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Strabismus surgery among Medicare beneficiaries: imputed rates of reoperation in the same calendar year
Digital Journal of Ophthalmology 2016
Volume 22, Number 1
March 15, 2016
DOI: 10.5693/djo.01.2016.02.001
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Christopher T. Leffler, MD, MPH | Department of Ophthalmology, VCU Medical Center, Virginia Commonwealth University, Richmond, Virginia
Allison Pariyadath, MD | Department of Ophthalmology, VCU Medical Center, Virginia Commonwealth University, Richmond, Virginia

Abstract
Purpose
To compare strabismus surgery reoperation rates in a large national database of provider payments when the adjustable-suture technique was available and not available.

Materials and Methods
Fee-for-service payments to Medicare providers for horizontal (CPT 67311) and vertical (CPT 67314) strabismus surgery in 2012 were analyzed to identify payments for reoperations in the same calendar year. The adjustable-suture technique was considered to be available to the patient if the patient’s surgeon billed for adjustable sutures during the year. We determined the association of reoperation with the availability of the adjustable-suture technique and with surgeon volume.

Results
Patients having horizontal muscle surgery had a rate of reoperation in 2012 of 4.1% (15 of 364 patients) when the adjustable technique was available, compared with 7.1% (77 of 1,082 patients) when the adjustable technique was not available (P = 0.047). Patients having vertical muscle surgery had a rate of reoperation in 2012 of 4.1% (8 of 196 patients) when the adjustable technique was available, compared with 8.3% (38 of 458 patients) when the adjustable technique was not available (P = 0.07). Having surgery in a high-volume surgical practice was not reliably associated with reoperation rates.

Conclusions
For patients having strabismus surgery, the availability of the adjustable-suture technique was associated with a lower reoperation rate in this large national database (compared with patients for whom the adjustable technique was not available). The difference was statistically significantly different from zero for horizontal muscle surgery but not for vertical muscle surgery.
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Introduction
Adjustable sutures can be used in strabismus surgery to permit refinement of ocular alignment in the immediate postoperative period. To our knowledge, only one small randomized controlled trial of adjustable versus conventional sutures has been performed.(1) This trial, in 45 patients, favored adjustable sutures.(1) In the absence of large randomized studies, reviewers have cited retrospective case series, which typically suggest better outcomes with adjustable sutures.(2) However, the small number of surgeons involved makes it uncertain whether the results can be generalized. Some single-center case series have no control group.(3,4) One study evaluated the results of strabismus surgery as a single surgeon switched from conventional to adjustable surgery.(5) Zhang et al studied two surgeons who frequently used adjustable sutures and one surgeon who did not.(6)

In order to evaluate the success of adjustable and conventional strabismus surgery with a national database, we evaluated reoperations for strabismus surgery in the database of Medicare payments to providers for 2012.(7)
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Materials and Methods
This study was approved by the VCU Office of Research Subjects Protection. We downloaded the database of Medicare payments for 2012.(7) This database is not a random sample. Rather, the database includes payment data for every practitioner in the country who received Medicare fee-for-service payments for a procedure at least 11 times in 2012. Each CPT code had to be paid to the provider for at least 11 beneficiaries for that particular CPT to be listed in the database under that provider. The database does not include Medicare Advantage Plan payments.

We evaluated rates of reimbursed reoperations in patients having one horizontal muscle strabismus surgery (CPT 67311), one vertical muscle surgery (CPT 67314), adjustable suture (CPT 67335), and surgery with scarring of extraocular muscles (eg, prior ocular injury, strabismus, or retinal detachment surgery) or restrictive myopathy (eg, dysthyroid ophthalmopathy; CPT 67332). These CPT codes were selected because they are the most commonly coded strabismus procedures. Other strabismus surgery codes were not used frequently enough to draw any meaningful conclusions.

The reoperation rate was determined from the numbers of beneficiaries and beneficiary service-days. For instance, if a given provider treated 12 beneficiaries with a particular CPT code in 2012 but there were 13 beneficiary service-days for this code, then 1 of the 12 beneficiaries had a reoperation. In other words, the reoperation rate is imputed, defined as “assigned a value by inference from the values to which it contributes.” The unit of analysis was the patient (beneficiary). If the surgeon received any payments for CPT 67335, then these 12 patients were counted in the group for whom the adjustable-suture technique was available. Otherwise, the 12 patients were counted in the group for whom the adjustable-suture technique was not available.

