Thursday, October 22, 2015

CT scan measurements are affected by the alignment of the shoulder to the scanner

The effect of sagittal rotation of the glenoid on axial glenoid width and glenoid version in computed tomography scan imaging.

These authors point out the challenging in aligning computed tomography (CT) scans of the shoulder to the axis of the scapula and glenoid.

Using 30 CT scans, they sought to determine the effect of sagittal rotation of the glenoid on axial measurements of anterior-posterior (AP) glenoid width and glenoid version attained by standard CT scan, comparing uncorrected (UNCORR) and corrected (CORR) CT scans.

The mean difference in glenoid version was 2.6% (2° ± 0.1°; P = .0222) and the mean difference in AP glenoid width was 5.2% (1.2 ± 0.42 mm; P = .0026) in comparing the CORR and UNCORR scans. 

Comment: This paper is important in that it emphasizes that any image is affect by the alignment of the subject to the camera. 

Before ordering imaging of an arthritic shoulder, it is important to decide what information is needed.  As a recent post emphasized, the glenoid pathoanatomy is complex - 'version' is only one of many key important aspects. In our practice, as pointed out earlier this week, preoperative attempts to characterize the glenoid shape do not affect the actual conduct of the glenoid preparation. The goal of glenoid reaming is simply to convert the glenoid surface to a single concavity with the removal of the smallest amount of bone. 
For this reason, we avoid the use of a 'guide' wire, in that it tends to force excessive removal of bone as shown below.

Instead we use a numbed reamer that allows the reamer to be positioned so that minimal bone is removed in the creation of a single concavity as shown in the Steve Lippitt figure below.

We can obtain all the information we need for total shoulder arthroplasty from a standard AP plain radiograph and from the 'truth' view, without the cost or radiation of a CT scan or the time for 3D reconstruction and correction of rotation.


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