Osseous variability around the elbow joint

Supracondylar process (Processus supracondylaris)

The supracondylar process is a variant located proximal to the elbow joint, sometimes referred to as the “processus supraepitrochlearis,” “epicondylic process,” or the “supracondylar spur,” which is a protrusion on the anteromedial surface of the humerus proximal to its medial epicondyle (4–6) (Fig. 1). Its length is in the range of 2–20 mm, with the longest described being around 20 mm by Camerlinck et al. (7). The term supracondylar tubercle is often used for its shorter variant (8) (Fig. 2). The supracondylar process may be connected to the medial epicondyle by the supracondylar ligament of Struthers (not to be confused with the arcade of Struthers, a thickened part of the brachial fascia that can cause entrapment of the ulnar nerve) (9). The prevalence of the supracondylar ligament of Struthers across various ethnic groups is unknown, but Gunther et al. postulated the incidence to be about 2% (10). It was first described in human by Sir John Struthers in 1854 (11). The supracondylar ligament of Struthers can be detected and seen during ultrasound examination. This appears as a thin hypoechoic linear structure extending from the apex of the supracondylar process to the medial epicondyle of the humerus (7). It is consistently present in the lower mammals, often in its ossified form called the entepicondylar foramen, beneath which the brachial vessels and the median nerve pass (12). As an example, it occurs constantly in the family Felidae (6). The ossified Struthers ligament has been reported in humans as well, although extremely rarely, as the supracondylar foramen (13).

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Fig. 1.

Radiographs of the supracondylar process in the anteroposterior and lateral projections.

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Fig. 2.

Scheme of the supracondylar process (on the left) and supracondylar tubercle (on the right).

In the American, European, and Indian populations, the overall prevalence of the supracondylar process has been reported to be <1% (8,14,15). According to the meta-analysis by Martin-Schütz et al., its prevalence is in the range of 0%–5.4%, with a pooled prevalence of 0.68% in a sample of over 26,000 elbow joints. This meta-analysis has also shown a statistically significant higher occurrence in women (16). Compression neuropathies are important clinically, especially the compression of the median nerve passing under the supracondylar ligament of Struthers (17,18). The symptoms are identical to those of the pronator syndrome, usually manifesting with a pain at the palmar side of the forearm and elbow that worsens during repeated finger grip or forearm pronation. It is often accompanied by hypoesthesia and skin numbness in the thenar region. However, the nocturnal pain typical of carpal tunnel syndrome is seldom present (18). The pronator syndrome can rarely cause compression of the brachial artery leading to ischemia, particularly during the elbow hyperextension (19). McCulloch et al. described a case where the distal insertion of the supracondylar ligament of Struthers joined the flexor carpi ulnaris muscle leading to the ulnar nerve compression (20). Kolb et al. presented two cases of a fracture of the supracondylar process. A four-year-old child was examined for pain above the left elbow after an injury wherein the supracondylar process was identified with a fracture in its middle. The fracture was treated conservatively with symptoms subsiding after it healed. The median nerve and vessels did not show symptoms of the entrapment. The second patient was a 12-year-old boy, who incurred direct trauma to his left elbow while wrestling with his brother. The injured elbow was swollen and painful, and the process was resected subperiosteally during the treatment (21). Case of the supracondylar process being confused with osteochondroma has also been reported (22).

Supratrochlear foramen (Foramen supratrochleare)

Another commonly described variation is the intra-articularly situated supratrochlear foramen (Figs 3 and 4). In English literature, it has been commonly referred to as a “septal aperture.” The most accepted theory of its origin is that it is a result of pressure atrophy leading to the perforation of bony septa dividing the olecranon and coronoid fossa induced by extreme overload upon hyperextension (23,24). It is more common in women and in the left side (25). The higher prevalence in women supports the elbow hyperextension theory as this is more common in women (26).

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Fig. 3.

Radiograph of the supratrochlear foramen.

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Fig. 4.

Scheme of the supratrochlear foramen.

The prevalence ranges between 0.3% in Greeks and 58% in Arkansas native Americans living in the south of the United States. In the European population, the overall prevalence is around 8% (27). The clinical relevance here is increased risk of the distal humeral fracture especially with fall on the outstretched elbow. A large supratrochlear foramen decreases the space for osteosynthesis implantation (24,28).

