Biomechanics of Wrist Complex

Aqeel , PTWRIST COMPLEX

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WRIST COMPLEXThe wrist (carpus) consists of two compound joints: the radiocarpal and

the midcarpal joints, referred to collectively as the wrist complex. Each joint proximalto the wrist complex serves to broaden the placement of the hand in space and toincrease the degrees of freedom available to the hand. The shoulder serves as adynamic base of support; the elbow allows the hand to approach or extend away fromthe body; and the forearm adjusts the approach of the hand to an object.

The major contribution of the wrist complex seems to be to controllength tension relationships in the multiarticular hand muscles and to allow fineadjustment of grip. The wrist muscles appear to be designed for balance and controlrather than for maximizing torque production.

The wrist complex as a whole is considered to be biaxial, with motions offlexion/extension (volar flexion/dorsiflexion) around a coronal axis, and radialdeviation/ ulnar deviation (abduction/adduction) around an anteroposterior axis.Normal ranges are 78 to 85 of flexion, 60 to 85 of extension, 15 to 21 of radialdeviation, and 38 to 45 of ulnar deviation.

Scientists proposed that the two joint, rather than single joint, systemof the wrist complex:o Permitted large ROMs with less exposed articular surface and tighter

joint capsules.o Had less tendency for structural pinch at extremes of

ranges.o Allowed for flatter multijoint surfaces that are more capable of

withstanding imposed pressures.

RadiocarpalJointThe radiocarpal joint is formed by the radius and radioulnar disc (triangularfibrocartilage complex [TFCC]) proximally and by scaphoid, lunate, andtriquetrum distally.The proximal radiocarpal joint surface has a single continuous biconcavecurvature that is long and shallow side to side (frontal plane) and shorter andsharper anteroposteriorly (sagittal plane).The proximal joint surface is composed of

The lateral radial facet that articulates with the scaphoid The medial radial facet that articulates with the lunate


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The TFCC that articulates predominantly with the triquetrum although italso has some contact with the lunate in the neutral wristThe TFCC consists of the radioulnar disc, a connective tissue wedge, and thevarious fibrous attachments. The disc is a fibrocartilaginous continuation of thearticular cartilage of the distal radius.The disc is connected medially via two dense fibrous connective tissue laminae.The upper lamina attaches to the ulnar head and ulnar styloid; the lowerlamina has connections to the sheath of the extensor carpi ulnaris and to thetriquetrum, hamate, and the base of the fifth metacarpal via fibres from theulnar collateral ligament.The so-called meniscus homolog is a region of irregular connective tissuethat lies within and is part of the lower lamina.The scaphoid, lunate, and triquetrum compose the proximal carpal row. Theproximal carpal row is the distal surface of the radiocarpal joint. The proximalcarpal row and ligaments together appear to present a single biconvex jointsurface that, unlike a rigid segment, can change shape somewhat toaccommodate to the demands of space between the forearm and hand.The pisiform, anatomically part of the proximal row, does not participate in theradiocarpal articulation. The pisiform functions entirely as a sesamoid bone,presumably to increase the moment arm (MA) of the flexor carpi ulnaris thatattaches to it.The curvature of the distal radiocarpal joint surface is sharper than the proximalsurface in both directions, making the joint somewhat incongruent.Joint incongruence and the angulation of the proximal joint surface result ina greater range of flexion than extension, and in greater ulnar deviation thanradial deviation for radiocarpal joint.

Midcarpal Joint:The midcarpal joint is the articulation between the scaphoid, lunate, andtriquetrum proximally and the distal carpal row composed of the trapezium,trapezoid, capitate, and hamate.It does not form a single uninterrupted articular surface, nor does it have itsown capsule, as does the radiocarpal joint. However, it is anatomicallyseparate from the radiocarpal joint and has a capsule and synovial lining that iscontinuous with each intercarpal articulation and may be continuous with someof the carpometacarpal articulations.The midcarpal joint surfaces are complex with an overall reciprocally concave-convex configuration.


