The Journal of Bone & Joint Surgery

The Journal of Bone & Joint Surgery, Volume 86, Issue 12

Scientific Articles | December 01, 2004

Prophylactic Pinning of the Contralateral Hip After Unilateral Slipped Capital Femoral Epiphysis

Mininder S. Kocher, MD, MPH¹; Julius A. Bishop, MD²; M. Timothy Hresko, MD¹; Michael B. Millis, MD¹; Young-Jo Kim, MD¹; James R. Kasser, MD¹

¹ Department of Orthopaedic Surgery, Children's Hospital, 300 Longwood Avenue, Boston, MA 02115. E-mail address for M.S. Kocher: mininder.kocher@tch.harvard.edu
² Harvard Medical School, 75 Francis Street, Boston, MA 02115

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The authors did not receive grants or outside funding in support of their research or preparation of this manuscript. They did not receive payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the authors are affiliated or associated.

Investigation performed at the Department of Orthopaedic Surgery, Children's Hospital, Boston, Massachusetts

The Journal of Bone and Joint Surgery, Incorporated

J Bone Joint Surg Am, 2004 Dec 01;86(12):2658-2665

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Abstract

Background: The management of the contralateral hip after unilateral slipped capital femoral epiphysis is controversial. The purpose of this study was to determine, with use of expected-value decision analysis, the optimal management strategy—prophylactic in situ pinning versus observation—for the contralateral hip.

Methods: Outcome probabilities were determined from a systematic review of the literature. Utility values were obtained from a questionnaire on patient preferences completed with use of a visual analog scale by twenty-five adolescent male patients without slipped capital femoral epiphysis. A decision tree was constructed, fold-back analysis was performed to determine the optimal treatment, and one and two-way sensitivity analyses were performed to determine the effect on decision-making of varying outcome probabilities and utilities.

Results: Observation was the optimal management strategy for the contralateral hip given the outcome probabilities and utilities that we studied (the expected value was 9.5 for observation and 9.2 for prophylactic in situ pinning, with a marginal value of 0.3). Increased rates of a late second slip favored prophylactic in situ pinning (the threshold probability was 27%). Risk-taking patients with a high utility for uncomplicated prophylactic in situ pinning favored prophylaxis (the threshold utility was 9.8).

Conclusions: The iatrogenic risks of treating a healthy patient or an uninvolved body part rarely outweigh the potential benefits unless the probability of the adverse event is likely and the consequences of the adverse event are very severe. In this decision analysis, the optimal decision was observation. In cases where the probability of contralateral slipped capital femoral epiphysis exceeds 27% or in cases where reliable follow-up is not feasible, pinning of the contralateral hip is favored. For a given individual patient, the optimal strategy depends not only on probabilities of the various outcomes but also on personal preference. Thus, we advocate a model of doctor-patient shared decision-making in which both the outcome probabilities and the patient preferences are considered in order to optimize the decision-making process.

Level of Evidence: Economic and decision analysis, Level III-1 (limited alternatives and costs; poor estimates). See Instructions to Authors for a complete description of levels of evidence.

Figures in this Article

The optimal management strategy for the contralateral hip after unilateral slipped capital femoral epiphysis is controversial. Some authors have advocated prophylactic in situ pinning of the contralateral hip, citing the high rate of contralateral slip, the potential for a silent contralateral slip, the risk of osteoarthritis, and the low risk of iatrogenic complications associated with modern pinning techniques^1-6. Others have advocated observation, citing the ability to detect slips with close observation and the iatrogenic risks of in situ pinning, including chondrolysis and osteonecrosis^7-9.

Expected-value decision analysis is a methodological tool, based in gaming theory, which allows for the quantitative analysis of decision-making under conditions of uncertainty^10-19. "Expected value" refers to the predicted consequences of a decision outcome, which is determined from the probability of that outcome and its utility. Utility is a subjective worth that a patient places on the specific outcome. The process of expected-value decision analysis involves the creation of a decision tree to structure the decision problem, the determination of outcome probabilities and utilities, fold-back analysis to determine the optimal decision-making strategy, and sensitivity analysis to determine the effect of varying outcome probabilities and utilities on decision-making. In this way, decision analysis may allow the clinician and the patient to optimize decision-making based on the best available evidence and patient preferences.

