Shoulder impingement describes the painful condition caused by decreased space between the acromion process of the scapula and the humeral head. For years, shoulder impingement syndrome has been a stand-alone diagnosis when, in reality, it is a symptom of something else. The subacromial space does not shrink on its own; something has to cause narrowing. Finding the root cause of impingement is vital to developing an appropriate treatment plan.
We often hear the “golf ball on a tee” analogy used to describe the relationship of the humerus to the glenoid fossa of the scapula in the shoulder. The position of the humeral head is maintained by the rotator cuff. There is an additional layer of movement protection afforded by the “roof” of the joint created by the acromion, coracoacromial ligament, and the coracoid process (also of the scapula). The space between the roof and the humeral head is the subacromial space. The subacromial space is home to the subacromial bursa and rotator cuff tendons. The insertion of the long head of the biceps tendon is close to this space, but not technically in it. In subacromial impingement syndrome, raising the arm compresses the structures in the space between the humeral head and the acromion.
Shoulder anatomy and biomechanics are complex at best. Elevation is created by the interaction of the rotator cuff and scapular stabilizer muscles as they work to keep the bony anatomy from colliding. The rotator cuff is well-known for its role in throwing, but arguably, its more important job is to maintain the position of the humeral head on the glenoid as it moves beneath the acromion.
Though there can be some variation in the ratio, it is widely accepted that there is about a 2-to-1 relationship between the humerus and scapula during elevation; this is known as scapulohumeral rhythm. Rather than focusing on the numbers, it may be more important to simply recognize that the scapula is quiet during the initiation of elevation, moves in coordination with the humerus through the mid-range of motion, and then becomes relatively stable again at angles above 90 degrees.
On the posterior side, the scapular stabilizers work in force couples to control movement. The upper trapezius, lower trapezius, and serratus anterior are responsible for upward rotation while the rhomboids, levator scapulae, and pectoralis minor control downward rotation.
With the proper scapular position comes maximal acromiohumeral distance, which is the measurement used to assess the subacromial space. This space is further calculated by the ocupation ratio percentage which considers the thickness of the supraspinatus tendon or subacromial bursa. Larger acromiohumeral distance and smaller occupation ratio mean more free space during movement and less likelihood of impingement occurring.
Types of shoulder impingement
Traditionally, subacromial impingement has been used to describe mechanical encroachment of the structures that live in the subacromial space. Mechanical encroachment, or the invasion of the subacromial space by the normal shoulder anatomy, occurs during the mid-range of motion and may create a “painful arc.”
Subacromial, or external impingement, is the most common type. This is where something external is causing the problem. This condition is also known as swimmer’s shoulder because of the constant repetition swimming entails. Subacromial impingement is further divided into primary and secondary categories.
Primary external impingement occurs when there are structural changes that decrease the subacromial space. Osteophyte formation, variants in acromion type, or an increase in the size of soft tissues can all narrow the space. Mechanical impingement refers to the compression or shear forces to soft tissues. In these cases, it is either a bone or ligament that is impinging on another soft tissue and creating pathological changes.
Secondary external impingement, describes a condition where function is affected by muscle imbalance or tissue tightness that leads to alterations in the position of the humeral head. In these cases, the humeral head moves upward, which decreases the space and compresses the soft tissues.
People with primary external impingement may not improve when mobility, strength, or proprioception are addressed because the fundamental cause of the problem is unchanged. That said, working on surrounding tissues can be an effective management strategy.
In a secondary external impingement, dysfunctional movement is the root cause of tissue compression. Repetitive activity/overuse, posture, imbalances in strength or flexibility, or poor proprioception are all possible causes. Research supports the idea that changes in movement patterns occur as a result of pain, rotator cuff dysfunction, or cause of dysfunction themselves. Movement changes may include a high riding humeral head due to rotator cuff weakness or poor scapular motion when tissue mobility, flexibility, or strength are problematic. Impingement caused by movement dysfunctions is often improved by rehabilitation that includes stretching and strengthening with a goal of restoring normal motion.
Internal (posterior) impingement, refers to a specific condition where the undersurface of the rotator cuff tendons are trapped between their attachment on the humerus and the posterior edge of the glenoid. Entrapment occurs when the arm is in 90 degrees of shoulder abduction and external rotation such that the greater tuberosity of the humerus butts up to the scapula.
Shoulder impingement causes
Shoulder impingement can be caused by several factors. Contributing factors include the quality of the soft tissues, age, flexibility, anatomical or osseous variations, joint mechanics, and posture. The typical patient with subacromial impingement syndrome is over 40-years-old and reports pain without any known trauma. These patients may recall a shoulder injury following a repetitive motion incident such as gardening with some probing questions. A “painful arc” is generally present when elevating their arm between 70 and 120 degrees, and lying on the involved side may be prohibited by pain.
