Monday, September 12, 2016

The arthritic glenoid, challenges in measurement of version and subluxation on CT scans

Radiographic characterization of the B2 glenoid: the effect of computed tomographic axis orientation.

These authors suggest that "accurate measurement of preoperative glenoid deformity is critical for decision-making and prognostication". They sought to determine whether glenoid version, inclination, and depth and humeral subluxation measurements on computed tomography (CT) scan slices oriented in the plane of the body differ from those oriented in the scapular plane and those obtained by automated 3-dimensional reconstruction software.

They studied 31 preoperative CT scans in patients undergoing total shoulder arthroplasty with Walch B2-type glenoids underwent a standardized measurement protocol by 3 observers. Glenoid version, inclination, and depth and humeral subluxation were measured on 2-dimensional CT images in the plane of the body, on 2-dimensional images in the plane of the scapula, and by a validated, automated 3-dimensional software program.

Correction of CT slice axis into the plane of the scapula decreased measured retroversion by 2.4° to 4.7° (P < .004) and inclination by 21° (P < .001). Whereas uncorrected version measurements do not differ from automated software measurements, corrected measurements do (P < .001). Whereas corrected inclination measurements do not differ from automated measurements, uncorrected measurements do (P < .001). Automated measurements differed from both corrected and uncorrected subluxation (P < .001 in both cases).

The images below show 'uncorrected' version measurement of 39.5 degrees (left) and "corrected" version measurement of 20.2 degrees (left).

They concluded that if CT images are not reoriented into the plane of the scapula, version and inclination will be significantly overestimated. In the setting of a retroverted, deformed glenoid, automated software may produce similar inclination measurements to corrected 2-dimensional CT, but it produces significantly altered measurements of version and subluxation.

Comment: This study shows that measuring preoperative glenohumeral pathoanatomy is more complex than previously thought, requiring consideration of the plane of the cut and reorientation to the plane of the scapula.

Rather than spending time, money and radiation exposure on CT scans and correction of the images into the plane of the scapula, we find that the preoperative 'truth' axillary x-ray (see this link) provides the necessary and sufficient images not only for the preoperative measurements but also for making the same measurements after arthroplasty as shown in this link.

The reproducibility of the 'truth' view is shown in the case of a patient we saw in the office today. Here are the films of the arthritic shoulder taken one year ago (see below)

to those taken today (see below)

Note the progressive deepening of the posterior concavity and the progressive loss of radiographic joint space. Note also that these films are taken with the arm elevated in the position of elevation in the plane of the scapula so that the functional decentering of the humeral head on the glenoid is revealed.