Plantar Aponeurosis

J. Anat. (1976), 121, 3, pp. 599-611 599With 8 figuresPrinted in Great Britain

Plantar aponeurosis and internal architectureof the ball of the foot*

FINN BOJSEN-M0LLER AND K. E. FLAGSTADDepartment of Biological Structure, University of Washington, School of Medicine,

Seattle, Washington 98195, and Anatomy Department C,University of Copenhagen, Universitetsparken, 1,

2100 Copenhagen 0, Denmark

(Accepted 26 July 1975)INTRODUCTION

The ball of the foot is exposed during walking and running to heavy tractionsand pressures which tend to slide the skin in relation to the deep structures and tocompress soft tissues such as nerves and vessels on their passage from the sole tothe digits. The area contains both fat and collagen fibres, the latter being concen-trated in the strong plantar aponeurosis which evidently sustains a protective function.The plantar aponeurosis springs from the calcaneus and proceeds to the forefoot,

where it divides into superficial fibres for the skin and deep fibres for the skeleton(Poirier, 1892; Loth, 1908; Henkel, 1913; Hicks, 1954). Anatomical textbooks arenot consistent in their accounts of where the divisions are located: thus someindicate that they are proximal to the metatarsophalangeal joints (Frohse & Frankel,1913; Basmajian, 1971), some at the joints (Testut & Latarjet, 1948; Lang & Wachs-muth, 1972; Gray, 1973), and some distal to the joints (Tondury, 1968; Snell, 1973).The matter has importance for the understanding of where the deep fibres areattached, and how the forces are intercepted.

In the relaxed state, the ball of the foot is a soft and pliable pad, the skin of whichcan be moved from side to side and proximo-distally. However, with the toes ex-tended and abducted, as during the push-off, the ball becomes tense and firm andno movements of the skin are allowed. It was the purpose of this investigation todescribe the structures underlying this mechanism.

MATERIALS AND METHODS

The description is based on dissection under the stereomicroscope, of 20 humanfeet fixed in formalin, and of 3 fresh specimens. Although attempts were made toselect normal looking feet, some degree of forefoot compression, from wearingshoes, was unavoidable. In order to study the neural elements (nerves and Paciniancorpuscles), and the course and attachments of the collagen fibres, feet from 4 fetusesaged 4-6 months were fixed in Lillie's neutral formalin and examined histologically.The specimens were decalcified in 5 % nitric acid with the aid of ultrasound; thenembedded in paraffin wax, sectioned and stained with haematoxylin and eosin.

* This work was supported in part by USPHS General Research Support Grant RR-05432 from theNational Institutes of Health.

FINN BOJSEN-M0LLER AND K. E. FLAGSTAD

OBSERVATIONS

The aponeurotic fibres of the ball of the foot are arranged in longitudinal, trans-verse and vertical bands and tracts. Together with fat bodies encapsulated betweenthe bands and tracts, they form a texture which ties the skin to the skeleton,allows longitudinal passage for vessels, nerves and tendons, and protects these belowthe weight-bearing heads of the metatarsals.

Proximal to the heads of the metatarsals the longitudinal fibres of the plantaraponeurosis separate into five superficial and ten deep tracts (Figs. 1, 3, 8). Of thesuperficial tracts, two course to the sides of the foot while the three intermediariesproceed in the direction of the middle toes. At the heads of the metatarsals theseparation is completed and the tracts are free to diverge, one reaching the intersticebetween the first and second toe, one approaching the base of the third toe and onethe interstice between the fourth and fifth toe. Anterior to the heads of the metatarsalsmost of the fibres are inserted into the skin of the ball (Figs. 5, 6), but others turntransversely into the plantar interdigital ligament which is deep to them. No fibresreach the skin crease between the ball and the toes. In the area of insertion only alittle fat is found between the aponeurosis and the dermis.The ten deep tracts form two marginal and eight intermediary sagittal septa

