Abstract
Background:
Studies about the influence of patient characteristics on mechanical failure of cups in total hip replacement have applied different methodologies and revealed inconclusive results. The fixation mode has rarely been investigated. Therefore, we conducted a detailed analysis of the influence of patient characteristics and fixation mode on cup failure risks.
Methods:
We conducted a case-control study of total hip arthroplasties in 4420 patients to test our hypothesis that patient characteristics of sex, age, weight, body mass index, and diagnosis have different influences on risks for early mechanical failure in cemented and uncemented cups.
Results:
Women had significantly reduced odds for failure of cups with cemented fixation (odds ratio = 0.59; 95% confidence interval, 0.43 to 0.83; p = 0.002) and uncemented fixation (odds ratio = 0.63; 95% confidence interval, 0.5 to 0.81; p = 0.0003) compared with that for men (odds ratio = 1). Each additional year of patient age at the time of surgery reduced the failure odds by a factor of 0.98 for both cemented cups (odds ratio = 0.98; 95% confidence interval, 0.96 to 0.99; p = 0.016) and uncemented cups (odds ratio = 0.98; 95% confidence interval, 0.97 to 0.99; p = 0.0002). In patients with cemented cups, the weight group of 73 to 82 kg had significantly lower failure odds (odds ratio = 0.63; 95% confidence interval, 0.4 to 0.98) than the lightest (<64 kg) weight group or the heaviest (>82 kg) weight group (odds ratios = 1.00 and 1.07, respectively). No significant effects of weight were noted in the uncemented group. In contrast, obese patients (a body mass index of >30 kg/m2) with uncemented cups had significantly elevated odds relative to patients with a body mass of <25 kg/m2 (odds ratio = 1.41; 95% confidence interval, 1.03 to 1.91) for early failure of the cups compared with an insignificant effect in the cemented arm of the study. Compared with osteoarthritis as the reference diagnosis (odds ratio = 1), developmental dysplasia (odds ratio = 0.52; 95% confidence interval, 0.28 to 0.97) and hip fracture (odds ratio = 0.38; 95% confidence interval, 0.16 to 0.92) were significantly protective in cemented cups.
Conclusions:
Female sex and older age have similarly protective effects on the odds for early failure of cemented and uncemented cups. Although a certain body-weight range has a significant protective effect in cemented cups, the more important finding was the significantly increased risk for failure of uncemented cups in obese patients. Patients with developmental dysplasia and hip fracture were the only diagnostic groups with a significantly decreased risk for cup failure, but only with cemented fixation.
Level of Evidence:
Therapeutic Level III. See Instructions to Authors for a complete description of levels of evidence.
Total hip arthroplasty is one of the most successful orthopaedic interventions ever developed1. Bulstrode characterized the increasing number of elective and emergency total hip arthroplasties as an orthopaedic epidemic taking place in all industrial nations with similar demographic changes2. Although modern component designs and fixation techniques have decreased the rn ate of prosthetic failure, the increasing number of primary procedures, particularly in younger patients, has resulted in a new wave of patients requiring joint revisions3,4. Three main factors influence the outcome and survival time of a primary total hip replacement: the surgeon's skills and experience5,6, the implant design and method of fixation6,7, and patient characteristics, such as sex, age, weight, underlying disease, and activity level7-9.
Because of the reduction in the infection rate by improved operative techniques, laminar flow operating rooms, and intravenous antibiotic prophylaxis10, mechanical failure remains the most common complication requiring revision9,11. To maximize the survival time of prostheses, and to educate patients about the long-term results of operative treatment, the influence of patient characteristics on the prosthetic survival must be considered. The literature provides a multitude of articles about this issue3,6,7,9,10,12-18.
