_{Published January 3, 2023}

Imaging findings in radiology are not always black and white; in between, there are multiple shades of gray. One of the key roles of the radiologist is to identify and differentiate true pathology from pathological mimics that can pose a diagnostic challenge in day-to-day clinical practice. As such, it is important to know the normal imaging anatomy of a joint and be aware of some of the typical anatomic variants that can occur. Most anatomic variants are asymptomatic and are seen as incidental findings on imaging. A few anatomic variants, however, can predispose to symptoms under specific circumstances and can cause pain, sensory loss, or restricted joint function. 

In advance of the 2023 ARRS Annual Meeting Categorical Course, “Pitfalls and Challenging Cases: How to Triumph and Make the Diagnosis,” this InPractice article focuses on anatomic variants occurring in and around the elbow and some of the potential pitfalls to be aware of when reading imaging studies. 

Normal Anatomy

The elbow is a complex synovial joint formed by the articulation of the distal humerus, proximal radius, and proximal ulna. The bones form three separate joints—the humeroulnar, radiocapitellar, and proximal radioulnar joints—contained within a single joint capsule lined with synovium and supported by groups of muscles, tendons, and ligaments [1–3]. The humeroulnar joint is a hinge joint formed between the humeral trochlea and the sigmoid notch of the ulna. The sigmoid notch of the ulna (also known as the trochlear notch or groove or semilunar notch) is a crescent-shaped depression along the proximal ulna lined with articular cartilage [1–4]. The radiocapitellar joint is the articulation between the humeral capitellum and radial head. The radial head has a central concavity that allows smooth articulation with the rounded, anteriorly directed capitellum or capitulum of the humerus. The surfaces of both the radial head and capitellum are covered with hyaline cartilage [1, 2]. The proximal radioulnar joint is formed by the radial head and radial notch (or lesser sigmoid notch) of the proximal ulna, which is a shallow depression distal and lateral to the coronoid process [1–3].  

Osseous Anatomic Variants 

Supracondylar Process 

The supracondylar process, or avian spur, is a congenital bony protuberance along the anteromedial aspect of the distal humerus. It ranges in size from 2 to 20 mm and is located 5–7 cm above the medial epicondyle. The spur grows toward the elbow, unlike an osteochondroma, which is directed away from the elbow. A supracondylar process is present in 1–3% of the population, more commonly found in men and boys and on the left [2, 3]. The spur may be connected to the medial epicondyle by a fibrous band called the Struthers ligament, which creates a fibroosseous tunnel. A supracondylar process is usually asymptomatic but can fracture or cause mechanical compression of the median nerve or brachial artery, where the neurovascular bundle passes through the fibroosseous tunnel formed by the supracondylar process and Struthers ligament. The symptoms include pain, paresthesia, and weakness in the distribution of the median nerve and can be aggravated by continuous movement or local compression at the elbow [1, 2]. 

A supracondylar process can be seen on lateral or oblique radiographs obtained with the elbow internally rotated and can be misinterpreted as a bony exostosis or osteochondroma. However, the typical location of the supracondylar process along the anteromedial aspect of the distal humerus and direction of growth toward the joint differentiate it from an osteochondroma, which typically occurs in the metaphysis and grows away from elbow (Fig. 1A). CT angiography can show the supracondylar process and deviation, compression, or thrombosis of the brachial artery (Fig. 1B). MRI and ultrasound can depict the Struthers ligament and any compression on the neurovascular bundle. Surgical decompression is usually the preferred approach over conservative treatment of patients who have symptoms [1–8]. 

Supratrochlear Foramen 

In the supratrochlear portion of the distal humerus, a thin plate of compact bone called the supratrochlear septum usually separates the olecranon and coronoid fossae. This bony septum can be opaque or radiolucent, is lined by a synovial membrane, and is present until approximately age 7. After this time, it is occasionally absorbed, forming a supratrochlear foramen (also known as the olecranon foramen, septal aperture, intercondylar foramen, and epitrochlear foramen) [3]. It is important to report the presence of a supratrochlear foramen (Fig. 2) and provide information about the shape and dimensions, because these factors can influence preoperative planning and surgical outcome. The presence of a supratrochlear foramen is commonly associated with a narrow medullary canal and anterior angulation of the distal humerus and gracile bones, which can predispose to fracture when instrumentation is placed in the distal humerus. Additionally, a large supratrochlear foramen may be misinterpreted as an osteolytic or cystic lesion, if the radiologist is unaware of this normal variant [9–12]. 

Pseudodefect of the Capitellum 

The capitellum is a hemispherical protuberance along the anterior and lateral aspects of the distal humerus. The anterior 180° of the capitellum is round, smooth, and covered by articular cartilage. As the capitellum curves distally and posteriorly, it tapers in width. The posterolateral aspect of the capitellum is devoid of articular cartilage and often has a rough and irregular appearance. A groove is normally present between the posterolateral aspect of the capitellum and the lateral epicondyle [3, 4].  

