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Biomechanical evaluation of wrist motor tendons after fractures of the distal radius
Biomechanical Evaluation of Wrist Motor Tendons After Fractures of the Distal Radius Jin Bo Tang, MD, Jaiyoung Ryu, MD, Shohei Omokawa, MD, JungSoo Han, PhD, Vincent Kish, Morgantown, WV We conducted a biomechanical study of changes in parameters of wrist motor tendons in fractures of the distal radius in 7 cadaveric extremities. Extra-articular distal radius fractures were simulated by distal radius osteotomy and fracture angulation was maintained by external fixators. Eight positions of the distal radius fractures were studied: dorsal angulation of 10°, 20°, 30°, and 40° and radial angulation of 5°, 10°, 15°, and 20°. Dorsal and radial angulation of the fractures were measured with respect to the shaft of the radius. Excursions of 5 principal wrist motor tendons extensor carpi radialis longus, extensor carpi radialis brevis, extensor carpi ulnaris, flexor carpi radialis, and flexor carpi ulnaris were recorded simultaneously with wrist joint angulation using a computer-assisted recording system. Data were collected from intact wrists and from wrists with fractures at each of 8 positions of angulation during wrist flexion and extension and radical and ulnar deviation. Moment arm of the wrist motor tendons was derived from tendon excursion and joint angulation. The results demonstrated that excursions and moment arms of principal wrist motor tendons are significantly affected by dorsal and radial angulation of distal radius fractures. Amplitude of changes in moment arms increased as the deformities became more severe. Statistical analysis revealed that dorsal angulation of 10° or more significantly affected moment arms of all the prime wrist motors. Dorsal angulation of 30° or 40° changed the moment arms greatly. Radial angulation of 5° did not affect moment arms of the tendons and angulation over 10° had a statistically significant effect on the tendons. We conclude that deformities of distal radius fractures have a significant influence on the biomechanics of the wrist motors. (J Hand Surg 1999;24A:121–132. Copyright © 1999 by the American Society for Surgery of the Hand.) Key words: Fracture, radius, wrist biomechanics, tendon
Fractures of the distal radius represent one of the most common skeletal injuries, accounting for approximately 10% of all body injuries.1,2 While many
From the Musculoskeletal Research Center, Department of Orthopedics, Health Sciences Center, West Virginia University, Morgantown, WV. Received for publication October 20, 1995; accepted in revised form June 17, 1998. 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: Jin Bo Tang, MD, Department of Orthopedics, Affiliated Hospital of Nantong Medical College, 20 W Temple Rd, Nantong 226001, Jiangsu, China. Copyright © 1999 by the American Society for Surgery of the Hand 0363-5023/99/24A01-0018$3.00/0
of the fractures are treated properly, these fractures are frequently associated with a high incidence of complications, with malunion ranking among the most common.3–7 In a large retrospective study of 2,132 distal radius fractures, Bacorn and Kurtze4 found that only 3% of patients had no permanent functional loss.4 A study of 565 distal radius fractures by Cooney et al7 revealed 177 (31%) had various bony or soft tissue complications. Although some studies have reported good long-term functional outcomes despite residual deformities,8 –10 many studies have shown that anatomic reduction leads to better functional outcome.1,2,6,7,11–20 In clinical practice, there is a lack of consensus with regard to the amount of acceptable deformity and the indiThe Journal of Hand Surgery 121
122 Tang et al / Wrist Motors in Distal Radius Fractures
cations for corrective osteotomy for malunion after distal radius fracture.21–26 Although treatment of fractures of the distal radius is a common subject of studies,27–29 biomechanical changes following distal radius fractures are poorly understood.30,31 Fracture deformities, function of the hand, joint kinematics, and joint loading patterns after distal radius fractures are closely dependent on kinetics of forearm muscles, especially the wrist motors. To clarify the impact of fracture deformities on the kinetics, we conducted a biomechanical study of the principal wrist motors in a cadaver model of simulated extra-articular distal radius fractures.
Materials and Methods Seven fresh-frozen cadaver upper extremities were harvested from 3 men and 2 women through amputation at the midhumeral level. The ages ranged from 54 to 72 years (average, 62 years). Wrist radiographs were obtained to eliminate bony deformities and obvious degenerative changes of the joints. Specimens with ligamentous laxity or with restriction in wrist motion were not used. Normal palmar tilt and radial inclination were measured in each specimen on anteroposterior and lateral x-ray films. Palmar tilt of these wrists was 11° 6 2°; radial inclination was 23° 6 4°.
Specimen Preparation After specimens were completely thawed at room temperature, the muscle mass of the arms was dissected, retaining the forearm interosseous membrane and elbow capsule. All tendons and ligaments within 6 cm proximal to the wrist were preserved. Dacron braided suture lines were sutured to the proximal ends of the cut tendons. The specimens were mounted to a Plexiglas jig using 2 screws, which transfixed the humerus to the vertical frame of the jig. K-wires were inserted in the thumb and fingers to immobilize the metacarpophalangeal and interphalangeal joints in extension to prevent unwanted motion. The elbow was kept in 90° of flexion and the forearm in neutral rotation by the forearm-supporting frame of the jig. The radius and ulna were fixed to the frame approximately 5 cm proximal to the wrist to prevent forearm rotation and translation.
Experimental Design The suture lines of the tendons were oriented along the natural direction of muscle pull and were passed through guide holes in the vertical frame close to
either the medial or lateral epicondyle of the elbow. To take up the slack in the tendons during wrist motion, each of the tendons was loaded with a weight of 250 g. Wires of the 5 prime wrist movers, extensor carpi radialis longus (ECRL), extensor carpi radialis brevis (ECRB), extensor carpi ulnaris (ECU), flexor carpi radialis (FCR), and flexor carpi ulnaris (FCU), were routed around the pulleys of a rotational potentiometer (model 3543s; Bourns Inc, Riverside, CA), which recorded the amount of voltage produced by the tendon excursions during wrist movement. Voltage was automatically converted to values of the tendon excursions by a software program. A dual-axis electrogoniometer (model 110; Penny and Giles Co, Santa Monica, CA) was mounted on the ulna and third metacarpal shaft to record degrees of wrist global motion. Tendon excursion values were calibrated to an accuracy of 0.1 mm and wrist angulation values to 0.1°. Tendon excursion and joint angulation were collected simultaneously by a data acquisition system and stored on a computer hard drive (PC 386; Zenith Data System, Milwaukee, IL).
Test Procedures Distal radius fractures were simulated by an osteotomy of the distal radius. Fracture angulation was maintained by external fixators. Using a Penning dynamic wrist fixator (EIB Medical System, Parsippany, NJ), the distal end of the radius was joined to the proximal portion of the radius. A 1.5-cm distal radius osteotomy was performed with an air osteotome starting 2 cm proximal to the distal radial articular surface. The distal interosseous membrane was incised 2 cm, without interference of the distal radioulnar joint. A Plexiglas external fixator was used to obtain the desired degree of angulation of the distal segment of the fracture. The Plexiglas fixator consisted of distal and proximal parts that allowed axial rotation. By rotating the distal part of the fixator, different degrees of dorsal or radial angulation were simulated when the fixator was applied to the radius laterally or dorsally (Fig. 1). Eight positions of distal radial angulation were studied: 10°, 20°, 30°, and 40° of dorsal angulation and 5°, 10°, 15°, and 20° of radial angulation. The fracture was angulated dorsally and laterally by setting the fracture in angulated position relative to the shaft of the radius (Fig. 2). X-ray films were taken to confirm that positions angulated by the external fixators accurately represented the desired extent of deformities. The Penning dynamic wrist fixator and the Plexiglas fixator fixed
The Journal of Hand Surgery / Vol. 24A No. 1 January 1999 123
Figure 1. The extra-articular fracture of the distal radius was simulated by osteotomy in the distal radius and angulation was maintained by an external fixation device. The desired grade of angulation in the sagittal plane and in the frontal plane was obtained by applying customized hinge Plexiglas fixators laterally or dorsally and setting the hinge fixators in appropriate angulation. (Top) Dorsal angulation of the fractures. (Bottom) Radial angulation of the fractures.
the fracture through planes perpendicular to each other. The dynamic wrist fixator maintained normal alignment of the fracture in planes perpendicular to simulated fracture deformities. Maintenance of the normal alignment in these planes was confirmed by x-ray films. The external fixation devices provided
satisfactory stability to the simulated fracture during testing. No dorsal or lateral displacement was added to the fractures during angulation of the fractures. In intact wrists and in each position of angulation, the wrists were moved passively along horizontal and vertical metal bars through flexion to extension
124 Tang et al / Wrist Motors in Distal Radius Fractures
in the sagittal plane and radial deviation to ulnar deviation in the frontal plane. As the joint moved, the excursion of each of the 5 motor tendons was recorded simultaneously and continuously with joint angulation for 10 seconds at a sampling rate of 10 Hz. Tendon excursions were measured in intact wrists and in each of the angulation positions of the simulated distal radius fractures.
