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Journal of Bodywork and Movement Therapies (2005) 9, 177–188

Bodywork and

Journal of

Movement Therapies

KEYWORDSphenoid;Cranial moCraniosacrCSR;SutherlandSphenobasondrosis

1360-8592/$ - sdoi:10.1016/j.j

�Tel.: +44 01E-mail addr

1Andrew CoNorwich, UK, wtional fluid mec

www.intl.elsevierhealth.com/journals/jbmt

HYPOTHESIS: CRANIAL BIOMECHANICS

The mechanics of cranial motion—thesphenobasilar synchondrosis (SBS) revisited

Andrew Cook�,1

Complementary Health Centre, 34 Exchange Street, Norwich NR3 2JY, UK

Received 7 July 2004; received in revised form 10 December 2004; accepted 11 December 2004

S

tion;al rhythm;

lesions;ilar synch-

ee front matter & 2005bmt.2004.12.002

603 485776/665173.ess: ovahimba@ntlworlok is a craniosacralith a background in ehanics.

Summary Building on Sutherland’s approach to sutures (‘‘form follows functionand function follows form’’) applied to the thicknesses—and hence flexibility—ofcranial bones, a new model of cranial motion has evolved. This places thesphenobasilar synchondrosis (SBS) as a primarily compressive–decompressive joint.SBS hinging is seen as an illusory artefact created as tissue rotates around a stableSBS. This article suggests that the apparent motion of the SBS instead takes place bya change in shape of the anterior body of the sphenoid, and that this motion isaccommodated by the superior orbital fissure. This new model can be used to derivecranial bone motion patterns directly from the assumption that the cranium changesits lateral diameter, and elegantly explains the well-known ‘‘four interlinked gears’’description of the occiput–sphenoid–vomer/ethmoid train. The model does notrequire sutures to be patent or membranous, since it applies equally well to ossifiedsuture relics.& 2005 Elsevier Ltd. All rights reserved.

Introduction

The aim of this paper is to explore osseous motionof the cranium from a biomechanical perspective.For the sake of simplicity, this has been done withas little reference as possible to driving mechan-isms for the ‘‘cranial rhythmic impulse’’ (CRI—alsocalled the ‘‘craniosacral rhythm or CSR)—physiolo-

Elsevier Ltd. All rights reserv

d.com (A. Cook).practitioner working inngineering and computa-

gical or otherwise. It is also intended that noassumptions should be made regarding the drivingforce behind the CRI—there is still no modelavailable for the CRI which agrees with bothmodern physiology and palpated phenomena. Con-sequently, the descriptions presented are a de-tailed clarification of Sutherland rather than a newmodel of cranial motion, and might be consideredas a further round of ‘‘digging on’’ (Sutherland,1998, p. 167). Cranial bone, periosteum, dura andtheir enclosed fluids, neural tissues and vascularstructures are part of a dynamic continuum ofvarying stiffness and elasticity. Their total motioncan theoretically be analysed and described by just

ed.

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examining the bony structures. This is simply anextension of the study of skeletal detail to under-stand the musculoskeletal system. Therefore, againfor reasons of simplicity, the following thesiscontains limited reference to the dura, falx andtentorium.

Historical context

It is over 80 years since Sutherland (1998) discoveredthe motion of cranial bones by inspection of adisarticulated skull and application of the principleof ‘‘form follows function and function followsform’’ to sutures. Sutherland’s original brilliantinsight that sutures are lines of relative ease ofmotion in the bony skull eventually evolved from aconcept centred on bones to a far more complex,holistic and subtle understanding of the bony, fascialand fluid nature of cranial motion, eventuallyencompassing the entire human body. The originalconcepts of sphenobasilar synchondrosis (SBS) mo-tion continue to be central to basic techniques inCraniosacral Therapy and Cranial Osteopathy (CST/CO), as does the concept of non-bony sutures, toaccommodate this model of cranial motion.

Does the cranium move?

The a priori basis for this paper is the palpatoryexperience shared by craniosacral therapists andcranial osteopaths that the cranium naturallymoves. Palpated cranial motion (the CRI) is usuallyrhythmic, moving in a recurring ‘‘Flexion–Exten-sion’’ (F–E) cycle, with the ‘‘expansive’’ phasehaving been historically (Sutherland, 1998) de-scribed as ‘‘Flexion’’, and the ‘‘contractive’’ phaseas ‘‘Extension’’. Flexion is most simply perceived asa lateral widening and an anterior–posterior (A–P)foreshortening of the skull. This motion is mostsimply described as a change in shape, with thecranium becoming more ‘‘round’’ (spherical) duringFlexion.2

Sutures as evidence of motion

The mere fact that sutures exist at all—even asrelics—could be considered evidence of cranial

2A capitalized ‘‘Flexion’’ is used to refer to motion, as in F–E,with ‘‘flexing’’ or ‘‘flexes’’ describing the bendability of cranialbones. Only the Flexion phase of motion is described in detailbecause the extension phase is simply the reverse motion. Alldescriptions of Flexion begin at the peak of Extension. Alldescriptions are of adult anatomy/physiology, unless otherwisestated.

