Wrist Biomechanics PDF

Dr. Bassem Khalifa Assistant Professor of Orthopedic Physical Therapy Wrist Biomechanics WRIST JOINT COMPLEX Axes and motions Review: Bones of the wrist Arthrokinematics Muscles that move the wrist Examples of muscle syne...

Dr. Bassem Khalifa Assistant Professor of Orthopedic Physical Therapy Wrist Biomechanics WRIST JOINT COMPLEX Axes and motions Review: Bones of the wrist Arthrokinematics Muscles that move the wrist Examples of muscle synergies in wrist function Carpal tunnel Axes and motions Close-packed Joint Axis Motion position Wrist lateral flexion / extension radio- carpal extension ulnar and radial A-P mid- deviation carpal Even though flexion and extension occur at both of the wrist's articulations,  most wrist extension occurs around the midcarpal joint's lateral axis. most wrist flexion occurs around the radiocarpal joint's lateral axis. Ulnar and radial deviation occur around an axis that passes through the capitate. Two wrist creases on the hand's palmar (or volar) surface are landmarks for the locations of the radiocarpal and midcarpal joints. Wrist arthrokinematics In open chain movement, the convex surfaces of the scaphoid and lunate move on the concave surfaces of the radius and ulna. during flexion: scaphoid/lunate roll anteriorly (toward palm) and glide posteriorly(toward dorsum) during extension: scaphoid/lunate roll posteriorly(toward dorsum) and glide anteriorly(toward palm). During ulnar deviation: scaphoid/lunate roll toward ulna and glide toward radius. During radial deviation: scaphoid/lunate roll toward radius and glide toward ulna. Muscles that move the wrist To predict a muscle's action, you must know: the joint(s) that the muscle crosses the axis/axes of each of those joints the muscle's line of application (LOA) local to each of the axes.  Extensors have LOA dorsal/posterior to wrist's lateral axis. Flexors have LOA ventral/anterior to wrist's lateral axis. Radial deviators have LOA on the radial side of the wrist's AP axis. Ulnar deviators have LOA on the ulnar side of the wrist's AP axis. Muscles of the wrist and hand I. Carpal muscles act at the wrist and, in some cases, the elbow. EXTENSOR CARPI RADIALIS LONGUS EXTENSOR CARPI RADIALIS BREVIS EXTENSOR CARPI ULNARIS FLEXOR CARPI RADIALIS FLEXOR CARPI ULNARIS PALMARIS LONGUS II. Extrinsic hand muscles act on the wrist and the digits Extrinsic hand mm that act on the second through fourth digits: EXTENSOR DIGITORUM EXTENSOR INDICIS (PROPRIUS) EXTENSOR DIGITI MINIMI (PROPRIUS) FLEXOR DIGITORUM SUPERFICIALIS FLEXOR DIGITORUM PROFUNDUS Extrinsic thumb muscles that act on the first digit: FLEXOR POLLICIS LONGUS EXTENSOR POLLICIS LONGUS EXTENSOR POLLICIS BREVIS ABDUCTOR POLLICIS LONGUS III. Intrinsic hand muscles act only the digits THENAR MUSCLES and ADDUCTOR POLLICIS HYPOTHENAR MUSCLES LUMBRICALES DORSAL INTEROSSEI PALMAR INTEROSSEI Muscles that act only at the wrist (and elbow):  1. EXTENSOR CARPI RADIALIS LONGUS 2. EXTENSOR CARPI RADIALIS BREVIS The muscles' names 3. EXTENSOR CARPI ULNARIS reflect their actions on 4. FLEXOR CARPI RADIALIS the carpus (wrist). 5. FLEXOR CARPI ULNARIS 6. PALMARIS LONGUS The only muscle in this group whose name does not suggest its action is palmaris longus. Although palmaris longus flexes the wrist, its line of application passes directly over the wrist's A-P axis, so that it possesses no moment arm to produce motion, either radial or ulnar deviation, in the frontal plane. Extrinsic hand muscles The muscles' names reflect their actions on the digits. However, each one crosses the wrist and produces movement in the sagittal plane around the wrist's lateral axis, and in the frontal plane around its A-P axis. EXTENSOR DIGITORUM (ED) FLEXOR POLLICIS LONGUS EXTENSOR INDICIS (EI) (FPL) EXTENSOR DIGITI MINIMI (EDM) EXTENSOR POLLICIS FLEXOR DIGITORUM LONGUS (EPL) SUPERFICIALIS (FDS) EXTENSOR POLLICIS FLEXOR DIGITORUM PROFUNDUS BREVIS (EPB) (FDP) ABDUCTOR POLLICIS LONGUS (APB) Effect of the