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The Journal of Bone and Joint Surgery 78:1004-14 (1996)
© 1996 The Journal of Bone and Joint Surgery, Inc.

The Effect of Superior Placement of the Acetabular Component on the Rate of Loosening after Total Hip Arthroplasty. Long-Term Results in Patients Who Have Crowe Type-II Congenital Dysplasia of the Hip*

MARK W. PAGNANO, M.D.{dagger}, ARLEN D. HANSSEN, M.D.{dagger}, DAVID G. LEWALLEN, M.D.{dagger} and WILLIAM J. SHAUGHNESSY, M.D.{dagger}, ROCHESTER, MINNESOTA

Investigation performed at the Department of Orthopedic Surgery, Mayo Clinic and Mayo Foundation, Rochester


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
A method for measurement of the true acetabular region and the approximate femoral head center as well as a classification consisting of four zones for assessment of the acetabular position of the acetabular cup were used to analyze the results of primary total hip arthroplasty with cement in 117 patients (145 hips). All patients had Crowe type-II congenital dysplasia of the hip. The mean age at the time of the arthroplasty was fifty-one years (range, fifteen to seventy-six years), and the mean duration of follow-up was fourteen years (range, two to twenty-two years). The initial position of the acetabular cup outside of the true acetabular region and outside of zone 1 (inferior and medial) was associated with an increase in the rates of loosening (p < 0.05) and revision (p < 0.04) of the femoral components. Cups that initially were more than fifteen millimeters superior to the approximate femoral head center, without lateral displacement, were associated with an increased rate of loosening (p < 0.001) and of revision (p < 0.04) of the femoral components as well as with an increased rate of loosening (p < 0.002) and of revision (p < 0.01) of the acetabular components. These findings suggest that superior positioning of the acetabular component, even without lateral displacement, leads to increased rates of loosening of the femoral and acetabular components. An attempt should be made to position the acetabular component in or near the true acetabular region.


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
Reports of total hip arthroplasty in patients who have congenital dysplasia of the hip have reflected a higher prevalence of complications and less durable results compared with those in patients who have had total hip arthroplasty for primary degenerative osteoarthrosis4-7,9,11,12,20,25-27,29,30. Total hip arthroplasty is a technically demanding procedure in a patient who has congenital dysplasia of the hip, as such patients often have distortion of the anatomy of the acetabulum and femur. The four categories of Crowe et al.7 for the classification of congenital dysplasia of the hip, based on the distance of migration of the femoral head out of the true acetabulum, correspond well with the magnitude and pattern of anatomical distortion. Operative strategies to overcome the anatomical variations in these patients remain controversial5,7,9,12,14,22.

The proper position of the acetabular component in a patient who has congenital dysplasia of the hip has not been determined. Some authors have recommended that the cup be placed in the true acetabular region to improve the long-term survival of the implant15,31. On the basis of a mathematical model of the hip joint, Johnston et al. found that "placement of the center of the acetabulum as far medially, inferiorly, and anteriorly as is anatomically possible is of prime importance in reducing the loads at the hip."15 Increased rates of loosening of both the femoral3,31 and the acetabular3,19,23,27 component have been associated with an initial position of the acetabular cup outside of the true acetabular region. Conversely, other authors have suggested that superior placement of the cup without concomitant lateral displacement has no detrimental effect on the longevity of the prosthesis19,25.

The purpose of the current study was to evaluate the long-term results of total hip arthroplasty with cement in patients who had Crowe type-II congenital dysplasia of the hip and to analyze specifically the effect of the initial position of the acetabular cup on the long-term durability of the femoral and acetabular components.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Six hundred and ninety-six primary total hip arthroplasties with cement were performed at the Mayo Clinic for the treatment of congenital dysplasia of the hip between March 1969 and December 1980. The preoperative radiographs were assessed to classify the hips according to the criteria of Crowe et al.7. Type I indicates less than 50 per cent subluxation of the femoral head from the acetabulum; type II, 50 to 74 per cent subluxation; type III, 75 to 100 per cent subluxation; and type IV, complete dislocation out of the acetabulum. Radiographs were not available for twenty-five hips, and anatomical distortion due to previous operations precluded classification of four additional hips. Of the remaining 667 hips, 319 were type I, 158 were type II, ninety were type III, and 100 were type IV. The results of arthroplasty in the type-III and IV hips have been reported previously26,27.

