Wrist kinetics after scapholunate dissociation: the effect of scapholunate interosseous ligament injury and persistent scapholunate gaps

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1 ELSEVIER Journal of Orthopaedic Research 2 (22) Journal of Orthopaedic Research Wrist kinetics after scapholunate dissociation: the effect of scapholunate interosseous ligament injury and persistent scapholunate gaps Jin Bo Tang *, Jaiyoung Ryu, Sholiei Omokawa, Stanley Wearden Abstract The purpose of this study was to investigate the effects of cutting of the scapholunate interosseous ligament (SLIL) and persistent widening of the scapholunate (SL) joint on changes in moment arms of the principal wrist motor tendons. In seven fresh frozen cadaveric upper extremities, excursions of the extensor carpi radialis longus (ECRL) and brevis (ECRB), extensor carpi ulnaris (ECU), flexor carpi radialis (FCR), and flexor carpi ulnaris (FCU) were recorded simultaneously with wrist joint angulation during wrist flexion-extension and radioulnar deviation. Tendon excursions were measured in intact wrists, then in the wrists with complete SLIL sectioning and in those with moderate or severe persistent SL joint widening. The data were converted to moment arms of the tendons. The results showed that moment arms of the ECRL and ECRB tendons after SLIL sectioning were, respectively, 1 1 f 6'!, and 15 C! 3% of those in the intact wrist. In the wrists with moderate or severe SL joint widening, moment arms of the flexors significantly increased (P <.1 and P <.1, respectively). During radioulnar deviation, moment arms of the ECRL, ECRB, ECU, and FCU tendons decreased after SLIL sectioning and the SL joint widening. However, moment arms of the FCR tendon significantly increased 123 f 23'%1 after the SLIL section, 133 i 38% after the moderate SL joint widening, and 138 i 24%) after the severe SL joint widening compared with those of the intact wrists. This study demonstrated that integrity of the SLIL and appropriate SL joint space are important for mechanics of wrist motor tendons. Loss of integrity of the SLIL and persistent SL joint widening increase mechanical effects of the radial side wrist motor tendons, which may contribute to the pathomechanics of scaphoid malrotation, scapholunate advanced collapse, and early osteoarthritis in the radioscaphoid joint interface seen in SL dissociation. The results also suggest that reduction of the displaced SL joint is imperative to the recovery of wrist kinetics after SL 11 Orthopaedic Research Society. Published by Elsevier Science Ltd. All rights reserved. Introduction Traumatic scapholunate dissociation represents a spectrum of instability caused by injury to the supporting carpal ligaments [ 1 1,191. This clinical entity presents pictures of scapholunate (SL) joint diastasis or rotatory subluxation of the scaphoid with sequential radiocarpal and intercarpal degenerative arthritis, and may pathologically range from disruption of a single scapholunate interosseous ligament (SLIL) to injuries to multiple ligaments [2,9, A variety of ligament * Corresponding author. Present address. Lahey Clinic Medical Center, Tissue Engineering Laboratory, 31 Mall Road, Burlington, MA 185. Tel.: +I ; fax: E-niuiI urldrrss: jbtang(ulrics.bwh.harvard.edu (J.B. Tang). injuries are associated with SL dissociation. The involved ligaments include the SLIL, the radioscapholuiiate ligament (RSLL), the radioscapholunate ligament (RSCL), the scaphotrapezial ligament complex (STLC), the dorsal radiocarpal ligament (DRCL), and the dorsal intercarpal ligament (DICL) [2,9,11-14,19,29]. Although severe forms of SL dissociation can not be produced without extensive loss of the ligament restraints, studies have demonstrated that the SLIL plays a major role in proximal scaphoid stability [15,17]. In a cadaver model, the SLIL sectioning produced kinematic changes of the wrist similar to those following the combined sectioning of the SLIL, RSLL, and RSCL [15,17]. Disruption of the SLIL results in separation of the motion between the scaphoid and lunate in the acute phase and the development of persistent widening of the SL joint as a late clinical sequence [13,19,3] /2/$ - see front matter 22 Orthopaedic Research Society. Published by Elsevier Science Ltd. All rights reserved PII: S7~6-~66(1)118-8

