JBJS | The Effect of Weight-Bearing on the Radiographic Measurement of the Position of the Femoral Head After Total Hip Arthroplasty*

The Journal of Bone & Joint Surgery, Volume 82, Issue 1

Articles | January 01, 2000

The Effect of Weight-Bearing on the Radiographic Measurement of the Position of the Femoral Head After Total Hip Arthroplasty*

K. DAVID MOORE, M.D.†, LACKLAND AIR FORCE BASE, TEXAS; ROBERT L. BARRACK, M.D.‡, NEW ORLEANS; CHRISTI J. SYCHTERZ, M.S.E.§, ALEXANDRIA; JASWIN SAWHNEY, M.D.‡, NEW ORLEANS, LOUISIANA; ANTHONY M. YANG, B.S.§; CHARLES A. ENGH, M.D.§, ALEXANDRIA, VIRGINIA

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Investigation performed at the Anderson Orthopaedic Research Institute, Alexandria, and Tulane University, New Orleans

The Journal of Bone & Joint Surgery. 2000; 82:62-9

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Abstract

Background: In most radiographic studies of polyethylene wear, investigators have used routine annual radiographs made with the patient in the supine position in order to measure penetration by the femoral head into the polyethylene liner. However, researchers have begun to question the effect of weight-bearing on the position of the head within the acetabular cup and, consequently, the effect of weight-bearing on measurements of penetration by the head. The purpose of the current study was to determine the effect of weight-bearing on the two-dimensional radiographic position of the femoral head within the acetabular cup.

Methods: Thirty-seven patients (forty-seven hips) who had had a total hip arthroplasty had radiographs made at one of two separate institutions. A set of anteroposterior radiographs was made for each patient: one radiograph was made with the patient supine and one was made with the patient upright bearing full weight on the replaced hip. At one of the institutions, a third anteroposterior radiograph was made with the patient in the same upright position but not bearing weight on the replaced hip. All measurements of the two-dimensional position of the head were performed by a single observer with use of a previously published computerized measurement system.

Results: Data from both institutions revealed that measurements of the position of the head on radiographs made with the patient supine were strongly and significantly correlated with measurements of the position of the head on radiographs made with the patient bearing weight (r² > 0.93, p < 0.001 for both regressions). Examination of the differences between the measurements revealed no bias for one set of measurements to consistently underestimate or overestimate the values derived with use of the other method. Moreover, we found a nearly perfect relationship between the measurements of the position of the head on radiographs made with the patient standing and bearing weight and those on radiographs made with the patient standing but not bearing weight (r² = 0.97, p < 0.001, slope = 0.99, intercept = 0.02 millimeter).

Conclusions: On the basis of these findings, there is no evidence that radiographs must be made with the patient bearing weight in order to accurately measure the position of the femoral head within the polyethylene liner.

Figures in this Article

Introduction

Introduction | Materials and Methods | Results | Discussion | References

Accelerated polyethylene wear is a well recognized complication of total hip arthroplasty that may lead to osteolysis or to catastrophic failure of the hip replacement, or both. Because of this potential problem, it has been recommended that orthopaedic surgeons closely evaluate the patient's radiographs at regular intervals. Various methods have been described for radiographic measurement of the amount of penetration by the femoral head into the polyethylene liner^{1,3-5,7,12,13,15,17}. These methods range from relatively simple measurements, which can be performed in any surgeon's office, to more sophisticated, computer-assisted techniques.

For accurate assessment of the results of some orthopaedic procedures, such as total knee arthroplasty and osteotomy, weight-bearing radiographs are considered to be necessary^16,24. However, weight-bearing radiographs are not routinely used to assess the results of total hip arthroplasty. Annual radiographs usually are made with the patient in the supine position. These non-weight-bearing radiographs are then used to determine penetration by the head into the polyethylene liner and to assess polyethylene wear. However, researchers have questioned the effect of weight-bearing on the position of the head within the acetabular cup and, therefore, the effect upon measurements of penetration by the head⁵. When a hip is loaded, as it is during weight-bearing, it can be assumed that the femoral head is maximally displaced into the acetabular component. It may not be correct, however, to assume that the femoral head remains in this same maximum position when the patient is supine and the hip is no longer loaded. The tension of the soft tissue and the hip capsule surrounding the hip replacement may not be sufficient to keep the head in the same position when the patient is supine. Evaluation of these issues requires a study with modern computer-assisted radiographic techniques for direct comparison of the position of the femoral head seen on radiographs made with the patient supine with that seen on radiographs made with the patient bearing weight. The purpose of the current study, therefore, was to determine the effect of weight-bearing on the two-dimensional radiographic position of the femoral head within the acetabular cup.