The purpose of this study was to determine, with use of expected-value decision analysis, the optimal management strategy—prophylactic in situ pinning or observation—for the contralateral normal hip after unilateral slipped capital femoral epiphysis.

Materials and Methods

Materials and Methods | Results | Discussion | Appendix | References

Institutional review board approval was obtained for this study. Analysis was performed with use of the DATA 3.5 software package (TreeAge Software, Williamstown, Massachusetts).

Outcome Probabilities

Outcome probabilities were determined from quantitative literature syntheses. The probability of a late contralateral slip after unilateral slipped capital femoral epiphysis was determined from a previously published quantitative synthesis by Castro et al.⁷. Those authors used Medline to search the literature for articles published on slipped capital femoral epiphysis, and they reviewed the references from the retrieved articles for additional data. Of the 325 articles reviewed, forty-five were used to establish the probability of late contralateral slip, and the data from seven were synthesized to determine the distribution of severity for the second slipped capital femoral epiphysis^3,4,8,20-24. Late contralateral slips were defined as mild (<30°), moderate (30° to 50°), or severe (>50°) on the basis of the lateral head-shaft angle as described by Southwick²⁰. The authors did not report additional data on complications associated with these slips.

The probabilities of complications after prophylactic in situ pinning and the grade of the contralateral slip (mild, moderate, or severe) were determined by a systematic review of the literature regarding slipped capital femoral epiphysis identified through Medline, Science Citation Index, textbooks, bibliographies, and discussions with colleagues. Our search strategy involved the following terms: "slipped capital femoral epiphysis (MeSH)" (search 1); "slipped upper femoral epiphysis (MeSH)" (search 2); "slipped capital femoral" (tw) (search 3); search 1 or 2 or 3 (search 4); limit human (search 5). The abstracts for all articles were reviewed by two authors. Articles with any possibility for inclusion were retrieved and reviewed by two authors. Explicit exclusion criteria included pinning with manipulative reduction or open reduction. A review of 239 articles yielded ten that reported on complications following prophylactic in situ pinning^1,2,25-32. Complications were defined as osteonecrosis, chondrolysis, superficial infection, deep infection, subtrochanteric fracture, hypertrophic scar, lateral irritation, loss of fixation of the capital femoral epiphysis, and a pin complication requiring a reoperation (not including reoperations following those complications already noted) (see Appendix). Data were also extracted and summarized from twenty articles that contained data regarding the probability of osteonecrosis and chondrolysis after mild, moderate, or severe slipped capital femoral epiphysis treated by pinning without manipulative reduction^25,29,33-50 (see Appendix). Additional data that addressed the severity of the slip and the probability of pin complications with treatment were also incorporated^51,52. It was assumed that the probability of infection, subtrochanteric fracture, hypertrophic scar, lateral irritation, and loss of fixation remained the same across all surgically treated hips, regardless of the severity of the slip.

Outcome Utilities

Utilities represent patient preferences for various disease states. Outcome utilities were obtained with a questionnaire that was answered with use of a visual analog scale by twenty-five adolescent boys between twelve and fourteen years old who were without a history of hip injury and were seen for an unrelated problem (a fracture, ankle sprain, scoliosis, knee ligament injury, or flatfeet) at a major children's hospital orthopaedic clinic. The questionnaire was also completed by the families of these patients. The questionnaire posed scenarios for the different outcomes and asked the subjects to rate these outcomes on a scale of 0 to 10 points, with 0 representing the worst possible medical outcome and 10 representing the best possible medical outcome. Additional questions or clarifications about the disease state were answered by the investigators. The scale was anchored at 0 by the outcome of observation followed by a severe late contralateral slip and the complication of osteonecrosis, and it was anchored at 10 by the outcome of observation without contralateral slip.

Fold-Back Analysis

A decision tree was constructed with one decision branch (decision node), ten chance branches (chance nodes), and forty-one outcomes (terminal nodes) (Fig. 1). Per convention, outcome utilities were placed to the right of the terminal nodes and outcome probabilities were placed under the terminal nodes. Fold-back analysis was performed by determining the weighted average of the outcome probabilities multiplied by the outcome utilities to determine the overall expected value of prophylactic in situ pinning and observation (on a scale of 0 to 10). The expected values were then compared to determine the optimal decision.