There are, however, important methodological differences between the various investigations that make comparisons difficult. Most authors have used revision for cup and/or stem loosening7 as the end point for their analyses, but others have also included symptomatic patients with radiographic evidence of loosening8. While some authors have agreed that demographic factors affect component loosening to a varying extent18, others have reported virtually identical rates of survivorship for the acetabular and femoral components that may merely be a result of different follow-up times in the studies7. In addition, many studies represent single-surgeon or single-department reports with only one type of prosthesis. This makes generalization of the findings difficult.
In order to further elucidate the influence of patient characteristics on mechanical failure of cups in total hip arthroplasty and to overcome some of the aforementioned methodological problems, we conducted a two-arm matched case-control study based on data from several centers and surgeons from various European countries that included different types of cups and both the cemented and the uncemented fixation mode. We hypothesized that patient demographics such as age, sex, body mass index, and the primary diagnosis affect the risks for early cup failure, depending on fixation mode. The study was designed to allow more general conclusions to be drawn about the principal research question without restricting conclusions to one specific type of prosthesis or surgical technique.
Consequently, we analyzed the patient characteristics of age, sex, weight, body mass index, and primary diagnosis regarding the risk they impose on mechanical failure of cemented and uncemented cups. Failure was considered as either the failure of ingrowth or ongrowth or the true loosening of a once well-fixed component. The defined end point for component survival was radiographic signs of failure or revision for aseptic failure of a component.
The study was designed as a matched-pairs case-control study with two arms in a ratio of 1:M (multiple controls were matched to each case), and we used the database of the Maurice E. Müller Research Center, formerly the Maurice E. Müller Center for Education and Documentation (MEM-CED).
Preoperative and postoperative clinical and radiographic data were prospectively documented with use of optically readable primary (A), revision (B), or follow-up (C) code sheets from consecutive primary total hip replacements, according to the principles of IDES (International Documentation and Evaluation System)19,20. Institutional review board approval at our center was not needed since the study utilized existing anonymous observational data.
The inclusion criteria were an underlying diagnosis of osteoarthritis, developmental dysplasia, inflammatory arthritis, fracture or osteonecrosis, a primary total hip arthroplasty, an age of more than twenty years at the time of surgery, and at least one follow-up examination with a complete set of radiographs (anteroposterior pelvic and lateral radiographs made preoperatively, immediately postoperatively, and at the time of follow-up) of the operatively treated side, or a documented first revision of the cup components for mechanical failure. Depending on the type of cup fixation, the case was referred to one of the two study arms (cemented or uncemented). Excluded were all patients from hospitals with fewer than fifty documented IDES A-code sheets and all patients with partial hip replacements, e.g., a femoral head prosthesis, a bipolar or double-cup prosthesis, and hemiarthroplasty surface replacements.
The follow-up form provides a section for the radiographic evaluation of the prosthetic components as well as for changes in bone or tissue structures (e.g., ossifications, change in cortical density, or cavitations)21. Radiographic assessment was performed on the basis of standardized anteroposterior pelvic and lateral radiographs with the MEM template for evaluation of total hip arthroplasty as a standardized measurement tool22. Component failure was defined by comparing the postoperative and follow-up radiographs for a broken cup25 or broken cement23and measuring superior and medial cup migration11, tilt23, and radiolucencies24.
The following binary (present or not present) variables were used: (1) continuous radiolucencies around the cup in zones 1, 2, and 314; (2) superior migration of =5 mm with a severe protrusion or a progressive tilt of the cup13,23,24; and (3) a fracture of the cup or the cement mantle after cemented fixation.
The revision diagnosis and exchanged components were recorded on the IDES B-forms (code sheets for revision).
Controls were defined as patients without radiographic signs of cup failure documented on the follow-up forms. Patients who had an early two-stage bilateral hip arthroplasty for whom one side qualified as the control for the other side and those who had clinical examinations after revision surgery were excluded as controls.
Patients and controls were matched on the basis of the following criteria: (1) a total hip arthroplasty performed in the same hospital; (2) a date of surgery that was within 2.5 years; (3) a follow-up examination performed within six months of the follow-up interval of the case; and (4) the same cup design, size, and material.