*One or more of the authors has received or will receive benefits for personal or professional use from a commercial party related directly or indirectly to the subject of this article. (Royalties have been received; stock is owned in companies manufacturing implants.) In addition, benefits have been or will be directed to a research fund, foundation, educational institution, or other nonprofit organization with which one or more of the authors is associated. No funds were received in support of this study.

†Wilford Hall Medical Center, Lackland Air Force Base, Texas 78236-5300.

‡Department of Orthopaedic Surgery, SL-32, Tulane University School of Medicine, 1430 Tulane Avenue, New Orleans, Louisiana 70112.

§Anderson Orthopaedic Research Institute, P.O. Box 7088, Alexandria, Virginia 22307.

†Wilford Hall Medical Center, Lackland Air Force Base, Texas 78236-5300.

‡Department of Orthopaedic Surgery, SL-32, Tulane University School of Medicine, 1430 Tulane Avenue, New Orleans, Louisiana 70112.

§Anderson Orthopaedic Research Institute, P.O. Box 7088, Alexandria, Virginia 22307.

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FIG1:: Fig. 1 Graph for the patients at the first institution, showing total penetration by the head measured on radiographs made with the patient supine and on radiographs made with the patient standing and bearing weight. Linear regression analysis revealed a strong and significant relationship (r² = 0.93, p < 0.001) with a slope of 0.87 and an intercept of 0.21 millimeter.

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FIG2:: Fig. 2 Graph for the patients at the second institution, showing total penetration by the head measured on radiographs made with the patient supine and on radiographs made with the patient standing and bearing weight. Linear regression analysis revealed a strong and significant relationship (r² = 0.97, p < 0.001) with a slope of 0.91 and an intercept of 0.12 millimeter.

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FIG3:: Fig. 3 Graph showing the relationship between penetration by the head measured on radiographs made with the patient standing and bearing weight and penetration measured on radiographs made with the patient standing but not bearing weight on the involved side. A strong correlation (r² = 0.97, p < 0.001) was revealed with a slope of 0.99 and an intercept of 0.02 millimeter.

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TABLE I: DESIGNS AND MATERIALS OF THE PROSTHETIC COMPONENTS

*The Duraloc, Tri-Lock, custom, and Trispike components were manufactured by DePuy (Warsaw, Indiana); the Arthropor components, by Joint Medical Products (Stamford, Connecticut); the Reflection components, by Smith and Nephew (Memphis, Tennessee); and the Harris-Galante and Trilogy components, by Zimmer (Warsaw, Indiana).

Institution	No. of Hips	Type of Acetabular Cup*	Material Used for Acetabular Shell	Material Used for Femoral Head
1	10	Duraloc	Titanium	Ceramic (3), cobalt-chromium (7)
1	6	Tri-Lock	Titanium	Ceramic (4), cobalt-chromium (2)
1	4	Arthropor	Titanium	Ceramic (1), cobalt-chromium (3)
1	4	Custom	Titanium	Ceramic (2), cobalt-chromium (2)
2	15	Duraloc	Titanium	Cobalt-chromium (15)
2	5	Reflection	Titanium	Cobalt-chromium (5)
2	1	Harris-Galante	Titanium	Cobalt-chromium (1)
2	1	Trilogy	Titanium	Cobalt-chromium (1)
2	1	Trispike	Cobalt-chromium	Cobalt-chromium (1)

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TABLE I: DESIGNS AND MATERIALS OF THE PROSTHETIC COMPONENTS

Institution	No. of Hips	Type of Acetabular Cup*	Material Used for Acetabular Shell	Material Used for Femoral Head
1	10	Duraloc	Titanium	Ceramic (3), cobalt-chromium (7)
1	6	Tri-Lock	Titanium	Ceramic (4), cobalt-chromium (2)
1	4	Arthropor	Titanium	Ceramic (1), cobalt-chromium (3)
1	4	Custom	Titanium	Ceramic (2), cobalt-chromium (2)
2	15	Duraloc	Titanium	Cobalt-chromium (15)
2	5	Reflection	Titanium	Cobalt-chromium (5)
2	1	Harris-Galante	Titanium	Cobalt-chromium (1)
2	1	Trilogy	Titanium	Cobalt-chromium (1)
2	1	Trispike	Cobalt-chromium	Cobalt-chromium (1)

