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Forearm rotation alters interosseous ligament strain distribution
Forearm Rotation Alters Interosseous Ligament Strain Distribution Theodore T. Manson, MS, H. James Pfaeffle, PhD, James H. Herndon, MD, Matthew M. Tomaino, MD, Kenneth J. Fischer, PhD, Pittsburgh, PA Recent interest in reconstruction of the interosseous ligament (IOL) of the forearm has led to questions concerning optimal placement of the reconstructive graft as well as the ideal rotational position of the forearm during graft tensioning. We therefore studied the strain distribution in the IOL to determine which fibers are strained in different positions of forearm rotation. Five cadaveric human forearms were subjected to compressive axial load (simulating power grip) and the strain values across the entire IOL were measured with the forearm in neutral, supination, and pronation. The strain distribution in the IOL changed with forearm rotation. The highest overall strain was found in neutral. In neutral and pronation, higher strain was observed in the proximal region of the IOL. In supination, however, higher average strain was seen in the distal region of the IOL. These results suggest that a reconstructive graft placed in the proximal region of the IOL and tensioned in neutral rotation would provide balanced constraint in different positions of forearm rotation. A graft placed in the distal region and tensioned in forearm neutral, however, may limit forearm rotation. (J Hand Surg 2000;25A: 1058 –1063. Copyright © 2000 by the American Society for Surgery of the Hand.) Key words: Interosseous membrane, interosseous ligament, distal radioulnar joint, radial head fracture, ligament reconstruction.
The interosseous membrane of the forearm plays an important role in forearm stability. The normal anatomy of the interosseous membrane is variable but is characterized in all cases by a strong central band (interosseous ligament [IOL]) with material From the Musculoskeletal Research Center, Department of Orthopaedic Surgery, University of Pittsburgh Medical Center, Pittsburgh, PA. Supported by the Orthopaedic Research and Education Foundation, the Whitaker Foundation, and the Albert B. Ferguson Foundation. Received for publication May 19, 2000; accepted in revised form May 26, 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 0363-5023/00/25A06-0007$3.00/0 doi: 10.1053/jhsu.2000.17869
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The Journal of Hand Surgery
properties similar to those of the patellar tendon.1–3 An Essex-Lopresti lesion that requires excision of the radial head may result in proximal migration of the radius if the IOL is torn.1,4 – 6 This proximal radial migration disrupts the distal radioulnar joint leading to wrist and elbow pain as well as diminished forearm rotation and wrist extension.4,5,7 Treatment of an Essex-Lopresti injury has focused on addressing the radial head fracture and crosspinning the radius and ulna to maintain acceptable radioulnar relationships during soft tissue healing. Variable results have been obtained with radial head excision and with prosthetic or allograft radial head replacement.5,8 –10 Inconsistent outcomes have led to an interest in concurrent IOL reconstruction to limit proximal radial migration and reduce the joint contact forces on a radial head prosthesis or allograft.7,11–13 There are little data, however, that address the optimal placement of the reconstructive
The Journal of Hand Surgery / Vol. 25A No. 6 November 2000 1059
graft, the proper graft tension, or the ideal rotational position of the forearm during graft tensioning. Studies of the normal biomechanics of the uninjured IOL may help to answer these questions. It has been shown that the IOL bears varying loads with different positions of forearm rotation.14,15 Skahen et al3 measured the strain at one location in the midsubstance of the IOL and found it to vary with forearm rotation, being highest in neutral. The IOL is a wide, thin ligament with a longer ulnar insertion than radial insertion.1,3 Consequently, it is unlikely that the strain distribution is uniform across the breadth of the ligament. An appreciation of strain distribution within the ligament would be of value in determining which ligament fibers are important to reconstruct, the forearm position in which to tension a reconstructive graft, and the location of the graft within the forearm. The objectives of this study, therefore, were to determine the strain distribution in the IOL when the forearm was loaded (simulating power grip) and to determine whether the strain distribution changes with forearm rotation (in pronation, neutral, and supination).
