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Adequate exposure of the proximal femur must be obtained.
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Use templating as a guide for neck resection and component sizing.
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Assess the patient’s bone stock before reaming and broaching.
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Assess the seating of the component using visual and auditory clues.
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Be cognizant of the possibility of iatrogenic fracture.
Achieving optimal cementless fixation of the femoral component is essential to the long-term success of the total hip arthroplasty. Optimal fixation affords the patient a predictable and durable result. Gentle, safe, and adequate exposure is required for proper component placement and for obtaining optimal initial fixation of the femoral component. Several successful cementless techniques exist, such as the proximal-fit tapered wedge, proximal fit-and-fill, distal fixation with a fully coated stem, and modular variations of these stems. The goal of a tapered stem ( Fig. 19-1 ) is to create a tight wedge-fit in the coronal plane using the tapered geometry of the stem. The component does not fill the stem in the sagittal plane. Intuitively, a tapered wedge provides more rotational stability than a component with a rod-shaped contour in an ovoid femoral canal. Another critical advantage of a tapered stem is its ability to subside, even after what appears to be firm impaction at the time of surgery, to a position of stability. This is not a feature of circular, rod-shaped fit-and-fill components.
COMPONENT DESIGN
The femoral component must be designed with several key characteristics in mind. These are the ability to achieve predictable, long-term fixation to the bone, close approximation to the modulus of elasticity of bone, offset options, optimal head-neck ratio, and variable neck lengths.
Fixation
Predictable, long-term fixation of cementless tapered stems in a variety of patient cohorts has been demonstrated in the recent literature.
Modulus of Elasticity
The primary materials used for femoral stems are cobalt-chromium and titanium alloys. Both alloys have demonstrated excellent mid- and long-term results. Theoretically, the modulus of elasticity of titanium more closely approximates that of bone and may lead to less thigh pain. However, this has yet to be borne out in clinical studies of this type of implant.
Offset
The femoral component should be available with standard and lateral offsets for each component size. The mechanical advantage of the gluteus medius and minimus is increased, and impingement of the femur against the pelvis is reduced with lateral offset components. Depending on how offset is achieved, the offset component may also shorten the leg while maintaining soft tissue tension. Thus, the surgeon is given more options for achieving both leg length equality and hip stability.
Head-Neck Ratio
Impingement of the femoral neck on bone or the acetabular component can lead to instability. One feature that may minimize impingement is reducing the neck diameter of the femoral stem, thereby maximizing the head-neck ratio. The ability to use a larger femoral head also combines to achieve this goal.
Neck Length
A graduated increase in neck length as the size of the femoral component increases helps to avoid the use of skirted femoral necks, thus reducing the likelihood of impingement ( Fig. 19-2 ).