This procedure resulted in two sets of matched cases and controls to analyze risk factors of cemented and uncemented cup failure. These two sets were derived from 1211 patient records for the cemented cup study and 3209 patient records for the uncemented study, with 1285 cemented cups (299 cases and 986 controls) and 3510 uncemented cups (510 cases and 3000 controls), respectively. Data collected between 1981 and 2003 in thirty hospitals in eight European countries (Austria, Belgium, Switzerland, Germany, France, Spain, the Netherlands, and Italy) were used for the study, and an average of 3.3 (cemented) and 5.9 (uncemented) controls per case were included. The percentage of male patients was 43.2% in the cemented study group and 50.9% in the uncemented group. Both hips were treated in 6.1% of the patients in the cemented group and 9.4% of the patients in the uncemented group, and 9.2% and 5.9% of the patients, respectively, had a previous operation other than a total hip arthroplasty on the affected side. The mean follow-up time was 5.2 years (range, 0.2 to 21.7 years) for the cemented cups and 3.8 years (range, 0.3 to 16.0 years) for the uncemented cups. Forty-three types of cemented cups and thirty-nine types of uncemented cups were included in the respective studies.
The patient characteristics were assessed by the clinician or derived from the patient interview and included age, sex, weight, height, and primary diagnosis. The body mass index was calculated, and patients were categorized according to the system of the National Institutes of Health (NIH)26 as normal weight if the body mass index was <25 kg/m2, overweight if it was 25 to 30 kg/m2, and obese if it was >30 kg/m2.
Statistical Analysis
The relationships between component failure or revision and patient characteristics were analyzed as a 1:M matched-pairs case-control study with use of multiple conditional multivariate logistic models. A first model was fitted to the data to investigate the overall effects of age and body mass index adjusted for sex and primary diagnosis. Age and body mass index were therefore included as continuous explanatory variables in the model, and sex, primary diagnosis (five diagnoses), and previous surgery (present or not present) were included as categorical variables. Similar models were used to estimate the effects of patient weight and height.
The results of continuous variables were interpreted as an estimate of the change in risk per unit of increase in each variable. Because it cannot be assumed that the relationship between predictors and outcome is always linear, additional models were evaluated with continuous explanatory variables categorized into groups. Body mass index was classified according to the NIH categories; age was classified into four groups: less than sixty years, sixty to sixty-nine years, seventy to eighty years, and more than eighty years. Weight was categorized into quartiles of the observed data. Results were interpreted as the risk difference compared with a reference level. The respective reference levels were normal weight (a body mass index of <25 kg/m2), an age of less than sixty years, and the lowest weight class. A further model was used to estimate the significance of all first-order interaction terms of explanatory variables. Apart from a significant interaction between body mass index and previous surgery in the uncemented study group (p < 0.05), no other interaction term appeared to be significant. As the number of hips with a previous operation was too small, a stratified analysis of the above-mentioned interaction could not be performed. Power calculations were performed in cases of nonsignificant associations27. For statistical tests, a p value of <0.05 was considered significant throughout the study.
Source of Funding
There was no external funding for this investigation that could have influenced the analysis or its findings.
Basic Demographic and Diagnostic Properties
Basic demographic and diagnostic properties of the study group are given in Table I.
Effects of Sex
The risks of cup failure were significantly lower for female than for male patients in both study arms (Tables II and III). Women had a risk of failure that was approximately 40% lower (odds ratio = 0.59, p = 0.0024) in the cemented group and 37% lower (odds ratio = 0.63, p = 0.0003) in the uncemented group.
