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Role of the forearm interosseous ligament: Is it more than just longitudinal load transfer?
Role of the Forearm Interosseous Ligament: Is it More Than Just Longitudinal Load Transfer? H. James Pfaeffle, PhD, Kenneth J. Fischer, PhD, Theodore T. Manson, MS, Matthew M. Tomaino, MD, Savio L-Y. Woo, PhD, Pittsburgh, PA, James H. Herndon, MD, Boston, MA The objective of our study was to measure 3-dimensional force vectors (magnitude and direction) acting in the forearm when load is applied to the hand and to measure the actual force in the interosseous ligament (IOL). Fourteen cadaveric forearms were loaded to 136 N of compression while special load cells measured force vectors in the forearm. Computer forearm models were used to display the 3-dimensional force vector directions. The study results showed that the radius bears most of the load at the wrist but load on the radius at the elbow is reduced because the IOL transfers load to the ulna between the wrist and the elbow. In addition to this role in longitudinal load transfer, our measurement of 3-dimensional forces allowed identification of transverse vectors which suggest that the IOL also functions to keep the radius and ulna from splaying apart. Our results imply that the IOL participates not only in longitudinal load transfer but also in the maintenance of transverse stability of the forearm during compressive load transfer from the hand to the elbow. (J Hand Surg 2000;25A: 683– 688. Copyright © 2000 by the American Society for Surgery of the Hand.) Key words: Interosseous ligament, forearm stability.
Compressive load transfer from the hand to the elbow has been studied by previous investigators. In 1960 Halls and Travill1 placed foil load transducers in the elbow joint and showed that the radius and ulna bore 57% and 43% of load at the elbow, respec-
From the Musculoskeletal Research Center, Department of Orthopaedic Surgery, University of Pittsburgh Medical Center, Pittsburgh, PA; and the Department of Orthopaedic Surgery, Harvard School of Medicine, Boston, MA. Supported by the Orthopaedic Research and Education Foundation, The Whitaker Foundation, and the Albert B. Ferguson Foundation. Received for publication July 23, 1999; accepted in revised form March 22, 2000. 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: Matthew M. Tomaino, MD, Musculoskeletal Research Center, Department of Orthopaedic Surgery, University of Pittsburgh Medical Center, PO Box 71199, Pittsburgh, PA 15213. Copyright © 2000 by the American Society for Surgery of the Hand 0363-5023/00/25A04-0028$3.00/0 doi: 10.1053/jhsu.2000.9416
tively, when a 72-kgf compressive load was applied to the hand. More recently, Rabinowitz et al2 found similar results for load transfer across the elbow using axial load cells. Palmer and Werner3 examined load transfer across the wrist with axial load cells in the distal radius and distal ulna and found that the radius carried approximately 80% of compressive load applied to the hand. The results of these studies imply that the interosseous membrane, which spans the space between the radius and ulna in the midforearm, participates in load transfer and relieves load on the radial head. Recent studies by Birkbeck et al4 and Markolf et al5 have quantified this effect of load transfer across the interosseous membrane on loads at the elbow. Hotchkiss et al6 showed that the central portion of the interosseous membrane, or the “central band,” is a strong ligamentous structure and advocated calling it the interosseous ligament (IOL). Skahen et al7 described the ligamentous portion of the interosseous membrane as a distinct central band The Journal of Hand Surgery 683
684 Pfaeffle et al / Role of the Forearm Interosseous Ligament
with smaller accessory bands sometimes present and used the term interosseous ligament complex. Studies from our laboratory have shown that the central band has biomechanical properties similar to that of ligaments.8 Therefore, we refer to this important structure as the IOL. Previous studies have assessed forces acting in the forearm bones as axial forces (1-dimensional), acting along the direction of the long axis of the forearm. As a result, only the presumed influence of the IOL on longitudinal forces in the radius and ulna at the elbow have been measured, but not the force in the IOL. The objective of our study was to measure both the magnitude and direction of 3-dimensional (3-D) force vectors in the forearm, including force in the IOL, when a compressive load is applied to the hand. Measurement of the magnitude and direction of forces in the forearm will improve our understanding of the role of the IOL in forearm compressive load transfer.
