Subacromial impingement syndrome: a musculoskeletal condition or a clinical illusion?

Abstract

Background: Subacromial impingement syndrome is considered by many to be the most common of the musculoskeletal conditions affecting the shoulder. It is based on a hypothesis that acromial irritation leads to external abrasion of the bursa and rotator cuff.
Objectives: The aim of this paper is to review the evidence for the acromial irritation theory and in doing so challenge the rationale for subacromial decompression.
Major findings: There is a body of evidence that suggests there is a lack of concordance regarding (i) the area of tendon pathology and acromial irritation, (ii) the shape of the acromion and symptoms, (iii) the proposal that irritation leads to the development of tendinitis and bursitis, and (iv) imaging changes and symptoms and the development of the condition. In addition, there is no certainty that the benefit derived from the surgery is due to the removal of the acromion as research suggests that a bursectomy in isolation may confer equivalent benefit. It is also possible that the benefit of surgery is due to placebo or simply enforces a sustained period of relative rest which may allow the involved tissues to achieve relative homeostasis. It is possible that pathology originates in the tendon and as such surgery does not address the primary pathoaetiology. This view is strengthened by the findings of studies that have demonstrated no increased clinical benefit from surgery when compared with exercise. Additionally, exercise therapy is associated with a substantially reduced economic burden and less sick leave.
Conclusion: As there is little evidence for an acromial impingement model, a more appropriate name may be ‘subacromial pain syndrome’. Moreover, surgery should only be considered after an appropriate period of appropriately structured rehabilitation.

Full text

Narrative Review
Subacromial impingement syndrome: a
musculoskeletal condition or a clinical
illusion?
Jeremy S Lewis
1,2,3,4
1
Therapy Department, Chelsea and Westminster Hospital,
2
Physiotherapy Department, St George’s Hospital,
3
Musculoskeletal Service, Central London Community Healthcare,
4
St George’s University of London, London,
UK
Background: Subacromial impingement syndrome is considered by many to be the most common of the
musculoskeletal conditions affecting the shoulder. It is based on a hypothesis that acromial irritation leads
to external abrasion of the bursa and rotator cuff.
Objectives: The aim of this paper is to review the evidence for the acromial irritation theory and in doing so
challenge the rationale for subacromial decompression.
Major findings: There is a body of evidence that suggests there is a lack of concordance regarding (i) the
area of tendon pathology and acromial irritation, (ii) the shape of the acromion and symptoms, (iii) the
proposal that irritation leads to the development of tendinitis and bursitis, and (iv) imaging changes and
symptoms and the development of the condition. In addition, there is no certainty that the benefit derived
from the surgery is due to the removal of the acromion as research suggests that a bursectomy in isolation
may confer equivalent benefit. It is also possible that the benefit of surgery is due to placebo or simply
enforces a sustained period of relative rest which may allow the involved tissues to achieve relative
homeostasis. It is possible that pathology originates in the tendon and as such surgery does not address
the primary pathoaetiology. This view is strengthened by the findings of studies that have demonstrated no
increased clinical benefit from surgery when compared with exercise. Additionally, exercise therapy is
associated with a substantially reduced economic burden and less sick leave.
Conclusion: As there is little evidence for an acromial impingement model, a more appropriate name may
be ‘subacromial pain syndrome’. Moreover, surgery should only be considered after an appropriate period
of appropriately structured rehabilitation.
Keywords: Shoulder, Subacromial impingement syndrome, Subacromial bursa, Acromioplasty, Subacromial decompression, Rotator cuff tendinopathy,
Shoulder posture
Background
The shoulder complex has a range of movement that
exceeds any other joint in the body and its main
function is to position the hand to affect functional
activities ranging from the performance of high
powered explosive activities, such as throwing base-
balls, to positioning the hand, often within the field of
vision, to perform highly complex prehensile tasks.
The shoulder is also used to place the hand so that
the upper limb may be used for weight bearing.
Musculoskeletal pathology involving the shoulder is
common, has the potential to adverse ly affect upper
limb function and is associated with substantial
morbidity that increases with age.
1–3
Of the wide
spectrum of musculoskeletal disorders affecting the
shoulder, subacromial impingement syndrome is
considered to be one of the most common.
4,5
This
condition is well recognized clinically, presenting as
antero-lateral shoulder pain experienced when the
arm is elevated.
4–7
Although numerous historic
references to subacromial pathology exist,
8–13
Neer
argued that abrasion by the under surface of the
anterior margin of the acromion onto the soft tissues
located anatomically in the space between the
humeral head and acromion leads to the symptoms
experienced in subacromial impingement syndrome.
4
He stated that this compression occurred principally
in forward elevation and described a clinical test, the
‘(Neer) impingement sign’ to reproduce the asso-
ciated symptoms.
5
The test involves restricting sca-
pular movement and forcing the arm into flexion
while the shoulder remains internally rotated.
