The human meniscus: A review of anatomy, function, injury, and advances in treatment: The Meniscus: Anatomy, Function, Injury and Treatment

Abstract

Meniscal injuries are recognized as a cause of significant musculoskeletal morbidity. The menisci are vital for the normal function and long-term health of the knee joint. The purpose of this review is to provide current knowledge regarding the anatomy and biomechanical functions of the menisci, incidence, injury patterns and the advancements in treatment options of meniscal injury. A literature search was performed by a review of PubMed, Google Scholar, MEDLINE, and OVID for all relevant articles published between 1897 and 2014. This study highlights the anatomical and biomechanical characteristics of the menisci, which may be relevant to injury patterns and treatment options. An understanding of the normal anatomy and biomechanical functions of the knee menisci is a necessary prerequisite to understanding pathologies associated with the knee. Clin. Anat., 2014. © 2014 Wiley Periodicals, Inc.

Full text

REVIEW
The Human Meniscus:
A Review of Anatomy, Function, Injury,
and Advances in Treatment
ALICE J.S. FOX,
1
* FLORIAN WANIVENHAUS,
2
ALISSA J. BURGE,
3
RUSSELL F. WARREN,
1
AND SCOTT A. RODEO
1
1
Laboratory for Soft Tissue Research, Hospital for Special Surgery, 535 East 70th Street, New York, New York
2
Department of Orthopaedics, Balgrist University Hospital, Forchstrasse 340, Zurich, Switzerland
3
Department of Radiology, Hospital for Special Surgery, 535 East 70th Street, New York, New York
Meniscal injuries are recognized as a cause of significant musculoskeletal mor-
bidity. The menisci are vital for the normal function and long-term health of the
knee joint. The purpose of this review is to provide current knowledge regard-
ing the anatomy and biomechanical functions of the menisci, incidence, injury
patterns and the advancements in treatment options of meniscal injury. A liter-
ature search was performed by a review of PubMed, Google Scholar, MEDLINE,
and OVID for all relevant articles published between 1897 and 2014. This study
highlights the anatomical and biomechanical characteristics of the menisci,
which may be relevant to injury patterns and treatment options. An under-
standing of the normal anatomy and biomechanical functions of the knee
menisci is a necessary prerequisite to understanding pathologies associated
with the knee. Clin. Anat. 00:000–000, 2014. V
C2014 Wiley Periodicals, Inc.
Key words: meniscus; menisci; knee; anatomy; biomechanics; injury; treatment
INTRODUCTION
Originally described as a vestigial structure (Sutton,
1897), the menisci are now known to be vital for the
normal functioning and longevity of the knee joint
(Fairbank, 1948; Burr and Radin, 1982; Arnoczky
et al., 1988; Arnoczky, 1992; Spilker et al., 1992; Roos
et al., 1998, 2001; Rodkey, 2000; McDermott and
Amis, 2006). The primary function of the meniscus is
to transmit load across the tibiofemoral joint by
increasing congruency, thereby decreasing the result-
ant stress placed on the articular cartilage. The menisci
also play a secondary role in shock absorption, stabil-
ity, lubrication, nutrition, and proprioception to the
knee joint (Kettelkamp and Jacobs, 1972; Seedhom,
1976; Seedhom and Hargreaves, 1979; Ahmed and
Burke, 1983; Mow et al., 2005; McDermott et al.,
2008; Chevrier et al., 2009; Englund et al., 2009).
Injuries to the menisci are recognized as a cause of
significant musculoskeletal morbidity (Fox et al.,
2012; Rath and Richmond, 2000). As a consequence
of its complex anatomical, biomechanical, and func-
tional characteristics, the menisci are prone to dam-
age and injury, particularly in contact-sport activities
but also in sedentary young or elderly patients. The
challenge remains to develop therapies and techni-
ques that will preserve the menisci’s distinct composi-
tion and function.
ANATOMY
Meniscal Etymology
The Latin word meniscus comes from the Greek
word m!
eniskos, meaning “crescent,” diminutive of
m!
en!
e, meaning “moon” (Fox et al., 2012).
*Correspondence to: A.J.S. Fox, MSc, Laboratory of Soft Tissue
Research, 535 East 70th Street, New York, NY 10021. Email:
foxa@hss.edu or ajsfox27@hotmail.com
Conflict of interest: Russell F. Warren, MD: Biomet (royalty) and
Ivy Sports (stock). Scott A. Rodeo, MD: Smith and Nephew
(consultant).
Received 22 May 2014; Accepted 23 July 2014
Published online in Wiley Online Library (wileyonlinelibrary.com).
DOI: 10.1002/ca.22456
V
V
C2014 Wiley Periodicals, Inc.
Clinical Anatomy 00:00–00 (2014)

Meniscal Phylogeny and Comparative
Anatomy
Many species, including all tetrapods, share a
genetic lineage reflected by the similar anatomic and
functional characteristics and asymmetries of the
knee which can be traced back more than 300 million
years (Haines, 1942; Mossman and Sargeant, 1983;
Dye, 2003). Approximately 1.3 million years ago, the
modern patellofemoral joint was established as homi-
nids evolved to a bipedal stance (Tardieu and Dupont,
2001). Tardieu analyzed the ontogenetic transition
from occasional bipedalism to habitual bipedalism.
The author observed that primates contain a medial
and lateral fibrocartilaginous meniscus, with the
medial meniscus being morphologically similar in all
primates (crescent shaped with two tibial insertions)
and the lateral meniscus being more variable in shape
(Tardieu, 1999). Unique to Homo sapiens is the pres-
ence of two tibial insertions—one anterior and one
posterior—indicating the permanent practice of full
extension movements of the knee joint during the
stance and swing phases of bipedal walking (Retterer,
1907; Vallois, 1914; Ricklin et al., 1983; Preuschoft
and Tardieu, 1996; Tardieu, 1999; Beaufils and Ver-
donk, 2010).
Embryology and Development
The menisci arise from a condensation of the inter-
mediate layer of mesenchymal tissue surrounding the
joint capsule. The characteristic shape of the lateral
and medial menisci is achieved between the 8th and
10th week of gestation (Gardner and O’Rahilly, 1968;
Gray, 1999). The developing menisci are highly cellular
and vascular, with a blood supply extending the entire
width and length of the menisci (Clark and Ogden,
1983). As the fetus continues to develop, there is an
increase in collagen content in a circumferential
arrangement with a concomitant decrease in cellularity
(Clark and Ogden, 1983; Carney and Muir, 1988).
Weight-bearing and joint motion during development
are important factors in determining the orientation of
the collagen fibers. By adulthood, only the peripheral
10 to 30% are vascular (Clark and Ogden, 1983).
Despite these histological changes, the proportion
of tibial plateau covered by the corresponding menis-
cus is relatively constant throughout fetal develop-
ment, with the medial and lateral menisci covering
approximately 51–74% and 75–93% of the surface
areas, respectively (Seedhom, 1976; Clark and
Ogden, 1983; Fukazawa et al., 2009).
Gross Anatomy
The menisci are crescent shaped wedges of fibro-
cartilage located on the medial and lateral aspects of
the knee (Figs. 1–3). The menisci enable effective
articulation between the concave femoral condyles
and the relatively flat tibial plateau. The menisci are
roughly triangular in cross section, covering one-half
to two-thirds of the articular surface of the corre-
sponding tibial plateau. Meniscal horns anchor the
menisci to the underlying subchondral bone of the tib-
ial plateau (Messner and Gao, 1998; Villegas et al.,
2008) (Fig. 3). These ligamentous structures transmit
sheer and tensile load from soft tissue into the bone
and decreasing contact area (Messner and Gao,
1998). In the medial meniscus, the anterior horn can
have a variable site of attachment, into either soft tis-
sue of bone, but a firm bony attachment to the flat
intercondylar region of the tibial plateau is most com-
mon (Berlet and Fowler, 1998). The posterior horn
attaches to the tibia just anterior to the insertion site
of the posterior cruciate ligament (PCL) (McKeon
et al., 2009; Palastanga and Soames, 2011). In the
lateral meniscus, the anterior horn inserts on the tibia
in front of the intercondylar eminence, just posterior
Fig. 1. Photographs of a right knee joint from a 64-year-old woman. (A) Anterior
view. (B). View toward lateral aspect. (C) View toward the medial aspect. LM, lateral
meniscus; ACL, anterior cruciate ligament; PCL, posterior cruciate ligament; MM,
medial meniscus.
2 Fox et al.

