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

from cadaver surgeries were recreated in a virtual environment; 2) the resection
parameters of the femora and tibiae were measured with respect to anatomical
references; 3) the variability of resection parameters were assessed; and 4) the impact of
surgical variability of alignment parameters on the mediolateral and anterior-posterior
dimensions of the resected knee were assessed.
3. MATERIAL & METHOD
A total of 11 cadaver specimens (8 Male, 3 Female; 8 Right, 3 Left legs) were
implanted with a single TKA system (NexGen, Zimmer Inc, Warsaw, IN) by three
experienced orthopedic surgeons. All specimens were pre-screened by x-ray to rule out
any gross congenital anomaly or deformity before implantation. A cruciate-retaining
(CR-Flex) femur was used in 5 cadavers and a posterior stabilized (LPS-Flex) in the
other 6 cadavers. Before the surgery was performed, the femur and tibia were potted
mid-shaft in fixation cylinders to be used in other internal studies. In one study (n=7)
anterior referencing instruments were used for the femur and posterior referencing (n=4)
in another. For all tibia bones, extramedullary instruments with standard surgical
parameters were used (7 degrees posterior slope, 0 degrees varus/valgus and 10 mm
resection depth). However all data will be pooled due to the low number of specimens.
Each specimen was scanned by clinical CT before and after TKA with approximately
0.4 mm in-plane resolution and 0.6 mm slice increments. The femora and tibiae were
segmented from the CT scans using commercial software 5 and the pre-operative scans
were registered to post-operative scans. The solid models of the associated TKA
implants were converted to a simulated CT scans and registered to the post-surgical CT
using the “Fast Rigid Registration” feature of 3d Slicer 6 which is based on the “Mattes
Mutual Information” registration metric of the Insight Segmentation and Registration
Toolkit 7 .
The femora, tibiae, and associated implants were imported into in-house software
(ZiBRA) with capabilities for annotating digital bone models and performing virtual
surgery. Bones were annotated using a series of automated algorithms, including
definition of reference planes (proximal/distal, coronal, and sagittal), as well as the
TKA reference landmarks and axes for the femur (mechanical axis, anatomical axis,
posterior plane, anterior cortex and most distal condyle) and the tibia (mechanical axis,
crest axis, and the lowest point of lateral plateau). A brief description of the
landmarking algorithms are listed below:
Tibia
Posterior plane – a plane perpendicular to the anatomic axis touching the posterior
medial and lateral border of the tibia
Proximal Entrance Point – A point at ¾ of the distance between the mid point of the
spines and the tibial tubercle
Mechanical axis- line connecting the proximal entrance point to to the center of the
distal articular surface
5
Amira (Visage Imaging Inc, San Diego CA) & Mimics (Materialise, Leuven, Belgium)
6
3d Slicer (http://www.slicer.org)
7
Insight Segmentation and Registration Toolkit (www.itk.org)