5
According to Neer, this manoeuvre causes the greater
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Correspondence to: Dr J S Lewis, Therapy Department, Chelsea and
Westminster Hospital NHS Foundation Trust, 369 Fulham Road, London
SW10 9NH, UK. Email: jeremy.lewis@chelwest.nhs.uk
ß
W. S. Maney & Son Ltd 2011
DOI 10 .1179/1743288X11 Y.0000000027
Physical Therapy Reviews 2011 VOL. 000 NO .000 1

tuberosity to impinge against the acromion. He
argued that 95% of rotator cuff tears are initiated
by impingement and that trauma may enlarge a tear
but is rarely the principal factor. Neer described three
stages of the impingement process. The first occurs in
people under 25 years of age and is associated with
tendinous oe dema and haemorrhage, and does not
require surgery. The second involves tendinitis and
occurs in people aged 25 to 40 and bursectomy and
coracoacromial ligament division should be consid-
ered after 18 months of conservative treatment. Neer
stated that in this group an acromioplasty is not
usually required. The third stage occurs in people
over 40 years of age and is associated with bone spurs
and tendon rupture and requires anterior acromio-
plasty. Neer stated that the reason rotator cuff tears
develop in some people and not others is principally
due to the shape of the acromion.
5
This hypothesis
was supported by Bigliani et al.
14
who described three
distinct morphological variations of acromial shape.
Bigliani et al.
14
argued that as a result of the shape
and the damage it would cause, those with a Type III
or hooked acromion were more likely to experience
subacromial impingement syndrome and suffer a
rotator cuff tear. Prior to Neer
4,5
presenting his
model, surgeons were performing complete acromio-
nectomies and lateral acromioplasties to alleviate the
symptoms. Basing his argument on intra operative
and cadaver observations, Neer
4,5
asserted that
removal of the inferior aspect of the anterior
acromion had greater efficacy. To augment the
procedure, he suggested that a partial resection of
the coracoacromial ligament together with surgery to
remove a hypertrophic acromioclavicular joint may
be required to arrest the impingement process.
Neer’s impingement model has been widely
embraced by surgeons, sports physicians and physical
therapists. So much in fact, that the percentage of
acromioplasties performed in New York State (USA)
alone has increased 254% in the 10 years from 1996 to
2006. The number of procedures has increased from 30
per 100 000 people (5571 operations) to 102 per
100 000 (19 743 operations).
15
Ketola et al.
16
reported
that the average cost of an acromioplasty and post-
surgical rehabilitation in Finland was J2961 (equiva-
lent to GB£2479, US$4017). In London, UK, an
average price for a series of quotations for private
subacromial decompression was GB£3500. In New
York State, a figure of US$4860 has been given. If this
is a representative amount, then staggeringly, the total
cost of performing acromioplasties in New York State
alone would be in the order of US$95 959 980. These
figures are not definitive and costs in some centres may
be less and in others higher. Nonetheless this is of
substantial economic burden and healthcare concern,
as Ketola et al.
16
have stated:
‘Arthroscopic acromioplasty provides no clinically
important effects over a structured and supervised
exercise programme alone in terms of subjective
outcome or cost-effectiveness when measured at
24 months. Structured exercise treatment should be
the basis for treatment of shoulder impingement
syndrome, with operative treatment offered judi-
ciously until its true merit is proven’.
This begs the question that if surgery aiming to
remove the cause of the impingement irritation (i.e.
the acromion) is no more clinically effective than a
substantially less expensive structured rehabilitation
programme,
16–18
then is the original hypothesis
correct, is the procedure valid, or is there an
alternative explanation for the symptoms?
Objectives
The aim of this paper is to review the evidence for the
acromial irritation theory and in doing so challenge
the rationale for the surgical removal of the inferior
aspect of the anterior acromion to remove the source
of symptoms.
Objective Findings
Area of pathology
If 95% of rotator cuff failure is caused by mechanical
irritation by the unde r surface of the acromion or
coracoacromial ligament,
5
then this should result in
abrasion to the superior (bursal side) surface of the
rotator cuff, especially the supraspinatus. Published
research disputes this. Payne et al.
19
reported that 39
(91%) partial thickne ss tears in 43 athletes were on
the inferior (articular or joint) side of the supraspi-
natus tendon with only 4 (9%) on the superior or
bursal side. In this series, 100% of those with non-
traumatic sho ulder pain had articular side tears.
Fukuda et al.
20
reported that in a study of 249
cadavera, 13% (n533) demonstrated partial thickness
tears. Of the partial thickness tears 82% were either
joint side or intra-tendinous (n527) and only 28%
(n56) were isolated to the upper bursal/acrom ial side.
Ozaki et al.
21
examined 200 shoulders from 1 00
cadavera and reported that a partial thickness tear
was observed in 69 specimens and that the major ity
involved the deeper articular side of the tendon. They
argued that the prevalence of tears increased with age
and occurred due to intrinsic degeneration and not
external (acromial) irritation. In a study of 306
rotator cuff specimens (from 153 cadavera) the
prevalence of partial thickness tears was 32%, with
histological and scanning electron microscopy sec-
tions demonstrating that the majority were either
intra-substance or occurred on the articular side of
the supraspinatus tendon, near the insertion.
