Intrapelvic protrusio acetabuli, a deformity of the hip joint in which the medial wall of the acetabulum invades the pelvic cavity with associated medial displacement of the femoral head, is caused by a variety of factors. The etiology of the deformity includes both primary idiopathic and secondary neoplastic, infectious, metabolic, inflammatory, traumatic, and genetic disorders1.
It was not until 1978 that protrusio acetabuli was added to the manifestations of Marfan syndrome. Höhle2 discovered protrusio acetabuli in two of his patients who had arachnodactyly, keel breast, ectopia lentis, and a familial incidence of protrusio acetabuli—all clinical manifestations of Marfan syndrome. Nevertheless, he stated that “the etiology and pathogenesis of idiopathic protrusio acetabuli is not yet explained in all aspects,” and he did not link Marfan syndrome to the development of protrusio acetabuli. In the same year, Steel3 reported seven cases of protrusio acetabuli, five of which were in patients who had all of the clinical manifestations of Marfan syndrome. Two years later, in 1980, Steel4 reported forty-six cases of Marfan syndrome and came to the conclusion that protrusio acetabuli was a constant finding in that disorder and that it frequently is very severe and associated with scoliosis. Wenger et al. confirmed this finding5, suggesting that protrusio acetabuli may be related to the same abnormal mesenchymal tissues that predispose patients to the development of scoliosis.
In 1896, the case of a five and a half-year-old girl was presented to the Société Médicale des Hôpitaux in Paris by Antoine Marfan6, a French pediatrician. The girl was reported to have atrophic muscles and a “remarkable” elongation of the bones, which was perfectly symmetrical and most notable in the hands (Fig. 1). The fingers were flexed by fibrous contractures, with an appearance similar to the legs of a spider (arachnodactyly, as Achard7 called this deformity in his patient). Marfan used the name dolichostenomelia, which is Greek for long, narrow limbs. By the time the patient was eleven years old, scoliosis, kyphosis, and a funnel-chest deformity had developed8. On the basis of the joint stiffness and the family history of convulsions, congenital cataracts, and an abortion in the first trimester, later analysis suggested that the patient probably had a deficiency of cystathionine synthetase, which is the biochemical derangement underlying homocystinuria9, the main differential diagnosis of Marfan syndrome.
Weve10 recognized the autosomal dominant inheritance of the syndrome, with a high penetrance but variable expressivity, and suggested that the basic cause of Marfan syndrome was a defect of tissue derived from embryonic mesoderm. He named the syndrome dystrophia mesodermalis congenital, typus Marfanis. McKusick11 labeled Marfan syndrome as a “heritable disorder of connective tissue.”
Since the detection of a mutation in the gene encoding the microfibrillar protein fibrillin-1 (FBN-1) mapped to 15q2112, more than 550 different mutations that are widespread throughout the gene have been found to cause Marfan syndrome13. Because of extensive variation in patient populations and limitations in screening techniques, an inability to detect these mutations in FBN-1 does not exclude the diagnosis of Marfan syndrome in a person who meets the clinical criteria14. Recently, mutations in the gene encoding transforming growth factor-β receptor 2 (TGFBR2) were associated with Marfan syndrome15.
During the twentieth century, other features of Marfan syndrome were added to complete the list of clinical criteria necessary for the diagnosis of the syndrome.
Ectopia lentis was first described in two tall, loose-jointed siblings by Williams in 187616 and was associated with Marfan syndrome in 191417. The ocular abnormality occurred in 60% of 160 patients with Marfan syndrome18, and usually it was bilateral and the lenses were displaced superotemporally. (In homocystinuria, the subluxated lenses rest downward.) Whenever Marfan syndrome is suspected, the patient must be evaluated with a slit-lamp examination with the pupils in mydriasis to look for ectopia lentis, with keratometry to measure the abnormally flat cornea, and with ultrasound to measure the increased axial length of the globe, which contributes to myopia, an increased risk of retinal detachment, and lens subluxation.
The cardiovascular manifestations, including dissection19 and dilatation of the ascending aorta20 and mitral valve prolapse21, are responsible for nearly all of the precocious deaths of patients with Marfan syndrome22. The routine use of echocardiography for the detection of these cardiovascular complications together with recent advances in treatment, including beta-blocker medications and cardiothoracic surgery, have enabled patients with Marfan syndrome to have a nearly normal life expectancy23.
Dural ectasia, a usually asymptomatic enlargement of the neural canal that is nearly always found in the lumbosacral region, is a common feature of Marfan syndrome24. Axial computed tomography or magnetic resonance imaging scans of the lumbosacral region are required for an adequate evaluation.