Data Analysis
Figure 2. Dorsal (top) and radial (bottom) angulation of distal radius fractures. The dorsal and radial angulations were measured as angulation of the distal part of the fracture relative to the shaft of the radius.
In this study, movement of the third metacarpal bone represented global motion of the wrist. After dorsal angulation of the distal radius, wrist extension increased and wrist flexion reduced. After radial angulation, radial deviation increased and ulnar deviation decreased. Flexion of over 45° and ulnar deviation of over 25°, however, were observed in all degrees of angulation in each specimen. Therefore, the tendon excursions used for analysis were those from 45° of flexion and 45° of extension and from 20° of radial deviation to 25° of ulnar deviation. Moment arm of the wrist movers was calculated according to the relationship that excursion (DE) along an arc equals the radius of the arc (R) times the subtended angle DU in the radian (1 radian 5 57°): R 5 DE/DU.32–35 Figure 3 illustrates how excursion is used to calculate the moment arms of the wrist motor tendons. The excursion of a tendon through 57° of joint motion is equal to its distance to the center of rotation (the moment arm). Since the wrist
Figure 3. The method of obtaining moment arm of a joint motor from its excursion and joint angulation. When the joint moves through an arc of 57° (1 radian), the distance of the motor tendon to the center of rotation is equal to its excursion through the joint motion of 57°. The distance from the tendon to the center of rotation is known as the moment arm. Therefore, moment arm of a tendon can be calculated by dividing excursion of the tendon by subtended degrees of joint motion in the radian.
The Journal of Hand Surgery / Vol. 24A No. 1 January 1999 125
Table 1. Excursion of the Prime Wrist Motors Through 45° of Wrist Flexion and 45° of Wrist Extension
Wrist Motors ECRL ECRB ECU FCR FCU
Dorsal Angulation
Radial Angulation
Normal Wrists
10°
20°
30°
40°
5°
10°
15°
20°
14.1 6 2.2 19.0 6 3.8 6.9 6 2.8 21.8 6 2.7 24.6 6 2.6
15 6 2 20 6 2 563 21 6 3 25 6 3
15 6 2 20 6 2 262 20 6 3 23 6 4
16 6 2 20 6 2 21 6 1* 18 6 3 23 6 4
16 6 3 20 6 2 21 6 1* 17 6 3 21 6 2
13 6 3 18 6 4 763 23 6 3 24 6 3
13 6 2 18 6 3 763 22 6 3 22 6 3
12 6 1 17 6 3 763 23 6 3 23 6 3
12 6 2 17 6 3 863 24 6 3 23 6 2
Data are expressed as mean values 6 SD. * The ECU tendon moved in the direction of wrist flexors when dorsal angulation was 30° or 40°.
resulting from dorsal or radial angulation were separately analyzed as described below.
range of motion is not a perfect sphere, moment arms determined by this study represent an approximation of calculation of average moment arms during tested ranges of wrist motion. Using custom-designed computer software, the values of moment arms of the prime wrist motors were derived from tendon excursions and wrist angulation. Because of large interspecimen variation of normal moment arms among tested intact wrists, moment arms of the tendons in the wrists with fracture angulation were adjusted by dividing their moment arm data by those of intact wrists. The percentage changes were used for analysis. This adjusted for differences in the sizes and configurations of the wrists between specimens. One-way ANOVA was used to determine statistical differences between wrists with displacements of the fractures and normal wrists. A difference of 5% was considered significant.
Effect of Dorsal Angulation Flexion and Extension Movement. Through the wrist flexion and extension arc of 90°, dorsal angulation of 10° to 40° resulted in an increase in excursions of the ECRL tendon by 1.3 to 2.1 mm (10% to 15%). The moment arm of the ECRL tendon increased by 0.8 to 1.3 mm. Excursions of the ECRB tendon increased only by 0.5 to 0.9 mm. The increase in moment arm was 0.3 to 0.6 mm. Excursions of the ECU tendon showed a dramatic change in dorsal angulation, decreasing by 1.9 to 7.3 mm (28% to 111%), while moment arms decreased by 1.1 to 5.2 mm. The ECU tendon glided in the direction of flexors when dorsal angulation exceeded 30°. A moderate decrease in excursions was observed in the FCR and FCU tendons: by 0.7 to 5.3 mm (3% to 24%) in the FCR tendon and by 1.3 to 3.7 mm (7% to 15%) in the FCU tendon. The moment arm of the FCR and FCU tendons decreased from 0.5 to 3.4 mm and from 1.0 to 2.4 mm, respectively. Statistical analysis revealed a significant difference in excursions and moment arms of the ECRL
Results Tables 1 to 4 list tendon excursions and moment arms of the prime wrist motors during wrist flexion and excursion and radial and ulnar deviation. Changes in tendon excursions and moment arms
Table 2. Excursion of the Prime Wrist Motors Through 20° of Wrist Radial Deviation and 25° of Wrist Ulnar Deviation Dorsal Angulation Wrist Motors ECRL ECRB ECU FCR FCU
Radial Angulation
Normal Wrists
10°
20°
30°
40°
5°
10°
15°
20°
15 6 3 962 15 6 4 662 11 6 3
14 6 2 962 16 6 3 662 11 6 3
14 6 2 863 17 6 3 762 963
13 6 2 762 17 6 3 662 862
12 6 1 662 16 6 3 561 862
17 6 3 10 6 2 15 6 3 662 10 6 2
17 6 2 10 6 2 16 6 3 762 962
17 6 2 10 6 1 16 6 3 863 862
17 6 2 11 6 2 15 6 3 862 862
Data are presented as mean values 6 SD.
126 Tang et al / Wrist Motors in Distal Radius Fractures
Table 3. Moment Arms of the Prime Wrist Motors in Wrist Flexion and Extension Motion Dorsal Angulation Wrist Motors ECRL ECRB ECU FCR FCU
Radial Angulation
Normal Wrists
10°
20°
30°
40°
5°
10°
15°
20°
961 12 6 2 462 14 6 2 16 6 2
10 6 1 13 6 2 13 6 2 13 6 2 16 6 2
10 6 1 12 6 1 161 13 6 2 15 6 2
10 6 1 12 6 1 061 12 6 2 14 6 2
10 6 2 12 6 1 21 6 1 11 6 2 13 6 2
862 11 6 3 562 14 6 2 15 6 2
861 11 6 2 462 14 6 2 15 6 2
861 11 6 2 562 14 6 2 15 6 2
861 11 6 2 562 15 6 2 14 6 2
Data are presented as mean values 6 SD.
mm (12% to 26%) when dorsal angulation was over 20°. Moment arms of the FCU tendon decreased by 1.9 to 3.8 mm. Excursions and moment arms of the ECRL tendon decreased significantly when dorsal angulation was over 20° (p , .01 and p , .001, respectively). Significant differences were also found in the ECRB tendon between fractures with 30° and 40° of dorsal angulation and intact wrists (p , .01 and p , .01, respectively). Excursions and moment arms of the FCU tendon were reduced significantly when dorsal angulation was greater than 20° (p , .05 and p , .01, respectively). Changes in excursions and moment arms of the ECU and FCR tendons were not statistically significant. Dorsal angulation of 10° had no significant effect on excursions and moment arms of the wrist motors (Fig. 5).
