Soft Tissue to Bone Healing in Rotator Cuff Repair - Structural Interfaces and Attachments in Biology - page 253

12.4.2 Tear Size
Tear size has also been shown to influence tendon healing; the larger the tear, the
lower the healing rate [ 25 ]. The threshold of tear size over which healing rates drop
significantly is unknown. Regardless, tears considered large or massive, involving
two or more tendons, have much lower healing potential. Care must be taken in
interpreting much of this data, however, as older individuals are more likely to have
larger tears, making age and tear size covariates. Larger numbers of patients must
be studied in order to accurately stratify the effect of these variables separately.
12.4.3 Muscle Degeneration
Muscle tissue is highly sensitive to its loading environment. When tendons detach
from their bony insertion sites, their respective muscles are unloaded and shortened,
leading to muscle atrophy and the accumulation of fibrous tissue and fat; this is
particularly apparent in the setting of a chronic tear [ 26 - 30 ]. Thus far, these changes
have been found to be irreversible [ 31 , 32 ]. Muscle degeneration has clinical
implications for the feasibility of the repair. The degenerated muscles are much less
compliant than normal muscles, and they often cannot be mobilized from their
retracted position to the anatomic insertion on the humeral head. In instances where
the tendon can be mobilized and repaired, the construct is under high tension, leading
to early failure of the repair. Ideally, rotator cuff tears should be repaired prior to the
onset of these chronic degenerative changes in clinically indicated patients.
12.4.4 Smoking
Smoking has been shown to have an effect on tendon healing. In a rodent model,
rats were administered nicotine through a subcutaneous, continuous delivery
osmotic pump [ 33 ]. High nicotine levels resulted in decreased cell proliferation
and decreased collagen I production in the healing tissue. Mechanical properties
were inferior to saline controls at all time points tested and histological signs of
inflammation persisted longer in the nicotine specimens. Mechanical testing
revealed increased stiffness at later time points (8 weeks). The clinical implication
of the increased stiffness is not yet known. Smoking has also been shown in a
human study to increase the likelihood of developing a rotator cuff tear; in patients
with shoulder pain, individuals who smoke have a significantly greater chance of
having a rotator cuff tear [ 34 ]. A more recent history of smoking, a greater quantity
of cigarettes smoked, and a higher number of years smoking are all associated with
rotator cuff tears. The extent to which these factors influence healing remains a
subject of investigation.