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Strains in the articular disk of the triangular fibrocartilage complex: A biomechanical study
Strains in the Articular Disk of the Triangular Fibrocartilage Complex: A Biomechanical Study Brian D. Adams, MD, Iowa City, IA, Kathy A. Halley, MSE, Burlington, The articular disk of the triangular
fibrocartilage
system in a cadaveric laboratory experiment. curred during pronation and supination was dependent upon forearm position. disk, with dorsoanterior
complex was studied using a video
Changes in disk configuration
and resulted
in a nonuniform
Strains occurred primarily
strains being negligible.
imaging
consistently
strain distribution
in the radioulnar
Strains wereconcentrated
of the disk and were highest with the forearm pronated. Application the distal radioulnar
VT
octhat
axis of the
in the radial portion
of a distraction
load to
joint to simulate the effect of axial wrist loading caused strains to increase
the most in the radial portion.
These findings suggest that joint distraction
pronation are important components traumatic tear that occurs
of the injury
mechanism
loading and forearm
for the most common type of
near the radial attachment of the disk.
(J Hand Surg
19!?3;18A:
919-925.)
The triangular fibrocartilage complex (TFCC) of the wrist is a multifunctional structure composed of several anatomical components.‘** The articular disk and the dorsal and anterior radioulnar ligaments comprise the horizontal portion of the complex, referred to as the triangular fibrocartilage proper, or TFC. The TFC forms a continuation of the distal radial articular surface from the disk’s attachment at the sigmoid notch to its apical attachment in the eccentric concavity of the ulnar head (fovea) and the projecting ulnar styloid. The TFC provides an interface between the ulnar head and ulnar carpus, with the biconcave shape accommodating the geometric incongruencies between the bony surfaces. The peripheral margins of the TFC (radioulnar liga-
From the Department of Orthopaedic Surgery, University of Iowa. lowa City, IA, and Department of Orthopaedics and Rehabilitation, University of Vermont, Burlington, VT. Supported by research grant #91-001 from the Orthopedic Research and Education Foundation. Received for publication Sept. 22, 1992; accepted in revised form Feb. 23. 1993. 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: Brian D. Adams, MD, Associate Professor. Division of Hand and Microsurgery, Department of Orthopaedic Surgery, University of Iowa. Iowa City. IA 52242.
ments) are thicker and composed of longitudinally oriented collagen fibers, structurally adapted to bear tensile loading. The central portion (articular disk) is thinner and the collagen fiber pattern has multiple obliquities to the surface, implying that variable loading conditions occur. 1.3 The multiplicity of TFC functions, including load bearing, load distribution, shock absorption, and joint stabilization, is related to the complexity of movements and loading in the forearm and wrist. During forearm motion, the distal radius undergoes both rotation and translation relative to the ulna, with the translational component occurring primarily at the extremes of pronation and supination.4-6 Since the TFC attachment to the ulnar head is near the axis of distal radioulnar joint motion, the TFC causes minimal resistance to forearm movement during the central portion of the motion arc.’ However, the discrepancy in the radii of curvatures between the articular surfaces of the distal radiotilnar joint allows translation to occur as the TFC and other soft tissues become taut at the extremes of motion.6*7 This translational motion produces asymmetrical loading in the TFC resulting in nonuniform strain distribution.‘,’ In addition, the separation of the articular surfaces of the distal radioulnar joint that occurs during power grip indicates that axial loading produces tension in the TFC.9 During injury The journal
of Hand Surgery
919
920
Adams and Halley / Strains in Articular
Disk of TFCC
loading conditions, one would expect that areas of higher strain would be at greater risk for the initiation and propagation of tears. Although the radioulnar ligaments have been the focus of several investigations and injury models, the mechanics of injury in the articular disk are poorly understood despite the frequency of treatment required for symptomatic tears in the disk. Increasing our knowledge of injury mechanics has important implications for prevention, treatment, and rehabilitation of disk tears. Some injury theories propose that susceptibility of injury and site of tear are related to forearm position at the time of loading. ‘.I0 Based on clinical histories of patients presenting with tears, and observations of TFC motion made during arthroscopy, a relationship seems probable. However, to test this hypothesis requires a better understanding of the effects of forearm motion and joint loading on the TFC. The purpose of this experiment was to study the relationship between surface strains in the articular disk and forearm position, with and without the application of a distal radioulnar joint distraction force to simulate the effects of joint separation that occurs during power grip.
