The Flexors and Extensors Donald Sammut Hand Surgeon KCH Upper Limb Anatomy 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$ Tension$ Length$ Length$ Passive$stretch$ Active$contracture$ 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$RADIALIS$