When a patient presents for revision hip surgery in the setting of massive acetabular bone loss, the surgeon should go through a check list of items to avoid potential pitfalls. At the initial encounter with the patient, a thorough history is performed. Most patients on average have had two or more prior acetabular revisions. Prior studies have shown that triflange cups are more commonly used in females than males.¹ A physical examination, documenting the patient’s prior incisions as well as the neurovascular status of the surgical leg, is important. Radiographs of the pelvis should include the AP and Judet views to determine the extent of the patient’s acetabular bone defects. A CT scan is obtained with 3D reconstructions and often a bone model on patients with severe bone loss disrupting either the acetabular rim, the anterior or posterior column, the medial wall, or significant migration of the failed component. Next, laboratory data include an infection workup (ESR, CBC, CRP), a hip aspiration for culture and fluid analysis, a nutrition assessment, an anemia workup if the patient hematocrit is less than 30, as well as the standard liver profile and electrolytes.

Major acetabular reconstruction is challenging for the operating surgeon regardless of which implant is selected. However, at our institution there are two groups of patients who have a higher complication rate after a complex acetabular reconstruction. The first is elderly patients who are malnourished. There is no obvious sign of malnutrition; however, laboratory data reveal low protein levels such as serum albumin, total protein, and iron transferrin. The second is undiagnosed or untreated patients with anemia. Blood loss tends to be much greater in these reconstructions because of the extended time of exposed pelvic bone bleeding. Furthermore, it is common for the femoral implant to be revised in these procedures adding to increased surgical blood loss. This increased surgical time often results in the need for intraoperative blood transfusion. Therefore, it is paramount that preoperative malnutrition and anemia be addressed and corrected prior to proceeding with the revision procedure.

Indications for the use of a custom triflange component will vary depending upon the individual surgeon’s comfort level with custom implants. Custom triflange acetabular components are commonly used in salvage procedures where standard heroic revision components have failed such as a “cup-antiprotrusio cage” construct or a large structural allograft. The classic indications for a triflange cup in revision total hip arthroplasty include Paprosky type 3B defects, pelvic discontinuity, and salvage of prior failed porous metal augments, failed structural allografts, or failed cup/antiprotrusio cage constructs.²

Most information regarding acetabular bone loss can be gleaned from a simple AP radiograph. At our institution, we loosely use the Paprosky defect classification system noted in³ Table 1. The important structures to identify are anterior and posterior columns, the superior weight-bearing dome, and the medial wall. These acetabular supporting structures are affected by osteolysis, implant migration, and the effects of bone cement. The extent of bone loss related to these structures can be inferred on plain radiographs because of the disruption of anatomic landmarks such as the teardrop and Kohler’s line. Also, the degree of superior and medial migration of the hip center is suggestive of the amount of bone loss. Judet views of the pelvis improve visualization of the anterior and posterior columns and can be helpful to assess these structures more easily. A Paprosky 3B defect is described as severe lysis with the loss of the teardrop, superior migration of the hip center > 2 cm, disruption of Kohler’s line, and severe lysis of the ischium (Figure 1). A CT scan with 3D reconstructions will show this degree of bone loss easily (Figure 2). A 1:1 bone model is created from the CT scan using stereolithography (Figure 3).
The reason we use the Paprosky system somewhat loosely is that there are cases where plain radiographs and CT scans are misleading. Often the bone surrounding an osteolytic lesion may look functional; however, the structural viability of that bone to support the load of an acetabular implant is compromised. Also, in patients where a prior structural allograft is present, this bone may look completely normal on radiographs but is completely nonviable at the time of revision surgery. Determining the viability of this allograft bone adjacent to a failed antiprotrusio cage or acetabular bone cement is difficult (Figure 4).

The decision to use a custom triflange cup is multifactorial. The surgeon must be well versed on complex acetabular revision surgery. The surgeon must have the availability of adequate staff in the operating room including a strong anesthesia team capable of managing significant blood loss if it occurs. The waiting time between the decision for surgery and when the custom implant will be available may not be optimal for the appropriate care of the patient. The total process time typically takes 8 to 16 weeks; however, with the advent of 3D printing, the wait time may be substantially reduced. Lastly, there are other ways to manage severe acetabular bone loss with off-the-shelf implants and cages that the individual surgeon may prefer.

The preparation and manufacturing of a custom triflange cup begins with a thin slice CT scan. Each manufacturer has their own proprietary protocol for this scan, so the surgeon should contact the manufacturer to provide the radiology team with this information. The protocol is usually some variant of 2-mm slices skipping 1 or 2 mm between slices. After the CT scan the implant manufacturer creates a 1:1 bone model of the hemipelvis. Typically, bone is removed from the model that is deemed nonsupportive using a cutoff level of Hounsfield unit of radiodensity. This region of bone is usually demarcated on the bone model. The bone model is void of any prior implants or hardware. Once the surgeon examines the hemipelvis model, the surgeon marks the regions on the model for flange placement (Figure 5). Additional bone can be marked for removal at this time to ease the placement of the implant during surgery. The minimal flange thickness is 6 mm based upon nonpublished finite element studies. This flange thickness allows for minimal contact between the hemisphere of the component and host bone in the true acetabulum. Minimizing this contact by design or by overreaming the acetabulum during surgery ensures that the cup will not restrict the intimate contact of all flanges on bone.

Generally, the greater the bone loss, the larger the iliac flange. The iliac flange provides the majority of the surface area for support of the implant. This flange can extend further, either in the anterior to posterior dimension or the superior dimension. However, the further
superiorly this flange extends, the more extensive the surgical dissection and the greater risk of superior gluteal neurovascular injury. Most surgeons prefer two rows of screws with at least three to four screws per row on the iliac flange. This ischial flange has a lateral and posterior surface. The author usually uses both surfaces for flange orientation. Ischial screw pullout is one of the reported complications after surgery and is probably related to underestimating the degree of ischial lysis. Therefore, most surgeons tend to secure the ischial flange with three or four screws. Some surgeons prefer locking screws for the ischium, whereas other surgeons have used bone cement in the ischial lytic lesions to improve fixation.⁴ The ischial flange should be rounded to avoid a sharp edge causing injury to the closely approximated sciatic nerve. This complication occurred in a patient early in the author’s experience with triflange cups at about 5 years postoperatively.⁵ The pubic flange provides a third arm of contact for the construct and helps to provide a reference point to orient the implant during surgery. The author has found that placing a screw in this flange is unnecessary and therefore has stopped using pubic screws without any change in the clinical or radiographic results.