Aim: Our goal was to evaluate the short term result of medial unicondylar arthroplasty in a valgus knee and to determine the reason for medial compartment osteoarthritis (OA) in a valgus knee.

Material and method: Between 1997–2001 312 unicondylar arthroplasties have been carried out at our department. 8 were lateral the others were all medial. Of the 304 medial 10 were done for a valgus knee. We examined the pre- and postoperative mechanical and anatomical axis. We rated the knees using the American Knee Society Knee Score on avarage at 5 years follow-up. Intra- and postoperative complications, revisions are noted.

Results: All of the knees had an anatomical axis of increased valgus. The mechanical axis differed, from 3 degrees of varus to 4 degrees of valgus.

8 of the patients rated the result excellent or good. 1 had an acceptable result, 1 had a revision.

Conclusion: Medial compartment OA can develop in a knee with an anatomical axis in valgus, and a mechanical axis of varus. It can be succesfully treated with medial unicondylar arthroplasty. The results are similar to medial unicondylar arthroplasty in a varus knee.