Sunday, April 24, 2016

Reverse shoulder arthroplasty results - the good and the bad of MCID

Causes of poor postoperative improvement after reverse total shoulder arthroplasty.

These authors analyzed 150 patients who underwent reverse total shoulder arthroplasty (RTSA) from 2007 to 2013. Patients with baseline and minimum 2-year postoperative American Shoulder and Elbow Surgeons (ASES) scores were included. Poor postoperative improvement was defined by the authors as a change in the ASES of less than 12 points using the standard deviation method (see this link). 20 patients had ASES score improvement < 12 points. 

Logistic regression revealed that male sex (adjusted odds ratio [OR], 7.9; P = .004), presence of an intact rotator cuff at the time of surgery (adjusted OR, 4.8; P = .025), depression (adjusted OR, 11.2; P = .005), a higher baseline ASES score (P < .001), and higher total number of medical comorbidities (P = .035) were associated with poor postoperative improvement after RTSA.

They concluded that evidence of better preoperative function, such as a higher baseline ASES score and intact rotator cuff at the time of surgery, correlated with poor postoperative improvement. In addition, male sex, depression, and total number of medical comorbidities also correlated with poor postoperative improvement. These results are similar to a prior study that used improvement of two in the Simple Shoulder Test as the metric for improvement (see this link).

Comment: This article again shows that the characteristics of the patient (sex, depression, comorbidities), as well as those of the shoulder (cuff integrity) have strong influences over the results of surgery.

When we talk about the results of surgery, we need to be careful to differential the outcome from the improvement (outcome-ingo). Consider the following hypothetical example of four patients having RTSA. The ingo is shown on the horizontal axis and the outcome on the vertical axis. The numbers to the left of the vertical lines indicate the difference between outcome and ingo.

The round patient and the square patient improved by the MCID used in this article and would be considered successes, but the diamond and triangle patients did not and would have been considered failures. Note however that the best outcomes were for the patients who did not improve by the MCID (the diamond and the triangle). Note also that better outcomes were observed for patients with higher ingos. We can only wonder which of the four patients would be most satisfied with the results of their surgery. This example shows the flaws in the use of MCID - (1) achieving the threshold value is likely to be less satisfactory if the ingo is low and (2) patients with a high ingo have a harder time achieving the MCID threshold for a success. In this study the baseline ASES scores were 53.7 ± 4.6 for the 'failure' group and  32.5 ± 1.6 for the 'non-failure' group. Interestingly, 68% of the "failure" group were satisfied with the outcome of surgery.

A complementary method for analyzing the results is to look at the amount of improvement (the outcome minus the ingo) divided by the maximal possible improvement (the maximum possible score  (100) minus the ingo).  Considered in this way, the round patient improved by 28% of the maximum possible in comparison to 44% for the square patient, 50% for the diamond patient and 100% for the triangle patient.

To us it seems easier to explain to a prospective patient that 'in the past patients having this procedure have regained an average of X% of their normal function back' as opposed to 'in the past Y% of patients have improved by the MCID'.


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