Avoiding Misclassification Bias with the Traditional Charnley Classification: Rationale for a Fourth Charnley Class BB C. Ro¨der,1 L.P. Staub,1 P. Eichler,1 M. Widmer,1 D. Dietrich,2 S. Eggli,3 U. Mu¨ller1 1
Institute for Evaluative Research in Orthopedic Surgery, University of Bern, Switzerland
2
Institute of Mathematical Statistics and Actuarial Science, University of Bern, Switzerland
3
Department of Orthopaedic Surgery, Inselspital, University of Bern, Switzerland
Received 3 November 2005; accepted 14 April 2006 Published online 4 August 2006 in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/jor.20224 ABSTRACT: Long-term follow up of patients with total hip arthroplasty (THA) revealed a marked deterioration of walking capacities in Charnley class B after postoperative year 4. We hypothesized that a specific group of patients, namely those with unilateral hip arthroplasty and an untreated but affected contralateral hip was responsible for this observation. Therefore, we conducted a study taking into consideration the two subclasses that make up Charnley class B: patients with unilateral THA and contralateral hip disease and patients with bilateral THA. A sample of 15,160 patients with 35,773 follow ups that were prospectively collected over 10 years was evaluated. The sample was categorized into four classes according to a new modified Charnley classification. Annual analyses of the proportion of patients with ambulation longer than 60 min were conducted. The traditionally labeled Charnley class B consists of two very different patient groups with respect to their walking capacities. Those with unilateral THA and contralateral hip disease have underaverage walking capacities and a deterioration of ambulation beginning 3 to 4 years after surgery. Those with bilateral THA have stable overaverage walking capacities similar to Charnley class A. An extension of the traditional Charnley classification is proposed, taking into account the two different patient groups in Charnley class B. The new fourth Charnley class consists of patients with bilateral THA and was labeled BB in order to express the presence of two artificial hip joints and to preserve the traditional classification A through C. ß 2006 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 24:1803–1808, 2006
Keywords:
charnley classification; bilateral total hip arthoplasty (THA); outcome
INTRODUCTION In 1972, Charnley introduced a patient classification for outcome assessment of low-friction hip arthroplasties.1 This classification takes into account patient-specific factors other than the total joint replacement that have a negative influence on walking capacity. Charnley stated: ‘‘1. Class A consists of patients with only one affected hip and no other condition interfering with walking. 2. Class B consists of patients with a bilateral hip disease, but the rest of the body normal and therefore not responsible for any defect in ability to walk. 3. Class C consists of patients with some factor contributing to failure to achieve normal locomotion, such as
Correspondence to: Urs Mu¨ller (Telephone: 0041-316315942; Fax: 0041-31-6315931; E-mail:
[email protected]) ß 2006 Orthopaedic Research Society. Published by Wiley Periodicals, Inc.
multi joint involvement, inflammatory arthritis, senility, hemiplegia, and cardiovascular or respiratory disability (Reference 1, p. 62).’’ For the most objective analysis about regained postoperative walking capacities, Charnley restricted his evaluation to patients in class A and those with bilateral hip replacement in class B because the status of the contralateral hip, the rest of the locomotor system, and other systemic factors can negatively bias the mobility improvement achieved with total hip arthroplasty (THA).1 We previously showed the major influence of the Charnley classification on the functional outcome after THA compared to other patient demographic factors such as sex, age, or underlying disease.2 Assessing its influence on walking capacity over a 15-year period, a pronounced loss of mobility was observed in Charnley class B, beginning in postoperative year 4. This decline was stronger than in the other Charnley groups and raised discussions about the influence of the subgroup with an untreated but diseased contralateral hip. JOURNAL OF ORTHOPAEDIC RESEARCH SEPTEMBER 2006
1803
1804
¨ DER ET AL. RO
For the current investigation, we hypothesized that the two patient groups in Charnley class B were significantly different with respect to their postoperative walking capacities and that patients with unilateral hip arthroplasty and a diseased contralateral hip are responsible for the loss of mobility in the Charnley class B. Similar suggestions have already been made for patients with total knee arthroplasty (TKA).3 In order to prove our hypothesis and quantify the differences between the two patient groups, we conducted a follow-up study about the walking capacities of patients in the three traditional Charnley classes and introduced a fourth Charnley class, BB, consisting of patients with bilateral total hip arthoplasty. The analysis focused on the annual postoperative number of patients with walking capacities greater than 60 min in each of the four groups over a 10-year course.
