JBJS | Characterization of Long-Term Femoral-Head-Penetration Rates : Association with and Prediction of Osteolysis*

The Journal of Bone & Joint Surgery, Volume 82, Issue 8

Articles | August 01, 2000

Characterization of Long-Term Femoral-Head-Penetration Rates : Association with and Prediction of Osteolysis*

James E. Dowd, M.D.†; Christi J. Sychterz, M.S.‡; Anthony M. Young, B.S.‡; Charles A. Engh, M.D.‡

Investigation performed at the Anderson Orthopaedic Research Institute, Alexandria, Virginia
*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. No funds were received in support of this study.
†Orthopaedic Associates of Virginia, 6275 East Virginia Beach Boulevard, Suite 300, Norfolk, Virginia 23502.
‡Anderson Orthopaedic Research Institute, P.O. Box 7088, Alexandria, Virginia 22307.

The Journal of Bone & Joint Surgery. 2000; 82:1102-1102

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Abstract

Background: We examined the relationship between long-term femoral-head-penetration patterns and osteolysis in a ten-year follow-up study of a well controlled patient population. The purposes of this study were to characterize the linearity of long-term head-penetration patterns over time, to describe the relationship between ten-year true wear rates and osteolysis, and to determine whether the occurrence of osteolysis at ten years could be predicted by penetration data obtained prior to five years.

Methods: Temporal femoral-head-penetration patterns were examined at a minimum of ten years after forty-eight primary total hip arthroplasties. The arthroplasties were performed with the use of an Arthropor acetabular cup (Joint Medical Products) and a thirty-two-millimeter-diameter cobalt-chromium femoral head (DePuy). Using a computer-assisted radiographic technique, we evaluated two-dimensional head penetration on serial annual radiographs. Linear regression analysis modeled penetration-versus-time data as a line for each patient. The slope of the regression line indicated the true wear rate for each patient. In a subgroup of thirty-four hips for which three annual radiographs had been made less than five years after the arthroplasty, we compared early head-penetration patterns with the later occurrence of osteolysis.

Results: For all forty-eight hips, the true wear rate averaged 0.18 millimeter per year (range, 0.01 to 0.44 millimeter per year) and temporal head-penetration patterns tended to be linear (mean r² = 0.91 ± 0.16). Osteolysis at ten years was strongly associated with increasing true wear rates (p < 0.001). Osteolysis did not develop in any of the nine hips with a true wear rate of less than 0.1 millimeter per year. However, osteolysis developed in nine (43 percent) of twenty-one hips with a rate between 0.1 and less than 0.2 millimeter per year, in eight of ten hips with a rate between 0.2 and 0.3 millimeter per year, and in all eight hips with a rate of greater than 0.3 millimeter per year. Evaluation of early true wear rates as a predictor of late osteolysis showed a similar relationship.

Conclusions: This study demonstrates that true wear rates tend to be constant and that increased true wear is significantly associated with osteolysis at ten years after the operation. A similar relationship was also found at the early follow-up interval, indicating that early true wear rates (determined from serial radiographs) might enable orthopaedists to predict if patients are at risk for the development of osteolysis.

Clinical Relevance: On the basis of these findings, we use temporal femoral-head-penetration data in our practice to evaluate polyethylene inserts in asymptomatic patients, to estimate the time to component wear-through, and to adjust the frequency of follow-up evaluations for monitoring the development of osteolytic lesions in at-risk patients.

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Introduction

Introduction | Materials and Methods | Results | Discussion | References

As component fixation in total hip arthroplasty has improved over the last decade, it has become increasingly apparent that a primary obstacle limiting the longevity of total hip prostheses is wear. The progressive removal of polyethylene material from the metal-polyethylene articulation results in thinning of the polyethylene insert as well as in the generation of microscopic polyethylene wear debris in the periarticular tissues. Clinical complications of the wear process include mechanical failure or wear-through of the polyethylene insert as well as a biological response to the wear debris.

