Maintenance of Technical Surgical Skills the Volume Outcome Relationship

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Perhaps the most widely quoted perspective for the role that specialist surgeons play in treating endocrine diseases stems from studies correlating surgical and hospital volume to patient outcomes. Low surgeon case-volume has been associated with higher complication rates in studies involving a vast array of specialties such as vascular, pediatric, colorectal, pancreatic, and endocrine surgery [18-24]. These studies suggest that surgeons who perform either a greater number of specific procedures or who dedicate a greater percentage of their overall practice to a specific procedure have better outcomes than surgeons whose practice is less active or focused. This association has held true with respect to mortality rates, complication rates, successful cure rates, length of hospital stay and healthcare charges in several studies, and endocrine surgery is no exception.

However, the studies that demonstrate this volume-outcome relationship have some limitations [22]. Although associations may be statistically significant, proving causality is much more difficult. For example, to prove a causal relationship, one must determine if high volume surgeons truly have better results or if they are simply drawing more referrals because they excel at those procedures [25]. In addition, despite efforts of case-mix adjustments in these studies, there may be selective referral of patients to high volume surgeons or centers. It is possible that the lower volume surgeons are actually performing more difficult procedures on sicker patients, but at this point there is no evidence to suggest this within endocrine surgery. The surgeon's volume-outcome association may be confounded by hospital volume, which may, in turn, be related to patient outcomes. This association between patient outcomes and hospital volume has been demonstrated in studies of pancreatico-duodenectomy, cholecystectomy, and coronary angioplasty [22,26-28], but has not been found for thyroid surgery. This may be due to the relatively young and otherwise healthy patient population that most commonly undergoes thyroid surgery; these patients are not heavily reliant on subspecialty care, complex equipment or monitoring [22]. Publication bias may also be an issue, because investigators in academic centers who conduct volume-outcome research may be more likely to publish results that support their role as referral centers.

Most volume-outcome studies that include results for community surgeons are based on large administrative databases that permit statistically valid estimates of associations between patient outcomes and surgeon volume [29]. While these databases offer powerful information, they have their limitations. They are often not set up to answer specific research questions. The information available may not include key variables that permit the most appropriate statistical assessment of study endpoints. An example is the extrapolation of treatment complications from hospital database "length of stay" information for patients who have had thyroid surgery. The main complications and endpoints in thyroid surgery studies are often confirmed after discharge - and after data collection has been completed. Complications such as recurrent or superior laryngeal nerve injury, persistent or recurrent hyperparathyroidism, permanent hypoparathyroidism, or wound infection may develop or be diagnosed after discharge, and therefore may not be accurately reflected in a discharge database. In addition, confounding issues, such as previous neck surgery or locally invasive malignancies, are significant risk factors for complications in thyroid operations that are often not available in databases not constructed specifically for analy sis of surgical outcome and complications. Other important patient information that allows accurate case-mix adjustment may not be optimal and is database dependent.

Two studies that used discharge databases examined the volume-outcome relationship in endocrine surgery. Chen et al. [30] used the Maryland inpatient discharge database to evaluate the state's experience with parathyroidec-tomy between 1990 and 1994. Their study confirmed the high cure rates and low morbidity and mortality rates associated with specialist surgeons that have been reported in previous studies. Database limitations did not permit the evaluation of cure rates or complication rates of the patients whose operations were performed outside of the endocrine center, but the analysis did show a significantly longer length of stay at these other hospitals (3.1 versus 1.3 days). The authors speculated that length of stay may be a proxy for operative complications, but although plausible, this is an assumption that has been both supported and criticized in the literature on surgical outcome. Sosa et al. used hospital discharge data from nonfederal acute care hospitals in Maryland to compare the results of endocrine surgeons with those of lower volume surgeons [22]. They found an association between high volume surgeons and decreased complication rates and shorter lengths of stay, which seemed strongest for the subgroups of patients who had more complex diagnoses or procedures, such as malignancies and total thy-roidectomies.

Although many studies suggest that increased surgical volume is associated with better clinical outcome, the concentration of endocrine surgery into the hands of specialist surgeons has not been widely adopted in the USA. When evaluating who was performing endocrine procedures (thyroid, parathyroid, and adrenal) in the USA, Saunders et al. evaluated surgeons based on the number of endocrine cases they performed and the percentage of each surgeon's practice that was endocrine in nature [7]. Surgeons whose endocrine experience comprised 25% or less of their practice performed 78% of all parathyroidectomies, 94% of all adrenalec-tomies, and 82% of all thyroidectomies (Figure 11.3) completed in the USA in the years 1988 through 2000 [7]. Surgeons for whom endocrine procedures comprised 75% or more of their practice performed only 5% of all parathy-

Figure 11.3 Contribution of individual surgeon quartiles to the total number of patients who underwent operation (calculated on percentage of practice calculation: quartile A: 0-25%; quartile B: 26-50%; quartile C: 51-75%; quartile D: 76-100%). (Reproduced from Saunders BD, Nainess RM, Dimick JB, et al. Who performs endocrine operations in the United States? Surgery. 2003 Dec; 134(6):928. Copyright 2003, with permission from Elsevier.)

