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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.
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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
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FINN BOJSEN-M0LLER AND K. E. FLAGSTAD
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
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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
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FINN BOJSEN-M0LLER AND K. E. FLAGSTAD
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
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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
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FINN BOJSEN-M0LLER AND K. E. FLAGSTAD
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
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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.
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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
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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
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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.
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