Because of the highly congruous articulation of the ulna and the trochlea of the humerus, there is significant inherent osseous stability. The ulnohumeral congruency is exemplified by the 180° arc of articular coverage (including the bear spot) in the greater sigmoid notch and the greater than 250° arc of articular coverage of the trochlea¹,² (Figure 1). These articulations allow the hinge-like movement achieved in the elbow, with the coronoid process limiting posterior and anteromedial instability. This inherent osseous stability also takes contribution from the radiocapitellar joint providing a secondary restraint to valgus instability.³ In addition, the articulation of the proximal radius with the ulna in the lesser sigmoid notch provides osseous stability while facilitating forearm pronosupination.

With these anatomic relationships established, new research has focused on the relationship between the radius and ulna to help guide implant design, selection, and surgical reconstruction techniques. The authors of a 2019 study analyzed 98 cadaver arms with CT three-dimensional (3D) reconstruction to investigate the variable anatomy of the radial notch of the ulna (lesser sigmoid notch).⁴ While finding highly variable anatomy, the study authors noted a trend of the notch extending laterally while moving distal in the notch. Although not clinically tested, these findings do have implications for radial head arthroplasty design. Radial head arthroplasty relies on articulation with the lesser sigmoid notch. In nonconforming total elbow arthroplasty constructs—which are commonly used—it is possible that the distal aspect may be overstuffed, leading to bony erosion or persistent pain.

A 2021 study investigated the relationship between the radial head and the tip of the coronoid.⁵ Using 80 3D reconstructions of cadaver upper extremity CT scans, the study authors found that the mean height of the coronoid from the base of the greater sigmoid notch was 3.6 cm, whereas the tip of the coronoid to the anterior aspect of the radial head was 4.5 mm. Knowledge of these relationships provides useful information when reconstructing the coronoid in elbows where the radial head is intact.

The lateral ligament complex—composed of the lateral ulnar collateral ligament (LUCL), the annular ligament, radial collateral ligament, and the accessory lateral collateral ligament—has proven crucial to elbow stability (Figure 2). Despite repeated investigations of the LUCL, granular detail of specific components—such as the annular ligament—remain poorly described. One study investigated the annular ligament in detail with specific focus on the superior, inferior, and anterior oblique bands.⁶ In reviewing cadaver specimens—both embalmed and fresh frozen—the study authors noted three layers to the lateral ligament complex: LUCL and radial collateral ligament, the superior and inferior oblique bands with the annular ligament, and the capsule. These bands broaden the lateral ulnar attachments of the annular ligament.

The medial ulnar collateral ligament (MUCL)—specifically the anterior bundle—has been a frequently investigated structure given its importance in throwing sports (Figure 3). However, in the setting of trauma, MUCL incompetence can also contribute to persistent instability. To better understand the effect in the setting of trauma, the authors of a 2019 study made a detailed assessment of the insertion of the MUCL relative to the tip of the coronoid in 84 embalmed elbows.⁷ They found a variable distance from the tip of the coronoid to the insertion of the MUCL (1.4 to 13.9 mm) with an average distance of 7.7 mm. Even for fractures outside of the anteromedial facet, based upon these findings, a fracture of the coronoid tip may extend into the insertion of the MUCL.⁸ One study provided an assessment of the origin and insertion sites of the three bundles: transverse, anterior, and posterior.⁹ It was noted that the transverse bundle—while inserting and originating from the same bone—had attachments to the anterior bundle in all 10 specimens evaluated. This suggests a biomechanical advantage provided by the transverse bundle in supporting the anterior bundle through valgus loads.

In attempts to understand the pathologic process for degenerative MUCL pathology, the authors of a 2019 study evaluated the vascular distribution of the MUCL.¹⁰ In 18 cadavers, the study authors consistently found dense vascularization in the proximal MUCL with hypovascularity distally. This provided a more detailed assessment of the MUCL vascular supply than had been previously described.¹¹ They also noted a consistent artery—naming it the recurrent flexor/pronator artery—traveling in line with the MUCL that appeared to contribute to the proximal ligament vascularity. These findings may provide explanation for the discrepancy in outcomes of nonsurgical management for proximal versus distal MUCL pathology.¹²

The most common pathologic tendons around the elbow are the biceps brachialis, triceps brachialis, and the extensor carpi radialis brevis (ECRB). In a 2020 study, 10 cadaver specimens were used to assess the lateral ligamentous complex and extensor tendon origins.¹³ The most notable finding in the cadaver analysis was the broad origin site for the ECRB. The elbows were examined in extension and the ECRB origin was found to extend distal to the radiocapitellar joint by 5.9 mm with
attachments to the capsule. As such, ECRB pathology and associated symptoms may extend distal to radiocapitellar joint.

