Hand Structure and Function

Structure & Function of the Hand

• The objectives are to discuss the osteology and joints of the hand.
• The objectives are to identify the functions of passive & active stabilizing features in the hand.
• The objectives are to describe the arthrokinematics of the joints of the hand.
• The objectives are to discuss the role of the muscles of the hand.
• The objectives are to identify the elements of optimal power grip.
• The objectives are to explain the differences between movements of the thumb and the 2nd - 5th fingers.
• The objectives are to discuss the functional significance of the hand’s arches.
• The objective is to describe the extensor mechanism and flexor pulleys of the hand.

The Hand

• The hand consists of 5 digits, each with multiple joints.
• The Carpometacarpal (CMC) joints are in the hand.
• The Metacarpophalangeal (MCP) joints are in the hand.
• The Interphalangeal (IP) joints are in the hand.
• Fingers 2 to 4 contain proximal interphalangeal (PIP) joints and distal interphalangeal (DIP) joints.
• The thumb has only one IP joint.

Metacarpals

• Metacarpals are numbered 1–5 from the radial to the ulnar side.
• The 1st metacarpal (thumb) is the shortest & most stout.
• The 2nd metacarpal is usually the longest.
• The remaining 3 metacarpals decrease in length as you move ulnarly.
• Bases of metacarpals are proximal and articulate with one or more carpal bones.
• The Metacarpal neck is proximal to the head.
• Heads of the 2nd - 5th metacarpals, “knuckles” are on the dorsal side.
• The distal end has a large convex head.
• Posterior tubercles function as attachment sites for MCP jt collateral ligaments.
• The 1st metacarpals differs plane than others.
• The 1st metacarpal is rotated approximately 90° medially with respect to others.
• The palmar surface of the 1st metacarpal faces the midline of the hand, allowing opposition of the thumb for grip & pinch.

Phalanges

• Proximal & middle phalanges have concave bases & convex heads.
• Palmar surfaces of phalanges are slightly concave longitudinally.
• The distal phalanx has a concave base and distal tuberosity.

Arches of the Hand

• The hand's concavity is supported by 3 arch systems: 2 transverse and 1 longitudinal.
• Control of the hand's concavity allows for grasping, gripping, and object manipulation.
• The proximal transverse arch is formed by the distal row of carpal bones.
• The proximal transverse arch is a static, rigid, and forms a portion of the carpal tunnel.
• Reinforcement is provided by contacts with other bones & intercarpal ligaments.
• Capitate serves as a keystone of the proximal transverse arch.
• The distal transverse arch passes through the MCP jts.
• Sides of the distal arch are mobile.
• Transverse flexibility with/in the hand occurs as 1st, 4th, and 5th metacarpals "fold” around more stable 2nd & 3rd metacarpals.
• The keystone is formed by the MCP jts of the second and third metacarpals.
• The longitudinal arch follows the general shape of the second and third rays.
• Its proximal end is firmly linked to carpals by the CMC jts, which provides longitudinal stability.
• The distal end of the longitudinal arch is very mobile.
• The keystone of longitudinal arch is the 2nd and 3rd MCP joints.

Thumb motions

• Thumb motions include flexion, extension, abduction, adduction and opposition

CMC Joints

• CMC joints are articulations of distal carpals with the base of metacarpals.
• CMC joints allow only gliding movements.
• Stability of CMC joints is enabled by anterior (palmar) CMC, posterior (dorsal) CMC, & intermetacarpal ligaments.
• Open-packed position: midway between flexion & extension.
• Close-packed position: fingers in full flexion.
• Mobility exists at the CMC joints.
• The 2nd & 3rd metacarpals are relatively immobile.
• Stability joints are the 1st, 4th & 5th which are more mobile.
• CMC joints allow for grasp of varying size/shape objects.
• The 4th and 5th CMC joints have greater mobility, observed by the metacarpal heads when making a full fist.
• CMC joints are supported by: joint capsules, dorsal CMC ligaments, palmar CMC ligaments, and Intermetacarpal ligaments.
• Dorsal ligaments are more developed and extensive than the palmar ligaments.
• The 2nd CMC joint involves the base of the 2nd MC with the distal trapezoid.
• The articulations with the capitate & trapezium is smaller.
• The 3rd CMC joint involves the base of the 3rd MC with distal capitate.
• The 4th CMC joint involves the base of the 4th MC with distal hamate; the articulation with the capitate is smaller.
• THe 5th CMC joint involves the base of the 5th MC with distal hamate.

