Protrusio acetabuli is a distortion of the hip where the acetabulum and femoral head migrate into the pelvic cavity. The disease can be idiopathic (primary) or can be secondary to a number of conditions, such as trauma, inflammation or other diseases. The cornerstone of diagnosis is anteroposterior radiography and the treatment varies by age. For the skeletally immature youth, a fusion of the triradiate cartilage may be recommended but is not frequently carried out because the protrusion does not always deteriorate. For young adults, valgus intertrochanteric osteotomy is performed. For older patients, total joint replacement is the procedure of choice.
Protrusio acetabuli is the protrusion of the acetabulum into the pelvic cavity. With the medial movement of the acetabulum, the femoral head also shifts into the pelvis. Although the condition was first recognized in 1816, it was not until 120 years later that it was classified into primary (idiopathic) and secondary disease[1]. By today, a large number of conditions are known to cause secondary protrusio acetabuli [1]; the category of primary protrusio acetabuli is maintained for cases whose origins cannot be attributed to other diseases. As diagnostic techniques keep improving, the number of primary cases keeps decreasing [2].
Infections by gonoccous and echinococcus as well as tuberculosis were found early on to cause protrusio acetabuli. The main inflammatory conditions that cause protrusio acetabuli are rheumatoid arthritis [3], ankylosing spondylitis [4], psoriatic arthritis, acute idiopathic chondrolysis, and Reiter’s syndrome. A number of metabolic diseases are also associated with protrusio acetabuli. Among these are osteogenesis imperfecta, osteoporosis, hyperparathyroidism, and Paget’s disease. Some genetic ailments associated with protrusio acetabuli are sickle cell disease [5], trisomy 18, Ehlers-Danlos syndrome, and Marfan’s disease. Neoplastic disease and trauma to the bone, such as fractures, can also result in protrusio acetabuli [6].
Primary protrusio acetabuli is caused by deficiency or destruction of the bone structure or developmental abnormalities. It can be the source of osteoarthritis; senile patients with protrusio acetabuli almost always report with osteoarthritis [7]. Primary protrusio acetabuli is most frequently bilateral and affects women more than men. Several familial cases have been reported which were compatible with an autosomal dominant inheritance pattern [2]. Primary protrusio acetabuli showcases mainly with stiffness, limitation of movement, and possibly pain [2].
Workup has to start with a thorough history (including family history), and careful physical examination of the hip. Blood tests (such as complete blood count, the presence of C-reactive protein) and synovial biopsy will aid in the diagnosis. An anteroposterior radiogram is the most usual diagnostic tool, using plain radiography. Some investigators advocate the use of the false profile view, which reveals excessive cartilage destruction [8]. Magnetic resonance arthrography is useful for the detection of structural changes in the early stages of the disease.
In young patients, the condition is difficult to diagnose because the radiological signs are not characteristic. The difficulty also lies in differentiating protrusio acetabuli from acute idiopathic chondrolysis. With complaints of stiffness and pain – and with secondary causes, such as inflammation and trauma excluded –young people would be diagnosed with idiopathic chondrolysis if they display loss of joint space. If they also exhibit signs of an acetabular protrusion in addition to the loss of joint space, the diagnosis is protrusio acetabuli. Some investigators regard protrusio acetabuli and idiopathic chondrolysis as the same condition [9]; however, for various reasons, this view is not universally accepted. For example, idiopathic chondrolysis does not show a female preponderance [2].
A critical measurement for diagnosis deduced from radiographs is the center-edge angle of Wiberg [2], which is between 25 and 40 degrees in normal hips. An angle of more than 46 degrees indicates protrusio acetabuli. However, several authors find the angle measurement inadequate for diagnosis; for example, angles greater than 46 degrees can also be found in the general population [10]. Another important indication of protrusio acetabuli is the extent to which the medial wall of the acetabulum shifts medially in relation to the ilioischial line (Kohler’s line). Some investigators require the acetabular wall to reach the ilioischial line, while others call for the line to be crossed to establish the diagnosis. Yet another widely used criterion is the arrangement of the teardrop. Using the above criteria for diagnosis requires the careful centering of radiographic images. Other anatomical criteria can be used. A grading of the protrusion is provided by determination of the distance of the acetabulum from the upper margin of the superior pubic ramus [11].