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1 Both authors: Department of Diagnostic Imaging, Brown University School of Medicine and Rhode Island Hospital, 593 Eddy St., Providence, RI 02903.
Received June 13, 2002;
accepted after revision August 6, 2002.
Address correspondence to G. A. Tung.
Abstract
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MATERIALS AND METHODS. Posterior labrocapsular tears identified on 24 MR arthrograms and surgically confirmed were evaluated for length of tear and labrocapsular avulsion. These examinations and a comparison cohort of 70 normal MR arthrograms with normal findings were also evaluated for humeral head position relative to the glenoid fossa. Medical records were reviewed for clinical diagnosis of posterior instability and history of shoulder trauma.
RESULTS. The position of the humeral head relative to the glenoid was significantly more posterior in patients with posterior labral tear than in patients with a normal posterior labrum (4.9 mm versus 0.7 mm; p < 0.0001). The mean length (± SD) of posterior labral tear was 15.9 ± 1.7 mm, and a direct correlation was found between tear length and posterior humeral translation (r = -0.65; p = 0.002). Posterior labral tears were significantly longer (18.6 vs 13.1 mm; p = 0.04), and posterior humeral translation was greater (6.4 vs 3.4 mm; p = 0.006) in patients with labrocapsular avulsion than in those without avulsion. Twelve (50%) of the patients with posterior labrocapsular tear had posterior instability, and 10 (83%) had a history of macrotrauma. On MR arthrography, the mean posterior humeral translation was greater (6.2 mm ± 0.08; p = 0.019), posterior labral tears were longer (19.4 mm ± 1.7; p = 0.0008), and labrocapsular avulsion was more common (83%; p = 0.0001) in patients with posterior instability than in patients who had a posterior labral tear but a clinically stable shoulder.
CONCLUSION. Clinical posterior instability is associated with excessive posterior humeral translation, long posterior labral tears, and posterior labrocapsular avulsion.
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There is no consensus about the critical lesion that produces posterior instability. An excessively retroverted or hypoplastic osseous glenoid, detached posterior labrum ("reverse Bankart" lesion), and capsular laxity have been reported in patients with recurrent posterior glenohumeral subluxation [4,5,6,7,8,9]. Posterior instability has been associated with hypoplasia and retroversion of the glenoid and increased retroversion of the humerus [5]. Weishaupt et al. [9] reported a 93% prevalence of posterior glenoid rim deficiency in 15 patients with posterior shoulder instability compared with 60% and 73% prevalence in patients with anterior and no instability, respectively. The prevalence of posterior labral tears is lower and more variable in patients with posterior instability than the prevalence of anteroinferior labrocapsular lesions in patients with anterior instability [2, 6, 10]. A lax capsule may also predispose to posterior instability; however, the surgical criteria for the diagnosis of capsuloligamentous laxity are not well defined and are based on subjective estimates of excessive joint capacity [11]. Most recently, Yu et al. [12] reported an injury in six athletes with posterior instability characterized on MR imaging by an avulsion of the attachment of the glenohumeral capsule and periosteum that produced a patulous recess beneath the detached labrum and periosteal sleeve. However, in general, few studies of MR imaging and MR arthrography of the posterior glenoid labrum and capsule have been performed, even though MR arthrography is widely held to be the most sensitive noninvasive examination for the evaluation of the labrocapsular complex. The objectives of this study were to investigate the relationship between tears of the posterior labrocapsular complex and humeral position relative to the glenoid fossa on MR arthrography and the presence and extent of posterior labrocapsular tears in patients with clinical posterior instability.
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Diagnostic Criteria
On the basis of the conventional clock-face description of location, tears
of the glenoid labrum from the 6-o'clock to 11-o'clock positions were
designated as posterior labral tears and were diagnosed primarily on axial
plane images
[13,14,15].
On MR arthrography, labral tear was diagnosed when there was a focal extension
of intraarticular contrast material into the fibrous portion of the labrum
(Fig. 1) or when the labrum was
detached or displaced [15,
16]. The craniocaudal length
of the tear was quantified from the product of the number of axial images on
which a tear was identified and the sum of slice thickness and interslice gap.
