Developmental anatomy

Ossification of the clavicle occurs around the fifth week of gestation from a mesenchymal or precartilaginous stage, often termed membranous or dermal bone. ^, Medial and lateral cartilaginous masses develop thereafter, resulting in linear growth of the clavicle, predominantly from the medial mass ( Fig. 20.2 ). A secondary center of ossification appears at the medial end of the clavicle at 18 to 20 years and unites to the remaining clavicle before 25 years of age. A similar secondary ossification center at the lateral end of the clavicle appears and unites in the 20th year of life. As many as three secondary ossification centers have been reported for the coracoid process; these centers and two for the acromion appear between 10 and 16 years of age and coalesce with the rest of the bony scapula between the ages of 20 and 25. ^,

Transverse section of an 11-week-old human fetus clavicle. Note the developing diaphysis and the lateral (l) and medial (m) cartilaginous masses. The shape at this early stage of development resembles the adult clavicle.

(From Ogata S, Uhthoff H. The early development and ossification of the human clavicle: an embryologic study. Acta Orthop Scand . 1990;61[4]:330–334.)

Pertinent osteology and joint architecture

The S-shaped clavicle tapers from being a cylindrical bone medially to a more flattened bone laterally ( Fig. 20.3 ). Van Tongel and colleagues evaluated the relationship of the acromion and distal clavicle articular facets in a normal population (34 females and 50 males) using bilateral three-dimensional (3D) computed tomography. In the coronal plane, the superior aspect of the lateral clavicle was found above the superior aspect of the medial acromion. In the axial plane, the most anterior aspect of the medial acromion was anterior to the distal clavicle ( Fig. 20.4 ). Another anatomic relationship previously defined is the distance between the superior aspect of the coracoid process and inferior aspect of the clavicle, which ranges from 11 to 13 mm. ^,

Osteology of the clavicle, from below. (A) The attachments for the trapezoid (6) and the conoid (8) ligaments are outlined. 1, Sternohyoid; 2, costal tuberosity; 3, pectoralis major; 4, subclavius; 5, deltoid; 7, trapezius muscle. (B) The inferior aspect of the clavicle. (C) Inferior surface of distal end of clavicle. Conoid tubercle (open arrow) is at the posterior-most aspect where the middle one-third “curves” into the distal one-third, serving as the attachment of the conoid ligament. Extending from the conoid tubercle laterally and anteriorly is the trapezoid ridge (dark arrow) , where the trapezoid ligament inserts.

(B, Modified from McMinn RMH, Hutchings RT. Color Atlas of Human Anatomy. Chicago: Year–Book Medical Publishers; 1985:93–94. C, From Renfree KJ, Wright TW. Anatomy and biomechanics of the acromioclavicular and sternoclavicular joints. Clin Sports Med. 2003;22[2]:219–237.)

(A) Normal superoinferior and (B) anteroposterior relationship of the medial acromion and distal clavicle of the acromioclavicular joint.

(From Bulkmans K, Peeters I, De Wilde L, et al. The relationship of the acromion to the distal clavicle in normal and symptomatic degenerated acromioclavicular joints. Arch Orthop Trauma Surg . 2020;140[4]:465-472.)

The convex lateral clavicle and concave medial acromion facets are covered with hyaline cartilage and articulate as a true diarthrodial joint. Transitional changes of hyaline cartilage to fibrocartilage have been reported to occur on both sides of the joint between the second and third decades of life. The AC joint orientation in the coronal plane is variable, ranging from nearly vertical to less frequent angulations approaching 50 degrees (i.e., from superior-lateral to inferior-medial) ( Fig. 20.5 ). Although less common, angulations from superior-medial to inferior-lateral have also been reported, leading to under-riding of the clavicle with respect to the acromial facet. In a cadaveric study that utilized dissections and computed tomography scanning, Colegate-Stone et al. identified three main 3D shapes of the AC joint: flat, oblique, and curved ( Fig. 20.6 ).

Sagittal plane orientation of the acromioclavicular joint shows great variability.

(A–B) Morphology of the acromioclavicular joint. Asterisks indicate equivalent points within the joints.

(A, From Colegate-Stone T, Allom R, Singh R, et al. Classification of the morphology of the acromioclavicular joint using cadaveric and radiologic analysis. J Bone Joint Surg Br. 2010;92[5]:743–746. B, Courtesy Steven Lippitt, MD.)