Rotator cuff pathology can be implicated in primary or secondary impingement. Swelling of the rotator cuff tendons may cause narrowing of the subacromial space in primary impingement. The rotator cuff can be implicated in secondary impingement if weakness interrupts the ability of the muscles to depress the humeral head during arm elevation to steer clear of the acromion. Disruption to the scapulohumeral rhythm may link scapular dyskinesis to secondary impingement. If the scapula does not upwardly rotate, posteriorly tilt, and externally rotate as a result of nerve entrapment, weakness, pain, or another cause, the scapula may not move out of the way of the humeral head during arm elevation.
Being diagnosed with shoulder impingement does not always provide the clarity patients are looking for. This can be a “catch-all’ diagnosis that fails to implicate a specific structure which is sometimes frustrating for patients. In my professional experience, this frustration can be mitigated by patient education. Relaying the complexities of the anatomy and biomechanics of the shoulder is often useful for helping patients understand why their pain may not be assigned to a single structure.
As with all other musculoskeletal conditions, the crux of the diagnosis of impingement is a thorough history and physical examination. Patients may present with posterior shoulder stiffness or instability. Special attention should be paid to the scapulohumeral rhythm to identify scapular dyskinesia, or abnormal movement. Hypermobility or instability of the glenohumeral joint should also be noted.
The most widely used shoulder impingement tests are the Neer, Jobe, and Hawkins-Kennedy tests. All of these tests are useful for concluding impingement is present but none are particularly good at isolating what structure is being impinged. Pain with both the Jobe and Hawkins tests is indicative of subacromial impingement only; if internal impingement is present the tests will be negative. The Neer test can identify the presence of subacromial or internal impingement depending on the location of the pain; anterior pain indicates subacromial impingement while posterior pain indicates internal impingement.
It is worth noting that the Jobe test, often referred to as the “empty can” test, can be paired with the “full-can” test to determine involvement of the rotator cuff. If the patient experiences pain in both test positions, rotator cuff pathology is present and likely contributing to impingement. If pain is only present in the empty can position, the test is positive for impingement. If pain is only present in the full can position, the test is positive for rotator cuff pathology. Taken together, if both are positive the impingement is likely being caused by rotator cuff pathology.
Scapular dyskinesia is a catch all term for aberrant movement at the scapula. Newer special tests, such as the scapular assistance test, can help differentiate patients with pain caused by scapular dysfunction from those with other causes of impingement. It may be clinically useful to use the scapular assistance test to rule out scapular dyskinesia as a source of pain when impingement is suspected.
On examination, a scapula that rests in more upward rotation is likely to be the result of a rotator cuff injury while a scapular that rests in downward rotation is likely to be the cause of dysfunction. In either case, physical therapy can help. Although most patients with impingement do not need imaging, it can provide insight into the source of the problem. Radiographs are used to examine the involved anatomical structures including the coraco-acromial arch, the acromioclavicular joint, the position of the humeral head, the greater tuberosity of the humerus, and normal variants such as the shape of the acromion. Arthritic changes can be visualized on radiographic imaging.
Radiographs are also useful for measuring the acromiohumeral distance. In most adults, this distance is 10 millimeters in men and 9.5 millimeters in women. Decreased acromiohumeral distance with no other specific anomaly visualized on imaging is indicative of rotator cuff pathology.
Though most injuries make activities of daily living challenging, the hardships associated with shoulder pain may be the worst. A combination of flexion, abduction and external rotation is necessary to reach overhead to style hair, pull a shirt on or off, and retrieve objects from high shelves or cabinets. Extension, adduction, and internal rotation combine to allow reaching behind the back to fasten a bra, tuck in a shirt, or remove a wallet from a back pocket.
The research agrees with common sense when it comes to shoulder pain and disability. A 2009 cross-sectional study by Lentz and colleagues investigated the relationship of pain, physical impairments, and pain-related fear to function. Range of motion measurements and outcome measures were collected on 142 patients with shoulder-related diagnoses. They reported range of motion and pain were correlated with higher levels of disability.
More recently, Anwer et al. investigated the relationship between pain, motion, and disability in patients with shoulder pathology. Sixty-four patients with a variety of diagnoses reported pain scores, functional ability, and range of motion measurements. In this cohort, flexion, abduction, and rotation range of motion were most associated with pain and disability.