extrinsic hand muscles on wrist flexion/extension Extrinsics to 2nd - 5th digits: Extrinsics to thumb Wrist extensors Wrist flexors Wrist extensors Wrist flexors* EDC FPL FDS EI EPL EPB FDP EDM APL *These muscles produce wrist flexion moments through very small moment arms. Kendall doesn't even list EPB as a wrist flexor/extensor. Effect of the extrinsic hand muscles on wrist radial/ulnar deviation Whether an extrinsic hand muscle produces wrist radial or ulnar deviation depends on its line of application's (LOA) position with regard to the wrist joint's A-P axis. Some muscles have LOA that pass directly over the wrist joint's AP axis, and so produce no frontal plane motion: EDC EI FDS FDP Others produce wrist movement in the frontal plane: EDM ulnarly deviates various extrinsic thumb muscles (EPL EPB APL FPL) radially deviate Examples of muscle synergies in wrist function I. palpate the ECRL as you make a fist or firmly grasp an object: role of the FDP: the only muscle that flexes the DIP joints. Also flexes the PIP, MP, and wrist. role of the FDS: flexes the MP and PIP joints; used for more forceful grasp role of wrist extensors (ECRB, ECRL, ECU): maintain appropriate length and tension (force) in finger flexors so they can produce strong grip. II. Open your fingers forcefully and observe that the wrist flexes automatically. role of the extensor digitorum: the only muscle that can open the fingers; also extends the wrist. role of the wrist flexors: maintain appropriate length and tension in finger extensors they can forcefully open the hand III. Palpate the ECU as you quickly move the thumb away from the second digit. Why is the ECU active during this maneuver? apl/epb move thumb but also radially deviate wrist; ecu acts as true synergist. IV. Carpal muscles act in helping synergies to produce: wrist flexion: wrist extension: ulnar deviation: radial deviation: WRIST PLANES OF MOTION o Joints involved ▪ radiocarpal ▪ intercarpal o Three axes of motion ▪ flexion-extension ▪ radial-ulnar deviation ▪ prono-supination  o Normal and function motion ▪ flexion (65 normal, 10 functional) ▪ 40% radiocarpal, 60% midcarpal ▪ extension (55 normal, 35 functional) ▪ 66% radiocarpal, 33% midcarpal ▪ radial deviation (15 normal, 10 functional) ▪ 90% midcarpal ▪ ulnar deviation (35 normal, 15 functional) ▪ 50% radiocarpal, 50% midcarpal WRIST BIOMECHANICS o Three biomechanic concepts have been proposed: o Link concept ▪ three links in a chain composed of radius, lunate and capitate ▪ head of capitate acts as center of rotation ▪ proximal row (lunate) acts as a unit and is an intercalated segment with no direct tendon attachments ▪ distal row functions as unit ▪ advantage ▪ efficient motion (less motion at each link) ▪ strong volar ligaments enhance stability ▪ disadvantage ▪ more links increases instability of the chain ▪ scaphoid bridges both carpal rows  ▪ resting forces/radial deviation push the scaphoid into flexion and push the triquetrum into extension ▪ ulnar deviation pushes the scaphoid into extension o Column concept ▪ lateral (mobile) column ▪ comprises scaphoid, trapezoid and trapezium ▪ scaphoid is center of motion and function is mobile ▪ central (flexion-extension) column ▪ comprises lunate, capitate and hamate ▪ luno-capitate articulation is center of motion ▪ motion is flexion/extension ▪ medial (rotation) column ▪ comprises triquetrum and distal carpal row ▪ motion is rotation o Rows concept ▪ comprises proximal and distal rows ▪ scaphoid is a bridge between rows ▪ motion occurs within and between rows CARPAL RELATIONSHIPS o Carpal collapse ▪ normal ratio of carpal height to 3rd metacarpal height is 0.54 o Ulnar translation ▪ normal ratio of ulna-to-capitate length to 3rd metacarpal height is 0.30  o Load transfer ▪ distal radius bears 80% of load ▪ distal ulna bears 20% of load ▪ ulna load bearing increases with ulnar lengthening ▪ ulna load bearing decreases with ulnar shortening WRIST LIGAMENTS o The ligaments