Orthopaedics & Traumatology: Surgery & Research (2009) 95S, S49—S59

ORIGINAL ARTICLE

Comparative anatomy of the knee joint: Effects onthe lateral meniscus

C. Javoisa, C. Tardieub, B. Lebelc, R. Seil d,C. Huletc,∗, the Société francaise d’arthroscopie

a Department of Orthopaedic Surgery and Sport Medicine, Clinique du Cours-Dillon, 1, rue Peyrolade, 31300 Toulouse, Franceb UMR 7179 ‘‘Mécanismes adaptatifs : des organismes aux communautés’’, USM 301 — EGB Department, CNRS, Department ofComparative Anatomy, National Museum of Natural History, 55, rue Buffon, 75005 Paris, Francec Department of Orthopaedics, Caen University Hospital Center, Avenue Côte-de-Nâcre, 14033 Caen, Franced Center of Musculoskeletal System, Sport Medicine and Prevention, Luxembourg Hospital Center, Clinic of Eich,

78, rue d’Eich, 1460 Luxembourg, Luxembourg

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Introduction

When replaced within the evolutive process of species,humans are primates, hominids sharing a close evolution-ary relationship with the great apes (gibbons, orangutans,gorillas and chimpanzees). The chimpanzee (s) delete is ourclosest living relative with whom we share a recent com-mon ancestor. This common ancestor is neither a chimp nora gorilla, nor a human. The study of fossil specimens andcomparative anatomy helped determine the time of splitbetween the main evolutive species. It is generally believedthat the chimpanzee-human split occurred about seven to10 million years ago [1,2]. More or less preserved fossilspecimens were recovered and give us a clearer picture ofthe human evolutionary line. The Australopithecus afaren-

sis currently name lucy, which lived between two and threemillion years ago, was discovered within Eastern Africa andis among the most famous and complete fossils ever found.

∗ Corresponding author.E-mail address: hulet-c@chu-caen.fr (C. Hulet).

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1877-0568/$ – see front matter © 2009 Elsevier Masson SAS. All rights redoi:10.1016/j.otsr.2009.09.008

Despite partial similarities between lateral and medialenisci in human beings, they display differences whichetter highlight the specific lateral meniscus pathology. Weelieved it was interesting to go back in time in order tonvestigate the anatomic and pathophysiological specificityf the lateral meniscus through the study of the comparativenatomy and the embryologic development.

omparative anatomy and species evolution

he first human ancestors were both of arboreal and ter-estrial origin. Humans are the only primates to use aermanent and exclusive bipedal gait [3,4]. A good knowl-dge of the knee phylogenetic evolution requires propernderstanding of the changes induced by the shift from therboreal to the terrestrial lifestyle and from quadrupedalismo bipedalism. The shift toward habitual bipedalism amongumans was associated with major anatomical changes [1].

The ‘‘tension’’ pelvis of arboreals evolved toward a‘pressure’’ pelvis with a shorter distance between theacroiliac and coxofemoral joints and a major widening ofhe sacrum. These morphological rearrangements of theelvis had to face the compromise between the child-

served.

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Figure 3 Femur of a chimpanzee, Cercopithecus and human.Bta

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igure 1 Change from quadrupedal to bipedal locomotionith acquisition of the erect position.

irth and bipedalism constraints. The straightening up ofhe trunk was combined with the acquisition of four spineurves, the lumbar curve being the result of a reverse incur-ation of the sacrum. In the hip region, the developmentf the gluteus maximus contributed to the straightening upf the spine. Primates’ prehensile foot was converted intoweight-bearing and propulsion foot with reduction of the

istance between the first and second rays, the appearancef two arches of the foot, the internal longitudinal arch andhe anterior transverse arch, and the horizontality of theibio-talar joint surface. In the knee region, the permanentnee flexion used in non-human arboreal and terrestrial pri-ates was converted into a complete weight-bearing kneeexion and extension specific to humans.

