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Extensor mechanism of the fingers. I. A quantitative geometric study
Extensor mechanism of the fingers. I. A quantitative geometric study A close-range stereophotogrammetric measurement system was used to determine the three-dimensional geometric characteristics of the extensor assembly in seven human finger specimens and five finger configurations. The numerical data obtained showed that, although changes in length of the different bundles are small, their spatial orientation varies considerably from one to another position. This information should help to improve the accuracy of models derived to understand the extensor assembly behavior in normal and pathological conditions. (J HAND SURG 1991;16A:1130-6.)
Marc Garcia-Elias, MD, Kai-Nan An, PhD, Lawrence Berglund, BS, Ronald L. Linscheid, MD, William P. Cooney III, MD, and Edmund Y.S. Chao, PhD Rochester, Minn.
In
recent years there has been increasing interest in the anatomical and functional complexities of the extensor mechanism of the fingers. The spatial relationships between its different components and their associated joints have been extensively studied by several authors.!" Quantitative description, however, has been limited. Schultz and colleagues," using a movie camera attached to the operating microscope, measured the projected angle between the intercrossing fibers of fresh cadaver specimens during simulated finger motion. That angle was found to change from 30 to 50 degrees as the finger was flexed from full extension. A similar study using photographs taken from the lateral side of cadaver specimens was reported by van Zwieten." In both studies, measurements were obtained from planar images; therefore, their results may not be accurate three-dimensionally. From the Orthopedic Biomechanics Laboratory, Department of Orthopedics, Mayo Clinic/Mayo Foundation, Rochester, Minn. Supported by Grant AR-17172, awarded by the National Institutes of Health. Received for publication June 21, 1990; accepted in revised form Aug. 11, 1990. No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. Reprint requests: Kai-Nan An, PhD, Biomechanics Laboratory, Department of Orthopedics, Mayo Clinic/Mayo Foundation, Rochester, MN
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,, Fig. 1. Schematic representation of the extensor assembly of the finger and the different markers (represented in letters) defining the different segments discussed in the text.
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Table I. Applied finger tendon loads (in grams) to produce and maintain the five finger positions Position
Interosseous
Lumbrical
Intrinsic minus Extended Midflexed Full flexed Intrinsic plus
125 250 250 500 750
125 500
Flexor Profundus
Extensor
1,250
1,500 500 250 750 250
250 750
250 500
Table II. Changes in length (mm) and orientation (degree) of different zones of the extensor mechanism (n = 7). Abbreviations as in Fig. I Intrinsic minus (I)
AB BC CD AF GC GF FE
HA GB (2)
GHA FGB FAC FGC GFD DFE
I
Extension
SD
Mean
14.2 32.8 11.2 49.3 36.2 33.9 17.1 18.0 16.2
1.8 4.8 1.8 3.7 1.9 1.3 4.2 3.3 1.9
14.4 32.5 10.6 50.9 35.9 37.1 18.2 16.7 17.1
Mean
SD
95.6 81.5 13.4 18.4 123.7 68.5
4.4 17.1 2.6 3.9 8.0 15.1
Mean
I
Midflexion
SD
Mean
2.2 5.2 1.6 4.2 1.7 2.6 4.3 3.1 2.3
14.7 33.1 10.9 50.8 35.1 35.3 17.7 16.5 16.2
Mean
SD
Mean
111.6 75.1 7.7 11.9 141.5 37.2
12.8 14.7 1.8 2.8 8.8 8.1
90.7 83.8 10.8 16.1 131.6 54.2
I
Full flexion
SD 2.1 4.9 I.7 3.6 1.4 1.8 3.9 2.0 1.5
Mean
I
Intrinsic plus
SD
Mean
I
SD
14.8 33.6 11.2 48.6 35.5 31.6 17.4 17.6 16.9
2.2 4.4 1.8 3.2 0.9 1.3 4.6 2.0 1.6
14.4 32.8 10.8 50.5 34.1 35.0 18.4 18.4 18.5
SD
Mean
SD
Mean
SD
9.0 16.9 1.9 3.4 7.1 11.8
90.4 85.9 13.8 18.9 114.8 70.1
7.8 16.0 2.7 3.6 8.5 12.6
103.6 81.3 8.1 12.8 139.8 39.5
IS
2.1 4.9 1.6 3.8 1.4 2.0 4.3 2.5 3.0
16 I.7 2.8 8.5 8.2
(I), Distances in millimeters.
(2), Angles in degrees.
An et aI., 11 using a biplanar stereoradiographic method, determined the three-dimensional (3-D) orientations and locations of the central slip and the two lateral bands with respect to the extended proximal interphalangeal (PIP) joint of IO fresh finger specimens. Unfortunately, they did not include information about the spatial orientation of either the intercrossing bundles or the transversely oriented fibers of the extensor hood. Besides, information was only given for a single finger configuration. To create an analytical model that could realistically describe the mechanical behavior of the extensor mechanism, numerical information about the spatial orientation of all its different components is required. An experimental model allowing accurate quantification of the 3-D structural arrangement of the extensor mechanism of the fingers has been developed. This article reports the results.
