Two-Dimensional Glenoid Version Measurements Vary with Coronal and Sagittal Scapular Rotation

Bryce, Chris D. MD; Davison, Andrew C. MS; Lewis, Gregory S. PhD; Wang, Li PhD; Flemming, Donald J. MD; Armstrong, April D. BSc(PT), MD, MSc, FRCSC

Journal of Bone & Joint Surgery - American Volume:
doi: 10.2106/JBJS.I.00177
Scientific Articles
Abstract

Background: Accurate analysis of osseous glenoid morphology is important in treating glenohumeral arthritis and instability. Two-dimensional computed tomography scans are used to evaluate glenoid alignment. Accuracy of this method is dependent on the angle of axial reconstruction in relation to the position of the scapula. The purpose of this study was to investigate the effect of scapular rotation in the coronal and sagittal planes on glenoid version as measured on two-dimensional images.

Methods: Computer-generated three-dimensional models of scapulae from computed tomography scans of thirty-six shoulders in whole-body cadavers were generated. The anatomic geometry of these models had been previously validated. The position of the scapulae relative to the gantry was determined. The three-dimensional models were rotated in 1° increments in the coronal and sagittal planes. Glenoid version was measured on two-dimensional images for each of the rotation increments. Version variability at each rotation increment was calculated.

Results: The anatomic glenoid version (independent of the resting position of the scapula) was an average (and standard deviation) of 2.0° ± 3.8° of retroversion. The average difference between anatomic glenoid version and clinical glenoid version (depending on the position of the scapula on the original computed tomography axial images) was 6.9° ± 5.6° (range, 0.1° to 22.5°). Version variability with coronal or sagittal rotation was significant for all degrees of rotation (p < 0.0001). Scapular abduction had the greatest effect on version variation and resulted in 0.42° of relative anteversion for every 1° of abduction in the coronal plane. In the sagittal plane, internal rotation resulted in relative anteversion.

Conclusions: Any malalignment of ≥1° of the scapula in the coronal or sagittal plane will create inaccuracies in measuring glenoid version. The plane of axial reconstruction should be aligned with the scapula when two-dimensional computed tomography images are used to measure glenoid version. These findings support the use of three-dimensional models to evaluate glenoid version.

Clinical Relevance: When computed tomography scans are made to evaluate glenoid version, the plane of axial reconstruction must be taken into account. In contrast, glenoid version measured on three-dimensional models is independent of scapular position.

Author Information

1Departments of Orthopaedics and Rehabilitation (C.D.B., A.C.D., G.S.L., and D.J.F.), Public Health Services (L.W.), and Radiology (D.J.F.), Penn State Milton S. Hershey Medical Center, Penn State College of Medicine, 500 University Drive, P.O. Box 850, Hershey, PA 17033

2Bone and Joint Institute, Penn State Milton S. Hershey Medical Center, Penn State College of Medicine, EC089, 30 Hope Drive, Building A, Hershey, PA 17033. E-mail address: aarmstrong@hmc.psu.edu

Article Outline

Glenoid orientation and morphology affect the stability and loading of the glenohumeral joint. The classic method of two-dimensional computed tomography of the shoulder has been used to evaluate the glenoid in the treatment of glenohumeral arthritis and instability1-4. Osteoarthritis of the shoulder frequently results in posterior glenoid wear and potentially acquired retroversion. Glenoid orientation may have to be corrected in total shoulder arthroplasty. A well-aligned glenoid component has been shown to allow for concentric loading at the glenohumeral joint5-8. In the setting of glenohumeral instability, accurate assessment of the glenoid is important to establish the proper course of treatment9.

Glenoid version is often used to describe the orientation of the glenoid. Glenoid version was defined by Friedman et al. in reference to the transverse axis of the scapula with use of two-dimensional computed tomography images1. The transverse axis of the scapula is the line between the midpoint of the glenoid fossa and the vertebral border of the scapula. Glenoid version is the angle between the line perpendicular to the transverse axis of the scapula and a line between the anterior and posterior margins of the glenoid. Unfortunately, this early description of glenoid version did not make any reference to the starting position of the scapula and failed to recognize that the position of the scapula is highly variable in human subjects. On two-dimensional computed tomography images, the points on the vertebral border, the anterior glenoid margin, and the posterior glenoid margin that define glenoid version may change with rotation of the scapula relative to the axial plane of the computed tomography image.

The purpose of this study was to investigate the effect of scapular rotation in the coronal and sagittal planes on the variation of glenoid version as measured on axial two-dimensional images. We hypothesized that scapular rotation in the coronal and sagittal planes would significantly alter glenoid version measurement in the axial plane.

