TRUE HEMISPHERIC OR PERIPHERALLY ENHANCED? IN VITRO ANALYSIS OF THE PRIMARY STABILITY OF TWO UNCEMENTED ACETABULAR PRESS-FIT CUPS

Abstract

Introduction: The geometry of uncemented press-fit ace-tabular cups is important in achieving primary stability to ensure bony ingrowth. This study compares the in vitro primary stability of two widely used designs.

Methods: The primary stability of two uncemented ace-tabular cup designs (true hemispheric and peripherally enhanced) with the same 52mm diameter and produced by the same manufacturer, was tested in vitro. Polyethylene blocks of low and high density -representing softer and harder bone- were reamed using the manufacturers’ reamers. The cups were seated using an Instron 5800R machine. Peak failure loads and moments during uniaxial pull-out and tangential lever-out tests were used as measures of primary stability. Eighty tests were performed.

Results: Low density substrate: no difference between the two designs for seating force or stability, with the substrate under-reamed by 2mm.

High density substrate: the cups could not be adequately seated with a 2mm under-ream. Seating was achieved with 1mm under-ream for the hemispheric and 1mm over-ream for the peripherally enhanced design. There was a statistically significant difference in seating forces, with the hemispheric cup requiring less force (6264±1535N vs 7858±2383N, p< 0.05). There was a statistically significant difference in the stability ratio of pull-out force to seating force, favouring the hemispheric cup.

Discussion: No difference was seen in the low density substrate between the 2 cups.

In the high density, the hemispheric design had better characteristics (lower seating force and higher pull-out force to seating force ratio) than the peripherally enhanced design, which are more favourable in clinical settings.