Elbow Imaging

Conventional Radiographs

Imaging evaluation of the elbow typically begins with conventional radiographs. Standard radiographic views of the elbow include anteroposterior (AP), lateral, and oblique projections ( Fig. 59.1 ). The AP and oblique views are obtained with the elbow extended and forearm supinated, permitting visualization of the proximal radioulnar and radiocapitellar articulations, medial and lateral epicondyles, and trochlea. The carrying angle of the elbow typically measures 12 to 15 degrees valgus in this anatomic resting position. The lateral view, obtained with the elbow flexed 90 degrees and the forearm in a neutral position, permits evaluation of the radiocapitellar and ulnotrochlear articulations, distal humerus, olecranon, and coronoid processes. An appropriately acquired lateral elbow radiograph clearly shows the ulnotrochlear articulation with minimal overlap of osseous structures. This ulnotrochlear space on the lateral view can be obscured with small amounts of rotation; therefore careful attention should be paid the obtaining well-positioned lateral elbow views. An anatomically positioned radial head is collinear with the capitellum. A dark radiolucent area is normally seen just anterior to the distal humerus, representing the anterior fat pad. Similarly, the posterior fat pad sign is a well-demarcated lucency posterior to the distal humeral metaphysis; its presence tends to indicate a pathologic process. Displacement of these fat pads, produced by an intra-articular effusion or hemarthrosis, resembles the spinnaker on a sailboat and is referred to as the sail sign .

Conventional radiographic projections are usually satisfactory for the initial evaluation of the elbow; however, additional projections may be utilized for structures that are difficult to evaluate on standard AP, lateral, and oblique projections. Fractures of the radial head are common and may be challenging to detect when nondisplaced. A specific radial head (radiocapitellar) view may better demonstrate the fracture, characterize the fracture pattern, and aid in classification. With the elbow flexed to 90 degrees, the beam is angled 45 degrees anteriorly from a true lateral. This projection provides an unobstructed view of the radial head without overlap of the proximal ulna. Other common fractures include those involving the epicondyles (especially the medial epicondyle in skeletally immature patients), distal humerus, and olecranon. The “terrible triad” injury includes fractures of the radial head and coronoid process, along with dislocation of the elbow joint. Conventional radiographs are usually sufficient for confirming concentric reduction of the elbow after closed manipulation.

Any patient presenting with non–activity-related pain or a mass at the elbow should first be evaluated with conventional radiographs. Radiographs offer a large amount of information and help guide further imaging workup.

Elbow trauma often involves a combination of osseous and soft tissue injury. When conventional radiographs do not reveal a problem or provide incomplete information, the choice of additional studies is guided by the history and physical examination. These additional studies may include CT, MRI, MR arthrography, fluoroscopy, and bone scan. Some relevant injuries and their imaging findings are highlighted in this chapter to aid the general orthopedic surgeon or sports medicine specialist.

Computed Tomography

Modern CT scanners allow for rapid image acquisition, high spatial resolution, and isovolumetric data sets that can provide multiplanar reformatting of the images. These qualities allow for excellent depiction of bone; they are useful for the detection of subtle fractures, periosteal reaction, and subtle bone lesions that may be difficult to appreciate on conventional radiographs. The high attenuation of cortical bone allows for segmentation and generation of three-dimensional volume-rendered images with minimal manual postprocessing. These images provide important visual information in the planning of complex fracture repairs. CT technology continues to improve, with dose reduction, metal reduction techniques, and quantitative technology. For example, dual-energy CT (DECT) can be used in the musculoskeletal system for the detection of uric acid crystals in gout. The foot and ankle are most commonly imaged with this DECT technique, but it can also be applied to the elbow in challenging diagnostic cases ( Fig. 59.2 ).

Magnetic Resonance Imaging

MRI provides excellent soft tissue contrast and is the modality of choice for evaluating soft tissue masses, ligament injury, tendon injury, nerve-related symptoms, and articular cartilage. It is also the modality of choice in evaluating clinically relevant nonspecific elbow pain not explained by conventional radiographs. There is no single MRI protocol for the elbow, and the MRI technique should be tailored to the specific clinical question in order to provide the best diagnostic information. T1-weighted, proton density and fluid-sensitive sequences are typically adequate for evaluating the elbow. T1-weighted images better depict osseous pathology and are preferred in the evaluation of a soft tissue mass and of the nerves. Ligaments, tendons, and cartilage are better depicted with proton density sequences. The term fluid sensitive refers to an MRI sequence that depicts the protons in fluid/edema as bright while the protons in fat appear dark. Examples of fluid-sensitive sequences include T2-weighted images with chemical fat saturation, intermediate proton density–weighted with chemical fat saturation, short tau inversion recovery (STIR), and Dixon water/fat separation. Intravenous gadolinium contrast is not typically required in elbow MRI but is often used for the evaluation of soft tissue masses, nerve pathology, or synovitis.

