Biomechanics of the Shoulder

Introduction

Well, everyone thinks that the biomechanics of the shoulder is highly complex. Yes, I can accept that shoulder biomechanics is complex, as studying shoulder biomechanics involves many bones, joints, ligaments, and muscles.

The shoulder area is one of the most complex regions in our body. The shoulder complex connects the upper extremity to the thorax, and it is made up of 3 synovial joints and one floating joint, and three bones. Both are mentioned below.

Joints 

The shoulder complex involves three synovial joints and one floating joint.

  • Glenohumeral joint
  • Acromioclavicular joint
  • Sternoclavicular joint
  • The Scapulothoracic joint – It also functions as a joint in the shoulder complex. Therefore, it is considered a ‘functional joint,’ but it is not a true joint.

four-joints-of-shoulder-complex

Bones 

  • Humerus
  • Clavicle
  • Scapula

Generally, in our neighbourhood and family, we have listened to and seen many peoples who suffer from many shoulders problems. The reason behind it is that the shoulder joint is highly mobile joint compared to any other joint in the human body. However, this mobility is compromised by stability as it is a less stable joint that predisposes to various injuries.

Therefore, basic principles and functional knowledge of the biomechanics of the shoulder complex is important for the proper diagnosis and rehabilitation of shoulder injuries in clinical practice. In this article, you’ll study the biomechanics of the shoulder joint, and this article will also provide you a detailed overview of shoulder biomechanics.

Osteology

Clavicle 

It is also known as the collar bone. The clavicle is a long S-shaped bone approx 15 cm long. It has two ends, one convex medial 2/3rd end, and the other concave lateral 1/3rd end. Hence the medial portion of the clavicle moves very little compared to the other parts of the shoulder girdle, and its primary function is to provide stability. 

clavicle-ends

The rounded medial end of the clavicle articulates with the sternum to form the sternoclavicular joint. While, the flat end of the clavicle articulates with the acromion to form the acromioclavicular joint. Let’s discuss some more biomechanical functions of the clavicle. 

  • It provides support to the upper extremity by resisting compressive forces. When the external force is applied to the clavicle during fall on extended arm and side-lying. The trapezius and pectoralis minor apply an internal muscular compressive force to the clavicle, which offers some shoulder girdle stability.
  • The clavicle also allows the extremity to move more freely about the thorax by placing the extremity away from the body axis.

Scapula

The scapula is a triangular bone that serves many biomechanical functions. It is three borders superior, medial, and lateral, two surfaces vertical and dorsal, three angles superior, inferior and lateral. The primary function of the scapula includes

  • By varying or changing the original position of the proximal humerus, the scapula increases the position available for the hand in the space. 
  • During functional activities of the hand, the scapula provides stability to the upper extremity.  
  • The posterior surface of scapula faces outward and provides ample surface area for the attachment of 10 major muscles.
  • The coastal (anterior) surface of the scapula faces the ribcages and provides a surface that can easily slide over the thorax.
  • For performing more specialized functions (increasing leverage), three bony protuberances are present – the acromion, the coracoid process, and the glenoid neck provide attachments for six more muscles. 

Scapular shape and muscular attachment are predominant for the stability and positioning of the humerus.

Humerus

The humerus is a long bone of the upper extremity it extends from the shoulder to the elbow. The proximal end of the humerus articulates with the scapula’s glenoid fossa, hence forming the glenohumeral joint. In contrast, the distal end of the humerus articulates with the radius and ulna at the elbow joint. 

Now let’s discuss the biomechanical functions of the humerus. The humerus serves many biomechanical functions 

  • It provides a wide range of motion for the hand and provides leverage for the upper extremity strength. 
  • The humerus proximal end provides attachment points for the four rotator cuff muscles and allows for increased mobility. 
  • The free mobility of the proximal end is contrasted to and influenced by the relative lack of mobility present at the distal end of the humerus.  
  • The flat triangular shape of the distal shaft provides free flexion and extension at the elbow joint.
  • The humeroulnar joint accepts and transmits tremendous torque to the glenohumeral joint in the form of humerus internal and external rotation. However, this humerus internal and external rotation is resisted by the rotator cuff, & other muscles, and the capsular structures. Sometimes occasionally, tremendous forces are imparted directly to the glenohumeral joint via the humerus.

Let’s talk about the anatomy and biomechanics of the joints that are involved in the shoulder complex. Here we’ll also discuss the osteokinematics (joint movement) and arthrokinematics (bone movement) of the shoulder complex.

Glenohumeral joint

The glenohumeral joint is a ball and socket type of synovial joint. It has 3 rotatory and 3 translatory degree of freedom. In glenohumal joint the head of humerus (convex surface) articulates with the glenoid fossa (concave surface) of the scapula. It has capsule and many other associated ligaments and bursae.

