MSK Session 2 - Shoulder and Upper Arm

Question 1

What are the muscles of the shoulder?

Answer 1

The muscles of the shoulder are associated with movements of the upper limb. They produce the characteristic shape of the shoulder, and can be divided into two groups:

- Extrinsic – originate from the torso, and attach to the bones of the shoulder (clavicle, scapula or humerus).

- Intrinsic – originate from the scapula and/or clavicle, and attach to the humerus.

Question 2

What are the intrinsic muscles of the shoulder?

Answer 2

• The intrinsic muscles (also known as the scapulohumeral group) originate from the scapula and/or clavicle, and attach to the humerus.
• There are six muscles in this group – the deltoid, teres major, and the four rotator cuff muscles (supraspinatus, infraspinatus, subscapularis and teres minor).

Question 3

Describe the structure, attachments, innervations and actions of the deltoid muscle.

Answer 3

• Structure: The deltoid muscle is shaped like the Greek letter delta – Δ. It can be divided into an anterior, middle and posterior part.

- Attachments: Originates from the scapula and clavicle, and attaches to the deltoid tuberosity on the lateral surface of the humerus.

- Innervation: Axillary nerve.

- Actions: The anterior fibres flex the arm at the shoulder, the posterior fibres extend the arm at the shoulder. The middle fibres are the major abductor of the arm – it takes over from the supraspinatus, which abducts the first 15 degrees.

Question 4

Describe the structure, attachments, innervation and actions of Teres Major.

Answer 4

• Structure: The teres major forms the inferior border of the quadrangular space – the ‘gap’ that the axillary nerve and posterior circumflex humeral artery pass through to reach the posterior scapula region.

- Attachments: Originates from the posterior surface of the inferior angle of the scapula. It attaches to the intertubercular groove of the humerus.

- Innervation: Lower subscapular nerve.

- Actions: Adducts at the shoulder and medially rotates the arm.

Question 5

What are the rotator cuff muscles?

Answer 5

• The rotator cuff muscles are a group of four muscles that originate from the scapula and attach to the humeral head.
• Collectively, the resting tone of these muscles acts to ‘pull’ the humeral head into the glenoid fossa.
• This gives the glenohumeral joint a lot of additional stability.
• The four rotator cuff muscles are supraspinatus, infraspinatus, subscapularis and teres minor.

Question 6

Describe the attachments, innervation and actions of supraprinatus.

Answer 6

• Attachments: Originates from the supraspinous fossa of the scapula, attaches to the greater tubercle of the humerus.

- Innervation: Suprascapular nerve.

- Actions: Abducts the arm 0-15°, and assists deltoid for 15-90°

Question 7

Describe the attachments, innervation and actions of infraprinatus.

Answer 7

• Attachments: Originates from the infraspinous fossa of the scapula, attaches to the greater tubercle of the humerus.

- Innervation: Suprascapular nerve.

- Actions: Laterally rotates the arm.

Question 8

Describe the attachments, innervation and actions of subscapularis.

Answer 8

• Attachments: Originates from the subscapular fossa, on the costal surface of the scapula. It attaches to the lesser tubercle of the humerus.

- Innervation: Upper and lower subscapular nerves.

- Actions: Medially rotates the arm.

Question 9

Describe the attachments, innervation and actions of teres minor.

Answer 9

• Attachments: Originates from the posterior surface of the scapula, It attaches to the greater tubercle of the humerus.

- Innervation: Axillary nerve

- Actions: Laterally rotates the arm.

Question 10

What are the extrinsic muscles of the shoulder?

Answer 10

The extrinsic muscles of the shoulder originate from the trunk, and attach to the bones of the shoulder – the clavicle, scapula or humerus. They are located in the back, and are also known as the superficial back muscles. The muscles are organised into two layers – a superficial layer and a deep layer.

Question 11

Which two muscles are in the superficial layer of the shoulder?

Answer 11

• Trapezius
• Latissimus Dorsi

Question 12

Describe the structure, attachments, innervations and actions of the trapezius.

Answer 12

• Structure: The trapezius is a broad, flat and triangular muscle. The muscles on each side form a trapezoid shape. It is the most superficial of all the back muscles.

- Attachments: Originates from the skull, nuchal ligament and the spinous processes of C7-T12. The fibres attach to the clavicle, acromion and the scapula spine.

- Innervation: Motor innervation is from the accessory nerve. It also receives proprioceptor fibres from C3 and C4 spinal nerves.

- Actions: The upper fibres of the trapezius elevate the scapula and rotates it during abduction of the arm. The middle fibres retract the scapula and the lower fibres pull the scapula inferiorly.

Question 13

Describe the structure, attachments, innervation and actions of the latissimus dorsi.

Answer 13

The latissimus dorsi originates from the lower part of the back, where it covers a wide area.

• Attachments: Has a broad origin – arising from the spinous processes of T6-T12, iliac crest, thoracolumbar fascia and the inferior three ribs. The fibres converge into a tendon that attaches to the intertubercular sulcus of the humerus.
• Innervation: Thoracodorsal nerve.
• Actions: Extends, adducts and medially rotates the upper limb.

Question 14

Which three muscles are in the deep layer of the shoulder?

Answer 14

• Levator Scapulae
• Rhomboid major
• Rhomboid minor

Question 15

Describe the structure, attachments, innervation and actions of the levator scapulae.

Answer 15

• Structure: The levator scapulae is a small strap-like muscle. It begins in the neck, and descends to attach to the scapula.

- Attachments: Originates from the transverse processes of the C1-C4 vertebrae and attaches to the medial border of the scapula.

- Innervation: Dorsal scapular nerve.

- Actions: Elevates the scapula.

Question 16

Describe the attachments, innervation and actions of the rhomboid major.

Answer 16

• Attachments: Originates from the spinous processes of T2-T5 vertebrae. Attaches to the medial border of the scapula, between the scapula spine and inferior angle.