Materials and Methods Computed Tomography Scan Digitization of the Interosseous Ligament Insertion Sites Five human cadaveric forearms (age range, 18 – 65 years) were dissected free of all soft tissues in the midforearm, except the IOL, and leaving the soft tissues surrounding the wrist and elbow intact. The IOL was distinguished and separated from the surrounding structures through careful dissection with backlighting. To register the computed tomography (CT; anatomic) geometry with the experimentally measured positions of the bones, Plexiglas registration blocks were mounted immediately adjacent to the IOL insertion sites on the radius and ulna (Fig. 1). Each forearm was placed in a clinical CT scanner (GE Genesis Highspeed Advantages 9800; General Electric Medical Systems, Waukesha, WI) and scanned under a standard bone protocol (80 kilovolt (peak), 140 mA, 1 second), at 1-mm intervals and 1-mm slice thickness using a field of view of 10 cm. Based on these parameters the spatial resolution of the scan geometry was 1 mm along the forearm axis and 0.2 mm transverse to the forearm axis. The bones and the IOL were reconstructed from the CT data as computer models using software developed in our laboratory (TT Manson. A new system for the seg-
Figure 1. A human forearm dissected down to the bone in the midforearm leaving the soft tissues around the joints intact. Plexiglas registration blocks were cemented adjacent to the insertion sites of the IOL.
mentation of medical images. Master’s thesis. Pittsburgh: University of Pittsburgh, 1998:102). The points on the radius and ulna that define the 3-dimensional shape and position of the insertions of the IOL were manually selected determined from the CT data set by digitizing the points on the interosseous ridge where the IOL inserts (Figs. 2, 3). In addition, the 3-dimensional position and orientation of the Plexiglas registration blocks (relative to the CT coordinate system) were determined by fitting planes to approximately 100 points on 3 block surfaces. The accuracy of the position and orientation of the registration blocks in the CT data set was found to be ⫾0.2 mm (in all directions) and ⫾0.2°.
Forearm Compressive Loading Each specimen was tested according to a compressive loading protocol designed for evaluation of force distributions in the normal forearm.16 This protocol included mounting miniature universal force-moment sensors (UFSs) in the distal radius and
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Manson et al / Forearm Rotation Alters IOL Strain
entation of the Plexiglas registration blocks (relative to the Microscribe coordinate system) were determined by fitting planes to the points from each surface. The accuracy of the position and orientation of the registration blocks using the Microscribe was found to be ⫾0.1 mm (in all directions) and ⫾0.1°. All registration block surfaces were digitized within 1 minute and no motion (creep) of the forearm bones was detected during digitization.
Analysis
Figure 2. Computed tomography scan of the midforearm showing the radius, ulna, IOL, and one of the registration blocks used for tracking the IOL insertion sites.
proximal ulna. Each UFS was rigidly implanted using a special technique that minimized changes in bone length and orientation.16 Each forearm was mounted in a materials testing machine (model 8521; Instron, Canton, MA) with the wrist in 20° extension and 15° ulnar deviation (simulating the power grip position), the elbow in full extension and neutral varus/valgus, and the forearm in neutral rotation. Custom fixtures allowed physiologic positioning of the specimen and locking of the hand and humerus in the desired position. The forearm was subjected to a preconditioning protocol (30 N applied at 0.5 Hz for 25 cycles) followed by a 136 N compressive load applied at 9 N/s and allowed to equilibrate for 1 minute at the target load (Fig. 4). The test was repeated with the specimen positioned in full supination and full pronation. The absolute measurements (°) for supination and pronation varied and was calculated using a screw axis displacement method from the kinematics data.17 The screw axis passed approximately through the radial head and distal radioulnar joint. During the experiment the positions of the radial and ulnar insertion sites of the IOL were determined before and after loading by digitizing the Plexiglas registration blocks with a spatial linkage (Microscribe 3DX; Immersion Corporation, San Jose, CA). Approximately 100 points on each of 3 block surfaces were collected continuously while moving the Microscribe tip over each surface. As with the CT data, the position and ori-
From the positions of the IOL insertion sites before and after loading the lengths of the ligament fibers were calculated for the loaded positions of neutral, supination, and pronation. The relative strain in the ligament fibers was calculated using the lengths of the fibers in the unloaded forearm in neutral rotation as the rest (reference) lengths. Strain along each fiber direction was assumed to be constant. For the purposes of analysis the mean fiber strain from the proximal half of the IOL was compared with the mean strain in the distal half of the IOL. Average fiber strains for each region (proximal and distal) were calculated for each position of forearm rotation. All strain values were normalized by the strain in the proximal region of the IOL in neutral rotation to account for interspecimen variability and provide a common base for comparison. No statisti-
Figure 3. Three-dimensional computer reconstruction of the IOL, radius, and ulna showing the IOL insertion site points on the radius and ulna digitized from the CT images. A subset of the actual number of points used in the analysis is shown for clarity.