GH is considered to be the most mobile joint and least stable joint in the human body. Less stability make this joint more prone to dislocation, therefore it is one of the most commonly dislocated joint in the human body.

Why Glenohumeral joint is the most mobile and least stable joint in the human body?

The concavity of the glenoid fossa is less intense than the convexity of the humeral head, meaning the articular surfaces aren’t fully congurent.

A fibrocartilagenous ring named glenoid labrum attaches to the margins of the glenoid fossa. The glenoid labrum somewhat increases the congurency, as it acts to deepen the glenoid fossa slightly.

The surface area of the humeral head is 4 times larger than the surface area of the glenoid fossa (4:1 ratio). It’s estimated that only 25 % of the humeral head articulates with the glenoid fossa at one time during motion.

The incongurent bony anatomy allows for the wide (immense) range of movement available in the shoulder joint but is also the man reason behind the lack of GH stability. Therefore, this lack of stability make the Glenohumeral joint more susceptible to instability, derangement and degenerative changes.

Therefore, joint security is provided by surrounding passive structure ( the capsule, labrum and the ligaments) and active structures ( muscles and their tendons).

Glenohumeral Joint Movements

As you have read above glenohumeral joint is the most mobile joint in the human body. It has three rotatory and three translatory degrees of freedom. There are two types of movement (osteokinematics and arthrokinematics) available at the glenohumeral joint. Osteokinematics means joint movement as it comes under rotatory motion, and arthrokinematics means bone movement as it comes under translatory motion.

Osteokinematics (joint movement) – 3 rotatory movement

  • Flexion / Extension
  • Adduction / Abduction
  • Medial Or Internal rotation / Lateral or External rotation 

Arthrokinematics (bone movement) – 3 translatory movement

  • Superior / Inferior Translation
  • Medial / Lateral Translation
  • Anterior / Posterior Translation

Let’s talk about the arthokinematics of the glenohumeral joint (shoulder complex). Arthokinematics means the motion of a part without reference to the force being applied to that part. for example – In case of clavicle, force is applied to the lateral part and movement happens in the medial part. 

As we have discussed above, the humeral head is four times larger than the glenoid fossa. Hence, it is estimated that only 25% of the humeral head articulates with the glenoid fossa at one time during motion. There comes only one point in the total ROM that allows an almost perfect fit between the joint surfaces while other positions are looser. Now let us talk about the loose-packed and closed-packed position of the glenohumeral joint. 

Loose-packed and Closed-packed position of the glenohumeral joint. 

The loose pack position permits better joint lubrication, less frictional forces on the joint surfaces, and permits more freedom of movement through a combined joint surface movement, which include spin, roll, and slide.

The position of almost fit is referred to as the “closed-packed” position. It is the position when the maximum articular surface is in contact with the concurrent ligamentous tension. The closed-packed position of the glenohumeral joint is full abduction and full external rotation. The joint is inherently stable in this position. 

Hence, the loose-packed position is of greater functional significance than the closed-packed position as the loose-packed position allows more freedom of movement. During the humerus elevation, the humeral head slides inferiorly, rolls posteriorly, and spins into internal rotation. 

In contrast, combined sliding, rolling, and spinning of the surfaces permit a greater range of motion than if only simple rolling of the surfaces is allowed. If any of the accessory movements are limited, the full range of motion will also be limited. 

Since the glenohumeral joint remains in a loose-packed position throughout most of its range, therefore the accessory movement of roll, glide, and spin can occur. Now let’s discuss the most important thing about the control of humeral movement.

Control of humeral movement

Several forces, most importantly muscular force, accomplish proper control over the humeral head placement. The rotator cuff muscles, mainly teres minor, infraspinatus, subscapularis, combine to accurately position the humeral head in the glenoid fossa throughout the range of motion. 

For the successful coordinated movement of the humeral head with normalized arthrokinematics, escaping an impingement situation requires the compatible co-contraction of the Rotator cuff tendons. Any deviation from the kinesthetic and muscular control may lead to improper biomechanics and joint pathology or trauma. This improper mechanics and pathology can lead to shoulder pain and discomfort and damage the other surrounding structures.

Acromioclavicular joint

The AC joint connects the scapula and the clavicle. It is generally referred to as a plane synovial joint. It has 3 rotatory and 3 translatory degress of freedom.

3 Rotatory Motions

  • Internal / External Rotation
  • Anterior / Posterior Tilting
  • Upward / Downward Rotation

3 Translatory Motions

  • Superior / Inferior Translation
  • Medial / Lateral Translation
  • Anterior / Posterior Translation

The AC joint has 2 major ligaments, a joint capsule and a joint disc that may be or may not be present. The AC joint provides stability and movement to the shoulder complex. It’s a commonly injured shoulder joint, which ranges from sprains to blunt tears, very occasionally requires surgery.

