Acromioclavicular joint
Encyclopedia
The acromioclavicular joint, or AC joint, is a joint
at the top of the shoulder
. It is the junction between the acromion (part of the scapula
that forms the highest point of the shoulder) and the clavicle
.
s:
Superior Acromioclavicular Ligament
This ligament is a quadrilateral band, covering the superior part of the articulation, and extending between the upper part of the lateral end of the clavicle and the adjoining part of the upper surface of the acromion.
It is composed of parallel fibers, which interlace with the aponeuroses of the Trapezius and Deltoideus; below, it is in contact with the articular disk when this is present.
Inferior Acromioclavicular Ligament
This ligament is somewhat thinner than the preceding; it covers the under part of the articulation, and is attached to the adjoining surfaces of the two bones.
It is in relation, above, in rare cases with the articular disk; below, with the tendon of the Supraspinatus
The Coracoacromial Ligament is a strong triangular band, extending between the coracoid process and the acromion.
It is attached, by its apex, to the summit of the acromion just in front of the articular surface for the clavicle; and by its broad base to the whole length of the lateral border of the coracoid process.
This ligament, together with the coracoid process and the acromion, forms a vault for the protection of the head of the humerus.
It is in relation, above, with the clavicle and under surface of the Deltoideus; below, with the tendon of the Supraspinatus, a bursa being interposed.
Its lateral border is continuous with a dense lamina that passes beneath the Deltoideus upon the tendons of the Supraspinatus and Infraspinatus.
The ligament is sometimes described as consisting of two marginal bands and a thinner intervening portion, the two bands being attached respectively to the apex and the base of the coracoid process, and joining together at the acromion.
When the Pectoralis minor is inserted, as occasionally is the case, into the capsule of the shoulder-joint instead of into the coracoid process, it passes between these two bands, and the intervening portion of the ligament is then deficient.
The Coracoclavicular Ligament serves to connect the clavicle with the coracoid process of the scapula.
It does not properly belong to the acromioclavicular joint articulation, but is usually described with it, since it forms a most efficient means of retaining the clavicle in contact with the acromion. It consists of two fasciculi, called the trapezoid ligament and conoid ligament.
These ligaments are in relation, in front, with the Subclavius and Deltoideus; behind, with the Trapezius.
attached to the superior
acromioclavicular ligament. This meniscus may be a blade of fibrocartilage that extends nearly halfway into the joint or it may form a complete disc that divides the joint into two parts. In other joints no synovial joint
is present with the joint being made by a pad of fibrous tissue attached to the outer end of the clavicle, and no articular cavity.
.
AC dislocation is particularly common in collision sports such as ice hockey
, football
, rugby
and aussie rules, and is also a problem for those who participate in swimming, horseback riding, mountain biking
, biking and snow skiing. The most common mechanism of injury is a fall on the tip of the shoulder or FOOSH (falls on outstretched hand).
AC dislocations are also graded from I to VI. Grading is based upon the degree of separation of the acromion from the clavicle with weight applied to the arm. Grade I is a tear of the AC ligament. It has the normal separation of <4mm. Grade II is a complete dislocation of AC ligament with partial disruption of coracoclavicular ligament. The AC gap is >5mm. Grades I and II never require surgery and heal by themselves, though physical therapy may be required. Grade III is complete disruption of AC and CC ligaments. On plain film the inferior aspect of the clavicle will be above the superior aspect of the acromion. This can also be assessed with an MRI scan, which will also demonstrate disruption of the coracoclavicular ligaments (the degree depending on the severity of AC joint disruption) as well as tearing of the joint capsule. The joint will be very tender and swollen on examination. Grade III separations most often do not require surgery and shoulder function should return to normal after 16–20 weeks. However, there will be some physical deformity of the shoulder with a noticeable bump resulting from the dislocation of the clavicle. Grades IV-VI are complications on a 'standard' dislocation involving a displacement of the clavicle, and will almost always require surgery.
(OA) of the AC joint is not uncommon. It may be caused by a prior trauma (secondary OA) or occur as a chronic degenerative disorder. In the latter cases the condition often co-exist with subacromial impingement
.
Joint
A joint is the location at which two or more bones make contact. They are constructed to allow movement and provide mechanical support, and are classified structurally and functionally.-Classification:...
at the top of the shoulder
Shoulder
The human shoulder is made up of three bones: the clavicle , the scapula , and the humerus as well as associated muscles, ligaments and tendons. The articulations between the bones of the shoulder make up the shoulder joints. The major joint of the shoulder is the glenohumeral joint, which...