motion. In the neonate, the cranium is still soft,and consists of thin membranous material surround-ing slightly harder ossification centres. The anteriorfontanelle (bregma in adults) is so thin that thecardiac pulse is visible. This soft structure of thebaby skull is a result of the need to negotiate thebirth canal. Bone growth ideally matches thevolume requirements of the brain as the centresof ossification gradually expand outwards and meetat the sutures. Some sutures, such as thosebetween the segments of the temporals andbetween the occipital squama, disappear entirelyin all adults during this ossification process. Withadulthood the bones harden, but still retainflexibility by virtue of their thinness and from thefact that they are alive, containing blood vesselsand nerves. The manual deformability of the ribscan be used as an analogue to give some impressionof the magnitude of internal or external forcesnecessary to create palpable cranial motion.

As bony plates expand towards each other, theintervening sutures shrink back to relic lines ofcomplex shape, persisting for some time asperiosteal/cartilaginous tissue (Upledger and Vre-devoogd, 1983, Appendix A). Hartman and Norton(2002) cite fresh-tissue autopsy and MRI (livingtissue) evidence that the SBS fuses in most adultsbefore the age of 19. Similarly, most vault suturesappear to ossify before the age of 30 in most adults.However, the evidence remains contradictory.Sutures retain more viscous mobility than surround-ing bones (Steenvoorden et al., 1990). Singer(1953) found that human sutures may remain openeven beyond the age of 60, but most histologicaland laboratory evidence of retained sutural mobi-lity comes from in vivo and dissected animal studiesrather than human studies. Byron et al. (2004)studied the effects of temporalis muscle strengthon the morphology of the sagittal suture, conclud-ing that ‘‘cranial suture connective tissue locallyadapts to functional demands of the biomechanicalsuture environment’’. Ogle et al. (2004) showedthat mechanical motion delays the ossification ofsutures.

The presence of Wormian bones (small ossicleswithin cranial suture lines) is hard to explain unlessmotion has to take place around a suture regardlessof the original locations of ossification sites orneonatal sutural lines. Conversely, the fact thatmost adult sutures are ossified relics indicates thateven if the presence of the suture is important toaccommodate motion, its morphology is less so.Unlike armour, most cranial bones do not overlapbut rather must be strongly abutted to provideadequate protection for the brain whilst notdamaging the dura. Therefore, motion is enhanced

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along sutural lines but the sutures are neverthelessquite tight, consisting of extensions to the perios-teum (Upledger and Vredevoogd, 1983). Theycannot open significantly or slide over each other;they can only provide some increased local bend-ability and flexibility. The great majority of sutureshave interlinking projections and indentations thatmechanically prevent sliding or shearing and assistthe absorption of impact forces (Jaslow, 1990).

Important atypical sutures

There are two very important exceptions to theabove description of sutures. Firstly, the superiororbital fissure lying between the greater and lesserwings of the sphenoid bone is special in that itremains open, being an exposed dural surface 1 or2mm wide and 15–20mm long. Secondly, thesutures around the squama of the temporal bonepossess some of the qualities of overlappingarmour. The way subdural arteries (visible asimpressions on the inner surface of the parietal)avoid the anterior section of the temporal squamasuggests there is a relatively large physical motionalong this line. I have no idea how much thispotential sliding overlap is reflected histologically.

Recognition of cranial motion

The CRI is not recognized in English-languagemedical texts, since there is no record of intracra-nial pressure (ICP) changes in the bandwidth of5–12 cycles/min (Davson and Segal, 1996); how-ever, some neurosurgeons have noted a similardural motion whilst carrying out operations (Up-ledger, 1995). Recent osteopathic studies haverelated the CRI to Traube–Meyer–Hering waves(Nelson et al., 2001; Sergueef et al., 2002). CST/CO laboratory measurements (Upledger, 1995) andcommon palpatory experiences suggest that parti-cularly mobile skulls change by up to 2mm inlateral diameter during the CRI (see, Adams et al.,1992, for a study of cranial motion in cats).Moskalenko et al. (1999) have measured cyclicchanges in cranial diameter up to 1mm withfrequencies between 6 and 14 cycles/min.

Deceptive simplifications

Cranial textbooks (e.g. Magoun, 1951) place a lot ofemphasis on sutural motion, providing highlysophisticated and exhaustive descriptions. How-ever, they do not fully include bone flexibility intheir models of cranial motion. This is for good

reason. Cranial bones are complex in shape, insutural morphology and in mobile relationship totheir neighbours. The whole cranium moves as acomplete and continuous unit/process along withthe membranes and fluids that surround it. Thetotal pattern of motion is complex, and is not easyto visualize or describe without isolating each boneor pair of bones. Regardless of whether the text-book authors intended this or not, one result ofisolating structures, emphasizing the sutures, andthe use of jargon suggestive of mechanical hingingis that the impression subliminally conveyed is oneof a relatively rigid bone that only moves at itsboundaries.

