The Flexors and Extensors

Donald Sammut Hand Surgeon

The$forearm$contains$the$powerful$grip$muscles$which$animate$thumb$and$digits$$ and$stabilize$the$wrist.$ $ All$precision$movement$of$the$digits$is$mediated$by$the$Intrinsic$musculature$while$ all$power$resides$in$the$forearm$ﬂexors$and$extensors.$

$ Although$it$appears$that$a$held$object$is$being$held$by$the$hand,$$ it$is$the$forearm$musculature$that$is$performing$the$work.$ $ When$an$object$or$tool,$such$as$a$tennis$racquet,$a$hammer,$is$grasped$by$the$hand,$$ The$tool$becomes$one$with$the$hand$and$forearm.$$ $ All$movement$in$space$is$then$mediated$by$the$larger,$more$proximal,$joints.$ $ These$movements$are$powerful$but$relatively$inaccurate$

Action$of$the$hand$can$be$:$ $ 1. With$an$object$clamped$in$the$hand$and$all$movement$proximal$ 2. With$proximal$muscle$and$joint$action$eliminated$and$the$hand$$ $intrinsic$muscles$performing$precision$movements$ $ Compare$writing$on$a$blackboard$with$writing$on$paper$on$a$ﬂat$surface$ $ The$surgeon$or$artist$who$requires$precision$movement$will$often$eliminate$ $more$proximal,$powerful$but$imprecise$muscles,$and$use$only$the$small,$precise$$ (but$relatively$weak),$small$muscles$in$the$hand.$$ The$elbow$and$forearm,$ $right$down$to$the$little$ﬁnger,$are$stabilised$on$a$surface,$and$only$the$interphalangeal$$ joints$are$animated,$to$perform$precision$work$ $ The$‘Peripherilisation’$of$grip:$$ More$distal$action$=$Precision$but$relative$weakness$ More$proximal$action$=$Strength$but$relative$imprecision$ $

The$‘Peripherilisation’$of$grip$ In$daily$use,$one$alternates$and$blends$the$two$forms$of$activity.$$ The$more$proximal$muscles$will$often$orientate$the$hand$in$space,$$ which$then$performs$$work$ $ This$‘orientation$in$space’$role$of$the$peripheral$muscles$also$optimizes$hand$action.$ Awkward$shaped$objects$or$work$in$inaccessible$places$can$be$rendered$more$eﬃcient$if$ The$hand$is$correctly$orientated$in$space.$$ $ For$instance,$pronation$and$supination$(more$proximal$actions)$are$often$essential$ for$the$eﬀective$performance$of$precision$(more$distal)$work,$such$as$writing,$$ reaching$for$change$or$small$objects.$.$

When$a$muscle$fibre$at$its$resting$length$is$detached$from$its$insertion,$it$shortens.$When$ it$is$stretched$to$its$maximum,$using$external$force,$it$is$in$a$state$of$tension$and$will$ spring$back$as$soon$as$it$is$released.$$ This$is$caused$by$the$elastic$behaviour$of$muscle,$and$is$independent$of$nerve$stimulus.$ Irrespective$of$muscle$contracture,$this$elastic$behaviour$would$tend$to$make$the$muscle$ assume$a$resting$length.$ $ If$one$descends$to$the$level$of$the$muscle$sarcomere,$the$maximum$force$is$generated$ when$there$is$maximum$overlap$between$the$interdigitating$fibres.$This$force$is$ diminished$if$the$plates$are$pulled$apart,$as$there$is$less$overlap$between$the$actin$and$the$ myosin.$$ Force$is$also$diminished$if$they$are$pulled$in$so$close$that$the$fibrils$bunch$up$together.$ $ $This$explains$the$shape$of$the$length/tension$curve$and$why$the$muscle$generates$most$ power$when$it$operates$at$the$peak$of$the$curve$i.e.$when$the$sarcomeres$are$at$heir$ optimum$overlap$position.$ Tension$ Passive$stretch$ Length$

Tension$ Active$contracture$ Length$ LengthUTension$curve$(The$Blix$Curve)$ The$tensionUlength$curve$shows$the$optimum$length$of$any$musculotendinous$unit.$The$ Unit$generates$most$power$when$working$at$this$length.$ Tension$ Resting$Length$

Length$ Given$that$there$is$a$resting$tone$and$elastic$recoil$in$the$muscle,$when$the$relaxed$hand$is$ flipped$over$into$supination$so$that$it$falls$with$the$wrist$in$extension,$the$flexor$tendons$ will$be$stretched$(over$the$extended$wrist)$and$the$tendency$for$the$flexor$muscles$to$stay$ at$the$resting$length$will$make$the$fingers$flex.$$ $ The$opposite$happens$when$the$relaxed$hand$is$flipped$over$into$pronation$and$the$wrist$ falls$into$flexion.$The$extensors$are$stretched$and$their$resting$length$makes$the$fingers$ extend.$$ $ This$tendency$of$the$relaxed$hand$to$flex$the$fingers$when$the$wrist$is$extended$and$to$ extend$the$fingers$when$the$wrist$is$flexed$is$known$as$the$TENODESIS$effect.$ $ $It$can$be$demonstrated$also$in$the$anaesthetised$patient$and$even$on$your$own$hand$if$ you$can$get$it$to$relax$as$you$go$from$pronation$to$supination$and$vice$versa$ The$Tenodesis$Effect$ In$the$hand:$ $ Flexion$of$the$digits$is$accompanied$by$extension$of$the$wrist.$The$digital$flexion$shortens$ the$muscle,$while$the$wrist$extension$lengthens$it.$ In$fact,$the$flexor$does$not$alter$in$length$from$full$extension$to$full$flexion$ This$enables$the$flexor$to$act$at$its$optimal$length$throughout$the$flexion$cycle$ $ Ditto$for$extension$ Extension$of$the$digit$is$accompanied$by$flexion$of$the$wrist$back$to$neutral.$Once$again,$ this$enables$the$extensor$to$act$at$its$optimal$length$throughout$the$extension$cycle.$ $ Generating$full$force$when$making$a$fist,$is$impossible$if$wrist$extension$is$prevented.$

