*C R McLean MRCS(Glasg)
AFRCSI, DPMSA, *J B Wood FRCS(Orth), *H Wynn-Jones MRCS ,*C White
MRCS and *A J Miller FRCS.
* Department of Orthopaedics
Mayday University Hospital, London Road, Croydon CR7 7YE, 020
8401 3193
Address for Correspondence
chrismclean@supanet.com |
ABSTRACT
We
report 10 cases of failure of the polyethylene liner at 3.5 to
11.75 years after total hip arthroplasty using the acetabular
cup system (ACS). In all cases the mode of failure was wear of
the superior rim of the liner, through to the metal backing,
with subsequent, localised liner fracture. All of these cases
were associated with the use of a 28mm head. The design of the
ACS is flawed as it lacks hemispherical geometry, causing rim
loading on thin polyethylene. All patients with this prosthesis
should be monitored for evidence of polyethylene wear.
J.Orthopaedics 2004;1(2)e2
Introduction
In 1999 Patel et al described the failure
of 5 cases of the uncemented acetabular cup system as a result
of excessive polyethylene wear [3]. In four of their five
cases, the femoral head component diameter size was 32mm. In
1994 Bono et al described failure in the same implant, by the
same mechanism, in 15 cases – all of which were associated with
the use of a 32mm diameter femoral head component [2]. In our
series we describe failure of the same implant, by the same
mechanism, however, in all 10 cases a 28mm diameter femoral head
component was used.
Patients and
method
Nine patients in this series underwent
modular, uncemented, total hip arthroplasty, using the titanium
alloy, porous coated, Trilock acetabular cup, with an ACS
polyethylene liner designed to press-fit into the Trilock cup
and uncemented AML femoral stem (DePuy, Leeds, Yorkshire, UK).
The liners were manufactured by ram extrusion and sterilised by
gamma irradiation in air. The femoral head components were made
from cobalt-chrome. All patients presented with symptoms of hip
discomfort and episodes of “hip-squeaking” with radiographic
evidence of excessive polyethylene wear. Osteolysis around the
acetabular component was radiographically evident in three of
these cases. One patient had a hybrid total hip arthroplasty
using the Trilock uncemented cup with ACS polyethylene liner and
a cemented MS-30 stem (Sulzermedica, Alton, Hampshire, UK). In
this case, in addition to the above symptoms, start-up pain was
also a feature. Radiographically there was gross osteolysis
around the cemented femoral component.
The mean age of the patients at the time of
primary arthroplasty was 55 years (range 41 – 65); there were 5
males and 5 females.
Acetabular abduction angles and presence of
osteolysis were recorded using anteroposterior pelvic x-ray. At
revision operation the mode of liner failure was noted.
Results
The time to failure was between 3.5 and
11.75 years – mean 7 years. Acetabular abduction angles were
between 42 and 54 degrees – mean 44 degrees. All patients had a
28mm diameter cobalt-chrome femoral head component. At revision
all polyethylene liners had evidence of excessive wear and
fracture at the superior rim. In nine cases the uncemented
acetabular component was solidly fixed, in the remaining case
the acetabular osteolysis was such that the acetabular component
was uncovered and loose. In three cases there was osteolysis of
the acetabular component, morselised bone allograft was used to
improve the bone stock, in one of these cases the loose
acetabular component was removed and an uncemented stemmed
acetabular component was inserted. In the one case where a
cemented femoral component was used this was found to be loose
and easy to remove. The remaining cement was removed and an
uncemented revision stem was implanted. Nine of the original
ten Trilock acetabular components were retained and new liners
were inserted.
Discussion
Porous or hydroxy-apatite coating on
metal-backed acetabular cups provides a metallic surface for
biological fixation by bony in-growth. Modularity of such
components allows for removal and exchange of liners as and when
required. It should be noted that, as with polyethylene liners
within a cement mantle, the diameter of the polyethylene is less
than that of the cup that houses it. The prosthetic femoral
head size will also affect the thickness of the polyethylene
liner. Polyethylene thickness is an important factor in the
determination of contact stresses that lead to surface damage
[1]. Sterilisation of polyethylene by gamma irradiation in air
may lead to oxidative degradation that may contribute to its
failure [3]. In this series, of the nineteen uncemented
components used, one required revision for aseptic loosening.
The only cemented component used, being a cemented femoral stem,
was revised for aseptic loosening. The remaining nine AML
femoral stems remained solidly fixed to bone. This may suggest
that the uncemented AML femoral component is capable of
withstanding the effects of polyethylene wear debris. We
recommend that all patients with a Trilock acetabular component
with an ACS liner be monitored – clinically and radiographically
– for signs of excessive polyethylene wear.
References
[1] Bartel DL, Bicknell VL, Ithaca MS and
Wright TM: The effect of conformity, thickness and material on
stresses in ultra-high molecular weight components for joint
replacement. J Bone Joint Surg Am 68: 1041, 1986
[2] Bono JV, Sanford L and Toussaint MD:
Severe polyethylene wear in total hip arthroplasty. Journal of
Arthroplasty 9: 119, 1994
[3] Patel J, Scott J, Radford JP: Severe
polyethylene wear in uncemented acetabular cup system
components. Journal of Arthroplasty 14: 635, 1999
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