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Research Papers

# Assessment by Finite Element Modeling Indicates That Surgical Intramuscular Aponeurotomy Performed Closer to the Tendon Enhances Intended Acute Effects in Extramuscularly Connected Muscle

[+] Author and Article Information
Can A. Yucesoy1

Biomedical Engineering Institute, Boğaziçi University, Bebek, Istanbul 34342, Turkey; Instituut voor Fundamentele en Klinische Bewegingswetenschappen, Faculteit Bewegingswetenschappen, Vrije Universiteit, De Biomedical 1081, 1081 HV Amsterdam, The Netherlands

Peter A. Huijing

Instituut voor Fundamentele en Klinische Bewegingswetenschappen, Faculteit Bewegingswetenschappen, Vrije Universiteit, De Biomedical 1081, 1081 HV Amsterdam, The Netherlands; Integrated Biomedical Engineering for Restoration of Human Function, Faculteit Constructieve Technische Wetenschappen, Universiteit Twente, De Vrijhof, Drienerlolaan 5, 7522 NB Enschede, The Netherlands

1

Corresponding author.

J Biomech Eng 131(2), 021012 (Dec 10, 2008) (9 pages) doi:10.1115/1.3005156 History: Received November 23, 2007; Revised September 22, 2008; Published December 10, 2008

## Abstract

The effects of location of aponeurotomy on the muscular mechanics of extramuscularly connected muscle were assessed. Using finite element modeling, extensor digitorum longus muscle of the rat was studied for the effects of aponeurotomy performed in each of three locations on the proximal aponeurosis: (1) a proximal location (case P), (2) an intermediate location (case I), and (3) a distal location (case D). Proximo-distal force differences were more pronounced for more proximal aponeurotomy. The location also affected proximally and distally assessed muscle length-force characteristics: (1) Muscle optimum length and active slack length shifted differentially to higher lengths, increasing slack to optimum length range (for D to P: distally by 15–44%; proximally by 2–6%). (2) Muscle forces decreased at all lengths (e.g., for D to P distal optimal $force=88–68%$ and proximal optimal $force=87–60%$ of intact values, respectively). Increased length range and force decreases were highest for case P, as were effects on muscle geometry: gap length within the proximal aponeurosis; decreased proximal fiber population pennation angle. Parallel, but not serial, heterogeneity of sarcomere length was highest in case P: (a) For the distal fiber population, sarcomere shortening was highest; (b) for the proximal population, sarcomeres were longer. It is concluded that if aponeurotomy is performed closer to the tendon, intended surgical effects are more pronounced. For bi-articular muscle, mechanics of both proximal and distal joints will be affected, which should be considered in selecting the location of aponeurotomy for optimal results at both joints.

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Topics: Force , Muscle , Fibers , Tendons

## Figures

Figure 1

Finite element modeling of extramuscularly connected EDL muscle and aponeurotomy. (a) The model of EDL muscle, modeled extramuscular connections, and locations of aponeurotomy. Details of the model and extramuscular connections were described elsewhere (13). In short, the nodes of the matrix mesh marked by a white “+” sign have connections to mechanical ground representing the muscles’ extramuscular connections. The nodes marked also by a black square show the stiffer proximal segment of extramuscular connections (6). The nodes marked by a circle indicate the different locations of aponeurotomy modeled (at both upper and lower faces of the model). (b) The nature of aponeurotomy (using transverse incisions) shown schematically. The dashed lines illustrate the modeled longitudinal muscle slice at the middle of the muscle belly. (c) Typical deformed shape after distal lengthening of modeled aponeurotomized muscle (case I is the example shown). The gap between the cut ends of the aponeurosis creates two distinct populations of muscle fibers: “proximal population” and “distal population.” The plane marked by dotted lines shows the interface between the two populations.