1st Carpometacarpal (CMC) Joint

• The 1st carpometacarpal is saddle-shaped.
• It consists of reciprocally shaped joint surfaces; articulation between the base of the 1st metacarpal & the distal trapezium.
• Motions at the 1st CMC jt include flexion & extension, adduction & abduction and opposition.
• The 1st CMC has a relatively large / lax joint capsule and its stability is related to muscle actions & ligamentous support.
• The open-packed position is midway between abduction & adduction and flexion & extension.
• The close-packed position is full opposition.

Ligaments of the 1st CMC Joint

• The radial collateral ligament is relatively thick and strong.
• It is taut with opposition, flexion, and abduction.
• The radial collateral ligament is a prime stabilizer of the opposed CMC joint.
• The ulnar collateral ligament is taut in abduction and extension.
• The anterior (palmar) oblique is a thin, weak ligament, and is taut in full extension.
• The posterior oblique is similar in function to the radial collateral ligament.

Arthrokinematics of 1st CMC Joint

• Flexion involves a Medial (ulnar) roll & slide of 1st metacarpal on the trapezium.
• Extension involves a Lateral (radial) roll & slide of 1st metacarpal on the trapezium.
• Abduction involves a Palmar roll & dorsal slide of the 1st metacarpal on the trapezium.
• Adduction involves a Dorsal roll & palmar slide of the 1st metacarpal on the trapezium.

MCP, PIP, & DIP Joints

• MCP = Metacarpophalangeal
• PIP = Proximal interphalangeal
• DIP = Distal interphalangeal

MCP Joints

• MCP Joints are formed between the convex heads of metacarpals and concave proximal surface of proximal phalanges.

Metacarpophalangeal Joints

• The 2nd - 5th metacarpals articulate with respective phalanges at biaxial joints.
• Widened, proximal bases articulate with carpals and one another.
• The joints allow flexion & extension, abduction & adduction.
• Flexion ROM progressively increases from 1st to 5th MCP.
• Joint capsules are lax and stability comes from collateral ligaments.
• MCP are generally lax in extension, become taut in approximately 70-90° of flexion.
• Open-packed position: slight flexion and close-packed position: full opposition (thumb)/full flexion (fingers).

Stability of MCP Joints

• Stability of MCP comes from Col lateral ligaments.
• They cross the joint in an oblique fashion.
• The dorsal cord of collateral ligaments are thicker and strong.
• Accessory portions are thinner, fan shaped and attach to palmar (volar) plate.
• Stability of MCP also comes from Volar (palmar) plates which include:
• Dense fibro cartilage
• Located on palmar aspect of joints.
• Attachments: base of proximal phalanx and neck of metacarpal.
• Volar Plates blend with volar aspect of capsule and resists excessive extension, adding to the joint stability.
• Flexible attachments of Volar Plates allow the plates to slide proximally on the metacarpal head w/o limiting motion.

Metacarpophalangeal Joints

• Deep transverse connects and loosely binds 2nd and 5th Metacarpals
• 3 Ligaments that merge in a wide, flat structure.

MCP Arthrokinematics

• For Flexions, the Proximal phalanx rolls & slides palmarly (anteriorly) on the metacarpals, and activates FPD.
• Flexion draws the dorsal capsule & cord portion of the radial collateral ligament which is relatively taut.
• For Extension, the Proximal phalanx rolls & slides dorsally on the metacarpal, which is then controlled by coactivation of ED & intrinsic finger muscles.
• The extended position draws volar plate taut & creates relative slack in cord portion of radial collateral ligament.
• In Abduction, the activation of 1st dorsal interosseus muscle (DI1).
• When in full abduction, the ulnar collateral ligament is taut & radial collateral ligament is slack.
• The proximal phalanx rolls & slides in the same direction dependent on the digit.
• The MCP joint of the thumb is a hinge joint and allows flexion & extension.
• Bony surfaces and ligamentous support (anterior/palmar and collateral ligaments) provide inherent stability.
• Articular surfaces are convex head of metacarpal & concave surface on base of phalanx.
• Volar plate increases articular surface area → allows for greater ROM.

1st MCP & IP Joint Arthrokinematics

• Sliding and rolling of phalanx occur in same direction at both joints.
• Flexion features roll & slide volarly.
• Extension features roll & slide dorsally.

Interphalangeal Joints

• These are hinge jts that allow flexion & extension.
• The shape of osseous surfaces contributes largely to stability.
• PIP is relatively stable in all positions.
• PIP capsules are composed of volar plate, collateral ligaments, & extensor expansion.
• Collateral ligaments are maximally taut at 25°finger flexion.
• DIP is less stable than PIP jts.
• Open-packed position: slight flexion and close-packed position: full extension.

