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 head-neck ratio increased definition diameter of femoral head/diameter of femoral neck importance larger head-neck ratios allow greater arc range of motion prior to impingement skirts can be avoided definition skirts are attachments used to extend the length of the femoral neck importance skirts decrease the head-neck ratio jump-distance is increased definition amount of translation prior to dislocation importance large femoral heads are seated deeper within the acetabulum, increasing jump-distance increase in jump-distance increases joint stability a larger femoral head will not compensate for abductor deficiency or a vertically positioned cup femoral offset see "soft tissue tensioning" below 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 5° - 25° abduction 30° - 50° medialization of the cup increases moment arm of the abductors (gluteus medius & gluteus minimus) increased moment arm leads to decreased joint reactive forces caveats surgical approach may affect optimal position of implants posterior approach should err towards more anteversion anterior approach should err towards less anteversion hypertrophy of the anterior inferior iliac spine may cause component impingement and instability complications excessive retroversion posterior dislocation excessive anteversion anterior dislocation excessive abduction (high theta angle, vertical cup) posterior superior dislocation eccentric polyethylene wear and late instability excessive adduction (low theta angle, horizontal cup) impingement in flexion inferior dislocation 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 decreased impingement decreased joint reaction force increases soft tissue tension without increasing leg length decreased offset may lead to instability abductor weakness gluteus medius lurch 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 stroke cerebellar dysfunction dementia MS Parkinson's myelopathy delirium alcoholism peripheral nervous system pathology that affects the peripheral nervous system spinal stenosis (gluteus medius is L5) peripheral neuropathy radiculopathy paralysis/paresis local soft tissue integrity trauma myoligamentous disruption deconditioning aging process poor health irradiation osteolysis collagen abnormalities myopathy malignancy infection