The Journal of Hand Surgery / Vol. 25A No. 6 November 2000 1061
Figure 4. (Left) The forearm mounted in the Instron machine during a compressive loading experiment. (Right) A closer view of the forearm showing the registration blocks used to track the positions of the radius and ulna during the experiment. Universal UFSs are also shown mounted in the radius and ulna as part of a concurrent experiment quantifying normal load distribution in the radius, ulna, and IOL. (Reprinted with permission.16)
cal comparisons were made due to the small number of specimens included in the study.
Results The strain distribution in the IOL changed with forearm rotation. Full forearm supination and pronation actually averaged 77° and 61°, respectively. In neutral and pronation a trend toward higher average strain in the proximal region of the IOL than in the distal region was observed (Fig. 5). Conversely, in supination, higher average strain was seen in the distal region of the IOL. The highest average regional strain overall was found in the proximal region of the IOL in neutral rotation. Strain across the entire ligament (averaging both regions) was highest in neutral; overall strains in supination and pronation were 73% and 50% of the neutral value, respectively.
Discussion The wide, flat, trapezoidal shape of the IOL, the geometry of the insertion sites, and the radioulnar
kinematics induce an uneven strain distribution that changes with forearm rotation. The results of this study, which quantified strain distribution across the entire breadth of the IOL, are consistent with the results of Skahen et al3 in which strain was quantified in one specific location of the IOL. Both studies showed the highest strain in neutral rotation. Likewise, the results for overall average strain in the ligament are consistent with studies quantifying the force in the IOL for each position of forearm rotation under load.14,15 In a study of potential IOL reconstructions Forster et al18 found a region of reconstructive graft isometry in the distal region of the IOL during unloaded forearm rotation. By contrast, in this study, in which the forearm was placed under compressive axial load, the distal fibers of the intact IOL were not found to be isometric with forearm rotation. This comparison suggests that the relative radioulnar relationships may change with application of load. The biomechanical model used for this study lim-
1062
Manson et al / Forearm Rotation Alters IOL Strain
Figure 5. Strain in the proximal and distal regions of the IOL in different positions of forearm rotation. The values shown were calculated from IOL insertion site positions with the forearm under 134 N of axial compressive load. The results are shown normalized by the strain in the proximal region of the IOL in neutral forearm rotation.
its our ability to make definitive statements regarding what occurs in the forearm clinically, since it does not account for active muscle forces. Because several muscles have an attachment on the IOL, these mus-
cles could affect the strain in the IOL. In addition, it is possible that implantation of the UFS in the radius and ulna affected the study results. Each UFS was constructed from aluminum, with a similar modulus
Figure 6. Proximal and distal functional bundles of the IOL are apparent from the strain distribution data.
