What is Normal Shoulder Anatomy?

The shoulder is one of the most complex joints in the body. Normal shoulder anatomy is made up of muscles, tendons, ligaments, cartilage, and bone that help facilitate the widest range of motion of any joint in the body. The shoulder blade (scapula) located posteriorly meets the collar bone (clavicle) located anteriorly to form the acromioclavicular joint (AC joint). The upper arm bone (humerus) meets the socket (glenoid) portion of the scapula to form the glenohumeral joint. Muscles, tendons, and ligaments work together to provide stability and provide shoulder range of motion and shoulder strength.

What Bones Make Up the Shoulder Joint?

The skeletal structure of the shoulder joint includes three main bones:

  • Scapula: Connects the arm to the trunk. The scapula, also known as the shoulder blade, is located on the upper back. It is a triangular shaped flat bone. The shoulder socket (glenoid) is a part of the scapula. The upper portion of the arm bone (head of the humerus) sits in the glenoid. The acromial process, which is an extension off of the upper part of the scapula, joins the outer end of the calvicle to form the AC joint.
  • Clavicle: Commonly known as the collarbone, the clavicle is a long, curved bone that spans from its attachment to the chest bone (sternum) out to the acromial process of the scapula, where the two make up the AC joint. A critical function of the clavicle is to help provide bony stability to the shoulder joint. Multiple muscles also connect onto the clavicle.
  • Humerus: The large upper arm bone has a round head at its upper portion which sits in the socket (glenoid) of the scapula. The glenoid is a very shallow socket. Because of this, the glenohumeral joint allows a very full shoulder range of motion. The shoulder joint is also very dependent on ligaments to hold the humeral head centered in the glenoid and thus provide stability to the joint.

What Muscles Make Up the Shoulder?

The rotator cuff, is made up of 4 muscles and the tendons attached to these muscles. These tendons attach to the top of the humerus as a “cuff” of tendon tissue that envelopes the humeral head. These rotator cuff muscles exert forces on the humeral head, allowing shoulder range of motion. The rotator cuff muscles also compress the humeral head into the glenoid, thus providing stability to the shoulder (glenohumeral) joint. The rotator cuff muscles consist of:

  • Supraspinatus
  • Infraspinatus
  • Subscapularis
  • Teres Minor

The deltoid muscle is a large muscle covering the front, back and side of the shoulder which also works in conjunction with the rotator cuff to move the shoulder.

Diagnostic Imaging of Shoulder Anatomy

Diagnostic imaging is useful when evaluating injuries to the shoulder. Typically, an x-ray is first obtained to confirm that there is no fracture of the shoulder bones. Proper alignment of the shoulder joint and AC joint can also be assessed. Xrays also show if there is narrowing of the joint space between the ball and socket as well as the presence of bone spurs, consistent with shoulder arthritis.

A Magnetic Resonance Imaging (MRI) shows the soft tissues of the shoulder, unlike an Xray. An MRI shows in high definition the tendons, muscles, cartilage, and ligaments of the shoulder as well as the bones. Rotator cuff tears, labral tears, impingement, and bicep tendon tears are all seen on an MRI.

Can you have shoulder pain without bruising or swelling?

While some sources of shoulder pain, such as a fractures or bicep tendon tears, may result in bruising and swelling, other shoulder conditions do not result in this. Common symptoms associated with shoulder conditions are:

  • Sharp or dull pain
  • Referred pain down the side or front of the arm
  • Pain at night
  • Decreased shoulder range of motion
  • Loss of shoulder strength

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What are some Common Shoulder Conditions?

The most common causes of shoulder pain include:

  • Subacromial Impingement, Subacromial Bursitis, and Rotator Cuff Tendonitis: These conditions are often a result of excessive, repetitive overhead activities, which generate inflammation of the rotator cuff tendons and the bursa, which overlies these tendons.
  • Rotator Cuff Tendon Tears: Both acute shoulder injuries, as well as normal wear and tear, cause rotator cuff tendon tears in the shoulder. These tendon tears may be complete, where the tendon has completely detached from its normal insertion point on the bone, or partial, where some portion of the tendon remains attached to the bone.
  • Shoulder Instability: This condition usually results from an acute injury. With a shoulder dislocation, the head of the humerus is suddenly completely forced out of the socket and the stabilizing ligaments of the shoulder joint tear. With a shoulder subluxation, the head of the humerus does not completely dislocate but comes part way out of the socket, with the stabilizing ligaments being stretched but not fully tearing. Both conditions may be just a one time event without further recurrences. In other cases, patients may suffer from chronic shoulder instability events that interfere with sports and even activities of daily living. Chronic shoulder instability can also lead to premature arthritis in the shoulder.

Dr. Cunningham is a shoulder specialist at Vail Summit Orthopaedics. He is an expert at diagnosing and treating a variety of shoulder conditions for patients in the Vail, Summit County, Aspen, and Denver, CO areas.

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Dr. Cunningham specializes in the treatment of knee, shoulder, and sports injuries.

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