22
This
study did not find a correlation between anatomical
bony variations and tendon failure and argued that
mechanical abrasion may not play an important part
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in the initial pathogenesis of degenerative rotator cuff
tears. Ellman
23
reported that partial thickness tears
were foun d in 15% (n520) of people undergoing
arthroscopic subacromial decompression (n5130).
He reported that when the findings of his observa-
tional work was combined with the findings of five
other studies
23
a total sample of 160 partial thickness
tears was produced, and of these the location was
reported in 126 cases. From these 126 cases, 76%
(n596) had articular side tears, 14% (n517) had
bursal side, and 10% (n513) had both. This repeated
and consistent finding that the predominance of
partial thickness tears occurs on the de eper articular
side of the tendon substantially challenges the
hypothesis that 95% of rotator cuff tears are caused
by acromial abrasion. Furthermore, Codman
10
al-
ready in 1934 observed that the rotator cuff de-
generates within the substance of the tendon or
frequently along the inferior margin of the tendon,
the side opposite the acromion, calling partial
thickness tears in this region, ‘rim rents’. He stated,
‘…I am confident that these rim rents accou nt for the
great majority of sore shoulders. It is my unproved
opinion that many of these lesions never heal,
although the symptoms caused by them usually
disappear after a few months’. The weight of evidence
supports Codman’s early observations that although
the acromion may be involved, the tendon may
structurally fail without direct mechanical irritation
from the overlying acromion. Supporting this con-
tention, Hashimoto et al.
24
observed diffuse degen-
erative changes involving tendon thinning, fibre
disorientation, myxoid and hyaline degeneration,
calcification, and chondroid metaplasia to be more
prominent in the middle and deeper rotator cuff
tendon layers, suggesting intrinsic tendon failure.
Variations observed in the morphology of the
supraspinatus tendon support these findings.
25
In a
histological and biomechanical investigation of 20
normal rotator cuff tendons, the deeper, non-
acromial, side fibres were reported to have a smaller
cross-sectional area than the superior acromial side
fibres.
25
In addition, when stretched to the point of
rupture, the deeper fibres were found to be more
vulnerable to tensile load than the bursal side fibres,
with the deeper fibres failing at approximately half
the tensile load of the failure point of the upper
acromial side fibres.
25
Supporting this finding, Bey
et al.
26
in a study of seven cadaveric shoulders
reported that when placed at 15, 30, 45 and 60u of
glenohumeral abduction, strain within the supraspi-
natus tendon increased with increasing joint eleva-
tion. At 60u elevation there was no significant
difference in strain between the superior, middle
and inferior portions of the tendon. However and
importantly, as the inferior fibres are comparatively
weaker,
25
Bey et al.
26
argued that the fibres in the
inferior region are relatively more susceptible to
failure in elevation. This is of clinical relevance as
many vocational activities and sporting pursuits
involve placing the shoulder in elevation for pro-
longed periods. As significantly more strain is placed
on the inferior fibres at 45 and 60u abduction than at
15 and 30u, and the deeper side fibres are relatively
weaker and fail earlier than the larger acromial side
fibres, it is arguable that the acromion has little or
nothing to do with the failure, which potentially may
result from the deeper side fibres passing their
physiological failure point.
As there is a lack of concordance between the area
of structural failure observed in the supraspinatus
tendon and the area predicted by an acromial
irritation model, others have suggested that the
observed joint side structural pathology is better
explained by external impingement between the
superior aspect of the glenoid fossa and the humeral
head. This alternative external irritation model has
been referred to as superior, postero-superior and
internal impingement.
27–30
However, robust evidence
required to support this model of impingement is
lacking, and as such it is possible that the deep side
tendon failure described in this model might not
result from external (extrinsic) impingement but may
result because of the heterogeneity of the fibre
distribution of the upper and lower aspects of the
supraspinatus tendon, together with the disparity of
tendon loading patterns during movement.
25,26,31
Recent reviews on the rotator cuff exploring these
and related issues have been published.
32,33
More than 70 years ago, Lindblom and Palmer
34
suggested that during shoulder abduction uneven
loads may be placed on the uppe r and lower aspects
of the tendon resulting in intratendinous shearing,
which may play a part in rotator cuff degeneration
and tears. This is relevant as the supraspinatus
tendon is made up of structurally independent
parallel fascicles
35,36
and movement will potentially
lead to different length tension relationships occur-
ring within and between the different fascicles. For
example, at the extreme of shoulder horizontal
abduction the anterior part of the tendon may be
relatively lengthened and the posterior shortened,
whereas at the extreme of horizontal adduction this
pattern may be reversed. As a greater range of
movement is required of the shoulder than any other
joint in the body it is conceivable that internal tendon
shearing may result that may predispose pathology
without the need for external compression on either
the superior or inferior aspects of the tendon. In
support of this, external irritation, in the form of an
Achilles tendon allograft wrapped around the left
acromion, in rats, did not lead to rotator cuff
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pathology. However, intrinsic overload in the form of
downhill eccentric running for 4, 8 and 16 weeks did.