and lateral to the anterior cruciate ligament (ACL)
insertion. The posterior horn attaches to the tibia in
between the insertion sites of the PCL and the poste-
rior horn of the medial meniscus (Fig. 3) (McKeon
et al., 2009). The outer rim of the menisci (also called
the red zone) is thick and convex and attached to the
knee joint capsule while the inner edge (also called
the white zone) is concave, thin and unattached
(Fig. 4) (Rath and Richmond, 2000).
Medial Meniscus
The medial meniscus is C-shaped and occupies
!60% of the articular contact area of the medial com-
partment (Figs. 1, 2, and 3A) (Clark and Ogden,
1983; Arnoczky et al., 1987; Thompson et al., 1991).
Its posterior horn is significantly wider than the ante-
rior horn, and the anteroposterior dimension is larger
than the mediolateral dimension. The anterior horn is
Fig. 2. Anatomical photographs of tibial plateau dem-
onstrating the relative size and attachments of the medial
and lateral menisci. (A) Superior view. (B) Posterior
view. ACL, anterior cruciate ligament; LPRA, lateral
meniscus posterior horn attachment; MPRA, medial
meniscus posterior horn attachment; PCL, posterior cru-
ciate ligament; SWF, shiny white fibers of posterior horn
of medial meniscus. (Reproduced with permission from
Johannsen AM, Civitarese DM, Padalecki JR, Goldsmith
MT, Wijdicks CA, LaPrade RF. Am J Sports Med, 2012, 40,
2342–2347). [Color figure can be viewed in the online
issue, which is available at wileyonlinelibrary.com.]
Fig. 3.(A) Anatomy of the meniscus viewed from
above (adapted image reprinted with permission from
Greis PE, Bardana DD, Holmstrom MC, Burks RT. J Am
Acad Orthop Surg. 2002, 10, 168–176; original from Pag-
nani MJ, Warren RF, Arnoczky SP, Wickiewicz T. The Lower
Extremity and Spine in Sports Medicine. 1995, p 581–
614, V
CMosby. (B) Axial view of a right tibial plateau
showing sections of the meniscus and their relationship to
the cruciate ligaments. AL, anterior horn lateral menis-
cus; AM, anterior horn medial meniscus; PCL, posterior
cruciate ligament; PL, posterior horn lateral meniscus;
PM, posterior horn medial meniscus. (Reproduced with
permission from Johnson RJ, Kettelkamp DB, Clark W,
Leaverton P. J Bone Joint Surg Am, 1974, 56, 719–729).
The Meniscus: Anatomy, Function, Injury and Treatment 3

firmly attached to the tibia anterior to the ACL, near
the intercondylar fossa. The posterior horn is attached
immediately anterior to the attachment of the PCL
(Figs. 2 and 3). The peripheral border of the medial
meniscus merges with the knee joint capsule. The
coronary ligament attaches to the meniscus to the
upper tibia (Rath and Richmond, 2000).
Lateral Meniscus
The lateral meniscus is almost uniformly circular
and in contrast to the medial meniscus, it is smaller
and considerably more mobile (Figs. 2 and 3A). It
also occupies a greater portion of the articular surface
(!80% vs. !60%) (Clark and Ogden, 1983; Arnoczky
et al., 1987; Thompson et al., 1991). The anterior
horn of the lateral meniscus is attached to the inter-
condylar fossa adjacent to the broad attachment
site of the ACL. The posterior horn is attached to the
PCL and medial femoral condyle through the menis-
cofemoral ligaments of Wrisberg (the posterior menis-
cofemoral ligament) and Humphrey (the anterior
meniscofemoral ligament) (Fig. 3A). It is also
attached to the popliteus tendon (Last, 1950).
Biochemistry of the Meniscus
The meniscus is composed of a dense extracellular
matrix (ECM) composed of primarily of water (72%)
and collagen (22%), interposed with cells (Ghadially
et al., 1983). Other constituents include glucosamino-
glycans (17%), DNA (2%), adhesion glycoproteins
(<1%), and elastin (<1%) (Herwig et al., 1984;
Makris et al., 2011). These proportions vary according
to age, injury, or pathological condition (Sweigart and
Athanasiou, 2001).
Collagen is the main fibrillar component of the
meniscus and varies in amount depending on region
within the meniscus. Collagens are primarily responsi-
ble for the tensile strength of the meniscus, contribut-
ing up to 75% of the dry weight of the ECM (Herwig
et al., 1984). In the red zone, type I collagen is pre-
dominant (80% composition by dry weight), with
other collagen variants (e.g., type II, III, IV, VI, and
XVIII) present in less than 1%. Type 1 collagen fibers
are oriented circumferentially, in the deeper layers of
the meniscus, parallel to the peripheral border
(Fig. 4). In the most superficial region of the menisci,
type 1 fibers are oriented in a more radial orientation.
Radially positioned “tie” fibers are also present in the
deep zone and woven between the circumferential
fibers to provide structural integrity (Bullough et al.,
1970; Yasui, 1978; Aspden et al., 1985; Beaupre
et al., 1986; Arnoczky et al., 1988; Fithian et al.,
1990; Skaags and Mow, 1990; Fox et al., 2012). In
the white zone, collagen (70% by dry weight) is com-
posed of only two types of collagen - types II (60%)
and I (40%) (Cheung, 1987). The collagen fibers are
heavily cross-linked and are ideal for transferring ver-
tical compressive load into “hoop stresses” (Voloshin
and Wosk, 1983).
Classification of meniscal cells is controversial, with
no uniform characterization accepted in the literature
(Nakata et al., 2001). Histological examination of
the inner white zone of the menisci reveals rounded
cells, that behave similarly to fibrochondrocytes or
chondrocyte-like cells (Fig. 4) (Verdonk et al., 2005).
Fig. 4. Anatomical variation in vascularization and cell population of the meniscus.
(Reproduced with permission from Makris EA, Hadidi P, Athanasiou KA. Biomaterials,
2011, 32,7411–7431). [Color figure can be viewed in the online issue, which is avail-
able at wileyonlinelibrary.com.]
4 Fox et al.