Materials and Methods A previously published experimental methodology was followed.9 X-rays of 14 human cadaveric forearms (age range, 45–70 years) amputated at the midhumerus were obtained to rule out bony pathology. Ulnar variance in these forearms ranged from 2 mm negative to 4 mm positive (mean, ⫹1 ⫾ 2 mm; 9 of the 14 specimens were ulnar positive). An experimental protocol approved by the institutional review board was followed for each forearm. Soft tissues overlying the central forearm and IOL (central band of the interosseous membrane) were removed. The membranous portions of the interosseous membrane and any accessory bands, as described by Skahen et al,7 were carefully dissected away leaving a consistent load transmitting central band in all specimens. Six degrees of freedom load cells, which are capable of measuring the 3 vector components of force acting on it and the 3 moments created by that force about its center, were used to measure the magnitude and direction of forces. Although not used in this study, the moment information can be used to calculate the point in space at which the force is acting. Custom, miniature 6 degrees of freedom load cells (model 600 series; Bertec Corp, Worthington, OH) were implanted in-line in the distal radius and proximal ulna. A custom vise was used to secure the bones during implantation, and a milling machine was used to resect a 30.86-mm segment of bone with highly accurate parallel osteotomies. A special im-
plantation technique that used threaded medullary implants cemented with polymethyl methacrylate and cross-stabilized with steel pins ensured rigid fixation of the load cells. Forearms were mounted on a materials testing machine (model 8541; Instron, Canton, MA) equipped with special clamps that gripped the metacarpals and the humerus and adjusted for any physiologic wrist, elbow, and forearm rotation position (Fig. 1). Forearms were placed in neutral rotation with the wrist in a position of power grip (15° ulnar deviation and 20° extension) and the elbow extended and in neutral varus–valgus alignment. These angles were set using a goniometer. A third 6 degrees of freedom load cell (model 350; ATI, Garner, NC) mounted on the Instron testing machine was used to measure the force applied to the hand. A preload to 5 N compression was applied followed by a cyclic compressive force of 30 N applied at 0.5 Hz for 25 cycles to precondition the forearm. A 136 N compressive load was then applied at 9 N/s and allowed to equilibrate for 1 minute on reaching the target load. Changes in wrist and elbow position were not observed during load application. The position and orientation of each 6 degrees of freedom load cell was then measured with a 3-D digitizing device (Microscribe 3-DX; Immersion Corp, San Jose, CA). One hundred to 150 points were digitized on 3 adjacent faces of Plexiglas registration blocks attached to each load cell. For the first 6 specimens this task was accomplished using a video motion analysis system. This information was used to mathematically align load cell coordinate systems for force vector calculations. Planes were fit to the points using a least-squares technique. Intersection of the 3 planes and the plane normal vectors provided the position and orientation of the load cell coordinate system. This technique was tested against a coordinate measuring machine and found to be accurate to within 0.1 mm and 0.1°. Experiments were repeated with the forearm in full pronation and full supination. Forearms were rotated and repositioned by loosening the clamps and then placed into the same wrist and elbow positions for each trial. Following testing, the force vectors acting in the distal ulna and proximal radius were calculated from force balances across the wrist and elbow. Force in the IOL was then calculated from the change in forearm forces from distal to proximal. An example of this calculation for forces across the wrist is as follows:
The Journal of Hand Surgery / Vol. 25A No. 4 July 2000 685
Figure 1. A forearm mounted on the materials testing machine. The 3-D digitizing device can be seen in the background. The right panel shows a close-up of the miniature universal force sensors and registration blocks. The blocks directly attached to the radius and ulna are part of another study.