Materials and Methods A video imaging system integrated with a microcomputer was used to study the articular disk. Six unmatched, fresh-frozen upper extremities from human cadavers were used in this laboratory experiment. Specimens were from young adult males, ranging from 26 to 37 years of age, with a mean of 33 years. Specimens were disarticulated through the radiocarpal and ulnocarpal joints with care to preserve the articular disk, radioulnar ligaments, and the proximal attachments of the ulnocarpal ligaments of the TFCC. Absence of preexisting injury to the TFCC on gross observation was required. Specimens were held in a custom jig with the elbow flexed at 90”. Rigid fixation to the jig was obtained with bone screws into the ulna and humerus. The ulna was mounted horizontal and the humerus was mounted vertical to the platform. A Steinmann pin inserted into the radial styloid was used to passively position the radius at desired degrees of forearm rotation. A stationary protractor was used to measure the angle of forearm pronation and supination. This experimental setup allowed an unrestricted arc of forearm motion and provided direct visualization of the distal articular surfaces of the radius and TFC. In order to track surface changes over the entire disk, black circular markers measuring 1.1 mm in diameter were glued to the surface with droplets of
cyanoacrylate. Four markers were applied to the distal radial articular surface adjacent to the TFC insertion at the edge of the sigmoid notch (ulnar margin of lunate fossa). An array of three rows of four markers each was placed in a grid-like fashion across the surface of the disk, spanning the area between the anterior and dorsal radioulnar ligaments and the radial and ulnar TFC attachment sites. Video images of the marker array were acquired with a high-resolution CCD video camera (Ultrachip, Javelin Electronics). The images were captured to a frame-grabber board (PC Vision Plus, Imaging Technologies, Inc.) mounted in a microcomputer. Image acquisition and analysis was controlled by imaging software (Optimas, BioScan, Inc.). Calibration of the system was performed with a high-precision ruler placed at the level of the markers. Camera field size was set to visualize only the TFC during a complete arc of forearm pronationsupination. The multiple tools and programming options provided by the software were used for image analysis. The black markers on each image were selected as screen objects based on a gray scale threshold set by the operator. Slight irregularities caused by optical glare were corrected to obtain marker uniformity. The perimeter of each marker was then outlined. The centroid of each marker (center of marker area) was found and written to a data tile. For visual clarification and anatomic reference of the strain analysis, a grid was created by connecting the centroids and overlaid on the anatomic image (Fig. 1). The distances between the 16 centroids, depicted by the line segments of the grid, were used to define 12 lengths (L) along the radioulnar axis and 9 lengths (T) along the dorsoanterior axis of the disk surface (Fig. 2A, B). In addition, the grid provided delination of nine regions (A) of the disk (Fig. 2C). A video image of the markers was acquired at every 10”of forearm motion, beginning with full pronation and moving to full supination. Neutral forearm position (0’) was defined as parallel alignment of the radioulnar axis of the distal radius with the longitudinal axis of the humeral shaft. Images were also obtained during forearm motion in the reverse direction, that is, supination to pronation, to determine if direction of motion affected results. The entire sequence was repeated to determine reproducibility of results. The experiment was then performed with the addition of a distraction load across the distal radioulnar joint. The load was applied to cause separation of the distal radioulnar joint surfaces to simulate the effect of power grip. Distraction was accomplished with a cable attached to the radial styloid by a bone screw. Tension was applied perpendicular to the
The Journal of Hand Surgery / Vol. 18A No. 5 September 1993
921
Figure 1. Video image of the ulnocarpal surface of the triangular fibrocartilage with markers in place. Forearm is in neutral position. Grid connecting marker centroids is overlaid. U, ulna; R, radius; P, paimar: D, dorsal; M, ulnar margin of distal radius.
Figure 2. Schematic of disk surface measurements. (A) distances (L) used in strain measurements (AL) along the radioulnar axis. (B) distances (T) used in strain measurements (AT) along the dorsopalmar axis. (C) Disk regions (A) used in measuring changes in area.
922
Adams and Halley / Strains in Articular Disk of TFCC
long axis of the forearm and parallel to the radioulnar axis of the distal radius. Cable alignment was changed for each forearm position by a pulley system to avoid causing joint torque. A load ramp from 0 to 22.2 N was applied by a custom load generator at a rate of 0.9N/s. Load output was recorded simultaneously with video acquisition by the computer in order to correlate images with load. Magnitudes of the 21 defined lengths (L and T) and the areas of the 9 regions (A) were measured for each image and stored to a data file. Strains in lengths (AL and AT) were calculated from changes in centroid locations between sequential images according to the equations: AL = (Li - L,)/L,
and
AT = (Ti - Tg)iTg
where Li and Ti are the respective distances between two centroids at i degrees of forearm motion and L, and Tg are the distances between the same centroids with the forearm in neutral position (gage length). Strains were recorded as a function of forearm position and applied load. Changes in areas (A A) were calculated for each of the regions, with neutral position as reference, by the equation: AA = (Ai - A,)/A, and recorded as a function of forearm position and applied load. The first derivative of total disk change was then calculated according to the equation: A’ = (Af - AI+ ,)/Ai where Af is the total area change at i degrees of forearm motion and A[+ 1 is the change at the next forearm position. The results demonstrate rate of change in total area during forearm motion, that is, rate of area change between forearm positions. System error is determined by the pixel density of the video frame buffer and the field of view. Given a 640 x 480 pixel density in the frame buffer and a 20 x 15 mm field of view, the resolution is 0.03 mm/ pixel. An object centroid is calculated by taking the average x and y values of points lying on the object perimeter. The minimum number of points needed to define a 1.1 mm object with this system is 64. Thus, error in calculating centroids was: +
