Using the National Joint Registry (UK) database, we compared the thickness of polyethylene (PE) and the level of constraint used during primary total knee arthroplasty (TKA) versus the revision of unicondylar knee arthroplasty (UKA) to TKA. A total of 251,803 TKA procedures and 374 revision UKA–TKA procedures between 2003 and 2009 were reviewed. The commonest PE size used in TKA was 10 mm, compared to 12.79 mm in the revision group. The use of constrained knee implant was required in 2.15% of primary TKA and 4.19% of UKA to TKA revisions. The revision of UKA to TKA is a more complex procedure compared to primary TKA, with a higher incidence of using constrained implants and thicker PE inserts. These findings may be useful for surgeons in their decision making.