The concept of an ‘‘axis of rotation’’ is also foundin cranial textbook descriptions, again subtlypushing towards an instinctive conceptual modelthat assumes rigidity. This can be seen in the recentpaper by Oleski et al. (2002). This groundbreakingstudy of cranial motion using X-rays employed thelesser wings of the sphenoid as a marker for thesphenoidal SBS angle. In doing so, it missed thepoint that the anterior sphenoidal body is com-posed of flexible bones that may move indepen-dently of the SBS. I do not believe that rigidity waspart of the authors’ conscious conceptual model,but rather that the subliminal implications of thecommonly used ‘‘axis of rotation’’ jargon were notfully recognized.

Furthermore, the fact that a four-gear train (e.g.Milne, 1995; Sills, 2002) is useful as a conceptualmodel for memorizing the relative motions of theocciput, sphenoid, vomer and ethmoid also pro-vides a mental smokescreen. The degree ofaccuracy of the motion description has removedattention from the actual processes behind thiscomplex relative motion, and there are so far nopublished descriptions of cranial bone motion thatadequately describe how or why this pseudo-gearmotion comes about.

Palpatory paradoxes

There are two clearly palpable motions of the CRIthat can be experienced as a paradox if the cranialbones are considered to be rigid, with most motionoccurring along sutures:

(a)

The fact that the cranium becomes morespherical on Flexion. To a palpator with nopreconceptions as to what the cranial struc-tures may be doing, this takes place by meansof a lateral expansion of the parietals andsuperior temporal squama, and a slight ante-rior–posterior foreshortening of the occiput–-

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frontal axis. Although the mechanism driving itis still under debate, the existence of thischange in dimension has been well demon-strated in several laboratory experiments citedby Upledger and Vredevoogd (1983) and Upled-ger (1995) and is easily palpated by anyone whocares to sit quietly holding a head in theirhands.

(b)

Although Sutherland emphasized the complex-ity and holistic nature of cranial motion,‘‘Sutherland’s SBS patterns’’ are a simplifieddescription of cranial motion, treating the SBSas a universal joint. As described in thissimplified model, the apparent motion of thesphenoid relative to the occiput is based onpalpation of the greater wings of the sphenoidas they move relative to the occipital squama.During Flexion the sphenoid body and occipitalsquama are apparently separating in an A–Pdirection as a hinging motion takes placearound their medial connection (the SBS).

If the sphenoid and occiput are truly moving astwo rigid bodies around the hinge of the SBS, (b)would lead to a lengthening of the cranium, while(a) experientially demonstrates a foreshortening ofthe cranium.

Adding bone flexibility

Milne (1995) points out the amazing flexibility andaliveness of the cranial bones. His description ofthe ethmoid as being like ‘‘the tiny head of a bird’sskeleton, such as you might find under a bush aftera hard winter’’ conveys both an evocative andaccurate image of delicate flexibility. Anyone whohandles a real or high-quality moulded disarticu-lated skull can also experience this flexibility tosome degree. Even so, in the ‘‘real’’ demonstrationskull, despite the obvious flexibility of this deadmaterial, the delicate bones have calcified andbecome hard far beyond their state in a living body.

It is proposed that the cranium behaves some-what as if constructed of cardboard, with thesutures acting as prefolded lines or perforations.The cardboard flexes, and the perforations facil-itate that flexing in a manner appropriate to thestiffness of the cardboard. In this case, it does notmatter whether the sutures are ossified relics orsoft periosteum—they are lines of relative ease ofbending in a semi-rigid structure. Any visible relicsuture—even if ossified—remains a line of dimin-ished rigidity, and so also remains a preferred site

for any flexing motion to occur, as described byJaslow (1990).

The superior 70% of the metopic suture usuallyossifies to the extent that it is invisible in mostadult skulls; hence, it does not retain its ease offlexing. If this were the case for the metopicsuture, it would presumably be the case for allsutures unless there were some biological require-ment for them to remain slightly weaker than thesurrounding bony plates. Conversely, if cranialbones were inflexible, sutures would have to beas extensive as in a neonate to allow motion;otherwise, the complex curved shapes wouldsimply lock against each other.

Analysing sutures

The function of the sutures in the motion of thecranial vault and most of the base can be readilyseen by inspecting sutural morphology, as describedin Magoun (1951). For example, the interdigitationsof the coronal suture allow a limited relativemotion to take place between the frontal andparietal bones. These interdigitations overlap indifferent ways along the suture. Starting at thebregma, the parietals are overlapped by thefrontal. Approximately 30mm laterally from thebregma, this overlap reverses, and the frontal isoverlapped by the parietals. It is well recognized(e.g. Magoun, 1951; Milne, 1995) that the coronaldome of the parietals moves in such a way as toflatten more than the dome of the frontal, and thissuture allows this relative difference in flexing totake place. In fact, the parietals are joinedmedially by the sagittal suture, which acts as ahinge line, causing a flattening of the apex of theskull. This is demonstrated in Milne (1995, vol. 2,p. 135).

