Discussion
We have described a novel technique that facilitates reconstruction of major osseous deficiencies of the pelvis. The technique involves cementation of a multihole, metal acetabular shell into a cage or ring, permitting use of modular liners. The modularity in this approach to major hip reconstruction provides stability-optimization options that are not available with APCs. So far, the technique has demonstrated more advantages than disadvantages, so the indications for its use would be whenever a cage is used for pelvic reconstruction. Traditional techniques involve cementing an APC into the cage or ring. Use of multihole, metal shells for this purpose has several theoretical advantages. Multiple holes and the textured surface allow more interdigitation of cement with cup than APCs do; this interdigitation may improve the durability of the cemented interface. Cement also extrudes through the holes of the cage to secure the cup to the pelvis, as is done with cementation of APCs. Introduction of trabecular metal shells may also provide an even more secure bond to the shell, compared with APCs, though durability of a cemented trabecular metal interface has not been established. In addition, mechanical alignment guides cannot fasten as securely onto some APCs.
Nonmodular, cemented, metal-backed acetabular components, which were commonly used in hip arthroplasties at one time, were abandoned because of their relatively high loosening rate and because of advantages noted with modular components.5 The nonmodular components had been developed because of their theoretical advantages of improved distribution of forces into the cement mantle.5,6 However, those models had a relatively smooth metallic surface, which probably did not bond as well to cement as the shells used with the technique described in this article.
Dislocations can occur because of inadequately placed cups. Metallic cups can be improperly positioned, as can APCs. An advantage of the technique we have described over APCs is that liners with raised rims can be inserted with the apex placed wherever needed to best address instability. Dislocations can also occur because of factors such as inadequate offset and cognitive impairments. Our technique allows use of offset liners and constrained liners. Although these options may not prevent further dislocations, they often mitigate instability issues. Constrained liners and lateralized liners can be easily placed, and elevated rims can be swiveled as needed for stability. As use of cementless, metal-backed, modular acetabular components is common in primary THAs, most surgeons are familiar with the modular liner options available with use of the technique described in this article.
In this setting, modular, metal acetabular shells have the advantage of allowing surgeons to use the alignment guides they are accustomed to using. Modularity is another significant advantage over APCs. When an APC wears down, the component must be extracted to permit implantation of a new APC. With metal shells, a worn liner can be exchanged relatively easily. Modularity also gives surgeons many more options for addressing instability. Elevated rims can be moved, head sizes can be changed, and lateralized or constrained liners can be implanted easily. By comparison, with APCs, stability can be addressed only by modifying the femoral component or taking hip precautions which restrict range of motion of the hip. Modification of the femoral component is not possible with nonmodular femoral components in place (Figure 5). A potential disadvantage of this technique is increased cost associated with use of another component.
This small series of patients has had an excellent rate of success with cementation of multihole, metal-backed acetabular components into a cage or ring. These components may offer more secure fixation than APCs to cement extruded into the multiple holes, and improved metallurgy, such as trabecular metal. Surgeons who want to use modular components may prefer this technique because it allows them to select from various liner options. Surgeons should consider this technique for patients who need major pelvic reconstruction, though a larger study with longer follow-up is needed to determine its long-term durability.
Although the novel technique we have described has been helpful in our experience, this study had several limitations—small series, retrospective study, relatively short follow-up, lack of control group and functional data—that may have affected its conclusions. Further study and follow-up are needed to better determine the utility of this technique in clinical practice.