Impact of Sagittal Rotation on Axial Glenoid Width Measurement in the Setting of Glenoid Bone Loss

For the entire cohort of 43 patients, the UNCORR scans underestimated the axial AP width (and thus overestimated GBL) in cut 1 (P =.003), and overestimated the axial AP width (and thus underestimated GBL) in cuts 3 to 5 (P < .001 for all) compared with that of the CORR scans. There was no significant difference between the UNCORR and CORR scans in cut 2 (P = .331).

For groups I (10%-14.9% GBL) and III (>20% GBL), the UNCORR scans underestimated the axial AP width (and thus overestimated anterior GBL) in cuts 1 and 2, while in cuts 3 to 5, the axial AP width was overestimated (GBL was underestimated) (Tables 2, 3). In Group II (15%-19.9% GBL), the axial AP width was underestimated (GBL was overestimated), while in cuts 2 to 5, the axial AP width was overestimated (GBL was underestimated). Overall, AP glenoid width was consistently underestimated in cut 1, the most caudal cut, while AP glenoid width was consistently overestimated in cuts 3 to 5, the more cephalad cuts.

Abbreviations: AP, anterior-posterior; GBL, glenoid bone loss.

Abbreviations: AP, anterior-posterior; CORR, corrected; UNCORR, uncorrected.

DISCUSSION

The principle findings of this study demonstrate that UNCORR conventional 2-D CT scans inaccurately estimate glenoid width as well as inaccurately quantify the degree of anterior 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. Therefore, the authors recommend correcting the orientation of the scapula in cases wherein clinical decisions are entirely based on 2-D CT, or using alternative methods for quantifying GBL, specifically in the form of 3-D reconstructions.

The use of axial imaging, with CT scans and/or magnetic resonance imaging, is growing in popularity for evaluation of both glenoid anatomy and GBL. Nevertheless, despite our improved ability to critically evaluate the glenoid using these advanced imaging modalities, the images themselves require scrutiny by clinicians to determine if the images accurately depict the true anatomy of the glenoid. As demonstrated by Gross and colleagues,¹⁵ conventional 2D CT scan protocols are not optimized to the anatomy of the glenohumeral joint, even in patients without GBL. Due to the alignment of the image relative to the plane of the scapula as opposed to the plane of the glenoid, UNCORR scans result in significantly different measurements of glenoid version (2.0° ± 0.1°) and AP glenoid width (1.2 mm  ± 0.42 mm) compared with corrected scans, requiring an average 20.1° ± 1.2° of correction to align the sagittal plane. In the present study involving the patients with GBL, we also found that conventional, UNCORR 2-D CT scan protocols inaccurately estimate glenoid width and the degree of anterior GBL. In particular, AP glenoid width was consistently underestimated in the more caudal cuts, while AP glenoid width was consistently overestimated in the more cephalad cuts. Thus, anterior GBL was overestimated (AP glenoid width was underestimated) in the more caudal cuts, whereas anterior GBL was underestimated in the more cranial cuts (AP glenoid width was overestimated). Given that approximately 1 mm of glenoid bone corresponds to approximately 4% of glenoid width,¹⁶ even subtle differences in the interpretation of GBL may lead to gross overestimation/underestimation of bone loss, with significant clinical implications.

In the anterior instability patient population, clinical decision-making is often based on the degree of GBL as determined by advanced imaging modalities. In addition to other patient-specific factors, including age, gender, activity level, type of sport, and number of prior dislocations and/or prior surgeries, the quantity of GBL will often determine which surgical procedure needs to be performed. Typically, patients with >20% to 25% anterior GBL are indicated for a glenoid reconstruction procedure, most commonly via the Latarjet procedure (coracoid transfer).^21-27 The Latarjet procedure remains an excellent technique for appropriately indicated patients, with historically good clinical outcomes and low recurrence rates. Complications associated with the Latarjet procedure, however, are not uncommon, including devastating neuropraxia of the axillary and musculocutaneous nerves, and occasionally permanent neurologic deficits.²⁸ Thus, it is critical to avoid overtreating patients with recurrent instability and GBL. As demonstrated by this study, depending on the cranial-to-caudal location on the glenoid, current 2-D CT techniques may underestimate AP glenoid width, resulting in an overestimation of GBL, potentially leading to the decision to proceed with glenoid bone reconstruction when such a procedure is not required. On the contrary, overestimation of AP glenoid width, which occurs in the more cephalad cuts of the glenoid, is perhaps more worrisome, as the resulting underestimation of GBL may lead to inadequate treatment of patients with recurrent instability. Certainly, one of the main risk factors for failed soft tissue shoulder stabilization is a failure to address GBL. If clinical decisions are made based on UNCORR 2-D CT scans, which are often inaccurate with respect to AP glenoid width by an average 2.86 mm ± 2.00 mm (equivalent to 12.6% ± 6.9% GBL) as determined in this study, patients who truly require osseous glenoid reconstructions may be indicated for only soft tissue stabilization, based on the underestimation of GBL.

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