The rats subject to overuse (running) demonstrated
an increase in cross-sectional area and reduced
maximal strain at all time points. A combination
group (allograft and overloa d) lead to the greatest
change, suggesting that compression potentiated
overload even though compression alone did not
produce pathology.
37–39
These findings suggest that
external compression is insufficient to cause pathol-
ogy unless there is a concomitant history of tendon
overload, suggesting that the primary pathoaetiology
occurs within the tendon. It is accepted that due to
differences in morphology and biomechanics, caution
is necessary with direct translation from animal
studies. Evolution of the human upper limb may
place biomechanical constraints on the modern
shoulder which make it less capable of sustaining
positions of elevation.
40
If this is the case, then work,
recreational and sporting activities performed above
90u of elevation may selectively affect the weaker,
more vulnerable joint side fibres without the need for
acromial compression.
Acromial shape
Based on a study of 140 shoulders in 71 cadavers,
Bigliani et al.
14
argued that three distinct shapes of
the acromion existed. These morphological variations
included a Type I (flat), Type II (curved) and Type III
or hooked acromion. If the acromion is responsible
for 95% of rotator cuff pathology and is the causat ive
mechanism of pathology in impingement syndrome,
then a definitive relationship between acromial shape,
pathology and symptoms sho uld exist with a Type II
or III more likely to predispose pathology. However,
research evidence has failed to demonstrate this.
In a study of 59 people without shoulder pain the
association between acromial morphology, age and
rotator cuff tears was investigated.
41
For people over
the age of 50 years, a 40% prevalence of asympto-
matic full thickness rotator cuff tears was identified in
this investigation. Based on the substantial number of
people with curved and hooked acromia who were
entirely asymptomatic, Worland et al.
41
concluded
that, ‘Surgeons should interpret radiologically hook-
ed or curved acromions as well as rotator cuff tears
diagnosed with ultrasound or other modalities with
caution’. In a study of 55 people who underwent
arthroscopic subacromial decompression (anterolat-
eral edge of the acromion resected together with
release and resection of the coracoacromial ligament
from the acro mion), the association between pre-
operative pain, clinical signs (Hawkins test, Neer
sign, Copeland impingement test) and satisfaction
with the severity of rotator cuff and acromial lesions
was investigated. At the 6 month follow-up no
significant correlation between pain and satisfa c-
tion and the severity of structural pathology was
identified.
42
Confirming this, after a study of 523
people unde rgoing arthroscopic or open shoulder
surgery, Gill et al.
43
reported no significant associa-
tion between acromial shape and rotator cuff pa-
thology in people over 50 years of age (n5192). A
highly significant correlation between age and rotator
cuff pathology existed and the researchers argued that
a Type III hooked acromial represents a degenerative
process rather than a morphological varia>tion as
described by Bigliani.
14,44
Although a relationship
between rotator cuff tears and acromial degeneration
appears to exist, this should be seen as an association,
rather than the acromion being implicated in (i.e. the
cause of) rotator cuff pathology.
An alternative explanation for the observed acro-
mial spurs is possible. Edelson and Taitz
45
observed
degenerative spur formation on the acromial insertion
of the coracoacromial ligament but not on the
coracoid side in 18% of 200 scapulae. When compared
with shoulder adduction, increasing ranges of shoul-
der elevation increase subacromial pressure.
46,47
The
coracoacromial ligament is more trapezoid in shape
with a smaller area of insertion on the acromial side
than the coracoid side. It is therefore possible that
superiorly directed pressure from below the ligament
will lead to relatively more tension on the acromial
insertion of the ligament than on the coracoid side due
to the smaller surface area of insertion on the former.
This potential increased stress on the bone may lead to
osteophyte formation. Supporting this hypothesis,
Chambler et al.
47
demonstrated in vivo (n55) that
tension in the coracoacromial ligament increased as
the arm was abducted. In an additional study
48
analysis of acromial bone spurs (n515) suggested that
the development of the spurs was a secondary
phenomenon. These studies suggest that tension in
the coracoacromial ligament is the probable mechan-
ism of acromial bone spur formation and that
acromial Type II (curved) and Type III (hooked) as
described by Bigliani
14,44
may not be inherited, but
may result from increased strain in the ligament
disproportionally affecting the acromial side.
Chronic strain in the coracoacromial ligament may
result from changes in the rotator cuff tendons that
may involve increased tendon volume, as well as from
failure of the rotator cuff to stop superior translation
of the humeral head during arm elevation.
49–53
Evidence for chronic strain exists, with free nerve
endings and neovascularity observed in coracoacro-
mial ligament samples from people undergoing
subacromial decompression.
54
This suggests that the
ligament may be a potential source of symptoms. The
coracoacromial ligament limits superior translation
of the humeral head
55–57
and as acromioplasty has
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been associated with increased anterosuperior trans-
lation of the humeral head,
52,57–59
the procedure itself
may be an iatrogenic cause of ligament strain.
Ligaments are structures that stabilize joint move-
ment and if disrupted they are replaced in an attempt
to recreate stability. Examples of this include patellar
tendon and hamstrings tendon grafts for anterior
cruciate knee ligament failure.