However, an unbiased analysis of the morphologyof the SBS does not yield an impression of motionother than possibly compression and decompres-sion. The osseous SBS is a flat surface about 1.5 cm2

in surface area. The two surfaces of the SBS areembedded in each other by short, sharp, wedge-like protrusions that cover the entire face of theSBS. The visible evidence suggests a highly stable‘‘design’’ that is resistant to bending, rotation andshear forces. This is somewhat at odds withdescriptions of the SBS contained in cranial text-books, which usually considers it to be a location ofsubstantial motion in the cranial base. A set ofrelease techniques for SBS motion patterns (e.g.Upledger and Vredevoogd, 1983; Upledger, 1987;Milne, 1995) is central to basic cranial practice(CSTA, 2002). These techniques are designed to

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release what are often referred to as the ‘‘Suther-land lesions’’, referred to henceforth as ‘‘SBS lesiontechniques’’.

An analysis of cranial mechanics

This paper is not long enough to analyse cranialmechanics in anything remotely comparable to thekind of detail achieved by Magoun (1951). With theexception of a few simplified examples (such as thecoronal suture description above), sutures will bedescribed in generic terms, and the reader isreferred to more authoritative texts (Magoun,1951; Upledger and Vredevoogd, 1983; Upledger,1987; Milne, 1995; Sills, 2002, 2004). Similarly,cranial bones flex in a highly complex pattern ofmotion, so only a brief description of these bones ispossible due to space restrictions. Instead, thereader is encouraged to play (carefully) with adisarticulated skull—of either real bone or flexibleplastic—to confirm the descriptions of motiongiven here and perhaps add to them. Based oncommon materials engineering principles, it isproposed that the flexibility of living bone can bequalitatively experienced by flexing disarticulatedbone or a plastic moulded skull. Therefore,although greater force is required to bend theseanalogues than to bend a living skull, wherever theosseous thicknesses are well preserved, so also isthe qualitative stress–flex response.

It is important to recognize the total synergybetween the amplitude of the CRI; the cranial bonethickness; the internal stress imposed on bones dueto compression, shear and rotation; sutural mor-phology; the location of cranial membranes (thetentorium and falx); and tensile forces imposed bymembranes and interlocked bones. Each one ofthese factors is dependent on all other factors.Furthermore, all this motion and transfer of tensionand compression happens within the context of thebony skull functioning as a means of protection forthe brain.

The main assumption in this analysis is thatbiological systems inherently self-optimize. From amechanical perspective, this means that the bodyreaches a compromise between weight, strengthand mobility, and that compromise is expressed ineach individual structural component. This is infact a rewording of Still’s aphorism, ‘‘form followsfunction and function follows form’’ (Still, 1899).

Putting this into practice, animals needing amore rigid frontal bone have a partially ossifiedanterior falx. If no motion were necessary, far moreinternal ribbing would be possible, and the mem-branous tentorium and falx would possibly ossify

like the parietals (which are membranous in origin),producing a cranium even more reminiscent of awalnut. The bony design of the cranium is alsorelated to ICP. The cranial ICP of somebody standingis less than atmospheric, with ‘‘neutral’’ ICPoccurring around the level of T1 (Davson and Segal,1996), so the skull has to resist a compressive force.When somebody is lying down or is upside down, ICPexceeds atmospheric, and the cranium has tocontain an expansive fluid force. Other lesssymmetrical stress-induced deformations also haveto be accommodated due to factors such as lyingwith the head on a surface, impacts, forces exertedby muscles such as temporalis or sternocleidomas-toid, and the weight of the cranium on thecondyles. These may be quite dynamic, and mayinclude compression, torsion and shear. All this day-to-day motion and variation in stress/strain pat-terns demands either rigidity (which is difficult toachieve in a biological structure) or built-incompliance.

Tensegrity structures

Tensegrity principles (Ingber, 1998) state that bonestransmit compressive forces, whereas membranestransmit tensile forces; also, that the compressiveand tensile components have a synergistic relation-ship. Hence, the bony skull accommodates externalpressures and internal compressive stresses by‘‘bottoming out’’ on its sutural surfaces. Likewise,the cranial falx and tentorium transmit tensileforces laterally and A–P; the cranial dura accom-modates internal pressures in a manner reminiscentof a balloon, placing an area-equalized expansiveforce on its surrounding and closely attached bonyprotective layer.

Rules of motion

The following basic rules are derived from ananalysis of cranial bone and suture flexibility usingthe principle of ‘‘form follows function and func-tion follows form’’ and tensegrity principles.

A.

The more a bone is subject to stress (torsional,shear, compressive or tensile), the thicker itbecomes. During normal physiological condi-tions, thick bones generally indicate a highstress load.

B.

Wherever bones have to protect vulnerablestructures they become thicker or wrap aroundthe structure in some way.

C.

Vulnerable structures are placed in locations ofminimal relative bone motion.

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D.

