The capsules are removed. _18_, Femur; _20_, patella; _21_, tibia.
(After Ellenberger-Baum, Anat. für Künstler.)
]
=Ligaments.=—The =lateral femoro-patellar ligaments=, external and
internal (Ligamentum femoro-patellare fibulare, tibiale), are two thin
bands which reinforce the capsule on either side. The external ligament
is fairly distinct; it arises from the external epicondyle of the femur
just above the lateral femoro-tibial ligament, and ends on the external
border of the patella. The internal ligament is thinner and is not
distinct from the capsule; it arises above the internal epicondyle, and
ends on the patellar fibro-cartilage.
The =patellar ligaments= (Ligamenta patellæ), also called the straight
ligaments of the patella, are three very strong bands which attach the
patella to the tuberosity of the tibia. The =external patellar ligament=
is attached above to the outer part of the anterior surface of the
patella, and below to the outer part of the tuberosity. It receives a
strong tendon from the biceps femoris muscle. The =middle patellar
ligament= extends from the front of the apex of the patella to the lower
part of the groove on the tuberosity of the tibia, a bursa being
interposed between the ligament and the upper part of the groove. The
=internal patellar ligament= is attached above to the patellar
fibro-cartilage, and ends on the tuberosity of the tibia, internally to
the groove. It is joined by the common aponeurosis of the gracilis and
sartorius. These so-called ligaments are, in reality, the tendons of
insertion of the quadriceps femoris muscle, and transmit the action of
the latter to the tibia; they also function similarly for the other
muscles attached to them as noted above.
It will be noticed that the upper attachments are further apart than
the lower ones, so that the ligaments converge below. The inner
ligament is especially oblique. The middle ligament is more deeply
placed than the others, and therefore cannot usually be felt
distinctly in the living animal.
[Illustration:
FIG. 158.—SEMILUNAR CARTILAGES AND CRUCIAL LIGAMENTS OF RIGHT STIFLE
OF HORSE.
_F_, Femoral ligament of external cartilage. (After Schmaltz, Atlas d.
Anat. d. Pferdes.)
]
The =femoro-tibial articulation= (Articulatio femoro-tibialis) is formed
between the condyles of the femur, the proximal end of the tibia, and
the interposed semilunar cartilages.
=Articular Surfaces.=—The condyles of the femur are slightly oblique in
direction. The articular surface of the outer one is more strongly
curved than that of the inner one; the latter is confluent below with
the inner ridge of the trochlea, while the narrow ridge which connects
the external condyle with the trochlea is usually non-articular. The
saddle-shaped surfaces of the condyles of the tibia are not adapted to
the femoral condyles, and are in contact with only a small part of them.
[Illustration:
FIG. 159.—PROXIMAL ENDS OF RIGHT TIBIA AND FIBULA OF HORSE, WITH
SEMILUNAR CARTILAGES AND CRUCIAL LIGAMENTS, EXTERNAL VIEW. (After
Schmaltz, Atlas d. Anat. d. Pferdes.)
]
The =semilunar cartilages= (Meniscus lateralis, medialis) are two
C-shaped or crescentic discs of fibro-cartilage which produce congruence
in the articular surfaces. Each has an upper concave surface adapted to
the condyle of the femur, and a lower surface which fits the
corresponding condyle of the tibia. The external cartilage does not
cover the outer and posterior part of the condyle, over which the tendon
of origin of the popliteus muscle plays. The peripheral border is thick
and convex, the central one very thin and concave. The fibrous ends or
=cornua= are attached to the tibia in front of and behind the spine. The
external cartilage has a third attachment by means of an oblique band
(Ligamentum femorale menisci lateralis) which passes from the posterior
cornu to the posterior part of the intercondyloid fossa of the femur.
The cornua of the internal cartilage (Ligamenta tibiæ anterius et
posterius menisci medialis) are attached in front of and behind the
inner eminence of the spine. The anterior cornu of the external
cartilage (Ligamentum tibiæ anterius menisci lateralis) is attached in
front of the outer eminence of the spine. The posterior cornu
bifurcates; the lower branch (Ligamentum tibiæ posterius menisci
lateralis) is inserted at the popliteal notch, the upper (Ligamentum
femorale menisci lateralis) in a small fossa in the extreme posterior
part of the intercondyloid fossa.