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Functionally, the carpals of distal row move as an almost fixed unit.The capitate and hamate are most strongly bound together with, at most, a smallamount of play between them. The union of the distal carpals also results innearly equal distribution of loads across the scaphoid-trapezium-trapezoid, thescaphoid-capitate, the lunate-capitate, and the triquetrum-hamate articulations.Together the bones of the distal carpal row contribute 2 of freedom to thewrist complex, with varying amounts of radial/ulnar deviation andflexion/extension credited to the joint. The excursions permitted by thearticular surfaces of the midcarpal joint generally favor the range ofextension over flexion and radial deviation over ulnar deviation the oppositeof what was found for radiocarpal joint.The functional union of the distal carpals with each other and with theircontiguous metacarpals not only serve the wrist complex, but also are thefoundation for the transverse and longitudinal arches of the hand.

Ligaments of the Wrist Complex:The ligamentous structure of the carpus is responsible not only for

articular stability, but also for guiding and checking motion between and amongthe carpals. The ligaments of the wrist complex are designated as either extrinsic orintrinsic ligaments. The extrinsic ligaments are those that connect the carpals tothe radius or ulnar proximally or to the metacarpals distally; the intrinsicligaments are those that interconnect the carpals themselves and are also known asintercarpal or interosseous ligaments. The intrinsic ligaments lie within thesynovial lining and therefore, must rely on synovial fluid for nutrition rather thancontiguous vascularized tissues, as do the extrinsic ligaments. Volar Carpal Ligaments:

On the volar surface of the wrist complex, the numerous intrinsic andextrinsic ligaments are variously described by either composite or separatenames.The composite ligament known as the volar radiocarpal ligament has beendescribed most commonly as having three distinct bands:

Radioscaphoid Radiotriquetral Radiocapitate

The composite ulnocarpal ligament arises from the TFCC and has beendescribed as having bands attaching to the lunate which is called ulnolunateand to the capitate either directly called ulnocapitate or indirectly viaulnotriquetral and capitotriquetral ligaments.


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Weak radial and ulnar collateral ligaments are part of the ligamentouscomplex. However, ulnar collateral is the part of the ill-defined tissues ofthe TFCC, whereas the radial collateral is considered as an extension ofthe volar radiocarpal ligament and capsule.There are two intrinsic ligaments in the wrist complex, which are as follows:

Scapholunate interosseous ligament Lunotriquetral interosseous ligament

Scapholunate interosseous ligament plays important role in the scaphoidstability and, therefore, stability of the wrist. Injury to this ligament leads toinstability of scaphoid.As an intrinsic ligament, it is largely avascular and, therefore, moresusceptible to degenerative change and less amenable to surgical repair.Lunotriquetral interosseous ligament maintains the stability between thelunate and triquetrum. Injury to this ligament leads to instability of lunate.

Dorsal Carpal Ligaments:Dorsally, the major wrist ligament is the dorsal radiocarpal ligament. Thisligament is obliquely oriented.The ligament as a whole converges on the triquetrum from the distal radius,with possible attachments along the way to the lunate and lunotriquetralinterosseous ligament.Scientist has suggested that the obliquity of the volar and dorsal

radiocarpal ligaments help offset the sliding of the proximal carpalcondyle on the inclined radius.Other ligament is dorsal intercarpal ligament that courses horizontally fromthe triquetrum to the lunate, scaphoid, and trapezium.The two dorsal ligaments together form a horizontal V that contributes toradiocarpal stability.

Movements of the Radiocarpal & Midcarpal Joints:Motions at the radiocarpal and midcarpal joints are caused by a ratherunique combination of active muscular and passive ligamentous and jointreaction forces.Although there are abundant passive forces on the proximal carpal row, nomuscular forces are applied directly to the articular bones of the proximal row,given that the flexor carpi ulnaris applies its force via the pisiform to the moredistal bones.


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The proximal carpals, therefore, are effectively a mechanical link between theradius and the distal carpals and metacarpals to which the muscular forces areactually applied.Scientists suggested that the proximal carpal row is an intercalated segment,a relatively unattached middle segment of the three-segment linkage.When compressive forces are applied across an intercalated segment, themiddle segment tends to collapse and move in the opposite direction from thesegments above and below.Eg Application of compressive muscular extensor forces across the biarticular

wrist complex would cause an unstable proximal carpal row to collapse intoflexion while the distal carpal row extended.

