What Is Shoulder Impingement Syndrome?

Introduction

Shoulder impingement is a condition of the shoulder involving compression and wearing of the rotator cuff muscles by bony structures. Repeated pinching and rubbing can cause inflammation, irritation, and damage to the muscle and tendon, resulting in pain and loss of strength.

The shoulder complex consists of several joints and soft tissues that work together to provide an extensive range of movement of the upper limb. Due to the complexity of the joint, the shoulder is prone to injury.  Impingement is the most common shoulder condition.¹

Relevant shoulder anatomy

The shoulder complex, consisting of four separate joints, connects the upper limb to the torso. The glenohumeral joint is the main joint, providing the majority of the movement of the arm. A ball-and-socket shape, this joint involves the ‘ball’ at the top of the humerus (the head of the humerus) and the ‘socket’ of the scapula (the glenoid fossa).

There are numerous soft tissues involved in the shoulder complex. These include muscles, tendons, ligaments, joint capsules (layers of connective tissue that surround joints), blood vessels, nerves, the glenohumeral joint labrum (a rim of fibrocartilage around the socket of the joint, serving to deepen it and increase its stability) and bursae (small sacs containing lubricating fluid which reduce friction between neighbouring structures). 

Several muscles play a role in the shoulder complex. Particularly relevant to impingement at this joint are the four muscles of the rotator cuff, which help to move the glenohumeral joint but also work together to create a stabilising effect, keeping the ball optimally centred in the socket during movement of the arm.² This is particularly important because the joint has a large ‘ball’ relative to a smaller, shallow ‘socket’, which allows for considerable movement but provides little stability.

Due to the layout of the shoulder complex, several of these soft tissues are located in a relatively narrow space between inflexible boundaries formed by bones, joints and ligaments.³ These structures are susceptible to impingement. 

What is shoulder impingement syndrome?

As the head of the humerus and the coracoacromial arch move in relation to each other during movement of the upper arm, the soft tissues that are located between them in the subacromial space are susceptible to compression and rubbing. This mechanical pressure is known as impingement. It can lead to these structures becoming irritated, inflamed and ultimately damaged. 

The soft tissue structures that are most commonly impinged include the subacromial bursa and the tendons of the four rotator cuff muscles, in particular, the tendon of the supraspinatus muscle.²

Stages of tissue damage due to impingement

Soft tissues which are repeatedly compressed can become irritated and inflamed. Inflammation is the body’s reaction to heal cell damage. 

If microtrauma and inflammation persist and become chronic, permanent changes to the structure and function of these tissues may occur, which can weaken them. This can result in partial or complete tears of tendons and muscles of the rotator cuff muscles. It can also lead to the development of extra bumps of bone growth called osteophytes or bone spurs on the acromion, which further narrows  the subacromial space.²

Types of shoulder impingement

Shoulder impingement syndrome is a collective term for several different pathologies, all of which have a similar mechanism of injury. Different structures can be involved and to different degrees of severity. There are several potential causes which may be present in isolation or collectively. 

Due to these variables, there are several different ways of classifying and categorising this condition. Further information on these can be found here.

Shoulder impingement signs and symptoms

Shoulder impingement syndrome typically presents with the following symptoms:

• Pain:
  • Gradual, insidious onset with no known trauma¹
  • Often brought on by raising and lowering the arm between 70 and 120 degrees (known as the ‘painful arc’), with forced movement of the arm above the head and by lying on the affected side⁴
  • Often experienced at night.⁵
• Felt at the tip and outside edge of the shoulder, radiating down the outside of the upper arm¹
• Reduction in shoulder muscle strength.⁵

Causes of shoulder impingement syndrome

Impingement can be due to several factors. These include a narrowing of the space available between the surfaces, pathology of the soft tissues within this space and abnormal biomechanics of the shoulder complex. These factors often are present in combination, and there is considerable overlap between them. 

Narrowing of the space available for soft tissues between bony surfaces increases the likelihood of impingement. This can be due to structural abnormalities of the bones or joints such as a ‘hooked’ acromion, the presence of osteophytes⁴ or structural changes resulting from trauma such as a fracture.²

Soft tissues of the shoulder complex, which become irritated and inflamed, may swell and expand in size due to the increased blood supply to the area. This leaves them susceptible to further impingement, with the potential to perpetuate the problem.⁶

Abnormal biomechanics

Shoulder biomechanics are complex and finely balanced, reliant on precise patterns of movement and synchrony of several components. Even small changes can result in disruption and dysfunction.

