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ponents, or Hinged prosthesis), and the need
for bone graft or metallic wedges. No studies
distinguish laterality in this context.
Incision
For revision of UKA to TKA the approach used
is ostensibly the same as the initial surgery.
Reusing the old incision for medial UKA is
uncomplicated, although a tibial tuberosity
osteotomy was necessary in 3.7% of cases in
the SFHG multicentre study [56], for patella
baha or for excessive adherence of patellar
tendon to bone. Skin problems have been
reported following revision of UKA to TKA
[26]. For the lateral UKA, the lateral approach
can be used with a lateral arthrotomy, again
with a tibial tubercle osteotomy if required –
this was utilised in 12% the SFHG study [56] or
using a subfascial dissection to effect a medial
knee arthrotomy [40]. Although some authors
have advocated mini-incisions for primary
lateral UKA[2-4], there is no role for minimally
invasive surgery in revision surgery.
Femur correction
In the largest study available, 90% of cases a
standard femoral implant was used [57]. This is
because femoral cuts for UKA are much more
conservative than for TKA (5-6mm as opposed
to 8-10mm respectively). Rarely is bone loss an
issue. Excessive cuts on the side of the removed
prosthesis must be avoided, as should avoiding
excessive internal or external rotation of the
cutting block for lateral or medial UKAs
respectively. Accurate referencing can be
achieved by placing a block the same thickness
as the removed prosthesis on the posterior
condyle, or even keeping the prosthesis in situ
during cutting block placement; both techniques
are effective for revision of both medial and
lateral UKAs. Planning restoration of offset can
be helped by referring to the index Operative
Report, or by obtaining a lateral view of the
contralateral knee. In the Saragaglia paper,
although bone loss is reported in 41% cases,
this “was rarely significant”; only two cases
used femoral augments, and only 18 of the 371
cases used a stemmed femoral prosthesis.
Tibial correction
Bone loss is inevitably the main problem with
revision of a failed UKA. A number of factors
are implicated in this:
i)
Resection level:
this can be influenced by
the surgeon, and by the operative technique,
as well as the patient – constitutional varus,
and by previous procedures. Varus
deformity, regardless of its cause, will
predispose to increased bone cuts (fig. 1).
ii)
Angle of resection:
in the AP plane this can
be affected by excessive varus or valgus
cuts, while in the lateral plane, a large tibial
slope may predispose to increasingly large
bony cuts (fig. 2).
iii)
Knee size:
proportionality of resection
when revising UKAs is as important, if not
more so, than for TKAs. Compared to a
large adult male knee, further bone
resection in the tibia of a female patient
with a very small knee quickly leads to
poorer quality bone stock and a smaller
surface area. No studies have reported the
size of either the implants or the
polyethylene insert involved for the primary
procedures or for revision procedures.
iv)
Other factors:
The presence of Granuloma
(fig. 3), Cement from the previous implant,
Bone Sclerosis (fig. 4), or previous surgery
(HTO or ACL Reconstruction) can all lead
to increased tibial resection as the surgeon
seeks stable, healthy bone as a base for the
tibial component, but the lawof diminishing
returns is very relevant here, as the biology
of the bone at deeper resection levels is
inferior, and may not yield the required
fixation.
Bone loss should be specifically assessed pre-
operatively by imaging, but intra-operative
assessment under direct visualization can only
be definitively confirmed following removal of
implants. Minimal bone resection is key, as the
quality of cancellous bone deteriorates in
proportion to the depth resected. The principles
remain; reconstructing bone stock while
preventing mechanical failure of the newly
implanted prosthesis, using bone graft
(morcellised or head allograft), metallic
augments, and cement or combination of all