An intercalated segment requires some type of stabilizing mechanism tonormalize combined midcarpal/radiocarpal motion and prevent collapse of themiddle segment (the proximal carpal row).The stabilization mechanism appears to involve the scaphoid and itsfunctional and anatomic connections both to the adjacent lunate and to thedistal carpal row.The stability of the proximal carpal row depends on the interaction of twoopposite tendencies when the carpals are axially loaded (compression across aneutral wrist); scaphoid tends to flex while the lunate and triquetrum tend toextend.These counterrotations within the proximal row are prevented by theligamentous structures (scapholunate interosseous & lunotriquetralinterosseous ligaments).Linking the scaphoid to the lunate and triquetrum will cause the proximalcarpals to collapse synchronously into flexion and pronation, while thedistal carpals move into extension and supination.

Flexion / Extension of the Wrist:During flexion/extension, the scaphoid seems to show the greatest motion ofthe three proximal carpals while the lunate moves least.The following sequence of events occurs during flexion/extension of thewrist:The motion begins with wrist in full flexion.

Active extension is initiated at the distal carpal row and theattached metacarpals by the wrist extensor muscles attached tothose bones.

The distal carpals (capitate, hamate, trapezium, and trapezoid)glide


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on the relatively fixed proximal bones (scaphoid, lunate, andtriquetrum).

Although the surface configurations of the midcarpal joint arecomplex, the distal carpal row effectively glides in the same directionas motion of the hand.

When the wrist complex reaches neutral (long axis of the 3rdmetacarpal in line with the long axis of the forearm), the ligamentsspanning the capitate and scaphoid draw the capitate and scaphoidtogether into a close-packed position.

Continued extensor force now moves the combined unit of thedistal carpal row and the scaphoid on the relatively fixed lunate andtriquetrum.

At approximately 45 of hyperextension of the wrist complex,the scapholunate interosseous ligament brings the scaphoid andlunate into close-packed position.

This unites all the carpals and causes them to function as asingle unit.

Wrist complex extension is completed as the proximal articular surface of the carpals move as a solid unit on the radius

and radioulnar disc. All ligaments become taut as full extension is reached and the

entire wrist complex is close-packed.Wrist motion from full extension to full flexion occurs in the reversesequence.

Radial Deviation / Ulnar Deviation of the Wrist:Radial deviation produces not only deviation of the proximal and distalcarpal radially, but simultaneous flexion of the proximal carpals andextension of the distal carpals.Ulnar deviation produces not only deviation of the proximal and distalcarpal ulnarly, but also simultaneous extension of the proximal carpalsand flexion of the distal carpals.During radial/ulnar deviation the distal carpals, once again, move as arelatively fixed unit, although the magnitude of motion between the bones ofthe proximal carpal row may differ.The magnitude of scaphoid flexion during radial deviation and extensionduring ulnar deviation is related to ligamentous laxity.Ligamentous laxity led to less binding of the scaphoid to the distal carpal


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row, therefore, more out-of-plane motion for the scaphoid.In full radial deviation, both the radiocarpal and midcarpal joints are inclose-packed position.The ranges of wrist complex radial and ulnar deviation are greatest whenthe wrist is in neutral flexion/extension.When the wrist is extended and is in close-packed position, the carpals areall locked and very little radial or ulnar deviation is possible.In wrist flexion the joints are loose-packed and the bones splayed.Further movement of the proximal row cannot occur and, as in extremeextension little radial or ulnar deviation is possible in the fully flexedposition.Wrist extension and ulnar deviation was found to be the position ofmaximum scapholunate contact.

Muscles of the Wrist Complex:The primary role of the muscles of the wrist complex is to provide a stable basefor the hand, while permitting positional adjustments that allow for optimallength-tension in the long finger muscles.The greatest interphalangeal flexor force occurs with ulnar deviation of thewrist (neutral flexion/extension), whereas the least force occurred with wristflexion (neutral deviation).The work capacity (ability of a muscle to generate force per unit of crosssection) of the wrist flexors is more than twice that of the extensors. The workcapacity of the radial deviators slightly exceeds that of the ulnar deviators.

