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

PARIS 2008 SFA ANNUAL MEETING SYMPOSIUM: THE LATERAL MENISCUS

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

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: [email protected] (C. Hulet).

Despite partial similarities between lateral and medialmenisci in human beings, they display differences whichbetter highlight the specific lateral meniscus pathology. Webelieved it was interesting to go back in time in order toinvestigate the anatomic and pathophysiological specificityof the lateral meniscus through the study of the comparativeanatomy and the embryologic development.

Comparative anatomy and species evolution

The first human ancestors were both of arboreal and ter-restrial origin. Humans are the only primates to use apermanent and exclusive bipedal gait [3,4]. A good knowl-edge of the knee phylogenetic evolution requires properunderstanding of the changes induced by the shift from thearboreal to the terrestrial lifestyle and from quadrupedalismto bipedalism. The shift toward habitual bipedalism amonghumans was associated with major anatomical changes [1].

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

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S50 C. Javois et al.

Figure 1 Change from quadrupedal to bipedal locomotionwith acquisition of the erect position.

birth and bipedalism constraints. The straightening up ofthe trunk was combined with the acquisition of four spinecurves, the lumbar curve being the result of a reverse incur-vation of the sacrum. In the hip region, the developmentof the gluteus maximus contributed to the straightening upof the spine. Primates’ prehensile foot was converted intoa weight-bearing and propulsion foot with reduction of thedistance between the first and second rays, the appearanceof two arches of the foot, the internal longitudinal arch andthe anterior transverse arch, and the horizontality of thetibio-talar joint surface. In the knee region, the permanentknee flexion used in non-human arboreal and terrestrial pri-mates was converted into a complete weight-bearing kneeflexion and extension specific to humans.

To promote better understanding of the knee jointanatomy in the human species, it appears instructive tomake a comparison with other primates and hominid fossils.Lucy’s skeleton combined with pieces from the A. Afarensis(three million years) included well-preserved femurs andtibias with intact meniscal tibial insertions. Lucy is bipedalwhen walking but also achieves arboreal displacements. Heradaptation to bipedalism included several decisive changesregarding pelvis and inferior limbs. (Fig. 1) [1,3]. Chim-panzees and all non-human primates exhibit abducted knees[4] (Fig. 2). Man is the only one to stand upright withadducted femurs [1,5,6,8—11]. Such evolution was markedby three skeletal modifications, which involved the femoraldiaphysis, the femoro-patellar joint and femoral condyles.

The femoral bicondylar angle enabled kneeadduction

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

Figure 2 Change from abducted to adducted knee in humans.

Figure 3 Femur of a chimpanzee, Cercopithecus and human.Both non-human primates exhibit a straight femur. Conversely,the human femur is obliquely inclined thus allowing theadducted position of the knee joint [1,8].

the knee joint line. The acquisition of both knee adduc-tion is linked to the development of a femoral bicondylarangle or femoral obliquity angle [3,5,6,12—14]. It is presentin all Australopithecus, which indicates these hominids hadbipedal habits. According to anthropologists, it representsone of bipedalism features [7,8]. The femoral bicondylarangle is defined as the angle between the diaphyseal axisand the perpendicular line to the infracondylar plane pass-ing through its middle that is the bottom of the trochleargroove. This angle occasionally differs from the mechanicalfemoral axis from 1◦ up to 8◦ (Fig. 3).

Tardieu analyzed the ontogenic development of thehuman femoral bicondylar angle through a sample of 25radiographs of the femur of zero to 13 year-old child takenfrom osteologic collections (Fig. 4) [2,10,11]. This angleis nil (or the angle value is zero) in the newborn child,(Fig. 4 upright insert) the femoral diaphysis remains strictlystraight; there is no angle of obliquity. The infradiaphysealplane, which separates the cartilaginous epiphysis from thediaphysis, is perfectly horizontal (Fig. 4). Femurs of a 7-month foetus (angle of obliquity: 0◦) and of four children,respectively six months (1◦), three years (5◦) and sevenyears old (9◦) suggest that this angle arises in the diaphy-seal region independently of growth of the distal epiphysis.A stronger medial metaphyseal appending is thus produced.This morphogenetic phenomenon is a diaphyseal characterarising independently of growth of the distal epiphysis. Theincrease in this angle occurs mostly between one and fouryears which closely parallels the developmental chronology


Comparative anatomy of the knee joint S51

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 kneesare 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◦.

