Abstract
Background:
The use of larger femoral heads has been proposed to reduce the risk of dislocation after total hip arthroplasty, but there is a lack of evidence to support this proposal. The aim of this multicenter randomized controlled trial was to determine whether the incidence of dislocation one year after total hip arthroplasty is significantly lower in association with the use of a 36-mm femoral head articulation as compared with a 28-mm articulation.
Methods:
Six hundred and forty-four middle-aged and elderly patients undergoing primary or revision arthroplasty were randomized intraoperatively to receive either a 36 or 28-mm metal femoral head on highly cross-linked polyethylene. Patients who were at high risk of dislocation (including those with dementia and neuromuscular disease) and those undergoing revision for the treatment of recurrent hip dislocation or infection were excluded. Patients were stratified according to other potential risk factors for dislocation, including diagnosis and age. Diagnosis of hip dislocation required confirmation by a physician and radiographic evidence of a dislocation.
Results:
Overall, at one year of follow-up, hips with a 36-mm femoral head articulation had a significantly lower incidence of dislocation than did those with a 28-mm articulation (1.3% [four of 299] compared with 5.4% [seventeen of 316]; difference, 4.1% [95% confidence interval, 1.2% to 7.2%]) when controlling for the type of procedure (primary or revision) (p = 0.012). The incidence of dislocation following primary arthroplasty was also significantly lower for hips with a 36-mm femoral head articulation than for those with a 28-mm articulation (0.8% [two of 258] compared with 4.4% [twelve of 275]; difference, 3.6% [95% confidence interval, 0.9% to 6.8%]) (p = 0.024). The incidence of dislocation following revision arthroplasty was 4.9% (two of forty-one) for hips with a 36-mm articulation and 12.2% (five of forty-one) for hips with a 28-mm articulation; this difference was not significant with the relatively small sample size of the revision group (difference, 7.3% [95% confidence interval, −5.9% to 21.1%]) (p = 0.273).
Conclusions:
Compared with a 28-mm femoral head articulation, a larger 36-mm articulation resulted in a significantly decreased incidence of dislocation in the first year following primary total hip arthroplasty. However, before a 36-mm metal-on-highly cross-linked polyethylene articulation is widely recommended, the incidence of late dislocation, wear, periprosthetic osteolysis, and liner fracture should be established.
Level of Evidence:
Therapeutic Level I. See Instructions for Authors for a complete description of levels of evidence.
Dislocation is the most common early complication following total hip arthroplasty and is one of the most common causes of early to intermediate-term revision of primary total hip arthroplasty1,2.
The use of larger femoral heads has been proposed as a means of reducing the risk of dislocation because larger-diameter articulations have a relatively larger femoral head-to-neck ratio, which increases hip motion before impingement between components occurs3,4. Larger femoral head implants require a greater amount of femoral head displacement before dislocation occurs within a well-oriented acetabular component4. However, concerns about polyethylene wear in larger-diameter articulations, such as those involving 36 or 40-mm femoral heads, have prevented their use with earlier generations of ultra-high molecular weight polyethylenes. The development of highly cross-linked polyethylenes has now made the use of larger articulations feasible in total hip arthroplasty, given that the articulations involving the newer polyethylenes have shown less wear than the previous generation of polyethylenes in hip-simulator studies5,6 and randomized controlled trials7-10.
Two nonrandomized cohort studies of primary arthroplasty in which larger (≥30-mm) articulations were compared with 28-mm articulations suggested that increased femoral head size may be associated with a decreased risk of dislocation11,12, whereas other studies have not conclusively shown this finding13,14.
There are two important issues that need to be addressed when determining the potential magnitude of the effect of articulation size on the incidence of dislocation following hip arthroplasty. First, the risk of dislocation may be influenced by a number of other factors, including patient-related factors (such as diagnosis14-16, age16,17, and sex15) and surgical technique. Dislocation is more common in association with the posterior approach11,17-19 and with a highly abducted acetabular component orientation4 and is less common following soft-tissue repair20,21. Second, the rate of hip dislocation is frequently under-reported, primarily because of inadequate follow-up22. As the number of data sources used to identify episodes of dislocation increases, the capture rate increases significantly22.
