REVIEW ARTICLE
Detection and Characterization of Tendon Abnormalities With Multidetector Computed Tomography Shadpour Demehri, MD,* Majid Chalian, MD,† Sahar J. Farahani, MD,* Elliot K. Fishman, MD,* and Laura M. Fayad, MD*
Abstract: With recent advances in multidetector computed tomography (MDCT) acquisition and reconstruction options, MDCT can now be used successfully for evaluating tendon abnormalities. In this article, MDCT protocol optimization for the imaging of tendons is underscored, and applications of MDCT for assessing tendon pathology are highlighted. Although our retrospective experience of CT imaging with 2-dimensional multiplanar reconstructions and 3-dimensional postprocessing techniques is reviewed, potential applications for newer CT technologies, including dual-energy CT and 4-dimensional CT imaging of the peripheral tendons, are also discussed. Key Words: multdetector CT, tendon, dual-energy CT, 4-dimensional CT (J Comput Assist Tomogr 2014;38: 299–307)
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lthough magnetic resonance imaging (MRI) and ultrasound are well established and widely used imaging modalities for the evaluation of tendons, multidetector computed tomography (MDCT) can also be a useful tool for the detection and characterization of tendon abnormalities. Familiarity with the normal and pathologic appearances of tendons is important to provide a comprehensive assessment during peripheral joint evaluation using CT images. Furthermore, in the care of patients who have metallic orthopedic hardware as well as in the setting of acute trauma, CT is particularly preferred as the modality of choice for the evaluation of osseous abnormalities. Therefore, assessment of the tendons at the time of imaging should be a component of the routine interpretation checklist. There is limited information and original research aimed at characterizing the normal and abnormal appearances of tendons using MDCT.1–6 In this article, we summarize the technical aspects for successful acquisition and display of MDCT images of the tendons. The normal MDCT appearance of tendons and abnormal appearances, including tendinopathy, tenosynovitis, and partial and complete tears of tendons, are discussed. Practical information for optimal acquisition, 3-dimensional (3D) postprocessing, and interpretation of noncontract MDCT images of the tendons is emphasized. Additionally, the potential role of newer CT technology with dual-energy CT (DE-CT) and 4-dimensional CT (4D-CT) is addressed.
From the *Russell H. Morgan Department of Radiology and Radiological Science, Johns Hopkins University School of Medicine, Baltimore, MD; and †Department of Radiology, University Hospitals Case Medical Center, Cleveland, OH. Received for publication July 27, 2013; accepted August 30, 2013. Reprints: Shadpour Demehri, MD, Musculoskeletal Radiology, Russell H. Morgan Department of Radiology and Radiological Science, Johns Hopkins University School of Medicine, 601 N Caroline St, JHOC 5165, Baltimore, MD 21287 (e‐mail: [email protected]). The authors declare no conflict of interest. Copyright © 2014 by Lippincott Williams & Wilkins
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INDICATIONS FOR MDCT Compared with CT, MRI offers superior contrast resolution for displaying soft tissue structures. However, in patients with metal hardware,7 the presence of MRI-incompatible implanted medical devices, and patients with a history of claustrophobia or obesity, MRI may be contraindicated and ultrasound or CT favored.8,9 Additionally, the wide availability and fast acquisition of MDCT has made CT a convenient modality for the assessment of the musculoskeletal system, with applications that continue to evolve. With regard to tendon assessment, CT is rarely a first-line technique (except where MRI is contraindicated), although CT arthrography is an established technique for assessing the tendons of the shoulder.10 Several advances have been introduced that result in improved visualization of the periarticular soft tissues by noncontrast CT: Robust 3D postprocessing with volume rendering (VR), DE-CT, as well as the feasibility of ultrafast image-acquisition for large body parts and 4D joint imaging using wide bore MDCT, can all potentially improve the demonstration of tendon pathology. Generally, potential exposure to ionizing radiation must be considered before MDCT is performed, but CT of the peripheral skeleton is subject to less effective radiation dose than that of CT of the torso.10 Table 1 summarizes the utility of MDCT in the assessment of tendon pathology.
TECHNICAL CONSIDERATIONS The optimal MDCT evaluation of tendons requires high spatial resolution as well as 3D postprocessing to improve the differentiation of normal tendons from abnormalities such as tendinopathy, tenosynovitis, and tendon rupture. The assessment of tendon structure is possible with MDCT by observation of changes in tendon attenuation and thickness change, continuity, as well as peritendinous synovial fluid accumulation. Multidetector CT can also be performed after the administration of intra-articular contrast, which enhances the morphologic assessment of many intra-articular and periarticular soft tissue structures, including the tendons. The administration of intravenous contrast is not routinely performed for the assessment of tendons, but can be used to improve the detectability of masses or inflammatory processes involving the tendons. The assessment of tendon motion is also possible with MDCT and is discussed further in the section regarding 4D-CT. Table 2 shows sample MDCT protocols for the evaluation of tendons. For the anatomic display of musculoskeletal structures by MDCT, the contrast resolution for assessment of the soft tissues can be improved by increasing the tube current and/or decreasing the pitch. By replacing previous CT image reconstruction algorithms such as filtered back projection with adaptive iterative dose reduction (AIDR), radiation exposure can be automatically reduced before the scan, ensuring that the optimal low dose is used for the specific diagnosis regardless of the size or shape of the patient. For routine clinical use, by implementing www.jcat.org
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TABLE 1. Utility and Limitations of MDCT in the Assessment of Tendon Abnormalities Utility/Indication for MDCT
Limitation
Contraindication to MRI, obesity, noncompliant patients with motion during image acquisition Presence of metal hardware close to the tendon
CT offers less contrast resolution than MRI and exposure to ionizing radiation
Assessment of tendinopathy (by changes in CT tendon morphology) Assessment of tendon tears (by changes in CT tendon attenuation and morphology) Assessment of tenosynovitis Assessment of intra-articular structures with CT arthrography* Assessment of peritendinous osseous abnormalities (spurs, avulsion fractures) Detection and characterization of gout with DE-CT Detection of tendon subluxation and adhesion to the adjacent structures during motion with 4D-CT
Although streak artifact can be reduced by metal-reduction CT acquisition protocols, tendons may remain obscured Atrophic and hypertrophic tendinopathy is detectable by CT, but a lack of morphologic change in the tendon size limits recognition of tendinopathy Complete tears are detectable; partial tears associated with no change in morphology are often undetected by CT or misdiagnosed as tendinopathy Septations in tenosynovitis characteristic of stenosing tenosynovitis are undetectable by CT CT arthrography is an invasive technique that offers limited visualization of nonarticular structures Bone marrow reactive changes (edema-like signal visible by MRI) are not visible by MDCT† Availability of DE-CT, diminished signal-to-noise and contrast-to-noise ratio, increase in radiation exposure Increased radiation exposure, increased data for postprocessing and archive needs, and longer scan time due to time consumed for patient's education and scan acquisition
*CT arthrography will not be discussed in this article, as noncontrast techniques are emphasized. †Bone marrow edema can be detected using dual-energy (DE) CT examination.