Figure 6 Pelvis of a chimpanzee, of Lucy (Australopithecusafarensis) and of human.

The prominence of lateral lip of femoral trochleaprevents any lateral dislocation of the patella

The chimpanzee distal femoral epiphysis has a flat trochleawith similar features to a femoro-patellar dysplasia. This flatsurface enables free patellar displacements during repetedknee rotation movements attributed to foot grasping duringarboreal displacements. In humans, the trochlea featuresa groove, the lateral lip being higher and more prominentanteriorly than the medial one. The femoral trochlea adaptsitself to act as a stabilizer for the femoro-patellar joint. Thisfeature is linked with the previous one and promotes medio-lateral patellar stability in the presence of a high femoralobliquity angle. It is interesting to observe that from fetaldevelopment, the cartilaginous structure of the inferior dis-tal epiphysis exhibits a flat trochlea in chimpanzees and adeep trochlear groove with prominence of the lateral lip inhumans Fig. 7.

The increased radius of curvature of the lateralfemoral condyle and tibial plateau facilitates fullextension of the knee joint

The anatomical shape of the lateral compartment bonestructures is another significant modification (Fig. 8). In non-human primates, the sagittal aspect of the lateral condyleis circular without any junction or condylotrochlear promi-nence. This circular shape therefore limits full extensionmovements of the knee joint. The tibia does not go up overthe trochlear surface since there is little or even no kneeextension movement.

In humans, the lateral condyle becomes elliptic, whichcorresponds to an increase in the radius of curvature in itsinferior part. According to Kapandji [15], the radius of curva-ture progressively increases from back to front up to the ‘‘t’’point and then begins to decrease. The section posterior tothe ‘‘t’’ point participates to the lateral femoro-tibial andthe anterior section participates to the femoro-patellar. Theradius of curvature increases femoro-tibial contact area infull extension [16]. This modification of the condyle shapereduces the stress applied on the knee particularly whenalmost in extension or in full extension.

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



Figure 7 Comparative view of the distal tibia epiphysis in humans, chimpanzees and various fossil specimens.

combination of a ‘‘elliptic’’ circular lateral condyle with avery convex lateral tibial plateau reduces the contact areabetween both articular surfaces. The lateral compartmentis very mobile and useful for arboreal displacements but lesscompatible with knee extension under loading conditions.This high convexity of the lateral plateau has dimin-ished in humans: It is slightly convex which increases thefemoro-tibial contact surface. The quadrupedal to bipedaltransition led to the practice of extension movements ofthe knee joint associated with reduction of the sustentationpolygon, knee adduction, deepening of the trochlear grooveand a change from a spherical to an elliptical profile ofthe lateral condyle, the knee requiring more stability whileremaining mobile.

What are the effects of these skeletal changes onthe lateral meniscus?

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

The lateral meniscus is variable in shape according to thetype of primate. A crescent-shaped lateral meniscus withone tibial insertion, anterior to the lateral tibial spine occursin lemuriforms, tarsius and orangutans. In gibbons, gorillasand chimpanzees, the lateral meniscus is ring-shaped andexhibits a single tibial insertion anterior to the lateral tibialspine (Fig. 9). A single notch is seen on the tibial bone sur-

Figure 8 Sagittal view of gorilla medial condyle (left), gorilla lateral condyle (center) and human lateral condyle (right, Anatomyof the menisci, R. Verdonk) ESSKA 2000 Basic Science Committee: Knee anatomy for orthopaedic surgeons. Note the difference inshape of the tibial plateau, which exhibits a concave medial surface and a convex lateral surface.



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 thepersistence, 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

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 10 Radiographic study of meniscal insertions.



Figure 11 Fat-saturation anteroposterior and lateral MRI of the menicofibular ligament anterior to the popliteal recess.

tions remain stable and show a narrow relationship withthe intercondylar eminences of the tibia. Insertions of themedial meniscus are closer to the peripheral articular sur-face whereas the lateral meniscus is narrower, its anteriorand posterior insertions being very close together. The ante-rior insertion of the lateral meniscus is slightly lateralizedcompared with the posterior insertion in the coronal plane.In the sagittal plane, these insertions are very close togethersince the anterior insertion is located in the middle of thelateral plateau whereas the posterior insertion is situatedat the junction between anterior 3/4, posterior 1/4. Let’skeep in mind the single insertion site of the lateral meniscusin the chimpanzee. Moreover, the lateral meniscus rests ona convex lateral tibial plateau unlike the medial meniscus.