60
It is therefore
surprising that the coracoacromial ligament, which
provides a stabilization role by preventing superior
translation of the humeral head,
61
has been exten-
sively sacrificed to retard or stop the subacromial
impingement, due to the belief that it is of relative
structural unimportance, when there is no conclusive
evidence to support the existence of primary external
impingement from this structure. It would be hard to
imagine that a surgeon would suggest, or a patient
would agree to, having the anterior cruciate ligament
removed to treat knee pain.
Posture and muscle imbalance
Physical therapists have also embraced the acromial
irritation model
62–65
and have argued that an
increased kyphosis, a change in scapular position
due to poor posture, uncontrolled scapular move-
ment (dyskinesis), or an imbalance in muscle activity
leads to subacromial impingement syndrome.
63,66–68
Even though concepts relating to pos ture and muscle
imbalance have existed for more than half a century it
is surprising how little evidence there is to support (i)
the existence of an ideal posture of the head, neck,
thorax, (ii) the existence of an ideal scapula r position
(i.e. the basis for scapula r setting exercises), (iii) that
uncontrolled scapular movement and dyskinesis is
always a primary problem, (iv) that postural devia-
tions and muscle length tension changes alter
scapular position in a consistent manner and sig-
nificantly lead to a detrimental effect of movement
and provoke impingement symptoms, (v) that reha-
bilitation can correct posture that is considered
abnormal, and (vi) the idea that this correction
leads to an improvement in function and a reduc-
tion in pain. There is evidence to challeng e these
concepts
69–76
which suggests that the certainty with
which this aspect of clinical practice is taught to
undergraduate and postgraduate students and
imparted to patients and clients requires robust
research enquiry. In addition, it is arguably inap-
propriate to suggest that an increased thoracic
kyphosis leads to restricted shoulder movement and
impingement based on studies that have unnaturally
restricted thoracic spine movem ent.
66,68
An assump-
tion implicit within the postural-muscle imbalance
model of assessment is that a forward head posture
and increased thoracic kyphosis observed during
static posture has a direct correlation on dynamic
movement, and that all scapulae have the same
geometric proportions and move in the same way on
the same shaped rib cage and thorax. This is simply
not correct
45,70,77,78
and as such, a one size fits all
approach is unlikely to be appropriate. Variations in
shoulder function may be dictated by variations in
structure, and the differences observed between
people with impingement syndrome may reflect a
range of normal values and not deviations from one
idealized normal posture. If this is correct it would
not be possible to identify pos tural deviations that
lead to subacromial impingement. Based on the
uncertainty of the current models of postural and
clinical assessment altern ative models of assessment
have been proposed;
69
however, these also require
clinical validation.
Tendinitis
Implicit within the three-stage impingement model
presented by Neer
4,5
is the association between the
mechanical abrasion caused by the acromion and the
ensuing microtrauma within the tendon leading to
tendon inflammation (tendinitis). The issue of tendon
inflammation is controversial. Although histological
studies have demonstrated substantial differences
between normal tendon and pathological tendon,
the evidence for the presence of cells classically
associated with inflammation is not robust. No
infiltration of neutrophils, lymphocytes or plasma
cells were identified in specimens taken from 12
subjects with rotator cuff disease during surger y.
79
Similarly, no inflammatory cells were identi fied in
bursal specimens (n58) also taken during surgery for
rotator cuff tendinopathy.
80
In another small study,
people with constant shoulder pain were more likely
to have lymphocyte infiltration in bursal tissue in
comparison to people with pain only on movement
who did not exhibit evidence of bursal inflammatory
cells.
81
There is distinct need for robust evidence from
appropriately designed research to better understand
if inflammation is part of the continuum of pathoae-
tiology of tendon and bursal pathology.
33
Without
this research an argument that acromial irritation and
the ensuring microtrauma leads to bursal and tendon
inflammation remains unsubstantiated.
The subacromial bursa
The subacromial bu rsa (SAB) separates the cora-
coacromial arch and deltoid above, and the rotator
cuff tendons below. Together with the other bursae
in the re gion, whose reported numbers ranging from
7/8 to 12, the SAB acts to r educe frictio n during
movement.
13,82
The SAB is innervated anteriorly by
the lateral pectoral nerve and posteriorly by the
suprascapu lar nerve.
83
The identification of mechan-
oreceptors and free nerve endings (Ad and C) in
bursal tissue suggests the SAB has a role in
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proprioception zand nociceptio n.
84
The presence of
nociceptors is highly relevant as high concentrations
of pro-inflammatory cytokines, pain mediating sub-
stances and matrix modify ing proteins have been
identified in bursal tissue of people whose shoul-
der pai n is exacerbated by shoulder elevation.
85–91
Higher shoulder pain scores were reported by those
found to have higher concentrations of the cytoki ne
interleukin-1beta an d the neuropeptide, substance P,
in their bursal tissue.
86
One clinical trial randomized people diagnosed
with subacromial impingement syndrome (n557,
mean age 47 years), to Group I: arthroscopic ac-
romioplasty and bursectomy or Group 2: arthro-
scopic subacromial bursectomy alone. Good results
were reported in both groups with no significant
differences identi fied between the two groups at a
mean follow-up of 2.5 years.