Volar Wrist Musculature:Six muscles have tendons crossing the volar aspect of the wrist and,therefore, are capable of creating a wrist flexion movement. These are asfollows:

Palmaris Longus (PL) Flexor Carpi Radialis (FCR) Flexor Carpi Ulnaris (FCU) Flexor Digitorum Superficialis (FDS) Flexor Digitorum Profundus (FDP) Flexor Pollicis Longus (FPL)

The first three of these muscles are primary wrist muscles. The last three areflexors of the digits with secondary actions at the wrist. All pass under theproximal flexor retinaculum of the wrist, except the palmaris longus andflexor carpi ulnaris.


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The positions of the flexor carpi radialis and flexor carpi ulnaris tendons atthe wrist indicate that the tendons can, respectively, radially deviate andulnarly deviate the wrist as well as flex.However, the flexor carpi radialis does not appear to be effective as aradial deviator of the wrist in an isolated contraction.Its distal attachment on the bases of the second and third metacarpals placesit in line with the long axis of the hand.Along with the palmaris longus, the flexor carpi radialis functions as a wristflexor with little concomitant deviation.The flexor carpi radialis is active during radial deviation. The flexor carpiradialis either augments the strong radial deviating force of the extensorcarpi radialis longus or offsets the extension also produced by the extensorcarpi radialis longus.The palmaris longus is a wrist flexor without producing either radial orulnar deviation. The muscle and tendon are absent unilaterally or bilaterallyin approximately 14% of people without any apparent strength or functionaldeficit.The flexor carpi ulnaris attaches to the pisiform, a sesamoid bone thatincreases the flexor carpi ulnaris moment arm for flexion.Through the pisiforms ligaments, the flexor carpi ulnaris acts on thehamate and fifth metacarpal, effectively producing flexion and ulnardeviation of the wrist complex.The flexor carpi ulnaris tendon crosses the wrist farther from the axis forwrist radial/ulnar deviation than does the flexor carpi radialis, so it is moreeffective in its ulnar deviation function than the flexor carpi radialis is inits radial deviation function.The flexor digitorum superficialis, flexor digitorum profundus, and flexorpollicis longus are predominantly flexors of the digits.As multijoint muscles, their capacity to produce an effective wrist flexionforce depends on a synergistic stabilizer to prevent full excursion of themore distal joints they cross.If these muscles attempt to act over both the wrist and the more distaljoints, they will become actively insufficient.The flexor digitorum superficialis and flexor digitorum profundus showvaried activity in wrist radial/ulnar deviation.The flexor digitorum superficialis seems to function more consistently as awrist flexor than does the profundus. This logical considering the flexor


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digitorum profundus is a longer, deeper muscle, crosses more joints and is,therefore, more likely to become actively insufficient.The position of the tendon of flexor pollicis longus suggests the ability tocontribute to both flexion and radial deviation of the wrist if its more distaljoints are stabilized.

Dorsal Wrist Musculature:The tendons of nine muscles cross the dorsum of the wrist complex. Three ofthe nine muscles are primary wrist muscles, whereas the other six are fingerand thumb muscles that may act secondarily on the wrist. These muscles areas follows: Extensor Carpi Radialis Longus (ECRL) Extensor Carpi Radialis Brevis (ECRB) Extensor Carpi Ulnaris (ECU) Extensor Digitorum Communis (EDC) Extensor Indicis Proprius (EIP) Extensor Digiti Minimi (EDM) Extensor Pollicis Longus (EPL) Extensor Pollicis Brevis (EPB) Abductor Pollicis Longus (APL)The tendons of all nine muscles pass under the extensor retinaculum that isdivided into six distinct tunnels by septa. The septa help stabilize the tendonson the dorsum of the hand and allow the muscles to be effective stabilizers ofthe wrist.The extensor carpi radialis longus and extensor carpi radialis brevistogether make up the predominant part of the wrist extensor mass.The extensor carpi radialis brevis is somewhat smaller than the extensorcarpi radialis longus, but has a more central location and generally showsmore activity during wrist extension activities.Extensor carpi radialis brevis is active during all grasp-and- release handactivities, except those performed in supination.The extensor carpi radialis longus has a smaller moment arm for wristflexion than does the extensor carpi radialis brevis.The extensor carpi radialis longus shows increased activity when eitherradial deviation or support against ulnar deviation is required, or whenforceful finger flexion motions are performed.The ongoing activity of the extensor carpi radialis brevis makes itvulnerable to overuse and is more likely than the quieter extensor carpiradialis longus to be inflamed in lateral epicondylitis.


