LIVRE JDG 2010

But it can be difficult to make the correct bone

cuts within the range of 1-2 mm.

Implant designs have their specific contact

point which is determined by the deepest part

of the bearing insert at a certain position mea-

sured at the anteroposterior distance of the

tibial plateau for instance at 60%. The problem

during surgery is to control this contact point

since the position of the femur on the tibia can-

not easily be measured at the back of the knee

joint. We therefore developed a simple tech-

nique to check the contact point in 90 degrees

of flexion by indirectly measuring the step off

between the distal cut of the femur and the

anterior edge of the tibia with a spacer in place

after all the bone cuts are made.

During laboratory tests of an anatomically-

designed cruciate retaining knee implant the

above described spacer technique was used to

check correct the PCL balancing during

implantation of the knee implant. Knee kine-

matics were measured passively and in a

muscle-loaded oxford rig simulating a weight

bearing squat.

The goal of this paper is to present prelimina-

ry data of some kinematic aspects of the

Journey CR implant design and explain the

PCL balancing technique and show some

results of this surgical approach.

MATERIALS AND

METHODS

For this study, eight fresh frozen full leg cadaver

specimen were used. The methodology followed

for the experiments was largely similar to the

detailed description given by Victor

et al.

[8].

Prior to the experiment, with the legs still fro-

zen, two bone pins were inserted bicortically in

both femur and tibia and frames with four

reflective spherical markers were fixed to the

pins. A CT scan of the full leg was then made

with the frames in place on a 64-row multide-

tector computed tomography (MDCT) scanner

(General Electric Lightspeed VCT, Milwaukee,

WI). Coordinate systems for femur and tibia

could be defined to describe the relative femo-

ro-tibial kinematics during the tests, based on

the tracked marker trajectories.

Twenty-four hours before the experiment, the

legs were taken from the freezer to thaw over-

night. With a computer assisted system

(PiGalileo) the mechanical axis of femur and

tibia were determined and laxity tests were

performed. Then, the femoral head and the

ankle were removed and the femur and tibia +

fibula were cut to a length of 32cm and 28cm

respectively. Both bones were cleaned and

embedded in aluminum fixtures with PMMA,

paying attention to proper alignment in the

coronal and sagittal planes. Afterwards, the

quadriceps tendon was dissected, stripped

from all muscle tissue and securely fixed in a

clamp. Also the medial and lateral hamstrings

tendons were dissected and suture wires were

attached to be able to load the hamstrings

during the experiment.

Finally, prototype components of a new

Journey CR implant (Smith & Nephew,

Memphis, TN) were implanted using naviga-

tion. With the computer assisted system the

knee prosthesis was implanted using a measu-

red resection technique resecting the same

amount of bone equal to the prosthesis thick-

ness in extension and flexion. A three degrees

external rotation jig was used to determine the

femur rotation. A bony island around the PCL

was preserved and all ligaments were intact

after finishing the bonecuts.

The surgical PCL balancing technique was

based on the preferred contact point in the

insert of the knee implant. The engineers of the

knee system can calculate per tibial base plate

size what the absolute distance from the

contact point to the anterior edge of the tibia is.

With a same size femur projected on the tibia

one can also calculate the distance of the distal

femur cut bone surface to the anterior edge of

the tibia. For the Journey Cruciate Retaining

knee implant tested the size 3-4 had a step-off

of 17mm, size 5-6 18mm, size 7-8 19mm

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