In contrast, the cells of the outer red zone have an
oval or fusiform appearance and are classified as
fibroblasts (Fig. 4) (Verdonk et al., 2005). A third cell
population has been identified in the superficial zone
of the meniscus. These cells are flattened and fusi-
form and lack cell extensions (Van der Bracht et al.,
2007). Although the exact purpose of these cells is
unknown, it has been suggested that they might be
specific progenitor cells with a regenerative capacity
(Van der Bracht et al., 2007).
Vascular Anatomy
The meniscus is a relatively avascular structure
with a limited peripheral blood supply. Branches of the
popliteal artery (medial and lateral inferior and middle
geniculate arteries) are the major blood vessels that
nourish each meniscus. Radial branches from a peri-
meniscal plexus enter the meniscus at intervals, with
a richer supply to the anterior and posterior horns
(Fig. 5) (Day et al., 1985). Vascularization is limited to
the peripheral 10–25% for the lateral meniscus and
10–30% for the medial meniscus, which has impor-
tant implications for healing (Arnoczky and Warren,
1982; Danzig et al., 1983; Harner et al., 2000). Endo-
ligamentous vessels from the anterior and posterior
horns travel a short distance into the substance of the
menisci to form terminal loops, providing a direct
route for nourishment (Danzig et al., 1983). The
remainder of the meniscus receives nourishment via
synovial diffusion or mechanical motion (Meyers et al.,
1988).
Neuroanatomy
The meniscus receives innervation via the recurrent
peroneal branch of the common peroneal nerve.
These fibers follow the blood supply and are found pri-
marily in the peripheral vascular zone covering the
outer third of the meniscus (Gardner, 1948; Kennedy
et al., 1982). Three distinct mechanoreceptors—
Ruffini endings (type 1), Pacinian (type II) and Golgi
tendon organs (type III)—have been identified within
the meniscus. These neural elements are found in
greater concentration in the meniscal horns (particu-
larly the posterior horn), and are important in joint
deformation and pressure, tension changes, and neu-
romuscular inhibition, respectively (Zimny, 1988).
Free nerve endings (nociceptors) can be found in the
horns and the outer two-thirds of the body of
the menisci (Wilson et al., 1969; Day et al., 1985;
Gronblad et al., 1985; Zimny, 1988; Zimny et al.,
1988; Assimakopoulos et al., 1992; Mine et al., 2000).
BIOMECHANICS AND FUNCTION
The complex functions of the meniscus are intri-
cately related to their composition, structure, and
morphology. The menisci perform many important
biomechanical functions. These functions include load
transmission (Fairbank, 1948; Walker and Erkman,
1975; Seedhom, 1976; Seedhom and Hargreaves,
1979; Fukubayashi and Kurosawa, 1980; Aspden
et al., 1985; Arnoczky et al., 1987), shock absorption
(Krause et al., 1976; Kurosawa et al., 1980; Voloshin
and Wosk, 1983; Arnoczky et al., 1987; Fithian et al.,
1990), stability (Markolf et al., 1976; Fukubayashi
et al., 1982; Levy et al., 1982, 1989; Shoemaker and
Markolf, 1986;), nutrition (Bird and Sweet, 1987,
1988; Renstrom and Johnson, 1990), joint lubrication
(Macconaill, 1932; Mac, 1946, 1950; Renstrom and
Johnson, 1990), and proprioception (Wilson et al.,
1969; Zimny et al., 1988; Assimakopoulos et al.,
1992; Jerosch et al., 1996; Messner and Gao, 1998;
Saygi et al., 2005; Akgun et al., 2008; Karahan et al.,
2010). They also serve to decrease contact stresses
and increase contact area and congruency of the knee
(Kettelkamp and Jacobs, 1972; Walker and Erkman,
1975).
Load Transmission
Studies with long-term follow-up of meniscectom-
ized knees have shown the importance of the menis-
cus in the functioning of the knee. Fairbank was first
to describe the direct load-bearing function of the
meniscus by describing the degenerative changes in
meniscectomized knees (Fairbank, 1948). Fairbank
described narrowing of the joint space, flattening of
the femoral condyle, and the formation of osteo-
phytes. Fairbank attributed these changes to the loss
of the meniscus (Fairbank, 1948). Since then, several
animal and clinical studies have confirmed Fairbank’s
thesis that the meniscus is an important protective,
load-bearing structure (Walker and Erkman, 1975;
Levy et al., 1989; Newman et al., 1989; Fukuda et al.,
Fig. 5. Frontal section of medial compartment.
Branching radial vessels from the perimeniscal capillary
plexus (PCP) can be observed penetrating the peripheral
border of the medial meniscus. Three zones are seen: (1)
RR, red-red area is fully vascularized; (2) RW, is at the
border of the vascular area; and (3) WW, white—white is
within the avascular area of the menicscus. F, femur; T,
tibia; PCP, perimeniscal capillary plexus. R, red. Original
image reprinted with permission from (Arnoczky SP, War-
ren RF. Am J Sports Med, 1982, 10, 90–95); adapted
image reprinted with permission from (Canale ST, Beaty
JH. Campbell’s Operative Orthopaedics, 2013, Elsevier).
[Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
The Meniscus: Anatomy, Function, Injury and Treatment 5

2000; Lee et al., 2006; Bedi et al., 2010, 2012, 2013;
Beveridge et al., 2011; Ode et al., 2012; Mononen
et al., 2013; Dong et al., 2014).
Biomechanical studies have demonstrated that
approximately 40–60% of load acting on the extended
knee joint is transmitted to the meniscus (65–70%
lateral and 40–50% medial) (Shrive et al., 1978; Dud-
hia et al., 2004). In flexion, this increases up to 90%
(Walker and Erkman, 1975). During weight bearing,
axial forces compress the menisci, resulting in “hoop”
(circumferential) stresses (Voloshin and Wosk, 1983).
Hoop stresses rely on the conversion of axial force
into tensile strain through the circumferential collagen
fibers of the meniscus (Fig. 6). The lateral meniscus is
displaced more than the medial meniscus during com-
pression, but because of the semilunar anatomy, load
is transmitted away from the center of the femoral
condyles resulting in tensile stress toward the tibial
plateau (Sweigart and Athanasiou, 2001).When
standing, the meniscus absorbs most of the load;
however, when the knee is in gait or stair climbing,
variations in contact stresses occur (Walker and Erk-
man, 1975; Gilbert et al., 2013). A recent cadaveric
study by Gilbert et al. found that during gait, peak
contact stresses of the medial plateau occurred in
areas of cartilage–cartilage contact, while on the lat-
eral meniscus peak contact stresses occurred under
the meniscus (Gilbert et al., 2013). During stair climb,
peak contact stresses of the medial meniscus were
located in the posterior aspect of the plateau, under
the meniscus. While in the lateral meniscus, during
the late phase of stair climb, peak contact stresses
were reported in the zone of cartilage–cartilage con-
tact (Gilbert et al., 2013).
Several studies have demonstrated that load is well
distributed when the meniscus is intact, however, its
removal results in a significant reduction in femoral
condyle contact area and a significant increase in con-
tact stress (Kettelkamp and Jacobs, 1972; Fukubaya-
shi and Kurosawa, 1980; Ahmed and Burke, 1983;
Radin et al., 1984; Radin and Rose, 1986; Bedi et al.,
2012). Several studies have reported that total lateral
meniscectomy results in a 40–50% decrease in con-
tact area and an increase in contact stress in the lat-
eral component (200–300% of what is considered
normal), which significantly increases the load per
unit area and may contribute to accelerated articular
cartilage damage and degeneration (Fairbank, 1948;
Fig. 6. Free body diagram of forces acting on the knee
meniscus during loading. During normal loading, the
meniscus is compressed by the downward force of the
femur. The meniscus deforms radially but is anchored by its
anterior and posterior horns (F
ant
and F
post
). During load-
ing, tensile, compressive, and shear forces are generated.
A tensile hoop stress (F
cir
) results from radial deformation,
while vertical and horizontal forces (F
v
and F
h
) result from
the femur compressing the curved superior surface of the
tissue. A radial reaction force (F
rad
) balances the femoral
horizontal force (F
h
). (Reproduced with permission from
Makris EA, Hadidi P, Athanasiou KA. Biomaterials, 2011,
32,7411–7431). [Color figure can be viewed in the online
issue, which is available at wileyonlinelibrary.com.]
6 Fox et al.