The aim of the present study was to examine the hypothesis that the incidence of dislocation at one year after total hip arthroplasty is significantly lower in association with a 36-mm femoral head articulation than with a 28-mm articulation. We undertook a randomized controlled trial in which a number of factors that may influence the risk of hip dislocation were controlled for by the study design and dislocation was tracked with a number of different methods.
The results of this trial are reported in accordance with CONSORT (Consolidated Standards of Reporting Trials) 2010 guidelines23. The study was undertaken as a multicenter, stratified, parallel-group randomized controlled trial involving fourteen hospitals (see Appendix). Consultants, or fellows or residents under their supervision, performed all procedures. The trial involved patients undergoing primary or revision total hip arthroplasty who were intraoperatively randomized to receive either a 28 or 36-mm femoral head articulation. Ethics approval was received from the institutional review board of every participating hospital. The trial is registered with the Australian New Zealand Clinical Trials Registry (ACTRN12609000678291).
Every patient who was scheduled to be managed with total hip arthroplasty by one of the collaborating surgeons was screened for inclusion in the trial. The reasons for, and the numbers of, preoperative exclusions are shown in the Appendix.
Eligible patients provided written informed consent if they were willing to participate in the trial. Patients were then stratified according to a number of factors to increase the likelihood that possible risk factors for dislocation would be distributed equally between patients randomized to a 36 or 28-mm femoral head articulation. The stratification and randomization procedures are described in detail in the Appendix.
The reasons for, and the numbers of, intraoperative exclusions for patients undergoing primary and revision procedures are shown in the Appendix. The randomization envelope was opened in the operating room after all exclusion criteria had been considered and it had been determined the patient was to be included. The envelope was opened after the acetabular component had been inserted and fixed with at least one screw but prior to the insertion of the stem. The patient received either a 36 or 28-mm articulation, according to the number in the envelope.
All arthroplasties were performed with use of uncemented acetabular components, which comprised a cluster three-holed acetabular shell (Trilogy; Zimmer, Warsaw, Indiana) fixed with one or two screws and a 10° elevated 36 or 28-mm-inner-diameter highly cross-linked polyethylene liner (Longevity; Zimmer). A cemented femoral stem was used for all primary arthroplasties (CPT; Zimmer). Either a cemented femoral stem (CPT; Zimmer) or an uncemented stem (ZMR; Zimmer) was used for revision arthroplasties. During the trial, the taper of the CPT femoral stem was changed from a 6° taper to a 12/14 taper by the manufacturer. When possible, each surgeon completed his allocated randomization block before commencing with the use of the 12/14 taper.
All primary arthroplasties were performed through a posterior surgical approach. Revision arthroplasties were performed through a posterior, transfemoral, or transtrochanteric approach. Repair of the capsule and external rotators was performed routinely during primary arthroplasties and, when possible, during revisions. The operative technique for insertion of the acetabular component through a posterior approach included reliance mainly on the alignment guide and confirmation by the surgeon’s judgment that the component was reasonably positioned.
Determination of the incidence of hip dislocation required the use of a number of different approaches to ensure that all dislocations were identified. Prior to discharge, each patient was provided with a Dislocation Card, to be given to any physician who subsequently treated the patient for dislocation, with instructions for that physician to notify the study coordinator of the dislocation. Case notes were reviewed to check for inpatient episodes of postoperative dislocation. The patient was then reviewed at six weeks to three months and at one year, and any complications were noted. In addition, at each visit, the patient completed a Hip Instability Questionnaire, which we had previously developed and validated, and a Hospital Visit Questionnaire. The former included the item “hip came out of joint and was put back in by a physician,” whereas the latter asked about all visits to an emergency room as well as any admissions. Dislocation was defined as an event requiring reduction by a physician or surgeon for which there was radiographic confirmation of a dislocation.
Patients, surgeons, and local study coordinators were not blinded to the articulation size received.
Radiographs showing the initial hip dislocation in every patient were assessed by one of the authors (D.W.H.) to determine the direction of dislocation. The position of the femoral head relative to the acetabular cup on the anteroposterior and lateral radiographs was used to determine the definite direction of the dislocation. If the lateral radiograph was unavailable or inadequate, the anteroposterior pelvic radiograph was used. The prominence of the lesser trochanter was compared to that of the contralateral side to determine the rotation of the femur and thereby the probable direction of dislocation. If only an anteroposterior hip radiograph was available, the prominence of the lesser trochanter was used to determine the possible direction of dislocation.
