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Anatomy
ANATOMY OF THE MENISCI
The knee is truly a remarkable structure when viewed
anatomically, biomechanically and physiologically. Consisting of the
relation between three articulations; patellofemoral, tibiofemoral, and
tibiofibular. Kettelkamp and Jacobs in 1972 stated that this triaxial joint
is often exposed to forces in excess of five times body weight per step
(Scauderi et al., 1996).
The distal femur articulates with the proximal tibia throughout its
range of motion in what appears to be incongruous situation.
The presence of the medial and lateral menisci, however, converts
this nonconforming geometry into a joint capable of sustaining significant
functional loading (Insall, 1984).
The meniscal apparatus (Segal and Jacob, 1989) :
The menisci are two cresentic lamellae of dense fibrous tissue
placed on condylar surface of the tibia inside the knee joint (Figure 1).
They differ in shape but have the same general form and structure.
Both end in anterior and posterior horns attached to anterior and posterior
intercondylar areas of the tibia respectively.
They are triangular in cross section. The upper surface is concave
from side to side and corresponds to femoral condyles. The lower surface
is nearly flat from side to side and corresponds to the tibial condyles. The
peripheral border is thick, convex and adheres to the capsule of the knee
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Anatomy
joint. The medial border is thin, concave, free and faces the intercondylar
notch.
Fig. (1) : The Meniscal apparatus
LMPL
TL
MMPL
MW
: Lateral Meniscopatellar Ligament
: Transverse Ligament
: Medial Meniscopatellar Ligament
: Meniscal Wall.
LM
: Lateral Meniscus
AHLM : Anterior Horn of Lateral Meniscus
PHLM : Posterior Horn of Lateral Meniscus
LDM : Lateral Dynamic Mooring
PMFL : Posterior Meniscofemoral Ligament
PHMM : Posterior Horn of Medial Meniscus
AHMM : Posterior Horn of Medial Meniscus
MDM : Medial Dynamic Mooring
MM : Medial Meniscus
After Segal and Jacob (1989)
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Anatomy
They are the only non-synovialised intra-articular structures. They
have numerous attachments, some of these are highly specific and
constant. These are :
1-
The transverse ligament : between the anterior horns of the menisci
and involved in the extensor apparatus through the infrapatellar fold.
2-
Meniscotibial thick bundles of fibrous tissue.
3-
Meniscocapsular attachment. This reinforces the medial and lateral
meniscopatellar ligaments.
They are divided into two zones (Figure 2). The peripheral zone
starting at the capsulo-synovial level and forming a quarter of the
meniscal area. It is vascular so called ‘parameniscal zone”. The second
zone forms three quarters of the meniscal area and called “articular zone”
because it is essentially fibrocartilagenous and capable of adapting with
articular needs.
Microscopically the menisci are composed primarily of collagen
fibers (98% type one) arranged mainly in circumferential manner with
few radial fibres primarily on the tibial surface, and few vertical or
oblique fibres connect upper and lower surfaces.
This characteristic orientation is probably a response to the over
whelming circumferential forces on the menisci. The radially directed
fibres resist excessive compression loads (Figure 3).
In addition to the collagen, the extracellular matrix includes
proteoglycans, glycoproteins, and elastin. Chondroitin sulfate is the
dominant
glycosaminoglycan
in
macromolecule.
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the
aggregating
proteoglycan
Anatomy
Fig. (2) : Zones of the Menisci
Cap
Syn
MW
: Capsule
: Synovial Membrane
: Meniscal Wall
After Segal and Jacob (1989)
Fig. (3) : Characteristic orientation of meniscal collagen fibres
After Segal and Jacob (1989)
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Anatomy
Stockwell in 1972 found that type I and II nerve endings
accompany the peripheral vascular supply of the menisci (Scuderi et al.,
1996). The midsubstance of the menisci is avascular, aneural and
alymphatic fibrocartilage consisting
of cells
(fibrochondrocytes)
surrounded by abundant extracellular matrix. The meniscal cells,
although typical chondrocytes, are called ‘fibrochondrocytes” because
they synthesize a fibrocartilage matrix rather than the hyaline cartilage
matrix typical of articular chondrocytes (Arnoczky, et al., 1988).
Each meniscus has specific anatomic features.
The medial meniscus (Fig. 4, 5) :
The medial meniscus is nearly semicircular in form, and broader
posteriorly than anteriorly. Its anterior end, or horn is attached to the
anterior intercondylar are of the tibia, in front of the anterior cruciate
ligament, its posterior fibres are continuous with the transverse ligament.
The anterior horn lies in front of the depression which is left on the
medial side of the upper part of the ligamentum patellae.
The posterior horn is fixed to the posterior intercondylar area of the
tibia between the attachments of the posterior horn of lateral meniscus
and the posterior cruciate ligament.
