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Anatomy of the Elbow Joint
This chapter discusses the normal anatomy of the elbow region. Abnormal and surgical anatomy is addressed in subsequent chapters of this book dealing with the pertinent condition.
Topical Anatomy and General Survey
The contours of the biceps muscle and antecubital fossa are easily observed anteriorly. Laterally, the avascular interval between the brachioradialis and the triceps, the so-called column, is an important palpable landmark for surgical exposures ( Fig. 2.1 ). Laterally, the tip of the olecranon, the lateral epicondyle, and the radial head also form an equilateral triangle and provide an important landmark for joint aspiration and elbow arthroscopy (see Chapters 39 and 80 ). The flexion crease of the elbow is in line with the medial and lateral epicondyles and thus actually reflects the joint axis and is 1 to 2 cm proximal to the joint line when the elbow is extended ( Fig. 2.2 ). The inverted triangular depression on the anterior aspect of the extremity distal to the epicondyles is called the cubital (or antecubital ) fossa .
The superficial cephalic and basilic veins are the most prominent superficial major contributions of the anterior venous system and communicate by way of the median cephalic and median basilic veins to form an “M” pattern over the cubital fossa ( Fig. 2.3 ).
The extensor forearm musculature originates from the lateral epicondyle and was termed the mobile wad by Henry. This forms the lateral margin of the antecubital fossa and the lateral contour of the forearm and comprises the brachioradialis and the extensor carpi radialis longus and brevis muscles. The muscles comprising the contour of the medial anterior forearm include the pronator teres, flexor carpi radialis, palmaris longus, and flexor carpi ulnaris. Henry has demonstrated that their relationship and location can be approximated by placing the opposing thumb and the index, long, and ring fingers over the anterior medial forearm. The dorsum of the forearm is contoured by the lateral extensor musculature, consisting of the anconeus, extensor carpi ulnaris, extensor digitorum quinti, and extensor digitorum communis.
Dermal innervation about the proximal elbow is quite variable, being provided by the lower lateral cutaneous (C5, C6) and medial cutaneous (radial nerve, C8, T1, and T2) nerves of the arm. The forearm skin is innervated by the medial (C8, T1), lateral (musculocutaneous, C5, C6), and posterior (radial nerve, C6–C8) cutaneous nerves of the forearm ( Fig. 2.4 ).
Osteology
Humerus
The distal humerus consists of an arch formed by two condyles that support the articular elements of the trochlea and capitellum ( Fig. 2.5 ).
Medial to the trochlea, the prominent medial epicondyle serves as a source of attachment of the medial ulnar collateral ligament and the flexor-pronator group of muscles. Laterally, the lateral epicondyle is located just proximal to the capitellum and is much less prominent than the medial epicondyle. The lateral ulnar collateral ligament and the supinator-extensor muscle group originate from the flat, irregular surface of the lateral epicondyle.
Anteriorly, the radial and coronoid fossae accommodate the radial head and coronoid process during flexion. Posteriorly, the olecranon fossa receives the tip of the olecranon.
In approximately 90% of individuals, a thin membrane of bone separates the olecranon and coronoid fossae. The medial supracondylar column is smaller than the lateral and explains the vulnerability of the medial column to fracture caused by trauma and some surgical procedures. The posterior aspect of the lateral supracondylar column is flat, allowing ease of application of contoured plates for fractures involving this structure. The prominent lateral supracondylar ridge serves as a site of attachment for the brachioradialis and extensor carpi radialis longus muscles anteriorly and for the triceps posteriorly ( Fig. 2.6 ). It is also an important landmark for many lateral surgical approaches, especially for the “column procedure” (see Chapters 11 and 54 ).
Proximal to the medial epicondyle, approximately 5 to 7 cm along the medial intramuscular septum, a supracondylar process may be observed in 1% to 3% of individuals. A fibrous band termed the ligament of Struthers sometimes originates from this process and attaches to the medial epicondyle. When present, this spur serves as an anomalous insertion of the coracobrachialis muscle and an origin of the pronator teres muscle. Various pathologic processes have been associated with the supracondylar process, including fracture and median and ulnar nerve entrapment (see Chapter 72 ).
