J. Anat. (1991), 177, pp. 127-134 127With 6 figures

Printed in Great Britain

Quantitative differences in the histology of the attachment zonesof the meniscal horns in the knee joint of Man

M. BENJAMIN, E. J. EVANS, R. DONTHINENI RAO, J. A. FINDLAYAND D. J. PEMBERTON*

Department of Anatomy, University of Wales College of Cardif, Cardif CF1 3 YF,Wales and * Department of Traumatic and Orthopaedic Surgery, Cardiff Royal

Infirmary, Newport Road, Cardif CF2 1SZ, Wales, UK

(Accepted 18 February 1991)

INTRODUCTION

Although the general features of attachment zones of tendons, ligaments and relatedstructures are well known (Cooper & Misol, 1970; Benjamin, Evans & Copp, 1986;Woo et al. 1988), little information is available about quantitative differences betweenattachment sites. Woo et al. (1988) have highlighted the need for such studies in theirrecent review. Among the factors likely to influence the proportion of tissues at anattachment zone is the force transmitted to the bone and the amount of movementallowed at the soft-hard tissue interface. We have previously reported differences inthe quantities of fibrocartilage and calcified tissue at the attachments of the quadricepstendon and the patellar ligament in Man and related these to mechanical differencesbetween the tendon and ligament (Evans, Benjamin & Pemberton, 1990, 1991). Wenow report differences in the quantity of fibrocartilage, the thickness of corticalcalcified tissue and the proportion of bone to marrow in the attachments of the medialand lateral menisci of the knee joint.

MATERIALS AND METHODS

Both horns and the underlying bone were removed from the lateral and medialmenisci from seven dissecting room cadavers of both sexes (ages 61-80 y). All theknees were devoid of gross pathological change and there were no discoid menisci. Thetissue was further fixed with 10% neutral buffered formol saline, decalcified in 2%nitric acid, dehydrated in graded alcohols, cleared in Inhibisol and embedded in 56 °Cparaffin wax. Sections (8 /tm thick) were collected at 500 ,um intervals throughout theblock. At each sample point, 2 sections were stained with haematoxylin and eosin and2 with Masson's trichrome. The intended place of sectioning was at right angles to thebone surface and along the length of the meniscal fibres. However, because of thetwisting of the horns of the lateral meniscus, some of the sections were inevitablyoblique. Where this was recognised (in the anterior horn of 1 cadaver and the posteriorhorn of 3), a correction factor of x 0 7 was applied to reduce the estimates of tissuethickness. This value assumed a plane of sectioning at 450 to that intended. It isunlikely that any of the sections departed from the correct plane by more than thisamount.The thickness of cortical calcified tissue (calcified fibrocartilage and lamellar bone)

was measured at 15 sites, equally spaced at approximately 500 ,um along the slide (Fig.1). The percentage of bone: bone marrow was estimated at each sample site by point

M. BENJAMIN AND OTHERS

Meniscalhorn Sections every 500 ,um

Uncalcifiedfibrocartilagemeasured at260 pm intervals

Two areas for pointcounting estimates ofbone: bone marrow

Fig. 1. The sampling and measuring protocol for estimating the thickness of the zones of uncalcifiedfibrocartilage (UF) and cortical calcified tissue (C) and for determining the percentage of bone: bonemarrow. BM, bone marrow; T, tidemark; SB, spongy bone.

128

Histology of meniscal horns 129counting volumetry at a magnification of 50. A quadratic lattice test system (1 cm2)was superimposed over an image projected from a Gillert & Sibert microscope. Twoareas (each 4 mm2) were measured per slide. One side of the square test system wasaligned along the tidemark, so that the bone was sampled to a depth of 2 mm at theattachment zone. The bone sampled thus included both the constituents of the corticalzone and the cancellous bone immediately beneath. Because of the greater difficulty ofmeasuring the thickness of uncalcified fibrocartilage, a larger number ofmeasurementswas taken (every 260 ,um) than for the cortical calcified tissue. Measurements weremade with a micrometer eyepiece from the tidemark to the furthest recognisablechondrocyte. For the purposes of this investigation, this was considered to be a roundor oval cell lying in a lacuna, and close to cells of similar type. Thus, any isolatedchondrocytes lying among fibroblasts were excluded. Statistical comparisons weremade with a one-tailed paired t test.

RESULTS

The arrangement of tissues at the attachment zone of the meniscal horns was similarto that in epiphyseal ligaments and tendons (Benjamin et al. 1986). Near theattachment site, a zone of uncalcified fibrocartilage was separated from the calcifiedtissues by a tidemark. Immediately deep to this was a zone of calcified fibrocartilageand then a thin shell of cortical lamellar bone. The tissues deep to the tidemark werecontinuous with the subchondral plate on those portions of the tibial plateau that werecovered by articular cartilage.