Kettelkamp and Jacobs, 1972; Walker and Erkman,
1975; Jones et al., 1978; Fukubayashi and Kurosawa,
1980; Baratz et al., 1986; Henning et al., 1987).
Shock Absorption
The shock absorbing capacity of the menisci has
been demonstrated by studies measuring the vibra-
tions in the proximal tibia resulting from gait. From
this, it has been shown that shock absorption is
approximately 20% less in knees without menisci
(Voloshin and Wosk, 1983). This function of the
menisci is associated with their viscoelastic properties,
the main component of which is the water content of
the tissue. Therefore, on impact, shock is absorbed by
frictional drag forces, which occur as the fluid escapes
the tissue.
Stability
The incongruous articulation between the convex
femoral condyles and flat tibial plateau is ameliorated
by the concave-shaped superior surface of each
meniscus (Brantigan and Voshell, 1941; Markolf et al.,
1976, 1981; Oretorp et al., 1979; Fukubayashi et al.,
1982; Levy et al., 1982; Shoemaker and Markolf,
1986; Allen et al., 2000). The firm attachment of the
medial meniscus to the tibia contributes to anterior
stability of the knee, and is more frequently torn (par-
ticularly in ACL-deficient knees) because it is less
mobile.
The intact meniscus limits excess motion in all
directions, contributing to the stability of the knee
joint (Arnoczky, 1992). Although the exact function of
the meniscofemoral ligaments (Wrisberg and Hum-
phrey) remains unknown, it is believed that in flexion
and internal rotation, the popliteal tendon retracts the
posterior horn, thus reducing entrapment of the lat-
eral meniscus between the femur and tibia. Joint sta-
bility is further facilitated by the soft tissue structures
of the knee joint capsule.
The role that the menisci play in joint stability can
best be demonstrated in studies investigating laxity in
ACL-deficient, meniscectomized or meniscus-torn
knees (Levy et al., 1982; Shoemaker and Markolf,
1986). Findings include greater anterior tibial transla-
tion in knees with a sectioned ACL and medial menis-
cectomy as compared with knees with only ACL
sectioning (Bargar et al., 1980). However, ACL sec-
tioning and lateral meniscectomy did not cause an
increase in anterior translation in contrast to medial
meniscectomy (Levy et al., 1982). Shoemaker and
Markolf stated that the posterior horn of the medial
meniscus is the most important structure resisting
anterior tibial force in the ACL-deficient knee (Shoe-
maker and Markolf, 1986). Allen et al. showed that
the resultant force in the medial meniscus of the ACL-
deficient knee increased by 52% in full extension and
by 197% at 60 degrees of flexion under a 134-N ante-
rior tibial load (Allen et al., 2000). More recently,
Musahl et al. reported that the lateral meniscus plays
a major role in the pivot-shift maneuver as lateral
meniscectomy increases translation and rotation and
increases the pivot shift (Musahl et al., 2010; Pearle,
2011). These significant changes in kinematics in the
ACL-deficient knee confirm the important role of the
menisci in knee stability.
Joint Lubrication and Nutrition
The menisci may also play a role in the lubrication
and nutrition of the knee joint. In a series of studies,
MacConaill, reported that the coefficient of friction of
the knee joint is increased by 20% following menis-
cectomy (Macconaill, 1932; Mac, 1946, 1950). The
precise mechanism(s) by which lubrication occurs
remains unknown; however some authors believe
that when the knee is loaded, the menisci compress
and circulate synovial fluid into the articular cartilage,
reducing the frictional forces during weight-bearing
and providing joint nutrition (Mac, 1950; Arnoczky
et al., 1988). The system of microcanals within the
meniscus that is located close to the blood vessels
communicates with the synovial cavity. It is believed
that these may provide fluid transport for lubrication
and nutrition (Bird and Sweet, 1987, 1988).
Proprioception
The menisci may serve a proprioceptive role as
suggested by the presence of mechanoreceptors in
the anterior and posterior horns of the menisci (Wil-
son et al., 1969; Zimny et al., 1988; Assimakopoulos
et al., 1992; Jerosch et al., 1996; Messner and Gao,
1998; Saygi et al., 2005; Akgun et al., 2008; Karahan
et al., 2010). Quick-adapting mechanoreceptors (e.g.,
Pacinian corpuscles) are thought to mediate the sen-
sation of joint motion, while slow-adapting receptors
(e.g., Ruffini endings and Golgi tendon organs) are
believed to mediate the sensation of joint position
(Reider et al., 2003). The identification of these neural
elements (located mostly in the middle and outer third
of the meniscus) indicates that the meniscus is capa-
ble of detecting proprioceptive information, thus play-
ing an important afferent role in the sensory feedback
mechanism of the knee (Kennedy et al., 1982; Skin-
ner et al., 1984; Aagaard and Verdonk, 1999; Gray,
1999; Karahan et al., 2010).
INJURY
Epidemiology
Meniscal injury is a common source of pain and dis-
ability of the knee that is frequently encountered by
orthopaedic surgeons, with a mean annual incidence
of 60–70 per 100,000 (Hede et al., 1990; Nielsen and
Yde, 1991; Majewski et al., 2006; Clayton and Court-
Brown, 2008). The overall male to female ratio for
meniscal tears ranges from 2.5:1 to 4:1, with a peak
incidence occurring in males between 21 and 30 years
of age and in girls and women between 11- and 20-
years old (Baker et al., 1985; Poehling et al., 1990;
Greis et al., 2002; Drosos and Pozo, 2004). Medial
meniscal tears (particularly in the stable knee or in
the chronic ACL-deficient knee) are more common
The Meniscus: Anatomy, Function, Injury and Treatment 7

