Shoulder Impingement Syndrome, a Common Affliction of the Shoulder: A Comprehensive Review

Abstract

The shoulder joint is the most mobile joint in the human body. It comprises a ball and socket structure that enables a series of functional movements to be carried out. These range from simple movements such as reaching for the top shelf to highly explosive overhead activities which involve throwing a baseball or serving with a tennis racquet as seen in athletes. Shoulder impingement syndrome (SIS) is one of the most common shoulder disorders seen in general practice. It is caused by a multitude of factors. Pathology can arise either from the rotator cuff tendon itself, structures external to this tendon or both in combination. The resultant shoulder pain with SIS not only causes distress but also limits the quality of life of patients. This article aims to provide a comprehensive review of the pathogenesis, aetiologies, clinical tests, investigations and management options for SIS based on current literature and research.

Full text

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REVIEW
Proceedings of Singapore Healthcare  Volume 23  Number 4  2014
Shoulder Impingement Syndrome, A Common Affliction of the Shoulder:
A Comprehensive Review
Justin De Yang Tien, MBChB, Andrew Hwee Chye Tan, MBBS, FRCS
Department of Orthopaedic Surgery, Singapore General Hospital, Singapore
ABSTRACT
The shoulder joint is the most mobile joint in the human body. It comprises a ball and socket structure that
enables a series of functional movements to be carried out. These range from simple movements such as reaching
for the top shelf to highly explosive overhead activities which involve throwing a baseball or serving with a tennis
racquet as seen in athletes. Shoulder impingement syndrome (SIS) is one of the most common shoulder disorders
seen in general practice. It is caused by a multitude of factors. Pathology can arise either from the rotator cuff
tendon itself, structures external to this tendon or both in combination. The resultant shoulder pain with SIS not
only causes distress but also limits the quality of life of patients. This article aims to provide a comprehensive
review of the pathogenesis, aetiologies, clinical tests, investigations and management options for SIS based on
current literature and research.
Keywords: Shoulder impingement syndrome, Subacromial impingement syndrome, Rotator cuff tedinopathy,
Rotator cuff tendinitis, Shoulder pain
INTRODUCTION
Shoulder pain is a disabling symptom frequently
encountered in primary care. The estimated
prevalence of shoulder complaints is 7–34%1
with about 14.7 new cases per 1000 patients per
year seen in clinics2. Of all the shoulder disorde rs,
shoulder impingement syndrome (SIS) is the most
commonly reported, accounting for 44–65% of all
shoulder pain complaints2. In simple terms, SIS can
be defined as a collection of shoulder symptoms
and signs caused by pathology within the rotator
cuff tendon itself (intrinsic) or structures external to
it (extrinsic), causing impingement in the narrowed
space between the acromion and humeral head3.
At times, both intrinsic and extrinsic pathologies
may occur in combination.
The multi-factorial aetiology of SIS makes it difficult
to clinically localise the lesion and formulate an
effective treatment plan. Given the high prevalence
and diagnostic challenges involved in this syndrome,
the aim of this review is to evaluate the pathogenesis,
aetiologies, diagnostic accuracy of clinical tests,
imaging modalities and various conservative and
surgical treatment options relating to SIS.
PATHOGENESIS
In 1972, Neer described three stages of the extrinsic
impingement process4. Stage-I impingement is
characterised by the oedema and haemorrhage
of the subacromial bursa and rotator cuff which
is usually seen in patients less than 25 years old.
Stage-II impingement demonstrates irreversible
changes such as fibrosis and tendinitis of the
rotator cuff seen in those aged 25–40 years old.
Stage-III impingement is characterized by more
chronic changes such as partial or complete-
thickness rotator cuff tears, usually seen in patients
aged more than 40 years old. From this three-
stage impingement classification, it is evident that
SIS is associated with rotator cuff tendinitis and
subacromial bursitis. However, the issue of tendon/
bursal inflammation is not without controversy
as a casual relationship between the extrinsic
impingement mechanism and the resultant
inflammation has not yet been firmly established.

