LECTURE
Dynamic carpal stability
Ronald L. Linscheid and James H. Dobyns
Department of Orthopedics, Mayo Clinic, Rochester, MN, USA(Received for publication on June 11, 2002)

Abstract. The term carpal instability is commonly used, but what carpal stability actually is has
not been deﬁned. Much of the mechanically complex wrist’s versatility is due to the intercalated threebone proximal carpal row. Landsmeer described the collapse tendency associated with intercalated
segments. The factors which provide static stability are the oblique alignment of the scaphoid, theobliquely aligned dorsal and palmar ligamentous complexs, the intrinsic perilunate ligaments, thetransiting transcarpal tendons and the negative intraarticular pressure. The proximal carpal row adjusts
its posture on the counterbalancing ﬂexion/pronation torque exerted by the scaphoid and the extension/supination torque exerted by the triquetrum. The dynamic factors are the compressive force
exerted across the joint acting on the joint surfaces and the effect of the bowstringing force provided by
the ﬂexor carpi radialis acting at the scaphoid tuberosity. The proximal carpal row has a tendency totranslate ulnarly along the ulnarly sloped radial articular surface while the distal row has a tendency toslide radially on the radially sloped lunatatotriqueteral distal articular surface. This activity produces
differential tension in the ligaments attaching to the triquetrum which effects an extension/supination
stance of the triquetrum. The force couple acting on the scaphoid effects the ﬂexion tendency of thescaphoid. The bowstringing of the ﬂexor carpi radialis also counteracts scaphoid ﬂexion. Alterations in
any of these factors may upset the delicate mechanical balance of the joint. (Keio J Med 51 (3): 140–
147, September 2002)
Key words: wrist, carpus, kinematics, stability, dissociation

Introduction
The condition of carpal instability with several of its
clinical manifestations was described by Destot over a
century ago.1 Renewed interest in the subject developed more recently when the term was introduced by
Dobyns in 1967.2–5 Carpal instability is at present best
described as a wrist unable to support physiologic loads
have altered kinematics or as is usually the case bothconditions.6 What actually constitutes carpal stability
has not been deﬁned or well described. What is appreciated is the fact that the wrist joint from a mechanicalstandpoint is very complex.7 It allows a wide range of
motion in two major planes and with its adjacent radioulnar joint permits a substantial arc of rotation aroundthe longitudinal axis of the forearm. There is also a
considerable allowance for stress absorption built into
its ligamentous constraints.
Much of the reason for the wrist’s versatility is
the intercalated three bone system that constitutes its

proximal carpal row. This also represents the Achilles’
heel of the wrist because of its tendency to undergoa mechanical collapse when ligamentous or bony constraints are interrupted. Landsmeer8 has described indetail the consequences of a biarticular system that
does not have one or more dynamic stabilizers for theintercalated segment.
From an evolutionary viewpoint minimizing muscle
bulk acrally allows the beneﬁt of the mass moment ofinertia to be located more proximally in the upper limb.
That there are no direct tendinous attachments into theproximal carpal row is therefore not surprising but begs
the question, how is carpal stability maintained.9
Static Stabilizing Factors
The factors that we recognize as contributing to
carpal stability are the shapes of the articular surfaces,
the constraints of the ligamentous systems, the obliquepositioning of the scaphoid lying athwart the midcarpal

Presented at the 1246th Meeting of the Keio Medical Society in Tokyo, February 8, 2002.
Reprint requests to: Dr. Ronald L. Linscheid, Department of Orthopedics, Mayo Clinic, 200 SW First Street, Rochester, MN 55905, USA
140