For example, if a posterior labral tear was observed on four images having
slice thickness of 3 mm and no gap, the tear was 12 mm in length. Posterior
labrocapsular avulsion was defined by the presence of intraarticular contrast
material that extended through a posterior labral tear posteromedially along
the posterior glenoid rim (Figs.
2,3A,3B,3C,4).
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A method for quantifying the position of the humeral head relative to the glenoid fossa was developed from methods described by Das et al. [17] and Weishaupt et al. [9] (Fig. 5). In those patients who had a posterior labral tear, humeral position was quantified on the axial image through the middle of the labral lesion in the following manner: First, a short line segment is drawn connecting the anterior and posterior tips of the osseous glenoid rim (line segment G). Next, a second longer line segment representing the plane of the scapular body (line segment SS) is drawn tangentially to the ventral or costal surface of the scapular body, through the midpoint of the glenoid line segment G, and is extended through the humeral head. The distance from the center of the humeral head to line segment SS represents the humeral translation distance. If the center of the humeral head was posterior to this line, we recorded the distance (measured in millimeters) as a negative value. If anterior, the distance was recorded as a positive value.
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To compare humeral position in the absence of a posterior labrocapsular tear, 70 MR arthrographic examinations were selected in which the posterior labrum and capsule were normal in appearance. These MR arthrograms were obtained during the same period as those of the study group. In these comparison examinations, humeral head translation was quantified on a single axial image 2 cm below the base of the scapular spine. On an axial image, the base of the scapular spine has a distinctive appearance that resembles a sea horse [18].
MR Arthrographic Technique
Formal approval from the institutional review board to perform MR
arthrography had been obtained when shoulder MR arthrography was first
performed at our hospital in 1994. A 2-mmol/L solution of gadodiamide was
prepared by diluting Omniscan (Nycomed, Amersham, Wayne, PA) in a saline
solution. To this diluted gadodiamide solution, we added 60% meglumine
diatrizoate (Reno-M-60; Squibb, New Brunswick, NJ), 1% lidocaine, and 1:1000
epinephrine. Between 12 and 20 mL of this contrast solution was carefully
injected into the glenohumeral joint under fluoroscopic guidance. The shoulder
was then placed through a full range of motion. Within 60 min of the
intraarticular instillation of contrast material, MR imaging of the shoulder
was performed on a 1.5-T magnet (Magnetom Vision; Siemens Medical Systems,
Erlangen, Germany) using a conventional, circularly polarized, flexible
surface coil. MR imaging was performed with the patient's arm adducted and the
hand in a neutral position. Fat-saturated T1-weighted conventional spin-echo
images of the shoulder were obtained in the axial, coronal oblique, sagittal
oblique, and axial oblique planes (TR range/TE, 780-800/12; field of view, 16
cm; matrix, 192 x 256; slice thickness, 3 mm; interslice gap, 0-1 mm;
acquisitions, 2). Before MR imaging was performed in the axial oblique plane,
the patient's arm was placed in an abduction—external rotation position.
Additionally, a T2-weighted turbo spin-echo sequence with fat-saturation was
performed in the coronal oblique plane (TR/effective TE, 3800/96; echo train
length, 7; field of view, 16 cm; matrix, 252 x 256; slice thickness, 3
mm; interslice gap, 10%; acquisition, 1).
Clinical Evaluation
Presurgical clinical evaluation, arthroscopy, or open surgery was performed
by one of five orthopedic surgeons whose subspecialty is shoulder orthopedics
or sports medicine. The clinical diagnosis of posterior instability is based
on history and physical examination, and includes one or more of the following
symptoms: reproduction of pain, instability, or apprehension when the arm is
in the provocative position of adduction, flexion, and internal rotation;
positive provocative clinical maneuvers, including posterior stress,
pivot-shift, jerk, or load and shifting tests; and greater than 1 cm of
translation of humeral head on the glenoid fossa as assessed on load-shift and
apprehension testing [19]. In
some patients, the physical examination was performed before and during
general anesthesia. All surgeons had knowledge of the results of MR
arthrography before surgery.