AIDR, one is able to reduce the effective dose, which ultimately leads to improved spatial resolution and excellent artifact (streak/beam hardening) reduction that are otherwise routinely present at low-dose acquisitions.11 In addition, with recently available denoising software,12 the contrast-to-noise and signalto-noise ratios of CT images can further improve without added radiation dose. The advent of isotropic voxels enables high spatial resolution volumetric imaging (3D-CT), and the transfer of the original CT acquisition into any other plane of choice; the latter allows viewing of the tendons from any desired projection, along with their relationship to adjacent osseous structures.
The main CT-based 3D techniques for the evaluation of tendons are VR, shaded surface display (SSD), and maximum intensity projection (MIP),1 which will be discussed later.
SSD and MIP Using SSD, tissues are defined by assigning a specific attenuation threshold to demonstrate the tissue types that are only above the chosen threshold. Because of the fundamental role of attenuation of threshold in the creation of SSD images, initial editing of the images may be required to eliminate unwanted tissues that may obscure the course and appearance of tendons, and therefore, can be very time-consuming. Other
TABLE 2. Sample MDCT Acquisition Protocols for Evaluation of Tendons Protocol Peak voltage, kVp Tube voltage, mA Rotation time, s Detector collimation, mm Slice thickness, mm Pitch IV contrast Temporal resolution, ms Image reconstruction† Reconstruction algorithms* Effective dose, mSv‡
Single-Energy 64-MDCT
DE-CT
4D-CT
100–140 (range) 250–350 0.5 0.6 0.75 0.9 Optional 150–300 AIDR MPR, 3D-CT 0.03–0.2
100 and 140* 75, 320 0.5 0.6 0.75 0.9 Optional — AIDR MPR, 3D-CT 0.04–0.3
80–120 80–125 0.5 0.6 0.5 0.9 None 500 AIDR 3D-CT 0.3–2.0
*MPR indicates multiplanar reconstruction. †AIDR algorithm is currently used for CT image reconstruction. ‡Radiation dose for CT acquisition of shoulder and hip joints were not included, and there is likely higher radiation exposures as these joints are closer to torso. The effective radiation dose (mSv) was calculated by multiplying the dose length product (DLP) reported from the CT scanner by the conversion factor of k = 0.0005 reported for distal extremities.
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disadvantages of SSD are its inaccurate display of tendon contour and size due to a potential suboptimal choice for the attenuation threshold.13 Maximum intensity projection is typically used to display blood vessels that demonstrate contrast material filling at CT or MR angiography. The tendons can also be demonstrated with MIP processing of CT images, although this technique is rarely used.14 Using this postprocessing method, the voxel with the maximum attenuation value will be selected. These maximum-attenuation voxels are retained and all other voxels will be eliminated from the CT data set. Similar to SSD, these voxels can represent much, but not all, of the data obtained during the CT acquisition. Maximum intensity projection images are not optimal methods for demonstrating other soft tissue structures, as only bones and tendons comprise the voxels with maximum attenuation values. Maximum intensity projection images have been suggested to be useful when metal is present because of the decreased image distortion from beam hardening artifact, although the VR technique remains adequate in the setting of metal.
Volume Rendering Volume rendering imaging has become a common CTbased 3D display method.4,5 With this technique, various tissues are defined by assigning a specific tissue type (such as a tendon) to a specific range of attenuations defined by Hounsfield units (HU). Each tissue will be assigned a certain color, based on its range of Hounsfield units using the histogram of the CT data set. Volume rendering imaging is particularly helpful in demonstration of soft tissue pathologies associated with osseous abnormalities. Due to the versatility of VR, this 3D technique is often favored.15–18
NORMAL TENDON ANATOMY By MDCT, normal tendons are relatively homogenous well-demarcated tubular structures.19 Tendons that are typically well visualized by CT include the extensor and flexor tendons of the wrist and hand, the hamstring tendons, the extensor tendons of the knee, as well as most tendons in the ankle.3 Some tendons are less discernible by noncontrast CT, such as the rotator cuff tendons, largely due to the conjoined configuration of adjacent tendons (eg, supraspinatus and infraspinatus tendons) and relative paucity of surrounding fat.
Detection of Tendon Abnormalities With MDCT
Using 3D postprocessing methods described previously, it is possible to simultaneously demonstrate the tendons and adjacent bones due to significant attenuation differences among the various soft tissues, tendons, and bone (fat approximately −100, muscle approximately 30, tendon approximately 90, and bone approximately 250 to 500 HU). The attenuation values of normal tendons are variable and can change with their size, due to partial volume effect as well as the CT acquisition parameters. For example, for tube kilovoltage (kVp) of 120 (the value used in routine clinical practice), the normal attenuation of tendons has been shown to range between 80 and 100 HU. However, using a lower kilovoltage will yield lower attenuation values for normal tendons, whereas using a higher kilovoltage will yield higher attenuation values for tendons.14 An important morphologic CT-based descriptive feature that can differentiate the normal from abnormal tendon is the relative size of the tendon in relationship to other adjacent nearby tendons, as small tendons may represent atrophic tendinopathy, whereas large tendons may reflect hypertrophic tendinopathy or a partial rupture. In addition, defining the course and margin of the peripheral tendons and associated structures such as the myotendinous junction, the osseous attachment site, and retinaculum are now feasible and should be sought on a clinical examination of the tendons.