All patients were asked to participate in our total joint registry and to return for radiographs and clinical evaluation at intervals of one, five, ten, fifteen, and twenty years after the total hip arthroplasty. At the time of this retrospective study of type-II hips, two patients (three hips) had died without returning for follow-up, four patients (six hips) had been lost to follow-up, and two patients (four hips) had been found to have concomitant rheumatoid arthritis and were therefore excluded. The remaining 117 patients (145 hips) were followed for a minimum of two years, and they form the basis of this report. One hundred and twelve patients (139 hips) were followed for at least five years; ninety-nine patients (117 hips), ten years; sixty-eight patients (eighty-three hips), fifteen years; and twenty-seven patients (thirty-two hips), twenty years. The mean duration of follow-up was fourteen years (range, two to twenty-two years).

There were twenty-one male patients and ninety-six female patients; the mean age at the time of the arthroplasty was fifty-one years (range, fifteen to seventy-six years). Seventy-four arthroplasties were performed on the right hip and seventy-one, on the left. Sixty-five hips had had operative procedures before the arthroplasty; these operations had included twenty-seven open reductions, thirteen femoral osteotomies, two pelvic osteotomies, eighteen shelf procedures, and five other procedures. Fifty-five patients had had a closed reduction of the dysplastic hip during childhood.

Operative Technique
During the years spanned by this study, hip arthroplasties at the Mayo Clinic were performed according to the philosophy that every attempt should be made to position the acetabular component in or near the true acetabulum. One hundred and thirteen transtrochanteric, twenty-four anterolateral, and eight posterior approaches were used. Standard Charnley components (Thackray, Leeds, England) were implanted in 100 hips. Miniature Charnley components (Thackray) were used in eleven hips (both the cup and the stem in two hips and the femoral component alone in nine hips). The remaining implants included fourteen T-28 components (Zimmer, Warsaw, Indiana), seven HD-2 components (Howmedica, Rutherford, New Jersey), seven Charnley-Muller components (DePuy, Warsaw, Indiana), four Aufranc-Turner components (Howmedica), one CAD component (Howmedica), and one Peterson-Charnley Mark-III component (Howmedica). All of the acetabular components were non-metal-backed, high-density polyethylene, with a mean outside diameter of forty-two millimeters (range, thirty-six to fifty-two millimeters). All acetabular and femoral components were inserted with cement. The operative approach and the type of component depended on the preference of the surgeon. No structural bulk acetabular autogenous grafts or allografts were used in any of the reconstructions.

Perioperatively, all patients received antimicrobial prophylaxis as well as warfarin for prophylaxis against deep venous thrombosis. Postoperatively, the patients remained supine in bed for a mean of four days (range, two to eleven days). The mean duration of hospitalization after the arthroplasty was twenty-three days (range, ten to forty-nine days).

Radiographic Evaluation
All radiographic measurements were made with a modified Müller template21,26. The template, which accounts for 15 per cent magnification, was modified to allow direct measurement, so that radiographic magnification was ignored; all measurements then were adjusted on the basis of the amount of magnification on the actual radiograph being measured. The magnification ratio for each radiograph was determined by measuring the diameter of the prosthetic femoral head and dividing it by the known diameter of the femoral head. This method allows definitive measurement of the position of the prosthesis and of the absolute magnitude of migration of the component from identifiable radiographic landmarks.

