Biomedical Engineering Reference
In-Depth Information
the SED increased by approximately 45% at the anteromedial site. The increased SED may affect
the cartilage signal pathways and potentially contribute to osteoarthritis (Gosset et al. 2008).
6.3.3 c
HanGe
of
S
treSS
d
irection
after
acl r
econStruction
ACL reconstruction further influenced the direction of stress in the bone. As shown in Figure 6.3,
tensile stress can be seen close to the femoral tunnel wall after ACL reconstruction, and reduced
compressive stress is apparent around the tunnel. From the trajectories of the tensile and compres-
sive stress vectors, it was confirmed that the creation of the bone tunnel interrupted the normal
load transmission in the knee and was responsible for the abnormal SED after surgery. The normal
compressive stress trajectory was blocked by the tunnel and the tensile stress increased around the
tunnel wall. The changes in stress orientation occurred near the intra-articular tunnel aperture,
which is a high-risk region for tunnel enlargement and osteoarthritis. Furthermore, these changes
in stress also influenced the mechanical environment within the cartilage. In both femoral and
tibial cartilages, SEDs were increased around the cartilage edge close to the intra-articular tunnel
apertures. Since there is a close relationship between the mechanical environment and cartilage
metabolism (Gosset et al. 2008), the postoperative abnormal SED may contribute to long-term car-
tilage degradation.
6.3.4 e
ffect
of
S
creW
m
aterial
on
Sed d
iStriBution
The influence of screw material on the redistribution in tibial SED was also investigated. Figure 6.4
indicates the coupled effect of both the tunnel and screw in altering the SED. When the modulus of
the screw is greater than that of cancellous bone, the screw played a dominant role in the redistribution
of the tibial SED. Conversely, when the modulus of the screw is lower than that of cancellous bone,
the bone tunnel became the dominant factor. Figure 6.4 shows that the abnormal SED alteration was
minimized when the screw modulus was equal to that of cancellous bone. This result implies that, on
the premise of sufficient fixation strength, using a screw with a modulus similar to that of bone could
decrease the risk of stress shielding, and thus could be positive for the surgical outcome.
6.3.5 d
iStriBution
of
Sed
in
i
interference
S
creW
The SED distribution in the screw was also nonuniform. The SED was low around the anterolat-
eral shaft of the screw, whereas it was high at the posteromedial shaft and the head of the screw
(Figure 6.5). Recently, there has been increasing interest in biodegradable interference screws, which
Compressive stress
Tensile stress
Cross section
Tensile stress
increased adjacent
to tunnel wall
(a)
(b)
FIgure 6.3
(See color insert.)
Compressive and tensile stress trajectories in a coronal section of the femur.
The red arrow indicates compressive stress, and the black arrow indicates tensile stress. (a) Intact knee. (b)
Knee after ACL reconstruction.
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