To promote better understanding of the knee jointnatomy in the human species, it appears instructive toake a comparison with other primates and hominid fossils.

ucy’s skeleton combined with pieces from the A. Afarensisthree million years) included well-preserved femurs andibias with intact meniscal tibial insertions. Lucy is bipedalhen walking but also achieves arboreal displacements. Herdaptation to bipedalism included several decisive changesegarding pelvis and inferior limbs. (Fig. 1) [1,3]. Chim-anzees and all non-human primates exhibit abducted knees4] (Fig. 2). Man is the only one to stand upright withdducted femurs [1,5,6,8—11]. Such evolution was markedy three skeletal modifications, which involved the femoraliaphysis, the femoro-patellar joint and femoral condyles.

he femoral bicondylar angle enabled knee

dduction

n chimpanzees and all non-human primates, the femoraliaphysis is straight: The femoral axis is perpendicular to

igure 2 Change from abducted to adducted knee in humans.

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oth non-human primates exhibit a straight femur. Conversely,he human femur is obliquely inclined thus allowing thedducted position of the knee joint [1,8].

he knee joint line. The acquisition of both knee adduc-ion is linked to the development of a femoral bicondylarngle or femoral obliquity angle [3,5,6,12—14]. It is presentn all Australopithecus, which indicates these hominids hadipedal habits. According to anthropologists, it representsne of bipedalism features [7,8]. The femoral bicondylarngle is defined as the angle between the diaphyseal axisnd the perpendicular line to the infracondylar plane pass-ng through its middle that is the bottom of the trochlearroove. This angle occasionally differs from the mechanicalemoral axis from 1◦ up to 8◦ (Fig. 3).

Tardieu analyzed the ontogenic development of theuman femoral bicondylar angle through a sample of 25adiographs of the femur of zero to 13 year-old child takenrom osteologic collections (Fig. 4) [2,10,11]. This angles nil (or the angle value is zero) in the newborn child,Fig. 4 upright insert) the femoral diaphysis remains strictlytraight; there is no angle of obliquity. The infradiaphyseallane, which separates the cartilaginous epiphysis from theiaphysis, is perfectly horizontal (Fig. 4). Femurs of a 7-onth foetus (angle of obliquity: 0◦) and of four children,

espectively six months (1◦), three years (5◦) and sevenears old (9◦) suggest that this angle arises in the diaphy-eal region independently of growth of the distal epiphysis.stronger medial metaphyseal appending is thus produced.

his morphogenetic phenomenon is a diaphyseal characterrising independently of growth of the distal epiphysis. Thencrease in this angle occurs mostly between one and fourears which closely parallels the developmental chronology

Comparative anatomy of the knee joint S51

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Figure 4 Femoral obliquity angle in osteologic samples ofinfant femurs.

of the acquisition of standing and walking. However, thisangle may vary significantly (6◦ to 14◦), the mean values ofthe studied populations ranging from 8◦ to 11◦. The variabil-ity of this angle might depend on each child skeletal loading.Females exhibit a higher angle correlated with a greaterinteracetabular distance. Conversely, radiographic analysisof nonwalking children shows a perfectly straight femoraldiaphysis and a bicondylar angle of 0◦ (Fig. 5). The obliquepositioning of the diaphysis and knee adduction relative tothe hip joint are not attributable to the femoral head-neckoffset. Therefore, in all mammals, the femoral diaphysis off-set is induced by the femoral head and neck while knees

are not adducted. Other mammals show no femoral obliq-uity angle as opposed to humans and their ancestors (Fig. 6)[1—3,13].

Figure 5 Anteroposterior radiograph of a non-walking childfemur. The femoral obliquity angle is of 0◦.