F XHAND ⫽ F XDISTAL ULNA ⫹ F XDISTAL RADIUS
ulna in each forearm rotation position was measured from the computer model.
F YHAND ⫽ F YDISTAL ULNA ⫹ F YDISTAL RADIUS F ZHAND ⫽ F ZDISTAL ULNA ⫹ F ZDISTAL RADIUS
Force vectors were calculated with respect to the coordinate system of the radial load cell, which had X, Y, and Z axes aligned in the dorsal–volar, radial– ulnar, and proximal– distal directions, respectively. The effect of forearm rotation on force magnitude in the IOL was statistically assessed using a repeatedmeasures 1-way ANOVA followed by multiple contrast testing. Force vectors were visualized in 3-D using computer forearm models generated from computed tomography scans of a representative forearm by plotting the geometry and force vectors together using the Tecplot software package (Amtec Engineering Inc, Bellevue, WA). The angle that the average IOL force vector made with the long axis of the
Results The radius was the primary load carrying bone at the wrist, but the IOL transferred load from the radius to the ulna between the wrist and the elbow. At the elbow, load on the radius was reduced (as shown in Fig. 2 for neutral rotation) and the ulna carried increased load. Forces in the IOL were significantly greater in neutral rotation compared with supination or pronation, with magnitudes of 15 ⫾ 11 N, 31 ⫾ 14 N, and 20 ⫾ 12 N (mean ⫾ SD) for pronation, neutral, and supination forearm positions, respectively (Fig. 3). The average IOL force vector was oriented along the direction of IOL fibers, and both axial and transverse forces acted on the radius and ulna (Fig. 4). The angle that the average IOL force vector made
686 Pfaeffle et al / Role of the Forearm Interosseous Ligament
Figure 2. Forearm force vector magnitudes in neutral rotation (mean ⫾ SD). The wrist and elbow bars show portions of load borne by the radius (open areas) and ulna (shaded areas) at these levels. Similar results were obtained in pronation and supination.
with the long axis of the ulna was 25°, 25°, and 21° in pronation, neutral rotation, and supination, respectively. The transverse forces exerted by the IOL were directed in such a way as to pull the radius and ulna together. Forces acting in the radius and ulna near the distal and proximal radioulnar joints generally displayed transverse components of force that opposed transverse IOL forces. These were directed in such a way as to load these joints in compression. All force vector results are given in Table 1.
Discussion In our study force vectors in the forearm were successfully measured when a 136 N compressive load was applied to the hand. It was important to measure forces in 3-D and characterize the directions of forearm forces because this allowed calculation of the force in the IOL. Our results show that the IOL exerts longitudinal forces on the radius and ulna. Indeed, as Birkbeck et al4 and Markolf et al5 have reported previously, we have shown that longitudinal force in the IOL acts to relieve load on the radial head at the elbow. Our results also agree with those of previous studies which show that the radius carries most of the wrist load and that the ulna carries more load at the elbow.1–5 Unlike previous studies, we have shown that forces in the radius and ulna at the elbow and wrist are not directed entirely longitudinally. Transverse force vectors are also present. Transverse forces in the IOL were directed in such a way as to pull the radius and ulna together at the proximal and distal
radioulnar joints, causing transverse forces across these joints. In the midforearm, transverse forces in the IOL were directed in such a way as to help prevent radioulnar splaying or bowing of the radius. These transverse forces that the IOL exerts on the radius and ulna may help provide transverse radioulnar stability and maintain the integrity of the forearm unit. Markolf et al5 also studied the influence of elbow varus–valgus moments on the amount of force transferred across the IOL. Although these effects were not systematically investigated in our study, our preliminary tests showed that elbow varus–valgus moments created by transverse hand loading strongly affected the magnitude of force in the IOL. Accordingly, we controlled this important variable by carefully maintaining the elbow in neutral varus–valgus alignment and applying only axial load to the hand. More studies are needed to assess the physiologic impact of these varus–valgus stresses at the elbow, especially under the influence of external forearm loads and internal muscle forces. Our findings are clinically important because they provide quantitative information describing the direction and magnitude of force in the IOL and at the distal and proximal radioulnar joints. An understanding of normal 3-D forces in the forearm will improve our ability to evaluate IOL reconstructions performed for longitudinal radioulnar dissociation. Skahen et al7 and Sellman et al10 have studied reconstruction of the IOL as a solution for the challenging clinical problem of longitudinal radioulnar dissociation following radial head excision. These investiga-
Figure 3. Magnitudes of force in the IOL for 136 N applied hand load. The IOL force was greatest in neutral rotation. * p ⬍ .05.