A key biomechanical factor in the prevention of impingement is the maintenance of the ball of the glenohumeral joint centrally in the socket throughout movements of the arm. This ensures that the space between the two surfaces is maintained and the soft tissue structures between them are not encroached upon. 

This optimal positioning relies on several factors. Most significantly, the stabilising rotator cuff muscles and the joint capsule. The likelihood of impingement can, therefore, be increased by weakness of the rotator cuff muscles and by tightness or laxity (looseness) of the joint capsule.

Other potential contributory factors include abnormal scapula movement patterns and certain postures affecting the spine and shoulder complex.²

Risk factors for shoulder impingement syndrome

Repetitive overhead movements increase the risk of developing this condition. Such actions are common in certain sports (such as swimming and volleyball) and occupations (such as painting and hairdressing). 

The likelihood of developing impingement syndrome increases with age, with a greater risk above the age of 40.⁴

Diagnosis of shoulder impingement syndrome

Thorough evaluation through the diagnostic process is essential to gain a clear understanding of which structures are involved and which factors have contributed to development in each case. This information will inform the treatment plan.

Diagnostic techniques include:

• Information from clinical history taking - with reporting of typical symptoms (as listed above)
• Physical examination - including observation, palpation, strength testing, range of movement testing, evaluation of biomechanics, use of clinical impingement tests
• Imaging techniques:
  • Ultrasound - to identify bursitis (inflamed bursa) and abnormalities of tendons
  • X-rays - to identify abnormalities in bony structures and the space between them 
  • Magnetic Resonance Imaging (MRI) - to provide greater detail to identify anomalies of soft tissues⁴

Treatment of shoulder impingement syndrome

Treatment for shoulder impingement aims to reduce symptoms and restore full function of the shoulder complex, including optimal range of movement, strength and biomechanics. The specific treatment plan in each case will address causative factors identified through the diagnostic process.²

Conservative (non-surgical) treatment of shoulder impingement

Conservative treatment is effective in 70-90% of shoulder impingement cases and is usually the initial choice.⁶ Options include

• Immobilisation
• Non-steroidal anti-inflammatory drugs (NSAIDs) 
• Cortisone injections 
• Acupuncture
• Therapeutic taping
• Ultrasound and other electrotherapy techniques
• Application of heat and/or cold 
• Exercise therapy
• Manual therapy⁴

Physiotherapy for shoulder impingement syndrome

Exercise therapy is effective in the treatment of this condition,⁵ and this is often delivered by physiotherapists. Programmes are usually progressive, involving several stages, each with different goals.²

Surgery for shoulder impingement

Surgical interventions may be recommended when conservative methods do not resolve the problem or in cases where the duration or severity of symptoms or the degree of structural damage and functional impairment is significant.⁴

Surgical procedures may be used to remove small areas of bone to give soft tissues more space, as well as to remove the bursa if inflamed. Rotator cuff tears can also be treated surgically. 

These procedures may be open or arthroscopic (keyhole). The specific procedure used will depend on the individual case.⁴ A programme of rehabilitation after shoulder impingement surgery is recommended.

Prevention of shoulder impingement syndrome

Maintaining good posture, rotator cuff strength, and joint flexibility can all help to prevent impingement at the shoulder. Orthopaedic surgeon and shoulder specialist Dr Scott Jacobson demonstrates a series of exercises for shoulder impingement prevention in this video: https://www.youtube.com/watch?v=ZugRJXmW-ik

FAQs

How common is shoulder impingement syndrome?

Shoulder impingement syndrome is the most common type of shoulder condition.¹ It is most frequently experienced by people over the age of 40.⁴

When should I seek medical care for shoulder impingement syndrome?

Symptoms of impingement include shoulder pain when raising the arm, particularly in the midsection of this movement, the ‘painful arc’ between 70 and 120 degrees, as well as when lying on the affected side.⁴ Shoulder impingement can occur without any known trauma. If you are experiencing these symptoms, consult a healthcare professional for advice. 

Summary

Shoulder impingement syndrome is a condition involving compression and rubbing  of soft tissues of the shoulder. The structures most frequently involved are the bursa in the subacromial space and the rotator cuff tendons, in particular, the tendon of the supraspinatus muscle.

There are several potential contributory factors to this condition, which often occur without any known trauma. These include structural anomalies and altered biomechanics. Risk factors for shoulder impingement include sports and occupations which involve repetitive overhead movements.

Treatment aims to reduce symptoms and restore the full function of the shoulder complex, including addressing relevant causative factors. Conservative (non-surgical) treatment is usually the initial choice.