Materials and methods Seven human fresh-frozen forearm specimens (three right and four left) free of disease or injujry, were obtained. The long fingers were disarticulated at the carpometacarpal (CMC) joint. Skin and subcutaneous tissue overlying the extensor apparatus was removed. The anatomy of the extensor assembly was studied using x 2.5 loupe magnification. Dissections were done to identify its different components. To facilitate their recognition, the beginning and ending of nine different segments were marked with small black sutures. These zones were defined as follows (Fig. I): I. Proximal central band (segment AB): Portion of the central band that receives attachments from the extensor hood. 2. Middle central band (segment BC): Central band with no attachments either from the hood or the intercrossing fibers.
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Fig. 2. A, Experimental set-up used in this project. Two fixed cameras were used to capture the position of 21 reference markers that defined a global coordinate system. B, Without moving the cameras, pictures were taken from the specimens in five different positions. This allowed calculation of the spatial location of markers attached to the surface of the extensor mechanism in each position. C, The different finger configurations obtained by means of free-hanging weights connected to the extrinsic and intrinsic tendons through a specially designed pulley system.
3. Distal central band (segment CD): Portion of the central band that includes fibers coming from the intrinsic muscles. 4. Central-to-lateral intercrossing fibers (segment AF): Deep collection of fibers arising from the central band that runs obliquely to join the lateral band at approximately the proximal interphalangeal (PIP) joint level.
5. Lateral-to-central intercrossing fibers (segment GC): Superficial fibers arising from the intrinsic tendons that join the distal central band at approximately the PIP level. 6. Proximal lateral band (segment GF): Portion of the lateral band distal to the point where the lateral-to-central fibers emerge, but proximal to the distal insertion of the central-to-lateral fibers.
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c
Fig. 3. Graphic display of the extensor assembly in five finger configurations. The corresponding finger position is shown superimposed to the computer-generated tendon representation: (A) Intrinsic minus, (B) extension, (e) rnidflexion, (D) full flexion, (E) intrinsic plus.
7. Distal lateral band (segment FE): Portion of the lateral band that includes fibers coming from the central-to-lateral bundle. 8. Proximal extensor hood (segment HA): The more proximal fibers of the extensor hood that arise exclusively from the distal tendon of the interosseous muscles. 9. Distal extensor hood (segment GB) : The more distal fibers of the extensor hood arising from the interosseous and lumbrical muscles. The fingers were rigidly mounted on a specially designed motion jig by means of two screws inserted laterally into the proximal phalanx, allowing free mo-
tion of the metacarpophalangeal (MP), the PIP and distal interphalangeal (DIP) joints. To simulate active motion, the free ends of both the extrinsic and intrinsic (interosseous and lumbrical) tendons were connected to variable loads (Table I) through a pulley system as to reproduce five different finger positions: (1) Intrinsic minus position (MP, 25 degrees of extension, PIP, 90 degrees of flexion, DIP, 45 degrees of flexion); (2) full extension (MP, PIP, and DIP, fully extended); (3) midflexion (MP and PIP, 45 degrees of flexion, DIP, 30 degrees of flexion); (4) full flexion (MP, PIP, and DIP, 90 degrees of flexion), and (5) intrinsic plus position (MP, 90 degrees of flexion , PIP and DIP, fully ex-
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tended). Loads to the tendons were proportional to the amount of electromyographic activity registered for each muscle and each finger position as reported by Landsmeer and Long." To accurately quantify the spatial location of the markers defining each segment, the following stereometric method was used. A calibration structure containing 21 control point markers were specially built (Fig. 2, A). The space location of each control point was previously measured using a vertical millingmachine with a digital read out (Futaba pulscale, Futaba Corp., Japan) with a resolution of 0.005 mm. That structure was photographed on slide film (Ektachrome 100 plus) by two cameras (Nikon N2020 35mm SLR) using micro lenses (55 mm AF Micro Nikkor). The two cameras were fixed to a frame at an angle of approximately 30 degrees with respect to the objective as shown in Fig. 2, B). Once photographed, the calibration structure was removed from the frame and the motion jig with the specimen attached to it was placed in that same location (Fig. 2, C). Care was taken to get all the markers inside the space previously defined by the calibrated structure. Photographs from each specimen in the five previously mentioned finger configurations were then obtained by the two fixed cameras. To obtain the projected X and Y film coordinates for each marker in each view, the films from both cameras were digitized utilizing a video camera with magnified image and a precision mechanical stage driven by step motors. The two sets of XY film coordinates of the 21 calibration markers and the tendon markers were used as input to a computer program which, using a modified direct linear transformation (DLT) method,":" allowed determination of the 3-D coordinates of the unknown tendon markers. With use of these coordinates, lengths and angles between bundles could be determined. These data were also used to obtain a graphical representation of the spatial location of the different segments in the extensor assembly, using a computer-aided design and drafting program (AutoCAD, Autodesk, Inc., Sausalito, Calif.). To determine the accuracy of this stereometric method, a rigid steel ruler was placed within the space defined by the calibration structure, photographed 13 times in different orientations by the two cameras. Two points of the ruler I inch apart from each other were then digitized and the distance between them calculated with the DLT method. The average experimental error when the two points were inside the control point distribution space was found to be of ±0.27 mm.