The Journal of Hand Surgery / Vol. 25A No. 4 July 2000 687
Figure 4. Computer visualization model reconstructed from computed tomography in pronation, neutral rotation, and supination. The magnitude and orientation of average 3-D force vectors are shown for the 14 forearms.
tors have reported that reconstruction of the IOL alone without radial head arthroplasty did not sufficiently restore forearm stability or stiffness. In the setting of radial head arthroplasty as a treatment for longitudinal radioulnar dissociation, reconstruction of the IOL may reduce load on the radial head and help stabilize the implant. The limitations of this study include the inability to separate forces at the distal and proximal radioulnar joints from the forces at the wrist and elbow joints, respectively. More work is required to assess
the complicated pressure distributions in these joints and complex interactions between these joints and the triangular fibrocartilage complex at the wrist. Studies were performed with limited wrist and elbow joint positions; hence, more studies are needed to quantify the effects of wrist and elbow joint position on forces in the forearm. Further work is needed to assess the effects of IOL reconstruction on radial head forces and stability and to establish recommendations regarding graft size, location, and technique. The methodology used in
Table 1. Three-Dimensional Forearm Force Vector Magnitude and Components Referred to a Local Anatomic Coordinate System Attached to the Radius Pronation
Neutral
Supination
Forearm Forces
X
Y
Z
Mag
X
Y
Z
Mag
X
Y
Z
Mag
Hand Distal ulna Distal radius IOL on radius Proximal ulna Proximal radius
⫺3 ⫺2 ⫺1 ⫺1 1 2
⫺5 ⫺3 ⫺2 4 6 ⫺2
⫺135 ⫺16 ⫺120 9 23 111
135 16 120 15 18 111
⫺5 ⫺3 ⫺2 1 4 1
⫺1 5 ⫺6 13 8 ⫺7
⫺134 ⫺16 ⫺118 26 41 91
134 19 118 31 43 92
0 3 ⫺3 1 ⫺1 1
2 5 ⫺3 6 1 ⫺3
⫺135 ⫺24 ⫺111 16 38 95
135 25 111 20 39 95
X, Dorsal; Y, ulnar; Z, distal; Mag, magnitude. Force in the IOL is given as the force exerted on the radius by the IOL. The forces on the radius and ulna represent net forces acting in these bones at the wrist and elbow. A negative force component is in a direction opposite to that given for the positive anatomic axis direction.
688 Pfaeffle et al / Role of the Forearm Interosseous Ligament
this study has allowed measurement of 3-D forces in the forearm and will be useful in future studies to advance our understanding of forearm function as well as to evaluate reconstructive options.
5.
6. The authors thank Damion Shelton for technical assistance.
7.
References 1. Halls AA, Travill A. Transmission of pressures across the elbow joint. Anat Rec 1960;150:243–248. 2. Rabinowitz RS, Light TR, Havey RM, et al. The role of the interosseous membrane and triangular fibrocartilage complex in forearm stability. J Hand Surg 1994;19A: 385–393. 3. Palmer AK, Werner FW. Biomechanics of the distal radioulnar joint. Clin Orthop 1984;187:26 –35. 4. Birkbeck DP, Failla JM, Hoshaw SJ, Fyhrie DP, Schaffler
8.