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The extensor carpi ulnaris extends and ulnarly deviates the wrist. It isactive not only in wrist extension, but also frequently in wrist flexion aswell.The extensor carpi ulnaris activity in wrist flexion adds an additionalcomponent of stability to the structurally less stable position of wristflexion. This is not needed on the radial side of the wrist that has moredeveloped ligamentous and skeletal control.The connection of the extensor carpi ulnaris tendon sheath to the TFCC alsoappears to help tether the extensor carpi ulnaris and prevent loss ofexcursion efficiency with bow-stringing.The effectiveness of the extensor carpi ulnaris as a wrist extensor is alsoaffected by forearm position. When the forearm is pronated, the crossing ofthe radius over the ulna causes a reduction in the moment arm of the extensorcarpi ulnaris, making it less effective as a wrist extensor.The extensor digiti minimi and the extensor indicis proprius insert into thetendons of the extensor digitorum communis and, therefore, have a commonfunction with the extensor digitorum communis.The extensor indicis proprius and extensor digiti minimi are capable ofextending the wrist, but wrist extension is credited more to the extensordigitorum communis.The extensor digitorum communis is a finger extensor muscle but functionsalso as a wrist extensor without radial or ulnar deviation.The abductor pollicis longus and extensor pollicis brevis are both capableof radially deviating the wrist.A synergistic contraction of the extensor carpi ulnaris may be required tooffset the unwanted wrist motion when the abductor pollicis longus andextensor pollicis brevis act on the thumb.When muscles producing ulnar deviators are absent, the thumb extrinsicsmay produce a significant radial deviation deformity at the wrist.

Wrist Joint Pathology:The scaphoid is the most frequently fractured of the carpal bones. Maximumstrain of the scaphoid occurs at neutral radial/ulnar deviation and wristextension, the position of a fall on the outstretched hand.The scaphoid is also involved in the most common carpal instability problemknown as scapholunate instability or radial perilunate instability.When injury to one or more ligaments attached to the scaphoid unlinks thelunate from the stabilizing influence of the scaphoid, the lunate and the


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attached triquetrum are left to act as an unconstrained intercalated segment.When ligamentous constraint on the scaphoid is reduced or removed, thescaphoid tends to follow its natural tendency to collapse into flexion on thevolarly inclined radius. The flexed scaphoid slides dorsally on the radius andsubluxes.Released from scaphoid stabilization, the lunate and triquetrum follow theirnatural tendency to extend, and the muscular forces applied to the distalcarpals cause them to flex on the extended lunate and triquetrum.The flexed distal carpals glide dorsally on the lunate and triquetrum,accentuating the extension of the lunate and triquetrum. This zigzag patternof the three segments (the scaphoid, the lunate and triquetrum, and the distalcarpal row) is known as dorsal intercalated segmental instability (DISI).The scaphoid subluxation may be dynamic, occurring only with compressiveloading of the wrist with muscle forces, or may become fixed or static.With subluxation of the scaphoid, the contact pressures at the radioscaphoidarticulation increase because the contact occurs over a smaller area.DISI, therefore, may result over time in degenerative changes at theradioscaphoid joint and then, ultimately, at the other intercarpal joints.With sufficient ligamentous laxity, the capitate may sublux dorsally off theextended lunate and migrate into the gap between the flexed scaphoid andextended lunate. This deformity is called scapholunate-advanced collapse(SLAC).The other form of carpal instability occurs when the ligamentous union of thelunate and triquetrum is disrupted through injury.The lunate and triquetrum together normally tend to move toward extension andoffset the tendency of the scaphoid to flex.When the lunate is no longer linked with the triquetrum, the lunate andscaphoid together fall into flexion, and the triquetrum and distal carpal rowextend. This ulnar perilunate instability is known as volar intercalatedsegmental instability (VISI).VISI and DISI illustrate the importance of proximal carpal row stabilization towrist function and of maintenance of the scaphoid as the bridge between thedistal carpal row and the two other bones of the proximal carpal row.

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Biomechanics of Wrist Complex