than lateral tears (81 and 19%, respectively). Lateral
meniscus tears are more common in association with
an acute ACL tear (range, 51–72%) (Poehling et al.,
1990).
Meniscal tears are generally caused by a combina-
tion of axial loading and rotational forces that result in
shear load on the meniscus (Browner et al., 2003).
Traumatic tears are usually associated with a known
insult to the knee and may be isolated or associated
with ligament or articular surface injury (Browner
et al., 2003). Traumatic tears generally occur in
younger, active individuals (Browner et al., 2003).
Degenerative tears may reflect cumulative stress and
correlate with the presence of associated chrondroma-
lacia (Browner et al., 2003).
Meniscal tears in children are commonly due to
trauma or congenital meniscal variants such as a dis-
coid meniscus or meniscal cysts. In adults, meniscal
tears are a result of trauma, degenerative disease or
a combination of both (Hirschmann and Friederich,
2009; Pujol and Boisrenoult, 2009; Verdonk and Ver-
erfve, 2009). Adult meniscal injuries predominantly
involve the medial meniscus, and are often associated
with concomitant ligament or cartilage lesions (Pujol
and Boisrenoult, 2009; Verdonk and Vererfve, 2009).
In addition, tears are more complex in adults as the
meniscus undergoes significant degeneration in the
course of a lifetime (Pujol and Boisrenoult, 2009). Not
surprisingly, there is an increase in the incidence of
meniscal tears with increasing age (Pujol and Boisre-
noult, 2009).
Symptoms produced by a meniscal tear are
typically pain and sometimes mild swelling. Less
commonly, “mechanical” symptoms such as locking,
catching, grinding, and giving away occur. The fre-
quency and severity of the symptoms varies according
to the size and mobility of the meniscal tear (Browner
et al., 2003).
CLASSIFICATION OF MENISCAL TEARS
Despite the International Society of Arthroscopy,
Knee Surgery and Orthopaedic Sports Medicine Com-
mittee publishing a standardized and validated system
to classify meniscal tears, no classification has been
universally accepted by the orthopaedic community
(Irrgang et al., 1998, 2006; Crawford et al., 2007;
Anderson et al., 2011). Tears are generally classified
according to observed tear patterns (seen at arthro-
scopy) or etiology of the injury and can be described
as either full-thickness or partial thickness, depending
on vertical depth of the tear (Figs. 7 and 8).
The authors follow the zone classification system
devised by Cooper, who provides consistent clinical
documentation of meniscal tear patterns and tear
locations (Cooper et al., 1990). In this system, the
menisci are divided into three radial zones anterior to
posterior and four circumferential zones extending
from the periphery to the inner aspect of the meniscus
(Fig. 9). Tears are further classified according to their
morphology and tear pattern relative to the tibial
plateau.
The main categories of meniscal tears include
vertical longitudinal, radial (transverse), horizontal
(cleavage), complex (degenerative), and bucket-
handle tears (Greis et al., 2002).
Vertical Longitudinal Tears
A vertical longitudinal tear occurs between the cir-
cumferential collagen fibers, parallel to the long axis
of the meniscus, perpendicular to the tibial plateau,
with the tear equidistant from the peripheral edge of
the meniscus (Jee et al., 2003). These tears trans-
verse the circumferential collagen fibers, resulting in
either two separate pieces of meniscus, or a single
portion of meniscus attached to the tibia in only one
location (Figs. 7A) (Tuckman et al., 1994; Magee
et al., 2002; Fox, 2007).
Vertical longitudinal tears are most commonly trau-
matic in origin and occur more often in younger indi-
viduals, with a peak incidence of 21–30 years of age
in mean and 11–20 years of age in women (Schepsis
and Busconi, 2006). They are noted more frequently
medially in isolated cases and laterally in association
with ACL tears (Schepsis and Busconi, 2006). The
incidence of vertical longitudinal tears ranges from 40
to 84% (Metcalf, 1981; Poehling et al., 1990; Dandy,
1990; Metcalf and Barrett, 2004). As these tears
occur between collagen fibers, the biomechanics of
the knee is therefore not always disrupted and these
Fig. 7. Common types of meniscal tears (Aand B) and; (C) depth of meniscal
tears. Image adapted and reprinted with permission from (Hauser RA, Philips HJ, Mad-
dela HS. J Prolother, 2010, 2, 416–437).
8 Fox et al.

tears may not be symptomatic (Mordecai et al.,
2014). Vertical longitudinal tears are commonly
repaired because they are amenable to suture
fixation.
Radial Tears
Radial (transverse) tears are vertical tears, which
often occur at the junction of the posterior and
middle thirds and extend from the inner free margin
toward the periphery, but can occur at other regions
(Figs. 7A). They may also occur in the midbody por-
tion of the lateral meniscus in younger patients. The
incidence of radial tears is approximately 14–15%
(Helms, 2002; Magee et al., 2002; Harper et al.,
2005). Radial tears are typically found in younger
patients, with a peak incidence of 11–20 years of age
in men and 51–70 years of age in women (Schepsis
and Busconi, 2006). Although relatively uncommon,
they are usually traumatic with the majority (!79%)
occurring in the posterior horn of the meniscus
(Helms, 2002; Magee et al., 2002; Harper et al.,
2005; Fox, 2007). Radial tear patterns may be associ-
ated with ACL rupture involving the posterior portion
or meniscal attachment (Wickiewicz, 1990; Vanhoe-
naker et al., 2007).
Radial tears disrupt the ability to distribute the
hoop stresses associated with weight bearing, and are
usually not reparable (Harper et al., 2005; Fox,
2007). Repairs of radial tears that extend to the
periphery have the potential to heal because of the
peripheral bloody supply (Schepsis and Busconi,
2006). Traditionally, repairs of radial tears were not
Fig. 9. Zone Classification of the Meniscus. Posterior
third zone of the medial menisci is lettered A. Moving in a
clockwise direction of thirds, the middle third zone is let-
tered B, and the anterior third zone is letter C, and so on. 0,
meniscosynovial junction (the periphery); 1, is the outer
third region of the menisci; 2, is the middle third region of
the menisci; and 3 is the inner third region of the menisci.
(Reproduced with permission from Schepsis AA, Busconi
BD. Sports Medicine, 2006, Lippincott Williams & Wilkins).
Fig. 8. Meniscal tear grading. The schematic dia-
grams on the top depict meniscal tear grades with corre-
sponding sagittal MR images on the bottom. Grade 0,
normal intact meniscus; Grade I, intrasubstance
globular-appearing signal not extending to the articular
surface; Grade II, linear increased signal patterns not
extending to the articular surface; Grade III, abnormal
signal intersects the superior and/or inferior articular sur-
face of the meniscus, an arthroscopically confirmable
tear. Red arrows indicate tear location. [Color figure can
be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
The Meniscus: Anatomy, Function, Injury and Treatment 9