9.
10.
M. The interosseous membrane affects load distribution in the forearm. J Hand Surg 1997;22A:975–980. Markolf KL, Lamey D, Yang S, Meals R, Hotchkiss R. Radioulnar load-sharing in the forearm: a study in cadavera. J Bone Joint Surg 1998;80A:879 – 888. Hotchkiss RN, An K-N, Sowa DT, Basta S, Weiland AJ. An anatomic and mechanical study of the interosseous membrane of the forearm: pathomechanics of proximal migration of the radius. J Hand Surg 1989;14A:256 –261. Skahen JR III, Palmer AK, Werner FW, Fortino MD. The interosseous membrane of the forearm: anatomy and function. J Hand Surg 1997;22A:981–985. Pfaeffle HJ, Tomaino MM, Grewal R, et al. Tensile properties of the interosseous membrane of the human forearm. J Orthop Res 1996;14:842– 845. Pfaeffle HJ, Fischer KJ, Manson TT, Tomaino MM, Herndon JH, Woo SL-Y. A new methodology to measure load transfer through the forearm using multiple universal force sensors. J Biomech 1999;32:1331–1335. Sellman DC, Seitz WH Jr, Postak PD, Greenwald AS. Reconstructive strategies for radioulnar dissociation: a biomechanical study. J Orthop Trauma 1995;9:516 –522.
Compressive load transfer from the hand to the elbow has been studied by previous investigators. In 1960 Halls and Travill1 placed foil load transducers in the elbow joint and showed that the radius and ulna bore 57% and 43% of load at the elbow, respec-
From the Musculoskeletal Research Center, Department of Orthopaedic Surgery, University of Pittsburgh Medical Center, Pittsburgh, PA; and the Department of Orthopaedic Surgery, Harvard School of Medicine, Boston, MA. Supported by the Orthopaedic Research and Education Foundation, The Whitaker Foundation, and the Albert B. Ferguson Foundation. Received for publication July 23, 1999; accepted in revised form March 22, 2000. 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: Matthew M. Tomaino, MD, Musculoskeletal Research Center, Department of Orthopaedic Surgery, University of Pittsburgh Medical Center, PO Box 71199, Pittsburgh, PA 15213. Copyright © 2000 by the American Society for Surgery of the Hand 0363-5023/00/25A04-0028$3.00/0 doi: 10.1053/jhsu.2000.9416
tively, when a 72-kgf compressive load was applied to the hand. More recently, Rabinowitz et al2 found similar results for load transfer across the elbow using axial load cells. Palmer and Werner3 examined load transfer across the wrist with axial load cells in the distal radius and distal ulna and found that the radius carried approximately 80% of compressive load applied to the hand. The results of these studies imply that the interosseous membrane, which spans the space between the radius and ulna in the midforearm, participates in load transfer and relieves load on the radial head. Recent studies by Birkbeck et al4 and Markolf et al5 have quantified this effect of load transfer across the interosseous membrane on loads at the elbow. Hotchkiss et al6 showed that the central portion of the interosseous membrane, or the “central band,” is a strong ligamentous structure and advocated calling it the interosseous ligament (IOL). Skahen et al7 described the ligamentous portion of the interosseous membrane as a distinct central band The Journal of Hand Surgery 683
684 Pfaeffle et al / Role of the Forearm Interosseous Ligament
with smaller accessory bands sometimes present and used the term interosseous ligament complex. Studies from our laboratory have shown that the central band has biomechanical properties similar to that of ligaments.8 Therefore, we refer to this important structure as the IOL. Previous studies have assessed forces acting in the forearm bones as axial forces (1-dimensional), acting along the direction of the long axis of the forearm. As a result, only the presumed influence of the IOL on longitudinal forces in the radius and ulna at the elbow have been measured, but not the force in the IOL. The objective of our study was to measure both the magnitude and direction of 3-dimensional (3-D) force vectors in the forearm, including force in the IOL, when a compressive load is applied to the hand. Measurement of the magnitude and direction of forces in the forearm will improve our understanding of the role of the IOL in forearm compressive load transfer.