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Fig. 1: The decision tree for observation versus prophylactic in situ pinning of the contralateral hip in slipped capital femoral epiphysis. Decision nodes are represented by a square, chance nodes are represented by a circle, and terminal nodes are represented by a triangle. Mean outcome utilities (0 to 10) are listed to the right of the terminal node. Outcome probabilities (0 to 1) are listed under the terminal node title. AVN = osteonecrosis.

Sensitivity Analysis

One-way sensitivity analysis was performed to model the effect on decision-making of varying the probability of a late second slip and the utility of prophylactic in situ pinning without complication.

Two-way sensitivity analysis was performed to model the effect on decision-making of simultaneously varying both the probability of a late second slip and the utility of prophylactic in situ pinning without complication.

Results

Materials and Methods | Results | Discussion | Appendix | References

Outcome Probabilities and Utilities

The outcome probabilities for prophylactic in situ pinning of the contralateral hip are listed in tables in the Appendix. For prophylactic in situ pinning of the contralateral hip, the probability of osteonecrosis was 0.2% and the probability of chondrolysis was negligible. For observation, the probability of contralateral slipped capital femoral epiphysis was 19%.

The outcome utilities for prophylactic in situ pinning of the contralateral hip are listed in tables in the Appendix. For prophylactic in situ pinning of the contralateral hip, the utility of osteonecrosis was 0.9, the utility of chondrolysis was 1.1, and the utility of uncomplicated pinning was 9.2.

Decision Analysis

Fold-back analysis identified observation as the optimal decision strategy (the expected value for observation was 9.5, the expected value for prophylactic in situ pinning was 9.2, and the marginal value was 0.3) (Fig. 1).

Sensitivity Analysis

One-way sensitivity analysis was performed to model the effect of varying the probability of a late second slip (Fig. 2) and the utility of prophylactic in situ pinning without complication (Fig. 3). Prophylactic in situ pinning was favored when the probability of a late second slip was >27% (Fig. 2). Prophylactic in situ pinning was favored when the utility of prophylactic in situ pinning without complication was >9.8 (Fig. 3).

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Fig. 2: One-way sensitivity analysis for the probability of contralateral slipped capital femoral epiphysis. The probability of contralateral slipped capital femoral epiphysis is varied on the x axis. The lines represent the expected value (EV) for the decision to manage with observation and the decision to use prophylactic in situ pinning. Above the threshold value (a probability of contralateral slipped capital femoral epiphysis of 27%), prophylactic in situ pinning is favored.

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Fig. 3: One-way sensitivity analysis for the utility of uncomplicated prophylactic in situ pinning. The utility of uncomplicated prophylactic in situ pinning is varied on the x axis. The lines represent the expected value (EV) for the decision to manage with observation and the decision to perform prophylactic in situ pinning. Above the threshold value (a utility of uncomplicated prophylactic in situ pinning of 9.8), prophylactic in situ pinning is favored.

Two-way sensitivity analysis was performed to model the effect of simultaneously varying the probability of a late second slip and the utility of prophylactic in situ pinning without complication (Fig. 4). The preferred decision path for the range of values of both the probability of contralateral slip and the utility of uncomplicated prophylactic in situ pinning is demonstrated graphically by the shaded areas.

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Fig. 4: Two-way sensitivity analysis for the probability of contralateral slipped capital femoral epiphysis (x axis) versus the utility of uncomplicated prophylactic in situ pinning (y axis). The labeled areas on the plot represent the preferred decision path for the range of values of both the probability of contralateral slip and the utility of uncomplicated prophylactic in situ pinning.

Discussion

Materials and Methods | Results | Discussion | Appendix | References

When diagnosed with unilateral slipped capital femoral epiphysis, the patient and the clinician must decide between prophylactic in situ pinning and observation for the contralateral normal hip. Optimal management of the contralateral hip after unilateral slipped capital femoral epiphysis is controversial^{1-3,5-7,9,21,23,31,32,53}. The principal advantage of observation is the generally good prognosis, while the principal advantage of prophylactic in situ pinning is the avoidance of the late contralateral slip with associated morbidity^1-4,7-9,54. The principal disadvantage of observation is the possibility of contralateral slipped capital femoral epiphysis with its subsequent risk of osteoarthritis, while the principal disadvantage of prophylactic in situ pinning is the exposure to an additional surgical procedure with potential complications such as osteonecrosis and chondrolysis^1-4,7-9,54.