We compared the likelihood of reoperation in patients having strabismus surgery when the adjustable technique was available with patients having surgery when the adjustable technique was not available.

Having surgery performed by a high-volume provider (who performed the procedure on at least 20 beneficiaries) was compared with having surgery from a lower-volume provider (11-19 beneficiaries). We excluded data from retinal oncologists, who might have been coding for muscle surgeries when detaching muscles to place radiotherapy plaques. Proportions were compared by the Fisher exact test, and the odds ratios with Wald’s 95% confidence intervals for reoperation with adjustable sutures are presented.
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Results
Patient Reoperation Rate
Overall, 6.3% of patients having horizontal muscle surgery, 7.0% of patients having vertical muscle surgery, 8.1% of patients having surgery for restrictive myopathy, and 4.6% of patients having adjustable surgery (28 of 613 patients) required reoperation in the same calendar year.

Patients having horizontal muscle surgery had a rate of reoperation in 2012 of 4.1% (15 of 364 patients) when the adjustable-suture technique was available, compared with a rate of 7.1% (77 of 1082 patients) when the adjustable-suture technique was not available (P = 0.047, Table 1, Figure 1).

Patients having vertical muscle surgery had a rate of reoperation in 2012 of 4.1% (8 of 196 patients) when the adjustable-suture technique was available, compared with a rate of 8.3% (38 of 458 patients) when the adjustable-suture technique was not available (P = 0.07, Table 1, Figure 1).

To further understand the results, we segregated the results according to whether the surgery involved scarring or restrictive strabismus (CPT 67332). For certain practices, the database did not record any payments for scarring or restrictive strabismus. Patients in such practices having horizontal muscle surgery had a rate of reoperation in 2012 of 3.2% when the adjustable technique was available, compared with a rate of 7.5% when the adjustable technique was not available (P = 0.01, Table 2, Figure 2).

Patients in such practices (without CPT 67332 payments) having vertical muscle surgery had a rate of reoperation in 2012 of 3.3% when the adjustable technique was available, compared with a rate of 7.2% when the adjustable technique was not available (P = 0.16, Table 2, Figure 2).

Patients having surgery for scarring or restrictive strabismus (CPT 67332) had a reoperation rate of 6.3% when the adjustable technique was available, compared with a rate of 9.4% when the adjustable technique was not available (P = 0.32, Table 1, Figure 2).

Patients having adjustable-suture surgery (CPT 67335) had a rate of reoperation rate in 2012 of 3.6% (15 of 411 patients) in practices that did not bill for scarring/restrictive strabismus, compared with a rate of 6.4% (13 of 202 patients) in practices that did bill for scarring/restrictive strabismus (P = 0.15, Figure 2).

Having surgery in a high-volume practice was not associated with a lower reoperation rate. For horizontal muscle surgery, the rate of reoperation in 2012 was 7.3% (58 of 793 patients) in a high-volume environment compared with 5.2% in a lower-volume environment (34 of 653 patients, P = 0.11). For vertical muscle surgery, the rate was 8.3% (28 of 337 patients) in a high-volume environment, compared with 5.7% (18 of 317 patients, P = 0.22) in a lower-volume environment.

Provider Suture Preference
In the preceding analysis of patient reoperation rate, the unit of analysis was the patient. In the following analysis of provider suture preference, the unit of analysis is the provider (surgeon). There was a clear divide in suture preference between practices. For instance, the majority of strabismus surgeons performing horizontal muscle surgery (CPT 67311) used only conventional sutures (58 of 78 surgeons, 74%). The data do not permit exact calculation of the frequency of adjustable-suture surgery among those who did use the technique. Of the 20 horizontal muscle surgeons who used adjustable sutures, 16 surgeons (80%) coded for adjustable sutures (CPT 67335) in as many beneficiaries as they coded one horizontal muscle surgery (CPT 67311). Because the latter is generally the most frequently coded strabismus procedure, those who do use adjustable sutures seem to employ the technique very often. A similar divide in suture preference was seen for vertical muscle surgery (CPT 67314).

Of the 20 adjustable horizontal muscle surgeons, 6 surgeons (30%) also coded for scarring or restrictive strabismus (CPT 67332). Of the 58 conventional horizontal muscle surgeons, 10 surgeons (17%, P = 0.33) also coded for scarring or restrictive strabismus (CPT 67332). Thus, these data do not provide evidence that the adjustable surgeons had a less challenging case load.
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Figure 1
Imputed rate of reoperation in 2012 for adjustable and nonadjustable horizontal and vertical muscle surgery among Medicare beneficiaries.