PATHOLOGICAL FINDINGS The most common tendon pathologies encountered include tenosynovitis, tendinopathy, partial or complete tears, and subluxation or dislocation. Soft tissue masses that are associated with tendons can also occur, although commonly include ganglion cysts. Bursae-related abnormalities such as retrocalcaneal or infrapatellar bursitis occur because of the intimate association of bursae and tendons.
Tendinopathy The mechanism of injury that leads to tendinopathy remains unclear. Many pathological changes may occur in tendinopathy, including a reduction in the number and rounding of fibroblasts, an increase in the content of proteoglycans, glycosaminoglycans, and water, and the hypervascularization and disorganization of collagen fibrils. There is a significant overlap between the MDCT appearance of normal tendon and tendinopathy, which is typically
FIGURE 1. A 70-year-old male patient with Achilles tendinopathy manifested by a thick tendon on sagittal (A) and axial (B) CT images. Convexity of the tendon on the anterior surface (arrow) and paratenonitis (arrowhead) is associated with tendinopathy. Figure 1 can be viewed online in color at www.jcat.org. © 2014 Lippincott Williams & Wilkins
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FIGURE 2. A 38-year-old female patient with peroneus longus and brevis tenosynovitis. Sagittal CT with bone windows (A), sagittal CT with soft tissue windows (B), and volume-rendered 3D-CT (C) show a bone spur due to prior healed fracture of distal fibula (arrow in A) and resulting tenosynovitis of the adjacent peroneal tendons (arrow in B). Three-dimensional CT for comparison also demonstrates peroneus longus and brevis tenosynovitis, manifested as decreased attenuation and thickening of these tendons. Figure 2 can be viewed online in color at www.jcat.org.
demonstrated by enlargement and heterogeneous attenuation of the tendon. Therefore, it is more likely to diagnose tendinopathy in larger tendons, such as the Achilles (Fig. 1) or patellar tendon. When internal tendon density approaches fluid attenuation, a partial tear should be suspected. However, again MDCT is not as accurate as MRI for differentiating tendinopathy from partial tendon tears in smaller peripheral tendons, as there is significant overlap in the CT features of tendinopathy and partial tears, namely morphologic changes and heterogeneous attenuation.
Tenosynovitis Disorders of the synovial membrane induce proliferation of the synovium and accumulation of fluid in the tendon sheath. By MDCT, the tendon appears as a hyperattenuating structure with a halo of low attenuation similar to fluid and/or thickening of the synovial membrane (Figs. 2 and 3). Multidetector CT allows detection of very small synovial collections. If intravenous contrast is administered during a CT examination, associated hypervascularity and inflammation can also be demonstrated. Similarly, the presence of fluid attenuation posterior to the Achilles tendon indicates paratenonitis (Fig. 1).
Tendon Tear Computed tomographic images have high spatial resolution, and along with 3D reconstructed images, even ruptures
in small tendons of the hands (Figs. 4 and 5) and feet are clearly discernible. The advantage of 3D-CT imaging over MRI is that the surgeon does not need to integrate multiple images (of MRI) and can readily recognize the site of the rupture as well as better delineate small osseous avulsions by a comprehensive 3D-CT image (Fig. 6); hence, surgical planning is facilitated, although 3D MR techniques are also currently increasingly available.20 Although less conspicuous compared to MRI or ultrasound,21 partial-thickness tears can also be diagnosed using MDCT, as evidenced by fluid attenuation within the tendon substance (Fig. 7).
Ganglion Cysts Ganglion cysts are one of the most common soft tissue masses to arise around a tendon. They appear as well demarcated and hypodense structures adjacent to a tendon and may communicate with the tendon sheath or joint.22 They do not demonstrate enhancement after contrast administration.
Other Soft Tissue Abnormalities Other soft tissue lesions that can affect tendons include neoplasms (Fig. 8) and infection (Fig. 9). The latter entities may involve a tendon as the primary site or may spread from adjacent structures. Also, processes with mineralization such as calcific tendinopathy (Fig. 10) and tumoral calcinosis
FIGURE 3. A 33-year-old male patient with persistent right-hand pain and swelling. Axial MDCT image (A) and axial MPR image (20-mm-thick section, B) demonstrate abnormal thickening and tenosynovitis of the flexor digitorum tendon of the long finger (arrow in A, arrowhead in B). The advantage of using thick section MPR views is the potential for enhanced demonstration of tendon pathology. Figure 3 can be viewed online in color at www.jcat.org.
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Detection of Tendon Abnormalities With MDCT
(Fig. 11) are better evaluated using MDCT (to detect mineralization) than MRI.
DUAL-ENERGY CT
FIGURE 4. A 52-year-old female patient with an intra-articular fracture of the distal phalanx. Coronal CT image of the right hand shows a complete tear of the flexor digitorum longus tendon of the little finger, with retraction (white arrow).
Dual-energy CT is either comprised of 2 orthogonally oriented rows of detectors, within a single gantry or a single row of detectors with alternating currents.5,14,23 The presence of 2 x-ray generation sources within this CT system offers the opportunity for independent data acquisition while the 2 CT tubes operate at 2 different energies (typically set at 100 and 140 kVp). Depending on the chemical composition of a structure, it will appear with different attenuation values at the 2 energy levels. For tendon evaluation, modest changes in the attenuation of tendons occur as the collagenous structures of tendons are disrupted (such as tendon tears), and these changes are accentuated by DE-CT. Dual-energy CT images have shown improved conspicuity of structures with high atomic numbers such as intravascular iodine, but other relatively dense structures composed of smaller atoms (eg, hydrogen, carbon, nitrogen, and oxygen) may not be as well seen because of reduced x-ray beam absorbance by the various tube energies. Dual-energy CT has recently emerged as a useful method for differentiating iodinated contrast from hemorrhage23 and it has been proven useful in the detection and characterization of mineralization or crystal deposition along the tendons in conditions such as early calcific tendinopathy or tophaceous gout (Fig. 12), compared with conventional CT.23
FIGURE 5. A 52-year-old female patient with history of left hip pain status post fall. Axial (A) and coronal (B) 2D MDCT images of the left hip demonstrate a heterogeneous hyperdense structure distal to the ischium (white arrows in A and B), which is confirmed as hamstring tendon rupture by corresponding axial (C) and coronal (D) inversion-recovery (inversion time, 150; TR/TE, 5850/46) MR images (white arrowheads in C and D). Note hematoma by coronal MR image (*). © 2014 Lippincott Williams & Wilkins
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FIGURE 6. A 70-year-old female patient with a complete tear of the Achilles tendon as seen on sagittal (A) and axial (B) MDCT images with tiny avulsed osseous fragment (white arrow). On axial views at the site of tendon rupture, there is diminished CT attenuation lower than tendon stump consistent with fluid/hematoma accumulation at the tear site (white arrowhead).
