Superior Pubic Ramus Screw Placement During Complex Acetabular Revision

Background:

Insertion of a superior pubic ramus screw may be indicated for the treatment of a chronic pelvic discontinuity when utilizing acetabular distraction in revision total hip arthroplasty (THA), especially in the setting of severe bone loss in the ischium. The aim of this procedure is to stabilize and prevent abduction failure of the acetabular component when utilizing acetabular distraction.

Description:

With the patient in the lateral decubitus position, the acetabulum is exposed from a standard posterior approach for a revision THA. The location of the superior pubic root is identified after making a recess within the anteroinferior capsule. In order to ensure that the appropriate trajectory is obtained, C-arm imaging (inlet view and orthogonal obturator outlet views) is utilized to safely predrill the screw trajectory into the superior pubic ramus. A Kirschner wire (K-wire) is then placed into the hole. With use of a metal-cutting burr on the back table, customized peripheral screw holes are placed and then the acetabular component is slid and impacted into place over the K-wire. After cup insertion, the K-wire is removed and the superior pubic ramus screw can be placed and confirmed on fluoroscopy.

Alternatives:

In general, chronic pelvic discontinuity requires surgical management with revision THA and has historically employed the use of a cup-cage construct, custom triflange implants, and/or jumbo acetabular cups with modular porous metal augments^1-5. With these treatment options, it is typically necessary to insert “kickstand” screws, which function to prevent abduction failure of the acetabular cup^4,5. However, in many cases of discontinuity, there may be severe ischial osteolysis, making ischial screw placement difficult or impossible. The superior pubic ramus, however, remains a reliable option that can be utilized for inferior screw fixation, even in cases of severe acetabular bone loss, and thus is especially beneficial in these difficult cases.

Rationale:

The technique of acetabular distraction was developed because of limitations with alternative techniques. This procedure achieves cementless biologic fixation and eventual discontinuity healing as a result of lateral or peripheral acetabular distraction and resultant medial or central compression across the pelvic discontinuity. Acetabular distraction allows for intraoperative customization and cement unitization of the acetabular construct. This procedure requires the use of a “kickstand” screw or of inferior screw fixation in order to prevent abduction failure of the cup. These screws may be placed into either the ischium or superior pubic ramus. If the patient has substantial ischial osteolysis, ischial screw fixation may not be possible. If not placed in a systematic manner, pubic ramus screws can be technically challenging, and incorrect placement can result in neurovascular injury. The present video article demonstrates a reproducible technical method for safely placing a screw in the superior ramus to aid in optimal fixation of the acetabular component in cases of pelvic discontinuity.

Expected Outcomes:

Thus far, short-term survivorship of acetabular distraction with use of a jumbo cup and kickstand screws has been about 95%. In the largest study to date assessing patients with chronic pelvic discontinuity who underwent revision THA with use of the acetabular distraction technique, only 1 of 32 patients required revision for aseptic loosening⁶. An additional 2 patients had evidence of radiographic loosening but did not undergo revision, and 3 had migration of the acetabular component into a more stable configuration. Radiographically, 22 of 32 patients in the study demonstrated healing of the discontinuity⁶. In another study assessing the survivorship of porous tantalum acetabular shells in revision THA to treat severe acetabular defects, patients with inferior screw fixation with superior pubic ramus or ischial screws had a significantly lower incidence of proximal translation of components compared with those without inferior screw fixation⁷.

Important Tips:

• For reproducible, successful execution of this technique, it is important to confirm correct placement of the acetabular retractors at the correct anatomical locations to ensure adequate surgical visualization of the acetabulum for easy identification of the superior pubic root.

• It is also critical to check placement of the drill via fluoroscopy with an inlet and obturator outlet views prior to drilling.

• The drill should be advanced on the oscillate setting to avoid inadvertently perforating the cortical bone and damaging surrounding neurovascular structures.

Acronyms and Abbreviations:

• OR = operating room

• f/u = follow-up

• vac = vacuum-assisted closure

• RSA = radiostereometric analysis

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