Soft Tissue-to-Bone Healing in Anterior Cruciate Ligament Reconstruction - Structural Interfaces and Attachments in Biology - page 275

13.4.4 Tunnel Position
The ideal position of tunnel placement is an important and highly debated topic in
current ACL reconstruction. Despite no universal consensus on the optimal tunnel
position or method of achieving it, tunnel misplacement is generally accepted to
be one of the main causes of failure of ACL reconstruction [ 56 - 58 ]. Several
biomechanical lab studies have shown that graft placement in a position closer to
the anatomic footprint results in better initial biomechanical stability and knee
kinematics [ 59 - 61 ]. There is also data that supports that separately reconstructing
the two functional bundles of the ACL (anteromedial and posterolateral) better
reproduces native knee kinematics and provides superior clinical results [ 62 - 68 ].
Much less is known about the effect of tunnel position and single vs. double bundle
reconstruction on graft healing and incorporation.
In an extra-articular rabbit model, a soft tissue graft was placed perpendicular to
the tibia to create an aperture with compressive forces on one side and tensile forces
on the other side [ 69 ]. The tensile end showed more abundant Sharpey's fibers early
and a direct type insertion with four zones after 6 months. On the compressive side,
there was chondroid formation and woven bone at the tendon-bone interface. In an
intra-articular goat model, Ekdahl et al. [ 70 ] evaluated the effects of two nonana-
tomic reconstructions and one anatomic reconstruction of one of the two bundles of
the native ACL. The anatomic tunnel placement group had less tibial tunnel enlarge-
ment and fewer osteoclasts within both the tibial and femoral tunnels. There was also
more vascularity, which has been previously shown to correlate with graft strength
[ 71 ] on the femoral side. Biomechanically, the anatomic reconstruction group had
less tibial translation and lower in situ forces than the nonanatomic tunnel groups.
13.4.5 Graft-Tunnel Motion and Rehabilitation
The effect of graft motion and subsequent mechanical loading during the healing
process is a complex interaction that is not fully understood. In their normal state,
ligaments and tendons show sensitivity to variable mechanical demands and tend to
demonstrate an increase in tensile modulus in response to increasing loads. Stress
deprivation has also been shown to be associated with a decline in mechanical
properties [ 30 , 72 - 75 ]. It is also well known that bone is sensitive to variable
mechanical loads. Stress deprivation leads to decreased bone mineral density and
resorption while increasing loads stimulate increased bone formation and improved
mechanical properties [ 76 - 78 ].
It is less clearly known how mechanical loads affect tendon-to-bone healing in
the setting of tendon repair and ligament reconstruction. In a canine flexor tendon
model, Thomopoulos et al. found that muscle loading significantly improved
biomechanical properties compared with unloaded repairs [ 75 ]. In contrast, the
same group found that early exercise after rotator cuff repair in a rat model led to a
diminished healing response compared to a group that was immobilized [ 74 ].