Revision THA — Cup-Cage Construct for Pelvic Discontinuity

Indications for Cup-Cage Reconstruction

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)
• 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)

Evidence Base

Cup-cage constructs were developed to address the high failure rate of jumbo cups and structural allografts in discontinuity. The porous tantalum cup provides excellent biological ingrowth potential while the titanium cage acts as a temporary load-sharing bridge.

Key Evidence

Paprosky Acetabular Defect Classification

Type I

Minor bone loss with intact rim and columns; standard primary cup sufficient.

Type II

Moderate bone loss with intact rim but deficient anterior or posterior wall.

• IIA: superior rim deficiency less than 50 percent
• IIB: superolateral rim deficiency
• IIC: superomedial rim deficiency with intact columns

Type III

Severe bone loss with deficient rim.

• IIIA: greater than 50 percent host bone loss but columns still supportive; superolateral migration less than 3 cm
• IIIB: greater than 60 percent host bone loss, superomedial migration greater than 3 cm, columns inadequate for support

Type IV — Pelvic Discontinuity

Complete separation of ilium from ischium/pubis. Two independent segments. Requires spanning fixation.

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 1-2 cm of distraction to restore column length
• Select porous tantalum cup 4-6 mm larger than the final reamer to achieve rim fit
• Choose cage size that allows two to three iliac screws and two bicortical ischial screws
• Plan extended trochanteric osteotomy length (usually 10-12 cm) for exposure and abductor advancement

Extensile Posterior Approach with Extended Trochanteric Osteotomy

Patient Positioning

Lateral decubitus position on a radiolucent table. The entire hemipelvis from iliac crest to mid-thigh must be prepped and draped to allow extensile exposure. A bump under the contralateral hip improves access to the posterior column.

Incision and Superficial Dissection

• Longitudinal incision centred over the greater trochanter, extending proximally along the iliac crest if needed
• Identify and protect the sciatic nerve immediately upon entering the deep fascia — it is often encased in scar from previous surgery
• Perform an extended greater trochanteric osteotomy (10-12 cm) with an oscillating saw; reflect the osteotomised fragment anteriorly with the attached abductors
• This exposes the entire acetabulum and both columns while preserving abductor continuity

Sciatic Nerve Protection

The sciatic nerve is identified at the greater sciatic notch and traced distally to the ischial tuberosity. A vessel loop is placed around it for gentle retraction. The nerve is mobilised from all scar tissue before any posterior column work. Intraoperative neuromonitoring (if available) provides additional safety during retraction.

Posterior Column Exposure

The posterior column is exposed from the acetabular rim to the ischial tuberosity. The quadrilateral surface is palpated to confirm discontinuity. Hohmann retractors are placed carefully under direct vision, avoiding the sciatic notch.

Anterior Column and Medial Wall

The anterior column is exposed by retracting the iliopsoas medially. The quadrilateral surface is inspected for fracture lines. The obturator nerve is identified and protected if anterior column screws are planned.

Step-by-Step Cup-Cage Reconstruction

Step 1: Exposure and Component Removal

Remove the existing acetabular component with curved osteotomes and a cup extraction device. Clear all fibrous tissue and cement from the defect. Send tissue for frozen section and culture to rule out infection.

Step 2: Assessment of Discontinuity

Manually stress the two columns under direct vision. Independent motion of the ilium and ischium confirms discontinuity. Measure the gap and assess bone quality for screw purchase.

Step 3: Reaming and Distraction Technique

Ream the acetabulum sequentially to the desired size, preserving as much host bone as possible. The acetabulum is then distracted 10-20 mm using laminar spreaders or a dedicated distractor placed between the ilium and ischium. This restores column length and creates a stable mechanical environment for the cup.

Step 4: Porous Tantalum Cup Insertion

Select a highly porous tantalum cup 4-6 mm larger than the final reamer. Impact the cup into the distracted acetabulum with 40-45 degrees of abduction and 15-20 degrees of anteversion. The cup should achieve rim contact superiorly and posteriorly. Confirm position with fluoroscopy (AP, iliac oblique, obturator oblique).

Step 5: Bone Grafting of Contained Defects

Any contained defects behind the cup are packed with morsellised allograft or autograft harvested from the femoral head or iliac crest. The graft is impacted through the cup screw holes before the cage is applied.

Step 6: Antiprotrusio Cage Placement

Select a titanium antiprotrusio cage sized to span from the ilium to the ischium. The cage is contoured to fit over the cup. The ischial flange is impacted into a slot created in the ischium. The iliac flange is seated against the ilium.

Step 7: Cage Fixation

• Place two to three 6.5 mm cancellous screws into the ilium under image guidance (superior and posterior trajectories)
• Place two bicortical 4.5 mm or 6.5 mm screws into the ischium (critical for load-sharing)
• Confirm all screws are extra-articular on multiple fluoroscopic views
• The cage should now bridge the discontinuity and offload the porous cup

Step 8: Liner Cementation and Bearing Surface

A polyethylene liner is cemented into the tantalum cup with antibiotic-loaded cement. A large-diameter head (36 mm or 40 mm) or dual-mobility construct is selected to maximise stability. The hip is reduced and stability tested through a full range of motion.

Step 9: Greater Trochanteric Osteotomy Fixation

The osteotomised greater trochanter is advanced distally if needed to restore abductor tension and fixed with two cables or a cable-plate construct. This step is critical for postoperative stability.

Step 10: Wound Closure and Drains

Layered closure over drains. A hip abduction brace is applied in the operating room.

Implant Selection Principles

• Porous tantalum cup: 2 mm wall thickness, 70-80 percent porosity, excellent frictional coefficient. Size 4-6 mm larger than reamer for rim fit.
• Titanium antiprotrusio cage: Multiple sizes with modular flanges. Reinforced ischial flange preferred. 6.5 mm cancellous screws for ilium; 4.5 mm or 6.5 mm cortical screws for ischium.
• Bearing surface: Highly cross-linked polyethylene with 36 mm or larger head. Dual-mobility liner if abductor insufficiency is severe. Constrained liner reserved for salvage only.

Complications

Postoperative Protocol

• Weight-bearing: Touch weight-bearing (10-15 kg) for 6 weeks, then 50 percent for another 6 weeks. Full weight-bearing after radiographic evidence of healing at 12 weeks.
• 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.