The Journal of Hand Surgery / Vol. 25A No. 6 November 2000 1063
of elasticity to bone, and was precisely and rigidly implanted. We think this minimizes the impact of UFS placement. Notwithstanding these limitations, the data provide new insight into strain patterns within the IOL. The IOL is a stiff ligament2; hence, small differences in strain between regions can reflect relatively large differences in restraining forces on the radius and ulna. The differential strain patterns observed between proximal and distal regions of the IOL reflect different functional fiber bundles (Fig. 6). These become either less or more taut depending on the position of forearm rotation and may be analogous, from an anatomic stand point, to the anteromedial and posterolateral bundles of the anterior cruciate ligament.19 –21 The data from this study have implications, therefore, regarding the location of a potential IOL reconstructive graft within the forearm. The results (Fig. 5) suggest that a graft placed in the proximal region of the compromised IOL and tensioned in the neutral position would provide balance between constraining force and forearm rotation range of motion. Conversely, a graft placed in the distal region and tensioned in forearm neutral may limit the extent of forearm rotation in supination. To address these issues further biomechanical studies are now being planned with cadaver specimens and computer models to determine the effects of various reconstructive graft positions on forearm kinematics. The authors would like to thank Damion Shelton for his technical assistance in generating the 3-dimensional computer reconstructions. In addition, the authors would like to express gratitude for funding received from the Albert B. Ferguson Foundation, the Orthopaedic Research and Education Foundation, and the Whitaker Foundation.
References 1. 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. 2. 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. 3. 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. 4. Essex-Lopresti P. Fractures of the radial head with distal radio-ulnar dislocation: report of two cases. J Bone Joint Surg 1951;33B:244 –247.
5. Edwards GS Jr, Jupiter JB. Radial head fractures with acute distal radioulnar dislocation: Essex-Lopresti revisited. Clin Orthop 1988;234:61– 69. 6. 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. 7. Hotchkiss RN. Injuries to the interosseous ligament of the forearm. Hand Clin 1994;10:391–398. 8. Morrey BF, Askew L, Chao EY. Silastic prosthetic replacement for the radial head. J Bone Joint Surg 1981;63A: 454 – 458. 9. Szabo RM, Hotchkiss RN, Slater RR Jr. The use of frozenallograft radial head replacement for treatment of established symptomatic proximal translation of the radius: preliminary experience in five cases. J Hand Surg 1997;22A: 269 –278. 10. Knight DJ, Rymaszewski LA, Amis AA, Miller JH. Primary replacement of the fractured radial head with a metal prosthesis. J Bone Joint Surg 1993;75B:572–576. 11. Sellman DC, Seitz WH Jr, Postak PD, Greenwald AS. Reconstructive strategies for radioulnar dissociation: a biomechanical study. J Orthop Trauma 1995;9:516 –522. 12. Skahen JR III, Palmer AK, Werner FW, Fortino MD. Reconstruction of the interosseous membrane of the forearm in cadavers. J Hand Surg 1997;22A:986 –994. 13. Wallace AL, Walsh WR, van Rooijen M, Hughes JS, Sonnabend DH. The interosseous membrane in radio-ulnar dissociation. J Bone Joint Surg 1997;79B:422– 427. 14. 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. 15. Birkbeck DP, Failla JM, Hoshaw SJ, Fyhrie DP, Schaffler M. The interosseous membrane affects load distribution in the forearm. J Hand Surg 1997;22A:975–980. 16. 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. 17. Kinzel GL, Hall AS, Hillberry BM. Measurement of the total motion between two body segments—I. Analytical development. J Biomech 1972;5:93–105. 18. Forster RI, Sharkey NA, Szabo RM. Forearm interosseous ligament isometry. J Hand Surg 1999;24A:538 –545. 19. Sakane M, Fox RJ, Woo SL-Y, Livesay GA, Li G, Fu FH. In situ forces in the anterior cruciate ligament and its bundles in response to anterior tibial loads. J Orthop Res 1997;15:285–293. 20. Norwood LA, Cross MJ. Anterior cruciate ligament: functional anatomy of its bundles in rotatory instabilities. Am J Sports Med 1979;7:23–26. 21. Girgis FG, Marshall JL, Al Monajem ARS. The cruciate ligaments of the knee joint: anatomical, functional and experimental analysis. Clin Orthop 1975;106:216 –231.
