Fixation - Orthopaedic Biomaterials in Research and Practice - page 336

fibrocartilage or fibrous tissue is formed owing to fibroblasts increasing
the production of collagen oriented in the tensile direction.
In a much more straightforward analysis, Perren theorized that strain
between 2% and 10% will lead to the secondary bone formation, while
strains below 2% were expected to result in primary bone healing and
those between 10% and 100% will lead to a sustained environment of
initial connective tissue. According to this theory, strain above 100%
will result in nonunion.
Gardner et al. proposed that high octahedral shear stress promotes
tissue proliferation and increases the size of the callus in all stages of
healing. They proposed that as bone matures, it is able to withstand a
higher degree of compressive dilational stress while still undergoing
endochondral ossification, limiting concerns about compressive hydro-
static stresses by Carter. Recently, Epari et al. studied the components of
fragment movement in osteotomized ovine tibiae that are beneficial for
fracture healing. They determined that high shear stability and moder-
ate, not maximal, axial stability are associated with high callus strength
and stiffness and a better healing outcome.
Problems
PROBLEM 13.1
Which of these are a characteristic of a porous coating that is NOT
expected to directly affect the rate of bone ingrowth?
A. Porosity
B. Fatigue strength
C. Frictional behavior
D. Stiffness
ANSWER:
B . Fatigue strength is important in terms of the failure of the actual coat-
ing material but does not have a direct effect on ingrowth.
PROBLEM 13.2
Comment on whether the following interfaces are considered Direct,
Graded, Compliant, or a combination of the three.
A. Cemented
B. Ingrown
C. Adhered
ANSWER:
Cemented, fully compliant; ingrown, combination of graded and com-
pliant; adhered, a direct bond with some compliant behavior.