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shortening of 1.1 cm to the measured value of
c_post. Here we could explain the difference
in a shortening of the tibia (T-JL) of 0.8 cm
while the femur stayed unchanged. The mea-
surement of the knee joint (F-T) also showed a
shortening of 1 cm which nearly matches the
lowering of the joint line of 0.8 cm and the leg
length of minus 1.1 cm. The AFTm was 188°
postoperatively considering this and assuming
an AFTm of 180° we would gain 0.2 cm in
length. This would reduce the shortening to
0.9 cm and we would have a close match with
our measured values (Table 4).
Group 3 (varus)
In group 3 (varus) we calculated a mean diffe-
rence of 0.9 cm between c_pre and c_post. The
smallest difference was 0 cm the largest 2 cm.
In only 4 cases the difference between c_post
and c_predictive was 5 mm or less (Table 4).
In case varus 2 c_post was 2 cm longer than
c_pre and 1.6 cm longer than c_predictiv.
Checking the data we can see that we lengthe-
ned the tibia (T-JL) 2 cm and in this way rai-
sed the joint line 2 cm. On the other side we
shortened the femur 0.2 cm. We actually mea-
sured 1.6 cm knee length postoperatively
which matches the 1.6 cm over length of the
whole leg (Table 4). If calculating only the
knee length pre and postoperatively we would
get a lengthening of 0.1 cm in the knee after
angel correction, this shows the small influen-
ce the angel correction has on this short dis-
tance and demonstrates quite nicely the useful-
ness of the knee length in cases were c_post
diverges from c_predictive.
In case varus 3 c_post is 0.7 cm longer than
predicted. Measuring the knee length we not a
lengthening of 0.8 cm resulting out of a leng-
thening of the femur of 0.6 cm and the tibia (T-
JL) of 0.3 cm which add up to 0.9cm only
0.1 cm out of the measured 0.8 cm and 0.2 cm
out of the predicted (Table 4).
In case varus 5 c_post is 0.8 cm longer than
predicted. Looking again at the knee length we
see that we measured a lengthening of 0.9 cm
as a result of lengthening the tibia (T-JL) of
1.1 cm (Table 4).
In case varus 6 c_post is 0.7 cm longer than
predicted. Looking at the knee length we mea-
sure a lengthening of 0.6 cm resulting out of
lengthening the tibia of 1.1 cm and shortening
the femur of 0.5 cm (Table 4).
In case varus 7 we measured a shortening of the
leg lengt c_post of 1.8 cm. The difference bet-
ween c_post to c_predictive is also 1.8 cm. The
AFTm was only corrected by 4° (from 170° to
174°) and therefore has nearly no effect on leg
length. If calculate it makes a lengthening of
0.2 cm. In this case we measured a shortening
of the knee length of 1.8 cm resulting out of a
shortening of the femur of 1.4 cm and the tibia
(T-JL) of 0.4 cm (Table 4).
In case varus 10 we measured a shortening of
0.4 cm between c_pre and c_post and c_ post
was 0.8 cm shorter than predicted. Expected
was a lengthening of 0.3 cm due to angel cor-
rection. Looking at knee length we measured a
shortening of 0.5 cm due to shortening the tibia
by 0.6 cm. If this shortening would not have
occurred c_post would only be 0.2 cm shorter
than predicted and therefore absolutely in the
range of measuring inaccuracy (Table 4).
Total group
Overlooking all of our measurements we see
that in 6 out of 10 cases in the valgus group we
can measure the predicted leg length. In the
normal group we measured the predicted value
in 8 out of 10 cases and in the varus group only
in 4 out of 10 cases. This equals that in 12 cases
the measured leg length diverged from the pre-
dicted value. Out of these 12 cases 8 cases sho-
wed a lengthening above the predicted due to
angle correction and 4 case a shortening of the
leg length. In the group with lengthening 5 of
the 8 showed the lengthening on the tibia, 2 on
tibia and femur and 1 only on the femur.
In the group showing a shortening 2 showed
the shortening only on the tibia and 2 showed
the shortening on femur and tibia.
CHANGES IN LENGTH AFTER TKA: ACCURACY OF A PREDICTIVE METHOD
77