This decision problem lends itself well to expected-value decision analysis because of well-defined treatment options and relatively discrete outcomes. In this decision analysis, we found that observation was the optimal decision path, given the outcome probabilities and utilities that we studied. This is in accordance with the vast majority of decision analyses and cost-effectiveness analyses in which a prophylactic treatment is considered^10-19. Unless the probability of the adverse event is relatively likely or the consequences of the adverse event are severe (i.e., life-threatening), the iatrogenic risks of treating a healthy patient or uninvolved body part rarely outweigh the potential benefits. Furthermore, prophylactic procedures must balance the ethical principle of benevolence through preventing an adverse event with the ethical principle of avoiding harm through iatrogenic injury.

Although observation was the preferred strategy, the difference in expected value between observation and prophylactic in situ pinning was relatively small (9.5 compared with 9.3). In the sensitivity analysis, we found that prophylactic in situ pinning was favored when the probability of contralateral slipped capital femoral epiphysis was >27%. In a systematic review of the literature, the probability of contralateral slipped capital femoral epiphysis that we used was 19%⁷. If the risk of contralateral slipped capital femoral epiphysis is believed to exceed 27%, then rational decision-making would favor prophylactic in situ pinning. In certain scenarios, such as slipped capital femoral epiphysis associated with endocrinopathies or renal failure, the probability of contralateral slipped capital femoral epiphysis likely exceeds 27%, and contralateral pinning would be the optimal decision path. In addition, some have suggested that the probability of a late "silent" slip exceeds 27%. If this is so, then prophylactic in situ pinning would also be favored. Also, in certain instances where close observation is not feasible because of unreliable follow-up, prophylactic in situ pinning may be the favored choice. In the sensitivity analysis, we also found that prophylactic in situ pinning was favored when the utility of uncomplicated prophylaxis was >9.8. Thus, patients who are risk takers may favor prophylactic in situ pinning, whereas patients who are risk averse may favor observation.

Schultz et al. described a decision analysis approach to the decision problem of prophylactic pinning compared with observation for the management of the contralateral hip in patients with slipped capital femoral epiphysis⁵³. They obtained outcome probabilities from a systematic review of the literature and obtained outcome utilities by using Iowa hip scores from the long-term outcome study of slipped capital femoral epiphysis by Carney et al.⁵⁴. Unlike this decision analysis and the vast majority of decision analyses that have found prophylactic treatment favored only in scenarios where the adverse event is very likely and has severe, life-threatening consequences, Schultz et al. found that prophylactic in situ pinning was the optimal decision path⁵³. However, a methodological issue with their decision analysis study was that the Iowa hip score was used for utilities, not for patient preferences. In formal decision analysis, utilities reflect patient preferences and are obtained from patients or potential patients with use of techniques of time trade-off, standard reference gambles, or visual analog scales^10-19. The use of an outcome instrument in lieu of patient preferences is not conventional^10-19. This is of paramount importance, since the goal of decision analysis is to quantify decision-making from the patients' perspective. Nevertheless, their use of a functional outcome measure is innovative and may represent decision-making from the perspective of hip function. In addition, Schultz et al. presented a very simplified decision model in which there was one decision node, six chance nodes, and ten terminal outcome nodes. The outcomes of superficial infection, deep infection, fracture, hypertrophic scar, lateral irritation, loss of fixation, and pin complication requiring reoperation were not considered. In our decision analysis, these outcomes are considered and there is one decision node, ten chance nodes, and forty-one terminal outcome nodes.

Despite these methodological differences, the major reason for the disparate conclusions between the decision analysis described by Schultz et al.⁵³ and our decision analysis is likely the estimated probability of a late contralateral slip in patients managed with observation. In the present study, we used a 19% estimated probability of contralateral slip based on a quantitative synthesis by Castro et al. from forty-five eligible studies of 325 studies that were reviewed⁷. Schultz et al. used a 14% estimated probability of contralateral slip based on a series from Hagglund³. However, for the 86% of patients without a contralateral slip during adolescence, Schultz et al. asserted that they had a 51% chance of a late "silent" slip based on the deformity demonstrated in the lateral radiographs made after long-term follow-up in the series by Hagglund³. Further understanding of the etiology of posterior displacement deformity of the femoral head, pistol-grip deformity, and femoral-acetabular impingement as it relates to slipped capital femoral epiphysis is essential in order to appreciate the potential sequelae of slipped capital femoral epiphysis. If these various deformities are a direct result of subclinical slipped capital femoral epiphysis, then prophylactic pinning is likely the rational approach because of the disability and risk of arthritis associated with these deformities. A prospective cohort study with routine radiographic surveillance until adulthood and then beyond would be necessary to establish the probability of a "silent" second slip during adolescence and the subsequent development of slip-related deformity.