Nevertheless, for the CT assessment of tendons, a recent study showed that the image quality of DE-CT for the demonstration of normal tendon structure is inferior to conventional CT images, even when radiation exposure is increased.5
FUNCTIONAL IMAGING: 4D-CT Musculoskeletal imaging has been predominantly based on the static, morphologic depiction of tendons using static CT and MRI. However, some tendon abnormalities such as subluxation or dislocation can only be detected using kinematic imaging. One option to depict joints in motion is to perform static CT, MRI, or ultrasound of a joint using small degrees of joint motion with passive positioning to detect subluxation not visible on radiographs. For example, patellofemoral maltracking has been assessed in this manner.24
The depiction of peripheral joints in motion under physiologic conditions has been termed kinematic imaging and has been previously performed with high diagnostic quality25; currently, kinematic imaging is technically feasible with widebore MDCT for 4D-CT imaging.26,27 Using a 320-row MDCT, 4D-CT imaging is performed as a 16-cm acquisition of the limbs during motion with temporal uniformity and high temporal resolution of 0.5 seconds. In our preliminary experience, the 4D-CT examination of distal peripheral joints (ankle and wrist) is associated with low radiation exposure (up to 1 mSv), and the depiction of abnormal motion of the peripheral tendons can be enhanced with a soft tissue reconstruction algorithm (Fig. 13).
CONCLUSIONS When MDCT is obtained for other reasons such as osseous trauma, it can also be useful to also assess the tendons,
FIGURE 7. A 17-year-old male patient with a history of gunshot wounds to the legs and comminuted fracture of left tibia. Sagittal multiplanar reconstruction (MPR) (5-mm section thickness) (A) and sagittal 3D volume rendered CT (B) demonstrate a high-grade tear (white arrows in A and B) of the anterior tibialis tendon of the ankle adjacent to the fracture site. In B, 3D-CT image shows the adjacent skeleton for a more comprehensive view, and a few remaining intact fibers (arrowhead in B) are identified, characterizing this tear as a high-grade partial tear, rather than complete tear. Figure 7 can be viewed online in color at www.jcat.org.
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FIGURE 8. A 23-year-old male patient with a history of neurofibromatosis. Coronal (A) and sagittal (B) volume rendered 3D-CT images demonstrate neurofibromas in close proximity to the wrist flexor tendon groups (arrows).
FIGURE 9. A 33-year-old woman with a history of intravenous drug abuse. Two-dimensional MPR (10-mm section thickness, A) and coronal volume rendered 3D-CT (B) images of right wrist obtained after intravenous contrast administration show subcutaneous stranding characteristic of cellulitis and an infected pseudoaneurysm of the radial artery (arrowheads), with stranding extending to flexor pollicis longus tendon (arrows). Figure 9 can be viewed online in color at www.jcat.org.
FIGURE 10. A 71-year-old male patient. Axial (A) and sagittal (B) MDCT images show mineralization characteristic of calcific tendinopathy (white arrow) of the rectus femoris tendon at its attachment site on the anterior inferior iliac spine. © 2014 Lippincott Williams & Wilkins
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FIGURE 11. A 49-year-old male patient with end-stage renal disease. Tumoral calcinosis around the left hip involving the greater trochanter, gluteus minimus, and medius tendons is shown with characteristic fluid-calcium levels by axial CT image of the pelvis.
FIGURE 12. A 69-year-old male patient with a history of peripheral vascular disease and hyperuricemia. MRP (10-mm section thickness) 2D-CT image both (A) feet in sagittal oblique projection, shows juxta-articular erosions with high-density material at the midfoot including at the naviculocuneiform joint (curved arrow) involving the flexor digitorum tendon (straight arrow). MPR (10-mm section thickness) 2D-CT image obtained by DE-CT (B) shows monosodium urate crystal deposition at the same site. Figure 12 can be viewed online in color at www.jcat.org.
and therefore, radiologists should be aware of the CT features of normal and abnormal tendons. Although there are some overlap between MDCT appearance of normal tendon, tendinopathy, and partial tendon tears, several advances in CT techniques, including 3D postprocessing methods as well as the emerging tools of DE-CT and functional kinematic imaging are promising approaches for the optimal assessment of tendons. Further investigations are mandated to determine the accuracy and specific applications of MDCT for diagnosing various tendon pathologies.
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FIGURE 13. A 28-year-old male patient with history of clicking and pain in the right medial wrist. Coronal views obtained from a 4D-CT study demonstrate transient subluxation of the extensor carpi ulnaris from the ulnar groove (arrow) during mild extension and ulnar deviation, and complete reduction (arrowhead) during flexion and radial deviation. Figure 13 can be viewed online in color at www.jcat.org.
