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Posterior Humeral Subluxation on MRI Not a Reliable Indicator of Posterior Instability

Key findings

  • A research team evaluated whether posterior humeral subluxation seen on magnetic resonance imaging (MRI) might be a reliable indication of posterior instability, but found that it is not.
  • Glenoid retroversion on MRI was significantly increased in patients with symptomatic posterior labral tears compared with a control group
  • Patients with posterior shoulder instability did not have increased posterior humeral subluxation on MRI
  • Posterior humeral subluxation on MRI did not reliably indicate the presence or absence of symptomatic posterior shoulder instability

Posterior shoulder instability is difficult to diagnose on physical examination, especially because patients usually can’t recall any precipitating traumatic event. A research team including Jon J. P. Warner, MD, chief of the Shoulder Center at Massachusetts General Hospital, evaluated whether posterior humeral subluxation seen on magnetic resonance imaging (MRI) might be a reliable indication of posterior instability, but found that it is not. The study was published in Arthroscopy.

In certain patients who undergo surgery for obvious posterior labral tears, there is concomitant clinical evidence of posterior instability, and MRI shows increased posterior humeral subluxation. To investigate the utility of MRI, the researchers identified 41 patients who had undergone shoulder capsulorrhaphy by one of two senior surgeons over a two-year period. Patients were included in the analysis if they had a posterior labral tear repair and had preoperative MRI or magnetic resonance arthrography (MRA). Exclusion criteria included pan-labral repairs, revision repairs, bony injuries treated by bone fixation and concomitant rotator cuff procedures.

The patients were matched by gender, laterality and age (within two years) to 41 control subjects who had another type of shoulder surgery by one of the same surgeons during the same period, and also had MRI or MRA. Exclusion criteria for controls included labral pathology, complaints of instability, and tears or repairs of the subscapularis.

Two independent reviewers measured glenoid version and percentage of humeral subluxation on the radiographic images. Inter- and intra-rater reliability were later judged to be excellent.

The average humeral subluxation was 56% in the posterior instability group and 54% in the control group, a difference that is not statistically significant. The average glenoid retroversion was 8.1° in the posterior instability group and 5.6° in the control group, which is statistically significant. Eighteen patients in the posterior instability group, compared with 10 in the control group, exhibited glenoid retroversion in excess of the normal -8°.

The researchers conclude that posterior humeral subluxation is not a reliable indicator of the presence or absence of symptomatic posterior shoulder instability. They draw no conclusion about the predictive utility of increased glenoid retroversion, explaining that the clinical relevance of the small difference between groups (2.5°) is unknown. They note that increased retroversion is a risk factor for development of posterior shoulder instability and for failure of soft-tissue posterior instability repair.

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