Periacetabular Osteotomy: What It Is & How It Works

Comprehensive Introduction and Pathophysiology of Acetabular Dysplasia

According to the prefix peri- (meaning “around”), a periacetabular osteotomy (PAO) is defined as a complex reconstructive pelvic osteotomy that involves dislodging the hip socket from its bony bed in the pelvis without distorting the normal pelvic ring anatomy. The fundamental objective is to reorient the dysplastic or malverted socket into a biomechanically optimized position, thereby reducing the deleterious effects of unfavorable loading conditions. Closure of the acetabular growth plate (the triradiate cartilage) is an absolute precondition for this procedure; performing a PAO in a skeletally immature patient risks catastrophic growth arrest and secondary deformity. Although the overarching purpose of all reconstructive pelvic osteotomies is the mitigation of joint-reactive forces, the PAO is unique in that it modifies only the orientation of the acetabulum while preserving the structural integrity of the true pelvis. Ideally, the site of the periacetabular osteotomy should be as close to the acetabulum as necessary to mobilize it, preserve its tenuous blood supply, and avoid catastrophic intra-articular joint penetration.

Historically, the evolution of the PAO includes several pivotal iterations, most notably the spherical or rotational osteotomies described by Eppright, Nynomiya, and Wagner, culminating in the polygonal Bernese osteotomy described by Ganz and colleagues. The osteotomy described by Eppright is barrel-shaped and oriented along an anteroposterior axis; while it allows for excellent lateral coverage, it is severely limited in the amount of anterior coverage that can be achieved. The Wagner type I osteotomy is a single, spherical cut providing simple rotatory displacement without lengthening, shortening, medialization, or lateralization. Its relative disadvantage is that the intact medial buttress of the quadrilateral plate prevents medialization of the joint, which is often critical for reducing joint-reactive forces. The Wagner type II osteotomy combines rotation of the isolated acetabular fragment with a lengthening effect, accomplished by placing an iliac bone graft in the cleft between the rotated acetabular fragment and the overlying ilium. The Wagner type III osteotomy involves both acetabular realignment and medialization, achieved by creating the basic spherical cut followed by an additional Chiari-like cut proximally, requiring complex fixation with tension Kirschner wires and a semitubular plate.

The Bernese osteotomy, introduced by Reinhold Ganz in 1988, revolutionized joint preservation surgery. It involves a series of straight, extra-articular cuts to separate the acetabulum from the pelvis. It remains the preferred acetabular procedure globally due to a multitude of biomechanical and surgical advantages. It can be performed through a single incision with highly reproducible cuts, allowing for massive multi-planar corrections including lateral rotation, anterior rotation, medialization of the hip center, and version correction. The construct is inherently stable because the posterior column remains intact, meaning minimal internal fixation is required and early ambulation without external immobilization is possible. Crucially, the vascularity of the acetabular fragment via the inferior gluteal artery is preserved, and concomitant arthrotomy can be performed without risk of devascularization. Furthermore, the shape of the true pelvis is not markedly changed, allowing women who become pregnant postoperatively to undergo normal vaginal delivery, and the procedure can be executed without violating the abductor mechanism, facilitating rapid functional recovery.

The pathogenesis necessitating PAO is primarily rooted in mechanical abnormalities that precipitate secondary osteoarthritis. In developmental dysplasia of the hip (DDH), a maloriented articular surface with deficient anterior or global coverage of the femoral head leads to decreased contact area. This results in excessive, eccentric loading of the anterosuperior rim, promoting the early onset of osteoarthritis. Acetabular retroversion, resulting from posterior wall deficiency, excessive anterior coverage, or both, also contributes significantly to joint degeneration. Abnormal contact between the proximal femur and the acetabular rim during terminal motion leads to impingement (pincer-type FAI), causing early chondral and labral lesions. This phenomenon is highly prevalent in young, physically active adults and is frequently observed in conditions like Legg-Calvé-Perthes disease and post-pediatric pelvic osteotomies. Because the posterior aspect of the acetabulum is subjected to high loads during activities of daily living, acetabular retroversion imposes theoretically greater unit loads on the available posterior cartilage. Joint hyperlaxity further exacerbates these mechanical mismatches, accelerating the degenerative cascade.