- Innervation: Dorsal scapula nerve.

- Actions: Retracts and rotates the scapula.

Question 17

Describe the attachments, innervation and actions of the rhomboid minor.

Answer 17

• Attachments: Originates from the spinous processes of C7-T1 vertebrae. Attaches to the medial border of the scapula, at the level of the spine of scapula.

- Innervation: Dorsal scapula nerve.

- Actions: Retracts and rotates the scapula.

Question 18

What are the muscles of the upper arm?

Answer 18

• The upper arm is located between the shoulder joint and elbow joint.
• It contains four muscles – three in the anterior compartment (biceps brachii, brachialis, coracobrachialis), and one in the posterior compartment (triceps brachii).

Question 19

What are the muscles in the anterior compartment?

Answer 19

• There are three muscles located in the anterior compartment of the upper arm – biceps brachii, coracobrachialis and brachialis.
• They are all innervated by the musculocutaneous nerve.
• Arterial supply to the anterior compartment of the upper arm is via muscular branches of the brachial artery

Question 20

Describe the structure, attachments, function and innervation of the biceps brachii.

Answer 20

• Structure: The biceps brachii is a two-headed muscle. Although the majority of the muscle mass is located anteriorly to the humerus, it has no attachment to the bone itself. As the tendon of biceps brachii enters the forearm, a connective tissue sheet is given off – the bicipital aponeurosis. This forms the roof of the cubital fossa and blends with the deep fascia of the anterior forearm.

- Attachments: Long head originates from the supraglenoid tubercle of the scapula, and the short head originates from the coracoid process of the scapula. Both heads insert distally into the radial tuberosity and the fascia of the forearm via the bicipital aponeurosis.

- Function: Supination of the forearm. It also flexes the arm at the elbow and at the shoulder.

- Innervation: Musculocutaneous nerve. The bicep tendon reflex tests spinal cord segment C6.

Question 21

Describe the structure, attachments, function and innervation of the coracobracialis.

Answer 21

• Structure: The coracobrachialis muscle lies deep to the biceps brachii in the arm.

- Attachments: Originates from the coracoid process of the scapula. The muscle passes through the axilla, and attaches the medial side of the humeral shaft, at the level of the deltoid tubercle.

- Function: Flexion of the arm at the shoulder, and weak adduction.

- Innervation: Musculocutaneous nerve.

Question 22

Describe the structure, attachments, function and innervation of the brachialis.

Answer 22

• Structure: The brachialis muscle lies deep to the biceps brachii, and is found more distally than the other muscles of the arm. It forms the floor of the cubital fossa.

- Attachments: Originates from the medial and lateral surfaces of the humeral shaft and inserts into the ulna tuberosity, just distal to the elbow joint.

- Function: Flexion at the elbow.

- Innervation: Musculocutaneous nerve, with contributions from the radial nerve.

Question 23

What are the muscles of the posterior compartment?

Answer 23

• The posterior compartment of the upper arm contains the triceps brachii muscle, which has three heads.
• The medial head lies deeper than the other two, which cover it.
• Arterial supply to the posterior compartment of the upper arm is via the profunda brachii artery.

Question 24

Describe the attachments, innervation and actions of the Triceps Brachii.

Answer 24

- Attachments: Long head – originates from the infraglenoid tubercle. Lateral head – originates from the humerus, superior to the radial grove. Medial head – originates from the humerus, inferior to the radial groove. Distally, the heads converge onto one tendon and insert into the olecranon of the ulna.

- Actions: Extension of the arm at the elbow.

- Innervation: radial nerve

Question 25

What is the cubital fossa?

Answer 25

The cubital fossa is an area of transition between the anatomical arm and the forearm. It is located as a depression on the anterior surface of the elbow joint.

Question 26

The cubital fossa is triangular in shape, and thus has three borders. What are they?

Answer 26

• Lateral border – The medial border of the brachioradialis muscle.

- Medial border – The lateral border of the pronator teres muscle.

- Superior border – An imaginary line between the epicondyles of the humerus.

Question 27

Describe the structure of the cubital fossa.

Answer 27

• The floor of the cubital fossa is formed proximally by the brachialis, and distally by the supinator muscle.
• The roof consists of skin and fascia, and is reinforced by the bicipital aponeurosis.
• Within the roof runs the median cubital vein, which can be accessed for venepuncture.

Question 28

What are the contents of the cubital fossa?

Answer 28

The contents of the cubital fossa include vessels, nerves and the biceps tendon (lateral to medial):

• Radial nerve – This is not always strictly considered part of the cubital fossa, but is in the vicinity, passing underneath the brachioradialis muscle. As is does so, the radial nerve divides into its deep and superficial branches.

- Biceps tendon – It runs through the cubital fossa, attaching to the radial tuberosity, just distal to the neck of the radius.

- Brachial artery – The brachial artery supplies oxygenated blood the forearm. It bifurcates into the radial and ulnar arteries at the apex of the cubital fossa.

- Median nerve – Leaves the cubital between the two heads of the pronator teres. It supplies the majority of the flexor muscles in the forearm.

Question 29

Describe the division of the back muscles.

Answer 29

The muscles of the back can be divided into three groups – superficial, intermediate and deep:

- Superficial – associated with movements of the shoulder.

- Intermediate – associated with movements of the thoracic cage.

- Deep – associated with movements of the vertebral column.

Question 30

Compare and contrast and describe the development the muscles of the back.

Answer 30

• The deep muscles develop embryologically in the back, and are thus described as intrinsic muscles.
• The superficial and intermediate muscles do not develop in the back, and are classified as extrinsic muscles.
• The superficial back muscles are situated underneath the skin and superficial fascia.
• They originate from the vertebral column and attach to the bones of the shoulder – the clavicle, scapula and humerus.
• All these muscles are therefore associated with movements of the upper limb.
• The muscles in this group are the trapezius, latissimus dorsi, levator scapulae and the rhomboids.
• The trapezius and the latissimus dorsi lie the most superficially, with the trapezius covering the rhomboids and levator scapulae.