HAND SURGERY
Finally, anteroposterior and lateral radiographs of the specimens were obtained to determine the spatial relationship of the tendon markers with respect to the underlying bony structures. Results The extensor mechanism presents a variable geometry during finger motion. Changes in length of its different parts may be small, but their spatial orientation varies substantially from one to another finger position. This is illustrated in Table II, which includes the means and standard deviations obtained for the distance and angle of various components of extensor mechanism at five finger configurations studied. A graphic representation of those geometrical changes for one of the specimens is shown in Fig. 3. The lateral bands (represented by the marker F in Fig. 1) were shown to migrate palmarly an average of 2.6 mm as the PIP joint flexed. In the sagittal plane, the shortest distance between the central band (represented by the segment CD) and the lateral band (represented by the marker F) averaged 7.7 mm (from 3.9 to 10.9 mm) for the intrinsic minus position, and of 7.8 mm (from 4.0 to 10.1 mm) for the full flexion position. Those values were found larger than the average 7.1 mm (from 6.9 to 7.5 mm) distance measured between the segment CD and the center of rotation of the PIP joint. The lateral bands, therefore, shift palmar to the axis of rotation in those finger positions. In extension or the intrinsic-plus position, by contrast, the lateral bands remained dorsal to the PIP axis of rotation (the distance between the central to the lateral bands averaged 5.2 mm for the finger extended, 6.1 mm for the midflexed finger and 5.5 mm for the intrinsic-plus position). The proximal angle between the lateral-tocentral and central-to-lateral intercrossing bundles (represented by the lines AF and GC) varies between finger positions, being widest during the intrinsic minus position (42.1 ± 4.1 degrees). In full flexion the crisscross angle was 38.6 ± 4 degrees; in midflexion, 35.7 ± 4 degrees; in extension, 34.5 ± 3.7 degrees; and during the intrinsic-plus position, 35.2 ± 4.2 degrees. Discussion This study has certain limitations. The extensor mechanism is not a network of well-defined cable structures as assumed by our model, but a more complex assembly of multidirectional fibers of different viscoelastic properties. To generate a workable model to
Vol. 16A, No.6 November 1991
evaluate its global geometry, however, we had to first simplify several structures, The lateral-to-central intercrossing fibers, for instance, a fan-like type of tendinous structure, 1.2.5.9 was assumed as a single collection of parallel fibers . Indeed, the more consistent fascicles of each bundle were selected to represent the different zones, which still resulted in a somewhat oversimplified model. The fact that we were more interested in quantitating relative changes rather than in the absolute values of different parameters, however, justified this approach. This study suggests that geometry changes in the extensor mechanism arc not due to changes in length of its different bundles, but to changes in their orientation. This is clearly demonstrated in Table II. Distances between markers change little between different positions, while angles between bundles show substantial changes. These results are not surprising. Schultz and colleagues? could not demonstrate the presence of elastic fibers in any part of the extensor assembly. Sarrafian et al . " recorded relatively small strains in its components during normal motion. Van Zwieten'" compared the intercrossing fibers system to a Chinese finger trap, where crisscrossing stiff fibers form a system which, if pulled longitudinally, becomes longer and at the same time, narrower. This study provides numerical data to support this view. .During PIP flexion, the lateral bands shift palmarly.I-~ Different structures have been said to limit that displacement; the triangular ligament," 8 the intercrossing fibers," and the spiral fibers. 10 There is controversy, however, . on whether under normal conditions these lateral bands remain always dorsal to the axis of rotation, as emphasized by some authors,": 17 or rather they shift pal marly to that axis, as others suggest. I. 6 In our study, the palmar migration of the lateral bands exceeded the distance between the central band and the axis of rotation of the PIP joint (assumed as the center of the curvature of the head of the proximal phalanx IS) in two finger positions; intrinsic minus, and full-flexion. The fact that these bundles are located palmar to the axi s of rotation, however, does not mean the existence of a flexor moment to the PIP joint. As already noted by Landsmeer,' Valentin," and van Zwieten,!? and later measured by Sarrafian et aI., 16 the lateral ba~ds during PIP flexion do not experience much tension, and therefore can not generate any significant flexor moment to the joint. If the central band is disrupted, the full force of the extrinsic and intrinsic mu scles is then transmitted to the lateral bands, thus generating a flexor moment
Extensor mechanism offingers. J
