Joints of the Wrist: Anatomy and Biomechanics

Joints of the Wrist

• The joints are the distal radioulnar, radiocarpal, midcarpal, and intercarpal joints.
• The wrist complex adjusts the length-tension relationships of hand muscles, allowing for fine grip adjustments.
• Structure and biomechanics can vary significantly.

Terminology

• Radial deviation equals abduction.
• Ulnar deviation equals adduction.
• Ventral/Volar/Palmar describes the anterior side.
• Dorsal (dorsum) refers to the posterior side.
• Medial is equivalent to ulnar.
• Lateral is radial.

Osteologic Features

• The radius features an ulnar tilt where its distal end angles ~25° toward ulnar (medial).
• The ulnar tilt enables greater ulnar deviation (UD) than radial deviation (RD).
• Radial deviation is limited by bony contact and the lateral carpals contacting the radial styloid.
• The radius also has palmar tilt and its distal surface angles ~10° in the palmar direction.
• Palmar tilt accounts for greater wrist flexion than extension.
• Fractures to the distal radius may impact the ulnar and/or palmar tilt affecting the DRUJ or radiocarpal joint function.
• The lateral (radial) aspect of the radius is longer than the ulnar aspect.
• Dorsal aspect of the radius is slightly longer than the volar aspect.

Triangular Fibrocartilage Complex (TFCC)

• The components of this complex are the articular disc, distal RU joint capsular ligaments, palmar ulnocarpal ligament, ulnar collateral ligament and the fascial sheath enclosing the ECU tendon.
• The distal RU joint capsular ligaments are the dorsal and volar RU ligaments.
• The Palmar ulnocarpal ligament is the ulnotriquetral and ulnolunate.
• The TFCC functions as an extension of the radius at the wrist.

Carpal Bones

• There are two rows, a proximal and a distal.
• The proximal row bones are the scaphoid, lunate, triquetrum, and pisiform.
• The distal row bones are the trapezium, trapezoid, capitate, and hamate.
• The distal row, bound tightly by strong ligaments, provides a stable base for articulation with the metacarpals
• The proximal row bones are joined relatively loosely.

Wrist Complex

• At the Radiocarpal (RC) joint, the radius and radioulnar disc articulate with the scaphoid, lunate, & triquetrum.
• At the Midcarpal (MC) joint, the scaphoid, lunate, & triquetrum articulate with the trapezium, trapezoid, capitate, & hamate.

Radiocarpal Joint

• The joint is formed by concave surface of distal radius, proximal carpal row, and RU disc of TFCC.
• The lunate and triquetrum articulate with TFCC.
• It is a Biaxial joint allowing for flexion and extension, and radial and ulnar deviation.
• The open packed position is in neutral and slight UD.
• The close-packed position is in full extension with RD.
• The proximal carpal bones interconnect by the scapholunate and lunotriquetral interosseous ligaments.
• The proximal carpal row & ligaments function together as single biconvex cartilage-covered joint surface.
• The pisiform is anatomically part of the proximal row, but not part of RC joint, it functions as sesamoid bone increasing the MA of the FCU tendon.
• Average ROM for wrist flexion is 80°, extension is 70°, radial deviation is 20° and ulnar deviation is 30°.
• With axial loading scaphoid & lunate receive ~ 80% of the load and the TFCC receives ~ 20% of the load.