Question 31

What is the shoulder joint?

Answer 31

• The shoulder joint (glenohumeral joint) is a ball and socket joint between the scapula and the humerus.
• It is the major joint connecting the upper limb to the trunk.
• It is one of the most mobile joints in the human body, at the cost of joint stability.

Question 32

Describe the structure and function of the articulating surfaces of the shoulder joint.

Answer 32

• The shoulder joint is formed by the articulation of the head of the humerus with the glenoid cavity (or fossa) of the scapula.
• This gives rise to the alternate name for the shoulder joint – the glenohumeral joint.
• Both the articulating surfaces are covered with hyaline cartilage – which is typical for a synovial type joint.
• The head of the humerus is much larger than the glenoid fossa, giving the joint inherent instability.
• To reduce the disproportion in surfaces, the glenoid fossa is deepened by a fibrocartilage rim, called the glenoid labrum.

Question 33

Outline the structure and function of the joint capsule in the shoulder.

Answer 33

• The joint capsule is a fibrous sheath which encloses the structures of the joint.
• It extends from the anatomical neck of the humerus to the border of the glenoid fossa.
• The joint capsule is lax, permitting greater mobility (particularly abduction).
• The synovial membrane lines the inner surface of the joint capsule, and produces synovial fluid to reduce friction between the articular surfaces.
• To reduce friction in the shoulder joint, several synovial bursae are present. A bursa is a synovial fluid filled sac, which acts as a cushion between tendons and other joint structures

Question 34

Identify which bursae in the shoulder are clinically important.

Answer 34

• Subacromial – Located inferiorly to the deltoid and acromion, and superiorly to the supraspinatus tendon and the joint capsule. It supports the deltoid and supraspinatus muscles. Inflammation of this bursa is the cause of several shoulder problems.

- Subscapular – Located between the subscapularis tendon and the scapula. It reduces wear and tear on the tendon during movement at the shoulder joint.

Question 35

Briefly, outline the structure and function of the ligaments in the shoulder joint.

Answer 35

• In the shoulder joint, the ligaments play a key role in stabilising the bony structures.
• The majority of the ligaments are thickenings of the joint capsule.

Question 36

Identify the key ligaments in the shoulder joint.

Answer 36

• Glenohumeral ligaments (superior, middle and inferior) – Consists of three bands, which runs with the joint capsule from the glenoid fossa to the anatomical neck of the humerus. They act to stabilise the anterior aspect of the joint.

- Coroacohumeral ligament – Attaches the base of the coracoid process to the greater tubercle of the humerus. It supports the superior part of the joint capsule.

- Transverse humeral ligament – Spans the distance between the two tubercles of the humerus. It holds the tendon of the long head of the biceps in the intertubercular groove.

Question 37

Identify another major ligament of the shoulder joint.

Answer 37

• The other major ligament is the coracoacromial ligament.
• Unlike the others, it is not a thickening of the joint capsule. It runs between the acromion and coracoid process of the scapula, forming the coraco-acromial arch.
• This structure overlies the shoulder joint, preventing superior displacement of the humeral head.

Question 38

Describe the neurovascular supply of the shoulder joint.

Answer 38

• Arterial supply to the glenohumeral joint is via the anterior and posterior circumflex humeral arteries, and the suprascapular artery. Branches from these arteries form an anastomotic network around the joint.
• The joint is supplied by the axillary, suprascapular and lateral pectoral nerves. These nerves are derived from roots C5 and C6 of the brachial plexus. Thus, an upper brachial plexus injury (Erb’s palsy) will affect shoulder joint function.

Question 39

Identify the primary movements of the shoulder joint.

Answer 39

As a ball and socket synovial joint, there is a wide range of movement permitted:

• Extension
• Flexion
• Abduction
• Adduction
• Medial Rotation
• Lateral Rotation

Question 40

Describe the flexion and extension of the shoulder joint.

Answer 40

• Extension (upper limb backwards in sagittal plane) produced by the posterior deltoid, latissimus dorsi and teres major.

- Flexion (upper limb forwards in sagittal plane) produced by the biceps brachii (both heads), pectoralis major, anterior deltoid and coracobrachialis.

Question 41

Describe the abduction and adduction of the shoulder joint.

Answer 41

• Abduction (upper limb away from midline in coronal plane) The first 0-15 degrees of abduction is produced by the supraspinatus. The middle fibres of the deltoid are responsible for the next 15-90 degrees. Past 90 degrees, the scapula needs to be rotated to achieve abduction – that is carried out by the trapezius and serratus anterior.

- Adduction (upper limb towards midline in coronal plane) produced by contraction of pectoralis major, latissimus dorsi and teres major.

Question 42

Describe the rotation of the shoulder joint.

Answer 42

• Medial Rotation (rotation towards the midline, so that the thumb is pointing medially) produced by contraction of subscapularis, pectoralis major, latissimus dorsi, teres major and anterior deltoid.

- Lateral Rotation (rotation away from the midline, so that the thumb is pointing laterally) produced by contraction of the infraspinatus and teres minor.

Question 43

Identify and describe the factors that contribute to the mobility of the shoulder joint.

Answer 43

• Type of joint – It is a ball and socket joint.

- Bony surfaces – Shallow glenoid cavity and large humeral head – there is a 1:4 disproportion in surfaces. A commonly used analogy is the golf ball and tee.

• Laxity of the joint capsule.

Question 44

Identify and describe the factors that contribute to the stability of the shoulder joint.

