Friday, November 29, 2013

Rotator cuff surgery, measuring outcome: the MCID vs the I/MPI

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

The minimal clinical important difference (MCID) is the smallest change in an outcome score for the treatment of a clinical condition that is meaningful and important for the patient. In order to determine the minimal clinically important difference for the Constant Score and cuff repair, the authors studied a prospectively 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 whether the shoulder was better or worse after the operation compared with the preoperative state? This 2-level question was used as an indicator of patient satisfaction to calculate the MCID for the Constant score.

The preoperative Constant score was 56.2 (SD 17.4) in male patients, and 48.2 (SD 15.6) in female patients. Postoperatively at 3 months, the scores were 65.1 (SD 16.1) in male patients, and 56.8 (SD 15.5) in female patients. At 1 year, the scores were 79.0 (SD 14.9) in male patients, and 71.0 (SD 14.3) 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); at 1 year, the satisfaction was 93.2% and 89.5%.

The 3-month mean change estimate of MCID was 10.4 points.

Interestingly, the pathology and surgical procedures were varied:  27% of the patients had "partial" cuff tears and were treated with "subacromial decompression", 13% had irreparable cuff tears and were treated with "partial" cuff repairs. 96% had involvement or the supraspinatus, 24% had involvement of the infraspinatus, and 38% had involvement of the subscapularis. 36% had a distal clavicle excision and 44% had biceps tendon surgery. So this is not a study of patients having cuff repair only. Even more interestingly, the mean change in Constant score was not affected by the severity of the cuff tear, working status of the patient, history of trauma.

A limitation of the MCID is that it it is 'fickle'. Even in in this study, different statistical approaches yielded 5 different values ranging from 2 to 16. Another limitation is that it needs to be estimated for each different diagnosis and each different outcome metric. A final limitation is that a defined value of MCID does not mean the same thing across the spectrum of values, for example an improvement of the Constant score from 0 to 11 is not the same thing as an improvement of the Constant score from 85 to 96; a patient with a preoperative score of 90 would not be capable of achieving the MCID of 10.4.

An alternative is the ratio of improvement to the maximal possible improvement (I/MPI). With this approach a change in Constant score from 0 to 11 would represent  11/100 or 11% of the possible improvement and a change from 85 to 96 would represent 11/15 or 73% of the maximal possible improvement. The I/MPI does not require complex statistics, can be applied to any pathology, and can be applied to any outcome measurement tool. Finally, it answers the question patients have, "How much better am I likely to be after this procedure?"

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