The$Extensors$ The$ Extensors$ THE$EXTENSORS$ $ Organised$in$3$groups$investing$the$back$of$the$forearm$ $ 1$Longitudinal:$The$Mobile$wad:$ $ $Brachioradialis$ $$$$$$$$Extensor$Carpi$Radialis$Longus$ $$$$$$$$Extensor$Carpi$Radialis$brevis$ $ 1$Longitudinal:$The$Common$Group:$ $Extensor$Digitorum$Communis$ $$$$$$$$Extensor$Carpi$Ulnaris$ $$$$$$$$Extensor$Indicis$Proprius$ $$$$$$$$Extensor$Digiti$Minimi$ $ 1$Oblique:$The$Group$to$the$Thumb $Abductor$Pollicis$Longus $ $$$ $$$$$$$$Extensor$Pollicis$Brevis$ $$$$$$$$Extensor$Pollicis$Longus$ $ THE$NERVE$OF$THE$EXTENSOR$COMPARTMENT$IS$THE$POSTERIOR$INTEROSSOUS$ NERVE$(FROM$THE$RADIAL).$THE$MOBILE$WAD$IS$SUPPLIED$DIRECTLY$BY$THE$RADIAL$ NERVE$$IN$THE$PROXIMAL$PART$OF$THE$ARM.$ $$$$$$$ The$ Extensors$ The Mobile Wad Brachioradialis ECRL ECRB The Crowd Extensor Digitorum communis Extensor Indicis Proprius Extensor Digiti Minimi Extensor Carpi Ulnaris

The Thumb Trio Abductor Pollicis Longus Extensor Pollicis Brevis Extensor Pollicis Longus The Mobile Wad Brachioradialis ECRL ECRB The Crowd Extensor Digitorum communis Extensor Indicis Proprius Extensor Digiti Minimi Extensor Carpi Ulnaris The Thumb Trio Abductor Pollicis Longus Extensor Pollicis Brevis Extensor Pollicis Longus

MOST$OF$THE$EXTENSOR$MUSCLES$ARISE$FROM:$ $ The$Common$extensor$Origin$$ The$Lateral$Epicondyle:$A$roughened$prominence$of$bone$at$the$lateral$distal$end$of$the$ humerus,$just$behind$the$capitulum$ $ The$Supracondylar$Ridge:$The$ridge$of$humerus$running$up$from$the$lateral$Epicondyle$ $ The$Lateral$Intermuscular$septum:$This$fibrous$sheet,$extends$laterally$as$a$partition$ between$flexor$and$extensor$muscles$and$gives$origin$to$both.$It$exists$as$an$extra$ platform$from$which$muscle$fibres$may$arise.$ Lateral$$ Supracondylar$ $ridge$

Lateral$Epicondyle/$ Common$extensor$$ origin$ Medial$ Lateral$ Intermuscular$$ Intermuscular$$ septum$ septum$ Medial Lateral Intermuscular Intermuscular septum septum

2cm$proximal$to$medial$epicondyle$of$humerus$ The$Mobile$Wad$ BR$ ECRL$ ECRB$ THE$MOBILE$WAD$ $ $ BRACHIORADIALIS:$FROM$DISTAL$HUMERUS$TO$DISTAL$RADIUS$ $ EXTENSOR$CARPI$RADIALIS$LONGUS:$DISTAL$HUMERUS$TO$BASE$2ND$METACARPAL$ $ EXTENBSOR$CARPI$RADIALIS$BREVIS:$DISTAL$HUMERUS$TO$BASE$3RD$METACARPAL$ $ Brachioradialis$ Extensor$Carpi$Radialis$Longus$ Extensor$Carpi$Radialis$Brevis$ ECRL$inserts$into$he$base$of$the$second$metacarpal$ ECRB$inserts$into$the$base$of$the$third$metacarpal$ $ These$tendons$provide$a$symmetrical$balance$to$the$insertion$of$Flexor$Carpi$Radialis$on$ the$palmar$side,$which$also$inserts$into$the$base$of$the$2nd$and$3rd$metacarpals.$ INSERTION$

BR$

ECRL$ ECRB$ Brachioradialis$ ECRL$ ECRB$ $ The$Mobile$wad$tendons$are$crossed$in$their$distal$parts$by$the$oblique$muscle$group$of$ muscles$coming$from$deep$(back$of$ulna,$radius$and$interosseous$membrane)$to$the$ thumb.$ $ The$cutaneous$terminal$branch$of$the$radial$nerve,$which$runs$beneath$Brachioradialis,$ emerges$between$Brachioradialis$and$ECRL,$to$run$superﬁcial,$just$deep$to$the$dermis$ and$also$crosses$over$the$distal$tendons$of$the$mobile$wad.$

ECRL$$ $ $ $$$$$$$$$$ECRB$

Brachioradialis$ The$Common$Ext$Origin$ $ Extensor$Digitorum$Communis$ Extensor$Carpi$Ulnaris$$ Extensor$Digiti$Minimi$ $ Extensor$Indicis$Proprius$ $ $ $ The$Common$extensor$origin$consists$of:$ The$Lateral$epicondyle$(front$and$back)$ The$adjacent$lateral$intermuscular$septum$ $ $ $ $ The$numerous$muscles$which$seek$origin$from$the$lateral$epicondyle$$jostle$for$a$foothold$ and$seek$extra$purchase$by$origin$from$the$lateral$intermuscular$septum.$ $ The$common$origin$muscles$arise$as$one$common$aponeurosis$(Aponeurosis+–+Flattened+ tendon)+from$which$the$individual$muscles$then$fan$out$to$their$destinations.$ $ This$aponeurosis,$originating$from$the$Lateral$epicondyle,$is$the$site$of$‘Lateral$ Epicondylitis’$or$‘Tennis$Elbow’$$which$is$an$inﬂammation$between$aponeurosis$and$bone.$ $ Common$extensor$origin/$ Lateral$epicondyle$

