Background: Subacromial corticosteroid injections are commonly used
in the nonoperative management of rotator cuff disease. The effects of
corticosteroid injection on injured rotator cuff tendons have not been
studied. Our aims were to characterize the acute response of rotator cuff
tendons to injury through the analysis of the type-III to type-I collagen
expression ratio, a tendon injury marker, and to examine the effects of
corticosteroid on this response.
Methods: Sixty Sprague-Dawley rats were randomly assigned to four
groups: control, tendon injury, steroid treatment, and tendon injury and
steroid treatment. Six rats served as sham controls. Unilateral tendon
injuries were created with full-thickness defects across 50% of the total
width of the infraspinatus tendon, 5 mm from its humeral insertion. Steroid
treatment with a single dose of methylprednisolone (0.6 mg/kg), equivalent to
that given to humans, was injected into the subacromial space under direct
visualization. Steroid treatment followed the creation of an injury in the
rats in the injury and steroid treatment group. At one, three, and five weeks
after the injury, the total RNA isolated from tendons was quantified with
real-time polymerase chain reaction with use of primers for type-I and
type-III collagen and ribosomal 18s RNA.
Results: The type-III to type-I collagen expression ratio remained
at baseline at all time-points in the control and sham groups. At one week,
the type-III to type-I collagen expression ratio increased more than fourfold
above the control level in the tendon injury group (p = 0.017) and the tendon
injury and steroid treatment group (p = 0.003). The ratio remained greater
than twofold above the control at three weeks in both groups (p = 0.003 and p
= 0.037) and returned to baseline at five weeks. Interestingly, the group that
had steroid treatment only showed an increase of >4.5-fold (p = 0.001) in
the type-III to type-I collagen expression ratio, without structural injury to
the tendon. This ratio returned to baseline levels by three weeks.
Conclusions: A single dose of corticosteroid does not alter the
acute phase response of an injured rotator cuff tendon in the rat. However,
the same steroid dose in uninjured tendons initiates a short-term response
equivalent to that of structural injury.
Clinical Relevance: These findings suggest that while a single
corticosteroid dose may have no long-term effects on tendon collagen gene
expression, collagen composition may be acutely altered by the injection.
Therapy and activity recommendations following subacromial corticosteroid
exposure should be made with the awareness of possible compromised rotator
cuff tendon properties.