Host response - Orthopaedic Biomaterials in Research and Practice - page 363

density found bone resorption from 17% to 28% in the cancellous bone
and 5% to 8% in the cortical bone. At 3 years after surgery, Schmidt et
al. (2004) found, using quantitative computed tomography, a decrease in
density of 14.3% in the femoral metaphysis along with a 17.3% decrease
in cortical bone in 24 patients with a Multicone uncemented tapered
stem. The same authors saw a slight decrease in resorption (14.2% and
15.5%, respectively), around the same stem with an HA-coated surface.

Short stems Neck-conserving, short-stem implants are intended to address issues of

proximal-distal mismatch, reduce stress shielding and overload, stimulate
increased bone formation around the device, encourage bony ingrowth,
and limit periprosthetic fractures. Lerch et al. (2012) analyzed bone
mineral density around the Metha short-stem implant in 25 consecu-
tive patients. In comparison to immediate postoperative baseline values,
BMD at 2 years decreased in the greater trochanter from 0.78 g/cm 2 to
0.72 g /cm 2 . BMD actually increased in the lesser trochanter by 12.9% and
increased in the calcar region by 6.1%. Most remodeling occured in the
distal calcar and lesser trochanter region (zone 6), along with the proxi-
mal calcar (zone 7). The authors concluded that this unique result was the
achievement of proximal load transfer with the Metha stem. Briem et al.
(2011) retrospectively examined radiographs of 155 patients treated with
the CFP (collum femoris preserving) prosthesis and observed a relative
loss of bone in the proximal region (osteopenic) with bone formation at
the distal stem. There were no signs of significant bone loss or osteolysis.
Similar to other stems, maximum bone remodeling typically occurs at 6
months after surgery and levels out after 1 year. Additional adaptation
will occur over the next 1 to 2 years. While short stems are bone conserv-
ing both from the surgical preservation and stress shielding standpoints,
this also implies that there may be less surface area in contact (and hence
frictional resistance) between the stem and adjacent bone. Therefore, con-
sideration should be given regarding relative motion of the stems.

Stress shielding
in TKa

The focus of stress shielding has been around the proximal femur, but
it is also known to occur anywhere in the presence of a stiff, foreign
implant, including behind the acetabular component and underneath the
metal tray of a tibial TKA component. Much less is known about density
response in these regions as bone density is more difficult to assess here
radiographically.

electrical stimulation, growth, and repair

All organisms are electrodynamic systems with large and stable gradi-
ents. The relationship between mechanical forces and endogenous cur-
rents has been well known since research on piezoelectric forces in bone
were published in 1953 by Yazuda, who had an interest in manipulation
of electromagnetism in bone. Deformed bone is electronegative rela-
tive to its undeformed state, and thus an electric current is produced.
The amplitude of the electrical potential is dependent on both the rate