Biomechanics of Wrist Complex

WRIST COMPLEX

Aqeel , PT

WRIST COMPLEX The wrist (carpus) consists of two compound joints: the radiocarpal and the midcarpal joints, referred to collectively as the wrist complex. Each joint proximal to the wrist complex serves to broaden the placement of the hand in space and to increase the degrees of freedom available to the hand. The shoulder serves as a dynamic base of support; the elbow allows the hand to approach or extend away from the body; and the forearm adjusts the approach of the hand to an object. The major contribution of the wrist complex seems to be to control length – tension relationships in the multiarticular hand muscles and to allow fine adjustment of grip. The wrist muscles appear to be designed for balance and control rather than for maximizing torque production. The wrist complex as a whole is considered to be biaxial, with motions of flexion/extension (volar flexion/dorsiflexion) around a coronal axis, and radial deviation/ 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 radial deviation, and 38° to 45° of ulnar deviation. Scientists proposed that the two – joint, rather than single – joint, system of 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.

RadiocarpalJoint

The radiocarpal joint is formed by the radius and radioulnar disc (triangular fibrocartilage complex [TFCC]) proximally and by scaphoid, lunate, and triquetrum distally. The proximal radiocarpal joint surface has a single continuous biconcave curvature that is long and shallow side to side (frontal plane) and shorter and sharper 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 2

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 The TFCC that articulates predominantly with the triquetrum although it also has some contact with the lunate in the neutral wrist The TFCC consists of the radioulnar disc, a connective tissue wedge, and the various fibrous attachments. The disc is a fibrocartilaginous continuation of the articular 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 lower lamina has connections to the sheath of the extensor carpi ulnaris and to the triquetrum, hamate, and the base of the fifth metacarpal via fibres from the ulnar collateral ligament. The so-called meniscus homolog is a region of irregular connective tissue that lies within and is part of the lower lamina. The scaphoid, lunate, and triquetrum compose the proximal carpal row. The proximal carpal row is the distal surface of the radiocarpal joint. The proximal carpal row and ligaments together appear to present a single biconvex joint surface that, unlike a rigid segment, can change shape somewhat to accommodate to the demands of space between the forearm and hand. The pisiform, anatomically part of the proximal row, does not participate in the radiocarpal articulation. The pisiform functions entirely as a sesamoid bone, presumably to increase the moment arm (MA) of the flexor carpi ulnaris that attaches to it. The curvature of the distal radiocarpal joint surface is sharper than the proximal surface in both directions, making the joint somewhat incongruent. Joint incongruence and the angulation of the proximal joint surface result in a greater range of flexion than extension, and in greater ulnar deviation than radial deviation for radiocarpal joint.

Midcarpal Joint:

The midcarpal joint is the articulation between the scaphoid, lunate, and triquetrum 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 its own capsule, as does the radiocarpal joint. However, it is anatomically separate from the radiocarpal joint and has a capsule and synovial lining that is continuous with each intercarpal articulation and may be continuous with some of the carpometacarpal articulations. The midcarpal joint surfaces are complex with an overall reciprocally concaveconvex configuration. 3

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Aqeel , PT

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 small amount of play between them. The union of the distal carpals also results in nearly equal distribution of loads across the scaphoid-trapezium-trapezoid, the scaphoid-capitate, the lunate-capitate, and the triquetrum-hamate articulations. Together the bones of the distal carpal row contribute 2° of freedom to the wrist complex, with varying amounts of radial/ulnar deviation and flexion/extension credited to the joint. The excursions permitted by the articular surfaces of the midcarpal joint generally favor the range of extension over flexion and radial deviation over ulnar deviation – the opposite of what was found for radiocarpal joint. The functional union of the distal carpals with each other and with their contiguous metacarpals not only serve the wrist complex, but also are the foundation 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 among the carpals. The ligaments of the wrist complex are designated as either extrinsic or intrinsic ligaments. The extrinsic ligaments are those that connect the carpals to the radius or ulnar proximally or to the metacarpals distally; the intrinsic ligaments are those that interconnect the carpals themselves and are also known as intercarpal or interosseous ligaments. The intrinsic ligaments lie within the synovial lining and therefore, must rely on synovial fluid for nutrition rather than contiguous vascularized tissues, as do the extrinsic ligaments.