= ‘*OF
= 0.00375 mm
Results Changes in disk configuration consistently occurred during forearm motion and resulted in a nonuniform distribution of strain (Fig. 3). Strain distribution was dependent upon forearm position but
Figure 3. Grids of a representative specimen in full pronation, neutral, and full supination (left to right). Note changes in disk configuration. was independent of the direction of forearm motion. Strains within the radial three regions of the disk (Al, A2, A3) behaved similarly during testing, as did strains within the central three (A4, A5, A6) and within the ulnar three regions (A7, A8, A9). Thus, to simplify the comparative analysis, the disk was divided into three portions: radial, central, and ulnar. Strains along the radioulnar axis (AL) decreased in all three portions of the disk during supination when compared to the neutral position. Greater decreases in strains occurred in the central and radial portions of the disk when each portion was compared to the ulnar portion (p < .05). During pronation, the reverse strain distribution was found when compared to supination (Fig. 4). Greater decreases in strains along the radioulnar axis occurred in the central and ulnar portions when each portion was compared to the radial portion, which remained unchanged or increased (p < .05). Thus, the radial portion of the disk behaved differently than the central and ulnar portions during forearm motion. Dorsoanterior axis strains were negligible (strains < 0.015) in all regions and in all forearm positions. Changes in area of a region consistently paralleled the radioulnar axis strains in that region (Fig. 5). For example, changes in area of region A5 paralleled strains in L6 and L7, while T2 and T5 remained unchanged. Therefore, disk configuration changes and the resulting strains were caused primarily by strains in the radioulnar axis of the disk. Rate of change in total disk area increased as full pronation and supination were approached (Fig. 6). This implies that strain rates and configuration changes occur more rapidly at the extremes of forearm motion. Although the radioulnar ligaments were not evaluated in this study, strains (AL) along the dorsal and anterior margins of the disk were calcu-
The Journal of Hand Surgery I Vol. 18A No. 5 September 1993
60 50 40 30 20
10
0
10 20 30 40 50 60
I -8’ 60 50 40 30 20 10 0 10 20 30 40 50 60
Forearm Position (degrees from neutral) Supination *
Pronation
-
923
Forearm
percent strains along the radioulnar axis. R, radial portion; C, central portion; U. ulnar portion of disk.
Figure 4. Average
lated. Strains along the dorsal margin decreased during supination, while strains along the anterior margin increased or were unchanged. The reverse strain pattern occurred during pronation: strains along the anterior margin decreased, while strains along the dorsal margin increased or were unchanged.
2r-.-----r
Position
(degrees
Pronation
4
from neutral) Supination
*
Average percent change in total disk area and percent rate of change in total disk area.
Figure 6.
When the load ramp was applied to cause distraction of the distal radioulnar joint, strains were preferentially increased in the same disk portions that demonstrated higher strains during forearm motion without loading fp < .02). Thus, additional load on the disk was nonuniformly distributed, with the majority distributed to the radial portion of the disk (Fig. 7). Discussion Contemporary advances in diagnostic imaging techniques and wrist arthroscopy have made it pos-
J
R
S t-
r -
a i
n
_,4L--60
50
40
30
20
10
0
10 20
30
40
50
60
Forearm Position (degrees from neutral)
a
Pronation
Supination
*
Average percent changes in areas (A) of the disk. R, radial portion; C, central portion; U, ulnar portion of disk. Figure 5.
Pronation
Neutral
Supination
Average percent strains in disk with 5 lb. distraction load applied to the distal radioulnar joint (unloaded joint used as baseline). R, radial portion: C. central portion; U, ulnar portion of disk. Figure 7.
924
Adams
and Halley
/ Strains
in Articular
Disk of TFCC
sible to regularly identify and treat lesions in the articular disk of the TFCC.‘O,” Although the care of TFCC injuries has greatly improved with these technologies, our knowledge of disk injury mechanics is sparse. Current injury theories are derived primarily from interpretations of clinical observations, gross anatomy dissections, and reviews of postmortem findings. Laboratory studies that support these theories are not available; however, some conclusions regarding distribution of stresses in the TFC have been made based on anatomical studies, force versus joint displacement analyses, and forearm axial load transmission experiments. 1~2,7,12The results provide strong evidence that both compressive and tensile loads are regularly borne by the TFC. Bowers has suggested that a load conversion mechanism may occur in the TFC during axial loading of the ulnocarpal joint.’ According to this mechanism, distraction of the distal radioulnar joint caused by axial loading is resisted by the TFC, thus converting some compressive stress in the disk to tensile stress in the radioulnar ligaments. In a radiographic analysis, Schuind et a1.9 demonstrated separation of the distal radioulnar joint surfaces during power grip, thus providing evidence that tension is generated in the TFC during axial loading of the wrist. Most investigators agree that the TFCC is the primary stabilizer of the distal radioulnar joint; however, controversy exists regarding the relative tension in the radioulnar ligaments of the TFC during forearm motion. In a biomechanical study, Schuind et al.* demonstrated greater tension in the anterior ligament in full supination and greater tension in the dorsal ligament in full pronation. Based on an anatomic study, Ekenstam et a1.7 claimed the opposite occurred in the ligaments. Both investigators, however, found reciprocal laxity in the other radioulnar ligament. In the present study, the changes found in the dorsal and anterior margins of the disk were consistent with the findings of Schuind et al.’ Despite this controversy, the translational component of distal radioulnar joint motion is considered a major reason for the nonuniform tension in the TFC. Translation is allowed by the inherent laxity in the distal radioulnar joint due to the difference in radii of curvatures between the radial and ulnar articular surfaces. 1*7Using computed tomography, translation is seen to occur primarily at the extremes of pronation and supination.4*6 In kinematic studies, the translational component is demonstrated by a slight but definite shift in the axis of rotation during forearm motion.’ The natural axis passes through the fovea of the ulnar head, which is a site of attachment for the TFC.’ Thus, the TFC can guide and restrain distal radioulnar joint motion during most
