In the clinical setting, MSK-US allows improved characterization of diseased tendon tissues . Painful tendons have structural abnormalities seen on MSK-US, which are typically localized tendon thickening with hypoechoic areas and altered vascularity [10-12]. Degenerative change is one of the characterizations of tendinopathy identified by the presence of the angio-fibroblastic hyperplasia on MSK-US . Neovascularization (47-100%), increased the thickness of the tendon (100%) with disorganized tendon structure (100%), formed calcific plaques (29-52%) and hypoechoic region (74-100%) are seen in the tendons with Achilles tendinopathy [14-18]. Additional MSK-US findings for tendinopathy are described as the follows.
Findings at each stage of tendinopathy can be correlated with pain. A positive correlation between neovascularization and subjective pain measurement has been reported by several authors [18,15,19]. The increased blood flow in injured tendons with focal hypoechoic lesions can be detected by a Color Doppler, as well as a Power Doppler, where as normal tendons have no blood flow detectible within them . In chronic Achilles tendinopathy, increased vascularity (neovascularization) has been demonstrated to be closely related to chronic tendon pain . The association with neovascularization inside and outside the tendon with chronic pain but not in the healthy tendons has been reported by Alfredson [19,20]
Increased tendon thickness is classified as a non-tear abnormality and may be associated with edema and the swelling of the tendon, which are relatively common symptoms of tendinopathy among middle aged recreational athletes [21-23]. Following clinical pain in the Achilles tendon, the tendon undergoes structural changes that are detected by MSK-US imaging. Tendon thickening is one of the most common MSK-US findings for the tendinopathy. The morphological characteristics of Achilles tendinosis can be viewed with MSK-US including an area with a high concentration of glycosaminoglycans and disorganized fiber pattern and arrangement in the tendon, however the amount of inflammatory cell infiltrates is the same as the acute phased . The alignment of the collagen fibers in healthy tendon is well organized, while the disorganized fibrillary pattern was found in the tendons with tendinopathy.
In comparison with MRI, MSK-US and surgical findings concluded that if the diameter of the hypoechoic region is more than 10 mm, it will be suggested a partial tear more than in pure tendinosis . The hypoechoic region is considered with loss of the fibrillar structure in chronic tendinopathy . Hypoechoic areas on MSK-US and the areas with increased signal intensity on MRI correspond to the areas of altered collagen fiber structure and increased interfibrillar ground substance (hydrophilic glycosaminoglycans) [27,28]. Percutaneous ultrasonic tenotomy (PUT) is a novel surgical technique with a minimally invasion for specific tendinopathy state which is recalcitrant tendinopathy with hypoechoic region . This machine is needed with MSK-US visually guidance that is being highly sensitive for hypoechoic lesions and MSK-US has proven to be a useful tool for localizing and targeting ablation of the pathologic tissue for properly suctioning scared tissue out [29,30].
Calcific tendinopathy is thought to be caused by poor oxygen supply within a tendon. Calcific tendinopathy has four stages: pre-calcific, calcific, resorptive, and post-calcific even though the MSK-US, as well as MRI, cannot distinguish those four stages by its visualization [33,34]. The stage of calcification is usually related to the consistency of the deposit: soft during the pre-calcific stage, hard during the calcific stage, and fluid during the early resorptive phase. Rotator cuff calcific tendinopathy, a common disease in daily clinic situation occurs in up to 7.5% of asymptomatic adults and up to 20% of painful shoulders, is frequently seen in women in their 40s and 50s [35-37]. It is believed that calcific tendinopathy is one of the reasons of highly disabling shoulder pain. The clinical studies have demonstrated that women aged between 30 and 60 years with subacromial pain syndrome and a calcific deposit of >1.5 cm in length have the highest chance of suffering from symptomatic calcific tendinopathy of the rotator cuff . The results showed that 7.8% of the asymptomatic patients were false positive as diagnosed by MRI calcific tendinopathy . US-guided percutaneous irrigation of calcific tendinopathy is widely accepted as the primary treatment and effective for a deposit calcification [33,39]. The hard calcification possibly will have to be removed out by surgical procedure and PUT with US guidance as well .