The earliest reports of polyethylene wear of which we are aware dealt with Charnley low-friction arthroplasty and involved measurements on a single follow-up radiograph without correction for magnification or for initial differences between the head and cup centers². Refinement in wear-measurement techniques led to a duo-radiographic technique that compared the position of the femoral head center on initial postoperative radiographs with its position on later follow-up radiographs^3,10. With these techniques, wear rates were calculated as the total head-penetration distance divided by the number of years in situ. More recently, two-dimensional and three-dimensional computer-assisted measurement techniques have been developed; these techniques are able to accommodate variations in pelvic position, acetabular component position, and radiographic beam center between serial radiographs^4,11,15,17. With a reported accuracy of 0.2 millimeter or better^4,11,15,17, these techniques provide a substantial improvement in the characterization of true component wear.

Several studies have used contemporary computer-assisted techniques to measure head-penetration rates of specific implant systems^1,6,18 at mid-term to long-term follow-up intervals. Still, a number of clinical questions regarding the polyethylene wear process remain unanswered. For example, when the effects of so-called bedding-in become negligible after the initial postoperative years, it is unknown whether head penetration remains constant over the life of the implant - that is, we do not know if the pattern of head penetration is linear over time. If the pattern is linear, it is still unknown if there is a quantifiable relationship between long-term head-penetration rates and the occurrence of osteolysis. Finally, if these two relationships exist, it remains unknown whether it is possible to use a component's early temporal penetration data to predict the occurrence of late wear-related complications.

In the present study, we combined reliable computer-assisted assessment of femoral head penetration and a ten-year serial follow-up of a well controlled patient population to address these questions. The purposes of the study were (1) to characterize the linearity of long-term head-penetration patterns over time, (2) to quantify the relationship between ten-year true wear rates and osteolysis, and (3) to determine whether the amount of head penetration and the occurrence of osteolysis at ten years could be predicted from penetration data obtained prior to five years.

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Fig. 1:: For most patients, the rate of head penetration between the second and tenth postoperative years was relatively constant. This graph demonstrates a strongly linear head-penetration pattern for a patient with an Arthropor porous-coated component (r² = 0.99).

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Fig. 2:: Four of the twelve hips with radiographic evidence of bead loss demonstrated a bimodal pattern of head penetration consisting of two apparent linear segments that increased in slope over time, as seen on this graph. Bead loss was first detected on this patient's eight-year postoperative radiograph.

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Fig. 3:: Graph of the amount of ten-year head penetration predicted from early true wear rates versus the amount of penetration measured at ten years from long-term radiographs. Although the predicted head-penetration and measured head-penetration values were significantly correlated (Pearson correlation: p < 0.001, r² = 0.55), there was a fair amount of scatter to the data, indicating that some early wear measurements were not as strongly predictive as others.

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Fig. 4:: Graph of predicted head penetration versus measured head penetration, excluding the eleven hips for which the early true wear rates were not linear (that is, early r² < 0.8). For the twenty-three hips that are represented in the graph, the relationship between predicted and measured head-penetration values was tighter (Pearson correlation: p < 0.001, r² = 0.73).

Materials and Methods

Introduction | Materials and Methods | Results | Discussion | References

Forty-eight hips in forty-seven patients met the following criteria for inclusion in the study. They had undergone a primary total hip arthroplasty with an Arthropor porous-coated acetabular component (Joint Medical Products, Stamford, Connecticut) that articulated with a thirty-two-millimeter-diameter cobalt-chromium femoral head. The cup had been inserted without cement, and at least one rim screw had been used to lock the polyethylene liner in place. The femoral component was a uniformly cobalt-chromium porous-coated Anatomic Medullary Locking (AML) stem (DePuy, Warsaw, Indiana) inserted without cement. The patients were required to have had a minimum ten-year clinical and radiographic follow-up that included at least four different postoperative evaluations. The initial evaluation was performed within the first few weeks postoperatively (at a mean of 2.0 weeks), and the most recent follow-up was performed at least ten years postoperatively (at a mean of 10.5 years). We evaluated an average of 6.5 (range, four to eleven) annual radiographs per hip. All of the polyethylene liners were 4.9 millimeters thick and had been sterilized with ethylene oxide. All of the acetabular and femoral components were considered stable according to the criteria of Engh et al.⁷ at the time of the most recent follow-up.

Of the forty-seven patients studied, twenty-seven were men and twenty were women. At the time of hip replacement, the average age was 56.4 years (range, thirty-five to seventy-six years) and the average weight was 79.7 kilograms (range, 45.8 to 129.3 kilograms).

