British Journal of Radiology (2005) 78, 1005-1008
© 2005 British Institute of Radiology
doi: 10.1259/bjr/27340997
Coracoid tip position on frontal radiographs of the shoulder: a predictor of common shoulder pathologies?
C U Schulz, MD1,
H Anetzberger, MD2 and
C Glaser, MD3
1 Department of Orthopaedic Surgery, Ludwig Maximilians Universitaet Munich, Germany, 2 Department of Orthopaedic Surgery, Technische Universitaet Munich, Germany and 3 Department of Radiology, Ludwig Maximilians Universitaet Munich, Germany
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Abstract
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The coracoacromial arch is a static anterior–superior stabilizer of the humeral head. Thus coracoacromial arch inclination, which varies depending on coracoid tip position, may be related to shoulder pathologies. Therefore, we retrospectively analysed coracoid tip positions in the true anterior posterior view of different shoulder pathologies: reference shoulders (n=27), shoulders with rotator cuff tear (supraspinatus tear n=29; subscapularis tear n=21) and shoulders with anterior glenohumeral instability (traumatic n=17; atraumatic n=14). In supraspinatus tear shoulders, the coracoid tip projected onto inferior glenoid half in 86% of cases (type I coracoid), extending more inferiorly compared with reference group (p=0.0002) or subscapularis tear shoulders (p<0.0001). In contrast, 78% of cases with subscapularis tear show the coracoid tip projected onto the superior glenoid half (type II coracoid). Atraumatic glenohumeral instabilities had a more superior coracoid tip position than traumatic instabilities (p=0.04), but no differences were observed on basis of coracoid type or in comparison with normal controls. We conclude that coracoid tip position is highly variable. Since type I coracoids are predominant in shoulders with supraspinatus tears and type II coracoids in shoulders with subscapularis tears, coracoid tip position may thus provide a simple diagnostic complement for a probable site of rotator cuff tears.
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Introduction
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The coracoacromial ligament, which at its medial end attaches to the tip of the coracoid process [1–3] is an important part of the subacromial arch and serves as a static anterior and superior stabilizer of the humeral head [4, 5]. Since morphology of the coracoid process shows differences in shape, length and direction [1] variations in coracoacromial ligament orientation are to be expected. Mechanical conditions of the glenohumeral joint may thus vary due to individual coracoid morphology and therefore may be related to shoulder pathologies such as rotator cuff disorders [6–8] or glenohumeral instabilities [4, 9].
In practice, little attention has been paid to the diagnostic value of the coracoid variants in common shoulder pathologies except in cases of the rare subcoracoid impingement [10]. However, further diagnostic criteria would be desirable, since diagnosis of rotator cuff pathologies or the cause of instabilities pose a problem even when using MRI [12, 13].
The true anterior posterior view of the shoulder, also known as glenoid fossa view, which projects a beam parallel to the glenoid surface and 15° inclined from superior to inferior, is one of the basic X-ray examinations in the diagnostic process of shoulder pathologies [11]. It is used routinely by shoulder surgeons and provides a simple method to describe coracoid morphology with respect to coracoid tip position, which has not been evaluated so far. Thus, if shoulder pathologies would correlate with coracoid tip position, an additional diagnostic supplement could be established which could relate to localizations of rotator cuff tears or suggest atraumatic causes of anterior glenohumeral instability.
Therefore, the aim of this study was to investigate the relationship between common shoulder pathologies and coracoid tip position in the true anterior posterior view of the shoulder.
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Table 1. Coracoid cavity ratio (CCR) measured in the true anterior–posterior view of the shoulder of different shoulder pathologies is defined as the ratio between inferior coracoid extension (D2) and glenoid height (D1) in %
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Patients
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Subjects in this study were selected from surgical reports of patients operated on in our department between 1999 and 2000 for isolated acromio-clavicular arthritis, rotator cuff pathologies or anterior glenohumeral instabilities. From this cohort of patients a true anterior posterior view of the shoulder was available in 119 patients [11]. 27 plain films were excluded because of inadequate radiographic projections: either due to missing overlap of anterior and posterior glenoid rims suggesting inadequate oblique positioning of the patients, or due to visually notable ovoid presentation of the humeral head suggesting excessively deviated superior or inferior beam positioning. In addition, shoulders (n=16) from healthy medical staff of our unit served as controls.