Clinical studies have shown macroscopic changes in the tendons with tendinopathy including tendon thickening, loss of mechanical properties, and pain . Recent studies demonstrated that degenerative changes have been occurred in the tendons with tendinopathy including disruption of collagen fibers, non-collagenous matrix such as matrix metalloproteinases (MMPs), glycosaminoglycans (GAG), fatty areolar tissue, calcification as well as neovascularization [24,31, 42-46 ]. The net results of tendinopathy are tendon degeneration, weakness, tearing, and pain . Although oral nonsteroidal anti-inflammatory medications (NSAIDs) have been used extensively for decades to treat pain associated with tendon overuse, the evidence suggests that NSAIDs are effective in relieving the pain associated with tendinopathy only in short term and many studies showed no improvement with NSAIDs [48-50].
It is well known that long-term NSAID use increases the risk of gastrointestinal, cardiovascular, and renal complications associated with these medications. Thus, physical therapy has been commonly used for the treatment of tendinopathy. In recent years, a new physical therapy modality used in the tendinopathy treatment is therapeutic ultrasound . MSK-US has been used in advanced tendinopathy to remove the calcified scared tissue . It is believed that calcification in the tendon can be a risk factor for leading the tendon tears even partial tear and rupture. Therefore, PUT via a TenexTM device (TenexHealth, Lake Forest, CA) has been established as a mostly new minimum invasive surgery for chronic particular case of tendinopathy and has been approved by FDA in the United States of American in 2011. The clinical results have shown that PUT can be used with a tiny skin incision and break the scared tissue for suctioning out [13, 14,18 ].
US-guided injection and procedures can place the needle tip on the exact target area of the tendons. The US-guided tendinopathy injection technique is taking the place of the primary tool instead of traditional blinded injection . The US imaging shows the exact location of nerve, vessels, and target soft tissue since ultrasound wave is penetrating subjects. Using a grey scale, color Doppler and power Doppler can also recognize vascular flow condition. These findings can be easy to understand the status of tendon, ligament, muscle, periosteum and bone surface. US-guided injection is more accurate than blinded injection even when it is done by the rookie physicians, with no X-ray exposure, cost-effective and repetitive use.
It is well known that animals and people get many of the same illnesses. Certain types of animals can stand in for humans with particular diseases. Medical research with animals is always required for conducting the clinical studies with humans. Tendinopathy is a prevalent tendon disorder that affects millions of Americans and costs billions of health care dollars every year. Current clinical treatments for tendinopathy are largely palliative because the precise cellular and molecular mechanisms of the disorder are not defined. Intensive and excessive mechanical loading is considered to be a major factor responsible for acute and chronic tendon injuries. Additionally, aging is also known to induce the degenerative changes of the tendons . Many studies focus on tendinopathy using animals [59-62], only several recent studies, however, revealed the potential of MSK-US usage in animal studies due to the size of the animal tendons and ultrasound sensitivity.
In clinical settings, the US frequencies in diagnostic radiology range are from 2 MHz to approximately 15 MHz. For deep abdomen, obstetric and gynaecological imaging, the US frequency is 2.5 MHz. For general abdomen, obstetric and gynaecological imaging, the US frequency is 3.5 MHz. For vascular, breast, pelvic imaging, ultrasound frequency is 5.0 MHz. For breast, thyroid, the US frequency is 7.5 MHz. For superficial masses and structures, the US frequency is 10 MHz. For musculoskeletal imaging, the US frequency is 15.0 MHz. It is known that high frequencies are used for the superficial body structures and low frequencies are used for those that are deeper. The animal tendons are much smaller than human tendons, the US frequency used for animal tendons should be much higher than that used for human. Some new MSK-US applications have been developed [63-65]. Currently, the highest US frequency can be 70 MHz (Vevo 3100, FUJIFILM SonoSite, Toronto, Canada).
Currently, a high-frequency MSK-US has been used for many animal studies. MSK-US-guided needle positioning near the sciatic nerve has been used to elicit compound muscle action potentials of the rats . The efficacy of intra-tendinous injection of PRP in treating tendinosis was also studied by MSK-US guided injection using a rat model . The structure and organization of the animals were also investigated by high frequency MSK-US .
The authors have declared that no any conflict of interest exists.
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