In modification of (A), wherever bones have tobend, or wherever they are not subject to largecompressive forces, they become thinner. Thisthinning is subject to a minimum requirementimposed by the protective function of the skull.

E.

Bone shape is also important for strength. Flatsurfaces (large radius of curvature) bend easilyand domed surfaces (small radius of curvature)resist deformation. ‘‘T’’- or ‘‘U’’-shaped sec-tions and solid triangular masses are alsoparticularly resistant to bending. These shapesare traditionally chosen by engineers for theirrigidity.

F.

Bending is also achieved by providing lines ofrelative thinness or even foramena insiderelatively thick or ribbed structures.

G.

Sutures can be similarly analysed to infer howmuch compression, shear or torsion they trans-mit. The thickness of the sutural face is relatedto the force transmitted.

H.

Figure 1 A biomechanical classification of suture types:

Case 1: Direct transfer of compressive forces, e.g. bregma,

Tensile forces from membranes acting internallyon the cranium cause compression of sutures,with some additional rotation, hinging, torsionand shear. The more complex the stressestransferred across the suture, the more complexis the suture design.

SBS, occipito-temporal sutures.

I. Case 2: One bone (usually B is external to A) requires freedomto expand, e.g. lateral parietal expanding away from frontal.Case 3: A sublaps and applies force to B, e.g. parietalexpands temporal squama.Case 4: Sliding surface (e.g. parieto-temporal suture.Case 5: Point of inversion/rotation, e.g. �25mm lateral tothe bregma.

Sutural faces are, on a gross level (ignoringdetailed patterns), perpendicular to the forcespassing through them. The direction of the faceof the suture can be used to determine in whichdirection the bone transfers forces to and fromits neighbours. On this structural basis, fivesimple suture classifications are proposed,shown in Fig. 1.

Not just in the bonesy

The above descriptions might possibly imply thatsutural motions and internal–external force vectorscan be large. This is not the case, as cranial suturesare synarthrotic. Regardless of the direction inwhich stress, strain and motion is transmitted, thesuture and the bony structures on either side areheld firmly—internally by the cranial dura, andexternally by the periosteum—and internal–exter-nal force vectors are likely to be minimized byadaptations of bone flexibility. Suture types 2, 3and 4 do not transmit much (if any) compression,but rather are expressions of hinging and slidingmotions contained by tensile forces within thesurrounding dura and periosteum. Type 5 suturesare zones of least relative motion on a suture, andare often associated with blood vessels crossingbetween cranial bones.

The above principles (A–I) and suture types (1–5)are applied below to the major cranial bones byusing them as guidelines to interpret the flexibilityof a disarticulated skull.

ParietalsEach parietal bone consists of a square plate with acentral dome. They are of fairly consistent thick-ness, with thinning at the inferior anterior corner(adjoining the sphenoid, temporals and frontal).The plain domed surface is quite difficult todeform, except in the case of a straightening ofthe coronal suture, which is accompanied by aslight increased doming (decreased radius ofcurvature) of the sagittal suture. This would implya lateral motion of the lateral anterior corner,which is confirmed by the local type 3 suture. Thisflexing can also be assisted by compressive forceson the bregma and its diagonal corner. Fortuitously,these two locations have type 2 sutures, confirming

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this distribution of forces. The widening of theanterior parietal increases the doming of thesagittal line and reduces the A–P dimension of theparietal. Compressive forces are transmitted A–Palong the line of the sagittal suture between thelambda and the bregma, both of which show type 1sutures.

OcciputThe occiput is much more solid than the parietal,having internal ribbing to anchor the tentorium andfalx. This ribbing is reminiscent of moulded plasticdesigns that have been deliberately stiffened. Thefact that the occiput is inflexible relative to theparietals is reflected in the complexity of theoccipitoparietal interdigitations. The anterior occi-put is even more massive, consisting of thecondyles and basilium. Nevertheless, the occiputcan flex along a single line of increased flexibilitypassing horizontally and laterally through the fora-men magnum, the sigmoid sinus and the condylarand hypoglossal canals, immediately posterior tothe condyles. The occiput will flex along this line(the internal angle between the squama and thebasilium decreases) if the squama is directlysubject to an anterior bending force or if the bowlof the squama is opened, both of which occur whenthe parietals open on Flexion. In practice, thesquama moves anteriorly, since the basilium is heldby the condyles. This Flexion/flexing can onlyhappen, structurally, if the occiput is pulledforwards (by the dura, falx and tentorium) againstthe parietals and temporals. The three type 1suture zones on the occiput are the SBS and lambda(transferring A–P compression in a sagittal plane tobalance tensile forces in the falx) and the occipi-tomastoid surfaces. The tentorium is stretched bylateral expansion of the parietals, and this tensionpulls the occiput forward, compressing the occipi-toparietal sutures, with the effect of expanding theparietals. Simultaneously, the internally directedtension of the tentorium on the parietals serves tolimit this expansion. This reciprocal and synergisticrelationship between tension and compression ischaracteristic of a tensegrity structure.

