Biomedical Engineering Reference
In-Depth Information
The use of acrylic dental bone cement to fix implants to bone reduced
loosening of implants and the introduction of a plastic acetabular component to
reduce frictional forces in the joint by Charnley revolutionized total joint
arthroplasty (Charnley, 1961). Charnley first started using polytetrafluoro-
ethylene (PTFE) but replaced it with UHMWPE due to high wear and cata-
strophic failures (Charnley, 1963). He called the material he used high density
polyethylene (HDPE) at the time because UHMWPE was classified among the
HDPEs at the time (Kurtz, 2009c), but it was nevertheless an ultrahigh
molecular weight polymer and was more wear resistant (Charnley and Cupic,
1973). Thus, the use of UHMWPE as a joint bearing surface material was
initiated.
3.3
Ultrahigh molecular weight polyethylene
(UHMWPE)
3.3.1
Structure and properties
UHMWPE is a linear polyolefin with a simple structure (± [CH
2
]
n
±) of a
saturated carbon backbone. While polyethylenes can come in a variety of forms
based mostly on their molecular weight and branch structure (very low density,
linear low density, low density, high density and ultrahigh molecular weight),
the molecular weight and linearity of UHMWPE render its morphology and its
mechanical/tribological properties interesting. They also present some
challenges in its processing and life as a long-term implantable medical device.
UHMWPE is a semicrystalline matrix embedded in an amorphous matrix
(Fig. 3.1) with 70±80% crystallinity in resin powder form and 50±60%
crystallinity in consolidated form (Table 3.1). When crystallized from the melt
at ambient pressures, it has an orthorhombic folded chain crystal structure
(crystal unit parameters a 7:4
Ê
, b 4:93
Ê
, c 2:534
Ê
; Bunn, 1939) and
crystalline lamellae thickness is approximately 10±50 nm. At room temperature,
UHMWPE is well above its glass transition temperature, where the mobility of
its chains is enhanced. The mobility of the chains in the crystalline phase can be
further enhanced by crystallization at high temperature and pressure (>160 ëC,
>300 MPa), where it can grow chain-extended crystals in the hexagonal phase
(a 8:46
Ê
, b 4:88
Ê
; Bassett and Turner, 1972; Bassett et al., 1974;
Wunderlich and Arakawa, 1964). In this phase, the lamellae can grow as thick as
several microns (Geil et al., 1964).
The mechanical properties of UHMWPE are strongly dependent on its
crystallinity and semicrystalline morphology. The tie-chains between crystalline
lamellae (Fig. 3.1) can span a very large distance and multiple lamellae, thus
making it a very strongly entangled network. Increasing crystallinity results in
increased yield strength and ultimate tensile strength (Brooks et al., 1999; Turell
and Bellare, 2004; Simis et al., 2006). High yield strength is combined with a