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that will eventually result in a boutonniere deformity. 17 According to Schultz and associates? the acute angle of intersection between the intercrossing fibers increases from 30 degrees to 50 degrees as the PIP joint flexes from a full extended po sition. Van Zwieten," by contrast, reported a decrease from 22 degrees to 18 degrees for the same condition. However, these two studies only reported on two-dimension al measurements. Our 3-D analysis showed this angle to increase significantly in five specimens (average , 6.2 degrees), while in the other, two remained almost unchanged; none of them showed a significant decrease. Whether or not these changes in the arrangement of the intercrossing fibers are important factors to explain the extensor physiology, as suggested by some authors," 10 remains unclear. REFERENCES I. Landsmeer JMF. The anatomy of the dorsal aponeurosis of the human finger and its functional significance. Anat Ree 1949;104:31-44. 2. Haines RW. The extensor apparatus of the finger. J Anat 1951;85:25 1-9. 3. Stack.HG. Muscle function in the fingers . J Bone Joint Surg 1962;44B:899-909. 4. Harris C, Rutledge GL. The functional anatomy of the extensor mechanism of the finger. J Bone Joint Surg 1972;54A:713-26. 5. Smith RJ. Balance and kinetics of the fingers under normal and pathological conditions. Clin Orthop 1974; 104:92-111. 6. Micks JE, Reswick JB. Confirmationof differential loading of lateral and central fibers of the extensor tendon. J HAND SURG 1981 ;6:462-7. 7. Valentin P. Physiology of extension of the fingers. In: Tubiana R, cd. The hand. Philadelphia: WB Saunders, 1981:389-98. 8. Zancolli EA. Anatomy and mechanics of the extensor apparatus of the fingers. In: Zancolli EA, ed, Structural and dynamic bases of hand surgery. Philadelphia: JB Lipincott, 1982:3-63. 9. Schultz RJ, Furlong J, Storace A. Detailed anatomy of the extensor mechanism at the proximal aspect of the finger. J HAND SURG 1981;6:493-8. 10. van Zwieten KJ. The extensor assembly of the finger in man and non-human primates [Doctoral thesis]. The Netherlands: University of Leiden, 1980. II. An KN, Chao EYS, Cooney WP, Linscheid RL. Normative model of human hand for biomeehanicalanalysis. J Biomech 1979;12:775-88. 12. Landsmeer JMF, Long C. The mechanismof finger control, based on electromyograms and location analysis. Acta Anat 1965;60:330-47.
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13. Hatze H. High-precision three-dimensional photogrammetric calibration and object space reconstruction using a modified DLT-approach. J Biomech 1988;7:533-8. 14. Abdel-Aziz YI, Karara HM. Direct linear transformation from comparator coordinates into object space coordinates in close-range photogrammetry. Transactions of the ASP Symposium on close-range photogrammetry. Falls Church, Virginia: American Society of photogrammetry, 1971:1-18. 15. Shapiro R. Direct linear transformation method for threedimensional cinematography. Res Quat 1978;49:197205.
16. Sarrafian SK, Kazarian LE, Topouzian LK, Sarrafian VK, Siegelman A. Strain variation in the components of the extensor apparatus of the finger during flexion and extension. A biomechanical study. J Bone Joint Surg 1970;52A:980-90. 17. Souter WA. The problem of boutonniere deformity. Clin Orthop 1974;104:116-33. 18. Micks JE, Reswick JB, Hager DL. The mechanism of the intrinsic-minus finger: a biomechanical study. J HAND SURG 1978;3:333-41.
Extensor mechanism of the fingers. II. Tensile properties of components Seven fresh human finger specimens have been studied to determine the differences in structural stiffness between different components of the extensor assembly. Differences were found to be significant, ranging from an average of 275 N/mm for the proximal and distal segments of the central band to an average of 40 N/mm for the central-to-lateral intercrossing fascicles. The terminal tendon of the lateral band was shown to be as stiff as the middle segment of the central band. (J HAND SURG 1991;16A:1136·40.)
Marc Garcia-Elias, MD, Kai-Nan An, PhD, Lawrence J. Berglund, BS, Ronald L. Linscheid, MD, William P. Cooney, MD, and Edmund Y. S. Chao, PhD, Rochester, Minn.
From the Orthopedic Biomechanics Laboratory, Department of Orthopedics, Mayo Clinic/Mayo Foundation, Rochester, Minn. This study was supported by Grant AR-17172 awarded by the Department of Health and Human Services, Public Health Service, National Institutes of Health, Baltimore, Md. Received for publication July 16, 1990; accepted in revised form May 14, 1991. Although none of the authors have received or will receive benefits for personal or professional use from a commercial party related directly or indirectly to the subject of !his article, benefits have been or will be received but are directed solely to a research fund, foundation, educational institution, or other nonprofit organization with which one or more of the authors are associated. Reprint requests: Kai-Nan An, PhD, Biomechanics Laboratory, Department of Orthopedics, Room C-053, Guggenheim Building, Mayo Clinic, Rochester, MN 55905. 3/1131540
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It
is generally accepted that finger abnormalities, such as swan-neck, boutonniere, and mallet finger deformities, are caused by an imbalanced distribution of forces in the different components of the extensor mechanism. I. 2 Quantitative information about the normal distribution of forces within this system, however, is still very limited. Sarrafian et aI.,3 using small strain gauges bonded to the surface ofthe extensor mechanism of fresh cadaver specimens, recorded the strain variation for different fascicles during flexion and extension. Assuming similar elastic properties for all these fascicles, they estimated the distribution of forces acting on specific segments of the extensor assembly." The present study was undertaken to validate this assumption.