Office records and procedure reports were reviewed for a history of any specific traumatic event that was believed by the patient or physician to have been the initial episode of posterior instability.
Statistical Analysis
Tests of statistical significance were performed using the unpaired
Student's t test, the chi-square test, and the nonparametric
Spearman's rank correlation coefficient. Significance values were calculated
with respect to a two-tailed alternative hypothesis, and the null hypothesis
was rejected with a p value of less than 0.05.
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Humeral Position Relative to the Osseous Glenoid
Seventy MR arthrographic examinations showing a normal posterior glenoid
labrum and capsule were performed in 45 males and 25 females with an average
age of 42.8 ± 2.1 years (range, 17-83 years). The right shoulder was
imaged in 36 patients and the left shoulder in 34 patients. Arthroscopic
confirmation of a normal posterior labrum was obtained in 17 patients (25%);
neither arthroscopy nor open surgery was performed in the other 53 patients.
In this group, the mean translation of the humeral head relative to the plane
of the scapular body was -0.7 ± 0.2 mm (range, -4.6 to 4.2 mm).
In the study group of 24 patients with posterior labral tear, mean translation of the humeral head was -4.9 ± 0.6 mm (range, -10.5 to -0.8 mm). Significantly greater posterior humeral translation was found in this group than in the comparison cohort with no labral tear (p < 0.0001; Table 1).
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Length of Tear
The mean length of the posterior labral tear was 15.9 ± 1.7 mm
(range, 7-31.5 mm). We found a direct correlation between the length of the
posterior labrocapsular tear and posterior translation of the humeral head
(r = -0.65, p = 0.002;
Fig. 6).
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Posterior Labrocapsular Avulsion
Posterior labrocapsular avulsion was identified in 12 (50%) of 24 patients
with posterior labral tear. Posterior humeral translation was greater in the
patients with labrocapsular avulsion (-6.4 ± 0.71 mm) than in those
without avulsion (-3.4 ± 0.68 mm; p = 0.006). The posterior
labral tear was also longer in those patients with avulsion of the posterior
labrocapsular complex (18.6 ± 1.8 mm) than in those with labral tear
but no capsular avulsion (13.1 ± 1.8 mm; p = 0.04).
Posterior Instability
Table 2 presents a
comparison of findings based on posterior glenohumeral instability in the
group of 24 patients with posterior labral tears. Patients with posterior
instability were younger (mean age, 25.5 ± 3.0 years; p =
0.0002) than those without posterior instability; the clinical history of 10
patients with posterior instability (83%) noted an instigating shoulder
trauma. On MR arthrography, posterior humeral translation was greater (-6.2
± 0.08 mm; p = 0.019), tears of the labrum were longer (19.4
± 1.7 mm; p = 0.008), and labrocapsular avulsion was more
prevalent (83%; p = 0.0001) in patients with posterior shoulder
instability. A labral tear 15 mm or longer had a sensitivity of 83% and
specificity of 83% for the diagnosis of posterior instability.
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Incompetence of the posterior capsule, whether from traumatic injury or congenital laxity, is recognized as a potential cause of recurrent posterior subluxation [5, 19]. Experiments on cadavers showed that selective incision of the posterior or inferior capsule promoted posterior glenohumeral subluxation [19, 27,28,29]. At surgery or arthroscopy, a stretched capsule or one that is stripped from the periosteum can be shown in some patients with posterior instability [5]. However, compared with the number of reports on the much more prevalent anterior instability, relatively few reports of imaging findings on posterior glenohumeral instability are found in the literature [9, 30]. On CT, ossification is occasionally identified in the posterior joint capsule (Bennett lesion) in throwing athletes with posterior shoulder pain and posterior labral tears [30]. The Bennett lesion is believed to result from a tear of the posterior capsule that produces bleeding and subsequent reactive heterotopic ossification [31]. Our study shows that lesions of the posterior capsule and glenoid labrum can be shown on MR arthrography in patients with clinical evidence of posterior instability. We found that patients with posterior instability were more likely than patients with labral tears but clinically stable shoulders to have focal avulsion of the posterior capsule adjacent to a posterior labral tear. Given its correlation with posterior humeral translation, capsular avulsion indicates a more severe injury than posterior labral tear alone. The proximity of posterior capsular avulsion to the posterior labral tear suggests that these two lesions may be related through a common mechanism of injury, such as macrotrauma or repetitive microtrauma. In our series, 63% of patients with posterior labrocapsular lesions recalled a discrete initial traumatic event; other researchers have reported a history of macrotrauma in approximately half of their patients with recurrent posterior shoulder instability [32].