tendon between intact wrists and wrists with 10° or 40° of dorsal angulation (p , .05, p , .01, and p , .001, respectively). Excursions and moment arms of the ECRB tendon increased significantly when dorsal angulation of the fracture was 10° (p , .05). Tendon excursions and moment arms of the ECU and FCR tendons showed a significant decrease at each position of angulation (p , .01 and p , .001). Excursions and moment arms of the FCU tendon decreased significantly when angulation was 30° or 40° (p , .05 and p , .001, respectively), but no significant difference was seen between normal wrists and wrists with 10° or 20° of dorsal angulation (Fig. 4). Radial and Ulnar Deviation. During radial and ulnar deviation of 45°, dorsal angulation of distal radius fractures resulted in a decrease of 0.2 to 2.6 mm (2% to 18%) in excursions of the ECRL tendon and of 0.2 to 2.7 mm (2% to 31%) in those of the ECRB tendon. The decrease of moment arm was 0.4 to 4.2 mm in the ECRL tendon and 0.2 to 3.4 mm in the ECRB tendon. Excursions of the ECU tendon increased by 1.0 to 2.3 mm (7% to 16%). Moment arms of the ECU tendon increased by 1.3 to 2.9 mm. Changes in tendon excursions of the FCR tendon varied largely, from an average increase of 0.7 mm (11%) to a decrease of 1.2 mm (20%). The FCU tendon showed a decrease in excursions by 1.5 to 2.8
Effect of Radial Angulation Flexion and Extension Movement. A decrease in the excursions of the ECRL tendon by 0.8 to 2.4 mm (6% to 17%) was seen during 90° of flexion and extension movement. The moment arm of the ECRL tendon decreased by 0.6 to 1.5 mm. Excursions of the ECRB tendon decreased by 1.1 to 2.2 mm (6% to 11%), with a corresponding decrease in moment
Table 4. Moment Arms of the Prime Wrist Movers in Wrist Radial and Ulnar Deviation Dorsal Angulation Wrist Motors ECRL ECRB ECU FCR FCU
Radial Angulation
Normal Wrists
10°
20°
30°
40°
5°
10°
15°
20°
19 6 2 11 6 2 19 6 5 863 14 6 4
19 6 3 11 6 2 20 6 4 763 14 6 4
18 6 3 10 6 3 22 6 4 862 12 6 4
16 6 2 962 21 6 3 763 11 6 3
15 6 2 862 21 6 4 662 10 6 3
21 6 3 13 6 3 19 6 4 863 12 6 3
21 6 3 13 6 2 20 6 4 963 12 6 3
21 6 3 13 6 2 20 6 4 10 6 3 12 6 3
22 6 3 14 6 2 19 6 4 10 6 3 11 6 3
Data are presented as mean values 6 SD.
The Journal of Hand Surgery / Vol. 24A No. 1 January 1999 127
Figure 4. Comparison of normalized values of moment arms of the primary wrist motor tendons during wrist flexion and extension in the fractures with dorsal angulation. Asterisks indicate columns with statistically significant differences compared with the values in normal wrists. DA, dorsal angulation (°).
arms of 0.7 to 1.4 mm. Changes in excursions were considerably less in the ECU and FCR tendons. The FCU tendon showed a decrease of 0.8 to 2.1 mm in excursion and a decrease of 0.6 to 1.4 mm in moment arms. Significant decreases in excursions and moment arms were seen in the ECRL tendon when radial angulation was over 10° (p , .05 and p , .01, respectively). Decreases in excursions and moment arms of the ECRB tendon were significant when 15° and 20° of radial angulation were produced (p , .05 for each comparison). The excursions and moment arms of the FCU tendon decreased significantly when angulation exceeded 10° (p , .01 and p , .001, respectively). Changes in excursions and moment arms of the ECU and FCR tendons were not statistically significant (Fig. 6). Radial and Ulnar Deviation. Radial angulation increased excursions of the ECRL tendon by 1.9 to 2.3 mm (13% to 16%) and those of the ECRB tendon by 1.4 to 2.3 mm (16% to 26%). The moment arm increased by 1.6 to 2.2 mm in the ECRL tendon and by 1.8 to 3.0 mm in the ECRB tendon. Changes in excursions of the ECU tendon were minimal, in-
creasing only by 0.4 to 1.2 mm. The FCR tendon showed an increase of 0.2 to 1.6 mm (3% to 12%) in tendon excursions and of 0.3 to 2.1 mm in moment arms. The FCU tendon showed a decrease of excursions by 1.1 to 1.9 mm (10% to 18%). The moment arms decreased by 1.4 to 2.2 mm. Excursions and moment arms of the ECRL tendon increased significantly with the varying degrees of radial angulation (p , .01 and p , .001, respectively). Excursions and moment arms of the ECRB tendon increased significantly when radial angulation was 10°, 15°, or 20° (p , .01 for each comparison). The ECU tendon was not significantly affected by radial angulation. Excursions and moment arms of the FCR and FCU tendons were influenced when radial angulation exceeded 10° (p , .05 and p , .01, respectively). Radial angulation of 5° did not affect excursions and moment arms of the ECRB, ECU, FCR, or FCU tendons (Fig. 7).
Discussion This study demonstrated that excursions and average moment arms of principal wrist motor tendons
128 Tang et al / Wrist Motors in Distal Radius Fractures
Figure 5. Moment arms of the wrist motor tendons during radial and ulnar deviation in the fractures with dorsal angulation. Asterisks indicate columns with significant differences compared with the moment arms of normal wrists. DA, dorsal angulation.
Figure 6. Values of moment arms of the wrist motor tendons during flexion and extension in simulated radial angulation. Columns with significant changes are indicted by asterisks. RA, radial angulation.
The Journal of Hand Surgery / Vol. 24A No. 1 January 1999 129
Figure 7. Moment arms of the wrist motor tendons during radial and ulnar deviation in simulated radial angulation. Columns with significant changes are indicated by asterisks. RA, radial angulation.
are significantly affected by angulation of distal radius fractures. Changes in excursions and moment arms increased as the simulated deformities became more severe. The moment arm is the effective adjustment for mechanical advantage of the joint mover.32 Since the prime wrist motors are the major dynamic source of wrist motion, the results of our study may imply that deformities of the distal radius fracture can influence hand function. The wrist motor tendons presented various patterns of moment arm changes in conjunction with fracture deformities. The most clinically relevant changes demonstrated by this study are summarized as follows: 1. Dorsal angulation had the most influence on the moment arms during wrist flexion and extension. Only when dorsal angulation together with wrist flexion and extension were simulated did all 5 prime wrist motors show significant changes in moment arms. 2. While 4 of 5 motors had changes in moment arms in cases of moderate radial angulation, slight radial angulation had almost no effect on moment arms of the wrist motors. 3. The major tendency of biomechanical alterations was a decrease in moment arms of the wrist motor tendons. All the flexors during flexion and exten-
sion and all the ulnar deviators during radial and ulnar deviation showed decreased moment arms. 4. Increased moment arms occurred in the radial wrist extensors and radial deviators when dorsal and radial angulation were simulated. 5. Fracture deformities had a greater influence on moment arms along the plane of wrist motion than on those in the perpendicular direction of wrist motion.
