Elbow anatomy and radiographic diagnosis of elbow fracture in children

INTRODUCTION — Knowledge of anatomy, normal bony development, and radiographic features of the pediatric elbow are essential to prompt recognition and treatment of elbow injuries in children. In most instances, plain radiographs are adequate to detect fractures that pose a threat to future growth and function. On occasion, additional modalities (eg, ultrasound, magnetic resonance imaging, or arthrography) are needed to identify and fully delineate elbow fractures, especially in infants and young children.

This review discusses the anatomy, plain radiographic views, and radiographic interpretation of the pediatric elbow. Findings pertinent to specific elbow fractures in children are discussed in more detail separately. (See "Supracondylar humeral fractures in children" and "Evaluation and management of condylar elbow fractures in children" and "Epicondylar and transphyseal elbow fractures in children".)

PERTINENT ANATOMY

Elbow joint — The elbow is a complex hinge joint composed of three separate articulations (figure 1) [1]:

●Ulnohumeral – Between the trochlea of the humerus and the olecranon of the ulna

●Radiohumeral – Between the capitellum of the humerus and the radial head

●Radioulnar – Between the proximal radius and ulna

Although the medial and lateral epicondyles are both extraarticular, the full articular surface of the distal humerus is intraarticular [2].

An anterior fat pad and a posterior fat pad are located between the capsule and the distal end of the humerus (figure 2) [2]. The appearance of these fat pads assists in the radiographic diagnosis of elbow fractures. (See 'Fat pads' below.)

Ossification centers — There are six growth centers within the elbow joint that usually appear in a consecutive manner within predictable age ranges [3]. Their appearance is usually earlier in girls than boys (table 1) [4]. These secondary ossification centers later fuse to create the mature bony elbow. Similar to the appearance of the ossification centers, fusion proceeds at relatively predictable age ranges and usually occurs earlier in girls than in boys (figure 3A-B) [3,4]. (See 'Normal ossification' below.)

Growth plate (physis) — Growing long bones in children are composed of the following segments: diaphysis (shaft), metaphysis (where the bone flares), physis (growth plate), and epiphysis (secondary ossification center) (figure 4). The physis is composed of cartilage and is a transition zone located between the epiphysis and diaphysis of an immature long bone. This is a highly metabolically active and rapidly changing area of bone where longitudinal growth of long bones occur [5-7].

The growth and change that occur at a growth plate promote rapid healing of fractures. However, injury to the physis itself can lead to asymmetric growth and subsequent deformity [6,8,9]. The epiphyseal cartilage cells stop duplicating at the end of puberty. The entire cartilage is eventually replaced by bone, and epiphyseal lines remain at the site [7]. In contrast to the proximal humeral physis, the distal humeral physis contributes little (15 to 20 percent) to the overall longitudinal growth of the humerus [10].

Vascular anatomy — The brachial artery is superficial to the brachialis muscle along the anteromedial aspect of the humerus. As the brachial artery passes anterior to the distal humerus, an extensive collateral circulation develops. As the artery extends into the forearm, it splits into the radial and ulnar arteries (figure 5).

The brachial artery is especially prone to injury following a supracondylar fracture. "Scraping" of the brachial artery by the bony fragment may result in intimal damage to the vessel; this injury may subsequently lead to thrombosis and vascular insufficiency. Because of the extensive collateral circulation present at the elbow, it is rare for the ischemia of the arm to occur from complete occlusion of the brachial artery. (See "Supracondylar humeral fractures in children", section on 'Pertinent anatomy'.)

Nerve anatomy — The median nerve crosses the elbow with the brachial artery [11]. The radial nerve runs between the brachialis and brachioradialis muscles before crossing the elbow and penetrating the supinator muscle. The ulnar nerve crosses the elbow posterior to the medial epicondyle (figure 5).

