32 Structure and Function
Figure 4.1 Frontal plane image of anex vivospecimen
showing the congruity provided by the menisci to the
femoral and tibial articulation. The wedge-shaped
menisci fill the space between the femoral and the tibial
condyles.
by the infrapatellar fat pad. A comprehensive
understanding of meniscal anatomy is integral
to understanding disease mechanisms, and for
avoiding iatrogenic damage to these important
structures during surgical treatment of stifle
disorders.
Structure and composition
The menisci consist of highly differentiated cells
of various phenotypes within an intricately
arranged extracellular matrix (ECM). Menis-
cal cells vary from fusiform, fibroblastic to
rounded, chondrocytic phenotypes, depending
on location. The morphological similarities of
these meniscal cells to those of other muscu-
loskeletal tissues suggest specific functions in
connective tissue synthesis and maintenance
for each phenotype. For example, cells in the
periphery of menisci are similar to those in lig-
ament and tendon, while cells in the central
region are similar to hyaline cartilage (Helio Le
Graverandet al. 2001). The ECM associated with
the cells in these different regions also mimics
the composition and function of the respective
tissues.
The ECM of menisci is composed primarily of
water, collagens, and proteoglycan aggregates
(Adams & Ho 1987; Stephanet al. 1998; Cook
et al. 1999; Nooneet al. 2002). Although menis-
cal tissue contains several different molecular
types of collagen, type I collagen accounts for
about 90% of the total collagen present (Eyre &
Wu 1983). Other types of collagen are also
present and demonstrate regional differences.
For example, type I collagen is more abundant
in the periphery, while type II is predominant in
the axial third of the meniscus (Cheung 1987).
This arrangement is most likely related to the
specific biomechanical functions of each region
(Bulloughet al. 1970). On the meniscal surfaces,
the collagen fibrils are randomly oriented and
form a mesh. Just beneath this layer the colla-
gen bundles show a more irregular orientation.
Deep to these layers in the peripheral meniscus,
the collagen fibers are organized in large bun-
dles, which are circumferentially arranged from
cranial to caudal attachments sites. Small radial
fibers, also called ‘tie fibers,’ are arranged across
the circumferential fibers and connect the abax-
ial region to the axial regions, where collagen
II fibers and proteoglycan predominate. The tie
fibers provide structural rigidity by resisting
the splitting force that arises from compressive
loading to the axial region and helping to trans-
fer it to a radial load, which the circumferential
fibers can resist.
Several types of proteoglycan exist in the
meniscus. In the adult dog a relatively constant
distribution of 60% chondroitin-6-sulfate, 25%
chondroitin-4-sulfate, 10% chondroitin, and 5%
dermatan sulfate is found (Adams & Muir
1981). The distribution of the proteoglycan
depends on the region of the meniscus, the inner
third being approximately 8% proteoglycan and
the outer third only 2% proteoglycan.Neurovascular anatomy
The menisci are relatively avascular. Both
menisci demonstrate a common pattern and
distribution of blood vessels that arise from
the medial and lateral genicular arteries
(Arnoczky & Warren 1983). Within the synovial
and capsular tissue of the stifle, an extensive
perimeniscal capillary plexus supplies the
peripheral border of the meniscus throughout