summary Four important variables that help determine the stability of THA component design component position soft-tissue tensioning soft tissue function Component Design Femoral component design large femoral heads decreased dislocation rates due to login to view 17 more bullets femoral offset see "soft tissue tensioning" below femoral neck-shaft angle increasing neck shaft angle (more valgus) compared to native anatomy can increase leg length and decrease offset decreasing neck-shaft angle (more varus) compared to native anatomy can decrease leg length and increase offset Acetabular component design elevated rim liner a posteriorly placed elevated rim liner may increase joint stability lateralized liner increases soft-tissue tension by increasing offset has been shown to increase the risk of acetabular component loosening Component Position Acetabular position recommendations anteversion login to view 1 more bullet abduction login to view 1 more bullet medialization of the cup increases moment arm of the abductors (gluteus medius & gluteus minimus) login to view 1 more bullet caveats surgical approach may affect optimal position of implants login to view 2 more bullets hypertrophy of the anterior inferior iliac spine may cause component impingement and instability complications excessive retroversion login to view 1 more bullet excessive anteversion login to view 1 more bullet excessive abduction (high theta angle, vertical cup) login to view 2 more bullets excessive adduction (low theta angle, horizontal cup) login to view 2 more bullets Femoral stem position recommendations 10°- 15° of anteversion caveats more difficult to adjust femoral component version in uncemented femoral components Combined version definition femoral component anteversion plus acetabular component anteversion recommendations 37 degrees Soft Tissue Tensioning Abductor Integrity gluteus medius serves as the major hip abductor muscle; insufficiency/tear leads to Trendelenburg gait gluteus maximus and/or fascia lata transfers can be utilized in chronic abductor insufficiency Restoration of offset definition perpendicular distance from femoral head center of rotation to the axis of the femur importance increased offset leads to login to view 3 more bullets decreased offset may lead to login to view 3 more bullets increasing offset improves hip stability techniques to increase offset increasing length of femoral neck decreasing neck-shaft angle medializing the femoral neck while increasing femoral neck length trochanteric advancement alteration of the acetabular liner (see "component design" above) Soft Tissue Function Three main factors controlling proper soft tissue function central nervous system pathology that affects the central nervous system login to view 8 more bullets peripheral nervous system pathology that affects the peripheral nervous system login to view 4 more bullets local soft tissue integrity trauma myoligamentous disruption deconditioning aging process poor health irradiation osteolysis collagen abnormalities myopathy malignancy infection