92
These findings clearly
suggest that the bursa is a significant pain generator
and that the addition of an acromioplasty may be
superfluous. Henkus et al.
92
argued that subacromial
impingement syndrome is largely an intrinsi c degen-
erative condition rather than an extrinsic mechanical
disorder. The importance of the SAB as a source of
shoulder pain is reinforced by studies that have
shown that injections reaching the SAB reduced pain
while injections targeting other structures increased
or did not change pain.
93
This may be a reason why
ultrasound guided injections appear to produce better
outcomes than non-guided injections.
94,95
It is also unclear whether treatment with corticos-
teroid and lidocaine is any more advantageous than
lidocaine in isolation.
96–99
Using analgesic injections
as a control, systemic (gluteal) corticosteroid injec-
tions have been report ed to be as effective as locally
guided corticosteroid injections
99
in the treatment of
impingement syndrome. However, the certainty of
this conclusion is questioned by studies that have
shown no added long term effect of analgesic over
steroid,
96,97
and no one has yet shown the added
benefit of the pharmacological substance injected
over the mechanical stimulation of the dry needle,
which in itself is frequently painful. The science
supporting the use of injection therapy for impinge-
ment syndrome (timing, volume, medications used,
direction of injection, post-injection advice, histolo-
gical effect on tissues) is not robust and requir es
ongoing investigation. In the UK, many physical
therapists perform injections and an increasing
number are performing ultrasound guided injections.
Alongside this change to scope of practice is a
requirement for further research, which is essential to
understand the histological and biochemical nature
of bursal pathology and pain and the relationship
between bursal pathology, tendon pathology and
shoulder pain.
Alternative explanations for the potential benefit
of surgery
Success rates of 80–90% following subacromial decom-
pression for impingement have been reported.
100–103
Neer
5
and those embracing his model argue that
removing the acromion removes the source of irrita-
tion. Henkus et al.
92
has clearly demonstrated this may
not be the case, as isolated removal of the bursa has
comparable effects to removing the acromion and
bursa. However, in addition to the suggestion that
bursectomy is more relevant than acromioplasty,
92
other, additional explanations for the beneficial results
reported following acromioplasty are entirely feasible.
In Australia, non-manual workers take on average
6 weeks to return to work following an acromioplasty
and 85% of manual workers take 3 months to return to
employment, with driving commencing at 29 days
post-surgery.
104
Comparable findings from the UK
suggest that non-manual workers return to work after
9 days, manual workers after 3 weeks and driving
recommences after 13 days.
105
These data clearly
demonstrate that there is a prolonged period of
substantial relative rest following the procedure and
to date no study has compared surgery and the ensuing
relative rest with comparable relative rest alone. This
highly relevant issue of relative rest was suggested by
Lewis
33
as an essential component of treatment for
rotator cuff tendinopathy in a reactive stage. This will
be referred to again later in this paper. In addition, it is
possible that subacromial decompression is a placebo.
Moseley et al.
106
reported that 180 people with painful
knee osteoarthrosis randomized to either (i) arthro-
scopic lavage, or (ii) arthroscopic debridement or (iii)
placebo surgery (skin incisions) reported the same
improvement at 2 year follow-up. This strongly
suggests that the benefit reported for people under-
going arthroscopic surgery for painful degenerative
knees may be entirely attributable to the placebo effect.
This is not the first time that the benefit of surgery has
been attributed to placebo.
107,108
Additionally, to
reduce the economic burden on healthcare systems, it
would be very appropriate to recommend an appro-
priately structured period of relative rest and a
supervised and graduated exercise programme before
surgery is considered as non-surgical care is at least of
equivalent clinical benefit.
16–18
Clinical diagnosis
A diagnosis of subacrom ial impingement syndrome is
initially made on the basis of clinical tests. Neer
5
introduced the Neer impingement sign and others
have proposed other tests to confirm or exclude
impingement under the acromion.
109,110
The clinical
tests are often supported by imaging investigations.
However, the ability for clinical tests and imaging
investigations to enable a clinician to confirm a
diagnosis of subacromial impingement syndrome is
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contentious,
69,111–113
as imaging investigation s ha ve
consistently demonstrated structural pathology in a
high percentage of people without sympto ms.
As discussed earlier there is a poor correla-
tion between acromial radiological changes and
symptoms.
41–43
In a study of 96 people without
shoulder symptoms 28% or those aged between 40
and 60 years and 54% of those aged above 60 years
had MRI evidence of a partial or full thickness
rotator cuff tear.
114
Milgrom et al.
115
reported in a
study of 90 people (age range 30 to 99 without
shoulder symptoms) that the incidence of full
thickness rotator cuff tears identified by ultrasound
increased with advancing age and that after the 5th
decade approximately 50% of people had asympto-
matic full thickness tears that did not affect function.
In a study that compared 42 people with impinge-
ment syndrome with 31 age matched symptom free
controls, Frost et al.
116
reported that 55% of the
symptomatic group and 52% of the asymptomatic
group had eviden ce of rotator cuff pathology on
imaging. They also reported that pathology related to
age and did not correlate with symptoms.