The peripheral border of the medial meniscus is attached to the
fibrous capsule of the knee joint and firmly adherent to the deep surface
of the tibial collateral ligament. By this capsular attachment the meniscus
is attached to the femur and tibia. The capsular attachment to the tibia is
lax, that to the femur is strong on the medial side forming the short
“internal collateral ligament”.
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Anatomy
Fig. (4) : The Medial Meniscus (After Duckworth, 1984)
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Anatomy
Fig. (5) : Relations of medial meniscus
QT
MFL
MMPL
MM
TCL
PL
GrM
ST
Sar
PA
: Quadriceps Tendon
: Medial Femoropatellar Ligament
: Medial Meniscopatellar Ligament
: Medial Meniscus
: Tibial Collateral Ligament
: Patellar Ligament
: Gracillis Muscle
: Semitendinosus
: Sartorius
: Pes Anserinus
MG
MPC
SM
MBC
: Medial head of Gastroenemius
: Medial Posterior Capsule
: Semimembranous Muscle
: Medial Back Corner (Zone of least
resistance)
MDM : Medial Dynamic Mooring
St TSM : Straight Tendon of
Semimembranous Muscle
RfTSM : Reflected Tendon of
Semimembranous Muscle
Fmid POL : Middle fibres of Posterior
Oblique Ligament
FLOW POL : Lower fibres of Posterior
Oblique Ligament
After Segal and Jacob (1989)
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Anatomy
The attachments of the medial meniscus are reinforced with
meniscomuscular attachments including the “medial dynamic mooring”
that connects the posterior horn of the meniscus to the straight head of the
semimembranosus muscle and associated structures.
The lateral meniscus (Figures 4 : 8) :
The lateral meniscus is nearly circular in form and covers a larger
portion of the articular surface than the medial meniscus. It has a uniform
breadth throughout its extent and grooved posterolaterally by the tendon
of the popliteus muscle which separates the meniscus from the fibular
collateral ligament (Figure 6).
Its anterior end, or horn is attached in front of the intercondlyar
eminence of the tibia behind and lateral to the anterior cruciate ligament
with which it blends partly. The anterior attachment is twisted so that its
free margin looks backwards and upwards. Also the anterior end is
resting on a slooping shelf of bone in the front of the lateral intercondylar
tubercle.
The posterior horn of the lateral meniscus is attached behind the
intercondylar eminence of the tibia in front of the posterior horn of the
medial meniscus (Figure 4) close to its posterior attachment it commonly
sends off a strong fasciculus called the posterior meniscofemoral
ligament or “ligament of Wrisberg”. Which passes upwards and medially
behind the posterior cruciate ligament to be inserted into the medial
condyle of the femur. The anterior meniscofemoral ligament or “ligament
of Humphry” is another oblique band that may arise from the posterior
part of the meniscus and pass to the medial condyle of the femur in front
of the posterior cruciate ligament (Figure 6).
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Anatomy
These meniscofemoral ligaments often constitute the sole
attachments of the posterior horn of the lateral meniscus. The tendon of
the popliteus muscle intervens between the lateral meniscus and the
fibular collateral ligament (Figure 7). The more medial part of the tendon
is inserted into the lateral meniscus. The mobility of the posterior horn of
the lateral meniscus is controlled by meniscofemoral ligaments and the
popliteus muscle tendon. The attachment of the peripheral surface of the
lateral meniscus to the capsule of the knee joint is interrupted at the rear
by the popliteus muscle tendon which crosses the posterior third of the
meniscus at a more or less oblique angle through the popliteal hiatus
(Figure 8).
Also a meniscomuscular attachment called the “lateral dynamic
mooring” connects the posterior horn of the lateral meniscus to the
popliteus muscle tendon and other structures.
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Anatomy
Fig. (6) : Attachment of lateral meniscus (After Sisk, 1992)
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Anatomy
Fig. (7) : Relations of lateral meniscus
LIMS
LPC
LFL
FCL.L
Fa
FCL-S
LBC
APL
PT
BT
: Lateral Intermuscular Septum.
: Lateral Posterior Capsule.
: Lateral Femorpatellar Ligament
: Fibular Collateral Ligament-long
: Fabella
: Fibular Collateral Ligament Short
: Lateral Back Corner
: Arcuate Posterior Ligament
: Popliteus Tendon
: Biceps Tendon
QT
FL
ITB
LMPL
LFc
ITA
LM
PL
: Quadriceps Tendon
: Fascia Lata
: Iliotibial Band
: Lateral Meniscopatellar Ligament
: Lateral Front Corner
: Ilioi Tibial Aponeurosis.
