Background and purpose. At the Ganz periacetabular osteotomy the osteotomized acetabular fragment is reoriented in an adducted, extended and rotated position. Two screws fixate the acetabular fragment and the patients are allowed 30 kg weight bearing immediately after surgery. We were interested in examining the stability of the reoriented acetabulum after the Ganz osteotomy and accordingly the migration of the acetabular fragment was assessed by radiostereometry.

Methods. Thirty two dysplastic patients (thirty two hips), twenty seven females and five males were included in the study. Median age was 39 (20–57) years. Radio-stereometric examinations were done at one week, four weeks, 8 weeks and six months. Data are presented as mean with 95% CI.

Results. Six months postoperatively, the acetabular fragment had migrated 0.7mm (0.4–1.0mm) medially, and 0.7mm (0.5–0.9 mm) cranially. Mean rotation in valgus direction was 0.5° (−0.1–1.0°). In other directions, migration was below 0.4. There was no statistical difference between migration 8 weeks and 24 weeks postoperative in translation or rotation.

Interpretation. This is the first paper dealing with radio-stereometric analysis in Ganz osteotomy. Due to the very limited migration, we find our postoperative partial weight-bearing regime safe.