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1 Department of Diagnostic Radiology and Organ Imaging, Chinese University of
Hong Kong, Sha Tin, N. T., Hong Kong.
2 Orthopaedics and Traumatology, Chinese University of Hong Kong, Hong
Kong.
Received September 3, 2002;
accepted after revision October 8, 2002.
Address correspondence to J. F. Griffith.
Abstract
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SUBJECTS AND METHODS. CT examinations were performed on 40 patients (average age, 31 years; range, 13–82 years), comprising 46 shoulders with anterior dislocation and 34 contralateral normal shoulders. Twenty shoulders in 10 healthy subjects were also examined. Both shoulders were examined simultaneously. Image reconstruction included oblique sagittal reformatted images en face to the glenoid fossa. Seven aspects of glenoid fossa shape and size were measured, including the cross-sectional area, maximum width, maximum height, and flattening of the anterior glenoid curvature.
RESULTS. Variable flattening of the anterior glenoid curvature was a feature in 42 (91%) of 46 dislocated shoulders although it was seen in only two (4%) of 54 normal shoulders. Anterior glenoid flattening increased exponentially with an increasing number of dislocations. Anterior glenoid flattening, decreased maximum glenoid width, and decreased maximum width-to-length ratio were the most useful measures of bone loss. Maximum glenoid width was smaller than on the contralateral side in 79% of patients with unilateral dislocation by an average of 3.0 mm (range, 0.1–10 mm) or 10.8% (range, 0.4–32%). Glenoid cross-sectional area was a less useful measure of glenoid bone loss.
CONCLUSION. Flattening of the anterior glenoid curvature is shown in most patients with anterior dislocation. In unilateral dislocation, a comparison of maximum glenoid width with that on the contralateral side was the best discriminator of moderate to severe glenoid bone loss.
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Preoperative quantification of glenoid bone loss would facilitate decision making as to the type of operative procedure required. Currently, shoulder imaging for anterior dislocation focuses primarily on labral and capsular abnormalities in addition to glenoid rim attrition but does not comprise a quantitative assessment of glenoid bone loss [5, 6, 7, 8, 9, 10]. Burkhart et al. [2] recently described a method of quantifying glenoid bone loss arthroscopically.
The aims of our study were to investigate whether CT could reveal the pattern of glenoid bone loss that occurs with anterior shoulder dislocation and to investigate methods of quantifying this bone loss.
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Both shoulders of 10 healthy subjects (three men, seven women; mean age, 38 years; range, 18–74 years) undergoing high-resolution CT of the upper thorax for unrelated reasons were also examined. Healthy subjects had sternal abnormalities (n = 2), sternoclavicular abnormalities (n = 5), and proximal humeral fractures (n = 3). No healthy subjects had a history of shoulder dislocation, chronic shoulder pain, or scapular symptoms.
The complete study cohort comprised a total of 100 shoulder joints, including 46 dislocating shoulders, 34 contralateral normal shoulders of patients with unilateral dislocation, and 20 shoulders of healthy subjects with no history of dislocation.
CT Protocol
An identical helical CT protocol was used for each examination (HiSpeed
Advantage scanner; General Electric Medical Systems, Milwaukee, WI). Both
shoulders were examined simultaneously. The scanning plane extended from the
superior aspect of the acromion to the inferior aspect of the glenoid fossa.
The following scanning parameters were applied: field of view, 48 cm; pitch,
1.0; collimation, 1 mm; kV, 120; mA, 200; with 1-mm image reconstruction and a
standard reconstruction algorithm. In healthy subjects, reconstruction of the
smaller field-of-view data sets (of the part under investigation) to a larger
field of view to include both glenoid fossae provided an identical data set to
study patients.
Image Analysis
Images were analyzed on a dedicated CT workstation (Advantage Windows
version 3.1; Sun Microsystems, Palo Alto, CA). Analysis included
reconstruction of oblique sagittal images en face to the glenoid
fossa. These images were obtained through successive reconstruction in the
oblique coronal and sagittal planes (i.e., double oblique reformations) (Figs.
1A,
1B,
1C and
1D). Surface-shaded
three-dimensional (3D) reconstructions of the shoulders were also
obtained.
