*smith&nephew; EP-FIT PLUS™

Foreword
Shells conceived on the basis of the press-fit principle offer a number of advantages for total
hip replacement, including ease of manipulation and a minimum degree of bone resection.
In an acetabulum that is not deformed, the implantation of a hemispherical press-fit shell
without oversizing results in a large percentage of the mechanical load being borne by the
central region of the acetabulum. Moreover, the peripheral ring – especially its lower section –
will in part remain free of loading. For this reason hemispherical shells often have a limited
stability. Additional measures to improve stability, for instance the use of screws or mandrels
and/or the use of shells with oversizing (Lachiewicz et al., 1989), can introduce other problems.
In contrast, a hemispherical press-fit shell will bring greater stability in a patient with protrusio
acetabuli. Here, the mechanical loading is primarily on the peripheral zone (Adler et
al., 1992). It is therefore a logical step to optimize the intrinsic stability of the press-fit
shell by reproducing the protrusio situation.
We have therefore developed a shell that is not spherical throughout, with a patented triple
radius profile and which has proven itself over years of use of the EPF-PLUS‚ and PLUS-FIT
acetabular cups. EP-FIT PLUS is the latest generation of both these cup systems with direct
anchorage for hard articulating surfaces and with a greater surface roughness.
After implantation of the shell, the Triple Radius Profile results in a gap of approximately 2 mm
between the roof of the shell and the hemispherical reamed acetabulum. The actual dimensions
of the gap will vary, however, depending on the quality and elasticity of the bone. This
results in a close contact in the peripheral zone of the shell and a relatively low contact in
the pole region, yielding a favorable prestressing of the wall of the acetabulum. These stress
gradients, declining from the rim towards the dome of the acetabulum, result in a large percentage
of the mechanical loading being transferred to the wall of the acetabulum. This is in
contrast to oversized hemispherical cups, where only a narrow area along the acetabular rim
is subject to loading, and so may result in overloading of the bone in this area.
In summary, the EP-FIT PLUS system has the advantage of reducing the risk of fracture of
the acetabulum during implantation, whilst maintaining excellent primary stability through
connection to bone over a large area along the wall of the acetabulum.
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