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Echelon Stems and Primary Total Hip Replacement Surgery

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Abstract

The Echelon Primary femoral stem is an extensively porous coated cylindrical cobalt chrome hip component. A prospective review of 392 Echelon stems revealed excellent survivorship of the stem, with a 99.3% survival rate for aseptic loosening and a 98.3% survival rate for revision for any reason at 8 years. Normalized Western Ontario and McMaster Universities Osteoarthritis Index and short-form health survey outcome scores were significantly improved and subsequently maintained after replacement, with 82% of patients obtaining a good or excellent result at the mean follow-up period. There have been no changes in manufacture or design of the stem during the review period.

Cementless total hip replacements (THRs) have been in use for over 40 years.¹ There are, however, few reports of their long-term performance^2,3 and, although short-term studies have reported promising results, a review of the Norwegian Arthroplasty register has reported a failure rate of uncemented femoral stems as high as 4.5% at 4.5 years.⁴ Despite this, the Swedish Hip register described a rapid transfer to uncemented stems, with use increased from 3% of implanted stems in 1997 to over 16% by 2006.⁵ With loosening identified as the primary problem in cemented hips,⁶ cementless stems are used as a potential option,² particularly in the younger patient.²

The Echelon series of stems was developed using conventional design concepts established from previous generations of uncemented stems while implementing novel design features that make it unique. The Echelon Primary stem is a cylindrical cobalt chrome stem, two-thirds of which is covered with a circumferential porous coating (Figures 1 and 2). Design features include a collar, oversized distal flutes for immediate rotational stability, and a polished distal tip to minimize end-of-stem thigh pain.⁷ Other features include a distal slot to ease insertion and reduce the risk of fracture and stem stiffness, a proximal flare to improve proximal fill, and a circulotrapezoidal neck to optimize range of motion (ROM). It is available in dual offsets with multiple 1-mm incremental stem diameters.

The purpose of this prospective review is to report the current medium-term outcome of a large series of Echelon Primary femoral stems. Although the durability of most uncemented femoral stems remains unknown, it is the aim of this study to investigate Echelon Primary femoral stem performance with regard to patient satisfaction and overall implant survival.

Materials and Methods

The Echelon Primary femoral stem was first introduced into regular use at our institution in 1998 and has since been the principal uncemented stem implanted. Use of this femoral prosthesis is based on a number of factors including age, quality of bone, and absence of any bony abnormality preventing its use. A cemented stem is used when age or bone stock is of concern. In addition, in the younger age group, hip resurfacing is offered when appropriate.

Surgery was performed by one of two staff surgeons using a posterior approach in the lateral decubitus position. The acetabular component and joint bearing were selected according to what was considered most appropriate for the patient by the surgeon. Each patient received perioperative prophylactic antibiotics and low-molecular-weight heparin deep vein thrombosis prophylaxis. Postoperative weight-bearing protocol varied between the two surgeons, with one allowing immediate full weight bearing and the other partial weight bearing for 6 weeks.

A prospectively updated registry recorded the data for all patients who received the Echelon Primary stem and included operative details, complications, failures, and outcome measures.

Between February 1998 and March 2007, 428 patients received the Echelon Primary stem. There have been no changes in manufacture or design of the stem during this period. Thirty-five patients were excluded from additional data collection in our registry due to follow-up impracticalities including geographical limitations (eg, reside out of province), mental illness, and language issues, leaving 392 patients in the registry available for analysis. Follow-up consisted of a 6-week and 3-month postoperative review followed by yearly visits thereafter.

Disease-specific outcome was measured using the normalized Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) (0, worst outcome; 100, optimum outcome),⁸ which assesses pain, stiffness, and physical function. The short-form health survey (SF-36) was used to assess mental function and physical quality of life.⁹ Assessments were made preoperatively, 3 months postoperatively, and then annually. Consistent with previous publications,¹⁰ the normalized WOMAC outcome variables were divided into three categories, poor (0-63), good (64-85), and excellent (86-100).

Along with the WOMAC index and SF-36 score, all patients undergoing THR at our institution receive a St. Michael’s outcome score. The score is a simple, validated outcome score, with a maximum measure of 25, that assesses pain, function, and ROM.^11,12 It is usefully compared with the Harris Hip Score by multiplying by a factor of 4.

