Biomedical Engineering Reference
In-Depth Information
fibrocartilage, and bone. There is a tidemark representing the mineralization front
between the calcified and uncalcified fibrocartilage zones. This gradual change in
tissue composition and biomechanical properties is believed to allow more efficient
load transfer between tendon and bone. Examples of direct insertions include: the
ACL, achilles tendon, patellar tendon, rotator cuff, and femoral insertion of the
medial collateral ligament (MCL).
Indirect insertions (fibrous entheses) include the tibial insertion of the MCL and
the deltoid insertion on the humerus. There is no fibrocartilage transitional zone and
the tendon fibers pass obliquely along the surface and insert over a broader area via
Sharpey's fibers [ 13 , 14 ]. In this way, the tendon collagen fibers become continuous
with the underlying bone.
13.3 Tendon Graft-to-Bone Healing in ACL Reconstruction
Healing of a tendon graft in a bone tunnel following ACL reconstruction takes place
in four phases: inflammatory phase, proliferative phase, matrix synthesis, and
matrix remodeling (Fig. 13.2 ). The inflammatory phase begins shortly after graft
implantation. There is recruitment of inflammatory cells and marrow-derived stem
cells to the tendon graft interface as early as 4 days [ 15 ]. These inflammatory cells
release cytokines and growth factors including transforming growth factor (TGF)-
beta and platelet-derived growth factor (PDGF). These cytokines may contribute to
the formation of a fibrous scar tissue interface between the bone and tendon graft
[ 15 ]. There is also an ingrowth of blood vessels and nerves that may be due to
growth factor stimulation or hypoxia [ 16 , 17 ]. The graft is covered by a vascular
synovial envelope by 6 weeks and the intrinsic vasculature matures gradually for at
least 5-6 months [ 18 ].
The acute inflammatory phase is followed by a period of proliferation of the
inflammatory and marrow-derived stromal cells. In the matrix synthesis phase, a
provisional matrix consisting of type III collagen and some type I collagen is
deposited. This is subsequently degraded by matrix metalloproteinases (MMPs)
and serine proteases, and new matrix with progressive bony ingrowth is deposited.
During matrix remodeling, the graft, interface tissue, and new bone remodel to
create a collagen fiber attachment between the graft and host bone [ 19 , 20 ].
In the normal uninjured ACL, the transition zone is a direct type insertion with a
fibrocartilagenous interface. The nature of the healing attachment between the
tendon graft and bone tunnel is debatable, with animal models demonstrating
both direct and indirect types of insertions. Some animal studies have demonstrated
chondrocytes present at the graft bone interface early in the healing process
[ 21 - 24 ]. Other studies have demonstrated an indirect attachment with the presence
of Sharpey's collagen fibers at the interface (Fig. 13.3 )[ 25 - 28 ].
In a dog model, Rodeo et al . evaluated the progression of collagen fiber conti-
nuity between the graft and host bone [ 25 ]. At 2 weeks, the earliest time point of
sectioning, there was evidence of a highly cellular, fibrous interface tissue between
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