Acetabular Components
The radiographs that were made immediately after the arthroplasty were measured to define an area labeled the true acetabular region (Fig. 1). To determine the true acetabular region, (1) a horizontal line is drawn through the inferior margin of the radiographic teardrop; (2) a second, parallel line is drawn above the first line at a distance equal to 20 per cent of the height of the pelvis on the anteroposterior radiograph; (3) a vertical line is drawn five millimeters lateral to the intersection of the Köhler line and the radiographic teardrop; and (4) a diagonal line is drawn superiorly and laterally at an angle of 45 degrees from the intersection of the vertical line and the inferior horizontal line to intersect the superior horizontal line. The triangular area enclosed by these lines defines the true acetabular region.



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  		Fig. 1 Diagram showing the true acetabular region (TAR) as the area enclosed by an isosceles triangle, with the height and width equal to 20 per cent of the height of the pelvis. The inferomedial corner of the true acetabular region is five millimeters lateral to the intersection of the Kohler line with the radiographic teardrop. The mid-point of the triangle's hypotenuse is defined as the approximate femoral head center (AFHC) and represents the normal center of rotation of the hip.

 
The mid-point of the diagonal line was defined as the approximate femoral head center and was used as the reference point to measure horizontal and vertical distances between the approximate femoral head center and the center of the prosthetic femoral head (Fig. 1). These distances were recorded for each hip. If the center of the head of the prosthesis was initially positioned superiorly or laterally outside of the true acetabular region, the acetabular component was defined as being outside of the true acetabular region.

The measurement was done with use of the true acetabular region and the approximate femoral head center because distortion of the anatomy by previous operations or bilateral congenital dysplasia often prevented direct measurement of the femoral head center in the contralateral hip. This method is easily reproducible and, as the measurements are based on the height of the pelvis as measured on the same radiograph, determination of the true acetabular region or the approximate femoral head center is not affected by radiographic magnification. This method also normalizes the measurements for each patient, regardless of his or her size, because it is based on the height of the pelvis. Despite correction for magnification, methods that rely on direct measurements from reliable landmarks, such as the radiographic teardrop, lead to underestimation or overestimation of the position of the acetabular cup in patients who have a small or large pelvis, respectively. For example, a point measured fifty millimeters superior and lateral to the radiographic teardrop in a large pelvis lies within the true acetabular region, whereas that same point lies completely outside of a small pelvis (Fig. 2). In our patients, there was a variance of sixty-two millimeters in the pelvic height, which translates to a difference of twelve millimeters in the height of the medial border of the true acetabular region.



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  		Fig. 2 Diagram showing the effect of variations in the pelvic height on the true acetabular region and on direct measurements from the teardrop. Direct measurement of the initial position of the acetabular cup from a landmark such as the radiographic teardrop can result in overestimation or underestimation of the position of the cup in patients who have a large or small pelvis, respectively, compared with normal. Point A, fifty millimeters superior and lateral to the teardrop in the large pelvis (left), is clearly in a different anatomical and functional position than the same point in the small pelvis (right).

 
In an effort to separate the combined effect of a superior and lateral position of the acetabular cup, a classification consisting of four zones was devised (Fig. 3). The zones were created by the intersection of a horizontal line and a vertical line through a point one centimeter superior and lateral to the approximate femoral head center. This point of division was chosen on the basis of the clinical axiom that placement of the cup one centimeter up and out (superior and lateral) is satisfactory. In this system, zone 1 lies inferior and medial; zone 2, superior and medial; zone 3, superior and lateral; and zone 4, inferior and lateral. Horizontal and vertical displacement was also measured from the radiographic teardrop to facilitate direct comparison with measurements obtained in previous studies25,31.



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  		Fig. 3 Diagram showing the four-zone system used to assess the initial position of the acetabular cup. The zones are created by the intersection of a horizontal and a vertical line through a point one centimeter superior and one centimeter lateral to the approximate femoral head center (AFHC). Zone 1 is inferior and medial; zone 2, superior and medial; zone 3, superior and lateral; and zone 4, inferior and lateral.