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TABLE II: DIFFERENCE BETWEEN MEASUREMENTS OF THE POSITION OF THE FEMORAL HEAD ON REPEATED RADIOGRAPHS*

*The values are given as the mean and the standard deviation. A p value of less than 0.05 indicates that the mean difference is significantly different from zero (one-sample t test).

	First Radiograph Made with the Patient Bearing Weight	Second Radiograph Made with the Patient Supine
First radiographmade with thepatient supine	0.04 ± 0.17 (p = 0.40)	-0.02 ± 0.13 (p = 0.60)
Second radiographmade with thepatient bearingweight	0.01 ± 0.11 (p = 0.81)	0.01 ± 0.18 (p = 0.80)

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TABLE II: DIFFERENCE BETWEEN MEASUREMENTS OF THE POSITION OF THE FEMORAL HEAD ON REPEATED RADIOGRAPHS*

*The values are given as the mean and the standard deviation. A p value of less than 0.05 indicates that the mean difference is significantly different from zero (one-sample t test).

	First Radiograph Made with the Patient Bearing Weight	Second Radiograph Made with the Patient Supine
First radiographmade with thepatient supine	0.04 ± 0.17 (p = 0.40)	-0.02 ± 0.13 (p = 0.60)
Second radiographmade with thepatient bearingweight	0.01 ± 0.11 (p = 0.81)	0.01 ± 0.18 (p = 0.80)

Materials and Methods

Introduction | Materials and Methods | Results | Discussion | References

Two groups of patients who had been managed at two different institutions were studied. The first group, which had been managed at the institution of the senior author (C. A. E.), consisted of seventeen patients (twenty-four total hip replacements). Patients who had a total hip replacement, inserted without cement, that had been in situ for more than two years and who had been seen for an annual clinical examination during a two-month period were selected for this investigation. Two different anteroposterior radiographs of the pelvis were made for each patient in this group. The first radiograph, a routine clinical anteroposterior radiograph of the entire pelvis, was made with the patient in the supine position and with both lower limbs in maximum internal rotation. If the patient was unable to keep the lower limbs internally rotated, positioning aids were placed around the feet to maintain internal rotation while the radiograph was being made. No other positioning devices were used. The second radiograph was made with the patient standing with the lower limbs in neutral rotation and bearing weight equally on both limbs. For both radiographs, the tube-to-film distance was 1.016 meters and the x-ray beam was centered on the pubic symphysis. The radiographic technique and the positioning of the patient were consistent, and all radiographs were made by the same technicians. The reproducibility of the positioning of the patient was subsequently studied (as described later) to ensure that this variable had no effect on the results. At the time of the investigation, the implants in this group had been in situ for a mean of 6.7 years (range, 2.6 to 12.0 years). The acetabular components included ten Duraloc cups (DePuy, Warsaw, Indiana), six Tri-Lock cups (DePuy), four Arthropor cups (Joint Medical Products, Stamford, Connecticut), and four custom porous-coated components (DePuy) (Table I). All femoral components were porous-coated and were inserted without cement.

The second group, which had been managed at Tulane University, consisted of twenty patients (twenty-three total hip replacements). Patients who had a well functioning total hip replacement with no radiographic signs of loosening, had a metal-backed acetabular component, and had been seen for an annual clinical examination during a four-month period were selected for study. Patients in whom a cobalt-chromium cup completely obscured the femoral head radiographically were excluded from analysis because visualization of part of the femoral head is necessary for computerized measurement of the position of the head. Three anteroposterior radiographs were made of the involved hip in each patient. The first radiograph was made with the patient in the supine position and the lower limbs internally rotated. The second radiograph was made with the patient standing (single-limb stance on the side with the replaced hip, with the limb in neutral rotation) and bearing full weight on the limb with the hip replacement. The third radiograph was made with the patient standing but not bearing weight on the limb with the hip replacement (single-limb stance on the contralateral side, with the limb in neutral rotation). For all radiographs, the tube-to-film distance was 1.016 meters and the x-ray beam was centered over the hip joint. No positioning devices were used. As in the first group of patients, the radiographic technique, technicians, and positioning of the patient were consistent for all radiographs. The components had been in situ for a mean of 1.7 years (range, 0.1 to 7.8 years). The acetabular components included fifteen Duraloc cups (DePuy), five Reflection cups (Smith and Nephew, Memphis, Tennessee), one Harris-Galante cup (Zimmer, Warsaw, Indiana), one Trilogy cup (Zimmer), and one Trispike cup (DePuy) (Table I). Sixteen of the femoral components had been inserted without cement and seven, with cement.

