Biomedical Engineering Reference
In-Depth Information
of polyethylene wear debris if they enter the joint space.
75-77
in order to
overcome these limitations, some iodine-containing monomers have been
investigated as potential substitutes for the inorganic radiopacifying agents,
with improved biological and mechanical characteristics.
71, 72, 78
Several commercial abCs also include antibiotics in their powder phase,
which after implantation progressively dissolve and leach to the surrounding
tissues, with the aim of avoiding post-operative infections. It seems clear
that the addition of antibiotics to abCs reduces their mechanical properties,
although the reduction depends very strongly on the amount of antibiotic
added.
56, 79-81
Other additives have also been investigated with the objective of improving
the mechanical properties of ABCs, by reinforcing the cement matrix either
with particles or with fibres. In relation to particle addition, three main
directions can be described. First, reinforcement with hard particles such
as glass beads
82, 83
or glass ceramic particles,
84
second, reinforcement with
tough or rubber toughened particles
61, 68, 85
and finally the reinforcement with
bioactive particles, such as inorganic bone and demineralized bone matrix
86,
87
and hydroxyapatite or other calcium phosphates, which could improve
both the mechanical properties of the bulk cement and the interfacial bone-
cement strength, by giving rise to a direct bonding between these bioactive
particles and the surrounding bone.
88-91
Fibre reinforcement of bone cements
has followed two main paths: reinforcement with metallic fibres92,
92, 93
and
reinforcement with polymeric fibres, including carbon fibres,
94-97
10.3.5 Biocompatibility
The three major concerns relating to the biological performance of ABCs are
exothermal polymerization of the cement, the residual liquid monomer and
the exposure of medical personnel to the fumes of the liquid monomer.
5
There is extensive discussion in the literature about the role of exothermic
polymerization in the necrosis of the surrounding bone. The amount of heat
released depends on the weight of reacting monomer in the mixture and the
peak temperature reached depends on the volume of cement and its surface
to volume ratio, in other words its thickness in service. More important than
the temperature reached by the cement is the temperature at the bone-cement
interface, since bone necrosis will occur if the temperature reaches values
over 56°C, which corresponds to the onset of coagulation of albumin.
98
However, it seems that this temperature should not be taken as a threshold
since cell damage is not only a question of temperature but also of time. A
wide variety of temperatures at the bone-bone cement interface have been
reported depending on the thickness and the position of the thermocouple.
Certain clinical complications have been associated with the use of bone
cement and particularly to monomer release.
99
A large amount of information