A:
Unilateral THA, contralateral hip not diseased (as in traditional classification) B: Unilateral THA, contralateral hip diseased (modified) BB: Bilateral THA (modified, new fourth Charnley class) C: Other condition affecting walking capacity (as in traditional classification) We carefully controlled for clinically impossible changes during the follow-up period (e.g., from Charnley class B to A). After implantation of the second THA, that is, a change to Charnley class BB, the follow-up period of the respective patient was reset to year 1. For the current study, a desired outcome was arbitrarily defined as patients with a walking capacity longer than 60-min without walking aids. The current study did not require institutional review board approval at our center, as it utilized existing anonymous observational data. Statistical Analysis
MATERIALS AND METHODS The current study is based on the IDES4,5 (International Documentation and Evaluation System) hip register of the Institute for Evaluative Research in Orthopaedic Surgery at the University of Bern. The history and administration of the IDES register were described in detail in previous articles.4,5 For the current study, all primary cases from 1970 to 1995 with one or more follow-up examinations were selected. All revisions and their respective follow ups were excluded. In order to have patients from all three Charnley classes, no exclusions regarding primary diagnosis, status of contralateral hip or locomotor system, or regarding comorbid conditions were made. The query resulted in 15,160 patients with 17,663 THAs. This data was derived from 52 centers in eight European countries (Switzerland, Austria, Germany, France, Belgium, Italy, The Netherlands, Spain). Of the study sample, all follow ups within the first 10 postoperative years were selected. In case of multiple follow-up visits within the same year, the one closest to the middle of the year was chosen and all others were discarded. Follow ups of symptomatic, radiographically loose primary prostheses were censored. This elimination procedure left 35,773 follow-up records for outcome assessment with the shortest follow up conducted 58 days after surgery. The average number of follow ups included per patient was 2.4, the median observational period was 5 years (IQR, 2–7 years). In order to assign patients to a Charnley class, the items ‘‘status of contralateral hip’’ and ‘‘walking capacity restriction’’ were used. If an item was missing on a primary or follow-up form, the case or the respective follow up was excluded (Fig. 1, Table 1). Based on the goal of the study, patients were classified according to a new, modified Charnley classification: JOURNAL OF ORTHOPAEDIC RESEARCH SEPTEMBER 2006
All follow ups were grouped based on time after surgery. Generalized estimation equations (GEE) were used to test the influence of patient characteristics (sex, age at operation, preoperative status) and time after surgery on the longitudinal course of occurrence of the desired outcomes. Annual calculations of frequencies of the desired outcome for the four patient groups—A, BB, B, and C— were performed. For each outcome variable and each follow-up year a Cochran–Mantel–Haenszel (CMH) test was used to compare the preoperative patient groups. Bonferroni–Holm adjustments for each outcome variable were set to account for multiple testing over the 10 follow-up years. Figures 2 and 3 graphically display 95% confidence intervals and the variability within the groups. The level of significance was set to 0.05 throughout the study. All statistical analyses were conducted using SAS 9.1 (SAS Institute Inc, Cary, NC).
RESULTS The study sample comprised 8015 female and 7145 male patients with a median age of 67 years (range, 21–93 years) and 65 years (range, 20–95 years), respectively. The median body mass index (BMI) was 26.1 kg/m2. The percentage of female patients ranged between 50.2% and 53.0%, and the median age increased from 65.9 to 72.2 years over the follow-up period. The proportion of patients in Charnley classes A and B decreased, whereas that in classes BB and C increased over the 10 years (Table 1). Using the traditional Charnley classification, Figure 2 shows a marked decline of patients in DOI 10.1002/jor
RATIONALE FOR A FOURTH CHARNLEY CLASS
1805
Figure 1. Selection process of study sample from complete IDES database.