Keio J Med 2002; 51 (3): 140–147

141

joint, the tendons surrounding the joint on their distalpassages and the other soft-tissue investments.10

ponent is the stronger of the two collagenous components.14,15

Articular surface morphology

Extrinsic (capsular) ligaments: The arrangements of
the ligaments of the wrist and their functions havereceived increasing scrutiny. The trajectories of the
ﬁbers follow oblique courses to the two major planesof motion thus allowing the movements somewhat
analogous to a universal joint. The dorsal capsule contains two major ligamentous inclusions that form a vee
shape. The proximal radio triquetral (RTL) component
arises distal to Lister’s tubercle and passes at an angleof approximately 20 to the lunate fossa to insert on thetriquetrum.16 The distal component, dorsal carpal ligament (DCL), passes over the neck of the capitate toinsert on the dorsodistal aspect of the scaphoid andtrapezium.
The palmar capsule consists of two major ligamentous inclusions.17,18 Both arise from a roughened
area on the palmar aspect of the radial tuberosity. The
radiolunate component, radiolunate ligament (RLL),
is the deeper of the two and passes at an angle ofapproximately 15 to the lunate fossa on to the palmar
pole of the lunate before reassembling as the strong
palmar aspect of the lunato-triquetral interoseous ligament (LTIL). This then intersects into the ulnolunate
and ulnotriquetral ligaments passing to the insertions
into the ulnar fovea and styloidal base (Fig. 1).
The more distal and superﬁcial component is often
referred to as the deltoid ligament. Radially the distal
aspect passes to the palmar ridge of the capitate with apartial insertion to the palmar aspect of the scaphoidwaist. The ulnar reappearance passes from the ulnar
aspect of the capitate ridge to insert on the palmar aspect of the triquetrum and then continues to the ulnotriquetral ligament. The proximal aspect of the deltoid
ligament passes over the capitate head and proceeds tothe thick conﬂuence of the two systems on the triquetrum.18

Radius: The shape and alignment of the distal radialarticular surfaces has changed little in comparison tohominid fossils and modern apes. The scaphoid fossa
is a shallow concavity with a palmar angulation of approximately 10 and an ulnar angulation of 30 . The
lunate fossa is deﬁned by the interfossal ridge radiallyand the attachment of the TFC ulnarly. It has a 12
palmar tilt and an ulnar angulation of 10 . The inclination of these surfaces relative to the axially directed
forces we believe plays a dynamic role in carpal stability.
Proximal carpal row: The radii of curvature of theproximal surfaces of both scaphoid and lunate areslightly less than that of their corresponding fossae on
the radius so that congruency in the joints is limited.
This allows for considerable translation as well as rotation during movement at the radiocarpal joint.11
The midcarpal surface of the scaphoid is angled
ulnarly and palmarly with respect to the head of thecapitate, while the midcarpal surface of the lunate isangled radially with respect to the capitate. The wedge
shape of the lunate in the frontal plane is of interest
when assessing dynamic factors in carpal stability because of the effect on translation of the surfaces. Thereis a distinct hamatal facet for the ulnar aspect of the
lunate in approximately one third of the population.12
The triquetro-hamatal joint has a semihelicoidal articular arrangement which is most congruent with ulnar
deviation and mild extension of the wrist.13
Ligamentous constraints
Intrinsic ligaments: The two primary intrinsic ligaments of the proximal carpal row (PCR) are the scapholunate and lunatotriquetral interosseous ligaments(SLIL and LTIL). These constrain the scaphoid, lunate
and triquetrum to act somewhat in concert under alternating forces acting on their surfaces during the various
motions of the wrist. Both have a C shaped sagittal
contour due to their origin around the convex proximal
perimeters of the bones. The SLIL has a central portion composed of ﬁbrocartilage that merges with thearticular cartilage of either bone. The dorsal and palmar components are collagenous with the former being
shorter, thicker and stronger than the latter. The
movement of the two bones tends to pivot largely about
the dorsal component. The LTIL allows smaller arcs ofmotion between lunate and triquetrum. It also has aﬁbrocartilagenous central aspect, but the palmar com-

Scaphoid alignment
The scaphoid is aligned at an angle of approximately
45 to the proximal and distal carpal rows in the sagittal
plane when the wrist is in a neutral position. We previously described this arrangement of the scaphoid andits stabilizing attachments as analogous to a ‘‘slider
crank mechanism’’ which imparted an oblique stabilizing support across the mid-carpal joint.5 Taleisnik suggested that the scaphoid may rock back and forth over
the radiocapitate ligament (RCL) at its waist and there
is much evidence to suggest that the alternating tension
betwen the proximal and distal elements of the ligament affects scaphoid angulation.17 The Proximal poleof the scaphoid articulating with the scaphoid fossa is

