Biomedical Engineering Reference
In-Depth Information
10
Bone Implant Design Using Optimization Methods
Paulo R. Fernandes, Rui B. Ruben, and Joao Folgado
10.1
Introduction
Total hip arthroplasty (THA) is a successful treatment for joint and bone diseases
as well as in fracture repair [1]. It involves the replacement of the natural joint
by an artificial one, composed of a femoral stem and an acetabular component as
shown in Figure 10.1. The femoral component can be classified as cemented or
uncemented depending on the mode of fixation. Cemented stems are fixed using
a bone cement layer between the femur and the implant, while cementless stems
are placed directly in contact with the bone. In this case, the biological fixation is
promoted by the stem coating such as hydroxyapatite or in general a porous surface.
In the first 10 years after surgery, the THA success is greater than 80%;
nevertheless, it is possible to indentify some causes of failure. Aseptic loosening is
the main cause for failure, being responsible for approximately 60% of revisions
[1, 4]. This failure can be due to mechanical problems such as excessive bone loss
[5], inefficient initial stability [6], or cement mantle fatigue [7]. Thigh pain is another
important mechanical cause for implant failure, with more than 6% of incidence
[4]. On the other hand, deep infection is the major biological problem with more
than 5% of prevalence [1, 4]. Actually, the majority of failures of the THA are related
to mechanical factors.
Regarding the mode of fixation, cemented stems are globally more durable than
cementless ones, with a 90% survival rate after 12 years. For uncemented implants
this rate is less than 80% [1]. However, some cementless stems achieve more
than 95% survival rate after 12 years. In fact, cementless stems with good initial
stability and osseointegration exhibit a superior performance [8]. Cemented stems
are preferred for elderly people, but, for patients less than 60 years old, the number
of uncemented stems has increased, particularly, in the past few decades. In the
early 1990s cementless stems represented only 20% of THA and in 2005 this value
increased to approximately 45% [1].
As stated above, the long-term implant success is strongly related to mechanical
factors. For cementless stems the initial mechanical conditions on bone-implant
interface (primary stability) are very important for success of the prosthesis. In