The most recognizable skeletal feature of Marfan syndrome is the increased length of the limbs as compared with the trunk, resulting in an arm span-to-height ratio of >1.05; this ratio may be exaggerated by scoliosis, which occurs in at least 60% of patients with Marfan syndrome and is generally thoracic and convex to the right. Further investigation reveals chest deformities caused by longitudinal overgrowth of the ribs, with pectus excavatum (funnel chest) found more frequently than pectus carinatum (pigeon chest). However, both deformities may be present in one patient and may change with growth25. Joint hypermobility, resulting in pes planus, is frequently present, but it is of little diagnostic specificity. In contrast, reduced extension at the elbows as a result of congenital joint contractures is accepted as an important diagnostic manifestation of Marfan syndrome. Arachnodactyly is suspected if the thumb, when maximally opposed within the clenched hand, projects beyond the ulnar border (thumb sign; Fig. 2) and the distal phalanges of the first and fifth digits of one hand overlap when the patient grasps the contralateral wrist (the wrist sign). However, these tests are subject to observer interpretation and may reflect the longitudinal laxity of the hand rather than arachnodactyly.
Since its recognition by Steel, many authors have stressed the importance of protrusio acetabuli in Marfan syndrome, which has a reported prevalence of 31% (in twenty-one patients26) and 100% (in twenty-two patients27). The deformity was accepted for inclusion in the International Nosology of Heritable Disorders of Connective Tissue28, Berlin, as a diagnostic manifestation of Marfan syndrome, although at that time it was regarded as less specific. When De Paepe et al.29 revised the diagnostic criteria for Marfan syndrome because of weaknesses in the Berlin Nosology, accentuated by the advent of molecular testing, they included protrusio acetabuli in the list of major skeletal criteria for diagnosis of the disorder. The revised criteria, which include a combination of major and minor clinical manifestations in the different affected organ systems as well as factors in the family and genetic history, provide the guidelines for diagnosing patients with Marfan syndrome.
Diagnosis of Protrusio Acetabuli
It is not unreasonable to call protrusio acetabuli a silent deformity30, as it remains asymptomatic until degenerative and irritative changes occur. When prolonged protrusio acetabuli results in secondary osteoarthritic changes—and this may not happen for years—the patient presents with progressive activity-related pain in the groin and stiffness1. A waddling gait31 may be noticed by the patient or relatives, and the patient may find it impossible to spread the limb or to stoop. Flexion contractures, accompanied by a forward tilt of the pelvis, may be present. The patient may create a so-called hollow back (hyperlordosis of the lumbar spine) to compensate for the pelvic tilt. Eventually, the deformity will progress to the point that all movements of the hip become restricted and walking is impossible.
Physical examination of the hip joint demonstrates a decrease in the range of motion, especially limitation of flexion and abduction of the femur. A positive Trendelenburg sign secondary to the shortened lever arm of the mechanically disadvantaged abductors may be found. In severe cases, a globular swelling projecting into the pelvis can be felt on abdominal, rectal, or vaginal palpation32. This globular swelling can interfere with normal vaginal delivery of a child so that a cesarean section is inevitable. An indentation in the head of a newborn from maternal protrusio acetabuli was once reported33; it disappeared in a couple of days. The end point of progression of protrusio acetabuli is the arrest of the femoral trochanter at the lateral margin of the pelvis, which makes further migration impossible.
Radiographic Findings (Table I)
Serial standard anteroposterior radiographs of the pelvis are essential for the diagnosis and the assessment of the severity and progression of protrusio acetabuli. When it is so severe that the acetabulum, projecting into the pelvis as a rounded, globular, dome-shaped mass, extends up to the sacroiliac joint and surrounds the neck of the femur as an “irregular serrated vegetative formation”32, the diagnosis of protrusio acetabuli is self-evident. In mild cases, it may be difficult to distinguish between a physiological deep acetabulum and true acetabular protrusion. Several methods for identifying protrusio acetabuli have been described.
In 1905, Köhler34 discussed the existence of the teardrop figure as a radiographic representation of acetabular contour and integrity. The anatomy of the teardrop, seen on the anteroposterior radiograph of the pelvis, was later outlined with metal markers to indicate its sinusoidal landmarks: “The lateral border of the pelvic teardrop, therefore, is made up of the cortical surface of the middle third of the acetabular fossa, commencing at the lunate surface proximally and extending to the cotyloid notch inferiorly. Continuation through the obturator foramen marks the inferior aspect of the roentgenographic structure. The medial aspect of the tear figure is made up of the cortical surface in the true pelvis, where a groove for the obturator vessels and nerve exists, continuing in the same plane as far as the arcuate line immediately posterior to the iliopectineal prominence.”35 Changes in the contour of this u-shaped radiographic landmark—particularly of the “dynamic” lateral border, in contrast to the stable medial line36—suggest a pathological process.