(Figs. 3, 5 and 8). The former reinforce the medial and lateral intermuscular septaof the sole while the latter, in the area proximal to the heads of the metatarsals,connect the plantar aponeurosis with the interosseous fascia, the fascia of the trans-verse head of the adductor hallucis and the deep transverse metatarsal ligament.There are two septa for each toe. They pass along the sides of the digital flexortendons, between these and the lumbrical muscles. The proximal extension of thesepta are thus limited by the origin of the lumbricals from the long flexor tendons,a distal lumbrical origin corresponding to a short septum and a proximal to a longerseptum (e.g. the septa on the tibial and fibular sides of the tendons to the second toe,respectively). Consistent with this, the septum on the fibular side of the flexorhallucis longus is the longest of the intermediary septa, as no lumbrical springs fromthis tendon. The fibres of the septa pass, from the plantar aponeurosis, anteriorlyand deeply to the above mentioned structures, into which they turn in both medialand lateral directions, thereby crossing the fibres from the neighbouring septa.Finally, the fibres continue into the plantar ligament of the metatarsophalangealjoints and on to the base of the proximal phalanges (Fig. 4).The lumbricals are covered by a fascia which splits off from the side of the respec-

tive sagittal septum (Figs. 7, 8). The fascia covering the first lumbrical is unique (1)in reaching from the plantar aponeurosis to the deep fasciae, (2) in being a longseptum, as it covers a unipennate muscle, and (3) in that it corresponds to themarginal septum, which in the hand bounds the central compartment on the radialside (Bojsen-Moller & Schmidt, 1974).In the area in which the plantar aponeurosis gives off the deep septa the retinacula

cutis form a series of transverse subcutaneous bands separated by fat (Figs. 1, 5).Medially and laterally some of the bands curve distally to join the longitudinal tractsof the aponeurosis. In sagittal sections it can be seen that the bands also containfibres which run an oblique course anteriorly and deeply from the dermis, penetrate

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Palmar aponeurosis and the ball of the foot 601

Fig. 1. Sole of right foot. The skin and subcutaneous fat have been removed to demonstratesubcutaneous transverse bands (rc) in the proximal part of the ball and the five superficial tracts(st) of the plantar aponeurosis (pa). Deep to their insertion into the skin of the distal part of theball are seen the lower edges ofsome ofthe transverse lamellae which form the plantar interdigitalligament (pil).

the plantar aponeurosis, and continue via the sagittal septa and the deep fasciae tothe bases of the proximal phalanges.

Anterior to the sagittal septa, and below each of the metatarsal heads, verticalfibres form a connective tissue cushion (Figs. 2, 3, 6, 7). The fibres spring from the

39 A NA 121


Fig. 2. Dissection of right foot. The plantar aponeurosis has been removed to demonstrate thesuperficial layer of the central compartment and the five submetatarsal cushions (cu). Thecushion under the head of the third metacarpal bone is very narrow in this specimen. Four fatbodies (fb) cover the digital nerves (dn) while they pass through the weight-bearing area of theball. The nerve for the third interstice takes an oblique course below the belly of the shortflexor (fdb) to pass behind the sagittal septa. One of the septa of the fourth toe is shown (ss).Note the mooring ligament (ml) which in the interdigital web arches from one fibrous flexorsheath to the next. ms, marginal septum.

602

Palmar aponeurosis and the ball of the foot

13 a _ :$'.,^': . ' .. .$S.Fig. 3. Dissection of right foot. The fat bodies and the contents of the central compartmenthave been removed to show the five submetatarsal cushions and the extent of the eight sagittalsepta. Three of these are marked (ss). The septum of the first lumbrical (Is) covers the shorttibial septum of the second toe. dn, digital nerve; ml, mooring ligament.

sides of the fibrous flexor sheaths, and from the plantar ligaments, and reach, aftera vertical course, the superficial fibres of the plantar aponeurosis, along with whichthey are inserted into the skin. Some of the vertical fibres, especially the proximalones, arch below their fibrous flexor sheath to form a few transverse bands across

39-2

603


Fig. 4. Sagittal section through the second ray of bones. Proximal to the head of the metatarsalbone the deep fibres of the plantar aponeurosis (pa) cross the two flexor tendons (flt) to con-tinue into the strong plantar ligament (pl) of the metatarsophalangeal joint. The plantar liga-ment is attached to a prominent facet on the base of the proximal phalanx. tah, transversehead of adductor hallucis.