The interosseous membrane of the forearm plays an important role in forearm stability. The normal anatomy of the interosseous membrane is variable but is characterized in all cases by a strong central band (interosseous ligament [IOL]) with material From the Musculoskeletal Research Center, Department of Orthopaedic Surgery, University of Pittsburgh Medical Center, Pittsburgh, PA. Supported by the Orthopaedic Research and Education Foundation, the Whitaker Foundation, and the Albert B. Ferguson Foundation. Received for publication May 19, 2000; accepted in revised form May 26, 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 0363-5023/00/25A06-0007$3.00/0 doi: 10.1053/jhsu.2000.17869
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The Journal of Hand Surgery
properties similar to those of the patellar tendon.1–3 An Essex-Lopresti lesion that requires excision of the radial head may result in proximal migration of the radius if the IOL is torn.1,4 – 6 This proximal radial migration disrupts the distal radioulnar joint leading to wrist and elbow pain as well as diminished forearm rotation and wrist extension.4,5,7 Treatment of an Essex-Lopresti injury has focused on addressing the radial head fracture and crosspinning the radius and ulna to maintain acceptable radioulnar relationships during soft tissue healing. Variable results have been obtained with radial head excision and with prosthetic or allograft radial head replacement.5,8 –10 Inconsistent outcomes have led to an interest in concurrent IOL reconstruction to limit proximal radial migration and reduce the joint contact forces on a radial head prosthesis or allograft.7,11–13 There are little data, however, that address the optimal placement of the reconstructive
The Journal of Hand Surgery / Vol. 25A No. 6 November 2000 1059
graft, the proper graft tension, or the ideal rotational position of the forearm during graft tensioning. Studies of the normal biomechanics of the uninjured IOL may help to answer these questions. It has been shown that the IOL bears varying loads with different positions of forearm rotation.14,15 Skahen et al3 measured the strain at one location in the midsubstance of the IOL and found it to vary with forearm rotation, being highest in neutral. The IOL is a wide, thin ligament with a longer ulnar insertion than radial insertion.1,3 Consequently, it is unlikely that the strain distribution is uniform across the breadth of the ligament. An appreciation of strain distribution within the ligament would be of value in determining which ligament fibers are important to reconstruct, the forearm position in which to tension a reconstructive graft, and the location of the graft within the forearm. The objectives of this study, therefore, were to determine the strain distribution in the IOL when the forearm was loaded (simulating power grip) and to determine whether the strain distribution changes with forearm rotation (in pronation, neutral, and supination).
Materials and Methods Computed Tomography Scan Digitization of the Interosseous Ligament Insertion Sites Five human cadaveric forearms (age range, 18 – 65 years) were dissected free of all soft tissues in the midforearm, except the IOL, and leaving the soft tissues surrounding the wrist and elbow intact. The IOL was distinguished and separated from the surrounding structures through careful dissection with backlighting. To register the computed tomography (CT; anatomic) geometry with the experimentally measured positions of the bones, Plexiglas registration blocks were mounted immediately adjacent to the IOL insertion sites on the radius and ulna (Fig. 1). Each forearm was placed in a clinical CT scanner (GE Genesis Highspeed Advantages 9800; General Electric Medical Systems, Waukesha, WI) and scanned under a standard bone protocol (80 kilovolt (peak), 140 mA, 1 second), at 1-mm intervals and 1-mm slice thickness using a field of view of 10 cm. Based on these parameters the spatial resolution of the scan geometry was 1 mm along the forearm axis and 0.2 mm transverse to the forearm axis. The bones and the IOL were reconstructed from the CT data as computer models using software developed in our laboratory (TT Manson. A new system for the seg-
Figure 1. A human forearm dissected down to the bone in the midforearm leaving the soft tissues around the joints intact. Plexiglas registration blocks were cemented adjacent to the insertion sites of the IOL.
mentation of medical images. Master’s thesis. Pittsburgh: University of Pittsburgh, 1998:102). The points on the radius and ulna that define the 3-dimensional shape and position of the insertions of the IOL were manually selected determined from the CT data set by digitizing the points on the interosseous ridge where the IOL inserts (Figs. 2, 3). In addition, the 3-dimensional position and orientation of the Plexiglas registration blocks (relative to the CT coordinate system) were determined by fitting planes to approximately 100 points on 3 block surfaces. The accuracy of the position and orientation of the registration blocks in the CT data set was found to be ⫾0.2 mm (in all directions) and ⫾0.2°.