Magnetic Resonance Arthrography

MR arthrography may improve the MRI evaluation of locking, limited range of motion (ROM), and ulnar collateral ligament (UCL) injury, but preference between standard MRI and MR arthrogram for these diagnoses varies. The instillation of dilute gadolinium contrast into the elbow may be guided by US or fluoroscopy, depending on availably and the comfort of the proceduralist. The patient is most commonly positioned prone with the elbow above the head, but the injection can also be administered with the patient seated adjacent to the fluoroscopy table and the arm placed on the table. With the elbow flexed at 90 degrees, the elbow is positioned as in a lateral radiograph. A 22-gauge needle is placed in the radiocapitellar joint and a small test of iodinated contrast is used to confirm intra-articular placement. The dilute gadolinium contrast is instilled under intermittent fluoroscopic guidance to ensure intra-articular placement. The MR arthrography sequences differ, as the exam relies heavily on T1-weighted fat-saturated images. This allows for the detection of any gadolinium contrast that may extend into or through a ligament tear, as in the evaluation of an UCL tear.

Ultrasound

US excels at dynamic soft tissue evaluation of the elbow joint and is able to provide an alternative for imaging around surgical hardware. Because of the complex mechanics of hinging and rotation, static imaging may fail to detect dynamic pathology. US has the benefit of high-resolution dynamic image evaluation without ionizing radiation and allows for direct patient communication and “real-time” examination. US is a powerful tool but is user-dependent, requiring a detailed anatomic knowledge and basic understanding of sonographic principles if the operator is to select the correct transducer and be aware of imaging artifacts and pitfalls. Image interpretation can be optimized by using high-frequency linear array transducers (10 to 20 MHz), utilizing standardized imaging acquisition planes and acquiring stored cine clips for subsequent review.

The use of US in the evaluation of the elbow is growing. Currently it is most often used in the evaluation of dynamic pain/snapping, tendon pathology, joint effusion, peripheral nerve evaluation, and for guiding percutaneous procedures about the elbow.

Fracture

The elbow features complex three-dimensional anatomy and overlapping structures that can make the detection of subtle fractures on conventional radiographs challenging. Interpretation of radiographs of the skeletally immature elbow for trauma requires knowledge of the ossification centers, the time line for development of the ossification centers, and an understanding of the order of growth plate closure. The mnemonic CRITOE can serve as an aid for remembering the order of ossification and closure (C, capitellum; R, radial head; I, internal/medial epicondyle; T, trochlea; O, olecranon; E, external/lateral epicondyle). Avulsion fractures are common injuries of the pediatric elbow, and knowledge of this pattern of growth plate closure helps in their identification.

CT or MRI can help to detect subtle fractures. CT is often preferred, given its availability and lower cost; however, MRI is also excellent for detecting nondisplaced fractures. CT with multiplanar reformatted images is the preferred modality in the presurgical planning of complex elbow fractures. Therefore complex fractures of the distal humerus and medial coronoid facet are commonly evaluated by CT for preoperative planning.

Fracture-Dislocation

Elbow dislocations are evident on conventional radiographs and are characterized by the position of the radius/ulna relative to the humerus. Typically these injuries are not subtle on radiographs, but the associated deformity and difficulty of positioning can limit the detection and characterization of fractures. Traction radiographs may help delineate the fracture pattern but may not be tolerated by all patients owing to the associated pain. CT better depicts complex fracture-dislocations and aids in fracture pattern characterization, degree of fragment displacement, articular surface involvement, and/or the presence of associated intra-articular fracture fragments, which may block concentric reduction and lead to persistent joint subluxation. Failure to identify fractures of stabilizing osseous structures such as the coronoid or radial head may lead to a poor functional outcome. Osseous three-dimensional reconstructions are commonly created from CT data for these complex fractures of the elbow to best demonstrate the relationship of the fracture fragments and for surgical planning. Finally, MRI is the best option for determining the pattern of soft tissue disruption in acutely unstable injuries or those that present with delayed complaints of subjective instability, which may require collateral ligament repair or reconstruction.

