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1 Mallinckrodt Institute of Radiology, Washington University School of Medicine,
510 S Kingshighway Blvd., St. Louis, MO 63110.
2 Department of Orthopaedics and Rehabilitation, Southern Illinois University
School of Medicine, Springfield, IL 62794.
3 Department of Orthopaedic Surgery, One Barnes-Jewish Hospital Plaza, St.
Louis, MO 63110.
Received July 30, 2004;
accepted after revision September 15, 2004.
Supported by a grant from Society of Radiologists in Ultrasound.
Abstract
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SUBJECTS AND METHODS. Seventy-one consecutive patients with shoulder pain who were prospectively studied with sonography had subsequent arthroscopy that showed a full-thickness or partial-thickness tear or intact cuff. For sonography and arthroscopy, the length or degree of retraction and width of a tear, when present, was recorded. When there were discrepant findings, representative images were jointly evaluated by the radiologist and orthopedic surgeon to determine the cause of the error.
RESULTS. Fifteen detection errors were found, including five misses (three < 5-mm subscapularis and two small partial-thickness tears), four errors inherent with the test (distinguishing large bursal side or extensive partial-thickness from full-thickness tears and tendinopathy from partial-thickness tears), three errors of an unknown cause, two due to misinterpretation, and one error inherent with the patient. Seventeen measurement errors occurred with full-thickness tears, 15 of those in patients with large or massive tears. Bursal thickening (n = 4), non-visualization of the torn tendon end (n = 2), nonretracted tear (n = 2), and complex tear (n = 1) contributed to the errors. Eight measurement errors occurred with partial-thickness tears. Difficulty distinguishing tendinopathy from partial-thickness tears (n = 3) and complex tears (n = 3) accounted for six errors.
CONCLUSION. Although infrequent, detection errors were due to limitations inherent with the test or misses. Limitations inherent with the patient and misinterpretation of the findings were rare. Most measurement errors occurred in patients with large or massive cuff tears.
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Imaging and Surgical Techniques
All sonograms were obtained with an Elegra scanner (Siemens Medical
Solutions) using a high-frequency, 7.5-9 MHz linear array transducer.
Real-time scanning of the shoulder was performed in a standardized fashion as
described by Teefey et al. [1].
In nearly all cases, harmonic imaging was performed with a transmit frequency
of 4.5 MHz.
A single subspecialty-trained shoulder surgeon performed all arthroscopic examinations and surgeries. Representative arthroscopic images and videotapes were made of all tears.
Imaging and Surgical Interpretation
One of two radiologists similarly experienced with musculoskeletal
sonography prospectively obtained and interpreted each sonogram. Established
criteria were used for the diagnosis of a full- or partial-thickness tear
[1,
3,
9]. For each sonogram, the
absence or presence of a full- or partial-thickness rotator cuff tear was
recorded. The length of a partial-thickness tear or degree of retraction of a
full-thickness tear (measured on longitudinal views oriented parallel to the
long axis of the cuff) and width (measured on transverse views oriented
perpendicular to the long axis of the cuff) were also recorded. When the width
was greater than 30 mm, an attempt was often made to measure it with extended
field-of-view imaging. The presence or absence of a subscapularis tendon tear
was recorded separately without measurements. Each radiologist also recorded
whether the sonogram was diagnostic, suboptimal but diagnostic, or
nondiagnostic.
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Data Analysis
Diagnoses and measurements made with sonography were compared with the
arthroscopic findings. Findings were compared for the presence or absence of a
full- or partial-thickness tear, tear length or retraction, and tear width.
Sonographic measurements within 5 mm of the measurement recorded at
arthroscopy were considered correct. Tears 30 mm or greater in width and those
retracted beneath the acromion were assigned a value of 30 mm.
When discrepant findings between sonography and arthroscopy were found, sonographic and representative arthroscopic images were retrospectively reviewed jointly by one radiologist and the orthopedic surgeon to determine the cause of the discrepancy. In one discrepant case, arthroscopic images were not available for review. Afterward, the causes for the detection errors (including subscapularis tears) were categorized as follows: inherent with the test (i.e., the study would have been interpreted the same way in retrospect based on the standard sonographic criteria for diagnosing a tear), inherent with the patient (i.e., an inability to perform the test because of patient limitations), misinterpretation of a finding (the abnormality was identified but misinterpreted), a miss (no abnormality detected), and an unknown cause. The causes for measurement errors were descriptive.
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Of the 19 partial-thickness rotator cuff tears, six diagnostic errors occurred. In two cases, the finding was missed. In these two cases, a review of the representative sonographic images showed a normal-appearing cuff (Figs. 1A, and 1B). The articular-side partial-thickness tears measured 5 x 15 mm and 15 x 5 mm (length x width) on arthroscopy. In the other four cases, full-thickness tears were diagnosed. In two of these cases, large bursal-side, partial-thickness tears were found on arthroscopy involving most of the cuff (in one case, more than 90% of the cuff fibers were torn) (Fig. 2) and in another case, a markedly thinned cuff with an extensive partial-thickness tear was found (Fig. 3). These three errors were categorized as inherent with the test. In the fourth case, the cause for the error was unknown. A review of the representative arthroscopic images showed no full-thickness tear; however, the sonographic images showed a small contour deformity of the cuff. All 19 sonograms were considered diagnostic.