Means of fixation or connection of the lateralmeniscus

Besides the meniscal horn insertions on the tibial plateau,other connections have been described.

These connections include the anterior intermeniscal lig-ament (AIL) of the knee or Winslow’s ligament. It doesnot present as a constant structure according to the typeof study [25,26,27]. In a cadaver study conducted byMarcheix et al. [25], this anterior ligament was presentin 100% of the cases and only in 80% of the cases wheninvestigated by MRI examination. It is 31.2 mm long andonly 1.8 mm wide. In another cadaver study conducted byTubbs et al., the Winslow’s ligament was found in only55% of the cases and demonstrated very similar dimen-sions (35.4 mm long and 2.5 mm wide). This ligament mightbe double (3.7% of cases) and various types are describedaccording to their associated insertion on the anteriormargin 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-cofibular ligament. Based on 50 cadaver dissections, he findsa meniscofibular ligament, which runs between the inferiormenisco-synovial junction of the midportion of the lateralmeniscus, anterior to the popliteal hiatus up to the artic-

Figure 12 Posterior anatomic view and posterior aspect of tibia and lateral meniscus featuring meniscofemoral ligament insertions(Anatomy of the PCL and the meniscofemoral ligament, A. Amis. ESSKA 2000 Basic Science Committee: Knee anatomy for orthopaedicsurgeons).



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 posteriorhorn 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

are commonly examined along with the PCL. They do notpresent as a constant structure. In an important study of theliterature, Gupte et al. [32] identified at least one MFL in 91%of the cases among 781 anatomic dissected cadaveric kneesfrom 13 studies. The aMFL was present in 48.2% of the casesand the pMFL was present in 70.4%. Meniscofemoral liga-ments were both identified in only 32% of the knees. Gupteet al. [32,33] suggest that these ligaments undergo degen-erative changes with age and are more commonly detectedin Caucasians than in Asians.

The insertion site of the aMFL is located on the medialfemoral condyle between the lower part of the insertion siteof the PCL and the cartilaginous edge of the medial femoralcondyle under the PCL. The insertion site of the p MFL ismore posterior and at the top of the femoral insertion of thePCL, above the PCL [30]. The length of the MFL ranges from21 to 27 mm according to gender and type of study. The pMFLis longer, ranging from 23 and 31 mm [32,33]. In an arthro-scopic observational study using the ‘‘meniscal tug test’’,Gupte et al. [35] examined the meniscofemoral ligamentinsertions and their attachment to the lateral meniscus.Technically, the hook pulls on the anterior meniscofemoralligament in order to realize a movement of the root of thelateral meniscus. The pMFL is less difficult to visualize sinceit is situated posteriorly to the PCL. Therefore, among 68knees, the anterior MFL could be identified in 68% of thecases [27] and the pMFL was only present in 15% of thecases. MRI studies of meniscofemoral ligaments report vari-able results [36] but are ancient (Fig. 13). The presence ofthe MFL is variable since the difficulty lies in passing in theMFL plane (Table 1).

The first descriptive anatomic studies described themeniscofemoral ligaments as being a ‘‘3rd cruciate liga-ment’’. These ligaments exhibit a high strength level andmodulus of elasticity compared with both bundles of thePCL (anterior bundle of the PCL 1620 N, 248 MPa, pos-terior bundle of the PCL 258N and 145 MPa [34]). Theirstrength is 30% of the PCL strength and identical to thatof the PCL posterior bundle. Biomechanical studies haverevealed that MFL have 30% of the last PCL strength.Moran et al. [38] assessed the tensile behavior of bothmeniscofemoral ligaments during flexion movements of theknee. The a MFL shows no tensile strength in extensionand starts at 10◦ in 20◦ of flexion up to its maximum at105◦ of flexion. Stress significantly increases during tib-ial external rotation movements. Conversely, the pMFLreveals a maximum tensile strength in extension, whichprogressively decreases while flexion increases. Its tensilestrength is nil in 80◦ of flexion. Stress when applied toboth MFL appears more complex. Initially, tension is greaterin extension then decreases in about 30◦ of knee flexionwhereas it increases significantly in full flexion. Accordingto the work of Amadi et al. [39], the presence of bothintact meniscofemoral ligaments reduces by 10% the lateral

Table 1 The mechanical properties of these two structures have been well defined [37].

