Biomedical Engineering Reference
In-Depth Information
• relative tangential displacement should be less than 25
µ
m;
•
gap between bone and stem must be lower than 150
µ
m [31];
•
contact tension stress limit is 0.8 MPa [31];
•
contact compression stress limit is 7.89 MPa [30];
•
shear stress should be inferior to the threshold values in Table 10.7, depending
on stiffness level [49].
Immediately after surgery the interface stiffness is zero, that is, coated inter-
face has contact with friction and uncoated surface has no friction. After each
step (remodeling model iteration) interface conditions, contact stress and relative
displacement, are computed at all interface nodes. If a certain point satisfies all
osseointegration conditions then interface stiffness increases by one level until
level 5 is reached. If at least one condition is not satisfied then, in that point,
interface stiffness goes back to level one (zero stiffness).
The remodeling model combined with the ingrowth algorithm is used to predict
the long-term bone adaptation and osseointegration of the optimized stems. Results
for this analysis are summarized in Tables 10.8 and 10.9.
In Table 10.8 the percentages of bone ingrowth are presented, comparing the
percentage of surface that verifies the conditions for bone ingrowth after the THA
and the long-term prediction of ingrowth. Note that 100% corresponds to bone
ingrowth on whole coated surface. It is possible to verify that all nondominated
points have more osseointegration just after the surgery than the initial shape
because of a more efficient primary stability. The differences between ''after
surgery'' and ''long-term'' illustrates the influence of bone remodeling on the
interface conditions and how these two processes are coupled. Thus, although all
nondominated points have more osseointegration just after surgery than initial
shape, in the long-term analysis it changes. In fact, the bone remodeling could in-
crease contact stresses to values above the limit. On the other hand, bigger amounts
of osseointegration promote a better load transfer and reduce the stress shielding.
Table 10.8
Osseointegration percentage after surgery and at long-term.
(
β
d
,
β
t
,
β
r
)
Totally coated
Half coated
After surgery (%)
Long-term (%)
After surgery (%)
Long-term (%)
(1,0,0)
73.6
85.8
62.1
78.4
(0,1,0)
57.2
75.4
19.6
55.1
(0,0,1)
54.3
44.0
33.7
66.7
(1/3,1/3,1/3)
71.3
85.5
47.8
58.9
(0.6,0.2,0.2)
73.3
85.8
57.6
62.9
(0.2,0.6,0.2)
70.5
81.3
50.4
60.3
(0.2,0.2,0.6)
69.0
79.3
42.9
57.4
Initial shape
61.6
81.0
39.5
70.8
