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Abstract
Background: In uni compartmental knee
replacement, the other compartments and knee ligaments are
largely untouched. Therefore, the knee kinematics after uni
compartmental replacement may also be unchanged. The results of
these uni compartmental knee replacements would depend on
maintaining alignment, so that the unaffected compartment is not
overloaded. The maintenance of the mechanical axis closer to the
centre of the knee also has a bearing on the rate of
polyethylene insert wear.
Methods: Uni compartmental knee replacements implanted
over a period of 15 years by one surgeon was reviewed and
analysed for the alignment using the mechanical axis. The tibial
plateau was assigned zones 0, 1, 2, C, 3, 4, 5. 0 being in
extreme varus and 5 being in extreme valgus. The appropriate
alignment was defined as varus or central for medial
unicompartmental knee replacements and valgus or central for
lateral uni compartmental knee replacements.
Results: 71% of the knees analysed had the ideal
alignment. 23% had an acceptable alignment. 6% had unacceptable
altered alignment. 30% of the knees with unacceptable altered
alignment had to undergo conversion to TKR.
Conclusions: Maintaining alignment in uni compartmental
knee replacements is of paramount importance, to prevent the
accelerated degeneration of the unaffected compartment.
Significant alteration of alignment results in poor results.
J.Orthopaedics 2006;3(2)e3
Introduction:
Replacement arthroplasty of
the knee is a successful procedure. The relief of pain and the
restoration of function can be dramatic, and the rate of
survival of the implants is long enough to satisfy most patients
requiring knee replacement (1, 9). It is difficult to justify
total knee replacement in patients with uni compartmental
disease. These patients are treated with upper tibial osteotomy
or uni compartmental knee replacement. The long-term outcome of
high tibial osteotomy has not been consistent, with a decrease
in the rate of survival to 75% at ten years and 65% at fifteen
years (13). Uni compartmental knee replacement is an attractive
alternative for this patient population (13).
The general consensus is
that total condylar (tricompartmental) knee arthroplasty
substantially changes the kinematic profile of the knee (12).
This may be attributed to several factors including the
differences between the geometry of the normal articular surface
of the knee and the replacement prosthesis, loss of the anterior
and/or posterior cruciate ligaments, and altered neuromuscular
patterns due to preexisting pathological conditions. Improved
kinematic profile with good long term survivorship at 10 yrs
makes uni compartmental knee replacements in uni compartmental
knee arthritis a very attractive proposition (8).
Theoretically, uni compartmental knee
replacement offers the potential to restore knee kinematics to
normal. The overall geometry of the knee is better preserved as
only one compartment is replaced. In addition, current
indications for a uni compartmental replacement necessitate the
presence of an intact anterior cruciate ligament (5, 8). Knee
kinematics after replacement with the Oxford uni compartmental
design may be similar to that of the normal knee (10).
Therefore, the ligamentous stability and soft-tissue balance of
the joint can be restored more closely to normal and would
influence the kinematic and kinetic profile of the knee. The
final alignment of the knee is a static reflection of these
factors. The objective of this study was to analyse the
mechanical axis in patients who have had uni compartmental knee
replacements from1989 to 2004.
Material and Methods :
  Between
1989 and 2004, 92 uni compartmental knee replacements were
performed by the same surgeon (PCM) using the same design of
prosthesis (Miller- Galante). 84 knees were medial uni
compartmental knee replacements and 8 knees were lateral uni
compartmental knee replacements. Of these 55 knees were
available for follow-up, 27 were dead and 4 lost to follow-up.
These 55 knees were subjected to long leg views to assess the
mechanical axis and the zone through which the mechanical axis
passed, as described by Kennedy and White. The mechanical axis
is defined as the line passing from the centre of the hip to the
centre of the ankle. The zone through which this line passes as
described by cartier and villiers is further assessed.
The above figures illustrate the mechanical
axis (on the right) and the zone of passing of the mechanical
axis on the left.
Results :
55 knees were assessed. All except one medial uni compartmental
knees were still in varus. The one medial uni compartmental knee
replacement which had an altered alignment was caused secondary
to a total hip replacement. 36 knees( 66 %) had the mechanical
axis pass through zone C or 2 .13 knees( 24 %) had the
mechanical axis pass through zones 1 and 0. 71% of the knees had
the ideal post- operative alignment we had aimed for. The
alignment over a long period of time, especially considering
that poly ethylene wear and component subsidence would have
resulted in alteration of alignment.
Discussion :
   Indications
for uni compartmental knee replacement include uni compartmental
disease (either medial or lateral), no evidence of substantial
patello femoral arthritis, and an intact, functioning anterior
cruciate ligament (7). In our series the radiographic changes in
the patello femoral joint were considered relevant only if they
produced clinical symptoms. Several authors have emphasized the
need for under correction of the deformity during uni
compartmental knee replacements in order to minimize the
degeneration in the unaffected compartment (4, 6). Alignment in
excessive varus in medial uni compartmental knee replacements
also result in accelerated wear and tibial component subsidence
(6). Most authors believe that the ideal alignment is with the
mechanical axis passing through Zone C or Zone 2 in case of
medial uni compartmental replacements and Zone C or Zone 3 in
case of lateral uni compartmental replacements (2, 6). Hence the
long term success of the surgery would be dependent on the
reproduction of the alignment. Collateral ligament laxity also
plays an important part in the maintenance of post operative
alignment, and needs to be assessed clinically and if necessary
with stress radiographs (2, 6). Although a release of the medial
collateral ligament has been proposed (12), it was not performed
in our series, as we were concerned about leaving the replaced
compartment lax, especially in cases of mobile bearing uni
compartmental knees as this may result in the dislocation of the
mobile meniscus (5). Moreover in our view if the knee required
significant soft tissue release a uni compartment knee
replacement may be an inappropriate procedure. Lateral uni
compartmental arthroplasty is much less common than medial uni
compartmental knee replacement (11). The slope of the lateral
tibial plateau is lesser than the medial tibial plateau and
hence the wear pattern is posterior as opposed to antero medial
in case of medial compartment. Lateral compartment replacements
should hence be placed with tibial resection made with less
posterior slope (11). Care should be taken to prevent alteration
of alignment while performing lateral uni compartmental knee
replacements. Majority of the revisions in our experience were
in lateral compartment disease, due to accelerated degeneration
in the uninvolved compartment, secondary to alterations in the
alignment.
Conclusion:
1) The maintenance of alignment in uni
compartmental knee replacement is of paramount importance in
preventing progression of osteoarthritis in the normal
compartment and wear in the prosthesis.
2) Appropriate alignment can be reproduced in majority (99%) of
the medial uni compartmental knees with adequate care before and
during surgery.
3) Lateral uni compartmental knee replacement require more care
partly because they are rarely performed and in our experience
more likely to result in an altered alignment.
4) Only 30% of the knees with unacceptable altered alignment
required conversion to TKR.
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