Ulnar$origin$of$$ Ext$Carpi$Ulnaris$

EXTENSOR$DIGITROUM$COMMUNIS$ $ Is$the$bulkiest$of$the$Extensor$muscles.$ It$divides$into$four$distinct$tendons$which$pass$through$Compartment$4$on$the$back$of$the$ wrist$and$then$ﬂare$out$to$the$dorsal$digital$expansions$of$each$digit,$and$then$on$into$the$ long$extensor$mechanism$(see$‘The$Hand’)$ $ The$tendons$share$the$4th$compartment$with$the$Extensor$Indicis$Proprius$tendon$and$ with$the$terminal$branch$of$the$Posterior$interosseous$nerve$which,$its$motor$ﬁbres$now$ spent,$ends$by$supplying$the$wrist$joint$with$sensation$and$proprioception.$ $ Extensor$ Digitorum$$ Communis$

Common$$ extensor$$ Origin$ $ $ $ $ Digital$ Extensor$ Expansions$ THE$EXTENSOR$CARPI$RADIALIS$ $ Forms$a$strong$tendon$which$is$an$important$stabilising$element$OFthe$head$of$the$ulna$ and$the$wrist.$ $ $It$passes$dorsal$to$the$ulnar$head,$in$the$groove$at$the$base$of$the$styloid$process$and$is$ ﬁrmly$bound$to$bone$here$by$its$sheath.$ $ It$inserts$into$the$base$of$the$5th$Metacarpal.$Despite$its$name,$it$bypasses$the$wrist$ entirely$ Extensor$ Carpi$$ Ulnaris$

Common$$ extensor$$ Origin/Ulna$ Subcut$border$ $ $ $ $ Meta$5$base$ THE$EXTENSOR$DIGITI$MINIMI$ $ A$small,$weak$muscle,$it$passes$through$its$own$dedicated$compartment$(Compartment$ 5)$and$inserts$onto$the$dorsal$expansion$of$the$little$ﬁnger$along$with$the$Ext$Dig$ Communis$to$the$little.$$ $ It$helps$provide$independent$extension$to$the$little$ﬁnger$(the$little,$index$and$thumb,$ have$their$owb$extensors,$independent$of$mass$action).$ Extensor$ Digiti$$ Minimi$

Common$$ extensor$$ Origin$ $ $ $ $ Digital$ Extensor$$ Expansion$ Little$ﬁnger$ THE$EXTENSOR$INDICIS$PROPRIUS$ $ Unlike$the$other$extensors,$this$does$not$arise$from$the$Humerus/common$extensor$origin$ but$from$the$back$of$the$distal$ulna$and$the$adjacent$interosseous$membrane.$ $ $It$courses$obliquely$deep$across$the$back$of$the$distal$forearm,$through$Compartment$4,$ which$it$shares$with$Ext$Dig$Communis,$and$inserts$on$the$ULNAR$side$of$he$dorsal$digital$ expansion$on$the$back$of$the$index.$ $ $ Extensor$ Indicis$ Proprius$

Ulna$and$$ Inteross.$$ Memb$ $ $ $ $ Digital$ Extensor$$ Expansion$ Index$ Extensor$ Indicis$ Proprius$

Ulna$and$$ Inteross.$$ Memb$ $ $ $ $ Digital$ Extensor$$ Expansion$ Index$

The$Thumb$Trio$ $ Extensor$Pollicis$Longus$ Abductor$Pollicis$Longus$ Extensor$Pollicis$Brevis$ $ THE$EXTENSOR$POLLICIS$LONGUS$ $ Also$arises$from$the$back$of$the$ulna$and$adjacent$interosseous$membrane.$Like$the$ Extensor$Indicis$Proprius$it$id$destined$for$a$radial$structure,$the$thumb,$and$this$must$run$ obliquely$from$ulnar$aspect$to$radial$aspect.$ $ In$fact,$it$sharply$increases$its$obliquity$by$changing$direction$around$the$Tubercle$of$ Lister,$on$the$back$of$the$distal$radius.$ At$this$point,$it$runs$in$its$own$tunnel,$Compartment$3.$ Just$beyond$this$point,$it$forms$the$ulnar$border$of$the$anatomical$snuﬀ$box.$ $ Its$action$is$to$extend$the$Interphalangeal$joint$of$the$thumb$(and$the$rest$of$the$thumb$ column)$as$well$as$provide$retropulsion.$ $ If$the$hand$is$laid$ﬂat$on$a$surface,$palm$down,$The$thumb$can$be$lifted$away$from$this$ surface.$This$is$retropulsion.$ Extensor$ Pollicis$$ Longus$

Ulna$and$$ Inteross.$$ Memb$ $ $ $ $ $ Distal$$ Phalanx$ Thumb$

ABDUCTOR$POLLICIS$LONGUS$ $ Is$a$powerful$muscle$which$arises$on$the$back$of$the$radius,$ulna$$and$intervening$ interosseous$membrane$.$ It$runs$in$Compartment$1,$the$most$radial$compartment,$in$company$with$Extensor$ Pollicis$brevis.$ $ Just$beyond$this$compartment$it$forms$the$radial$border$of$the$anatomical$snuﬀ$box$ $ It$inserts$into$the$base$of$the$$thumb$metacarpal.$It$provides$strong$abduction$(Moving$ the$thumb$away$from$the$palm$at$90°$to$the$palm)$and$is$an$essential$early$player$in$ opening$the$thumb$to$reach$for$an$object,$and$to$initiate$opposition.$ $ Its$tendon$is$frequently$multiple.$ $ Abductor$ Pollicis$$ Longus$

