Biomedical Engineering Reference
The significance and translation of these findings to clinical practice is yet to be
determined. There is data to support that early aggressive rehabilitation protocols
may contribute to tunnel enlargement and theoretically more stress at the tendon-
bone interface [ 81 , 82 ]. There is also good data supporting early motion, early
weight bearing, and closed chain kinetic exercises in successful ACL rehabilitation
protocols [ 83 - 87 ]. More studies are needed to understand how different postopera-
tive rehabilitation protocols influence biologic healing and clinical outcomes.
13.5 Techniques for Improving Graft Healing
The slower and more variable rates of incorporation of tendon grafts used in ACL
reconstruction have generated a strong interest in strategies to augment and improve
the healing process. These include growth factors, biomaterial augmentation,
biologic modulation, cell therapy, biophysical modalities, and gene therapy [ 13 ].
13.5.1 Growth Factors
Bone formation is critical for tendon graft-to-bone healing and the addition of
growth factors has been shown to improve the graft healing process. In a sheep
model, PDGF increased vascular density and collagen fibril number at 6 and 12
weeks and increased load-to-failure at 6 weeks [ 88 ]. In an intra-articular canine
model, transforming growth factor-
1) improved the density of perpen-
dicular collagen fibrils and load-to-failure vs. controls [ 89 ].
In a rabbit model, Anderson et al. used a type I collagen sponge carrier that
contained a mixture of bovine bone-derived bone morphogenetic protein (BMP)-2,
1, and fibroblastic growth factor [ 90 ]. Compared
to controls, the treatment group had significantly higher load-to-failure values and
more extensive new bone and cartilage formation at the graft bone interface. Other
growth factors including vascular endothelial growth factor (VEGF) [ 91 ] and
granulocyte colony stimulating factor [ 92 ] have shown improved histological
and biomechanical properties when introduced at the tendon-bone interface.
Calcium phosphate has a chemical composition similar to bone and has been used as
a bone void filler and osteoconductive material in the setting of fractures with bone
loss. These properties make it an attractive option as an augment to the tendon-bone
interface to encourage bone formation and inhibit fibrous tissue deposition.
Various forms have been investigated in lab studies and show encouraging early