Introduction                       

Protrusio acetabuli, also known as arthrokatadysis, was first described by the German pathologist Otto1 in 1824 as a result of remodeling of weak, medial acetabular bone after multiple, recurring stress fractures. Standard anteroposterior (AP) radiograph of the pelvis is critical both to make the diagnosis and to stage the severity of protrusio (Fig. 28.1). The center-edge angle, described by Wiberg2 to grade acetabular dysplasia, of over 40 degree is diagnostic of protrusio acetabuli. Armbuster et al3 considered protrusio to be present if the medial wall of the acetabulum protruded medial to the Ilioischial line (Kohler’s line) by 3 mm in males or 6 mm in females (Figs 28.2A and B).

 

Etiology                         

 

 

Protrusio acetabuli can be primary or secondary. Idiopathic, or primary1,4 protrusio acetabuli is the diagnosis reserved for patients in whom no causative factors can be found.

 

Figure 28.1: Radiographic landmarks used in the diagnosis of protrusio acetabuli; Kohler’s line and angle of Wiberg

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Total Hip Arthroplasty

 

Figures 28.2A and B: Protrusio is present if the medial wall of the acetabulum protrudes medial to the Ilioischial line

 

Figure 28.3: Secondary protrusio in a patient with rheumatoid arthritis

 

Primary protrusio affects young patients and is generally diagnosed at adult age. Its etiology in childhood is attributed to changes in tri-radiate cartilage ossification. The expression Otto-pelvis is used only in primary acetabular protrusion. McCollum et al5 reported that protrusion is progressive and only stops when the great trochanter touches acetabular border.

Secondary6-9 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 connective tissue causation, there is a qualitative deficiency of the bone. The usual causes are rheumatoid arthritis (Fig. 28.3), ankylosing spondylitis (Fig. 28.4), osteoarthritis, chronic renal osteodystrophy, osteoporosis, Paget’s disease, neoplasias, surgery sequale, trauma, infections, and even Marfan’s syndrome. Protrusio has been reported in 20-25% patients with rheumatoid arthritis.7 It generally progresses at the rate of approximately 2 mm per year. However, rapidly progressive type is

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Figure 28.4: Secondary protrusio in a patient with ankylosing spondylitis. Note fusion of sacroiliac joint

Figure 28.5: Secondary severe protrusio in a case with hemiarthroplasty

 

Total Hip Replacement for Protrusio Acetabuli

 

also seen and 7.5 mm increase in 40 days has been seen. Sudden onset of hip pain and significant decrease in ambulatory capacity should alert an orthopedic surgeon about rapid progression of protrusio. This rapid progression is more common in patients on steroids.

It can also be seen following hemiarthroplasty or total hip arthroplasty (Fig. 28.5) because of the medial migration of cup resulting from pressure, sepsis, excessive reaming, fracture of the medial wall, or mechanical loosening with resultant destruction of the medial wall. In secondary acetabular protrusion, healing of the medial wall seldom occurs spontaneously and frequently there is no bone support. In Sotelo-Garza and Charnley studies,10 the incidence of primary protrusio acetabuli was of 75.3% and secondary, due to causes such as rheumatoid arthritis was 18.7%.

 

Matter          of           Concerns                    

For an arthroplasty surgeon, protrusio is a matter of concerns. These concerns are mainly because there are increased stresses on the medial wall. Also there is poor implant coverage as well as anchorage. These concerns are both mechanical and biological.

 

MECHANICAL CONCERNS

Bone can be inherently structurally impaired to provide stable prosthesis fixation. Preoperative limitation of movement range can be problematic for exposure and dislocation of hip. There is higher risk of impingement leading to subluxation or dislocation, or, wear. Limb length discrepancy is also a problem. Abductor insufficiency is also seen because of short abductor lever arm. These mechanical concerns may be enhanced as protrusio can be progressive also.

 

BIOLOGICAL CONCERNS

There may be failure to achieve ingrowth into porous implant particularly if implant is rim fit and is not seated properly leading to inadequate bony contact. In case of cemented acetabulum, there may be lack of stable interface with cement.

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Why      is      Correction      Important?               

The correction of protrusio is important because in protruded acetabulum, cortical bone stresses on the medial part of the pelvic wall increase with medial placement of acetabular component. Also, migration of femoral head is both medial and superior, therefore, depth of the socket is oblong in shape. This requires careful reaming to create convergence of acetabular rim without preferentially reaming superior rim and causing a secondary segmental defect. So lateralization of acetabulum is important as lateralization of acetabulum increases femoral offset and decreases tendency for impingement.11,12 It also restores bone stock for future revisions, if needed.