The limitations of decision analyses involve the methods from which probabilities and utilities were obtained. In general, the most accurate, stable, and robust estimates of outcome probabilities are derived from a meta-analytic synthesis of randomized clinical trials, the highest level of clinical evidence. In this analysis, we obtained outcome probabilities from a quantitative review of the literature. Because of the lack of randomized clinical trials in slipped capital femoral epiphysis, we included observational studies. A truly comprehensive search strategy was not employed. Probabilities regarding complications from prophylactic in situ pinning included some series with multiple pin fixation, and thus they may not be reflective of complications from modern techniques with a single pin. The probabilities regarding osteonecrosis and chondrolysis according to the severity of the contralateral slip were obtained from heterogeneous series including unilateral and contralateral slips, acute and chronic slips, and stable and unstable slips^25,29,33-50. Although the contralateral slip may be acute or chronic, and stable or unstable, the actual prevalence of these categories for contralateral slips may vary from the distribution in these series, thus introducing bias.

The determination of outcome utilities also has limitations. Utility is a subjective worth that a patient places on the specific outcome. This can be very difficult to operationalize and to quantify. In general, the most robust and valid estimates of patient-derived utilities are derived from complex qualitative methods, such as the standard reference gamble or time trade-offs, in which patients are asked to gamble or choose between health states usually referenced to death. In this study, we determined patient-derived utility values from a direct scaling method using a visual analog scale because of the complexity of the standard reference gamble and the difficulty of referencing to death for slipped capital femoral epiphysis. Although the use of a visual analog scale for the determination of utility values is less rigorous than the standard reference gamble, this technique has been corroborated methodologically¹⁷. We obtained utility values from a questionnaire given to adolescent male patients and their families without a history of slipped capital femoral epiphysis because the study population for utility assessment in decision analysis should be comprised of individuals who may potentially face the decision being studied. By convention, patients who have already sustained the clinical problem and have undergone the decision process are usually not studied because their assessment of probability and utility for the various outcomes may have been altered by their experience. A decision analysis based on utility values from patients who have had treatment for slipped capital femoral epiphysis may yield differing conclusions. Thus, our results are generalizable to this population, and they may not be applicable to patients with different demographic patterns. Female patients were not questioned because of the lower prevalence of slipped capital femoral epiphysis in females; however, this may have introduced some bias to the utility values since slips do occur in females. Although specific scenarios were posed and individual questions regarding these scenarios were answered, and although the disease states were anchored at 0 for observation followed by a severe late contralateral slip with the complication of osteonecrosis and at 10 for observation without a contralateral slip, the individuals completing the questionnaires may not have had deep insight into the subtleties and implications of the various disease states. Regardless of how outcome probabilities and utilities are determined, they are nevertheless considered point estimates in decision analysis and sensitivity analyses are therefore performed to assess how decision-making changes over a range of these values. Finally, we attempted to determine probabilities and utilities for discrete outcome events; however, certain outcomes, such as osteoarthritis, represent a spectrum of involvement and disability.

In conclusion, the results of this study may help us to better understand the decision between prophylactic in situ pinning and observation that is faced by a patient with a unilateral slipped capital femoral epiphysis. The overall optimal decision path is observation. When the probability of contralateral slip exceeds 27% or when reliable follow-up is not feasible, pinning of the contralateral hip is favored. For a given individual patient, the optimal strategy depends not only on the probabilities of the various outcomes but also on personal preference. Thus, there may not be one right answer for all patients. Issues such as short-term compared with long-term complications and individualized probabilities of late contralateral slip should be discussed. Risk-averse patients with a low utility even for uncomplicated prophylactic in situ pinning will likely favor observation, while risk-taking patients who are particularly mindful of the possibility of bilateral involvement and long-term degenerative joint disease may opt for prophylactic in situ pinning. We therefore advocate a model of shared decision-making in which the physician and the patient are jointly involved in the medical decision-making process, considering both outcome probabilities and patient preferences.

