To date, soft-tissue scanning has relied on standard 2-dimensional scanners which do not provide any depth or volume measurement. The latest improvements—including high frequency transducers and much improved image resolution capabilities of ultrasound scanners—have dramatically improved multidimensional ultrasonography and has demonstrated it’s ability in the detection of shoulder rotator cuff (RTC) tears.
Epidemiology of Shoulder Pain
There are only a few areas of the body whose biological dysfunction can cause greater personal and societal costs than the shoulder. Pain in the shoulder is a common condition in America and is responsible for an enormous human burden. In 2000, the direct costs for the treatment of shoulder pathology totaled in excess of 7 billion dollars.1 The economic burden, at a societal level, is surpassed only by low back pain as the most costly condition to treat. Persistent shoulder pain can be related to numerous causes including bursitis, arthritis, tendonitis, capsulitis, impingement, and rotator cuff disease among the more common reasons for shoulder pain and/or weakness. Some of these conditions have complex etiologies but a differential diagnosis is possible when clinicians combine medical history, physical examination, and diagnostic imaging data.
Rotator cuff disorders are the most common cause of shoulder pain with an estimated prevalence of approximately 10%.2 It is estimated that, in general, non-specific shoulder pain has a one year prevalence estimate in some countries between 20-50%, with many community-based study estimates of having shoulder pain within a lifetime at between 14-21%.3 Sport-specific rates of shoulder injury are also available and indicate that certain sports, such as football, have significantly higher rates of shoulder trauma. Using the NFL injury surveillance system, the rate of shoulder injuries in quarterbacks between the time period 1984 and 2001 was estimated to be 15.2% and represented the second most injured region of the body.4 Injuries were classified as either trauma or repetitive strain related, with rotator cuff syndrome (tendonitis) being the most common shoulder problem reported. In collegiate players, rotator cuff injury is also not uncommon. In a recent cohort study of 336 elite college players, approximately 50% had a history of shoulder injuries with the most problematic positions being quarterbacks and defensive backs. Rotator cuff injuries represented about 12% of the total injuries reported by the players.5
With shoulder injury rates being so significant in the U.S. general population, workforce, and sporting milieus, it is not surprising that the clinical community has sought to develop innovative and cost-effective ways in which to image the shoulder and rotator cuff in general. The use of MRI as a means of viewing shoulder anatomy and pathology is well recognized. Magnetic resonance imaging has proven to be a very useful tool in determining the presence or absence of trauma or disease to the various shoulder structures. MRI has been used to estimate the severity of rotator cuff tearing and the general integrity of the rotator mechanism.6 Having said that, not all hospitals and medical centers have access to an MRI—not to mention the high cost, patient inconvenience, and recognized contraindications (metallic implants/pacemakers) to MRI. The high incidence of shoulder pain, specifically rotator cuff dysfunction, has led to the search for more cost-effective means of evaluating key shoulder structures such as the RTC conjoint tendon which seems to be implicated in many cases of shoulder pain.
The shoulder is a complex structure that is actually comprised of four separate articulations including; glenohumeral, acromioclavicular, sternoclavicular, and scapulothoracic joints. Motion of the arm on the trunk involves coordinated movements of all these joints. For the purposes of this report, the important muscular elements forming the rotator cuff are the four muscles that converge into the cuff tendon itself. The subscapularis arises from the deep surface of the scapula and inserts onto the lesser tuberosity. The supraspinatus, infraspinatus, and teres minor arise from the posterior surface of the scapula and insert onto the greater tuberosity, from superior to inferior respectively.
When a detailed view of the rotator cuff tendon is seen, it becomes apparent as to how the RTC can be damaged during a fall, overuse, and/or age-related degenerative changes (delamination). The RTC tendon is a relatively flat structure that passes under the bony acromion and rigid coracromial ligament arch. With prolonged and repetitive upper extremity activities, there is cuff irritation leading to possible tendon hypertrophy (swelling) within a normally very narrow space, that being under the coracromial arch. The resultant microtrauma to the cuff can lead to shoulder pain.7 Unfortunately, tendons do not have a very good blood supply and such is the case with the RTC common tendon whose watershed area (region of greatest vascularity) is typically that portion of the tendon lying directly below the acromion. This is the site most likely to be damaged and subject to the subsequent morphological alterations (fibrosis) that follow. Chronic irritation of the RTC tendon has been known to cause more permanent structural changes in the tendon including atrophy, fibrosis, calcification, and impingement leading to RTC tearing.
Multidimensional Ultrasound Imaging
The use of diagnostic ultrasound imaging (DUI) for the detection of rotator cuff disorders and disease has become increasingly popular as studies performed using higher resolution scanners have more consistently demonstrated very good overall accuracy rates.
Unlike lab-based testing, where established sensitivity and specificity values might be considered fixed properties of a test, diagnostic orthopedic testing is inherently different. In orthopedic testing, the tests are performed differently by different examiners across the country. This adds greater variability to those factors that eventually determine the derived values of the Bayesian model. This individual psychomotor aspect to diagnostic testing—combined with variation within scanners (man-machine interaction)—will heavily influence the final outcome of diagnostic accuracy. As a result, global reports on the validity of DUI reflect variance in sensitivity, specificity, positive and negative predicitive values.