ARUP; ISBN: 978-0-9562121-5-3 - CMBBE 2012 - Cardiff University

Crest axis – a best fit line to the silhouette curve of the anterior mid tibia in the sagittal
plane
Lateral condyle low point – lateral AP contour (20% of ML Width) midpoint projected
to tibia surface
Neutral axis – line connecting the tibial tubercle with the PCL attachment point
Femur
Distal anatomic axis – line connecting the centers of best fit circles to the periosteal
contours at cross-sections cut at 1/3 and 1/6 of the femur length from distal.
Mechanical axis – line connecting the head center to the lowest point of the trochlear
groove silhouette curve
Posterior plane – plane tangent to most posterior points of medial and lateral condyle
Distal plane -– plane tangent to most distal points of medial and lateral condyle
Most Distal Condyle – the further point of the medial and lateral condyle along the
mechanical axis
Anterior cortex – most anterior point above the femoral condyle
The implant surfaces that were in contact with bones were used to construct the surgical
resection planes, assuming the effect of the cement mantle is negligible. The surgical
TKA resection parameters were assessed by measuring the resection planes with respect
to the anatomical references, including:
1) tibial varus/valgus (V/V) angle: the coronal angle between the resection plane
and the plane perpendicular to the mechanical axis of tibia;
2) tibial posterior slope: the sagittal angle between the resection plane and the
tibial crest axis;
3) tibial resection depth: the length from the lowest point of the lateral plateau to
the resection plane along the direction of the tibial crest axis border;
4) femoral flexion angle: the sagittal angle of the distal anatomical axis with respect
to the distal resection plane;
5) femoral varus/valgus (V/V) angle: the coronal angle of the mechanical axis with
respect to the distal resection plane;
6) femoral internal/external (I/E) angle: the transverse angle between the posterior
tangent plane and the implant AP axis;
In order to understand the robustness of key TKA dimensions to the variability of
resection parameters, first the average resection parameters across all bones were found.
Resection contours were then recreated for all bones using the average parameters. The
ML and AP dimensions of the contours were measured as shown in Figure 1A for both
the actual resection contour and the contour generated with the mean parameters. For
the tibia the ML width (ML) was defined as the mediolateral length of the contour
bounding box, which was constructed in a 2D coordinate system with the Y axis
oriented along the neutral rotational axis. The medial and lateral AP dimensions (MAP
and LAP) were defined as the AP lengths of the contour, measured at either 25% or
75% of the ML.
The mean resection parameters were then perturbed by ±1 standard deviations for the
varus/valgus angle and posterior slope individually. This resulted in an additional 44
contours which were visually checked to rule out poor quality, giving a total 39 proofed
tibial contours. The variations of ML and AP dimensions due to surgical variability in
tibial alignment were investigated by assessing the magnitude of deviations between