Properties of natural materials - Orthopaedic Biomaterials in Research and Practice - page 122

indicating rheumatoid arthritis as an independent risk factor for frac-
tures. Significant microarchitectural changes in cancellous bone can
be observed after osteoarthritis, leading to a very heterogeneous archi-
tecture. Osteoarthritic trabecular bone is thicker and denser, as well as
more plate-like, but has lower mechanical strength.

effects of
antiresorptives

Antiresorptive therapy includes bisphosphonates and estrogen replace-
ment. Over the past two decades, bisphosphonates have been used to
help arrest bone loss and to treat osteoporosis, by limiting the dissolution
of the principal bone mineral, that is, hydroxyapatite. The mechanism
is achieved through encouraging osteoclasts, which destroy bone, to
undergo cell death and thereby preventing bone turnover. This improves
bony microarchitecture by preventing loss of trabecular volume and
suppressing activation of remodeling, thereby adding mass to trabecular
bone and halting cortical bone erosion on the corticoendosteal surfaces.
However, bone formation does not appear to continue beyond filling
remodeling sites previously created. Thus, bisphosphonates do not add
bone mass or change the macroarchitecture or shape of bone, outside
of limiting the effect of increasing resorption. Bisphosphonate drugs
include alendronate, etidronate, ibandronate, risedronate, zoledronate,
and pamidronate. In contrast, other drugs function anabolically instead
by stimulating osteoblasts, which build new bone.
Estimates of human bone strength after the use of bisphosphonates
suggest that vertebral strength may be increased by up to 10% after 3
months. Similar changes have also been observed for femoral bone,
with 3.6% and 4.8% improvement after 1 and 2 years of treatment,
respectively. Consequently, bisphosphonates are also highly effective in
decreasing the incidence of hip and spine fractures by up to 30% to 50%.
Increased bone strength can be explained almost entirely by increases
in bone mineral density, but the increased mechanical properties are
partly offset by reduced bone toughness. Differences occur, depending
on drug, dosage, and duration of use.
Sex hormones, such as estrogen for women and testosterone for men,
have a protective effect on bone. If hormone production is altered because
of surgery, illness, or early menopause in women, hormone replacement
can be used to replace hormones that the body should be making or used
to make. Such therapy can include replacing calcitonin, which acts on
the skeleton to regulate the amount of calcium in the blood. Supplements
of estrogen or estrogen with progesterone may also be taken.

PROBLEM 5.3
What is the primary mechanism for bisphosphonates on bone remodeling?

A. Creating bone throughout, including beyond remodeling sites

B. Arresting osteoclasts to prevent bone turnover

C. Changing the macroarchitecture of bone

D. Activating osteoblasts