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Of the eight subscapularis tendon tears, four diagnostic errors were found. A review of the sonographic images in three cases showed no tear. All three of these errors were categorized as misses. In all three cases, the subscapularis tears were < 5-mm partial-thickness tears of the superior aspect of the tendon as noted on arthroscopy. Sonographic visualization of the subscapularis tendon was hindered by limited external rotation in the fourth case and was the only case considered nondiagnostic. This error was categorized as inherent with the patient.
Two subscapularis tendon tears were diagnosed with sonography in patients with intact subscapularis tendons on arthroscopy (Fig. 4). In one of these cases, a tear involving the supraspinatus and infraspinatus tendons was noted to extend over the rotator interval on arthroscopy although it did not involve the subscapularis tendon. The arthroscopic images were not available for review in the second case. Review of the sonographic images showed a small defect in the subscapularis tendon. The first diagnostic error was categorized as a misinterpretation of the findings and the second as from an unknown cause.
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Size and Extent of the Tears
Of the 45 full-thickness rotator cuff tears correctly diagnosed on
sonography, 11 errors were in tear retraction and six were errors in
tear-width measurements. Retraction was overestimated in eight cases and
underestimated in three. In all but one case, large or massive tears (20-45 mm
in width) were found on arthroscopy. In two of the three cases in which
retraction was underestimated, the medial torn ends were retracted beneath the
acromion. In three of the cases in which tear retraction was overestimated and
in one case in which it was underestimated, a review of the sonograms showed
bursal thickening. In another case, review of the arthroscopic images showed a
complex, irregularly shaped tear. In the remaining four cases, a cause for the
errors could not be determined. Tear width was underestimated in three cases
and overestimated in three. In five of these cases, tear widths measured 20-30
mm on arthroscopy. In two of the three cases in which tear width was
underestimated, portions of the tears were nonretracted. A review of the
arthroscopic and sonographic images showed no apparent reasons for the
remaining four errors.
Of the 13 partial-thickness tears that were correctly diagnosed with sonography, two errors had occurred in tear length and six in tear-width measurements. In one of the two cases in which tear-length measurements were underestimated, a review of the arthroscopic images showed a complex tear with multiple, separate components and in the other case, a review of the sonograms showed a hypoechoic region that was larger than what was measured. In two of the six cases in which tear widths were underestimated, a review of the arthroscopic showed irregularly shaped tears. In two other cases, a review of the sonograms showed apparent hypoechoic regions on transverse images that were larger than what was measured (Fig. 6). In the remaining two cases, a cause for the measurement errors could not be determined.
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We found that one third of our diagnostic errors were inherent with the test or patient and beyond the control of the radiologist. One of these errors occurred because of difficulty distinguishing an extensive partial-thickness tear from a full-thickness tear on sonography (Fig. 3). The error occurred because of the substantial loss of cuff substance and compressibility of the few remaining fibers with the transducer. One study reported that an extensive partial-thickness tear involving greater than 50% of the cuff substance can mimic a full-thickness tear by virtue of its compressibility [1]. Large partial-thickness bursal-side tears can also be mistaken for full-thickness tears (Fig. 2). A large bursal-side cuff tear, by virtue of its size and location, will appear on sonography as a contour deformity and simulate a full-thickness tear, as in two of our cases. This difficulty in distinguishing partial- and full-thickness tears has recently been shown as the primary cause of interobserver variability between two experienced observers [12]. Clinically, a failure to distinguish extensive partial-thickness from full-thickness tears may not be important because orthopedic surgeons generally treat extensive partial-thickness tears or large bursal-side tears as if they were full-thickness ones.
Difficulty distinguishing tendinopathy from an articular-side partial-thickness tear was another source of error inherent with the test but occurred only once. As stated earlier, differentiating these two entities can be problematic. Only one error was inherent with the patient; this error occurred when sonographic visualization of the subscapularis tendon was hindered by a limited range of motion, as the patient was unable to externally rotate his arm to evaluate the tendon. This was the only case in our study in which such a limitation occurred, indicating that it is rare.
We were unable to determine the causes of the diagnostic errors in three cases. One occurred in a patient with a partial-thickness tear; MRI and sonography showed a contour deformity in the cuff, suggestive of a full-thickness tear. The second error occurred in a patient thought to have a partial-thickness subscapularis tear; unfortunately, arthroscopic images were not available for review. In the third case, a patient with adhesive capsulitis was diagnosed with a small full-thickness tear, also reported in an MRI study (Fig. 5). Although it is difficult to explain these errors, it is possible that small tears may have been missed on arthroscopy.
