A paired t test was used to test the error of rotation or the difference between version at neutral and the rotated version. The theoretical difference was set at zero to mean that the error should be zero if rotation does not cause significant changes in version measurement. In addition, 95% confidence intervals were calculated for version variation at all degrees of rotation. We also conducted regression analysis to see how much version variability would be caused if the scapula was rotated by 1°. That is, we fit a straight regression line to describe the relationship between scapular rotation and version variability. All regression analyses were done with the constraint that 0° of rotation resulted in 0° of version variability.
Source of Funding
There was no external funding for this study.
The in situ position of the scapulae with the cadavers supine on the scanner bed was an average (and standard deviation) of 38.8° ± 8.9° (range, 26.3° to 63.6°) of horizontal adduction in the transverse plane, 1.9° ± 8.1° of abduction (range, 15.9° of coronal adduction to 21.8° of coronal abduction) in the coronal plane, and 16.4° ± 5.5° (range, 1.6° to 28.1°) of internal rotation in the sagittal plane.
The anatomic glenoid version was an average of –2.0° ± 3.8° (range, −8.8° to +7.6°). The clinical glenoid version based on the orientation of the original computed tomography images was an average of 3.8° ± 8.0° (range, −8.5° to +22.6°). The average difference between anatomic glenoid version (independent of the resting position of the scapula) and clinical glenoid version (dependent on the position of the scapula at the time of computed tomography acquisition) was 6.9° ± 5.6° (range, 0.1° to 22.5°).
Version measured with coronal or sagittal rotation of the scapula was significantly different from the anatomic version for each specimen for all degrees of rotation (p < 0.0001). Two-dimensional midglenoid axial images for a single specimen rotated in the sagittal and coronal planes are shown in Figures 6-A and 6-B, respectively. For this particular case, the anatomic glenoid orientation was 2.4° of retroversion with the scapula positioned in the neutral position in the coronal and sagittal planes. When this scapula was rotated in the sagittal plane, glenoid version measured 4.7° of retroversion at 10° of external rotation and 9.7° of anteversion at 30° of internal rotation. When the scapula was rotated in the coronal plane, glenoid version measured 10.6° of anteversion at 20° of abduction and 4.6° of retroversion at 20° of adduction.
Figure 7 shows the average version variation for scapulae rotated in the sagittal plane. Internal rotation resulted in relative anteversion, and external rotation resulted in relative retroversion. Regression analysis for scapular rotation in the sagittal plane showed that every 1° of scapular internal rotation and external rotation resulted in 0.2° (p < 0.001) of version variability. The effect of internal rotation on version variability was similar (p = 0.78) to that of external rotation on version variability. At 30° of internal rotation, average version variation was +6.4° ± 4.0° (range, –0.2° to +17.4°). At 10° of external rotation, average version variation was –2.0° ± 1.3° (range, –4.7° to 0.8°).
Average version variation for scapulae rotated in the coronal plane is shown in Figure 8. Scapular abduction resulted in relative anteversion, and adduction resulted in relative retroversion. Regression analysis for scapular rotation in the coronal plane showed that every 1° of scapular abduction led to 0.42° (p < 0.001) of version variability, whereas every 1° of scapular adduction resulted in 0.16° (p < 0.001) of version variability. The effect of abduction on version variability was significantly stronger (p < 0.001) than the effect of adduction on version variability. At 20° of scapular abduction and adduction, the mean version variation was +9.4° ± 3.1° (range, +3.5° to +14.6°) and –2.4° ± 1.1° (range, –4.7° to –0.3°), respectively.
Assessing the orientation and morphology of the glenoid is important when treating certain conditions of the shoulder such as instability and arthritis. For example, in total shoulder arthroplasty, implanting the glenoid component in a more retroverted position causes eccentric loading and may lead to early failure5-8,18. Also, in the operative treatment of glenohumeral instability, accurate evaluation of the osseous detail of the glenoid is important in determining the need to address osseous abnormalities compared with relying solely on soft-tissue procedures19,20.
Two-dimensional computed tomography has been a standard modality for evaluating the osseous morphology of the glenoid1-4. However, acquisition of the two-dimensional images in these earlier works made no reference to the starting orientation of the scapula. The highly variable orientation of the scapula between subjects was not taken into account. When measuring glenoid version from two-dimensional computed tomography scans, it was critical that the orientation of the scapula in which the images were acquired was recognized. Any malalignment of the scapula in the coronal or sagittal planes of ≥1° would create significant inaccuracies in measuring glenoid version.
While we were able to show significant inaccuracies in glenoid version, the results from this study were meaningless unless there was clinical relevance. Although the amount of tolerable glenoid version measurement error has not been demonstrated precisely in the current available literature as far as we know, the degree at which error in glenoid version measurement becomes clinically important has been suggested in studies investigating the effects of glenoid component malalignment in shoulder arthroplasty. Nyffeler et al. showed that the best centering of the humeral head and distribution of forces occurred when the glenoid component was implanted at neutral version7. They demonstrated 0.5 mm of humeral head displacement for every degree of altered glenoid component version. Farron et al. used finite element analysis to predict increased stress in bone and cement with the glenoid component implanted in 5° of retroversion, with an exponential increase in micromotion at the bone-cement interface when the glenoid component was implanted in >10° of retroversion5. The average difference between anatomic glenoid version (independent of the resting position of the scapula) and clinical glenoid version (dependent on the position of the scapula on original computed tomography axial images) in the present study was 7° (range, 0.1° to 22.5°). Clinically important inaccuracies in glenoid version measurement were probable with this degree of error.