Intra-articular disk

The intra-articular disk is of variable shape and size and separates the AC joint either partly (meniscoid) or completely into two halves ( Fig. 20.7 ). ^, In the cadaveric study by Emura and colleagues (52 shoulders), three types of AC joint morphology were described based on the presence or absence of the articular disk: type I, complete disk (3.8%); type II, incomplete disk (25%); and type III, absent disk (71.2%). In another cadaveric study of 38 shoulders by Hatta et al., the intra-articular disk was absent in 24% of specimens and meniscoid-like in the remainder. The intra-articular disk is composed of fibrocartilage superiorly and dense connective tissue inferiorly. Although it is presumed to function in a manner similar to the knee meniscus (i.e., load distribution and joint stability), its actual role is unknown. The articular facets of the lateral clavicle and medial acromion may be intrinsically incongruent, in which case the intra-articular disk morphology is the conforming intervening structure (see Fig. 20.7 ). In younger patients, it is a structure potentially at risk for an acute injury, including contusion or tearing.

Coronal section through the acromioclavicular (AC) joint reveals a complete articular disk (AD) that completely separates the articular cavity. The upper part of the AD is thicker than the lower part. The acromial facet of the clavicle (AFC) and the clavicular facet of the acromion (CFA) are both convex. Cl, Clavicle.

(Modified from Emura K, Arakawa T, Miki A, et al. Anatomical observations of the human acromioclavicular joint. Clin Anat. 2014;27:1046–1052.)

Ligaments

Coracoclavicular (trapezoid and conoid) ligaments

Despite their morphologic differences, the coracoclavicular (CC) ligaments (i.e., trapezoid and conoid ligaments) have similar dimensional, viscoelastic, and structural properties. ^, Both ligaments attach to the superior aspect of the angular region of the coracoid process ( Fig. 20.8 ). The trapezoid ligament footprint extends along the anterolateral base of the coracoid ending just posterior to the pectoralis minor tendon attachment, and projects toward the clavicle in a superior, anterior, and slightly lateral direction to attach at the trapezoid line on the inferior clavicle (see Fig. 20.3 ). The conoid ligament has a posteromedial footprint on the angle of the coracoid and expands in a superior and slightly medial direction to insert on the conoid tubercle ( Figs. 20.8 and 20.9 ). Here, the lateral one-third and medial two-thirds of the clavicle join to form the posterior curve of the clavicle; at the curve’s apex, the conoid tubercle can be identified on the posteroinferior clavicle, just posterior and medial to the most medial aspect of the trapezoid line. On the inferior surface of the clavicle, points of attachment for the CC ligaments are delineated by a slightly prominent tubercle and a more subtle line for the conoid and trapezoid ligaments, respectively (see Fig. 20.3 ).

(A–B) The coracoclavicular ligaments with their footprint on the superior left coracoid process. cla , Clavicle; con , conoid ligament; cor , coracoid process; tra , trapezoid ligament.

(From Salzmann G, Paul J, Sandmann G, et al. The coracoid insertion of the coracoclavicular ligaments: an anatomic study. Am J Sports Med. 2008;36[12]:2392–2397.)

(A) Anterior view of the right shoulder demonstrating the coracoacromial ligament (CAL) , oblique orientation of the trapezoid ligament (T) , and the relative vertical orientation of the conoid ligament (C) . (B) Posterosuperior view of the same cadaveric specimen demonstrating the posterior insertion of the conoid ligament.

In a cadaveric study of 120 scapulae, Rios et al. found that the average distance from the lateral edge of the clavicle to the medial edge of the conoid tubercle was 46.3 mm (standard deviation, 5.1 mm); the average distance to the center of the trapezoid tuberosity was 24.9 mm (standard deviation, 3.8 mm). These authors also determined a ratio of the measured distance from the lateral edge of the clavicle to the respective ligament reference point and the total clavicle length (0.17 for the trapezoid ligament and 0.31 for the conoid ligament). Salzmann et al. defined coracoid anatomic landmarks that facilitated identification of the dimensions and orientation of the insertional footprint of the CC ligaments (see Fig. 20.8 ). Knowledge of these landmarks may prove useful intraoperatively when anatomic reconstructions are being considered.