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Linscheid RL and Dobyns JH: Mechanical basis of carpal stability

Fig. 2 Schematic sagittal representation of the radio-scapho-trapezial
complex and the position of the ﬂexor carpi radialis. The cut end of
the radiocapitate ligament is seen beneath the scaphoid waist where it
acts like a fulcrum about which the scaphoid may pivot. The scaphoidinherently seeks to rotate into ﬂexion due to the force couple represented by the dorsal arrow at the radioscaphoid joint and the palmar
arrow at the scaphotrapezial joint. These forces induce a clockwise
torque around the pivot point over the scaphoid waist. The variable
tension within the ﬂexor carpi radialis (FCR) may be divided into
an axial component (cos Q) and a perpendicular component (sin Q).
The latter balances the ﬂexion tendency by supporting the scaphoidtiberosity.

Fig. 1 Exploded schematic representation of the wrist as seen fromabove. The palmar capsular ligaments are represented as seen ﬁrst at
the radius, then beneath the proximal carpal row (PCR) and lastly at
the distal carpal row (DCR). The proximal ligamentous arch is darker
than the distal arch (deltoid ligament). Differential tensions may be
induced in the ligaments by translations occurring between the PCR
and DCR. (lateral arrows). Vertical arrow represents joint compression force.

dorsal to the RCL fulcrum while the distal scaphoid
articulates with the trapezium palmar to the fulcrum.
This arrangement may be considered a ‘‘force couple’’that inherently induces ﬂexion of the scaphoid10 (Fig.
2).
Proximally the scapholunate interosseous ligament(SLIL) provides a pivot point for the scaphoid througha very strong attachment on the dorsal pole of the
lunate.15 This connection inﬂuences the angulation of
the lunate in the sagittal plane. Distally the scaphoidis supported on its palmar aspect by the strong veeshaped palmar scapho-trapezial ligament (STL).19 On
the ulnopalmar aspect of the scaphoid tuberosity there
is also a strong attachment for the tendon sheath of the
ﬂexor carpi radialis (FCR), which supplies a dynamic
stabilizing force.20,21 See below.
Transcarpal tendons and soft tissue investments
The tension in the tendons on either side of the wrist
which are held in close association to the radius by thedorsal and palmar retinacula provide some support tothe capsule as to a les ser extent does the rest of the soft
tissue investment. Subatmospheric intraarticular pressure may also help stabilize the joint.

Dynamic Factors in Carpal Stability
Ulnar translation
The compresive forces acting across the wrist are
well distributed in the neutral position. At the radiocarpal level approximately 80% of the axial joint compressive force is directed onto the scaphoid and lunate
fossae of the radius and 20% onto the ulnocarpal
joint.22 When forces are measured directly in the cadaver radiocarpal joint by means such as Fuji ﬁlm
inserts this distribution of force in the scaphoid fossa
averages about 60% and that of the lunate fossa 40% of
the measured force on the radius.23 More proximally at
the midcarpal joint the scaphoid is considered to transmit approximately 40% and the capititohamate block
60% of the axially directed force.24 This leads to some
confusion because the distribution of compressive
forces follows the bony columns. As the angular position of the wrist changes the force distribution will alsovary.
Forces acting on oblique surfaces may be decomposed into vectors normal to or tangential to the joint
surface. In the frontal plane the compressive force acting on the wedge shaped lunate devolves into an ulnarlydirected tangential force on both its distal and proximal
surfaces which favors ulnar translation, of the lunotriquetral block. The tangential component of force
acting on the capitohamate block however is opposite
in direction and causes the capitate to slide radially andintrude into the scapholunate space.24–30 The proximal
scaphoid trapped between the capitate and scaphoidfossa of the radius is therefore under additional stressnormal to its surfaces.

