Reconstruction of Acetabular Bone Deficiencies Using the Antiprotrusio Cage

CHAPTER 50 Reconstruction of Acetabular Bone Deficiencies Using the Antiprotrusio Cage



James Purtill, Khalid Azzam



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.1


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.2 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.3


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.4 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.5 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.6,7 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)8:


image Type I—segmental bone loss

image Type II—cavitary bone loss

image Type III—combined (segmental plus cavitary) bone loss

image Type IV—pelvic discontinuity

image Type V—hip arthrodesis

image

FIGURE 50-1 Acetabular bone defect classification according to the American Academy of Orthopedic Surgeons.


(Redrawn from Macheras GA, Baltas D, Kostakos A, et al: Management of large acetabulum bone defects. Acta Orthop Traumatol Hell 54:1, 2003.)


Berry and colleagues9 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.


Paprosky10 classified acetabular defects as follows (Fig. 50-2):


image Type I—undistorted intact rim, small focal areas of contained osteolysis, Kohler line intact, no structural graft needed

image 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
image 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)

image 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

image 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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The Cementless Tapered Stem Femoroacetabular Osteoplasty Revision Total Hip Arthroplasty Preoperative Radiographic Evaluation and Classification of Defects Revision Total Hip Replacement The Cemented Stem

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Mar 9, 2016 | Posted by in Reconstructive surgery | Comments Off on Reconstruction of Acetabular Bone Deficiencies Using the Antiprotrusio Cage

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