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Cup-cage construct provides options for large defect acetabular revisions
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In most acetabular revisions, an uncemented hemispherical socket with screws can be used (Figures 1 and 2). At the Mayo Clinic in Rochester, Minn., we routinely use morselized allograft for small cavitary defects and we use screws in 100% of our revision cases. However, in the past 15 years, we have learned that not all ingrowth surfaces are equal. There are limitations to the use of uncemented hemispheres alone, such as cases that involve huge deficiencies in which an allograft bears most of the cup, pelvic discontinuity, and in pathologic bone.
Figure 1. In this radiograph showing dislocation, the patient has a loose socket with superior lateral bone loss.
Figure 2. A postoperative radiograph demonstrating acetabular reconstruction with structural allograft and an uncemented socket with secondary screws is shown here.
Images: Trousdale RT
In the early- and mid-1990s, we migrated to using large antiprotrusio cages during revision total hip arthroplasty with massive defects. However, limitations in containing cages include the lack of osteointegration and potential loosening if the socket bears enough load over time. We have abandoned the use of cages alone in our acetabular revisions.
We, and other centers, have migrated to using highly porous metals and we have predominantly used tantalum for acetabular revision cases with marked bone loss, pelvic discontinuity and salvage of failed cages. In patients with massive defects, selected pelvic discontinuities, and in pathologic bone, we have now migrated to using the cup-cage concept.
Figure 3. This patient had a dislocated hip with a malpositioned socket and marked superior and medial acetabular bone loss.
Figure 4. Marked acetabular bone loss can be seen.
Figure 5. Surgeons performed an acetabular reconstruction with a highly porous metal cup with secondary screw fixation.
Figure 6. The cage is placed external to a highly porous metal socket.
Figure 7. This postoperative radiograph
illustrates a cup-cage reconstruction.
The concept of this technique is to utilize an uncemented highly porous metal implant against as much native host bone as possible using secondary screws for fixation of the socket. If fixation with screws does not provide enough fixation, we put a cage over the top of the acetabular component securing screws through the cage, through the highly porous metal cup in the bone, as well as screws in the ilium and into the ischium (flange fixation). Occasionally, we cut off the ischial flange and just provide screws through the cup and onto the ileum depending on the anatomy of the bone. The steps are typical for any revision surgery: remove the old implant; thoroughly clean the bone and sequentially ream to fit in an uncemented hemisphere. Morselized allografts, structural allografts or metal augments can be used to obtain optimal stability of the hemisphere if needed. The hemisphere is impacted into place, secured with multiple screws through screw holes and occasionally through the tantalum socket. We then, if needed, place the cage on the external aspect of the hemisphere, put screws through the cage through the tantalum cup into the host bone and put as many screws into the ileum as can be obtained. This provides secondary fixation of the uncemented acetabular component until hopefully we get osteointegration into the cup. One can put the ischial flange down into the ischium or alternatively cut the ischial flange off and just use screws into the cup and into the ileum (Figures 3 through 7).
In conclusion, our current attitude is to use uncemented hemispheres in most of our revision cases. In scenarios where routine uncemented hemispheres are going to be unreliable, as in patients with massive defects, pelvic dissociation or pathologic bone, we use highly porous metal screws and augment that with the use of a cage if extra fixation is needed.
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Disclosures: Lewallen works for Mayo Clinic and develops implants for Zimmer, OrthoSonic and Medtronic in association with Mayo Clinic. Trousdale has no conflicts related directly to this paper; however, he receives royalties from DePuy, Mako and Wright Medical.