Effects of Patient Age
The mean age at the time of the primary surgery was 63.9 years in the cemented group and 65.3 years in the uncemented group. When age was analyzed as a continuous variable, the risks for cemented cup failure decreased significantly (p = 0.01) and by a factor of 0.98 per additional year of patient age at the time of surgery. For uncemented cups, the risks decreased similarly (odds ratio = 0.98, p = 0.0002), i.e., the older the patients were at the time of the intervention, the lower the chance for mechanical failure of the cemented and uncemented cup. Similarly, age-related risks for cup failure decreased in an almost linear fashion across the age groups of less than sixty years, sixty to sixty-nine years, seventy to eighty years, and more than eighty years in both study arms. In the uncemented group, we found two age groups that displayed significantly lower risks than the reference group (an age of less than sixty years): the seventy to eighty-year-old group (odds ratio = 0.54; 95% confidence interval, 0.39 to 0.74) and the group of those older than eighty years (odds ratio = 0.34; 95% confidence interval, 0.17 to 0.68). Similarly, for cemented cups, one age group had a significantly different failure risk than the reference group of less than sixty years, namely, patients with an age of seventy to eighty years (odds ratio = 0.58; 95% confidence interval, 0.37 to 0.91) (Tables II and III). The mean ages in the various diagnostic groups were 66.9 years for patients with osteoarthritis, 58.9 years for those with developmental dysplasia, 59.1 years for those with inflammatory arthritis, 64.9 years for those with femoral neck fracture, and 61.7 years for those with osteonecrosis.
Effects of Body Weight
The average body weight was 71.7 kg for the cemented group and 74.2 kg for the uncemented group. For both fixation modes, the body-weight group of 73 to 82 kg had the lowest failure risks. These risks increased for both the patients who weighed <73 kg and those who weighed >82 kg. In the uncemented study arm, the differences between the various weight groups were not significant. In the cemented study arm, however, the patients in the 73 to 82-kg weight group displayed a significant risk reduction of about 37% (odds ratio = 0.63; 95% confidence interval, 0.4 to 0.98) compared with the reference group of patients who weighed <64 kg. When weight was analyzed as a continuous variable, risks were increased per additional kilogram of body weight, but the difference was not significant (p = 0.62 for cemented cups and p = 0.52 for uncemented cups) (Tables II and III).
Effects of Body Mass Index
The mean body mass index at the time of surgery was 26.3 kg/m2 in the cemented group and 26.6 kg/m2 in the uncemented group. The distribution of patients across the body-mass-index groups is provided in Table IV. The risks of uncemented cup failure increased by a factor of 1.03 (odds ratio = 1.03, p = 0.04) for each additional unit of body mass index over 25 kg/m2. Also, the risk profile of uncemented cup failure related to body mass index appeared to increase linearly across the overweight and obese body-mass-index groups, and obese patients had a significantly (=40%) increased risk of uncemented cup failure compared with the normal weight patients (odds ratio = 1.41; 95% confidence interval, 1.03 to 1.91). Risk differences related to body mass index for the cemented cups were small and not significant (Tables II and III).
Effects of the Primary Diagnosis
The distribution of diagnoses in the two study groups is given in Table I. Osteoarthritis as the most common primary diagnosis for total hip arthroplasty was used as the reference diagnosis against which the other diagnoses were compared. A significant and approximately 48% lower risk of cemented cup failure (odds ratio = 0.52; 95% confidence interval, 0.28 to 0.97) was detected in patients with developmental dysplasia. Risks were even lower in patients with a femoral neck fracture (odds ratio = 0.38; 95% confidence interval, 0.16 to 0.92). No significantly altered risk patterns regarding cemented cup failure were observed for the other primary diagnoses (Table V). A matched-pairs analysis of the postoperative mobility levels of the various diagnostic groups showed that the odds to achieve a walking capacity of greater than sixty minutes were, with one exception, significantly lower in all groups in comparison with the reference group with osteoarthritis (odds ratio = 1). For achieving this capacity, patients with developmental dysplasia displayed an odds ratio of 0.63 (95% confidence interval, 0.46 to 0.85); those with inflammatory arthritis had the lowest odds ratio of 0.28 (95% confidence interval, 0.17 to 0.46); those with a femoral neck fracture had an odds ratio of 0.74 (95% confidence interval, 0.54 to 1.01), which was not significant; and patients with osteonecrosis had an odds ratio of 0.49 (95% confidence interval, 0.34 to 0.7).