We examined 108 shoulders (48 left and 60 right) from 28 females and 80 males aged between 18 years and 79 years. On the basis of pathological finding, subjects were divided in three groups: (I) control group, consisting of stable shoulders with isolated acromio-clavicular arthritis (n=11) and normal shoulders (n=16), (II) rotator cuff group, consisting of shoulders with isolated tear of the supraspinatus (n=29) or shoulders with subscapularis tears with or without rotator interval defect (n=21), and (III) anterior glenohumeral instability group, consisting of shoulders with traumatic, unilateral (n=17) or atraumatic, bilateral glenohumeral instability (n=14).
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Methods
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A standardized analysis of each plain film was performed to evaluate the inferior dimensions of the coracoid with respect to the superior edge of the glenoid cavity (Figure 1). Tip of the coracoid process projecting on the inferior half of the glenoid surface (type I coracoid, Figure 2a) was distinguished from a variant with coracoid tip projecting on the superior half of the glenoid surface (type II coracoid, Figure 2b). For quantitative analysis the relationship between glenoid cavity height (D1) and inferior dimensions of the coracoid with respect to superior glenoid edge (D2) was measured (coracoid cavity ratio (CCR)) to minimize errors due to interindividual variations in size or plain film magnification factor.
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Figure 1. Measurement of coracoid tip position in the true anterior–posterior view of the shoulder. Glenoid cavity plane (line 1, tangent on the inferior and superior glenoid rim); glenoid cavity height (D1, distance between inferior and superior glenoid rim); parallel of line 1 at level of coracoid tip (line 2); inferior coracoid extension (D2, distance between superior glenoid rim and coracoid tip).
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Figure 2. Coracoid variants on the true anterior posterior view of the shoulder (a) type I coracoid: coracoid tip (arrow) projects at level of the inferior glenoid half (b) type II coracoid: coracoid tip (arrow) projects at level of the superior glenoid half.
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The statistical analysis was performed by Graph Pad Prism 3.0 (Graph Pad Software, San Diego, CA). The level of significance was defined as p<0.05. Sensitivity [real positive/(real positive+false negative)] and specificity [real negative/(false positive+real negative)] of an inferior versus a superior coracoid tip position were calculated for shoulders with rotator cuff tears.
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Results
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Only 4 out of 29 cases with supraspinatus tear and 6 out of 17 cases with traumatic glenohumeral instability showed a coracoid tip projecting within the superior half of the glenoid. In contrast, only 6 out of 21 cases with subscapularis tear and 4 out of 14 cases with atraumatic glenohumeral instability had a coracoid tip projecting within the inferior half of the glenoid. Thus, distribution of type I and II coracoids significantly varies between shoulders with supraspinatus tears and shoulders with subscapularis tears (p<0.0001, Fisher exact test) whereas no significant variation was observed in shoulders with traumatic compared with atraumatic glenohumeral instabilities (p<0.0732, Fisher exact test).
Only shoulders with isolated supraspinatus tear were characterized by higher CCR compared with normal controls (p=0.0002, unpaired t-test). In contrast, shoulders with subscapularis tear or shoulders with glenohumeral instability did not differ from normal controls.
CCR was lower in shoulders with subscapularis tear when compared with shoulders with supraspinatus tear (p<0.0001, unpaired t-test). Furthermore, CCR was also lower in shoulders with atraumatic anterior glenohumeral instability compared with post-traumatic instabilities (p=0.0395, unpaired t-test).
Type I coracoid had a sensitivity of 86% and a specificity of 71% for diagnosis of a supraspinatus tear. Type II coracoid had a sensitivity of 71% and a specificity of 86% for diagnosis of a subscapularis tear.