Insertion site area (mm2) Tensile strength (N) Modulus of elasticity (MPa) Length (mm)

aMFL 14.7 ± 14.8 300 ± 155 281 ± 239 20.7 ± 3.9pMFL 20.9 ± 11.6 302 ± 158 227 ± 128 23.0 ± 4.3



femoro-tibial strain and limits the tibial posterior transla-tion.

The pMFL tension increases during knee extension whilethe aMFL loosens. The pMFL tension decreases in kneeflexion while the aMFL develops tension [39]. The aMFL sta-bilizes the posterior horn of the lateral meniscus in kneeflexion while the pMFL acts as a stabilizer in knee exten-sion. Therefore, during knee flexion, both MFL move theposterior horn of the LM anteriorly and interiorly. Theymake a significant contribution to limiting its posterior dis-placement. Gupte et al. [40] have described an antagonisticrelationship between the meniscofemoral ligaments and themeniscofibular ligaments anteriorly to the popliteal hiatus.The meniscofibular ligament pulls the post portion poste-riorly and interiorly whereas the traction exercised by theMFL is located anteriorly and inferiorly and more proximallythat is anteriorly in the anteroposterior direction. Thesemeniscofemoral ligaments prevent the natural posterior dis-placement during knee flexion.

Therefore, during tibial internal rotation in a flexed knee,the MFL pulls the posterior portion anteriorly and inferiorlywhereas the meniscofibular ligaments maintain it posteriorlythus protecting it from the lateral femoral condyle.

In extension, during axial compression, the stress appliedpushes the lateral meniscus outside. The compressive forceschange into shearing forces and are transmitted by theMFL to the circumferential fibers of the lateral meniscus.In case of excessive stress, one of the elements from theMFL-meniscal root and post portion chain might disrupt andinvolve a meniscal or ligamental tear.

These meniscofemoral ligaments are the secondaryrestraint to posterior tibial translation after the PCL but theyalso act as an important stabilizer of the horn and the lateralmeniscal root when this latter is subjected to compressivestress on the lateral tibial plateau convexity.

Finally, the lateral meniscus is integrated in thisanatomo-functional entity, which is the posterolateral cor-ner of the knee. The major structures of the posterolateralcorner of the knee include the popliteofibular ligament, thepopliteus muscle, the lateral collateral ligament, the bicepsfemoris and the posterior joint capsule.

Meniscal kinetics

Therefore, the whole insertion means and the convexity ofthe lateral tibial plateau provide better understanding of themeniscal displacements. The lateral meniscus is subjectedto greater movements than the medial meniscus.

Up to 90◦ of knee flexion, Vedi et al. [40] have demon-strated that the lateral meniscus displaces 9.5, 3.7 and5.6 mm posteriorly for the anterior portion, the centralportion and the posterior portion respectively. The naturalposterior meniscal movement during knee flexion is greater[41] for the lateral meniscus than for the medial one andmight reach up to 10 mm for the anterior horn. Yao et al. [41]have recently studied the posterior translation with over130◦ of knee flexion. The overall in vivo posterior translationis 8.2 ± 3.2 mm for the lateral meniscus and 3,3 ± 1,5 mm forthe medial one. The anterior horns show a greater posteriortranslation (LM : 10.2 mm, MM : 6.5 mm) than the posteriorhorns (LM: 6.2 mm, MM : 3.1 mm). There is a significant dif-

Figure 14 Dynamic MRI of backward displacement of ant andpost portion of menisci during over 120◦ of flexion.

ference of about 5 mm between the average translations ofboth menisci.

We conducted a study in five healthy knees; the kneewas initially extended then progressively flexed, by incre-ments of 30◦ up to more than 100◦ of flexion. For eachlateral position, we measured the backward displacementof the anterior and posterior portions regarding the circlerepresenting the posterior femoral radius of curvature cor-responding to the higher diameter of the lateral femoralcondyle [42]. Movement of the posterior and anterior por-tion was measured from extention to full knee flexion. Thelateral meniscus shows a greater posterior translation thanthe medial meniscus. The posterior translation is 15 and16 mm for LM anterior and posterior portions respectivelyand only 8 and 5 mm for MM anterior and posterior portions(Fig. 14).