recommended; however, more recent studies have
indicated that, with more advances repair techniques,
clinical success may be achieved after full-thickness
radial tear repairs (particularly of the posterior horn of
the lateral meniscus) (Wickiewicz, 1990; Schepsis and
Busconi, 2006).
Horizontal Tears
Horizontal (cleavage/longitudinal) tears are parallel
to the tibial plateau, dividing the meniscus into inferior
and superior segments (Figs. 7B) (Jee et al., 2003).
Tears can not only extend to the articular surface of
the meniscus in the region of the free margin (non-
vascularized portion of meniscus), but also on the tib-
ial and femoral side of the meniscus. These tears
occur in all groups but are seen most frequently in
older patients, with a peak incidence of 31–50 years
in men and 51–60 years of age in women (Schepsis
and Busconi, 2006). Horizontal tears occur most com-
monly in the posterior aspect of the medial meniscus.
When this tear pattern is noted in the lateral menis-
cus, it may be associated with lateral meniscal cysts
or osteoarthritis (Outerbridge, 1961; Aichroth, 1996;
Philips, 2003; Schepsis and Busconi, 2006).
The mechanism of injury is thought to be second-
ary to shear forces between the superior and inferior
surfaces of the meniscus that tend to cause separa-
tion of the upper and lower segments of the tear. Ini-
tial tears may be confined to the substance of the
meniscus without meniscal surface communication
(Schepsis and Busconi, 2006). Repeated traumatic
episodes may result in an intrasubstance tear, which
may extend to the articular surface or may occur in
the horizontal plane (Schepsis and Busconi, 2006).
Repeated applied load to a meniscal tear can result in
tear propagation, fragment displacement, and edge
instability. This can lead to mechanical symptoms
such as locking and catching with increased pain and
effusion. Excision of the unstable portions of the
meniscus is performed as these tears are typically not
repairable (Schepsis and Busconi, 2006).
Complex (Degenerative) Tears
Complex tears either have two or more tear config-
urations or are not easily categorized into a specific
type of tear (Jee et al., 2003; Fox, 2007). This is the
most common of all meniscal lesions, accounting for
nearly 30% of all tears with a peak incidence of 41–
50 years of age in men and 61–70 years of age in
women (Schepsis and Busconi, 2006). However,
atraumatic degenerative flap tears have been identi-
fied by magnetic resonance imaging (MRI) and arthro-
scopy in patients as young as 20 years of age.
Complex tears may or may not be associated with any
history of trauma and may have an insidious onset.
Complex degenerative tears are frequently seen in
association with other degenerative changes within
the joint. In addition, complex degenerative tears
usually have minimal to no healing potential and gen-
erally are not amenable to repair (Schepsis and
Busconi, 2006).
Bucket-handle Tears
A bucket-handle tear of the meniscus is a vertical or
oblique tear with longitudinal extension toward the
anterior horn in which the inner fragment is frequently
displaced toward the intercondylar notch (Singson
et al., 1991). The term bucket-handle is derived from
the appearance of the tear, in which the inner displace-
ment fragment of the meniscus resembles a handle,
and the peripheral nondisplaced portion has the appear-
ance of a bucket (Singson et al., 1991). Bucket-handle
tears often involve the entire meniscus but can also
involve only the posterior horn and body or a single
horn of the meniscus and are common in ACL deficient
knees (Ruff et al., 1998). It is the most common type of
displaced “flap” tear, occurring in approximately 10–
26% of patients (Lecas et al., 2000; Watt et al., 2000;
Ververidis et al., 2006) and more commonly involves
the medial meniscus (Magee and Hinson, 1998; Watt
et al., 2000; Dorsay and Helms, 2003; Ververidis et al.,
2006). The inner flipped portion of the meniscus can
remain intact or it can be disrupted.
MECHANISM OF INJURY
The mechanism of injury of meniscal tears can be
generally categorized as occurring during a sporting
activity (e.g., soccer, rugby football), a nonsporting
activity (e.g., squatting), or nonactivity. In a recent
study of 392 patients from an unselected population,
Drosos and Pozo found that only one-third (32.4%) of
patients incurred their meniscal injury in a sports-
related activity, while over one third (38.8%) occurred
due to nonsporting activities and nearly one third
(28.8%) of patients count not identify any specific
event or incident which resulted in an injury (Drosos
and Pozo, 2004).
Injury to the meniscus during sporting activities can
be further defined as secondary to contact or noncon-
tact mechanisms, with the latter being the most com-
mon. In the young athlete, contact (sports)-related
meniscal tears may result from excessive application
of force to the meniscus while in older patients, degen-
eration makes the meniscus particularly susceptible to
injury. The mechanism of injury typically involves a
twisting or shearing motion, with a varus or valgus
force directed to a flexed knee (Hayes et al., 2000).
Contact with another player typically does not occur,
nor does lunging or landing awkwardly. Patients typi-
cally report taking a single “wrong step” (Vanhoenaker
et al., 2007). In noncontact (sports)-related injuries,
common mechanisms include cutting, decelerating or
landing from a jump (Rath and Richmond, 2000).
Age has been suggested as a risk factor of meniscal
injury. Drosos and Pozo found that the average age of
the sporting, nonsporting and nonactivity groups was
33, 41, and 43 years, respectively (Drosos and Pozo,
2004). The differences may reflect a higher represen-
tation of the general population in recreational sports
and of age-related degenerative changes within the
meniscus, which make it more vulnerable to injury
(Drosos and Pozo, 2004). These findings confirm the
opinion of other authors, which state that after the
10 Fox et al.

third decade of life, degenerative changes start to
diminish the elasticity and increase the susceptibility
of the meniscus to injury (Noble and Hamblen, 1975;
Smillie, 1978).
Gender has also been suggested to be a risk factor
for meniscal injury. Several studies have reported
than men are four times more likely to incur a menis-
cal injury than women (Baker et al., 1985; Casteleyn
et al., 1988; Drosos and Pozo, 2004). This may be
due to the subtle anatomical and physiological charac-
teristics of the meniscus, differences in normal daily
activities, sports participation, prior participation in
sports and differences in occupation, which can result
in different rates of microtrauma and degeneration of
the meniscus (Drosos and Pozo, 2004).
IMAGING MODALITIES
Standard Radiography
Although unable to demonstrate pathology of the
meniscus, radiographs of the knee are able to exclude
bony pathologies and assess the concomitant presence
of degenerative changes. Standing weight-bearing radi-
ographs (anteroposterior at 0", posteroanterior at 45"
(“Rosenberg view”), lateral, Merchant views) can show
a reduction of the joint space width, loose bodies, chon-
drocalcinosis, osteophytes, subchondral bone cysts,
sclerosis, and other degenerative changes (Rosenberg
et al., 1988; Maffulli et al., 2010; Robinson, 2010).
Magnetic Resonance Imaging
Magnetic Resonance Imaging is a valuable imaging
method for diagnosing meniscal tears, with an accu-
racy range of 82–95% (Mandelbaum et al., 1986;
Reicher et al., 1986; Muellner et al., 1997; Yan et al.,
2011; Subhas et al., 2012; Sharifah et al., 2013).
Accuracy is defined as a measure of closeness in
detecting a quantity to that quantity’s actual (true)
value. Sensitivity and specificity of MRI are 93 and
88%, respectively for medial meniscal tears, and 79
and 95%, respectively for lateral meniscal tears (Oei
et al., 2003). Sensitivity and specificity are statistical
measures that describe the proportion of actual posi-
tives that are correctly identified and the proportion of
negatives that are correctly identified, respectively.
Spin-echo or fast spin-echo proton density with or
without fat saturation, T1, and gradient echo are the
most commonly used sequences (Helms, 2002).
The MRI grading system classifies tears based on
their appearance on an MRI scan (Fig. 8). Grade 0
represents an intact, normal meniscus. Grade I and
Grade II signals do not intersect the superior or infe-
rior articular surface of the meniscus, but may repre-
sent meniscal degeneration. A Grade III signal
intersects the superior and/or inferior articular surface
of the meniscus and represents a tear (Fig. 8).
Arthroscopy
Diagnostic arthroscopy has become the gold stand-
ard for assessing meniscal injuries and determining
the feasibility of a successful repair (Maffulli et al.,
2012). A probe is used to characterize the size of the
tear, degree of instability, quality of the tissue and
zone of the tear (i.e., red–red, red-white, and white–
white) and the width and integrity of the meniscal rim
is evaluated (Jackson, 2008). Once the diagnostic
arthroscopy is complete, the treating surgeon should
determine the appropriate treatment.
ADVANCES IN TREATMENT
The goal of surgical intervention for meniscal tears
is to relieve pain, facilitate preinjury level daily living
activities, and prevent early degeneration of the knee
joint. In the past, total meniscectomy was the gold
standard because the meniscus was considered a
functionless remnant vestige (Sutton, 1897). In his
landmark article, Fairbank radiologically examined
postmeniscectomized knees, describing the femoral
condylar flattening and narrowing of the joint space
that occurred over time (Fairbank, 1948). However,
despite this evidence, total meniscectomy remained
the widespread treatment of mensical tears until the
1970s.
Over the past four decades, with the general adop-
tion of arthroscopy, there has been an improvement
in surgical techniques to assess and treat meniscal
pathology. These improvements along with an
enhanced understanding of the anatomic structure
and biomechanical function of the menisci has led to a
shift toward preservation of the menisci to avoid the
degenerative results that follow its removal (Jackson,
1968; Appel, 1970; Johnson et al., 1974; Lutfi, 1975;
McGinity et al., 1977; Hoch et al., 1983; Northmore-
Ball et al., 1983; Voloshin and Wosk, 1983; Allen
et al., 1984; Ghosh et al., 1990; Berjon et al., 1991;
Abrams et al., 2013). Partial meniscectomy is still
indicated if the tear cannot be satisfactorily sutured
(Sommerlath, 1991; Shelbourne and Carr, 2003).
Meniscal preservation includes leaving small or partial
tears, partial meniscectomy and meniscus repair tech-
niques. Meniscal allograft transplantation and the use
of synthetic implants have been described in the liter-
ature and have shown promising results in sympto-
matic patients that have undergone a partial, subtotal
or total meniscectomy (Brophy and Matava, 2012).
Meniscal Repair
Meniscal repair techniques have been developed
and refined over the years. A combination of techni-
ques may be used to adequately stabilize a particular
meniscal tear. Common to every type of meniscal
repair is preparation of the meniscus and the local
environment. Any loose or frayed fragments of menis-
cus should be removed and the opposing edges are
rasped to define the meniscal edge and promote the
healing response (Ochi et al., 2001; Laible et al.,
2013). It is also recommended that abrasion of the
local synovium be routinely performed.
Only certain types of tears are feasible for meniscal
repair due to the limited vascular supply of the menis-
cus. Tear morphologies such as flaps, radial tears, and
The Meniscus: Anatomy, Function, Injury and Treatment 11