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igure 6 Pelvis of a chimpanzee, of Lucy (Australopithecusfarensis) and of human.

he prominence of lateral lip of femoral trochlearevents any lateral dislocation of the patella

he chimpanzee distal femoral epiphysis has a flat trochleaith similar features to a femoro-patellar dysplasia. This flat

urface enables free patellar displacements during repetednee rotation movements attributed to foot grasping duringrboreal displacements. In humans, the trochlea featuresgroove, the lateral lip being higher and more prominent

nteriorly than the medial one. The femoral trochlea adaptstself to act as a stabilizer for the femoro-patellar joint. Thiseature is linked with the previous one and promotes medio-ateral patellar stability in the presence of a high femoralbliquity angle. It is interesting to observe that from fetalevelopment, the cartilaginous structure of the inferior dis-al epiphysis exhibits a flat trochlea in chimpanzees and aeep trochlear groove with prominence of the lateral lip inumans Fig. 7.

he increased radius of curvature of the lateralemoral condyle and tibial plateau facilitates fullxtension of the knee joint

he anatomical shape of the lateral compartment bonetructures is another significant modification (Fig. 8). In non-uman primates, the sagittal aspect of the lateral condyles circular without any junction or condylotrochlear promi-ence. This circular shape therefore limits full extensionovements of the knee joint. The tibia does not go up over

he trochlear surface since there is little or even no kneextension movement.

In humans, the lateral condyle becomes elliptic, whichorresponds to an increase in the radius of curvature in itsnferior part. According to Kapandji [15], the radius of curva-ure progressively increases from back to front up to the ‘‘t’’oint and then begins to decrease. The section posterior tohe ‘‘t’’ point participates to the lateral femoro-tibial andhe anterior section participates to the femoro-patellar. Theadius of curvature increases femoro-tibial contact area inull extension [16]. This modification of the condyle shape

educes the stress applied on the knee particularly whenlmost in extension or in full extension.

In non-human primates, the lateral tibial plateau has aery convex shape, with regard to the lateral condyle. The

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Figure 7 Comparative view of the distal tibia epiph

ombination of a ‘‘elliptic’’ circular lateral condyle with aery convex lateral tibial plateau reduces the contact areaetween both articular surfaces. The lateral compartments very mobile and useful for arboreal displacements but lessompatible with knee extension under loading conditions.his high convexity of the lateral plateau has dimin-

shed in humans: It is slightly convex which increases theemoro-tibial contact surface. The quadrupedal to bipedalransition led to the practice of extension movements of

he knee joint associated with reduction of the sustentationolygon, knee adduction, deepening of the trochlear groovend a change from a spherical to an elliptical profile ofhe lateral condyle, the knee requiring more stability whileemaining mobile.

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igure 8 Sagittal view of gorilla medial condyle (left), gorilla laterf the menisci, R. Verdonk) ESSKA 2000 Basic Science Committee: Khape of the tibial plateau, which exhibits a concave medial surface

n humans, chimpanzees and various fossil specimens.

hat are the effects of these skeletal changes onhe lateral meniscus?

rimate and mammal knee joint has two menisci [17—19].he medial meniscus is delete identical in all species, is C-haped and features a double tibial insertion: an anteriornd a posterior insertion [1,3,13].

The lateral meniscus is variable in shape according to theype of primate. A crescent-shaped lateral meniscus with

ne tibial insertion, anterior to the lateral tibial spine occursn lemuriforms, tarsius and orangutans. In gibbons, gorillasnd chimpanzees, the lateral meniscus is ring-shaped andxhibits a single tibial insertion anterior to the lateral tibialpine (Fig. 9). A single notch is seen on the tibial bone sur-

al condyle (center) and human lateral condyle (right, Anatomynee anatomy for orthopaedic surgeons. Note the difference inand a convex lateral surface.

Comparative anatomy of the knee joint S53

lateral meniscus and reinforce the knee extension stabilitynecessary for habitual bipedalism.

Ontogeny of meniscal characters

The lateral and medial menisci originate from the same mes-enchymatous embryologic tissue of the knee. It consists ofsemilunar fibrocartilage, which appears by the 4th month ofthe gestational development.

The meniscus has a rich blood supply particularly in themeniscal horn region [20], which evolves during gestationaldevelopment. It is initially limited to the outer third ofthe medial or lateral meniscus but progresses up to theperipheral 2/3 of the whole meniscus at birth. The avascularpopliteal recess appears at the beginning of the fetal stage.

