Cup-Cage Constructs
David G. Lewallen
Ashton H. Goldman
Key Concepts
Major acetabular bone loss may make it difficult or even impossible to achieve stable fixation of a large uncemented porous acetabular component against the remaining host bone.
Unless a motionless interface between the porous acetabular component and bone is achieved and maintained over the first several months post surgery, bone ingrowth will not occur and the risk of implant migration and failure dramatically increases.
Chronic pelvic discontinuity is associated with increased failure rates due to cup loosening when standard cup fixation with screws is attempted even when adjunctive augments or posterior column plating is added.
Charnley zone 3 fixation of the inferior portion of the acetabular component to the posterior column and base of the ischium is important in all acetabular reconstructions but is critical in cases of massive bone loss or pelvic discontinuity.
The cup-cage technique involves the addition of an antiprotrusio cage over the top of an uncemented acetabular component, which greatly increases the stability of the initial fixation and thereby increases the chances of successful bone ingrowth to the cup over the ensuing weeks post surgery (Figure 27.1).
When a full cup-cage construct is elected, the ischial flange typically is driven into a bony slot in the ischium. Alternatively, the flange can be placed on the outer surface of the ischium, but this requires additional dissection and can increase the risk of sciatic nerve injury.
A half cup-cage construct is an alternative in which the ischial flange is removed and the iliac flange is used alone. The half cup-cage technique is technically easier, allows room for screws instead of a flange for fixation into the ischium, and can be used more easily in combination with distraction of any associated pelvic discontinuity.
Sterile Instruments and Implants
Standard retractors and hip instruments.
Hemispherical acetabular component instruments and trial and real components in a full range of sizes up to 80 mm.
Acetabular porous metal augment trials in a variety of sizes and thicknesses to aid in intraoperative assessment, sizing, and planning of final construct.
Cancellous bone graft to fill any small remaining gaps, defects, or nonunion sites in the acetabulum and to fill the fenestrations of any augments used.
Antiprotrusio cages in a range of sizes with corresponding malleable trials to aid in bending and fitting of the real component. Fixation screws for the corresponding antiprotrusio cage.
 Figure 27.1 ▪ A, Preoperative radiograph of massive Paprosky type 3B bone loss with segmental medial acetabular defect and pelvic discontinuity from migrated cup. B, Postoperative anteroposterior (AP) radiograph of the reconstructed hip showing 2 large medial acetabular augments and bone graft spanning the large medial defect and supporting a jumbo cup fixed with multiple screws and stabilized by a full cage over the top. C, Follow-up radiograph at 6 years post surgery showing solid fixation without implant migration, hardware failure, or radiolucencies. |
Surgical Approaches
Any of the standard potentially extensile surgical approaches can be used per surgeon preference similar to any revision total hip arthroplasty (THA).
Posterolateral and direct lateral approaches are the most commonly used at our institution, either alone or in combination with an extended trochanteric osteotomy as needed. The exposure is chosen based on exposure requirements on the femoral side and on the acetabular side.
When a direct lateral approach is used, care must be taken to elevate but not split the gluteus medius too far proximally when exposing the lateral aspect of the ilium above the acetabular cavity to avoid injury to branches of the superior gluteal nerve running to the anterior portion of the muscle.
When using the posterolateral approach, the superior gluteal nerve is at risk at the sciatic notch at the proximal end of the posterior column, and injury at this level places innervation to the entire gluteus medius at risk.
Preoperative Planning
An organized assessment of the degree of bone loss on the acetabular side is strongly recommended in cases of major bone loss, and this is ideally based on careful x-ray analysis of key anatomic features.
The Paprosky classification of acetabular bone defects is helpful in anticipating operative findings and for planning treatment options.
This classification scheme involves measurement of the amount of cup migration, direction of that migration, status of Kohler line and the tear drop (when medial deficiency and migration of the cup has occurred), and critical assessment of the status of the posterior column as noted on the cross-table lateral radiograph, iliac and obturator oblique (or Judet) views, and occasionally adjunctive computed tomography scanning (Figure 27.2A-C).
The cup-cage technique is most useful when more severe bone defects as seen in Paprosky type 3A (cup up and out) or 3B defects (cup up and in) make stable fixation of a cementless hemispherical porous cup tenuous despite multiple screws and any adjunctive fixation such as augments.
Sometimes, decreased residual host bone quality (not just bone deficiency) will play a role in prompting use of a cup-cage to provide more secure fixation of the acetabular component.
However, the most common current indication for use of a half or full cup-cage is an associated pelvic discontinuity.
Figure 27.2 ▪ A, Preoperative AP radiograph of the left hip showing radiolucencies at the acetabular implant-bone interface and a bony step off medial to the cup involving the iliopectineal line suggestive of a fracture and possible complete pelvic discontinuity. B, Cross-table lateral view showing the posterior column and a questionable fracture line visible. C, Obturator oblique radiograph more clearly showing the fracture line medially. D, Example of intraoperative appearance of the transverse fracture line of a pelvic discontinuity, which was visible only after removal of the overlying fibrous tissue highlighted here by the arrow where it is exiting the posterior wall of the acetabulum.
The location and size of the bone defects, size of the patient, and quality of the bone remaining can be difficult to judge despite careful assessment of imaging studies before surgery.Related posts:
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