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Protrusio Acetabuli: Diagnosis and Treatment

McBride, Mark T. MD; Muldoon, Michael P. CDR, MC, USN; Santore, Richard F. MD; Trousdale, Robert T. MD; Wenger, Dennis R. MD

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Journal of the American Academy of Orthopaedic Surgeons: March 2001 - Volume 9 - Issue 2 - p 79-88
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Abstract

Protrusio acetabuli, also known as arthrokatadysis, was first described by the German pathologist Otto in 1824. A translated excerpt of Otto’s original report provides a vivid description:

The right acetabulum protrudes into the pelvis like half an orange … at the crest corresponding to the center of the acetabular fossa, is an irregular circular defect, measuring one and one-half inches … supplying communication between the acetabulum and abdominal cavity … both the inner aspect of the acetabulum and the head of the femur are devoid of cartilaginous covering, and have the abraded and polished appearance of some gouty joints.1

Idiopathic, or primary, protrusio acetabuli is the diagnosis reserved for patients in whom no causative factors can be found. It is a diagnosis of exclusion; as more underlying disease states associated with this condition are recognized, the num- ber of diagnoses of primary protrusio acetabuli continues to decline. In those cases in which an underlying cause is identified, the term secondary protrusio acetabuli is used. Numerous processes are now known to cause protrusio acetabuli (Table 1).

Table 1
Table 1:
Causes of Secondary Protrusio Acetabuli

The prevalence of the disease process is unknown, but the natural history of those with the condition appears to be one of inexorable progression of the deformity, corresponding deterioration of function, and increase in pain. This is predictable on the basis of Pauwels’ theories of hip biomechanics.2 The major contributors to the forces across the hip are (1) body weight,

(2) distance from the center of the femoral head to the midline, and

(3) femoral neck-shaft angle. Using basic newtonian statics and a freebody diagram, one may calculate the various vector forces acting on the hip. Egan et al3 calculated the typical orientation for the jointreaction force to be 69 degrees from horizontal during the stance phase of gait. McCollum et al4 observed that protrusio acetabuli occurs at 65 degrees from the horizontal in patients with rheumatoid arthritis and concluded that the axis of migration was nearly the same as that of the joint-reaction force during stance.

Etiology

There are a number of theories regarding the etiology of primary protrusio acetabuli. Eppinger postulated that the condition was secondary to a chondrodystrophy wherein the three plates of the triradiate cartilage remain unfused, allowing protrusion of the femoral head medially into the pelvis.1 Alexander5 observed that an inward bulge of the acetabulum was normal in children between the ages of 4 and 8 years, and favored the theory of Eppinger. He stated that the “protrusion is reversible, diminishing stresses after the age of 8 years, permitting corrections in the majority of children.” He believed that protrusio in the adult is the result of failure of this normal correction.

Macdonald6 reported on four generations of one family affected by protrusio acetabuli. One index subject in the first generation had the condition, as did all three members of the second generation. Most members of the ensuing generations had borderline cases. A familial study by Ventruto et al7 reinforced the concept that protrusio may well be an inherited disease. Sherlock8 recently published a case study indicating a link between acute idiopathic chondrolysis and primary protrusio acetabuli. In that study, computed tomographic scans demonstrated areas of focal osteonecrosis of the femoral head, as well as degenerative changes in the acetabulum, in patients with idiopathic protrusio acetabuli.

Many of the early reported cases were secondary to untreated septic arthritis, particularly gonococcal and tuberculous infections. The condition is also known to develop in patients with defective collagen, such as those with Marfan or Ehler Danlos syndrome, osteogenesis imperfecta, or trisomy 18.9–11 There are numerous reports in the literature of patients with acetabular deficiency secondary to metastatic cancer, as well as in those with neurofibromatosis (prevalence of up to 20%).12 In inflammatory arthritides, protrusio is very common, occurring in as many as 15% of those with rheumatoid arthritis in the hip and 33% of those with ankylosing spondyli- tis,13,14

There are some who feel that in many cases diagnosed as primary protrusio acetabuli an overlooked subtle metabolic abnormality is actually the cause. Certainly, osteo- malacia and Paget’s disease have been identified as etiologic factors. The incidence of protrusio acetabuli in patients with documented osteomalacia may be as high as 50%.14 This is in sharp contrast to the incidence in patients with osteoarthritis, which is generally estimated to be about 5%. More information with respect to bone metabolism and histomorphometry in patients with idiopathic protrusio may reveal additional risk factors for acquiring this condition.