Charnley class B who walk longer than 60 min after postoperative year 4. Figure 3 shows the proportion of patients with walking capacity greater 60 min in the four Charnley classes (modified) over a 10-year followup period. While the proportion of patients with walking capacities longer than 60 min ranged from 56.3% to 76.2% in Charnley class A and from 52.8% to 67.7% in the newly defined Charnley class BB, it DOI 10.1002/jor
only ranged from 21.2% to 50.8% in the refined Charnley class B with all patients having a contralaterally diseased but untreated hip. In Charnley class C, between 9.4% and 17.9% of patients could walk longer than 60 min (Table 2, Fig. 3). CMH tests, controlled for sex and age, showed highly significant differences (p < 0.0001) between at least two classes in all 10 followup years. JOURNAL OF ORTHOPAEDIC RESEARCH SEPTEMBER 2006
1806
¨ DER ET AL. RO
Table 1. Demographic Characteristics and Allocation to the Four Charnley Groups Per Followup Year Follow-up year
FU absolute FU rate (%) Median age Female (%) A (%) BB (%) B (%) C (%)
1
2
3
4
5
6
7
8
9
10
5012 28.4 65.9 53.0 56.4 21.8 9.0 12.8
9158 51.8 67.4 50.9 51.2 25.9 7.8 15.1
5227 29.6 66.8 51.3 45.8 31.1 8.1 15.0
2815 15.9 67.3 50.2 37.2 40.3 6.8 15.7
3321 18.8 68.6 51.2 39.7 36.3 6.7 17.3
4077 23.1 70.3 51.3 40.5 34.2 5.6 19.7
2253 12.8 69.9 50.9 37.2 38.0 5.5 19.3
1429 8.1 70.2 51.9 31.5 45.5 5.5 17.5
1419 8.0 71.9 52.0 30.7 42.7 4.7 21.9
1062 6.0 72.2 51.6 27.3 46.9 3.0 22.8
DISCUSSION The current study hypothesized that patients with unilateral total hip arthroplasty and a contralaterally diseased hip have a significantly different postoperative mobility status than patients with bilateral total hip arthroplasty. Both these patient groups make up class B of the traditional Charnley classification. When reporting the outcome of THA, the status of the contralateral hip, the overall status of the locomotor system, and other systemic conditions can bias the observed walking capacity. In order to take these covariables into
account, the outcome of THA should be reported separately for the different Charnley classes. As Charnley did himself, for the most objective assessment of re-establishment of mobility, only the Charnley class A and class B patients with bilateral total hip arthroplasty should be considered.1 However, this recommendation which Charnley appended to the presentation of his classification system is mostly not respected. Studies that explicitly focus on the influence of the Charnley classification on outcome make use of the Charnley classes A,B, and C without splitting class B into two subgroups. In addition, an assessment
Figure 2. The proportion of patients with walking capacity greater than 60 min in the three Charnley classes (traditional) and in the complete study population (average) over a 10-year period. JOURNAL OF ORTHOPAEDIC RESEARCH SEPTEMBER 2006
DOI 10.1002/jor
RATIONALE FOR A FOURTH CHARNLEY CLASS
1807
Figure 3. The proportion of patients with walking capacity greater than 60 min in the four Charnley classes (modified) and in the complete study population (average) over a 10-year follow-up period.
Charnley classification more obvious, we analyzed the data applying the traditional three-level grouping (Fig. 2) and re-analyzed the data introducing a new fourth Charnley class (BB), representing patients with bilateral THA (Fig. 3). The study has weaknesses and strengths that need to be considered. Only those patients were assessed who actually underwent follow-up examination, whereas patients not attending follow-up visits may have changed the treatment center
of the contralateral hip which is needed to make this distinction (THA or diseased but untreated) is not reported.6,7 For proving our hypothesis and demonstrating the magnitude of the influence of a diseased yet untreated contralateral hip, we evaluated a large patient sample over a 10-year course regarding the proportion of patients with walking capacities greater than 60 min in each follow-up year. In order to make differences to the traditional
Table 2. Percentage (and Confidence Intervals) of Patients in Each Followup Year who Achieved the Desired Outcome (Ambulation >60 min.), Depending on Their Charnley Class Charnley A Year 1 2 3 4 5 6 7 8 9 10
% 56.3 68.2 72.1 76.2 74.5 70.4 70.4 61.1 65.5 63.3
DOI 10.1002/jor
Charnley BB
95% CI 54.6 66.8 70.3 73.5 72.1 68.1 67.2 56.4 60.8 57.5
58.0 69.5 73.9 78.7 76.9 72.5 73.5 65.6 69.9 68.9
% 52.8 59.9 63.3 67.5 67.7 63.9 61.7 61.2 54.5 62.9
Charnley B