Ulna,$Radius$ $and$$ Inteross.$$ Memb$ $ $ $ $ Base$of$Meta$$ Thumb$ EXTENSOR$POLLICIS$BREVIS$ $ Arises$from$the$back$of$the$radius$and$interosseous$membrane.$ It$runs,$with$APL,$in$Compartment$1,$(the$most$radial$compartment)$and$inserts$into$the$ base$of$the$proximal$phalanx$of$the$thumb.$It$extends$the$MP$joint.$ $ Variations$are$frequent$–$the$most$frequent$is$an$extension$of$its$insertion$into$the$distal$ phalanx.$ $ $ $ Extensor$ Pollicis$$ Brevis$

Radius$ $and$$ Inteross.$$ Memb$ $ $ $ $ Thumb$P1$ EPL$ APL$ EPB$

INTERSECTION$SYNDROME$ $ $ The$thumb$muscles$APL$and$$EPB$come$from$deep$to$superﬁcial$and$cross$the$path$of$the$ mobile$wad,$particularly$the$ECRL$and$ECRB.$$ $ When$these$muscles$hypertrophy$and$are$used$heavily$and$frequently,$the$plane$between$ the$two$groups$(between$thumb$muscles$and$ECR$tendons)$may$become$inﬂamed.$This$is$ painful$and$the$swelling$of$the$overlying$muscles$in$their$inelastic$fascia$is$also$painful.$ $ This$condition$is$common$in$sportsmen$who$perform$strong,$repetitive,$wrist$action,$such$ as$Rowers.$ $ Intersection$Syndrome$

DE$QUERVAIN’S$TENOSYNOVITIS$(also:$TENOVAGINITIS)$ $ The$APL$and$EPB$run$in$Compartment$1,$a$tight$ﬁt.$ $ Strong,$repeated$abduction$of$the$wrist$can$cause$these$tendons$to$inﬂame$inside$this$ tight$tunnel.$ $ Pain$is$transmitted$along$the$thumb$and$radial$part$of$the$wrist.$ $ This$condition$is$common$in$those$who$perform$frequent$heavy$lifting,$particularly$if$this$ involves$radial$abduction$of$the$wrist.$ $ De$Quervain’s$$ Tenosynovitis$ De$Quervain’s$Tenosynovitis$

The$Anatomical$Snuff$Box$ THE$ANATOMICAL$SNUFF$BOX$ So+named+because+a+pinch+of++snuff+or+tobacco+was+placed+in+it+and+then+sniffed+ $ $ This$is$the$name$given$to$the$hollow$bounded$by$the$APL/EPB$on$one$side$and$the$EPL$on$ the$other$side.$ $ In$the$roof$lie$branches$of$the$cutaneous$radial$nerve$(easily$injured$and$readily$forming$a$ tender$neuroma)$ $ The$floor$is$bony$and$consists,$in$sequence,$of:$ Radial$Styloid$ Scaphoid$ Trapezium$$ Base$of$thumb$metacarpal$ $ The$snuff$box$is$traversed$by$the$radial$artery$on$its$way$between$forearm$and$hand$(it$ leave$the$snuff$box$by$passing$between$the$two$heads$of$the$1st$Dorsal$Interosseous$ $ APL$

EPB$

EPL$ Distal$to$the$snuff$box,$the$long$extensors$are$held$in$axis$on$the$back$of$the$thumb$by$the$ dorsal$expansion$(as$in$the$long$digits).$ $ This$is$stabilised$by$insertion$of:$ $ On$the$ulnar$side:$The$Adductor$Pollicis$ $ On$the$radial$side:$The$abductor$pollicis$brevis.$ $ $ $ The$Dorsal$Digital$Expansion$ Add P Abd PB The$Extensor$Retinaculum$ THE$EXTENSOR$RETINACULUM$ $ Is$a$broad$sheet$of$fibrous$tissue$which$forms$a$band$across$the$tendons$on$the$back$of$ the$wrist$and$distal$forearm.$$ $ It$binds$the$extensors$down$to$bone:$ $ It$stops$bowstringing$of$the$tendons$when$the$wrist$is$extended$ The$space$between$it$and$distal$Radius/ulna$is$divided$by$septa$so$that$each$tendon/group$ of$tendons,$is$held$in$alignment$which$best$uses$its$line$of$action.$ This$is$important$since$the$convexity$of$the$back$of$the$wrist$would$otherwise$allow$ subluxation$of$the$tendons$when$the$wrist$flexes.$ $ $ The$Extensor$Retinaculum$ ECU

EPL APL/ EPB ECRB EDC ECRL EDM EIP There$are$SIX$extensor$compartments:$ $ Compartment$1:$ $APL$and$EPB$ $ Compartment$2: $$ECRL$and$ECRB$ $ Compartment$3:$ $EPL$ $ Compartment$4:$ $EDC$and$EIP$ $ Compartment$5:$ $EDM$ $ Compartment$6:$ $ECU$ $ 2$ 3 1 4 5 6

Abductor Pollicis Longus Extensor Pollicis Brevis Extensor$Carpi$Radialis$Longus$ Extensor$Carpi$Radialis$Brevis$

2$ Extensor$Pollicis$Longus$ 3$ Extensor$Digitorum$Communis$ Extensor$Indicis$Proprius$

