JOURNAL OF MORPHOLOGY 202:165-172 (1989)

Neural Elements in the Cruciate Ligaments and Menisci of the KneeJoint of the American Alligator, Alligator mississippiensis

CAROLE S. WINK, RUTH M. ELSEY, MICHELE ST. ONGE, ANDMARILYN L. ZIMNYDepartment of Anatomy, Louisiana State University Medical Center, NewOrleans, Louisiana 70112 (G.S. w.,M.S.O., M.L.Z.); Louisiana Departmentof Wildlife and Fisheries, Rockefeller Wildlife Refuge, Grand Chenier,Louisiana 70643 (R.M.E.)

ABSTRACT Knee joints from adult, juvenile, hatchling, and embryonic (full term)American alligators were dissected to reveal the cruciate ligaments and the medialand lateral menisci. Two anterior cruciate (major and minor), a posterior cruciate, anintermeniscal, and a meniscofemoralligament were identified. In addition, we founda fourth internal ligament which has not been reported previously. Menisci andligaments from left knees were fixed in formalin and processed for routine histologicalobservation. Those from right knees were stained in bulk by using a gold chloridemethod and were either frozen and sectioned at 100 ).Lm on a sliding microtome orwere processed for paraffin sections at 30 ).Lm. The morphology of the collagenous,cartilaginous, and vascular constituents of the tissues was similar to that of the dog,cat, and human. Nerve fibers were observed in all tissues sampled. Structuresresembling Golgi tendon organs and Pacinian corpuscles were identified, reinforcingthe theory that neural elements within cruciate ligaments and menisci may provideafferent input that affects the function of the knee joint.

The cruciate ligaments in knees of tetrapodsare situated in the middle of the joint and arecalled "cruciate" because they cross each otherlike the lines of the letter x (Gray, '73). Mostmammals have two cruciates. In reptiles, amphib-ians, and birds there may be one, two, or threecruciate ligaments. (Haines, '42). Cartilaginouslamellae (menisci) are also present in knees oftetrapods and vary in size and shape from dis-coid to crescentic. There may be one meniscus,as in most lizards (Rewcastle, '80), or there maybe paired structures, as in the crocodile andmammals (Haines, '42). Ligamentous "horns"are located at the ends of the meniscus with afibrocartilagenous "body" in between (O'Connor,'76). Functionally, ligaments bind bones to-gether and menisci have been called "thrustpads" (MacConaill, '32, '50), devices to increasethe congruity of the opposing articular surfaces(Barnett et al., '61), and biological shock absorb-ers (Seedhom et al., '74). Recently, nerve fibersnot associated with blood vessels (Kennedy etal., '74, '82) and specialized receptors (mechan-oreceptors) have been observed in the anteriorcruciate ligament of the human (Schultz et al.,'84; Zimny et al., '86) and in the menisci of thedog (O'Connor, '76), cat (O'Connor and Me-Connaughey, '78), and human (Wilson et al., '69;

e 1989 ALAN R. LISS, INC.

Day et al., '85; MacKenzie et al., '85; Zimny et al.,'88). These investigators postulated that intra-articular tissues are capable of afferent input tothe central nervous system, which may deter-mine or modify movement at the joint.

To our knowledge there has been no report ofnerves in the intra-articular tissues of reptiles.The present study was undertaken to describethe morphology of and identify neural elementsin the cruciate ligaments and menisci of the kneejoint of the alligator, Alligator mississippiensis.

MATERIALS AND METHODS

Hind limbs and/or knee joints from freshlykilled adult, juvenile, and hatchling alligatorswere removed and frozen. Fertile alligator eggswhich had been incubated for 55 days (full term)were collected just before hatching and refriger-ated. Later, the embryonic alligators were re-moved from the eggs and the frozen limbs andknee joints from the other animals were thawed.Movements of the knee were analyzed after theskin and muscles were removed from the hindlimbs. Then all the knee joints were carefullydissected, using a dissecting microscope whennecessary. Photographs were taken and draw-ings made during the dissection to demonstrategross structures. After the gross morphology was

166 c.s. WINK ET AL.