The axis of rotation in the wrist was previously determined to lie within the head of the capitate.36 Kauer,37 however, demonstrated that there are 2 shifting instant centers of rotation. It was noted that the head of the capitate is not spherical, but rather 3 separate facets for scaphoid, lunate, and hamate articulations.37 During wrist radial and ulnar deviation, there are also subtle changes in relative positions between proximal and distal carpal rows and positions of the bones within the proximal row.38 Therefore, wrist motion restricted to a plane is not a perfect sphere and does not take place around a fixed center of rotation. Instantaneous moment arms of these tendons may differ at each wrist joint angulation. Consequently, our method of determining moment arms of the wrist motors during wrist motion in this study
130 Tang et al / Wrist Motors in Distal Radius Fractures
represents an approximate calculation. The moment arms determined in this study represent average moment arms of these motor tendons during the tested ranges of wrist motion. By alternative methods of calculation, it will be possible to determine changes of moment arms of wrist motors at specific wrist joint motion.39 Dorsal and radial angulations are the dominant patterns in displacement of fractures of the distal radius and important parameters with which to judge reduction. This study revealed that dorsal angulation of only 10° significantly altered mechanics of the wrist motors. Further increases in dorsal angulation proportionately increased the alterations. Dorsal angulation decrease the moment arms of the flexors (the FCR and FCU tendons) and even reversed the effect of the ECU tendon. Increasing moment arms of the ECRL and ECRB tendons are expected to produce increased torque to dorsally displace the fractures. These findings suggest that normal anatomic alignment of the fractures is important to the stability of fracture reduction and to maintaining balanced torque of wrist motor tendons for wrist motion. For malunited fractures of the distal radius, Fernandez24,26 believes that a symptomatic extraarticular malunion causing dorsal angulation over 25° to 30° is an indication for corrective osteotomy. The results of our study revealed that dorsal angulation over 30° induced changes in moment arms of all the wrist motor tendons in a considerably large amplitude. The changes were mostly as large as 25% to 110% of the moment arms in the intact wrists. Radial angulation of 5° was shown to have no significant influence on the majority of wrist motor tendons. Radial angulation of 10° or more influenced the wrist motor tendons. These results support the clinical impression that allowable changes in radial inclination should not exceed 10°. A number of clinical studies have reported that deformities of malunited fractures positively correlate with functional deficits, such as decreases in power or inability to perform activities of daily living.11–20 In a review of 154 fractures of the distal radius, Solgaard16 reported that the number of patients with excellent and good results decreased significantly as the amount of dorsal angulation increased. McQueen and Caspers17 found that grip strength, performance of activities of daily living, and motion were significantly worse in wrists with dorsal angulation over 12° than in wrists with 10° or less of dorsal angulation. Jenkins and MintowtCzyz19 showed a significant correlation between a
decrease in radial inclination of the radial articular surface and a decrease in grip strength. Porter and Stockley,1 Frykman,6 and Cooney et al7 reported that a decrease in grip power has a close relationship with severity of fracture deformity. It was speculated that a mechanical base for clinical findings is the disturbance of the mechanical advantages of wrist motor tendons by fracture deformities.40,41 This study demonstrated that either dorsal or radial angulation decreased the moment arm of the flexor tendons. The decrease of moment arms in the wrist flexors places the muscles at a mechanical disadvantage. Activities of daily living depend on muscle power as well as coordinated joint motion.40,41 Angulation of the fractures decreased moment arms of the tendons for motion in the perpendicular direction. On the other hand, angulation of the fractures increased tendon moment arms for the motion along the direction of the deformity. We speculate that these changes may distort normal patterns of wrist motion and contribute to the inability of wrists to perform activities of daily living. The potential for dorsal subluxation of the carpus to accompany fractures of the distal radius has been recognized for many years.42 Midcarpal instability caused by malunited fractures of the distal radius was also noted in young, active patients who tend to have ligamentous laxity.43 One of the common features presenting in these accompanying carpal malalignments is that main parts of the carpus translate dorsally with respect to the long axis of the radius. The mechanical unbalance of wrist motor tendons noted in this study may constitute an important pathomechanical factor in the formation of associated carpal deformities. Dorsal angulation of the fractures generates increased torque in the extensor aspect and decreased torque in the flexor aspect. This combination of changes around the wrist favors translation or twisting of the inherently unstable wrist joint, resulting in carpal instability. In a biomechanical study of wrist load patterns, Short et al30 demonstrated that the center of pressure in the radioscaphoid and ulnocarpal joint migrates dorsally as the distal radius is dorsally angulated to varying degrees. Porgue et al31 demonstrated a dorsal shift in the scaphoid and lunate high-pressure areas by angulating the distal fragment of the fractures. The results of our study revealed an increase in the effect of the ECRL and ECRB tendons and a decrease in the effect of the flexor tendons. Palmer and Werner found that the load in the lunate fossa could be decreased by increasing radial inclination through distal radial wedge osteotomy
The Journal of Hand Surgery / Vol. 24A No. 1 January 1999 131
(presented at the 43rd Annual Meeting of the American Society for Surgery of the Hand, 1988). On the other hand, Porgue et al31 showed increased load in the lunate fossa when radial inclination is decreased. Our work demonstrated that radial angulation leads to an increase in moment arms of the radial side motor tendons. The kinetic changes found in this study are consistent with the load distribution demonstrated by previously reported studies. Short et al30 also found a shift in force transmission from the radius to the ulna when dorsal angulation of the distal radial fragment increases. The present study revealed that the moment arms of the radial side motors decreased as dorsal angulation increased. Our study was performed on a cadaveric model of Frykman I fracture without injury to the ulnar side of the wrist joint. The location of the ECU tendon is confined within a separate, firm compartment attached to the ulna. Therefore, the ECU tendon exhibited the most dramatic changes in moment arm during wrist flexion and extension as dorsal angulation was simulated. The relationship between the ECU tendon and the axes of wrist radial and ulnar deviation, however, was well protected. No significant changes in moment arms of the ECU tendon resulted from radial angulation or radial and ulnar deviation. It should be realized that involvement of the ulnar styloid is a frequent component of distal radius fractures. The moment arm of the wrist motor tendons in the case of ulnar styloid injury, especially that of the ECU tendon, may differ from the results demonstrated in this study. This study focused on the impact of 2 basic components of fracture deformities, the results of which will enable us to move toward understanding the biomechanics of the wrist in varying clinical situations. Clinically, the deformities may involve multidirectional angulation, shortening of the radius, and injury of the ulnar styloid, the triangular fibrocartilage complex, and the distal radioulnar joint.6,44 Further studies of this subject should be performed in a model that combines 2 or more of the factors noted above.
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thousand cases from the New York State Workman’s Compensation Board. J Bone Joint Surg 1953;35A:643– 658. Milch H. Treatment of disabilities following fractures of the lower end of the radius. Clin Orthop 1963;29:157–163. Frykman G. Fracture of the distal radius including sequelae. Acta Orthop Scand 1967;108:1–155 (suppl). Cooney WP, Dobyns JH, Linscheid RL. Complications of Colles’ fractures. J Bone Joint Surg 1980;62A:613– 619. Cassebaum WH. Colles’ fracture: a study of end results. JAMA 1950;143:963–965. Dowling JJ, Sawyer B. Comminuted Colles’ fractures: evaluation of method of treatment. J Bone Joint Surg 1961;43A:657– 668. Smaill BG. Long-term follow-up of Colles’ fractures. J Bone Joint Surg 1965;47B:80 – 85. Gartland JJ, Werley CW. Evaluation of healed Colles’ fractures. J Bone Joint Surg 1951;33A:895–907. Lindstrom A. Fractures of the distal end of the radius. Acta Orthop Scand 1969;41:1–118 (suppl). Sarmiento A, Pratt GW, Berry NC, Sinclair WF. Colles’ fracture, functional bracing in supination. J Bone Joint Surg 1975;57A:311–317. Ver der Linden W, Ericson R. Colles’ fracture. How should its displacement be measured and how should it be immobilized? J Bone Joint Surg 1981;63A:1285–1288. Stewart HD, Innes AR, Burke FD. Factors affecting the outcome of Colles’ fracture: an anatomical and functional study. Injury 1985;16:289 –295. Solgaard S. Function after distal radius fracture. Acta Orthop Scand 1988;59:39 – 42. McQueen M, Caspers J. Colles’ fracture: does the anatomical results affect the final function? J Bone Joint Surg 1988;70B:649 – 651. Bickerstaff DR, Bell MJ. Carpal malalignment in Colles’ fractures. J Hand Surg 1989;14B:155–160. Jenkins NH, Mintowt-Czyz WJ. Mal-union and dysfunction in Colles’ fracture. J Hand Surg 1988;13B:291–293. Villar RN, Marsh D, Rushton N, Greatorex RA. Three years after Colles’ fracture. J Bone Joint Surg 1987;69B: 635– 638. Altissimi M, Antenucci R, Fiacca C, Mancini GB. Longterm results of conservative treatment of fractures of the distal radius. Clin Orthop 1986;206:202–210. Sarmiento A, Latta LL. Fractures of the distal radius. In: Closed functional treatment of fractures. Berlin: SpringerVerlag, 1991:339 –380. Herndon JH. Distal radius fractures: nonsurgical treatment options. In: Instructional course lectures, American Academy of Orthopedic Surgeons. St Louis: CV Mosby, 1993: 67–72. Fernandez DL. Correction of post-traumatic wrist deformity in adults by osteotomy, bone-grafting, and internal fixation. J Bone Joint Surg 1982;64A:1164 –1178. Af Ekenstem FW, Hagert CG, Engkvist O, Tornvall AH, Wilbrand H. Corrective osteotomy of mulunited fractures of the distal end of the radius. Scand J Plast Reconstr Surg 1985;19:175–187. Fernandez DL. Radial osteotomy and Bowers arthroplasty for malunited fractures of the distal end of the radius. J Bone Joint Surg 1988;70A:1538 –1551.