Sonographic findings were rarely misinterpreted. In one case, a contour deformity was present on sonography, but an extensive partial-thickness tear was diagnosed rather than a full-thickness tear due to the perceived presence of intact bursal-side fibers during real-time scanning. One study reported that extensive partial-thickness tears can produce a cuff defect [1]. Nevertheless, had the cuff defect, an important criterion for the diagnosis of a full-thickness tear, been interpreted as such, the error would not have occurred. The only other error occurred in a patient thought to have a subscapularis tear (Fig. 4). The arthroscopic images showed a supraspinatus tear that extended over the rotator interval but did not involve the tendon. This subtle distinction was almost certainly the cause for the error. MRI also diagnosed a subscapularis tear in this case.
We found that most of the retraction-measurement errors in patients with full-thickness tears occurred with large or massive tears. Although it was difficult to determine the exact cause of the discrepant measurements in every case, we speculate that several factors may have contributed. First, because large and massive tears can be irregularly shaped, it is doubtful that both sets of measurements were obtained at the same location in these cases. In addition, when bursal thickening was present, it was more difficult to determine the precise location of the torn tendon ends. Shoulders were also positioned differently during sonography and surgery. During sonography, the shoulder was extended and the elbow flexed and directed medially; this may have increased the distance between the torn tendon ends and may have caused local muscle groups to contract, resulting in further medial retraction. At surgery, the humeral head was reduced to an anatomic position, allowing the torn ends of the cuff to return to their relaxed positions. An inability to visualize the medial torn tendon end beneath the acromion was another cause for discrepant measurements. In such cases, an accurate measurement could not be obtained. Nevertheless, reporting the location of the torn tendon end as "retracted beneath the acromion" provided useful information to the orthopedic surgeon.
Although far fewer tear-width measurement errors for full-thickness tears occurred, the tears were again large or massive in the majority of cases. We found it difficult to measure large or massive tear widths along a curved surface. In some cases, we used extended field-of-view imaging to better show the width of a large or massive tear and overcome the usual limitations of a small field of view. Nevertheless, an accurate measurement could not be obtained because the resultant image was often foreshortened. In those cases in which tear widths were underestimated, arthroscopic images showed that a portion of the tears were nonretracted; these portions were most likely not included in the measurements accounting for the errors.
Few errors occurred when measuring partial-thickness tear lengths; however, tear widths were underestimated on sonography in nearly one half of the cases. The irregular shape of the cuff tear was thought to be responsible for the incorrect sonographic measurements in two cases due to difficulty measuring such tears at the same location as arthroscopy. In two other cases, review of the sonographic images showed hypoechoic regions on transverse views that were larger than what was measured (Fig. 6). Difficulty distinguishing tendinopathy from the tear most likely accounted for these errors.
All but one of the sonographic studies was considered diagnostic. Body habitus was not a limiting factor in our study, nor was it a cause of subdiagnostic or nondiagnostic studies. The one nondiagnostic study occurred in a patient with limited external rotation of his arm and only hindered evaluation of the subscapularis tendon. Our study was limited by the fact that the joint review was done months after the arthroscopic and sonographic examinations were completed rather than immediately after surgery. Although the sonographic and arthroscopic studies were performed and measurements obtained prospectively, we were nevertheless unable to determine the exact cause of some of the measurement errors and can only speculate as to the reasons. Had the joint review been done earlier, we may have been able to determine the exact causes of the measurement errors in more cases.
Although all of these patients had MRI as part of another study [8], we did not determine if the diagnostic errors made with sonography were the same errors made with MRI. However, we did note that many of the cases that were problematic for sonography were also problematic for MRI [8]. This primarily reflects the difficulty that both tests have in distinguishing extensive partial-thickness or large bursal-side tears from full-thickness tears and tendinopathy from partial-thickness tears. Given this overlap in diagnostic capabilities, the choice as to which test to order depends on several factors. Based solely on diagnostic accuracy, there is ample evidence to support the use of either test in a patient with a suspected rotator cuff tear [8, 13]. Thus, when determining which test to order, other factors besides accuracy should be considered, including regional expertise with a particular test, the importance of ancillary clinical information (labral, capsular, ligamentous, or bone pathology), patient tolerance, and cost. We found that the patients' perception of the test was also an important consideration; in our experience, sonography is preferred to MRI by most patients [14].
In summary, we have shown that detection errors, although infrequent, will occur when obtaining a shoulder sonogram, primarily due to limitations inherent with the test or misses in patients with small partial-thickness tears. From a clinical perspective, these errors should not have a significant impact on patient care. Most measurement errors for full-thickness tears occurred in patients with large or massive tears, errors that may not be rectifiable.
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  A. Al-Shawi, R. Badge, and T. Bunker The detection of full thickness rotator cuff tears using ultrasound J Bone Joint Surg Br, July 1, 2008; 90-B(7): 889 - 892. [Abstract] [Full Text] [PDF]
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