Risk differences for uncemented cup failure were not significant for any of the primary diagnoses (Table VI). A post hoc power analysis demonstrated a power of >0.9 for the two diagnostic groups of developmental dysplasia and osteonecrosis. For the diagnostic groups of inflammatory arthritis and femoral neck fracture, the power was 0.13 and 0.1, respectively.
Different Principles of Uncemented Fixation
In order to find out if all uncemented cups included in the study had the same risk profiles for mechanical failure, we classified them according to their fixation method. The classification resulted in five different categories: a press-fit metal shell with polyethylene liner (e.g., a St. Narbor shell; Sulzer Medica, Winterthur, Switzerland), a press-fit polyethylene cup with coating (e.g., an RM cup; Mathys, Bettlach, Switzerland), a press-fit titanium cup with screw fixation (e.g., SL titanium shell; Sulzer Medica), a threaded titanium cup (e.g., Weill cup; Sulzer Medica), and an expansion shell (e.g., CLS expansion shell; Sulzer Medica). For each category, the influence of sex, age, diagnosis, and body mass index was analyzed. Since the number of observations in the subgroups became small, age and body mass index were only analyzed as continuous variables and diagnosis was analyzed as a factor per se, not in its various outcomes. As for the uncemented group as a whole, we found a significantly elevated failure risk per increased unit of body mass index in the press-fit metal shells (1404 cups) (odds ratio = 1.11, p = 0.02). In the categories of press-fit polyethylene cups with coating (215 cups) and press-fit cups with screws (795 cups), the odds were also increased but not to a significant extent. In contrast, the categories of threaded titanium smooth cups (257 cups) and the expansion shell systems (839 cups) displayed decreased risks with increasing body mass index; however, the decrease was also not to a significant extent. In the category of press-fit cups with screw fixation, female sex had the largest, protective influence on mechanical failure, but it was not significant (odds ratio = 0.3, p = 0.066). No other covariate showed significant influences. The small group sizes and the wide confidence intervals indicate that there is a high likelihood for a beta error in many cells, i.e., these analyses lack power.
Since the first total hip replacements were performed in the 1960s, acetabular component fixation was dominated by cemented cups, the major problems of which were progressive loosening and so-called cement disease, i.e., polymethylmethacrylate-induced osteolysis. Thus, impetus was given to promoting uncemented cup fixation. Although in revision situations, uncemented cup fixation is considered the method of choice by most surgeons, the absence of long-term results and inhomogeneous or small study groups have led to ongoing discussions about optimal cup fixation in the treatment of primary osteoarthritis28-30.
Considering these controversies, we analyzed the influence of patient characteristics on the failure of cemented and uncemented mechanical cups.
Radiographic evidence of mechanical failure of a cup and actual revision of a cup for mechanical failure were selected as the outcome measures. Failure risks were estimated with use of prospectively collected data and multivariate conditional logistic regression models for data analysis. This procedure allowed an extensive investigation of various potential predictors of cup failure and facilitated generalization of the results because the studies were performed over a long period of time at many centers and with many different prostheses included.
Age
We found advanced age to be a protective factor against mechanical failure of a cup in both study groups. With a risk reduction for cemented and uncemented cups of 2.2% and 2.3%, respectively, for each additional year of patient age, the results for the groups were similar. The seemingly small but significant risk reduction becomes more meaningful if it is translated into a five-year age difference between two recipients of a total hip replacement over the age of sixty years: the older person has about a 10% lower risk for early mechanical failure of the cup; at a ten-year difference in age, the difference becomes 18% (i.e., 0.9810). In most other studies, younger age has had a negative influence on the durability of total hip prostheses, and it represents an unfavorable condition for component survivorship, especially for patients who receive a cemented cup7,31-33. With few exceptions12,16, the literature agrees on the protective role of advanced age in the etiology of mechanical failure; therefore, the surgeon may want to advocate total hip arthroplasty at the latest possible point in time. However, the outcome perspective should not be forgotten. Younger patients and those with a better preoperative functional status benefit from a higher degree of regained mobility34, but they also expose the prosthesis to higher physical strains35, which in turn may lead to higher failure rates.