According to a previously published protocol that used specially designed computer software to analyze anteroposterior radiographs, we determined two-dimensional femoral-head penetration at annual intervals¹⁷. Briefly, a computer system, a digitizer tablet, and specially designed software were used to measure femoral head penetration into the polyethylene liner on an anteroposterior radiograph. After aligning the radiograph on the digitizer tablet so that the interteardrop line was parallel to the edge of the tablet, a computer operator digitized at least five points around the circumference of the prosthetic head and at least five points around the circumference of the acetabular cup. The computer software, after correcting for magnification and distance away from the center of the x-ray beam, then fit circles to these points and determined their centers. The movement of the head center relative to the cup center was then used to calculate the amount and direction of femoral head penetration into the polyethylene liner. The amount of penetration seen on the immediate postoperative radiograph was designated as the zero position; subsequent movement from this initial point was defined as head penetration. All computer-assisted measurements were made by a single observer to eliminate interobserver variability. This measurement system had been previously validated with the use of polyethylene liners machined to known amounts of wear; its reported accuracy was ±0.19 millimeter¹⁷.

Linear regression analysis was used to model temporal head-penetration data as a line. The slope of the regression line represented the true wear rate of the component; the intercept represented head penetration due to the bedding-in process^9,18. The r² value for regression indicated how closely the wear data for each patient approximated a straight line (r² values range from zero to one). An r² value of 1.0 indicated a perfectly linear head-penetration pattern.

Since a previous study had documented beads shearing off Arthropor porous-coated cups¹⁹, we were aware that this third-body debris could enter the articular space and affect wear patterns and rates^8,12. Thus, we examined all annual radiographs for evidence of loose porous-coating beads. Any evidence of bead-shedding and the year of its first occurrence were recorded. A nonparametric test for two independent samples was used to examine the difference in true wear rates between patients who had radiographic evidence of bead loss and those who did not.

A single surgeon blinded to wear rates then evaluated all serial radiographs for the presence of osteolytic lesions. We defined osteolysis as a localized area of decreased radiographic bone density that measured 1.5 square centimeters or larger. Typically, osteolytic lesions in the pelvis and the greater trochanter were characterized by so-called punched-out areas devoid of trabecular bone, usually with a sclerotic border. Femoral osteolytic lesions were also characterized by progressive erosion of the endosteal surface. For data analysis, osteolysis was recorded simply as either present or absent for each hip.

To examine the relationship between ten-year true wear rates and osteolysis, we grouped patients into four categories according to whether the true wear rate was less than 0.1 millimeter per year, between 0.1 and less than 0.2 millimeter per year, between 0.2 and 0.3 millimeter per year, or greater than 0.3 millimeter per year. Chi-square analysis was used to determine if there was a significant difference in the presence or absence of osteolysis among the four groups.

To determine if early head-penetration data (obtained less than five years after the operation) could be used to predict the amount of head penetration and the occurrence of osteolysis at ten years, a subgroup of thirty-four patients was examined. This subgroup consisted of patients who had had three annual radiographs made during the first five postoperative years. Linear regression analysis was used to model temporal penetration data from these three radiographs and to determine a true wear rate at this early interval (at a mean of 3.7 years postoperatively). The slope and intercept values from the regression analysis were used to calculate (that is, to predict) the amount of head penetration that would occur at ten years. Pearson correlation was used to examine the relationship between predicted and measured ten-year head-penetration values. Early true wear rates also were compared with the later occurrence of osteolysis. With use of the four previously listed categories, the hips were grouped by their early true wear rates, and chi-square analysis was used to determine if there was a difference in the occurrence of osteolysis among these groups.

Results

Introduction | Materials and Methods | Results | Discussion | References

The ten-year true wear rate for the forty-eight hips averaged 0.18 ± 0.10 millimeter per year (range, 0.01 to 0.44 millimeter per year). As determined by the computer software, the acetabular components had an average abduction angle of 42.8 ± 6.2 degrees and an average of 16.4 ± 6.8 degrees of anteversion. Review of the radiographs revealed evidence of bead loss from the ingrowth surface of the acetabular component in twelve hips. Evidence of bead-shedding was first detected radiographically at an average of 6.5 years (range, one to eight years) postoperatively. The average true wear rate for the hips with evidence of bead-shedding was 0.21 millimeter per year (range, 0.10 to 0.35 millimeter per year) compared with 0.18 millimeter per year (range, 0.01 to 0.44 millimeter per year) for the hips with no visible evidence of bead-shedding (p = 0.23).

