Biomedical Engineering Reference
In-Depth Information
The result is that cast immobilization leads to improved healing; this has led to
further studies investigating the role of the mechanical environment in healing.
Cast immobilization is difficult to enact in a clinical setting because it depends on
patient compliance. Additionally, it does not completely remove all forces across
the healing insertion because some motion is possible and muscle forces are still
present. This led several investigators to probe the effect of complete removal of
muscle loading on healing. A rat rotator cuff acute repair model used Botulinum
toxin A injections to reversibly paralyze the supraspinatus muscle with and
without cast immobilization [ 136 ]. Botulinum toxin and cast immobilization
reduced the structural and mechanical properties of the healing insertion,
indicating that complete removal of loading was unfavorable to healing. This
result was corroborated by a study investigating healing of canine flexor tendon-
to-bone repairs [ 137 ]. This suggests that some muscle loading is necessary for
optimal healing.
A separate series of studies investigated the role of mechanical loading on
ACL healing in rats. Ligaments were surgically replaced by flexor digitorum
longus tendons in a bone tunnel. An in vivo joint loading system was used to
apply controlled axial loading daily to the healing insertion [ 138 ]. In a compari-
son of immobilization and cyclic loading, the loaded group had increased inflam-
matory macrophages at the tendon-bone interface and decreased bone formation
in the bone tunnel, but no differences in the biomechanical properties of the
interface [ 139 ]. A separate group of animals was used to evaluate a period of
immobilization followed by loading [ 140 ]. This study demonstrated an increase in
bone formation and load-to-failure compared to immobilization and early cyclic
loading groups. This suggests that a period of immobilization followed by loading
is beneficial to healing of ACL reconstructions. A separate study investigated the
effect of immobilization followed by exercise in a rat rotator cuff model. Immo-
bilization followed by exercise reduced the desired mechanical performance of
repaired tendons compared to immobilizationfollowedbycageactivity[ 141 ]. In
contrast to this, a recent report using a rat Achilles tendon rupture model indicated
that exercise early in the healing process combined with unloading by hindlimb
suspension improved healing compared to tendons that were unloaded without
exercise [ 142 ].
Taken together, the above results indicate that the healing tendon-to-bone
insertion is influenced by loading environment. The conflicting results indicate
that modulating the loading environment is just one factor among many that affect
healing outcomes. One conclusion that may be drawn from the above studies is that
complete unloading is detrimental to healing, either through chemical paralysis,
hindlimb suspension, or tendon transection away from the repair site. Furthermore,
excessive exercise is also harmful to the healing process. Optimal healing seems to
require a precise degree of loading that is extremely difficult to accurately predict
and control in laboratory or clinical settings (Fig. 11.10 ).
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