Ö Volar Carpal Ligaments: On the volar surface of the wrist complex, the numerous intrinsic and extrinsic ligaments are variously described by either composite or separate names. The composite ligament known as the volar radiocarpal ligament has been described most commonly as having three distinct bands:  Radioscaphoid  Radiotriquetral  Radiocapitate The composite ulnocarpal ligament arises from the TFCC and has been described as having bands attaching to the lunate which is called ulnolunate and to the capitate either directly called ulnocapitate or indirectly via ulnotriquetral and capitotriquetral ligaments. 4

WRIST COMPLEX

Aqeel , PT

Weak radial and ulnar collateral ligaments are part of the ligamentous complex. However, ulnar collateral is the part of the ill-defined tissues of the TFCC, whereas the radial collateral is considered as an extension of the 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 scaphoid stability and, therefore, stability of the wrist. Injury to this ligament leads to instability of scaphoid. As an intrinsic ligament, it is largely avascular and, therefore, more susceptible to degenerative change and less amenable to surgical repair. Lunotriquetral interosseous ligament maintains the stability between the lunate and triquetrum. Injury to this ligament leads to instability of lunate.

Ö Dorsal Carpal Ligaments: Dorsally, the major wrist ligament is the dorsal radiocarpal ligament. This ligament 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 lunotriquetral interosseous ligament. Scientist has suggested that the obliquity of the volar and dorsal radiocarpal ligaments help offset the sliding of the proximal “carpal condyle” on the inclined radius. Other ligament is dorsal intercarpal ligament that courses horizontally from the triquetrum to the lunate, scaphoid, and trapezium. The two dorsal ligaments together form a horizontal V that contributes to radiocarpal stability.

Movements of the Radiocarpal & Midcarpal Joints:

Motions at the radiocarpal and midcarpal joints are caused by a rather unique combination of active muscular and passive ligamentous and joint reaction forces. Although there are abundant passive forces on the proximal carpal row, no muscular 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 more distal bones. 5

WRIST COMPLEX

Aqeel , PT

The proximal carpals, therefore, are effectively a mechanical link between the radius and the distal carpals and metacarpals to which the muscular forces are actually 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, the middle segment tends to collapse and move in the opposite direction from the segments above and below. Eg – Application of compressive muscular extensor forces across the biarticular wrist complex would cause an unstable proximal carpal row to collapse into flexion while the distal carpal row extended. An intercalated segment requires some type of stabilizing mechanism to normalize combined midcarpal/radiocarpal motion and prevent collapse of the middle segment (the proximal carpal row). The stabilization mechanism appears to involve the scaphoid and its functional and anatomic connections both to the adjacent lunate and to the distal carpal row. The stability of the proximal carpal row depends on the interaction of two opposite tendencies when the carpals are axially loaded (compression across a neutral wrist); scaphoid tends to flex while the lunate and triquetrum tend to extend. These counterrotations within the proximal row are prevented by the ligamentous structures (scapholunate interosseous & lunotriquetral interosseous ligaments). Linking the scaphoid to the lunate and triquetrum will cause the proximal carpals to “collapse synchronously” into flexion and pronation, while the distal carpals move into extension and supination.

Ö Flexion / Extension of the Wrist: During flexion/extension, the scaphoid seems to show the greatest motion of the three proximal carpals while the lunate moves least. The following sequence of events occurs during flexion/extension of the wrist: The motion begins with wrist in full flexion.  Active extension is initiated at the distal carpal row and the attached metacarpals by the wrist extensor muscles attached to those bones.  The distal carpals (capitate, hamate, trapezium, and trapezoid) glide 6

Aqeel , PT on the relatively fixed proximal bones (scaphoid, lunate, and triquetrum). WRIST COMPLEX

 Although the surface configurations of the midcarpal joint are complex, the distal carpal row effectively glides in the same direction as motion of the hand.  When the wrist complex reaches neutral (long axis of the 3rd metacarpal in line with the long axis of the forearm), the ligaments spanning the capitate and scaphoid draw the capitate and scaphoid together into a close-packed position.  Continued extensor force now moves the combined unit of the distal carpal row and the scaphoid on the relatively fixed lunate and triquetrum.  At approximately 45° of hyperextension of the wrist complex, the scapholunate interosseous ligament brings the scaphoid and lunate into close-packed position.  This unites all the carpals and causes them to function as a single 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 reverse sequence.

