Uncemented Acetabular Components

Key Concepts

Choosing an uncemented acetabular component over a cemented acetabular component for total hip arthroplasty (THA) is largely surgeon preference in the majority of patients.
The vast majority of acetabular components (cups) implanted in the United States are uncemented owing to excellent clinical outcomes, decreased operating time, ability to add adjuvant screw fixation for primary stability, and versatility of use with the availability of differing head diameter and bearing surface liner options.
Advantages of uncemented acetabular components:
- Survivorship: Excellent long-term survivorship in clinical series and registries
- Versatility: Can be used in nearly any case
- Decreased operating time
- Acetabular version or hip center of rotation can be altered after cup insertion by changing the cup position or by using different liner options
- Ease of modular liner exchange if that should be needed for hip stability or other reasons
Disadvantages of uncemented acetabular components:
- Risk of acetabular fracture when impacting component with a press-fit.
- Bone needs to grow into the cup for long-term stability.
- Conventional uncemented cups are contraindicated in irradiated, necrotic, and allograft bone. This problem has been mostly solved with highly porous uncemented cups.
The goal of implanting an uncemented acetabular component is to obtain adequate component stability against adequate bone that will support the component and to obtain long-term fixation by bone ingrowth.
- To aid in initial stability, underreaming the acetabular bone by 1 to 2 mm smaller than the acetabular component will create hoop stresses between the component and bone, stabilizing the component with this press-fit.
- Adding acetabular screws can also increase initial fixation to the acetabular bone and should be used when adequate press-fit is not achieved.
- Almost all uncemented cups have external surfaces that are coated with 3-dimensional matrices that lead to bone ingrowth and/or roughened surfaces that lead to bone ongrowth. Three-dimensional porous surfaces have a strong track record of resisting loosening even if osteolysis occurs.
Two main types of uncemented acetabular components exist for primary and revision THA (Figure 6.1).
- Hemispherical acetabular components have a constant radius of curvature throughout the entire component. Obtaining a press-fit with a hemispherical cup will create relatively uniform hoop stresses at all contact points between the cup and bone.
- Elliptical acetabular components have an increasing radius of curvature of the outer surface of the cup when comparing the center of the component to the edges of the component. Obtaining a press-fit with an elliptical cup will create stronger hoop stresses at the periphery of the cup and weaker hoop stresses at the center of the cup. These cups are usually reamed line to line as the periphery of the cup is usually thicker than the last reamer used.

Sterile Instruments and Implants

Routine hip retractors
Uncemented acetabular component (with instrumentation)

Positioning

Surgeon preference for primary THA; the author utilizes a posterolateral approach with the patient in the lateral decubitus for primary and revision THA.

Surgical Approaches

Uncemented acetabular components can be inserted through any THA approach. Please refer to Chapters 2, 3, 4, 5.

Preoperative Planning

Template acetabular component size and acetabular version. It can be useful to template cup diameter on a cross-table lateral view in addition to an anteroposterior (AP) view (Figures 6.2 and 6.3). An uncemented cup gets most of its press-fit from the anterior and posterior walls, which are best seen on the cross-table lateral view. This is especially helpful in cases in which superolateral defects exist, whereby templating from the AP radiographs might lead to overestimation of cup diameter. Placing
the templated acetabular component from the lateral radiograph on the AP radiograph should give the surgeon an idea as to whether appropriate coverage laterally will be achieved without bone grafting or superior placement of the acetabular component. Getting an idea of the patient’s preoperative acetabular version on this lateral view will also allow the surgeon to judge acetabular component version intraoperatively (Figure 6.4).

Figure 6.1 ▪ Hemispherical cups (shells) have a constant radius of curvature from the center of the cup toward the periphery (dotted line), whereas elliptical cups have a radius of curvature that increases from the center of the cup toward the periphery. In this example, if the acetabulum was reamed with a 54-mm reamer, a 54-mm hemispherical cup would have no press-fit, whereas a 54-mm elliptical cup would have 2 mm of press-fit at the periphery where the diameter is 56 mm and no press-fit at the center of the cup where the diameter is 54 mm. (With permission from Haidukewych GJ, Jacofsky DJ, Hanssen AD, Lewallen DJ. Intraoperative fractures of the acetabulum during primary total hip arthroplasty. J Bone Joint Surg Am. 2006;88(9):1952-1956.)

Figure 6.2 ▪ Preoperative templating on a cross-table lateral reproduces an accurate cup size as this shows the anterior and posterior walls, which provide most of the stability when obtaining a press-fit.

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