Linearity of Long-Term Penetration Patterns

Despite a fortyfold variability in wear rates across the entire group, most individual head-penetration patterns between the second and tenth postoperative years were linear (Fig. 1). This was indicated by a mean r² value of 0.91 ± 0.16. Ninety-four percent (forty-five) of the forty-eight hips had an r² value of greater than 0.80, and 65 percent (thirty-one) of the forty-eight had a value of greater than 0.95. However, four of the twelve hips with radiographic evidence of bead loss did not exhibit this pattern. These hips demonstrated a bimodal pattern of head penetration consisting of two apparent linear segments that increased in slope over time (Fig. 2). The thirty-six hips without radiographic evidence of bead-shedding were less likely to demonstrate this pattern.

Relationship Between Long-Term Penetration Rates and Osteolysis

Fifty-two percent (twenty-five) of the forty-eight hips in our study, all of which were treated with a first-generation cup and a thirty-two-millimeter-diameter head, had osteolysis. We found that the presence of osteolytic lesions was strongly and significantly associated with increased true wear rates (p < 0.001). At the ten-year follow-up evaluation, no osteolysis was seen in the nine hips with a true wear rate of less than 0.1 millimeter per year. In contrast, osteolysis occurred in nine (43 percent) of the twenty-one hips with a true wear rate between 0.1 and less than 0.2 millimeter per year, in eight of the ten hips with a rate between 0.2 and 0.3 millimeter per year, and in all eight hips with a rate of greater than 0.3 millimeter per year. Furthermore, osteolysis was present in all twelve hips with radiographic evidence of third-body debris (bead-shedding).

Predictive Ability of Early Penetration Data

For the thirty-four hips used to compare early and late wear, no significant difference existed between true wear rates determined from the three early radiographs and those determined for the entire ten-year series of radiographs. At a mean of 3.7 years, the true wear rates averaged 0.18 ± 0.13 millimeter per year; at a mean of 10.5 years, they averaged 0.19 ± 0.10 millimeter per year (p = 0.44, nonparametric test for two related samples). Similarly, the average amount of ten-year head penetration predicted from the early true wear rates was not significantly different from the average amount of penetration measured at ten years (2.01 compared with 2.06 millimeters, p = 0.50). Although the predicted head-penetration and measured head-penetration values were significantly correlated (Pearson correlation: p < 0.001, r² = 0.55) (Fig. 3), there was a fair amount of scatter to the data, indicating that some early wear measurements were not as strongly predictive as others.

Interestingly, eleven of the thirty-four hips had an early temporal head-penetration pattern that was not highly linear - that is, the r² value for the penetration-versus-time data was less than 0.8. For these eleven hips, the ten-year head-penetration value that was predicted by the early true wear rate was not reflective of the actual ten-year head penetration measured radiographically (Pearson correlation: p = 0.91, r² = 0.002). Excluding these eleven hips, the relationship between predicted and measured head-penetration values was much tighter (Pearson correlation: p < 0.001, r² = 0.73) (Fig. 4). For the remaining twenty-three hips, the best-fit line between predicted and measured values had a slope of 1.02 and an intercept of -0.03 millimeter.

There was a significant relationship between an increased rate of later occurrence of osteolysis and a high early true wear rate (p = 0.05). Osteolytic lesions developed in three of the eleven hips with an early true wear rate of less than 0.1 millimeter per year compared with seven of the thirteen hips with an early rate between 0.1 and 0.3 millimeter per year and seven of the ten hips with an early rate of greater than 0.3 millimeter per year.