4$ Extensor$Digiti$Minimi$

5$ Extensor$Carpi$Ulnaris$

6$ THE$DORSL$DIGITAL$EXPANSION$ $ From$the$constraints$of$the$Extensor$Compartments,$the$long$extensors$are$next$directed$ at$the$dorsal$digital$expansions.$$ $ These$consist$of$triangular$sheets$of$Fibrous$tissue,$either$side$of$the$extensor$tendon,$ over$the$back$of$the$MP$joint.$Transverse$fibres$cross$these$bands$and$are$known$as$ sagittal$fibres.$ $ Their$function$is$to$hold$the$long$extensors$in$perfect$alignment$over$the$crest$of$the$MP$ joint.$ $ Without$these$bands$either$side,$the$extensor$tendon$subluxes$on$one$or$other$side$of$the$ joint$prominence.$It$may$even$sublux$beyond$the$axis$of$rotation$of$the$joint$and$will$act$ as$a$flexor,$rather$than$an$extensor.$ $ The$dorsal$expansion$receives$insertion,$either$side,$of$the$palmar$and$dorsal$interossei.$ $ $ $ $

Dorsal$Digital$Expansion$

THE$NERVE$OF$THE$EXTENSOR$COMPARTMENT$ $ Is$the$Posterior$interosseous$nerve,$a$terminal$branch$of$the$radial$nerve./$ $ $ The$radial$nerve$leaves$the$posterior$cord$of$the$Brachial$Plexus$and$travels$in$the$spiral$ groove$on$the$back$of$the$humerus.$ $ It$appears$in$the$ﬂexor$compartment$around$the$lateral$border$of$the$humerus$by$piercing$ the$lateral$intermuscular$septum.$$ At$this$point$it$supplies$the$Brachioradiualis,$The$ECRL$and$the$ECRB$(The$Mobile$Wad).$ $ It$lies$here$in$the$plane$between$brachioradialis$and$Brachialis$and$can$be$seen$in$the$ depths$of$this$plane$$if$these$two$muscles$are$separated.$ $ $

The Radial Nerve

Biceps and Brachialis

The Mobile Wad

BENEATH$BRACHIORADIALIS,$THE$RADIAL$NERVE$DIVIDES$INTO$ITS$TWO$TERMINAL$ BRANCHES$ $ THE$POSTERIOR$INTEROSSEOUS$NERVE$C5,6,7,8$ This$aims$to$reach$the$back$of$the$forearm.$It$does$so$by$curling$round$the$neck$of$the$ radius$and$passes$between$the$two$heads$of$supinator$(which$it$supplies).$ It$then$lies$on$the$posterior$aspect$of$the$Interosseous$membrane,$in$the$valley$between$ the$muscle$groups$(just$like$the$anterioR$interosseous$on$the$other$side$of$the$ membrane).$ It$supplies$all$the$remaining$extensors.$ It$ends$with$some$residual$sensory$fibres,$by$travelling$through$compartment$4$and$ supplying$the$wrist/carpus$with$proprioception.$ $ $ THE$TERMINAL$CUTANEOUS$BRANCH$C6,7,8$ This$progresses$distally$beneath$Brachioradialis,$then$emerges$to$become$superficial$ between$Brachioradialis$and$ECRL.$It$runs$in$the$subcutaneous$fat,$spreading$into$many$ end$branches$which$mediate$cutaneous$sensation$to$the$radial$half$of$the$back$of$the$ hand.$ It$is$easily$injured$by$pressure$from$a$heavy$watch,$bracelet,$or$handcuffs$or$in$surgical$ procedures$on$this$area.$ $

The Posterior Interosseous Nerve

The$Flexors$

The$Forearm$musculature$is$invested$with$deep$fascia$which$forms$ an$inelastic$sleeve$of$fibrous$tissue$encasing$the$muscle$mass.$ $ This$sleeve$is$divided$into$compartments:$ The$Mobile$Wad$ The$Extensor$compartment$ The$flexor$compartment$ $ $ Each$of$these$compartments$is$relatively$inelastic.$Any$pathology,$such$as$bleeding$from$a$ fracture,$can$cause$the$contents$of$the$compartment$to$swell$and$the$pressure$to$rise$ such$that$it$impairs$blood$supply.$ $ This$is$a$serious$emergency$which$is$called$COMPARTMENT$SYNDROME.$It$presents$with$ severe$pain,$especially$provoked$by$passive$stretch$of$the$muscles$in$the$affected$ compartment.$ It$requires$a$high$index$of$suspicion,$early$surgery$and$release$of$the$fascia$to$relax$ tension.$ $ $ Ulna$ Radius$ PL$ FCR$ BR$

FCU$

FDS$

PT$ FDP$

ECRL$ Ulna$ Sup$ Radius$

ECU$ EDM$ EDC$ MOBILE$$ VOLAR$ WAD$

DORSAL$

Even$in$the$cadaver$limb$it$is$easy$to$see$how$restraining$the$ﬁrm$deep$fascia$is$on$the$ contained$muscles.$Removal$of$the$fascia$produces$$relaxation$of$the$entire$muscle$group.$ $

ORGANISATION$OF$THE$FLEXOR$MUSCLE$COMPARTMENT$ $ Superficial$group:$ Pronator$Teres$ Flexor$Carpi$Radialis$ Palmaris$Longus$ Flexor$Digitorum$Superficialis$$ Flexor$Carpi$Ulnaris$ Most$of$these$take$part/whole$origin$from$the$common$flexor$origin.$These$muscles$cross$ the$elbow$joint$ $ $ Deep$Group$ The$Flexor$Pollicis$Longus$ The$flexor$digitorum$communis$ The$Pronator$Quadratus.$ These$lie$in$the$deep$plane$of$the$forearm,$on$radius$and$ulna$and$interosseous$ membrane.$These$muscles$do$not$cross$the$elbow$joint.$ $ $ $ $ The$Superficial$Flexors$ The$Common$Flexor$Origin$ $ $ $ Pronator$Teres*$ Flexor$Carpi$Radialis$ Palmaris$Longus$ Flexor$Digitorum$Superficialis$*$ Flexor$Carpi$Ulnaris$ $ $ $ *$Dual$origin$ The$Deep$Flexors$ $ Flexor$Pollicis$Longus$ Flexor$Digitorum$Profundus$ $ Pronator$Quadratus$ $

