Introduction: During total hip arthroplasty various femoral stem offsets are available and the femoral stem can be placed in either varus or valgus. The overall effect of this is to increase or decrease the functional offset at the hip joint. Many authors have investigated the effects of the functional offset upon the reconstructed hip joint. To our knowledge no studies have concentrated on the effects, if any, upon the loading and function of the knee joint. The aim of this study was to investigate the effects, if any, of reducing functional offset at the knee.

Materials and methods: To study the effects of alterations in functional offset during hip arthroplasty, a biomechanical computer model was constructed. Normal lower limb anthropometric measurements available in the literature were used within this program. The model thus constructed calculated the effect of different functional offsets upon moments about the knee in stationary standing on one leg. The model also allowed for different varus/valgus placement of the stem.

Results: Reducing prosthetic neck length reduces the moment arm created by the ground reaction force about the knee axis. Placing the stem in valgus reduces the moment arm. This reduction depends upon the length of the neck.

Discussion: In the one legged stance, the line of weight is offset in the coronal plane from the AP axis of the knee joint producing an adducting moment about the knee, balanced by tension in the lateral collateral ligament of the knee and the iliotibial tract. Any reduction in this moment arm would alter the loading of the knee, altering the value of the force on the lateral compartment of the knee. Indeed, it is possible to develop a situation where the knee moment arm creates an abducting moment about the knee, increasing the loading of the lateral compartment of the knee. This may lead to valgus malalignment of the knee.

During total hip arthroplasty various femoral stem offsets are available. Additionally, the femoral stem can be placed in either varus or valgus. The overall effect of this is to increase or decrease the functional offset at the hip joint. To our knowledge no studies have concentrated on the effects of these variations in offset, if any, upon the loading and function of the knee joint. The aim of this study was to investigate the effects, if any, of reducing functional offset at the knee.

A computer model was constructed to study the effects of variations in functional offset in different anatomical settings.