 
The anteroposterior radiographs were assessed for radiolucent lines in the three zones of the acetabulum described by DeLee and Charnley8. The modified Muller template was used to assess the magnitude and direction of migration of the acetabular component, any change in the obliquity of the acetabular cup, and the magnitude of radiolucent lines. Loosening of the acetabular component was classified according to the criteria of Hodgkinson et al.14. Type-0 cups were associated with no radiolucency; type-1, with radiolucency in zone 1; type-2, with radiolucency in zones 1 and 2; type-3, with a complete radiolucent line in all three zones; and type-4, with a change in the position of the cup or a fracture of the cement mantle. Type-3 or 4 acetabular components were considered loose.

Femoral Components
The anteroposterior and oblique radiographs were used to assess radiolucency at the bone-cement and cement-prosthesis interfaces of the femoral component1. The location and magnitude of the radiolucent lines were recorded for the seven zones described by Gruen et al.10. Osteolysis or fracture of the cement was also recorded according to those zones. The criteria of Harris et al. were used to assess radiographic evidence of loosening13. Definite loosening was indicated by evidence of a fracture of the cement or prosthesis or by migration of the component; probable loosening, by a complete radiolucent line at the bone-cement interface; and possible loosening, by a radiolucent line that was incomplete but that encompassed more than 50 per cent of the bone-cement interface. We defined loosening of the femoral component as definite or probable loosening according to this grading scheme. Additional measurements included the distance of the cement distal to the tip of the femoral prosthesis and the axial alignment of the femoral prosthesis as determined on the anteroposterior radiographs.

Functional Evaluation
The clinical portion of the Mayo Clinic hip-scoring system17, which evaluates clinical and radiographic parameters, was used to assess function. With this system, a maximum of 80 points—40 points for pain, 20 points for function (distance walked and use of walking aids), and 20 points for mobility and muscle power—is assigned.

Statistical Methods
The cumulative probabilities of revision and loosening were estimated with use of the Kaplan-Meier product-limit method16, thereby taking into consideration the time from the operation until the event of interest. In the analysis of time until revision, hips were censored at the time of the revision. In the analysis of time until loosening, hips that had loosening were censored at the time that loosening was recognized and hips that did not have loosening were censored at the time of the most recent radiograph. The survivorship curves for various subgroups were compared with use of the log-rank test.


   Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 

Clinical
The clinical portion of the hip score (maxiumum, 80 points) was a mean of 33 points (range, 0 to 70 points) preoperatively. This score improved to a mean of 73 points (range, 44 to 80 points) at one year postoperatively and to a mean of 74 points (range, 40 to 80 points) at the five-year follow-up evaluation. At the most recent follow-up evaluation, the mean score was 69 points (range, 13 to 80 points).

Complications
Six patients had intraoperative complications; these included perforation of the femur in two patients and a fracture of the femoral shaft, a fracture of the acetabulum, a sciatic-nerve injury, and protrusion of the acetabular component through the medial wall of the acetabulum in one patient each. Nineteen patients had at least one early postoperative complication; there were five superficial infections, three hematomas, one deep venous thrombosis, two pulmonary emboli, five sciatic-nerve palsies, and five urinary-tract infections. Long-term complications occurred in ten patients: three had a fracture of the femoral component; two, a nonunion of the greater trochanter; and one each, a dislocation, a fracture of the femoral shaft, a deep infection, massive osteolysis of the acetabulum, and osteolysis of the femur and acetabulum.

Reoperation
Twenty-eight hips had a reoperation: three, for aseptic loosening of the acetabular component; six, for aseptic loosening of the femoral component; fourteen, for aseptic loosening of both components; three, for a fracture of the femoral stem; one, for deep infection; and one, for heterotopic ossification2. At the latest follow-up evaluation, twenty-five femoral components and eighteen acetabular components had been revised.

Radiographic Analysis

Acetabular Components
At the most recent follow-up evaluation, eighteen acetabular components had been revised for loosening and an additional sixty-eight were radiographically loose (Figs. 4 and 5).



