How is the 'offset' of a prosthetic hip implant defined and why is it important?

• A. Lateral distance between center of rotation and femoral shaft axis; influences soft tissue tension and abductor function and risk of dislocation
• B. Vertical distance from femoral head to acetabular rim
• C. The distance from the stem tip to the greater trochanter
• D. The width of the prosthetic cup

Answer: (A)

Offset is the lateral distance between the hip joint center (center of rotation) and the axis of the femoral shaft. This spacing sets the abductor lever arm: a larger offset increases the lever arm, so the gluteal muscles can generate hip abduction torque with less muscle force, improving gait and joint stability and reducing dislocation risk. Restoring the native offset helps reproduce normal soft tissue tension and abductor function; too little offset shortens the lever arm, increasing demand on the abductors and raising the risk of limp or instability, while too much offset can over-tension soft tissues. The other distances described (vertical distance from the femoral head to the acetabular rim, stem tip to the greater trochanter, or cup width) do not define offset.