According to a previously documented technique, a single observer used a specially designed computer system to determine the two-dimensional position of the femoral head within the acetabular cup on anteroposterior radiographs²². With this system, points around the periphery of the prosthetic head and the edge of the metal shell were digitized. After correction for magnification and distance from the center of the x-ray beam, a computer algorithm fit circles to these points and determined their centers. The distance between the centers of the prosthetic head and the metal shell defined the position of the femoral head. Validation of this measurement system with use of polyethylene liners machined to a known amount of wear has been documented previously²². Because our goal was to compare the position of the femoral head on radiographs made with the patient bearing weight with the position of the femoral head on radiographs made with the patient supine at a discrete point in time, it was not necessary to determine the initial postoperative location of the head within the metal shell. In other words, we did not examine each patient's immediate postoperative radiographs and then examine the patient's most recent follow-up radiographs to determine the temporal penetration by the head within the shell after the operation. Instead, we examined only the most recent radiographs (made with the patient in different positions) to determine how the position of the head had changed under the different loading conditions.

A Wilcoxon signed-rank test for two related samples was used to compare the mean amount of penetration by the head within the cup under different loading conditions. Linear regression analysis was used to further assess the relationship between measurements of penetration by the head on radiographs made with the hip in the loaded and unloaded conditions. In each instance, the dependent variable was penetration by the head measured on the weight-bearing radiograph; the independent variable was penetration measured on either the radiograph made with the patient supine or on the radiograph made with the patient standing but not bearing weight on the side with the hip replacement. With use of regression analysis, a slope of 1.0 and an intercept of 0.0 millimeter would indicate that the values for penetration by the head as measured on the two different radiographs were identical. A power analysis showed that for a study power of 90 percent and a correlation coefficient of 0.80, a minimum sample of only nine hips would be needed to demonstrate a significant correlation.

We also assessed, with use of the statistical method of Bland and Altman², the agreement between measurements performed on the radiographs made with the patient standing and on those made with the patient supine. This method, which was developed to assess agreement between two methods of clinical measurement, was used to examine the mean difference between the two measurements and the standard deviation of the differences. A one-sample t test was used to determine whether these mean differences were significantly different from zero.

To examine the reproducibility of the positioning of the patient and to ensure that this variable had no effect on the results, an additional group of thirteen patients (seventeen hips) were tested and retested. The acetabular components, which included thirteen Duraloc cups (DePuy), three Arthropor cups (Joint Medical Products), and one Reflection cup (Smith and Nephew), had been in situ for a mean of 4.6 years. For eleven of these hips, anteroposterior radiographs of the pelvis were made according to the first institution's protocol (described earlier)—the patient supine and then standing and bearing weight on the involved side. The patient then was repositioned to make these radiographs again. For the remaining six hips, radiographs were made according to the second institution's protocol (described earlier)—the patient supine, then standing while bearing weight on the involved side, and then standing but not bearing weight on the involved side. The patient then was repositioned so that a second, identical set of radiographs could be made. Statistical analysis of the data for this group was identical to that described for the other two groups.