Detailed Surgical Anatomy and Biomechanics

The basic anatomy around the hip dictates the surgical approach and the safe execution of the osteotomy cuts. The clinically relevant surface anatomy consists of several superficial bony prominences. Anteriorly, the prominent anterosuperior iliac spine (ASIS) and anterior inferior iliac spine (AIIS) serve as critical landmarks and insertion points for the sartorius and the direct head of the rectus femoris, respectively. The greater trochanter and the posterior superior iliac spine (PSIS) are palpable on the posterolateral aspect. The proximal femur and the acetabulum constitute a highly constrained bony articulation classified histologically as a synovial (diarthrodial) joint, morphologically as an enarthrodial (ball-and-socket) joint, and biomechanically as polyaxial. The acetabulum is formed by the confluence of the ischium, ilium, and pubis, which fuse at the triradiate cartilage by 15 to 16 years of age. It is oriented approximately 45 degrees caudally and 15 degrees anteriorly, is hemispherical in shape, and covers 170 degrees of the femoral head.

The intra-articular and capsular anatomy provides secondary static stabilization. The articular surface is horseshoe-shaped and lined with hyaline cartilage, except at the central acetabular notch. The acetabular labrum, a fibrocartilaginous structure, runs circumferentially around the periphery, increasing the depth of the bony socket and creating a fluid seal that contributes to joint stability. It attaches to the articular cartilage via a thin transition zone of calcified cartilage, while the nonarticular side attaches directly to the bone. Only the peripheral one-third of the labrum has a rich blood supply (from obturator, superior gluteal, and inferior gluteal branches), and pain fibers are most concentrated anteriorly and anterosuperiorly. The transverse acetabular ligament connects the anterior and posterior portions of the labrum, and the ligamentum teres originates here, inserting into the fovea of the femoral head. The joint capsule extends like a sleeve to the base of the femoral neck, reinforced by three major ligaments: the inverted Y-shaped iliofemoral ligament of Bigelow (anterior, tightens with extension), the pubofemoral ligament (inferomedial, tightens with extension and abduction), and the ischiofemoral ligament (posterior, spirals upward to blend with the zona orbicularis, tightening with extension). Consequently, the hip joint is least stable in flexion, where these capsular ligaments slacken. Normal hip range of motion includes abduction/adduction (50/0/30 degrees), internal/external rotation (40/0/60 degrees), and flexion/extension (15/0/120 degrees).

The muscular and neurovascular anatomy surrounding the hip is incredibly complex, with 27 muscles crossing the joint. The primary flexors include the iliacus, psoas, iliocapsular, pectineus, rectus femoris, and sartorius. Extensors comprise the gluteus maximus, hamstrings, and adductor magnus. Abductors include the gluteus medius, minimus, and tensor fascia lata, while adductors consist of the adductor brevis, longus, gracilis, and anterior adductor magnus. The external rotators (piriformis, quadratus femoris, gemelli, and obturators) provide dynamic posterior stability. The blood supply originates from the common iliac arteries. The internal iliac system gives rise to the superior and inferior gluteal arteries and the obturator artery, which supply the pelvic viscera, parts of the bony pelvis, and the acetabulum. The external iliac artery becomes the femoral artery, giving off the medial and lateral femoral circumflex arteries (MFCA and LFCA). The deep branch of the MFCA is the primary blood supply to the femoral head, coursing between the pectineus and iliopsoas, along the inferior border of the obturator externus, and perforating the capsule posteriorly to give rise to superior retinacular vessels.

Neurologic anatomy is divided into the lumbar and lumbosacral plexuses. From the lumbar plexus (L1-L4), the femoral nerve passes under the inguinal ligament on the anteromedial side of the iliopsoas. The lateral femoral cutaneous nerve (LFCN) emerges from the lateral border of the psoas major, crosses the iliacus obliquely toward the ASIS, and divides into anterior and posterior branches supplying the skin of the anterolateral thigh. The obturator nerve runs directly under the pubic bone. From the lumbosacral plexus (L4-S3), the sciatic nerve exits via the greater sciatic foramen. The superior gluteal nerve exits the suprapiriform space; injury here results in an abductor lurch (Trendelenburg gait). The inferior gluteal nerve exits the infrapiriform space; palsy leads to weakness in hip extension, causing difficulty rising from a seated position or climbing stairs. Mastery of these neurovascular pathways is non-negotiable for the safe execution of the deep pelvic osteotomies.