FrontalThe frontal dome is roughly the same thickness asthe parietal over most of its area, with substantialthickening along the superciliary arch (eyebrows),culminating in a triangular mass of bone on thelateral aspect of the eye socket. This is proximal tothe suture contacting the superior surface of thegreater wings of the sphenoid. This sutural surfaceis internally domed and has a particularly smallradius of curvature. The centrally located glabella

contains the frontal sinus, and consists of a thin,complex, open box section facing inferiorly towardsthe ethmoid. It is reinforced posteriorly by thecrista galli. The supraorbital plates are not onlythin but also domed; they are thickened by a seriesof small lumps directly above the centre of theorbit. The ethmoid notch (leading to the frontalsinus and glabella box section) provides a line ofease of motion to the frontal dome. The fact thatthe inferior segment of the metopic suture remainsopen reflects the proximity of the ‘‘weak’’ line ofthe ethmoid notch. Experientially, the frontalwidens during Flexion. Classical cranial descriptions(Sutherland, 1998; Magoun, 1951) even refer to thefrontal hinging around the metopic suture much asthe parietals hinge around the sagittal suture.When the frontal dome widens laterally, thecoronal line is pulled inferiorly, becoming moreflattened, though less so than the parietals. Theethmoid notch widens and there is a slighttendency for the ethmoid and supraorbital platesto be lifted superiorly, though this direction ofmotion has some ambiguity. All the sutural surfacesof the frontal bone face posteriorly or externally.The posterior-facing suture at the bregma trans-mits an arch of A–P compression along the sagittalline. Posterior tension of the falx on the crista galli(meeting anterior compressive forces at the breg-ma and glabella) is a possible mechanism fordeformation of the frontal bone. This would alsobe confirmed by ossified animal falx structureswere it assumed that cranial stress patterns werefunctionally similar between species.

TemporalThe temporal bone is a very solid triangular ridgeconnected to a very flexible squama, whichaccommodates parietal expansion (Fig. 2). Themechanical relationship of the petrous ridge to theocciput and sphenoid is shown in Fig. 3. Its suturesdo not allow any vertical displacement to occurrelative to either the sphenoid or occiput. How-ever, there is a ridge along the line of the bonyedge (rather than cutting across it) medial to themastoid that interlocks with a groove in theocciput, allowing the temporal to rotate in thislocation relative to the occiput. This is the closestarrangement in the cranium to that of a rotatinggear face; it is necessary due to the solidity of thetruly rotational axis of the petrous ridge. Inaddition to its protective role for the inner ear,this massive ridge provides compressive and tor-sional strength. The interior motion of the tempor-al is somewhat more complex. Medially, it containsthe cerebral artery, the pulsing of which is some-times palpable. Posteriorly, there is a thick and

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Figure 2 Flexible adaptive motion of the parietals and temporals (not to scale, motions exaggerated).

Figure 4 Shape change of anterior sphenoid body boxsection during Flexion (coronal section).

Figure 3 Transfer of compressive forces from occiput tosphenoid via the temporal body.

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stable sutural connection backed up by thick bonein both the occiput and temporal, with little roomfor motion beyond a very slight rocking action thathinges around the suture. The sphenotemporalsuture contains the jugular foramen, which furtherincreases the flexibility of this line. The anteriorcompression from the mastoid is transferred in awedge-like motion by a type 1 suture to the lateralSBS and inferio-posterior edge of the greater wingof the sphenoid.

SphenoidThe sphenoid is a complex composite of manydifferent structural properties, so the followingbiomechanical description is necessarily incom-plete. The solid basilar portion is less than 1 cmlong, and supports the posterior horns of the sellaturcica. The occipitosphenoid basilium is coveredon its superior surface by the basilar venous plexus.

The anterior sphenoid is a thin, pentagonal boxsection surrounding the sphenoid sinus, easilydeformed along its diagonal axis (Fig. 4). Itssuperior lateral angles are thickened, containingthe optic foramina and the roots of the lesserwings. The lesser wings are thin and highly flexible.The inferior lateral aspects of the anterior body arealso thickened, and merge into the roots of thepterygoid and greater wings. The pterygoid wingscomprise a strong channel (‘‘U’’) section that is alsosolidly anchored to the basilium. The greater wingis a complex, anterio-inferiorly domed structurecomprised of the middle cranial fossa and thecavernous sinus, by which it is medially filled. Thesuperior lateral surfaces of the greater wings(which abut the frontal bone inferiorly) aretriangular and ‘‘T’’-shaped in section. The extremetips are thin and flexible. The superior orbitalfissure is extended by a line of accommodation

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passing through the foramen rotundum and pter-ygoid canal. The line of the foramen ovale andforamen spinosum is similarly more flexile. Thesetwo lines work with the compliance of the anteriorbody to give the sphenoid the greatest flexibility ofany bone in the cranium, whilst still allowing it toretain substantial A–P compressive strength. Inpractice, these flexile lines allow the greater wingsto move anteriorly in Flexion. The anterior compo-nent of this motion is restricted by the curvedsurface of the middle fossa, and the tips of thewings move anteriorly and laterally in a twistingmotion. This also moves the medial apex of thetriangular sphenofrontal suture slightly posteriorly.This compound motion parallels the detailed mo-tion of the lower surface of the frontal. As theanterior body and greater wings flex, the sellaturcica is also deformed, and the resultant ‘‘milk-ing’’ action on the pituitary is a well-recognizedphenomenon in CST/CO.

