CHAPTER 8 Periacetabular Osteotomy

KEY POINTS

Pearls

With an approach through the anterior superior iliac spine, this osteotomy anticipates the bone quality.

The lateral femoral cutaneous nerve would be better protected when medially retracted with sartorius muscle.

Subperiosteal dissection at the superolateral pubic ramus protects the obturator nerve.

The pubic osteotomy must be medial to the iliopectineal eminence.

Free mobility of the fragment is essential.

Lack of mobility is more likely to be because of an incomplete osteotomy.

The patient should be well positioned to obtain a radiograph in the correct anteroposterior pelvic view.

If the lateral femoral cutaneous nerve is damaged in the course of the procedure it is better to resect and cauterize it because of the morbidity associated with its recovery.

Use of an oscillating saw minimizes the likelihood of uncontrolled fracture in the ilium cut.

Hip flexion-adduction protects anteromedial vascular structures (femoral, obturator, superior gluteal).

Hip extension protects the sciatic nerve.

Releasing the anterior rectus femoris from the anterior inferior iliac spine reduces the risk of femoral nerve injury.

The center of rotation should be corrected by medializing the acetabular fragment.

According to the prefix peri (meaning “around” or “about”), a periacetabular osteotomy (PAO) is defined as an osteotomy that involves dislodging the hip socket from its bony bed in the pelvis without distorting the normal pelvic anatomy. The socket is then reoriented in a more appropriate position, reducing the deleterious effects of some unfavorable conditions. Therefore, closure of the acetabular growth plate is a precondition.

Although the purpose of all reconstructive pelvic osteotomies is the same, the PAO modifies the orientation of the acetabulum only. Ideally, the site of the periacetabular osteotomy should be as close to the acetabulum as needed to mobilize it and as far as needed to preserve the blood nutrition and to avoid joint penetration.

Following the definition, PAO includes the spherical or rotational procedures described by Eppright, Wagner, and Nynomiya ¹^–⁴; the polygonal Bernese operation described by Ganz⁵; and the modifications to these procedures described by others.^6,⁷

The osteotomy described by Eppright is barrel-shaped along an anteroposterior axis. This osteotomy allows for excellent lateral coverage but achieves only limited anterior coverage.

The Wagner type I procedure is a single spherical osteotomy and simple rotatory displacement without lengthening, shortening, medialization, or lateralization. The relative disadvantage of this procedure, because it only involves a simple acetabular realignment, is that the intact medial buttress of the quadrilateral plate prevents medialization of the joint.

The Wagner type II procedure is a spherical acetabular osteotomy that involves a combination of rotation of the isolated acetabular fragment with a lengthening effect. This is accomplished by placing an iliac bone graft in the cleft between the rotated acetabular fragment and the overlying ilium.

The Wagner type III procedure is a spherical acetabular osteotomy that involves both acetabular realignment and medialization. It is accomplished by performing a basic spherical acetabular osteotomy that is followed by an additional Chiari-like cut proximally. Fixation is usually achieved with a special construct of tension Kirschner wires connected by a semitubular plate.

The Bernese osteotomy ⁵ involves a series of straight cuts to separate the acetabulum from the pelvis. It is the acetabular procedure preferred by many centers for several reasons:

1. It can be done through one incision with a series of straight, relatively reproducible, extra-articular cuts. It allows for large corrections of the osteotomized fragment in all directions that are needed, including lateral rotation, anterior rotation, medialization of the hip center, and version correction. It is inherently stable in part because the posterior column remains intact.

2. Minimal internal fixation is required.

3. Early ambulation with no external immobilization is possible.

4. The vascularity of the acetabular fragment through the inferior gluteal artery is preserved.

5. Arthrotomy can be done without risk of further devascularization of the osteotomized fragment.

6. The shape of the true pelvis is not markedly changed, allowing women who become pregnant after the procedure to have normal vaginal delivery.

7. It can be done without violation of the abductor mechanism, facilitating a relatively rapid recovery.

ANATOMY

The basic anatomy around the hip consists of the superficial surface anatomy and deep bony, muscular, and neurovascular structures. The clinically relevant surface anatomy of the hip consists of several superficial bony prominences. The anterior landmarks are the prominent anterior superior iliac spine and the anterior inferior iliac spine. These landmarks serve as insertion points for the sartorius and direct head of the rectus femoris, respectively. The greater trochanter and the posterior superior iliac spine also are easily identifiable on the posterolateral aspect of the hip. The proximal femur and the acetabulum constitute a very stable and constrained bony articulation, which can be classified with regard to the following:

Histology—synovial (diarthrodial)

Morphology—enarthrodial (ball and socket)

