Engineering Fibrous Tissues and Their Interfaces with Bone - Structural Interfaces and Attachments in Biology - page 330

microCT imaging revealed mineral deposition was present in areas of high retrovi-
ral coating only. This experiment demonstrates proof-of-concept evidence for a
means to create a single scaffold that can exhibit properties of a native interface
region containing graded mineralization.
Phasic and gradient scaffolds both aim to recreate the tendon/ligament to bone
interface. Phasic scaffolds are limited by the lack of continuous transitions between
different zones. However, the gradient scaffolds developed to date pose the prob-
lem of requiring gene transfection agents to achieve the desired biological outcome.
While promising early results have been obtained, for both techniques, reestablish-
ment and regeneration of the interface region has not been successfully shown
in vivo , therefore additional experiments are required. Effects of biological, physi-
cal, and chemical stimulation on these constructs also have not been explored [ 137 ].
Therefore, significant further work in this area is required before these approaches
could be implemented clinically for interface regeneration.
15.5.3 Soluble Factors
The growth factor BMP-2 has been shown to stimulate tendon-/ligament-bone
interface healing [ 141 - 143 ]. BMP-2 was incorporated into a poly(ethylene glycol)
(PEG)/hyaluronic acid hydrogel with periosteal progenitor cells to direct fibrocar-
tilaginous attachment and new bone formation in a healing tendon model [ 141 ].
This construct was implanted in the knee joints of white rabbits for up to 6 weeks.
After this time, RT-PCR revealed that osteogenic markers of collagen type I and
osteopontin were present, and the cartilage specific markers of collagen type II and
aggrecan were expressed. Histological findings demonstrated that in hydrogels with
BMP-2, the graft interface contained organized collagen fibers and Sharpy-like
fibers after 3 weeks [ 141 ]. This study suggests improved graft fixation may be
achieved clinically with a BMP delivered from a PEG-based hydrogel.
Another study aimed to develop a method for improving the integration of grafts
for the replacement of ACL. Semitendinosus tendon grafts were infected with a
retroviral adenovirus-BMP-2. These tendon grafts were implanted and replaced the
native ACLs of rabbits [ 142 ]. Over the course of 8 weeks after surgery, a four-zone
interface was histologically visible and higher failure loads and stiffness levels
when compared to grafts without BMP-2 were observed [ 142 ]. Overall, this study
demonstrates the potential of BMP-2 gene transfer to improve the integration of
tendon grafts to bone after reconstruction surgery.
Delivery of soluble factors has been a recent advance in engineering fibrous
tissue interfaces and, as the above studies indicate, holds promise in enhancing
current graft fixation and providing possible means to create fully integrated tissue-
engineered bone-ligament-bone grafts. However, further experiments are required
to better understand the signaling pathways involved and the effects of temporal
combinations of growth factors on tissue formation, bone tunnel healing, and graft
fixation in order to further optimize these approaches.