The prevalence of posterior labral tears in patients with posterior instability is lower and more variable than the prevalence of anteroinferior labrocapsular tears in patients with anterior instability. Altchek et al. [10] reported 10 posteroinferior labral tears in a series of 40 shoulders, 16 of which were unstable posteriorly, and Papendick and Savoie [6] reported 20 labral detachments in 41 shoulders with posterior instability. Conversely, posterior labral tears can be shown in clinically stable shoulders. On MR arthrography, Palmer and Caslowitz [16] reported 13 posterior labral tears with variable superior extension in 84 stable shoulders, and posterior labral tears were identified on physical examination in 12 patients without posterior instability. Our study suggests that integrity of the posterior labrum is important for maintaining normal congruity between the humeral head and glenoid fossa and that this can be assessed on routine MR arthrography in the axial plane. In a group of patients with no signs of posterior instability, we found that the center of the humeral head was usually within 1 mm of the plane of the scapular spine on MR arthrography in the axial plane. In contrast, the center of the humeral head was translated 4 mm more posteriorly on average in patients with posterior labral tears than in those without tears. However, we suggest that it is the length of the tear that has a more important correlation with instability than the simple presence of a posterior labral tear. Patients with posterior instability had posterior labral tears of greater length than those with stable shoulders, and there was a direct correlation between tear length and posterior humeral translation. In a selected population of patients with posterior labral tear, a tear of 15 mm or greater in craniocaudal length on MR arthrography had a sensitivity and specificity of 83% for posterior instability. Weishaupt et al. [9] found that a posteroinferior defect in glenoid shape with a craniocaudal length of 12 mm or more predicted posterior instability with a sensitivity of 87% and a specificity of 83%. Our findings support the contention of Weishaupt et al. that it is the extent of the abnormal glenoid articulation that is critical to the pathogenesis of posterior shoulder instability. Long posterior labral tears, particularly when associated with an avulsed capsule, may promote posterior instability much like that of a retroverted bony glenoid.
Our study has several limitations. First, this investigation does not assess the prevalence of posterior tears or capsular avulsion in all patients with posterior instability because it was a retrospective study and the presence of a posterior labral tear was the major inclusion criterion. Findings in numerous other studies of posterior instability show that it is not necessary to have a posterior labral tear or deficient posterior capsule for the shoulder to be unstable [5,6,7, 10]. Second, arthroscopic confirmation of a normal posterior labrum was not available in 53 of 70 patients in the comparison cohort of stable shoulders. However, the rate of false-normal findings on MR arthrography for labral lesions is generally accepted to be low [16]. Third, humeral position relative to the glenoid fossa is in part determined by the degree of shoulder rotation. In MR arthrography, the shoulder position is neutral but small degrees of rotation may affect relative humeral position. More important, we do not know the effect of arthrography on physiologic humeral position. Negative pressure within the glenohumeral joint is thought to provide resistance to humeral translation and is dissipated when the volume of the capsule is increased [33].
In conclusion, the position of the humeral head relative to the glenoid fossa and the integrity of the posterior labrocapsular complex can be evaluated on routine MR arthrography. A long posterior labral tear, particularly in combination with avulsion of the posterior joint capsule, is associated with excessive posterior translation of the humerus and may predispose to recurrent posterior subluxation and instability.
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