: Lateral Meniscus
: Patellar Tendon
After Segal and Jacob (1989)
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Anatomy
Fig. (8) : Relation of lateral meniscus to capsule of knee joint
Cap
PH
PT
LM
MM
MW
: Capsule
: Popliteal Hiatus
: Politeal Tendon
: Lateral Meniscus
: Medial Meniscus
: Meniscal Wall
After Segal and Jacob (1989
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Anatomy
* BLOOD SUPPLY OF THE MENISCI
The blood supply to medial and lateral menisci originates
predominately from lateral and medial genicular arteries which are
branches from the popliteal artery. Branches from these vessels give rise
to perimeniscal capillary plexus within the synovial membrane and
fibrous capsule of the knee joint as Arnoczky and Warren found in 1982
in an experimental study on dogs. This plexus supplies the peripheral
border of the menisci throughout their attachment to the joint capsule.
These perimeniscal capillaries are oriented predominately in a
circumferential pattern with radial branches directed toward the center of
the joint (Scuderi et al., 1996).
These vessels penetrate the meniscal stroma for a short distance
and terminated in small capillary loops. The degree of vascular
penetration is about 15% to 20% of the width of the meniscus.
The posterolateral portion of the lateral meniscus immediately
adjacent to popliteus muscle tendon, has no attachment to the capsule of
knee joint and thus it is devoid of any penetrating perimeniscal vessels.
The synovial tissue gives a small reflection throughout the peripheral
border of the medial meniscus on both its femoral and tibial surface. This
“synovial fringe” extends for about one millimeter over the articular
surfaces of the meniscus and contains small vessels freely anastomosing
with one another. This vascular synovial tissue is intimately adherent to
the articular surface of the meniscus but does not contribute vessels into
the meniscal tissues (Fig. 9).
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Anatomy
Microvasculature of the menisci
Fig. (9) : Superior aspect of medial, A, and lateral, B, menisci following vascular
perfusion with India ink and tissue clearing using modified Spaltheholz
technique. Note vascularity at periphery of meniscus, as well as at
anteior and posterior horn attachments. Absence of peripheral
vasculature at posterolateral corner of lateral meniscus (arrow)
represents area of passage of popliteal tendon (After Arnoczky and
Warren, 1982).
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Anatomy
Also another synovial fringe is present over the peripheral articular
surfaces of the lateral meniscus except over the area adjacent immediately
to the popliteus muscle tendon.
The anterior and posterior horn attachment of both menisci are
covered also with a layer of vascular synovial tissue continuous with the
vascular synovial sheath which surrounds the cruciate ligaments. Vessels
within this synovial covering give branches to the ligamentous
attachments of the menisci. These vessels penetrate the meniscal stroma
of the anterior and posterior horns for one to two millimeters and
terminate in small capillary loops.
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Anatomy
EMBRYOLOGIC DEVELOPMENT
OF THE MENISCI
Each limb develops from a mass of mesoderm derived from
“lateral plate mesoderm” and “migrated myotomes”.
This “limb bud” becomes flattened and rotates. The lower limb bud
rotates internally so big toe comes medially.
The bones develop in the core of the limb bud as masses of
mesenchyme which becomes chondrified, then ossify. The joints are
formed as spaces left unchondrified between the individual bones. They
change into cavities which become later-on filled with synovial fluid.
There is a staging system for embryologic development depends on
the external appearance of the embryo and not on its length or age.
The staging system includes 23 stages or “horizons”. The leg bud
appears during the 13th horizon (28 days). In horizon 18 (37 days)
chondrification of the femur, tibia and fibula begins, along with early
differentiation of the patella and patellar ligament (Streeter, 1951).
The knee joint develops in the last 10 days of the embryologic
period from the blastemal interzone.
The patella, anterior cruciate ligament, posterior cruciate ligament
and both menisci develop in horizon 22 (45 days) (Figure 10).
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Anatomy
Fig. (10) : The progression of knee joint development from the mesenchymal skeleton
(horizon XVI) to the interzone (horizon XX). The menisci appear during horizon
XXII, and by horizon XXII discrete cavities and the synovium are present
(After Scuderi et al., 1996).
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Anatomy
The menisci become clearly defined by the 8 th week of
development as a collection of fibroblasts. With further development they
become more collagenous, gradually allowing for circumferential
orientation of the collagen bundles. At birth blood vessels are present
through much of the menisci then become limited to periphery at
midadolescence (Hosea et al. 1991).
The menisci were remnants of muscles that arose intra-articularly
and were useless in humans (Sutton, 1897). From an evolutionary
perspective, the development of the menisci represented the adaptation of
humans to a bipedal existence (Scuderi et al., 1996).
Although the amphibians were the first animals to exhibit
meniscus-like structures, the tetrapedal and bipedal mammals have the
most developed fibrocartilagenous menisci (Scuderi et al., 1996).
The crescent shape of the menisci prevails in most animals except
the horse, which has discoid menisci (Insall, 1984).
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