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Lateral unicompartment knee arthroplasty (UKA)
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In the frontal plane, the radius of curvature is
generally convex, symmetric and sufficiently
wide to allow adequate stress distribution on
the tibial component, reducing the
risk of
excessive loading
.
Somelaboratorieshavedevelopedasymmetrical
medial/lateral condyles which purpose is to
avoid the patellofemoral conflict. However, the
major challenging is the need of more
instrumental boxes.
Classically, two concepts were described:
femoral prosthesis performed by “resection”
(“cut”) and by “resurfacing”.
The term “resurfacing” or “resection” in the
UKA procedure defines the gesture to be done
on the anterior and distal femoral condyle,
since a cut is usually done on the posterior
femoral condyle which is without wear.
Systems of resection (“cut”)
The UKA critics have emphasized that the
sacrifice of subcondral bone could be a factor
of prosthesis loosening and also, a factor of
bone loss. These systems seek to ensure a distal
femoral cut perpendicular to the mechanical
axis of the femur performed by intra or
extramedullary guides.
Although, the intramedullary guide is more
accurate than extramedullary ones, the classic
morbidity related to the catheterism of femoral
diaphyis (fat embolism and cortical bone
damage) is a disadvantage and hence, the mini-
invasive approach is considered.
On the other hand, the extramedullary guides
have no additional morbidity, but they are less
accurate than intramedullary ones.
Systems of resurfacing
These systems are well-matched to the concept
of mini-invasive surgery. The main advantages
of these systems are preservation of the
subchondral bone, which allow solid fixation to
the prosthesis there are no guide and the
ancillary is compact.
Usually, one or two pegs or sagittal fin are used
to the femoral component fixation.
Certain systems called “resurfacing” perform a
gradual and variable drilling of the femoral
condyle according to the extension gap; it could
lead to bone loss such that they are closer to the
bottom in the system of resection than
resurfacing.
However, both systems (“resection” or
“resurfacing” UKR) have their indication in
lateral UKR.
In our point of view, the origin of the tibial and
femoral deformity guides this decision.
Therefore, a resurfacing UKR to build the
lateral femoral condyle has been indicated
when hypoplasia of the lateral femoral condyle
exists. However, if the lateral femoral condyle
is normal and the valgus is on the tibial side,
the resection UKR should be performed,
because resurfacing UKR could lead to
hyperstructure of femoral condyle, tibial
overcut and lower joint line which is source of
pain and worse functional results.
Tibial component
The main causes of UKA failure due to wearing
and loosening of the tibial plateau are well
known.
The thickness of the polyethylene
It plays an important role in case of wear of
polyethylene caused by flow or creep, where
sustained stress produces the polyethylene
deformation, which is less important with a
thicker polyethylene. In the UKA, a high
density polyethylene can be used. A minimum
thickness of 6mm is recommended to limit the
risk of wearing by creep with no metal back
and 9mm thickness, in case of metal bac.