The median and radial nerves are susceptible to injury following supracondylar fractures after elbow hyperextension [11]. The ulnar nerve is susceptible to injury when a direct blow to the posterior aspect of the elbow or supracondylar fracture with elbow hyperflexion occurs [12]. (See "Supracondylar humeral fractures in children", section on 'Complications'.)

PLAIN RADIOGRAPHIC VIEWS — Radiographs should be obtained in children who present with localized tenderness, deformity, swelling, or limited movement of the elbow and known or suspected trauma. Plain radiographs should be obtained in at least two views: an anterior-posterior view in extension and a lateral view at 90 degrees of flexion (image 1) [6].

●The anterior-posterior view shows the epicondyles (medial and lateral) and the articular surfaces (radiocapitellar and ulnotrochlear).

●The lateral view shows the relationship between the bones of the distal humerus and the proximal forearm and the relationship of the anterior and posterior fat pads to the humerus.

A third view, the lateral oblique view, shows the radiocapitellar joint, medial epicondyle, radioulnar joint, and the coronoid process. This view can be helpful in diagnosing subtle lateral condyle fractures and assessing displacement but is not generally routinely obtained [3].

PLAIN RADIOGRAPH INTERPRETATION — Interpretation of the pediatric elbow radiograph is challenging due to the different stages of ossification and predominance of cartilage present prior to ossification [13].

Several radiographic signs and lines have been described to assist with the radiographic diagnosis of elbow fractures. These radiographic signs are reliable only if the elbow has been properly positioned for the radiographs; the importance of a true lateral view in particular cannot be overemphasized (image 1 and figure 6 and image 2) [1].

Normal ossification — There are six ossification centers in the pediatric elbow (table 1) [1]. The following acronym is a useful memory aid for recalling the typical radiographic order of appearance (figure 7):

●C – Capitellum

●R – Radial head

●I – Internal (medial) epicondyle

●T – Trochlea

●O – Olecranon

●E – External (lateral) epicondyle

The first ossification center, the capitellum, appears at about one year of age and each subsequent ossification site in the mnemonic appears approximately in a two year progression (ie, 1, 3, 5, 7, 9, and 11 years). However, this timing is highly variable and markedly differs by sex. The exact mean age, range of appearance, and degree of difference by sex is presented in the table (table 1). These secondary ossification centers later fuse to create the mature bony elbow. If it is difficult to distinguish the normal growth center from a fracture fragment, comparison views of the uninjured elbow may be helpful [1].

Anterior humeral line — In the normal elbow, a line drawn through the anterior cortex of the humerus intersects the capitellum in its middle third (image 1) [14]. Because the most common mechanism of injury to the elbow is hyperextension, posterior displacement of the distal humerus is common when a fracture occurs. In these cases, the anterior humeral line passes through the anterior third of the capitellum or fails to intersect it (image 3) [6].

Detection of abnormalities of the anterior humeral line in children less than two years of age is complicated by the variable rates of ossification of the capitellum; comparison views can be helpful in uncertain cases [1].

Radial head alignment — The radial head should point toward the capitellum on all views [3]. The clinician should suspect a lateral condyle fracture or Monteggia fracture when radial head dislocation is evident (image 1 and image 4).

Fat pads — Displacement of fat pads on radiographs is an indirect indicator of hemorrhage or an elbow joint effusion (figure 8). In the setting of known or suspected trauma, the presence of an abnormal fat pad sign should be considered a marker of an occult fracture and an indication for careful immobilization and close follow-up (image 5) [1].

Anterior fat pad — An anterior fat pad can be a normal variant when it appears as a narrow radiolucent strip superior to the radial head and anterior to the distal humerus. A wide anterior fat pad, however, is known as a "sail" sign and may be indicative of an occult fracture (figure 8) [6]. In one study of 197 children (25 percent with fractures) undergoing elbow radiographs in a pediatric emergency department, an abnormal anterior fat pad, as interpreted by a pediatric radiologist, had a sensitivity of 96 percent and a positive predictive value of 64 percent for an occult elbow fracture [15]. In contrast, the presence of a posterior fat pad in these patients had a sensitivity of 88 percent and a positive predictive value of 79 percent.