The histological composition of the menisci is iden-tical (75% water, 20% of type I collagen fibers and 5%de GAG). Macroscopically, it is usually constituted of tri-angular structures, the medial meniscus is C-shaped andthe lateral meniscus is ring shaped. Menisci are 35 mm indiameter, mean length when measured from the periph-eral rim is 110.86 ± 13.18 mm for medial meniscus and111.15 ± 11.07 mm for lateral meniscus [21]. The meancoverage rate of medial meniscus on the tibial articularsurface is 64% (51 to 74%) while lateral meniscus shows ahigher mean coverage rate of 84% (75 to 98%), such val-ues remain stable during the whole gestational developmentand growth period [22]. Kohn and Moreno [21] have studiedthe anatomical meniscal insertions in 46 preserved humancadaver tibias of mean age 35 years. The anterior insertionsite surface area of the medial meniscus is 139 ± 43 mm2

while the posterior insertion surface area is 80 ± 10 mm2.Lateral meniscus measurements are different: 93 ± 25 mm2

for its anterior insertion and 115 ± 51 mm2 for its posteriorinsertion. Anatomic insertions of both medial menisci weredefined from standard lateral and A/P knee radiographs andrecently studied by Wilmes et al. [23,24]. He could there-fore accurately determine reproducible landmarks for thelatitude and longitude of both medial and lateral menisciby focusing on the insertion point of the anatomical horns.Based on 20 cadaver tibias and lateral and A/P views, hecould determine the position in the latero-medial axis (Xaxis) and anteroposterior axis (Y axis) (Fig. 10). These posi-

Figure 9 Effects on the lateral meniscus. Posterior insertionspecific to humans.

face and the posterior border of the external tibial plateauappears shorter and very abrupt. A crescent-shaped lateralmeniscus with two tibial insertions, one anterior and oneposterior to the lateral spine, is found in humans.

In humans, there are two tibial insertions of the lateralmeniscus. The posterior border of the lateral tibial plateauis long, discontinuous and notched by the posterior insertionof the lateral meniscus. Lucy (A. afarensis) exhibits a singleinsertion of the lateral meniscus on the tibia, which suggestsgreater knee mobility and the practice of arboreal locomo-tion. In arboreal primates with a bent-knee posture, theshape of the lateral compartment accounts for the impor-tant mobility of the lateral meniscus. Major anteroposteriormovements of the lateral meniscus around its single inser-tion site [1,3,13] are observed. A meniscofemoral ligamentis present in all primates.

Conversely, the changes observed in humans restrict theanteroposterior movement of the lateral meniscus and rein-force the knee extension stability necessary for habitualbipedalism. Such posterior insertion in humans contributesto prevent the lateral meniscus from an extreme ante-rior gliding during frequent extension. The lateral meniscusis pulled strongly anteriorly during medial rotation of thefemur on the tibia. As in extension, this posterior attach-ment of the lateral meniscus limits the anterior movement.This feature is specific to the human species compared tothe whole of living mammals. The development of menis-cofemoral ligaments also contributes to the stability of thelateral meniscus during flexion, extension and rotationalmovements of the knee joint. The modification of the tibialinsertion of the lateral meniscus caused the appearance inhumans of ring or crescent-shaped menisci with single inser-tion. The clinical entity known as ‘‘discoid lateral meniscus’’is by far the most common morphological anomaly of the lat-eral meniscus in humans, which represents 1.5 to 4.6% of thecases. These shapes and insertions are attributable to theevolution of our species: and therefore genetically deter-mined. Discoid menisci observed in humans result from the

persistence, in some cases, of the tendencies seen in gib-bons, gorillas and chimpanzees, our closest living relativephylogenetically [19]. The changes observed in the humanspecies help reduce the anteroposterior movement of the Figure 10 Radiographic study of meniscal insertions.

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igure 11 Fat-saturation anteroposterior and lateral MRI of t

ions remain stable and show a narrow relationship withhe intercondylar eminences of the tibia. Insertions of theedial meniscus are closer to the peripheral articular sur-

ace whereas the lateral meniscus is narrower, its anteriornd posterior insertions being very close together. The ante-ior insertion of the lateral meniscus is slightly lateralizedompared with the posterior insertion in the coronal plane.n the sagittal plane, these insertions are very close togetherince the anterior insertion is located in the middle of theateral plateau whereas the posterior insertion is situatedt the junction between anterior 3/4, posterior 1/4. Let’seep in mind the single insertion site of the lateral meniscusn the chimpanzee. Moreover, the lateral meniscus rests onconvex lateral tibial plateau unlike the medial meniscus.

eans of fixation or connection of the lateraleniscus

esides the meniscal horn insertions on the tibial plateau,ther connections have been described.