Figure 2

The isometric muscle length-force curves of modeled aponeurotomized and intact EDL muscles. Active and passive isometric forces of aponeurotomized muscles with the aponeurotomy modeled at proximal (case P), intermediate (case I), and distal (case D) locations. (a) Effects on distally exerted muscle forces. (b) Effects on proximally exerted muscle forces. To quantify the reduction in muscle force due to different aponeurotomy cases, active and passive isometric forces of the intact muscle are considered: All sets of data are normalized for optimum distal force of intact muscle. Active slack lengths (dotted arrows) and optimum lengths (solid arrows) are marked to indicate location of aponeurotomy dependent shift to a higher length.

Figure 3

The proximo-distal total force differences for modeled aponeurotomized and intact EDL muscles. Total (ΔFmt) proximo-distal force differences (Fdist−Fprox) of aponeurotomized muscles with the aponeurotomy modeled at proximal (case P), intermediate (case I), and distal (case D) locations. Proximo-distal force differences are normalized for force difference of intact muscle encountered at muscle length=30.7 (indicated with dotted line).

Figure 4

Fiber direction strains within modeled muscles aponeurotomized at different locations. (Left) The strain distributions within the fiber mesh of active aponeurotomized muscles for low muscle length (i.e., 25.2mm). (Right) The strain distributions within the fiber mesh of active aponeurotomized muscles for high muscle length (i.e., 31.2mm). Contour plots are shown for the aponeurotomy modeled at proximal (case P, i.e., (a) and (d)), intermediate (case I, i.e., (b) and (e)), and distal (case D, i.e., (c) and (f)). The dotted line contour indicates passive muscle geometry at the initial length. The local fiber direction, as well as the proximal and distal ends of the muscle, is indicated (f). The decreased pennation angle of the most proximal muscle fibers in case P compared to case D is shown (f). “Zone of very short sarcomeres” is indicated in (a) and (d). MX, MN, X, and Y represent maximal and minimal strains and the global coordinates.

Figure 5

Length of the gap within the EDL proximal aponeurosis as a function of muscle length. The effects of location of aponeurotomy on the length of the gap between the two cut ends of the proximal aponeurosis are shown. The data are normalized for maximum gap length of case P. Note that for each case EDL length is expressed as a function of deviation from its own muscle optimum length (0mm).

Figure 7

Fiber direction stresses within modeled aponeurotomized muscles. (Left) The stress distributions within the fiber mesh of active aponeurotomized muscles for low muscle length (i.e., 25.2mm). (Right) The stress distributions within the fiber mesh of active aponeurotomized muscles for high muscle length (i.e., 31.2mm). Contour plots are shown for aponeurotomy modeled at proximal (case P, i.e., (a) and (d)), intermediate (case I, i.e., (b) and (e)), and distal locations (case D, i.e., (c) and (f)). The dotted line contour indicates passive muscle geometry at the initial length. The local fiber direction, as well as the proximal and distal ends of the muscle, is indicated (f). MX, MN, X, and Y represent maximal and minimal stresses and the global coordinates.

Figure 6

Distributions of mean fiber direction strain and stress within fascicles of modeled aponeurotomized muscles. Mean fiber direction strains (left panel: (a) and (b)) were used to assess the heterogeneity of mean sarcomere lengths of different fibers within the muscle (i.e., parallel distribution of sarcomere lengths). Mean fiber direction stress (right panel: (c) and (d)) was used to assess the overall potential of active force exertion. (e) Definition of fascicle numbers within muscle geometry (case I is shown). Each fascicle interface is indicated (from 1 to 7). Mean fiber direction strains and stresses were calculated at nodes of the myofiber elements in series (i.e., within the fiber mesh) representing a muscle fascicle. Note that for the fascicle corresponding to the location of aponeurotomy (e.g., fascicle 4 for case I), the mean of nodal strain or stress values includes the values calculated for the two cut ends of the proximal aponeurosis in addition to the strain or stress values calculated for the remainder of three nodes (i.e., a mean of five strain values). For other fascicles, however, the mean includes only the strains or stresses in the four nodes for the same fascicle interface.

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