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Discussion
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This retrospective study was undertaken to establish whether variations of coracoid morphology, with respect to coracoid tip position in frontal view of the shoulder, could serve as a diagnostic supplement in routine evaluation of common shoulder disorders.
The true anterior posterior view of the shoulder was chosen for evaluation because it is an established examination in patients with shoulder pathologies. Even though the position of the patient and angle of the beam are defined and coracoid tip position can easily be assessed, there are several limitations which should be considered. In practice, patient positioning is difficult because of individual physiognomic variations, such as degree of kyphotic posture or muscle status which may influence real beam angle. Furthermore, anatomical differences in the glenoid fossa, such as glenoid version or glenoid tilt, are frequent and may thus affect the measurements [14]. Alternative two-dimensional reconstructions by MRI or CT are biased because they are operator dependent. Also, three-dimensional CT, which theoretically should be more exact, also has limitations. It is complicated and unsuitable for use in daily routine because of higher costs and exposure to irradiation.
Variations of the acromial shape [15] and traction osteophytes at the acromial insertion of the coracoacromial ligament have already been suggested as related aetiological factors for impingement syndrome [3, 6]. However, coracoid variants have rarely been considered as a factor except for in rare subcoracoid impingement, where coracoid tip position is closer to the neck of the glenoid in the coronal plane [6, 7, 10, 16, 17].
Our data on coracoid tip position in the frontal plane of the shoulder suggest a link between common shoulder pathologies and variations of relative coracoid length, which can be described as type I and II coracoids (Figure 2
) In addition, type and relative inferior extension of coracoid variants differed significantly in rotator cuff tears at different sites. A similar trend, which was statistically insignificant, was also observed in shoulders with anterior glenohumeral instability of varying causes.
Alterations in orientation of the subacromial arch due to a more inferiorly or superiorly positioned medial insertion of the coracoacromial ligament probably play a role in rotator cuff pathology. From a functional perspective, a more inferior coracoid tip position indicates a reduction of the subacromial space at the site of the greater humeral tuberosity, which is the area of supraspinatus tendon insertion [2, 3]. Prevalence of supraspinatus tears in presence of type I coracoid in 86% of cases supports a hypothesis of mechanical impingement between the supraspinatus tendon and coracoacromial arch (Figure 2a).
In contrast, a more horizontal course of the subacromial arch should reduce static anterior–superior stabilization of the humeral head [4, 5] and thus increase mechanical stress on the subscapularis tendon, which is thought to be an essential active anterior stabilizer of the humeral head [19]. Accordingly, 78% of shoulders with subscapularis tear had a high coracoid tip position, suggesting an association between decreased anterior humeral head stability and subscapularis pathologies. This would further explain the prevalent variant of a type II coracoid in shoulders with atraumatic anterior glenohumeral instability in 71% (Figure 2b).
Even under standardized conditions, 23% of the available plain films had to be excluded because of inadequate projections suggesting that true anterior–posterior view of the shoulder should be controlled by fluoroscopy. However, measurement of relative coracoid length may only be a rough estimate. Despite these limitations, appreciation of coracoid type in shoulders with diagnosed rotator cuff tears revealed 86% sensitivity and 71% specificity for supraspinatus tear shoulders (type I) and 71% sensitivity and 86% specificity for subscapularis tear shoulders (type II). These data are comparable with values obtained by MRI in rotator cuff abnormalities suggesting an association between biceps tendon tears and probable site of rotator cuff tears [20].
We therefore conclude that coracoid tip position is highly variable. Depending on whether it is projecting at the level of the inferior or superior half of the glenoid, coracoid tip position may predict the site of rotator cuff tear. Therefore, evaluation of coracoid type on the true anterior–posterior view of the shoulder may be an additional diagnostic tool in shoulder pathologies.
Received for publication September 2, 2004.
Revision received May 11, 2005.
Accepted for publication May 20, 2005.
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Abstract
Introduction