Materials and Methods A previously published experimental methodology was followed.9 X-rays of 14 human cadaveric forearms (age range, 45–70 years) amputated at the midhumerus were obtained to rule out bony pathology. Ulnar variance in these forearms ranged from 2 mm negative to 4 mm positive (mean, ⫹1 ⫾ 2 mm; 9 of the 14 specimens were ulnar positive). An experimental protocol approved by the institutional review board was followed for each forearm. Soft tissues overlying the central forearm and IOL (central band of the interosseous membrane) were removed. The membranous portions of the interosseous membrane and any accessory bands, as described by Skahen et al,7 were carefully dissected away leaving a consistent load transmitting central band in all specimens. Six degrees of freedom load cells, which are capable of measuring the 3 vector components of force acting on it and the 3 moments created by that force about its center, were used to measure the magnitude and direction of forces. Although not used in this study, the moment information can be used to calculate the point in space at which the force is acting. Custom, miniature 6 degrees of freedom load cells (model 600 series; Bertec Corp, Worthington, OH) were implanted in-line in the distal radius and proximal ulna. A custom vise was used to secure the bones during implantation, and a milling machine was used to resect a 30.86-mm segment of bone with highly accurate parallel osteotomies. A special im-
plantation technique that used threaded medullary implants cemented with polymethyl methacrylate and cross-stabilized with steel pins ensured rigid fixation of the load cells. Forearms were mounted on a materials testing machine (model 8541; Instron, Canton, MA) equipped with special clamps that gripped the metacarpals and the humerus and adjusted for any physiologic wrist, elbow, and forearm rotation position (Fig. 1). Forearms were placed in neutral rotation with the wrist in a position of power grip (15° ulnar deviation and 20° extension) and the elbow extended and in neutral varus–valgus alignment. These angles were set using a goniometer. A third 6 degrees of freedom load cell (model 350; ATI, Garner, NC) mounted on the Instron testing machine was used to measure the force applied to the hand. A preload to 5 N compression was applied followed by a cyclic compressive force of 30 N applied at 0.5 Hz for 25 cycles to precondition the forearm. A 136 N compressive load was then applied at 9 N/s and allowed to equilibrate for 1 minute on reaching the target load. Changes in wrist and elbow position were not observed during load application. The position and orientation of each 6 degrees of freedom load cell was then measured with a 3-D digitizing device (Microscribe 3-DX; Immersion Corp, San Jose, CA). One hundred to 150 points were digitized on 3 adjacent faces of Plexiglas registration blocks attached to each load cell. For the first 6 specimens this task was accomplished using a video motion analysis system. This information was used to mathematically align load cell coordinate systems for force vector calculations. Planes were fit to the points using a least-squares technique. Intersection of the 3 planes and the plane normal vectors provided the position and orientation of the load cell coordinate system. This technique was tested against a coordinate measuring machine and found to be accurate to within 0.1 mm and 0.1°. Experiments were repeated with the forearm in full pronation and full supination. Forearms were rotated and repositioned by loosening the clamps and then placed into the same wrist and elbow positions for each trial. Following testing, the force vectors acting in the distal ulna and proximal radius were calculated from force balances across the wrist and elbow. Force in the IOL was then calculated from the change in forearm forces from distal to proximal. An example of this calculation for forces across the wrist is as follows:
The Journal of Hand Surgery / Vol. 25A No. 4 July 2000 685
Figure 1. A forearm mounted on the materials testing machine. The 3-D digitizing device can be seen in the background. The right panel shows a close-up of the miniature universal force sensors and registration blocks. The blocks directly attached to the radius and ulna are part of another study.