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AETIOLOGY
According to Neer, the subacromial space is defined
inferiorly by the humeral head and superiorly by
the coracoacromial arch which comprises three
structures: the under surface of the anterior third
of the acromion, coracoacromial liagament (CAL)
and coracoid process5. The acromioclavicular joint
(ACJ) is situated superior and posterior to the
CAL5 (Figs. 1 and 2). As defined earlier, SIS can be
caused by intrinsic (intratendinous) or extrinsic
(extratendinous) factors or both3.
Intrinsic Factors
The “intrinsic impingement” theory postulates that
degenerative changes due to age, overuse, trauma
or tension overload result in partial or full thickness
tears of the rotator cuff tendon, thereby causing SIS.6
Two prospective studies7,8 have demonstrated a
statistically significant increase in prevalence of
full-thickness rotator cuff tears with increasing
age. Yamaguchi et al. also found that the average
size of a symptomatic tear was 30% greater than
an asymptomatic tear8. As the tear size was shown
to be a factor in symptomatic development,
they recommended yearly interval monitoring
for tear size progression in patients undergoing
non-operative treatment for symptomatic or
asymptomatic full thickness rotator cuff tears.
Acromion Coracoacromial ligament
Coracoacromial arch
Coracoid process
Subacromial-subdeltoid
bursa
Fig. 2. The structures comprising the coracoacromial arch.
Clavicle
Acromion
Scapula
Glenoid fossa
Humeral head
Greater tuberosity
Humerus
}Subacromial
space
Bursa
Fig. 1. The structures associated with the subacromial space.
Supraspinatus

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In sports that require overhead motion such as
swimming, throwing or racquet sports, athletes
tend to overuse their rotator cuff tendons, causing
inflammation and thickening to the tendons
(tendinitis) and subacromial bursa (bursitis), this
contributes to the impingement process. This
overuse phenomenon can also be seen in jobs that
require frequent heavy lifting of goods.
Extrinsic Factors
Neer’s theory of “extrinsic impingement” described
the mechanical compression of the supraspinatus
tendon by the inferior under surface of the anterior
third of the acromion, the CAL and ACJ5. This can
occur as a result of anatomical factors, abnormal
rotator cuff and scapular musculature or poor body
posture6. Other causes include an os acromiale
(unfused distal acromial epiphysis) and posterior
capsule tightness6.
Anatomical factors include acromial morphology
variations, and degenerative changes at the inferior
surface of the acromion, ACJ or CAL. Bigliani et al.
described three distinct acromial morphologies:
Type I (flat), Type II (curved) and Type III (hooked)
acromion9 (Fig. 3). He argued that due to the shape
and resultant damage, the Type II and Type III
acromions had a greater predisposition to a rotator
cuff tear and hence SIS9. This classification has
been widely criticised due to poor inter-observer
reliability10. More recent studies11,12 have not
shown a significant association between acromial
morphology and rotator cuff pathology. Gill et al.
found a significant correlation between age and
rotator cuff pathology and argued that the Type III
acromion was more likely a result of a degenerative
process (acquired) rather than a morphological
variation (congenital)12. According to Neer, arthritic
changes to the ACJ can occur with age, causing
joint space narrowing and osteophyes to form at
the distal clavicle and acromion articulation5. The
CAL can also cause stress-induced acromial spurs
on the undersurface to form due to the higher
tension on the acromial insertion of the CAL as
compared to the coracoid side13. A thickened CAL
caused by repeated strain overtime can also cause
narrowing of the subacromial space.
Weak or dysfunctional rotator cuff musculature can
cause superior translation of the humeral head14,
thereby narrowing the subacromial space, resulting
in SIS. Scapular musculature serves to stabilise and
rotate the scapula during movements. During
overhead arm movements, weak scapular muscles
fail to elevate the scapula and acromion sufficiently,
causing impingement of the underlying rotator
cuff muscles15.
Furthermore, a slouched posture involves an
increase in thoracic kyphosis, downwardly rotated,
anteriorly tilted and protracted scapula, thereby
decreasing glenohumeral joint flexion, elevation
and abduction range16. In light of this, proper
postural assessment and scapular and rotator cuff
muscle strengthening through physiotherapy may
help improve impingement symptoms.