The primary function of the AC joint is to permit the scapula to rotate in the 3 dimensions during arm motion, so that upper extremity movement increases. Some other functions include

  • To permit the scapula additional range of motion on the thorax.
  • The AC joint permits for the adjustment of the scapula outside the initial plane of the scapula so as to follow the changing shape of the thorax as the arm motion happens.
  • It allows the transmission of forces from the upper extremity to the clavicle.

Sternoclavicular joint

The SC joint is a plane synovial joint forms between the manubrium of the sternum and the clavicle bone. It is only a true joint that connects the shoulder complex and upper extremity to the axial skeleton. It has three rotatory and three translatory degrees of freedom. 

3 Rotatory Motions

  • Elevation / Depression
  • Protraction / Retraction
  • Anterior / Posterior Rotation

3 Translatory Motions

  • Superior / inferior Translation
  • Medial / Lateral Translation
  • Anterior / Posterior Translation

The SC joint has a joint disc, a synovial capsule, and three major ligaments. The motion of the clavicle at the sternoclavicular joint inevitably produces motion of the scapula under normal functional conditions.

The reason behind the clavicle and scapula motion is that the scapula is attached to the lateral end of the clavicle at the acromioclavicular joint. Likewise, the motion of the scapula often results in the motion of the clavicle in the SC joint. 

The saddle shape of the articular surfaces of both the clavicle and sternum is very small. Therefore the sternoclavicular joint is often classified as a plane synovial joint.

The manubrium of the sternum and medial end of the clavicle is incongruent means that there is very little direct contact between their articular surfaces. 

The superior part of the medial clavicle doesn’t contact the manubrium of the sternum but serves as the attachment points for both the interclavicular ligament and the sternoclavicular disc. Let’s talk about the sternoclavicular disc.

Sternoclavicular disc

  • It is a fibrocartilagenous disc that increases the congruence between the incongruent articular surfaces.
  • The upper part of the SC disc is connected to the posterosuperior clavicle, while the lower portion is connected to the first costal cartilage and the manubrium sternum, and the anterior and posterior part of the fibrous sternoclavicular capsule. 
  • The disc transects the SC joint space diagonally, dividing the joint into two separate cavities. During shoulder movement, the disc acts as a hinge or pivot point for the medial end of the clavicle.

Scapulothoracic Joint

When the scapula articulates with the thoracic trunk (ribs), the space between them forms the scapulothoracic joint. It is not an actual anatomic joint as in this joint, no union of a bony segment occurs and it doesn’t have any ligaments and joint capsule. It has three rotatory and two translatory degrees of freedom. 

3 Rotatory Motions

  • Upward / Downward rotation,
  • Internal / External rotation
  • Anterior / Posterior tilting

2 Translatory Motions

  • Elevation / Depression
  • Protraction / Retraction

The scapulothoracic junction is a connection between the anterior surface of the scapula and the superior lateral surface of the thoracic wall. They are separated by the subscapularis muscle, which lies on the anterior surface of the scapula and the serratus anterior muscle, which attaches to the thoracic wall. There is a fascial space between these two muscles, which is filled with the loose connective tissue that facilitates the gliding movement of the scapula. 

The articulation between the scapula and the thorax depends on the integrity of the acromioclavicular and the sternoclavicular joint. These three joints (SC, AC, ST) are strictly linked to each other. 

    • The Sternoclavicular joint and the acromioclavicular joint are interdependent with the ST joint movement as the scapula is connected to the lateral end of the clavicle by its acromion process through the acromioclavicular joint. While the clavicle, in turn, is connected to the axial skeleton at the manubrium sternum via the sternoclavicular joint. 
    • Any motion of the scapula on the thorax must result in the movement at either the AC joint, or the SC joint, or both. Finally, this makes the functional scapulothoracic joint a part of a truly closed chain with the AC and SC joints and the thorax. 

Takeaway

I hope that this article has helped you in reducing the complexity in understanding the biomechanics of the shoulder joint. Well, understaning biomechanics of the shoulder is very important for the proper diagnosis and rehabilitation of the shoulder injuries in clinical practise.

In contrast, the more you research in depth in underestanding the basic principles and functional knowledge of the shoulder biomechanics. The more you can can properly diagnose and rehabilitate the shoulder injures in your clinical practise. Hope this article has helped you in understaning the shoulder biomechanics. Share your views in the below comment box.

Also Read 

Biomechanics of knee joint – Tibiofemoral joint and Meniscus

Anterior Cruciate Ligament – Anatomy And Biomechanics

Posterior Cruciate Ligament – Anatomy And Biomechanics

Medial Collateral Ligament – Anatomy And Biomechanics

Lateral Collateral Ligament – Anatomy And Biomechanics

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