Cup-cage/half cage for chronic pelvic discontinuity

Introduction

Chronic pelvic discontinuity and major acetabular bone loss cause difficulty achieving a stable motionless interface with a hemispherical uncemented acetabular component, the key to obtaining bone ingrowth. If biologic fixation is not achieved, implant failure and migration will likely occur. Chronic pelvic discontinuity has a clear association with acetabular component loosening when a standard hemispherical cup with screws is utilized. Adjunctive posterior column plating or augment use in conjunction with a hemispherical cup has also been associated with high failure rates.

The cup-cage technique utilizes a porous acetabular component inserted against the host bone and an antiprotrusio cage applied over the top for adjunctive fixation. Ischial fixation is achieved by anchoring the inferior aspect of the antiprotrusio cage either into a slot created in the ischium just lateral to the hemispherical cup or by additional screw fixation with the inferior flange over the outer surface of the ischium. When the full cage is applied to the outer ischium, care must be taken to protect the sciatic nerve, as the nerve is near the construct at this level.

The half cup-cage is a similar technique that involves utilizing a helicoidal rasp or other high-speed metal cutting burr to remove the ischial phalange of the antiprotrusio cage, then applying it to only the ilium over the top of the porous socket. This approach to fixation is technically simpler to utilize. This technique can be utilized in combination with distraction for pelvic discontinuity. This technique also requires less dissection along the ischium and is less likely to result in sciatic nerve injury.

Premise: Regardless of the reconstruction technique employed, fixation in Charnley zone three is critical in chronic pelvic discontinuity to optimize healing of the remaining inferior posterior column and ischium to the porous acetabular component – this avoids abduction failure of the component.

History

Antiprotrusio cages were developed in the 1990s by multiple companies and were used in the setting of chronic pelvic discontinuity, massive bone loss, or pathologic bone. These cages were applied directly to the host bone or graft without an ingrowth surface, and the acetabular bearing was cemented into these cages. Cage reconstruction demonstrated good initial stability in select patients, but because of a lack of osseointegration often developed mid-term to long-term mechanical failure.

The cup-cage technique was initially reported in 2005. It consisted of osseointegration of the remaining host bone with a tantalum revision cup with screws and the addition of an antiprotrusio cage applied over the top of the cup to improve initial stability while achieving biologic fixation. The antiprotrusio cage is secured to the ischium with the inferior flange in a full cage construct and the ilium with screws in both full and half cup-cage constructs. The shared loading by the cage allows cup osseointegration in addition to remodeling of any bone deficits that were grafted behind the cup.

The cage is only functional during this early osseointegration phase, at which time the revision cup bears the mechanical load, leading to diminished mechanical fatigue and failure of the cage. We recommend the application of cup-cage constructs instead of a hemispherical socket in patients with Paprosky type III-A and III-B deficiencies. The authors also prefer this reconstructive technique in destructive periacetabular secondary tumors, native acetabular fractures, and prior pelvic radiation. Due to intraoperative modularity, the decision can be made at the time of surgery for the application of this technique after evaluating the extent of the periacetabular bone defects.

Indications, contraindications, and alternatives

Indications for cup-cage reconstruction include cases where a hemispherical cup alone or hemispherical cup and augment cannot obtain stable acetabular fixation for biologic ingrowth. This reconstructive method is ideal when the host bone is unsupportive of a hemispherical cup, such as in chronic pelvic discontinuity, massive bone loss, or pathologic bone from radiation or primary or secondary tumors. This technique should be utilized when high failure rates are anticipated with a standard hemispherical socket.

Contraindications for the cup-cage technique include rare cases of severe ilium and ischium bone loss where alternative methods such as the custom triflange technique are needed to bridge the defect. Additionally, in cases with less severe bone loss where 50% or more host bone contact with an acetabular component is possible, cup-cage reconstruction is likely unnecessary, and a simpler reconstruction may be utilized.

Commonly utilized alternative reconstructive options include the use of custom triflange acetabular components, a porous jumbo cup with modular porous augments, a porous acetabular shell with a posterior column plate, the acetabular distraction technique, or the application of an antiprotrusio cage against host bone without the ability to achieve osseointegration. Early and mid-term results for anti-protrusio cages have been satisfactory, but long-term results demonstrate a high mechanical failure rate. However, this may still be the best option in resource-poor settings (see Chapter 13 ). Custom triflange acetabular component reconstruction utilizes similar methods of reconstruction as the cup-cage, with the goal of osseointegration, and works as a non-modular cup-cage, rigidly bridging periacetabular defects with the goal of unitizing the ilium, ischium, and pubis (see Chapter 19 ).