The$anterior$view$of$the$forearm$shows$the$ﬂexor$muscle$mass$but$also$the$mobile$wad$ which$invests$the$radial$border$of$the$forearm$and$is$therefore$visible$from$the$front.$ $ $ $

Organisation$of$the$Superficial$Flexors:$ $ $ The$five$flexors$that$take$origin$from$the$Common$Flexor$Origin$fan$out$from$the$medial$ epicondyle.$ If$the$heel$of$the$hand$is$placed$on$the$opposite$medial$epicondyle,$palm$down$on$the$ forearm,$the$digits$point$along$the$five$superficial$muscles.$ $ Thumb:$ $Pronator$$Teres$ Index:$ $Flexor$Carpi$Radialis$ Middle: $$Palmaris$Longus$ Ring: $$Flexor$Digitorum$superficialis$ Little: $$Flexor$Carpi$Ulnaris$ $ $ $

PT$

FCR$

FCU$ PL$

FDS$ THE$PRONATOR$TERES$ $C6,7$ $ This$has$dual$origin$ One$head$arises$from$the$Common$ﬂexor$Origin$and$from$the$supracondylar$ridge$ The$deeper$smaller$head$arises$from$the$coronoid$process$of$the$ulna$ $ The$two$heads$clasp$the$median$nerve$which$enters$the$forearm$in$this$plane.$ $ They$both$insert$into$the$most$prominent$part$of$the$convexity$of$the$radius$ $ $ The$median$nerve$can$be$compressed$inside$the$pronator$teres$muscle,$and$mimic$carpal$ tunnel$syndrome.$ $ Pronator$$ Teres$

1.Common$Flexor$$ origin$and$above$ Medial$Epicond.$ 2.$Medial$Coronoid$ $ $ $ $ Midway$point$radius$

THE$FLEXOR$CARPI$RADIALIS $C6,7$ $ Arises$from$the$common$flexor$origin$–$a$fleshy$belly$which$produces$a$thick,$rounded$ tendon.$ $ It$travels$down$the$radial$border$of$the$forearm$(the$radial$artery$is$just$deep$to$its$radial$ border).$ It$enters$the$hand$over$the$tubercle$of$the$scaphoid$which$enables$it$to$change$direction$ and$point$into$the$deep$palm.$It$then$passes$through$its$own$fibrous$tunnel$in$the$lee$of$ the$ridge$of$trapezium$and$inserts$into$the$palmar$bases$of$the$2nd$and$3rd$metacarpals.$ $ Its$action$is$to$stabilise,$flex,$and$radially$abduct$the$wrist$ $ Flexor$ Carpi$ Radialis$

Common$Flexor$$ origin$ $ $ $ $ $ Base$of$Metacarpals$ Index$and$Middle$

Insertion$of$Flexor$Carpi$Radialis$ THE$PALMARIS$LONGUS $C7,8$ $ This$muscle$is$absent$in$some$15%$of$subjects.$It$has$a$short$belly$and$a$long,$thin$tendon$ (features$of$phylogenetic$degeneration.$It$is$supposed$to$have$been$more$useful$in$our$ ancestors,$when$grasping$tree$branches$and$walking$on$all$fours$may$have$been$the$ norm)$ It$enters$the$hand$on$the$palmar$surface$of$the$Flexor$retinaculum$ Its$fibres$are$in$continuity$with$the$palmar$aponeurosis$which$fans$across$the$palm$and,$ via$dual$slips,$inserts$into$the$bases$of$the$proximal$phalanges.$ It$is$a$weak$flexor$of$the$proximal$phalanges.$ $ It$is$functionally$unimportant.$Those$in$whom$it$is$absent,$show$no$functional$deficit.$ It$is$a$convenient$harvest$tendon$when$a$tendon$graft$is$required.$ $ $ $ Palmaris$ Longus$

Common$Flexor$$ origin$ $ $ $ $ $ Palmar$Aponeurosis$ $

THE$FLEXOR$DIGITROUM$SUPERFICIALIS $C7,8$ $ This$has$dual$origin$ One$head$arises$from$the$Common$flexor$Origin$,$the$medial$ligament$of$the$elbow,$and$ the$coronoid$process$of$the$ulna$ One$head$arises$from$the$whole$length$of$the$anterior$oblique$line$of$the$radius.$ $ A$fibrous$arch$connects$the$two$heads,$beneath$which$the$Median$nerve$dips$into$the$ forearm$musculature.$ $ This$bulky$muscle$give$rise$to$four$individual$tendons$which$pass$through$the$carpal$ tunnel$and$into$the$flexor$sheaths$in$the$digits.$Each$tunnel$forms$a$chiasma,$or$division,$ through$which$the$flexor$digitorum$profundus$proceeds.$The$FDS$inserts$into$the$full$ length$of$the$middle$phalanx$of$the$digit.$ $ $ $ Flexor$ Digitorum$ Superficialis$

HumeroUUlnar$Origin:$ a.$Common$Flexor$$ $$$$$Origin$ b.$Ulnar$Coronoid$ $ Radial$origin:$ Anterior$Radial$Border$ $ $ $ $ Index,$Middle,$ Ring,$little.$ Middle$Phalanx$ $