PIP Joints

• Articulation is b/w proximal phalanges & base of middle phalanges.
• The head of the prox phalanx has 2 condyles separated by a shallow groove.
• The middle phalanx has 2 shallow concave facets separated by central ridge.
• The joint capsule is reinforced by radial & ulnar collateral ligaments, and the cord portion of collateral ligaments at PIP jt limits ABD & ADD.
• The accessory portion blends with and reinforces palmar plate.
• Proximal-lateral regions of palmar plate at PIP jts thicken longitudinally, forming fibrous tissue called check-rein ligaments.
• Check-rein ligaments reinforce proximal attachments of palmar plate, assisting in resisting hyperextension.

Volar (Palmar) Plate

• This is a mobile, thickened fibrocartilaginous structure.
• Functions as restraint to hyperextension at IP joints.
• Improves mechanical advantage of flexor tendons at initiation of PIP flexion.
• Increases surface area of joints, allowing for greater ROM.

DIP Joints

• Articulation is b/w the heads of middle phalanges & bases of distal phalanges.
• DIP joint structure & tissues are similar to PIP structure, with one exception, no check-rein ligaments.

Kinematics & Arthrokinematics of PIP & DIP Joints

• Flexion of MCP, PIP, & DIP joints is greater in the more ulnar digits.
• PIP joints allow very little hyperextension.
• MCP & DIP joints allow some hyperextension.
• Nearly constant passive tension exists in collateral ligaments at IP joints throughout ROM.
• Arthrokinematics at the DIP & PIP joints are similar.
• Concave base of middle phalanx rolls & slides in palmar direction during PIP flexion. For DIP flexion, the concave base of distal phalanx rolls & slides in palmar direction.
• Passive tension created in dorsal capsule helps guide the arthrokinematics.

Musculature of the Hand

• Extrinsic flexor tendons of digits enclosed in fibro-osseous tunnels.
• Ulnar synovial surrounds FDS and PDP tendons.
• Radial synovial extends with FPL
• Muscles of the hand can be categorized as extrinsic or intrinsic.
• Intrinsic muscles have both proximal and distal attachments within the hand.
• Extrinsic muscles have proximal attachments in the forearm or epicondyles of the humerus.
• Most hand movements require coordination between extrinsic & intrinsic muscles and muscles of the wrist.

Intrinsic Muscles of the Hand

• Thenar eminence muscles: abductor pollicis brevis, flexor pollicis brevis, and opponens pollicis.
• Hypothenar eminence muscles: abductor digiti minimi, flexor digiti minimi, opponens digiti minimi, and palmaris brevis.
• Other Intrinsic muscles: adductor pollicis (2 heads), lumbricals (4), interossei (4 palmar & 4 dorsal).

Extrinsic Muscles of the Hand

• Flexors of the digits: flexor digitorum superficialis, flexor digitorum profundus, flexor pollicis longus.
• Extensors of the fingers: extensor digitorum, extensor indicis, and extensor digiti minimi.
• Extensors of the thumb: extensor pollicis longus, extensor pollicis brevis, and abductor pollicis longus.

Extrinsic Flexors of the Digits

• Flexor digitorum superficialis (FDS) primary action: flexes the PIP joints and assists in flexing all joints it crosses.
• Flexor digitorum profundus (FDP) primary action is to flex the DIP and assist in flexing all joints it crosses.
• The flexor pollicis longus (FPL) primary action is flexion of thumb IP joint.
• Flexor pulleys are embedded within each digital sheath and the digital synovial surrounds flexor tendons deep to flexor pulleys and extends from distal palmar to DIP Joints.

Flexor Digitorum Superficialis (FDS)

• The muscle belly is just deep to the 3 wrist flexors & pronator teres. They consists 4 tendons that cross the wrist & enter palmar aspect of hand.
• Each tendon splits at level of proximal phalanx, which allows passage of FDP.
• The split parts partiallyreunite, then cross the PIP joint, and attaches on sides of palmar aspect of middle phalanx.

Flexor Digitorum Profundus (FDP)

• The muscle belly exists in deepest muscular plane of forearm, deep to FDS
• Each tendon passes through split FDS & attaches to palmar aspect of base of distal phalanx
• It is the only flexor of DIP joints and flexes all joints it crosses.
• The DFP can control index finger relatively independently of other profundus tendons however interconnection of other 3 tendons limits isolated DIP jt flexion of a single finger.