Discussion

Introduction | Materials and Methods | Results | Discussion | References

Polyethylene wear and wear-related complications have become a principal reason for revision of a total hip arthroplasty. Complete wear-through of the polyethylene bearing surface, catastrophic failure of the insert, and wear-debris-mediated osteolysis are among the most common indications for revision total hip replacement in patients with long-term clinical follow-up. Consequently, analysis of head penetration into the polyethylene liner remains paramount to the study of total hip arthroplasty. From previous studies of femoral head penetration, we have found that computer-assisted measurement methods, examination of serial annual radiographs, and linear regression modeling are necessary techniques for the analysis of the polyethylene wear process^{4,9,11,15,17,18}.

Using these techniques, we discovered four useful findings regarding the wear process and the development of osteolysis. First, we found that, between the second and tenth postoperative years, head penetration is approximately constant - that is, head-penetration patterns are linear over time. Although one might postulate that head-penetration rates would accelerate on late follow-up after a polyethylene insert had thinned substantially, we found no evidence of such a phenomenon. Because wear patterns are linear, our results suggest that orthopaedists can use radiographically determined head-penetration patterns to assess when complete liner wear-through is likely to occur. This information may be useful for determining how frequently a patient should return for follow-up examinations or when to schedule a patient for a revision operation.

Second, we found a strong and significant relationship between long-term true wear rates and the occurrence of osteolysis (p < 0.001). Osteolysis did not develop in any of the nine hips with a ten-year true wear rate of less than 0.1 millimeter per year. In contrast, osteolysis was evident in nine of the twenty-one hips with a ten-year true wear rate between 0.1 and less than 0.2 millimeter per year, in eight of the ten hips with a rate between 0.2 and 0.3 millimeter per year, and in all eight hips with a rate of greater than 0.3 millimeter per year. Although the occurrence of periprosthetic osteolysis is multifactorial, this relationship supports the hypothesis that osteolysis is a particle-related phenomenon. Our data suggest that osteolysis is directly related to the number of microscopic debris particles generated; low true wear rates may represent a tolerable level of polyethylene particle production, whereas increased true wear rates may represent an amount of particle generation that could overwhelm the periprosthetic tissues, eventually resulting in osteolysis. While we are not the first to demonstrate a relationship between wear and osteolysis^1,5,13,14, our study improved upon previous reports because it involved an examination of serial radiographs of a well controlled patient population with a more refined measurement method. This allowed us to determine long-term true wear rates (not simply head-penetration rates) with the use of more precise methods and, thus, to determine a more precise relationship between wear and osteolysis.

Third, although long-term head-penetration patterns were strongly linear, we were not always able to accurately predict ten-year penetration data from five-year penetration data. Although the relationship between predicted and measured ten-year head penetration was highly significant (p < 0.001, Fig. 3), there was a fair amount of scatter to the graph owing to two problems that hampered our ability to predict long-term data accurately. The first problem was third-body wear debris. As demonstrated in this study, the introduction of third-body debris into the articular space has the potential to alter head-penetration patterns (Fig. 2), making predictive analyses less reliable. The second problem concerned the susceptibility of early penetration measurements to error. Since early values for head penetration typically are small, they are more susceptible to measurement error. Thus, regression analysis using three early radiographic data points was not always strongly linear. We found, however, that when the early head-penetration pattern of a hip was strongly linear (that is, when r² was more than 0.8), the early penetration data predicted the amount of penetration at the time of long-term follow-up more accurately (Fig. 4). These data suggest that the more linear the penetration pattern is before five years, the more linear it will remain and the more reliably it will predict future component performance.

Finally, we found that high early true wear rates were significantly related to the occurrence of osteolysis at ten years (p = 0.05). Osteolysis later developed in three of eleven hips that had an early true wear rate of less than 0.1 millimeter per year compared with seven of thirteen hips with a rate between 0.1 and 0.3 millimeter per year and seven of ten hips with a rate of greater than 0.3 millimeter per year. Owing to the problems with predictive analyses described above, the relationship between early true wear rates and osteolysis was not as strong as that between ten-year wear rates and osteolysis. However, our data suggest that orthopaedic surgeons can still use early head-penetration patterns and early true wear rates as a guide to assess a patient's risk for the development of osteolysis after long-term in vivo implantation. Patients with a true wear rate of less than 0.1 millimeter per year have a substantially lower risk of osteolysis developing than patients with a wear rate of greater than 0.3 millimeter per year.