Figure 2
Imputed rate of reoperation in 2012. A, adjustable; H, horizontal; R, restrictive/scarring; V, vertical. The key format is “Surgery type:practice type.” A preceding minus sign (?) indicates absence of a clinical feature. Groups 1-4: practice not paid for restrictive myopathy (:?R [Table 2]). Groups 5-6: surgery for restrictive myopathy (R: [Table 1]). Groups 7-8: adjustable surgery (A:).

Discussion
In patients having strabismus surgery, the availability of the adjustable-suture technique tended to be associated with a lower reoperation rate, although the association was not always significant. The strength of the study was the inclusion of a large number of patients from a national database.

In order to code at least 11 horizontal or vertical strabismus surgeries in a given year from just one payor, all surgeons contributing patients to the dataset had to have substantial practices. Nonetheless, one might assume that the patients having surgery with the highest-volume surgeons (>20 beneficiaries) might have lower reoperation rates. No such association was seen.

The study does not address reoperation rates in the lowest-volume settings (1-10 surgeries annually). Conceivably, reoperation rates might be higher with less experienced surgeons. If a certain surgical volume threshold is required to attain proficiency with adjustable sutures, it is possible that patients having adjustable sutures in extremely low-volume environments might experience particularly high reoperation rates. However, these suggestions are speculative, and the dataset simply does not address the question.

Our study was similar to others that have looked at the availability of a medical resource on patient outcomes. For instance, studies have evaluated the association of patient mortality with the availability of paramedics at the scene of cardiac arrest,(8) the availability of external defibrillators with police responders,9 the availability of helicopter evacuation in wartime,(10-11) and the availability of increased perioperative invasive hemodynamic monitoring and high-risk postoperative surveillance for arthroplasty patients.(12) For each of these studies, the unit of analysis was the patient.(8-12) The calculation was the number of patients with an unfavorable outcome (death) divided by the total number of patients.(8-12) Moreover, the exposure was the general availability of the medical resource rather than its use for a specific patient.(8-12) The studies did not evaluate which patients were evacuated by helicopter,(10-11) had the defibrillator placed on their chest or activated,9 received invasive monitoring,(12) or had specific interventions performed by a paramedic.(8)

Similarly, in our study of strabismus surgery outcomes, the unit of analysis is the patient. We determined the number of patients with an unfavorable outcome (reoperation) divided by the total number of patients. The exposure variable was the availability of the adjustable-suture technique, regardless of whether the technique was applied in each specific patient’s case.

There are some advantages to this type of approach. Consider the previously mentioned mortality studies.(8-12) It is easy to accept that the availability of paramedics,(8) external defibrillators,(9) helicopter evacuation,(10,11) and high-risk postoperative surveillance(12) could decrease patient mortality, compared with the rate for patients for whom these resources are not available. On the other hand, it is also easy to imagine that the specific patients who have interventions performed by paramedics, are shocked by the defibrillator, are evacuated by helicopter, or are assigned to high-risk postoperative surveillance might have a higher mortality rate than comparison patients who receive none of these interventions. The reason is that patients at higher risk are more likely to receive these interventions. Therefore, looking at the availability of the medical resource, rather than its actual use in specific patients, might give a better idea of the effects of the medical resource on outcomes.

With respect to strabismus, an alternate study design, which associates outcomes with the suture technique used for specific patients, might reveal spurious associations related not to the effect of the suture but rather to baseline differences in patient risk. When the adjustable-suture technique is available, it is used in most cases, but not in all cases. Patient factors undoubtedly influence the decision to place, or not to place, an adjustable suture. For instance, patients requiring only oblique muscle surgery typically do not have an adjustable suture used. Perceived ability of the patient to tolerate adjustment, thyroid disease, patient age, or history of previous surgeries might influence the decision. In one study, adjustable sutures were used more frequently in patients with prior surgery or restrictive or paralytic strabismus.(13) Such patient-specific data was not available in the present dataset.

Let us denote the patients in whom adjustable sutures are typically used group A, and the remaining patients group B. Comparison of all patients receiving adjustable sutures with those receiving conventional sutures compares: (1) group A patients receiving adjustable sutures with (2) group A patients for whom adjustable sutures were not available, plus all group B patients. This comparison neglects consideration of the patient factors that influenced the technique selected. These groups’ baseline clinical pictures are not the same. In other words, patients who receive adjustable sutures cannot be assumed to have the same baseline risk of reoperation as those who do not receive adjustable sutures. In our study, the primary analysis compared: (1) patient groups A and B when the adjustable technique was available, with: (2) patient groups A and B when the adjustable technique was not available. Therefore, we presumably compared similar groups of patients.

