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VIERNES / FRIDAY
190
LIGAMENT BALANCING IN
THE VALGUS KNEE
Leo A. Whiteside, MD
Missouri Bone and Joint Center St. Louis,
Missouri, (USA)
The cornerstone to correct ligament balancing is correct varus
and valgus alignment in flexion and extension. For alignment
in the extended position, fixed anatomic landmarks such
as the intramedullary canal of the femur and long axis of the
tibia are accepted. When the joint surface is resected at an
angle of 5° to 7° valgus to the medullary canal of the femur
and perpendicular to the long axis of the tibia, the joint surfaces
are perpendicular to the mechanical axis of the lower extremity,
and roughly parallel to the epicondylar axis. In the flexed position,
anatomic landmarks are equally important for varusvalgus
alignment. Incorrect varus-valgus alignment in flexion
not only malaligns the long axes of the femur and tibia, but
also incorrectly positions the patellar groove both in flexion
and extension.
Finding suitable landmarks for varus-valgus alignment has
led to efforts to use the posterior femoral condyles, epicondylar
axis, and anteroposterior (AP) axis of the femur. The posterior
femoral condyles provide excellent rotational alignment
landmarks if the femoral joint surface has not been worn or
otherwise distorted by developmental abnormalities or the
arthritic process. However, as with the distal surfaces, the
posterior femoral condylar surfaces sometimes are damaged
or hypoplastic (more commonly in the valgus than in the varus
knee) and cannot serve as reliable anatomic guides for alignment.
The epicondylar axis is anatomically inconsistent and
in all cases other than revision total knee arthroplasty with
severe bone loss, is unreliable for varus-valgus alignment in
flexion just as it is in extension. The AP axis, defined by the
lateral border of the posterior cruciate ligament posteriorly
and the deepest part of the patellar groove anteriorly, is highly
consistent, and always lies within the median sagittal plane
that bisects the lower extremity, passing through the hip,
knee, and ankle. When the articular surfaces are resected
perpendicular to the AP axis, they are perpendicular to the
AP plane, and the extremity can function normally in this plane
throughout the arc of flexion.
In the valgus knee with significant posterior deformity or erosion,
the posterior femoral condyles are unreliable as rotational
alignment landmarks, and the anteroposterior axis provides
a reliable landmark for rotational alignment of the femoral
surface cuts.
TECHNIQUE FOR FEMORAL BONE RESECTION
Intramedullary alignment instruments usually are used for
the femoral resection. The distal femoral surfaces are resected
at a valgus angle of 5-7°. A medialized entry point generally
is advised because the distal femur curves toward valgus in
the valgus knee. The current technique is to reference the
resection from the distal medial femoral surface. The distal
femoral cutting guide is seated on the distal surface of the
medial femoral condyle, which is resected equal to the
thickness of the distal condylar surface of the implant. If the
distal lateral femoral condylar surface is deficient, considerably
less is resected from the lateral surface than from the medial
surface, and in many cases of a severe valgus angle, no
bone is present to resect from the distal lateral surface. Seating
on bone is necessary on the lateral distal side, but this
can be accomplished with the anterior lateral bevel surface.
In cases of severely deficient lateral femoral condylar bone
stock, the anterior bevel surface is the only bony contact for
the distal lateral surface of the femoral component. This
leaves a gap that is filled with bone graft between the distal
bone surface and the inner surface of the implant on the
lateral side. When the posterior flange and the anterior bevel
surfaces are seated on viable bone, the distal defect can be
treated as a contained defect and needs no structural grafting.
Rotational alignment of the distal femoral cutting guide is
adjusted to resect the anterior and posterior surfaces perpendicular
to the anteroposterior axis of the femur. The AP axis
is drawn and the femoral cutting guides are aligned to make
the cuts perpendicular to this line. In the valgus knee this almost
always results in much greater posteromedial than
posterolateral femoral condylar resection.
TECHNIQUE FOR TIBIAL BONE RESECTION
Intramedullary alignment instruments are used to resect the
proximal tibial surface perpendicular to its long axis. Like the
femoral resection, resection of the proximal tibial surface is
based on the height of the intact medial bone surface. A maximum
thickness of 10 mm is removed from the medial tibial
plateau, which often leaves the defect on the lateral side of
the tibia that requires a bone graft. Use of a long-stem tibial
component and screws in the tibial tray securely fixes the tibial
component, and obviates the use of fixed structural bone graft.
LIGAMENT RELEASE TECHNIQUE AND DIFFERENTIAL
BALANCING
Stability is assessed first in flexion by holding the knee at 90°
flexion and maximally internally rotating the extremity to stress
the medial side of the knee, then maximally externally rotating
the extremity to evaluate the lateral side of the knee. A medial
opening greater than 4 mm, and a lateral opening greater than
5 mm, is considered abnormally lax, whereas an opening less
than 2 mm on either side is considered abnormally tight. The
knee then is extended and stability is assessed in full extension
by applying varus and valgus stress to the knees. A medial
opening greater than 2 mm and a lateral opening greater
than 3mm is considered abnormally lax, whereas an opening
less than 1 mm on either side is considered abnormally tight.
TIGHT LATERALLY IN FLEXION ONLY
In knees that are too tight laterally in flexion, but not in extension,
the LCL is released in continuity with the periosteum