Supplementum 3+4/2007 - Společnost pro pojivové tkáně

Fig. 1. The bone remodeling process (according to our model) creates cortical and cancellous bone
from the homogeneous density distribution even if only the effect of external forces is considered.
Compare the calculation a) with the Rtg. image b). It is evident that mechanic (dynamic) loading
not only significantly influences the bone remodelling process – resorption or formation of bone in
a given element – but also determines the shape, thickness, and emplacement of cortical bone. The
structure of osteons corresponds to the isostress lines c).
tribution mainly around the prosthesis.
In other words, the joint implant creates
a very new condition for natural remodeling
processes.
The preliminary numerical simulation
shows that the concentration of the new
bone and the bone elastic constants in the
surroundings of the living bone-implant
interface substantially depend on the history
and intensity of the loading, drug delivery,
and nutrition. The clinical experience
indicates that dynamical loading (above the
threshold level 1500–2500 microstains, e.g.
Frost 1987), especially walking with a characteristic
time period about 1 s, influences
the whole process of bone remodeling
after approximately 3 months.
Great unknown in the joint-replacement
problem is how will the bone respond
in terms of remodelling to new stress-strain
field in bone after the replacement. Usually
there is considerable resorption in the
vicinity of implant (especially in proximal-
-medial and proximal-lateral part of femur)
but in some cases there is also a significant
deposition of bone in specific sites that
strengthens the imposition of prosthesis in
bone. Fig. 2 shows one example when adequate
physical activity (50-year-old man at
the time of operation, approx. 10 thousand
gaits per day) guarantees sufficient bone
density for a long time (Fig. 2b – the same
man after 6 years from operation).
Our research group tries to give some
insight into this problematics. Despite the
complexity, when not only person-specific
gene expression together with diet
and activity that he performs but also the
choice of material for prosthesis, angle of
insertion and hollow created plays a role,
the same type of response – the same pattern
of density distribution – after month
from operation is obtained as in clinical
observation – Fig 2c.
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