In vitro testing of the primary stability of the VerSys enhanced taper stem: a comparative study in intact and intraoperatively cracked femora

The Zimmer VerSys enhanced taper uncemented stem was tested in vitro for pr imary stability. Six stems were implanted in six composite femora. Three fe mora were unintentionally damaged by cracking of the bone during stem inser tion and press-fit. A previously validated protocol was used to load the sp ecimens cyclically and to record locally the rotational and axial bone/impl ant relative motions in terms of elastic motion and permanent migration. For the undamaged femora, the VerSys stem showed excellent primary stabilit y, in terms of both elastic motions (less than 9 mum) and permanent migrati ons (less than 6 mum), and in both axial and rotational directions, compari ng favorably with the literature. Intraoperatively-induced proximal cracks did influence the primary stability of the stem in terms of permanent migra tion. As cracks may easily be produced in the diaphyseal canal during inser tion of cementless stems, which rely for primary stability on conical fitti ng into the canal, great care must be taken in preparation of the canal and insertion of the stem.