Weight and Body Mass Index
Many studies have underlined the role of weight in the progression of osteoarthritis36,37. Similarly, body weight is widely considered a risk factor for loosening of a total hip arthroplasty component. However, this relationship remains controversial since many studies have found no or no clear-cut association between increased weight and prosthetic failure35,38.
The decreased walking activity of overweight and obese patients39,40 may counterbalance the increased stresses on the prosthetic components and interfaces. This could at least partially explain why weight has had no consistent effect on implant loosening and wear in clinical studies41,42. An additional explanation may be that the difference between weight as an absolute, and body mass index as a relative, measure was often not considered31. Therefore, we examined the influence of both variables on cup failure. In uncemented cups, an increasing body mass index consistently increased risks, producing a significant effect in obese patients. As with age, a seemingly small risk increase (odds ratio = 1.03) per additional body-mass-index unit is cumulative, resulting in a 16% risk difference between patients with a body mass index of 25 kg/m2 and those with a body mass index of 30 kg/m2, and a 34% risk difference between patients with a body mass index of 25 kg/m2 and those with a body mass index of 35 kg/m2. In contrast, inconsistent and nonsignificant risk patterns were found in the cemented group. As initial stability is necessary to achieve osseous integration and permanent fixation of uncemented cups, the higher stresses that obese patients exert on the prostheses may compromise early osseointegration and lead to delayed or incomplete achievement of stability with increased risks for implant failure. Studies measuring the early stability of different fixation modes of acetabular components have not described significant differences in early postoperative stability. However, the uncemented group in the study by Digas et al. had a median weight of 68 kg, certainly not representative of obese patients, although body mass index was not reported43. With the same odds ratios of 1.003, the analysis of increased absolute body weight alone revealed a slightly detrimental influence in both fixation groups. Worth mentioning in our study is a distinct risk reduction of >30% for cemented cups in patients weighing 73 to 82 kg compared with both the lighter and heavier groups. The 73 to 82-kg weight group probably represents patients with no underlying systemic disease such as rheumatoid arthritis affecting the bone stock, a good balance between body mass index and activity level, and good mobility and coordination (i.e., rare or no falls) who are the most ideal candidates for the implant. In a previous analysis that was not stratified by fixation principle, we showed a protective effect of increased body weight, whereas the body mass index had no significant influence44. The effect of obesity on osteoarthritis is an increasing concern as, over the last decade, obesity in western and westernizing countries has more than doubled and the proportion continues to climb37,45. Accordingly, it is likely that more and more overweight and obese patients will demand total hip arthroplasty in the future.
Sex
The current study revealed that women had an approximately 40% lower risk of cup failure in both study groups. The literature is not consistent, and neither are our own previous but unstratified findings44, but the majority of authors have reported similar conclusions. Berry et al. found that men have a more than a twofold increased risk of cup loosening7, and Nercessian et al. found male sex to be a significant risk factor for the development of osteolysis46. Schmalzried et al. attributed the increased rate of cup wear to the amount of use rather than to time after the operation, and they identified sex as the most important surrogate for activity and the most potent variable affecting wear47. Other authors, however, have found no increased revision rates in men after correction for weight17. In light of the magnitude of this effect and from the point of view of component failure, it seems less relevant to delay surgery in female patients or to advise them to lose one or several units of body mass index before surgery if they are not morbidly obese.
