Uncovering secrets of the beauty bone: A comprehensive review of anatomy and regional anesthesia techniques of clavicle surgeries

The clavicle is the most frequently fractured bone in humans. General anesthesia with or without Regional Anesthesia (RA) is most frequently used for clavicle surgeries due to its complex innervation. Many RA techniques, alone or in combination, have been used for clavicle surgeries. These include interscalene block, cervical plexus (superficial and deep) blocks, SCUT (supraclavicular nerve + selective upper trunk) block, and pectoral nerve blocks (PEC I and PEC II). The clavipectoral fascial plane block is also a safe and simple option and replaces most other RA techniques due to its lack of side effects like phrenic nerve palsy or motor block of the upper limb. We present a comprehensive review of anatomy and RA techniques of clavicle surgeries. This review will help readers understand the functional anatomy and nature of clavicle fractures, and apply an algorithmic approach to procedure-specific blocks for complexly innervated structures like clavicle. Research Article Uncovering secrets of the beauty bone: A comprehensive review of anatomy and regional anesthesia techniques of clavicle surgeries Kartik Sonawane1*, Hrudini Dixit2, J.Balavenkatasubramanian3 and Palanichamy Gurumoorthi4 1Junior Consultant, Department of Anesthesiology, Ganga Medical Centre & Hospitals, Pvt. Ltd., Coimbatore, Tamil Nadu, India 2Fellow in Regional Anesthesia, Department of Anesthesiology, Ganga Medical Centre & Hospitals, Pvt. Ltd., Coimbatore, Tamil Nadu, India 3Senior Consultant, Department of Anesthesiology, Ganga Medical Centre & Hospitals, Pvt. Ltd., Coimbatore, Tamil Nadu, India 4Consultant, Department of Anesthesiology, Ganga Medical Centre & Hospitals, Pvt. Ltd., Coimbatore, Tamil Nadu, India Received: 31 March, 2021 Accepted: 07 April, 2021 Published: 10 April, 2021 *Corresponding author: Dr. Kartik Sonawane, Junior Consultant, Department of Anesthesiology, Ganga Medical Centre & Hospitals, Pvt. Ltd. Coimbatore, Tamil Nadu, India, E-mail:


Introduction
The name 'clavicle' is derived from the Latin word Clavicula, which means "little key" because it rotates along its axis like a key when the shoulder is abducted. Fractures of the clavicle are relatively common, accounting for up to 10% of all fractures [1]. Clavicle surgeries can be done under Regional Anesthesia (RA) with or without general anesthesia (GA), depending on the fracture site. Analgesia for clavicle fractures can be challenging for anesthesiologists due to the complex and varied innervation in this region.
Many RA techniques, either individually or in combinations, have been used for perioperative analgesia for clavicle surgeries.

Functional anatomy of clavicle
The clavicle, or collarbone, is also referred to as "beauty bone" because of its prominent body location. It is the only long bone that lies horizontally in the body. The shape of the clavicle appears elongated 'S' due to its unique anterior and posterior curvatures. It can appear long and slender, thick and short, or any combination due to its complex shape. It consists of spongy cancellous bone within the compact bone shell, forming a periosteal collar. It is the only membranous bone outside the skull and is uniquely pierced through-and-through by the Supraclavicular Nerve (SCN) [12]. Thus, it is also referred to as dermal bone. It follows the "rule of two," i.e., two in number, having two primary ossification centers, two borders, two curvatures, two ends, two surfaces, and two articulations/ joints. (Figure 1A).

• Two primary ossification centers:
The clavicle is the first bone to begin the ossification process during the development of the embryo (5 th -6 th weeks of gestation) and one of the last bones to finish ossification (21)(22)(23)(24)(25) years of age).  2. Inferior surface: It is rough, with many prominent lines formed due to muscles and ligamentous attachment. It has three bony landmarks: the sternal facet forming the sternoclavicular joint, the costal tuberosity for the costoclavicular, and the conoid tubercle for conoid ligaments attachments.
• Two Articulations/Joints: It is also called a 'double plane joint' as a layer of articular cartilage separates two joint cavities.
1. Acromioclavicular (AC) joint is a plane synovial joint formed by the acromial end of the clavicle and the acromion of the scapula, connected by the acromioclavicular ligament. It supports the function of muscles that move the clavicle. Primarily, it allows elevation and depression of the shoulder joint. Blood supply to the clavicle is mainly derived from the thoracoacromial and suprascapular arteries, and rarely from the internal mammary artery.
There is a rich blood supply in the middle third and no supply on the posterior or inferior surfaces, leading to an increased risk of nonunion in high-energy midshaft fractures. The detailed innervation of the clavicle is discussed in the later section of this article.

