Elevated proximal tibial strains following unicompartmental knee replacement—A possible cause of pain

Simpson, D.J.; Price, A.J.; Gulati, A.; Murray, D.W.; Gill, H.S.

doi:10.1016/j.medengphy.2009.02.004

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Volume 31, Issue 7 , Pages 752-757, September 2009

Elevated proximal tibial strains following unicompartmental knee replacement—A possible cause of pain

D.J. Simpson
- Nuffield Department of Orthopaedic Surgery, University of Oxford, Oxford, OX3 7LD, UK
- Corresponding author at: Botnar Research Centre, University of Oxford, Nuffield Orthopaedic Centre, Oxford, OX3 7LD, UK. Tel.: +44 1865 227679; fax: +44 1865 227671.
A.J. Price
- Nuffield Department of Orthopaedic Surgery, University of Oxford, Oxford, OX3 7LD, UK
- Nuffield Orthopaedic Centre, Oxford, OX3 7LD, UK
A. Gulati
- Nuffield Department of Orthopaedic Surgery, University of Oxford, Oxford, OX3 7LD, UK
D.W. Murray
- Nuffield Department of Orthopaedic Surgery, University of Oxford, Oxford, OX3 7LD, UK
- Nuffield Orthopaedic Centre, Oxford, OX3 7LD, UK
H.S. Gill
- Nuffield Department of Orthopaedic Surgery, University of Oxford, Oxford, OX3 7LD, UK

Received 29 September 2008; received in revised form 23 January 2009; accepted 10 February 2009. published online 12 March 2009.

Abstract

Unexplained pain is an important complication of both total knee replacement and unicompartmental knee replacement. After unicompartmental knee replacement the most common site for the pain is antero-medial over the proximal tibia. The reason for this is not clear; however it may be due to high bone strain.

A validated finite element model of a proximal tibia implanted with a fully congruent unicompartmental knee replacement was used to investigate the effect that certain implantation parameters had on the surface strains of the tibia. The tibial tray was positioned neutrally, and also mal-aligned separately in the sagittal and coronal planes. Different amounts of tibial tray overhang and underhang, and different resection levels were modelled. All models were compared to an intact tibia and the strain on the exterior cortex compared for a peak load condition measured in-vivo during a step-up activity.

Following implantation the bone strain in the proximal tibia increased by 40%. There were no comparable increases in strain with different amounts of mal-alignment in the sagittal plane. There was a comparable increase in strain with a tibial tray overhang of 3mm or greater, and excessive varus mal-alignment.

This study has demonstrated that there is a large increase in strain, antero-medially on the proximal tibia, following implantation with a unicompartmental knee replacement. This may be the cause of antero-medial pain. As the bone remodels over time this strain will decrease, which probably explains why the pain usually settles within 12 months after surgery. However, certain errors in implantation result in strain values that might lead to degenerative remodelling and/or increased micro-damage of the bone; this may explain why the pain progressively worsens in some cases.

Keywords: Unicompartmental, Knee, Finite element, Contact mechanics, Pain, Strain