[Modular reconstruction in acetabular revision with antiprotrusio cages and metal augments : the cage-and-augment system]

Abstract

Objective: Restore primary center of rotation and reconstruct extensive bone defects in hip revision surgery with a modular off-label implant combined with antiprotrusion cage and metal augment, thus, achieving improved hip function.

Indications: Large segmental acetabular defects with nonsupportive columns (Paprosky type 3a and 3b) in cup loosening or Girdlestone situation. In case of pelvic discontinuity posterior column-plating is possible.

Contraindications: Persisting hip infection and severe systemic disorders impairing achievement of secondary stability through bony integration of metal augment.

Surgical technique: Posterolateral (if dorsal column plating) or other approach. Remove loose implant and granulation tissue with sufficient exposure of bleeding bone. Size acetabular defect with trial components of augment and appropriate antiprotrusio cage. Fixation of selected metal augment with screws. Fill additional acetabular defects with morsellized bone graft. Open a slot into the ischium to fix the distal flange of the cage. If necessary, bend both flanges according to patient's anatomy. Enter the ischium with distal flange and gradual impaction of the antiprotrusio ring. Final stabilization of the ring with several screws aiming at the posterior column or the acetabular dome. Inject cement between ring and augment to stabilize the construction and avoid metal wear. Final cement fixation of a polyethylene liner or a dual-mobility cup into the antiprotrusio ring. In pelvic discontinuity with major instability osteosynthesis of the dorsal column can be performed prior to cementation.

Postoperative management: Prophylaxis of periprosthetic infection, DVT and heterotopic ossification. Physical therapy with partial weight bearing (20 kp) for 6 weeks; in discontinuity initial wheel chair mobilization.

Results: Since 2008, 72 off-label implantations of a combined antiprotrusio cage and a Trabecular Metal™ Augment were performed. A total of 44 patients (46 operations) were investigated at 38.8 (36-51) months postoperatively. In all, 36 patients had a bone defect according to Paprosky type 3a/b and in 3/4 patients with pelvic discontinuity additional osteosynthesis was performed. The WOMAC score increased from 39.8 (8.7-75) points preoperatively to 57.9 (16.7-97.9) points at follow-up. Migration or failure of implant components was not observed. In 11 % of dislocations and 11 % periprosthetic infections surgical revision was necessary.