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igure 12 Posterior anatomic view and posterior aspect of tibia andAnatomy of the PCL and the meniscofemoral ligament, A. Amis. ESSKurgeons).

enicofibular ligament anterior to the popliteal recess.

These connections include the anterior intermeniscal lig-ment (AIL) of the knee or Winslow’s ligament. It doesot present as a constant structure according to the typef study [25,26,27]. In a cadaver study conducted byarcheix et al. [25], this anterior ligament was present

n 100% of the cases and only in 80% of the cases whennvestigated by MRI examination. It is 31.2 mm long andnly 1.8 mm wide. In another cadaver study conducted byubbs et al., the Winslow’s ligament was found in only5% of the cases and demonstrated very similar dimen-ions (35.4 mm long and 2.5 mm wide). This ligament mighte double (3.7% of cases) and various types are describedccording to their associated insertion on the anteriorargin of the meniscal horn and on the anterior capsule

27].Immediatly anterior to the popliteal recess, Bozkurt et al.

28] examined more specifically the presence of the menis-ofibular ligament. Based on 50 cadaver dissections, he findsmeniscofibular ligament, which runs between the inferiorenisco-synovial junction of the midportion of the lateraleniscus, anterior to the popliteal hiatus up to the artic-

lateral meniscus featuring meniscofemoral ligament insertionsA 2000 Basic Science Committee: Knee anatomy for orthopaedic

Comparative anatomy of the knee joint

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Figure 13 MRI view of the anterior (at the bottom) and pos-terior (at the top) meniscofemoral ligament.

ular surface of the upper portion of the fibulotibial joint.The average thickness of this ligament is 3.84 mm (2.6 to6.1 mm) and varies according to the orientation of the fibu-lotibial joint. It thus promotes the stability of the midportionduring rotational movements of the tibia under the femur,knowing that the midportion of the LM is subjected to stresson the block of the lateral tibial plateau convexity. Obaid etal. [29] could detect this ligament during fat saturation MRIsequences. MRI shows a curvilinear or straight hyposignalstructure of variable thickness originating from the inferiorborder of the lateral meniscus and running to the fibu-lar articular surface (Fig. 11). It corresponds to the deeplayer of the lateral collateral ligament and is always locatedanterior to the popliteal hiatus. In 152 MRI retrospectivelyanalyzed, it was clearly identified in 42.5% of the cases,this prevalence reaches 63% after injection of colored liquidin the posterolateral recess. The meniscofibular ligament isalso identified in 80% of the subjects and visible in almosthalf of the knee MRI in daily practice according to a recentstudy published by Bozkurt et al. [28] and Obaid et al. [29].Let’s underline the presence of a free, non-fixed poplitealrecess or lateral collateral ligament, which is avascular.

Another specificity of the lateral meniscus is the presenceof two meniscofemoral ligaments: the ligament of Humphrey(aMLF) passes anterior to the PCL and the ligament of Wris-berg (pMLF) passes posterior to the PCL (Figs. 12 and 13).

These two ligamentous structures stretch from posterior

horn of the lateral meniscus to the lateral aspect of themedial femoral condyle. Amis, Masouros et al. and Gupteet al. [30—34] performed a thorough anatomical and biome-chanical analysis of these meniscofemoral ligaments, which

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Table 1 The mechanical properties of these two structures have

Insertion site area (mm2) Tensile strength (

aMFL 14.7 ± 14.8 300 ± 155pMFL 20.9 ± 11.6 302 ± 158

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re commonly examined along with the PCL. They do notresent as a constant structure. In an important study of theiterature, Gupte et al. [32] identified at least one MFL in 91%f the cases among 781 anatomic dissected cadaveric kneesrom 13 studies. The aMFL was present in 48.2% of the casesnd the pMFL was present in 70.4%. Meniscofemoral liga-ents were both identified in only 32% of the knees. Gupte

t al. [32,33] suggest that these ligaments undergo degen-rative changes with age and are more commonly detectedn Caucasians than in Asians.

