Subsidence of callotasis zone in distraction osteogenesis after external fixator removal, measured by RSA

BACKGROUND AND PURPOSE: In clinical practice, achieved lengthening of a callotasis zone should be maintained after the external fixator has been removed. The common understanding has been that the regenerated bone may subside. To investigate this, we used high-resolution radiostereometric analysis (RSA) with accurate measurement of the lengthening zone. PATIENTS AND METHODS: We assessed the longitudinal subsidence of a callotasis zone after removal of the external fixator in distraction osteogenesis in 16 patients who underwent 17 segmental lengthening operations on the tibia (n = 9) or femur (n = 8). Median lengthening was 32 (6–80) mm. RSA was performed at the end of the consolidation period before the external fixation device was removed, and this was later repeated at a median time of 11 (4–32) weeks after frame removal. RESULTS: A minimal median longitudinal change of 0.01 (–0.28 to 0.60) mm across the lengthening zone occurred in uncomplicated cases. INTERPRETATION: Our results indicate that no subsidence of clinical interest occurs after external frame removal.