Answer 44

• Rotator cuff muscles – These muscles surround the shoulder joint, attaching to the tubercles of the humerus, whilst also fusing with the joint capsule. The resting tone of these muscles act to ‘pull’ the humeral head into the glenoid cavity.

- Glenoid labrum: This is a fibrocartilaginous ridge surrounding the glenoid cavity. It deepens the cavity, reducing the risk of dislocation.

- Ligaments – The ligaments act to reinforce the joint capsule, and forms the coraco-acromial arch.

Question 45

What is the scapula?

Answer 45

• The scapula is also known as the shoulder blade.
• It articulates with the humerus at the glenohumeral joint, and with the clavicle at the acromioclavicular joint. In doing so, the scapula connects the upper limb to the trunk.
• It is a triangular, flat bone, which serves as a site for attachment for many (17!) muscles.

Question 46

Describe the structure, function and componentns of the costal surface of the scapula.

Answer 46

• The anterior surface of the scapula is termed ‘costal’, this is because it is the side facing the ribcage.
• This side of the scapula is relatively unremarkable, with a concave depression over most of its surface, called the subscapular fossa. The subscapularis muscle, one of the rotator cuff muscles, originates from this side.
• Originating from the superolateral surface of the costal scapula is the coracoid process. It is a hook-like projection, which lies just underneath the clavicle.
• The short head of the biceps brachii and the pectoralis minor attach here, while the coracobrachialis muscle originates from this projection.

Question 47

Describe the structure, function and components of the lateral surface of the scapula.

Answer 47

• The lateral surface of the scapula faces the humerus. It is the site of the glenohumeral joint, and of various muscle attachments.

- Glenoid fossa – A shallow cavity, which articulates with the humerus to form the glenohumeral joint. The superior part of the lateral border is very important clinically, as it articulates with the humerus to make up the shoulder joint, or glenohumeral joint.

- Supraglenoid tubercle – A roughening immediately superior to the glenoid fossa, this is the place of attachment of the long head of the biceps brachii.

- Infraglenoid tubercle – A roughening immediately inferior to the glenoid fossa, this is the place of attachment of the long head of the triceps brachii.

Question 48

Describe the structure, function and components of the posterior surface of the scapula.

Answer 48

• The posterior surface of the scapula faces outwards. It is a site of attachment for the majority of the rotator cuff muscles of the shoulder.

- Spine – The most prominent feature of the posterior scapula. It runs transversely across the scapula, dividing the surface into two.

- Infraspinous fossa – The area below the spine of the scapula, it displays a convex shape. The infraspinatus muscle originates from this area.

- Supraspinous fossa – The area above the spine of the scapula, it is much smaller that the infraspinous fossa, and is more convex in shape. The supraspinatus muscle originates from this area.

- Acromion – projection of the spine that arches over the glenohumeral joint and articulates with the clavicle.

Question 49

Describe the structure and function of the clavicle.

Answer 49

• The clavicle (collarbone) extends between the sternum and the acromion of the scapula.
• The clavicle is a slender bone with an ‘S’ shape. Facing forward, the medial aspect is convex, and the lateral aspect concave. It can be divided into a sternal end, a shaft and an acromial end.
• It is classed as a long bone, and can be palpated along its length. In thin individuals, it is visible under the skin.
• The clavicle has three main functions:

I. Attaches the upper limb to the trunk.

II. Protects the underlying neurovascular structures supplying the upper limb.

III. Transmits force from the upper limb to the axial skeleton.

Question 50

What are the bony landmarks of the clavicle?

Answer 50

• Sternal (medial) End
• Shaft
• Acromial (lateral) End

Question 51

Describe the structure and function of the sternal end of the clavicle.

Answer 51

• The sternal end contains a large facet – for articulation with the manubrium of the sternum at the sternoclavicular joint.
• The inferior surface of the sternal end is marked by a rough oval depression for the costoclavicular ligament (a ligament of the SC joint).

Question 52

Describe the structure and function of the shaft of the clavicle.

Answer 52

The shaft of the clavicle acts a point of origin and attachment for several muscles – deltoid, trapezius, subclavius, pectoralis major, sternocleidomastoid and sternohyoid.

Question 53

Describe the structure and function of the acromial end of the clavicle.

Answer 53

• The acromial end houses a small facet for articulation with the acromion of the scapula at the acromioclavicular joint.
• It also serves as an attachment point for two ligaments:

I. Conoid tubercle – attachment point of the conoid ligament, the medial part of the coracoclavicular ligament.

II. Trapezoid line – attachment point of the trapezoid ligament, the lateral part of the coracoclavicular ligament.

• The coracoclavicular ligament is a very strong structure, effectively suspending the weight of the upper limb from the clavicle.

Question 54

Describe the structure and function of the humerus.

Answer 54

• The humerus is the bone that forms the upper arm, and joins it to the shoulder and forearm.
• The proximal region articulates with the scapula and clavicle, forming part of the shoulder joint.
• Distally, the humerus articulates with the forearm bones (radius and ulna), to form the elbow joint.
• The humerus acts as an attachment site for many muscles and ligaments, resulting in various raised roughening on the bony surface.

Question 55

Describe the structure of the proximal region of the humerus as well as its anatomical features.

Answer 55

• The proximal region of the humerus articulates with the scapula to form the glenohumeral joint (shoulder joint).
• The important anatomical features of the proximal humerus are the head, anatomical neck, surgical neck, greater and lesser tubercles and intertubercular sulcus.

Question 56

What is a tubercle?

Answer 56

A tubercle is a round nodule, and signifies an attachment site of a muscle or ligament.

Question 57

Describe the structure and function of the anatomical features of the proximal region of the humerus.