Keio J Med 2002; 51 (3): 140–147

143

the distal pole of the scaphoid and resisting the innate
tendency of the scaphoid to collapse in ﬂexion underload21 (Fig. 3).
In this connection the action of the FCR is analogous
to that of the posterior tibial tendon at the foot where
its tension protects the integrity of the plantar ‘‘spring’’ligament. The overall action of the transcarpal tendons
acting on the ﬂexed scaphoid induces a palmodorsaltranslatory force on the PCR.
The persistence of the 12 palmar slope of the distal
radial articular surface acts to resist this dorsal translatory tendency of the PCR.
Relevance to Carpal Kinematics
Fig. 3 Top: Frontal schematic wrist joint. Dotted lines represents theinitial condition of the wrist at rest. Solid lines represent the wrist
under increased compressive force. The lunato-triquetral block is displaced ulnarward and the DCR radialward. This reenforces scaphoidcompression., bottom: Ulnar translation of the triquetrum increases
tension in the radio-lunate ligament. Radial translation of the capitate
relaxes the radio-capitate ligament (RCL) and tensions the triquetrocapitate ligamenrt (TCL). left: TCL tension extends and supinates
triquetrum. right: Capitate action on the scaphoid induces ﬂexion/
pronation.

As the capito-hamate block translates radially on the
distal surface of the lunato-triquetral block it has a tendency to reduce tension in the radio capitate ligamentand increase tension in the capito-triquetral ligament.
The former, as it relaxes, allows increasing scaphoid
ﬂexion and the latter induces lunatotriquetral extension. Differential tensioning of the carpal ligaments is
obviously affected by the force applied by the transcarpal tendons. Movements of the wrist also alter theinterligamentous tensions (Fig. 2).
The lunate is very unstable. Its rotational moment
depends to a large extent on the points of contact between the capitate on its distal surface and the lunate
fossa on its proximal surface. The capitate contact tends
to lie slightly below the equator of the lunate when
the wrist is in a neutral position inducing the lunate to
mildly ﬂex relative to the capitate.
Dynamic effect of the FCR
In a series of MRI images taken with the wrist in
neutral ﬂexion, extension and both radial and ulnar
deviation the angle subtended by the bowstringing of
the FCR depending on wrist position varied from 25 to45 as it passed the scaphoid tuberosity. A vector distribution of the tension in the tendon suggests the dorsally directed component accounts for 40% to 70% of
the total tension. By virtue of the bow stringing effecton the scaphoid tuberosity the FCR is capable ofhelping to dynamically stabilize the wrist by supporting

The gross motions of the wrist are a composite of theindividual motions of the eight carpal bones. It is commonly recognized that the scaphoid tends to induce
ﬂexion of the PCR while the triquetrum is responsible
for a counterbalancing extension effect.31 The exact
mechanisms by which these tendencies effect carpal
kinematics is unclear.
The bones of the distal carpal row are articulatedwith ﬂattened congruent surfaces held together by both
short intrinsic and extrinsic ligaments, allowing but afew degrees of motion.10,11,18,32–34 The proximal row
by contrast allows considerable scapholunate movement and modest lunotriquetral motion. In ﬂexion andextension motion (FEM) the angulation at the midcarpal and radiocarpal joints occurs concurrently and ina nearly equiangular fashion. The scaphoid by necessityof its length must angulate further than either row.10,32
In radioulnar deviation (RUD) the intercarpal
motions are more complex. The distal row supinates
several degrees with radial deviation while the proximalrow pronates. The reverse is true on ulnar deviation.35,36 The proximal row also undergoes a conjunctrotation ﬂexing approximately 15–20 on radial deviation (RD) and extending 15–20 on ulnar deviation(UD). It is readily seen that the scaphoid must ﬂex
some 30 to 40 during radial deviation to effectively
shorten its intercarpal length. This rotation is transmitted to the lunate largely through the dorsal aspect of
the SLIL. Additionally increasing tension in both the
PCL and RTL occurs dorsally as the distal scaphoid
ﬂexes and the triquetrum displaces ulnarly. This encourages lunotriquetral ﬂexion as well.
In ulnar deviation (UD) the proximal carpal row
extends. The reason the lunate and triquetrum extend
has been the subject of much debate. The partial helicoid surfaces of the triquetrum and hamate approacheach other in a very congruent match at full UD. Weber
described a low and high position of the hamate as
being responsible for the alternate ﬂexion and extension of the triquetrum.13 He felt displacement of the144