Ulnar Variance

• The length of the ulna in relation to the radius impacts wrist function.
• Ulnar variance refers to the relative lengths of distal articular surfaces of radius & ulna.
• Neutral ulnar variance (ulnar zero) is when the difference between ulnar and radial length is 0 - 1mm
• Ulnar negative variance (negative ulnar variance) is a short ulna when compared with the radius at the distal end.
• Ulnar positive variance (positive ulnar variance) occurs when the distal ulna is long in relation to the distal radius.
• Ulnar variance impacts distribution of compressive loads at wrist
• With neutral ulnar variance 80% of loading on the radius and 20% on the ulna
• Positive ulnar variance decreases radial load and increases ulnar load Negative ulnar variance increases radial load and decreases ulnar load
• Factors that can alter ulnar variance: congenital, post fracture, DRUJ injuries, or growth plate disturbance
• Ulnar Variance is associated with changes in TFCC thickness
• Thicker TFCC is associated with negative ulnar variance
• Thinner TFCC is associated with positive ulnar variance
• A small amount of ulnar movement occurs with open chain RU joint motion.
• The ulnar head moves distally and dorsally with pronation.
• The ulnar head moves proximally and volarly with supination.
• Compressive loading when the forearm is pronated may cause greater stress through the TFCC in those with + UV

Radiocarpal Capsule and Ligaments

• This joint is a somewhat loose and strong joint capsule.
• The joint is reinforced by capsular & intercapsular ligaments.
• Most ligaments crossing the RC joint contribute to stability at the midcarpal joint.
• RC joint has no muscles that act on this joint alone
• The FCU is the only muscle attaching to any of the bones of proximal carpal row.
• Forces acting on pisiform by the FCU are not translated to triquetrum, but transfer to hamate & 5th metacarpal via ligaments.
• RC motions result from forces applied by ligaments & muscles attached to distal carpal row & metacarpals

Midcarpal Joint

• This joint is formed proximally by the scaphoid, lunate, & triquetrum and distally by the trapezium and trapezoid.
• Fibrous capsule & synovial lining is continuous with each intercarpal articulation ,and may be continuous with some carpometacarpal CMC joints.
• Joint surfaces are complex Overall, reciprocally concave-convex configuration.
• The distal row moves functionally as a unit.
• The open-packed position is neutral or slight flexion with UD.
• The close-packed position is full extension with UD.

Carpal Bones

• The Scaphoid forms synovial joints with four other carpals and the radius.
• Its proximal pole articulates with scaphoid facet of radius.
• The distal pole, with slightly rounded surface, articulates with the trapezium and trapezoid projecting obliquely in palmar direction about 30°.
• The distal-medial surface is deeply concave and articulates with the lateral half of head of capitate
• A small medial facet articulates with the lunate.
• The Lunate is the central bone of the proximal row.
• It is wedged b/w scaphoid & triquetrum, making it an inherently unstable carpal bone lacking muscle attachments & strong ligamentous support
• The proximal surface articulates with radius, the distal is deeply concave and articulates with the medial ½ of head of capitate and part of hamate.
• The Triquetrum is the most ulnarly positioned carpal that is palpable and distally located to the ulnar styloid.
• The lateral surface articulates with hamate and the facet on palmar surface articulates with pisiform.

Pisiform

• The pisiform articulates loosely with the triquetrum which makes it easily movable and palpable as well as embedded in the FCU tendon
• The Capitate is the largest carpal bone located centrally in the wrist.
• Its head is the proximal surface and articulates with the scaphoid and lunate
• It is stabilized between the trapezoid and hamate, and the distal surface is rigidly joined to base of 3rd metacarpal allowing the capitate and 3rd metacarpal to function as single column giving longitudinal stability to the wrist and hand
• The Axis of rotation for all wrist motions passes through the capitate.

Trapezium

• The Trapezium has a proximal surface that is slightly concave & articulates with the scaphoid.
• Its distal surface is saddle-shaped and articulates with base of 1st metacarpal forming the 1st carpometacarpal CMC joint (saddle joint allowing wide ROM for thumb motion).
• The tubercle on palmar surface is attachment site for transverse carpal ligament and the medial to Palmer tubercle is groove for the FCR tendon.
• The Trapezoid is a small bone wedged b/w capitate & trapezium.
• It has a slightly concave proximal surface that articulates with scaphoid and a relatively firm articulation with the base of 2nd metacarpal

Hamate

• The Hamate has hook projections from the palmar surface.
• Hook of the hamate is attachment site for transverse carpal ligament.
• The base (distal surface) articulates with the 4th & 5th metacarpals providing mobility mobility to ulnar aspect of hand (esp with cupping of hand)
• The apex (proximal surface) contacts the lunate and wedges between capitate.