In summary, secondary protrusio acetabuli can have an inflammatory cause or a noninflammatory cause (i.e., a metabolic, neoplastic, or connective-tissue disorder). Inflammatory causes lead to destruction and weakening of the bone surrounding the hip with resultant migration along the joint-reaction vector. In cases with metabolic or connectivetissue causation, there is a qualitative deficiency of the bone. The thin medial acetabular wall has less strength than the better supported bone in the superior portion of the joint, and when it falls below the threshold of strength required to withstand the medial component of the joint-reaction force, secondary protrusio occurs. This results in the medial pattern of migration seen in these cases. Once the joint-reaction vector has migrated medial to the ilioischial line, the rate of progression increases.

Diagnosis

History and Physical Examination

As with all other medical problems, a thorough history and physical examination are critical. A family history of similar or related problems should be well documented. Because numerous syndromes and disease states are associated with protrusio acetabuli, it is important to do a complete review of systems in addition to the musculoskeletal system. Of particular importance are the neurologic and cardiovascular examinations. This will greatly enhance the likelihood of identifying any underlying cause and thereby avoiding embarkation on an injudicious treatment plan. The symptoms of idiopathic protrusio frequently first develop in adolescence; therefore, this condition should be considered in the differential diagnosis for hip pain in the teenager,15,16

Patients with protrusio acetabuli typically present with complaints of activity-related pain in the groin region as well as stiffness. Occasionally, the presenting complaint is knee pain. Arising from a seated position is a frequent cause of exacerbation. There is often a decrease in both active and passive range of hip motion, especially in abduction. Pain may occur with active straight leg raise or at extremes of motion. There may be a positive Trendelenburg sign secondary to the shortened lever arm of the mechanically disadvantaged abductors. With ambulation, there may be an antalgic gait, a Trendelenburg gait, or both. Physical manifestations of related disease processes may or may not be present.

Radiographs

Standard anteroposterior (AP) and lateral radiographs of the pelvis are critical both to make the diagnosis and to stage the severity of protrusio (Fig. 1). Many early authors expressed their dissatisfaction with the absence of a coherent grading system. The center-edge angle, described by Wiberg to grade acetabular dysplasia, has generally been adopted; an angle of over 40 degrees is diagnostic of protrusio acetabuli.17 Armbuster et al18 considered protrusio to be present if the medial wall of the acetabulum protruded medial to the ilioischial line by 3 mm in males or 6 mm in fe- males and noted that mild coxa vara is not uncommon. Sotelo-Garza and Charnley19 used the ilioischial line on an AP radiograph of the pelvis as a reference point from which to measure the location of the acetabulum. This distance was used to designate the condition as mild (1 to 5 mm), moderate (6 to 15 mm), or severe (>15 mm). Gates et al20 compared multiple measurements in a series of patients with protrusio and discovered that the teardrop is the most consistent landmark, varying little with minor degrees of pelvic obliquity. They recommended utilizing an X-Y coordinate system based on the hip center in relation to the teardrop as the most useful way to assess and track progression of protrusio.

Figure 1
Figure 1:
Radiographic landmarks used in the diagnosis of protrusio acetabuli: A = ilioischial (Kohler) line; B, iliopectineal line; C = acetabular wall; D = interteardrop line; E = normal center-edge angle; F = abnormal center-edge angle indicative of protrusio acetabuli.

Laboratory Studies

It is important to obtain basic blood tests to facilitate identification of a possible underlying cause. These include complete blood cell count, erythrocyte sedimentation rate, rheumatoid factor, antinuclear antibody, and serum chemistry determinations. If concern regarding an inflammatory etiology is engendered by the results of this workup, a synovial biopsy may be required to make a definitive diagnosis.