Variations
• Males have longer and more massive clavicles than females.
• The right clavicle is usually stronger and shorter than the left clavicle [13,14].
• An autosomal dominant congenital disability like cleidocranial dysplasia is associated with a partly or completely absent clavicle.
The levator claviculae muscle is present in 2-3% of people.
It originates from the upper cervical vertebrae's transverse processes and inserts over the lateral half of the clavicle [15,16].

Functions of the clavicle [17]
• Adaptation to perform tasks: The well-developed clavicles and scapulae in humans and other brachiating mammals are an adaptation to perform tasks using hands well away from the body and climb, swing and grasp distant objects.
• Acts as a strut: The clavicle helps to position the arm away from the trunk by providing a stable strut between the arm and the axial skeleton. It allows the muscles to generate adequate power to perform reaching and elevation tasks.
• Scapular rotation: Out of the total 60° rotation of the scapula during the abduction of the arm, 30° comes purely from the lateral clavicle, and the remaining 30° comes from angulation of the whole clavicle and scapula together at the sternoclavicular joint.

Fascias related to the clavicle (Figure 1C)
Deltoid fascia: It is a fibrous membrane over the outer aspect of the deltoid muscle. Superiorly, it is attached to the clavicle, acromion, and the crest of the scapular spine. Inferiorly, it is continuous with the brachial fascia of the arm. Posteriorly, it is continuous with the posterior infraspinatus fascia. Anteriorly, it unifies with the pectoral fascia at its frontal-most edge.
Pectoral fascia: It is a thin layer of fascia covering the pectoralis major muscle. It is attached medially to the sternum superiorly to the clavicle, and inferolaterally, it is continuous with the fascia of the thorax, shoulder, and axilla. At its most lateral section, it forms the floor of the axilla as the axillary fascia.

Axillary fascia:
The classic bowl-like appearance of the armpit results primarily via the action of axillary fascia in tethering the skin to the floor of the axilla. It is frequently referred to as the suspensory ligament of the axilla. In females, it is pierced by the tail of the breast (aka the tail of Spence). This fascia surrounds the latissimus dorsi muscle laterally and envelops the pectoralis minor muscle medially before becoming the clavipectoral fascia at the upper edge of it.

Clavipectoral fascia:
The clavipectoral fascia is a strong fascia situated posterior to the clavicular part of the pectoralis major muscle. It extends superiorly from the clavicle, medially from the costochondral joints, and superolaterally from the coracoid process. Inferiorly, it converges in the axilla, where it acts as a protective structure over the neurovascular structure of the axilla. It lies between the pectoralis minor and subclavius muscles, protecting the axillary nerve and vessels. Superiorly, it splits to enclose the subclavius muscle before attaching to the clavicle. Its posterior layer fuses with the deep cervical fascia, which connects the omohyoid muscle to the clavicle. Medially, it is attached to the first rib before blending with the fascia over the first two intercostal spaces. Laterally, it is attached to the coracoid process before blending with the coracoclavicular ligament. The fascia often thickens to form a whiter and denser band, the costocoracoid ligament/membrane, between the first rib and coracoid process. Inferiorly, the fascia becomes thin, splits around pectoralis minor, and descends to blend with the axillary fascia and laterally with the fascia over the short head of biceps. It is pierced by CALL [cephalic vein, artery (thoracoacromial), lateral pectoral nerve, lymphatics]. The clavipectoral fascia completely surrounds the clavicle, and the nerve endings of the clavicle penetrate this fascia [18].
• There is a bimodal distribution of the clavicle fractures, with the two peaks being younger men (due to sports injury) and patients older than 55 years of age (due to osteoporosis and fall) [21].

Mechanism:
• Clavicle fractures are often caused by a direct impact on the outer aspect of the shoulder, typically from a fall, injury and trauma in motor vehicle accidents or sports.
• Clavicle fractures represent 95% of fractures seen during childbirth while passing through the birth canal [24].
• It can easily be fractured due to impact to the shoulder from the force of falling on outstretched arms or by a direct hit [25].

Classification
Allman classification system [26] is based on the anatomical location of the fracture, as shown in Figure 1B.
A. Type I involves the middle third of the clavicle, which constitutes around 75% of all clavicular fractures. Being the region with the narrowest cross-sectional diameter and lack of muscular and ligamentous attachment, the middle third of the clavicle becomes more vulnerable to the bending movements exerted by the structures attached. They are generally stable but may be associated with significant deformity.
B. Type II involves the lateral third of the clavicle and constitutes around 20% of all clavicular fractures. They are more common in elderly patients or those with comorbidities, although they do occur in all age groups. When displaced, these types are often unstable.
C. Type III involves the medial third of the clavicle and constituting around 5% of all clavicular fractures. It is commonly associated with multi-system polytrauma.
It can be associated with neurovascular compromise, pneumothorax, or hemothorax.