Answer 57

• The head of the humerus projects medially and superiorly to articulate with the glenoid cavity of the scapula. The head is connected to the tubercles by the anatomical neck, which is short in width and nondescript.
• The greater tubercle is located laterally on the humerus. It has an anterior and posterior face. The greater tubercle serves as attachment site for three of the rotator cuff muscles – supraspinatus, infraspinatus and teres minor.
• The lesser tubercle is much smaller, and more medially located on the bone. It only has an anterior face. It is a place of attachment for the last rotator cuff muscle – subscapularis.

Question 58

Describe the separation of the greater and lesser tubercles.

Answer 58

• Separating the two tubercles is a deep depression, called the intertubercular sulcus, or groove.
• The tendon of the long head of biceps brachii runs through this groove.
• The edges of the intertubercular sulcus are known as lips.
• Tendons of the pectoralis major, teres major and latissimus dorsi attach to the lips of the intertubercular sulcus.
• The surgical neck runs from the tubercles to the shaft of the humerus.

Question 59

Describe the structure and function of the shaft of the humerus.

Answer 59

• The shaft of the humerus contains some important bony landmarks such as the deltoid tuberosity and radial groove, and is the site of attachment for various muscles.
• On the lateral side of the humeral shaft is a roughened surface where the deltoid muscle attaches. This is known is as the deltoid tuberosity.
• The radial groove is shallow depression that runs diagonally down the posterior surface of the humerus, parallel to the deltoid tuberosity. The radial nerve and profunda brachii artery lie in this groove.

Question 60

Which muscles on the shaft of the humerus attach to the humerus?

Answer 60

The following muscles attach to the humerus:

- Anteriorly: Coracobrachialis, deltoid, brachialis, brachioradialis

- Posteriorly: Medial and lateral heads of the triceps

Question 61

Describe the structure of the distal region of the humerus.

Answer 61

• The distal part of the humerus articulates with the ulna and radius at the elbow joint.
• Here, the bone adopts a flattened, almost 2-D shape.

Question 62

Describe the lateral and medial supraepicondylar ridges.

Answer 62

• The lateral and medial borders of the humerus form medial and lateral supraepicondylar ridges.
• The lateral supraepicondylar ridge is more roughened, as it is the site of attachment for many of the extensor muscles in the posterior forearm.

Question 63

Describe the lateral and medial epicondyles.

Answer 63

• Immediately distal to the supraepicondylar ridges are the lateral and medial epicondyles – projections of bone.
• Both can be palpated at the elbow (the medial more so, as it is much larger).
• The ulnar nerve passes into the forearm along the posterior side of the medial epicondyle, and can also be palpated there.

Question 64

Describe the trochlea, capitulum and the depressions of the distal region of the humerus.

Answer 64

• The trochlea articulates with the ulna.
• It is located medially, and extends onto the posterior of the bone.
• Lateral to the trochlear is the capitulum, which articulates with the radius.
• Also found on the distal portion of the humerus are three depressions, known as the coronoid, radial and olecranon fossae.
• They accommodate the forearm bones during movement at the elbow.

Question 65

Outline Traumatic Anterior Dislocation.

Answer 65

• Anterior dislocations (in which the humeral head is displaced anteriorly in relation to the glenoid), account for as many as 95-98% of shoulder dislocations.
• The reason is that the muscular and ligamentous support anterior to the humeral head is much less robust than the substantial muscular and bony support afforded posteriorly by the rotator cuff and scapula.

Question 66

Outline Traumatic Posterior Dislocation.

Answer 66

• Traumatic Posterior Dislocation: Posterior shoulder dislocations make up a small minority of total shoulder dislocation cases, accounting for 2-4% of presentations.
• However, because of a low level of clinical suspicion and insufficient imaging, they are often missed.
• Approximately half of posterior shoulder dislocations go undiagnosed on initial presentation. This can be causes by electric shock or a seizure.

Question 67

Describe ACJ Dislocation.

Answer 67

• Acromioclavicular joint
• A simple joint with six different ways of dislocating
• The coracoclavicular ligaments are important
• Some need surgery

Question 68

Outline Clavicle fractures

Answer 68

• 80% of cases are undisplaced
• 20% of cases are displaced
• A function of the clavicle is to transmit forces from the upper limb to the axial skeleton. Thus, the clavicle is the most commonly fractured bone in the body.
• Fractures commonly result from a fall onto the shoulder, or onto an outstretched hand.
• The most common point of fracture is the junction of the medial 2/3 and lateral 1/3.
• After fracture, the lateral end of the clavicle is displaced inferiorly by the weight of the arm, and medially, by the pectoralis major. The medial end is pulled superiorly, by the sternocleidomastoid muscle.
• The suprascapular nerves (medial, intermedial and lateral) may be damaged by the upwards movement of the medial part of the fracture. These nerves innervate the lateral rotators of the upper limb at the shoulder – so damage results in unopposed medial rotation of the upper limb – the ‘waiters tip’ position.

Question 69

Describe the process of impingement.

Answer 69

• Low painful arc
• Tender over tuberosity
• Hawkins test +ve
• Ultrasound scan

I. Bursitis

II. Tendinopathy

III. Dynamic impingement

• Treatment

I. Steroid and physio

II. Surgical decompression.

Question 70

Outline calcific supraspinatus tendonitis.

Answer 70

• Degenerative condition
• Calcium hydroxyapatite deposit
• Subacromial impingement if large
• If bursts - acute calcific tendinitis

I. 2 day history

II. Rapidly progressive pain

III. 10/10 severity

IV. Resolves 1-2 weeks

Question 71

Outline the rotator cuff tears.

Answer 71

• Full thickness large/massive tears

I. Impingement signs

II. Supraspinatus test weak

III. Infraspinatus weak & ER lag

IV. Subscapularis push off & belly press weak

V. Progressive functional loss with size of tear

VI. Open repair, augmentation with allograft

• Rotator cuff arthropathy

I. Elderly

II. Cuff not repairable

III. Glenohumeral arthritis

IV. Acromial erosion +/- fracture

V. Reverse anatomy shoulder replacement

Question 72

Describe the structure, function and classification of skeletal muscle cells.