Figure 11.3 Contribution of individual surgeon quartiles to the total number of patients who underwent operation (calculated on percentage of practice calculation: quartile A: 0-25%; quartile B: 26-50%; quartile C: 51-75%; quartile D: 76-100%). (Reproduced from Saunders BD, Nainess RM, Dimick JB, et al. Who performs endocrine operations in the United States? Surgery. 2003 Dec; 134(6):928. Copyright 2003, with permission from Elsevier.)

roidectomies, 3% of all adrenalectomies, and 3% of all thyroidectomies in this same time period. In a study of thyroidectomies, 78.6% of surgeons who perform these procedures did fewer than ten thyroid operations per year (Figure 11.4) [22].

There are, however, geographic regions in the USA where the bulk of endocrine surgeries are performed in specialist centers. In Maryland, between the years 1990 and 1994, an increasing percentage (8% to 21%) of the state's parathy-roidectomies were being performed by a single

Figure 11.4 Summary of the distribution of thyroid surgeons and cases by the four surgeon volume groups. (Reproduced with permission from Sosa et al. [22], page 323.)

endocrine surgeon [30]. Similarly, between 1991 and 1996, the highest volume thyroid surgeons in Maryland (those performing over 100 cases per year) noted an increased referral pattern as their share of thyroidectomies increased from 11.9% (1991-1993) to 17.6% (1994-1996) [22]. Overall, those surgeons performing 30 or more thyroidectomies completed 42.6% of the total thyroidectomies in the state, while comprising only 6.5% of the surgeons (Figure 11.4) [22].

Some studies do not support the "volume-outcome" relationship in endocrine surgery. These looked at smaller series, at trainees' experience, and at general surgeons in community practices [9,25,31-33]. Even studies that find an association between surgical volume and clinical outcome must be interpreted with caution. Sosa et al. showed there was no significant difference for rates of hypoparathyroidism between any of the surgeon volume categories, but did find an increased rate (1.5% versus 0.4%) of recurrent laryngeal nerve injury after thyroidectomy in the lowest volume group (fewer than 10 thyroidectomies per year) as compared to the highest volume group (over 100 thyroidectomies per year) [22]. There was, however, no significant difference in nerve injury rates between the mid-volume groups who performed between 10 and 100 thyroid operations per year and those who performed over 100 (0.5-0.8% versus 0.4%). Perhaps this is suggestive of a threshold annual number of cases where the volume-outcome relationship exists and above which the relationship weakens.

The results from Sosa et al. must be considered in light of the differences in the patient populations. Surgeons who operated on more than 100 patients with thyroid disease per year were performing more total thyroidectomies (versus lobectomy), treating more malignant (versus benign) conditions and had a significantly younger patient population than the lower volume surgeons [22].The finding that the highest volume group treated the more challenging and complex patient population may support one of two views: First, that the small difference in recurrent laryngeal nerve injury rates and the absence of difference in the hypoparathyroidism rates are biased towards the null, that is, that the difference in complication rates would be greater if the lower volume surgeons performed more complex cases.

Second, and conversely, that appropriate selection of thyroid cases based on the surgeon's training and experience (and appropriate referrals based on case complexity) enables safe endocrine surgery to be performed by all surgeons who maintain their skills with an adequate number of cases. Both are probably true to some extent, although more definitive conclusions cannot be made without clinical evidence.

The threshold at which one becomes a "high volume" surgeon is variable and to some extent, arbitrary. Some published thresholds for "high volume" surgeons reflect this variability: parathyroidectomy, over 50 cases per year [34], carotid endarterectomy, 30 or more cases per year [35], thyroid surgery, over 20 cases per year (P. Haigh, 2004, personal communication), colorectal procedures, over 10 per year [36]. This variability in case number suggests that it may be more than just pure annual volume of cases that determines surgical expertise and subsequent clinical outcome for patients. Clinical outcomes are a multifactorial entity with patient selection, appropriate choice of procedure, referral pattern, institutional impact, and surgical skill all part of this surgical outcome puzzle.

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