Overall Ligamentous Support of Wrist

• Ligamentous structures are responsible for articular stability and for guiding and checking motion between and among the carpals.
• Extrinsic ligaments connect carpals to radius or ulna proximally OR carpals to metacarpals distally and provide majority of wrist stability.
• Intrinsic ligaments attach carpal bones to other carpal bones (Aka interosseus or intercarpal ligaments), are largely avascular and serve as rotational restraints of carpals

Volar Ligaments

• The volar ligaments are generally stressed with wrist extension.
• There are three bands to the Volar radiocarpal ligament: Radioscaphocapitate (radiocapitate); Short & long radiolunate (radiolunotriquetral); Radioscapholunate,.
• The Radial collateral ligament is Extension of volar radiocarpal ligament & capsule:. Ulnocarpal ligament complex of TFCC & associated ligaments.
• The Scapholunate ligament maintains scaphoid stability
• The Lunotriquetral ligament maintains stability b/w lunate & triquetru

Dorsal Ligaments

• The dorsal ligaments are stressed/taut with wrist flexion
• The Dorsal radiocarpal is an Obliquely oriented ligament and converges on triquetrum from distal radius.
• This may have possible attachments to lunate & lunotriquetral ligament
• The Dorsal intercarpal ligament runs horizontally from triquetrum to the lunate, scaphoid, & trapezium forming a horizontal V shape contributing to stability as well stabilizing the scaphoid during wrist ROM.

Arthrokinematics

• Synchronous convex-on-concave rotation occurs at radiocarpal & midcarpal joints.
• During Wrist extension at the radiocarpal joint, the convex surface of lunate rolls dorsally on radius & slides palmarly and the roll directs lunates distal surface dorsally, toward direction of extension
• During wrist extension at the midcarpal joint the head of capitate rolls dorsally on lunate and slides in a palmar direction
• During the radiocarpal joint, the convex surface of the lunate rolls palmarly. With Wrist Flexion and slides dorsally. The lunate's distal surface is directed palmarly.
• During Wrist Flexion at the the Midcarpal joint head of the capitate rolls palmarly on the lunate and slides dorsally..

Joint Stability and ROM

• During full wrist extension the palmar RC ligaments & muscles are lengthened helping to stabilize wrist in its close-packed position by having stability in full wrist extension which is functionally important for activities that require loading the wrist/hand

• During full wrist flexion the dorsal RC ligaments & muscles lengthened are lengthened.

• During the radiocarpal joint in which the scaphoid, and lunate and triquetrum roll at Midcarpal joint rolls in the unar direction and slide redially and the the midcarpal joint the Capitate rolls slightly redially

• Radial deviation

• Radiocarpal joint

  • Scaphoid, lunate, and triquetrum roll in a radial direction and slide ulnarly.
  • Limited RD at this jt due to carpal impingement with radial styloid majority of RD occurs at midcarpal jt.
  • Midcarpal joint
  • Capitate rolls in a radial direction and slides ulnarly.

Loading Through Hand/Wrist

• Compressive force through the hand is transmitted primarily through the wrist
• Compressive forces pass through RC joint to radius
• Central band of the interosseous membrane is pulled taut transferring a large portion of force to the ulna
• Compressive forces cross through HU & HR joints
• Forces are transmitted up humerus to shoulder

Musculature of the wrist

• Designed for balance & control rather than for maximizing torque
• Allows for adjustments to length-tension relationship of extrinsic hand muscles
• Wrist extensors function largely to position hand for gripping activities
• FDP & FDS have significant MA as wrist flexors, wrist extensors must counterbalance this torque during grip activities
• For optimal grip, wrist position is 30-35° extension, which is the position of optimal length-tension relationship of finger flexors