Results

Introduction | Materials and Methods | Results | Discussion | References

The mean two-dimensional position of the head measured on radiographs made with the patient bearing weight was not significantly different from that measured on radiographs made with the patient supine (1.34 compared with 1.33 millimeters; p = 0.44). The mean difference (and standard deviation) between these two measurements (that is, the bias²) was 0.01 ± 0.23 millimeter; this value was not significantly different from zero (p = 0.68). A bias of zero indicates that one method does not consistently result in an underestimation or overestimation of the values derived with the other method². In addition, because these differences were normally distributed, 95 percent of all of the difference values could be expected to fall between the mean value (0.01 millimeter), plus or minus two standard deviations (0.46 millimeter). These limits, referred to as the limits of agreement², implied that for 95 percent of all cases the difference between the position of the head measured on a radiograph made with the patient bearing weight and that measured on a corresponding radiograph made with the patient supine will fall between -0.45 and +0.47 millimeter. These data showed that the position of the femoral head within the acetabular cup did not differ significantly, regardless of whether the patient was standing and bearing weight (with the lower limbs in neutral rotation) or was supine (with the lower limbs internally rotated).

The regression data from both institutions were remarkably similar. We constructed a graph showing total penetration by the head with the patient supine versus that with the patient bearing weight for the patients at the first institution (Fig. 1). Linear regression analysis revealed a strong and significant relationship (r² = 0.93, p < 0.001) with a slope of 0.87 (95 percent confidence interval, 0.76 to 0.98) and an intercept of 0.21 millimeter (95 percent confidence interval, 0.01 to 0.41 millimeter). The data for the patients at the second institution also were analyzed (Fig. 2). Again, there was a strong and significant relationship between penetration by the head measured on radiographs made with the patient supine and penetration measured on radiographs made with the patient bearing weight (r² = 0.97, p < 0.001) (Fig. 2). The slope of the line was 0.91 (95 percent confidence interval, 0.84 to 0.98), and the intercept was 0.12 millimeter (95 percent confidence interval, 0.02 to 0.22 millimeter).

The mean position of the head measured on radiographs made with the patient standing and bearing weight (1.07 millimeters) was the same as that measured on radiographs made with the patient standing without bearing weight (1.07 millimeters; p = 0.65). There was no bias for one set of measurements to underestimate or overestimate the values derived with the other method (mean difference, 0.00 ± 0.14 millimeter). The range for the limits of agreement (-0.28 to +0.28 millimeter) indicated extremely good agreement between the methods. In the second group of patients, we also analyzed the relationship between penetration by the head measured on radiographs made with the patient standing and bearing weight and penetration measured on radiographs made with the patient standing but not bearing weight (Fig. 3). A significant relationship (r² = 0.97, p < 0.001), with a slope of 0.99 (95 percent confidence interval, 0.91 to 1.06) and an intercept of 0.02 millimeter (95 percent confidence interval, -0.08 to 0.12 millimeter), was revealed. This nearly perfect relationship indicates that, with the patient in the same upright position, the weight-bearing status does not affect measurements of penetration by the head.

In the group of seventeen hips that were used to test reproducibility, there was almost no difference in the position of the head measured on the two radiographs made with the patient supine (mean difference, -0.02 ± 0.13 millimeter) or between the two radiographs made with the patient bearing weight (mean difference, 0.01 ± 0.11 millimeter) (Table II). Similarly, there was no almost no difference in the position of the head between the initial radiograph made with the patient bearing weight and the initial radiograph made with the patient supine (mean difference, 0.04 ± 0.17 millimeter) or between the repeat radiographs made with the patient in these two positions (mean difference, 0.01 ± 0.18 millimeter) (Table II). None of these mean differences were significantly different from zero (p = 0.60, 0.81, 0.40, and 0.80, respectively), indicating that the positioning of the patient was reproducible and did not affect the results.

Discussion

Introduction | Materials and Methods | Results | Discussion | References

Severe polyethylene wear is a major problem after total hip replacement^{6,8,10,11,14,18-20,25}. In an attempt to reduce wear, manufacturers of orthopaedic devices have begun to develop new types of polyethylene with superior mechanical properties and greater resistance to oxidative degradation. As components made from new polyethylenes become more widely used in total hip replacement, their in vivo performance over time will require close monitoring. Currently, the best method of which we are aware for objectively assessing in vivo polyethylene wear is computerized radiographic measurement. Since radiographic methods will continue to be used to assess the wear performance of new polyethylenes, any issue related to the accuracy of these methods is of paramount importance. One such issue is the effect of weight-bearing on the position of the femoral head within the polyethylene liner. It has been suggested that the femoral head may not be in the position of maximum penetration on radiographs made when the patient is supine and not bearing weight⁵. Because there have been few published studies in which the findings on weight-bearing and non-weight-bearing radiographs have been compared, the effect of the loading condition on the position of the femoral head within the acetabular cup has remained uncertain.