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  		Fig. 4 Graph showing the Kaplan-Meier16 cumulative probability of revision of the acetabular component, with 95 per cent confidence intervals. THA = total hip arthroplasty.

 


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  		Fig. 5 Graph showing the Kaplan-Meier16 cumulative probability of revision or loosening of the acetabular component, with 95 per cent confidence intervals. THA = total hip arthroplasty.

 
As seen on the initial postoperative radiographs, 127 acetabular components were positioned inside of the true acetabular region and eighteen were outside of this area. A comparison between the acetabular components positioned inside and those positioned outside of the true acetabular region revealed no significant difference in the rate of loosening or revision (p = 0.42, log-rank test). There was also no significant difference in the rate of loosening or revision between the components located lateral to the approximate femoral head center and those placed medial to that point (p = 0.50). With use of the four-zone classification described earlier, seventy-seven cups were in zone 1, forty-nine were in zone 2, twelve were in zone 3, and seven were in zone 4. There was no significant difference in the rate of loosening or revision among the four zones (p > 0.22, log-rank test).

Thirty-five cups were initially placed more than fifteen millimeters superior to the approximate femoral head center. Comparison of these superiorly placed cups with all other cups revealed a significant difference in the rate of loosening (p < 0.001) and revision (p < 0.05). The superiorly placed cups were not found to be positioned significantly more laterally than the other cups in this study (p = 0.54). Superior displacement has been previously defined as placement more than thirty-five millimeters proximal to the interteardrop line, with the range of normal lateral displacement being thirty to forty-nine millimeters25. With use of these criteria, thirty-one of the thirty-five cups were superiorly displaced without lateral displacement, and these cups were significantly associated with an increase in the rates of loosening (p < 0.002) and revision (p < 0.01).

Femoral Components
At the latest follow-up evaluation, twenty-five femoral components had been revised and an additional eighteen were considered radiographically loose (Figs. 6 and 7).



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  		Fig. 6 Graph showing the Kaplan-Meier16 cumulative probability of revision of the femoral component, with 95 per cent confidence intervals. THA = total hip arthroplasty.

 


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  		Fig. 7 Graph showing the Kaplan-Meier16 cumulative probability of revision or loosening of the femoral component, with 95 per cent confidence intervals. THA = total hip arthroplasty.

 
The rates of loosening (p < 0.05) and revision (p < 0.04) of the femoral components associated with the eighteen acetabular components that had been initially positioned outside of the true acetabular region were significantly increased compared with those associated with the 127 acetabular components initially positioned inside of this region (Figs. 8-A, 8-B, 8-C, 9-A, 9-B and 9-C). The zone in which the acetabular component had been positioned also was significantly associated with loosening of the femoral component; cups in zone 1 were associated with a lower rate of loosening of the femoral components than were cups in zones 2, 3, and 4 (p < 0.04). The rates of loosening (p < 0.01) and revision (p < 0.04) of the femoral components in association with the cups that had been initially placed in zone 2 were significantly higher than those associated with the cups initially placed in zone 1.



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  		Figs. 8-A, 8-B, and 8-C: Radiographs made before and after a total hip arthroplasty with the initial position of the cup within the true acetabular region. Fig. 8-A: Preoperative radiograph showing Crowe type-II congenital dysplasia in a fifty-five-year-old woman.

 


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  		Fig. 8-B: Early postoperative radiograph showing placement of the acetabular cup in the true acetabular region and close to the approximate femoral head center (zone 1).

 


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  		Fig. 8-C: Eighteen years postoperatively, there is no evidence of loosening of either the femoral or the acetabular component.

 


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  		Figs. 9-A, 9-B, and 9-C: Radiographs made before and after a total hip arthroplasty with the initial position of the cup outside of the true acetabular region. Fig. 9-A: Preoperative radiograph showing Crowe type-II congenital dysplasia in a sixty-two-year-old woman.

