An intramedullary lengthening device (110) configured for placement inside bone having a first bone section (102) and a second bone section (104) comprises a housing (112) configured for attachment to one bone section and a distraction shaft configured for attachment to the other bone section. A permanent magnet (134) which is rotatable relative to the housing (112) is coupled to a lead screw in an internal cavity of the distraction shaft (114). A thrust bearing (138) in the housing (112) protects the permanent magnet from one or more of compressive and tensile stresses.
