SURFACE AND MATERIAL ANALYSIS OF ISOELASTIC SYSTEMS REMOVED FROM FEMUR

Abstract

A stiff prosthetic stem in the proximal femur alters the load-bearing model af the bone and leads to changes of the lamellar organization, known as “stress shielding”.

To avoid this problem in the 70th a stem with a higher elastic modulus was developed and implanted in patiens (isoelastic stem of Mathys and Morscher).

A group of 17 isoelastic stems of this type which have remained in the femur from 6 to 8 years, were removed and are the object of the study. Analysis of the polyacetile surface was carried out with a low enlargement microscope. The stems were then radiographed and cut with a low speed saw to investigate structural changes between the polyacetile cover and the steinless steel core.

Two type of material lesions were observed:

• - loss of polyacetile material from the surface (wear)

• - structural yielding lesions.

Distribution and frequency of each type of lesion was registerd in relation to four zones of the stem: a) cone with the metallic head; b) proximal stem (metaphyseal); c) body of the stem (diaphyseal); a)apex of the stem.

Wear of the polyacetile stem is evidence that no fixation of the stem was achieved, therefore a fraction of the load energy was dissipated at the prosthesis-endosteal surface.

However the presence of structural yielding lesions demonstrate that another fraction of the load energy was carried out by the stem. The topography of these lesions allowed to understand the biomechanical behavior of the “isoelastic” stem inside the femur with vertical longitudinal (piston) and rotational in the horizontal plane movements.

Failure of polyacetile and decoupling of polyacetile cover and stainless steel core were the other main findings.

The theory of an isoelastic stem is appealing by a biomechanical point of view, however two practical problems caused failure in this series: 1) lack of fixation at the implant-bone interface; 2) failure of materials.