Posterior fat pad — The posterior fat pad is not visible in normal children but usually appears as a radiolucency posterior to the distal humerus and adjacent to the olecranon fossa in children with a distal humeral fracture (image 5) [6].The presence of a posterior fat pad on radiograph indicates an effusion of the elbow joint caused by significant trauma [16]. The sensitivity of a posterior fat pad for an occult fracture varies significantly in the literature with the largest study suggesting a sensitivity of approximately 88 percent [15].

Carrying angle — With the arms in anatomic position (eg, extended with palms facing forward), the normal carrying angle of the elbow is approximately 5 to 15 degrees, thus placing the hands and forearms slightly away from the body.

Varus deformity of the elbow, also called cubitus varus, results in the forearm pointing towards the body when the arm is anatomic position with the elbow sticking out ("gunstock deformity") (figure 9). This abnormality may be seen after inadequate alignment of a displaced supracondylar fracture. (See "Supracondylar humeral fractures in children", section on 'Complications'.)

Valgus deformity of the elbow, also called cubitus valgus, results in an exaggerated carrying angle. This abnormality may occur after inadequate union or alignment of lateral condyle fractures (image 6).

Several angles are used to estimate the carrying angle radiographically [17]:

●Baumann angle – Although some variation in definition exists within the literature, the angle originally described by Baumann is defined as the angle between the physeal line of the lateral condyle of the humerus and the long axis of the humeral shaft (figure 10) [17,18]. The normal angle is between 75 and 80 degrees; a larger angle suggests that a varus deformity exists. This angle may be helpful in determining the adequacy of reduction for displaced distal humeral fractures.

●Humeral-ulnar angle – This angle is formed by a longitudinal line along the humeral shaft that is bisected by a line that is longitudinal to the ulna and is regarded as the most accurate in determining the true carrying angle of the elbow (normal value: 5 to 15 degrees) (figure 11) [17].

●Medial epicondylar epiphyseal angle – This angle describes a line down the axis of humerus that intersects with a line along the medial epicondylar physis and is 34 to 42 degrees in normal children [19].

FURTHER IMAGING — Additional imaging (eg, ultrasound, magnetic resonance imaging [MRI], arthrography) is rarely needed to manage children with elbow fractures but may be helpful in selected circumstances (eg, young children and infants, distal humeral transphyseal fractures, intraarticular soft tissue foreign bodies, condylar fractures). Consultation with a radiologist or orthopedic surgeon will generally be obtained before deciding upon the need for further studies and/or selecting the most appropriate modality. In situations where appropriately expert consultants are not available to assist in the diagnosis of these subtle injuries, the emergency physician may place the patient in a posterior splint and arrange for follow-up.

Ultrasound — Infants and young children with elbow injuries may have significant injury of the unossified epiphyses in the elbow region. Ultrasound may be helpful to identify fractures, physeal injuries, and dislocations that are not apparent on plain radiographs [20,21].

Magnetic resonance imaging — Although plain radiography is adequate to identify significant elbow fractures in most patients, MRI may delineate additional features that may impact management, especially in children with lateral condyle fractures [22-24].

Arthrography — In difficult cases, arthrography has been a useful adjunct to help delineate condylar fractures with intraarticular extension [25,26].

ADDITIONAL INFORMATION — Several UpToDate topics provide additional information about fractures, including the physiology of fracture healing, how to describe radiographs of fractures to consultants, acute and definitive fracture care (including how to make a cast), and the complications associated with fractures. These topics can be accessed using the links below:

●(See "General principles of fracture management: Bone healing and fracture description".)

●(See "General principles of fracture management: Fracture patterns and description in children".)

●(See "General principles of definitive fracture management".)

●(See "General principles of acute fracture management".)

