minimal clinically important difference (MCID) vs I/MPI

Investigating minimal clinically important difference for Constant score in patients undergoing rotator cuff surgery

There has been a recent fascination with the minimal clinically important difference as a unit of treatment effectiveness. These authors prospectively analyzed collected cohort of 802 consecutive shoulders with arthroscopically treated partial- or full-thickness rotator cuff tears.. The Constant score was measured preoperatively and at 3 months and 1 year postoperatively. At follow-up visits, the patients were asked a simple 2-stage question: Is the shoulder better or worse after the operation compared with the preoperative state? This single 2-level question was used as an indicator of patient satisfaction and as an anchor to calculate the MCID for the Constant score.

The preoperative Constant score averaged 53.1 (SD 17.2) in all patients, 56.2 (SD 17.4) in male patients, and 48.2 (SD 15.6) in female patients. Postoperatively at 3 months, the scores were 61.7 (SD 16.4) in all patients, 65.1 (SD 16.1) in male patients, and 56.8 (SD 15.5) in female patients.  At 3 months postoperatively, 92.2% of male patients and 87.2% of female patients were satisfied with the outcome (P= .027). The 3-month mean change estimate of MCID was 10.4 points.

Comment: As surgeons, our goal is to restore as much of the patient's lost function and comfort as possible. The problem with the MCID is that an improvement of 10 points from a Constant score of 10 to 20 is not the same as an improvement of 10 points from a Constant score of 60 to 70, in that it does not reflect the amount of improvement in relation to the improvement possible.  As an alternative, we have used the I/MPI or percent of possible improvement as metric for measuring the result of treatment. The I/MPI is easily calculated as:

(score at the time of follow-up - score before surgery) 

divided by 

(perfect score - score before surgery) 

The beauty of the I/MPI  is that it can be used for any scoring system (Constant, SST, WOMAC, UCLA,  ASES, etc). Thus, taking the example above, if a perfect score is 100 and the patient records 60 before and 70 after surgery (improving by the MCID), the I/MPI  is

(70 - 60)/(100 - 60) 

or 

25% (the patient improved 25% of the possible improvement).

On the other hand, if the patient records 10 before and 20 after surgery (again improving by the MCID), the I/MPI  is

(20-10)/(100 - 10) 

or 

11% (the patient improved 11% of the possible improvement).

The I/MPI  (percentage of possible improvement) has the following advantages:

(1) It measures what is most important to the patient (how much better is the shoulder after treatment).

(2) It enables comparison of studies done using different outcome metrics (i.e. 'normal' is converted to 100%)

(3) It avoids the problem of having different MCIDs for each condition (arthritis, instability, cuff disease), for different genders, and for different instruments (ASES, SST, Constant). 

Try it, you will like it! 

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The results of shoulder arthroplasty - how to measure. Impact of Total Shoulder Arthroplasty on Generic and Shoulder-Specific Health-Related Quality-of-Life MeasuresJBJS

The JBJS recently published: Impact of Total Shoulder Arthroplasty on Generic and Shoulder-Specific Health-Related Quality-of-Life Measures

This is an important article. It emblemizes the shift from evaluating results of shoulder joint replacement by physician-measured metrics (e.g. range of motion) to patient-measured metrics (e.g. comfort and function) as well as overall well-being. In this systematic review, the SF 36 self-assessment of overall physical function was highly significantly improved - meaning that the improvement in the shoulder resulted in a significant benefit to overall functioning. They also found highly improved scores on three shoulder-specific metrics: the Constant, American Shoulder and Elbow Surgeons, and the  Simple Shoulder Test .*

The value of patient self-assessment is that it focuses attention on what is important to the individuals we treat. As the authors point out, it is essential to use the same metric before and after treatment so that the difference can be measured.

We use the SF 36 not only as a benchmark for the patient's status before treatment, but also as an indicator of health issues that may be correlated with the outcome of surgery, such as emotional and social functioning. We use the Simple Shoulder Test because it is the only one of the three shoulder-specfic metrics in this study that does not require the patient to return to the office for an examination - by increasing the convenience of followup for the patient, it has the advantage of reducing the number of patients lost to followup (the downfall of many clinical studies).

The value of this study extends far beyond showing that the reports confirm the value of shoulder arthroplasty. In our opinion, its greatest value is in establishing patient self-assessment metrics (the SF 36 and the SST) by which we can determine which patients are NOT benefitting from shoulder arthroplasty so we can ask, as Codman said "Why Not?".  It is often stated that '85% of patients receive a good or excellent result'. We should use the tools described there to learn what is it about that other 15% that led to suboptimal results and what can we do to reduce that number. Only by studying failures, as we have tried to do here and here, can we learn out to get better. Imagine a world in which shoulder surgeons shared the data on their patients who did not improve their SST and SF 36 scores after shoulder joint replacement with a goal of identifying the features common to those shoulders and individuals!

*The authors state that the minimal clinically important difference has not been defined for the Simple Shoulder Test. Actually, this definition is included in our recent publication: The Prognosis for Improvement in Comfort and Function After the Ream-and-Run Arthroplasty for Glenohumeral Arthritis: An Analysis of 176 Consecutive Cases. To quote from that manuscript: "We utilized an alternative approach in evaluating the benefit of shoulder arthroplasty by determining the percentage of the total possible improvement realized by the patient. In this calculation, the improvement realized by the patient is divided by the total possible improvement for the patient (i.e., the difference between the maximal possible score on the SST [12 points] and the preoperative score). We refer to this as the I/MPI (improvement / maximal possible improvement). Thus, the percentage of the total possible improvement is calculated as:

(SST total score at the time of follow-up - SST total score before surgery X 100%) 

divided by 

(12 points - SST total score before surgery) 

For purposes of this analysis, we defined the MCID as an improvement of 30% of the total improvement possible. Thirty percent is the highest percent improvement required for an MCID in the shoulder literature. It is of interest that since the average preoperative SST score was 4 points, the average maximal possible improvement was 8 points. Thirty percent of this value would be 2.4 points, a value that lies between the values separately proposed by Roy et al. and Tashjian et al. The value of the method selected for our analysis lies in the fact that it avoids the ceiling effect, and the MCID is normalized by the maximal possible improvement for each patient, rather than consisting of a fixed value applied to all patients irrespective of their maximal
possible improvement."

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