With two different impingement theories, it is
difficult to ascertain cause and effect. Did the
intrinsic tendon degeneration or the extrinsic
Type I Acromion Type II Acromion Type III Acromion
FLAT ACROMION CURVED ACROMION HOOKED ACROMION
Type II and III acromions result in decreased subacromial space.
Both are associated with rotator cuff tears.
Fig. 3. The three different structural types of acromion.

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structures cause the impingement process in the
first place? Confusion regarding the impingement
aetiology can occur due to overlap of factors.
For example, athletes in addition to rotator cuff
tendinitis caused by overuse may also have
underlying glenohumeral instability. By the time
they present with shoulder symptoms, it is clinically
challenging for the physician to ascertain if the
rotator cuff tendinitis was caused by overuse activity
or due to subtle glenohumeral instability. In this
case, glenohumeral instability as the primary cause
should be ruled out first using imaging studies17
before diagnosing overuse.
External vs. Internal Impingement
Impingement syndrome is an umbrella term which
can also be classified into external (outlet) versus
internal (inlet). External impingement also known
as subacromial/shoulder impingement refers to any
pathology or structures encroaching in and hence
narrowing the subacromial space. This forms the
main focus of our article. However, this should not be
confused with a more subtle internal impingement;
which refers to any pathology affecting structures
within the glenohumeral joint space itself. These
structures include the under-surface (articular
side) of the supraspinatus tendon, infraspinatus
tendon and posterior-superior glenoid labrum. In
1992, Walch described the internal impingement
process as a condition caused by repetitive contact
of the posterior aspect of the greater tuberosity
of the humeral head with the posterior-superior
aspect of the glenoid labrum18. As a result, this
impinges and damages both the articular side of
the supraspinatus tendon and the glenoid labrum
in the process18. The classic thrower’s position
typically seen in baseball players; with the arm in
90 degrees abduction and full external rotation
causes this internal impingement18. This clinical
entity is beyond the scope of this article.
CLINICAL EVALUATION AND DIAGNOSIS
History and Physical Examination
Shoulder impingement syndrome has a tendency
to be overdiagnosed as the primary cause of
shoulder pain. A careful and thorough history is
essential in ruling out more subtle or sinister causes.
Patients often complain of pain in the anterolateral
aspect of the shoulder. This site corresponds to
the insertion of the supraspinatus tendon at the
greater tuberosity of the humerus. Pain is made
worse with overhead movements such as reaching
for the top shelf or in athletes involved in throwing,
racquet sports or swimming. The onset of the pain
in SIS can be acute in a recent traumatic shoulder
injury or more chronic if the impingement is due to
osteophytes. Other associated symptoms include
pain when lying on the affected shoulder, weakness
and loss of arm function.
On physical examination, it is important to check
for any signs of shoulder swelling, deformity,
tenderness, muscle wasting or stiffness. Both active
and passive range of shoulder motion should be
tested. Abduction of the arm in the 60–120° angle
range causes the greater tuberosity of the humerus
to impinge on the under surface of the acromion,
causing anterolateral shoulder pain. This is the
painful arc sign which can indicate a minor rotator
cuff injury or a supraspinatus tendinitis.
Special Clinical Tests
Two systematic reviews19,20 found low specificities
each for Hawkins-Kennedy test, Neer’s sign and
empty can test for the diagnosis of SIS and did not
recommend individual tests for pathognomonic
diagnosis of SIS.
Park et al. evaluated eight well known clinical
tests to determine their diagnostic values to help
distinguish between bursitis, partial-thickness
rotator cuff tears and full-thickness rotator cuff
tears (n=913)21. For the diagnosis of SIS, the best
combination of three tests was a positive: Hawkins-
Kennedy test, painful arc sign and weakness in
external rotation with arm at the side (best post-
test probability 95%)21. To diagnose a full-thickness
rotator cuff tear, the best combination of three
positive tests was: painful arc sign, drop arm sign
and weakness in external rotation (best post-
test probability 91%)21. As shown above, using
a combination of tests as opposed to tests in
isolation tends to provide a more reliable result.
This is because the rotator cuff unit is a dynamic
structure where all the muscles and tendons work
synergistically in a compound movement. Rotator
cuff tendons do not function as separate entities22.
