Anthony M. Young

Characterization of long-term femoral-head-penetration rates. Association with and prediction of osteolysis.

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.

Total hip arthroplasty in patients 50 years and younger.

Because the young patient with a total hip arthroplasty is presumed to place increased demands on a reconstruction for a longer period than the average patient with a hip replacement, long-term results are expected to be inferior. To determine the efficacy of total hip replacements in this population, the current authors reviewed the long-term results of patients who were 50 years and younger who had cementless total hip arthroplasties at their institution, and reviewed the literature on total hip arthroplasty in younger patients. The results from the study population and the literature were encouraging. At the author’s institute, during the past 20 years, 561 hip replacements were done on 488 patients in this age group, using extensively porous-coated cobalt-chromium stems matched with beaded, press-fit acetabular components of cobalt-chrome or titanium. Using the Kaplan-Meier technique, the survival rates for femoral and acetabular components, using any revision as an end point, were 89′ at 10-year followup and 60′ at 15-year followup. A subset of the authors’ patients who were 40 years and younger (256 hips, 223 patients) had slightly inferior results, with 85′ 10-year survivorship and 54′ 15-year survivorship, using any revision as an end point. A comprehensive literature review also showed that long-term success can be achieved with cemented or cementless total hip arthroplasties in young patients. Because some reconstructions exhibited inferior results in younger patients, the authors recommend that surgeons be much more critical of the components used in these patients and allow long-term data to guide their decisions.

Radiographic signs of osseointegration in porous-coated acetabular components

There currently is no direct method to radiographically determine osseointegration of an uncemented, porous-coated acetabular component. We defined five radiographic signs for detecting acetabular osseointegration: (1) absence of radiolucent lines; (2) presence of a superolateral buttress; (3) medial stress-shielding; (4) radial trabeculae; and (5) an inferomedial buttress. We assessed their ability to predict acetabular osseointegration by reviewing the postprimary and prerevision radiographs from a series of 119 total hip arthroplasties that had revision surgery. Each sign had a high positive predictive value for the presence of bone ingrowth (range, 92.2-96.3%). The absence of radiolucent lines, presence of superolateral buttresses, and presence of medial stress-shielding were the most sensitive signs for indicating bone ingrowth. Ninety-seven percent of the cups with three to five signs were bone ingrown, whereas 83% of the cups with one or no signs were unstable. When three or more signs were present, the positive predictive value of the radio-graphic test was 96.9%, the sensitivity was 89.6%, and the specificity was 76.9%. The five signs of acetabular osseointegration reliably predicted osseointegration, especially when used in combination. They can be used as a tool for radio-graphically assessing the status of patients with porous-coated uncemented cups.Level of Evidence: Diagnostic study, Level III (Study of nonconsecutive patients; without consistently applied reference “gold” standard). See the Guidelines for Authors for a complete description of levels of evidence.

Clinical consequences of stress shielding after porous-coated total hip arthroplasty.

From a series of 223 extensively porous-coated total hip arthroplasties, 208 hips had radiographic followup at a minimum of 2 years, which could be evaluated for radiographic evidence of stress-shielding. These patients had a mean 13.9-year followup (range, 2–18 years). We compared the outcome of 48 total hip arthroplasties that had radiographically evident stress-shielding with 160 total hip arthroplasties that did not have radiographically visible stress-shielding or that had less severe stress-shielding. Stress-shielding was more likely in females, patients with a low cortical index, and patients with larger stems. At the most recent followup, patients with stress-shielding had a lower mean walking score than patients without stress-shielding and less osteolysis. No patients with stress-shielding had femoral loosening, implant fractures, or loss of porous coating. The revision rate was 13% (six hips) among hips with stress-shielding and 21% (33 hips) among hips without stress-shielding. Fifteen-year survivorship was 93% among hips with stress-shielding and 77% among hips without stress-shielding. Stress-shielding did not produce adverse consequences in these extensively porous-coated total hip arthroplasties.