Marc Garcia-Elias, MD, Kai-Nan An, PhD, Lawrence Berglund, BS, Ronald L. Linscheid, MD, William P. Cooney III, MD, and Edmund Y.S. Chao, PhD Rochester, Minn.
In
recent years there has been increasing interest in the anatomical and functional complexities of the extensor mechanism of the fingers. The spatial relationships between its different components and their associated joints have been extensively studied by several authors.!" Quantitative description, however, has been limited. Schultz and colleagues," using a movie camera attached to the operating microscope, measured the projected angle between the intercrossing fibers of fresh cadaver specimens during simulated finger motion. That angle was found to change from 30 to 50 degrees as the finger was flexed from full extension. A similar study using photographs taken from the lateral side of cadaver specimens was reported by van Zwieten." In both studies, measurements were obtained from planar images; therefore, their results may not be accurate three-dimensionally. From the Orthopedic Biomechanics Laboratory, Department of Orthopedics, Mayo Clinic/Mayo Foundation, Rochester, Minn. Supported by Grant AR-17172, awarded by the National Institutes of Health. Received for publication June 21, 1990; accepted in revised form Aug. 11, 1990. No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. Reprint requests: Kai-Nan An, PhD, Biomechanics Laboratory, Department of Orthopedics, Mayo Clinic/Mayo Foundation, Rochester, MN
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,, Fig. 1. Schematic representation of the extensor assembly of the finger and the different markers (represented in letters) defining the different segments discussed in the text.
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Table I. Applied finger tendon loads (in grams) to produce and maintain the five finger positions Position
Interosseous
Lumbrical
Intrinsic minus Extended Midflexed Full flexed Intrinsic plus
125 250 250 500 750
125 500
Flexor Profundus
Extensor
1,250
1,500 500 250 750 250
250 750
250 500
Table II. Changes in length (mm) and orientation (degree) of different zones of the extensor mechanism (n = 7). Abbreviations as in Fig. I Intrinsic minus (I)
AB BC CD AF GC GF FE
HA GB (2)
GHA FGB FAC FGC GFD DFE
I
Extension
SD
Mean
14.2 32.8 11.2 49.3 36.2 33.9 17.1 18.0 16.2
1.8 4.8 1.8 3.7 1.9 1.3 4.2 3.3 1.9
14.4 32.5 10.6 50.9 35.9 37.1 18.2 16.7 17.1
Mean
SD
95.6 81.5 13.4 18.4 123.7 68.5
4.4 17.1 2.6 3.9 8.0 15.1
Mean
I
Midflexion
SD
Mean
2.2 5.2 1.6 4.2 1.7 2.6 4.3 3.1 2.3
14.7 33.1 10.9 50.8 35.1 35.3 17.7 16.5 16.2
Mean
SD
Mean
111.6 75.1 7.7 11.9 141.5 37.2
12.8 14.7 1.8 2.8 8.8 8.1
90.7 83.8 10.8 16.1 131.6 54.2
I
Full flexion
SD 2.1 4.9 I.7 3.6 1.4 1.8 3.9 2.0 1.5
Mean
I
Intrinsic plus
SD
Mean
I
SD
14.8 33.6 11.2 48.6 35.5 31.6 17.4 17.6 16.9
2.2 4.4 1.8 3.2 0.9 1.3 4.6 2.0 1.6
14.4 32.8 10.8 50.5 34.1 35.0 18.4 18.4 18.5
SD
Mean
SD
Mean
SD
9.0 16.9 1.9 3.4 7.1 11.8
90.4 85.9 13.8 18.9 114.8 70.1
7.8 16.0 2.7 3.6 8.5 12.6
103.6 81.3 8.1 12.8 139.8 39.5
IS
2.1 4.9 1.6 3.8 1.4 2.0 4.3 2.5 3.0
16 I.7 2.8 8.5 8.2
(I), Distances in millimeters.
(2), Angles in degrees.
An et aI., 11 using a biplanar stereoradiographic method, determined the three-dimensional (3-D) orientations and locations of the central slip and the two lateral bands with respect to the extended proximal interphalangeal (PIP) joint of IO fresh finger specimens. Unfortunately, they did not include information about the spatial orientation of either the intercrossing bundles or the transversely oriented fibers of the extensor hood. Besides, information was only given for a single finger configuration. To create an analytical model that could realistically describe the mechanical behavior of the extensor mechanism, numerical information about the spatial orientation of all its different components is required. An experimental model allowing accurate quantification of the 3-D structural arrangement of the extensor mechanism of the fingers has been developed. This article reports the results.
