Fig. 1.1
Illustration depicting the bony anatomy of the clavicle, with anterior and posterior views showing both articular ends and the important muscle and ligament attachments along its length. (Borrowed from Rockwood C, Matsen F, Wirth M, Lippitt S. The Shoulder, Volume 1, page 33–100, 4th edition. Saunders; Philadelphia, PA. 2009)
The average length of the clavicle is around 15 cm, but this varies according to laterality, gender, ethnicity, and body height [6]. Overall, the clavicle is longer, wider, and thicker in men [7]. Mathieu et al. [8] defined the hourglass morphology of the clavicle using anatomical and two-dimensional CT (computed tomography) study. The study demonstrated intramedullary canal narrowing in the middle of the clavicle, in addition to a more pronounced medial curvature and larger epiphyseal diameter in men, in the axial plane. The left clavicle is wider and longer than the right one at most intervals [7]. The cortex of the clavicle is thinnest on the dorsal side at the acromial end and on the ventral side of the sternal end [2]. The importance of understanding the three-dimensional morphology of the clavicle as a prelude to establishing consistent therapeutic criteria for various clavicular pathologies cannot be understated.
Muscular Attachments
Several muscles attach to the clavicle along its length. The platysma is a thin broad muscle that originates in the subcutaneous tissue around the clavicle and attaches to the base of the mandible and angle of mouth. It is superficial to the cervical fascia. Although the platysma does not originate or attach on the clavicle, it must be divided when surgically approaching the middle one-third of the clavicle, such as during open reduction and internal fixation using a plate and screws construct. The superior surface of the clavicle is smooth and subcutaneous. The clavicular head of the pectoralis major muscle originates anteriorly on the medial two-thirds. Posterior to the pectoralis major, the clavicular head of the sternocleidomastoid originates at the middle one-third of the clavicle. Toward the lateral end of the clavicle, the origin of the deltoid muscle spans the whole anterior surface from superior to the inferior.
The only muscle that inserts on the superior surface of the clavicle is the upper portion of the trapezius muscle, which attaches posteriorly at the lateral end of the clavicle. In contrast to the superior surface of the clavicle, its inferior surface is rough, owing to a number of ligamentous attachments. Two muscles attach to the inferior surface the clavicle, the sternohyoid, and the subclavius muscles. The sternohyoid muscle originates on the most medial end of the inferior clavicle, its origin continues over the posterior surface of the sternoclavicular ligament and the sternum. In the middle one-third of the clavicle, the subclavius muscle inserts in a groove known as the subclavius groove , which serves as an attachment site of the clavipectoral fascia (Fig. 1.2) [4].
Fig. 1.2
Illustration depicting the anatomy of the medial clavicle and the sternoclavicular joint and demonstrating the robust ligamentous structures around the joint including the sternoclavicular, costoclavicular, and interclavicular ligaments. The subclavius muscle is seen inserting on the inferior surface of the clavicle along its groove. (Borrowed from Lee JT, Campbell KJ, Michalski MP, Wilson KJ, Spiegl UJ, Wijdicks CA, Millett PJ. Surgical anatomy of the sternoclavicular joint: a qualitative and quantitative anatomical study. J Bone Joint Surg Am, 96(19): e166, 2014)
Ligamentous Attachments
Medially, and posterior to the sternohyoid muscle origin, there is often a rough depression referred to as the rhomboid fossa which, as discussed earlier, serves as the attachment site of the costoclavicular ligament. Laterally on the inferior surface of the clavicle, the conoid and trapezoid ligaments form the coracoclavicular ligaments. The conoid tubercle serves as the attachment of the conoid ligament. Directly lateral to the conoid tubercle is the trapezoid line that serves as an attachment to the trapezoid ligament [5]. We will elaborate further on these important static stabilizers in the section on the anatomy of the acromioclavicular joint.
Neurovascular Anatomy
The anterior curve of the medial two-thirds of the clavicle provides a rigid arch under which the great vessels emerge as they exit the mediastinum toward the axilla. The subclavian and axillary vessels, the brachial plexus, and the lung are all located immediately posterior to the medial third of the clavicle. Hence the clavicle also functions as a bony protector of the neurovascular structures at the thoracic outlet .
Robinson et al. [9] used high definition photography of sagittal cadaveric sections to identify the neurovascular structures in closest proximity to the clavicle. On the medial aspect, the subclavian vein was at 4.8 mm from the posterior cortex of the clavicle. In the middle one-third of the clavicle, the brachial plexus was the closest structure, lying at an average of 15.2 mm from the posterior and inferior cortices. The subclavian vessels and the brachial plexus trunks and divisions were located more than two centimeters from the lateral clavicle.