Answer 72

• There are three types of muscle cells: skeletal, cardiac and smooth.
• Skeletal and cardiac are classed as striated muscle due to the ordered arrangement of the myofibrillar apparatus producing a characteristic banding pattern when viewed under the microscope.
• Skeletal muscles function to permit movement and are usually contracted voluntarily and consciously.
• Muscular contractions also help maintain body posture by stabilising joints even when there is no obvious movement.

Question 73

Describe the role of skeletal muscle in heat generation.

Answer 73

• A significant amount of body heat is also produced by skeletal muscle as this tissue is inefficient at converting chemical energy into mechanical work.
• Estimates vary, but about 80% of the chemical energy is lost as heat as a by-product of muscle activity.
• This inefficiency is utilised by the physiological process of shivering to raise core body temperature by the involuntary activation of skeletal muscle.

Question 74

Describe the numerical and structural distribution of skeletal muscles in the body.

Answer 74

• There are about 640 skeletal muscles within the human body, most of which are present as bilateral pairs (e.g. left and right deltoid muscles).
• These muscles vary in their overall size, the angles at which the muscle fibres are oriented and their contractile properties.
• The organization and composition of each muscle is related to the functions it performs.

Question 75

Briefly describe the different arrangements of skeletal muscle.

Answer 75

• Muscle fibres within muscles can be arranged in several different ways.
• The most common arrangement is for the fibres to run parallel to the force-generating axis.
• There are three main categories of parallel muscles: strap, fusiform and fan shaped.

I. Strap muscles (e.g. sartorius) are, as the name suggests, shaped like strap or belt with the fibres running longitudinally, parallel to the direction of contraction.

II. Fusiform muscles (e.g. biceps brachii) are cylindrical and are wider in the centre with tapering off at the ends.

III. Fan shaped muscles (e.g. pectoralis major) have fibres that converge at one end.

Question 76

What are pennate muscles? Classify them.

Answer 76

• Pennate muscles have one or more aponeuroses running through the muscle body from the tendon and the fascicles of muscle fibres attach to these aponeuroses at an angle (called the pennation angle) to the direction of movement.
• These can be unipennate where all the fascicles are on the same side as the tendon, bipennate with fascicles on both sides of a central tendon or multipennate where a central tendon branches.

Question 77

What are circular muscles?

Answer 77

• Finally, there are also circular muscles where the fibres form concentric rings around a sphincter or opening.
• Such muscles attach to skin, ligaments and fascia of other muscles rather than to bone. E.g. orbicularis oculi around the eye and orbicularis oris around the mouth.

Question 78

What are agonist and antagonist muscles?

Answer 78

• The prime muscle (or muscles) responsible for a particular movement are referred to as the agonist(s) whereas the muscles which oppose this movement are called antagonists.
• For example, for flexion at the elbow joint, brachialis and biceps brachii are the agonists whereas triceps brachii opposes this movement and so is the antagonist. This opposition of the action of agonists by antagonists is required for the fine control of movement.

Question 79

What are synergists?

Answer 79

• Muscles that act to assist the prime mover are called synergists (e.g. brachioradialis and pronator teres in elbow flexion).
• Acting alone they cannot perform the movement of the agonist but their angle of pull assists the agonist.

Question 80

What are neutralisers?

Answer 80

• Neutralisers prevent the unwanted actions that an agonist can perform.
• For example, the rotator cuff muscles stabilise the glenohumeral joint whilst biceps, (whose long head acts to cause shoulder flexion) acts to cause flexion at the elbow joint.

Question 81

What are fixators?

Answer 81

• Fixators (stabilisers) act to hold a body part immobile whilst another body part is moving.
• In most activities, proximal joints are stabilised whilst distal joints move.
• An example is stabilising the shoulder whilst flexing the elbow. The fixators active in elbow flexion are the muscles that stabilise the position of the scapula and those that stabilise the shoulder joint.
• NB. Neutralisers prevent the unwanted actions of a muscle; fixators stabilise a joint.

Question 82

Briefly outline muscle contraction.

Answer 82

• When a muscle contracts, there is not necessarily a shortening of the muscle associated with the contraction.
• This concept may seem contradictory at first but the term ‘contraction’ refers to the active cycling of cross-bridges between the actin thin filaments and the myosin thick filaments within the sarcomeres, rather than what is happening to the muscle on a gross physical level.

Question 83

What is isotonic contraction?

Answer 83

Isotonic contraction (lit. ‘equal tension’) is where the tension within the muscle remains constant and the length changes. There are two types of isotonic contractions: concentric and eccentric.

Question 84

Describe concentric and eccentric contraction.

Answer 84

• Concentric contraction is where the muscle shortens, for example when flexing the elbow to lift a load, the biceps shortens.

- Eccentric contraction refers to active contraction of muscles whilst they are lengthening.

Classic examples of this are walking, when the quadriceps (knee extensors) are active just after heel strike whilst the knee flexes, or setting an object down gently (the arm flexors must be active to control the fall of the object).

Question 85

What is isometric contraction?

Answer 85

Isometric contractions (lit. ‘equal length’) occur when the load against the muscle equals the contractile force being generated, e.g. holding a weight in a fixed position, or trying to push a brick wall. In both these cases the muscle is generating tension but is not shortening.

Question 86

What is passive stretch?

Answer 86

- Passive stretch is a fourth type of muscle ‘contraction’. As the name implies, the muscle is being lengthened while in a passive state (i.e. not being stimulated to contract).

• An example of this would be the pull felt in the hamstrings whilst touching the toes.

Question 87

What are the structures responsible for passive stretch?