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

Reflexion on physiopathology of lateralmeniscus lesions

The lateral meniscus provides good articular congruency inthe femoro-tibial compartment, which is hostile because ofits opposed convexities. It acts as a knee stabilizer, particu-larly via the interconnections of the posterolateral corner ofthe knee, and shows a great capacity to move due to its phy-logenetic evolution. The lateral compartment of the knee iscommonly presented to be the mobile knee compartment(that of mobility). Stress applied to the lateral meniscusis mostly exerted on the anterior and posterior portions asreported in the work of Moyen et al. [43]. Its location andmorphology contribute to the transformation of compressiveforce to shearing force as confirmed by the SFA study in 1996[43]. The lateral meniscus acts as a shock absorber whiledistributing compressive stress circumferentially. Its propri-oceptive receptors are located in the peripheral 2/3 andin the anterior and posterior portions. The possible mecha-nisms of injury and their consequences in terms of excessive



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 comparedto 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

confirmed by clinical studies on the analysis of meniscallesions in a stable knee.

In case of torn ACL, the translation is excessive and theblock movement associated with high shear force will exerton the posterior portion of the lateral meniscus. In such con-ditions, the meniscofibular ligaments, the meniscofemoralligaments — according to the degree of knee flexion — andthe root of the lateral meniscus are highly exposed to therisk of injury.

West et al. [44], in 2004, have reported this type ofmeniscal root injury combined with tear of the ACL. Thisinjury was identified in 12.4% of the cases. Brody et al. [45]well described the MRI features of meniscal roots in T1 andT2. Among 264 MRI performed in patients with torn ACL, 9.8%of lateral meniscal root injuries and only 3% of medial menis-cal root injuries were observed. Meniscal extrusion wasfound in six out of the 26 lateral meniscal injuries (23%). Theabsence of MFL was commonly found to be associated withmeniscal extrusion when there was a lateral meniscal injury.It underlines the difficulty to obtain clear visualization ofmeniscal roots and meniscofemoral ligaments [46,47,48,49].

De Smet et al. [50] defined MRI criteria using six imag-ing planes (three coronal and three sagittal projections)(Fig. 16) acquired with fat-saturated T2-weighted MR imagesto provide proper visualization of the meniscal roots. Threemillimetres thick sections should be successively used with1.5 mm interpolation. This technique combined with thor-ough analysis of six thin slices proves helpful in the detectionof lateral meniscus root injuries.

Ahn et al. [51] have shown that these injuries commonlyoccur in the coronal plane and cause a tear of the poste-rior portion of the lateral meniscus, which loses its tibialattachment (Fig. 17). This lesion is defined as being situ-ated within 1 cm from the tibial insertion site. The reportedprevalence of these injuries in his study is only 6.7% of the432 ACL grafts. Ahn et al. [51] believe meniscus repair shouldbe performed whenever possible, even in case of lesionsin the white-white area. Arthroscopic meniscal healing was

Figure 16 Six MRI images for analysis of meniscal roots and detection of lateral meniscus root lesions.



Figure 17 Arthroscopic view of a lateral meniscus root lesion.

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

Conclusion

The use by the Homo sapiens of exclusive bipedalismprovides better knowledge of the anatomical differencesbetween both knee compartments. Consequences on the lat-eral meniscus are greater than those observed on the medialmeniscus, which remains identical whatever, the species.The lateral meniscus double insertion is a unique featureof the Homo sapien. Along with anatomic study and bycombining dissections with recent imaging investigations,the use of modern imaging systems, digital radiography,especially in MRI, provides reliable landmarks to facilitatemeniscal allografts. Since modern imaging techniques pro-vide good understanding of the functional anatomy, kneekinematics and more particularly that of lateral meniscusmight be properly assessed. It provides better understand-ing of meniscus biomechanics and helps determine itslesional mechanisms. Accurate imaging protocols should bedevelopped to offer better analysis of MFL (especially theposterior one), anterior cruciate ligament injuries (ACL) andmeniscus roots in order to quantify the relationships withmeniscal extrusion. By defining in a more accurate and com-prehensive manner the ACL lesion associated injuries, wewill improve our therapeutic indications and techniques ofmeniscus preservation.

Conflict of interest

The present authors had no conflict of interest regarding thispublication.

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