Reconstruction of Acetabular Bone Deficiencies Using the Antiprotrusio Cage

CHAPTER OUTLINE
- Relevant Surgical Anatomy 370
  - Vessels 370
  - Nerves 371
- Classification of Acetabular Bone Defects 371
- Cage Designs 373
- Indications for the Use of Antiprotrusio Cages 373
- Preoperative Planning 373
- Imaging 373
- Laboratory Tests 373
- Surgical Approach 373
- Cage Insertion Technique 374
- Uncemented Cup-Cage Construct 374
- Postoperative Care 374
- Outcomes 374
- Advantages 375
- Disadvantages 375
- Complications 375

One of the most challenging aspects of acetabular revision is managing bone loss. Bone loss occurs in a variety of locations, and bony defects are of variable size. The goal is to create a stable construct capable of providing long-term stability of an acetabular component.

Most acetabular defects can be reconstructed with an uncemented hemispherical cup with screws with or without bone graft. Significant acetabular defects may require structural grafts, a bilobed cup, a trabecular metal cup with or without augments, an acetabular protrusio cage, or a cup-cage construct depending on the type of bone loss. An unresolved issue in revision total hip arthroplasty (THA) is acetabular reconstruction when extensive bone loss is significant enough to exceed the limits of large hemispherical cups.

The antiprotrusio cage (APC) provides a large contact area between the implant and remaining pelvic bone, distributes force over a large area, and decreases the likelihood of implant migration. It also allows the treatment of large bone defects with morcelized or bulk bone grafts, bridging gaps in native bone and thus protecting bone graft from forces that might contribute to failure. The APC provides fixation above and below areas of pelvic discontinuity, thereby allowing simultaneous treatment of the discontinuity and the failed acetabular component.

RELEVANT SURGICAL ANATOMY

The knowledge of relevant anatomy is essential before any procedure but in particular before placement of an APC.

Vessels

The external iliac artery and vein are immobile and lie close to the medial wall of the acetabulum. Because of their proximity to the surgical field, they are at highest risk for injury during acetabular revision surgery. The artery is at less risk for damage than the vein owing to its thicker wall and increased distance from bone.

The common femoral artery lies anterior and medial to the hip capsule. Only the iliopsoas lies between the vessel and capsule at this point. The femoral vein lies medial to the artery and is less likely to be injured.

The most commonly cited mechanism of injury of the femoral artery is by way of the anterior retractor that is placed during the surgical approach. The surgeon must avoid damage by keeping the retractor close to bone, using a blunt-tipped device, and avoiding strong retraction over the lip of the acetabulum.

Nerves

Sciatic Nerve

The anatomic course of the sciatic nerve places it at risk for injury by posterior acetabular retractors and power reamers. The sciatic nerve (L4-S3) arises from the sacral plexus and is the largest nerve in the body. It is located anterior and medial to the piriformis muscle just proximal to where it will emerge through the greater sciatic notch.

The sciatic nerve continues vertically between two layers of muscle. The outer layer is formed by the gluteus maximus and the piriformis (sometimes the nerve passes through the piriformis or posterior to it). The inner layer is formed by the superior gemellus, the obturator internus, the inferior gemellus, and the quadratus femoris. Complex acetabular reconstructions, especially when a triflanged cage is used, pose an additional risk of injury to the sciatic nerve.

Femoral Nerve

The femoral nerve (L2, L3, L4) descends through the fibers of the psoas major and emerges from the lower part of its lateral border. The nerve then passes down behind the iliac fascia, beneath the inguinal ligament, and into the thigh. The femoral nerve is the most lateral structure within the femoral triangle. It lies on the psoas muscle belly at the approximate midpoint between the anterior superior iliac spine and pubic tubercle. It is anterior to the acetabulum and consequently is primarily at risk during capsule dissection, especially in an ilioinguinal approach.

Obturator Nerve

The obturator nerve (L2, L3, L4) descends through the fibers of the psoas major and emerges from its medial border. It then passes behind the common iliac vessels, on the lateral side of the ureter, and runs along the lateral wall of the lesser pelvis, above and in front of the obturator vessels. The nerve then enters the thigh by passing through the obturator canal located in the upper part of the obturator foramen. It then divides into an anterior and a posterior branch. Obturator nerve injury in THA appears to be a rare complication.

CLASSIFICATION OF ACETABULAR BONE DEFECTS

The American Academy of Orthopedic Surgeons classifies acetabular bone deficiencies into the following five categories ( Fig. 50-1 ) :

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Type I—segmental bone loss
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Type II—cavitary bone loss
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Type III—combined (segmental plus cavitary) bone loss
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Type IV—pelvic discontinuity
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Type V—hip arthrodesis

Berry and colleagues subclassified the pelvic discontinuity as type IVa if it was associated only with cavitary bone loss, as type IVb if it was associated with segmental or combined (cavitary and segmental) bone loss, and as type IVc if it was associated with previous irradiation of the pelvis with or without cavitary or segmental bone loss.

Paprosky classified acetabular defects as follows ( Fig. 50-2 ):

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Type I—undistorted intact rim, small focal areas of contained osteolysis, Kohler line intact, no structural graft needed
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Type II—distorted rim with intact columns, hemispherical acetabulum, <3 cm of superomedial or superolateral migration, small areas of osteolysis at the ischium <7 mm below the obturator line, Kohler line intact; if graft needed, it is needed for augmentation and not for structural purpose
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  Type IIA—superomedial migration, center of acetabulum is <3 cm above the obturator line, Kohler line is intact; if graft needed, it is for augmentation only, not for structural purpose (must have a superior rim capable of containment)
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  Type IIB—superolateral migration, center of acetabulum is <3 cm above the obturator line, Kohler line is intact, superior rim is disrupted less than one third of circumference; structural graft might be needed
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  Type IIC—medial migration only, intact rim, Kohler line is disrupted; if graft needed, it is needed for augmentation or structural purpose as a buttress (in case the medial membrane is not supportive of particulate graft)
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Type III—distorted rim with insufficient supportive columns, hemispherical acetabulum, >2 cm of superolateral or superomedial migration, severe areas of osteolysis at the ischium >7 mm below the obturator line; graft needed for structural purpose
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  Type IIIA—superolateral migration, center of acetabulum is >3 cm above the obturator line, must have >40% to 60% of host bone to support uncemented cup; defect is more than one third but less than one half of circumference, usually located between 10-o’clock and 2-o’clock positions; Kohler line is intact; graft needed for structural purpose
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  Type IIIB—superomedial migration, center of acetabulum is >3 cm above the obturator line, has <40% of host bone; defect is more than one half of circumference, usually located between 9-o’clock and 3-o’clock positions; severe areas of osteolysis at the ischium >15 mm below the obturator line; Kohler line is disrupted; graft needed for structural purpose; may or may not have pelvic discontinuity; usually requires a cage