Similar conditions to shoulder impingement
The differential diagnosis of shoulder impingement is complicated in that there are several underlying pathologies that may be related to impingement. Some of the pathologies that need to be ruled out include frozen shoulder (adhesive capsulitis), rotator cuff tears/tendinopathy, scapular dyskinesia, instability, biceps tendonitis/osis, labral tears, and glenohumeral internal rotation deficit.
The signs and symptoms of most shoulder conditions are quite similar. Just to muddy things up a little more, pain may be caused by more than one structure or pathology at a time.
Patients with shoulder conditions may report the following regardless of diagnosis:
Pain with overhead motion
Pain when reaching out to the side
Inability to lie or sleep on the involved side
Tenderness at the front of the shoulder or mid-humeral region
Aching at night
Pain when reaching behind the back
Weakness or stiffness of the involved side
As a result of similarities in injury presentation it is possible that depending who is doing the diagnosing, a patient could be diagnosed with: shoulder impingement, subacromial bursitis, subacromial impingement, internal impingement, shoulder tendinopathy, biceps tendonitis, rotator cuff tendonitis, or rotator cuff tear.
Shoulder impingement treatments
When it comes to treatment, there is strong evidence and anecdotal support for delaying surgery in favor of conservative management. Despite inadequate research support, subacromial decompression (acromioplasty) surgery remains the most common arthroscopic procedure at the shoulder. A period of three to six months of conservative management should be employed prior to even considering surgery. The patient should be educated on activity modification that includes avoiding overhead movement, heavy mechanical loading, and repetitive motion.
A 2017 systematic review and meta-analysis in the British Journal of Sports Medicine concluded that exercise should be used in the treatment of this condition and that non-steroidal anti-inflammatories and corticosteroids were better than placebo for controlling pain.
Shoulder impingement exercises
Exercise selection for patients with shoulder impingement typically involves targeting the rotator cuff and scapular stabilizers. Choosing exercises that strengthen the posterior muscles reciprocal inhibition of the pectoral muscles which may have the added benefit of increasing postural awareness and decreasing rounded shoulders. Most of these patients also benefit from improved thoracic rotation to decrease the stress through their shoulder joints.
Several studies have examined exercise intervention. A 2017 systematic review and meta-analysis investigated the use of specific vs. general exercise in the treatment of subacromial impingement syndrome. Six randomized controlled trials were included in the review, and all the included studies used resistive exercises targeting proprioception, rotator cuff and/or scapular muscle strength, or stretching. The results suggest there is no evidential support for the use of specific vs. general exercises for targeting pain, function, range of motion, or strength.
Though exercise has been shown effective in the treatment of shoulder pathology, appropriate mode and/or dosage parameters have not been established. Heron et al. sought to discover what type of progressive loading was most effective. A panel of experienced therapists chose three loading strategies after an extensive literature review. They chose a minimally loaded range of motion exercises, open kinetic chain loading, and closed kinetic chain loading. All three groups improved with regard to pain and functional ability but no group was better than another. This may suggest that simply doing something to load the rotator cuff is the most important guideline to follow and that repeated activation of these muscles alone may have the ability to decrease pain.
Shoulder impingement and massage therapy
The length-tension relationship of muscles is of utmost importance at the shoulder joint. This is one of the reasons we emphasize postural awareness in patients with shoulder pathology. The scapula alone has 17 muscular attachments which should make it easy to understand why proper positioning is critical to healthy shoulder function. A primary focus of tissue work should be ensuring the scapula is able to freely move throughout full shoulder range of motion. One of the easiest ways to examine muscle length in the shoulder, and thus direct your treatment, is back-to-wall scaption. The patient stands with their feet six to eight inches from the wall and the low back pressed into the wall. They are instructed to raise their arms overhead until they reach the wall behind them or their low back can no longer stay pressed into the wall.
They may also demonstrate forward head posture near the end range of available motion. If they are unable to reach the wall, this indicates tightness in the rhomboids, pectorals, or latissimus dorsi muscles. This position can be used for post-treatment assessment to determine the effectiveness of the intervention. You may also suggest moving in and out of this position repeatedly as a supportive exercise.
Patients with shoulder impingement tend to have tight external rotators (limited internal rotation motion) and benefit from body work that addresses the rotator cuff muscles. When working in the axilla to target subscapularis, it is important to educate the patient that discomfort may be normal and that they should communicate to you anything that is intolerable.
Shoulder impingement, one thought to stand alone, is now regarded as a multi-factorial diagnosis. It is important to identify the cause of the impingement to ensure your treatment plan is targeting the appropriate tissues and leading the patient to full, pain-free use of their arm and shoulder.