We found that the position of the femoral head within the acetabular cup did not substantially change between radiographs made with the patient standing and bearing weight (with the lower limbs in neutral rotation) and those made with the patient supine (with the lower limbs internally rotated). It appears that, in these patients, the tension of the hip capsule and the soft tissue surrounding the implant was sufficient to keep the head in nearly the same position whether the hip was loaded or unloaded.

Despite this finding, we recognize that some acetabular liners can have multiple wear tracks. In a retrieval study, Yamaguchi et al. documented multiple wear vectors in thirty-one of 104 polyethylene liners²⁶. It is possible that changes in femoral or pelvic rotation can shift the femoral head from one wear track into another, thus changing the position of the femoral head as measured radiographically. A study by Horne et al. supports the complex nature of penetration by the head into the polyethylene liner and the effect of femoral rotation on measurements of penetration⁹. Those authors found that maximum internal and external rotation of the femur increased the measured volumetric wear by 35 percent; therefore, they concluded that the diameter of the wear track was larger than that of the femoral head. Although the cause of multiple wear vectors remains unknown and in vivo detection of multiple wear vectors is not yet possible, it is necessary to be aware that measurements of the position of the head may be inconsistent regardless of the patient's weight-bearing status.

Of additional interest is the comparison of measurements of penetration performed on radiographs made with the patient standing and bearing weight and those performed on radiographs made with the patient standing without bearing weight. In our comparison, neither the rotation of the pelvis nor that of the femur changed between the two radiographs. The extremely close correlation (r² = 0.97), coupled with a slope of 0.99 and an intercept of 0.02 millimeter, indicated that with the patient in the same upright position weight-bearing had no effect on two-dimensional measurements of the position of the head.

Contrary to our findings, Smith et al. recently reported that weight-bearing affects measurements of penetration by the femoral head²¹. Those authors examined twenty hips at a mean of twelve years postoperatively and found that the rate of wear measured on radiographs made with the patient supine represented a significant underestimation of measurements made on radiographs made with the patient bearing weight. We are uncertain why the findings of Smith et al. differed from our findings. Perhaps the differences between the two studies can be explained by such variables as the method of measuring penetration by the femoral head, the internal rotation of the lower limbs when the radiographs were made with the patient supine, the use of components inserted without cement, or the operative technique. We are encouraged by our findings because our series included patients who were operated on and examined radiographically at two separate institutions, indicating that these results are not unique to one particular surgical center. Although we measured penetration by the head with a modern, validated, computer-assisted radiographic technique²², more complex analyses (such as radiostereometric analysis) ultimately may be needed to corroborate our findings.

We acknowledge that some of the patients in our study had rather short-term follow-up (fifteen of the forty-seven hips were followed for less than two years). However, it would be incorrect to assume that little penetration by the femoral head occurred in these patients and, thus, that measurements of the position of the head would be difficult to obtain and inaccurate. The measurement error for the technique used in our study was 0.19 millimeter, and we agree that penetration of less than 0.19 millimeter would be difficult to assess. However, low values for penetration are not directly related to a short duration of follow-up. Patients with short-term follow-up can have a large amount of penetration because of the bedding-in process (that is, creep or settling-in of the liner into the metal shell). Conversely, patients with long-term follow-up can have little measurable penetration. In the patients at the first institution, who were followed for 2.6 to 12.0 years, penetration by the head ranged from 0.4 to more than 4.0 millimeters (Fig. 1), with a fair distribution of measurements throughout that range. Clearly, penetration by the head in these hips was measurable (that is, it was greater than the error of the system) and, thus, comparable. Similarly, in the patients at the second institution, penetration by the head ranged from 0.3 to 4.0 millimeters (Fig. 2). Although these measurements were not distributed as evenly as those for the first group of patients, all of the magnitudes were measurable and comparable. Therefore, we think that the inclusion of patients who had a short duration of follow-up was justified.

Despite the extremely close correlation between the positions of the head under different loading conditions, there are limitations to two-dimensional measurement techniques. A two-dimensional measurement of the position of the head can underestimate the true three-dimensional position. However, in a recent study of two and three-dimensional measurements of penetration by the head of 202 total hip prostheses, the two-dimensional measurements were found to be nearly identical to the three-dimensional measurements for 95 percent of the hips²³. On the basis of that study, we believe that the findings in the current study would not have differed substantially if the position of the head had been measured three-dimensionally.