The position of the acetabular component was assessed on the most recent anteroposterior pelvic radiograph that had been made prior to the dislocation. Inclination and anteversion of the acetabular component were measured with use of EBRA (Ein-Bild-Roentgen-Analyse) (EBRA-CUP, University of Innsbruck, Innsbruck, Austria).
Statistical Analysis
With use of a power of 80% and a two-sided alpha of 0.05, initial sample size estimates indicated that a total sample size of 650 patients would be required to detect a significant and clinically important reduction in the incidence of dislocation at one year from 8% in the 28-mm articulation group to 3% in the 36-mm articulation group, if such a difference were to exist. A planned interim analysis by an independent data-monitoring committee indicated adequate power for our data, even allowing for a 5% rate of patient attrition, and therefore a decision was made to stop recruitment after 644 patients had been randomized.
Poisson regression with, first, main effects of type of procedure (primary or revision arthroplasty) and articulation size (36 or 28 mm) and, second, type of stem (CPT 12/14, CPT 6°, or ZMR) and articulation size, was used to examine whether the primary outcome measure, the incidence of dislocation one year following total hip arthroplasty, was affected by articulation size. Log of the total number of patients was used as offset. An analysis with use of a Cox model was also undertaken to take into account the observed experience of patients who were lost to follow-up within the first year, either through death, revision, reoperation, or other reasons. Differences between means were assessed with use of an independent-samples t test, and differences in proportions were assessed with use of chi-square tests.
Source of Funding
The present study was funded by the National Health and Medical Research Council of Australia and Zimmer. Funds were used for salary support of research staff and for research-related activities. The funding sources had no role in study design, data collection and analysis, the decision to publish, or the preparation of the manuscript.
Patients were recruited from September 2001 to June 2007. The numbers of patients who were assessed for eligibility, who were excluded preoperatively or intraoperatively, and who were randomized and included in the analyses are shown in Figure 1. Three patients received the wrong articulation size. These errors were due to breaches of protocol, with the envelope being opened prior to confirmation of the availability of all required components of the prosthesis and the required component in the allocated size subsequently being identified as not being available. These patients were included in the analysis according to their allocated articulation size; none of these patients had a dislocation. Seven (1.1%) of the 644 patients were lost to follow-up at one year and were excluded from the analysis. Another twenty-two patients were also excluded, ten because they died before the one-year follow-up without having a dislocation and twelve because they had undergone revision arthroplasty or reoperation, for reasons other than dislocation, that involved a change of implant or potential damage to the hip as a result of the surgery, which may have altered their risk of dislocation.
The incidence of dislocation at one year following hip arthroplasty was significantly lower in patients with a 36-mm femoral head articulation than in patients with a 28-mm articulation. One year following primary or revision arthroplasty, four (1.3%) of 299 hips with a 36-mm articulation and seventeen (5.4%) of 316 hips with a 28-mm articulation had dislocated (Table I). Controlling for the type of procedure (primary or revision), the articulation size was significantly related to dislocation (χ2 = 6.4, p = 0.012), with a significantly lower incidence of dislocation at one year in hips with a 36-mm articulation than in those with a 28-mm articulation. The incidence of dislocation at one year following primary hip arthroplasty was also significantly lower in hips with a 36-mm articulation than in those with a 28-mm articulation (0.8% [two of 258] compared with 4.4% [twelve of 275]) (χ2 = 5.1, p = 0.024). One year following revision arthroplasty, the incidence of dislocation was not significantly different between hips with a 36-mm articulation and those with a 28-mm articulation (4.9% [two of forty-one] compared with 12.2% [five of forty-one]) (χ2 = 1.2, p = 0.273), most likely because of an insufficient number of revision procedures in the trial to achieve adequate power for this comparison. A Cox model stratified by the type of procedure (primary or revision) confirmed a lower risk of dislocation for hips with a 36-mm articulation during the first year following arthroplasty, taking into account the observed experience of patients who were subsequently lost to follow-up during the first year either through death, revision, reoperation, or another reason (p = 0.005). Controlling for the type of femoral stem, articulation size was significantly related to dislocation (χ2 = 6.4, p = 0.012). However, there was no significant difference in the incidence of dislocation between articulation sizes within any of the three stem types when considered individually, likely because of the smaller sample sizes in the individual analyses (Table I).
