Shoulder Injuries Part 1 - Five Joints to Consider Not Just One
Updated: 6 days ago
Introduction to Shoulder Injuries
Your shoulder is an incredibly complex system that is made up of multiple osseous structures, muscles, tendons, ligaments, fascia, and neurological and vascular structures. The scapulae (shoulder blade) alone has seventeen different muscles that connect into it. These muscles, soft tissues, and osseous structures of the shoulder act together to create a balance of forces that provide both mobility and stability. When this exquisite balance is disrupted, the shoulder quickly becomes prone to injury and dysfunction.
Your Shoulders... Inherently Unstable
Our shoulders are designed to provide an optimum range-of-motion, but at the cost of stability. In fact, when compared to other joints in your body, the shoulder girdle is actually quite unstable. When shoulder injuries occur, this inherent instability immediately affects a variety of other anatomical structures within the body.
When attempting to resolve a shoulder injury, it is essential to understand anatomical inter-relationships, relative motions, and kinetic chain links between the various soft-tissue and osseous structures of the shoulder. So let’s start with the shoulder joints.
Note: We have a lot of information to share with you about this complex topic. In this first section we will focus upon the shoulder joints. Subsequent blogs in this series will cover other affected structures, diagnosis and solutions. So stay tuned, or move forward to the section that interests you!
Anatomy and Bio-mechanics of the Shoulder Joints
When people think of the shoulder, they often think it is just one joint; in reality we must consider five different joints.
The shoulder girdle consists of five critical joints, three of which are standard joints, and two are physiological joints:
Scapulothoracic Joint. (physiological joint)
Subacromial Joint. (physiological joint)
Understanding the inter-relationships between the structures of these five joints will play a critical role in being able to successfully resolve shoulder injuries.
It is important to remember that no joint works in isolation, a restriction or dysfunction in one area always creates compensation somewhere else in the body. These compensations along the joints kinetic chain must often be addressed for full resolution of a shoulder injury.
1. The Glenohumeral (GH) Joint
The glenohumeral joint (shoulder joint) is a ball-and-socket joint that is filled with synovial fluid. During movement, synovial fluid acts to reduce friction between the articular cartilage of the synovial joints. The glenohumeral joint permits flexion, extension, adduction, abduction, and internal and external rotation of the arm.
Practitioners will often find glenohumeral instability during shoulder joint examinations and translation tests. Research has shown that extended periods of shoulder instability (glenohumeral instability) can start a cycle of micro-trauma, and result in secondary impingement syndromes that often cause chronic shoulder pain.
Anterior shoulder instability (laxity of the anterior capsule) is often related to problems in the posterior shoulder capsule, which can have a considerable impact upon the other structures of the shoulder’s kinetic chain.
When the posterior shoulder capsule becomes tight, it affects the IGHL (Inferior Glenohumeral Ligament). This ligament helps to maintain your shoulder’s position in the joint and acts as a supporting hammock or sling for the humerus (humeral head). When your IGHL does not function the way it should, your arm changes position, which can then result in a variety of impingement problems. (1,2)
The glenohumeral joint has some very interesting fascial connections. For example, the subscapular fascia is continuous with the rhomboid fascia, which then inserts into the glenohumeral joint. Any restrictions or forces generated along this fascial line could affect the function of the glenohumeral joint.(1)
2. The Acromioclavicular (AC) Joint
The acromioclavicular (AC) joint is located between the acromion process of the scapula and the lateral end of the clavicle.
The AC joint is a ‘gliding’ or ‘plane type’ of synovial joint.
The acromion of the scapula rotates on the acromial end of the clavicle.
Though the AC joint is a gliding joint, it actually acts as a pivot point to increase scapular motion, which in turn increases arm rotation. This motion of the AC Joint is often described as scapular movement with respect to the clavicle.
