Summary Acetabular protrusio, or arthrokatadysis, is a rare osteological condition that may be primary (idiopathic) or secondary to other causes such as infectious, inflammatory, metabolic, traumatic, or genetic etiologies, leading to protrusion of the acetabulum and femoral head into the pelvis Diagnosis is made by a lateral center edge angle greater than 40 degrees or protrusion of the medial wall of the acetabulum past the ilioischial line (by 3 mm in men or 6 mm in women) Total Hip Arthroplasty (THA) is frequently employed for adult patients with acetabular protrusio and end-stage osteoarthritis, though it requires specific consideration of the patient's abnormal anatomy Epidemiology Incidence 5% in the osteoarthritis population 15% in patients with rheumatoid arthritis 33% in patients with ankylosing spondylitis up to 55% in patients with osteogenesis imperfecta up to 20% in patients with neurofibromatosis up to 50% in patients with osteomalacia Etiology Pathophysiology multiple proposed mechanisms chondrodystrophy triradiate cartilage remains unfused, allowing medial femoral head protrusion inflammatory destruction or non-inflammatory weakening of the medial acetabular wall, leading to insufficient bone stock required to withstand medially directed joint reaction forces Associated conditions idiopathic acetabular protrusio secondary acetabular protrusio genetic predisposition (Stickler syndrome, Trisomy 18, Ehlers-Danlos syndrome, Marfan syndrome, Sickle Cell Disease, Trichorhinophalangeal syndrome) infection (gonococcus, echinococcus, staphylococcus, streptococcus, mycobacterium tuberculosis) inflammatory conditions (infection, rheumatoid arthritis, ankylosing spondylitis, juvenile rheumatoid arthritis, psoriatic arthritis, Reiter's syndrome, osteolysis) neoplasms (radiation-induced osteonecrosis, metastatic carcinoma, neurofibromatosis, hemangioma) connective tissue disorders metabolic conditions (osteomalacia, Paget's disease, osteogenesis imperfecta, acrodysostosis, hyperparathyroidism) Classification Hirst Classification Men Women Type I 3-8 mm between acetabular and ilioischial lines 6-11 mm between acetabular and ilioischial lines Type II 8-13 mm between acetabular and ilioischial lines 12-17 mm between acetabular and ilioischial lines Type III over 13 mm between acetabular and ilioischial lines with fragmentation over 17 mm between acetabular and ilioischial lines with fragmentation Presentation History/Symptoms activity-related groin pain hip stiffness knee pain difficulty or pain with rising from a seated position systemic symptoms consistent with/suspicious for an underlying condition Physical exam pain with log roll diminished active and passive hip range of motion vascular Trendelenburg sign Trendelenburg or antalgic gait additional findings characteristic of underlying conditions Imaging Radiographs pelvic radiographs AP lateral lateral center edge angle > 40 degrees measured by the angle formed between a vertical line through the center of the femoral head and a line from the center of the femoral head to the lateral edge of the acetabulum protrusion of the medial wall of the acetabulum beyond the ilioischial line by over 3 mm in males by over 6 mm in females utilization of the Paprosky classification may guide acetabular reconstruction strategies Diagnosis made with pelvic radiographs demonstrating protrusion of the medial wall of the acetabulum beyond the ilioischial line by 3 mm in men 6 mm in women lateral center edge angle > 40 degrees THA requires careful planning and adjuvants to address specific concerns in this patient population goals of surgery address medial or supero-medial migration of the femoral head address the altered abductor lever arm center of rotation may be determined by Ranawat's triangle method address the insufficient medial acetabular wall surgical exposure may require trochanteric osteotomy (single plane osteotomy , trochanteric flip osteotomy , or extended trochanteric osteotomy) for acetabular exposure more anteriorly based sciatic nerve in patients with acetabular protrusio requires careful dissection and positioning of the hip in max extension and 90 degrees of hip flexion difficult hip dislocation requires circumferential release of the capsule and excision of osteophytes may require in situ neck cut or resection using drill and bone knife hip center of rotation requires restoration of a more inferior and lateral hip socket deviation of prosthesis center of rotation leads to a 24% increase in aseptic cup revision risk for every 1 mm deviation from native center of rotation acetabular socket preparation ream anterosuperiorly and posteroinferiorly, but not medially graft may be used to address acetabular defects recommended in cases of protrusion > 5 mm with a thin medial wall autograft (femoral neck, iliac crest, femoral head, trochanter) synthetic bone graft reverse reaming implants cemented cup osteopenic bone inflammatory arthropathies significant bony defects requires wire mesh to separate cement mantle from underlying bone graft cementless cup good residual bone stock reconstruction plate used in post-traumatic scenarios, especially those where pelvic discontinuity is present anti-protrusio cages/devices for significant defects outcomes improvement in Harris Hip scores and Japanese Orthopedic Association scores after surgery up to 97% rate of graft incorporation mean time to graft union of 3-6 months 72-90% implant survival rate at 12 years Specific Complications sciatic nerve injury due to medial migration of femoral head requires careful positioning of the hip in extension and the knee in 90 degrees of flexion limb length discrepancy secondary to normalization of hip center of rotation fracture 1.2% 90-day periprosthetic fracture rate in large cohort studies risk factors deficient acetabular medial wall difficult dislocation requiring increased force blood transfusion up to 28.5% incidence of blood transfusion risk factors larger surgical approach longer operative time older patients with more medical co-morbidities are more susceptible to anemia