The distal biceps tendon has seen renewed study over the past decade with the contention that there are distinct insertion sites for the short and long heads of the biceps¹⁴ (Figure 3). The clinical significance of this distinction remains unknown, with authors advocating for repair of an isolated short head rupture.¹⁵ To further the possibility of routine endoscopic biceps treatment, anatomic landmarks to aid in endoscopy have been evaluated.¹⁶ In 20 cadavers, a bare area on the radial tuberosity was described in all cases. In all cases, the tendon was encased in a bursal sheath but had a variable number of bundles (two to five). In a separate analysis of 11 cadavers, the proximal radioulnar space at the level of the distal biceps insertion was evaluated.¹⁷ It was noted that the space through which the native distal biceps tendon passes between the radius and ulna narrows in pronation, especially distally. Any thickening of the distal biceps tendon, native or surgically, may create an impingement within this space in pronation.

Management of the triceps tendon in distal humerus open reduction and internal fixation or elbow arthroplasty remains variable and is a topic of debate. In a 2021 histologic analysis of 17 cadaver specimens to assess the footprint of the triceps tendon and its relationship to bony landmarks, a smaller insertional footprint was found with histologic analysis in comparison with historical findings and the study authors’ own gross measurements.¹⁸ Specifically, the distal to proximal footprint was 10.9 mm compared with the previously reported 13 mm.¹⁹ This provided that the distance from the tip of the olecranon to the insertion of the triceps averaged 16.7 mm. This updated knowledge provides surgeons reassurance during removal of pathologic processes of the olecranon tip and obtaining extensive exposure in surgery for degenerative or traumatic pathologies.

The amount of elbow motion required to perform common daily tasks has historically been described as 30° to 130° of elbow flexion, 50° of supination, and 50° of pronation.²⁰ However, with modern-day tasks, such as holding a phone to the ear, greater flexion and forearm pronation are required.²¹ A systematic review has confirmed the greater need of flexion than previously
reported to achieve modern-day tasks (>140°).²² A 2020 study revisited this topic for children and adolescents, finding that for most common tasks the initial ranges historically reported were accurate. However, for modern-day tasks—telephone and keyboard use—a need for greater forearm pronation (up to 65°) and elbow flexion (approaching 150°) was similar to that of the adult population.²³ It is now clear that not all desired tasks can be performed within the range described historically. Rather, a graduated increase in functional tasks is seen with greater flexion and pronation. Whether patients can make accommodations when unable to achieve these end-ranges has yet to be determined.

Passing through the geometric centers of the trochlea and capitellum, the center of rotation for flexion and extension is static throughout functional ranges of motion. Accurate identification of the flexion-extension axis is critical when applying dynamic hinged fixators or reconstructing collateral ligaments. The authors of a 2019 study revisited the center of rotation about the elbow with a focus on the relationship between the center of rotation and the medial epicondyle.²⁴ This was performed to aid in accurate placement of the humeral bone tunnel for anterior bundle of the MUCL reconstruction. The center of rotation, defined by the trochlea, was predictably found on the distal aspect of the anterior medial epicondyle. This center of rotation was slightly posterior and proximal to the center of the trochlea when viewing from medial to lateral. The distance from the ulnohumeral joint to the center of rotation line was 14.3 mm in the sagittal plane. This knowledge may guide a surgeon during an MUCL reconstruction when the native MUCL is not visible in hopes of achieving isometric reconstruction.

The carrying angle of the elbow is normally defined as the degree of cubitus valgus with the elbow in anatomic position—extension and supination. It has been established that the carrying angle, or amount of valgus, decreased with elbow flexion.²⁵ An increased carrying angle has been implicated as an independent predictor of subsequent injury in pitchers.²⁶ The difference in carrying angle between injured (n = 8) and noninjured pitchers (n = 24) was limited (17.5° versus 13.1°) and may not be clinically applicable. To further investigate this, the authors of a 2019 study reported on 52 pitchers for a single organization who were followed for a season.²⁷ Although a greater carrying angle was found in the dominant arm of pitchers, no statistical difference was observed in the carrying angle of injured versus noninjured pitchers.

Stability of the elbow is provided through bony constraint, static soft-tissue stabilizers, and dynamic stabilizers.²⁸ Dynamic stabilizers have primarily been thought to include the brachialis and triceps. In a 2019 cadaver simulation of an injury resulting in lateral collateral ligament complex and common extensor tendon incompetence, the anconeus as a dynamic stabilizer was tested. Tensioning the anconeus through its anatomic line of pull, the effect seen from lateral collateral ligament and common extensor tendon disruption was reversed.²⁹ On the opposing side of the elbow, the medial elbow joint space was analyzed with ultrasonography in 22 healthy males with intact ulnar collateral ligaments.³⁰ The medial elbow joint space enlarged significantly with valgus stress. However, under the same stress with maximal grip contraction, the medial elbow joint space was no different than the space without valgus stress. This suggests the common flexor tendon—likely primarily flexor carpi ulnaris and flexor digitorum superficialis—dynamically contributes to elbow stability.