Axes of movement—poliaxial

The acetabulum is formed by the confluence of the ischium, ilium, and pubis, which are usually fused by 15 to 16 years of age. It is orientated approximately 45 degrees caudad and 15 degrees anteriorly. Its hemispherical shape covers 170 degrees of the femoral head. The articular surface is horseshoe-shaped and completely lined with hyaline cartilage, except at the acetabular notch. The acetabular labrum is a fibrocartilaginous structure that runs circumferentially around the periphery of the acetabulum. It increases the depth of the bony acetabulum and contributes to the great stability of the hip joint by helping to create a negative intra-articular pressure in the joint.^8,⁹ The labrum is attached to the acetabular articular cartilage via a thin transition zone of calcified cartilage layer on the articular side. The nonarticular side of the labrum is directly attached to bone. Only the peripheral one third or less of the labrum has a rich blood supply. The sources of this blood supply are branches from the obturator, superior gluteal, and inferior gluteal arteries.⁹ Pain fibers have been identified within the labrum and are most concentrated anteriorly and anterosuperiorly.¹⁰

The transverse acetabular ligament connects the anterior and posterior portions of the labrum. The ligament teres originates from the transverse ligament over the acetabular notch and inserts into the fovea of the femoral head.

The proximal femur is formed by the femoral epiphysis and the trochanteric apophysis, both of which ossify by 16 to 18 years of age. The femoral head is approximately two-thirds of a sphere and is covered with hyaline cartilage except at the foveal notch. The angle between the shaft and the neck is approximately 125 degrees, with 15 degrees of anteversion related to the posterior femoral condyles.

The joint capsule attaches to the margins of the acetabular lip and to the transverse ligament and extends like a sleeve to the base of the femoral neck. Three major ligaments reinforce it. The iliofemoral ligament of Bigelow lies anteriorly and has an inverted-Y shape. It tightens with hip extension. The pubofemoral ligament covers the inferior and medial aspect of the hip joint capsule. It tightens with hip extension and abduction. The ischiofemoral ligament lies posteriorly, and its fibers spiral upward to blend with the zone orbicularis, a band that courses circumferentially around the femoral neck. It also tightens with extension, which explains why some degree of hip flexion increases capsular laxity.¹¹ The hip joint is least stable in the flexed position, when the capsular ligaments are slack. Normal hip range of motion includes abduction and are slack adduction (50/0/30 degrees), internal and external rotation (40/0/60 degrees), and flexion and extension (15/0/120 degrees).

The muscular attachments surrounding the hip are extensive, with a total of 27 muscles crossing the joint. The primary flexors are the iliacus, psoas, iliocapsular, pectineus, rectus femoris (direct and indirect heads), and sartorius. The extensors are the gluteus maximus, semimembranosus, semitendinosus, biceps femoris (short and long heads), and adductor magnus (ischiocondyle part). The abductors are the gluteus medius, gluteus minimus, tensor fasciae latae, and iliotibial band. The adductors are the adductor brevis, adductor longus, gracilis, and the anterior part of the adductor magnus. The external rotators are the piriformis, quadratus femoris, superior gemellus, inferior gemellus, obturator internus, and obturator externus.

The blood supply to the hip originates from the common iliac arteries, which diverge and descend lateral to the common iliac veins and slightly posterior and medial to the common iliac veins. At the pelvic brim, the common iliac artery divides into the internal and external iliac arteries. From the internal iliac system the superior and inferior gluteal arteries and the obturator artery supply the psoas major and quadratus lumborum muscles, the pelvic viscera, and parts of the bony pelvis.

The acetabulum receives its blood supply from branches of the superior and inferior gluteal arteries, the pudendal artery, and the obturator anastomoses, all of which are branches of the internal iliac artery. The external iliac artery continues to follow the iliopsoas muscle, first medially then anteriorly. It exits the pelvis under the inguinal ligament and becomes the femoral artery. The iliopectineal arch divides the space between the inguinal ligament and the coxal bone. The lacuna musculorum, which is lateral to the iliopectineal arch, contains the iliopsoas muscle and femoral nerve. The lacuna vasorum, which is medial to the iliopectineal arch, contains the femoral artery and vein. From the external iliac system, the medial and lateral femoral circumflex artery anastomoses around the proximal femur. The medial femoral circumflex artery has three main branches: the ascending, the deep, and the trochanteric. The deep branch is the primary blood supply to the femoral head. Its course starts between the pectineus and iliopsoas tendon along the inferior border of the obturator externus. A trochanteric branch sprouts off at the proximal border of the quadratus femoris to the lateral trochanter. Posteriorly, the deep medial femoral circumflex artery enters between the proximal border of the quadratus femoris and inferior gemellus and travels anterior to the obturator internus and superior gemellus, where it perforates the capsule. It then gives rise to two to four superior retinacular vessels intracapsularly. The deep branch of the medial femoral circumflex artery has several anastomoses: with the descending branch of the lateral femoral circumflex artery at the base of the femoral neck; with the deep branch of the superior gluteal artery at the insertion of the gluteus medius; with the inferior gluteal artery along the inferior border of the piriformis, posterior to the conjoined tendon; and with the pudendal artery near the retroacetabular space. The lateral femoral circumflex artery, metaphyseal artery, and medial epiphyseal artery all contribute little to the vascularity of the femoral head.