. It is the junction between the acromion (part of the scapula
Scapula
In anatomy, the scapula , omo, or shoulder blade, is the bone that connects the humerus with the clavicle ....
that forms the highest point of the shoulder) and the clavicle
Clavicle
In human anatomy, the clavicle or collar bone is a long bone of short length that serves as a strut between the scapula and the sternum. It is the only long bone in body that lies horizontally...
.
Function
The AC joint allows the ability to raise the arm above the head. This joint functions as a pivot point (although technically it is a gliding synovial joint), acting like a strut to help with movement of the scapula resulting in a greater degree of arm rotation.Ligaments
The joint is stabilized by three ligamentLigament
In anatomy, the term ligament is used to denote any of three types of structures. Most commonly, it refers to fibrous tissue that connects bones to other bones and is also known as articular ligament, articular larua, fibrous ligament, or true ligament.Ligament can also refer to:* Peritoneal...
s:
- The acromioclavicular ligamentAcromioclavicular ligamentThe Acromioclavicular ligament is part of the acromioclavicular joint. It is divided into two parts: superior and inferior.-Superior Acromioclavicular Ligament:...
, which attaches the clavicle to the acromion of the scapula.
Superior Acromioclavicular Ligament
This ligament is a quadrilateral band, covering the superior part of the articulation, and extending between the upper part of the lateral end of the clavicle and the adjoining part of the upper surface of the acromion.
It is composed of parallel fibers, which interlace with the aponeuroses of the Trapezius and Deltoideus; below, it is in contact with the articular disk when this is present.
Inferior Acromioclavicular Ligament
This ligament is somewhat thinner than the preceding; it covers the under part of the articulation, and is attached to the adjoining surfaces of the two bones.
It is in relation, above, in rare cases with the articular disk; below, with the tendon of the Supraspinatus
- The coracoacromial ligamentCoracoacromial ligamentThe Coracoacromial Ligament is a strong triangular band, extending between the coracoid process and the acromion.It is attached, by its apex, to the summit of the acromion just in front of the articular surface for the clavicle; and by its broad base to the whole length of the lateral border of the...
, which runs from the coracoid process to the acromion.
The Coracoacromial Ligament is a strong triangular band, extending between the coracoid process and the acromion.
It is attached, by its apex, to the summit of the acromion just in front of the articular surface for the clavicle; and by its broad base to the whole length of the lateral border of the coracoid process.
This ligament, together with the coracoid process and the acromion, forms a vault for the protection of the head of the humerus.
It is in relation, above, with the clavicle and under surface of the Deltoideus; below, with the tendon of the Supraspinatus, a bursa being interposed.
Its lateral border is continuous with a dense lamina that passes beneath the Deltoideus upon the tendons of the Supraspinatus and Infraspinatus.
The ligament is sometimes described as consisting of two marginal bands and a thinner intervening portion, the two bands being attached respectively to the apex and the base of the coracoid process, and joining together at the acromion.
When the Pectoralis minor is inserted, as occasionally is the case, into the capsule of the shoulder-joint instead of into the coracoid process, it passes between these two bands, and the intervening portion of the ligament is then deficient.
- The coracoclavicular ligamentCoracoclavicular ligamentThe Coracoclavicular Ligament serves to connect the clavicle with the coracoid process of the scapula.It does not properly belong the acromioclavicular joint articulation, but is usually described with it, since it forms a most efficient means of retaining the clavicle in contact with the acromion...
, which consists of two ligaments, the conoidConoidIn geometry, a conoid is a Catalan surface all of whose rulings intersect a fixed line, called the axis of the conoid. If all its rulings are perpendicular to its axis, then the conoid is called a right conoid....
and the trapezoid ligamentTrapezoid ligamentThe trapezoid ligament, the anterior and lateral fasciculus, is broad, thin, and quadrilateral: it is placed obliquely between the coracoid process and the clavicle....
s.
The Coracoclavicular Ligament serves to connect the clavicle with the coracoid process of the scapula.
It does not properly belong to the acromioclavicular joint articulation, but is usually described with it, since it forms a most efficient means of retaining the clavicle in contact with the acromion. It consists of two fasciculi, called the trapezoid ligament and conoid ligament.
These ligaments are in relation, in front, with the Subclavius and Deltoideus; behind, with the Trapezius.
