ARUP; ISBN: 978-0-9562121-5-3 - CMBBE 2012 - Cardiff University

of-freedom. The numerical-experimental comparison through the linear regression with
a previous experimental study presented a correlation value of R 2 = 0.8925, with a slope
of 0.9357 and an interception value of 2.3258 [16, 18].The diagrams plotted in Fig. 4
are examples of the comparison the natural femur with the implanted one in both health
and loosening scenarios, namely “all glued” or “with friction”, respectively.
Minimal Principal Strain (μstrain)
0
-2000
-4000
-6000
-8000
-10000
-12000
-14000
-16000
-18000
-20000
-22000
Line 1, at 45o A B
Journey PFJ (all glued) Natural Femur Journey PFJ (with friction)
DISCUSSION
Fig. 4 Diagrams of Minimal Principal Strain
The objective of this work was to study the structural effects of the implantation of PF
prosthesis, on femur cancellous bone. According to the knowledge of the authors, there
are no published biomechanical or clinical studies examining the strain behavior in
cancellous bone or even changes in mineral density of the cancellous bone after PFA.
The hypothesis here considered is based on the fact that the nature/magnitude of PF
cyclic loads, with contemporary Journey PFJ prosthesis, generates strain/stress levels at
bone that exceeds the fatigue strength of these materials. Ideally, bone stress levels will
be low enough to avoid loosening but high enough to prevent significant bone atrophy.
The FE models developed for this study were compared with a previous PF in-vitro
study [16, 17]. Two cement-bone interfaces were considered in order to replicate two
clinical scenarios. Chronologically, the “health scenario” may be considered as a
clinical condition at short/mid-term after surgery, and the “loosening scenario” as a
mid/long-term clinical scenario. In the “health scenario”, where the cement was
considered rigidly bonded (“all glued”) to the bone, a generalized strain reduction on
cancellous bone was observed, at the three different daily activities. This phenomenon,
known by stress-shielding, can be explained by the load sharing capacity of the implant.
The loads previously applied to the trochlea surfaces are shifted, after the implantation,
to the surrounding regions (bridge effect), unloading the cancellous bone under
prosthesis. The extents of these strain reductions stood between -1 to -12 times in
comparison with the strain levels in the natural femur. Such findings appoint a risk of
cancellous bone resorption for loads in range of daily activities, since when bone loads
are reduced or eliminated, bone mass is reabsorbed (Wolff’s law) [12, 21, 22]. In the
loose scenario, where the cement was considered not bonded to the bone (“with
friction”), at the three different daily activities, a localized increase on cancellous bone
strain was observed. These increases represent several times (1 to 12 times) the strains
observed in the natural condition. The failure process in the cancellous bone tissue can
occur because of overload and usually a fatigue mode. Reasonably, bone is expected to
suffer fatigue-failure if the induced strains approach 60 to 80% of the yield strength [13,
23]. These strain levels may occur if the normal maximum strains/stresses in the
Minimal Principal Strain (μstrain)
0
-2000
-4000
-6000
-8000
-10000
-12000
-14000
-16000
Line 3, at 90o A B
Journey PFJ (all glued) Natural Femur Journey PFJ (with friction)