ARUP; ISBN: 978-0-9562121-5-3 - CMBBE 2012 - Cardiff University

COMBINED BONE-IMPLANT FIXATION: A PRE-CLINICAL HIP
STEM EVALUATION
A. Completo 1 , J. Simões 1 , F. Fonseca 1 , A. Ramos 1 , C. Relvas 1 , M. Oliveira 1 ,
J. Pereira 1
1. ABSTRACT
Most of orthopaedic implants used in articular reconstruction are fixed to the host bone
using bone-cement (cemented implants) or are press fixed directly to bone (press-fit
implants). The main disadvantages of cemented implants [1] are: limited cement fatigue
life, bone osteolysis and high bone destruction if revision is necessary. For press-fit
implants the major limitation is low initial stability that is important for bone
growth/adhesion on the implant surface and is related to implant loosening. Reduced
bone stock due to the larger press-fit implant dimensions, in order to improve stability,
is another disadvantage of this type of implants. Therefore, in order to reduce the main
disadvantages of both implant types, a new implant fixation concept was developed and
tested. This fixation concept presents the ability of contact/fixation of the implant with
host bone through two different interfaces: a press-fit implant-bone interface and an
implant-cement-bone interface. The cemented interface is provided by bone cement
housed in cavities on the implant surface. After placing the implant inside the bone
cavity, the cement is injected into the implant, which flows through the surface cavities
by a set of inner canals. This concept allows good initial stability supplied by the
cement, which is important to promote bone growth/adhesion at the implant-bone
interface. The implant-bone interface promotes a secure long-term support, reducing the
load carried by cement in the cavities and the risk of cement fatigue damage. Another
advantage relatively to full cemented implants has to do with revision, since less bone
damage will occur because cement is only restrained in the cavities with lower contact
area with bone. In the present study, a new hip femoral stem was developed based on
the described combined fixation concept and evaluated pre-clinically.
2. METHODS
In a first phase the hip stem geometry (Fig. 1) was defined using modeling and
numerical tools, assessing the following design parameters : (1) the maximization of the
contact area between the implant and cancellous bone; (2) ensuring a sufficient initial
implant stability in order to promote bone growth/adhesion at the implant-bone
interface: (3) maintain cancellous bone strains at acceptable physiologic levels; (4)
assure that implant and cement stress levels are under the fatigue limits of the materials;
(5) reassure a easy bone implantation movement without risk of bone damage.
However, the length of stem defined by the distal cancellous bone limit was considered
a fixed design parameter. All design parameters considered in the study were evaluated
for two implant-bone interface conditions: one representing the immediate post-
1 Biomechanics Research Group, Mechanical Engineering Department, University of Aveiro, Campus de
Santiago, Aveiro, Portugal.