Given the relatively small number of patients with larger acetabular cup diameters, the relationship between femoral head size, cup diameter, and dislocation risk could not be determined in this study (Table II). However, three of the twenty-one patients with a dislocation had a 28-mm articulation in an acetabular cup with a diameter of at least 62 mm, representing a radius mismatch of at least 17 mm, which previously was identified as a risk factor for dislocation24.
In both the primary and revision arthroplasty groups, the patients who were randomized to a 36-mm articulation were similar to those who were randomized to a 28-mm articulation (Tables III and IV).
Overall, seventeen (81%) of the twenty-one hips that dislocated within one year after primary or revision arthroplasty had a 28-mm articulation (Table V). The majority (nine) of the fourteen hips that dislocated after primary arthroplasty did so within thirty days after surgery, whereas hips that dislocated after revision arthroplasty showed a tendency to dislocate later. Approximately one-third of dislocating hips redislocated. Within the first year after hip arthroplasty, revision surgery for the treatment of recurrent dislocation was required in two of the fourteen hips that dislocated after primary arthroplasty and three of the seven that dislocated after revision arthroplasty; another hip that had dislocated after primary arthroplasty was revised because of failed closed reduction of the dislocation.
Of the fourteen first dislocations that occurred after primary arthroplasty, three were classified as definitely posterior, seven were classified as probably posterior, one was classified as possibly posterior, and three were classified as probably anterior. Of the seven first dislocations that occurred after revision arthroplasty, two were classified as definitely posterior, two were classified as probably posterior, two were classified as anterior, and one was classified as having an indeterminate direction.
The median inclinations of the acetabular components used for primary and revision total hip arthroplasties with a 28-mm articulation that subsequently dislocated were 44° (range, 34° to 52°) and 45° (range, 41° to 51°), respectively, and the median anteversions were 15° (range, 7° to 32°) and 16° (range, 10° to 22°), respectively. The two hips with a 36-mm articulation that dislocated after a primary procedure both had an inclination of 48° and anteversions of 5° and 7°. The two hips with a 36-mm articulation that dislocated after a revision procedure both had an inclination of 43° and an anteversion of 10°.
The purpose of the present randomized controlled trial was to determine whether a larger (36-mm) femoral head articulation significantly reduced the incidence of dislocation within the first year following total hip arthroplasty in comparison with a 28-mm articulation. The results of this trial indicated that the incidence of dislocation within one year after primary arthroplasty was five times lower in patients with a 36-mm articulation (0.8%) than in those with a 28-mm articulation (4.4%); this difference was both clinically important and statistically significant.
The use of larger femoral head implants for total hip arthroplasty has been increasing during the last decade25-27, largely on the basis of the premise that larger articulations are efficacious for preventing dislocations. Our trial showed that a larger articulation significantly reduced the risk of dislocation following primary arthroplasty. The number of patients undergoing revision arthroplasty as part of the trial was relatively small, and therefore the difference in the incidence of dislocation between the 36 and 28-mm articulations did not attain significance. It should be noted, however, that initial sample size calculations estimated the total number of patients required for an analysis of the effect of articulation size on the incidence of dislocation rather than the numbers required to examine the effects in the primary and revision arthroplasty groups independently.
Our conclusion that a larger articulation decreased the risk of dislocation following total hip arthroplasty supports the findings of two cohort studies11,12 as well as those of two registry studies that showed a decreased risk of revision for dislocation after total hip arthroplasty with larger articulations18,28.
Although we have been able to determine the short-term benefits of a larger, 36-mm metal-on-polyethylene articulation in total hip arthroplasty, specifically in terms of decreasing the incidence of dislocation up to one year following arthroplasty, what is best at one year may not be best at ten years. This needs to be emphasized because the use of a larger articulation in a metal-on-polyethylene bearing is not without potential risks. In an acetabular component of a given outer diameter, a 36-mm liner will of necessity be thinner than a 28-mm liner, particularly at the rim. The polyethylene thickness for a 36-mm liner in an acetabular component with an outer diameter of 50 mm is 6.7 mm at the pole and 5.8 mm at 45°. This may increase wear or even wear-through compared with the smaller-diameter liner, although the findings of simulator studies have been encouraging29,30. However, even if cross-linking improves wear resistance, the mechanical properties of highly cross-linked polyethylenes are reduced, leading to increased fracture potential of such liners, irrespective of the inner diameter31,32.
