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Revisions of UKA with TKA; Medial versus Lateral UKA
209
Baker
et al
from the UK & Wales JR was the
first to assess the impact of laterality on the
failure of UKA. They demonstrated that Lateral
UKAs account for 6% of UKAs, that the
midterm survival rates of lateral and medial
UKRs are equivalent, and that the pattern of
failure was similar for both medial and lateral
UKAs. The only factors to influence outcome,
consistent for both medial and lateral UKA
were patient age and ASA status at initial
surgery, with younger age and higher ASA
associated with lower survival [6].
Learning Curve-experience and
lessons learned
Timeline failures are another confounding
factor; revision of implants inserted between
1975 and 1985 had more significant bone loss
than more recent revisions, likely related to
less precise earlier ancillary equipment [7, 47].
Other early failures (1984-1998) are associated
with thin or oxidation-prone polyethylene
bearings [15, 16, 18], or poor operative
technique [38]. Early polyethylene bearing
dislocations for lateral UKAs were cited by
Gunther
et al.
[21], but rates were reduced
following introduction of techniques to prevent
dislocation.
Comparing the results of revision of UKA to
TKA is difficult given the multifactorial nature
of any failed prosthesis. Several authors found
inferior results for conversion of UKA to TKA
than for those with a primary TKA [7, 10, 12,
27, 47, 49]. Revision for pain in the absence of
a clear diagnosis is not as successful as when a
cause of failure has been identified [29].
Register studies by virtue of their size give more
quantitative than qualitative data; Lewold’s
paper from the Swedish register focuses on
outcome, and breaks down the indication for
revision of UKA in 1135 patients, but the end
point is revision for any reason, including – in
addition to conversion to TKA – interventions
such as exchange of either the femoral, tibial
andor polyethylene components, or contralateral
UKA, PFR, & mensicectomy [36].
Length of follow up
Length of follow up is extremely heterogeneous
due to a combination of study design and
phraseology. Joint Register studies can follow
outcomes of type of prosthesis over a 20 year
period [36], while others follow cohorts
operated on over defined – sometimes very
lengthy – periods and analyze outcomes for
that timeframe [39, 61]. Others follow specific
implant failure cohorts from (e.g. all UKAs
performed between certain dates) for defined
period post-operatively [13, 52], often with a
view to measuring outcome scores at specific
intervals post-operatively. Some authors
specifically look at short term outcomes for a
given cohort [9, 62], while others report on
timeframe from initial procedures [43], others
from the time of revision or failure [60]. One
study reports on a series of primary UKAs
performed over a 15 year period, with a mean
follow up range of two years, with a mean
interval between primary and revision surgery
of 5 years [55]. Evidently, the reported
timeframe from each study must be assessed in
the context of the studies respective aim, which
makes distinguishing the effect of time to
revision of UKA and laterality difficult.
Implants used as a surrogate for
difficulty of revision
The difficulty of assessing the technical
challenge associated with revision cases for
UKA to TKA has been alluded to. Another
method of measuring the difficulty of revision
is analyzing the type of implants, the use of
bone graft, and metallic augments as a surrogate
for difficulty of revision. Table 3 highlights
studies that have described these features. Bone
loss is common, and requires filling in 33% to
77% of revision cases [40, 46, 57]. Lack of
uniformity in the description of classification
of bone loss means poor comparison between
studies, however the treatment options for a
lack of bone stock, or for ligamentous imbalance
may be inferred from the type of prosthesis
used (CR, PS or Revision/Stemmed com