of the arc of forearm motion without causing significant distortion of TFC anatomy or resistance to joint motion. However, as soft tissues become taut at the extremes of pronation and supination, gliding along the incongruous articular surfaces occurs and results in asymmetrical changes in TFC configuration .7*8One purpose of this study was to quantify, by region and by direction, surface strains in the disk due to these configuration changes. Traumatic injuries to the TFCC usually result from an acute rotational injury to the forearm, an axial load and distraction injury to the ulnar border of the forearm, or a fall on the pronated outstretched upper extremity. ‘,‘O Although strains cannot be directly related to injury mechanics, the present results are consistent with previously reported clinical and histological findings. According to several authors, the most common site of a traumatic tear is located 2-3 mm ulnar to the radial attachment of the TFC and oriented anterior to dorsa1.3.11.‘3.14According to Palmer’s classification, this tear is a class 1A lesion.” The site corresponds to the junction of short, thick, and radially oriented collagen fibers emanating from the radius with the remaining interweaving and obliquely oriented fibers in the central disk.3 As collagen fibers tend to align with principle stresses, the transition in fiber arrangement suggests that a distinct change in functional requirements and material properties occurs in this region. In addition, disparity in tissue architecture combined with asymmetrical loading will result in locally high stress gradients with an attendant increased risk of injury. The results of this study demonstrate that strains occur primarily in the radioulnar axis of the disk during forearm motion and distal radioulnar joint distraction. Progressively negative strains were consistently found across the disk during supination. During pronation, strains in the radial portion of the disk remained unchanged or increased. Thus, tissue tension due to forearm motion is reduced across the entire disk in supination while the radial portion of the disk is under increased tension in pronation. Furthermore, strains were preferentially distributed to the radial portion of the disk when tensile loads were applied to simulate distal radioulnar joint separation that occurs during wrist axial loading. These findings suggest that forearm pronation and joint distraction loading are important components of the injury mechanism for the most common type of traumatic tear (class 1A) and perhaps for the radial avulsion injury (class ID). Although this conclusion is consistent with current knowledge of disk injury, there are additional forces acting simultaneously on the disk during wrist loading activities that influence
The Journal of Hand Surgery i Vol. 18A No. 5 September 1993 925 overall tissue response. In addition, this laboratory experiment studied the horizontal components of the TFCC without the influence of the ulnocarpal ligaments. The full implications of this study to injury mechanics will depend upon the results of further investigations that evaluate other disk loading conditions and anatomical contraints.
7.
References
9.
I. Bowers WH. The distal radioulnar joint. In: Green D. ted). Operative hand surgery. 2nd ed. New York: Churchill Livingstone. 1988:939-89. 7_. Palmer AK. Werner FW. The triangular fibrocartilage complex of the wrist: anatomy and function. J Hand Surg 1981;6:153-62. 3. Chidgey LK, Dell PC. Bittar ES, Spanier SS. Histologic anatomy of the triangular fibrocartilage. J Hand Surg 1991;16A:1084-100. R, 4 Cone RO. Szabo R. Resnick D, Gelberman Talesnik .I, Gilula LA. Computed tomography of the normal radioulnar joints. invest Radio1 1983:lS: 541-5. f King GJ, McMurty RY, Rubenstein JD, Gertzbein _ SD. Kinematics of the distal radioulnar joint. J Hand Surg 1986:1 lA:798-804. 6. Wechsler R, Wehbe MA, Rifkin MD, Edeiken J,
8.
10.
II. 12. 13.
14.
Branch HM. Computed tomography diagnosis of distal radioulnar subluxation. Skeletal Radio] l987;16: 1-5. af Ekenstam F. Hagert CG. Anatomical studies on the geometry and stability of the distal radioulnar joint. Stand J Plast Reconstr Surg 1985:19: 17-25. Schuind F. An KN, Berglund L et al. The distal radioulnar ligaments: a biomechanical study. J Hand Surg 1991~16A:1106-14. Schuind FA, Linscheid RL. An KN, Chai EYS. Changes in wrist and forearm configuration with grasp and isometric contraction of elbow flexors. J Hand Surg 1992;17A:698-703. Palmer AK. Triangular fibrocartilage complex lesions: a classification. J Hand Surg 1989:14A: 594-606. Osterman AL, Terrill RG. Arthroscopic treatment of TFCC lesions. Hand Clin 1991:7:277-81. Palmer AK, Werner FW. Biomechanics of the distal radioulnar joint. Clin Orthop 1984;187:26-35. Reinus WR, Hardy DC. Totty WG. Gilula LA. Arthrographic evaluation of the carpal triangular fibrocartilage complex. J Hand Surg 1987A;12: 495-503. Osterman AL. Arthroscopic debridement of triangular fibrocartilage complex tears. Arthroscopy 1990:h: 120-4.
fibrocartilage
system in a cadaveric laboratory experiment. curred during pronation and supination was dependent upon forearm position. disk, with dorsoanterior
complex was studied using a video
Changes in disk configuration
and resulted
in a nonuniform
Strains occurred primarily
strains being negligible.
imaging
consistently
strain distribution
in the radioulnar
Strains wereconcentrated
of the disk and were highest with the forearm pronated. Application the distal radioulnar
VT
octhat
axis of the
in the radial portion
of a distraction
load to
joint to simulate the effect of axial wrist loading caused strains to increase
the most in the radial portion.