We reiterate that one strength of the current study is the remarkable similarity among the results obtained for two different groups of patients examined radiographically at two separate institutions. This similarity is due in part to the consistency of the radiographic technique at each institution and to the positioning of the patient. We stress that a reproducible radiographic technique is essential for any study of penetration by the femoral head.

In conclusion, when radiographs were made with the patient in the same upright position, there was no difference in the measurements of the position of the head with use of different loading conditions of the hip. Moreover, the position of the head within the acetabular cup did not change substantially when the patient went from a standing, weight-bearing position (with the lower limbs in neutral rotation) to a supine position (with the lower limbs internally rotated). It appears that the hip capsule and the soft tissue surrounding the implant were sufficient to keep the head in nearly the same position when load was removed.

References

Introduction | Materials and Methods | Results | Discussion | References

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TABLE I: DESIGNS AND MATERIALS OF THE PROSTHETIC COMPONENTS

Institution	No. of Hips	Type of Acetabular Cup*	Material Used for Acetabular Shell	Material Used for Femoral Head
1	10	Duraloc	Titanium	Ceramic (3), cobalt-chromium (7)
1	6	Tri-Lock	Titanium	Ceramic (4), cobalt-chromium (2)
1	4	Arthropor	Titanium	Ceramic (1), cobalt-chromium (3)
1	4	Custom	Titanium	Ceramic (2), cobalt-chromium (2)
2	15	Duraloc	Titanium	Cobalt-chromium (15)
2	5	Reflection	Titanium	Cobalt-chromium (5)
2	1	Harris-Galante	Titanium	Cobalt-chromium (1)
2	1	Trilogy	Titanium	Cobalt-chromium (1)
2	1	Trispike	Cobalt-chromium	Cobalt-chromium (1)

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TABLE I: DESIGNS AND MATERIALS OF THE PROSTHETIC COMPONENTS

Institution	No. of Hips	Type of Acetabular Cup*	Material Used for Acetabular Shell	Material Used for Femoral Head
1	10	Duraloc	Titanium	Ceramic (3), cobalt-chromium (7)
1	6	Tri-Lock	Titanium	Ceramic (4), cobalt-chromium (2)
1	4	Arthropor	Titanium	Ceramic (1), cobalt-chromium (3)
1	4	Custom	Titanium	Ceramic (2), cobalt-chromium (2)
2	15	Duraloc	Titanium	Cobalt-chromium (15)
2	5	Reflection	Titanium	Cobalt-chromium (5)
2	1	Harris-Galante	Titanium	Cobalt-chromium (1)
2	1	Trilogy	Titanium	Cobalt-chromium (1)
2	1	Trispike	Cobalt-chromium	Cobalt-chromium (1)

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TABLE II: DIFFERENCE BETWEEN MEASUREMENTS OF THE POSITION OF THE FEMORAL HEAD ON REPEATED RADIOGRAPHS*

*The values are given as the mean and the standard deviation. A p value of less than 0.05 indicates that the mean difference is significantly different from zero (one-sample t test).

	First Radiograph Made with the Patient Bearing Weight	Second Radiograph Made with the Patient Supine
First radiographmade with thepatient supine	0.04 ± 0.17 (p = 0.40)	-0.02 ± 0.13 (p = 0.60)
Second radiographmade with thepatient bearingweight	0.01 ± 0.11 (p = 0.81)	0.01 ± 0.18 (p = 0.80)

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TABLE II: DIFFERENCE BETWEEN MEASUREMENTS OF THE POSITION OF THE FEMORAL HEAD ON REPEATED RADIOGRAPHS*

*The values are given as the mean and the standard deviation. A p value of less than 0.05 indicates that the mean difference is significantly different from zero (one-sample t test).

	First Radiograph Made with the Patient Bearing Weight	Second Radiograph Made with the Patient Supine
First radiographmade with thepatient supine	0.04 ± 0.17 (p = 0.40)	-0.02 ± 0.13 (p = 0.60)
Second radiographmade with thepatient bearingweight	0.01 ± 0.11 (p = 0.81)	0.01 ± 0.18 (p = 0.80)