Impact of Sagittal Rotation on Axial Glenoid Width Measurement in the Setting of Glenoid Bone Loss
TAKE-HOME POINTS
- Standard 2-D CT scans of the shoulder are often aligned to the plane of the body as opposed to the plane of the scapula, which may challenge the ability to accurately measure glenoid width and GBL.
- Underestimations of GBL may lead to insufficient treatment of clinically meaningful GBL, thereby increasing the risk of instability recurrence; whereas overestimations of GBL may lead to unnecessary treatment, subjecting patients to increased surgical morbidity.
- AP glenoid width was consistently underestimated in uncorrected axial cut 1, the most caudal cut.
- AP glenoid width was consistently overestimated in uncorrected axial cuts 3 to 5, the more cephalad cuts.
- CORR 2-D CT scans or a 3-D reconstruction can help in accurately defining the anterior GBL in patients with shoulder instability.
Historically, plain radiographs have been the mainstay for evaluating the glenohumeral joint, including Grashey and axillary views, allowing clinicians to evaluate the congruency of the glenohumeral joint and to assess bone loss on both the glenoid and humeral head.1,10 While large, acute fractures of the glenoid are fairly evident on radiographs, including the Grashey view,11 shoulders with chronic and/or attritional anterior GBL are more difficult to evaluate, and often do not provide the information necessary to guide surgical decision-making.
Computed tomography (CT) of the shoulder has become the most commonly utilized imaging modality in the evaluation of patients with shoulder instability associated with GBL. Standard 2-dimensional (2-D) CT scans of the shoulder are often aligned to the plane of the body as opposed to the plane of the scapula/glenoid, as standard protocols often fail to account for the anterior sagittal rotation of the scapula/glenoid, similar to the disadvantage of standard radiographs. While 3-dimensional (3-D) CT reconstructions eliminate the effect of gantry angles, and thus allow for an en face view of the glenoid, 3-D reconstructions are not always available, and cannot always be measured.12-14 Thus, improved methodology for utilizing standard 2D scans is warranted, as the ability to correctly align the axial CT scan to the axis of the glenoid may allow for more accurate GBL measurements, which will ultimately impact surgical decision-making. Recently, Gross and colleagues15 reported the effect of sagittal rotation of the glenoid on axial measurements of anterior-posterior (AP) glenoid width and glenoid version in normal glenoids, without bone loss, and found that the mean angle of correction needed to align the sagittal plane was 20.1° ± 1.2° of rotation. To the authors’ knowledge, this same methodology has not been applied to patients with clinically meaningful anterior GBL. Given that the average glenoid width in human shoulders is 24.4 mm ± 2.9 mm,16 1 mm of glenoid bone loss (GBL) corresponds to approximately 4% of the glenoid width, and thus even subtle differences in the interpretation of GBL may have substantial clinical implications. Therefore, the purpose of this study is to determine the effect of sagittal rotation of the glenoid on axial AP glenoid width measurements in the setting of clinically significant anterior GBL.
METHODS
This study was approved by Massachusetts General Hospital Institutional Review Board. A retrospective review of consecutive patients with a diagnosis of anterior shoulder instability between 2009 and 2013 was conducted. Inclusion criteria comprised patients with a minimum of 10% anterior GBL, an available CT scan of the affected shoulder, and no history of prior ipsilateral surgeries. Exclusion criteria comprised evidence of degenerative changes to the glenoid and/or humeral head, as well as prior ipsilateral shoulder surgery. Sixty consecutive patients were originally identified as having anterior shoulder instability, and 17 were excluded based on the inclusion/exclusion criteria, leaving 43 patients (43 shoulders) available for inclusion. Shoulder CT scans from all 43 patients were reformatted utilizing open-source DICOM software (OsiriX MD, version 2.5.1 65-bit) multi-planar reconstruction (MPR).
CT PROTOCOL
All patients underwent a standard glenohumeral CT scan using a Siemens Sensation 64 Scanner (Siemens), a 64-detector scanner. Scans were acquired with 0.6 mm of collimation, 140 kV, and 300 mA-seconds. Slice thickness was set to 2 mm. All patient information was de-identified for analysis.
The uncorrected (UNCORR) scans were defined as the default orientation on the scanner. In the UNCORR scans, the axial, coronal, and sagittal views were oriented relative to the scanner gantry table, as opposed to the anatomy of the glenoid. The corrected (CORR) CT scans were aligned in all 3 planes relative to the glenoid face, and thus the cuts were perpendicular to the long axis of the glenoid.15 This resulted in sagittal cuts perpendicular to the 12-o’clock to 6-o’clock axis in the sagittal plane (Figure 1). 
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