1

I L

TP3Fig. 1. Drawing of posterior surface of alligator knee joint

in extension; femur above, tibia below. MM, medial meniscus;LM, lateral meniscus; A, posterior meniscofemoralligamentfrom medial meniscus; B, posterior menisofemoralligamentfrom lateral meniscus; F, fibula. Arrow indicates femorofib-ular disc. Capsule has been removed.

Fig. 2. Photograph of posterior surface of knee joint aftercapsule, menisci, and meniscofemoral ligaments have beenremoved. M, medial condyle of femur; L, lateral condyle offemur; A, anterior cruciate major ligament; P, posterior cruci-ate ligament; X, fourth cruciate ligament. Arrow indicatesanterior cruciate minor ligament.

Fig. 3. Drawing of anterior surface of knee joint; capsulehas been removed. The tibia is fully flexed under the femur sothat only the anterior portion of the tibial plateau (TP) isvisible in this end-on view. LC, lateral condyle of femur; MC,medial condyle of femur; IL, intermeniscal (transverse) liga-ment. Arrow indicates anterior meniscofemoralligament frommedial meniscus.

Fig. 4. Drawing of anterior surface of knee joint (fullyflexed as in Fig. 3) after the capsule, the intermeniscal, andthe anterior meniscofemoral ligaments have been removed.LC, lateral condyle of femur; MC, medial condyle of femur;LM, lateral meniscus; MM, medial meniscus; A, anteriorcruciate major ligament; B, posterior cruciate ligament; C,anterior cruciate minor ligament.

NEURAL ELEMENTS IN ALLIGATOR KNEE JOINT

\..\GAMEN T

7MEDIAL MENISCUS

LA TERA L ME NISCUSFig. 5. Micrograph of longitudinal section of posterior

cruciate ligament from an adult alligator. Note parallel bun-dles of collagen fibers (B), fibroblasts (arrowheads), and bloodvessel (V). Masson's trichrome stain. x 400.

Fig. 7. Drawing of a horizontal section of medial andlateral menisci from a hatchling alligator. AH, anterior hornof lateral meniscus; PH, posterior horn of medial meniscus.Stippled areas of menisci were supplied with blood vessels.Clear areas of menisci were avascular. x 10.

Fig. 6. Micrograph of a horizontal section of posteriorportion of the medial meniscus from a hatchling alligator.Note ligamentous posterior horn (PH). Masson's trichromestain. x80.

167

.:

891

NEURAL ELEMENTS IN ALLIGATOR KNEE JOINT 169determined, internal ligaments and menisci wereremoved from the joints of the adult, juvenile,and hatchling alligators. Large menisci were cutinto three pieces. Small menisci were left whole.Tissues from the left knees were fixed in 10%neutral-buffered formalin and processed by rou-tine histological methods. Paraffin sections 10~m thick were stained with hematoxylin andeosin or Masson's trichrome stain.Tissues from right knees were stained in bulk

by using a modification (Zimny et al., '85) of agold chloride method (Gairns, '30) to demon-strate nerve elements. Some of the tissues werefrozen and sectioned on a sliding microtome at100 ~m. The remaining tissues were dehydratedand embedded for paraffin sectioning at 30 ~m.All sections were viewed with the light micro-scope.

RESULTSMovements of the knee joint

Motion of the knee of the alligator resembledthat of the crocodile (Haines, '42). As the kneewas extended, the femur glided and rolled for-ward on the tibia. The last few degrees of exten-sion were accompanied by medial rotation of thefemur about its longitudinal axis, similar to the"screw home" movement in the human (Basma-jian, '76). Flexion was accompanied by gliding,rolling, and lateral rotation of the femur on thetibia. The head of the fibula articulated with afacet on the lateral femoral condyle and movedslightly in all planes as the knee was flexed andextended. The fibula was not connected to thetibia by a strong interosseus membrane or otherstrong ligaments and moved independently ofthe tibia, as it does in the crocodile (Haines, '42).

Gross anatomyA photograph and drawings of the dissected

knee joint of the alligator are shown in Figures

Fig.8. Micrograph of a horizontal section oflateral menis-cus from hatchling alligator. Note fibrocartilaginous bundles(B) arranged in an irregular pattern. Masson's trichromestain. x 200.