Materials and methods Seven human fresh-frozen forearm specimens (three right and four left) free of disease or injujry, were obtained. The long fingers were disarticulated at the carpometacarpal (CMC) joint. Skin and subcutaneous tissue overlying the extensor apparatus was removed. The anatomy of the extensor assembly was studied using x 2.5 loupe magnification. Dissections were done to identify its different components. To facilitate their recognition, the beginning and ending of nine different segments were marked with small black sutures. These zones were defined as follows (Fig. I): I. Proximal central band (segment AB): Portion of the central band that receives attachments from the extensor hood. 2. Middle central band (segment BC): Central band with no attachments either from the hood or the intercrossing fibers.
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Fig. 2. A, Experimental set-up used in this project. Two fixed cameras were used to capture the position of 21 reference markers that defined a global coordinate system. B, Without moving the cameras, pictures were taken from the specimens in five different positions. This allowed calculation of the spatial location of markers attached to the surface of the extensor mechanism in each position. C, The different finger configurations obtained by means of free-hanging weights connected to the extrinsic and intrinsic tendons through a specially designed pulley system.
3. Distal central band (segment CD): Portion of the central band that includes fibers coming from the intrinsic muscles. 4. Central-to-lateral intercrossing fibers (segment AF): Deep collection of fibers arising from the central band that runs obliquely to join the lateral band at approximately the proximal interphalangeal (PIP) joint level.
5. Lateral-to-central intercrossing fibers (segment GC): Superficial fibers arising from the intrinsic tendons that join the distal central band at approximately the PIP level. 6. Proximal lateral band (segment GF): Portion of the lateral band distal to the point where the lateral-to-central fibers emerge, but proximal to the distal insertion of the central-to-lateral fibers.
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c
Fig. 3. Graphic display of the extensor assembly in five finger configurations. The corresponding finger position is shown superimposed to the computer-generated tendon representation: (A) Intrinsic minus, (B) extension, (e) rnidflexion, (D) full flexion, (E) intrinsic plus.
7. Distal lateral band (segment FE): Portion of the lateral band that includes fibers coming from the central-to-lateral bundle. 8. Proximal extensor hood (segment HA): The more proximal fibers of the extensor hood that arise exclusively from the distal tendon of the interosseous muscles. 9. Distal extensor hood (segment GB) : The more distal fibers of the extensor hood arising from the interosseous and lumbrical muscles. The fingers were rigidly mounted on a specially designed motion jig by means of two screws inserted laterally into the proximal phalanx, allowing free mo-
tion of the metacarpophalangeal (MP), the PIP and distal interphalangeal (DIP) joints. To simulate active motion, the free ends of both the extrinsic and intrinsic (interosseous and lumbrical) tendons were connected to variable loads (Table I) through a pulley system as to reproduce five different finger positions: (1) Intrinsic minus position (MP, 25 degrees of extension, PIP, 90 degrees of flexion, DIP, 45 degrees of flexion); (2) full extension (MP, PIP, and DIP, fully extended); (3) midflexion (MP and PIP, 45 degrees of flexion, DIP, 30 degrees of flexion); (4) full flexion (MP, PIP, and DIP, 90 degrees of flexion), and (5) intrinsic plus position (MP, 90 degrees of flexion , PIP and DIP, fully ex-
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tended). Loads to the tendons were proportional to the amount of electromyographic activity registered for each muscle and each finger position as reported by Landsmeer and Long." To accurately quantify the spatial location of the markers defining each segment, the following stereometric method was used. A calibration structure containing 21 control point markers were specially built (Fig. 2, A). The space location of each control point was previously measured using a vertical millingmachine with a digital read out (Futaba pulscale, Futaba Corp., Japan) with a resolution of 0.005 mm. That structure was photographed on slide film (Ektachrome 100 plus) by two cameras (Nikon N2020 35mm SLR) using micro lenses (55 mm AF Micro Nikkor). The two cameras were fixed to a frame at an angle of approximately 30 degrees with respect to the objective as shown in Fig. 2, B). Once photographed, the calibration structure was removed from the frame and the motion jig with the specimen attached to it was placed in that same location (Fig. 2, C). Care was taken to get all the markers inside the space previously defined by the calibrated structure. Photographs from each specimen in the five previously mentioned finger configurations were then obtained by the two fixed cameras. To obtain the projected X and Y film coordinates for each marker in each view, the films from both cameras were digitized utilizing a video camera with magnified image and a precision mechanical stage driven by step motors. The two sets of XY film coordinates of the 21 calibration markers and the tendon markers were used as input to a computer program which, using a modified direct linear transformation (DLT) method,":" allowed determination of the 3-D coordinates of the unknown tendon markers. With use of these coordinates, lengths and angles between bundles could be determined. These data were also used to obtain a graphical representation of the spatial location of the different segments in the extensor assembly, using a computer-aided design and drafting program (AutoCAD, Autodesk, Inc., Sausalito, Calif.). To determine the accuracy of this stereometric method, a rigid steel ruler was placed within the space defined by the calibration structure, photographed 13 times in different orientations by the two cameras. Two points of the ruler I inch apart from each other were then digitized and the distance between them calculated with the DLT method. The average experimental error when the two points were inside the control point distribution space was found to be of ±0.27 mm.
