Fixation - Orthopaedic Biomaterials in Research and Practice - page 325

where the subscript SF refers to the stem-fixation construct, M SF and M B
are the bending moments, and I SF and I B are the areal moments of inertia
of the stem-fixation system and the bone, respectively. Thus, it is clear
that the condition E S = E B is neither a necessary nor a sufficient solution
to the problem. Furthermore, since M SF decreases distally and concur-
rently M B increases, satisfying the appropriate condition at more than a
few points along the shaft is probably impossible.
At locations 3 and 3 ′, lateral and medial, respectively, no design can
restore normal stresses so long as there is any stem-bone contact. This is
the case since, below the limit of the natural cancellous bed (line C - C ′),
all stress in the intact femur is carried within the cortex. Introduction of
stress by intramedullary contact must change either the magnitude or the
direction of the stress resultant.
At locations 4 and 4 ′, stresses in the implanted femur must always
be larger than those in the intact femur. This reflects a greater bending
moment, produced as a consequence of the proximal stiffening effect
of the stem. For stresses to be normal at this level, (1) there must be no
stem-bone contact below C - C ′ and (2) the stemixation construct stiff-
ness above C - C ′ must be near to that of the cancellous bone it replaces;
that is,
E SF ≈ E cancellous bone ≪ E cortical bone
Point 2 is the design consideration leading to the so-called “isoelas-
tic” stems; however, they remain far too stiff to satisfy this condition and
extend distal to line C - C ′, with continuing stem-bone contact.
Similar arguments lead to the same conclusions for intramedullary
stem fixation in other skeletal locations.
role of the interface in fixation
It should be clear from the previous discussion that the three simulta-
neous conditions of no relative motion and normal stresses on and in
bone, stated as the goals of fixation, cannot be met by a two-phase
(stem-bone) rigidly bonded system. Therefore, the fixation system must
act to “decouple” the implant from the adjacent bone while providing for
stability and stress distribution and transmission.
Types of bond systems
The bonding of materials with dissimilar mechanical properties is a
classical problem in engineering and has been solved in a number of
ways. There are three generic approaches, illustrated in Figure 13.6. (In
this case, the implant is assumed to be made of a titanium-base alloy
[modulus = 100 GPa], but the arguments would be similar for higher-
stiffness materials.)