Differences in the thickness of the zones of cortical calcified tissue and uncalcifiedfibrocartilage, and in the percentage of bone: bone marrow between the two menisciare illustrated in Figs 2-3 and the quantitative data are summarised in Figs 4-6. Forall three sets of measurements, the values were significantly greater (P < 0-01 orP < 0-05) in the lateral than the medial meniscus. Whereas the differences in both thecortical zone thickness and in the percentage of bone: bone marrow were mostpronounced in the anterior horns, the differences between the amounts of uncalcifiedfibrocartilage were largely attributable to differences in the posterior horns.

DISCUSSION

In an earlier study (Evans et al. 1990), we proposed that differences in the amountof fibrocartilage at the insertion of the quadriceps tendon and at the patellar and tibialattachments of the patellar ligament were related to differences in mobility at theseattachment zones during joint movement. Where there was a significant change inangle between the tendon/ligament and the bone, there was a prominent zone ofuncalcified fibrocartilage at the attachment site. Where there was little change in angle,fibrocartilage was less conspicuous. The slight increase in stiffness that we presumeresults from the presence of cartilage matrix may reduce the risk of fibre fraying. Wesuggested that there was more bone at the insertion of the quadriceps tendon becausethe reaction of the patella against the femur ensures that a greater maximum force isexerted at this attachment site than at either end of the patellar ligament (Evans et al.1991).There are similar quantitative differences in the attachment zones of the knee joint

menisci, even though all four horns are attached to the tibia within a short distanceof each other. Indeed, the anterior horns are linked by a transverse ligament. It issuggested that the greater thickness of uncalcified fibrocartilage in the lateral meniscus

M. BENJAMIN AND OTHERS

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130

Histology of meniscal horns 131

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rottio.urig lexonandemeniscushesaesta mhenltras nsusiusslaeFirog. 4. Difeenein the thickness ofathe zoeob naluidfbocriaeithe medialandcsisdslcehoglaerlymnsi(envle+SE)Thereare sigifian diffeence beteethe posterlmnsusmr orilthornsothe twoal

meicTP 0)hemeanarvnfralue forinothe menisci(P co005)landbetee theaneirmnosterintortheeorns ofnthelaeral Theniscust(P raiu 001)aueftelteaodlemasta

teltrlmeniscissldiacwrsasdethred knee movesg frovmetful ethensinetofeion. but thernmfteditealmeniscusmvs onlyefloew millietresandithe latenrallgmeniscs 1rcm.rKaandjit

(18)as eonssthegratera diplceenofathe lateral(bmeniscus. Hees saysrpsaraenfothataeneit ,moesatwiiedafiboartaslthe; L,lmedial both;duinfldexinanaxenin,adduig.xaroaion. DuinUnaeflexibonatiandexensF)ionhoeeostateuostat the lateralhmenicu isdhoseisplacedthougthe 12tramm)ihn thesagital plamnie,ut.Thesmeda meniscuapsise displacedmthrogoaenly.

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the lateral meniscus is deformed mor duin moeenso thCnejit honof th latral mniscs ar closr toethe an .tscrnayliaetsaeoela;i

Fig 2.Thezon ofcorica caciiedtisue arrws)is oreproinnt eneth he nteiorhor othe~~~~~~~~~~~~~~ laea a0hntemda b eics hs htgah r ae rmtesm ne

CE,~~~~~~~~~~~~~~cacfe0ircrilg;L,lmla bn;T ieakFig. 3. Ucalcifiedfibrocartlage (UF ismr0 osiuu tteatcmn ftepseirhr

of te ltera (a thn th meial(b) enicus Thee potogaph ar takn fom he sme neeT,tidemark.~ ~ ~~0

M. BENJAMIN AND OTHERS

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UMean value

for meniscus

Posterior horn

EE

n

-0

CoCo

0

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Un

CDu

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Lateral Medialmeniscus meniscus

Fig. 5. Differences in the thickness of the zone of cortical calcified tissue in the medial and lateralmenisci (mean values + SEM). There are significant differences between the anterior horns of the twomenisci (P < 0 05) and between the mean values for the menisci (P < 0 05).

is pulled backwards during flexion of the knee joint by the tendon of popliteus and bythe meniscofemoral ligaments (Last, 1990). Unlike the medial meniscus, the lateral isnot constrained by any attachment to a collateral ligament and its coronary ligamentsare deficient posteriorly near the tendon of popliteus. Because this tendon and themeniscofemoral ligaments pull on the posterior horn of the lateral meniscus and as theanterior horn is partly blended with the anterior cruciate ligament, it is probable thatthe posterior horn is the more mobile. Our observation that there is more uncalcifiedfibrocartilage at the attachment of the posterior horn is thus consistent with thesuggestion that greater quantities of this tissue are present at more 'mobile'attachment sites.