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or reduced multiplanar reconstruction image quality? AJR Am J Roentgenol. 2008;191:1386–1390. 6. Magnusson SP, Langberg H, Kjaer M. The pathogenesis of tendinopathy: balancing the response to loading. Nat Rev Rheumatol. 2010;6:262–268. 7. Ohashi K, El-Khoury GY, Bennett DL, et al. Orthopedic hardware complications diagnosed with multi-detector row CT. Radiology. 2005;237:570–577. 8. Fayad LM, Johnson P, Fishman EK. Multidetector CT of musculoskeletal disease in the pediatric patient: principles, techniques, and clinical applications. Radiographics. 2005;25:603–618. 9. Fritz J, Fishman EK, Corl F, et al. Imaging of limb salvage surgery. AJR Am J Roentgenol. 2012;198:647–660. 10. Fritz J, Fishman EK, Small KM, et al. MDCT arthrography of the shoulder with datasets of isotropic resolution: indications, technique, and applications. AJR Am J Roentgenol. 2012;198:635–646. 11. Prakash P, Kalra MK, Kambadakone AK, et al. Reducing abdominal CT radiation dose with adaptive statistical iterative reconstruction technique. Invest Radiol. 2010;45:202–210. 12. Demehri S, Salazar P, Steigner ML, et al. Image quality improvement using an image-based noise reduction algorithm: initial experience in a phantom model for urinary stones. J Comput Assist Tomogr. 2012;36:610–615.
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Detection and Characterization of Tendon Abnormalities With Multidetector Computed Tomography Shadpour Demehri, MD,* Majid Chalian, MD,† Sahar J. Farahani, MD,* Elliot K. Fishman, MD,* and Laura M. Fayad, MD*
Abstract: With recent advances in multidetector computed tomography (MDCT) acquisition and reconstruction options, MDCT can now be used successfully for evaluating tendon abnormalities. In this article, MDCT protocol optimization for the imaging of tendons is underscored, and applications of MDCT for assessing tendon pathology are highlighted. Although our retrospective experience of CT imaging with 2-dimensional multiplanar reconstructions and 3-dimensional postprocessing techniques is reviewed, potential applications for newer CT technologies, including dual-energy CT and 4-dimensional CT imaging of the peripheral tendons, are also discussed. Key Words: multdetector CT, tendon, dual-energy CT, 4-dimensional CT (J Comput Assist Tomogr 2014;38: 299–307)
A
lthough magnetic resonance imaging (MRI) and ultrasound are well established and widely used imaging modalities for the evaluation of tendons, multidetector computed tomography (MDCT) can also be a useful tool for the detection and characterization of tendon abnormalities. Familiarity with the normal and pathologic appearances of tendons is important to provide a comprehensive assessment during peripheral joint evaluation using CT images. Furthermore, in the care of patients who have metallic orthopedic hardware as well as in the setting of acute trauma, CT is particularly preferred as the modality of choice for the evaluation of osseous abnormalities. Therefore, assessment of the tendons at the time of imaging should be a component of the routine interpretation checklist. There is limited information and original research aimed at characterizing the normal and abnormal appearances of tendons using MDCT.1–6 In this article, we summarize the technical aspects for successful acquisition and display of MDCT images of the tendons. The normal MDCT appearance of tendons and abnormal appearances, including tendinopathy, tenosynovitis, and partial and complete tears of tendons, are discussed. Practical information for optimal acquisition, 3-dimensional (3D) postprocessing, and interpretation of noncontract MDCT images of the tendons is emphasized. Additionally, the potential role of newer CT technology with dual-energy CT (DE-CT) and 4-dimensional CT (4D-CT) is addressed.
From the *Russell H. Morgan Department of Radiology and Radiological Science, Johns Hopkins University School of Medicine, Baltimore, MD; and †Department of Radiology, University Hospitals Case Medical Center, Cleveland, OH. Received for publication July 27, 2013; accepted August 30, 2013. Reprints: Shadpour Demehri, MD, Musculoskeletal Radiology, Russell H. Morgan Department of Radiology and Radiological Science, Johns Hopkins University School of Medicine, 601 N Caroline St, JHOC 5165, Baltimore, MD 21287 (e‐mail: [email protected]). The authors declare no conflict of interest. Copyright © 2014 by Lippincott Williams & Wilkins
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INDICATIONS FOR MDCT Compared with CT, MRI offers superior contrast resolution for displaying soft tissue structures. However, in patients with metal hardware,7 the presence of MRI-incompatible implanted medical devices, and patients with a history of claustrophobia or obesity, MRI may be contraindicated and ultrasound or CT favored.8,9 Additionally, the wide availability and fast acquisition of MDCT has made CT a convenient modality for the assessment of the musculoskeletal system, with applications that continue to evolve. With regard to tendon assessment, CT is rarely a first-line technique (except where MRI is contraindicated), although CT arthrography is an established technique for assessing the tendons of the shoulder.10 Several advances have been introduced that result in improved visualization of the periarticular soft tissues by noncontrast CT: Robust 3D postprocessing with volume rendering (VR), DE-CT, as well as the feasibility of ultrafast image-acquisition for large body parts and 4D joint imaging using wide bore MDCT, can all potentially improve the demonstration of tendon pathology. Generally, potential exposure to ionizing radiation must be considered before MDCT is performed, but CT of the peripheral skeleton is subject to less effective radiation dose than that of CT of the torso.10 Table 1 summarizes the utility of MDCT in the assessment of tendon pathology.