degenerative tears are generally not repaired (Laible
et al., 2013; Taylor and Rodeo, 2013). In a prospec-
tive study using second-look arthroscopies, Scott
et al. reported the highest rates of healing in menisci
that had a narrow peripheral meniscal rim (range 0–
2 mm) (Scott et al., 1986). Typically, longitudinal
tears that are less than 3 cm in length and within the
peripheral zone of the meniscus are amenable to
repair (Taylor and Rodeo, 2013). Currently, it is
unclear whether the timing of repair affects success
(Tengrootenhuysen et al., 2011; Laible et al., 2013).
Age of patient has been reported to be a factor in
capacity to heal. Laible et al. reported an improved
healing response in younger patients following
meniscal repair (Laible et al., 2013). In the event that
a ligamentous (e.g., ACL) injury has occurred con-
comitantly with the meniscal tear, it is recommended
that reconstruction be performed to improve func-
tional stability of the knee (Nepple et al., 2012).
Meniscal repair in conjunction with ACL reconstruc-
tion, orthopaedic surgeons can expect an estimated
>90% clinical success rate at 2-year follow-up (War-
ren, 1990; Cannon and Vittori, 1992; Guisasola et al.,
2002; Pujol et al., 2008; Pujol and Beaufils, 2009;
Toman et al., 2009; Ghodadra et al., 2012; Fu et al.,
2013; Yan et al., 2014). Conversely, meniscal recon-
struction in the ligamentous stable knee, without con-
comitant ACL reconstruction resulted in a 50–67%
clinical success rate (Cannon and Vittori, 1992; Daniel
et al., 1994; Shelbourne and Klotz, 2006). It is
believed that simultaneous ACL reconstruction with
meniscal repair achieves better meniscal healing
because of intra-articular bleeding from the surgically
exposed tunnels (Ochi et al., 2001; Guisasola et al.,
2002; Scotti et al., 2009) and the improved stability
of knee (Shelbourne et al., 1996).
Several repair techniques facilitating suture alone
or repair devices has been described in the literature.
The inside-out and outside-in techniques use suture
that is either first passed on either side of the tear
through the meniscus using a cannulated needle and
passed out of the joint capsule (inside- out technique)
or the other way round (outside- in technique). The
main risk of this technique is to cause injury to neuro-
vascular structures and the difficulty in accessing the
anterior portion of the medial and lateral menisci (Lai-
ble et al., 2013). Newer generation repair devices
allow all-arthroscopic meniscal repair by facilitating
anchors located outside the joint capsule and sliding
knots that can be tensioned by the surgeon for secure
tear repair. Meniscal horn tears may be fixed back to
the tibial plateau either using suture anchors in the
bone or using a transosseous suturing technique with
sutures brought through bone tunnels.
Typically, postoperative recovery following meniscal
repair is slow (approximately 4 months) due to the
need to protect the healing tissue. In an attempt to
optimize the healing capacity of the meniscus, a num-
ber of augmentation techniques have been investi-
gated. These include trephination (Zhang et al.,
1995), fibrin clots (Arnoczky et al., 1988; Henning
et al., 1990; van Trommel et al., 1998), and platelet-
rich plasma (PRP) (Delos and Rodeo, 2011). Trephina-
tion connects a meniscal lesion in the avascular zone
to the peripheral blood supply via a vascular access
channel and allows vascular ingrowth and enhances
the healing potential when combined with suture
repair of the lesion (Zhang et al., 1995). Autogenous
fibrin clots, a precipitated clot material produced by
agitation of whole blood, may contain platelet-derived
growth factor and fibronectin that may act as chemo-
tactic and mitogenic stimuli of reparative cells and
provide a scaffolding to support a reparative response
of meniscal lesions (Arnoczky et al., 1988). PRP may
initiate the healing cascade by releasing growth fac-
tors from the alpha and dense granules located in the
platelet cytoplasm. These growth factors lead to cellu-
lar chemotaxis, angiogenesis, collagen matrix synthe-
sis, and cell proliferation (Delos and Rodeo, 2011).
Future studies are required to determine the long-
term effects of these augmentation techniques on
meniscal healing.
Meniscal Allograft Transplantation
Milachowski et al. were the first to publish a clinical
study using meniscal allograft transplantation in
painful postmeniscectomized patients (Milachowski
et al., 1989). Since then, meniscal transplantation has
become an accepted management option for select
young symptomatic patients who have undergone a
subtotal or total meniscectomy (Fig. 10) (Verdonk
et al., 2013). Patients who develop symptoms of pain
and swelling due to early degenerative changes fol-
lowing meniscectomy are the typical candidates for
this procedure.
A study by Verdonk et al, reported that 75–90% of
patients experienced fair to excellent functional results
after meniscal allograft transplantation (Verdonk
et al., 2013). The authors also found that second-look
arthroscopies reported good healing of the peripheral
rim to the joint capsule in the majority of patients,
although shrinkage of the transplant was observed in
some cases (Verdonk et al., 2013). The clinical survi-
vorship 10 years postoperatively is estimated to be
70% for medial and lateral allografts (Verdonk et al.,
2013). In a systematic review of the literature by Her-
gan et al., the expected outcome following meniscal
allograft transplantation should be a painless knee
during activities of daily living. The patients should be
informed that expectations of returning to sports
should not be overestimated (Hergan et al., 2011).
Recovery time is approximately 6 months.
The primary indication for a meniscal transplant is
pain localized to the involved compartment (Brophy
and Matava, 2012). At the time of transplantation,
there should be only mild pre-existing arthrosis and
no focal chondral lesion higher than Grade III (Brophy
and Matava, 2012; Verdonk et al., 2013). A stable
knee joint in correct axial alignment should be
ensured. If necessary, ligamentous reconstruction
and/or corrective osteotomies should be performed.
Axial mal-alignment may result in excessive compres-
sive loads on the allograft leading to graft failure
(Rijk, 2004). A size-matched meniscus implant with a
size tolerance of 5% should be used (Brophy and
Matava, 2012). Commonly, the periphery of the
12 Fox et al.