Acromioclavicular ligaments

A relatively weak, thin capsule encloses the AC joint. Distinct AC ligaments, named by their location—anterior, posterior, superior, and inferior—augment the strength of the capsule. In an anatomical study of 63 cadaveric shoulders, Salter and colleagues found that the superior AC ligament was better developed and thicker (range, 2 mm to 5.5 mm) than the inferior capsular ligaments, which could not be consistently measured due to poor definition and interdigitation with the coracoacromial ligament ( Fig. 20.10 ). More recently, Nakazawa et al. confirmed that there is a well-developed superoposterior bundle that runs obliquely at an average of 30 degrees (i.e., relative to the AC joint surface) from the anterior part of the acromion to the posterior distal clavicle. This is in contrast to the anteroinferior bundle, which was found to be thin and narrow ( Fig. 20.11 ). Stine and Vangsness analyzed the ligamentous insertions of the AC ligaments in 28 cadaveric shoulders. On the acromial side, the capsular ligament begins, on average, 2.8 mm (range, 2.3 to 3.3 mm) from the medial acromion and begins on the lateral clavicle a mean of 3.5 mm (range, 2.9 to 3.9 mm) from the distal clavicle. Medial resections greater than 15 mm will begin to compromise the trapezoid ligament insertional footprint. Such information is crucial for surgeons to understand when considering a bipolar resection of the AC joint to address symptomatic arthritic pathology.

(A) Coronal section through the acromioclavicular joint. Note the thick superior capsular ligament (arrow) and the less well-developed inferior capsular ligament (arrowheads) . (B) Coronal section through the acromion process. Note the confluence of the fibers of the inferior capsular ligament and the coracoacromial ligament (arrow) .

(From Salter EG, Nasca RJ, Shelley BS. Anatomical observations on the acromioclavicular joint and supporting ligaments. Am J Sports Med. 1987;15:199–206.)

Orientation of the superoposterior bundle of the acromioclavicular ligament. (A) The white solid outline shows the superoposterior (SP) bundle, and the black dotted lines outline the clavicle and the scapula. (B) Posterior view of the right acromioclavicular joint is shown. The scapular spine was resected to allow visualization of the SP bundle, as indicated by the solid outline . (C) Superior view of the right acromioclavicular joint. The anteroinferior bundle is indicated by the white outline , the SP bundle by the black outline . Acr , Acromion; Cl , clavicle; Lat , lateral; Post , posterior; SS , scapular spine; Sup , superior.

(From Nakazawa M, Nimura A, Mochizuki T, et al. The orientation and variation of the acromioclavicular ligament: an anatomic study. Am J Sports Med. 2016;44[10]:2690–2695.)

Biomechanical studies have revealed that anterior restraint of the distal clavicle is predominantly afforded by the anterior-inferior AC ligaments and conoid ligament; posterior restraint by the superior-posterior AC ligaments and the trapezoid ligament; superior restraint by the conoid ligament and AC joint; and compression restraint by the trapezoid ligament. ^, The results of the in vitro studies of Dawson et al. demonstrate the importance of the CC ligaments for equally restraining both anterior-posterior (AP) and superior-inferior movements, while the restraint of the AC ligaments was threefold greater in the AP plane than the superior-inferior plane. In their study, joint compression contributed to joint stability in a reverse linear fashion, signifying the importance, at least experimentally, of the distal clavicle articulation with the acromion. ^, Other investigators have confirmed the importance of both the AC and CC ligaments in resisting horizontal translation of the AC joint.

Muscular attachments

The anterior head of the deltoid muscle originates from the lateral one-third of the clavicle (i.e., anterosuperior, anterior, and anteroinferior aspects) and anterior acromion process. The trapezius inserts onto the posterosuperior aspect of the lateral clavicle and medial aspect of the acromion process. The trapezius as well as the deltoid aponeurosis blend with the superior capsular ligaments of the AC joint, thus providing additional support to the AC joint on its superior aspect. The actions of these two muscles contribute to the stability of the AC joint and aid the suspension of the upper limb from the clavicle and the axial skeleton.

Neurovascular structures

Familiarity with the subcoracoid neurovascular structures is essential when surgical repair or reconstruction of the AC joint is undertaken. Tom et al. examined the relationship of bony landmarks of the shoulder, including the AC joint and the coracoid tip to the musculocutaneous nerve, posterior and lateral cords of the brachial plexus, and the axillary artery. The musculocutaneous nerve was the furthest and the lateral cord the closest structure from both landmarks; based on these findings, a 30-mm “safe zone” exists around the anteromedial tip of the coracoid process. Surgeons should also be familiar with the close proximity of the suprascapular artery and nerve when working around the base of the coracoid.