Effect of Acetabular Modularity on Polyethylene Wear and Osteolysis in Total Hip Arthroplasty

Background: Debris from polyethylene wear causes osteolysis. In this study, we examined the effect of acetabular liner modularity on polyethylene wear and osteolysis. Methods: We compared forty-one hips (thirty-nine patients) treated with a nonmodular, porous-coated acetabular component with a matched group of forty-one hips (forty patients) treated with a modular acetabular component. The groups were matched by patient gender and age, type of polyethylene material, method of polyethylene sterilization, femoral head size and manufacturer, and stem manufacturer. The mean follow-up period was 5.3 years (range, 3.8 to 6.8 years) for the nonmodular group and 5.5 years (range, 3.8 to 8.0 years) for the modular group. Using serial radiographs and a computer-assisted method, we measured two-dimensional head penetration into the polyethylene liner. Temporal head-penetration data and linear regression analysis were used to calculate the true wear rates. Results: The nonmodular acetabular components demonstrated a lower, but not a significantly lower, mean true wear rate than did the modular components (0.11 compared with 0.16 mm/yr, p = 0.22), and they were associated with a significantly lower rate of osteolysis (2% compared with 22%, p = 0.01). In addition, the true wear rates of the nonmodular components were less variable than those of the modular components. The 95% confidence interval for the wear rates of the nonmodular components (0.08 to 0.13 mm/yr) was nearly half that of the modular group (0.11 to 0.20 mm/yr). Conclusions: The lower and more consistent true wear rates of the nonmodular components could be attributed to the fact that these cups were designed to have greater liner-shell conformity, greater liner thickness, and less liner-shell micromotion than modular components. These design factors could have favorably altered the stress distribution throughout the liner and could have thereby decreased wear. Although nonmodular components may present a partial solution to the problems of wear and osteolysis, they pose a disadvantage when a failed liner in a bone-ingrown acetabular component needs to be revised.

Dislocation after polyethylene liner exchange in total hip arthroplasty

Little has been published on the outcomes of polyethylene liner exchanges for wear or osteolysis. We assessed 24 patients from our clinic who had an isolated polyethylene liner exchange for wear or osteolysis with retention of the acetabular shell and femoral stem. At a mean 56-month follow-up time, 6 hips (25%) had dislocated. Of these, 2 underwent repeat surgery for recurrent dislocation; 1 had 3 dislocations; 1 had 2 dislocations; and 2 had single dislocations. Seventeen of the remaining patients answered our questionnaire: 13 recovered faster from the polyethylene liner exchange than from the primary arthroplasty; 4 believed that their hips were not as stable, 4 believed the hip did not function as well; and 1 underwent revision for cup loosening. We conclude that polyethylene liner exchanges, with or without femoral head exchange for wear or osteolysis, are associated with a high risk of dislocation and possible decrease in function.

Correlation between early and late wear rates in total hip arthroplasty with application to the performance of Marathon cross-linked polyethylene liners

Laboratory simulations are typically used to assess the performance of new bearing surfaces for total hip arthroplasty. However, the ability of in vitro studies to accurately predict clinical wear performance remains uncertain. Using computer-assisted radiographic measurement techniques, this study found that the average wear rate based on early (2-3 years) clinical follow-up is representative of the average long-term wear rate for a population. Based on early wear data, Marathon polyethylene liners, cross-linked with 5.0 Mrad of gamma irradiation, are wearing at a mean rate of 0.08 mm/yr. This rate is about half that of non-cross-linked polyethylene but represents a more modest wear reduction than in vitro studies have predicted. If 5 to 10-year Marathon wear data corroborate our short-term results, early clinical wear data should be used to validate hip simulator studies.

Effect of terminal sterilization with gas plasma or gamma radiation on wear of polyethylene liners.