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Seven measures of glenoid fossa area and shape were performed: the largest width of the glenoid fossa on axial images (Figs. 2A, 2B), the maximum glenoid fossa length on reformatted images (Figs. 3A, 3B), the maximum glenoid fossa width on reformatted images (Figs. 3A, 3B), the ratio of the maximum glenoid fossa width to the maximum glenoid height, the glenoid fossa cross-sectional area by continuous and point tracing on reformatted image (Fig. 4), the glenoid fossa cross-sectional area by summation of sequential 1-mm axial widths (Figs. 2A, 2B), and the presence and length of flattening of the anterior glenoid curvature (Figs. 5, 6, 7, 8, 9, 10, 11).
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Three-dimensional CT images were graded using five categories (noninterpretable, poor, fair, good, and very good) according to visualization of the anterior glenoid rim. "Noninterpretable" referred to complete nonvisualization of the anterior glenoid rim, and "very good" referred to complete visualization.
Each analysis (two shoulders) required approximately 50 min to complete, with 30 min required for axial measurements and 20 min required for reformatted image reconstruction, measurements, and 3D reconstruction.
Statistical Analysis
A Student's t test was used to test for significant difference
among subgroups. Two-sided p values of less than 0.05 were considered
statistically significant. Pearson's test was used to assess the strength of
association between the number of dislocations sustained and the degree of
anterior flattening and reduction in maximum glenoid width.
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Glenoid Dimensions in Patients with Shoulder Dislocation
The maximum glenoid width, the glenoid width-to-length ratio, and the
cross-sectional area measured en face to the glenoid fossa were all
significantly smaller on the affected than on the nonaffected side in patients
with unilateral dislocation (Table
2). The degree of reduction of maximum diameter of the glenoid
fossa correlated moderately with the number of dislocations (r =
0.424, p = 0.028). No correlation was found between the reduction in
cross-sectional area or glenoid width-to-length ratio and the number of
dislocations.
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No statistical difference was found for the largest axial width, maximum glenoid length, and summated assessment of glenoid cross-sectional area. The range of measured values and the perceived reduction on the affected side are shown in Table 2.
Glenoid Contour in Healthy Subjects and in Patients with Anterior
Dislocation
The normal glenoid has a smooth rounded anterior contour (Figs.
2A,
2B) that was readily seen on
reformatted images en face to the glenoid fossa
(Fig. 5).
Flattening of the anterior glenoid curvature was a feature of 42 (91%) of 46 dislocated shoulders (Figs. 6, 7, 8, 9, 10, 11). This feature was seen in only two (4%) of 54 normal shoulders. Flattening of the anterior curvature resulted in an anterior straight line of variable length. Mild flattening was seen at the midpoint of the anterior contour glenoid (Figs. 6 and 7). With increasing severity, the anterior contour became progressively flattened (Figs. 8 and 9) until eventually the anterior curvature was no longer apparent (Fig. 10). With even further bone loss, the anterior edge of the glenoid contour became concave (Fig. 11).
The greater the degree of anterior flattening, the longer the length of the anterior straight line. The anterior straight line, when present, varied in length from 5.7 to 32.1 mm (Figs. 7, 8, 9, 10, 11, 12). The length of the anterior straight line was uniformly distributed across the study cohort (Table 3). A moderate correlation was noted between the number of dislocations and the length of the anterior straight line (r = 0.61, p < 0.001). The length of the anterior straight line (i.e., the degree of flattening of the anterior glenoid curvature) tended to increase exponentially with the number of anterior dislocations sustained (Fig. 12). The amount of anterior flattening sustained per dislocation was greatest with the first few dislocations and tended to lessen with later dislocations.
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Healed glenoid rim fractures of varying sizes were present in 10 (22%) of the 46 patients with shoulder dislocation. Two of these fractures were located at the 3- to 4-o'clock position on the anterior glenoid curvature, four at the 3- to 5-o'clock position, two at the 4- to 5-o'clock position, and two at the 5- to 6-o'clock position.
Three-Dimensional CT Reconstructions
Of 3D CT reconstructions, 27 were considered to be of noninterpretable
quality, 14 poor, 15 fair, 26 good, and 18 very good quality.
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To our knowledge, ours is the first imaging-based study to explore quantification of bone loss in anterior shoulder dislocation. Our study shows that appreciable, although variable, glenoid bone loss is present in most patients with anterior shoulder dislocation. Flattening of the anterior curvature was a feature of 42 (91%) of 46 dislocated shoulders, although this feature was present in only two (4%) of 54 normal shoulders (Figs. 6, 7, 8, 9, 10, 11). This frequency of bone loss is more than has been previously reported on radiographic [12] or cross-sectional [5, 6, 7, 8, 9, 10] imaging studies. In a study of 83 patients with anterior shoulder dislocation, comprehensive radiographic examination (comprising anteroposterior projections in internal and external rotation, axillary, West Point, Stryker notch, and Didiee projections) revealed a fracture of the anterior glenoid rim in 17 patients (21%) [12]. In a CT arthrographic study of 54 shoulders with anterior dislocation, "glenoid rim attrition or fracture" was a feature in nine patients (17%) [5].