We acknowledge that the head-penetration patterns and relationships described in this study could be specific to the components that we examined. In these hips, all of the polyethylene liners were sterilized with ethylene oxide and, therefore, should not have exhibited the subsurface oxidation peaks that have been described for components sterilized with gamma irradiation in air¹⁶. The absence of the subsurface white bands of oxidized polymer chains may have contributed to the uniform wear characteristics demonstrated in our population. Furthermore, all of the polyethylene liners were 4.9 millimeters thick, regardless of the diameter of the surrounding metal shell, and all of the femoral components had the same thirty-two-millimeter-diameter head size and manufacturer. These uniform characteristics also could have influenced the head-penetration patterns described for these components. Moreover, the use of a large-diameter femoral head and a thin polyethylene liner - two variables associated with increased volumetric wear - may explain why osteolysis developed in such a large percentage of the hips in our study and why it was so strongly associated with wear (p < 0.001). Because thirty-two-millimeter-diameter heads and thin polyethylene liners sterilized with ethylene oxide are no longer used frequently, the results of this study may not be directly applicable to components being implanted today. As a result, continued examination of head-penetration patterns of other implant systems is necessary to determine if the results presented in this study are applicable to all implant types.

The clinical relevance of the findings of this study is twofold. First, using true wear rates calculated from temporal head-penetration patterns, we have been able to evaluate thinning polyethylene inserts in asymptomatic patients. At late follow-up intervals, we frequently see patients who do not have osteolysis and are asymptomatic but whom we believe will have wear-through of their polyethylene liner eventually. When a patient has a first-generation Arthropor component, we estimate the time to liner wear-through and, when necessary, appropriately schedule a revision operation. Typically, we obtain polyethylene characteristics from the manufacturer and schedule a revision operation when the patient has an estimated two millimeters of polyethylene thickness left. Second, using computer-assisted radiographic technique, we continually monitor the early head-penetration data of our patients who have had a total hip arthroplasty. When a patient has a high early wear rate (greater than 0.3 millimeter per year) and an early penetration pattern that is highly linear, we adjust the frequency of follow-up evaluations to monitor the development of osteolytic lesions. When a patient has a low early wear rate, we continue with standard postoperative care but with the understanding that there are variables (such as third-body debris) that could alter the time-course of head penetration and undermine predictive analyses.

References

Introduction | Materials and Methods | Results | Discussion | References

Berger, R. A.; Jacobs, J. J.; Quigley, L. R.; Rosenberg, A. G.; and Galante, J. O.: Primary cementless acetabular reconstruction in patients younger than 50 years old. 7- to 11-year results. Clin. Orthop.,344: 216-226, 1997.344216 1997 [PubMed]

Charnley, J., and Cupic, Z.: The nine and ten year results of the low-friction arthroplasty of the hip. Clin. Orthop.,95: 9-25, 1973.959 1973 [PubMed]

Charnley, J., and Halley, D. K.: Rate of wear in total hip replacement. Clin. Orthop.,112: 170-179, 1975.112170 1975 [PubMed]

Devane, P. A.; Bourne, R. B.; Rorabeck, C. H.; Hardie, R. M.; and Horne, J. G.: Measurement of polyethylene wear in metal-backed cups. I. Three-dimensional technique. Clin. Orthop.,319: 303-316, 1995.319303 1995 [PubMed]

Devane, P. A.; Bourne, R. B.; Rorabeck, C. H.; MacDonald, S.; and Robinson, E. J.: Measurement of polyethylene wear in metal-backed acetabular cups. II. Clinical application. Clin. Orthop.,319: 317-326, 1995.319317 1995 [PubMed]

Devane, P. A.; Horne, J. G.; Martin, K.; Coldham, G.;; and Krause, B.: Three-dimensional polyethylene wear of a press-fit titanium prosthesis. Factors influencing generation of polyethylene debris. J. Arthroplasty,12: 256-266, 1997.12256 1997 [PubMed]

Engh, C. A., Jr.; Culpepper, W. J., II; and Engh, C. A.: Long-term results of use of the anatomic medullary locking prosthesis in total hip arthroplasty. J. Bone and Joint Surg.,79-A: 177-184, Feb 1997.79-A177 1997