Flexor Pollicis Longus

• It lies deepest muscular plane of forearm, just lateral to FDP and crosses the wrist & attaches distally to palmar side of base of distal phalanx of thumb.
• FPL exerts a substantial flexion torque at MCP & CMC joints of thumb, and can flex the wrist.
• The radial synovial sheath extends with FPL tendon to its distal insertion and the extrinsic flexor tendons of digits are enclosed in fibro-osseous tunnels (fibrous digital sheaths).

Flexor Structures of Hand

• The Flexor pulleys are embedded within each digital sheath; the digital synovial sheaths surround flexor tendons deep to flexor pulleys, extending from distal palmar crease to DIP joints.
• They supply nutrients and lubricate enclosed tendons, which ultimately reduces friction between FDS and FDP tendons.

Flexor Pulleys

• Five annular pulleys exists for each finger (A1 to A5)
• Major pulleys: A2 and A4 attach to shafts of proximal & middle phalanges.
• Minor pulleys: A1, A3, & A5 attach directly to volar plate at each of the joints within a finger.

Three cruciate pulleys (C1 to C3)

• Cruciate pulleys are less distinct and features thin, flexible fibers that crisscross over tendons at regions where digital sheaths bend during flexion.
• Annular & oblique ligaments of thumb acts as pulleys for FPL tendon.
• The Shape of pulleys allows finger flexion without pinching of the pulleys and allows for pressure distribution on tendons and sheath.
• The vincula are folds of synovial membrane that carry blood vessels; vincula are able to provide direct vascularization of FDS and FDP tendons.
• Flexor pulleys hold underlying tendons relatively close to joints, preventing bowstringing of tendons during muscle contractions.
• Severing or overstretching of major (A2 or A4) pulleys alters the mechanics of finger flexion a and is often observed in elite rock climbers.
• Extensor muscles must contract synergistically for extrinsic finger flexors to isolate function at a single joints such that for example FDS creates flexion torque across all joints it crosses when it contracts
• To isolate function at PIP, the Extensor Digitorum (ED) contracts to stabilizes the MCP jt and wrist.

Extrinsic Extensors of fingers

• The Extensor digitorum (ED) and interconnecting tendons are interconnected by several junctura tendinae. The Extensor Indicies (EI) and Extensor Digiti Minimi (EDM) exist. The Extensor Digiti Minimi is commonly supported by two tendons.
• Distal to wrist, extensor tendons lack defined digital sheath or pulley system, instead, become integrated into fibrous expansion of connective tissues called extensor mechanism.
• The Extensor mechanism acts as primary distal attachment for ED, EI, & EDM,as well as most intrinsic muscles acting on fingers.

Extensor Mechansim of the Fingers

• Small part of ED attaches to dorsal base of proximal phalanx
• Remaining flattens into central band where ED extends distally attaching it to dorsal base middle phalanx
• It is the central and lateral base of the distal phalanx that allows ED to transfer force distally throughout entire finger
• Extensor mechanisms act with other active components such as dorsal interossei, palmar interossei, & lumbricals whose origin sites at the sides of MCP Joints and insertion sites includes direct placement in proximal phalanxes as well as in the dorsal hood or in tendons where they contribute to fibers in central and lateral bands.

Extensor Mechnais of the Fingers

• Lumbricals are connected to FDP and distally to lateral band.
• In oblique retinacular attachment the pulley must attach to lateral side.
• Dorsal hood most prominent feature, helps to transmit force to IP joint through transverse and oblique fibers.

Oblique Reinacular Ligaments

• Runs dorsally over the Distal Interphalangeal joint, the orientation across helps coordinate them to extend between joints and creates balance.

Couplings of PIP and DIP Motion

• Active and Passive interactions help to connect motion between the joints.

Musculature of the Muscle: Extrinsic and Intrinsic

• Extrinsic: FPL, EPB, EPL, and Abductor Pollicis Longus.
• Intrinsic: Muscle is used for Opponens pollicis by holding metacarpal within abduction, and flexed joints. If thumb gently used, then is helps support APB. Then muscle activity exceeds in oppositions of fingers and activity is then exceeded by firm motions.

Power Grips

• In gripping finger assume at sustained flexion that depends of the weight, height and size of an object.
• Palms that support such action create palmar arches, and in gripping thumb is adducted to add force.
• Cylindrical Grips that are activated and activated depend of wrist.
• Splerical Grip relays on extrinsic muscle activity.
• Hook Grips activated the fingers but FDP, FDS is used and thumb does not work.

Precision Handling

• Pad to Pad, Tip to Tip, Side to Side gripping that adds strength and support.