Metals - Orthopaedic Biomaterials in Research and Practice - page 189

Table 7.9 fatigue strength of orthopaedic alloys

Fatigue strength
(10 7 cycles, R = −1)
(MPa)

Ultimate strength
(MPa)

Material σ f /σ u

Stainless alloys
F 55 a 480 240 0.5

F 55 cw 655 310 0.47

Ortron 90 a 834 459 0.55

Ortron 90 cw 1035 640 0.62

Co-base alloys
F 75 c 793 310 0.39

F 75 pm 1300 765 0.61

F 75 f 1400 793 0.56

F 90 f 860 485 0.56

F 90 pm, f 1240 825 0.6

F 562 cw 1200 500 0.42

F 799 f 1400 900 0.64

Ti and Ti-base alloys
F 67 af 550 240 0.44

F 136 f, a 985 520 0.53

Note: a, Annealed; af, as fabricated; cw, cold worked; pm, powder metal-
lurgical; f, forged; data from various sources, all in air.

Modification of surface geometry

Current interest in the replacement of poly(methyl methacrylate) bone
cements with implant fixation by direct biologic means has led to the
development of numerous techniques to modify the surfaces of ortho-
paedic implants. There are four principal types of surface structural
modification in use in implant fabrication today (some are in routine use;
others are still in the pre-market approval stage).

Cast structures . Devices that are cast may easily have a grooved,
pebbled, or beaded surface formed during initial fabrication. The
advantage of such surfaces is that they have the same composition
and properties of the base material; the disadvantage is that they
must have very simple geometry with no real possibility for inter-
nal porosity (Figure 7.10).
Sintered structures. Cast and forged devices may have a surface coat-
ing attached by sintering. This involves placing beads or parti-
cles in close approximation to the surface and then heating both
components to produce diffusion bonds, both within the coating
and between the coating and the surface. This is the most popular