In reality, it is difficult to truly isolate one muscle
or tendon using each individual clinical test alone,
although it is possible to provoke more pain at the
specific muscle or tendon tested. Furthermore,
concomitant pathology affecting other adjacent
structures such as the subacromial bursa which
contain nociceptors (innervations from lateral
pectoral nerve, suprascapular nerve) can cause
pain23, thereby affecting the reliability of the tests.

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Imaging Studies
The three main imaging modalities for the shoulder
in SIS are plain film radiographs, ultrasonography
(US) and magnetic resonance imaging (MRI) or
arthrography (MRA).
Routine radiographs involve three standard views.
These include the antero-posterior (AP) view,
scapular Y view and axillary view. AP radiographs
may demonstrate a narrowed subacromial space,
subacromial osteophytes, sclerosis of the acromion
undersurface (“sourcil” sign) and subchondral
sclerosis or cyst formation in the greater tuberosity
of the humerus3,14.
Ultrasonography and MRI are both useful in
excluding partial and full-thickness rotator cuff
tears. This important information not only has
prognostic value but also guides orthopaedic
surgeons in their choice method of surgical repair
(open or arthroscopic). Two prospective studies had
demonstrated that US and MRI had comparable
accuracy for identifying and measuring the size
of partial and full-thickness rotator cuff tears24,25.
Compared to MRI, US tends to be more operator
dependent but is not claustrophobic, is less costly
and more easily accessible as it can be performed
in a clinic or at the bedside. A study comparing
patient satisfaction on both US and MRI showed
that most patients prefer US as an imaging
modality for shoulder pain26. Iannotti et al. had
demonstrated MRI to have great diagnostic value
in distinguishing a normal tendon from one with
tendinitis and impingement signs, sensitivity and
specificity were 93% and 87% respectively27.
However, another study evaluating the MRI results
of 96 asymptomatic individuals revealed a high
prevalence of rotator cuff tears in all age groups28.
The overall prevalence of tears was 34%, of which
15% were full-thickness tears and 20% were partial
thickness tears28. The study also found a significant
correlation of rotator cuff tears with increasing age
in individuals who had normal, painless shoulder
function28. These findings highlight the potential
dangers of diagnosing rotator cuff tears and
dictating surgery on the basis of MRI alone, with-
out complementing the results with the overall
clinical picture.
Magnetic resonance arthrography is useful for
assessing the glenoid labrum and glenohumeral
ligaments for ruling out bankart lesions and
glenohumeral instability respectively. The
contrast (gadolinium) enhancement enables good
visualisation of intra-articular anatomy to detect
any damage. A full-thickness rotator cuff tear is diag-
nosed if the contrast dye leaves the glenohumeral
joint and enters the subacromial space. For the
detection of rotator cuff pathology, MRI is less
sensitive than MRA due to its inability to detect
partial-thickness tears and associated soft-tissue
injuries29. Although minimally invasive, arthroscopy
is currently the only technique which enables
direct visualisation of all the glenohumeral joint
structures. Once a lesion is detected on a magnified
screen, the surgeon can also subsequently choose
to treat it.
MANAGEMENT PLAN
Conservative Methods
Patient Education
Patients should be educated on the potential early
warning signs of impingement through advice and
educational leaflets. Early intervention in the form of
activity modification and adequate rest can prevent
further deterioration of pain, strength and function.
In the early stages of impingement, patients should
generally avoid activities which involve raising the
arm over the head such as reaching, lifting, cleaning,
climbing or other activities which may aggravate
the pain. The main message to convey to patients
is a gradual return of shoulder function within the
limits of pain.
Physiotherapy
Physiotherapy for SIS can involve multiple
interventions ranging from simple advice to
structured rehabilitative exercises, manual joint
mobilisations, acupuncture and electrotherapy30.
The main goals of a structured shoulder exercise
program are to relieve pain, restore joint range of
motion, increase strength, improve proprioception
and promote healing30. A quantitative and
qualitative systematic review and meta-analysis of
16 studies (n=1162) demonstrated a statistically
significant benefit in terms of pain and function with
multiple exercises involving, stretching, scapular
stabilising and rotator cuff strengthening for the
shoulder30. These findings highlight the importance
of a structured and varied exercise program being
incorporated as part of shoulder rehabilitation in
SIS patients.
