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 3 mm 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.