Professional baseball pitchers have been reported to
pitch up to 165 km/hour and demonstrate internal
rotation velocities during pitching of 6100 to 6940u/
second.
117,118
These examples of extremely high levels
of function may be achieved even in the presence of
structural pathology. Miniaci et al.
119
reported that
14 professional baseball pitchers without shoulder
symptoms demonstrated rotator cuff changes in their
throwing (79%) and non-throwing (86%) shoulders
and labral changes (79%) in both shoulders.
Professional baseball pitchers and tennis players
without symptoms demonstrated partial and full
thickness tears (40%), glenohumeral joint effusions
(90%) and excess subacromial fluid (48%) in their
dominant shoulder and remained asymptomatic at a
5 year follow-up.
120
In addition, the sourcil (eye-
brow) sign observed radiologically as sclerosis on the
under surface of the acromion and consider ed to be
an indictor of rotator cuff pathology due to increased
pressure (impingement) was found not to correlate
with clinical signs of impingement, rotator cuff tears,
or age, and did not aid diagnosis in 175 people with
shoulder pain.
121
Lewis et al.
122
demonstrated that
neovascularity may be present in both the sympto-
matic and asymptomatic shoulders of people diag-
nosed with unilateral rotator cuff pathology.
These imaging studies demonstrate that high
percentages of people without symptoms will have
evidence of structural failure and at present there is
no clinical certainty that imaging abnormalities are
the cause of the presenting impingement symptoms.
Observation of structural pathology identified in
radiographs, ultrasound, MRI and arthroscopy are
frequently employed as the gold standard comparator
in studies designed to test the diagnostic accuracy,
sensitivity, specificity, positive and negative predictive
values, and positive and negative likelihood ratios.
However, if the gold standard is not robust (i.e.
people wi thout shoulder symptoms have structural
failure
114–116,119,122
) then a concomitant high percen-
tage of false positives would not provide the
confidence required by a clinician to make a diagnosis
with any certainty.
This is of major concern as surgery may be
recommended to people diagnosed with impingement
syndrome based on clinical tests and supported by
imaging findings that are currently incapable of
conclusively confirming such a diagnosis.
69,111,112
The consequence of this is that for a substantial
percentage of people the surgery may be unnecessary,
inappropriate and unwarranted. This is clearly high-
lighted by the number of studies that do not show
added benefit of surgery over non-surgical care.
16–18
In addition, surgery carries risks, such as infections,
and is substantially more expensive.
16,123
Therefore,
an appropriate and defensible argument is that until a
robust method of confirming a diagnosis is obtain-
able, and until clear evidence concerning the pathol-
ogy is available, surgery should only be offered after
an appropriate period of appropriate non-surgical
care.
Tendinopathy
Shalabi et al.
50
performed an MRI investigation of
the Achilles tendon (n544 from 22 people, 30
symptomatic and 14 asymptomatic tendons) imme-
diately before and within 30 minutes of an intense
bout of concentric (bilateral heel raises) and/or
eccentric (6 sets of 15 repetitions) gastrosoleus
exercises. They reported a 12% increase in tendon
volume in the eccentrically loaded symptomatic
Achilles tendons and a 17% increase in the
concentrically loaded tendons (mixed symptomatic
and asymp tomatic) Achilles tendons. There was a
20% increase in t endon volume in the concentrically
loaded asymptomatic Achilles tendons. Rats sub-
ject to a tendon overload programme have also
demonstrated an increase in rotator cuff cross-
sectional area.
37
Increased rotator cuff tendon
volume as a result of unaccustomed activity or
activity at an intensity that surpasses the physiolo-
gical limit of the tendon (which will be highly
variable between and within individuals) may lead
to increased upward pressure on the acromion and
coracoacromial ligament. This increased strain in
the l igament is a possible aetiological mechanism
for acromial spur formation and as such the
acromial osteophyte may not be the primary
problem but secondary to the increased tendon
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volume. If the increased volum e and resulting
ligamentous strain occurs at a subclinical level
from bursts of activity or spor t a s pur m ay de vel op
over time but may remain asymptomatic over a
lifetime and explain the poor correlation between
acromial shape and symptoms.
41–43
Overuse tendinopathies involving the la teral epi-
condyle, patellar, adductor and Achilles tendons
may occur without impingement from external
structures such as adjacent bony surfac es, and this
may also be the case for the rotator c uff. It is
possible that the external irritation accentuates the
tendon failure
37–39,124
but it is unlikely that it is the
prim ary cause. The failure is lik ely to be due to a
combination of factors including: relative overload,
gene tics, nutritional and life style variabl es,
32
and
the rotator cuff tendon failure may be seen as a
continuum of pathology.
33
The initial symptomatic
stage of the continuum of pathology has been
term ed a reactive tendon,
33,125
which may be cha-
racterized by increased tenocyte numbers.
126
In-
creased expression of the large negatively charged
proteoglycan aggrecan is observed in painful overuse
tendinopathy.
