Copyright © 2006 by The Journal of Bone and Joint Surgery, Inc.
Commentary & Perspective
Commentary & Perspective by
Thomas P. Schmalzried, MD
Joint Replacement Institute at Orthopaedic Hospital, Los Angeles, California
Because of concerns regarding the rising costs of medical technology, the financial orientation of this study is timely and of interest. The authors are to be commended on the thoughtful construction and concise description of their model as well as their detailed analysis. I believe their conclusions are well supported by the results and that they provide a fair and balanced discussion. They are honest about the limitations of their method, which are set by the validity and applicability of the input data on implant cost and clinical outcomes. Thus, the results and conclusions of this study are valid based on these cost inputs and these outcome inputs in this model. The outcome inputs, which are based on historical data, may not accurately reflect the outcomes of the current generation of technologies in the current generation of patients. The authors are honest about the potential impact of unknown clinical outcomes that can only be determined by long-term clinical studies of the current generation of technologies, and the model can be updated as relevant information becomes available. Finally, and very importantly, they differentiate between this type of financial decision analysis for a population of patients and implant selection for an individual patient.
Historical data from published reports were used as inputs for the clinical outcomes and the failure rates because there are little long-term data on the implant technologies that are currently in use. The authors use historical data from cemented Charnley total hips to weight the risk of failure as a function of patient age. It should be recognized that the mechanism(s) and rates of failure for that prosthetic design may not be relevant to the current generation of implants that includes predominantly cementless acetabular components and is showing a trend toward the use of cementless femoral components. It should also be appreciated that the attitudes, expectations, and activities of the current generation of patients may differ from the historical cohorts.
Patient age at the time of surgery is the central input variable of this model. Patient age influences at least two other variables in the model: (1) the potential number of years of additional life and (2) the risk of failure. Patient age is frequently used as a surrogate for patient activity, or the use of the prosthesis, which is the true variable that influences wear and arthroplasty survival1. The use of age as a surrogate for activity is appropriate in large cohort studies, but there is considerable variability in the activity of individual patients regardless of age. Some older patients may be very active2 (Fig. 1). On this basis, patient age should not be the dominant factor for implant selection in individual patients.
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Fig. 1 Scatterplot of age versus average number of steps per day, showing a high degree of variability. (Reprinted, with permission, from Schmalzried TP, Szuszczewicz ES, Northfield MR, Akizuki KH, Frankel RE, Belcher G, Amstutz HC. Quantitative assessment of walking activity after total hip or knee replacement. J Bone Joint Surg Am. 1998;80:54-9.) |
The model assumes no benefit from the articulation in terms of improved implant survival during the first five years, although it is recognized by the authors that failures associated with any bearing surface could occur at any time. This may be a practically important limitation of their method because certain bearing surfaces may be more at risk for early failure than others. For example, there are differences in the range of available bearing diameters for polyethylene, cross-linked polyethylene, metal-on-metal and ceramic-on-ceramic. Since the risk of dislocation has been demonstrated to be lower with larger bearing diameters3 and the relative risk of dislocation has been reported to be 1.3 times higher in patients who are seventy years of age or older4, the increased cost of a larger diameter bearing may be justified by a reduction in dislocation risk, especially in patients over seventy years of age.
I believe cost-effectiveness should be a consideration in implant selection. I also agree with the authors that the validity of their findings needs to be confirmed by long-term clinical experience. The benefits and risks (including cost) of arthroplasty technologies should be evaluated and then applied on a patient-by-patient basis. General health status, dislocation risk, and the type and amount of anticipated physical activity should be considered as well as patient age5. Similar to this cost-effectiveness analysis, our clinical decision-making should be updated as new information becomes available.
*The author did not receive grants or outside funding in support of his research for or preparation of this manuscript. The author received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity (DePuy, Johnson and Johnson). In addition, a commercial entity (Piedmont Fund of the Los Angeles Orthopaedic Hospital Foundation) paid or directed, or agreed to pay or direct, benefits to a research fund, foundation, educational institution, or other charitable or nonprofit organization with which the author is affiliated or associated.
References
1. Schmalzried TP, Shepherd EF, Dorey FJ, Jackson WO, dela Rosa M, Fa'vae F, McKellop HA, McClung CD, Martell J, Moreland JR, Amstutz HC. Wear is a function of use, not time. Clin Orthop Relat Res. 2000;381:36-46.
2. Schmalzried TP, Szuszczewicz ES, Northfield MR, Akizuki KH, Frankel RE, Belcher G, Amstutz HC. Quantitative assessment of walking activity after total hip or knee replacement. J Bone Joint Surg Am. 1998;80:54-9.
3. Berry DJ, von Knoch M, Schleck CD, Harmsen WS. Effect of femoral head diameter and operative approach on risk of dislocation after primary total hip arthroplasty. J Bone Joint Surg Am. 2005;87:2456-63.
4. Berry DJ, von Knoch M, Schleck CD, Harmsen WS. The cumulative long-term risk of dislocation after primary Charnley total hip arthroplasty. J Bone Joint Surg Am. 2004;86:9-14.
5. Schmalzried TP. How I choose a bearing surface for my patients. J Arthroplasty. 2004;19 (8 Suppl 3):50-3.
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