Forearm Compressive Loading Each specimen was tested according to a compressive loading protocol designed for evaluation of force distributions in the normal forearm.16 This protocol included mounting miniature universal force-moment sensors (UFSs) in the distal radius and
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Manson et al / Forearm Rotation Alters IOL Strain
entation of the Plexiglas registration blocks (relative to the Microscribe coordinate system) were determined by fitting planes to the points from each surface. The accuracy of the position and orientation of the registration blocks using the Microscribe was found to be ⫾0.1 mm (in all directions) and ⫾0.1°. All registration block surfaces were digitized within 1 minute and no motion (creep) of the forearm bones was detected during digitization.
Analysis
Figure 2. Computed tomography scan of the midforearm showing the radius, ulna, IOL, and one of the registration blocks used for tracking the IOL insertion sites.
proximal ulna. Each UFS was rigidly implanted using a special technique that minimized changes in bone length and orientation.16 Each forearm was mounted in a materials testing machine (model 8521; Instron, Canton, MA) with the wrist in 20° extension and 15° ulnar deviation (simulating the power grip position), the elbow in full extension and neutral varus/valgus, and the forearm in neutral rotation. Custom fixtures allowed physiologic positioning of the specimen and locking of the hand and humerus in the desired position. The forearm was subjected to a preconditioning protocol (30 N applied at 0.5 Hz for 25 cycles) followed by a 136 N compressive load applied at 9 N/s and allowed to equilibrate for 1 minute at the target load (Fig. 4). The test was repeated with the specimen positioned in full supination and full pronation. The absolute measurements (°) for supination and pronation varied and was calculated using a screw axis displacement method from the kinematics data.17 The screw axis passed approximately through the radial head and distal radioulnar joint. During the experiment the positions of the radial and ulnar insertion sites of the IOL were determined before and after loading by digitizing the Plexiglas registration blocks with a spatial linkage (Microscribe 3DX; Immersion Corporation, San Jose, CA). Approximately 100 points on each of 3 block surfaces were collected continuously while moving the Microscribe tip over each surface. As with the CT data, the position and ori-
From the positions of the IOL insertion sites before and after loading the lengths of the ligament fibers were calculated for the loaded positions of neutral, supination, and pronation. The relative strain in the ligament fibers was calculated using the lengths of the fibers in the unloaded forearm in neutral rotation as the rest (reference) lengths. Strain along each fiber direction was assumed to be constant. For the purposes of analysis the mean fiber strain from the proximal half of the IOL was compared with the mean strain in the distal half of the IOL. Average fiber strains for each region (proximal and distal) were calculated for each position of forearm rotation. All strain values were normalized by the strain in the proximal region of the IOL in neutral rotation to account for interspecimen variability and provide a common base for comparison. No statisti-
Figure 3. Three-dimensional computer reconstruction of the IOL, radius, and ulna showing the IOL insertion site points on the radius and ulna digitized from the CT images. A subset of the actual number of points used in the analysis is shown for clarity.
The Journal of Hand Surgery / Vol. 25A No. 6 November 2000 1061
Figure 4. (Left) The forearm mounted in the Instron machine during a compressive loading experiment. (Right) A closer view of the forearm showing the registration blocks used to track the positions of the radius and ulna during the experiment. Universal UFSs are also shown mounted in the radius and ulna as part of a concurrent experiment quantifying normal load distribution in the radius, ulna, and IOL. (Reprinted with permission.16)
cal comparisons were made due to the small number of specimens included in the study.