the region. No fibres spring from the surface of the flexor sheaths, which are coveredby fat entrapped inside the cushions. The sesamoid bones of the hallux are containedwithin the medial cushion. Between the cushions, four compartments about 5-8 mmdeep are left for the passage of digital vessels and nerves and the lumbrical muscles.In each compartment the structures are covered by an elongated, encapsulated fatbody which extends throughout the weight-bearing area (Fig. 2). The fat bodies areseparated from the cushions by sagittal planes of cleavage which can be used fortheir demonstration during dissection.

Anterior to the heads of the metatarsals, and in the interdigital webs, the plantarinterdigital ligament forms six to eight transverse lamellae separated by fat (Figs.2, 5, 6). The lamellae extend from one side of the foot to the other and, in sagittalsections, are seen to be convex anteriorly. Superficially, they are connected with thesuperficial tracts of the plantar aponeurosis, and in between these their free loweredges can be seen when the skin is dissected away (Fig. 1). In the depth they areattached to the fibrous flexor sheaths and to a mooring ligament which archesbetween the proximal phalanges. The fibres of this mooring ligament reach from thetibial side of one phalanx to the fibular side of the next, forming a crossing beloweach flexor sheath.

In the sole, the digital nerves and vessels are protected by fat from contact with

604

Palmar aponeutrosis and the ball of the foot

Fig. 5. Sagittal section through the second interstice showing the fibular side of the second meta-tarsal bone and toe. The fat has been removed to show the fibrous structures of the ball. Thesagittal septum (ss), which is seen proximal to the head of the metatarsal bone, connects theplantar aponeurosis (pa) with the fascia of the transverse head of the adductor hallucis (tah)and the plantar ligament (pi). Note the anterior slope of the fibres of the subcutaneous transversebands (rc) as they course from the dermis to the deep structures. The vertical fibres (vf) aresituated below the head of the metatarsal bone. The superficial fibres of the plantar aponeurosisinsert in the skin at the distal part of the ball. Deep to the insertion the plantar interdigitalligament (pil) forms a series of transverse lamellae connected to the mooring ligament (ml)and the proximal phalanx. A digital nerve (dn) crosses the posterior edge of the sagittal septum.Compare with Figure 6.

the plantar aponeurosis. On their way to the digits they must cross behind the deepsepta before they can enter the compartments between the submetatarsal cushions.The nerves for the second and third interstices pass close to the sharp proximal edgeof the lateral septum of the respective toes, and that for the third interstice is oftenseen to be bent around the edge (Figs. 2, 5, 6). In each compartment, the digitalnerve and its accompanying vessels pass below the transverse metatarsal ligament,having the fat body superficially.

Pacinian corpuscles, 1 5-2 mm long, solitary or in groups of three or four, are

605


4

hc

14

4d

'e onsertion of longitudinal fibres

Vertical fibres I Transverse metata rsal ligamentFig. 6. Drawing of a sagittal section through the second interstice showing the internal archi-tecture of the three areas of the ball of the foot. The sagittal septum is attached to the proximalphalanx through the transverse metatarsal ligament and the plantar ligament of the joint. Thevertical fibres and the lamellae of the plantar interdigital ligament are attached to the proximalphalanx through the fibrous flexor sheath.

*

Fibrous flexor sheath - Plantar ligament--Transverse metat.arsal ligament

First lumbrical and septum-...:, e,

:~~~~~~~~~~~~~~~~~. Vertical fibres

Fat bodyFig. 7. Transverse section through the heads of the second and third metatarsal bone showingthe course of the fibres in the submetatarsal cushions and around the joint. Fat covers thefibrous flexor sheath inside the cushion and the digital nerves and vessels between the cushions.

found alongside the digital nerves and their accompanying arteries. The corpusclesare numerous in the area below the transverse metatarsal ligament and where thenerves pass deep to the mooring ligament of the interdigital web. Solitary corpusclesare also found in the covering fat bodies and in the subcutaneous fat.The transverse head of the adductor hallucis springs from the posterior border of