Diagnosis
The most frequent underlying diagnosis for total hip replacement, osteoarthritis, served as the reference diagnosis in this study. Patients with osteonecrosis had the most highly elevated risk of failure of a cemented cup, but the risk difference was not significant. No other diagnostic group had a significantly elevated risk compared with the reference group. However, patients with developmental dysplasia and femoral neck fracture had significantly lower risks of failure in the cemented study arm. Reports in the literature have noted poor durability of cemented hip replacements, especially of the acetabular component, in patients with osteonecrosis48. The results of hip arthroplasty vary in patients with osteonecrosis of the femoral head. The main reason may be the etiology of femoral head necrosis in this heterogeneous patient group. Analysis of total hip arthroplasty outcome in relation to the etiology of femoral head necrosis led to the assumption that patients with steroid-induced osteonecrosis and osteonecrosis with underlying bone diseases (renal osteodystrophy or sickle-cell hemoglobinopathy) have the highest rates of loosening. Moreover, these patients are mostly much younger than those in other etiologic groups and are therefore more active and often need bilateral total hip arthroplasty48. We can partially confirm these findings, but the average age of sixty-one years in the osteonecrosis group indicates that our patients were older and had lower activity levels compared with patients who had osteoarthritis. This may explain the elevated, but insignificant, risk differences in that diagnostic group in the current study. Our findings of a substantially decreased failure risk in patients with developmental dysplasia of the hip are in contrast with most of the literature, which has found that the cemented cups in particular perform poorly7,29,49,50. Only a large Norwegian study with a study design similar to the current analysis found no significant differences among the various diagnoses51. The investigators did not, however, differentiate between cups and stems or between fixation modes. In contrast, an analysis of the Swedish hip register revealed the highest rate of aseptic loosening in the patients with childhood diseases (80%) compared with patients with osteoarthritis (75%)52. Our findings may be partially explained by the significantly lower activity pattern of the developmental dysplasia group, although they had the youngest mean age of all of the diagnostic groups. The same Swedish study showed the lowest revision rates for aseptic loosening in patients with a fracture (63%), and accordingly, the ten-year survival rates were >90% for male and female patients. In the current study, these results can at least be confirmed for cemented cups as the group of relatively older patients showed the lowest of all failure risks combined with a lower level of activity that was of borderline significance. Finally, there also is controversy about patients with inflammatory arthritis as those who are younger are thought to have a higher risk of failure31,50, but those who are older are not7,50. Schmalzried and Huk found lower walking activities in these patients, and our results revealed the lowest walking capacities in this sample of relatively young patients (mean age, 54.6 years)42. Poor bone stock is most often stated as the underlying reason for loosening in these patients; however, with the extremely low activity levels, they did not have significantly altered failure risks in the current analysis.
Regarding the risk patterns for uncemented cup fixation, the results varied and were not significant. The power analysis revealed an extremely low power level for the subgroups with inflammatory arthritis and femoral neck fracture. It is doubtful whether any study or registry can enroll the necessary numbers of patients to achieve a sufficient power level. To find conclusive answers, it would be necessary to collect and pool data in international registries that use the same data collection forms and database structures. Today, only the Scandinavian registries may be able to achieve such collaboration.
Weaknesses and Strength
This study represents a matched case-control design nested into a prospective data collection effort. Many European centers have documented their total hip arthroplasties with use of a common database and consistent terminology within the framework of IDES. Nevertheless, the chosen study end points and the study design entail certain potential problems that need to be considered. While a revision for aseptic failure is a definite and frequently used end point, radiographic signs of failure indicate the need for a revision procedure, which may actually be delayed or never be undertaken for reasons such as waiting times in certain health-care systems, a lack of symptoms, and a patient who is too ill to undergo surgery or is unwilling to consent for a reoperation. By using both end points, we may have created higher failure rates or increased the number of cases with consequently increased calculated risks.
The reliability of the data collected was ascertained to be as good as possible by a preselection of only the hospitals whose data are used for scientific evaluations. These comprise about two-thirds of all participating hospitals that are distinguished by a long-term commitment and a certain required number of documented cases. Moreover, the IDES system itself (as a tool) makes documentation of total and partial hip arthroplasties possible so that all hip replacement interventions are recorded.
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