Ultrasonographic Evaluation of Canine Supraspinatus Calcifying Tendinosis

Introduction

Calcifying tendinosis is a chronic lesion commonly found in tendons such as the supraspinatus, patellar, and common calcaneus.^1,2 The diagnosis of supraspinatus tendinosis is usually made from mediolateral, craniocaudal, or skyline radiograph projections of the shoulder joint. Such radiographs reveal egg-shaped, rounded, or irregularly delineated mineralization near the greater tubercle of the humerus.^3–7 Radiographs cannot provide parenchymal details. Tendon ultrasonography is a simple, noninvasive, and cost-efficient tool that can aid in the diagnosis of supraspinatus calcifying tendinosis. In fact, ultrasonography is one of the most sensitive and accurate techniques for tendon evaluation.^8–11

Canine supraspinatus calcifying tendinosis is an uncommon condition, and literature on the applicability of ultrasonography for evaluation of this disease is scarce. The purpose of this report is to highlight the importance of ultrasonography in the evaluation of canine supraspinatus calcifying tendinosis and to describe the ultrasonographic appearance of the condition in 33 tendons.

Materials and Methods

From 1996 to 2007, 31 dogs with radiographic evidence of supraspinatus tendinosis, characterized by calcified lesions or increased radiopacity above the greater tubercle of the humerus, underwent shoulder ultrasonographic examinations. Two dogs showed bilateral evidence of the disorder, thus 33 shoulders were included in this study. All dogs were patients of the Department of Veterinary Clinical Science, Clinic for Small Animal Surgery, of the Justus-Liebig University, Giessen, Germany.

All dogs were premedicated with a combination of diazepam^a (1 mg/kg) and atropine sulfate^b (0.05 mg/kg) administered IV. General anesthesia was induced and maintained with IV ketamine hydrochloride^c (3 mg/kg) and xylazine hydrochloride^d (0.3 mg/kg). General anesthesia or sedation was necessary because all dogs underwent additional examination of the bicipital tendon, requiring extensive outward rotation of the shoulder joint, which would otherwise not be possible due to pain. To standardize the procedure, all dogs underwent general anesthesia.

The dogs were placed on a table and positioned in lateral recumbency with the affected limb on top. The area to be examined, which included the craniolateral and medial regions of the shoulder, was prepared for the ultrasound examination. The ultrasonographic examination of the supraspinatus and biceps brachii tendons was performed in a standardized manner as described in the literature.^10,12 A 10–12 MHz multifrequency linear transducer^e was used.

For the ultrasonographic evaluation of the supraspinatus tendon of the shoulder, the forelimb was positioned neutrally (i.e., 135° flexion). The transducer was placed longitudinally over the supraspinatus muscle, cranially to the spine of the scapula and moved distally to the musculotendinous junction and further to its tendinous insertion on the greater tubercle of the humerus. Transverse planes were obtained by rotating the transducer counterclockwise and scanning from the proximal to distal structures.^10,11 Ultrasonography of the biceps brachii tendon followed the examination in all shoulders. The tendinous parenchyma was considered normal if a hypoechoic and homogenous region could be noticed on longitudinal and transverse views from the musculotendinous junction to the tendon insertion on the greater tubercle of the humerus and if no evidence of either calcification or heterogeneity were visualized (Figure 1).

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Results

The ultrasonographic appearance of the supraspinatus tendon varied considerably. The results of the ultrasound examinations have been summarized in Table 1.

Table 1 Ultrasonographic Findings of Supraspinatus Calcifying Tendinosis in 33 Canine Tendons

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A heterogeneous tendinous pattern (Figure 2A) was observed in nine tendons and was the only alteration in two of those tendons. The heterogeneous pattern occurred in combination with mineralized deposits (hyperechoic areas or points) in the other seven tendons. Hyperechoic points (Figure 2B) at the musculotendinous junction were observed in four tendons. Two of those tendons also showed a heterogeneous tendinous pattern. A high proportion of supraspinatus tendons (n = 27) showed an enlarged hyperechoic area, typically due to mineralization. In 20 tendons, the larger hyperechoic areas showed distal acoustic shadowing (Figure 3A), which were the most common findings. Most of the calcifications were ultrasonographically visualized as single structures, except in three cases where multiple calcified particles were noticed. The size of the mineralized areas ranged from 2 mm to 20 mm. They were classified as either round or irregularly delineated. A hypoechoic area surrounding the calcification was observed in six tendons, suggesting the presence of effusion, probably due to of an inflammatory process (Figure 3B). Color Doppler ultrasonography was performed in only five patients, and no evidence of vascularization was observed.

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Biceps brachii tenosynovitis was diagnosed in 6 of 33 shoulders studied. None had any evidence of contact with the calcifications observed in the supraspinatus tendon. A possible contact between calcifications in the supraspinatus tendon and the bicipital tendon sheath was observed in three shoulders, which showed no signs of biceps brachii tenosynovitis.