These findings suggest that joint distraction
pronation are important components traumatic tear that occurs
of the injury
mechanism
loading and forearm
for the most common type of
near the radial attachment of the disk.
(J Hand Surg
19!?3;18A:
919-925.)
The triangular fibrocartilage complex (TFCC) of the wrist is a multifunctional structure composed of several anatomical components.‘** The articular disk and the dorsal and anterior radioulnar ligaments comprise the horizontal portion of the complex, referred to as the triangular fibrocartilage proper, or TFC. The TFC forms a continuation of the distal radial articular surface from the disk’s attachment at the sigmoid notch to its apical attachment in the eccentric concavity of the ulnar head (fovea) and the projecting ulnar styloid. The TFC provides an interface between the ulnar head and ulnar carpus, with the biconcave shape accommodating the geometric incongruencies between the bony surfaces. The peripheral margins of the TFC (radioulnar liga-
From the Department of Orthopaedic Surgery, University of Iowa. lowa City, IA, and Department of Orthopaedics and Rehabilitation, University of Vermont, Burlington, VT. Supported by research grant #91-001 from the Orthopedic Research and Education Foundation. Received for publication Sept. 22, 1992; accepted in revised form Feb. 23. 1993. 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: Brian D. Adams, MD, Associate Professor. Division of Hand and Microsurgery, Department of Orthopaedic Surgery, University of Iowa. Iowa City. IA 52242.
ments) are thicker and composed of longitudinally oriented collagen fibers, structurally adapted to bear tensile loading. The central portion (articular disk) is thinner and the collagen fiber pattern has multiple obliquities to the surface, implying that variable loading conditions occur. 1.3 The multiplicity of TFC functions, including load bearing, load distribution, shock absorption, and joint stabilization, is related to the complexity of movements and loading in the forearm and wrist. During forearm motion, the distal radius undergoes both rotation and translation relative to the ulna, with the translational component occurring primarily at the extremes of pronation and supination.4-6 Since the TFC attachment to the ulnar head is near the axis of distal radioulnar joint motion, the TFC causes minimal resistance to forearm movement during the central portion of the motion arc.’ However, the discrepancy in the radii of curvatures between the articular surfaces of the distal radiotilnar joint allows translation to occur as the TFC and other soft tissues become taut at the extremes of motion.6*7 This translational motion produces asymmetrical loading in the TFC resulting in nonuniform strain distribution.‘,’ In addition, the separation of the articular surfaces of the distal radioulnar joint that occurs during power grip indicates that axial loading produces tension in the TFC.9 During injury The journal
of Hand Surgery
919
920
Adams and Halley / Strains in Articular
Disk of TFCC
loading conditions, one would expect that areas of higher strain would be at greater risk for the initiation and propagation of tears. Although the radioulnar ligaments have been the focus of several investigations and injury models, the mechanics of injury in the articular disk are poorly understood despite the frequency of treatment required for symptomatic tears in the disk. Increasing our knowledge of injury mechanics has important implications for prevention, treatment, and rehabilitation of disk tears. Some injury theories propose that susceptibility of injury and site of tear are related to forearm position at the time of loading. ‘.I0 Based on clinical histories of patients presenting with tears, and observations of TFC motion made during arthroscopy, a relationship seems probable. However, to test this hypothesis requires a better understanding of the effects of forearm motion and joint loading on the TFC. The purpose of this experiment was to study the relationship between surface strains in the articular disk and forearm position, with and without the application of a distal radioulnar joint distraction force to simulate the effects of joint separation that occurs during power grip.