Appendix

Materials and Methods | Results | Discussion | Appendix | References

Tables presenting the articles used in the systematic review, and listing outcome probabilities and utilities for both pinning in situ and observation, are available with the electronic versions of this article, on our web site at jbjs.org (go to the article citation and click on "Supplementary Material") and on our quarterly CD-ROM (call our subscription department, at 781-449-9780, to order the CD-ROM). ?

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Ward WT, Stefko J, Wood KB, Stanitski CL. Fixation with a single screw for slipped capital femoral epiphysis. J Bone Joint Surg Am.1992;74: 799-809. Erratum in: J Bone Joint Surg Am. 1993;75:1255-6.74799 1992 [PubMed]

Greenough CG, Bromage JD, Jackson AM. Pinning of the slipped upper femoral epiphysis—a trouble-free procedure? J Pediatr Orthop.1985;5: 657-60.5657 1985 [PubMed][CrossRef]

Lynch GJ, Stevens DB. Slipped capital femoral epiphysis. Treatment by pinning in situ. Clin Orthop.1987;221: 260-6.221260 1987

Schultz WR, Weinstein JN, Weinstein SL, Smith BG. Prophylactic pinning of the contralateral hip in slipped capital femoral epiphysis: evaluation of longterm outcome for the contralateral hip with use of decision analysis. J Bone Joint Surg Am.2002;84: 1305-14.841305 2002

Carney BT, Weinstein SL, Noble J. Long-term follow-up of slipped capital femoral epiphysis. J Bone Joint Surg Am.1991;73: 667-74.73667 1991

Topics

epiphyses, slipped ; hip region ; hip joint ; decision analysis ; sensitivity analysis

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Robert Poss, MD

Posted on April 26, 2005

Editor's Note

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The authors have been invited to respond to the letter by Dr Schultz et al, but to date, have not done so.

W. Randall Schultz

Posted on April 11, 2005

Prophylactic Pinning of the Contralateral Hip in SCFE

Private Prace

To the Editor:

We would like to thank Dr. Kocher for his fine work(1) in response to our decision analysis on prophylactic pinning of slipped capital femoral epiphysis(2). This is an important and controversial topic that warrants thoughtful, scientific debate prior to adopting generalized standards of care.

Although it may appear that the conclusions of our studies are vastly different, the reality is the treatment options re: observation vs. prophylactic pinning are very nearly equivalent in statistical probability of outcome. In fact this is, in part, why decision modeling is such a powerful tool which allows one to create models for predicting outcome based on multiple probabilities that we otherwise could not compute.

The major differences in our studies relate to different probability data used in the two studies. The most obvious of these is the probability of late contralateral slip. This is a controversial subject and some authors feel that it is a leading cause of degenerative arthritis of the hip in older patients(3). Hagglundâ€™s data suggest that the rate of unrecognized mild slips is very high(4,5). Anthropometric skeletal studies confirm this finding(3), but it is difficult to say with certainty if the anatomic changes seen are the result of a primary slip or secondary changes of degenerative arthritis.

We also have concerns about the probabilities of complications. Most of these data were taken from isolated case reports. It is not clear from your paper (1) where the denominator comes from to arrive at these numbers and provide probabilities. In the Greenough paper(6) which was cited as one source of probability data the complications were seen with pins that are no longer in widespread clinical use today or viewed as standard of care(7). In addition, recent studies suggest that avascular necrosis is a phenomenon almost exclusive to unstable slips and not an iatrogenic phenomenon(8). With the advent of computer assisted technology the probabilities for iatrogenic complications related to hardware placement may even be further reduced(9).

Dr. Kocher is absolutely correct in his explanation of decision analysis. We chose to perform what is more appropriately termed an â€œexpected value analysisâ€ for several reasons. First, utility analysis is a challenging and difficult task in the adult population and virtually impossible in the pediatric population. A true utility is not obtained from a visual analogue scale because there is no relative quantification of the value of a particular health state. Standard gamble and time trade off methods are required to obtain true utility values. Both methods require abstract thought, usually beyond the facility of the subjects studied in this model.