 


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  		Fig. 9-B: Early postoperative radiograph showing placement of the acetabular cup outside of the true acetabular region but without far lateral displacement (zone 2).

 


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  		Fig. 9-C: Twelve years postoperatively, there is extensive osteolysis around both the femoral and the acetabular component. Both components were subsequently revised.

 
Superior placement of the cup more than fifteen millimeters proximal to the approximate femoral head center, without lateral displacement, was a significant predictor of increased rates of loosening (p < 0.001) and revision (p < 0.04) of the femoral components; an initial position of the cup of more than thirty-five millimeters superior to the interteardrop line also was associated with an increase in the rates of loosening and revision of the femoral component (p < 0.004 for both).

The type of femoral component was found to be a significant predictor of loosening; Charnley components were less likely to loosen than other components (p < 0.02). The so-called offset of the femoral component (the horizontal distance from the center of the prosthetic femoral head to the longitudinal axis of the femoral stem) may be a factor in subsequent loosening of the prosthesis. Additional analysis of the femoral components with a so-called limited offset (the mini-Charnley, Charnley-Müller, HD-2, Aufranc-Turner, and CAD components) revealed higher rates of loosening and revision of the femoral components (p < 0.001) as well as of the acetabular components (p < 0.002). The various types of femoral components were distributed similarly inside and outside of the true acetabular region and within the four zones; therefore, these factors did not confound the observation of an association between the initial position of the cup and loosening of the femoral component. Similarly, the patient's age and weight and the alignment of the femoral prosthesis were not confounding variables (log-rank test).


   Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
Total hip arthroplasty for the treatment of congenital dysplasia or complete dislocation of the hip is more technically demanding and is associated with a higher rate of complications than is total hip arthroplasty for the treatment of primary degenerative osteoarthrosis of the hip4-7,9,11,12,20,25-27,29,30. Classification of congenital dysplasia of the hip, on the basis of the degree of distortion of the anatomy and of subluxation of the femoral head, seems useful for predicting the difficulty of the operative procedure and strategies used to address these anatomical variations7,9,26,27. The current study was undertaken in an effort to determine the effect of the position of the acetabular component on the long-term durability of both femoral and acetabular components in total hip arthroplasty. Our study comprised a unique group of patients who had the same diagnosis (Crowe type-II congenital dysplasia of the hip7) and who therefore had similar acetabular bone stock. None of these patients needed a structural bone graft to obtain coverage of the acetabular component. Despite a philosophical bias, during the years encompassed by this study, toward positioning of the acetabular component in the true acetabular region, variation in the center of rotation of the hip still occurred. That variation allowed us to analyze the effect of the initial position of the acetabular component on eventual loosening of the acetabular and femoral components.

Hips with severe subluxation of the femoral head (between 75 and 100 per cent; Crowe type III) and those with complete dislocation (Crowe type IV) have deficient acetabular bone stock, and structural bone-grafting or placement of the acetabular component proximal to the true acetabulum is often needed to obtain adequate coverage of the acetabular component7,9,11,12,24-28,30. Total hip arthroplasty in patients who have less than 50 per cent subluxation of the hip joint from the acetabulum (Crowe type I) is technically comparable with total hip arthroplasty in patients who have primary degenerative osteoarthrosis of the hip joint9. Hips with subluxation of between 50 and 74 per cent (Crowe type II) have reasonable preservation of acetabular bone stock in the region of the true acetabulum, and placement of the acetabular component near the anatomical position is usually possible without the use of structural bonegrafting9. The location of the hip center may vary in patients with preserved acetabular bone stock if the acetabular reamer is directed superiorly and excessive bone is removed29.

