Subsidence, tip, and hump micromovements of noncoated ribbed femoral prostheses.

The micromovements of 18 ribbed femoral prostheses were measured for two years postoperatively. Repeated roentgen stereophotogrammetric examinations (RSA) were used to measure distal migration of the center of the prostheses and movements of the prosthetic hump and tip. During the first six postoperative months, an increasing number of prostheses migrated significantly. Migration was first observed at the tip. Two years after the operation, one prosthesis had been revised because of pain. Thirteen of 17 remaining prostheses had subsided, 12 displayed hump and 16 tip migration, and eight had rotated about the longitudinal axis of the femur. In these hips, the instant center of rotation ran obliquely through the greater trochanter or the isthmus of the femur. Clinical follow-up examination and measurements on conventional roentgenograms disclosed a correlation between pain, the extent of sclerosis surrounding the prostheses, and the distal migration according to RSA measurements.