Results The strain distribution in the IOL changed with forearm rotation. Full forearm supination and pronation actually averaged 77° and 61°, respectively. In neutral and pronation a trend toward higher average strain in the proximal region of the IOL than in the distal region was observed (Fig. 5). Conversely, in supination, higher average strain was seen in the distal region of the IOL. The highest average regional strain overall was found in the proximal region of the IOL in neutral rotation. Strain across the entire ligament (averaging both regions) was highest in neutral; overall strains in supination and pronation were 73% and 50% of the neutral value, respectively.
Discussion The wide, flat, trapezoidal shape of the IOL, the geometry of the insertion sites, and the radioulnar
kinematics induce an uneven strain distribution that changes with forearm rotation. The results of this study, which quantified strain distribution across the entire breadth of the IOL, are consistent with the results of Skahen et al3 in which strain was quantified in one specific location of the IOL. Both studies showed the highest strain in neutral rotation. Likewise, the results for overall average strain in the ligament are consistent with studies quantifying the force in the IOL for each position of forearm rotation under load.14,15 In a study of potential IOL reconstructions Forster et al18 found a region of reconstructive graft isometry in the distal region of the IOL during unloaded forearm rotation. By contrast, in this study, in which the forearm was placed under compressive axial load, the distal fibers of the intact IOL were not found to be isometric with forearm rotation. This comparison suggests that the relative radioulnar relationships may change with application of load. The biomechanical model used for this study lim-
1062
Manson et al / Forearm Rotation Alters IOL Strain
Figure 5. Strain in the proximal and distal regions of the IOL in different positions of forearm rotation. The values shown were calculated from IOL insertion site positions with the forearm under 134 N of axial compressive load. The results are shown normalized by the strain in the proximal region of the IOL in neutral forearm rotation.
its our ability to make definitive statements regarding what occurs in the forearm clinically, since it does not account for active muscle forces. Because several muscles have an attachment on the IOL, these mus-
cles could affect the strain in the IOL. In addition, it is possible that implantation of the UFS in the radius and ulna affected the study results. Each UFS was constructed from aluminum, with a similar modulus
Figure 6. Proximal and distal functional bundles of the IOL are apparent from the strain distribution data.
The Journal of Hand Surgery / Vol. 25A No. 6 November 2000 1063
of elasticity to bone, and was precisely and rigidly implanted. We think this minimizes the impact of UFS placement. Notwithstanding these limitations, the data provide new insight into strain patterns within the IOL. The IOL is a stiff ligament2; hence, small differences in strain between regions can reflect relatively large differences in restraining forces on the radius and ulna. The differential strain patterns observed between proximal and distal regions of the IOL reflect different functional fiber bundles (Fig. 6). These become either less or more taut depending on the position of forearm rotation and may be analogous, from an anatomic stand point, to the anteromedial and posterolateral bundles of the anterior cruciate ligament.19 –21 The data from this study have implications, therefore, regarding the location of a potential IOL reconstructive graft within the forearm. The results (Fig. 5) suggest that a graft placed in the proximal region of the compromised IOL and tensioned in the neutral position would provide balance between constraining force and forearm rotation range of motion. Conversely, a graft placed in the distal region and tensioned in forearm neutral may limit the extent of forearm rotation in supination. To address these issues further biomechanical studies are now being planned with cadaver specimens and computer models to determine the effects of various reconstructive graft positions on forearm kinematics. The authors would like to thank Damion Shelton for his technical assistance in generating the 3-dimensional computer reconstructions. In addition, the authors would like to express gratitude for funding received from the Albert B. Ferguson Foundation, the Orthopaedic Research and Education Foundation, and the Whitaker Foundation.
References 1. 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. 2. 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. 3. 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. 4. Essex-Lopresti P. Fractures of the radial head with distal radio-ulnar dislocation: report of two cases. J Bone Joint Surg 1951;33B:244 –247.
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