Pelvic innervation involves the lumbar (L1 to L4) and lumbosacral (L5 to S3) plexuses. The femoral nerve is located on the anteromedial side of the iliopsoas muscle and passes under the inguinal ligament as it enters the thigh. The lateral cutaneous nerve emerges from the lateral border of the psoas major at about its middle and crosses the iliacus muscle obliquely, toward the anterior superior iliac spine. It then passes under the inguinal ligament and over the sartorius muscle into the thigh, where it divides into an anterior and a posterior branch. The anterior branch becomes superficial about 10 cm below the inguinal ligament and divides into its own branches, which are distributed to the skin of the anterior and lateral parts of the thigh as far as the knee. The terminal filaments of this nerve frequently communicate with the anterior cutaneous branches of the femoral nerve and with the infrapatellar branch of the saphenous nerve; together, these nerves form the patellar plexus. The posterior branch, on the other hand, pierces the fascia lata and subdivides into filaments that pass backward across the lateral and posterior surfaces of the thigh and innervate the skin from the level of the greater trochanter to the middle of the thigh. The obturator nerve is located in the fascia directly under the pubic bone. The femoral and obturator nerves also travel with their arteries anteriorly and medially, respectively.

The sciatic nerve travels without any significant arterial counterpart out to the greater sciatic foramen with the posterior femoral cutaneous and other small nerves to the short external rotators. The superior gluteal nerve exists the pelvis via the suprapiriform portion of the sciatic foramen along with the superior gluteal vessels.

Palsy results in abductor lurch, or a Trendelenburg gait. The inferior gluteal nerve exists the pelvis via the infrapiriform portion of the sciatic foramen along with the superior gluteal vessels. Palsy results in difficulty in rising from a seated position and climbing stairs owing to weakness of hip extension.

PATHOGENESIS

Biomechanical principles for development of osteoarthritis of the hip generally are based on the calculations of force transmission: cartilage degeneration is thought to be initiated by concentric or eccentric overload.^12,¹³ The mechanical cause of osteoarthritis is secondary to several conditions. In developmental dysplasia of the hip, a maloriented articular surface with deficient anterior or global coverage of the femoral head and decreased contact area leads to excessive and eccentric loading of the anterosuperior portion of the hip and subsequently promotes the development of early osteoarthritis in the joint.¹⁴^–¹⁹ Acetabular retroversion can result from posterior wall deficiency, excessive anterior coverage, or both, and is an etiologic factor in osteoarthritis.²⁰^–²⁴ Abnormal contact between the proximal femur and the acetabular rim that occurs during terminal motion of the hip leads to lesions of the acetabular labrum and/or the adjacent acetabular cartilage. This phenomenon is more common in young and physically active adults in whom these early chondral and labral lesions continue to progress and result in degenerative disease. It has been reported in a variety of hip conditions more commonly than has been previously noted. These conditions include the dysplasias,²⁵ Legg-Calvé-Perthes disease,^26,²⁷ and postpelvic osteotomies.²⁸ The posterior aspect of the acetabulum is subjected to high loads during the activities of daily living.^27,^29,³⁰ With acetabular retroversion, theoretically greater unit loads are imposed on the available posterior cartilage, and this increased load may be responsible for the development of osteoarthritis of the hip.²⁷ Patients with joint hyperlaxity as in Down syndrome³¹ and neurogenic hip dysplasia³² have hips with a substantial structural deformity that predisposes the hip to dynamic instability, localized joint overload, impingement, or a combination of these factors, and this results in intra-articular disease and premature secondary osteoarthritis.

PREDISPOSING ILLNESSES

There is a relationship between the anatomy of the hip joint and the development of degenerative joint disease.

In femoroacetabular impingement due to overcoverage of the acetabulum (i.e., retroversion), the repeated insult leads to degenerative arthritis, rendering a joint-preserving procedure much less predictable and the quality of the results dependent on the extent of cartilage damage.

Patients with developmental dysplasia of the hip (dysplasia without subluxation) are usually identified because of an incidental finding of dysplasia on a radiograph or because they become symptomatic. Evidence exists that supports the idea that dysplasia will result in degenerative joint disease in adults, particularly in females.³³ Increased contact stresses at the joint interface are postulated as being the cause of articular degeneration.³⁴ Dysplasia with hip subluxation usually leads of significant degenerative changes around the third or fourth decade of life.^27,³⁵ The prevalence of osteoarthritis by the age 50 years has been reported to be 43%³³ to 50%¹⁹ among patients who have dysplasia and 53%³⁶ among patients with Perthes disease. Using a technique that respects the blood supply to the acetabular fragment and promotes an adequate reorientation can modify the natural history of the osteoarthritis. The improvement of the insufficient coverage of the femoral head, reduction of mediolateral displacement, and correction of the version of the fragment are the main tasks to abolish deleterious malalignments of the hip.