Fig. 9. Micrograph of a horiwntal section of the innerportion of lateral meniscus from hatchling alligator. Notehyaline cartilage (arrows) and fibrocartilage (F). Masson'strichrome stain. x 200.

Fig. 10. Micrograph of a horiwntal cross section of lateralmeniscus from hatchling alligator. I, inner (medial); 0, outer(lateral). Note nerves (arrows) and receptor (R) entering fromperiphery. Gold chloride, frozen section. x 40.

Fig. 11. Micrograph of a cross section of the intermeniscalligament (anterior) from adult alligator. Note nerves (ar-rows), entering at periphery. Gold chloride, paraffin. x80.

1-4. The morphology of the joint is similar tothat of the crocodile (Haines, '42). The medialand lateral menisci, the head of the fibula, thefemorofibular disc, and the two posteriormeniscofemoralligaments were seen on the pos-terior surface of the joint, immediately deep tothe capsule (Fig. 1). The femorofibular disc was afibrocartilaginous pad continuous with the lat-eral meniscus and bound to the femur and fibulawith ligaments, as in the crocodile (Haines, '42).It occupied a position similar to that of thecyamella in lizards and mammals (Pearson andDevin, '21; Rewcastle, '80). After removing thesestructures, the cruciate ligaments could be seen(Fig. 2). The larger anterior cruciate ligament(major) and the posterior cruciate had attach-ments similar to those in the crocodile (Haines,'42) and the human; the smaller anterior cruci-ate (minor) was located medial to the posteriorcruciate, as in the crocodile (Haines, '42). Inaddition, there was a fourth internal ligament,which has not, to our knowledge, been describedbefore. It extended from the medial condyle ofthe tibia, anterior to the posterior cruciate liga-ment, to the posteromedial surface of the lateralfemoral condyle, inferior to the attachment ofthe anterior major cruciate ligament. After re-moving the capsule from the anterior surface ofthe joint, a meniscofemoral ligament from themedial meniscus and a stout intermeniscal(transverse) ligament could be identified (Fig.3). Fibers from the tendon ofthe femoro-tibialismuscle blended with the former. When theseligaments were removed, portions of the cruciateligaments could be seen (Fig. 4).

HistologyThe histology of the meniscofemoral and cru-

ciate ligaments was similar to that of the dog(O'Connor, '76) and cat (O'Connor and McCo-nnaughey, '78) (Fig. 5). Posteriorly, the medialmeniscus attached to the lateral tibial condyleby a ligamentous band (posterior horn), andanteriorly the lateral meniscus attached to theintercondylar region of the tibial plateau by aligamentous anterior horn (Figs. 6, 7). The re-mainder of the menisci (bodies) were composedlargely of fibrocartilage with the collagen bun-dles arranged in an irregular pattern (Fig. 8), asdescribed by O'Connor (,76) and O'Connor andMcConnaughey (,78) in the dog and cat. Alongthe inner borders of the menisci, patches of hya-line cartilage replaced the fibrocartilage (Fig. 9).Blood vessels were seen in the meniscal hornsand in the anterior and posterior portions of themenisci; the middle of each meniscus was largelyavascular (Fig. 7).

OLl

NEURAL ELEMENTS IN ALLIGATOR KNEE JOINT 171

Nerves were seen in all of the tissues sampled.They appeared to enter from the periphery andfollow the connective tissue septa of the tissue(Figs. 10,11). Some of the nerve fibers appearedto end as free nerves (Fig. 12), while othersended in receptors (Figs. 13-15). The morphol-ogy of some of the receptors resembled Golgitendon organs (Figs. 13, 14) and some resembledPacinian corpuscles (Fig. 15) as described in thecat (O'Connor and McConnaughey, '78), dog(O'Connor, '76), and human (Kennedy et al., '82;Day et al., '85; Zimny et al., '86, '88).