Linscheid RL and Dobyns JH: Mechanical basis of carpal stability

Both major palmar ligament complexes merge andinsert into the triquetrum proximoradially to its articular surface for the pisiform. These insertions may bethought of as the ‘‘triquetral force nucleus’’, an analogy
to the metacarpophalangeal joint ‘‘force nucleus’’ ofZancolli.37 The tension in the TCL is adjusted by the
relative translation of the PCR on the DCR. When the
TCL is tensed it induces an extension/supination moment on the triquetrum.
Fig. 4 The triquetrum is seen from its radial aspect with the TCL
attached to its palmar surface. The cut end would be attached to thepalmar ridge of the capitate. left: The TCL appears slack and the triquetrum is relatively ﬂexed., middle: The TCL under increased tension has slightly extended the triquetrum., right: The TCL is taut andthe triquetrum is extended and supinated.

contact area between the triquetrun and hamate ulnovolarly imparted an extension moment to the triquetrum (low position) during UD whereas the triquetrum
was in a high position during radial deviation (RD).
Using the description of dynamic carpal stability dependent on differential tensioning of the ligamentous
components under the inﬂuence of ulnar translation
cited above, it seems reasonable to offer a further explanation of the conjunct ﬂexion and extension of theproximal row.
Radial deviation consists of simultaneous but independent radial angulation of both the distal and proximal carpal rows. The instantaneous center of rotation(COR) for the composite movement is represented bytightly grouped foci in the neck of the capitate.28,35 The
PCR angulates radially while also ﬂexing and pronating. This is due to the compression of the scaphoid between the scaphoid fossa of the radius and the STT
joint. This movement is transmitted to the lunate andtriquetrum through the SLIL and LTIL. At the sametime the radial angulation of the DCR on the PCR
relaxes the TCL, while dorsally increasing tension in
the RTL. This favors triquetral ﬂexion as well. The
DCR supinates slightly on the PCR.
On ulnar deviation both the distal and proximal
row angulate ulnarly around the COR that lies in thecapitate neck. The radial displacement of the lunate
reduces tension in the RLL; but tension in the RCL,
particularly in its distal aspect, increases as its capitate
insertion moves ulnarly. This also increases the supportof the RCL under the scaphoid waist. At the same time
the tension in the TCL increases as the capito-triquetral
distance increases. The tension in the TCL produces anextension supination moment on the triquetrum. Thisin turn extends the lunate on the head of the capitate
carrying the capitate contact dorsally on the distal facetof the lunate. The tension in the RTL dorsally isrelaxed, so as not to inhibit the motion (Fig. 4).

Relevance to the Common Wrist Instabilities
The positions seen in the common wrist instabilities
give some suggestions as to what dynamic factors may
provide stability in the normal wrist. If one considers
the angulations in the sagittal plane that occur in the
major so called instability problems as dorsiﬂexed
intercalated segment instabilities (DISI) or volarﬂexed
intercalated segment instabilities (VISI) some clues tounderstanding the deformities are provided.
DISI conditions
Scapholunate dissociation (SLD): In SLD lunate extension and scaphoid ﬂexion give rise to the increased
scapholunate angle. This is accompanied by an increased ﬂexion of the capitolunate angle (CLA) which
is seen as a DISI posturing of the wrist. A less recognized dorsal translation of the distal carpal row often
occurs with the angulation. This dorsal translation
pushes the proximal pole of the scaphoid against thedorsal rim of the radius.5,38–40 This dorsal translationof the capitate is likely augmented by the bowstringing
action of the FCR. The capitate contact area shifts
dorsally on the distal surface of the lunate aggravating
extension in the lunato-triquetral block.
A more important factor in inducing lunatotriquetral
extension is probably the increased ulnar translationof the lunato-triquetral block relative to the triquetrocapitate block.17 This may be seen as an extended
scapholunate gap and is due to the release of the SLIL.
The net result of these motions is to increase the tension in the triquetro-capitate ligament (TCL) which inturn induces an extension/supination moment in the
lunatotriquetral segment of the proximal row (Fig. 4).
Scaphoid non-union and malunion: A similar angulardeformity is seen at the capitolunate area in non-unions
and malunions of scaphoid waist fractures. An apex
dorsal angulation at the fracture site and dorsal translation of the distal on the proximal fragment is often
readily observable on tomographic studies. This deformity often progresses with time as the palmar corticesof the fragments erode. The mechanism of deformity is
the same as it is in SLD.5,41–46