The insertion site of the aMFL is located on the medialemoral condyle between the lower part of the insertion sitef the PCL and the cartilaginous edge of the medial femoralondyle under the PCL. The insertion site of the p MFL isore posterior and at the top of the femoral insertion of theCL, above the PCL [30]. The length of the MFL ranges from1 to 27 mm according to gender and type of study. The pMFLs longer, ranging from 23 and 31 mm [32,33]. In an arthro-copic observational study using the ‘‘meniscal tug test’’,upte et al. [35] examined the meniscofemoral ligament

nsertions and their attachment to the lateral meniscus.echnically, the hook pulls on the anterior meniscofemoraligament in order to realize a movement of the root of theateral meniscus. The pMFL is less difficult to visualize sincet is situated posteriorly to the PCL. Therefore, among 68nees, the anterior MFL could be identified in 68% of theases [27] and the pMFL was only present in 15% of theases. MRI studies of meniscofemoral ligaments report vari-ble results [36] but are ancient (Fig. 13). The presence ofhe MFL is variable since the difficulty lies in passing in theFL plane (Table 1).

The first descriptive anatomic studies described theeniscofemoral ligaments as being a ‘‘3rd cruciate liga-ent’’. These ligaments exhibit a high strength level andodulus of elasticity compared with both bundles of theCL (anterior bundle of the PCL 1620 N, 248 MPa, pos-erior bundle of the PCL 258N and 145 MPa [34]). Theirtrength is 30% of the PCL strength and identical to thatf the PCL posterior bundle. Biomechanical studies haveevealed that MFL have 30% of the last PCL strength.oran et al. [38] assessed the tensile behavior of botheniscofemoral ligaments during flexion movements of the

nee. The a MFL shows no tensile strength in extensionnd starts at 10◦ in 20◦ of flexion up to its maximum at05◦ of flexion. Stress significantly increases during tib-al external rotation movements. Conversely, the pMFLeveals a maximum tensile strength in extension, whichrogressively decreases while flexion increases. Its tensiletrength is nil in 80◦ of flexion. Stress when applied to

n extension then decreases in about 30 of knee flexionhereas it increases significantly in full flexion. According

o the work of Amadi et al. [39], the presence of bothntact meniscofemoral ligaments reduces by 10% the lateral

been well defined [37].

N) Modulus of elasticity (MPa) Length (mm)

281 ± 239 20.7 ± 3.9227 ± 128 23.0 ± 4.3

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emoro-tibial strain and limits the tibial posterior transla-ion.

The pMFL tension increases during knee extension whilehe aMFL loosens. The pMFL tension decreases in kneeexion while the aMFL develops tension [39]. The aMFL sta-ilizes the posterior horn of the lateral meniscus in kneeexion while the pMFL acts as a stabilizer in knee exten-ion. Therefore, during knee flexion, both MFL move theosterior horn of the LM anteriorly and interiorly. Theyake a significant contribution to limiting its posterior dis-lacement. Gupte et al. [40] have described an antagonisticelationship between the meniscofemoral ligaments and theeniscofibular ligaments anteriorly to the popliteal hiatus.he meniscofibular ligament pulls the post portion poste-iorly and interiorly whereas the traction exercised by theFL is located anteriorly and inferiorly and more proximally

hat is anteriorly in the anteroposterior direction. Theseeniscofemoral ligaments prevent the natural posterior dis-lacement during knee flexion.

