Biomedical Engineering Reference
In-Depth Information
Fig. 11.11
Basic unit of polyethyleneterephtalate (PET). By courtesy of Springer Verlag [40,
p. 168]
Fig. 11.12
Basic unit of
polyamide; above the brace
0
f
0
the amide group. By
courtesy of Springer
Ve r l a g [ 40, p. 173]
11.5
Polyamide
Polyamide was marketed as Nylon (PA 66) and Perlon (PA 6) since 1937. The fiber-
forming abilities are excellent due to interchain hydrogen bonding, with its -N- and
-C
O groups an obvious marriage, and a high degree of crystallinity increasing
tensile strength in the fiber direction (Fig. 11.12). The -(CH
2
)
n
- can be substi-
tuted by benzene C
6
H
6
as in PET (Fig.
11.11
). One variant is made by Dupont and
known as Kevlar
R
. Its specific strength is five times that of steel. They exhibit good
hemocompatibility. They are hygroscopic (water adsorption of 1-3.5%) and water
molecules act as plasticizer.
D
11.6
Was the Isoelastic Concept a Good Idea?
Mid of the 1990s, the production and use of this prosthesis stopped. First generation
implants were not that big a success (subsidence, slight varus tilt, stem and metallic
head fracture due to imperfect design). These inconveniences were perfectly met
by the subsequent improvements (longer stem, better initial fixation, larger neck
shaft angle) and the results on cohorts of patients with third generation implants
proved very satisfactory after two years follow-up [350]. Niinimki et al. reported in
1994 that only half of the 71 cases were satisfactory after 7 years follow-up [365].
Ali et al. report on 111 hips of patients from 30 to 70 years old after a follow-up
time of 5-12 years and found an average Harris score of 80, a high incidence of
lateral migration, 3 were revised for aseptic loosening, 6 radiographically loose and
osteolysis noted around 2 hips. The authors continued to believe in the principle of
isoelasticity but improvements in design and material of the femoral component are
required [366].
