Radiographs of 33 patients who had an expandable prosthesis inserted after tumor resection in the distal femur were evaluated. We hypothesized that, when a sleeve was present, there was greater stem migration and cortical reaction. The thickness of medial and lateral cortices of the tibia was measured at 6 months and on the last follow-up radiograph. The distance from the edge of the prosthesis to the cortical edge was also compared. When a sleeve was present, there was greater stem migration (P < .001) and a greater increase in lateral cortical thickness (P < .001). There was a higher revision rate when a sleeve was used, but this was not statistically significant. There was no difference in function according to the Toronto Extremity Salvage Score (TESS) between the 2 groups. This is the first study to demonstrate that the presence of a polyethylene sleeve is associated with a tendency for the tibial prosthesis to migrate laterally and cause a greater sclerotic reaction.