127,128
Due to its negative charge aggre-
can attracts and retains water, which explains the
swelling observed in acute Achilles tendinopathy.
50
The non-steroidal anti-inflammatory drug, ibuprofen,
appears to inhibit the synthesis of aggrecan
129
and
may be an appropriate treatment at this stage.
Additionally, glucocorticoids have been shown to
inhibit tenocyte proliferation,
130
which may explain
the benefit ascribed to corticosteroid injections for
the shoulder in some people.
98
However as stated, the
long term efficacy and potential detrimental effects of
corticosteroid injections for the shoulder require on
going investigation.
The other pathological stages associated with rota-
tor cuff tendinopathy (disrepair and degeneration)
33
may have an associated element of reactivity. When
reactivity is present tendon thickening and swe lling
is possib le. If this pathoaetiology is accurate and if
the pathology is correctly explained by intrinsic
tendon failure as a consequence of relative ove rload
then it simply may be the swollen tendon pushing up
and not the acr omion pushing down that is the cause
of the problem. If this hypothesis is correct, then an
acromiopl asty will not treat the primary problem
(i.e. intrinsic tendon failure) or provide appropriate
init ial management for the condition. If relative rest
and appropr iate reloading strategies are principal
factors in tendon rehabilitation it is possible that a
major benefit of an acromioplasty is enforced
relative rest .
104,105
If this is correct, the associate
expe nse, potential risks and lack of appropr iately
targeted treatment question its utility as a first line
treatment option.
Surgery versus non-surgical management
As mentioned, reports of 80–90% success following
subacromial decompression for impingement have
been published.
100–102
When acromioplast y was com-
pared with conservative care (physiotherapy exercises
and pain relief) surgery appeared to be no more
beneficial clini cally at 6, 12 or 48 months.
16–18
As
elucidated earlier there is no certainty that the benefit
relates directly to the stated aim of the surgery (i.e.
removal of the acromion) and benefit may be derived
from the bursectomy, the period of post-surgical
relative rest, and potentially placebo. Relative rest is
of relevance as Cook and Purdam
125
in a generic
model of overuse tendinopathy, have suggested that
tendon load management and reduction in frequency
and/or intensity of tendon load is important during
the reactive phase. Relative rest may also be
important in the reactive stage of rotator cuff
tendinopathy.
33
Relative rest may allow the tendon
to attain relative homeostasis, by reducing the up
regulation of tenocytes that may be characteristic of a
reactive tendon and thereby reduce the associated
swelling before a graduated and appropriately con-
structed rehabilitation programme is instigated. It
may be possibly to enhance the exercise prescription
that has been utilized in clinical trials by more
effectively targeting the stage of the rotator cuff
tendinopathy.
33,131
In addition, consideration of the
varying effects exercise may have on subacromial
pressure
46
is relevant. To further reduce upward
humeral head translation and tendon compression,
avoidance of internal rotation in the early stages of
rehabilitation may be appropriate. Although uncer-
tainty exists,
132
it may be possible to enhance the
effect of exercise by including manual therapy in the
treatment package.
133,134
These issues relating to
rehabilitation need to be appropriately scrutinized
through robust research investigations.
Conclusion
Subacromial impingement syndrome is considered by
many to be the most common of the musculoskeletal
conditions affecting the shoulder. It is based on a
hypothesis that acromial irritation leads to external
abrasion of the bursa and rotator cuff. Subacromial
decompressive surgery aims to remove the source of
this irritation. There is however a body of evidence
that suggests there is a lack of concordance regarding
(i) the area of tendon pathology and acromial
irritation, (ii) the shape of the acromion and
symptoms, (iii) the proposal that the irritation leads
to the development of tendinitis and bursitis, and (iv)
imaging changes and symptoms and the development
of the condition. In addition, there is no certainty
that any benefit derive d from the surgery is due to the
removal of the acromion as research suggests that a
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bursectomy in isolation may confer equivalent
benefit. It is also possible that the benefit of surgery
is that it simply enforces a sustained period of relative
rest which may allow the involved tissues to achieve
relative homeostasis. It is possible that pathology
originates in the tendon and as such surgery does not
address the primary problem. This view is strength-
ened by the findings of studies that have demon-
strated no increased clinical benefit from surgery
when compared with exercise, with exercise therapy
being associated with a substantially reduced eco-
nomic burden and less sick leave. Evidence based
healthcare involves the integration of clinical exper-
tise, patient values and best research evidence. To
provide the research evidence required, surgeons
performing acromioplasties need to demonstrate that
it is the acromioplasty that is beneficial and not the
enforced reduction in acti vity or the possibility of
placebo. As there is little evidence for an acromial
impingement model a more appropriate name may be
‘subacromial pain syndrome’. Moreover, surgery
should only be considered after an appropriate period
of appropriately structured conservative treatment.
Acknowledgements
Aspects of the information contained in this paper
were presented as a key note lecture entitled
‘Subacromial impingement syndrome. A musculos-
keletal condition or a clinical illusion?’ at the 11th
International Conference of Shoulder and Elbow
Surgeons, Edinburgh, Scotland, UK, 5–8 September
2010.
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