Exhaustive Indications and Contraindications

Patient selection is the most critical determinant of long-term success in periacetabular osteotomy. The primary indication for a PAO is symptomatic developmental dysplasia of the hip in a skeletally mature patient (closed triradiate cartilage). Patients typically present with insidious onset groin pain, mechanical symptoms such as catching or locking (indicative of secondary labral tearing), and fatigue with prolonged standing or walking. Radiographic indications include a lateral center-edge angle (LCEA) of less than 20 degrees, an anterior center-edge angle (ACEA) of less than 20 degrees, an increased acetabular inclination (Tönnis angle > 10 degrees), and an unbroken Shenton's line. Another major indication is symptomatic acetabular retroversion causing pincer-type femoroacetabular impingement (FAI) that is not amenable to arthroscopic rim trimming, characterized radiographically by a positive crossover sign, prominent ischial spine sign, and posterior wall sign.

Conversely, strict contraindications exist to prevent early failure and rapid conversion to total hip arthroplasty (THA). The most absolute contraindication is an open triradiate cartilage; performing a PAO in this setting will cause premature closure and severe secondary growth deformities (such patients are better served by a Salter or triple innominate osteotomy). Advanced osteoarthritis, defined as Tönnis grade 2 or 3 with significant joint space narrowing (< 2 mm) or the presence of large subchondral cysts, is a strong contraindication, as the articular cartilage is already irreversibly compromised. Inflammatory arthropathies, such as rheumatoid arthritis or ankylosing spondylitis, are also contraindications due to the systemic nature of the cartilage degradation.

Relative contraindications require nuanced clinical judgment. Patients over the age of 40 generally have poorer outcomes and higher rates of conversion to THA, though physiologic age and cartilage quality (assessed via delayed gadolinium-enhanced MRI of cartilage, dGEMRIC) can sometimes justify the procedure in older cohorts. Severe joint incongruency that does not reduce or improve on dynamic functional radiographs (abduction and internal rotation views) suggests that reorienting the acetabulum will not establish a concentric joint space. Furthermore, patients with profound connective tissue disorders (e.g., severe Ehlers-Danlos syndrome) may experience recurrent instability despite optimal bony correction due to extreme capsular hyperlaxity.

Clinical Decision Matrix for PAO

Pre-Operative Planning, Templating, and Patient Positioning

Meticulous pre-operative planning is the cornerstone of a successful Bernese periacetabular osteotomy. The planning phase begins with a comprehensive radiographic series. A standardized anteroposterior (AP) pelvis radiograph is mandatory to assess the LCEA, Tönnis angle, extrusion index, and the presence of retroversion markers (crossover sign, ischial spine sign). A false profile view of Lequesne and de Seze is critical for evaluating anterior coverage (ACEA) and anterior joint space narrowing. Dunn views or cross-table lateral radiographs of the proximal femur are necessary to evaluate for concomitant cam-type morphology, which may require simultaneous osteochondroplasty via an arthrotomy to prevent secondary impingement once the acetabulum is reoriented.

Advanced imaging has become the standard of care in modern PAO planning. A high-resolution computed tomography (CT) scan with 3D surface rendering allows the surgeon to visualize the complex pathomorphology of the dysplastic acetabulum. Virtual surgical planning (VSP) software can be utilized to simulate the osteotomy cuts, calculate the exact degrees of multi-planar rotation required, and anticipate the degree of medialization needed to optimize the hip center of rotation. Magnetic resonance imaging (MRI) or MR arthrography (MRA) is indispensable for evaluating the soft-tissue envelope. It provides detailed assessment of labral tears, chondral delamination, and hypertrophy of the ligamentum teres

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