Discussion

Tendon ultrasonography has been widely applied for the diagnosis of calcifying processes, inflammatory or degenerative disorders, and traumatic injuries in tendons.^13–16 The technique became the noninvasive gold standard method for human shoulder tendinous evaluation over the last decade.^16,17 In dogs, ultrasonographic evaluation of the shoulder is mostly used for the diagnosis of biceps brachii tenosynovitis.¹⁰

The ultrasonographic technique used in this study followed previously published instructions.^12,18 The technique was reproducible, simple, quick to perform, and highly sensitive.^19–21 Mineralization, tendinous inhomogeneity, and/or calcified points could be observed in all supraspinatus tendons that were suspected of calcifying tendinosis of the supraspinatus muscle based on the radiographs. A hyperechoic focus could also be due to spectral artifact formation; therefore, good scanning technique is imperative and correlation of the ultrasonographic and radiologic findings is necessary.

Tendinous inhomogeneity in humans has been described previously.¹⁴ Tendinous inhomogeneity can follow degeneration due to age or diffuse calcium deposition and can be misdiagnosed as partial tendon rupture.²⁰ In the current study, the supraspinatus tendon was inhomogeneous in nine cases and either small hyperechoic points or larger hyperechoic areas were noted in seven cases. No sign of either partial or complete tendinous rupture was observed. In fact, all tendons could be evaluated completely, indicating the integrity of the fibers.

Most of the tendons (27 cases) revealed a calcified area in the supraspinatus tendon, which could be seen either on transverse or longitudinal planes. Those observations were already described in the literature.¹² This study confirmed that the calcified areas can be considerably variable in size, ranging from 2 mm to 20 mm. Nevertheless, the calcification size seems to have no clinical relevance.⁵

The shape of the calcifications varied considerably from round to irregular. This is in contrast to descriptions found for human supraspinatus calcifying tendinosis, which reported them as punctiform, arched, nodular, or even cystic.²² Distal acoustic shadowing was found in 20 of the dogs included in the current study. In human medicine, distal acoustic shadowing is classified as dense, slight, or absent during tendon evaluation. The appearance correlates with the composition of the deposits, which has an influence on the choice of a treatment, such as extracorporeal shock wave therapy, puncture, or surgical excision.^23,24 A similar conclusion could not be drawn in the current study because no invasive technique to determine the composition of the calcifications was performed. Further studies should be performed to determine if a correlation between the composition of the tendinous calcifications and their ultrasonographic appearance in dogs exists.

The presence of a hypoechoic area surrounding the calcification was evident in only six tendons. Five of those dogs presented for lameness in the respective forelimb caused by shoulder pain without any other orthopedic disorder. All other dogs included in this study also showed forelimb lameness, but they all had a concurrent orthopedic disorder diagnosed and considered the cause of the lameness, such as panosteitis or fragmented coronoid process of the ulna. As previously mentioned in the literature, the ultrasonographic appearance in conjunction with the clinical evaluation is useful to identify an active inflammatory process, helping the clinician to differentiate between clinically relevant supraspinatus calcifying tendinosis and an incidental finding.² Although additional patients should be evaluated to confirm this correlation, the authors assume that a calcification in the supraspinatus tendon with a surrounding hypoechoic area is a sign of an active inflammatory process and is related to clinical signs.

Color Doppler ultrasonography has been used for the evaluation of human patellar and Achilles tendinosis, and it is reported that vascularization is associated with local pain.^25–27 In human supraspinatus calcifying tendinosis, color Doppler ultrasonography is also applied and is helpful for the determination of the phase of calcification (formation or resorption). It is known that the resorption phase is related to clinical signs of pain.^22,24 No studies regarding the use of color Doppler during canine supraspinatus tendinosis examinations were found while preparing this article. The authors evaluated the last five tendons included in this study, and despite mineralization, they showed no signs of vascularization. That said, the authors believe that further studies involving a greater number of patients should be performed to obtain conclusive data.

Although all studied cases were suspected of having supraspinatus tendinosis based on the radiographs, the biceps brachii tendon was also evaluated because it is proposed that a deeply located supraspinatus tendon calcification can lead to tenosynovitis of the biceps tendon due to its tendon sheath contact.^1,5 Concurrent bicipital tenosynovitis was identified in 6 of 33 shoulders, but none of the calcified areas in the supraspinatus tendon were close to the biceps brachii tendon sheath based on ultrasonography. A possible contact between the calcified area of the supraspinatus tendon and the biceps brachii tendon sheath was suggested for three other tendons in which no signs of bicipital tenosynovitis were seen. Ultrasonography of the biceps brachii tendon requires maximal abduction and outward rotation of the shoulder and can result in an unphysiologic situation, which might lead to misinterpretation. To confirm the hypothesis that supraspinatus tendon calcification deeply located can lead to bicipital tenosynovitis, the authors suggest further studies in which imaging can be performed in a physiologically positioned forelimb, such as MRI.

Conclusion

This study described the ultrasonographic appearance of supraspinatus calcifying tendinosis. Ultrasonography proved to be applicable, easy to perform, and confirmed all radiographically suspected cases. Ultrasonography provided detailed information about size, shape, location, acoustic shadowing, and presence of an inflammatory process surrounding the calcification. Additional studies using color Doppler ultrasonography should be performed to evaluate its ability to diagnose canine supraspinatus calcifying tendinosis. Additionally, the authors believe that tendon ultrasonography is not the best method to evaluate the relationship between calcifications found on supraspinatus tendon and the biceps brachii tendon sheath because the forelimb positioning for the evaluation differs greatly from the physiologic positioning.