Oral Non-Steroidal Anti-inflammatories (NSAIDs)
Oral NSAIDs include aspirin, ibuprofen and

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naproxen. Although there are reports indicating
there is little or no inflammation involved in rotator
cuff tedinopathies, many studies31 have shown
that oral NSAIDs seem to improve pain and clinical
outcomes in the short term. This could be due to the
decreased pain which allows the patient to carry
out physiotherapy exercises which strengthen the
rotator cuff muscles. A randomised double-blinded
placebo-controlled trial carried out on 100 patients
with a painful shoulder showed oral naproxen
(P=0.02) was superior to a placebo in the treatment
of painful shoulder32. We recommend a short
course of NSAIDs for two weeks as there are long
term gastrointestinal side effects associated with
its use. If longer term use is required, coverage with
proton-pump inhibitor drugs such as omeprazole
can be instituted.
Corticosteroid VS NSAID Injections
When more conservative measures have failed,
corticosteroid injections together with local
anaesthetic can also provide symptomatic
relief and improved function due to its anti-
inflammatory mechanism. A randomised double-
blinded control trial on 58 patients compared
the subacromial injection of tenoxicam (NSAID)
with methylprednisolone (Corticosteroid) in
SIS patients33. Both groups were injected with
lignocaine. Outcome measures were assessed
using the Constant-Murley Shoulder Score (CMSS),
Disability of Arm, Shoulder and Hand (DASH) and
the Oxford Shoulder Score (OSS). At six weeks post-
injection, CMSS was significantly greater in the
methylprednisolone group than the tenoxicam
group (p=0.003)33. Improvement in DASH score was
also statistically significant at week 2 (p<0.01), week
4 (p<0.01) and week 6 (p<0.02) post injection33.
Oxford Shoulder Score improvement was also
consistently greater in the corticosteroid group but
not statistically significant at week 6 (p=0.055)33.
These findings suggest that corticosteroid
injections are more effective than NSAID injections
in improving shoulder outcomes in the short term.
However, there is a lack of evidence in the study
to suggest the long term effectiveness of both
injection groups. Another more recent randomised
control trial on 32 SIS patients showed that single
ketorolac (NSAID) injections demonstrated better
outcomes than triamcinolone (Corticosteroid)
injections at four weeks follow-up34. Evidence from
both studies may be conflicting but both do indeed
show that NSAIDs and corticosteroids are effective.
Long-term use of corticosteroid injections can cause
immunosuppression, spontaneous tendon rupture,
localised osteoporosis, skin depigmentation around
injection site and should thus be administered with
caution33,34. In patients who are concerned about
the long term side effects of corticosteroids, NSAIDs
may be a safer alternative.
Other Conservative Treatments
Extracorporeal shock-wave therapy for rotator cuff
tendinitis has not shown to be of any additional
benefit when compared to placebo in recent
studies35,36. Similarly, ultrasound therapy comb-
ined with exercise and NSAIDs was of little or no
benefit when compared to placebo in another
study37. More large scale trials are needed to con-
firm the value of ultrasound therapy in improving
pain and functional outcomes. As these treatments
become commercialised in the private healthcare
sector, patients have to approach these modalities
with caution.
Surgical Methods
After a 3- to 6-month trial of failed conservative
measures, surgical intervention is the next
treatment option. To increase the subacromial space
(subacromial decompression), the anterior inferior
third of the acromion is resected (acromioplasty)
together with either a release or removal of the
CAL3. Sometimes, a posterior capsule release may
also be performed. ACJ resection is only done if the
joint is tender or if osteophytes are contributing
to impingement. With the advent of shoulder
arthroscopy, open acromioplasty is gradually
becoming more obsolete.
Arthroscopic Subacromial Decompression
In 1985, Ellman devised this alternative technique.