The$Flexor$Digitorum$superficialis$is$itself$divided$into$superficial$and$deep$parts.$ $ The$superficial$part$gives$rise$to$the$tendons$of$$middle$and$ring$fingers$ $ The$deep$part$gives$rise$to$the$tendons$of$index$and$little$ $ $ $ $ The$deep$part$is$frequently$and$variably$digastric,$with$an$INTERMEDIATE$TENDON$ between$its$proximal$and$distal$parts.$This$tendon$can$be$well$formed$and$lies$adjacent$to$ the$median$nerve$on$the$deep$surface$of$the$muscle.$ $ $ $ The$Intermediate$Tendon$ Middle$and$Ring$tendons$ $ Dual$Origin$$ $HumeroUUlnar$ $Radial$ $ $ $ $ Index$and$Little$Tendons$ HumeroUUlnar$ $ (often$with$Intermediate$$ Tendon)$ $ THE$FLEXOR$CARPI$ULNARIS $C8,T1$ $ This$arises$by$two$heads$ $ One$head$arises$from$the$common$flexor$origin$ One$head$arises$from$a$wide$aponeurosis$on$the$subcutaneous$surface$of$the$posterior$ ulna.$ A$fibrous$arch$connects$the$two,$beneath$which$the$ulnar$nerve$enters$the$forearm$(from$ its$brief$sojourn$in$the$extensor$compartment).$The$ulnar$nerve$runs,$with$the$ulnar$artery,$ beneath$the$muscle$all$the$way$to$the$wrist.$ $ Its$tendon$receives$muscle$fibres$virtually$as$far$down$as$the$wrist.$ $ It$inserts,$via$the$pisiform$bone,$which$is$a$sesamoid,$and$the$pisoUhamate$and$$ pisoUmetacarpal$ligament,$into$the$hamate$and$the$5th$metacarpal.$ $ It$stabilises,$flexes$and$ulnar$abducts$the$wrist.$ $ $ $ Flexor$ Carpi$Ulnaris$ $ Common$$ Flexor$Origin$ +$ Medial$Olecranon$ And$Proximal$2/3$ Ulna,$Subcut$.$ border$ $ $ Pisiform$ (5th$Metacarpal$ +$ Hamate)$

Insertion$of$Flexor$Carpi$Ulnaris$

PisoUHamate$ PisoUMetacarpal$ Ligament$ Ligament$ LONG$TENDON$STABILISATION$OF$THE$WRIST$ $ All$around$the$circumference$of$the$wrist$are$long,$string$tendons$which,$when$tensed,$ stabilise$the$carpus$on$the$radius.$ $ Thus$U$Starting$on$the$radial$border$and$in$sequence:$ $ Abductor$Pollicis$longus$ Flexor$Carpi$Radialis$ Flexor$Carpi$Ulnaris$ Extensor$Carpi$Ulnaris$ Extensor$Carpi$Radialis$Brevis$ Extensor$Carpi$Radialis$Longus$ $ Form$a$full$circle$of$strong$guy$ropes$around$the$wrist.$At$the$$moment$of$impact$of$a$ punch,$all$of$these$tendons$(along$with$all$flexors$and$extensors)$are$tense$and$taut.$This$is$ the$main$$extrinsic$stabilisiing$element$of$the$wrist,$without$which$the$carpus$would$ buckle$on$the$relatively$flat$platform$of$the$distal$radius.$ $ $ FCR$ FCU$ Radius$ Ulna$ PL$ FCR$ FCU$ FDS$ The$Deep$Flexors$ $ Flexor$Pollicis$Longus$ Flexor$Digitorum$Profundus$ (Pronator$Quadratus)$ $ $ THE$DEEP$FLEXORS$ $ Three$deep$flexors$lie$along$the$front$of$the$radius,$ulna$and$interosseous$membrane.$ $ Two$Flexors:$$ The$Flexor$Digitorum$Profundus$ The$Flexor$Pollicis$Longus$ Cost$the$back$of$the$radius$and$ulnar,$and$interosseous$membrane,$leaving$a$narrow$ valley$down$the$back$of$the$membrane$in$which$run$the$anterior$interosseous$nerve$and$ artery,$which$supply$these$muscles$ $ One$pronator:$ The$pronator$Quadratus:$ This$square$muscle$joins$radius$and$ulna$in$the$distal$1/3rd$$ $ THE$FLEXOR$POLLICIS$LONGUS $C8,T1$ $ This$arises$from$the$anterior$surface$of$the$radius$and$adjacent$interosseous$membrane.$ It$forms$a$round,$strong$tendon$and$enters$the$hand$as$the$most$radial$structure$in$the$ carpal$tunnel.$ It$is$the$only$long$flexor$to$the$thumb.$It$enters$the$thumb$flexor$sheath$and$inserts$into$ the$distal$phalanx.$ $ It$is$supplied$by$the$anterior$interosseous$nerve,$a$branch$of$the$median.$Testing$the$FPL,$ by$resisting$flexion$of$the$interphalangeal$joint,$tests$for$high$median$nerve$compression.$ $ $ $ $ $ Flexor$ Pollicis$ Longus$

Radius$and$$ Interosseous$$ membrane$ $ $ $ $ Distal$Phalanx$ Thumb$ $ THE$FLEXOR$DIGITORUM$PROFUNDUS $C7,8$ $ This$the$most$powerful$of$the$forearm$muscles.$ It$arises$from$the$olecranon$process$of$the$ulna,$the$anterior$surface$of$the$ulna$and$the$ interosseous$membrane.$ $ The$tendons$which$emerge$from$the$muscle$belly$are$meshed$together,$with$the$index$ tendon$separating$earlier$than$the$others.$This$means$that$flexion$is$‘mass$action’.$It$is$ impossible$to$flex$middle,$ring$or$little$without$flexing$the$neighbours.$The$index$is$more$ independent.$ $ These$tendons$pass$through$the$carpal$tunnel$and$flare$out$towards$the$flexor$sheaths$in$ each$digit.$ Inside$the$flexor$tendon$sheath$the$profundus$passes$through$the$chiasma$of$the$ superficialis$to$reach$its$destination$–$the$base$of$the$distal$phalanx.$Action:$It$produces$ strong,$full$rollUup$into$flexion,$of$DIPJ,$PIPJ,$and$MCPJ.$ $ In$the$bare$area$between$Carpal$tunnel$and$Flexor$sheath,$the$FDP$tendons$give$rise$to$ the$lumbricals.$Each$lumbrical$travels$on$the$radial$side$of$its$own$digit,$to$insert$into$the$ extensor$tendon.$ $ The$nerve$supply$is$shared:$ Median$for$the$radial$two$digts$ Ulnar$for$the$ulnar$two$digits$ $ Flexor$ Digitorum$$ Profundus$