Neurovascular injury
a) Type I fractures are associated with injury to the SCN (intermediate branch), which pierces the clavicle in 1% to 4% of the population and innervates the proximal half of the clavicle [27].
b) The ulnar nerve is the most commonly injured peripheral nerve in clavicle fractures as it is close to the middle and medial third junction.
c) The brachial plexus lies as close as 12 mm from the medial end of the clavicle [28]. Thus, direct injury to the vessels and brachial plexus can occur at the time of fracture or iatrogenically during surgery with 0-15% incidence [29,30]. bearing. An open reduction and internal fixation will be necessary for fracture nonunion, usually performed at 2-3 months post-injury.

RA techniques for clavicle surgeries
The pain generating structures in clavicle surgeries include the skin over the incision area and richly innervated periosteum.
So, the planned RA technique should cover all the innervations of these pain generators. Due to the complex innervation of the clavicle, a 3-step algorithmic approach, 'Identify-Select-Combine' [31] can be applied to determine the most suitable procedure-specific RA option.

Identifying target nerves for clavicle surgery
The clavicle derives its sensory innervation from the cervical plexus and the brachial plexus. Due to the superficial and anterior location of the clavicle, all the innervations come from the anterior rami of the spinal nerves (C3-C7) without any contribution from the posterior rami. The clavicle receives its innervation from the nerves lying in the vicinity, either supplying skin, muscles, joints, or ligaments. The anterosuperior portion of the clavicle is bare and lies directly under the skin and subcutaneous tissue. The cutaneous branches supplying skin over the clavicle, i.e., supraclavicular nerves, also supply the clavicle at different levels. The procedure-specific RA option should cover all essential innervations of skin (dermatomes), muscles (myotomes), and bone (osteotomes) of the clavicle (Figure 2) as per the surgery involved.
The sensory innervation of the clavicle and the overlying skin is not clearly identified, and it varies depending on the source between C3 and C6 [4,[32][33][34]. The C3-C5 roots contribute to the dermatomal innervations, whereas the C3-C7 roots contribute to the myotomal and osteotomal innervations. Four muscles arise from, and two muscles insert into the clavicle, as shown in Table 1.
The periosteum of any bone is richly innervated by the nerve fibers that stem from different sources [33]    The target nerves to cover dermatomes, myotomes, and osteotomes of the surgical areas are shown in Table 2.

Selecting procedure-specific target blocks
A procedure-specific approach should be applied to target specific nerves as per the surgical steps to involve the innervation (Table 3) of all the pain generating components.
Since all nerves supplying the clavicle enters the clavipectoral fascia surrounding it, it is possible to block these nerves by depositing drug between this fascia and the periosteal collar.
Such a fascial plane block is a relatively simple and safe option available for awake clavicle surgeries.

Combining selected target blocks
After selecting all the target blocks, the following combinations can be considered as per the patient's suitability and the surgery involved.
A. Although the most common RA technique used in clavicle surgery is interscalene block [2], it is not without its undesirable effects or complications, such as phrenic nerve palsy, recurrent laryngeal nerve block, Horner syndrome, total spinal anesthesia, and cardio/neurotoxicity [37,38]. Furthermore,

Single-injection
interscalene blockade now has one of the highest rates of post-blockade nerve dysfunction, with an incidence ranging from 3% to 10% [38]. The combination of superficial and deep cervical plexus block [4] is also associated with unwanted side effects. The cervical plexus block is given mainly to target the SCN. However, the complications associated with the entire cervical plexus blockade can be avoided by targeting the SCN selectively under ultrasound guidance.     Single-injection approach -Interscalene Block + Superficial cervical plexus block B.
Single-injection approach -Selective superior trunk block + Selective supraclavicular nerve block C.
Dual-injection approach -Clavipectoral fascial plane block with probe kept longitudinally over the clavicle D.
Dual-injection approach -Clavipectoral fascial plane block with probe kept transversely over the clavicle  Table 4 and their choices as per the type of surgery are shown in Table 5.
The CPB requires further discussion and studies to address the following concerns: 1. Hyperalgesia and pain perceptions: The analgesic efficacy of CPB may get affected due to primary hyperalgesia in the damaged tissues, leading to a change in pain perception, which has to be studied further.
2. Change in pH: After the injury, local tissue inflammation causes a change in the pH, altering the local anesthetic effect.
3. Effect of myofascial plane disruption and loss of potentiality on the success of the block. Using the novel "Identify-Select-Combine" approach, a procedure-specific RA technique for a complexly innervated structure like a clavicle can be planned. There is still much to explore in the field of RA for clavicular surgery.

Acknowledgments
We thank Dr. Dheenadhayalan J., Senior Consultant, Orthopedics and Traumatology, for sharing surgical information and radiological images. We also thank 3D4Medical from Elsevier for granting permission to use and modify images from the Essential Anatomy Application.