606

IC

lb

4k

4

4

Pa/mar aponeurosis and the ball of the foot 607

Mooring ligament

Digital nerve and artery

C u s h l o l sFirst lumbricaland septum

// ~~~Transverse metatarsal/ ligament

N ~~~~~~~~~Transverse headof adductor hallucis

Sagittal septa

ta8A Z !s t Marginal se ptUmFlexor hallucis longusMedial plantar nerve

'1; xe 2 |,0 | - Flexor digitorum brevisAponeurosis plantaris

/

~ ~ ~ ~ ~ ~ ~ ~ .........

Fig. 8. Plantar aspect of the central compartment and the structures forming the ball of thefoot. Of the plantar interdigital ligament, only the mooring ligament is shown.

FINN BOJSEN-MOLLER AND K. E. FLAGSTADthe transverse metatarsal ligament and from the lateral sagittal septa of the plantaraponeurosis, i.e. the septa for the third, fourth and, usually, the fifth ray. The musclecan thus act as an adductor for the hallux and, with a reversed fixed point, as atensor of the plantar aponeurosis and the skin (Fig. 8).

DISCUSSION

The present observations on the insertions of the plantar aponeurosis and of thecourse of the plantar interdigital ligament correspond to those of Henkel (1913),whose detailed description and naming of the parts of the two ligaments seems tohave been overlooked by the authors of modern textbooks. The plantar interdigitalligament was given its name by Poirier (1892): it was called the natatory ligamentby Henkel (1913), following Grapow (1887). In the Nomina Anatomica (1968) itis called the superficial transverse metatarsal ligament. However, it is deep to theplantar aponeurosis and distal to the metatarsal region, and so we have used Poirier'sterm as it is descriptively more accurate.On the basis of its internal structure the ball of the foot can be divided into three

transverse areas: (1) a proximal, with the subcutaneous transverse bands and thedeep sagittal septa of the plantar aponeurosis, (2) an area below the heads of themetatarsals, with the submetatarsal cushions and the fat bodies and (3) a distalarea, with the insertions into the skin of the longitudinal fibres of the plantaraponeurosis and, deep to these, the plantar interdigital ligament.The three areas seem adapted to three different mechanical functions. During

push-off the heel is raised and the anterior part of the ball is in contact with theground. The skin tends to slide anteriorly but is prevented from this by the longi-tudinal fibres of the plantar aponeurosis, which insert exactly in this area, and whichtransmit the forces to the calcaneus. The arcuate form of the mooring ligament ofthe interdigital web limits the spreading of the toes and allows at the same timeindividual extension of a digit.During braking the heel is lowered and the posterior area of the ball has the ground

contact. In this area the retinacula cutis are developed into transverse bands, ofwhich some fibres slope anteriorly and deeply to be connected, via the sagittal septaand the transverse metatarsal ligament, to the proximal phalanges. The posteriorlydirected forces exerted on the skin can thus be transferred to the skeleton throughthis system of fibres.The intermediate area of the ball is the weight-bearing part and each of the five

metatarsals takes part of the load. Firm cushions support the heads of the metatar-sals and in the intervals, as was noted by Frohse & Frankel (1913), fat bodies protectthe nerves and vessels as they pass below the transverse metatarsal ligament. Insidethe cushions the fibrous flexor sheaths are covered by a layer of fatty tissue, anarrangement which probably protects the tendons also.An extension and an abduction in the metatarsophalangeal joints will tense the

plantar interdigital ligament in the web, the vertical fibres in the cushions, and byway of the sagittal septa, the retinacula cutis in the posterior area of the ball as wellas the plantar aponeurosis itself. This explains why the ball becomes tense and firmand why the movements of the skin become restricted. The plantar aponeurosis exerts

608

Palmar aponeurosis and the ball of the foothere a function similar to that of the palmar aponeurosis of the hand during apower grip (Bojsen-M0ller & Schmidt, 1974). The fibres of the ball are stout, butthe strain on the system can also be very heavy, for instance, during running and/orduring a twist on the ball of the foot.