TECHNICAL CONSIDERATIONS The optimal MDCT evaluation of tendons requires high spatial resolution as well as 3D postprocessing to improve the differentiation of normal tendons from abnormalities such as tendinopathy, tenosynovitis, and tendon rupture. The assessment of tendon structure is possible with MDCT by observation of changes in tendon attenuation and thickness change, continuity, as well as peritendinous synovial fluid accumulation. Multidetector CT can also be performed after the administration of intra-articular contrast, which enhances the morphologic assessment of many intra-articular and periarticular soft tissue structures, including the tendons. The administration of intravenous contrast is not routinely performed for the assessment of tendons, but can be used to improve the detectability of masses or inflammatory processes involving the tendons. The assessment of tendon motion is also possible with MDCT and is discussed further in the section regarding 4D-CT. Table 2 shows sample MDCT protocols for the evaluation of tendons. For the anatomic display of musculoskeletal structures by MDCT, the contrast resolution for assessment of the soft tissues can be improved by increasing the tube current and/or decreasing the pitch. By replacing previous CT image reconstruction algorithms such as filtered back projection with adaptive iterative dose reduction (AIDR), radiation exposure can be automatically reduced before the scan, ensuring that the optimal low dose is used for the specific diagnosis regardless of the size or shape of the patient. For routine clinical use, by implementing www.jcat.org
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TABLE 1. Utility and Limitations of MDCT in the Assessment of Tendon Abnormalities Utility/Indication for MDCT
Limitation
Contraindication to MRI, obesity, noncompliant patients with motion during image acquisition Presence of metal hardware close to the tendon
CT offers less contrast resolution than MRI and exposure to ionizing radiation
Assessment of tendinopathy (by changes in CT tendon morphology) Assessment of tendon tears (by changes in CT tendon attenuation and morphology) Assessment of tenosynovitis Assessment of intra-articular structures with CT arthrography* Assessment of peritendinous osseous abnormalities (spurs, avulsion fractures) Detection and characterization of gout with DE-CT Detection of tendon subluxation and adhesion to the adjacent structures during motion with 4D-CT
Although streak artifact can be reduced by metal-reduction CT acquisition protocols, tendons may remain obscured Atrophic and hypertrophic tendinopathy is detectable by CT, but a lack of morphologic change in the tendon size limits recognition of tendinopathy Complete tears are detectable; partial tears associated with no change in morphology are often undetected by CT or misdiagnosed as tendinopathy Septations in tenosynovitis characteristic of stenosing tenosynovitis are undetectable by CT CT arthrography is an invasive technique that offers limited visualization of nonarticular structures Bone marrow reactive changes (edema-like signal visible by MRI) are not visible by MDCT† Availability of DE-CT, diminished signal-to-noise and contrast-to-noise ratio, increase in radiation exposure Increased radiation exposure, increased data for postprocessing and archive needs, and longer scan time due to time consumed for patient's education and scan acquisition
*CT arthrography will not be discussed in this article, as noncontrast techniques are emphasized. †Bone marrow edema can be detected using dual-energy (DE) CT examination.
AIDR, one is able to reduce the effective dose, which ultimately leads to improved spatial resolution and excellent artifact (streak/beam hardening) reduction that are otherwise routinely present at low-dose acquisitions.11 In addition, with recently available denoising software,12 the contrast-to-noise and signalto-noise ratios of CT images can further improve without added radiation dose. The advent of isotropic voxels enables high spatial resolution volumetric imaging (3D-CT), and the transfer of the original CT acquisition into any other plane of choice; the latter allows viewing of the tendons from any desired projection, along with their relationship to adjacent osseous structures.
The main CT-based 3D techniques for the evaluation of tendons are VR, shaded surface display (SSD), and maximum intensity projection (MIP),1 which will be discussed later.
SSD and MIP Using SSD, tissues are defined by assigning a specific attenuation threshold to demonstrate the tissue types that are only above the chosen threshold. Because of the fundamental role of attenuation of threshold in the creation of SSD images, initial editing of the images may be required to eliminate unwanted tissues that may obscure the course and appearance of tendons, and therefore, can be very time-consuming. Other
TABLE 2. Sample MDCT Acquisition Protocols for Evaluation of Tendons Protocol Peak voltage, kVp Tube voltage, mA Rotation time, s Detector collimation, mm Slice thickness, mm Pitch IV contrast Temporal resolution, ms Image reconstruction† Reconstruction algorithms* Effective dose, mSv‡
Single-Energy 64-MDCT
DE-CT
4D-CT
100–140 (range) 250–350 0.5 0.6 0.75 0.9 Optional 150–300 AIDR MPR, 3D-CT 0.03–0.2
100 and 140* 75, 320 0.5 0.6 0.75 0.9 Optional — AIDR MPR, 3D-CT 0.04–0.3
80–120 80–125 0.5 0.6 0.5 0.9 None 500 AIDR 3D-CT 0.3–2.0
*MPR indicates multiplanar reconstruction. †AIDR algorithm is currently used for CT image reconstruction. ‡Radiation dose for CT acquisition of shoulder and hip joints were not included, and there is likely higher radiation exposures as these joints are closer to torso. The effective radiation dose (mSv) was calculated by multiplying the dose length product (DLP) reported from the CT scanner by the conversion factor of k = 0.0005 reported for distal extremities.
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disadvantages of SSD are its inaccurate display of tendon contour and size due to a potential suboptimal choice for the attenuation threshold.13 Maximum intensity projection is typically used to display blood vessels that demonstrate contrast material filling at CT or MR angiography. The tendons can also be demonstrated with MIP processing of CT images, although this technique is rarely used.14 Using this postprocessing method, the voxel with the maximum attenuation value will be selected. These maximum-attenuation voxels are retained and all other voxels will be eliminated from the CT data set. Similar to SSD, these voxels can represent much, but not all, of the data obtained during the CT acquisition. Maximum intensity projection images are not optimal methods for demonstrating other soft tissue structures, as only bones and tendons comprise the voxels with maximum attenuation values. Maximum intensity projection images have been suggested to be useful when metal is present because of the decreased image distortion from beam hardening artifact, although the VR technique remains adequate in the setting of metal.
Volume Rendering Volume rendering imaging has become a common CTbased 3D display method.4,5 With this technique, various tissues are defined by assigning a specific tissue type (such as a tendon) to a specific range of attenuations defined by Hounsfield units (HU). Each tissue will be assigned a certain color, based on its range of Hounsfield units using the histogram of the CT data set. Volume rendering imaging is particularly helpful in demonstration of soft tissue pathologies associated with osseous abnormalities. Due to the versatility of VR, this 3D technique is often favored.15–18
NORMAL TENDON ANATOMY By MDCT, normal tendons are relatively homogenous well-demarcated tubular structures.19 Tendons that are typically well visualized by CT include the extensor and flexor tendons of the wrist and hand, the hamstring tendons, the extensor tendons of the knee, as well as most tendons in the ankle.3 Some tendons are less discernible by noncontrast CT, such as the rotator cuff tendons, largely due to the conjoined configuration of adjacent tendons (eg, supraspinatus and infraspinatus tendons) and relative paucity of surrounding fat.
