Observations on the initial stability of acetabular components in total hip arthroplasty. An experimental study

It was the purpose of the study to investigate the initial stability, with or without screws, of 3 acetabular components with press-fit anchoring, by measuring bone-prosthesis micromovements during the application of physiological loading on the hip simulated in the laboratory. The prostheses, already used clinically, were characterized by different shapes and coatings. For the purposes of the study a total of 30 acetabula were used in 10 human pelves in polyurethane and 5 human pelves preserved at -20 degrees. The pelves were assembled on a hydraulic bench test with a steel jig that could be oriented, and they were cemented with polymethylmethacrylate. Three electromagnetic transducers with sensitivity of up to 1 micron (+ 500 microns) were used to measure the micromovements between the prosthesis and the acetabular rim in its three anatomical quadrants. Bone-prosthesis micromovements were recorded during 5 consecutive load tests, from 0 to 2.39 kN (244 kg). Experimental studies have shown that bone-prosthesis micromovement that exceeds 150 microns obstructs bone integration. The most significant micromovement was observed for all of the prostheses, without accessory screws, in the iliac quadrant, but only the hemispherical one with a semi-smooth surface in zirconium oxide surpassed the threshold of 150 microns. Prostheses with a porous surface demonstrated good stability (102 +/- 33 microns and 94 +/- 36 microns, respectively). None of the prostheses demonstrated micromovement exceeding 90 microns in the area corresponding to the pubis and the ischium. The use of 2 accessory screws sensitively increased the stability of all of the prostheses on the ilium, reducing the average micromovement by 40 microns. Reduction of micromovement was less on the pubis and on the ischium.