The objectives of total hip arthroplasty in a protrusio acetabuli can be summarized as follows:

• Strengthen medial wall and restore acetabular integrity

• Lateralize acetabular component to restore hip biomechanics and center of hip rotation

• Ensure acetabular component coverage

• Secure rigid prosthesis fixation

• Reconstruct the defect

A careful preoperative planning and vigilance at every level preoperatively, intra-operatively and postoperatively are essential for a successful outcome of total hip arthroplasty in a protrusio case.

 

Total Hip Arthroplasty

 

Preoperative                   Planning                   

The key consideration before proceeding with operative treatment is identification and treatment of any underlying disease process.9 This is important as they may have an impact upon patient’s general health, anesthetic management, prosthetic longevity and long-term functional status. The aim of preoperative planning should be assessment of severity of protrusio, bone stock and the potential means of restoring normal hip biomechanics. Attention should also be given towards femoral side as restoration of normal center of hip can result in relative lengthening of that limb. So, it is important to choose neck resection level and neck length carefully. Also, medial and vertical offset should be restored to optimize abductor strength, stability and range of motion.

 

SEVERITY OF PROTRUSIO

Severity of protrusio can be assessed by measuring the migration of femoral head past the Ilioischial line (Kohler’s line). Sotelo-Garza and Charnley10 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). Similarly an increase in center-edge angle of Wiberg can be measured radiographically.

 

BONE STOCK

Assessment of quality as well as quantity of bone stock is an integral part of preoperative planning for total hip arthroplasty for protrusio. Additional deformities that may pose difficulties in reconstruction should be carefully looked for. As the main deforming forces are in the direction of femoral neck, the usual defect is a combined medial and superior defect. These defects can be adequately assessed on standard anteroposterior and lateral radiographs of pelvis and hip. However, whenever additional defects are suspected as in posttraumatic acetabular fractures, Judet views or computed tomography should be done for proper assessment. The presence of an acetabular bone defect can make fixation of the acetabular component more challenging and in certain cases dictate the need for more specialized components or supplemental internal fixation. Developing a strategy for dealing

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with acetabular bone defects requires not only an assessment of the size of the defect, but also the location of the defect, and whether the defect has resulted in a structural deficiency that will diminish support for an acetabular component.

Total Hip Replacement for Protrusio Acetabuli

 

The main assessment should be whether adequate peripheral bony contact can be achieved with a hemispherical acetabular shell to have an initial stability of the cup or not. If this cannot be achieved due to some bony defect, then correction of this defect should be the primary focus, followed by reconstruction of acetabulum.

 

ALTERED HIP BIOMECHANICS

Acetabular orientation can also change in case of protrusio defect due to fixed pelvic obliquity or flexion of pelvis. One particularly important point is to recognize that soft tissue contracture will change the acetabular position when the patient moves from the standing or supine position to the lateral decubitus position on the operating table. The change in acetabular position is important to recognize so that the surgeon does not underestimate the angle of inclination of the socket, or the degree of anteversion when trying to place components in the proper position.

It is also important to restore the center of rotation for optimum biomechanics of hip. Acetabular tear drop is an important landmark.5 Another more reliable and commonly used method is Ranawat method.

 

RANAWAT’S TRIANGLE

 

 

Ranawat et al12 described a method (Fig. 28.6) to locate the correct anatomical position of the acetabulum in deformed hips and to enable any variation of the position of the acetabular component following hip arthroplasty to be assessed on roentgenograms. Parallel horizontal lines are drawn at the level of the iliac crests and the Ischial tuberosities and are connected by a perpendicular passing through a point (A) located five millimeters lateral to the intersection of Kohler’s and Shenton’s lines. The length of the perpendicular between the parallel lines is equal to the height of the pelvis and one-fifth of this equals the height of the acetabulum. A second point (B) is located on the perpendicular superior to point A, at a distance equal to one-fifth of the perpendicular line. From B, a perpendicular is erected laterally to point C, so that distance BC equals distance AB. Joining points A and C completes

 

Figure 28.6: Ranawat’s method to identify the correct position of acetabulum

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Total Hip Arthroplasty

 

Figure 28.7: Preoperative templating is useful. Choose a size with peripheral rim fit

 

the isosceles triangle, indicating the correct position of the acetabulum to be reconstructed. In a normal hip the superior border of the triangle will pass through the superior aspect of the subchondral bone of the acetabulum. The hypotenuse of the triangle will be equal to the diameter of the mouth of the acetabulum.

