Revision THA — Cup-Cage Construct for Pelvic Discontinuity

Indication. A cup-cage construct is used for revision total hip arthroplasty when chronic pelvic discontinuity and severe acetabular bone loss leave the ilium and ischium mechanically separate, so that no biologically fixed cup — however large — can gain stable purchase on both columns at once. The construct is a salvage for the deficient acetabulum, not a routine revision option. Absolute indications. - Chronic pelvic discontinuity (AAOS Type IV / Paprosky Type IV) with greater than 3 cm superomedial migration and independent column motion on stress testing

• Paprosky IIIB acetabular defect (greater than 60 percent host bone loss) with associated discontinuity that precludes stable jumbo cup or augment reconstruction
• Failed previous reconstruction with cage fracture, nonunion or recurrent migration Relative indications. - Paprosky IIIA defect with borderline discontinuity where augments alone are judged insufficient for column stability
• Severe contained defects requiring structural support while bone graft incorporates
• Patient factors favouring a single definitive reconstruction over staged grafting procedures Contraindications. - Absolute — active periprosthetic joint infection (must be eradicated first); insufficient bone stock for any fixation (pelvic dissociation with complete column loss); a patient unable to comply with protected weight-bearing
• Relative — acute discontinuity after periprosthetic fracture (consider ORIF first); severe abductor deficiency with high dislocation risk (consider constrained or dual-mobility options) Why a cup-cage, and not the alternatives. The defining problem in discontinuity is that the ilium and ischium move independently, so any device fixed to only one segment will migrate. The options differ in how — and whether — they span that gap:

Preoperative planning checklist. - Obtain Judet oblique radiographs (obturator and iliac) plus AP pelvis and cross-table lateral

• CT scan with metal artefact reduction and 3D reconstruction to quantify bone loss and confirm discontinuity
• Infection workup: CRP, ESR, aspiration if inflammatory markers elevated
• Abductor assessment: Trendelenburg gait, abductor lag sign, MRI for muscle quality if available
• Leg-length discrepancy measurement and templating for restoration
• Vascular surgery consultation if intrapelvic hardware or medial migration greater than 4 cm Templating principles. - Identify the true acetabular centre using the teardrop and ilioischial line
• Plan for 1 to 2 cm of distraction to restore column length
• Select a porous tantalum cup 4 to 6 mm larger than the final reamer to achieve rim fit
• Choose a cage size that allows two to three iliac screws and two bicortical ischial screws
• Plan the extended trochanteric osteotomy length (usually 10 to 12 cm) for exposure and abductor advancement Setup. Lateral decubitus on a radiolucent table, with the entire hemipelvis from iliac crest to mid-thigh prepped and draped to allow extensile exposure; image intensification is available throughout.

The goal is to expose both columns through an extensile posterior approach, confirm and stabilise the discontinuity by distraction, impact a porous tantalum cup, and then bridge the defect with a titanium antiprotrusio cage fixed into the ilium and ischium so the construct carries load while the cup osseointegrates and the discontinuity heals. The exposure is laid out in full as the first steps below (and in depth on the extended iliofemoral approach and posterior (Moore/Southern) approach to the hip pages).

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Implant selection. The three components each have a defined job in the construct:

Postoperative protocol. Protected weight-bearing for 12 weeks is the single most important non-operative determinant of success — early full weight-bearing is the commonest cause of cage fracture and nonunion.

• Hip precautions: posterior hip precautions for 12 weeks; abduction brace for 6 weeks.
• Follow-up imaging: AP pelvis and Judet views at 6 weeks, 3 months, 6 months, 1 year, then annually.
• Dislocation prevention: abductor strengthening programme from week 6; avoid adduction and internal rotation. Complications.

Rationale. Cup-cage constructs were developed to address the high failure rate of jumbo cups and structural allografts in discontinuity. The highly porous tantalum cup provides excellent biological ingrowth potential while the titanium cage acts as a temporary load-sharing bridge that offloads the cup until osseointegration occurs and the discontinuity heals. Once healing is confirmed, the bearing surface — not the cage — drives long-term survival.

Paprosky acetabular defect classification. The classification drives the reconstruction: the breakpoint for a cage is a IIIB defect with inadequate columns, and discontinuity (Type IV) demands spanning fixation.