Treatment

The key consideration before proceeding with operative treatment is identification and treatment of any underlying disease process. Selection of the most appropriate surgical option is based on the patient’s age and skeletal maturity and the extent of degenerative changes visualized on plain radiographs.

Skeletally Immature Patients

Patients with protrusio acetabuli of noninflammatory origin who have an open triradiate cartilage may be considered for surgical tri- radiate closure. Steel21 reported a series of 22 patients with Marfan’s syndrome and protrusio acetabuli. Eleven patients (21 hips) underwent surgical triradiate closure. The center-edge angle, the appearance of the teardrop, and the relationship of the acetabulum to the ilioischial line were evaluated on plain radiographs of 19 hips at maturity. On the basis of these factors, 12 of the 19 hips were considered to be restored to normal, 4 were reduced to “acetabular deepening,” and 3 were unchanged.

Depending on the neck-shaft angle of the proximal femur, surgical triradiate closure may be combined with valgus intertrochanteric osteotomy (VITO) in an attempt to establish more normal hip biomechanics. Figure 2 demonstrates successful surgical treatment of protrusio acetabuli in a skeletally immature patient who underwent combined triradiate epiphysiodesis and proximal femoral valgus osteotomy. The 12-year-old girl had no evidence of a secondary cause for the condition, and radiographic examination revealed a center-edge angle of 45 degrees.

Figure 2
Figure 2:
A, AP radiograph of the pelvis of a 12-year-old girl with idiopathic protrusio acetabuli. B, Radiograph obtained 2 years after triradiate cartilage epiphysiodesis and VITO.

Adolescent or Young Adult Patients

In the skeletally mature young adult with idiopathic protrusio acetabuli, the treatment options are very limited. Historically, surgical approaches included resection arthroplasty and arthrodesis. An acetabuloplasty was described by Smith-Peterson to treat the condition,22 and a hanging hip procedure was devised by Voss.17 Pauwels2 first described VITO for protrusio and documented several successful results, providing a biomechanical rationale for an osteotomy to relieve medializing forces on the affected hip (Fig. 3).

Figure 3
Figure 3:
Effects of VITO on the medializing force (vector Q) in the hip (CCD = femoral neck-shaft angle). (Adapted with permission from Pauwels F [ed]: Biomechanics of the Normal and Diseased Hip. Berlin: Springer-Verlag, 1976, p 245.)

No other series in which VITO was used for protrusio was reported until 1973, when Rosemeyer et al23 showed good to excellent results in 21 of 25 hips at 6-year follow-up. A good result was correlated with young age as well as with minimal preoperative arthritis.

In the series of Hooper and Jones,17 of 57 patients treated by a variety of means, 9 underwent valgus osteotomy. The authors reported fair or poor results in 7 of the 9 patients with 2- to 7-year follow-up and concluded that the only operation with a predictable outcome was total hip arthroplasty (THA). In a 1978 study, Verburg and Elzenga24 reported good improvement in all 6 patients, with markedly improved function in the 3 patients evaluated at a mean follow-up interval of 3 years. These authors noted, as had Rosemeyer et al,23 that the best results were obtained in patients who were young and had minimal arthritic changes.

Poss25 has written that the keys to valgus osteotomy are lateralization of the femur to restore mechanical alignment and soft-tissue releases, particularly of the psoas tendon, to help effect lateralization and to improve motion of the hip. The amount of valgus correction is typically determined on the basis of the amount of preoperative adduction present. Generally, a correction of 20 to 30 degrees is desirable. Excessive correction can lead to abductor contracture. A trapezoid-shortening osteotomy has been recommended as well, to minimize the limb-length inequality that occurs when a large correc- tion is performed (Fig. 4). Increased abductor tension can be reduced by an opening-wedge osteotomy of the greater trochanter if necessary.