Overgaard37 agreed with Köhler that the teardrop figure is altered in protrusio acetabuli, secondary to medial migration of the lateral wall resulting first in closing, then crossing, and finally reversal of the teardrop (Fig. 3).
Friedenberg38 used the center-edge angle, introduced by Wiberg39 as a measurement of acetabular development or the degree of displacement of the femoral head, to measure inward protrusion of the femoral head into the deepened acetabulum. Wiberg noted that this angle, which is formed by a vertical line drawn through the center of the femoral head and a line drawn from the center through the lateral edge of the acetabular roof (Fig. 4), ranges from 20° to 46° in normal adults, with an average of 36°. In children, a range of 15° to 40° with an average of 30° (identical in both sexes) can be considered normal, with an increase in the angle up to the age of fifteen years40. An angle of >40° raises the suspicion of a deep acetabulum. The center-edge angle is useful for monitoring the progression of protrusio acetabuli. However, some authors have found the center-edge angle to be unreliable because it is altered by the angle of incidence of the x-ray beam and because of the large spread of values according to age group41.
The ilioischial line (or Köhler line)—the tangential view of the cortex of the quadrilateral plate to its termination at the obturator groove, continued as the sharp cortical posterior descending limb of the obturator foramen (Fig. 5)—is an indication of the integrity of the cortical quadrilateral plate of the pelvis. Crossing of the acetabular line medial to the ilioischial line, by >3 mm in men, >6 mm in women42, >1 mm in boys, and >3 mm in girls41, is regarded by some as the most reliable measurement of protrusio acetabuli as it is not altered by the position of the femoral head or the angle of incidence of the x-ray beam. Others thought that this parameter alone is not accurate for determining true protrusio acetabuli and concluded that the best single radiographic indicator of protrusio acetabuli is the crossing of the acetabular and the iliopectineal line26—i.e., the cortical ridge forming the lower boundary of the greater pelvis. According to one study26, displacement of the femoral head medial to the ilioischial line is the least reliable parameter for determining protrusio acetabuli, although it was used by Hastings and Parker43.
Schaap44 found it striking that in hardly any of his patients was the angle between the neck of the femur and the shaft larger than 120° (125° is generally accepted as the normal angle), so that he found a mild degree of coxa vara in his study when he had expected coxa valga. Brailsford45 attributed the coxa vara to the femoral head sinking into the deeper acetabulum. He also suggested the existence of hypertrophy of the greater trochanter.
Currently, most authors accept the diagnosis of protrusio acetabuli if findings derived with at least two of the measurement methods suggest it (Table I). Alexander46 did not use the center-edge angle but did evaluate whether the crossing of the teardrop and the femoral head reached “the straight line,” with some underdiagnosis as a result. Wenger et al.5, in their report on protrusio acetabuli in Marfan syndrome, used a combination of the center-edge angle and the crossing of the teardrop by the femoral head to establish the diagnosis. Kuhlman et al.47 used the crossing of the ilioischial line by the acetabular line, the center-edge angle, and the crossing of the teardrop by the ilioischial line or the femoral head as the criteria for diagnosing protrusio acetabuli. The diagnosis was accepted only if the first finding was present and one other criterion was met.
Treatment of Protrusio Acetabuli
Rest in bed was promoted in early reports because it was believed that this removed the thrust of the femur on an already weakened acetabulum32. Nonoperative treatment in the form of forcible stretching with the patient under anesthesia and weight-extension on an abduction frame was used to overcome the contractures around the hip30. Palliative measures such as the use of local heat in the form of diathermy and reeducation regarding active movements were employed in an attempt to retain painless movement for long periods. Even though nonoperative management of protrusio acetabuli cannot arrest the progression of the processes that cause it, exercises are currently suggested to improve the function of the hip; however, it is advised that patients avoid excessive physical stress48. Vigorous stretching to decrease concomitant loss of hip abduction due to protrusio acetabuli can produce bilateral fatigue fracture of the femoral neck49 in a patient with Marfan syndrome.
In 1936, Smith-Petersen50 reported the first operative procedure for the treatment of protrusio acetabuli—i.e., acetabuloplasty. By resecting anterior parts of the acetabular margin and the femoral head, this procedure removes areas of osseous contact that prevent free motion of the hip and cause irritation within the joint. It also removes a sufficiently large portion of the sensitive joint capsule. Several surgical options are now available to arrest progression, relieve pain, and restore the function of the hip. The treatment of choice is age-specific.
