Biomechanics of the Finger

I. Schöffl

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Anatomy: Pulley System

• 4 anular fibroosseous pulleys (A1, A2, A4, A5) • 1 anular pulley inserting on the palmar plate (A3) • 3 cruciform pulleys (C1, C2, and C3)

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Anatomy: Pulley System

• FDP runs through the pulleys A2, A3, and A4 • FDS only runs through the pulleys A2 and A3 => The A4 pulley is not loaded by the FDS tendon.

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Histology: Pulley System

• The pulley consists of three layers: 1. Inner layer: connective tissue

with collagen running perpendicular to the digital axis

2. Middle layer: dense connective tissue with elastin fibrils also perpendicular to the digital axis

3. Outermost layer: loose connective tissue with numerous vessels.

• The inner layer secrets hyaluronic acid.

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Histology: Pulley System

•The inner layer has multiple folds and creases.

Hyaluronic acid

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Crimping and Hanging Fingerposition

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Biomechanics: Pulley System

Roloff, Schöffl et al. J Biomech 2005

)2

cos(**2

TTFP

is a function of :

- (PIP-angle)

- Position of the pulley

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Concentric vs. Eccentric Motion

Concentric Motion consists of Flexing of

the finger.

Eccentric Motion consists of Extending the

previously flexed finger.

Eccentric Movements are always stronger

than concentric Movements as a

consequence of the allignment of the

muscle fibres.

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The Importance of the Eccentric Motion

Birds and Bats show a Tendon locking mechanism

leading to a high strength deficit enabling them to

hang effortlessly on their closed fingers.

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Importance of the Eccentric Motion

Same histology in humans?

Possible pathomechanism for pulley ruptures:

„opening of the hand“ when tired or when loosing a

foothold.

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Material and Methods of Anatomic Study

FDS/FDP1:

Hanging = 1:1

Crimp grip = 1:2

1 Vigouroux et al., Journal of Biomechanics 40, 2007

2 Schweizer et al., Journal of Biomechanics 36, 2003

Eccentric

Concentric

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Measuring of the Forces acting underneath the pulley System

Specifically designed force transducers: • 1 finger with trasducer under A2 pulley • 1 finger with transducer under A2 and A4 pulley

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Results: Forces acting underneath the pulleys

0

20

40

60

80

100

120

140

0 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 64

Zeit (s)

Kra

ft (

N)

A 2 M es sung

A 2 Rechnung

A 4 M es sung

A 4 Rechnung

0

20

40

60

80

100

120

140

160

0 4 8 12 16 20 24 28 32 36 40

Zeit (s)

Kra

ft (

N)

A 2 M es sung

A 2 Rechnung

Schöffl et al., J Sports Tech., 2009

A2 Measurement

A2 Calculation

A4 Measurement

A2 Calculation

A2 Calculation

A2 Measurement

Forc

e (

N)

Forc

e (

N)

Time (sec)

Time (sec)

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Results: Hanging vs. Crimp Grip Position during Concentric Movement

Verteilung Erstruptur

0

10

20

30

40

50

60

70

80

Hängend Aufgestellt

Fingerposition

Ru

ptu

rereig

nis

(%

)

A2

A4

Alternativ

Schöffl et al. 1, J. of Biomechanics, 2009

First Event

Crimp Grip Hanging

Prim

ary

Failure

in

%

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Results: Hanging vs. Crimp Grip Position during Concentric Movement

Schöffl et al. 1, J. of Biomechanics, 2009

The forces acting on the pulleys at the moment of rupture are higher on the pulleys for crimp but comparable for FDP and fingertip.

The distance between pulleys at the moment of rupture is lower for cimp grip (pulleys approach each-other).

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Discussion: Hanging vs. Crimp Grip Position

A pulley rupture was rare in the hanging finger

position as a consequence of lower forces acting

on the pulleys.

Fp2

Fp4

Fp2

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Discussion: Hanging vs. Crimp Grip Position

A pulley rupture was rare in the hanging finger

position as a consequence of lower forces acting

on the pulleys.

No A4 pulley rupture in the hanging finger

position because of the lesser influence of FDS.

FDP = 1

FDS = 1 Total= 1

FDP = 1

FDS = 1/2

Total = 1 1/2

Total = 1 Total = 2

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Discussion: Hanging vs. Crimp Grip Position

However, the forces at the first event were

comparable. Thus the fact that the pulleys

approach each other in the crimp grip is a

protective factor for other injuries.

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Discussion: Hanging vs. Crimp Grip Position

Slopers are better than Crimps

Verteilung Erstruptur

0

10

20

30

40

50

60

70

Exzentrisch Konzentrisch

Belastungsform

Ru

ptu

rereig

nis

(%

)

A2

A4

Alternativ

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Results: Concentric vs. Eccentric Motion

First Event

Concentric Eccentric

Prim

ary

Failure

in %

Schöffl et al. 1, J. of Biomechanics, 2009

Loading condition

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Results: Concentric vs. Eccentric Motion

Schöffl et al. 1, J. of Biomechanics, 2009

At the moment of pulley rupture comparable forces on the pulleys but lower in FDP and fingertip = Less force needed for pulley rupture in eccentric movement.

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Discussion: Concentric vs. Eccentric Motion

Ruptures occured at far lower forces for the

Eccentric Motion => Friction underneath the

pulleys also relevant in humans

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Discussion: Concentric vs. Eccentric Motion

Ruptures occured at far lower forces for the

Eccentric Motion => Friction underneath the

pulleys also relevant in humans

A2 rupture was more common

in the Eccentric Motion. This reflects

the injuries we observe.

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Discussion: Concentric vs. Eccentric Motion

Ruptures occured at far lower forces for the

Eccentric Motion => Friction underneath the

pulleys also relevant in humans

A2 rupture was more common

in the Eccentric Motion. This reflects

the injuries we observe.

Beware with difficult

moves when being

tired!

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Thank you for your attention