DISCUSSION

There appears to be variation in the numberof cruciate ligaments among reptiles. Haines (,42)reported three in the crocodile and one in Vara-nus varius. Rewcastle (,80) found two in Vara-nus bengalensis. It is unclear why Furst (,03)and Nauck ('38) observed only two cruciate liga-ments in the knee joint of Alligator mississippi-ensis. One possible explanation is that they mayhave dissected joints from small, very youngalligators. In the present study the four in-terosseous ligaments of the knee joint were dif-ficult to distinguish in small joints, even with adissecting microscope. Also, in some of the jointsthere were connecting fibers between the ante-rior cruciate major and the fourth ligament whichmade it difficult to delineate the two. O'Connorand McConnaughey ('78) reported great varia-tion from joint to joint in the morphology of thelateral meniscus of the cat. It is possible thatmorphological variations exist in alligator joints.Perhaps in the animals we sampled the fourthligament had split off from the anterior cruciatemajor ligament. In Chelonia and Anura there isonly one massive internal ligament between thetibia and femur and rotary movements are mini-mal. Haines (,42) suggested that in species withmultiple cruciates, the single ligament split intotwo or more as rotation at the knee joint in-creased. Rewcastle (,80), however, reported thatin lizards, knee joints with two cruciate liga-

Fig. 12. Micrograph of nerve fibers in the intermeniscalligament from an adult alligator. Smallest fibers (arrows) areapproximately 1.0 uix: in diameter. Gold chloride, paraffin.x 200.

Fig. 13. Micrograph of Golgi tendon organ in medialmeniscus from adult alligator. Gold chloride, paraffin. x 200.

Fig. 14. Micrograph of Golgi tendon organ in lateral me-niscus from hatchling alligator. Gold chloride, frozen section.x 400.

Fig. 15. Micrograph of Pacini an corpuscle (arrow) in pos-terior cruciate ligament from adult alligator. Gold chloride,frozen section. x 200.

~

ments are strictly hinge joints, with no rotationoccurring. Our findings support those of Haines(,42). Rotation occurred in the knee joint of theAmerican alligator, as in the crocodile.

Haines ('42) described the menisci of the croc-odile as "massive structures" and pictured themas circular discs occupying most of the tibialcondylar surfaces. In the present study, the lat-eral meniscus in hatchling and juvenile alligatorswas discoid and the medial meniscus was crescen-tic. In adult alligators both menisci were crescen-tic, resembling those of the human and the dogand cat (O'Connor, '76; O'Connor and McCo-nnaughey, '78). Menisci of the alligator differedfrom those of the latter species in that there wereonly two meniscal horns instead of four (Fig. 7).

To our knowledge this is the first histologicaldemonstration of neural elements in tissues ofthe reptilian knee joint. This finding supportsthe theory that a joint in which gliding, rotation,and/or rolling occurs is likely to be stabilized andprotected by muscular reflex activity intitiatedin whole or in part by afferent input from thetissues of the joint, regardless of the taxonomicstatus, locomotor style, or size of the animal(O'Connor, '76; O'Connor and McConnaughey,'78). Flexion and extension of the knee joint inthe alligator is accompanied by gliding, rolling,and rotation, as it is in man and other mammals.

Although we did not quantify the types ofreceptors found in this study, it appeared thatGolgi tendon organs were more numerous thanPacinian corpuscles. Golgi tendon organs in thealligator (Figs. 14, 15) resembled the spiral end-ings of muscle spindles, similar to those in themedial meniscus of the human (Zimny et al.,'88).

In conclusion, the knee joint of the alligator issimilar to that of the crocodile and some mam-mals in that it has multiple cruciate ligamentsand two semilunar cartilages (the menisci) inter-posed between the femur and tibia. It differsfrom the joints of other species in that it has fourcruciate ligaments instead of two or three, andthe menisci are attached to the intercondylarregion of the tibial plateau by two ligamentoushorns instead of four. The cruciate ligamentsand menisci appear to be innervated with nervefibers and receptors which resemble those seenin mammals. Thus, additional support is givento the theory that sensory input from the intra-articular tissues of joints may be of importanceto the function of the joint.

ACKNOWLEDGMENTS

The authors wish to thank Ted Joanen, TomHess, Tom Moorman, and Larry McNease ofthe Louisiana Department of Wildlife and Fish-eries for assistance in sampling the alligators.