95% CI 49.9 58.0 60.9 64.7 65.1 61.3 58.4 57.4 50.4 58.5
55.6 61.9 65.6 70.2 70.4 66.4 65.0 65.0 58.5 67.1
% 29.3 37.3 44.5 50.8 39.9 33.5 35.4 24.6 22.5 21.2
Charnley C
95% CI 25.5 33.9 39.8 43.6 33.6 27.4 27.2 15.3 13.5 8.9
33.4 40.8 49.3 57.9 46.5 49.9 44.4 35.4 34.0 38.9
% 10.6 14.4 16.8 15.4 15.4 17.9 17.3 10.1 12.8 9.4
95% CI 8.4 12.6 14.3 12.2 12.6 15.3 13.9 6.6 9.3 6.1
13.1 16.3 19.6 19.0 18.6 20.7 21.1 14.5 17.0 13.8
JOURNAL OF ORTHOPAEDIC RESEARCH SEPTEMBER 2006
1808
¨ DER ET AL. RO
because of compromised outcomes. Hence, a certain amount of selection bias may be possible. In addition, only few patients had a complete record of 10 follow ups. On the other hand, a large patient sample was assessed over a 10-year course. The outcomes of all patients were reported using a consistent terminology derived from a broad expert consensus.8 By design, the current study is not of a survival analysis type but an assessment of affiliation to a Charnley class and mobility at distinct postoperative points in time. Consequently follow-up rates are less important compared to the absolute number of observations in each year. Because the latter ones were large, even in the very late follow-up periods, conclusions could be drawn on a sound basis with small confidence intervals. Although the hypothesis of our study was already considered by Charnley and may intuitively be confirmed by many clinicians, the literature does not provide articles that explicitly focus on the topic or suggest a modification of the traditional Charnley classification for THA patients. This may be due to a lack of clinical data in other large registry databases that mainly collect demographic and implant information and do not record disease specific functional status. The Charnley classification is also in use in total knee arthroplasty (TKA), and one article suggests a modification of the classification for enabling a better discrimination of patients with bilateral joint involvement in class B.3 Our hypothesis of clinically relevant and significantly different walking capacities in patients with unilateral THA and a contralateral hip affection and those with bilateral THA was confirmed. Instead of p values, we provided figures with 95% confidence intervals. That way the reader gets both an impression about statistical significance and magnitude of the differences between the four groups. Based on our findings, we suggest a modification of the Charnley classification. In order not to change the traditional terminology (A, B, C) and to express the presence of bilateral THA in the newly separated group, we have come up with the
JOURNAL OF ORTHOPAEDIC RESEARCH SEPTEMBER 2006
label BB. In the future, walking capacity improvement should be independently reported for the four patient groups of the modified Charnley classification to avoid the currently existing misclassification bias for patients in Charnley class B.
CONCLUSION Patients with bilateral THA have significantly better walking capacities than patients with unilateral THA and an untreated contalaterally diseased hip. In order to better account for these differences, a modification of the traditional Charnley classification and creation of a fourth Charnley class (BB) for patients with two artificial hip joints is proposed.
ACKNOWLEDGMENTS No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article.
REFERENCES 1. Charnley J. 1972. Long-term results of low-friction arthroplasty of the hip performed as a primary intervention. J Bone Joint Surg Br 54:61–76. 2. Ro¨der C, Parvizi J, Eggli S, et al. 2003. Demographic factors affecting long-term outcome of a total hip arthroplasty. Clin Orthop 417:62–73. 3. Dunbar J, Robertsson O, Ryd L. 2004. What’s all that noise? Acta Orthop Scand 75:119–126. 4. Paterson D. 1993. The International Documentation and Evaluation System (IDES). Orthopedics 16:11–14. 5. Ro¨der C, Eggli S, EL-Kerdi A, et al. 2003. The International Documentation and Evaluation System (IDES)— 10-years experience. Int Orthopedics 27:259–261. 6. Brinker M, Lund P, Cox D, Barrack R. 1996. Demographic biases found in scoring instruments of total hip arthroplasty. J Arthroplasty 7:820–830. 7. Garellick G, Malchau H, Herberts P. 1998. Specific or general health outcome measures in the evaluation of total hip replacement. J Bone Joint Surg Br 80:600–606. 8. Johnston RC, Fitzgerald RH, Harris WH, et al. 1990. Clinical and radiographic evaluation of total hHip replacement. J Bone Joint Surg Am 72:161–168.
DOI 10.1002/jor
