Biomedical Engineering Reference
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Fig. 13.5 Immunohistochemical staining for ED1+ macrophages on axial section of the tibial
tunnel from animals subjected to ( a ) immediate loading and ( b ) delayed loading. The delayed-
loading specimen demonstrates significantly reduced quantity of catabolic ED1+ macrophages. A
similar finding of decreased number of osteoclasts, identified by tartrate-resistant acid phosphatase
(TRAP) staining, is noted in the delayed loading specimen ( d ) vs. the immediate loading specimen
( c ) (adapted, with permission, from ref. [ 80 ])
Animal models of ACL reconstruction have also shown variable effects of early
loading on graft healing. In a rabbit model, Sakai et al. showed that histologic graft
incorporation and graft load-to-failure were improved in animals that were
immobilized vs. a group that was allowed normal cage activity postoperatively [ 79 ].
Rodeo et al. showed an inverse relationship between graft motion in different locations
of the bone tunnel and histologicmarkers of healing [ 32 ]. Using a rat model, Bedi et al.
evaluated the effects of cyclical axial loading producing approximately 2% strain
performed at various time points [ 80 ]. Animals were assigned to immobilization or
loading immediately, on postoperative day 4, or on postoperative day 10. The delayed
loading groups demonstrated improved mechanical and biologic healing parameters
including increased load-to-failure, increased new bone formation inside the bone
tunnel, higher levels of anabolic ED2+ macrophages and reduced tendon-bone inter-
face fibrous tissue, catabolic ED1+ macrophages, and osteoclasts (Fig. 13.5 ). The
authors hypothesized that the application of physiologic loads may bemost effective if
initiated after resolution of the acute inflammatory reaction to surgical trauma.
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