Vascular Structures
The axillary vein joins the subclavian vein behind the medial one-third of the clavicle. The external jugular vein joins the subclavian vein at its confluence with the internal jugular vein at the level of the omohyoid fascia. Distal to that junction, the external jugular vein is joined on its lateral aspect by the transverse cervical and scapular veins and on its medial aspect by the anterior jugular vein. This confluence usually lies behind the angle formed by the sternocleidomastoid muscle and the clavicle [10]. The subclavian artery takes the same path as the brachial plexus lying in an anterior and inferior position progressing into the arm through the interval between the anterior and middle scalene muscles. It is divided into three segments in relation to the insertion of the scalenus anterior muscle. The vertebral artery branches out in the first segment, and the costocervical trunk and thyrocervical trunk branch out in the second segment. No branches are found in the third segment. The thyrocervical trunk gives rise to the transverse scapular artery and the suprascapular artery, which are mostly found in dissections involving the shoulder girdle (Fig. 1.3) [11].
Fig. 1.3
Schematic representation of the main vascular branches of the subclavian artery arising around the clavicle. The subclavian artery continues as the axillary artery distally to supply the entire upper extremity. (Borrowed from Netter FH. Atlas of Human Anatomy, page 430, 6th edition. Saunders, Philadelphia, PA. 2014)
Neurological Structures
The brachial plexus lies posterior and superior to the subclavian artery at the level of the clavicle (Fig. 1.4). It forms from C5 through T1 cervical roots and exits the neck through the interval between the anterior and middle scalene. The five cervical roots form three trunks that separate into the anterior and posterior divisions. As the brachial plexus crosses beneath the clavicle it gives rise to three cords. The medial cord derives from the eighth cervical and first thoracic roots and forms a branch of the median nerve, the medial pectoral nerve, the entire ulnar nerve, and the medial cutaneous nerve. A lateral cord deriving from the fifth, sixth, and seventh cervical roots forms the lateral pectoral nerve, musculocutaneous nerve, and another branch of the median nerve. Both the medial and lateral cords are anterior whereas the posterior cord of the plexus forms the upper and lower subscapular, thoracodorsal, axillary, and radial nerves. The medial cord passes between the first rib and medial third of the clavicle. It is vulnerable to compression in this location so much so that severe trauma can lead to proximal ulnar neuropraxia [12].
Fig. 1.4
Diagram of the brachial plexus, highlighting its complexity and formation from roots, trunks, divisions, cords, and branches. The clavicle serves as a solid barrier protecting the plexus from traumatic injury. (Borrowed from Agur AM, Dalley AF. Grant’s Atlas of Anatomy, page 488, 13th edition. Lippincott, Williams and Wilkins. 2013)
Supraclavicular Nerves
The supraclavicular nerves emerge from C3 and C4 nerve roots of the cervical plexus and divide more distally into medial, intermediate, and lateral rami . Havet et al. [13] conducted a cadaveric study on the anatomy of the supraclavicular nerves describing the path of the intermediate and lateral rami from a common trunk behind the posterior border of the sternocleidomastoid. The intermediate ramus courses below platysma and divides into two or three terminal branches. The lateral ramus courses in a frontal plane toward the acromion process crossing the anterior belly of the trapezius. This ramus is probably the nerve that supplies the acromioclavicular joint as reported by Ebraheim et al. [14]. In about 6–10% of cases, the shaft of the clavicle has an accessory osseous canal, called the canalis nervi supraclavicularis, which allow a ramus of the medial supraclavicular nerve to traverse to the chest [15].
Information regarding the anatomy of the supraclavicular nerves may help in preventing potential injury to these branches during surgery about the clavicle, especially during open reduction and internal fixation of midshaft clavicle fractures . The supraclavicular nerves are pure sensory nerves that supply sensation over the clavicle, the anteromedial shoulder, and the proximal chest. Although the precise locations of the principal rami and terminal branches are variable, there are safe zones extending 2.7 cm from the medial end of the clavicle and 1.9 cm from the lateral end that are devoid of any suprascapular nerve branches, according to a cadaveric study by Nathe et al. [16]. Nevertheless, preserving branches of the supraclavicular nerve remains a major challenge during the surgical approach to the clavicle because these lie in close proximity to its anterior surface. Even when the branches are identified they are often injured from repetitive stretch due to retraction or manipulation. As a result, incisional and chest wall numbness are reported in 10–29% of patients following plating of clavicular fractures [16].
Blood Supply to the Clavicle
The main blood supply of the clavicle is periosteal involving mainly the anterior and superior surfaces [13]. A secondary blood supply is intramedullary and canalicular, but this is of secondary importance. The prominent periosteal perfusion underscores the importance of minimizing dissection and avoiding periosteal stripping during the treatment of clavicle fractures, whether by intra-medullary or plate fixation.
Clavicle Biomechanics
The clavicle serves as a point of insertion for two muscles: the trapezius and subclavius and the point of origin for four muscles: the deltoid, pectoralis, sternocleidomastoid, and sternohyoid. However, the biomechanical role of the clavicle, in terms of overall shoulder motion and strength is not entirely understood. Proponents of total clavicular resection during tumor surgery or after multiple failed fracture surgeries point to the absence of substantial functional deficits postoperatively, if claviculectomy is performed with careful muscle repair.