●(See "General principles of fracture management: Early and late complications".)

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: General management of pediatric fractures" and "Society guideline links: Upper extremity, thoracic, and facial fractures in children" and "Society guideline links: Acute pain management".)

INFORMATION FOR PATIENTS — UpToDate offers two types of patient education materials, "The Basics" and "Beyond the Basics." The Basics patient education pieces are written in plain language, at the 5^th to 6^th grade reading level, and they answer the four or five key questions a patient might have about a given condition. These articles are best for patients who want a general overview and who prefer short, easy-to-read materials. Beyond the Basics patient education pieces are longer, more sophisticated, and more detailed. These articles are written at the 10^th to 12^th grade reading level and are best for patients who want in-depth information and are comfortable with some medical jargon.

Here are the patient education articles that are relevant to this topic. We encourage you to print or e-mail these topics to your patients. (You can also locate patient education articles on a variety of subjects by searching on "patient info" and the keyword(s) of interest.)

●Basics topic (see "Patient education: Elbow fracture (The Basics)")

SUMMARY

●Elbow joint – The elbow is a complex hinge joint composed of three separate articulations (figure 1) and (see 'Elbow joint' above):

•Ulnohumeral - Between the trochlea of the humerus and the olecranon of the ulna

•Radiohumeral - Between the capitellum of the humerus and the radial head

•Radioulnar - Between the proximal radius and ulna

●Normal ossification – In children, there are six ossification centers that vary in appearance depending on the age and sex of the child (table 1). These secondary ossification centers later fuse to create the mature bony elbow (figure 3A-B). The following acronym is a useful memory aid for recalling their radiographic order of appearance (figure 7). (See 'Ossification centers' above and 'Normal ossification' above.)

•C - Capitellum

•R - Radial head

•I - Internal (medial) epicondyle

•T - Trochlea

•O - Olecranon

•E - External (lateral) epicondyle

●Radiographic views – Radiographs should be obtained in children who present with localized tenderness or swelling of the elbow and known or suspected trauma. Plain radiographs should be obtained in at least two views: an anterior-posterior view in extension and a lateral view at 90 degrees of flexion (image 1 and figure 6). (See 'Plain radiographic views' above.)

Comparison views of the unaffected elbow may be helpful to distinguish a fracture from a normal ossification center in infants and young children with condylar or epicondylar elbow fractures. (See 'Normal ossification' above.)

●Anterior humeral line – In the normal elbow, a line drawn through the anterior cortex of the humerus intersects the capitellum in its middle third (image 1). Because the most common mechanism of injury to the elbow is hyperextension, posterior displacement of the distal humerus is common when a fracture occurs, and the anterior humeral line will pass through the anterior third of the capitellum or fail to intersect it (image 3). (See 'Anterior humeral line' above.)

●Radial head alignment – The radial head should point toward the capitellum on all views [3]. The clinician should suspect an elbow dislocation, lateral condyle fracture, or Monteggia fracture when radial head dislocation is evident (image 1 and image 4). (See 'Radial head alignment' above.)

●Fat pads – An anterior fat pad and a posterior fat pad are located between the capsule and the distal end of the humerus (figure 2). Visualization of the posterior fat pad or displacement of the anterior fat pad on radiographs is an indirect indicator of hemorrhage or an elbow joint effusion often arising from an occult fracture (figure 8 and image 5). (See 'Fat pads' above.)

●Other imaging – Additional imaging (eg, ultrasound, magnetic resonance imaging, arthrography) is rarely needed to manage children with elbow fractures but may be helpful in selected circumstances (eg, young children and infants, intraarticular soft tissue foreign bodies, condylar fractures, distal humeral transphyseal fractures). Consultation with a radiologist or orthopedic surgeon with appropriate pediatric expertise will generally be obtained before deciding upon the need for further studies and/or selecting the most appropriate modality. (See 'Further imaging' above.)