For the design of shoes it seems important to make sure that extension and abduc-tion of the toes are unimpaired. The movements are essential for proper functioningof both the ball of the foot, as shown in this paper, and the plantar aponeurosisin its support of the longitudinal arch of the foot, as shown by Hicks (1954). Theinner sole of the shoe must provide enough friction to transfer a deceleration to theball of the foot even with a smooth stocking in between. Otherwise the foot willslide anteriorly in the shoe and the forces will be transferred via the anterior endsof the clawed toes.

Morton's metatarsalgia is pain associated with a neurofibroma of the commondigital nerve for the third interstice (Betts, 1940). This nerve differs from the otherdigital nerves in that it commonly arises from both plantar nerves. It is thus lessmobile in its proximal part and it is, therefore, exposed to stretch and entrapmentagainst the transverse metatarsal ligament during extension of the toes (Betts, 1940;Bickel & Dockerty, 1947; Kopell & Thompson, 1960). The disease is reported to berelated to the wearing of ill-fitting and high-heeled shoes. Posterior fixation of thenerve is evidently not the only factor in the pathogenesis as neurofibromas also occurin cases with a single origin of the nerve as well as in other digital nerves (Bickel &Dockerty, 1947; Mann, 1973; Curtiss, 1973).From Figures 2 and 5 it can be appreciated that the digital nerves for the second

and third interstice, after an oblique course below the belly of the short flexor ofthe digits, come into a narrow contact with the sharp proximal edges of the sagittalsepta. The nerve for the third space actually makes a bend on the last septumbefore it proceeds into its compartment. The nerve seems even more exposed toentrapment at this site in cases where it misses an anchorage to the lateral plantarnerve, because a contraction of the short flexor will pull it medially. Shoes which,during the push-off, prevent proper extension and abduction of the toes, will alsoprevent forward sliding of the septa with the transverse metatarsal ligament andthus augment the risk of metatarsalgia developing.

Solitary, and groups of, Pacinian corpuscles were found along the digital nervesand arteries below the transverse metatarsal ligament and between this ligamentand the mooring ligament of the web. This is the classical site where the corpuscleswere found and described by Pacini (Pacini, 1840; Henle & Kolliker, 1844). It maybe worth noting that they seem to be concentrated in the area of the ball which isweight-bearing during both normal and tiptoe standing.

SUMMARY

On the basis of its internal structure, the ball of the foot can be divided into threetransverse areas, each with a different mechanical function: (1) an area proximal tothe heads of the metatarsals in which the retinacula cutis are developed into a seriesof transverse bands, and in which the deep fibres of the plantar aponeurosis formten sagittal septa connected to the deep transverse metatarsal ligament and through

609

FINN BOJSEN-M0LLER AND K. E. FLAGSTADthis to the proximal phalanges of the toes, (2) an area below the heads of the metatar-sals in which vertical fibres from the joint capsules and the sides of the fibrous flexorsheaths form a cushion below each metatarsal head, and in which fat bodies coverthe digital nerves and vessels in their passage between the cushions, and (3) a distalarea which comprises the interdigital web. The superficial fibres of the plantaraponeurosis are inserted into the skin of this distal area, and deep to them theplantar interdigital ligament forms a series of transverse lamellae connected to theproximal phalanges by a mooring ligament which arches from one fibrous flexorsheath to the next. When the metatarsophalangeal joints are extended, the fibresof the three areas are tensed and the skin is anchored firmly to the skeleton. Thedirection of the fibres in the distal and proximal area promotes the transfer offorces exerted on the skin during push-off and braking respectively, while the inter-mediate area is adapted to bear the weight of the body. A concentration of Paciniancorpuscles is found along the digital nerves in the weight-bearing area below thetransverse metatarsal ligament. The nerves for the second, and especially forthe third, interstice are close to or in contact with the sharp proximal edges of thesagittal septa.

The authors wish to thank Mr Finn Eldon for the care with which he executed thedrawings.