Therefore, during tibial internal rotation in a flexed knee,he MFL pulls the posterior portion anteriorly and inferiorlyhereas the meniscofibular ligaments maintain it posteriorly

hus protecting it from the lateral femoral condyle.In extension, during axial compression, the stress applied

ushes the lateral meniscus outside. The compressive forceshange into shearing forces and are transmitted by theFL to the circumferential fibers of the lateral meniscus.

n case of excessive stress, one of the elements from theFL-meniscal root and post portion chain might disrupt and

nvolve a meniscal or ligamental tear.These meniscofemoral ligaments are the secondary

estraint to posterior tibial translation after the PCL but theylso act as an important stabilizer of the horn and the lateraleniscal root when this latter is subjected to compressive

tress on the lateral tibial plateau convexity.Finally, the lateral meniscus is integrated in this

natomo-functional entity, which is the posterolateral cor-er of the knee. The major structures of the posterolateralorner of the knee include the popliteofibular ligament, theopliteus muscle, the lateral collateral ligament, the bicepsemoris and the posterior joint capsule.

eniscal kinetics

herefore, the whole insertion means and the convexity ofhe lateral tibial plateau provide better understanding of theeniscal displacements. The lateral meniscus is subjected

o greater movements than the medial meniscus.Up to 90◦ of knee flexion, Vedi et al. [40] have demon-

trated that the lateral meniscus displaces 9.5, 3.7 and.6 mm posteriorly for the anterior portion, the centralortion and the posterior portion respectively. The naturalosterior meniscal movement during knee flexion is greater41] for the lateral meniscus than for the medial one andight reach up to 10 mm for the anterior horn. Yao et al. [41]

ave recently studied the posterior translation with over

30◦ of knee flexion. The overall in vivo posterior translations 8.2 ± 3.2 mm for the lateral meniscus and 3,3 ± 1,5 mm forhe medial one. The anterior horns show a greater posteriorranslation (LM : 10.2 mm, MM : 6.5 mm) than the posteriororns (LM: 6.2 mm, MM : 3.1 mm). There is a significant dif-

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igure 14 Dynamic MRI of backward displacement of ant andost portion of menisci during over 120◦ of flexion.

erence of about 5 mm between the average translations ofoth menisci.

We conducted a study in five healthy knees; the kneeas initially extended then progressively flexed, by incre-ents of 30◦ up to more than 100◦ of flexion. For each

ateral position, we measured the backward displacementf the anterior and posterior portions regarding the circleepresenting the posterior femoral radius of curvature cor-esponding to the higher diameter of the lateral femoralondyle [42]. Movement of the posterior and anterior por-ion was measured from extention to full knee flexion. Theateral meniscus shows a greater posterior translation thanhe medial meniscus. The posterior translation is 15 and6 mm for LM anterior and posterior portions respectivelynd only 8 and 5 mm for MM anterior and posterior portionsFig. 14).

These observations are similar in the five studied knees;n all cases, when the knee is placed in over 90◦ of flexion,he posterior portion of the lateral meniscus moves poste-iorly to the tibial plateau whereas the posterior portion ofhe medial meniscus remains directly above the posteriordge of the medial tibial plateau.

eflexion on physiopathology of lateraleniscus lesions

he lateral meniscus provides good articular congruency inhe femoro-tibial compartment, which is hostile because ofts opposed convexities. It acts as a knee stabilizer, particu-arly via the interconnections of the posterolateral corner ofhe knee, and shows a great capacity to move due to its phy-ogenetic evolution. The lateral compartment of the knee isommonly presented to be the mobile knee compartmentthat of mobility). Stress applied to the lateral meniscuss mostly exerted on the anterior and posterior portions aseported in the work of Moyen et al. [43]. Its location andorphology contribute to the transformation of compressive

orce to shearing force as confirmed by the SFA study in 199643]. The lateral meniscus acts as a shock absorber while

istributing compressive stress circumferentially. Its propri-ceptive receptors are located in the peripheral 2/3 andn the anterior and posterior portions. The possible mecha-isms of injury and their consequences in terms of excessive

Comparative anatomy of the knee joint

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Figure 15 Physiopathological mechanisms of lateral meniscallesions.

load transmission might be anticipated. Stress applied tothe medial compartment is thus mainly compressive. Withinthe lateral compartment, the compressive stress associatedwith a greater displacement subjects the lateral meniscusto shear forces applied to the lateral tibial plateau con-vexity. During a traumatism associating valgus, flexion andexternal rotation, the opposition between the thrust of thelateral condyle reinforced by the anterior traction of themeniscofemoral ligaments and the posterior traction of themeniscofibular ligament and of the capsule result in a pos-terior subluxation of the lateral tibial plateau, especiallyin knee flexion. The convexity of the lateral tibial plateauacts as a block on a narrow and mobile meniscus compared

to the popliteal recess, which induces a shear movement(Fig. 15).