Location of the center of rotation of the hip as far medially, inferiorly, and anteriorly as possible has been determined, with a mathematical analysis model, to be optimum for the reduction of forces across the hip joint15. Placement of the acetabular component in the anatomical location during total hip arthroplasty theoretically should reduce the prevalence of loosening of the components. A number of reports have documented an association between the initial position of the acetabular component and loosening of the acetabular or femoral component3,18-23,31. Those studies have often included patients with variable diagnoses, such as a previous hip arthroplasty3,18,25,31, poor bone quality in association with rheumatoid arthritis19,23, and all types and degrees of severity of congenital dysplasia of the hip20,25.

A seven-zone system for assessment of the position of the acetabular component was previously proposed for analysis of the effect of the location of the hip center on the outcome of total hip arthroplasty31. With use of this system in a series of 116 total hip arthroplasties, a superior or lateral center of rotation was associated with an increased rate of loosening of the femoral component31. Although a trend toward loosening of the acetabular component was observed, this was not significant (p > 0.05). In an effort to separate the effects of lateral and superior placement of the acetabular component, Russotti and Harris compared the results of thirty-seven total hip arthroplasties in which the hip center was three times more superior than normal with the results in a control group of fifty total hip arthroplasties in which there was no superior displacement25. The two groups had similar magnitudes of lateral displacement. The lack of an association between the height of the hip center and loosening of the acetabular component suggested that lateral displacement is the detrimental factor and that superior, non-anatomical placement without lateral displacement is acceptable. These findings were in agreement with those in another study, in which horizontal displacement of the acetabular component was found to be an adverse factor and isolated superior displacement was considered unrelated to loosening of the acetabular component20.

Identification of the true center of rotation of the hip by extrapolation from that of the contralateral hip often is not possible because of distortion due to previous operative procedures or bilateral congenital dysplasia. Methods of measurement and classification that have been used to determine the center of the hip joint under these circumstances have been based on the absolute distance from the radiographic teardrop3,20,26,31. However, there may be a difference in this distance when small and large patients are compared (Fig. 2).

The method of measurement described in the current report allows determination of a true acetabular region, normalized according to the height of the pelvis. This measurement is independent of the radiographic magnification and is easily reproducible as it is also based on the radiographic teardrop. The approximate femoral head center that is created is therefore a normalized reference point that facilitates accurate measurement of the vertical and horizontal displacement of the center of the reconstructed hip after total hip arthroplasty. Even with meticulous preoperative use of a template and careful intraoperative placement of the acetabular component, the center of the prosthetic hip frequently is not placed at the true femoral head center during a total hip arthroplasty. To accommodate that variation in the position of the cup, the four-zone system described earlier was developed. This system allows separation of the effects of superior, lateral, and combined superior and lateral displacement.

The indications for the use of cement for fixation of both the femoral and the acetabular component are currently undergoing extensive review. The present study suggests that the position of the acetabular component is an important parameter affecting long-term fixation of both the cup and the stem when cement is used. These results are probably due to the increase in forces across the hip joint produced by major alterations in the position of the cup. The detrimental effects of a non-anatomical position of the cup as demonstrated in this study are particularly relevant to cemented femoral or acetabular components. Although femoral or acetabular components implanted without cement were not specifically examined in our study, major alterations in the position of the cup may well be detrimental to the longevity of those components as well.

In summary, with use of the method described in the present report, a reference point for the hip center, normalized according to the patient's size, can be reasonably and reproducibly determined. This reference point is independent of radiographic magnification. The position of the cup as determined with the four-zone system was associated with the risk of eventual loosening of the components in our patients. When possible, efforts should be made to position the acetabular component in the true acetabular region so that the total hip arthroplasty produces an anatomical center of the hip without marked superior or lateral displacement.


   Footnotes
 
*No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. No funds were received in support of this study.

{dagger}Mayo Clinic, 200 First Street, S.W., Rochester, Minnesota 55905. Please address requests for reprints to Dr. Hanssen.


   References
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

  1. Brand, R. A.; Pedersen, D. R.; and |and |Yoder, S. A.: How definition of "loosening" affects the incidence of loose total hip reconstructions. Clin. Orthop., 210: 185-191, 1986.
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