Stevens et al. [13] studied 11 patients with anterior shoulder dislocation and described the appearances and benefits of 3D reconstruction of CT images in assessing glenoid osseous defects. Oblique sagittal and oblique coronal reconstructions were obtained but did not add any further information. In our study, we found that, due to beam hardening artifacts, acceptable 3D reconstruction of CT images was not universally achievable; the images were of poor or noninterpretable quality in 41% of shoulders. Conversely, good-quality coronal reconstruction images en face to the glenoid fossa were uniformly obtainable and yielded the most useful information.
Our study indicates that glenoid bone loss in recurrent anterior dislocation probably takes two distinct forms. The first form is the anterior or anteroinferior glenoid rim fracture that results from either an avulsion or an impaction during dislocation. This type of injury was seen in 22% of patients in our study, a frequency comparable to that of previous studies [5]. The second form is anterior bone loss that most likely results from a compressive injury as the dislocated humerus subsequently impacts the anterior glenoid rim [4]. The finding that some degree of anterior curvature flattening is observed in most patients with anterior dislocation supports this mechanism. Earliest bone loss was observed in the midpoint of the anterior glenoid curvature (i.e., the most anterior portion of the glenoid fossa). With more severe glenoid bone loss, progressive flattening of the anterior glenoid curvature became apparent until eventually no anterior convexity was present. Even further glenoid bone loss resulted in a concavity to the midpoint of the anterior glenoid margin. Because glenoid bone loss was observed mainly anteriorly rather than anteroinferiorly, the "inverted pear" appearance (the glenoid fossa shape typically ascribed to severe bone loss [1]) was not observed in our study.
The pattern of changing glenoid shape with increasing bone loss suggests that the main cause of glenoid bone loss in recurrent dislocation tends to be an anterior compressive injury rather an anterior or anteroinferior avulsion fracture injury (Figs. 6, 7, 8, 9, 10, 11).
An exponential relationship was found between the degree of anterior flattening (as evidenced by the length of the anterior straight line) and the number of dislocations sustained (Fig. 12). The first few dislocations had a relatively greater effect on the degree of anterior flattening. This finding may be a reflection of the triangular outline of the glenoid tapering toward the neck of the glenoid (Figs. 1A, 1B, 1C, 1D and 2A, 2B). Early dislocations likely impact a relatively thin rim of bone, whereas later dislocations encounter an increasingly thick bony rim. Although the degree of anterior flattening correlated broadly with the number of dislocations (r = 0.61, p < 0.001), a linear relationship between the two was not found. For example, one patient with an anterior straight line of 6.2 mm reported 10 episodes of dislocation, although another patient with an anterior straight line of 27.2 mm reported only a single dislocation (Fig. 12). It may be that some patients with "dislocation" are experiencing subluxation, because it is sometimes difficult to distinguish clinically between these two entities. Another explanation is that anterior flattening may be related to the force of the dislocation sustained. In this respect, the aforementioned patient with a single dislocation and an anterior straight line of 27.2 mm suffered a particularly violent dislocation during a high-velocity motorcycle accident.
Although flattening of the anterior glenoid curvature provides tangible evidence of glenoid bone loss, this finding is limited in objectively quantifying the amount of bone loss incurred. Burkhart et al. [2] described an effective arthroscopic method of measuring anterior bone loss that necessitates identification of the bare spot, a small 3-mm area in the glenoid fossa devoid of articular cartilage. The bare spot lies approximately 11 mm equidistant from the anterior, posterior, and inferior glenoid rims. If a calibrated arthroscopic probe is placed across the glenoid from a posterior portal, the distance from the posterior glenoid rim to the bare spot and from the bare spot to the anterior glenoid rim should be equal. With increasing glenoid bone loss, the anterior distance becomes progressively smaller, allowing quantification of glenoid bone loss.