Motion of the faciomaxilliary complex

As the anterior body deforms (Fig. 4), the pterygoidwings move laterally. The angle of the rostrumchanges as the most anterior part of the sphenoidbody flexes whilst the basilar part remains rigid. Thisdeformation naturally results in the classicallydescribed apparent superior rotation of both vomerand ethmoid and the widening of the maxilla (Figs. 4and 5). The simultaneous widening of the anteriorbody also follows the ethmoid notch of the frontal.

Recent medical research on sheep (e.g. Mollanjiet al., 2003; Bozanovic-Sosic et al., 2001) hasidentified that up to 70% of cerebrospinal fluid(CSF) flows out from the craniosacral system (CSS)through the cribriform plate, passing into theethmoid and subsequently into the cervical lym-phatics. This significantly revises the historicalassumption in cranial and medical textbooks that

Figure 5 Shape change of anterior sphenoid body boxsection during Flexion (A–P section), showing contra-rotation of vomer and ethmoid relative to motion ofgreater wings.

all CSF is absorbed by the arachnoid villi. In thislight, the arachnoid villi are secondary drainagepoints, and may also possibly be recategorized asemergency relief valves for the accommodation ofpressure peaks due to blows to the head. Never-theless, there is a lot of variation and ambiguity inmeasurements of CSF production and reabsorption(Davson and Segal, 1996) in terms of the proportionof total flow that may be attributed to each site.The CSS may have considerable flexibility as to theproportion of CSF exiting from each drainage site.These new findings lend importance to the rhyth-mic motion of the cribriform plate and ethmoid asimplied by a CRI.

Discussion

Considering all of the above analysis of howcompression and tension are transferred withinthe cranium, the SBS seems to be a location ofstability around which the more flexible structuresappear to rotate. The phenomenon of ‘‘arcing’’described by Upledger (1990) and the principle of‘‘fulcrums’’ described by Sutherland (1998) andSills (2002, 2004) give concrete examples of this indaily CST/CO practice.

The authors view is that the SBS compressesduring Flexion, and that its function is to transferA–P compressive stress through the cranial base asthe vault moves, and to resist any bending, torsionor shear forces arising from motion of the cranium.This is in marked contrast to the simplified ‘‘SBSlesion’’ approach, which considers the SBS to be alocation of motion.

The tendency for tissues to visibly align in thedirection of stress/strain is seen in the completehierarchy of scales from alignment of actin fibres ata cellular level3 to Wolff lines in bone. Arbuckle(1994, p. 226) described stress fibres in thetentorium and falx, which give some clue as tothe direction of stress they are subjected to. It isinteresting that the basilium has virtually no fibres,again suggesting that there is no motion throughthis area; whereas, the medial aspect of thecavernous sinus has a distinctly fibrous connectionto the margins of the superior orbital fissure, andthere is a very clear lateral band of fibres runningalong the line of the lesser sphenoid wings.

3The importance of this to bodywork theory has beenhighlighted in recent research by Sultan et al. (2004), whichshows how the rheological ‘‘dashpot’’ effect commonly noted incranial work is possibly caused by tensegrity structures in cellsand tissues. Ingber (2003) has described a cellular model whichconnects the mechanical (cellular-level tensegrity) properties ofhuman tissue to a wide range of common diseases.

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Stress, tension and compression patternsthrough sutures

It might be easy to assume from the above thatcranial motion is a complex, inseparable motion ofinterdependent membrane and bone, with noobvious causality. However, an analysis of stressdirection across suture surfaces suggests that thereis a very clear transfer of tension and compressionthrough the cranial system during an F–E cycle.There must always be a tension in some fascialstructures, balanced by a compression in someosseous sutures, forming a tensegrity network.Tension and compression are not in a constantreciprocal relationship between themselves, butrather are subject to a constantly shifting relation-ship within the context of the whole system (e.g.see the description of occipital motion above). Thisconcept of alternating driving forces (instead of awhole-motion effect) is analogous to the local non-equilibrium cycles in dissipative structures, out-lined in Ho (1998).

During Flexion, lateral expansion of the parietalsnaturally pulls the anterior and posterior sectionsof the cranium together, compressing all A–P facingsutures including the SBS. The forward motion ofthe occipital squama caused by tension in the falx istransmitted osseously through the rigid temporalbody onto the SBS and flexible inferior greaterwings of the sphenoid. From the wedging action ofthe temporal, compression is passed both ante-riorly and contralaterally through the sphenoid tocomplete the sagittal A–P compression circle. Thelinking of the tentorium and falx at the straightsinus ensures that any lateral expansion is trans-lated into an A–P tension in the falx, and any A–Ptension causes a relaxation of the tentorium—atruly ‘‘reciprocal tension’’ arrangement.