It is now well established that the menisci play an important role in transmittingbody weight from the femur to the tibia (Walker & Erkman, 1975). The lateralmeniscus is particularly significant - hence its greater quantity of cortical calcified

132

Histology of meniscal horns

Anterior horn

[lnMean valuefor meniscus

E Posterior horn

50 -

40 -

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Lateral Medialmeniscus meniscus

Fig. 6. Differences in the percentage of bone: bone marrow between the medial and lateral menisci(mean values + SEM). There are significant differences between the anterior horns of the two menisci(P < 0-05) and between the mean values for the menisci (P < 0 05).

tissue and its larger percentage of bone: bone marrow. However, it is less easy toexplain why the differences in calcified tissue between the menisci are largelydifferences between the anterior horns. It is possible that the blending of the anteriorhorn of the lateral meniscus with the anterior cruciate ligament is significant. If theforces transmitted through the cruciate ligament are partly relayed to the tibia via theanterior horn of the lateral meniscus, this could well result in a thicker zone of corticalcalcified tissue and a greater percentage of bone: bone marrow. Indeed, a similar resultmight be seen if there was no attachment of a meniscus immediately next to theanterior cruciate ligament, because it seems unlikely that there would be a suddendiminution in the amount of calcified tissue at the periphery of such a powerfulligament.

133

M. BENJAMIN AND OTHERS

SUMMARY

The attachment zones of the meniscal horns of 7 dissecting room cadavers wereexamined by routine histology. All the knees were devoid of gross pathological changeand no discoid menisci were included. Significant differences are reported in thethickness of the zones of uncalcified fibrocartilage and cortical calcified tissue (calcifiedfibrocartilage and underlying lamellar bone) and in the percentage of bone: bonemarrow. There was a thicker zone of uncalcified fibrocartilage and a greater quantityof calcified tissue at the horns of the lateral than the medial meniscus. The differencesin uncalcified fibrocartilage were largely attributable to the posterior horns, but thevariations in calcified tissue mainly reflected differences between the anterior horns. Itis suggested that the greater mobility of the lateral meniscus and the blending of itsanterior horn with the anterior cruciate ligament are important factors accounting forthe quantitative differences in the meniscal attachment zones.

We wish to thank Gareth Watkins and Derek Scarborough for their much valuedhelp in cutting and staining the sections.

REFERENCES

BENJAMIN, M., EVANS, E. J. & Copp, L. (1986). The histology of tendon attachments to bone in man. Journalof Anatomy 149, 89-100.

BRANTIGAN, 0. C. & VOSHELL, A. F. (1941). The mechanics of the ligaments and menisci of the knee joint.Journal of Bone and Joint Surgery 23A, 44-66.

COOPER, R. R. & MISOL, S. (1970). Tendon and ligament insertion. Journal of Bone and Joint Surgery 42A,1-20.

EVANS, E. J., BENJAMIN, M. & PEMBERTON, D. J. (1990). Fibrocartilage in the attachment zones of thequadriceps tendon and the patellar ligament of man. Journal of Anatomy 171, 155-162.

EVANS, E. J., BENJAMIN, M. & PEMBERTON, D. J. (1991). Variations in the amount of calcified tissue at theattachments of the quadriceps tendon and patellar ligament in man. Journal of Anatomy 174, 145-151.

KAPANDJI, I. A. (1987). The Physiology of the Joints, 5th ed, Vol. 2, pp. 94-96. Edinburgh: ChurchillLivingstone.

LAST, R. J. (1990). Last's Anatomy. Regional and Applied (ed. R. M. H. McMinn), 8th ed. Edinburgh:Churchill Livingstone.

WALKER, P. S. & ERKMAN, M. J. (1975). The role of the menisci in force transmission across the knee. ClinicalOrthopaedics and Related Research 109, 184-192.

WOO, S., MAYNARD, J., BUTLER, D., LYON, R., TORZILLI, P., AKESON, W., COOPER, R. & OAKES, B. (1988).Ligament, tendon, and joint capsule insertions to bone. In Injury and Repair of the Musculoskeletal SoftTissues (ed. S. L.-Y. Woo & J. A. Buckwalter), pp. 133-166. Park Ridge, Illinois: American Academy ofOrthopaedic Surgeons.

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