Displaced mid-clavicular fracture: pull of sternocleidomastoid
When the middle third of the collarbone breaks and the ends move out of line (a displaced mid-clavicular fracture), the neck muscle sternocleidomastoid lifts the piece nearer the breastbone.
Remember
In a displaced mid-clavicular fracture, sternocleidomastoid lifts the medial fragment (the piece nearer the breastbone), while the weight of the arm and pectoralis major (the large chest muscle) pull the lateral fragment (the piece nearer the shoulder) down and inwards, giving a dropped shoulder.
Memory aid
Up at the neck end, down and in at the arm end.
It reminds you that the neck muscle lifts the piece nearest the neck, while the outer piece goes down and in.
The clavicle (collarbone) is a strut that holds the shoulder out from the chest. A mid-clavicular fracture is a break through its middle third, which is where about 80% of clavicle fractures occur, usually after a fall onto an outstretched hand or a blow to the shoulder.
The fracture is displaced when the two ends move out of line. The bone can then no longer keep the muscle pulls and the weight of the arm in balance, so each piece moves according to what is attached to it.
The medial fragment is the piece nearer the breastbone. Sternocleidomastoid, the thick muscle on the side of the neck that runs down towards the inner end of the collarbone, pulls it upwards.
The lateral fragment is the piece nearer the shoulder. The rest of the bone no longer holds it up, so the weight of the arm drags it downwards. Pectoralis major, the large chest muscle that pulls the arm in towards the body, draws it medially, towards the midline.
Together these forces give a dropped shoulder, with the arm held adducted (in towards the side of the body).
The sharp bone ends can occasionally push through the skin. That would make an open fracture and increase the chance of infection. Most of these fractures heal in a sling to restrict movement, and a displaced fracture may need surgery to realign the fragments.
Key facts
- Medial fragment
- pulled upwards by sternocleidomastoid.
- Lateral fragment
- lowered by the weight of the arm and drawn medially by pectoralis major.
- Appearance
- a dropped shoulder with the arm held adducted.
- Frequency
- about 80% of clavicle fractures involve the middle third.
- Treatment
- most are treated in a sling, and some displaced fractures need surgery to realign the fragments.
Exam traps
- Pectoralis major (the large chest muscle) does not lift the medial fragment of a displaced mid-clavicular fracture, because in that injury it draws the lateral fragment medially.
- Subclavius, a muscle lying directly under the clavicle, does not explain upward displacement of the medial fragment, because it anchors and depresses the clavicle, the opposite of elevating it.
- Trapezius, although its upper fibres elevate the scapula, does not lift the medial fragment of a displaced mid-clavicular fracture, because it works on the shoulder blade, whereas the muscle that attaches to the medial clavicle and lifts that fragment is sternocleidomastoid.
Read the full notes
Tested in 6 SimpleMed+ questions
Practice it in Musculoskeletal System, with the answer and a full explanation after every question.
Practice these questions