We chose, instead, to assume the perspective of a person at middle age that would have experienced the long-term outcomes of a slipped epiphysis. We believe this is essential to the reasoning behind performing these operations in children - for long-term gain. To bring short-term complications into the model distorts the results of the model. Many of these short-term complications are reversible and do not result in long-term disability.

If short-term complications are to be considered then one must perform Markhov modeling(10) within the decision analysis. This allows time in a particular health state to be considered. One cannot compare utilities for temporary health states such as superficial infections or even fracture with utilities for long-term health states such as degenerative arthritis of the hip. In a perfect model we would be able to calculate the utility associated with short-term and long-term disabilities and the different treatment options would accumulate utility based on time spent in that particular health state â€“ essentially Quality Adjusted Life Years(QALYâ€™s).

This type of model is achievable and may provide us with the most accurate assessment of the benefits associated with each treatment option but the major obstacle will remain the assessment of utility in the pediatric population using traditional utility assessment tools (time trade off or standard gamble).

It is still our opinion that long-term disability remains the overriding concern when treating children with Slipped Capitol Femoral Epiphysis and we stand by our assertion that prophylactic pinning provides superior probability of better hip function over the long term.

Sincerely,

W. Randall Schultz MD, MS

James N. Weinstein, DO, MS

Stuart L. Weinstein, MD

Brian G. Smith, MD

References:

1. Kocher, M. S.; Bishop, J. A.; Hresko, M. T.; Millis, M. B.; Kim, Y. J.; and Kasser, J. R.: Prophylactic pinning of the contralateral hip after unilateral slipped capital femoral epiphysis. J Bone Joint Surg Am, 86-A(12): 2658-65, 2004.

2. Schultz, W. R.; Weinstein, J. N.; Weinstein, S. L.; and Smith, B. G.: Prophylactic pinning of the contralateral hip in slipped capital femoral epiphysis: evaluation of long-term outcome for the contralateral hip with use of decision analysis. J Bone Joint Surg Am, 84-A(8): 1305-14, 2002.

3. Goodman, D. A.; Feighan, J. E.; Smith, A. D.; Latimer, B.; Buly, R. L.; and Cooperman, D. R.: Subclinical slipped capital femoral epiphysis. Relationship to osteoarthrosis of the hip. J Bone Joint Surg Am, 79(10): 1489-97., 1997.

4. Hagglund, G.; Hannson, L. I.; and Sandstrom, S.: Slipped capital femoral epiphysis in southern Sweden. Long-term results after nailing/pinning. Clin Orthop, (217): 190-200., 1987.

5. Hagglund, G.; Hansson, L. I.; Ordeberg, G.; and Sandstrom, S.: Bilaterality in slipped upper femoral epiphysis. J Bone Joint Surg Br, 70(2): 179-81., 1988.

6. Greenough, C. G.; Bromage, J. D.; and Jackson, A. M.: Pinning of the slipped upper femoral epiphysis--a trouble-free procedure? J Pediatr Orthop, 5(6): 657-60., 1985.

7. Kenny, P.; Higgins, T.; Sedhom, M.; Dowling, F.; Moore, D. P.; and Fogarty, E. E.: Slipped upper femoral epiphysis. A retrospective, clinical and radiological study of fixation with a single screw. J Pediatr Orthop B, 12(2): 97-9, 2003.

8. Tokmakova, K. P.; Stanton, R. P.; and Mason, D. E.: Factors influencing the development of osteonecrosis in patients treated for slipped capital femoral epiphysis. J Bone Joint Surg Am, 85-A(5): 798-801, 2003.

9. Perlick, L.; Tingart, M.; Wiech, O.; Beckmann, J.; and Bathis, H.: Computer-assisted cannulated screw fixation for slipped capital femoral epiphysis. J Pediatr Orthop, 25(2): 167-70, 2005.

10. Weinstein, M. C.: Clinical decision analysis. Edited, xiv, 351 p., Philadelphia, Saunders, 1980.

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Mininder S. Kocher, Julius A. Bishop, M. Timothy Hresko, Michael B. Millis, Young-Jo Kim, James R. Kasser; Prophylactic Pinning of the Contralateral Hip After Unilateral Slipped Capital Femoral Epiphysis. The Journal of Bone & Joint Surgery. 2004 Dec;86(12):2658-2665.

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