Femoral offset is the distance from the center of rotation of the femoral head to a line dissecting the long axis of the femur. In case of total replacement hip the the offset is considered as the distance from the center of rotation of the femoral head to a line bissecting the long axis of the stem.
Normal femoral offset varies between 30 amd 60 mm.
Image Credit: http://www.traumazamora.org/articulos/offset/offset.html
A decrease in femoral offset would move the femur closer to the pelvis medially. This can lead to impingement of greater trochanter in extremes. Medial movement would also result in soft tissue relaxation. Both of these factors can lead to instability of the implant and possible dislocation.
Moreover, when the offset decreases, greater force is required by the abductor muscles to balance the pelvis and resultant force across the hip joint also increases resulting in greater wear and tear.
An increase in femoral offset moves the femur laterally resulting in decrease chances of impingement, a better tension in soft tissues and better stability.
A change in femoral offset does not affect the leg length and thus provides a measure to make adjustment without altering the leg length the leg.
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.
In total hip replacement surgery, the offset needs to be determined preoperatively for better planning of the surgery.
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