Figure 4
Figure 4:
A, AP radiograph of the pelvis of a 19-year-old man with bilateral protrusio. B, Radiographic appearance 2 years after VITO.

Clinical Experience With VITO

The combined experience with VITO at the Mayo Clinic, Children’s Hospital San Diego, Sharp Memorial Hospital, and Naval Medical Center San Diego are summarized in Table 2. A total of 19 hips in 12 patients were treated with VITO for primary protrusio. One skeletally immature patient was treated with triradiate epiphysiodesis as well. All patients had a thorough preoperative workup to rule out inflammatory disease and metabolic bone disease. The patient group consisted of 8 women and 4 men, ranging in age from 18 to 47 years. Clinical follow-up was 2 to 33 years. Eight hips in 5 patients were revised to total hip replacement. The interval from index osteotomy to primary THA ranged from 10 months to 15 years (Fig. 5). All patients except the 2 with less than a 3-year interval between osteotomy and THA were satisfied with the decision to undergo osteotomy. One patient required revision of bilateral THAs 21 and 26 years after the initial procedures.

Table 2
Table 2:
Data on 12 Patients Treated With VITO
Table 2
Table 2:
continued
Figure 5
Figure 5:
A, AP radiograph of the pelvis obtained 16 years after bilateral VITO. B, Patient subsequently underwent bilateral THA. This radiograph was obtained 1 year after that procedure.

A variety of fixation devices were used to stabilize the intertrochanteric osteotomies, including AO blade-plates, Harris plates, sliding compression screws, and, in the earliest cases, triflange nail and a side-plate and Wainwright spines. Iliopsoas tendon releases were performed in 11 hips, and in 6 patients trapezoidal osteotomies were used to shorten the femur.

One patient had a nonunion that required bone grafting but went on to heal uneventfully; no further surgery other than hardware removal was necessary. One patient who underwent reoperation for inadequate correction did relatively well, but the occurrence of more symptoms necessitated reoperation. The third patient with bilateral osteotomies underwent revision for a recurrent flexion deformity; healing eventually occurred, but THA was required 6 years after the initial osteotomy. Abduction contractures were treated with iliotibial band lengthening at the time of hardware removal in 1 patient.

Although this series represents a relatively small number of patients with varying follow-up who were treated at different institutions, some pertinent observations can be made. Clearly, there is a subgroup of patients with protrusio acetabuli who cannot be effectively managed with osteotomy. On the basis of this study as well as the work of Verburg and Elzenga24 and of Rosemeyer et al,23 the VITO procedure should not be performed on patients who are over age 40 years or who have significant degenera- tive changes visualized on plain radiographs. The longest survivorship after this procedure was associated with decreased hip motion and diminished exercise tolerance. However, the patients remained satisfied with their results and felt that they would have gone on to further surgery earlier if they had not undergone the osteotomy. Interestingly, limited preoperative motion does not preclude a satisfactory outcome, as several longterm survivors were quite stiff preoperatively. There were no difficulties encountered in conversion to THA after VITO, and survivorship in this group mirrors the results reported in the literature for primary cemented THAs.

Older Adult Patients

The surgical options for the older adult with protrusio acetabuli are VITO or some form of arthroplasty. The results of VITO in this age group are less predictable than in younger patients, especially if there is radiographic evidence of arthritis. Wilson and Scott26 and Torisu et al27 reported relatively poor results after use of bipolar hemiarthroplasty combined with bone grafting. In both studies there was a high percentage of continued medial and superior migration at intermediate follow-up. The authors expressed concern regarding the longevity of this approach.

Total hip arthroplasty is the recommended treatment for the older adult with protrusio acetabuli and degenerative changes. Sotelo-Garza and Charnley19 reported on the use of THA to treat protrusio in 253 hips and found that the outcome of cemented THA for protrusio was not different from that of other primary THAs at a minimum follow-up interval of 60 months. Ranawat et al28 reported on 35 hips with protrusio acetabuli secondary to rheumatoid arthritis that had been treated with cemented THA and had been followed up for an average of 4.3 years. They reported loosening in 16 of 17 hips reconstructed with the cup center more than 10 mm from the anatomic center. Of the 13 hips reconstructed with the cup center within 5 mm of the anatomic center, none was loose.

