This study aimed to determine if a difference exists between the mental well being of patients prior to total knee, medial unicompartmental and patellofemoral joint replacement and then assessed its effect on outcome. One hundred and thirteen patients were identified (41 TKR, 37 UKR and 35 PFJR). All patients had mental health assessed pre-operatively using SF-12. The reduced WOMAC (rWOMAC) score was used pre-operatively and then at 8 and 24 months post-operation to assess outcome. Reduced pre-operative mental wellbeing was found in 54% of the patients. There was no statistically significant difference demonstrated between the mean mental component scores (MCS) between the three operation groups. The mean rWOMAC at 24 months in the MCS60). Pre-operative MCS had a significant effect, such that reduced mental wellbeing was associated with a worse 24 month outcome.