Older adults with protrusio acetabuli and substantial arthritis can be treated effectively with total hip arthroplasty with non-structural bone-grafting of the medial cavity. Sotelo-Garza and Charnley51 found very satisfactory results in their study of 253 total hip replacements in patients with protrusio acetabuli. The immediate and late complications were no different from those in patients with ordinary cases of osteoarthritis and rheumatoid arthritis. Furthermore, Sotelo-Garza and Charnley did not find noticeable differences between the results of total hip arthroplasties performed with bone graft and those done with cement alone. However, in a later report52, patients treated with cement alone had the highest rate of loosening of the acetabular component, which is the most common cause of long-term failure of total hip arthroplasty. The addition of metal backing to the component, together with lateral positioning of the prosthesis, reduces the stresses responsible for the loosening53. Lateral positioning (the Heywood technique54) is based on the biomechanical principle that, in protrusio acetabuli, the upward inclination of the acetabular roof increases the force that drives the head into the softened acetabular floor (Fig. 6). Heywood suggested that, in order to prevent inward migration of the acetabular component, the prosthesis should be positioned laterally near the acetabular rim. He preferred to use living bone from the femoral head rather than synthetic materials for grafting the acetabular floor to reinforce the medial wall of the acetabulum. Autogenous and homologous bone grafts, stabilized by a metal prosthesis when necessary, appeared to be equally effective56. Ranawat and Zahn57 recommended that autogenous bone graft be used when protrusio acetabuli is >5 mm with a thin but intact medial wall and that it be reinforced with additional fixation devices when a grossly deficient medial wall is present. The approach that is most widely used today is filling of the defect with morselized graft followed by insertion of a porous-coated metal cup58 for the acetabular reconstruction. To facilitate initial dislocation of the femoral head, a trochanteric osteotomy or an in situ osteotomy of the femoral neck—to allow mobilization of the femur—followed by removal of the femoral head is sometimes required.
Since the original description of the technique by Pauwels59, valgus intertrochanteric osteotomy has become an alternative to total hip arthroplasty, especially for a select group of patients who are younger than forty years old and have minimal arthritis. Depending on the amount of preoperative adduction, a valgus correction of 20° to 30° reduces the transverse vector of the forces acting on the hip joint, and driving the femoral head into the acetabulum, and this results in long-term pain relief and restoration of function. Furthermore, a valgus intertrochanteric osteotomy can delay the need for total hip arthroplasty for a decade or more, without negatively affecting the ability to perform such an arthroplasty later in life1.
In 1978, Steel3 not only related Marfan syndrome to protrusio acetabuli but also, on the basis of the observed effects of traumatic closure of the triradiate physis in the growing pelvis and of operative closure of this growth plate in growing animals, proposed operative closure of the triradiate physis to reverse or arrest the progression of protrusio acetabuli. To avoid invading what Steel termed the papyraceous articular cartilage of the acetabulum at the confluence of the three limbs, the physis is curetted in the pubic area and in the vertical flange, and on each side of each limb; a small cortical graft taken from the crest of the ilium is slotted in the bone to bridge each of the three limbs of the physis. This operative intervention should be reserved for children with Marfan syndrome in the age range of eight to ten years who have documented progression of acetabular deepening.
The question now arises regarding when to operate on a patient with Marfan syndrome who is diagnosed as having protrusio acetabuli. Is an operation on a patient with Marfan syndrome indicated the moment protrusio acetabuli is discovered on a radiograph; or is it more advisable to operate only when progression either is verified on serial radiographs or is expected on the basis of a family history of symptomatic protrusio; or, finally, should an operation be performed only when the deformity has become symptomatic?
Fast et al.60 suggested postponing operative treatment until hip symptoms make further delay unavoidable. In addition to the radiographic examination, a good physical assessment and evaluation of clinical symptoms should be performed in order to predict the need for operative intervention1. Yule et al.61 suggested that, to avoid unnecessary irradiation, radiographic examination should not be performed routinely but be done only if identification of protrusio acetabuli would change the final diagnosis to Marfan syndrome. Steel27 described the indication for his operative technique as “a full-blown diagnosis of MFS, between the ages of 8 and 10, with a family history of protrusio and progression of the CEA, collapse of the teardrop, and hip symptoms.” Finally, as we stated previously62, we believe closure of the triradiate cartilage to be indicated in patients with Marfan syndrome when the triradiate cartilage is still open and the protrusio acetabuli appears to be progressing even if the patient has no hip symptoms or limitation of motion. This early intervention may abate the inevitable loss of motion and the future pain of osteoarthritis in these carefully selected patients with Marfan syndrome. It is reasonable to perform a simple, early procedure to arrest or reverse acetabular protrusio, rather than waiting for progression, which requires more complex procedures at a later time.
The authors did not receive grants or outside funding in support of their research for or preparation of this manuscript. They did not receive payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the authors are affiliated or associated.
Investigation performed at Shriners Hospitals for Children, Honolulu, Honolulu, Hawaii
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