To quantify bone loss, one needs an indication as to how much bone was present ab initio. CT enables both shoulders to be examined simultaneously. In unilateral dislocation, simultaneous examination allows ready comparison of glenoid dimensions in the dislocating shoulder with the contralateral normal side. In the small number of healthy subjects studied, the range of normal values for measured glenoid fossa width and cross-sectional area measurements on reformatted oblique sagittal images varied up to 1.6 mm and 12%, respectively. How much this variation represents measurement error or a true physiologic side-to-side difference in glenoid size is not clear.
When quantifying bone loss, because variation of side-to-side measurements was mild, both glenoid fossae were initially considered to be of equal size and shape (Table 2). Applying this assumption, the best indicator of glenoid bone loss in patients with unilateral dislocation was a reduction in the maximum width of the glenoid fossa. In 79% of dislocated shoulders, the maximum width of the glenoid fossa (measured on reformatted images en face to the glenoid fossa) was less than on the contralateral side (Table 2). The observed reduction in the maximum glenoid width concurs with anterior glenoid bone loss and is consistent with the arthroscopic findings of Burkhart et al. [2]. The degree of reduction in the maximum glenoid width correlated moderately with the number of dislocations sustained (r = 0.424, p = 0.028). No reduction in glenoid length compared with normal was observed in dislocating shoulders (Table 2).
The next best indicator was a decreasing glenoid width-to-length ratio. In normal shoulders, the glenoid width was found to be slightly more then two thirds of the glenoid length, giving a glenoid width-to-length ratio of approximately 0.7 (Table 2). Reduction in glenoid width with a constant length results in a decreased glenoid width-to-length ratio. The glenoid cross-sectional area (measured on oblique sagittal reformatted images en face to the glenoid fossa) was a less useful indicator of bone loss. Summated glenoid cross-sectional area, maximum glenoid length, or the largest axial glenoid width were not useful indicators of glenoid bone loss. Because of anterior scapular tilt, the standard axial CT plane runs anterosuperiorly to posteroinferiorly relative to the long axis of the glenoid (Figs. 2A, 2B). As a result, the anterior point of the largest axial width is likely to lie superiorly to the area of bone loss.
On the basis of the findings of this study, when assessing glenoid bone loss with CT the routine use of reformatted images en face to the glenoid fossa is recommended. When glenoid bone loss is mild, as indicated by mild flattening of the anterior glenoid curvature, comparison with the opposite glenoid is not likely to be beneficial because mild variations in side-to-side measurements do exist. When bone loss is moderate or severe, with prominent flattening of the anterior glenoid curvature, comparison with the opposite side is recommended in patients with unilateral dislocation to assess the reduction in maximum width. If moderate to severe bone loss exists bilaterally, quantification of bone loss should be an approximation based on the outline of the glenoid fossa. In practice, MR imaging, and not CT, is the preferred means of evaluating shoulder dislocation. The multiplanar capability of MR imaging may make it ideally suited to acquiring images en face to the glenoid fossa, providing a direct assessment of glenoid contour.
This study addresses potential methods of quantifying glenoid bone loss on CT. The study has three main limitations. First, the number of healthy subjects is small, reflecting the paucity of patients undergoing high-resolution upper body CT examinations. Second, in predicting bone loss, we assumed that both glenoid fossae were equal in size, although mild variation in side-to-side measurements has been shown to exist in healthy subjects. Third, no arthroscopic correlation with glenoid bone defects or cadaveric correlation of gle noid measurements was undertaken.
Longitudinal studies are required to determine the critical level of bone loss required to necessitate a bone graft procedure rather than a Bankart repair and to determine what bearing bone loss has on procedure outcome. A 25% loss in glenoid axial depth has been suggested as a threshold value in one study, although this figure has not been prospectively validated [2]. In a further study, osseous glenoid defects of varying sizes were created on cadaveric scapulae, and the effect on the force required to induce dislocation was tested [3]. A defect resulting in a glenoid width of less than 21% of glenoid length was considered to induce instability. A potential limitation of this latter study is that defects were created on the anteroinferior margin of the glenoid, whereas our study indicates that, in recurrent dislocation, principal bone loss occurs anteriorly rather than anteroinferiorly.
In conclusion, an oblique sagittal image aligned en face to the glenoid fossa should be routinely acquired when assessing anterior shoulder dislocation. Flattening of the anterior glenoid curvature is shown in most patients with anterior dislocation and increases exponentially with increasing number of dislocations. In unilateral dislocation, when glenoid bone loss is moderate to severe, a comparison of maximum glenoid width with that of the contralateral side proved to be the best indicator of glenoid bone loss.
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