132 Tang et al / Wrist Motors in Distal Radius Fractures 27. Cooney WP, Linscheid RL, Dobyns JH. External pin fixation for unstable Colles’ fracture. J Bone Joint Surg 1979;61A:840 – 845. 28. Melone CP. Open treatment for articular fractures of the distal radius. Clin Orthop 1986;202:103–111. 29. Seitz WH, Froimson AI, Lab R, Shapiro JD. Angmented external fixation of unstable distal radius fractures. J Hand Surg 1991;16A:1010 –1016. 30. Short WH, Palmer AK, Werner FW, Murphy DJ. A biomechanical study of distal radial fractures. J Hand Surg 1987;12A:529 –534. 31. Porgue DJ, Viegas SF, Patterson M, et al. Effects of distal radius malunion on wrist joint mechanics. J Hand Surg 1990;15A:721–727. 32. Brand PW. Clinical mechanics of the hand. St Louis: CV Mosby, 1985:192–309. 33. Amstrong TJ, Chaffin DB. An investigation of the relationship between displacements of the finger and wrist joint and the extrinsic finger flexor tendons. J Biomech 1977;11:119 –128. 34. Kline SC, Moore JR. The transverse carpal ligament. An important component of the digital flexor pulley system. J Bone Joint Surg 1992;74A:1478 –1485. 35. Kiritsis PG, Kline SC. Biomechanical changes after carpal tunnel release: a cadaveric model for comparing open, endoscopic, and step-cut lengthening techniques. J Hand Surg 1995;20A:173–180. 36. Youm Y, McMurtry RY, Flatt ARE, Gillespie TE. Kine-
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matics of the wrist. I. An experimental study of radial-ulnar deviation and flexion-extension. J Bone Joint Surg 1978; 60A:423– 431. Kauer JMG. The mechanism of the carpal joint. Clin Orthop 1986;149:9 –20. Seradge H, Sterbank PT, Seradge E, Owens W. Segmental motion of the proximal carpal row: their global effect on wrist motion. J Hand Surg 1990;15A:236 –239. Tang JB, Ryu J, Han JS, Omokawa S, Kish V, Wearden S. Biomechanical changes of the wrist flexor and extensor tendons following loss of scaphoid integrity. J Orthop Res 1997;15:69 –75. Agee JM. Mechanics of tendons that cross the wrist. In: Brand PW, Hosllister A, eds. Clinical mechanics of the hand. 2nd ed. St Louis: CV Mosby, 1993:371–378. Simon SR, Alaranta H, An KN, et al. Kinesiology. In: Simon SR, ed. Orthopedic basic science. St Louis: CV Mosby, 1992:519 –549. Posner MA, Ambrose L. Mulunited Colles’ fractures: correction with biplanar closing wedge osteotomy. J Hand Surg 1991;16A:1017–1026. Taleisnik J, Watson K. Midcarpal instability caused by malunited fractures of the distal radius. J Hand Surg 1984; 9A:350 –357. Mohanti RC, Kar N. Study of triangular fibrocartilage of the wrist joint in Colles’ fractures. Injury 1980;11:321– 324.
Fractures of the distal radius represent one of the most common skeletal injuries, accounting for approximately 10% of all body injuries.1,2 While many
From the Musculoskeletal Research Center, Department of Orthopedics, Health Sciences Center, West Virginia University, Morgantown, WV. Received for publication October 20, 1995; accepted in revised form June 17, 1998. 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: Jin Bo Tang, MD, Department of Orthopedics, Affiliated Hospital of Nantong Medical College, 20 W Temple Rd, Nantong 226001, Jiangsu, China. Copyright © 1999 by the American Society for Surgery of the Hand 0363-5023/99/24A01-0018$3.00/0
of the fractures are treated properly, these fractures are frequently associated with a high incidence of complications, with malunion ranking among the most common.3–7 In a large retrospective study of 2,132 distal radius fractures, Bacorn and Kurtze4 found that only 3% of patients had no permanent functional loss.4 A study of 565 distal radius fractures by Cooney et al7 revealed 177 (31%) had various bony or soft tissue complications. Although some studies have reported good long-term functional outcomes despite residual deformities,8 –10 many studies have shown that anatomic reduction leads to better functional outcome.1,2,6,7,11–20 In clinical practice, there is a lack of consensus with regard to the amount of acceptable deformity and the indiThe Journal of Hand Surgery 121
122 Tang et al / Wrist Motors in Distal Radius Fractures
cations for corrective osteotomy for malunion after distal radius fracture.21–26 Although treatment of fractures of the distal radius is a common subject of studies,27–29 biomechanical changes following distal radius fractures are poorly understood.30,31 Fracture deformities, function of the hand, joint kinematics, and joint loading patterns after distal radius fractures are closely dependent on kinetics of forearm muscles, especially the wrist motors. To clarify the impact of fracture deformities on the kinetics, we conducted a biomechanical study of the principal wrist motors in a cadaver model of simulated extra-articular distal radius fractures.
Materials and Methods Seven fresh-frozen cadaver upper extremities were harvested from 3 men and 2 women through amputation at the midhumeral level. The ages ranged from 54 to 72 years (average, 62 years). Wrist radiographs were obtained to eliminate bony deformities and obvious degenerative changes of the joints. Specimens with ligamentous laxity or with restriction in wrist motion were not used. Normal palmar tilt and radial inclination were measured in each specimen on anteroposterior and lateral x-ray films. Palmar tilt of these wrists was 11° 6 2°; radial inclination was 23° 6 4°.
Specimen Preparation After specimens were completely thawed at room temperature, the muscle mass of the arms was dissected, retaining the forearm interosseous membrane and elbow capsule. All tendons and ligaments within 6 cm proximal to the wrist were preserved. Dacron braided suture lines were sutured to the proximal ends of the cut tendons. The specimens were mounted to a Plexiglas jig using 2 screws, which transfixed the humerus to the vertical frame of the jig. K-wires were inserted in the thumb and fingers to immobilize the metacarpophalangeal and interphalangeal joints in extension to prevent unwanted motion. The elbow was kept in 90° of flexion and the forearm in neutral rotation by the forearm-supporting frame of the jig. The radius and ulna were fixed to the frame approximately 5 cm proximal to the wrist to prevent forearm rotation and translation.
Experimental Design The suture lines of the tendons were oriented along the natural direction of muscle pull and were passed through guide holes in the vertical frame close to
either the medial or lateral epicondyle of the elbow. To take up the slack in the tendons during wrist motion, each of the tendons was loaded with a weight of 250 g. Wires of the 5 prime wrist movers, extensor carpi radialis longus (ECRL), extensor carpi radialis brevis (ECRB), extensor carpi ulnaris (ECU), flexor carpi radialis (FCR), and flexor carpi ulnaris (FCU), were routed around the pulleys of a rotational potentiometer (model 3543s; Bourns Inc, Riverside, CA), which recorded the amount of voltage produced by the tendon excursions during wrist movement. Voltage was automatically converted to values of the tendon excursions by a software program. A dual-axis electrogoniometer (model 110; Penny and Giles Co, Santa Monica, CA) was mounted on the ulna and third metacarpal shaft to record degrees of wrist global motion. Tendon excursion values were calibrated to an accuracy of 0.1 mm and wrist angulation values to 0.1°. Tendon excursion and joint angulation were collected simultaneously by a data acquisition system and stored on a computer hard drive (PC 386; Zenith Data System, Milwaukee, IL).