The AC Joint is very vulnerable to trauma and degenerative change. (3)
3. The Sternoclavicular (SC) Joint
The sternoclavicular joint is located between the manubrium of the sternum and the first costal cartilage. Technically, the sternoclavicular joint is a saddle-type of synovial joint.
The SC joint is the only attachment point of the upper limb onto the axial skeleton.
The SC joint allows movement of the clavicle across three planes, with most movement occurring in the anteroposterior and vertical planes.
The most common problem with the SC joint are instabilities caused by injuries and osteoarthritic changes.
4. The Scapulothoracic (Scapulocostal) Joint
The scapulothoracic joint is a 'physiological joint', one that is musculo-tendinous in nature. It is formed by an articulation of the anterior scapula and the posterior thoracic rib cage. The scapulothoracic joint is primarily formed by the muscles of the trapezius, rhomboids, and the serratus anterior.
The gliding movements performed by the scapulothoracic joint include elevation, depression, retraction, and protraction as well as superior and inferior rotation of the scapula.
The movement of the scapulothoracic joint is generally due to a combination of SC and AC joint motion.
The scapulothoracic joint increases the ability to elevate the arm while providing a stable base for the controlled motions between the humeral head and glenoid fossa. (4)
The most common problems encountered by the scapulothoracic joint are due to abnormal motion patterns. Abnormal motion patterns of the shoulder blade (scapular dyskinesis) can result in considerable dysfunction. We will discuss this in more detail in part two of "Shoulder Injuries ".
5. The Subacromial (Suprahumeral) Joint
The subacromial joint is a ‘physiological joint’. The subacromial joint is formed by an articulation of the coracoacromial ligament and the head of the humerus.
The joint is created by the space between the humerus and the acromion process of the scapula.
The subacromial joint is occupied primarily by the subacromial bursa and the tendon of the supraspinatus.
Subacromial Impingement Syndrome (SIS) presents as anterior-lateral shoulder pain when the patients arm is elevated. This syndrome is classified as a symptomatic irritation of the subacromial structures between the coraco-acromial arch and the humeral head during elevation of the arm.
Want to learn more on shoulder joint anatomy and bio-mechanics
Check out our shoulder anatomy video. In this video we cover the bones (osseous structures) of the Shoulder Girdle.
CONCLUSION - PART 1
As you can see with its five joints, the shoulder girdle is a complicated structure. Dysfunctions within any of the five joints will result in compensations and abnormal motion throughout the entire shoulder complex.
In Part 2 of "Shoulder Injuries" we go over some of the soft tissue structures that connect directly into the shoulder blades (scapulae). We will see how restrictions in any one structure can easily cascade, and cause development of abnormal motion patterns (dyskinesis), impingement syndromes, and a host of other problems.
REFERENCES - PART 1
Burkhart SS, Morgan CD, Kibler WB. The disabled throwing shoulder: spectrum of pathology: Part I: pathoa-natomy and biomechanics. Arthroscopy 2003;19:404-420
Shuenke, Micheal (2010). Thieme Atlas of Anatomy: General Anatomy and Musculoskeletal System. New York: Everbest Printing Ltd. ISBN 978-1-60406-286-1.
Levangie PK, Norkin CC. Joint Structure and Function : A Comprehensive Analysis. 4th ed. India: JAYPEE; 2006.
Levangie, P.K. and Norkin, C.C. (2005). Joint structure and function: A comprehensive analysis (4th ed.). Philadelphia: The F.A. Davis Company.
Stecco, Carla; Stecco, Carla. Functional Atlas of the Human Fasical System. Elsevier Health Sciences.
DR. BRIAN ABELSON DC.
Dr. Abelson believes in running an Evidence Based Practice (EBP). EBP's strive to adhere to the best research evidence available, while combining their clinical expertise with the specific values of each patient.
Dr. Abelson is the developer of Motion Specific Release (MSR) Treatment Systems. His clinical practice in is located in Calgary, Alberta (Kinetic Health). He has recently authored his 10th publication which will be available later this year.
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