Answer 87

• The structures responsible for passive tension are outside of the cross-bridge itself since muscle activation is not required.
• The protein titin is now known to be important in the contraction of striated muscle tissues.
• Titin connects the Z line to the M line in the sarcomere.
• It contributes to force transmission at the Z line and resting tension in the I band region.
• Titin limits the range of motion of the sarcomere in tension, thus contributing to the passive stiffness of muscle.

Question 88

Describe motor neurons in the skeletal muscle.

Answer 88

• Each skeletal muscle is supplied by a number of motor neurons which stimulate the muscle fibres to contract.
• The type of motor neurons innervating skeletal muscle fibres are called α-motor neurons and the cell bodies of these neurons are either located in the ventral horn of the spinal cord, for muscles of the limbs and trunk, or in the motor nuclei of the brainstem for the muscles of the head and face.
• The axons of α-motor neurons leave the central nervous system and form part of a peripheral nerve, to supply the muscle fibres of a skeletal muscle.

Question 89

Describe the key features of a neuromuscular junction.

Answer 89

• The neuromuscular junction is a specialised synapse between an a motor neurone and a skeletal muscle fibre.
• An action potential arriving at the synapse triggers opening of voltage-gated calcium ion channels.
• The resulting increase in intracellular Ca²⁺ causes vesicles containing acetylcholine (ACh) to release their contents into the synaptic cleft.
• Acetylcholine activates nicotinic ACh receptors (a type of ligand gated ion channel) in the muscle fibre plasma membrane resulting in an influx of sodium ions and depolarisation of the muscle fibre membrane potential.
• This local depolarisation activates voltage sensitive sodium channels resulting in the generation of an action potential in the muscle fibre.
• Acetylcholine is rapidly broken down in the synaptic cleft by the enzyme acetylcholinesterase.

Question 90

What is a motor unit?

Answer 90

• A motor unit is defined as an α-motor neuron and the group of individual muscle fibres that it innervates.
• Any single muscle fibre is innervated by only one α-motor neuron, but each α-motor neuron can innervate a number of different muscle fibres.
• The number of muscle fibres in a motor unit varies between different muscles and depends on the function of the muscle.
• Muscles that perform precise fine movements, such as inferior rectus which moves the eyeball have around ten muscle fibres in each unit motor unit whereas powerful muscles, where fine control is less important, such as gastrocnemius may have several thousand muscle fibres in each motor unit.

Question 91

What is muscle tone?

Answer 91

• All muscle has some degree of baseline tone (degree of tension) due to the elasticity of the muscle tissue and low levels of motor neuron activity.
• Skeletal muscle tone is controlled by motor control centres in the brainstem.
• The locus coeruleus, which contains noradrenergic cells, projects ascending axons to spinal motor neurons, where it facilitates muscle tone.
• Muscle tone is lost in REM sleep when the locus coeruleus cells shut off.
• A lack of skeletal muscle tone is known as hypotonia. This could result from damage to the motor cortex or cerebellum or spinal cord. Alternatively, there could be degeneration of the muscle itself (myopathy).

Question 92

Describe how the contractile force of muscles is controlled.

Answer 92

• The number of muscle fibres per motor unit and the proportion of the different fibre types varies between muscles depending on their function.
• The contractile force produced by a muscle depends on two factors:

I. The size principle and the rate code. The size principle simply means that small motor neurons are recruited before large ones. In general, this means that motor units with slow type I fibres are recruited first followed by those containing mostly fast IIa fibres and then those containing fast IIX fibres.

II. The Rate code refers to the frequency at which the muscle fibres are stimulated by their α-motor neuron. Consecutive action potentials in a repetitive train result in summation giving a slightly larger force with each contraction. Eventually a limit is reached where no further force can be produced termed tetany.

Question 93

Outline Malignant hyperthermia.

Answer 93

• Malignant hyperthermia is a rare life-threatening condition triggered by some volatile anaesthetic agents and succinylcholine, a neuromuscular blocking agent.
• In genetically susceptible individuals such drugs can lead to an uncontrolled increase in oxidative metabolism and increase in body temperature, which can be fatal if not immediately treated.
• Polymorphism in the ryanodine receptor is the most common genetic cause and this receptor is activated by some volatile anaesthetic agents leading to a massive increase in intracellular calcium from intracellular stores.
• The muscle cell SERCA pump (which consumes ATP in its action) is therefore working at a dramatically increased rate ultimately leading to excessive heat production.
• Such patients can be treated with dantrolene, a muscle relaxant which antagonises the ryanodine receptor.

Question 94

Identify and describe the sources of energy for muscle contraction.

Answer 94

• Very little ATP is stored in muscle fibres and this is sufficient for only a few seconds of contraction. Additional short term stores in the form of creatine phosphate can be used to rapidly replenish ATP, but again this is only sufficient to provide immediate energy for an initial burst of activity lasting a few seconds.
• Further energy to supply the contractile apparatus with the ATP it requires must come from glycolysis (anaerobic or aerobic) and oxidative metabolism (aerobic). Glycolysis can typically provide ATP sufficient for maximum muscle contraction for around 30-40 seconds.

Question 95

What are the advantages and disadvantages of glycolysis.

Answer 95

• The advantage of using glycolysis for immediate rapid activity is that this pathway can function under anaerobic conditions and so is not reliant on the blood supplying oxygen at the rate required for maximal contraction.
• Anaerobic glycolysis is therefore typically used in fast type II muscle fibres to produce short duration maximum force movements such as jumping or sprinting over a short distance.
• The disadvantage of using glycolysis under anaerobic conditions is that lactate is produced from pyruvate by the action of lactate dehydrogenase.

Question 96

What is the effects of the accumulation of lactate?

Answer 96

The accumulation of lactate is associated (although not necessarily the direct cause) with acidification of the muscle cell environment leading to “cramps” and underlies one of the reasons why maximal activity such as sprinting rapidly fatigues muscle.

Question 97

Describe glycolysis under anaerobic and aerobic conditions.