His study performed a 1–3 year follow up on the
pre and post-operative outcomes of 50 consecutive
arthroscopic subacromial decompression (ASD)
cases38. Eighty per cent of the cases had stage II
impingement without rotator cuff tear and 20%
had full-thickness rotator cuff tears. Patients were
evaluated according to the UCLA shoulder rating
scale which assesses pain, function, range of
motion, strength and patient satisfaction38. The
majority of the cases (88%) were rated satisfactory
and the remainder (12%) unsatisfactory38. Arthro-
scopic subacromial decompression is now the gold
standard as arthroscopy allows direct visualisation
inside the glenohumeral joint to detect other
pathologies and the option to treat on the spot39.

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For ASD to be successful, the diagnosis has to be
a primary mechanical (extrinsic) impingement
process. Arthroscopic subacromial decompression
is usually indicated for young and active patients
with stage II impingement syndrome who are
eager to resume their daily or sporting activities.
Older patients may also undergo ASD if a series
of conservative measures fail to address the
structural impingement. Complications of ASD
include acromial fractures or insufficient acro-
mion or osteophyte removal, necessitating a
revision procedure3.
The operation can be carried out either in the
lateral decubitus position or beach-chair position.
The latter being favoured by surgeons as this not
only reduces the risk of neurological injury3, but
also enables easy manipulation of the humerus
during surgery. In the procedure, three incisions
or portals are created around the shoulder. The
anterior portal is for saline inflow or outflow,
posterior portal for camera viewing and lateral
portal for instrumentation. Alternatively, two
portals can also be created around the shoulder40.
The anterior portal is used as an instrumentation
portal and the posterior portal for the arthroscope40.
The arthroscope is introduced in the glenohumeral
joint which is filled with saline to allow visualisation.
The subacromial space can also be viewed on screen
and any inflamed subacromial bursa removed via a
bursectomy. A high speed burr is deployed to trim
the anterior inferior undersurface of the acromion
to prevent impingement. The idea is not to shorten
the acromion undersurface but to smoothen it by
shaving off any bony spur irregularities. If necessary,
the distal 1cm of the clavicle together with any
surrounding osteophytes can be resected with the
surgical burr. Over time, the space between the
acromion and clavicle fills with scar tissue which
strengthens and stabilises the ACJ.
There has been much clinical debate regarding
the routine release or resection of the CAL. Studies
on cadaveric shoulders with deficient rotator cuffs
showed post-operative proximal migration of the
humeral head after CAL resection41. One clinical
study of 10 patients who had revision surgery
following failed symptomatic relief after ASD
with partial resection of the CAL was observed
to have complete regeneration of the CAL which
exhibited histology indistinguishable from normal
ligament41. The study raised the possibility of the
CAL regeneration as being the cause of ASD failure
but concluded that this was part of the normal
healing process and was unlikely. Nonetheless, the
study did not recommend the routine excision of
the CAL in SIS patients with rotator cuff disruption
as this causes proximal migration of the humeral
head thereby causing shoulder instability. In
addition to an ASD, the rotator cuff tendons can
also be inspected and repaired if necessary.
Surgical versus Conservative Methods
Although there have been reports documenting
the favourable clinical outcomes of surgical
intervention, much of the current literature
evidence fails to show any significant differences
between surgical and conservative treatment
for SIS. A systematic review of four randomised
controlled trials failed to demonstrate any
differences in outcome between both surgical and
conservative treatment groups with SIS42. However,
several studies43 have also documented favourable
clinical outcomes for SIS patients undergoing
surgical intervention who were non-responsive to
conservative treatment. The take-home message is
that surgery should only be considered after a trial
of conservative measures has been fully exhausted.
The mechanism of impingement, the clinical
history and tests, imaging results, responsiveness
to conservative treatment as well as patient’s
wishes should be considered before proceeding
with surgical intervention for SIS.
CONCLUSION
As SIS can present with other shoulder pathologies,
the clinician has to keep an open mind with a list
of differential diagnoses. It is also important to
identify the underlying pathology causing SIS as
this determines the management plan. SIS has a
good prognosis as non-operative and operative
treatments are usually successful in addressing the
impingement mechanism. This is made possible
if early clinical evaluation, prompt diagnosis and
treatment is undertaken.
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