Materials and Methods A video imaging system integrated with a microcomputer was used to study the articular disk. Six unmatched, fresh-frozen upper extremities from human cadavers were used in this laboratory experiment. Specimens were from young adult males, ranging from 26 to 37 years of age, with a mean of 33 years. Specimens were disarticulated through the radiocarpal and ulnocarpal joints with care to preserve the articular disk, radioulnar ligaments, and the proximal attachments of the ulnocarpal ligaments of the TFCC. Absence of preexisting injury to the TFCC on gross observation was required. Specimens were held in a custom jig with the elbow flexed at 90”. Rigid fixation to the jig was obtained with bone screws into the ulna and humerus. The ulna was mounted horizontal and the humerus was mounted vertical to the platform. A Steinmann pin inserted into the radial styloid was used to passively position the radius at desired degrees of forearm rotation. A stationary protractor was used to measure the angle of forearm pronation and supination. This experimental setup allowed an unrestricted arc of forearm motion and provided direct visualization of the distal articular surfaces of the radius and TFC. In order to track surface changes over the entire disk, black circular markers measuring 1.1 mm in diameter were glued to the surface with droplets of
cyanoacrylate. Four markers were applied to the distal radial articular surface adjacent to the TFC insertion at the edge of the sigmoid notch (ulnar margin of lunate fossa). An array of three rows of four markers each was placed in a grid-like fashion across the surface of the disk, spanning the area between the anterior and dorsal radioulnar ligaments and the radial and ulnar TFC attachment sites. Video images of the marker array were acquired with a high-resolution CCD video camera (Ultrachip, Javelin Electronics). The images were captured to a frame-grabber board (PC Vision Plus, Imaging Technologies, Inc.) mounted in a microcomputer. Image acquisition and analysis was controlled by imaging software (Optimas, BioScan, Inc.). Calibration of the system was performed with a high-precision ruler placed at the level of the markers. Camera field size was set to visualize only the TFC during a complete arc of forearm pronationsupination. The multiple tools and programming options provided by the software were used for image analysis. The black markers on each image were selected as screen objects based on a gray scale threshold set by the operator. Slight irregularities caused by optical glare were corrected to obtain marker uniformity. The perimeter of each marker was then outlined. The centroid of each marker (center of marker area) was found and written to a data tile. For visual clarification and anatomic reference of the strain analysis, a grid was created by connecting the centroids and overlaid on the anatomic image (Fig. 1). The distances between the 16 centroids, depicted by the line segments of the grid, were used to define 12 lengths (L) along the radioulnar axis and 9 lengths (T) along the dorsoanterior axis of the disk surface (Fig. 2A, B). In addition, the grid provided delination of nine regions (A) of the disk (Fig. 2C). A video image of the markers was acquired at every 10”of forearm motion, beginning with full pronation and moving to full supination. Neutral forearm position (0’) was defined as parallel alignment of the radioulnar axis of the distal radius with the longitudinal axis of the humeral shaft. Images were also obtained during forearm motion in the reverse direction, that is, supination to pronation, to determine if direction of motion affected results. The entire sequence was repeated to determine reproducibility of results. The experiment was then performed with the addition of a distraction load across the distal radioulnar joint. The load was applied to cause separation of the distal radioulnar joint surfaces to simulate the effect of power grip. Distraction was accomplished with a cable attached to the radial styloid by a bone screw. Tension was applied perpendicular to the
The Journal of Hand Surgery / Vol. 18A No. 5 September 1993
921
Figure 1. Video image of the ulnocarpal surface of the triangular fibrocartilage with markers in place. Forearm is in neutral position. Grid connecting marker centroids is overlaid. U, ulna; R, radius; P, paimar: D, dorsal; M, ulnar margin of distal radius.
Figure 2. Schematic of disk surface measurements. (A) distances (L) used in strain measurements (AL) along the radioulnar axis. (B) distances (T) used in strain measurements (AT) along the dorsopalmar axis. (C) Disk regions (A) used in measuring changes in area.
922
Adams and Halley / Strains in Articular Disk of TFCC
long axis of the forearm and parallel to the radioulnar axis of the distal radius. Cable alignment was changed for each forearm position by a pulley system to avoid causing joint torque. A load ramp from 0 to 22.2 N was applied by a custom load generator at a rate of 0.9N/s. Load output was recorded simultaneously with video acquisition by the computer in order to correlate images with load. Magnitudes of the 21 defined lengths (L and T) and the areas of the 9 regions (A) were measured for each image and stored to a data file. Strains in lengths (AL and AT) were calculated from changes in centroid locations between sequential images according to the equations: AL = (Li - L,)/L,
and
AT = (Ti - Tg)iTg
where Li and Ti are the respective distances between two centroids at i degrees of forearm motion and L, and Tg are the distances between the same centroids with the forearm in neutral position (gage length). Strains were recorded as a function of forearm position and applied load. Changes in areas (A A) were calculated for each of the regions, with neutral position as reference, by the equation: AA = (Ai - A,)/A, and recorded as a function of forearm position and applied load. The first derivative of total disk change was then calculated according to the equation: A’ = (Af - AI+ ,)/Ai where Af is the total area change at i degrees of forearm motion and A[+ 1 is the change at the next forearm position. The results demonstrate rate of change in total area during forearm motion, that is, rate of area change between forearm positions. System error is determined by the pixel density of the video frame buffer and the field of view. Given a 640 x 480 pixel density in the frame buffer and a 20 x 15 mm field of view, the resolution is 0.03 mm/ pixel. An object centroid is calculated by taking the average x and y values of points lying on the object perimeter. The minimum number of points needed to define a 1.1 mm object with this system is 64. Thus, error in calculating centroids was: +