REFERENCES

BASMAJIAN, J. V. (1971). Grant's Method of Anatomy. 8th edition, pp. 383-389. Baltimore: The Williams& Wilkins Company.

BETTS, L. 0. (1940). Morton's metatarsalgia: neuritis of the fourth digital nerve. Medical Journal ofAustralia 1, 514-515.

BICKEL, W. H. & DOCKERTY, M. B. (1947). Plantar neuromas, Morton's toe. Surgery, Gynecology andObstetrics 84, 111-116.

BOJSEN-M0LLER, F. & SCHMIDT, L. (1974). The palmar aponeurosis and the central spaces of the hand.Journal of Anatomly 117, 55-68.

CURTISS, P. H. (1973). Neurologic diseases of the foot. In Foot Disorders (ed. N. F. Giannestras), 2ndedition, pp. 493-516. Philadelphia: Lea and Febiger.

FROHSE, F. & FRANKEL, M. (1913). Die Muskeln des mnenschlichen Beines. In Handbuch der Anatomiedes Menschen (ed. K. von Bardeleben), Band 2, Abt. 2, Teil 2B, pp. 415-693. Jena: G. Fischer.

GRAPOW, M. (1887). Die Anatomie und physiologische Bedeutung der Palmar Aponeurose. Archiv firAnatomie und Physiologie, Anatomische Abteilung, pp. 143-158.

GRAY'S ANATOMY (1973). (ed. R. Warwick and P. L. Williams), 35th British edition, pp. 579-580.Philadelphia: W. V. Saunders & Co.

HENKEL, A. (1913). Die aponeurosis plantaris. Archiv fiur Anatomie undPhysiologie, Anatomische Abteil-ung, Supplemenit-Band, pp. 113-123.

HENLE, J. & K0LLIKER, A. (1844). Ueber die Pacinischen Korperchen an deni Neri'en des Menschen und derSicugethiere, pp. 1-40. Zurich: Meyer und Zeller.

HICKS, J. H. (1954). The mechanics of the foot. 11. The plantar aponeurosis and the arch. Journal ofAnatomy 88, 25-30.

KOPELL, H. P. & THOMPSON, W. A. L. (1960). Peripheral entrapment neuropathies of the lower extremity.New England Journal of Medicine 262, 56-60.

LANG, J. & WACHSMUTH, W. (1972). Bein und Statik, pp. 413-418. Berlin, Heidelberg, New York:Springer-Verlag.

LOTH, E. (1908). Die Aponeurosisplantaris in der Primatenreihe. Morphologische Jahrbuch 38, 194-322.

MANN, R. A. (1973). Diseases of the nerves of the foot. In DuVries' Surgery of the Foot (ed. Verne T.Inman), 3rd edition. Saint Louis: C. V. Mosby Co.

NOMINA ANATOMICA (1968), p. 44. Amsterdam: Excerpta Medica Foundation.

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Palmar aponeurosis and the ball of the foot 611PACINI, F. (1840). Nuovi Organi, scoperti nel corpo umano. Pistoja. Cited from A. Kollmann (1885).Der Tastapparat des Fusses von Mensch und Affen in seinen Beziehungen zuLm Nervensystem. Archivfiur Anatomie und Physiologie. Anatomische Abteiling, pp. 56-99.

POIRIER, P. (1892). Traite d'Anatontie Humlaine, vol. 2, p. 296. Paris: L. Battaille et Cie.SNELL, S. (1973). Clinical Anatomy for Medical Students, pp. 557-558, Boston: Little, Brown & Co.TESTUT, L. & LATARJET, A. (1948). Trait.d'Anatonmie Huinaine, 9th edition, vol. 1, pp. 1206-1210. Paris:

G. Doin et Cie.TONDURY, G. (1968). Bewegungsapparat. In A. Rauber and F. Kopsch: Lehrbich lind Atlas deer Anatonmie

des Menschen (ed. W. Bargmann, H. Leonhardt and G. Tondury), 20th edition, vol. 1, pp. 412-414.Stuttgart: G. Thieme.

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Plantar Aponeurosis