Occult Fracture

Persistent tenderness over a suspected injury site despite negative findings of conventional radiographs may be an indication for further evaluation in select cases. The posterior fat pad sign in the setting of trauma corresponds to an occult fracture in more than 75% of patients. The radial head is the most frequent site of an occult fracture. Fractures of the coracoid process are best visualized on well-positioned lateral radiographs; however, oblique views may also be helpful in detection and characterization. CT or MRI better detect non-displaced fractures and are indicated with high clinical suspicion of a fracture and negative radiographs. Fluid-sensitive (fat-suppressed T2-weighted or inversion recovery) sequences are the most sensitive for detecting radiographically occult fractures. Fractures show a linear pattern of signal change, with decreased signal on T1-weighted images and increased signal on T2-weighted images ( Fig. 59.3A and B ). Proton density images are commonly utilized in examining the elbow, but T1 images better demonstrate the linear signal abnormality of a fracture (see Fig. 59.3A ). In contrast, osseous contusion produces a nonspecific diffuse increase in signal on T2-weighted images without a discrete fracture line (see Fig. 59.3C ).

Stress Fracture

The olecranon is the most common stress fracture site, and this condition is most often seen in baseball players. When the history and physical examination suggest it, a CT, MRI, or bone scan may be diagnostic. A CT scan typically reveals a subtle fracture line in the olecranon, often with a linear area of sclerosis. Findings on MRI resemble those of occult fractures described previously. A bone scan would reveal abnormal radiotracer uptake at the site of repetitive stress.

Ulnar Collateral Ligament Injury

The UCL is an important valgus stabilizer of the elbow and is particularly vulnerable to injury in athletes whose sport involves throwing, such as baseball pitchers and javelin throwers. Interpretation of valgus stress radiography in athletes can be challenging. Several investigators have shown increased valgus laxity in the dominant arm of asymptomatic persons. When the index of clinical suspicion for a UCL injury is high, MRI provides important additional information. The UCL is a vertically oriented structure coursing between the medial epicondyle and the coronoid process that is uniformly of low signal intensity; it is best depicted on coronal MRI. The UCL is composed of anterior, posterior, and transverse bands. The anterior bundle provides the most valgus elbow stabilization. MRI may detect full-thickness tears of the anterior bundle, with increased signal intensity often found within and adjacent to the discontinuous ligament on fat-suppressed T2-weighted images as a result of edema and/or hemorrhage ( Fig. 59.5A ). MRI is less reliable for the detection of partial-thickness tears. Timmerman et al. found MRI to be 100% sensitive for full-thickness tears but only 14% sensitive for partial-thickness tears. Administration of intra-articular contrast material improves sensitivity in detecting partial-thickness UCL tears (see Fig. 59.5B ). Schwartz et al. reported 86% and 100% sensitivity and specificity, respectively, for the detection of partial-thickness UCL tears with MR arthrography. Variability in the insertion of the anterior bundle, however, complicates MRI interpretation of partial-thickness tears. The distal ulnar attachment of the anterior bundle may insert anywhere between 1 mm of the articular margin of the coronoid process and 3 mm distal to the sublime tubercle of the ulna. This characteristic can create a small recess on MR arthrography along the medial margin of the coronoid process. Partial tears of the distal insertion of the UCL are diagnosed by intra-articular gadolinium extending deep to the insertion, referred to as the “T-sign” (see Fig. 59.5B ). Consequently distinguishing between normal anatomy and a pathologic partial undersurface tear at the attachment of the distal ligament can be challenging ; therefore clinical correlation is paramount.

US examination of the UCL can aid in the diagnosis of a tear and can evaluate for instability with valgus stress. The UCL is best depicted at 70 degrees of flexion with the transducer placed in a longitudinal manner to the long axis of the ligament ( Fig. 59.6 ). In this position, the anterior band of the UCL should appear taut, with a tear demonstrated as focal hypoechogenicity with fiber disruption. The medial joint margin and UCL can be dynamically evaluated under valgus stress for determining laxity/insufficiency, with greater than 2 mm of widening compared with the contralateral elbow being considered an abnormality of the anterior band of the UCL. The prevalence of UCL thickening, irregularity, and laxity complicates the US evaluation of the UCL in pitchers, and these abnormalities tend to progress over time.