HAND SURGERY
Finally, anteroposterior and lateral radiographs of the specimens were obtained to determine the spatial relationship of the tendon markers with respect to the underlying bony structures. Results The extensor mechanism presents a variable geometry during finger motion. Changes in length of its different parts may be small, but their spatial orientation varies substantially from one to another finger position. This is illustrated in Table II, which includes the means and standard deviations obtained for the distance and angle of various components of extensor mechanism at five finger configurations studied. A graphic representation of those geometrical changes for one of the specimens is shown in Fig. 3. The lateral bands (represented by the marker F in Fig. 1) were shown to migrate palmarly an average of 2.6 mm as the PIP joint flexed. In the sagittal plane, the shortest distance between the central band (represented by the segment CD) and the lateral band (represented by the marker F) averaged 7.7 mm (from 3.9 to 10.9 mm) for the intrinsic minus position, and of 7.8 mm (from 4.0 to 10.1 mm) for the full flexion position. Those values were found larger than the average 7.1 mm (from 6.9 to 7.5 mm) distance measured between the segment CD and the center of rotation of the PIP joint. The lateral bands, therefore, shift palmar to the axis of rotation in those finger positions. In extension or the intrinsic-plus position, by contrast, the lateral bands remained dorsal to the PIP axis of rotation (the distance between the central to the lateral bands averaged 5.2 mm for the finger extended, 6.1 mm for the midflexed finger and 5.5 mm for the intrinsic-plus position). The proximal angle between the lateral-tocentral and central-to-lateral intercrossing bundles (represented by the lines AF and GC) varies between finger positions, being widest during the intrinsic minus position (42.1 ± 4.1 degrees). In full flexion the crisscross angle was 38.6 ± 4 degrees; in midflexion, 35.7 ± 4 degrees; in extension, 34.5 ± 3.7 degrees; and during the intrinsic-plus position, 35.2 ± 4.2 degrees. Discussion This study has certain limitations. The extensor mechanism is not a network of well-defined cable structures as assumed by our model, but a more complex assembly of multidirectional fibers of different viscoelastic properties. To generate a workable model to
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evaluate its global geometry, however, we had to first simplify several structures, The lateral-to-central intercrossing fibers, for instance, a fan-like type of tendinous structure, 1.2.5.9 was assumed as a single collection of parallel fibers . Indeed, the more consistent fascicles of each bundle were selected to represent the different zones, which still resulted in a somewhat oversimplified model. The fact that we were more interested in quantitating relative changes rather than in the absolute values of different parameters, however, justified this approach. This study suggests that geometry changes in the extensor mechanism arc not due to changes in length of its different bundles, but to changes in their orientation. This is clearly demonstrated in Table II. Distances between markers change little between different positions, while angles between bundles show substantial changes. These results are not surprising. Schultz and colleagues? could not demonstrate the presence of elastic fibers in any part of the extensor assembly. Sarrafian et al . " recorded relatively small strains in its components during normal motion. Van Zwieten'" compared the intercrossing fibers system to a Chinese finger trap, where crisscrossing stiff fibers form a system which, if pulled longitudinally, becomes longer and at the same time, narrower. This study provides numerical data to support this view. .During PIP flexion, the lateral bands shift palmarly.I-~ Different structures have been said to limit that displacement; the triangular ligament," 8 the intercrossing fibers," and the spiral fibers. 10 There is controversy, however, . on whether under normal conditions these lateral bands remain always dorsal to the axis of rotation, as emphasized by some authors,": 17 or rather they shift pal marly to that axis, as others suggest. I. 6 In our study, the palmar migration of the lateral bands exceeded the distance between the central band and the axis of rotation of the PIP joint (assumed as the center of the curvature of the head of the proximal phalanx IS) in two finger positions; intrinsic minus, and full-flexion. The fact that these bundles are located palmar to the axi s of rotation, however, does not mean the existence of a flexor moment to the PIP joint. As already noted by Landsmeer,' Valentin," and van Zwieten,!? and later measured by Sarrafian et aI., 16 the lateral ba~ds during PIP flexion do not experience much tension, and therefore can not generate any significant flexor moment to the joint. If the central band is disrupted, the full force of the extrinsic and intrinsic mu scles is then transmitted to the lateral bands, thus generating a flexor moment
