Meniscal Structure and Function 35proteoglycans influences the viscoelastic behav-
ior of the meniscus because their negative
charges counteract fluid flow.
The material properties of the meniscus are
also very important for its behavior in tension.
Variations in tensile stiffness and strength are
found in different regions, and relate to local
differences in collagen fiber ultrastructure and
fiber bundle orientation. Intact tie fibers are
also very important as they restrain motion
between circumferential fibers and improve the
stiffness characteristics of meniscus (Bullough
et al. 1970). The importance of the integrity of
the collagen network cannot be overempha-
sized, because disruption of this precise colla-
gen ultrastructure results directly in alterations
in tissue biomechanics, predisposing to tearing
and a clinically significant loss of meniscal func-
tion (Thiemanet al. 2009).
Meniscal function
The material properties of the meniscus are
closely related to its function. The combina-
tion of low compressive stiffness and low per-
meability suggests that the menisci are highly
efficient shock absorbers in the stifle. This is
crucial for the activities of dogs. When the
menisci are subjected to loading during weight-
bearing activities, they act as ‘firm pillows’
between the femoral and tibial condyles. Based
on their viscoelastic properties the wedge-
shaped menisci also adapt to the incongru-
ent articular surfaces until fluid flow ceases as
equilibrium is reached. Repeated compressive
loading and unloading of menisci create a cir-
culation pathway that is important for tissue
nutrition and joint lubrication (Arnoczkyet al.
1980).
Another important concept for understand-
ing the role of the meniscus as a shock absorber
is the hoop tension theory. Shriveet al. (1978)
proposed that the meniscus is able to absorb
high loads across the joint by converting the
compressive forces into radially directed forces.
This theory has been confirmed in dogs by
direct measurements of hoop strain and con-
tact pressures of the medial meniscus (Pozzi
et al. 2010a). As the meniscus is loaded, the
cranial and caudal attachments of the meniscus
are tensioned, along with the circumferential
fibers, to prevent the meniscus from extruding
(Figure 4.4). The tension developed in the
circumferential fibers from the cranial to the
caudal attachments is called ‘hoop tension.’ The
integrity of both collagen network and meniscal
attachments is critical for the hoop tension to
develop. A transection of the meniscus or its
attachments disrupts completely the primary
functions of the meniscus because it eliminates
the hoop tension (Pozziet al. 2008; Pozziet al.
2010b).
The meniscus contributes to stifle stability
and joint kinematics by enhancing congruity
between the convex tibial plateau and the round
femoral condyle. The effect of the meniscus
on joint congruity is evident when assessing
meniscal motion during flexion–extension. Both
menisci translate more than 13 mm caudally
on average as the stifle goes from extension
to flexion. Meniscal displacement maintains
congruity during the pronounced roll-back of
the femoral condyle (Park et al. 2016). This
increased congruity is important for joint sta-
bility. In the normal stifle, the CrCL is con-
sidered a primary restraint, while the menisci
act as secondary stabilizers. However, in the
CrCL-deficient stifle the menisci become pri-
mary stabilizers of the uncontrolled translation
and rotation of the joint. The caudal pole of
the medial meniscus is particularly important in
providing this stability, and as such is at highest
risk of injury if excessive femoro-tibial motion
is present, such as in the CrCL-deficient stifle
(Pozziet al. 2006).
The contributions of the menisci to joint con-
gruity and stability in both intact and CrCL-
deficient stifles suggest that a functionally intact
meniscus should be preserved whenever pos-
sible. This approach may be especially impor-
tant in the CrCL-deficient stifle, which may ben-
efit significantly from the stabilizing effects of
the meniscus for improving already disrupted
joint kinematics. In a weight-bearing radio-
graphic study evaluating joint stability after tib-
ial plateau leveling osteotomy (TPLO), dogs
with an intact meniscus had a normal femoro-
tibial alignment while dogs that underwent
meniscectomy had a small degree of tibial sub-
luxation (Kimet al. 2012). For meniscal preser-
vation it becomes crucial to protect the menisci
by stabilizing the joint with a technique that
most optimally reestablishes normal kinemat-
ics, neutralizing both cranio-caudal and rota-
tional instability. On the other hand, it could be