The database does not contain the specific clinical information for each case, such as patient comorbidities, preoperative alignment, or prior strabismus surgery. Advocates for conventional sutures might argue that the availability of the adjustable-suture technique was associated with lower reoperation rates because the patients in the adjustable-suture practices are less challenging. We have no reason to believe this to be true, and the proportion of adjustable surgeons’ practices that were paid for scarring or restrictive strabismus does not support the statement.

Over 80% of Medicare beneficiaries qualify on the basis of age over 65, whereas the remainder qualify on the basis of disability. Restriction to a homogeneous population strengthens the study, because patient groups in each practice were likely similar. On the other hand, it cannot be assumed that the results will apply to other age groups, especially children.

The relative difficulty of performing adjustable-suture surgery undoubtedly has influenced the fact that 74% of the strabismus surgeons in this study did not use them. Presumably, the lower-volume surgeons not reflected in this database might be even less likely to use adjustable sutures. One reviewer noted that “the learning curve for accurate decision making during suture adjustment may span a decade or more.”(2) The specific technical steps that can be challenging have been reviewed.(3)

The study was based on payments to providers for reoperations. Provider coding might be inaccurate or incomplete.(14) A reoperation must indicate that the physician and patient do not believe that the first surgery has attained the final treatment goals. (It is possible that a staged approach to surgery was planned from the start.) In fact, reoperation rates were suggested as a primary outcome measure in a recent review.(15) Reoperations for which payment was denied were not reflected. Some patients might consider their surgery unsuccessful, but would forgo reoperation, obtain reoperation in a subsequent year, or have reoperation performed by a different provider. Although many studies have used a particular cutoff for postoperative ocular alignment as a measure of success (eg, within 10 prism diopters [PD] for horizontal surgery), this metric is imperfect. Some patients with residual deviation of >10 PD are satisfied with the outcome, if the deviation is only occasional, does not involve diplopia, and is a major improvement over their preoperative state. Likewise, some patients with a small deviation are unsatisfied, especially if they experience diplopia. Ideally, a goal-directed analysis can be used to evaluate surgical success.(16)

As in previous studies, the follow-up time was limited and/or varied between patients. The range of follow-up times has been 7 days to 12 weeks,(6) 6-110 weeks,(4) 2 months,(3) 3 months,(5) and 3-48 months.(13) Some studies have simply excluded patients if the duration of follow-up was less than a specified period, such as 6 weeks4 or 3 months.(5,13) Another study selected charts “based on availability” and then excluded patients if there was “insufficient information in the chart.”(6)

A previous study noted similar success rates at 2 months and at the final follow-up time available.(4) Would the present study capture more or fewer reoperations than a study analyzing outcomes at 2 months? The proportions might be similar, because both studies would capture reoperations between 1 and 60 days for 10 months of the year (January-October). Only the study with 2-month follow-up would capture reoperations between 31 and 60 days for the month of December. Only the present study would capture reoperations between 61 and 90 days for 9 months of the year (January-September), reoperations between 91 and 120 days for 8 months of the year (January-August), and reoperations between 121 and 150 days for 7 months of the year (January-July). The important point, however, is not whether the present study would capture slightly more or slightly fewer reoperations than another study but whether all patient groups within the present study were treated identically. Because the patient groups having surgery when adjustable sutures were available and unavailable were treated identically, the present analysis is less likely to be biased.

The study cannot provide a definitive answer to the value of adjustable sutures, but we have illustrated an approach that could be extended to other insurance databases.(17,18) The data might help inform the design of future studies and trials by providing inputs for power calculations. As our study draws from a large national source, and all surgeries were evaluated by the same metric, the study provides an informative look at the association of the availability of the adjustable-suture technique with reoperation after strabismus surgery.

For patients having strabismus surgery, the availability of the adjustable-suture technique was associated with a lower reoperation rate in this large national database (compared with patients for whom the adjustable-suture technique was not available). The difference was statistically significantly different from zero for horizontal muscle surgery but not for vertical muscle surgery.
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Acknowledgements
No authors have any conflicts of interest related to this study.
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References
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Table 1
Imputed rates of reoperation in 2012 according to surgery and practice type

Table 2
Table 2. Imputed rates of reoperation in 2012 in practices not paid for scarring or restrictive strabismus (CPT 67332)