Detection of Tendon Abnormalities With MDCT
Using 3D postprocessing methods described previously, it is possible to simultaneously demonstrate the tendons and adjacent bones due to significant attenuation differences among the various soft tissues, tendons, and bone (fat approximately −100, muscle approximately 30, tendon approximately 90, and bone approximately 250 to 500 HU). The attenuation values of normal tendons are variable and can change with their size, due to partial volume effect as well as the CT acquisition parameters. For example, for tube kilovoltage (kVp) of 120 (the value used in routine clinical practice), the normal attenuation of tendons has been shown to range between 80 and 100 HU. However, using a lower kilovoltage will yield lower attenuation values for normal tendons, whereas using a higher kilovoltage will yield higher attenuation values for tendons.14 An important morphologic CT-based descriptive feature that can differentiate the normal from abnormal tendon is the relative size of the tendon in relationship to other adjacent nearby tendons, as small tendons may represent atrophic tendinopathy, whereas large tendons may reflect hypertrophic tendinopathy or a partial rupture. In addition, defining the course and margin of the peripheral tendons and associated structures such as the myotendinous junction, the osseous attachment site, and retinaculum are now feasible and should be sought on a clinical examination of the tendons.
PATHOLOGICAL FINDINGS The most common tendon pathologies encountered include tenosynovitis, tendinopathy, partial or complete tears, and subluxation or dislocation. Soft tissue masses that are associated with tendons can also occur, although commonly include ganglion cysts. Bursae-related abnormalities such as retrocalcaneal or infrapatellar bursitis occur because of the intimate association of bursae and tendons.
Tendinopathy The mechanism of injury that leads to tendinopathy remains unclear. Many pathological changes may occur in tendinopathy, including a reduction in the number and rounding of fibroblasts, an increase in the content of proteoglycans, glycosaminoglycans, and water, and the hypervascularization and disorganization of collagen fibrils. There is a significant overlap between the MDCT appearance of normal tendon and tendinopathy, which is typically
FIGURE 1. A 70-year-old male patient with Achilles tendinopathy manifested by a thick tendon on sagittal (A) and axial (B) CT images. Convexity of the tendon on the anterior surface (arrow) and paratenonitis (arrowhead) is associated with tendinopathy. Figure 1 can be viewed online in color at www.jcat.org. © 2014 Lippincott Williams & Wilkins
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FIGURE 2. A 38-year-old female patient with peroneus longus and brevis tenosynovitis. Sagittal CT with bone windows (A), sagittal CT with soft tissue windows (B), and volume-rendered 3D-CT (C) show a bone spur due to prior healed fracture of distal fibula (arrow in A) and resulting tenosynovitis of the adjacent peroneal tendons (arrow in B). Three-dimensional CT for comparison also demonstrates peroneus longus and brevis tenosynovitis, manifested as decreased attenuation and thickening of these tendons. Figure 2 can be viewed online in color at www.jcat.org.
demonstrated by enlargement and heterogeneous attenuation of the tendon. Therefore, it is more likely to diagnose tendinopathy in larger tendons, such as the Achilles (Fig. 1) or patellar tendon. When internal tendon density approaches fluid attenuation, a partial tear should be suspected. However, again MDCT is not as accurate as MRI for differentiating tendinopathy from partial tendon tears in smaller peripheral tendons, as there is significant overlap in the CT features of tendinopathy and partial tears, namely morphologic changes and heterogeneous attenuation.
Tenosynovitis Disorders of the synovial membrane induce proliferation of the synovium and accumulation of fluid in the tendon sheath. By MDCT, the tendon appears as a hyperattenuating structure with a halo of low attenuation similar to fluid and/or thickening of the synovial membrane (Figs. 2 and 3). Multidetector CT allows detection of very small synovial collections. If intravenous contrast is administered during a CT examination, associated hypervascularity and inflammation can also be demonstrated. Similarly, the presence of fluid attenuation posterior to the Achilles tendon indicates paratenonitis (Fig. 1).
Tendon Tear Computed tomographic images have high spatial resolution, and along with 3D reconstructed images, even ruptures
in small tendons of the hands (Figs. 4 and 5) and feet are clearly discernible. The advantage of 3D-CT imaging over MRI is that the surgeon does not need to integrate multiple images (of MRI) and can readily recognize the site of the rupture as well as better delineate small osseous avulsions by a comprehensive 3D-CT image (Fig. 6); hence, surgical planning is facilitated, although 3D MR techniques are also currently increasingly available.20 Although less conspicuous compared to MRI or ultrasound,21 partial-thickness tears can also be diagnosed using MDCT, as evidenced by fluid attenuation within the tendon substance (Fig. 7).
Ganglion Cysts Ganglion cysts are one of the most common soft tissue masses to arise around a tendon. They appear as well demarcated and hypodense structures adjacent to a tendon and may communicate with the tendon sheath or joint.22 They do not demonstrate enhancement after contrast administration.
Other Soft Tissue Abnormalities Other soft tissue lesions that can affect tendons include neoplasms (Fig. 8) and infection (Fig. 9). The latter entities may involve a tendon as the primary site or may spread from adjacent structures. Also, processes with mineralization such as calcific tendinopathy (Fig. 10) and tumoral calcinosis
FIGURE 3. A 33-year-old male patient with persistent right-hand pain and swelling. Axial MDCT image (A) and axial MPR image (20-mm-thick section, B) demonstrate abnormal thickening and tenosynovitis of the flexor digitorum tendon of the long finger (arrow in A, arrowhead in B). The advantage of using thick section MPR views is the potential for enhanced demonstration of tendon pathology. Figure 3 can be viewed online in color at www.jcat.org.
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Detection of Tendon Abnormalities With MDCT
(Fig. 11) are better evaluated using MDCT (to detect mineralization) than MRI.
DUAL-ENERGY CT
FIGURE 4. A 52-year-old female patient with an intra-articular fracture of the distal phalanx. Coronal CT image of the right hand shows a complete tear of the flexor digitorum longus tendon of the little finger, with retraction (white arrow).