Test Procedures Distal radius fractures were simulated by an osteotomy of the distal radius. Fracture angulation was maintained by external fixators. Using a Penning dynamic wrist fixator (EIB Medical System, Parsippany, NJ), the distal end of the radius was joined to the proximal portion of the radius. A 1.5-cm distal radius osteotomy was performed with an air osteotome starting 2 cm proximal to the distal radial articular surface. The distal interosseous membrane was incised 2 cm, without interference of the distal radioulnar joint. A Plexiglas external fixator was used to obtain the desired degree of angulation of the distal segment of the fracture. The Plexiglas fixator consisted of distal and proximal parts that allowed axial rotation. By rotating the distal part of the fixator, different degrees of dorsal or radial angulation were simulated when the fixator was applied to the radius laterally or dorsally (Fig. 1). Eight positions of distal radial angulation were studied: 10°, 20°, 30°, and 40° of dorsal angulation and 5°, 10°, 15°, and 20° of radial angulation. The fracture was angulated dorsally and laterally by setting the fracture in angulated position relative to the shaft of the radius (Fig. 2). X-ray films were taken to confirm that positions angulated by the external fixators accurately represented the desired extent of deformities. The Penning dynamic wrist fixator and the Plexiglas fixator fixed
The Journal of Hand Surgery / Vol. 24A No. 1 January 1999 123
Figure 1. The extra-articular fracture of the distal radius was simulated by osteotomy in the distal radius and angulation was maintained by an external fixation device. The desired grade of angulation in the sagittal plane and in the frontal plane was obtained by applying customized hinge Plexiglas fixators laterally or dorsally and setting the hinge fixators in appropriate angulation. (Top) Dorsal angulation of the fractures. (Bottom) Radial angulation of the fractures.
the fracture through planes perpendicular to each other. The dynamic wrist fixator maintained normal alignment of the fracture in planes perpendicular to simulated fracture deformities. Maintenance of the normal alignment in these planes was confirmed by x-ray films. The external fixation devices provided
satisfactory stability to the simulated fracture during testing. No dorsal or lateral displacement was added to the fractures during angulation of the fractures. In intact wrists and in each position of angulation, the wrists were moved passively along horizontal and vertical metal bars through flexion to extension
124 Tang et al / Wrist Motors in Distal Radius Fractures
in the sagittal plane and radial deviation to ulnar deviation in the frontal plane. As the joint moved, the excursion of each of the 5 motor tendons was recorded simultaneously and continuously with joint angulation for 10 seconds at a sampling rate of 10 Hz. Tendon excursions were measured in intact wrists and in each of the angulation positions of the simulated distal radius fractures.
Data Analysis
Figure 2. Dorsal (top) and radial (bottom) angulation of distal radius fractures. The dorsal and radial angulations were measured as angulation of the distal part of the fracture relative to the shaft of the radius.
In this study, movement of the third metacarpal bone represented global motion of the wrist. After dorsal angulation of the distal radius, wrist extension increased and wrist flexion reduced. After radial angulation, radial deviation increased and ulnar deviation decreased. Flexion of over 45° and ulnar deviation of over 25°, however, were observed in all degrees of angulation in each specimen. Therefore, the tendon excursions used for analysis were those from 45° of flexion and 45° of extension and from 20° of radial deviation to 25° of ulnar deviation. Moment arm of the wrist movers was calculated according to the relationship that excursion (DE) along an arc equals the radius of the arc (R) times the subtended angle DU in the radian (1 radian 5 57°): R 5 DE/DU.32–35 Figure 3 illustrates how excursion is used to calculate the moment arms of the wrist motor tendons. The excursion of a tendon through 57° of joint motion is equal to its distance to the center of rotation (the moment arm). Since the wrist
Figure 3. The method of obtaining moment arm of a joint motor from its excursion and joint angulation. When the joint moves through an arc of 57° (1 radian), the distance of the motor tendon to the center of rotation is equal to its excursion through the joint motion of 57°. The distance from the tendon to the center of rotation is known as the moment arm. Therefore, moment arm of a tendon can be calculated by dividing excursion of the tendon by subtended degrees of joint motion in the radian.
The Journal of Hand Surgery / Vol. 24A No. 1 January 1999 125
Table 1. Excursion of the Prime Wrist Motors Through 45° of Wrist Flexion and 45° of Wrist Extension
Wrist Motors ECRL ECRB ECU FCR FCU
Dorsal Angulation
Radial Angulation
Normal Wrists
10°
20°
30°
40°
5°
10°
15°
20°
14.1 6 2.2 19.0 6 3.8 6.9 6 2.8 21.8 6 2.7 24.6 6 2.6
15 6 2 20 6 2 563 21 6 3 25 6 3
15 6 2 20 6 2 262 20 6 3 23 6 4
16 6 2 20 6 2 21 6 1* 18 6 3 23 6 4
16 6 3 20 6 2 21 6 1* 17 6 3 21 6 2
13 6 3 18 6 4 763 23 6 3 24 6 3
13 6 2 18 6 3 763 22 6 3 22 6 3
12 6 1 17 6 3 763 23 6 3 23 6 3
12 6 2 17 6 3 863 24 6 3 23 6 2
Data are expressed as mean values 6 SD. * The ECU tendon moved in the direction of wrist flexors when dorsal angulation was 30° or 40°.
resulting from dorsal or radial angulation were separately analyzed as described below.
range of motion is not a perfect sphere, moment arms determined by this study represent an approximation of calculation of average moment arms during tested ranges of wrist motion. Using custom-designed computer software, the values of moment arms of the prime wrist motors were derived from tendon excursions and wrist angulation. Because of large interspecimen variation of normal moment arms among tested intact wrists, moment arms of the tendons in the wrists with fracture angulation were adjusted by dividing their moment arm data by those of intact wrists. The percentage changes were used for analysis. This adjusted for differences in the sizes and configurations of the wrists between specimens. One-way ANOVA was used to determine statistical differences between wrists with displacements of the fractures and normal wrists. A difference of 5% was considered significant.
Effect of Dorsal Angulation Flexion and Extension Movement. Through the wrist flexion and extension arc of 90°, dorsal angulation of 10° to 40° resulted in an increase in excursions of the ECRL tendon by 1.3 to 2.1 mm (10% to 15%). The moment arm of the ECRL tendon increased by 0.8 to 1.3 mm. Excursions of the ECRB tendon increased only by 0.5 to 0.9 mm. The increase in moment arm was 0.3 to 0.6 mm. Excursions of the ECU tendon showed a dramatic change in dorsal angulation, decreasing by 1.9 to 7.3 mm (28% to 111%), while moment arms decreased by 1.1 to 5.2 mm. The ECU tendon glided in the direction of flexors when dorsal angulation exceeded 30°. A moderate decrease in excursions was observed in the FCR and FCU tendons: by 0.7 to 5.3 mm (3% to 24%) in the FCR tendon and by 1.3 to 3.7 mm (7% to 15%) in the FCU tendon. The moment arm of the FCR and FCU tendons decreased from 0.5 to 3.4 mm and from 1.0 to 2.4 mm, respectively. Statistical analysis revealed a significant difference in excursions and moment arms of the ECRL
Results Tables 1 to 4 list tendon excursions and moment arms of the prime wrist motors during wrist flexion and excursion and radial and ulnar deviation. Changes in tendon excursions and moment arms
Table 2. Excursion of the Prime Wrist Motors Through 20° of Wrist Radial Deviation and 25° of Wrist Ulnar Deviation Dorsal Angulation Wrist Motors ECRL ECRB ECU FCR FCU
Radial Angulation
Normal Wrists
10°
20°
30°
40°
5°
10°
15°
20°
15 6 3 962 15 6 4 662 11 6 3
14 6 2 962 16 6 3 662 11 6 3
14 6 2 863 17 6 3 762 963
13 6 2 762 17 6 3 662 862
12 6 1 662 16 6 3 561 862
17 6 3 10 6 2 15 6 3 662 10 6 2
17 6 2 10 6 2 16 6 3 762 962
17 6 2 10 6 1 16 6 3 863 862
17 6 2 11 6 2 15 6 3 862 862
Data are presented as mean values 6 SD.