Background: Although terminal sterilization with gamma radiation in air promotes cross-linking, which improves wear resistance, it also results in free radicals, which can oxidize and degrade the mechanical properties of polyethylene liners used for total hip arthroplasty. For this reason, non-cross-linked polyethylene components have also been sterilized with chemical surface treatments, such as gas plasma. In this study, we tested the hypothesis that conventional polyethylene liners cross-linked by sterilization with gamma radiation in air had better in vivo wear performance than non-cross-linked liners sterilized with gas plasma. Methods: We retrospectively reviewed the wear rates in a series of hips treated with a Duraloc 100 cup, a 28-mm femoral head, and an Enduron liner that had been sterilized with either gamma radiation (sixty-one hips followed for a mean of 5.2 years) or gas plasma (sixty-three hips followed for a mean of 3.9 years). The irradiated liners had been stored with access to ambient oxygen for an average of 1.0 year (range, 0.05 to 5.72 years) prior to implantation. Multiple linear regression was used to assess the effect of the sterilization method on the wear rate while accounting for the possible influence of other factors, including liner geometry, femoral head material, patient gender, cup abduction angle, and age at surgery. Results: The polyethylene liners that had been sterilized with gamma radiation in air had a significantly lower wear rate than did the gas-plasma-sterilized liners (0.097 compared with 0.19 mm/yr, p < 0.001). The sterilization method (p < 0.001) and age at surgery (p = 0.001) were the only factors that we analyzed that correlated with the wear rate. Conclusions: The in vivo wear of conventional polyethylene liners that had been sterilized with gamma radiation in air was, on the average, 50% less than that of non-cross-linked liners sterilized with gas plasma. Level of Evidence: Therapeutic study, Level III-2 (retrospective cohort study). See Intructions to Authors for a complete description of levels of evidence.

Effect of radiographic quality on computer-assisted head penetration measurements.

Even the most sophisticated computer-assisted radiographic techniques of measuring femoral head penetration into the polyethylene liner depend on the quality of the radiograph being evaluated, which varies greatly in clinical settings. The authors of this study sought to determine how the accuracy and reproducibility of three commercially available computer-assisted measurement systems differed when measuring optimal radiographs (with sharply defined component edges) and suboptimal radiographs (with less well defined edges). Using three computer-assisted measurement systems, the authors measured head penetration on simulated and clinical hip radiographs. All systems calculated head penetration as the movement of the head center relative to the cup center. To define the periphery of the prosthetic head and cup, one method (System One) used the human eye and a digitizing tablet, whereas the other two methods (System Two and System Three) used digital edge detection algorithms. For simulated hip radiographs, error was calculated as the absolute value of the difference between the known amount of head penetration, determined by a coordinate measuring machine, and the amount of penetration determined by the software. Three way analysis of variance showed a significant difference in absolute error among the three measurement techniques. System One had a significantly smaller absolute error (0.11 ± 0.06 mm) than did System Two (0.25 ± 0.25 mm) and System Three (0.19 ± 0.13 mm). In addition, three-way analysis of variance showed that optimal radiographs were associated with a significantly lower absolute error (0.14 ± 0.09 mm) than were suboptimal radiographs (0.23 ± 0.22 mm). For optimal radiographs, there was no significant difference in error among the three measurement methods; all systems were accurate and reproducible. However, for suboptimal radiographs absolute error increased and varied widely, and a significant difference among the methods existed. These data show the susceptibility of head penetration measurements to radiographic technique and underscore the importance of good quality radiographs for all analyses of head penetration.

Second-generation Porous-coated Cementless Total Hip Arthroplasties Have High Survival

We retrospectively reviewed 157 consecutive total hip arthroplasties performed with Prodigy® stems and Duraloc® cups to see whether design modifications made to these components would improve their clinical performance as compared with first-generation porous-coated components. At an average of 6.7 years postoperatively, 145 hips were available for followup. The data suggested encouraging clinical and radiographic performances for these second-generation components. With only two hips (1.4%) in this population being revised, survivorship analysis was 99% 5 years postoperatively. Bone ingrowth was evident in 99% of the stems, and all cups were stable at last followup. Large osteolytic lesions were observed in three acetabula and seven femurs with all femoral lesions being confined to the proximal Gruen Zones 1 or 7. Polyethylene wear averaged 0.10 ± 0.14 mm/year. Ninety-eight percent of patients reported satisfaction with the procedure, and 7% reported activity-limiting pain. Early followup of the Prodigy-Duraloc® combination in primary cementless total hip arthroplasties showed the Prodigy® stem had similarly good clinical results compared with its predecessor, the AML stem, and the Duraloc® cup was superior to its first-generation predecessor, the anatomic medullary locking cup with Acetabular Cup System (ACS) liner. Level of Evidence:Therapeutic Level III (retrospective cohort). See the Guidelines for Authors for a complete description of levels of evidence.