Descriptive statistics for patient demographics and functional scores were computed using Microsoft Excel (Microsoft Corporation, Redmond, Ca.). Comparison of pre- and postoperative functional scores was conducted using a paired sample t test using Excel and the statistical software package SPSS (v.13, SPSS Inc, Chicago, Ill.). The statistical software package Stata (Statacorp, College Station, Tex.) was used to conduct survival analysis.

Results

A total of 392 patients (135 women) underwent Echelon primary femoral stem hip replacement. The mean age was 58.1 years (SD, 11.1; range, 20-87). Body mass index averaged 30.5 kg/m² (SD, 5.8; range, 17.7-58.2). Fourteen patients underwent bilateral procedures, 189 patients underwent right THR, and 189 underwent left THR. Preoperative diagnoses are listed in Table 1.

The majority of patients received a Reflection uncemented acetabular component (91%) and an ultrahigh molecular-weight polyethylene liner (76.5%) (Table 2), although the highly cross-linked polyethylene is now used with increased frequency (31% of hips since 2005). The majority of femoral heads were cobalt chrome (79.3%).

Complications are listed in Table 3. The mean overall follow-up was 42 months (range, 3-100 months). One patient was lost to follow-up (12 months), and there has been 1 death (3 months). Two femoral stems required revision due to loosening at 13 months and 29 months, postoperatively. Radiographic evidence of stem loosening was noted in 3 additional patients; however, none have undergone revision at latest follow-up. Kaplan Meier survivorship for the Echelon Femoral stem with revision for aseptic loosening as endpoint is shown in Figure 3. The survival at 100 months (8 years, 4 months) is 99.3% (95% CI: 97.1-99.8). Three acetabular revision procedures have been performed after multiple dislocations at 13, 18, and 22 months postsurgery. Therefore, with revision for any reason as the endpoint, the Kaplan Meier survivorship is 98.3% at 100 months (95% CI, 95.9-99.3) as shown in Figure 4. Two deep infections were diagnosed; however, neither required revision. Both were successfully treated/suppressed with debridement and antibiotics, and both patients retained their prostheses. A total of 4 periprosthetic fractures have occurred in the series. One intraoperative fracture was successfully treated with a circlage wire. The remaining 3 femoral fractures were secondary to trauma, with each successfully managed without component revision.

Pre- and postoperative outcome scores, along with 95% CIs, are shown in Table 4. A preoperative WOMAC score was available for 345 of the 392 patients, with mean score of 43.5 (95% CI, 41.6-45.4). At the 3-month postoperative review, the mean WOMAC score was significantly increased to 74.54 (95% CI, 72.7-76.3) (P < .001) and by 1 year to 84.3 (95% CI, 80.5-88.1). In subsequent years, the normalized WOMAC score remained at a plateau of approximately 80. Using categorical levels of the normalized WOMAC score, a mean score of 80 is borderline good/excellent, whereas a detailed assessment within the 4-year follow-up period (mean follow-up, 42 months) reveals an outcome of 18% poor, 31% good, and 51% excellent. Similar improvements were identified in the St. Michaels’s scores, with a preoperative mean score of 14.5 (95% CI, 13.1-15.0) increased to 18 (95% CI, 17.7-18.3) (P < .01) at 6 weeks; 20.2 (95% CI, 19.9-20.5) (P < .001) at 3 months; and 22 (95% CI, 21.7-22.4) by 1 year. The mean St. Michael’s score continued to improve with a year 8 score of 24 (95% CI, 22.2-25.0). General health assessment using the SF-36 showed an improvement in the physical component score from 33.1 (95% CI, 32.3-33.9) preoperatively to 42.6 (95% CI, 41.7-43.6) (P < .001) at 3 months and 48.19 (95% CI, 44.2-52.2) at latest follow-up. Mental function scores from the SF-36 increased from 48.7 (95% CI, 47.6-49.9) to 51.4 (95% CI, 50.3-52.4) (P < .001) and 53.5 (95% CI, 50.3-56.6), respectively (Table 5).

Discussion

Although reports on the long-term follow-up of uncemented stems are limited,^2,3 survival results are promising. A recent review of 11,516 primary cementless stems within the Norwegian arthroplasty register, using aseptic loosening as the endpoint, reported that all currently used stem designs performed excellently with 96% to 100% survival at 10 years.³ However, with regard to revision for any reason, there were significant differences among stem designs, with long-term survivorship at 15 years varying from 29% to 97%³ with many reports lacking long-term data beyond 7 years. An additional issue is frequent change in stem manufacture and design that may significantly confound results and outcomes.