F XHAND ⫽ F XDISTAL ULNA ⫹ F XDISTAL RADIUS
ulna in each forearm rotation position was measured from the computer model.
F YHAND ⫽ F YDISTAL ULNA ⫹ F YDISTAL RADIUS F ZHAND ⫽ F ZDISTAL ULNA ⫹ F ZDISTAL RADIUS
Force vectors were calculated with respect to the coordinate system of the radial load cell, which had X, Y, and Z axes aligned in the dorsal–volar, radial– ulnar, and proximal– distal directions, respectively. The effect of forearm rotation on force magnitude in the IOL was statistically assessed using a repeatedmeasures 1-way ANOVA followed by multiple contrast testing. Force vectors were visualized in 3-D using computer forearm models generated from computed tomography scans of a representative forearm by plotting the geometry and force vectors together using the Tecplot software package (Amtec Engineering Inc, Bellevue, WA). The angle that the average IOL force vector made with the long axis of the
Results The radius was the primary load carrying bone at the wrist, but the IOL transferred load from the radius to the ulna between the wrist and the elbow. At the elbow, load on the radius was reduced (as shown in Fig. 2 for neutral rotation) and the ulna carried increased load. Forces in the IOL were significantly greater in neutral rotation compared with supination or pronation, with magnitudes of 15 ⫾ 11 N, 31 ⫾ 14 N, and 20 ⫾ 12 N (mean ⫾ SD) for pronation, neutral, and supination forearm positions, respectively (Fig. 3). The average IOL force vector was oriented along the direction of IOL fibers, and both axial and transverse forces acted on the radius and ulna (Fig. 4). The angle that the average IOL force vector made
686 Pfaeffle et al / Role of the Forearm Interosseous Ligament
Figure 2. Forearm force vector magnitudes in neutral rotation (mean ⫾ SD). The wrist and elbow bars show portions of load borne by the radius (open areas) and ulna (shaded areas) at these levels. Similar results were obtained in pronation and supination.
with the long axis of the ulna was 25°, 25°, and 21° in pronation, neutral rotation, and supination, respectively. The transverse forces exerted by the IOL were directed in such a way as to pull the radius and ulna together. Forces acting in the radius and ulna near the distal and proximal radioulnar joints generally displayed transverse components of force that opposed transverse IOL forces. These were directed in such a way as to load these joints in compression. All force vector results are given in Table 1.
Discussion In our study force vectors in the forearm were successfully measured when a 136 N compressive load was applied to the hand. It was important to measure forces in 3-D and characterize the directions of forearm forces because this allowed calculation of the force in the IOL. Our results show that the IOL exerts longitudinal forces on the radius and ulna. Indeed, as Birkbeck et al4 and Markolf et al5 have reported previously, we have shown that longitudinal force in the IOL acts to relieve load on the radial head at the elbow. Our results also agree with those of previous studies which show that the radius carries most of the wrist load and that the ulna carries more load at the elbow.1–5 Unlike previous studies, we have shown that forces in the radius and ulna at the elbow and wrist are not directed entirely longitudinally. Transverse force vectors are also present. Transverse forces in the IOL were directed in such a way as to pull the radius and ulna together at the proximal and distal
radioulnar joints, causing transverse forces across these joints. In the midforearm, transverse forces in the IOL were directed in such a way as to help prevent radioulnar splaying or bowing of the radius. These transverse forces that the IOL exerts on the radius and ulna may help provide transverse radioulnar stability and maintain the integrity of the forearm unit. Markolf et al5 also studied the influence of elbow varus–valgus moments on the amount of force transferred across the IOL. Although these effects were not systematically investigated in our study, our preliminary tests showed that elbow varus–valgus moments created by transverse hand loading strongly affected the magnitude of force in the IOL. Accordingly, we controlled this important variable by carefully maintaining the elbow in neutral varus–valgus alignment and applying only axial load to the hand. More studies are needed to assess the physiologic impact of these varus–valgus stresses at the elbow, especially under the influence of external forearm loads and internal muscle forces. Our findings are clinically important because they provide quantitative information describing the direction and magnitude of force in the IOL and at the distal and proximal radioulnar joints. An understanding of normal 3-D forces in the forearm will improve our ability to evaluate IOL reconstructions performed for longitudinal radioulnar dissociation. Skahen et al7 and Sellman et al10 have studied reconstruction of the IOL as a solution for the challenging clinical problem of longitudinal radioulnar dissociation following radial head excision. These investiga-
Figure 3. Magnitudes of force in the IOL for 136 N applied hand load. The IOL force was greatest in neutral rotation. * p ⬍ .05.