Radius
The radial head articulates with the capitellum. It exhibits a cylindrical symmetrical shape with a depression in the midportion to accommodate the capitellum. The osseous contour of the radial head, on the other hand, actually is more elliptical in shape, with a major and minor axis. The disk-shaped head is secured to the ulna by the annular ligament ( Fig. 2.7 ). Distal to the radial head, the bone tapers to form the radial neck, which, along with the head, is vulnerable to fracture. The radial tuberosity marks the distal aspect of the neck and has two distinct parts ( Fig. 2.8 ). The anterior surface is covered by a bicipitoradial bursa protecting the biceps tendon during full pronation. However, it is the rough posterior aspect that provides the site of attachment of the biceps tendon. During full pronation the tuberosity is in a dorsal position; this allows repair of a ruptured biceps tendon through a posterior approach (see Chapter 63 ) and is helpful to determine axial alignment of proximal radial fractures. In addition to the bicipital radial bursa, several other potential bursae have also been described about the elbow ( Fig. 2.9 ).
Ulna
The proximal ulna provides the greater sigmoid notch (incisura semilunaris), which serves as the major articulation of the elbow that is responsible for its inherent stability ( Fig. 2.10 ). The cortical surface of the coronoid process serves as the site of insertion of the brachialis muscle and of the oblique cord. Medially, the sublime tubercle serves as the insertion site of the medial ulnar collateral ligament. The triceps tendon attaches to the posterior aspect of the olecranon process.
On the lateral aspect of the coronoid process, the lesser semilunar or radial notch articulates with the radial head and is oriented roughly perpendicular to the long axis of the bone. Distal to this, the supinator crest serves as the site of attachment to the supinator muscle. On this crest, a tuberosity occurs that is the site of insertion of the lateral ulnar collateral ligament.
Elbow Joint Structure
Articulation
The elbow joint articulation is classified as a trochoginglymoid joint. The ulnohumeral joint resembles a hinge (ginglymus), allowing flexion and extension. The radiohumeral and proximal radioulnar joint allows axial rotation or a pivoting (trochoid) type of motion ( Chapter 3 ).
Humerus
The trochlea is the hyperboloid, pulley-like surface that articulates with the semilunar notch of the ulna covered by articular cartilage through an arc of 300 degrees ( Fig. 2.11 ). The medial contour is larger and projects more distally than does the lateral portion of the trochlea (see Fig. 2.5 ). The two surfaces are separated by a groove that courses in a helical manner from an anterolateral to a posteromedial direction.
The capitellum is almost spheroidal in shape and is covered with hyaline cartilage, which is approximately 2 mm thick anteriorly. A groove separates the capitellum from the trochlea, and the rim of the radial head articulates with this groove throughout the arc of flexion and during pronation and supination.
In the lateral plane, the orientation of the articular surface of the distal humerus is rotated approximately 30 degrees anteriorly with respect to the long axis of the humerus ( Fig. 2.12 ). The center of the concentric arc formed by the trochlea and capitellum defines the flexion axis and is on a line that is coplanar to the anterior distal cortex of the humerus. In the transverse plane, the articular surface and axis of rotation are rotated outward approximately 5 degrees referable to the epicondylar line ( Fig. 2.13 ), and in the frontal plane, it is tilted approximately 6 degrees in valgus (see Fig. 2.5 ).
Proximal Radius
Hyaline cartilage covers the depression of the radial head, which has an angular arc of about 40 degrees, as well as approximately 240 degrees of articular cartilage that articulates with the ulna, hence approximately 120 degrees of the radial circumference is not articular and amenable to open reduction internal fixation (ORIF) for fracture (see Fig. 2.7 ). The lesser sigmoid fossa forms an arc of approximately 60 to 80 degrees, leaving an excursion of about 180 degrees for pronation and supination. The anterolateral third of the circumference of the radial head is void of cartilage. This part of the radial head lacks subchondral bone and thus is not as strong as the part that supports the articular cartilage; this part has been demonstrated to be the portion most often fractured. The head and neck are not colinear with the rest of the bone and form an angle of approximately 15 degrees with the shaft of the radius, directed away from the radial tuberosity (see Fig. 2.8 ).
Proximal Ulna
In most individuals, a transverse portion of nonarticular cartilage divides the greater sigmoid notch into an anterior portion comprising the coronoid and the posterior olecranon ( Fig. 2.14 ).
In the lateral plane, the sigmoid notch forms an arc of about 190 degrees. The contour is not a true hemicircle but rather is ellipsoid. This explains the articular void in the midportion.
The orientation of the articulation is approximately 30 degrees posterior to the long axis of the bone ( Fig. 2.15 ). This matches the 30-degree anterior angulation of the distal humerus, providing stability in full extension. In the frontal plane, the shaft is angulated from about 1 to 6 degrees lateral to the articulation ( Fig. 2.16 ). This angle contributes, in part, to the variation of the carrying angle, which is discussed in Chapter 3 .