Articular surfaces

If meticulously measured, every anatomical structure will have some extent of variability. It is worth mentioning the variability of the trochlear notch (incisura trochlearis), which consists of two facets. This norm is referred to as type I, which was found in 60% cases in the study by Totlis et al. The two facets of the trochlear notch might also be partially (type II) or completely (type III) fused. If this is the case and the articular surface is not completely separated (the remaining 40% of cases), the olecranon osteotomy increases the risk of secondary osteoarthritis (29).

The head of the radius is quite uniform with a few quantitative deviations in its morphology described mostly for the purpose of the radial head replacement (30,31).

Accessory ossicles

As with other joints of the upper and lower limbs, there are several accessory (supernumerary) bones around the elbow. Even though they are less frequent around the elbow than at the hand or foot, it is important to know them well to be able to differentiate the pathological and traumatic conditions that might mimic them. We were able to describe six types of accessory ossicles around the elbow joint. Nevertheless, the only proof of their congenital origin is a statistical analysis showing no significant relation to age (hypothesizing that degenerative changes would be more common in older patients). The accessory ossicles are defined based on the following criteria: (i) a regular ovoid shape; (ii) smooth margins; and (iii) a regular cortical to medullary ratio throughout the circumference. All three criteria must be fulfilled for a diagnosis of a true accessory ossicle (32).

Two types of accessory ossicles, the os subepicondylare mediale and the os subepicondylare laterale, are located under the respective epicondyles in the flexor and extensor muscle groups. The os subepicondylare mediale is the most common accessory ossicle around the elbow, occurring in 0.46% of the studied population (32). This accessory ossicle has also been previously referred to as the “medial epicondyle accessory ossicle” or “paratrochlear bone” (33,34). The literature does not describe any clinically relevant conditions concerning these two accessory bones. Cubital tunnel syndrome caused by the ulnar nerve irritation has been described only in the presence of an ossicle of the traumatic etiology in a case report presented by Poelstra et al. (35).

Then, there are two sesamoid (intra-tendinous) bones: the os sesamoideum tricipitale within the triceps brachii muscle tendon and the os sesamoideum brachiale within the brachialis muscle tendon.

The os sesamoideum tricipitale has previously been described as “sesamum cubiti” by Pfitzner and later as “patella cubiti” by Kienböck (36,37). It is still unclear whether those two bones are separate entities. According to the review by Mittal et al., there are only 18 cases described in the literature (38), but several cases could be missing (39–46). Through the last century, there have been numerous articles arguing the etiology of this ossicle congenital versus traumatic (37,38,42,47).

An in-depth analysis of these cases will be needed to differentiate other pathological conditions from what could be considered a true patella cubiti / os sesamoideum tricipitale. One example would be a case of congenital pseudarthrosis presented by Burge and Benson, Pouliquen et al., and Ramseir and Exner (48–50), which differ from the rest. The literature describes many different symptoms associated with these as elbow flexion or extension deficit, edema, weakness, and pain (38,41,51,52). Mittal et al. recommend avoiding surgical treatment in patients reporting only stiffness at the elbow (38). Patella cubiti is also present in birds, bats, and rodents (47).

The os sesamoideum brachiale has also been described by the terms “accessory coronoid,” “accessory coronoid ossicle,” and “os coronoides” (33,34,53). As this ossicle is usually small in diameter, it is hard to apply the criteria described above, and are therefore difficult to differentiate it from type I fracture of the coronoid process of the ulna as classified by Regan and Morrey (32,54,55).

The last two accessory bones are intra-articular, less frequent, and often symptomatic. Ventrally the os supratrochleare anterius is rarer with only a few cases previously described, and dorsally the os supratrochleare posterius (53). The os supratrochleare anterius has been previously referred to as “os cubiti anterius,” “Type A antecubital bone,” “fabella cubiti,” and “os supratrochleare ventral” (34,53,56,57), while the more commonly present os supratrochleare posterius only has one synonym, “os supratrochleare dorsale” (57). The os supratrochleare posterius was first described in an article published by Hirsh in 1927 and to this date, there are more than 30 cases described in the literature (53,58). In most cases, it presented with pain, progressive restriction, and limitation of elbow extension. A secondary osteoarthritis may also develop due to its repeated contact with olecranon; for this reason, it is often necessary to surgically resect it (53,59). Earlier, this used to be misdiagnosed as osteochondritis dissecans of the supratrochlear septum (59,60).