Dual-energy CT is either comprised of 2 orthogonally oriented rows of detectors, within a single gantry or a single row of detectors with alternating currents.5,14,23 The presence of 2 x-ray generation sources within this CT system offers the opportunity for independent data acquisition while the 2 CT tubes operate at 2 different energies (typically set at 100 and 140 kVp). Depending on the chemical composition of a structure, it will appear with different attenuation values at the 2 energy levels. For tendon evaluation, modest changes in the attenuation of tendons occur as the collagenous structures of tendons are disrupted (such as tendon tears), and these changes are accentuated by DE-CT. Dual-energy CT images have shown improved conspicuity of structures with high atomic numbers such as intravascular iodine, but other relatively dense structures composed of smaller atoms (eg, hydrogen, carbon, nitrogen, and oxygen) may not be as well seen because of reduced x-ray beam absorbance by the various tube energies. Dual-energy CT has recently emerged as a useful method for differentiating iodinated contrast from hemorrhage23 and it has been proven useful in the detection and characterization of mineralization or crystal deposition along the tendons in conditions such as early calcific tendinopathy or tophaceous gout (Fig. 12), compared with conventional CT.23
FIGURE 5. A 52-year-old female patient with history of left hip pain status post fall. Axial (A) and coronal (B) 2D MDCT images of the left hip demonstrate a heterogeneous hyperdense structure distal to the ischium (white arrows in A and B), which is confirmed as hamstring tendon rupture by corresponding axial (C) and coronal (D) inversion-recovery (inversion time, 150; TR/TE, 5850/46) MR images (white arrowheads in C and D). Note hematoma by coronal MR image (*). © 2014 Lippincott Williams & Wilkins
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FIGURE 6. A 70-year-old female patient with a complete tear of the Achilles tendon as seen on sagittal (A) and axial (B) MDCT images with tiny avulsed osseous fragment (white arrow). On axial views at the site of tendon rupture, there is diminished CT attenuation lower than tendon stump consistent with fluid/hematoma accumulation at the tear site (white arrowhead).
Nevertheless, for the CT assessment of tendons, a recent study showed that the image quality of DE-CT for the demonstration of normal tendon structure is inferior to conventional CT images, even when radiation exposure is increased.5
FUNCTIONAL IMAGING: 4D-CT Musculoskeletal imaging has been predominantly based on the static, morphologic depiction of tendons using static CT and MRI. However, some tendon abnormalities such as subluxation or dislocation can only be detected using kinematic imaging. One option to depict joints in motion is to perform static CT, MRI, or ultrasound of a joint using small degrees of joint motion with passive positioning to detect subluxation not visible on radiographs. For example, patellofemoral maltracking has been assessed in this manner.24
The depiction of peripheral joints in motion under physiologic conditions has been termed kinematic imaging and has been previously performed with high diagnostic quality25; currently, kinematic imaging is technically feasible with widebore MDCT for 4D-CT imaging.26,27 Using a 320-row MDCT, 4D-CT imaging is performed as a 16-cm acquisition of the limbs during motion with temporal uniformity and high temporal resolution of 0.5 seconds. In our preliminary experience, the 4D-CT examination of distal peripheral joints (ankle and wrist) is associated with low radiation exposure (up to 1 mSv), and the depiction of abnormal motion of the peripheral tendons can be enhanced with a soft tissue reconstruction algorithm (Fig. 13).
CONCLUSIONS When MDCT is obtained for other reasons such as osseous trauma, it can also be useful to also assess the tendons,
FIGURE 7. A 17-year-old male patient with a history of gunshot wounds to the legs and comminuted fracture of left tibia. Sagittal multiplanar reconstruction (MPR) (5-mm section thickness) (A) and sagittal 3D volume rendered CT (B) demonstrate a high-grade tear (white arrows in A and B) of the anterior tibialis tendon of the ankle adjacent to the fracture site. In B, 3D-CT image shows the adjacent skeleton for a more comprehensive view, and a few remaining intact fibers (arrowhead in B) are identified, characterizing this tear as a high-grade partial tear, rather than complete tear. Figure 7 can be viewed online in color at www.jcat.org.
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FIGURE 8. A 23-year-old male patient with a history of neurofibromatosis. Coronal (A) and sagittal (B) volume rendered 3D-CT images demonstrate neurofibromas in close proximity to the wrist flexor tendon groups (arrows).
FIGURE 9. A 33-year-old woman with a history of intravenous drug abuse. Two-dimensional MPR (10-mm section thickness, A) and coronal volume rendered 3D-CT (B) images of right wrist obtained after intravenous contrast administration show subcutaneous stranding characteristic of cellulitis and an infected pseudoaneurysm of the radial artery (arrowheads), with stranding extending to flexor pollicis longus tendon (arrows). Figure 9 can be viewed online in color at www.jcat.org.
FIGURE 10. A 71-year-old male patient. Axial (A) and sagittal (B) MDCT images show mineralization characteristic of calcific tendinopathy (white arrow) of the rectus femoris tendon at its attachment site on the anterior inferior iliac spine. © 2014 Lippincott Williams & Wilkins
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FIGURE 11. A 49-year-old male patient with end-stage renal disease. Tumoral calcinosis around the left hip involving the greater trochanter, gluteus minimus, and medius tendons is shown with characteristic fluid-calcium levels by axial CT image of the pelvis.
FIGURE 12. A 69-year-old male patient with a history of peripheral vascular disease and hyperuricemia. MRP (10-mm section thickness) 2D-CT image both (A) feet in sagittal oblique projection, shows juxta-articular erosions with high-density material at the midfoot including at the naviculocuneiform joint (curved arrow) involving the flexor digitorum tendon (straight arrow). MPR (10-mm section thickness) 2D-CT image obtained by DE-CT (B) shows monosodium urate crystal deposition at the same site. Figure 12 can be viewed online in color at www.jcat.org.
and therefore, radiologists should be aware of the CT features of normal and abnormal tendons. Although there are some overlap between MDCT appearance of normal tendon, tendinopathy, and partial tendon tears, several advances in CT techniques, including 3D postprocessing methods as well as the emerging tools of DE-CT and functional kinematic imaging are promising approaches for the optimal assessment of tendons. Further investigations are mandated to determine the accuracy and specific applications of MDCT for diagnosing various tendon pathologies.
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FIGURE 13. A 28-year-old male patient with history of clicking and pain in the right medial wrist. Coronal views obtained from a 4D-CT study demonstrate transient subluxation of the extensor carpi ulnaris from the ulnar groove (arrow) during mild extension and ulnar deviation, and complete reduction (arrowhead) during flexion and radial deviation. Figure 13 can be viewed online in color at www.jcat.org.
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