Hop, J. D.; Callaghan, J. J.; Olejniczak, J. P.; Pedersen, D. R.; Brown, T. D.; and Johnston, R. C.: Contribution of cable debris generation to accelerated polyethylene wear. Clin. Orthop.,344: 20-32, 1997.34420 1997 [PubMed]

Isaac, G. H.; Dowson, D.; and Wroblewski, B. M.: An investigation into the origins of time-dependent variation in penetration rates with Charnley acetabular cups - wear, creep or degradation?. Proc. Inst. Mech. Eng., Part H - J. Eng. Med.,210: 209-216, 1996.210209 1996

Livermore, J.; Ilstrup, D.; and Morrey, B.: Effect of femoral head size on wear of the polyethylene acetabular component. J. Bone and Joint Surg.,72-A: 518-528, April 1990.72-A518 1990

Martell, J. M., and Berdia, S.: Determination of polyethylene wear in total hip replacements with use of digital radiographs. J. Bone and Joint Surg.,79-A: 1635-1641, Nov 1997.79-A1635 1997

Morscher, E. W.; Hefti, A.; and Aebi, U.: Severe osteolysis after third-body wear due to hydroxyapatite particles from acetabular cup coating. J. Bone and Joint Surg.,80-B(2): 267-277, 1998.80-B(2)267 1998

Nashed, R. S.; Becker, D. A.; and Gustilo, R. B.: Are cementless acetabular components the cause of excess wear and osteolysis in total hip arthroplasty?. Clin. Orthop., 317: 19-28, 1995. 31719 1995 [PubMed]

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Shaver, S. M.; Brown, T. D.; Hillie, S. L.; and Callaghan, J. J.: Digital edge-detection measurement of polyethylene wear after total hip arthroplasty. J. Bone and Joint Surg.,79-A: 690-700, May 1997.79-A690 1997

Sutula, L. C.; Collier, J. P.; Saum, K. A.; Currier, B. H.; Currier, J. H.; Sanford, W. M.; Mayor, M. B.; Wooding, R. E.; Sperling, D. K.; Williams, I. R.; Kasprzak, D. J.; and Surprenant, V. A.: Impact of gamma sterilization on clinical performance of polyethylene in the hip. Clin. Orthop.,319: 28-40, 1995.31928 1995 [PubMed]

Sychterz, C. J.; Engh, C. A., Jr.; Shah, N.; and Engh, C. A., Sr.: Radiographic evaluation of penetration by the femoral head into the polyethylene liner over time. J. Bone and Joint Surg.,79-A: 1040-1046, July 1997.79-A1040 1997

Sychterz, C. J.; Engh, C. A., Jr.; Yang, A. ; and Engh, C. A.: Analysis of temporal wear patterns of porous-coated acetabular components: distinguishing between true wear and so-called bedding-in. J. Bone and Joint Surg.,81-A: 821-830, June 1999.81-A821 1999

von Knoch, M.; Sychterz, C. J.; Engh, C. A., Jr.; and Engh, C. A., Sr.: Incidence of late bead shedding from uncemented porous coated cups. A radiographic evaluation. Clin. Orthop.,342: 99-105, 1997.34299 1997 [PubMed]

Topics

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Charnley, J., and Cupic, Z.: The nine and ten year results of the low-friction arthroplasty of the hip. Clin. Orthop.,95: 9-25, 1973.959 1973 [PubMed]

Charnley, J., and Halley, D. K.: Rate of wear in total hip replacement. Clin. Orthop.,112: 170-179, 1975.112170 1975 [PubMed]

Livermore, J.; Ilstrup, D.; and Morrey, B.: Effect of femoral head size on wear of the polyethylene acetabular component. J. Bone and Joint Surg.,72-A: 518-528, April 1990.72-A518 1990

Martell, J. M., and Berdia, S.: Determination of polyethylene wear in total hip replacements with use of digital radiographs. J. Bone and Joint Surg.,79-A: 1635-1641, Nov 1997.79-A1635 1997

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James E. Dowd, Christi J. Sychterz, Anthony M. Young, Charles A. Engh; Characterization of Long-Term Femoral-Head-Penetration Rates Association with and Prediction of Osteolysis*. The Journal of Bone & Joint Surgery. 2000 Aug;82(8):1102-1102.

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