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Materials and Methods

Model Generation

Forty computed tomography scans of the shoulders in twenty whole-body human cadavers (eleven male and nine female donors ranging in age from sixty-three to ninety-eight years at the time of death) were performed with a Philips Medical Systems scanner (Bothell, Washington). The cadavers were placed on the scanner bed in the supine position with the arms at the side. Two-dimensional computed tomography images were acquired in 3-mm increments with use of a bone algorithm to enhance cortical margins. Cadavers were imaged from the seventh cervical vertebra encompassing the entire scapula. On review of the computed tomography images, four scans were excluded from the study because of glenohumeral osteoarthritis. The computed tomography scans of the remaining thirty-six shoulders were imported into Mimics software (version 8.0; Materialise, Leuven, Belgium) to generate three-dimensional scapular models. The anatomic geometry of these three-dimensional scapular models had been previously validated10. The models were without degenerative changes of the glenoid or other structural changes. Triangular surface meshes of the three-dimensional scapular models were exported as a Polygon File Format (PLY) into MATLAB (The MathWorks, Natick, Massachusetts) to manipulate scapular orientation and calculate glenoid version.

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Coordinate System

A coordinate system was developed individually for each scapula to allow for rotation in the coronal and sagittal planes. The coordinate system was based on the plane of the scapula defined by three hand-picked points (Fig. 1): 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)11-14. The origin of the reference coordinate system was the center of the glenoid fossa (P2). The coronal plane was the plane of the scapula. The transverse plane was the plane orthogonal to the coronal plane and colinear with the scapular axis, i.e., the line between the center of the glenoid fossa (P2) and the point at the intersection of the scapular spine and medial border of the scapula (P3). The sagittal plane was then the cross product of the coronal and transverse planes or the plane orthogonal to these two planes.

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In Situ Position of the Scapulae

The in situ position of each scapula was found with respect to the orientation of the computed tomography gantry with use of custom MATLAB computer code. Three planes defined the orientation of the gantry. The transverse plane of the gantry was the axial plane in which the images were acquired. The coronal plane of the gantry was parallel to the bed of the computed tomography scanner and was perpendicular to the transverse plane of the gantry. The sagittal plane of the gantry was the plane perpendicular to both the transverse and coronal planes of the gantry. Figure 2 illustrates the method for finding the orientation of the scapula with respect to the orientation of the gantry. The transverse, coronal, and sagittal angles of the scapula with respect to the gantry were described as a sequence of Euler and Cardan rotations15,16. This approach is a de facto standard for precise description of three-dimensional orientation of the scapula and other body segments15,17. The proposed standard from the International Society of Biomechanics for the description of the scapular-thoracic orientation was adapted to describe the scapular-gantry orientation15,16. Coordinate directions of the scapula and gantry were as defined above.

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Calculating Anatomic and Clinical Glenoid Version

Anatomic glenoid version was defined as the true version of the glenoid independent of the resting position of the scapula. Clinical glenoid version was defined as the apparent version of the glenoid, which is dependent on the position of the scapula in relation to the orientation of the computed tomography scan axial images. With the three-dimensional scapular model oriented neutral in the coordinate system described above, the anatomic glenoid version was found by taking a two-dimensional slice of the scapula in the transverse plane at the midglenoid level as determined by P2. Three points were found with use of a custom MATLAB computer code: the most medial point of the scapula, P4 (P4 is the same point as P3, defined above, when there is no rotation of the scapula in the coronal plane); the most posterior point on the glenoid surface, P5; and the most anterior point on the glenoid surface, P6. The transverse axis line was defined as the line between the vertebral border of the scapula (P4) and the center of the glenoid (P2)1. Version was calculated as the angle between the transverse axis line and a line perpendicular to the line between the anterior (P6) and posterior margins (P5) of the glenoid (Fig. 3). Anteversion was assigned positive values, and retroversion was assigned negative values. The apparent clinical glenoid version (not reoriented to neutral) based on the original computed tomography images was also measured by finding the midglenoid axial slice in its original orientation and calculating version as above with use of MATLAB.

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Calculating Version Variability

Version variation was then found by virtually rotating the scapula within the reference coordinate system through the use of coordinate transformations in MATLAB. The scapulae were rotated about the origin (P2) in the coronal and sagittal planes independently (Fig. 4). With rotation in the coronal plane, P4 intersected the medial border of the scapula above or below the point P3 that was used to measure anatomic version above (Fig. 5). In the coronal plane, positive rotation corresponded to clinical adduction, with negative rotation corresponding to abduction. In the sagittal plane, positive rotation corresponded to clinical internal rotation, with negative rotation corresponding to external rotation. The scapulae were rotated in the coronal direction from 20° of abduction to 20° of adduction at 1° increments with sagittal rotation held constant at 0°. The scapulae were then rotated in the sagittal plane from 10° of external rotation to 30° of internal rotation at 1° increments with coronal rotation held constant at 0°. At each 1° increment, the version was recalculated on the midglenoid two-dimensional transverse outline (Fig. 3). Version variability was defined for each increment of rotation as the difference between the version with the scapula rotated and the anatomic version. Positive values of version variability corresponded to relative anteversion, and negative values corresponded to relative retroversion.

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Statistical Analysis

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.

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Source of Funding

There was no external funding for this study.

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Results

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.

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Discussion

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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