Crowninshield et al29 used finiteelement analysis to identify the variables affecting stress on the acetabulum after reconstruction for protrusio acetabuli. They discovered that medial cup placement led to high medial stresses, whereas anatomic placement resulted in decreased medial stresses. Furthermore, they found that reinforcement of the medial wall with cement and wire mesh was not effective, but that a metal-backed component was effective, due to the superior stress distribution of the metal cup. Their data also suggested that a metal protrusio ring device more reliably transfers stress from the medial wall to the rim than does a flanged protrusio cup.

In clinical series, Bayley et al30 and Gates et al20 confirmed the importance of restoring the hip to an anatomic center. They noted that 50% of reconstructed hips with a cup center more than 10 mm from the anatomic hip center had failed.

Use of bone graft is the optimal method of cup lateralization. Heywood31 described a technique whereby the femoral head was used as a graft. The interior of the femoral head was scooped out, and a reverse reamer was utilized to denude the remaining cartilage from the articular side, which was then put into the defect of the protruding acetabulum to restore the hip center. This “concavoconvex”graft was press-fitted into the medial wall defect, and an all-polyethylene cup was then cemented into the reconstructed socket in the standard fashion. He reported excellent shortterm results with this technique in a series of nine hips.

McCollum et al4 reported the results obtained by using a similar technique in a series of 39 hips with an average follow-up of nearly 5 years. On serial radiographic follow-up, all grafts had incorporated without evidence of loosening or migration. Subsequently, numerous other authors have demonstrated nearly universal incorporation of morcellized or fragmented grafts, both with and without cement. This results in restoration of the anatomic center and provides medial bone stock for potential future revision surgery with less graft preparation time.

The approach most widely used today is to fill the defect with morcellized graft and then to use a porous-coated metal cup for the acetabular reconstruction (Fig. 6). Porous-coated “deep profile” components may be used without bone graft in mild cases. On occasion, antiprotrusio cages may be required for a primary reconstruction. Ranawat and Zahn32 have recommended the following guidelines: In cases in which the protrusion is less than 5 mm, bone graft is not required. When the protrusion is greater than 5 mm and there is an intact medial wall, bone graft without augmentation devices is appropriate. If there is gross deficiency of the medial wall, bone graft with consideration of additional fixation devices (hemispherical noncemented cup with screw supplementation or antiprotrusio ring) is indicated. The choice of surgical approach is determined by surgeon preference; however, a trochanteric osteotomy is sometimes required in severe cases to facilitate initial femoral head dislocation. An alternative is an in situ neck cut to allow mobilization of the femur, followed by removal of the femoral head with use of a corkscrew.

Figure 6
Figure 6:
A, AP radiograph of the pelvis of a 48-year-old man with bilateral protrusio. B, Film obtained 18 months after THA shows complete incorporation of medial bone grafts.

Summary

Idiopathic protrusio acetabuli is a rare cause of osteoarthritis in young adults. Further study of the genetic, histomorphometric, and anatomic factors that may contribute to its development is warranted. The secondary causes of protrusio must be identified and managed before embarking on surgical treatment. Synovial biopsy may be required for diagnosis in some cases.

The surgical treatment options are age-specific. In the skeletally immature patient with progressive protrusio acetabuli, triradiate fusion has been demonstrated to arrest progression or improve the condition in the majority of reported cases. Valgus intertrochanteric osteotomy is effective for a select group of young patients with protrusio. Long-term pain relief and restoration of function can be achieved in properly selected individuals. Furthermore, osteotomy can delay the need for a THA for a decade or more. For the older adult with significant arthritis as well as protrusio, THA with nonstructural bone grafting of the medial wall cavity can be effective. This approach achieves two important objectives: (1) restitution of bone stock and (2) lateralization of the hip center to the anatomic position to ensure the greatest chance of a durable long-term outcome.

References

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