Biomedical Engineering Reference
In-Depth Information
3
2
1
CoNiCrMo
0
0
15
25
35
Reduction (%)
45
55
65
75
FIGURE 1.3
Relationship between ultimate tensile and the amount of cold work for CoNiCrMo alloy. (Adapted
from Devine, T.M. and Wulff, J. 1975.
J. Biomed. Mater. Res.
9, 151-167.)
long service life without fracture or stress fatigue. Such is the case for the stems of the hip joint prosthe-
ses. This advantage is better appreciated when the implant has to be replaced, since it is quite difficult
to remove the failed piece of implant embedded deep in the femoral medullary canal. Furthermore, the
revision arthroplasty is usually inferior to the primary surgery in terms of its function due to poorer
fixation of the implant.
The mechanical properties required for CoCr alloys are given in Table 1.4. As with the other alloys,
the increased strength is accompanied by decreased ductility. Both the cast and wrought alloys have
excellent corrosion resistance.
Experimental determination of the rate of release of nickel from the CoNiCrMo alloy and 316L stain-
less steel in 37°C Ringer's solution showed an interesting result. Although the cobalt alloy has more
initial release of nickel ions into the solution, the rate of release was about the same (3 × 10
−10
g/cm
2
/
day) for both alloys (Richards Manufacturing Company, 1980). This is rather surprising since the nickel
content of the CoNiCrMo alloy is about three times that of 316L stainless steel.
The metallic products released from the prosthesis because of wear, corrosion, and fretting may
impair organs and local tissues.
In vitro
studies have indicated that particulate Co is toxic to human
osteoblast-like cell lines and inhibits synthesis of type-I collagen, osteocalcin, and alkaline phosphatase
in the culture medium. However, particulate Cr and CoCr alloys are well tolerated by cell lines without
significant toxicity. The toxicity of metal extracts
in vitro
have indicated that Co and Ni extracts at 50%
TABLE 1.4
Mechanical Property Requirements of Co-Cr Alloys
Wrought CoNiCrMo (F562)
Solution
Annealed
Cast
CoCrMo
(F75)
Wrought
CoCrWNi
(F90)
Property
Cold-Worked and Aged
Tensile strength (MPa)
655
860
793-1000
1793 min.
Yield strength
(0.2% offset) (MPa)
450
310
240-655
1585
Elongation (%)
8
10
50.0
8.0
Reduction of area (%)
8
—
65.0
35.0
Fatigue strength (MPa)
a
310
—
—
—
Source:
Adapted from ASTM. 1992.
Annual Book of ASTM Standards
,
Vol. 13,
Medical Devices and
Services
,
F75-F87, p. 42; F90-F87, p. 47; F562-F84, p. 150. Philadelphia, PA: ASTM.
a
From Semlitch, M. 1980.
Eng. Med.
9, 201-207.