 

PREOPERATIVE TEMPLATING (FIG. 28.7)

Preoperative templating is important as it can ascertain the need for larger cup to attain rim fit if required. It can also tell the need for cage. Also, it helps in minimizing any preoperative surprises. Opposite hip should also be templated for proper hip biomechanics and cup positioning.

 

Surgical Considerations13-15

 

Proper exposure of the difficult acetabulum is one of the most challenging and important aspects of acetabular reconstruction in protrusio. A lot of intraoperative problems may be encountered in protrusio hips. Hence, it has been aptly named as “Problem in depth”. It is very essential to be aware of all anatomic alterations to prevent any iatrogenic complication. During primary total hip replacement with a protrusio deformity, it may be difficult to dislocate the hip. Sometimes, even visualization of the neck may be difficult. In such cases a neck cut in situ is helpful. The head can simply be broken down in situ with osteotomes and

removed.

The exposure often entails excessive soft tissue dissection. As the exposure is limited, sometimes the trochanteric osteotomy is also needed which may have its own set of complications including nonunion or implant problems. Finally, closure of the difficult hip includes appropriate tensioning of the abductor mechanism when an osteotomy has been performed.

In protrusion, the sciatic nerve is closer to the femoral neck than in hips without protrusio and must be protected during exposure. Failure to restore normal lateral offset may cause the greater trochanter to impinge on the anterior edge of the acetabulum, leading to posterior instability. Medial wall of the acetabulum is usually thin or may be partially membranous, and should not be penetrated. It is essential not to deepen the acetabulum while reaming; surgeon should ream in order to obtain a good peripheral fit.

In cases of posttraumatic protrusio, there are additional problems of ankylosis, scarring and tethering of sciatic nerve, retained hardware and fracture nonunion and necrotic bone. The pearl is to wait till fracture heals before planning hip arthroplasty.

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Total Hip Replacement for Protrusio Acetabuli

 

Figures 28.8A and B: A Grade I protrusio in a 72-year old male treated by cement augmentation alone

 

Treatment Options

 

The surgical tools employed in acetabular reconstruction for protrusio have changed with the evolution from cemented to cementless acetabular components. Nevertheless, restoration of the hip center can be accomplished by a variety of surgical techniques:

• Cement augmentation

• Bone grafts:

  • Auto or allografts

  • With cemented socket ± wire mesh

  • With bipolar prosthesis

  • With cementless cup

• Support rings:

  • Burch-Schneider’s

  • Muller’s

 

CEMENT AUGMENTATION

 

 

If the patient is very elderly, an acetabular cup can be cemented on top of a large cement mantle (Figs 28.8A and B). Cement augmentation can be combined with placement of wire mesh. If the cup is not pushed in too hard, this build-up of mesh and cement will lateralize it which is to the desired level.16 However, there are conflicting reports regarding this method. There are also concerns of thermal osteolysis of medial wall caused by curing of cement and early loosening (Figs 28.9A and B).

 

Figures 28.9A and B: Cemented cup for protrusio at one year and 6 years follow-up showing good position initially but loosening at late follow-up

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Total Hip Arthroplasty

 

Figures 28.10A and B: Protrusio cage used in a revision setting

 

Sotelo-Garza and Charnley10 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 total hip arthroplasties at a minimum follow-up interval of 60 months. Ranawat et al12 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 al17 used finite element 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.

 

MESH OR PROTRUSIO RINGS

Protrusio rings are available that rest on the pelvis outside the acetabulum and prevent further sinkage. Juggling the cage or, ring, cement and cup is just a little difficult and unless care is exercised, some version malposition can occur. The ring seldom sits down absolutely flush on the bone and occasionally impingement with the femoral neck can occur. A protrusio cup with built in metal flanges is somewhat easier to use, but again the problem of a flush fit may arise. These rings or cages are more useful in revision settings (Figs 28.10A and B).

Jasty18 has reported that patients with defects through the central acetabular wall had an

unacceptably high rate of acetabular component loosening when mesh alone was used without bone graft. When used alone without bone graft, there is again a high incidence of osteolysis and loosening. This is mainly because of inadequate restoration of hip center.

Also, there are differences in modulus of elasticity with different implants.

Deep, egg shaped ingrowth protrusio sockets are not really designed for primary cases and are intended to be used for revision when the patient’s own bone is not available.16

 

BONE GRAFT

Use of bone graft is the optimal method of cup lateralization. It can be used to build-up the defect for both cemented as well as cementless acetabulum. Several reports19,20 on the use of medial bone graft in reconstruction of acetabular protrusion have led to general consensus that bone grafting is more likely to arrest the progression of the medial migration than acetabular reconstruction without the use of bone graft when implanting any type of fixed

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socket, support mesh, or acetabular ring. The reason is that all types of cups tend to migrate slowly into the bone. If the floor is not bone grafted or left thin, over time, the cup may migrate to such an extent that it breaks through the inner wall and ends up with a central fracture dislocation. So, particularly if the patient is young, the floor should be rebuilt and the cup lateralized, thus restoring both floor thickness and center of rotation of the hip.