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that will eventually result in a boutonniere deformity. 17 According to Schultz and associates? the acute angle of intersection between the intercrossing fibers increases from 30 degrees to 50 degrees as the PIP joint flexes from a full extended po sition. Van Zwieten," by contrast, reported a decrease from 22 degrees to 18 degrees for the same condition. However, these two studies only reported on two-dimension al measurements. Our 3-D analysis showed this angle to increase significantly in five specimens (average , 6.2 degrees), while in the other, two remained almost unchanged; none of them showed a significant decrease. Whether or not these changes in the arrangement of the intercrossing fibers are important factors to explain the extensor physiology, as suggested by some authors," 10 remains unclear. REFERENCES I. Landsmeer JMF. The anatomy of the dorsal aponeurosis of the human finger and its functional significance. Anat Ree 1949;104:31-44. 2. Haines RW. The extensor apparatus of the finger. J Anat 1951;85:25 1-9. 3. Stack.HG. Muscle function in the fingers . J Bone Joint Surg 1962;44B:899-909. 4. Harris C, Rutledge GL. The functional anatomy of the extensor mechanism of the finger. J Bone Joint Surg 1972;54A:713-26. 5. Smith RJ. Balance and kinetics of the fingers under normal and pathological conditions. Clin Orthop 1974; 104:92-111. 6. Micks JE, Reswick JB. Confirmationof differential loading of lateral and central fibers of the extensor tendon. J HAND SURG 1981 ;6:462-7. 7. Valentin P. Physiology of extension of the fingers. In: Tubiana R, cd. The hand. Philadelphia: WB Saunders, 1981:389-98. 8. Zancolli EA. Anatomy and mechanics of the extensor apparatus of the fingers. In: Zancolli EA, ed, Structural and dynamic bases of hand surgery. Philadelphia: JB Lipincott, 1982:3-63. 9. Schultz RJ, Furlong J, Storace A. Detailed anatomy of the extensor mechanism at the proximal aspect of the finger. J HAND SURG 1981;6:493-8. 10. van Zwieten KJ. The extensor assembly of the finger in man and non-human primates [Doctoral thesis]. The Netherlands: University of Leiden, 1980. II. An KN, Chao EYS, Cooney WP, Linscheid RL. Normative model of human hand for biomeehanicalanalysis. J Biomech 1979;12:775-88. 12. Landsmeer JMF, Long C. The mechanismof finger control, based on electromyograms and location analysis. Acta Anat 1965;60:330-47.
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13. Hatze H. High-precision three-dimensional photogrammetric calibration and object space reconstruction using a modified DLT-approach. J Biomech 1988;7:533-8. 14. Abdel-Aziz YI, Karara HM. Direct linear transformation from comparator coordinates into object space coordinates in close-range photogrammetry. Transactions of the ASP Symposium on close-range photogrammetry. Falls Church, Virginia: American Society of photogrammetry, 1971:1-18. 15. Shapiro R. Direct linear transformation method for threedimensional cinematography. Res Quat 1978;49:197205.
16. Sarrafian SK, Kazarian LE, Topouzian LK, Sarrafian VK, Siegelman A. Strain variation in the components of the extensor apparatus of the finger during flexion and extension. A biomechanical study. J Bone Joint Surg 1970;52A:980-90. 17. Souter WA. The problem of boutonniere deformity. Clin Orthop 1974;104:116-33. 18. Micks JE, Reswick JB, Hager DL. The mechanism of the intrinsic-minus finger: a biomechanical study. J HAND SURG 1978;3:333-41.
Extensor mechanism of the fingers. II. Tensile properties of components Seven fresh human finger specimens have been studied to determine the differences in structural stiffness between different components of the extensor assembly. Differences were found to be significant, ranging from an average of 275 N/mm for the proximal and distal segments of the central band to an average of 40 N/mm for the central-to-lateral intercrossing fascicles. The terminal tendon of the lateral band was shown to be as stiff as the middle segment of the central band. (J HAND SURG 1991;16A:1136·40.)
Marc Garcia-Elias, MD, Kai-Nan An, PhD, Lawrence J. Berglund, BS, Ronald L. Linscheid, MD, William P. Cooney, MD, and Edmund Y. S. Chao, PhD, Rochester, Minn.
From the Orthopedic Biomechanics Laboratory, Department of Orthopedics, Mayo Clinic/Mayo Foundation, Rochester, Minn. This study was supported by Grant AR-17172 awarded by the Department of Health and Human Services, Public Health Service, National Institutes of Health, Baltimore, Md. Received for publication July 16, 1990; accepted in revised form May 14, 1991. Although none of the authors have received or will receive benefits for personal or professional use from a commercial party related directly or indirectly to the subject of !his article, benefits have been or will be received but are directed solely to a research fund, foundation, educational institution, or other nonprofit organization with which one or more of the authors are associated. Reprint requests: Kai-Nan An, PhD, Biomechanics Laboratory, Department of Orthopedics, Room C-053, Guggenheim Building, Mayo Clinic, Rochester, MN 55905. 3/1131540
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is generally accepted that finger abnormalities, such as swan-neck, boutonniere, and mallet finger deformities, are caused by an imbalanced distribution of forces in the different components of the extensor mechanism. I. 2 Quantitative information about the normal distribution of forces within this system, however, is still very limited. Sarrafian et aI.,3 using small strain gauges bonded to the surface ofthe extensor mechanism of fresh cadaver specimens, recorded the strain variation for different fascicles during flexion and extension. Assuming similar elastic properties for all these fascicles, they estimated the distribution of forces acting on specific segments of the extensor assembly." The present study was undertaken to validate this assumption.