Musculature of the Wrist: Extensors

• Prime movers: the Extensor carpi radialis longus (ECRL), Extensor carpi radialis brevis (ECRB) and the Extensor carpi ulnaris (ECU)
• Secondary Movers: the Extensor digitorum (ED), Extensor indicis (EI) and the Extensor digiti minimi (EDM) and Extensor pollicis longus (EPL)

Extensor Compartments of the Wrist

• First, Extensor pollicis brevis & abductor pollicis longus
• Second, Extensor carpi radialis longus & extensor carpi radialis brevis
• Third, Extensor pollicis longus
• Fourth, Extensor digitorum & extensor indicis
• Fifth, Extensor digit minimi
• Sixth, Extensor carpi ulnaris
• Retinaculum prevents tendons from "bowstringing" during wrist movements.

Musculature of the Wrist: Flexors

• Primary Movers: the Flexor carpi radialis (FCR), Flexor carpi ulnaris (FCU) and the Palmaris longus (if present)
• Secondary Movers: the Flexor digitorum profundus (FDP), Flexor digitorum superficialis (FDP), Flexor pollicis longus (FPL) and the Abductor pollicis longus (APL) when wrist is in neutral position
• Wrist flexors can produce ~ 70% > isometric torque than wrist extensors. Is related to greater total CSA of wrist flexor muscles peak wrist flexion torque occurs at ~ 40° flexion
• the FCU has greatest wrist flexion torque potential based on MA due to FCU & CSA
• During active wrist flexion the FCR & FCU act together as synergists to oppose each other's RD and UD functions.

Radial Deviators

• Radial deviator muscles Generate ~15% >isometric torque than ulnar deviators. Active wrist extension is typically coupled with RD.
• In neutral wrist position: the ECRL & APL have largest product of CSA & MA for RD torque
• the EPB that has greatest MA of all radial deviators, however Small CSA results in relatively small force and torque production
• APL & EPB provide stability to radial side of the wrist

Muscles In Radial and Ulnar Deviation

• Extensor carpi radialis longus, Extensor carpi radialis brevis, Extensor pollicis longus, Extensor pollicis brevis,Flexor carpi radialis and Abductor pollicis longus are the muscles In Radial Deviation
• Extensor carpi ulnaris, Flexor carpi ulnaris, Flexor digitorum profundus, Flexor digitorum superficialis and Extensor digitorum are the muscles In Ulnar Deviation

Flexor Retinaculum: Transverse Carpal Ligament

• The Flexor Retinaculam functions for the attachment site for thenar & hypothenar muscles , helps maintain transverse carpal arch and acts as restraint against bowstringing of extrinsic flexor tendons and protects Median nerve
• Spans region between pisiform, hamate, scaphoid, & trapezium creating the carpal tunnel, has Proximal attachments at the tubercle of scaphoid & pisiform, has Distal attachments at the hook of the hamate & tubercle of trapezium
• Structures deep to retinaculum are Flexor digitorum superficialis (FDS) tendons , Flexor digitorum profundus (FDP) tendons, Flexor pollicis longus (FPL) is found here, and Flexor carpi radialis (FCR) and the Median nerve

Other Ligaments

• Dense fibrous is structure found just deep to subcutaneous tissue Continuous with palmaris longus tendon & fascia covering thenar & hypothenar muscles attaches distally to transverse metacarpal ligaments & flexor tendon sheaths and provide some protection to ulnar artery& nerve, digital vessels & nerves and splits in to
• Four slips As it travels towards fingers
• Functions are passageway for ulnar nerve & artery into hand is Located superficial to flexor retinaculum, Between hook of the hamate & pisiform
• Its roof is by Palmar carpal ligament, Palmaris brevis muscle and by
• Palmar aponeurosis, its Floor it by Transverse carpal ligament, Pisohamate ligament and by Pisometacarpal ligament