Total Hip Replacement for Protrusio Acetabuli

 

Use of bone graft is a must if component stability is in doubt or compromised, if there is large superior wall defect, if there is lack of medial support, or, translation of center of rotation is more than 4 cm. Use of bone graft in protrusio may not be required if translation of center of rotation is less than 2 cm or lateralization of the cup is possible.

The use of a supporting mesh in conjunction with medial bone graft and a cemented socket for treatment of protrusio was reported by McCollum21,22 in 32 patients with 2- to 8-year follow-up, and later at 10.9- to 17.4-year follow-up. The technique included a corticocancellous graft placed medially with gelfoam at its periphery to prevent cement extrusion. A fine Vitallium mesh was placed over the bone graft, and an acetabular prosthesis was cemented into the mesh. When the medial wall was found to be completely absent, a protrusio ring was used. On serial radiographic follow-up, all grafts had incorporated without evidence of loosening or migration.

Heywood23 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 concavo-convex 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 short-term results with this technique in a series of nine hips.

Both structural and particulate grafts24 (Figs 28.11A and B) can be used for bone grafting. Structural grafts are larger in diameter (2-3 cm diameter and 0.5 cm thick), while particulate grafts are very small and malleable, and conform to the irregular beds. They are usually obtained from the femoral head (Figs 28.12A to D) of the same patient (autograft) or from allograft. The particulate bone graft is placed medially against the deficient medial wall. The graft can be compacted by using a spherical reamer spinning in reverse to provide a firm platform over which the socket is placed. They are intimately opposed and provide sufficient compression. Particulate graft also provides resistance to cement penetration. Numerous authors have demonstrated nearly universal incorporation of morselized or particulate 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 today9 is to fill the defect with morselized graft and then to use a porous-coated metal cup for the acetabular reconstruction (Figs 28.13 and 28.14). The uncemented, porous socket is dependent upon rigid fixation and contact with viable

 

Figures 28.11A and B: Both morselized and structural bulk graft can be used to fill the defect

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Total Hip Arthroplasty

 

Figures 28.12A to D: (A) Particulate graft is prepared from femoral head usually from the same patient, (B) with the help of a bone mill.

1. which is converted into morselized graft,

    

    

2. This is compacted into the defect with the help of reverse reamers or specially designed impactors

    

   Figures 28.13A and B: A case of severe protrusio treated by particulate graft with cementless socket fixation. Particulate graft consolidate to provide a good bone stock if revision is needed in future

    

   host bone, as bone will not grow into the areas of the porous socket directly adjacent to the bone graft.25,26 It is also important to delay the weight bearing for about two months to allow graft consolidation. With cemented cups, the technique useful in osteoporotic bone, immediate results are gratifying; however, late lucencies appear leading to loosening.

   Ranawat and Zahn27 have recommended that 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.

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   Summary                        

   Surgical technique of total hip arthroplasty is demanding in case of protrusio. Careful preoperative planning and evaluation is vital for successful outcome. The aim in protrusio surgery is to achieve two important objectives: (1) restitution of bone stock, and,

   (2) lateralization of the hip center to the anatomic position to maximize the chances of successful long-term outcome.

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   Total Hip Replacement for Protrusio Acetabuli

    

   Figures 28.14A and B: Protrusio caused by a bipolar prosthesis. This was treated with bone grafting and cementless socket with screw fixation

    

   Illustrative                      Case                     

   A 48-year-old female with rheumatoid arthritis presented with bilateral hip pain and limitation of hip range of motion particularly abduction. Anteroposterior X-ray of pelvis, (Fig. 28.15A) showed bilateral inflammatory arthritis of hip with moderate grade protrusio on both sides. Total hip arthroplasty was planned bilaterally. Patient was operated for both sides in a single stage. We used both femoral heads and prepared particulate graft with the help of bone mill. The grafts were impacted into the defect with the help of reverse acetabular reamers. Cementless sockets were implanted on top of grafts with additional screw fixation. A good integration with excellent functional outcome is seen at 5 years follow-up (Fig. 28.15B).

