Experimental study of bone-implant contact with a parabolic acetabular component (Hofer-Imhof)

Incomplete bone contact with threaded acetabular components is usually attributable to the tapered thread design and sclerotic bone, and might be the reason for early loosening. The self-tapping flat threads of the Hofer-Imhof cup allow accurate insertion without incarceration, even in sclerotic bone. To demonstrate the effectiveness of this threaded cup design based on digitized measurements of bone contact area, six Hofer-Imhof cups were implanted into three human cadavers. The bone contact area at the threads was digitally analyzed on cadaver-explanted cups cut into horizontal and coronal sections. In cups fixed with ideal form fit, bone contact was 31.6% (range 8-55%) at the threads and 28.5% (range 0-49%) between the threads on average. In cups with incomplete bone contact secondary to insufficient reaming, the bone contact area averaged 27.4% (range 10-44%) at the threads. In sclerotic bone, the highest bone contact at the threads was 69%. The threads were the first part of the cup to achieve primary bone contact. Factors contributing to poor contact were incomplete reaming, sections which passed through the incisura of the acetabulum, and incomplete contact with the medial aspect of the acetabulum. The aim was to achieve complete bone contact. Because of the higher contact areas observed in sclerotic bone, some sclerotic bone should remain after reaming.