Uncemented femoral stems are available in two primary designs: the proximally coated stem with tapered geometries and the extensively coated cylindrical stem with straight geometries such as the Echelon Primary stem. Regardless of stem design, Engh reported survival greater than 95% at 15 years,¹³ and in a separate review using a cylindrical stem, he reported a stem survival rate, with revision for aseptic loosening as the endpoint, to be 98.9% at 15 years.¹⁴ Similarly, McLaughlin and Lee, using a cementless tapered stem, reported their stem survival to be 87% at 22 years.¹⁵ With registry reviews and series such as these, and with the continued concern about loosening of cemented implants, it is understandable that an “evolution favoring cementless fixation” continues.¹⁶

Investigation of the Echelon Primary stem in this series identifies a survival of over 99% at 100 months with respect to aseptic loosening and over 98% for the same period with revision for any reason. Although our study was only medium-term, with a mean follow-up of 42 months, the results are excellent with regard to overall implant survival. Unlike many stems in the Scandinavian registries,³ the Echelon Primary stem now has supportive survival data beyond 7 years. Also, importantly, there were no changes in stem design or manufacture during our study period, which eliminates this confounding factor from our results.

As would be expected with any successful primary joint replacement, disease-specific outcome scores showed significant improvement between pre- and postoperative periods. This improvement was sustained with a mean normalized WOMAC score of 84.2 at 6 years and a mean St. Michael’s score of 24 at 8 years. At the mean follow-up and using categorical levels of the normalized WOMAC, 82% of patients obtained a good or excellent outcome. Significant improvement was also seen in general health assessment after an Echelon primary hip replacement, with sustained improvements in both physical and mental function through latest follow-up in this series.

The current success and predicted continued long-term survival of the Echelon Primary stem are attributable in large part to its design as an extensively coated cylindrical stem. The advantage of such a stem is solid fixation within the consistent cannular portion of the diaphysis, which can be obtained with a simple and reproducible insertion technique. The long-term clinical results using a cylindrical stem have been studied extensively by Engh^14,17,18 as well as by Whiteside and Easley.¹⁹ The success of any uncemented stem ultimately is dependent on sufficient implant fixation.¹⁹ Whiteside and Easley concluded that the most important factor for achieving rigidity of fixation is a tight distal fit, which, in turn, reduces micromotion of the entire implant. To obtain and optimize this distal fit, the Echelon Primary stem is available in many sizes in 1-mm increments. The availability of many sizes also eliminates the need to overream the femur, thus preserving bone stock, which is important particularly in the younger patient. A tight distal fit also changes the stress pattern in the femur, which in turn causes narrowing of the medullary canal, again an important factor considering that aging tends to expand the medullary cavity.²⁰ Also incorporated within the Echelon Primary stem design is a collar. Although a collar on a cylindrical stem has not been shown to prevent proximal micromotion and is not as important as distal fit, it has been shown to be beneficial in high-load patients (>100 kg), preventing excessive settling or fracture even with tight distal fixation.¹⁹

Conclusion

This prospective review of the Echelon Primary femoral stem revealed excellent survivorship of the stem at 8 years, with a 99.3% survival rate for aseptic loosening and a 98.3% survival rate for revision for any reason. Patient outcome scores were significantly improved and subsequently maintained following joint replacement, with 82% of patients obtaining a good or excellent result in the mean follow-up period. There had been no changes in manufacture or design of the stem during the review period.

References

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Authors

From the Division of Orthopaedics, Department of Surgery, St. Michael’s Hospital, Toronto, Ontario, Canada.

Drs Schemitsch is a consultant for Smith & Nephew and received research grant support from Smith & Nephew. Drs Lewis, Olsen, and Waddell have no relevant financial relationships to disclose.

The authors thank Kerry Ann Griffith-Cunningham and Jane Morton from the Division of Orthopaedics at St. Michael’s Hospital for their kind assistance in the data collection for this study.

Correspondence should be addressed to James P. Waddell, MD, FRCSC, St. Michael’s Hospital, 30 Bond Street, Toronto, ON, Canada M5B 1W8.