    

    

   Figures 28.15A and B: (A) Bilateral inflammatory arthritis with protrusio (B) A good osteointegration at 5 years follow-up with no evidence of loosening

    

    

    

   References                       

   1. Pomeranz MM. Intrapelvic protrusion of the acetabulum (Otto pelvis). J Bone Joint Surg Am 1932;14:663-86.

   2. Hooper JC, Jones EW. Primary protrusion of the acetabulum. J Bone Joint Surg Br 1971;53:23-9.

      Total Hip Arthroplasty

       

   3. Armbuster TG, Guerra J Jr, Resnick D, et al. The adult hip: An anatomic study. Part I. The bony landmarks. Radiology 1978;128:1-10.

   4. Alexander C. The etiology of primary protrusio acetabuli. Br J Radiol 1965;38:567-80.

   5. Gates HS III, Poletti SC, Callaghan JJ, McCollum DE. Radiographic measurements in protrusio acetabuli. J Arthroplasty 1989;4:347-51.

   6. McCollum DE, Nunley JA, Harrelson JM. Bone grafting in total hip replacement for acetabular protrusion. J Bone Joint Surg 1980;62A:1065-73.

   7. Hastings DE, Parker SM. Protrusio acetabuli in rheumatoid arthritis. Clin Orthop 1975;108:76-83.

   8. Duthie RB, Harris CM. A radiographic and clinical survey of the hip joint in seropositive rheumatoid arthritis. Acta Orthop Scand 1965;40:346.

   9. McBride MT, Muldoon MP, Santore RF, et al. Protrusio Acetabuli: Diagnosis and Treatment. J Am Acad Orthop Surg 2001;9:79-88.

   10. Sotelo-Garza A, Charnley J. The results of Charnley arthroplasty of the hip performed for protrusio acetabuli. Clin Orthop 1978;132:12-8.

   11. Coventry MB. The treatment of fracture dislocation of the hip by total hip arthroplasty. J Bone Joint Surg 1974;56A:1128-34.

   12. Ranawat CS, Dorr LD, Inglis AE. Total hip arthroplasty in protrusio of rheumatoid arthritis. J Bone Joint Surg 1980;62A:1059-65.

   13. Gross AE. The difficult socket. Instruct Course Lect 2000;49:57-61.

   14. Gross AE, Allan DG, Catre M, et al. Bone grafts in hip replacement surgery. The pelvic side. Orthop Clin North Am 1993;24:679-95.

   15. Hardinge K, Williams D, Etienne A, et al. Conversion of fused hip to low friction arthroplasty. J Bone Joint Surg 1977;59B:385-92.

   16. Cameron HU. Deformity. Cameron HU (Ed). The technique of total hip arthroplasty. St. Louis Mosby 1992.p.198-236.

   17. Crowninshield RD, Brand RA, Pedersen DR. A stress analysis of acetabular reconstruction in protrusio acetabuli. J Bone Joint Surg Am 1983;65:495-9.

   18. Jasty M, Harris WH. Results of total hip reconstruction using acetabular mesh in patients with central acetabular deficiency. Clin Orthop 1988;237:142-9.

   19. Ebert FR, Hussain S, Krackow KA. Total hip arthroplasty for protrusio acetabuli: A 3- to 9-year follow-up of the Heywood technique. Orthopedics 1992;15:17-20.

   20. Hirst P, Esser M, Murphy JC, et al. Bone grafting for protrusio acetabuli during total hip replacement. J Bone Joint Surg 1987;69B:229-33.

   21. Gates HS, McCollum DE, Poletti SC, et al. Bone grafting in total hip arthroplasty for protrusio acetabuli. J Bone Joint Surg 1990;72A:248-51.

   22. McCollum DE, Nunley JA, Harrelson JM. Bone grafting in total hip replacement acetabular protrusion. J Bone Joint Surg 1980;62A:1065-73.

   23. Heywood AWB. Arthroplasty with a solid bone graft for protrusio acetabuli. J Bone Joint Surg 1980;62B:332-6.

   24. Kinzinger PJ, Karthaus RP, Slooff TJ. Bone grafting for acetabular protrusion in hip arthroplasty. 27 cases of rheumatoid arthritis followed for 2 to 8 years. Acta Orthop Scand 1991;62:110-2.

   25. Heekin RD, Engh CA, Vinh T. Morselized allograft in acetabular reconstruction: a post-mortem retrieval analysis. Clin Orthop 1995;319:184-90.

   26. Greis PE, Kang JD, Silvaggio V, et al. A long-term study on defect filling and bone ingrowth using a canine fiber metal total hip model. Clin Orthop 1992;274:47-59.

   27. Ranawat CS, Zahn MG: Role of bone grafting in correction of protrusio acetabuli by total hip arthroplasty. J Arthroplasty 1986;1(2):13