634
H. T. Leong et al.
DISCUSSION
This systematic review and meta-analysis examined
factors that were associated with increased risk of
rotator cuff tendinopathy. Overall, 22 potential risk
factors were identified. Pooled analyses provided
strong evidence of age over 50 years and diabetes
being associated with increased risk of rotator cuff
tendinopathy. In addition, moderate evidence showed
that working with the shoulder above 90° was associ-
ated with increased risk of rotator cuff tendinopathy.
The aetiology of rotator cuff tendinopathy is multi-
factorial, and has been classified as intrinsic, extrinsic
or a combination of both (50). Our finding showed that
age above 50 years appears to be a significant factor
in the intrinsic aetiology of rotator cuff tendinopathy
among the working population, which is consistent
with the knowledge regarding age-related degenerative
changes in ageing rotator cuff tendons (36, 44, 51, 52).
Biomechanical studies showed that there is decreased
ultimate strain, decreased ultimate load, decreased
elasticity, and decreased overall tensile strength in
ageing tendons (53–57). In addition, microscopic and
biochemical pathological changes showed degenera-
tion of the tenocytes and collagen fibres, accumulation
of lipids and ground substance (glycosaminoglycans)
in ageing tendons (51). Nevertheless, there is no
consensus as to whether pathological changes in the
rotator cuff tendon are primarily due to ageing or a
secondary consequence with mechanical overuse/
overloading (50).
The findings of the current study showed that syste-
matic disorders, such as diabetes, have been identified
as risk and associated factors for rotator cuff tendi-
nopathy among the general and working populations
(41–44). Recent evidence showed that patients with
diabetes had a 2.11-fold higher risk of rotator cuff
disorders compared with those without diabetes (58).
Previous studies have shown that hyperglycaemia as-
sociated with diabetes affects tendon collagen cross-
linking (59) and reduces proteoglycan content (60). In
addition, other studies also reported that people with
diabetes demonstrated limited shoulder joint mobility,
reduced shoulder muscle strength and impaired shoul-
der function compared with non-diabetes controls (61);
and sonographic evaluation also revealed rotator cuff
degeneration among patients with diabetes (62). Thus,
strict control of diabetes with lifestyle/diet modifica-
tion and careful monitoring of load progression during
exercises may be indicated for the prevention and
management of rotator cuff tendinopathy in diabetic
patients (63). Nevertheless, other systematic disorders,
such as cardiovascular disease (34) and osteoarthritis
(42), were also reported to be associated with rotator
cuff tendinopathy. However, the number of studies
www.medicaljournals.se/jrm
investigating these factors is small and the studies
were largely heterogeneous; therefore, conducting a
meta-analysis was impossible.
For the extrinsic mechanism, the mechanical theory
has been proposed to be the main aetiology of rota-
tor cuff tendinopathy. Micro-injuries in the tendon
resulting from overload can lead to matrix and cell
changes, altered mechanical properties of the tendon,
and contribute to failed healing of the tendon (2, 19).
Work with the shoulder above 90° was revealed to be
associated with increased risk of rotator cuff tendino-
pathy among the working population (36, 41, 42, 49).
Evidence from biomechanical studies supports these
findings, and it is reported that the intramuscular pres-
sure in the rotator cuff muscles increases with extreme
arm elevation (64). In addition, overhead activities may
mechanically compress or irritate the subacromial tis-
sues and tendon structures between the humeral head
and the acromion, resulting in rotator cuff tendinopathy
(65). Moreover, other mechanical work-related factors,
such as heavy manual work (42), repetitive work (39,
44), high frequency of work (39, 45, 46), high force
exposure of work (39, 45) and vibration work (45,
47, 48), have been identified to be associated with
increased risk of rotator cuff tendinopathy. Based on
our findings, extrinsic factors that overload or mecha-
nically compress or irritate the subacromial tissues and
tendon structures between the humeral head and the
acromion may contribute to rotator cuff tendinopathy.
Psychological factors, such as high psychological
demand (40, 44, 49), low job control/decision authority
(40, 44, 46), poor safety commitment (40), job dissa-
tisfaction (34), and low co-worker support and work
with temporary workers (41), have been reported to be
risk and associated factors for rotator cuff tendinopa-
thy among the working population (37). Pooled data
analysis was conducted for high psychological demand
(36, 44, 45, 49). However, our results showed a trend
that high psychological demand might be associated
with increased risk of rotator cuff tendinopathy and
the between-study heterogeneity was high (I 2 = 64%).
The current findings may suggest that the awareness of
psychological variables may be associated with rotator
cuff tendinopathy. Clinicians should consider using
validated screening tools for detection of the presence
of psychological variables, and addressing individual
psychological variables and the underlying cognitive
behaviour should be considered as the management and
prevention of rotator cuff tendinopathy (66).
The current systematic review and meta-analysis
had several limitations due to the inherent biases of
the included studies. Thus, caution should be taken
when interpreting the findings. First, publication bias
may be present, as only English language studies
were included and only 3 databases were searched.