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near as possible to the JL. The idea was that in
this manner a lengthening that occurred main-
ly in the knee joint could be differentiated bet-
ween a lengthening of the leg based on the
change of the angle gamma. This should be so
because the nearer together the points T and F
are the less influence changes in the angle
gamma will have on the absolute distance
measured. F was defined 35 cm distal to D on
the mechanical axis of the femur. T was defi-
ned 25 cm proximal to the centre of the ankle
on the tibial mechanical axis.
After defining all points on pre and postopera-
tive radiographs the distances a_pre, a_post,
b_pre, b_post, c_pre, c_post and gamma pre
and gamma post were measured and put in to a
table. Then c_pre calc, c_post calc and
c_predictive were calculated with the formulas:
CHANGES IN LENGTH AFTER TKA: ACCURACY OF A PREDICTIVE METHOD
73
c_pre calc = √
a
2
+ b
2
- 2
ab
cos
γ
,
a = a_pre, b = b_pre,
γ
=
γ
_pre
c_post calc = √
a
2
+ b
2
- 2
ab
cos
γ
,
a = a_post, b = b_post,
γ
=
γ
_post
c_predictive = √
a
2
+ b
2
- 2
ab
cos
γ
,
a = a_pre, b = b_pre,
γ
=
γ
_post
Table 1: Postoperativ length compared with predictive leg length.
All measurements are given in cm.