When we placed the whole-body cadavers on the scanner bed, the ranges of orientation of the scapulae were approximately 25° in the sagittal plane, 40° in the coronal plane, and 35° in the transverse plane. This range of resting scapular positions among specimens results in varying degrees of error in version measurement. Average scapular position in the sagittal plane was 16.4° of internal rotation, but it ranged from 1.6° to 28.1° of internal rotation. Figure 7 shows that 16.4° and 28.1° of internal rotation of the scapula with respect to the gantry were expected to result in errors of approximately 3.5° and 6° of relative anteversion, respectively. The coronal position of the scapulae relative to the gantry averaged 1.9° of abduction but ranged from 15.9° of adduction to 21.8° of abduction. Figure 8 shows that with 20° of abduction, version error was approximately 9° of relative anteversion, and with 20° of adduction, version error was approximately 2° of relative retroversion. On the basis of the average coronal and sagittal in situ position of the scapulae, we expected the average clinical glenoid version based on the original computed tomography images to be more anteverted compared with the average anatomic glenoid version. This was confirmed, as the average clinical glenoid version was 3.8° of anteversion compared with the average anatomic glenoid version of 2.0° of retroversion.
We know of one other study in the peer-reviewed literature that has investigated the variability of glenoid version as measured on computed tomography scans21. Those investigators used computed tomography scans of ten dry scapulae to investigate version variability while manually rotating the scapulae in the coronal plane at neutral, 10° of adduction, 10° of abduction, and 20° of abduction. It was found that version varied by as much as 10° with coronal rotation of the scapula. We were able to confirm the finding that version varies significantly with scapular rotation. Using computer modeling, we were able to investigate the effect of scapular rotation in greater detail. When the scapula was abducted or internally rotated, glenoid version measurements were more anteverted. When the scapula was in adduction or external rotation, the glenoid version measurements were more retroverted.
The images from a modern computed tomography scanner are usually made helically, and the data are therefore a volume data set. Images are typically displayed in the traditional axial plane, but the benefit of multidetector computed tomography technology is that the images can be reconstructed in any arbitrary plane. With older equipment, a technologist would have had to have been more concerned about perfectly positioning the patient in the scanner to obtain images that could accurately measure glenoid version. This would be nearly an impossible task given the range of scapular flexion and rotation that results from variation in body habitus. On a modern helical scanner, a computed tomography technologist could theoretically provide a surgeon a single “axial” two-dimensional image generated from a three-dimensional volume set by making the plane of the image oblique through set osseous landmarks. This single two-dimensional image could then be used to measure anatomic glenoid version.
We suggest that anatomic glenoid version can be found in the clinic from two-dimensional axial images that have been properly reconstructed by the computed tomography technologist. Properly oriented axial reconstructions can be created on the basis of the plane of the scapula and the transverse axis of the scapula (Fig. 1). Given a three-dimensional volume set, the computed tomography technologist chooses the three points that define the plane of the scapula. These points include the most distal point of the inferior scapular angle (P1), the center of the glenoid fossa (P2), and the point at the vertebral border where the scapular spine intersects the medial border of the scapula (P3). The line between the center of the glenoid fossa (P2) and the point at the vertebral border where the scapular spine intersects the medial border of the scapula (P3) also defines the scapular axis. Axial reconstructed images are then created in the plane perpendicular to the plane of the scapula that includes the scapular axis.
One of the strengths of this study is that the anatomic geometry of the three-dimensional models has been previously validated10. We believed that validation of the three-dimensional models was a critical first step so that we could be confident that any references to anatomic variations were representative of the real situation. A potential weakness of this study is that the orientation of the scapula in situ may not be representative of the real situation as it reflects cadaver position, which may be more constraining, but we still believed that even with this limitation it reflected well the idea of the potential for significant variability in scapular orientation.
The scapula has three-dimensional geometry. Multiple studies have examined the complex anatomy of the scapula and shown that there is substantial variability of scapular dimensions and shapes among individuals22-25. Other studies have demonstrated the variability of glenoid shape, indicating that glenoid version differs between axial planes superior and inferior to the midplane4,26,27. With this variability, it is challenging to measure glenoid version consistently among different individuals with use of two-dimensional imaging modalities, such as computed tomography or plain radiographs. Most of the previous literature on the measurement of glenoid version has made little reference to the position of the scapula or has presumed that the scapula was appropriately aligned. Therefore, any reference to glenoid version without reference to scapular position is not reliable. We demonstrated that just 1° of coronal or sagittal rotational variation in alignment will not reflect true anatomic version. For two-dimensional images to be accurate, the computed tomography technician or radiologist must have a standard protocol in place to orient the angle of axial reconstruction in relationship to the position of the scapula. Orthopaedic surgeons who rely on this information need to be aware of the potential error and misguidance. Therefore, we agree with others who have stated that reconstructed three-dimensional images enable more precise measurement of glenoid version12,28. It should also be recognized that the definition of true version on a three-dimensional model relies on the accurate identification of the plane of the scapula.
In conclusion, these findings support the use of three-dimensional models to evaluate glenoid version relative to the plane of the scapula, as they do not rely on the position of the scapula. Two-dimensional images also allow for accurate measurement of glenoid version, provided that the proper plane of axial reconstruction is taken into account.
Investigation performed at the Department of Orthopaedics and Rehabilitation, Penn State Milton S. Hershey Medical Center, Penn State College of Medicine, Hershey, Pennsylvania
Disclosure: The authors did not receive any outside funding or grants in support of their research for or preparation of this work. Neither they nor a member of their immediate families received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity.
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