Keio J Med 2002; 51 (3): 140–147

Scapho-trapezio-trapezoidal degenerative joint disease (STT-DJD): A third DISI type deformity associated with isolated degenerative arthritis of the scapho
trapezio trapezoidal (STT) joint is less commonly recognized and seems counterintuitive to the expectationthat the scaphoid inherently tends to undergo ﬂexion
when destabilized. In this condition however the scaphoid length is slowly dimished as the articular cartilage
in the STT joint is eroded. The scaphoid increasingly
adopts an extension stance as the STT component of
the scaphoid force couple is diminished. The additional
dynamic factor is the dorsal displacement the bowstringed FCR tendon effects on the scaphoid tuberosity.
Over time this induces lunate extension as well.20,21
VISI conditions
Lunatotriquetral dissociation: If the destabilizationof the scaphoid is responsible for DISI deformities it
seems reasonable that destabilization of the triquetrum
is responsible for VISI deformities. Lunatotriquetraldissociation (LTD) and carpal instability nondissociative (CIND) believed to be on the basis of congenital
ligamentous laxity are the prime examples of this.47
The key injury in the former is the ligamentous failureof the lunato-triquetal interosseous ligament (LTIL)
particularly the strong palmar ﬁbers which are nolonger able to transmit an extension moment from thetriquetrum to the lunate. In the well delineated LTD
the intact scapholunate component of the proximal row
ﬂexes as one while the triquetrum extends and appears
to displace proximally. The triquetro capititate ﬁbers
may be torn as well in those instances when there is aprominent carpal supinatation as well. Prominence of
the ulnar head dorsally is often an associated ﬁnding.
The dorsal radiotriquetral ligament may be responsible
for the proximal migration and extension of the triquetrum.48–53
In the CIND condition a clue to the mechanism of
deformity may be noted in the so-called ‘‘catch-up
clunk’’. This occurs in the transition of the wrist fromulnar to radial deviation or vice-versa. During this motion under the image intensiﬁer the lunate is seen to
hesitate and then instananeously jerk onto the lunate
fossa while ﬂipping from its extended position in ulnar
deviation to the ﬂexed position of radial deviation. This
conjunct rotation normally occurs as a smooth integrated motion rather than an unpleasant catastrophic
jerk. If gentle pressure is applied to the triquetrum
to radially displace the proximal row a more normal
motion may be obtained.54,55 This observation also led
to noting that an ulnar minus status of the distal ulna
may be an etiologic factor in some cases. The laxity of
the RLL may allow the lunate to slip far enough ulnarlythat it edges off the lunate fossa and sinks proximally145

onto the soft TFCC. When the wrist is ulnarly deviating
the lunate must then be lifted over the edge of the
lunate fossa to slide radially. It is at that point that the‘‘catch-up clunk’’ occurs.55,56
Carpal Ulnar translation: This condition emphasizes
the inherent tendency of the carpus to ulnarly translate.
This occurs when the radial ligamentous attachments at
the radial styloid or the capitate are lost. The entire
carpus slides ulnarly down the radial slope producing a
gap in the radioscaphoid fossa and translation of the
lunate onto the triangular ﬁbrocartilage (TFC).57 The
midcarpal joint may collapse into either a VISI or DISI
posture.
Adaptive carpus: One further condition that hasreceived much attention and a much more aggressive
treatment strategy is the carpal deformity seen in radial
malunions and often referred to as the adaptive carpus.58,59 The distal fragment of the radial metaphysis is
dorsally and radially angulated. The greater the angulation the more severe is likely the result. The dorsal
angulation of the radial articular surface induces the
carpus to translate dorsally. To compensate for theradial angulation the wrist is positioned in ulnar deviation. As this occurs primarily at the midcarpal joint
the natural tendency for the lunate and triquetrum toextend occurs and the captitate displaces dorsally on
the lunate. The palmar carpal ligaments become tautholding the deformity in place. This precludes any
effective dynamic restitution beyond osseous realignment.
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