126 Tang et al / Wrist Motors in Distal Radius Fractures
Table 3. Moment Arms of the Prime Wrist Motors in Wrist Flexion and Extension Motion Dorsal Angulation Wrist Motors ECRL ECRB ECU FCR FCU
Radial Angulation
Normal Wrists
10°
20°
30°
40°
5°
10°
15°
20°
961 12 6 2 462 14 6 2 16 6 2
10 6 1 13 6 2 13 6 2 13 6 2 16 6 2
10 6 1 12 6 1 161 13 6 2 15 6 2
10 6 1 12 6 1 061 12 6 2 14 6 2
10 6 2 12 6 1 21 6 1 11 6 2 13 6 2
862 11 6 3 562 14 6 2 15 6 2
861 11 6 2 462 14 6 2 15 6 2
861 11 6 2 562 14 6 2 15 6 2
861 11 6 2 562 15 6 2 14 6 2
Data are presented as mean values 6 SD.
mm (12% to 26%) when dorsal angulation was over 20°. Moment arms of the FCU tendon decreased by 1.9 to 3.8 mm. Excursions and moment arms of the ECRL tendon decreased significantly when dorsal angulation was over 20° (p , .01 and p , .001, respectively). Significant differences were also found in the ECRB tendon between fractures with 30° and 40° of dorsal angulation and intact wrists (p , .01 and p , .01, respectively). Excursions and moment arms of the FCU tendon were reduced significantly when dorsal angulation was greater than 20° (p , .05 and p , .01, respectively). Changes in excursions and moment arms of the ECU and FCR tendons were not statistically significant. Dorsal angulation of 10° had no significant effect on excursions and moment arms of the wrist motors (Fig. 5).
tendon between intact wrists and wrists with 10° or 40° of dorsal angulation (p , .05, p , .01, and p , .001, respectively). Excursions and moment arms of the ECRB tendon increased significantly when dorsal angulation of the fracture was 10° (p , .05). Tendon excursions and moment arms of the ECU and FCR tendons showed a significant decrease at each position of angulation (p , .01 and p , .001). Excursions and moment arms of the FCU tendon decreased significantly when angulation was 30° or 40° (p , .05 and p , .001, respectively), but no significant difference was seen between normal wrists and wrists with 10° or 20° of dorsal angulation (Fig. 4). Radial and Ulnar Deviation. During radial and ulnar deviation of 45°, dorsal angulation of distal radius fractures resulted in a decrease of 0.2 to 2.6 mm (2% to 18%) in excursions of the ECRL tendon and of 0.2 to 2.7 mm (2% to 31%) in those of the ECRB tendon. The decrease of moment arm was 0.4 to 4.2 mm in the ECRL tendon and 0.2 to 3.4 mm in the ECRB tendon. Excursions of the ECU tendon increased by 1.0 to 2.3 mm (7% to 16%). Moment arms of the ECU tendon increased by 1.3 to 2.9 mm. Changes in tendon excursions of the FCR tendon varied largely, from an average increase of 0.7 mm (11%) to a decrease of 1.2 mm (20%). The FCU tendon showed a decrease in excursions by 1.5 to 2.8
Effect of Radial Angulation Flexion and Extension Movement. A decrease in the excursions of the ECRL tendon by 0.8 to 2.4 mm (6% to 17%) was seen during 90° of flexion and extension movement. The moment arm of the ECRL tendon decreased by 0.6 to 1.5 mm. Excursions of the ECRB tendon decreased by 1.1 to 2.2 mm (6% to 11%), with a corresponding decrease in moment
Table 4. Moment Arms of the Prime Wrist Movers in Wrist Radial and Ulnar Deviation Dorsal Angulation Wrist Motors ECRL ECRB ECU FCR FCU
Radial Angulation
Normal Wrists
10°
20°
30°
40°
5°
10°
15°
20°
19 6 2 11 6 2 19 6 5 863 14 6 4
19 6 3 11 6 2 20 6 4 763 14 6 4
18 6 3 10 6 3 22 6 4 862 12 6 4
16 6 2 962 21 6 3 763 11 6 3
15 6 2 862 21 6 4 662 10 6 3
21 6 3 13 6 3 19 6 4 863 12 6 3
21 6 3 13 6 2 20 6 4 963 12 6 3
21 6 3 13 6 2 20 6 4 10 6 3 12 6 3
22 6 3 14 6 2 19 6 4 10 6 3 11 6 3
Data are presented as mean values 6 SD.
The Journal of Hand Surgery / Vol. 24A No. 1 January 1999 127
Figure 4. Comparison of normalized values of moment arms of the primary wrist motor tendons during wrist flexion and extension in the fractures with dorsal angulation. Asterisks indicate columns with statistically significant differences compared with the values in normal wrists. DA, dorsal angulation (°).
arms of 0.7 to 1.4 mm. Changes in excursions were considerably less in the ECU and FCR tendons. The FCU tendon showed a decrease of 0.8 to 2.1 mm in excursion and a decrease of 0.6 to 1.4 mm in moment arms. Significant decreases in excursions and moment arms were seen in the ECRL tendon when radial angulation was over 10° (p , .05 and p , .01, respectively). Decreases in excursions and moment arms of the ECRB tendon were significant when 15° and 20° of radial angulation were produced (p , .05 for each comparison). The excursions and moment arms of the FCU tendon decreased significantly when angulation exceeded 10° (p , .01 and p , .001, respectively). Changes in excursions and moment arms of the ECU and FCR tendons were not statistically significant (Fig. 6). Radial and Ulnar Deviation. Radial angulation increased excursions of the ECRL tendon by 1.9 to 2.3 mm (13% to 16%) and those of the ECRB tendon by 1.4 to 2.3 mm (16% to 26%). The moment arm increased by 1.6 to 2.2 mm in the ECRL tendon and by 1.8 to 3.0 mm in the ECRB tendon. Changes in excursions of the ECU tendon were minimal, in-
creasing only by 0.4 to 1.2 mm. The FCR tendon showed an increase of 0.2 to 1.6 mm (3% to 12%) in tendon excursions and of 0.3 to 2.1 mm in moment arms. The FCU tendon showed a decrease of excursions by 1.1 to 1.9 mm (10% to 18%). The moment arms decreased by 1.4 to 2.2 mm. Excursions and moment arms of the ECRL tendon increased significantly with the varying degrees of radial angulation (p , .01 and p , .001, respectively). Excursions and moment arms of the ECRB tendon increased significantly when radial angulation was 10°, 15°, or 20° (p , .01 for each comparison). The ECU tendon was not significantly affected by radial angulation. Excursions and moment arms of the FCR and FCU tendons were influenced when radial angulation exceeded 10° (p , .05 and p , .01, respectively). Radial angulation of 5° did not affect excursions and moment arms of the ECRB, ECU, FCR, or FCU tendons (Fig. 7).
Discussion This study demonstrated that excursions and average moment arms of principal wrist motor tendons
128 Tang et al / Wrist Motors in Distal Radius Fractures
Figure 5. Moment arms of the wrist motor tendons during radial and ulnar deviation in the fractures with dorsal angulation. Asterisks indicate columns with significant differences compared with the moment arms of normal wrists. DA, dorsal angulation.
Figure 6. Values of moment arms of the wrist motor tendons during flexion and extension in simulated radial angulation. Columns with significant changes are indicted by asterisks. RA, radial angulation.