The lesser sigmoid notch consists of a depression with an arc of about 70 degrees and is situated just distal to the lateral aspect of the coronoid and articulates with the radial head.
Carrying Angle
The so-called carrying angle is the angle formed by the long axes of the humerus and the ulna with the elbow fully extended ( Fig. 2.17 ). In men, the mean carrying angle is 11 to 14 degrees, and in women, it is 13 to 16 degrees. Furthermore, the carrying angle is approximately 1 degree greater in the dominant than nondominant side.
Joint Capsule
The anterior capsule inserts proximally above the coronoid and radial fossae ( Fig. 2.18 ). Distally, the capsule attaches to the anterior margin of the coronoid medially as well as to the annular ligament laterally. Posteriorly, the capsule attaches just above the olecranon fossa, distally along the supracondylar bony columns. Distally, attachment is along the medial and lateral articular margin of the sigmoid notch. The greatest capacity of the elbow, 25 to 30 mL, occurs at about 80 degrees of flexion.
The anterior capsule is normally a thin transparent structure, but significant strength is provided by transverse and obliquely directed fibrous bands.
Plica synovalis
A fold of the anterior capsule, the plica synovalis, is invariably present but is of variable prominence. Duparc credits Testut with the original description in 1928, but the clinical relevance as the cause of a snapping elbow is credited to Miyazaki et al. in 1958. It courses from proximal to distal and obliquely from lateral to medial. In so doing, it crosses the joint obliquely over the radial head and neck and inserts into the anterior distal capsule near the lesser sigmoid notch ( Fig. 2.19 ). While a normal structure, it can become thickened and in so doing produces the well-recognized symptom complex recognized as a snapping elbow. It has also been implicated in tennis elbow–like symptoms in those without the classic snapping sensation (see Chapter 59 ).
The anterior capsule is, of course, taut in extension but becomes lax in flexion. The joint capsule is innervated by highly variable branches from all major nerves crossing the joint, including the contribution from the musculoskeletal nerve ( Fig. 2.20 ).
Ligaments
The collateral ligaments of the elbow are formed by specialized thickenings of the medial and lateral capsules.
Medial Collateral Ligament Complex
The medial collateral ligament consists of three parts: anterior, posterior, and transverse segments ( Fig. 2.21 ). The anterior bundle is the most discrete component, the posterior portion being a thickening of the posterior capsule, and is well defined only in about 90 degrees of flexion. The transverse component (ligament of Cooper) appears to contribute little or nothing to elbow stability.
The ligament originates from a broad anteroinferior surface of the epicondyle. The ulnar nerve rests on the posterior aspect of the medial epicondyle but is not intimately related to the fibers of the anterior bundle of the medial collateral ligament itself. This has obvious implications with regard to the treatment of ulnar nerve decompression by medial epicondylar ostectomy. A more obliquely oriented excision might be most appropriate to both decompress the ulnar nerve and preserve the collateral ligament origin. On the lateral projection, the origin of the anterior bundle of the medial collateral ligament is precisely at the axis of rotation at the anterior, inferior margins of the medial epicondyle ( Fig. 2.22 ). The posterior bundle inserts along the midportion of the medial margin of the semilunar notch. The width of the anterior bundle is approximately 4 to 5 mm compared with 5 to 6 mm at the midportion of the fan-shaped posterior segment. Recently ultrasound assessment has proved helpful in further documenting the dimensions of these structures.
The function of the ligamentous structures is discussed in detail in the following. Clinically and experimentally, the anterior bundle is clearly the major portion of the medial ligament complex and has been divided into anterior, posterior, and deep medial subcomponents.
Lateral Ligament Complex
Unlike the medial collateral ligament complex, with its rather consistent pattern, the lateral ligaments of the elbow joint are less discrete, and individual variation is common. Our investigation has suggested that several components make up the lateral ligament complex: (1) the radial collateral ligament; (2) the annular ligament; (3) a variably present accessory lateral collateral ligament; and (4) the lateral ulnar collateral ligament. These observations have now been confirmed by others. The current thinking is to consider the complex to be roughly in a “Y” shape, the arms of which attach to the anterior and posterior aspects of the semilunar notch ( Fig. 2.23 ).
Radial collateral ligament
This structure originates from the lateral epicondyle and is actually a complex of several components ( Fig. 2.24 ). Its superficial aspect provides a source of origin for a portion of the supinator muscle. The length averages approximately 20 mm with a width of approximately 8 mm. This portion of the ligament is almost uniformly taut throughout the normal range of flexion and extension, indicating that the origin of the ligament is very near the axis of flexion.
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