Ulna$and$$ Interosseous$$ membrane$ $ $ $ $ Distal$Phalanges$ Digits$F2$–F5$ $ Mass$Tendon$of$FDP$ SeparateTendons$of$FDS$ FDP$ FPL$ FCU$ FCR$ PL$

FDS$

FDP$ FPL$

Insertion$of: $Flexor$Digitorum$Superﬁcialis$ $$$$Flexor$Digitorum$Profundus$

THE$PRONATOR$QUADRATUS $C8,T1$ $ This$Square$shaped$muscle$arises$from$the$distal$1/4th$of$the$anterior$surface$of$the$ulna$ and$inserts$into$the$same$area$on$the$front$of$the$radius.$ $ $Despite$its$square,$flat$appearance,$it$is$thick$and$powerful,$since$$its$fibres$arise/$insert$ into$a$wide$area$on$the$front$of$the$radius.$ $ It$is$a$powerful$pronator$of$the$forearm,$especially$with$the$elbow$flexed$when$Pronator$ Teres$is$less$powerful.$ Pronator$$ Quadratus$

Distal$ulna$ $ $ $ $ Distal$radius$ $ FDP$ FPL$

PQ$

FPL$ FDP$ PQ$

FPL$ FDP$ The$organisation$of$Finger$flexors$ $ One$can$consider$three$layers$of$$flexors,$in$sequence,$starting$with$the$superficial:$ $ 1$ $The$Palmaris$Longus$inserts,$via$the$continuity$of$the$Palmar$Aponeurosis,$into$the$ base$of$the$proximal$phalanges.$It$is$a$very$weak$flexor$and$to$be$discounted$in$terms$of$ function$and$power$ $ 2$ $The$Flexor$Digitorum$superficialis,$which$inserts$into$the$middle$phalanx$of$the$digit$ $ 3$ $The$Flexor$digitorum$profundus,$which$inserts$into$the$distal$phalanx$of$the$digits$ $ $ $ PL

FDS

FDP The$Most$superﬁcial,$The$Palmaris$Longus/Palmar$aponeurosis$inserts$ﬁrst$ The$next$layer$inserts$next$ The$deepest$layer$inserts$most$distal.$ $ $ This$means$that$each$layer$must$let$the$deeper$layers$through.$ $ In$fact,$each$layer$opens$in$the$form$of$chiasmata,$to$let$through$the$next$tendon:$ $ The$Palmar$aponeurosis$lets$thrpugh$the$FDS$and$FDP$ The$FDS$lets$through$the$FDP$in$its$digital$Chiasma$ $ $ PL

FDS

FDP

P3 The$functional$beneﬁt$of$this$rather$counterUintuitive$arrangement,$is$that$each$layer$acts$ as$a$retainer$of$the$one$beneath,$providing$additional$support$against$bowstringing$of$the$ tendons,$much$as$the$more$formal$pulleys$do$in$the$ﬂexor$sheath$in$the$digit$(see$‘The$ Hand’).$ $ $ PL

FDS

FDP Flexion$and$extension$of$the$digits$is$effective$mainly$because$there$is$a$system$of$ stabilisation$of$the$wrist,$which$provides$a$firm$platform,$and$reference$point$for$the$ fingers$to$flex$and$extend.$ $ The$wrist$flexors$and$extensors$ech$have$their$own$particular$actions$on$the$wrist,$e.g.$The$ FCR,$will$flex$the$wrist$and$also$produce$radial$abduction.$ $ But$the$more$important$function$of$these$wrist$tendons$is$stabilisation.$They$often$act$in$ concert,$one$antagonist$acting$with$an$agonist$to$produce$stability,$rather$than$ movement.$ $

Although$we$speak$of$an$‘action’$of$a$muscle,$this$never$happens$in$isolation.$Never$does$ a$muscle$act$without$the$collaboration$of$a$number$of$others,$sometimes$surprisingly$ numerous,$even$for$the$simplest$action.$ $

As$an$example$of$this$multiUmuscled$action$and$collaboration,$consider$the$simple$action$ of$Abduction$of$the$little$ﬁnger$

This$involves$the$seemingly$isolated$action$of$the$Abductor$Digit$Minimi$

The$abductor$Digiti$Minimi$arises$from$the$pisiform$bone,$which$is$a$mobile$sesamoid$in$ the$tendon$of$the$Flexor$Carpi$Ulnaris$

In$order$for$the$ADM$to$have$a$stable$platform,$the$pisiform$must$be$stabilised.$$ So$the$FCU$contracts,$to$stabilise$the$pisiform$

If$the$FCU$were$to$contract$unopposed,$the$wrist$would$ﬂex$

To$counter$the$ﬂexion,$the$Extensor$Carpi$Ulnaris$also$contracts$

If$FCU$and$ECU$were$to$contract$unopposed,$this$would$produce$ulnar$abduction$of$the$ wrist$

To$counter$Ulnar$abduction,$the$Flexor$Carpi$Radialis$contracts$

If$FCR$were$to$contract$unopposed,$this$would$produce$ﬂexion$of$the$wrist$

To$counter$ﬂexion,$the$Abductor$Pollicis$longus$and$the$ECRL$and$ECRB,$all$contract$ $ $

The$apparently$simple$action$of$abduction$of$the$little$ﬁnger$requires$the$contraction$and$ synchronous$collaboration,$of$and$number$of$muscles$and$the$stabilisation$of$a$number$of$ bones:$ 6$Long$tendons$ 8$Carpal$bones$ 4$Metacarpals$ $ $ $ Ab$DM$action$

6$Long$tendons$ 8$Carpal$bones$ 4$Metacarpals$ www.donaldsammut.com