Answer 97

• Glycolysis under anaerobic conditions is also very inefficient in that only 2 molecules of ATP are produced when the pathway terminates at pyruvate.
• Glycolysis under aerobic conditions however allows the pyruvate produced to enter mitochondria where it is oxidized by the tricarboxylic acid (TCA) cycle to produce the high- energy electron donors utilised by the electron transport chain allowing much more ATP to be produced per molecule of glucose.
• For prolonged activity, however, such as running for several miles, the muscle needs to switch to beta oxidation of fatty acids released from triacylglycerol stored in adipose tissue.

Question 98

Outline the contraction of muscle fibres in relation to the type of fibre.

Answer 98

• Skeletal muscle fibres differ in the speed at which they contract, the amount of force that they generate and in their energy requirements/susceptibility to fatigue.
• Different types of muscle fibre differ in the isoforms of contractile proteins they express, most notably the myosin heavy chain.

Question 99

Outline the contraction in Type I muscle fibres.

Answer 99

• Slow muscle fibres (Type I) express the type I myosin heavy chain, they contract relatively slowly and produce low amounts of force.
• Type I fibres are however extremely resistant to fatigue due to their high mitochondria content and use of oxidative metabolism to produce the ATP they consume.

Question 100

Outline the contraction in Type II muscle fibres.

Answer 100

• Fast muscle fibres (Type II) contract relatively quickly and produce large amounts of force.
• There are two types of fast muscle fibres in human skeletal muscle termed fast IIA and fast IIX.
• The fast IIX muscle fibres are classified as Fast Glycolytic, while IIA fibres are Fast Oxidative/Glycolytic.
• Type IIA fast fibres therefore represent an intermediate form of muscle fibre between oxidative Type I fibres which produce low force but are fatigue resistant and the fast IIX fibres which rely on glycolytic metabolism, produce greater amounts of force and fatigue rapidly.

Question 101

Describe the effect of the type of muscle fibre on the nature of the motor unit

Answer 101

• The muscle fibres making up each motor unit tend to be all of the same contractile type, so each motor unit is either fast, intermediate or slow contracting.
• Slow motor units are very well-vascularised, with a high myoglobin content and can maintain contractions for long periods of time.
• These motor units are therefore typically found in postural muscles and those used for low intensity long duration activities such as walking.
• The soleus muscle in the leg for example is composed almost exclusively of slow type I muscle fibres.
• Fast type II muscle fibres have a glycolytic metabolism, and produce ATP quickly to enable a fast, powerful contraction but they also fatigue quickly.
• Most muscles are composed of a mixture of different fibre types and motor unit types and the proportion of fast to slow fibres depends on the typical function of the muscle.

Question 102

Outline the clinical relevance of rotator cuff tendinitis.

Answer 102

• Rotator cuff tendonitis refers to inflammation of the tendons of the rotator cuff muscles.
• This usually occurs secondary to repetitive use of the shoulder joint.
• Over time, this causes degenerative changes in the subacromial bursa, and the supraspinatus tendon. This increases friction between the structures of the joint.
• The characteristic sign of rotator cuff tendonitis is the ‘painful arc’– pain in the middle of abduction, where the affected area comes into contact with the acromion.

Question 103

Outline the clinical relevance of testing the accessory nerve.

Answer 103

• The most common cause of accessory nerve damage is iatrogenic (i.e. due to a medical procedure).
• In particular, operations such as cervical lymph node biopsy or cannulation of the internal jugular vein can cause trauma to the nerve.
• To test the accessory nerve, trapezius function can be assessed. This can be done by asking the patient to shrug his/her shoulders

Question 104

Outline the clinical relevance of rupture of the biceps tendon.

Answer 104

• A complete rupture of any tendon in the body is rare. However, the long head of the biceps brachii is one of the more common tendons to rupture.
• This produces a characteristic sign on flexing the elbow – a bulge where the muscle belly is, called the ‘Popeye Sign’.
• The patient would not notice much weakness in the upper limb due to the action of the brachialis and supinator muscles.

Question 105

Outline the clinical relevance of brachial pulse and blood pressure as well as a venepuncture.

Answer 105

• The brachial pulse can be felt by palpating immediately medial to the biceps tendon in the cubital fossa.
• When measuring blood pressure, this is also the location in which the stethoscope must be placed, to hear the korotkoff sounds.
• The median cubital vein is located superficially within the roof of the cubital fossa.
• It connects the basilic and cephalic veins, and can be accessed easily – this makes it a common site for venepuncture.

Question 106

Outline the clinical relevance of a mid-shaft fracture.

Answer 106

• A mid-shaft fracture could easily damage the radial nerve and profunda brachii artery, as they are tightly bound in the radial groove.
• The radial nerve innervates the extensors of the wrist. In the event of damage to this nerve, the extensors will be paralysed. This results in unopposed flexion of the wrist occurs, known as ‘wrist drop’.
• There is also some sensory loss over the dorsal (posterior) surface of the hand, and the proximal ends of the lateral 3 and a half fingers dorsally.

Question 107

Outline the significance of supracondylar and medial epicondyle fractures

Answer 107

• Supracondylar fractures and medial epicondyle fractures are common fracture types of the distal humerus.
• A supraepicondylar fracture occurs by falling on a flexed elbow.
• It is a transverse fracture, spanning between the two epicondyles.
• Direct damage, or swelling can cause interference to the blood supply of the forearm from the brachial artery.
• The resulting ischaemia can cause Volkmann’s ischaemic contracture - uncontrolled flexion of the hand, as flexor muscles become fibrotic and short. There also can be damage to the median, ulnar or radial nerves.
• A medial epicondyle fracture could damage the ulnar nerve, a deformity known as ulnar claw is the result.
• There will be a loss of sensation over the medial 1 and 1/2 fingers of the hand, on both the dorsal and palmar surfaces.