The =joint capsule= is attached to the margin of the tibial articular
surface, but on the femur the line of attachment is for the greater part
about half an inch from the articular margin. It is also attached to the
convex borders of the semilunar cartilages and to the crucial ligaments.
It is strong posteriorly, but in front it practically consists only of
the synovial layer. There are two synovial sacs, corresponding to the
double nature of the articular surfaces; they do not usually
communicate, and each is partially divided into an upper and a lower
compartment by the semilunar cartilage. The inner sac pouches upward
about half an inch over the condyle of the femur. The external sac
invests the tendon of origin of the popliteus muscle, and also pouches
downward about three or four inches (ca. 8 to 10 cm.) beneath the
peroneus tertius and long extensor muscles (Fig. 156). As stated above,
the outer sac sometimes communicates with the femoro-patellar joint
cavity, and the inner sac usually, if not always, does so in the adult.
[Illustration:
FIG. 160.—LEFT STIFLE JOINT OF HORSE, INTERNAL VIEW IN EXTREME
EXTENSION.[38]
_18_, Femur; _20_, patella (base); _21_, tibia; _d_, internal
semilunar cartilage. (After Ellenberger-Baum, Anat. für Künstler.)
]
=Ligaments.=—There are four of these—two lateral and two crucial.
The =internal lateral ligament= (Ligamentum collaterale tibiale) is
attached above to the prominent internal epicondyle of the femur, and
below to a rough area below the margin of the internal condyle of the
tibia.
The =external lateral ligament= (Ligamentum collaterale fibulare) is
somewhat thicker; it arises from the upper depression on the external
epicondyle, and ends on the head of the fibula. It covers the tendon of
the origin of the popliteus muscle, and a bursa is interposed between
the lower part of the ligament and the margin of the external condyle of
the tibia.
The =crucial ligaments= are two strong rounded bands situated mainly in
the intercondyloid fossa of the femur, between the two synovial sacs.
They cross each other somewhat in the form of an X, and are named
according to their tibial attachments. The =anterior crucial ligament=
(Ligamentum cruciatum anterius) arises in the central fossa on the
tibial spine, extends upward and backward, and ends on the upper part of
the inner surface of the external condyle of the femur. The =posterior
crucial ligament= (Ligamentum cruciatum posterius) is internal to the
preceding, and is somewhat larger. It is attached to an eminence at the
popliteal notch of the tibia, is directed upward and forward, and ends
in the anterior part of the intercondyloid fossa of the femur.
[Illustration:
FIG. 161.—RIGHT STIFLE JOINT OF HORSE; SAGITTAL SECTION PASSING
THROUGH OUTER PART OF INNER RIDGE OF TROCHLEA AND INTERCONDYLOID
FOSSA.
_1_, Anterior crucial ligament; _2_, posterior crucial ligament; _3_,
posterior cornu of internal semilunar cartilage; _4_, femoral
ligament of external semilunar cartilage; _5_, articular artery;
_6_, part of vastus internus.
]
It may be added that these ligaments do not lie in a sagittal plane,
but are somewhat twisted across each other; outward rotation of the
leg untwists and slackens them.
=Movements.=—The principal movements of the stifle joint as a whole are
flexion and extension. In the ordinary standing position the articular
angle (behind) is about 140° to 150°. Flexion is limited only by contact
of the leg with the thigh if the hock is also flexed. Extension is
incomplete, _i. e._, the femur and tibia cannot be brought into the same
straight line. Rotation is limited, and is freest during semiflexion.
The patella glides on the femoral trochlea upward in extension, downward
in flexion.
Extension is checked mainly by tension of the crucial and lateral
ligaments. In extreme extension, which is accompanied by slight
outward rotation of the leg, the patella can be pushed upward and
inward so that its fibro-cartilage hooks over the upper end of the
inner ridge of the trochlea, but it will not remain there unless held
in position. When pressure is removed, the base of the patella tips
forward and the cartilage lies upon the most prominent part of the
trochlear ridge. During flexion, which is accompanied by slight inward
rotation of the leg, the condyles of the femur and the semilunar
cartilages glide backward on the tibia; the movement of the external
condyle and cartilage is greater than that of the inner one. In
extreme flexion the patellar and posterior crucial ligaments are
tense; the other ligaments are relaxed. The movement of the patella is
gliding with coaptation, _i. e._, different parts of the opposing
articular surfaces come into contact successively. Only a narrow
transverse strip (ca. 1.5 to 2 cm. wide) of the patella is in contact
with the trochlea at a time.