= ‘*OF
= 0.00375 mm
Results Changes in disk configuration consistently occurred during forearm motion and resulted in a nonuniform distribution of strain (Fig. 3). Strain distribution was dependent upon forearm position but
Figure 3. Grids of a representative specimen in full pronation, neutral, and full supination (left to right). Note changes in disk configuration. was independent of the direction of forearm motion. Strains within the radial three regions of the disk (Al, A2, A3) behaved similarly during testing, as did strains within the central three (A4, A5, A6) and within the ulnar three regions (A7, A8, A9). Thus, to simplify the comparative analysis, the disk was divided into three portions: radial, central, and ulnar. Strains along the radioulnar axis (AL) decreased in all three portions of the disk during supination when compared to the neutral position. Greater decreases in strains occurred in the central and radial portions of the disk when each portion was compared to the ulnar portion (p < .05). During pronation, the reverse strain distribution was found when compared to supination (Fig. 4). Greater decreases in strains along the radioulnar axis occurred in the central and ulnar portions when each portion was compared to the radial portion, which remained unchanged or increased (p < .05). Thus, the radial portion of the disk behaved differently than the central and ulnar portions during forearm motion. Dorsoanterior axis strains were negligible (strains < 0.015) in all regions and in all forearm positions. Changes in area of a region consistently paralleled the radioulnar axis strains in that region (Fig. 5). For example, changes in area of region A5 paralleled strains in L6 and L7, while T2 and T5 remained unchanged. Therefore, disk configuration changes and the resulting strains were caused primarily by strains in the radioulnar axis of the disk. Rate of change in total disk area increased as full pronation and supination were approached (Fig. 6). This implies that strain rates and configuration changes occur more rapidly at the extremes of forearm motion. Although the radioulnar ligaments were not evaluated in this study, strains (AL) along the dorsal and anterior margins of the disk were calcu-
The Journal of Hand Surgery I Vol. 18A No. 5 September 1993
60 50 40 30 20
10
0
10 20 30 40 50 60
I -8’ 60 50 40 30 20 10 0 10 20 30 40 50 60
Forearm Position (degrees from neutral) Supination *
Pronation
-
923
Forearm
percent strains along the radioulnar axis. R, radial portion; C, central portion; U. ulnar portion of disk.
Figure 4. Average
lated. Strains along the dorsal margin decreased during supination, while strains along the anterior margin increased or were unchanged. The reverse strain pattern occurred during pronation: strains along the anterior margin decreased, while strains along the dorsal margin increased or were unchanged.
2r-.-----r
Position
(degrees
Pronation
4
from neutral) Supination
*
Average percent change in total disk area and percent rate of change in total disk area.
Figure 6.
When the load ramp was applied to cause distraction of the distal radioulnar joint, strains were preferentially increased in the same disk portions that demonstrated higher strains during forearm motion without loading fp < .02). Thus, additional load on the disk was nonuniformly distributed, with the majority distributed to the radial portion of the disk (Fig. 7). Discussion Contemporary advances in diagnostic imaging techniques and wrist arthroscopy have made it pos-
J
R
S t-
r -
a i
n
_,4L--60
50
40
30
20
10
0
10 20
30
40
50
60
Forearm Position (degrees from neutral)
a
Pronation
Supination
*
Average percent changes in areas (A) of the disk. R, radial portion; C, central portion; U, ulnar portion of disk. Figure 5.
Pronation
Neutral
Supination
Average percent strains in disk with 5 lb. distraction load applied to the distal radioulnar joint (unloaded joint used as baseline). R, radial portion: C. central portion; U, ulnar portion of disk. Figure 7.
924
Adams
and Halley
/ Strains
in Articular
Disk of TFCC
sible to regularly identify and treat lesions in the articular disk of the TFCC.‘O,” Although the care of TFCC injuries has greatly improved with these technologies, our knowledge of disk injury mechanics is sparse. Current injury theories are derived primarily from interpretations of clinical observations, gross anatomy dissections, and reviews of postmortem findings. Laboratory studies that support these theories are not available; however, some conclusions regarding distribution of stresses in the TFC have been made based on anatomical studies, force versus joint displacement analyses, and forearm axial load transmission experiments. 1~2,7,12The results provide strong evidence that both compressive and tensile loads are regularly borne by the TFC. Bowers has suggested that a load conversion mechanism may occur in the TFC during axial loading of the ulnocarpal joint.’ According to this mechanism, distraction of the distal radioulnar joint caused by axial loading is resisted by the TFC, thus converting some compressive stress in the disk to tensile stress in the radioulnar ligaments. In a radiographic analysis, Schuind et a1.9 demonstrated separation of the distal radioulnar joint surfaces during power grip, thus providing evidence that tension is generated in the TFC during axial loading of the wrist. Most investigators agree that the TFCC is the primary stabilizer of the distal radioulnar joint; however, controversy exists regarding the relative tension in the radioulnar ligaments of the TFC during forearm motion. In a biomechanical study, Schuind et al.