The abrupt change in contour of the articular capitellum at the junction with the lateral epicondyle can create a pseudodefect of the capitellum on coronal MR images. This finding is more pronounced in contrast to the smooth articular surface of the radial head or in the presence of a joint effusion where fluid outlines the groove. The pseudodefect can simulate an osteochondral lesion, particularly when there is accompanying fibrocystic change. However, the two can easily be distinguished by their anatomic location on sagittal images: the pseudodefect will be along the posterior nonarticular surface of the capitellum (Fig. 3), whereas an osteochondral lesion will be located along the anterior capitellum and is commonly accompanied by subchondral cystic change, bone marrow edema, and a joint effusion [1–4]. 

Pseudodefect of the Trochlear Groove

The trochlear groove is constricted at the junction of the olecranon and coracoid process with inward tapering that results in subtle cortical notches peripherally, which can simulate cortical disruption on sagittal MR images. These pseudodefects of the trochlear groove can simulate a fracture (Fig. 4). However, the well-defined margins, absence of bone marrow edema, and midtrochlear location differentiate the pseudodefect from a true fracture, which usually extends into the medullary cavity [1, 2, 13]. 

Transverse Trochlear Ridges 

The midtrochlear groove is partially or completely traversed by a nonarticular transverse bony ridge at the junction of the olecranon process and coronoid process. The bony ridge is devoid of articular cartilage and best seen on sagittal MR images. The bony elevation may be misinterpreted as an intraarticular osteophyte or sequela of a healed fracture; however, the absence of bone marrow edema and characteristic location help differentiate the midtrochlear groove from pathology [1, 3, 4, 13]. 

Accessory Ossicles  

Patella cubiti—A patella cubiti (also known as an os sesamum cubiti or os sesamoideum tricipitale) is a rare sesamoid bone located within the distal triceps brachii tendon that develops when part of the olecranon or the entire olecranon remains separated from the proximal ulna [3]. On radiographs, the patella cubiti is seen as a well-corticated ossicle with a smooth contour adjacent to the olecranon process. The ossicle can mimic an ununited avulsion fracture of the olecranon tip or a nonunited olecranon apophysis. However, a history of trauma and the presence of irregular sclerotic margins in both the parent bone and fracture fragment favors a chronic fracture. In preadolescent or adolescent athletes involved in overhead throwing activities, such as baseball, a widened physis and sclerotic margins on imaging favor an ununited olecranon apophysis [14–16]. A fractured olecranon enthesophyte or calcium hydroxyapatite deposition in the distal triceps tendon can also mimic a patella cubiti [3].

Os supratrochleare dorsale—The os supratrochleare dorsale is an accessory ossicle found in the olecranon fossa; it is commonly seen in the dominant arm of men and boys age 15–40 [1, 17]. Its appearance has been described as a medallion on a frontal radiograph. This finding is characterized by a thick sclerotic border of bone around an ossicle related to stress changes with a thin rim of surrounding lucency due to deepening of the olecranon fossa. Repetitive impaction by the olecranon process during elbow extension can lead to osseous remodeling or fragmentation of the os with secondary osteoarthritis. It is important to differentiate the os supratrochleare dorsale from its common mimickers, such as patella cubiti, intraarticular joint body, and fragmentation of the olecranon tip in patients with valgus extension overload. Surgical removal is the treatment of choice of both os supratrochleare dorsale and the joint body [1, 17].

Prominent Radial Tuberosity 

The radial tuberosity along the ulnar aspect of the proximal radius serves as the attachment site of the distal biceps tendon. Cancellous bony trabeculae are sparse subjacent to the radial tuberosity and may produce an exuberant or bubbly osseous prominence that can simulate a pathologic lucent lesion when seen en face on radiographs. An adjacent osseous fossa anterolateral to the biceps tendon insertion can also appear lucent when seen en face. Knowledge of the distal biceps insertional anatomy can prevent mistaking this physiologic structure for a lucent bone lesion [18].

Offering 18 total CME credits for ARRS members, the “Pitfalls and Challenging Cases: How to Triumph and Make the Diagnosis” Categorical Course will also tackle challenging cases in abdominal, chest, and neuroradiology. Didactic lectures will emphasize clinical scenarios and interpretative skills across a true diversity of findings, enhancing the radiologist’s role in patient management. Together, we look forward to presenting more information—especially about nonosseous normal variants in elbow imaging—during the “Pitfalls in Upper Extremity in MSK Imaging” session at the ARRS Annual Meeting Categorical Course. Please join us and other expert faculty in Honolulu, virtually, or on demand to help diagnose your future shoulder, wrist, and hand imaging cases, too.  

References

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