of the wrist include ▪ extrinsic ligaments ▪ bridge carpal bones to the radius or metacarpals ▪ include volar and dorsal ligaments ▪ intrinsic ligaments ▪ originate and insert on carpal bones ▪ the most important intrinsic ligaments are the scapholunate interosseous ligament and lunotriquetral interosseous ligament o Characteristics ▪ volar ligaments are secondary stabilizers of the scaphoid ▪ volar ligaments are stronger than dorsal ligaments ▪ dorsal ligaments converge on the triquetrum o Space of Poirier ▪ center of a double "V" shape convergence of ligaments ▪ central weak area of the wrist in the floor of the carpal tunnel at the level of the proximal capitate ▪ between the volar radioscaphocapitate ligament and volar long radiolunate ligament (radiolunotriquetral ligament) ▪ wrist palmar flexion ▪ area of weakness disappears ▪ wrist dorsiflexion ▪ area of weakness increases ▪ in perilunate dislocations, this space allows the distal carpal row to separate from the lunate  ▪ in lunate dislocations, the lunate escapes into this space EXTRINSIC LIGAMENTS o Volar radiocarpal ligaments ▪ radial collateral ▪ radioscaphocapitate ▪ at risk for injury with excessively large radial styloid ▪ from radial styloid to capitate, creating a sling to support the waist of the scaphoid ▪ preserve when doing proximal row carpectomy ▪ acts as primary stabilizer of the wrist after PRC and prevents ulnar drift ▪ long radiolunate ▪ also called radiolunotriquetral or volar radiolunate ligament ▪ counteracts ulnar-distal translocation of the lunate ▪ abnormal in Madelung's deformity ▪ referred to as Vickers ligament ▪ radioscapholunate ▪ Ligament of Testut and Kuentz ▪ only functions as neurovascular conduit ▪ not a true ligament ▪ does not add mechanical strength ▪ short radiolunate  ▪ stabilizes lunate o Volar ulnocarpal ligaments ▪ ulnotriquetral ▪ ulnolunate ▪ ulnocapitate o Dorsal ligaments ▪ radiotriquetral ▪ also referred to as dorsal radiocarpal ligament (DRC) ▪ must also be disrupted for VISI deformity to form (in combination with rupture of lunotriquetral interosseous ligament rupture) ▪ dorsal intercarpal (DIC) ▪ radiolunate ▪ radioscaphoid INTRINSIC (INTEROSSEOUS) LIGAMENTS o Proximal row ▪ scapholunate ligament ▪ primary stabilizer of scapholunate joint ▪ composed of 3 components ▪ dorsal portion ▪ thickest and strongest ▪ prevents translation ▪ volar portion ▪ prevents rotation ▪ proximal portion  ▪ no significant strength ▪ disruption leads to lunate extension when the scaphoid flexes ▪ creating DISI deformity ▪ lunotriquetral ligament ▪ composed of 3 components ▪ dorsal ▪ volar ▪ strongest ▪ proximal ▪ disruption leads to lunate flexion when the scaphoid is normally aligned ▪ creating VISI deformity (in combination with rupture of dorsal radiotriquetral rupture) o Distal row ▪ trapeziotrapezoid ligament ▪ trapeziocapitate ligament ▪ capitohamate ligament o Palmar midcarpal ▪ scaphotrapeziotrapezoid ▪ scaphocapitate ▪ triquetralcapitate ▪ triquetralhamate Carpal tunnel: The "strut" that maintains the tunnel's shape is the flexor retinaculum, also called the transverse carpal ligament or the volar carpal ligament. This ligament connects the scaphoid and trapezium on the hand's radial side with the hamate on the ulnar side. The carpal tunnel contains (from radial to ulnar side): FCR tendon FPL tendon median nn. tendons of FDS and FDP also contains vascular structures Repetitive motion can produce a tenosynovitis in the tendon sheaths of the long flexor muscles. This, in turn, can increase hydrostatic pressure in the tunnel, causing compression damage to median nerve. "Carpal tunnel syndrome's" impairments include pain and paresthesia in the distribution of the median nerve. They also include weakness of muscles innervated by the median nerve, the thenar muscles, and the first and second lumbricales. The tunnel's contents are also prone to compression injury due to trauma, congenital stenosis, acromegaly, or hormonal changes.