* demonstrated greater tension in the anterior ligament in full supination and greater tension in the dorsal ligament in full pronation. Based on an anatomic study, Ekenstam et a1.7 claimed the opposite occurred in the ligaments. Both investigators, however, found reciprocal laxity in the other radioulnar ligament. In the present study, the changes found in the dorsal and anterior margins of the disk were consistent with the findings of Schuind et al.’ Despite this controversy, the translational component of distal radioulnar joint motion is considered a major reason for the nonuniform tension in the TFC. Translation is allowed by the inherent laxity in the distal radioulnar joint due to the difference in radii of curvatures between the radial and ulnar articular surfaces. 1*7Using computed tomography, translation is seen to occur primarily at the extremes of pronation and supination.4*6 In kinematic studies, the translational component is demonstrated by a slight but definite shift in the axis of rotation during forearm motion.’ The natural axis passes through the fovea of the ulnar head, which is a site of attachment for the TFC.’ Thus, the TFC can guide and restrain distal radioulnar joint motion during most
of the arc of forearm motion without causing significant distortion of TFC anatomy or resistance to joint motion. However, as soft tissues become taut at the extremes of pronation and supination, gliding along the incongruous articular surfaces occurs and results in asymmetrical changes in TFC configuration .7*8One purpose of this study was to quantify, by region and by direction, surface strains in the disk due to these configuration changes. Traumatic injuries to the TFCC usually result from an acute rotational injury to the forearm, an axial load and distraction injury to the ulnar border of the forearm, or a fall on the pronated outstretched upper extremity. ‘,‘O Although strains cannot be directly related to injury mechanics, the present results are consistent with previously reported clinical and histological findings. According to several authors, the most common site of a traumatic tear is located 2-3 mm ulnar to the radial attachment of the TFC and oriented anterior to dorsa1.3.11.‘3.14According to Palmer’s classification, this tear is a class 1A lesion.” The site corresponds to the junction of short, thick, and radially oriented collagen fibers emanating from the radius with the remaining interweaving and obliquely oriented fibers in the central disk.3 As collagen fibers tend to align with principle stresses, the transition in fiber arrangement suggests that a distinct change in functional requirements and material properties occurs in this region. In addition, disparity in tissue architecture combined with asymmetrical loading will result in locally high stress gradients with an attendant increased risk of injury. The results of this study demonstrate that strains occur primarily in the radioulnar axis of the disk during forearm motion and distal radioulnar joint distraction. Progressively negative strains were consistently found across the disk during supination. During pronation, strains in the radial portion of the disk remained unchanged or increased. Thus, tissue tension due to forearm motion is reduced across the entire disk in supination while the radial portion of the disk is under increased tension in pronation. Furthermore, strains were preferentially distributed to the radial portion of the disk when tensile loads were applied to simulate distal radioulnar joint separation that occurs during wrist axial loading. These findings suggest that forearm pronation and joint distraction loading are important components of the injury mechanism for the most common type of traumatic tear (class 1A) and perhaps for the radial avulsion injury (class ID). Although this conclusion is consistent with current knowledge of disk injury, there are additional forces acting simultaneously on the disk during wrist loading activities that influence
The Journal of Hand Surgery i Vol. 18A No. 5 September 1993 925 overall tissue response. In addition, this laboratory experiment studied the horizontal components of the TFCC without the influence of the ulnocarpal ligaments. The full implications of this study to injury mechanics will depend upon the results of further investigations that evaluate other disk loading conditions and anatomical contraints.
7.
References
9.
I. Bowers WH. The distal radioulnar joint. In: Green D. ted). Operative hand surgery. 2nd ed. New York: Churchill Livingstone. 1988:939-89. 7_. Palmer AK. Werner FW. The triangular fibrocartilage complex of the wrist: anatomy and function. J Hand Surg 1981;6:153-62. 3. Chidgey LK, Dell PC. Bittar ES, Spanier SS. Histologic anatomy of the triangular fibrocartilage. J Hand Surg 1991;16A:1084-100. R, 4 Cone RO. Szabo R. Resnick D, Gelberman Talesnik .I, Gilula LA. Computed tomography of the normal radioulnar joints. invest Radio1 1983:lS: 541-5. f King GJ, McMurty RY, Rubenstein JD, Gertzbein _ SD. Kinematics of the distal radioulnar joint. J Hand Surg 1986:1 lA:798-804. 6. Wechsler R, Wehbe MA, Rifkin MD, Edeiken J,
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II. 12. 13.
14.
Branch HM. Computed tomography diagnosis of distal radioulnar subluxation. Skeletal Radio] l987;16: 1-5. af Ekenstam F. Hagert CG. Anatomical studies on the geometry and stability of the distal radioulnar joint. Stand J Plast Reconstr Surg 1985:19: 17-25. Schuind F. An KN, Berglund L et al. The distal radioulnar ligaments: a biomechanical study. J Hand Surg 1991~16A:1106-14. Schuind FA, Linscheid RL. An KN, Chai EYS. Changes in wrist and forearm configuration with grasp and isometric contraction of elbow flexors. J Hand Surg 1992;17A:698-703. Palmer AK. Triangular fibrocartilage complex lesions: a classification. J Hand Surg 1989:14A: 594-606. Osterman AL, Terrill RG. Arthroscopic treatment of TFCC lesions. Hand Clin 1991:7:277-81. Palmer AK, Werner FW. Biomechanics of the distal radioulnar joint. Clin Orthop 1984;187:26-35. Reinus WR, Hardy DC. Totty WG. Gilula LA. Arthrographic evaluation of the carpal triangular fibrocartilage complex. J Hand Surg 1987A;12: 495-503. Osterman AL. Arthroscopic debridement of triangular fibrocartilage complex tears. Arthroscopy 1990:h: 120-4.