Maximus Gage
The acromion is a bony projection on the scapula (shoulder blade). It is an important landmark in the skeletal system and serves as a muscle attachment point that is essential to the function of the shoulder joint. The acromion forms the acromioclavicular joint with the clavicle (collarbone). Two major muscles, the deltoid and trapezius, attach to the acromion. The deltoid abducts the arm at the shoulder, while the trapezius functions to elevate and depress the shoulder.
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What You'll Learn
The deltoid muscle
The deltoid is supplied by the thoracoacromial artery (acromial and deltoid branches), the circumflex humeral arteries, and the profunda brachii artery (deltoid branch). It is innervated by the axillary nerve, which originates from the anterior rami of the cervical nerves C5 and C6. The axillary nerve is sometimes damaged during surgical procedures, which can lead to deltoid muscle dysfunction.
The deltoid is one of the two major muscles that attach to the acromion, the other being the trapezius muscle. The acromion is a large, somewhat triangular or oblong bony projection on the superior end of the scapula (shoulder blade). It forms the summit of the shoulder and articulates with the clavicle (collar bone) to create the acromioclavicular (AC) joint. This joint is essential to the function of the shoulder, allowing for a wide range of arm movements.
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The trapezius muscle
The muscle is involved in many movements of the shoulder girdle, head, neck, and upper back, and is crucial in maintaining and adjusting posture. It is also involved in active movements like side bending and turning the head, elevating and depressing the shoulders, and internally rotating the arm. The trapezius works in conjunction with several other muscles to produce coordinated movements, primarily involving the scapula. Some key muscle pairings include the rhomboids, levator scapulae, serratus anterior, deltoid, and latissimus dorsi.
The trapezius is susceptible to overuse, injuries, and nerve damage, which can cause pain in the upper back, neck, or the back of the head. Other symptoms of trapezius issues include limited mobility, decreased range of motion, muscle weakness, neck and shoulder stiffness, and swelling or tenderness in the shoulders, neck, or back.
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The acromion and the scapula
The acromion is a bony projection on the scapula (shoulder blade). It is a crucial part of the shoulder joint, forming the acromioclavicular (AC) joint with the clavicle (collarbone). The AC joint is one of four joints that make up the shoulder complex. It is a gliding or planar synovial joint, allowing the scapula to move in multiple directions relative to the rest of the body. This significantly increases the mobility of the arm at the shoulder.
The acromion is a continuation of the scapular spine and hooks over anteriorly. It projects laterally and then curves forward and upward to overhang the glenoid fossa. The acromion is also involved in the formation of the pectoral girdle, along with the coracoid process. This structure is highly specialised and varies among different species. In humans, the acromion extends laterally over the shoulder joint, providing an attachment point for several important muscles.
Two major muscles attach to the acromion: the deltoid and the trapezius. The deltoid abducts the arm at the shoulder and originates along the acromion, the spine of the scapula, and the clavicle. It then crosses the shoulder joint and inserts on the deltoid tuberosity of the humerus. The trapezius muscle, on the other hand, arises from the cervical and thoracic vertebrae and inserts on the acromion and spine of the scapula.
The AC joint is surrounded by a thin capsule reinforced by superior, inferior, anterior, and posterior AC ligaments. The deltoid and trapezius muscles attach to the superior aspect of the AC joint, strengthening the ligaments and adding stability to the joint. The AC joint is susceptible to trauma and degenerative changes due to its small, incongruent surfaces, which result in large forces per unit area.
In some cases, the acromion may not fully fuse with the scapula, resulting in a condition known as os acromiale. This was observed in skeletons from the Mary Rose shipwreck, attributed to extensive archery practice during childhood, which prevented bony fusion. However, os acromiale rarely causes pain and is usually asymptomatic.
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The acromion in turtles
The acromion is a bony process on the scapula (shoulder blade) that forms the summit of the shoulder. In humans, the acromion gives attachment to some fibres of the deltoid muscle.
In turtles, the acromion is part of the shoulder girdle, which has a unique triradiate morphology. The shoulder girdle of turtles has undergone a transformation during evolution, resulting in a distinct structure compared to other vertebrates. The acromion in turtles is shorter than in their Late Triassic ancestors. The base of the acromion is concave, and in larger, probably adult specimens, it bears a smaller, more pronounced, rounded depression.
The development of the turtle's shell is a result of skeletal shifts and muscular origami. The muscle plate in the lower half of the turtle's body folds inwards along a line called the carapacial ridge, which eventually forms the edge of the shell. The second pair of ribs grow outwards and swing forwards over the shoulder blade, bringing it within the confines of the rib cage. This process twists the serratus anterior muscle, which connects the shoulder blade to the second pair of ribs, but it retains its attachment to both bones.
The unique arrangement of muscles in turtles also contributes to the differences in their musculature compared to other vertebrates. For example, the latissimus dorsi muscle, which connects the top of the forearm to the back in mice and chickens, attaches to the front part of the developing shell in turtles. Additionally, the pectoral or chest muscles in turtles connect to the plastron, the bottom half of the shell, instead of the sternum as in chickens and mice. These variations in muscle attachment and function can be influenced by the external environment, and turtles exhibit differences in muscle use during locomotion in water and on land.
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The acromioclavicular joint
The AC joint is formed by the junction of the lateral end of the clavicle and a facet on the acromion process of the scapula. The articular facets are considered incongruent as they vary in configuration and inclination from individual to individual. They may be flat, reciprocally concave-convex, or reversed (convex-concave). Due to its small and incongruent surfaces, the AC joint is extremely susceptible to both trauma and degenerative changes. Degeneration of the AC joint occurs with age, and the formation of osteophytes increases in frequency over time.
The AC joint capsule and ligaments surrounding the joint work together to provide passive stability and maintain the integrity of the joint. The capsule is thin and lined with a synovial membrane. It is strengthened by capsular ligaments, both inferiorly and superiorly, which are reinforced through attachments from the deltoid and trapezius muscles. The deltoid and trapezius muscles originate along the acromion, with the deltoid muscle abducting the arm at the shoulder. The trapezius muscle arises from the cervical and thoracic vertebrae and inserts on the acromion and spine of the scapula.
The AC joint is innervated by articular branches of the suprascapular and lateral pectoral nerves, which arise from the brachial plexus. Arterial supply to the AC joint is provided by the suprascapular and thoracoacromial arteries. AC joint dislocation, or a separated shoulder, occurs when the two articulating surfaces of the joint are separated, often from a direct blow or fall on an outstretched hand. Ligament rupture during dislocation can result in a more serious injury. Management of AC joint dislocation depends on injury severity and impact on quality of life, ranging from conservative treatments such as ice and rest to surgical interventions.
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Frequently asked questions
The acromion is a small projection of the scapula (shoulder blade) that extends anteriorly from the spine of the scapula. It forms the acromioclavicular joint (ACJ) with the lateral third of the clavicle.
The deltoid and trapezius muscles attach to the acromion.
The acromion is an important landmark of the skeletal system and a muscle attachment point that is essential to the function of the shoulder joint.
The acromioclavicular joint (ACJ or AC joint) is formed by the junction of the lateral clavicle and the acromion process of the scapula. It is a gliding or plane-style synovial joint that holds the scapula and clavicle together.
The acromion provides stability to the AC joint by allowing the scapula to glide in many directions relative to the rest of the body, increasing the mobility of the arm at the shoulder.