meniscal allograft is sutured to the remaining periph-
eral rim or the joint capsule. The meniscal horns are
fixed with either small, attached bone plugs (the sen-
ior authors’ preference) or soft tissue fixation with
sutures (Fig.10).
Contraindications for allograft implantation are
advanced arthrosis, obesity, synovial disease, inflam-
matory arthritis, significant osteoarthritis (OARSI
Grade 3–4), and previous joint infection (Verdonk
et al., 2013). Drawbacks include the limited number
of available grafts, cost, graft sizing, effects of sterili-
zation and preservation on biomechanical strength of
the graft, and the risk of disease transmission (Brophy
and Matava, 2012; Verdonk et al., 2013). Currently,
no level I evidence exists to support the role of a
meniscus transplant in halting the progression of
osteoarthritis (Hergan et al., 2011).
Synthetic Implants
Synthetic scaffolds are emerging as a promising
alterative to meniscal allograft transplantation for par-
tial meniscal replacement in symptomatic patients.
The Menaflex Collagen Meniscal Implant (Regen Bio-
logics, Hackensack, NJ) and the Actifit polyurethane
scaffold (Orteq, London, UK) are currently used in
clinical studies outside of the U.S. The goal of these
resorbable scaffolds is to allow in-growth of meniscal
tissue and thereby create a regenerated meniscus
over time formed by host tissue (Verdonk et al.,
2013).
The Menaflex is composed of Collagen I isolated
from bovine Achilles tendons, treated with hyaluronic
acid, chondroitin sulphate, glycosaminoglycans and
cross-linked with formaldehyde. The scaffold is
sterilized with gamma irradiation. The result is a
sponge like structure with a pore size ranging from 75
to 400 microns (Stone et al., 1997). The surgical tech-
nique includes preparation of the implant bed, confir-
mation of the blood supply, rehydration, and sizing of
the implant, followed by the securing of the implant to
the remaining meniscus using suture (Stone et al.,
1997). The implant requires a meniscal rim and intact
anterior and posterior meniscal horns for attachment
(Rodkey et al., 1999). Rodkey et al. presented a large
randomized trial comparing the collagen meniscal
implant with partial medial meniscectomy (Rodkey
et al., 2008). The authors of this study were able to
demonstrate that patients with prior partial medial
meniscectomy regained significantly more of their lost
activity and significantly fewer reoperations were nec-
essary (Rodkey et al., 2008). Conversely, patients
with an acute trauma to the meniscus without prior
surgery did not show a significant difference com-
pared to the control group. Zaffagnini et al. presented
a nonrandomized cohort study of 33 patients with
medial meniscus injuries and minimum 10 year
follow-up (Zaffagnini et al., 2011). Patients them-
selves decided if they preferred partial medial menis-
cectomy alone or with implantation of a collagen
meniscal implant. At a minimum of 10 years postop,
the patients that underwent scaffold implantation
showed significantly lower pain scores, higher activity
level and significantly less medial joint space narrow-
ing on radiographs than the control group (Zaffagnini
et al., 2011).
The Actifit meniscus implant is a slowly degrading
polymer-polycaprolactone and urethane porous scaf-
fold with high interconnectivity. The scaffold has been
shown to improve contact area and pressure in a
cadaveric ovine model (Maher et al., 2011). Verdonk
Fig. 10. Meniscal Allograft Transplantation. (A)
Medial meniscal allograft transplantation with anterior
(large arrow) and posterior (small arrow) bone plugs
through transosseous tunnels. A combination of sutures
placed arthroscopically and open fix the meniscal allograft
to the peripheral rim. (B) Lateral meniscal allograft trans-
plantation using a bone bridge. The bone bridge is
secured in the tibial trough (large arrow) using transoss-
eous sutures (small arrow). A combination of arthroscopi-
cally and open placed sutures fix the meniscal allograft to
the peripheral rim. Image reprinted with permission from
(Sekiya JK, Ellingson CI. J Am Acad Orthop Surg, 2006,
14, 164–174).
The Meniscus: Anatomy, Function, Injury and Treatment 13

and colleagues were able to demonstrate on MRI 3
months after implantation tissue ingrowth and vascu-
lar perfusion in 35 of 43 patients (Verdonk et al.,
2011). A second-look arthroscopy 1 year following
surgery showed integration of the implant in 43 out of
44 patients. Patients with irreparable medial and lat-
eral meniscal defects showed a statistically significant
improvement in pain and activity scores 6 months
after implantation of the Actifit scaffold compared to
baseline (Verdonk et al., 2012).
Several other implants have been tested in animal
studies including implants derived from porcine small
intestinal submucosa (Cook et al., 2006), polycapro-
lactone and hyaluronan-derived polymer reinforced
with polylactic acid fibers or polyethylene terephtha-
late net (Chiari et al., 2006), Kevlar reinforced poly-
carbonate-urethane implants (Zur et al., 2011), and
ultrahigh-molecular-weight polyethylene fiber rein-
forced polyvinyl-alcohol hydrogel implants (unpublished
data).
The current challenges of implant design are the
fixation (particularly of total meniscal implants), the
material properties, and surface characteristics. Good
fixation of the graft to the tibia and joint capsule is
mandatory to minimize extrusion of the implant. The
material properties of the implant should be engi-
neered to match the compressive and tensile proper-
ties of the native meniscus. The implant surface
characteristics are important to minimize chondral
damage to the femur und tibia.
CONCLUSIONS
The menisci are integral to the normal function of
the knee joint and play an important role in load dis-
tribution, shock absorption, stability, lubrication, and
proprioception. Injuries to the menisci are recognized
as a common cause of significant musculoskeletal
morbidity. The unique and complex structure of the
menisci makes treatment and repair challenging for
the patient, the surgeon and the physical therapist.
Preservation of the menisci’s distinctive composition
and organization is paramount to knee joint health.
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