Variability
An X-ray study of 100 shoulders in US soldiers found considerable variation in the size and shape of the joint. The articular surfaces were notably different in size and form. On some they are separated by a meniscusMeniscus
The meniscus is the curve in the upper surface of a liquid close to the surface of the container or another object, caused by surface tension. It can be either convex or concave. A convex meniscus occurs when the molecules have a stronger attraction to each other than to the material of the...
attached to the superior
acromioclavicular ligament. This meniscus may be a blade of fibrocartilage that extends nearly halfway into the joint or it may form a complete disc that divides the joint into two parts. In other joints no synovial joint
Synovial joint
A Synovial joint, also known as a diarthrosis, is the most common and most movable type of joint in the body of a mammal. As with most other joints, synovial joints achieve movement at the point of contact of the articulating bones....
is present with the joint being made by a pad of fibrous tissue attached to the outer end of the clavicle, and no articular cavity.
Injuries
A common injury to the AC joint is dislocation, often called AC separation or shoulder separation. This is not the same as a "shoulder dislocation," which refers to dislocation of the glenohumeral jointGlenohumeral joint
The glenohumeral joint, or shoulder joint, is a multiaxial synovial ball and socket joint and involves articulation between the glenoid fossa of the scapula and the head of the humerus...
.
AC dislocation is particularly common in collision sports such as ice hockey
Ice hockey
Ice hockey, often referred to as hockey, is a team sport played on ice, in which skaters use wooden or composite sticks to shoot a hard rubber puck into their opponent's net. The game is played between two teams of six players each. Five members of each team skate up and down the ice trying to take...
, football
American football
American football is a sport played between two teams of eleven with the objective of scoring points by advancing the ball into the opposing team's end zone. Known in the United States simply as football, it may also be referred to informally as gridiron football. The ball can be advanced by...
, rugby
Rugby union
Rugby union, often simply referred to as rugby, is a full contact team sport which originated in England in the early 19th century. One of the two codes of rugby football, it is based on running with the ball in hand...
and aussie rules, and is also a problem for those who participate in swimming, horseback riding, mountain biking
Mountain biking
Mountain biking is a sport which consists of riding bicycles off-road, often over rough terrain, using specially adapted mountain bikes. Mountain bikes share similarities with other bikes, but incorporate features designed to enhance durability and performance in rough terrain.Mountain biking can...
, biking and snow skiing. The most common mechanism of injury is a fall on the tip of the shoulder or FOOSH (falls on outstretched hand).
AC dislocations are also graded from I to VI. Grading is based upon the degree of separation of the acromion from the clavicle with weight applied to the arm. Grade I is a tear of the AC ligament. It has the normal separation of <4mm. Grade II is a complete dislocation of AC ligament with partial disruption of coracoclavicular ligament. The AC gap is >5mm. Grades I and II never require surgery and heal by themselves, though physical therapy may be required. Grade III is complete disruption of AC and CC ligaments. On plain film the inferior aspect of the clavicle will be above the superior aspect of the acromion. This can also be assessed with an MRI scan, which will also demonstrate disruption of the coracoclavicular ligaments (the degree depending on the severity of AC joint disruption) as well as tearing of the joint capsule. The joint will be very tender and swollen on examination. Grade III separations most often do not require surgery and shoulder function should return to normal after 16–20 weeks. However, there will be some physical deformity of the shoulder with a noticeable bump resulting from the dislocation of the clavicle. Grades IV-VI are complications on a 'standard' dislocation involving a displacement of the clavicle, and will almost always require surgery.
Osteoarthritis
OsteoarthritisOsteoarthritis
Osteoarthritis also known as degenerative arthritis or degenerative joint disease, is a group of mechanical abnormalities involving degradation of joints, including articular cartilage and subchondral bone. Symptoms may include joint pain, tenderness, stiffness, locking, and sometimes an effusion...
(OA) of the AC joint is not uncommon. It may be caused by a prior trauma (secondary OA) or occur as a chronic degenerative disorder. In the latter cases the condition often co-exist with subacromial impingement
Subacromial bursitis
Subacromial bursitis is a condition caused by inflammation of the bursa that separates the superior surface of the supraspinatus tendon from the overlying coraco-acromial ligament, acromion, coracoid and from the deep surface of the deltoid muscle...
.
See also
- Shoulder girdle (Pectoral girdle)
- Glenohumeral jointGlenohumeral jointThe glenohumeral joint, or shoulder joint, is a multiaxial synovial ball and socket joint and involves articulation between the glenoid fossa of the scapula and the head of the humerus...
(Shoulder joint) - ShoulderShoulderThe human shoulder is made up of three bones: the clavicle , the scapula , and the humerus as well as associated muscles, ligaments and tendons. The articulations between the bones of the shoulder make up the shoulder joints. The major joint of the shoulder is the glenohumeral joint, which...
- Sternoclavicular joint