Dislodgment of a polyethylene insert from the tibial baseplate
following total knee arthroplasty is an uncommon event. To our knowledge, it
has been described in only four reports in the English-language
literature1-4.
The extremely low rate of this complication contrasts with the frequent
dissociation of polyethylene liners from total hip replacements, as reported
at our institution5
and by others6,
occurring in association with so-called first-generation uncemented, modular
acetabular cup designs.
The purpose of the present report was threefold: (1) to present the
assessment and management of a patient who had a dislodged polyethylene insert
following total knee replacement, (2) to review the literature on this rare
complication, and (3) to discuss possible mechanisms of failure. The patient
was informed that data concerning the case would be submitted for
publication.
Afifty-five-year-old man (height, 170 cm; weight, 109 kg; body mass
index, 38) had undergone a cruciate-retaining left total knee arthroplasty
(Duracon; Howmedica, Rutherford, New Jersey) for the treatment of
posttraumatic arthritis. He returned to his orthopaedic surgeon seven years
after surgery because of a new onset of knee pain and swelling. A diagnosis of
patellofemoral degeneration of an unresurfaced patella was made, and the
patient underwent revision surgery for resurfacing of the patella. According
to the operative note that was obtained, delamination and cold flow
deformation were noted at the posteromedial aspect of the 9-mm tibial
polyethylene liner, suggesting excessive posterior medial loading of the
insert. The patella was resurfaced, and a new 19-mm tibial insert was
implanted.
Six months later, the patient presented with spontaneous onset of knee
pain, swelling, instability, and clicking due to dislodgment of the
polyethylene insert from the tibial baseplate. During further revision
surgery, the posterior metallic rims and the anterior locking tab of the
baseplate were intact. A new Duracon 19-mm anteroposterior lipped
semistabilized polyethylene liner was implanted, and the patient had an
uneventful recovery.
Within six months after the latest operation, the patient felt the knee
suddenly become unstable. Although no injury was reported at the time, he had
continued to work as a truck driver and often jumped from a height of 1 m to
get out of his truck.
The patient was referred to our institution. On physical examination, the
dislodged polyethylene insert was palpable within the suprapatellar pouch.
Global knee laxity was present, and metal-on-metal crepitus was evident on
passive motion. Radiographs confirmed the tibial insert to be in the
suprapatellar pouch (Fig.
1).
The patient underwent a third revision. The modular liner was found lying
in the suprapatellar pouch. The posterior cruciate ligament was found to be
intact, with adequate tension. The tibial and femoral components were revised
to a constrained condylar knee (NexGen LCCK; Zimmer, Warsaw, Indiana) with an
uncemented diaphyseal stem augmentation. This knee replacement design was
chosen because it offers metal screw placement through the polyethylene post
to strengthen the post and to provide additional fixation of the liner to the
tibial tray. The patient subsequently ceased working as a truck driver, and
the knee was functioning well at the time of the latest follow-up.
The retrieved, dislodged liner and the tibial baseplate were analyzed
macroscopically and under scanning electron microscopy (Quanta 600
Environmental SEM; FEI, Hillsboro, Oregon). Macroscopic examination of the
tibial baseplate revealed polishing of the backside surface, apparently
produced by articulation with the cobalt-chromium femoral component after
dislodgment (Fig. 2). The
locking mechanism, consisting of two posterior grooves to accommodate the
polyethylene retaining lips of the insert and an anterior locking tab that
provides a snap-fit lock, was intact (Fig.
2). Macroscopic examination of the polyethylene insert revealed
that the posterior retaining lips and the anterior locking groove were intact.
Abrasion, pitting, and cement embedding were evident, particularly in the
posterior third of the liner. The posteromedial edge showed compression
deformation affecting the articulating surface and the surface in contact with
the baseplate (Fig. 3).
Scanning electron microscopy of the liner demonstrated mild wear of the
posterior retaining lips and peripheral backside wear, suggesting that there
was micromotion between the liner and the baseplate.
Metal-backed tibial components were introduced in the 1980s. Bartel
et al. demonstrated that a metallic baseplate that is bonded to the insert
reduces the tensile stresses in the polyethylene when compared with
all-polyethylene
designs7 and that
metal backing reduces the maximum compressive stresses on the bone under
load8. Tibial
modularity evolved from this concept, offering the future option of isolated
liner exchange and the use of inserts of differing geometry and constraint.
Concerns related to backside wear of modular implants
persist9. Locking
mechanism failure is a more catastrophic, although exceedingly rare,
event.
There have been few reports of dissociation of modular polyethylene liners
from tibial baseplates. Ries reported the case of a patient with quadriceps
insufficiency in whom a constrained condylar insert (Insall/Burstein II;
Zimmer) had dissociated from the
baseplate1. This
finding was thought to be due to failure of the posterior locking mechanism,
resulting in posterior lift-off. Davis et al. reported the cases of two
patients who had dissociation of a modular PCA cruciate-retaining component
(Howmedica)2. The
failure was believed to have resulted from anterior lift-off of the insert
from the metal baseplate. Hedlundh et al. described the case of a patient who
had a traumatic, late dissociation of the polyethylene surface of a
cruciate-retaining Kinemax knee replacement
(Howmedica)3.
Finally, Poulter and Ashworth reported the case of a patient who had
dislodgment of a cruciate-retaining AGC insert (Biomet, Warsaw, Indiana),
which had been preassembled in the
factory4.
Interestingly, three of these five
dislodgments2,3
occurred in knees with a locking mechanism similar to the one in our patient
and four occurred in knees with a cruciate-retaining replacement.
Scanning electron microscopy of the liner in the present study demonstrated
backside wear produced by micromotion between the liner and the baseplate,
which may have contributed to the dislodgment. Examination of the articulating
surface of the insert suggested that posterior femoral translation and point
loading in the posteromedial corner produced anterior cantilevering of the
liner. The tension of the posterior cruciate ligament was adequate, making it
unlikely that a tight posterior cruciate ligament increased the posterior
stress on the insert. This dislodgment could not have been initiated
posteriorly as the polyethylene retaining lips were intact.
In this young, obese truck driver who continued to jump from heights, the
locking mechanism was being continually challenged. It may be argued that this
liner had been incorrectly seated during the second revision, leading to a
dislodgment only six months later. However, the liner's intact posterior
retaining lips, the presence of peripheral backside wear, and the absence of
scratching on the backside surface discredit this theory.
In our patient, the dislodgment occurred in two instances, with two
different inserts, on the same tibial baseplate. Tradonsky et al. examined the
in vitro lever-out and push-out strengths required to dislodge the liner from
the metal shell in a study of eight non-implanted acetabular components with
five different locking
mechanisms10.
Repeated testing on the same shell with a new polyethylene insert revealed
progressive weakening of the locking mechanism. In the case of our patient, it
is conceivable that the titanium anterior locking tab in the tibial baseplate
may have fatigued because of micromotion and the previous dislodgment.
Although exceedingly rare, dislodgment of a modular tibial liner should be
considered as a mechanical cause of failure following total knee arthroplasty.
Revision of tibial and femoral components is required when metal-on-metal
contact has produced articulating surface and/or locking mechanism damage.
When there is potential for a new dislodgment or when motion between the old
baseplate and a new insert is encountered during revision surgery, strong
consideration should be given to revising the baseplate. We believe that the
new tibial baseplate should have enhanced polyethylene insert fixation with a
screw or metal clip. ?
Note: The authors thank Stephen Doty, PhD, and Tony Labissiere
for the scanning electron microscopy study. They also thank Nicole Taveras,
BS, for assistance in the preparation of the manuscript.