Wear has been used as a surrogate measure of osteolysis with previous generations of polyethylene implants. Given the same rate of linear wear, volumetric wear will be greater in a larger articulation. However, the relationship between head penetration, volumetric wear of highly cross-linked polyethylene, and osteolysis is not yet well defined33.
The major strength of our randomized trial was the ability to control for other variables that may affect the risk of dislocation. We chose to exclude patients who had certain characteristics that, although not common, could significantly increase the risk of dislocation and could affect the results if not equally distributed across the 36 and 28-mm articulation groups. Importantly, patients who were to undergo revision were excluded if revision was being undertaken because of recurrent dislocation or infection. Patients were stratified by other factors that were also considered possible risk factors for dislocation.
One limitation of our study is that seven (1.1%) of the 644 patients were lost to follow-up at one year and that six of these patients had received a 36-mm articulation. For patients who had been lost to follow-up, reviews of hospital records and, when available, local physician records suggested that no dislocations had occurred. However, as it could not be confirmed that no dislocations had occurred, the patients were treated as having been lost to follow-up and were excluded from the analysis.
In our randomized trial, the incidence of dislocation in the first year after primary total hip arthroplasty with a 28-mm articulation was 4.4%. Although this figure is at the upper end of the range of incidences reported in large cohort studies, two factors are likely to have influenced this finding. First, the incidence of dislocation is known to be higher in association with a posterior approach11,17-19. Second, the reported incidence is higher when patients are routinely followed and when the number of methods used to track dislocation increases22.
In conclusion, the present randomized trial showed that a larger articulation significantly reduced the incidence of dislocation in the first year after total hip arthroplasty with a metal-on-highly cross-linked polyethylene articulation. It must be emphasized that before a 36-mm metal-on-highly cross-linked polyethylene articulation is widely recommended, particularly in younger patients or those at lower risk of dislocation, the incidence of late dislocation, wear, periprosthetic osteolysis, and acetabular liner fracture needs to be established.
Tables showing a description of the study centers and the reasons for the preoperative and intraoperative exclusion of patients from the study and additional paragraphs describing the stratification and radomization preocedures are available with the online version of this article as a data supplement at jbjs.org.
Note: The Large Articulation Study Group: D. Howie, O. Holubowycz (Chief Investigators). Royal Adelaide Hospital: D. Howie, B. Allen, S. Brumby, M. Chehade, R. Clarnette, A. Comley, A. Mintz, R. Montgomery, A. Pohl, T. Savvoulidis, B. Solomon, J. van Essen (surgeons), A. Standen (Local Study Coordinator [LSC]). St. Andrew’s Hospital: D. Howie (surgeon), M. Bennier (LSC). Glenelg Hospital: D. Howie (surgeon), M. Bennier (LSC). Modbury Hospital: S. Brumby (surgeon), S. Pannach (LSC). Whyalla Hospital: T. Savvoulidis (surgeon). Royal North Shore Hospital: A. Ellis, S. Ruff (surgeons), E. Cole (LSC). St. John of God and Ballarat Base Hospitals: J. Nelson (surgeon), C. Gear (LSC). Geelong Hospital: S. Williams, R. Angliss (surgeons), S. Beattie, U. Farago, C. Gleeson, A. Vanderveen (LSCs). St. Vincent’s Hospital Melbourne: A. Dunin, B. Love (surgeons), M. Dowsey, M. Farley (LSCs). Maroondah Hospital: D. Booth, P. Gard (surgeons), J. Walsh (LSC). Royal Bournemouth Hospital: R. Middleton (UK surgeon coordinator), T. Wainwright, S. Sargeant (LSCs). Southampton General Hospital: D. Dunlop, J. Latham (surgeons), A. Wakefield (LSC). Ninewells Hospital: B. Clift, D. Rowley (surgeons), L. Johnston, L. McCormack (LSCs).
The authors gratefully acknowledge Dr. Nancy Briggs, from the Discipline of Public Health at the University of Adelaide, for undertaking statistical analysis of the data; Mr. Stuart Callary, from the Department of Orthopaedics and Trauma at the Royal Adelaide Hospital, for assessing acetabular component position; and the Joint Replacement Nurses at the Royal Adelaide Hospital for assistance with the trial.
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