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Jumbo cup and modular porous metal augments for chronic pelvic discontinuity
Background
Chronic pelvic discontinuity, separation of the superior ilium from the inferior ischiopubic segment through the acetabulum with loss of host bone, is one of the most challenging issues to manage with respect to acetabular revision surgery. It is estimated that chronic pelvic discontinuity is the cause of up to 5% of revision total hip arthroplasty (THA) procedures. Construct survivorship is a function of the degree of bone loss and the presence of a chronic pelvic discontinuity at the time of treatment. In this setting, achieving adequate acetabular component stability is particularly difficult; the amount of remaining bone stock, the residual bone’s biologic in-growth potential for cementless implants, and the healing potential of the discontinuity are three important factors that must be taken into consideration. ,
There are many options available for the treatment of chronic pelvic discontinuity, including the use of a porous metal jumbo cup with modular augments. Porous metal implants facilitate bony in-growth and promote long-term stability via biological fixation. Promising results have been shown in revision acetabular reconstruction with severe bone loss using porous metal cups with and without augments. A literature review showed that porous metal cups and augments have the lowest re-revision rates and loosening in the early to mid-term and that reconstruction with an antiprotrusio cage is the most reported technique.
The Australian Orthopaedic Association National Joint Registry reports a very high re-revision rate of 36.3% for all causes ten years after the first revision THA. In an effort to improve outcomes, the authors have focused their research on improving acetabular component stability. Radiographic studies have established that early cup stability is a predictor for survivorship in both studies of primary and revision THA. Proximal translation greater than 1 mm within the first 24 months correctly identified 81% of cups to be either loose or not loose at re-revision surgery. An early predictive migration threshold of loosening enables surgical techniques to be evaluated objectively with fewer patients and much shorter clinical follow-up.
Using the most accurate radiographic measurement method available, radiostereometric analysis (RSA), the authors have repeatedly reviewed the results of sequential surgical technique changes over time. A number of cups in our initial series were found to migrate greater than the acceptable early migration threshold, and cups that treated larger bone defects were associated with a larger amount of early migration. This was a common finding in a scoping review of all RSA studies of revision cups. Following a review of RSA results and fixation failure in our initial series, the surgical technique was modified to incorporate the use of inferior screw fixation in cases considered necessary by the surgeon. RSA results confirmed there was improved stability with the use of inferior screws.
More recently, the authors have used a combination of RSA and computed tomography (CT) imaging and determined that adequate press-fit fixation or three-point fixation is associated with acceptable amounts of early migration. In this chapter, we discuss our current surgical technique using porous tantalum cups with modular augments to treat chronic pelvic discontinuity, including lessons learned during the refinement of the technique over a long period. Premise: This technique relies upon using a porous metal jumbo cup in conjunction with modular porous metal augment(s) while obtaining three-point fixation and placing long ‘intramedullary’ screws into the superior and inferior pelvis.
Surgical technique
Preoperative planning
A thorough understanding of the bone defect pattern to be reconstructed is required for proper surgical planning of these complex cases. In addition to standard X-rays in all cases, we perform a metal artifact reduction sequence pelvic CT. Surgical planning is performed preoperatively and corroborated intraoperatively after implant removal and exposure of the residual acetabular bone stock.
This is an algorithmic approach to our planning process for reconstructing large acetabular defects ( Fig. 21.1 ). If the defect allows preparation and insertion of a spherical cup, the entire acetabular defect is reamed progressively to the required size using incremental reamer sizes, starting with the largest reamer that fits. If interference fit between the anterosuperior and posteroinferior columns is possible, the component is inserted, and fixation is augmented with a minimum of two screws. We recommend ‘intramedullary’ fixation with long screws in reliable bone rather than multiple short screws. The screws are preferentially inserted into the sacro-acetabular buttress or ilio-acetabular buttress, which is achieved by palpating the greater sciatic notch and the anterior column/buttress to guide the screw trajectory into robust bone.
If the defect does not allow interference fit between the columns, which is common in the setting of chronic pelvic discontinuity, reliable component contact still needs to be achieved with the ilium, ischium, and pubis. The cup should be stabilized to the ilium and at least one of the ischium and/or pubis with long ‘intramedullary’ screws. Screw combinations can include: two screws in the sacro-acetabular buttress and one screw in the ilio-acetabular buttress and at least one ‘intramedullary’ screw in the pubis extending to the pubic tubercle or one ‘intramedullary’ screw in the ischium aimed towards the ischial tuberosity. In cases where contact between the cup and ilium is very minimal, the iliac fixation should be augmented with the flange of a cage through a cup-cage construct. If the residual acetabular defect cannot be spanned by a single cup, a bespoke implant is constructed with augments or a two-cup construct to achieve optimal contact with host bone (cup-ilium, cup-ischium, cup-pubis), and these are individually fixed to the bone segments with screws. For very large defects, separate augments are first compressed to the superior and inferior hemipelvis prior to the two components being linked together by a jumbo cup with cement and screws.
Required equipment
Our technique for reconstruction of chronic pelvic discontinuity cases is performed with jumbo porous metal revision cups with or without porous metal augments [Trabecular Metal™ Acetabular Revision System (TMARS) , Zimmer Biomet, Warsaw, IN]. We remove the stainless-steel reinforcement ring from the trabecular metal revision cup using a blunt osteotome to allow marginal screw placement. For porous metal augments, we use (1) commercially available augments from the TMARS range, (2) segments of trabecular metal revision cups, and (3) small femoral cones. All cups and augments used are fashioned using a metal cutting wheel and a metal cutting burr to best fit the defect. The implants are compressed to the ilium, ischium, and pubis using long intramedullary screws. When the contact between the ilium and the augment/cup is minimal so that only two to three screws can be accommodated, we consider augmenting the iliac fixation with a cage, from which we remove the inferior flange, to provide additional iliac fixation in a perpendicular plane. The decision is based on the quality of inferior fixation achieved. In cases with severe ischial osteolysis, we consider reinforcing the fixation of an ischial augment with a 2.7-mm plate hooked over the augment and extending distally over the anterolateral aspect of the inferior pubic ramus.
The cup and augments are united into one composite using a combination of cement and screws. A metal cutting burr or a rigid drill bit is required to drill holes through cups and augments in the desired position for ideal screw fixation. Once the metal acetabular component composite is secured, a polyethylene liner is cemented in place. Last, we aim to use the least constrained articulation possible to maintain a stable hip. In cases where abductor deficiency is diagnosed preoperatively, i.e., preceding trochanteric escape, we use either a dual mobility or constrained liner. Otherwise, we use a standard lipped liner or snap-fit semi-constrained liner without a ring locking mechanism. For these large defects, the final extent of the defect is unknown until the failed implant and scar tissue is removed and the defect is re-staged despite thorough preoperative planning. It is important to have all of these options available for use when performing revision surgery on patients with chronic pelvic discontinuity.
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