I. Schöffl. Biomechanics of the Finger

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1 I. Schöffl Biomechanics of the Finger

2 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) 2

3 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. 3

Histology: Pulley System The pulley consists of three layers: 1. Inner layer: connective tissue with collagen running perpendicular to the digital axis 2.

4 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. 4

5 Histology: Pulley System The inner layer has multiple folds and creases. Hyaluronic acid 5

6 Crimping and Hanging Fingerposition 6

7 Biomechanics: Pulley System F P 2* TT *cos( ) 2 is a function of : - (PIP-angle) - Position of the pulley 7 Roloff, Schöffl et al. J Biomech 2005

8 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. 8

9 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. 9

10 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. 10

11 Material and Methods of Anatomic Study FDS/FDP 1 : Hanging = 1:1 Crimp grip = 1:2 Eccentric Concentric 1 Vigouroux et al., Journal of Biomechanics 40, Schweizer et al., Journal of Biomechanics 36,

12 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 12

13 Force (N) Force (N) Results: Forces acting underneath the pulleys AA2 2 MMeasurement es s ung AA2 2 Rec Calculation hnung AA4 4 MMeasurement es s ung AA2 4 Rec Calculation hnung Time Zeit (s) (sec) Schöffl et al., J Sports Tech., AA2 2 Measuremen M es s ung A 2 Rec hnung A2 Calculation Zeit (s) Time (sec)

14 Primary Failure in % Results: Hanging vs. Crimp Grip Position during Concentric Movement 80 First Event Verteilung Erstruptur A2 A4 Alternativ Hanging Hängend Fingerposition Crimp Aufgestellt Grip Schöffl et al. 1, J. of Biomechanics,

15 Results: Hanging vs. Crimp Grip Position during Concentric Movement 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). 15 Schöffl et al. 1, J. of Biomechanics, 2009

16 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. F p2 F p4 F p2 16

17 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 = 2 Total = 1 Total = 1 1/2 17

18 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. 18

19 Discussion: Hanging vs. Crimp Grip Position Slopers are better than Crimps 19

20 Primary Failure in % Results: Concentric vs. Eccentric Motion 70 First Event Verteilung Erstruptur A2 A4 Alternativ Eccentric Exzentrisch Belastungsform Loading condition Concentric Konzentrisch Schöffl et al. 1, J. of Biomechanics,

21 Results: Concentric vs. Eccentric Motion 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. Schöffl et al. 1, J. of Biomechanics,

22 Discussion: Concentric vs. Eccentric Motion Ruptures occured at far lower forces for the Eccentric Motion => Friction underneath the pulleys also relevant in humans 22

23 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. 23

24 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! 24

25 Thank you for your attention 25

Elements of the Physics of Pulley Injury (draft)

Elements of the Physics of Pulley Injury (draft) I. Introduction. Rock climbers have long been plagued with finger injuries due to various climbing activities, almost all of which include crimping at some