DISCUSSION: of the construct until histological anchoring

DISCUSSION:

            Tunnel
enlargement is a consequence of impaired graft healing16 and graft
healing depends on a secure fixation technique that will not allow graft to
move within the bone tunnel during everyday activities. Thus, graft healing,
fixation technique and tunnel enlargement are inter-related. Graft fixation is
the weak link of the construct until histological anchoring of the graft in the
bone tunnel occurs23,24. In animal models, it appears that grafts
with bone plugs (6 weeks) achieve histological incorporation earlier than soft
tissue grafts (4 months)25. Wieler et al26 showed in
animal studies that reducing the relative motion between tendon and wall of the
tunnel promotes the ingrowth of an intervening fibrous layer and bony
trabeculae. They used biodegradable IFS to reduce shear forces on the tendon within
the tunnel. In a biomechnical study, Ishibashi et al27 demonstrated
that graft fixation at the aperture reduces anterior tibial translation
compared with extra-articular fixation techniques. This improved stability can
be attributed to an overall shortening of the fixation construct leading to
reduced elastic deformation. The total length of a fixation button-graft
construct is much longer than the normal ACL. This leads to longitudinal motion
of the graft in the tunnel or the ‘bungee effect’15. Buelow11,
Fauno28, Iorio14 have all concluded that there was a
significant reduction of tunnel widening using fixation points close to the
joint compared to the system where the distance between fixation points is
long. In our study also, there was significantly less tunnel enlargement
(p-value) with the use of interference screw as compared to fixation button. We
have observed tunnel enlargement in the early post-operative period (2 weeks)
probably because we have used extraction drilling29,30. A possible
solution to this problem may be to drill 1 mm smaller than measured graft
diameter and then enlarge to appropriate diameter with a tunnel dialator29.
Another possible cause for early tunnel enlargement may be the early
aggressive, brace-less rehabilitation protocol that we have followed. Several
studies5,14,31,32 have shown that non-aggressive rehabilitation can
reduce micro-motion of the graft in the bone tunnel and thereby reducing
synovial bathing effect which may result in tunnel enlargement. Measuring bone
tunnels by radiographs can underestimate the size of bone tunnels. CT scan
accurately images the boundaries of the intra-osseous tunnels and can give
axial cross-sections. CT scans are not influenced by factors of magnification
and knee positioning14. Fink et al12 and Harris et al33
reported that enlargement occurs particularly within first 6 weeks after
operation and no further increase is observed 2 years after operation. We have
also noted maximum tunnel enlargement during first 2 months after surgery.

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