomical axes used as reference for implantation.

Fig. 2

Scatter graphs for both implants showing the mean number of acoustic emission (AE) hits measured at each load compared with finite element model-predicted volume of cancellous bone elements with compressive (minimum principal) strain < -3000 µε (AP, all polyethylene; MB, metal backed).

Fig. 3

Scatter graphs showing digital image correlation (DIC) measured cortical bone vertical strain along an anteromedial line for 8 mm all-polyethylene (AP) and metal-backed (MB) implants (inset) and predicted finite element model (FEM) vertical strain data at nodes along the same line (inset).

Volume of cancellous bone elements with compressive (minimum principal) strain < -3000 µε (a) and < -7000 µε (b) for both metal-backed (MB) and all-polyethylene (AP) implants of 6 mm to 10 mm thickness.

Volume of cancellous bone elements with tensile strain (maximum principal strain) > 3000 µε (a) and > 7000 µε (b) for both metal-backed (MB) and all-polyethylene (AP) implants of 6 mm to 10 mm thickness.

Fig. 6

Mid-coronal oblique contours of the cancellous bone for each 8 mm implant at total load of 4170 N (medial load 2500 N). Strain > -50 µε appears pale grey, strain < -7000 µε appears black.

Fig. 7

Medial aspect contour of the outer surface of cancellous bone for each 8 mm implant. Strain > -50 µε appears pale grey, strain < -7000 µε appears black.

Fig. 8

Axial compressive (minimum principal) contours of the upper surface of cancellous bone for implants of different thickness at a 2502 N total load (1500 N medial load). Strain > -50 µε appears pale grey, strain < -7000 µε appears black.