1. Diagnosis incorrect
- other local (intra-articular eg meniscal tear)
- referred (hip or spinal pathology)
2. Right diagnosis but wrong surgery
- osteotomy for advanced OA
3. Right surgery performed inadequately
- poorly performed - inadequate fixation for osteotomy
- incomplete - didn't address all pathology / incomplete correction of deformity
4. Recognized complication of well performed surgery
5. Well performed surgery with inadequate rehabilitation
- patient motivation
- wrong rehab
- right rehab done poorly
6. Progression of pathology or disease process
- progression of OA after osteotomy