However, the clavicle functions as a strut that allows shoulder motions such as cross body adduction and internal rotation, and also serves as a bony arch that protects major neurovascular structures. In fact, the clavicle rotates 45° with full shoulder abduction but for only 5°–8° relative to the acromion due to the synchronous scapuloclavicular motion [17]. By assuming the role of a strut, the clavicle permits the scapulohumeral and the scapulothoracic muscles to work at their optimal lever arms, thereby improving muscle efficiency and minimizing energy expenditure and muscle fatigue.
In addition, the clavicle serves a suspensory role that prevents the shoulder from drooping down due to the weight of the arm and avoids the recruitment of stabilizing muscles such as the trapezius from overcompensating to level both shoulder girdles. Thus, it largely contributes to the superior shoulder suspensory complex from which the upper extremity is suspended [18]. As a result, the clavicle is clearly an integral part of the kinetic chain responsible for an effective throwing mechanism starting with the hips and abdominal core muscles moving to the scapula, which is partly stabilized by the clavicle, and ending with a well-centered glenohumeral shoulder motion [19].
Embryology and Fetal Development
Prenatal Development
The clavicle is the first bone to ossify in the developing embryo. In the embryonic period, the mesenchymal cells of the clavicular blastema separate into medial and lateral groups and then differentiate into osteoblasts following the fifth week of gestation [20]. Thereafter, primary ossification begins. The clavicle undergoes intra-membranous ossification initially, in contrast to axial long bones, which undergo endochondral ossification. The medial and lateral ossification centers fuse during week 7 of gestation. Subsequently, longitudinal growth of the clavicle becomes cartilaginous similar to axial long bones; acromial and sternal ossification centers lead to the formation of the acromioclavicular and sternoclavicular joints [20]. By studying cadaveric transverse and coronal sections, Ogata and Uhthoff [21] confirmed these observations and noted that by week 8 of gestation, which corresponds to the beginning of the fetal period, the endochondral ossification and longitudinal growth of the clavicle has already commenced and the S-shaped morphology of the clavicle has been established [21]. These authors also determined that the less prominent medial growth center contributed more to overall clavicle length and curvature than the lateral growth center.
Periosteal bone formation is another determinant of clavicle development. It results in an increase in girth or cross-sectional area and subsequent remodeling of the clavicle. Garzon-Alvarado et al. [22] took a novel approach to the study of embryonic clavicle development and redefined it according to interplay between systemic, local biochemical, and mechanical factors. The first phase happens during the fifth week of gestation whereby mesenchymal cells forming the blastema at the center of the clavicle are prepared for ossification. The second phase during the sixth and seventh week dictates progression of ossification from the center to periphery following the same model whereby local mechanical and biochemical cues at the sternal and acromial ends promote cartilage formation and endochondral ossification.
Post-natal Development
As mentioned above, most of the longitudinal growth of the clavicle derives from the medial epiphysis, through the process of endochondral ossification [21]. The ossification of the medial growth plate starts at approximately 18 years of age and fuses with the remainder of the clavicle between the ages of 22 and 25 years. The lateral epiphysis ossification is more vaguely defined and often confused with a lateral clavicle fracture owing to a poorly demarcated border with the shaft of the clavicle.
Joints : Acromioclavicular (AC) and Sternoclavicular (SC) Joints
The Acromioclavicular Joint
Anatomical and Functional Considerations
The acromioclavicular, or AC, joint is the sole articulation between the clavicle and the scapula, excepting the rare true coracoclavicular joint arising in less than 1% of the population. The acromioclavicular joint is a synovial diarthrodial joint with a meniscal homologue that usually degenerates by the fourth decade. The acromioclavicular joint height is 9 mm and its antero-posterior width is 19 mm, but this is variable in size. The width of the acromioclavicular joint is between 1 and 3 mm. Park et al. [23] used ultrasonography to demonstrate that the joint space is minimal during cross-body adduction and active compression maneuvers so that these appear to be the most specific in diagnosing acromioclavicular pathology. The acromioclavicular joint is inclined 50° in the axial plane and 12° in the coronal plane whereby the distal clavicle is oriented posterior and lateral while the acromion is medial and anterior [24]. Colegate-Stone et al. [25] analyzed cadaveric, CT, and radiographic data to classify the 3-D morphology of the AC joint as either flat, oblique, or curved. The volume of the acromioclavicular joint is quite small, given its linear measurements. According to Edelson et al. [26], the joint may accept 0.5–1.2 mL of fluid when injected under sonographic guidance (Fig. 1.5).
Fig. 1.5
Ultrasound image displaying the width and depth of the acromioclavicular (AC) joint. The AC joint was found to absorb around 1 mL of fluid upon repeated injections under ultrasound guidance. (Borrowed from Edelson G, Saffuri H, Obid E, Lipovsky E, Ben-David D. Successful injection of the acromioclavicular joint with use of ultrasound: anatomy, technique, and follow-up. J Shoulder Elbow Surg, 23(10): e243–250, 2014)
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