Ö Radial Deviation / Ulnar Deviation of the Wrist: Radial deviation produces not only deviation of the proximal and distal carpal radially, but simultaneous flexion of the proximal carpals and extension of the distal carpals. Ulnar deviation produces not only deviation of the proximal and distal carpal ulnarly, but also simultaneous extension of the proximal carpals and flexion of the distal carpals. During radial/ulnar deviation the distal carpals, once again, move as a relatively fixed unit, although the magnitude of motion between the bones of the proximal carpal row may differ. The magnitude of scaphoid flexion during radial deviation and extension during ulnar deviation is related to ligamentous laxity. Ligamentous laxity led to less binding of the scaphoid to the distal carpal 7

WRIST COMPLEX

Aqeel , PT

row, therefore, more out-of-plane motion for the scaphoid. In full radial deviation, both the radiocarpal and midcarpal joints are in close-packed position. The ranges of wrist complex radial and ulnar deviation are greatest when the wrist is in neutral flexion/extension. When the wrist is extended and is in close-packed position, the carpals are all 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 extreme extension little radial or ulnar deviation is possible in the fully flexed position. Wrist extension and ulnar deviation was found to be the position of maximum scapholunate contact.

Muscles of the Wrist Complex:

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

Ö 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 as follows:  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 are flexors of the digits with secondary actions at the wrist. All pass under the proximal flexor retinaculum of the wrist, except the palmaris longus and flexor carpi ulnaris. 8

WRIST COMPLEX

Aqeel , PT

The positions of the flexor carpi radialis and flexor carpi ulnaris tendons at the wrist indicate that the tendons can, respectively, radially deviate and ulnarly deviate the wrist as well as flex. However, the flexor carpi radialis does not appear to be effective as a radial deviator of the wrist in an isolated contraction. Its distal attachment on the bases of the second and third metacarpals places it in line with the long axis of the hand. Along with the palmaris longus, the flexor carpi radialis functions as a wrist flexor with little concomitant deviation. The flexor carpi radialis is active during radial deviation. The flexor carpi radialis either augments the strong radial deviating force of the extensor carpi radialis longus or offsets the extension also produced by the extensor carpi radialis longus. The palmaris longus is a wrist flexor without producing either radial or ulnar deviation. The muscle and tendon are absent unilaterally or bilaterally in approximately 14% of people without any apparent strength or functional deficit. The flexor carpi ulnaris attaches to the pisiform, a sesamoid bone that increases the flexor carpi ulnaris moment arm for flexion. Through the pisiform’s ligaments, the flexor carpi ulnaris acts on the hamate and fifth metacarpal, effectively producing flexion and ulnar deviation of the wrist complex. The flexor carpi ulnaris tendon crosses the wrist farther from the axis for wrist radial/ulnar deviation than does the flexor carpi radialis, so it is more effective in its ulnar deviation function than the flexor carpi radialis is in its radial deviation function. The flexor digitorum superficialis, flexor digitorum profundus, and flexor pollicis longus are predominantly flexors of the digits. As multijoint muscles, their capacity to produce an effective wrist flexion force depends on a synergistic stabilizer to prevent full excursion of the more distal joints they cross. If these muscles attempt to act over both the wrist and the more distal joints, they will become actively insufficient. The flexor digitorum superficialis and flexor digitorum profundus show varied activity in wrist radial/ulnar deviation. The flexor digitorum superficialis seems to function more consistently as a wrist flexor than does the profundus. This logical considering the flexor 9

WRIST COMPLEX

Aqeel , PT

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 to contribute to both flexion and radial deviation of the wrist if its more distal joints are stabilized.

Ö Dorsal Wrist Musculature: The tendons of nine muscles cross the dorsum of the wrist complex. Three of the nine muscles are primary wrist muscles, whereas the other six are finger and thumb muscles that may act secondarily on the wrist. These muscles are as 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 is divided into six distinct tunnels by septa. The septa help stabilize the tendons on the dorsum of the hand and allow the muscles to be effective stabilizers of the wrist. The extensor carpi radialis longus and extensor carpi radialis brevis together make up the predominant part of the wrist extensor mass. The extensor carpi radialis brevis is somewhat smaller than the extensor carpi radialis longus, but has a more central location and generally shows more activity during wrist extension activities. Extensor carpi radialis brevis is active during all grasp-and- release hand activities, except those performed in supination. The extensor carpi radialis longus has a smaller moment arm for wrist flexion than does the extensor carpi radialis brevis. The extensor carpi radialis longus shows increased activity when either radial deviation or support against ulnar deviation is required, or when forceful finger flexion motions are performed. The ongoing activity of the extensor carpi radialis brevis makes it vulnerable to overuse and is more likely than the quieter extensor carpi radialis longus to be inflamed in lateral epicondylitis. 10