Consequently, the temporal body literally pushesthe inferior sphenoid anterior and slightly inferior,creating the forward motion of the faciomaxillarycomplex via the pterygoid wings. Of course, theremay also be a circular route of tension that alsopulls the pterygoid wings from deep midlinestructures outside the cranium (Myers, 2001).

Practical implications for treatmentprotocols

Very few of the ideas presented here are new.Perhaps, the defining feature of this model ofcranial motion is that it approaches the concept ofa mobile skull from the perspective of osseousflexibility rather than from that of sutural articula-tion. As it is, the function and shape of the sutures

is far easier to understand once their role asboundaries between mobile and flexible cranialbones is fully appreciated. Similarly, the idea of anon-moving SBS is not a big conceptual step, but ismore a matter of being aware how jargon mightaccidentally imply incorrect mechanisms that maythen come to be taken at face value. This flexiblemodel of cranial motion sits well with the recenttrend towards an emphasis on an adaptive ap-proach rather than fixed technique (Kern, 2001;Sills, 2004). Since the SBS is primarily an immobilecompression joint, there is also virtue in focusingon the SBS as a location of stored energy. Theauthor’s experience is that the SBS naturallybecomes a much clearer perceptual energetic focusonce the more complex model of motion has beenassimilated and the SBS is conceptually decoupledfrom motion of the greater wings. Redefining theSBS as an immobile joint also places it more clearlyas a Zero Balancing ‘‘Foundation Joint’’ as definedin Smith (1989).

It could be argued that there is no need tovisualize anything more complex than simplifiedSBS lesion patterns (e.g. Upledger and Vredevoogd,1983) because the motions necessary to release theanterior sphenoid body are still achieved. However,Sutherland (1998) stated clearly that, ‘‘For theperfection of skill required in cranial diagnosis andtechnique, it is necessary, primarily, to possess aperfect anatomical–physiological mental picture’’.Because the sphenoid is the location of greatestflexibility in the midline structures, placing greatimportance on its freedom of motion is in no waycompromised by this revised model; however, thefunction of sphenoid mobilization techniques mustbe viewed slightly differently.

A final word

Perhaps, the most satisfying aspect of this evolvedflexibility model for cranial motion is that sphenoi-dal motion—palpatory or otherwise—is not anecessary assumption. All that needs to be assumedis that the cranium simply changes shape. Fromthat point, all one needs to do is ask thequestion—How does that happen? Everythingelse—all the commonly accepted cranial bonemotions (with the obvious exception of SBS hin-ging)—come about as a logical conclusion from thefact of a semi-rigid structure accommodatingmotion. As has been stated previously, ossified relicsutures will also act as lines of preferential folding,so life-long retention of membranous sutures is nota necessary prerequisite of cranial motion. This isan unexpected and welcome relief in that it

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removes a lot of hoops and hurdles from the processof technical (medical) justification of CST/COtechniques while answering some of the validquestions raised by Hartman and Norton (2002).From this point, a simple and repeatable demon-stration of cranial dimension change (e.g. as usedby Moskalenko et al., 1999) would constitute a verygood proof of the functionality of the sutures and ofthe potential usefulness of the standard CST/COtechniques.

Conclusions

It is possible to explain the motion of cranial bonesby use of a model that presupposes flexibility ofbones, a mechanical structure based on tensegrityprinciples, and with an overriding assumption thatbiological structures self-optimize. This evolvedmodel applies the principle of ‘‘form followsfunction and function follows form’’ to bonethickness in addition to suture patterns. Itsconclusions—with regard to cranial work—includethe possibility that the SBS is an essentiallycompressive–decompressive joint rather than aprimary source of motion.

This flexible bone model successfully accountsfor recognized palpated patterns of cranial bonemotion in a logically consistent manner andresolves commonly glossed over questions aboutdifferent aspects of cranial motion in the vicinity ofthe SBS. In particular, it fully accounts for theapparent ‘‘quadruple-gear’’ motion of the occi-put–sphenoid–vomer/ethmoid complex. The appar-ent motion of the SBS is probably an arcingphenomenon in the rest of the cranium aroundthe relatively immobile basilium. Palpated sphe-noid motion would therefore be a result ofdeformation of the anterior body and flexing ofthe greater wings of the sphenoid. This mobility ofthe sphenoid is made possible by the presence ofthe sphenoidal sinus and orbital fissures. Much ofthe above analysis is based on simple playing with adisarticulated skull.

Acknowledgements

I am grateful to Mij Ferret for his helpful andapposite (as always!) comments, to James Nortonfor constructive dialogue on this model in its earlystages of development and to Keith Farvis and BillFerguson for their support and for putting it intopractice. It almost goes without saying that my CSTteachers have contributed enormously to this by

providing a structure whilst encouraging me to findmy own way. I would particularly like to thankZhixing Wang, who helped to release the creativeenergy, which produced these ideas.

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