172MacKenzie, W.G., S.S. Shim, B. Day, and G. Leung (1985)

The blood and nerve supply of the knee meniscus in man.Anat. Rec. 211:115A-116A.

Nauck, E.T. (1938) Extremitatenskelett der Tetrapoden. InL. Bolk, E. Geppert, E. Kallius, and WW. Lubosch (eds):Handbuch der Vergleichenden Anatomie der Wirbeltiere.Berlin: Urban and Schwarzenberg.

O'Connor, B.L. (1976) The histological structure of dog kneemenisci with comments on its possible significance. Am. J.Anat.141:407-417.

O'Connor, B.L., and J.S. McConnaughey (1978) The struc-ture and innervation of cat knee menisci and their relationto a "sensory hypothesis" of meniscal function. Am. J.Anat.153:431-442.

Pearson, K., and A.G. Davin (1921) On the sesamoids of theknee joint. Biometrika 13:133-174, 350-400.

Rewcastie, S.C. (1980) Form and function in lacertilian kneeand mesotarsal joints; a contribution to the analysis ofsprawling locomotion. J. Zoo!. (Lond.) 191:147-170.

Schultz, R.A., D.C. Miller, C.S. Kerr, and L. Micheli (1984)Mechanoreceptors in human cruciate ligaments. J. BoneJoint Sur.g. 66A:1072-1076.

Seedhom, B.B., D. Dowson, and V. Wright (1974) Functionsof the menisci. A preliminary study. J. Bone Joint Surg.56B:381-382.

Wilson, A.S., P.G. Legg, and J.C. McNeur (1969) Studies onthe innervation of the medial meniscus in the human kneejoint. Anat. Ree. 165:486-492.

Zimny, M.L., M. St. Onge, and M. Shutte (1985) A modifiedgold chloride method of the demonstration of nerve end-ings in frozen sections. Stain Techno!. 60:305-306.

Zimny, M.L., M. Schutte, and E. Dabezies (1986) Mechanore-ceptors in the human anterior cruciate ligament. Anat. Rec.214:204--209.

Zimny, M.L., D.J. Albright, and E. Dabezies (1988) Meehan-oreceptors in the human medial meniscus. Acta Anat.(Basel) 133:35-40.

C.S. WINK ET AL.

Appreciation is also extended to Garbis Kerim-ian for the photography and Iretha Robinson fortyping the manuscript. This project was fundedin part by the Louisiana Department of Wildlifeand Fisheries.

LITERATURE CITED

Barnett, C.H., D.V. Davies, and M.A. MacConaill (1961) Thestructure and function of tibrocartilages within vertebratejoints. J. Anat. 88:363-368.

Basmajian, J.V. (1976) Primary Anatomy. Baltimore: TheWilliams and Wilkins Company.

Day, B., W.E. MacKenzie, S.S.Shim, and G. Leung (1985)The vascular and nerve supply of the human meniscus.Arthroscopy 1:58-62.

Furst, C.M. (1903) Der Musculus Popliteus und sein Sehne.Lund: Malmstrom.

Gairns, F.W. (1930) A modified gold chloride method for thedemonstration of nerve endings. Q. J. Microsc. Sci. 74:151-155.

Gray, H.G. (1973) Anatomy of the Human Body. 29th Ameri-can Edition. C.M. Goss, ed. Philadelphia: Lea and Febiger.

Haines, R. W. (1942) The tetrapod knee joint. J. Anat. 76:270-301.

Kennedy, J.C., HW. Weinberg, and A.S. Wilson (1974) Theanatomy and function of the anterior cruciate ligament asdetermined by clinical and morphological studies. J. BoneJoint Surg. 56A:223-235.

Kennedy, J.C., I.J. Alexander, and K.C. Hayes (1982) Nervesupply of the human knee and its functional importance.Am. J. Sports Med.10:329-335.

MacConaill, M.A. (1932) The function of intraarticular fibro-cartilages. J. Anat. 66:210-214.

MacConaill, M.A. (1950) The movement of bones and joints;3. The synovial fluid and its assistants. J. Bone Joint Surg.30B:244--252.

~

.