Aqeel , PT The extensor carpi ulnaris extends and ulnarly deviates the wrist. It is active not only in wrist extension, but also frequently in wrist flexion as well. The extensor carpi ulnaris activity in wrist flexion adds an additional component of stability to the structurally less stable position of wrist flexion. This is not needed on the radial side of the wrist that has more developed ligamentous and skeletal control. The connection of the extensor carpi ulnaris tendon sheath to the TFCC also appears to help tether the extensor carpi ulnaris and prevent loss of excursion efficiency with bow-stringing. The effectiveness of the extensor carpi ulnaris as a wrist extensor is also affected by forearm position. When the forearm is pronated, the crossing of the radius over the ulna causes a reduction in the moment arm of the extensor carpi ulnaris, making it less effective as a wrist extensor. The extensor digiti minimi and the extensor indicis proprius insert into the tendons of the extensor digitorum communis and, therefore, have a common function with the extensor digitorum communis. The extensor indicis proprius and extensor digiti minimi are capable of extending the wrist, but wrist extension is credited more to the extensor digitorum communis. The extensor digitorum communis is a finger extensor muscle but functions also as a wrist extensor without radial or ulnar deviation. The abductor pollicis longus and extensor pollicis brevis are both capable of radially deviating the wrist. A synergistic contraction of the extensor carpi ulnaris may be required to offset the unwanted wrist motion when the abductor pollicis longus and extensor pollicis brevis act on the thumb. When muscles producing ulnar deviators are absent, the thumb extrinsics may produce a significant radial deviation deformity at the wrist. WRIST COMPLEX

Wrist Joint Pathology:

The scaphoid is the most frequently fractured of the carpal bones. Maximum strain of the scaphoid occurs at neutral radial/ulnar deviation and wrist extension, the position of a fall on the outstretched hand. The scaphoid is also involved in the most common carpal instability problem known as scapholunate instability or radial perilunate instability. When injury to one or more ligaments attached to the scaphoid unlinks the lunate from the stabilizing influence of the scaphoid, the lunate and the 11

Aqeel , PT attached triquetrum are left to act as an unconstrained intercalated segment. When ligamentous constraint on the scaphoid is reduced or removed, the scaphoid tends to follow its natural tendency to collapse into flexion on the volarly inclined radius. The flexed scaphoid slides dorsally on the radius and subluxes. Released from scaphoid stabilization, the lunate and triquetrum follow their natural tendency to extend, and the muscular forces applied to the distal carpals 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 pattern of the three segments (the scaphoid, the lunate and triquetrum, and the distal carpal row) is known as dorsal intercalated segmental instability (DISI). The scaphoid subluxation may be dynamic, occurring only with compressive loading of the wrist with muscle forces, or may become fixed or static. With subluxation of the scaphoid, the contact pressures at the radioscaphoid articulation increase because the contact occurs over a smaller area. DISI, therefore, may result over time in degenerative changes at the radioscaphoid joint and then, ultimately, at the other intercarpal joints. With sufficient ligamentous laxity, the capitate may sublux dorsally off the extended lunate and migrate into the gap between the flexed scaphoid and extended lunate. This deformity is called scapholunate-advanced collapse (SLAC). The other form of carpal instability occurs when the ligamentous union of the lunate and triquetrum is disrupted through injury. The lunate and triquetrum together normally tend to move toward extension and offset the tendency of the scaphoid to flex. When the lunate is no longer linked with the triquetrum, the lunate and scaphoid together fall into flexion, and the triquetrum and distal carpal row extend. This ulnar perilunate instability is known as volar intercalated segmental instability (VISI). VISI and DISI illustrate the importance of proximal carpal row stabilization to wrist function and of maintenance of the scaphoid as the bridge between the distal carpal row and the two other bones of the proximal carpal row. WRIST COMPLEX

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