If the ACL is intact, the translation is not excessive andthe shearing force will exert on the central portion com-pared with the popliteal recess. Such phenomenon has been

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Figure 16 Six MRI images for analysis of meniscal roo

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onfirmed by clinical studies on the analysis of meniscalesions in a stable knee.

In case of torn ACL, the translation is excessive and thelock movement associated with high shear force will exertn the posterior portion of the lateral meniscus. In such con-itions, the meniscofibular ligaments, the meniscofemoraligaments — according to the degree of knee flexion — andhe root of the lateral meniscus are highly exposed to theisk of injury.

West et al. [44], in 2004, have reported this type ofeniscal root injury combined with tear of the ACL. This

njury was identified in 12.4% of the cases. Brody et al. [45]ell described the MRI features of meniscal roots in T1 and2. Among 264 MRI performed in patients with torn ACL, 9.8%f lateral meniscal root injuries and only 3% of medial menis-al root injuries were observed. Meniscal extrusion wasound in six out of the 26 lateral meniscal injuries (23%). Thebsence of MFL was commonly found to be associated witheniscal extrusion when there was a lateral meniscal injury.

t underlines the difficulty to obtain clear visualization ofeniscal roots and meniscofemoral ligaments [46,47,48,49].De Smet et al. [50] defined MRI criteria using six imag-

ng planes (three coronal and three sagittal projections)Fig. 16) acquired with fat-saturated T2-weighted MR imageso provide proper visualization of the meniscal roots. Threeillimetres thick sections should be successively used with

.5 mm interpolation. This technique combined with thor-ugh analysis of six thin slices proves helpful in the detectionf lateral meniscus root injuries.

Ahn et al. [51] have shown that these injuries commonlyccur in the coronal plane and cause a tear of the poste-ior portion of the lateral meniscus, which loses its tibialttachment (Fig. 17). This lesion is defined as being situ-

ted within 1 cm from the tibial insertion site. The reportedrevalence of these injuries in his study is only 6.7% of the32 ACL grafts. Ahn et al. [51] believe meniscus repair shoulde performed whenever possible, even in case of lesionsn the white-white area. Arthroscopic meniscal healing was

ts and detection of lateral meniscus root lesions.

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igure 17 Arthroscopic view of a lateral meniscus root lesion.

bserved in all cases and the lateral meniscus root, due to itswo insertions (MFL and tibial insertion) uneases the repairrocedure compared with a mid-segment vertical lesion.hese injuries should be investigated with growing interest.

onclusion

he use by the Homo sapiens of exclusive bipedalismrovides better knowledge of the anatomical differencesetween both knee compartments. Consequences on the lat-ral meniscus are greater than those observed on the medialeniscus, which remains identical whatever, the species.he lateral meniscus double insertion is a unique featuref the Homo sapien. Along with anatomic study and byombining dissections with recent imaging investigations,he use of modern imaging systems, digital radiography,specially in MRI, provides reliable landmarks to facilitateeniscal allografts. Since modern imaging techniques pro-

ide good understanding of the functional anatomy, kneeinematics and more particularly that of lateral meniscusight be properly assessed. It provides better understand-

ng of meniscus biomechanics and helps determine itsesional mechanisms. Accurate imaging protocols should beevelopped to offer better analysis of MFL (especially theosterior one), anterior cruciate ligament injuries (ACL) andeniscus roots in order to quantify the relationships witheniscal extrusion. By defining in a more accurate and com-rehensive manner the ACL lesion associated injuries, weill improve our therapeutic indications and techniques ofeniscus preservation.

onflict of interest

he present authors had no conflict of interest regarding thisublication.

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