The Journal of Hand Surgery / Vol. 24A No. 1 January 1999 129
Figure 7. Moment arms of the wrist motor tendons during radial and ulnar deviation in simulated radial angulation. Columns with significant changes are indicated by asterisks. RA, radial angulation.
are significantly affected by angulation of distal radius fractures. Changes in excursions and moment arms increased as the simulated deformities became more severe. The moment arm is the effective adjustment for mechanical advantage of the joint mover.32 Since the prime wrist motors are the major dynamic source of wrist motion, the results of our study may imply that deformities of the distal radius fracture can influence hand function. The wrist motor tendons presented various patterns of moment arm changes in conjunction with fracture deformities. The most clinically relevant changes demonstrated by this study are summarized as follows: 1. Dorsal angulation had the most influence on the moment arms during wrist flexion and extension. Only when dorsal angulation together with wrist flexion and extension were simulated did all 5 prime wrist motors show significant changes in moment arms. 2. While 4 of 5 motors had changes in moment arms in cases of moderate radial angulation, slight radial angulation had almost no effect on moment arms of the wrist motors. 3. The major tendency of biomechanical alterations was a decrease in moment arms of the wrist motor tendons. All the flexors during flexion and exten-
sion and all the ulnar deviators during radial and ulnar deviation showed decreased moment arms. 4. Increased moment arms occurred in the radial wrist extensors and radial deviators when dorsal and radial angulation were simulated. 5. Fracture deformities had a greater influence on moment arms along the plane of wrist motion than on those in the perpendicular direction of wrist motion.
The axis of rotation in the wrist was previously determined to lie within the head of the capitate.36 Kauer,37 however, demonstrated that there are 2 shifting instant centers of rotation. It was noted that the head of the capitate is not spherical, but rather 3 separate facets for scaphoid, lunate, and hamate articulations.37 During wrist radial and ulnar deviation, there are also subtle changes in relative positions between proximal and distal carpal rows and positions of the bones within the proximal row.38 Therefore, wrist motion restricted to a plane is not a perfect sphere and does not take place around a fixed center of rotation. Instantaneous moment arms of these tendons may differ at each wrist joint angulation. Consequently, our method of determining moment arms of the wrist motors during wrist motion in this study
130 Tang et al / Wrist Motors in Distal Radius Fractures
represents an approximate calculation. The moment arms determined in this study represent average moment arms of these motor tendons during the tested ranges of wrist motion. By alternative methods of calculation, it will be possible to determine changes of moment arms of wrist motors at specific wrist joint motion.39 Dorsal and radial angulations are the dominant patterns in displacement of fractures of the distal radius and important parameters with which to judge reduction. This study revealed that dorsal angulation of only 10° significantly altered mechanics of the wrist motors. Further increases in dorsal angulation proportionately increased the alterations. Dorsal angulation decrease the moment arms of the flexors (the FCR and FCU tendons) and even reversed the effect of the ECU tendon. Increasing moment arms of the ECRL and ECRB tendons are expected to produce increased torque to dorsally displace the fractures. These findings suggest that normal anatomic alignment of the fractures is important to the stability of fracture reduction and to maintaining balanced torque of wrist motor tendons for wrist motion. For malunited fractures of the distal radius, Fernandez24,26 believes that a symptomatic extraarticular malunion causing dorsal angulation over 25° to 30° is an indication for corrective osteotomy. The results of our study revealed that dorsal angulation over 30° induced changes in moment arms of all the wrist motor tendons in a considerably large amplitude. The changes were mostly as large as 25% to 110% of the moment arms in the intact wrists. Radial angulation of 5° was shown to have no significant influence on the majority of wrist motor tendons. Radial angulation of 10° or more influenced the wrist motor tendons. These results support the clinical impression that allowable changes in radial inclination should not exceed 10°. A number of clinical studies have reported that deformities of malunited fractures positively correlate with functional deficits, such as decreases in power or inability to perform activities of daily living.11–20 In a review of 154 fractures of the distal radius, Solgaard16 reported that the number of patients with excellent and good results decreased significantly as the amount of dorsal angulation increased. McQueen and Caspers17 found that grip strength, performance of activities of daily living, and motion were significantly worse in wrists with dorsal angulation over 12° than in wrists with 10° or less of dorsal angulation. Jenkins and MintowtCzyz19 showed a significant correlation between a
decrease in radial inclination of the radial articular surface and a decrease in grip strength. Porter and Stockley,1 Frykman,6 and Cooney et al7 reported that a decrease in grip power has a close relationship with severity of fracture deformity. It was speculated that a mechanical base for clinical findings is the disturbance of the mechanical advantages of wrist motor tendons by fracture deformities.40,41 This study demonstrated that either dorsal or radial angulation decreased the moment arm of the flexor tendons. The decrease of moment arms in the wrist flexors places the muscles at a mechanical disadvantage. Activities of daily living depend on muscle power as well as coordinated joint motion.40,41 Angulation of the fractures decreased moment arms of the tendons for motion in the perpendicular direction. On the other hand, angulation of the fractures increased tendon moment arms for the motion along the direction of the deformity. We speculate that these changes may distort normal patterns of wrist motion and contribute to the inability of wrists to perform activities of daily living. The potential for dorsal subluxation of the carpus to accompany fractures of the distal radius has been recognized for many years.42 Midcarpal instability caused by malunited fractures of the distal radius was also noted in young, active patients who tend to have ligamentous laxity.43 One of the common features presenting in these accompanying carpal malalignments is that main parts of the carpus translate dorsally with respect to the long axis of the radius. The mechanical unbalance of wrist motor tendons noted in this study may constitute an important pathomechanical factor in the formation of associated carpal deformities. Dorsal angulation of the fractures generates increased torque in the extensor aspect and decreased torque in the flexor aspect. This combination of changes around the wrist favors translation or twisting of the inherently unstable wrist joint, resulting in carpal instability. In a biomechanical study of wrist load patterns, Short et al30 demonstrated that the center of pressure in the radioscaphoid and ulnocarpal joint migrates dorsally as the distal radius is dorsally angulated to varying degrees. Porgue et al31 demonstrated a dorsal shift in the scaphoid and lunate high-pressure areas by angulating the distal fragment of the fractures. The results of our study revealed an increase in the effect of the ECRL and ECRB tendons and a decrease in the effect of the flexor tendons. Palmer and Werner found that the load in the lunate fossa could be decreased by increasing radial inclination through distal radial wedge osteotomy
The Journal of Hand Surgery / Vol. 24A No. 1 January 1999 131
(presented at the 43rd Annual Meeting of the American Society for Surgery of the Hand, 1988). On the other hand, Porgue et al31 showed increased load in the lunate fossa when radial inclination is decreased. Our work demonstrated that radial angulation leads to an increase in moment arms of the radial side motor tendons. The kinetic changes found in this study are consistent with the load distribution demonstrated by previously reported studies. Short et al30 also found a shift in force transmission from the radius to the ulna when dorsal angulation of the distal radial fragment increases. The present study revealed that the moment arms of the radial side motors decreased as dorsal angulation increased. Our study was performed on a cadaveric model of Frykman I fracture without injury to the ulnar side of the wrist joint. The location of the ECU tendon is confined within a separate, firm compartment attached to the ulna. Therefore, the ECU tendon exhibited the most dramatic changes in moment arm during wrist flexion and extension as dorsal angulation was simulated. The relationship between the ECU tendon and the axes of wrist radial and ulnar deviation, however, was well protected. No significant changes in moment arms of the ECU tendon resulted from radial angulation or radial and ulnar deviation. It should be realized that involvement of the ulnar styloid is a frequent component of distal radius fractures. The moment arm of the wrist motor tendons in the case of ulnar styloid injury, especially that of the ECU tendon, may differ from the results demonstrated in this study. This study focused on the impact of 2 basic components of fracture deformities, the results of which will enable us to move toward understanding the biomechanics of the wrist in varying clinical situations. Clinically, the deformities may involve multidirectional angulation, shortening of the radius, and injury of the ulnar styloid, the triangular fibrocartilage complex, and the distal radioulnar joint.6,44 Further studies of this subject should be performed in a model that combines 2 or more of the factors noted above.
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