Femoral Offset
J. Roderick Davey, M.D.,
F.R.C.S.(C) Head, Division of
Orthopaedic Surgery Toronto Western Hospital Toronto, Ontario Definition: Femoral
offset is the distance from the center of rotation of the femoral head to a line
bissecting the long axis of the femur. The offset of the femoral component is
the distance from the center of rotation of the femoral head to a line
bissecting the long axis of the stem. Mechanical Effects: Femoral
offset can be decreased following total hip arthroplasty compared to the
preoperative anatomy. A decrease in femoral offset moves the femur closer to
the pelvis, which can result in impingement at the extremes of movement.
Moving the femur medially also results in soft tissue laxity. Both of these
problems can cause instability and possible dislocation. Increasing femoral
offset which moves the femur laterally will decrease impingement and improve
soft tissue tension resulting in better stability without lengthening the
leg. Biomechanical Effects: When
the offset decreases, greater force is required by the abductor muscles to
balance the pelvis, which may result in a Trendelenburg limp. Resultant force
across the hip joint also increases with decreased offset which could result in
greater ployethylene wear and loosening over time. An increase in femoral
offset decreases the force required by the abductor muscles to balance the
pelvis, which will improve gait. As well, resultant force decreases with
increased offset, which may result in less wear and loosening over time. Previous
studies have shown that the stress in the bone, cement, and femoral component
are not adversely affected by an increase in femoral component offset. The
placement of the acetabular component, acetabular size, neck osteotomy
location, prosthetic neck length, and offset can all be determined by
preoperative templating. Varus femurs will tend to have increased femoral
offsets while valgus femurs will tend to have decreased femoral offsets. In order to achieve greater offset the surgeon has
two options: 1)
Use a longer neck component and make a lower osteotomy of the femoral
neck. 2)
Use a femoral component with increased femoral offset. The disadvantage of the first option is that more
bone has to be removed and the longer head neck component may have a skirt
which can decrease range of movement and impinge on the polyethylene
resulting in wear. Clinical Need: Studies indicate that femoral offset can vary
between 30 amd 60 mm. Preoperative templating indicates that a high offset
component may be needed in approximately 60% of cases. Intraoperative trial
reduction using the regular and high offset neck trials can identify
situations where the high offset component increases stability even though
the need was not demonstrated on templating.  Previous Lecture |