The Journal of Hand Surgery / Vol. 25A No. 4 July 2000 687
Figure 4. Computer visualization model reconstructed from computed tomography in pronation, neutral rotation, and supination. The magnitude and orientation of average 3-D force vectors are shown for the 14 forearms.
tors have reported that reconstruction of the IOL alone without radial head arthroplasty did not sufficiently restore forearm stability or stiffness. In the setting of radial head arthroplasty as a treatment for longitudinal radioulnar dissociation, reconstruction of the IOL may reduce load on the radial head and help stabilize the implant. The limitations of this study include the inability to separate forces at the distal and proximal radioulnar joints from the forces at the wrist and elbow joints, respectively. More work is required to assess
the complicated pressure distributions in these joints and complex interactions between these joints and the triangular fibrocartilage complex at the wrist. Studies were performed with limited wrist and elbow joint positions; hence, more studies are needed to quantify the effects of wrist and elbow joint position on forces in the forearm. Further work is needed to assess the effects of IOL reconstruction on radial head forces and stability and to establish recommendations regarding graft size, location, and technique. The methodology used in
Table 1. Three-Dimensional Forearm Force Vector Magnitude and Components Referred to a Local Anatomic Coordinate System Attached to the Radius Pronation
Neutral
Supination
Forearm Forces
X
Y
Z
Mag
X
Y
Z
Mag
X
Y
Z
Mag
Hand Distal ulna Distal radius IOL on radius Proximal ulna Proximal radius
⫺3 ⫺2 ⫺1 ⫺1 1 2
⫺5 ⫺3 ⫺2 4 6 ⫺2
⫺135 ⫺16 ⫺120 9 23 111
135 16 120 15 18 111
⫺5 ⫺3 ⫺2 1 4 1
⫺1 5 ⫺6 13 8 ⫺7
⫺134 ⫺16 ⫺118 26 41 91
134 19 118 31 43 92
0 3 ⫺3 1 ⫺1 1
2 5 ⫺3 6 1 ⫺3
⫺135 ⫺24 ⫺111 16 38 95
135 25 111 20 39 95
X, Dorsal; Y, ulnar; Z, distal; Mag, magnitude. Force in the IOL is given as the force exerted on the radius by the IOL. The forces on the radius and ulna represent net forces acting in these bones at the wrist and elbow. A negative force component is in a direction opposite to that given for the positive anatomic axis direction.
688 Pfaeffle et al / Role of the Forearm Interosseous Ligament
this study has allowed measurement of 3-D forces in the forearm and will be useful in future studies to advance our understanding of forearm function as well as to evaluate reconstructive options.
5.
6. The authors thank Damion Shelton for technical assistance.
7.
References 1. Halls AA, Travill A. Transmission of pressures across the elbow joint. Anat Rec 1960;150:243–248. 2. Rabinowitz RS, Light TR, Havey RM, et al. The role of the interosseous membrane and triangular fibrocartilage complex in forearm stability. J Hand Surg 1994;19A: 385–393. 3. Palmer AK, Werner FW. Biomechanics of the distal radioulnar joint. Clin Orthop 1984;187:26 –35. 4. Birkbeck DP, Failla JM, Hoshaw SJ, Fyhrie DP, Schaffler
8.
9.
10.
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