Biomedical Engineering Reference
In-Depth Information
of proximal bone, the success of THA is influenced by problems of interface
instability as the formation of fibrous tissue around the prosthesis and the inclusion
of appropriate interface bone/implant conditions still present open problems. In
fact, the majority of the bone adaptation models found in the literature do not
distinguish between the interfacial and the periprosthetic adaptation.
This chapter discusses the effects of interface and periprosthetic adaptation and
the derivation of bone adaptation models based on the hypothesis of an optimal bone
topology. The final purpose of this work is to present an adaptation model for the
bone-prosthesis interface, capable of being used in conjunction with formulations
oftheperiprostheticadaptation.Toaccountforthesparsenatureofboneingrowth
that is known to occur
in vivo
, the interfacial model is based on the application of a
mixture rule, in which different kinds of tissues may be present in the same region
of the interface. Thus, at each interfacial point, two kinds of interfacial conditions
could coexist in different proportions: ingrowth (bonded bone) and fibrous tissue
(encapsulated bone). Constitutive laws were derived for each one of the interfacial
conditions, and appropriate evolution equations were defined for the relative
quantities. These evolution equations are dependent on biomechanical criteria
related to local micromovements based on experimental results reported in the
literature. For the periprosthetic bone volume, an optimization-based remodeling
model is employed for the bone density evolution. The bases of these propositions
were developed in [31-33].
In the next section, a brief discussion on the biomechanics of bone adaptation is
presented. Section 2.3 deals with constitutive models for bone and the interfacial
region. The adaptation model is presented in Section 2.4. Numerical experiments
are shown in Section 2.5, leaving Section 2.6 for final remarks.
2.2
Mechanical Adaptation of Bone
The process of bone tissue adaptation, also known as
remodeling
, has typically
been separated into external and internal remodeling. In reality, they both occur
simultaneously [34]. In external remodeling, the material properties are assumed to
be fixed, while the geometry changes as a function of time. In internal remodeling,
the geometry is assumed to be fixed, whereas the material properties are a
function of time. The internal remodeling is currently more studied, since it
represents mechanisms relevant to the evaluation of the durability of the prosthetic
replacement [35].
The bone adaptation in the interfacial region has frequently been related to
theories of bone repair, since during the preparation of the cavity for the prosthesis
the bone tissue at this location is damaged. The periimplant repair is a complex
process that involves many cellular and extracellular events. The repair is influenced
by a variety of factors including the type of bone (cortical or trabecular), blood
supply, location of the implant, severity of trauma at the implantation site, degree
of fixation during the repair, and sex and age of the patient.