2 216 J. B. Tung r! ul. I Journcrl oj Orrhopurdic Rrseurch 7 ( Knowledge of wrist biomechanics after various extents of SL dissociation is important for understanding the clinical sequelae of the disease and for determining treatment options. In the past two decades, interest has been seen in ligament anatomy [5,6,13,2,27,3], carpal kinematic changes [5,1,15,17], load transfer characteristics [%], and contact pressure of the radiocarpal joint [7] in this disease. However, the moment arms of the wrist movers after SL dissociation have not been studied. We know of no information about how SL dissociation affects moment arms of wrist motor muscles and, consequently, how these changes further contribute to the mechanism responsible for the formation of chronic status of the disease. The purpose of this study was to investigate changes in moment arms of the principal wrist motor tendons following loss of integrity of all the components of the SLIL (including palmar, proximal, and dorsal subregions) and the persistent widening of the SL joint space in a cadaveric model. We hypothesized that sectioning of the SLIL and widening of the SL joint space would significantly change moment arms of the principal wrist motor tendons. Materials and methods Seven fresh frozen cadaver upper extremities (three right and four left) were harvested from four men and three women. The ages ranged from 5 to 76 years, with an average of 64 years. Approval of the Institutional Review Board was obtained before the use of human sub.jects. These persons died of diseases unrelated to the hands. Radiographs excluded specimens with skeletal abnormalities. There was no ligamentous laxity of the wrist or restriction in wrist motion. These specimens were thawed overnight at room temperature before the study. The soft tissues were stripped from the upper arms to 4 cm above the wrist, preserving the forearm interosseous membrane, elbow capsule, and ligaments. All tendons and ligaments were preserved within 6. cm proximal to the wrist. Care was taken to protect the extensor and flexor retinacula of the wrist. Dacron braided Ethilon sutures (Ethicon, Sonierville. NJ. USA) were sutured to the proximal ends of the cut tendons. The specimens were mounted with two screws to a custom jig. The screws firmly transfixed the humerus to the vertical frame of the jig. Motions of metacarpophalangeal (MP) and interphalangeal (IP) joints of the thumbs and fingers were prevented by insertion of Kirschner wires into the phalanx and metacarpal bones. The elbow was kept in 9" of flexion, and the forearm was maintained in neutral rotation by the forearm supporting frame of the jig. The radius and ulna of the forearm were respectively fixed to the frame to prevent forearm rotation. E.xperimenta1 setup 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 epicondyles of the elbow. Each of the tendons was loaded by a weight of 25 g to take up the slack during wrist motion. Five prime wrist motor tendons were tested, the extensor carpi radialis longus (ECRL) and brevis (ECRB), extensor carpi ulnaris (ECU), flexor carpi radialis (FCR), and flexor carpi ulnaris (FCU). The Ethicon suture connected to each tendon was routed around the pulley of a rotational potentiometer (Model 3543s, Bourns, Riverside, CA, USA), which recorded the amplitude of voltage produced by the tendon excursions during wrist movement (Fig. 1). The voltage was automatically transferred to values of the tendon excursions by computer software. A dual-axis electrogoniometer (Model 11, Penny and Giles, Santa Monica, CA, USA) mounted onto the distal radius and third metacarpal shaft monitored global motion of the wrist. Tendon excursion and joint angulation were simultaneously collected by a data acquisition system and stored in a computer (PC 586, Zenith Data System, Milwaukee, IL, USA). The values of joint angles were calibrated to an accuracy of.1", and values of tendon excursions were rounded to.1 mm. System accuracy was tested for measurement of joint angulation and tendon excursion. The goniometer was moved on a gauge biock through 9" 1 times. The recorded angulation was 9.4 f.5". The accuracy of the excursion measurement was tested by measuring1 mm length in a micrometer caliper 1 times. The measured excursion was 1.2*.4 mrn. Computer monitor I Metal bar Angular display k Mounting jig Fig. 1. The experimental setup for measurement and recording of excursions of wrist motor tendons

3 JB. Tang et ul. I Journul of Orthopedic Reseurch 2 (22) The wrists were passively moved along horizontal and vertical metal bars, respectively, through full range of flexion to extension in the sagittal plane and full radial to ulnar deviation in the frontal plane. As the joint moved, excursion of each motor tendon was recorded with joint angulation for 1-s periods, with a sampling rate of 1 Hz. The data were obtained in each of the following experiment stages: Intact wrist. The excursions of the wrist motor tendons of specimens in normal wrists were recorded first, serving as the control for the analysis of changes due to the SLIL section or SL joint widening. SLIL sectioning. Through a dorsal approach distal to the third extensor compartment, the SLIL was sharply transected with a No. 1 surgical blade. Care was taken to section the entire SLIL including its dorsal, proximal, and pahar subregions completely and not to injure the RSLL, RSCL, DRCL, and DICL. Moderate SL,joint widening. After repetitive stretching of the SL joint space with a silicone wedge spacer 15-2 times, a smooth, round silicone spacer 2.5 mm thick was inserted in the SL joint space. Severe SL joint w3idening. A silicone spacer 4. mm thick was placed to the joint space after repetitive stretching of the gap to 4. mm with a wedge spacer. Data unalysis Although the range of global motion of the wrist was restricted to some extent in specimens with widening of the SL joint, the motion exceeded 6" of flexion, 6" of extension, 2" of radial deviation, and 3" of ulnar deviation in all specimens. Therefore, tendon excursions used for analysis were those from 6" of flexion to 6" of extension, and from 2" of radial deviation to 3" of ulnar deviation. The moment arms of these tendons were calculated according to the relationship that excursion along an arc equals the radius of a circle times the subtended angle in the radian [3,4,21, The values of moment arms of the principal wrist movers were derived from the tendon excursion data using customized software. The individual specimens presented large variation in normal tendon excursions and moment arms. Therefore, tendon excursions and moment arms in the wrist with SLIL sectioning or SL joint widening were adjusted for the normal condition. Percentage changes of the moment arms in pathological conditions were used in the analysis of the significance of the changes. The changes in tendon excursions and moment arms were analyzed by one-way analysis of variance (ANOVA). The statistical significance level was set at P <.5 for all tests. Turkey-Kramer honest significant difference tests were used to determine the significance in percentage changes of tendon excursions and moment arms after SLIL sectioning or SL joint widening when the F test of ANOVA was significant. No further post hoc tests were run when the F test was not significant. Results Effect of the SLIL section As shown in Table 1, excursion of the ECRL tendon over 12" of flexion+xtension increased by an average of 2. mm. The moment arms increased by.9 mm (Table 2). The moment arm of the ECRL tendon after the SLIL section was 11 f 6% (mean i SD) of that in the intact wrist. Excursion of the ECRB tendon increased by 1.4 mm, and the moment arm increased by.7 mm after the SLIL section (Tables 1 and 2). The moment arm of the ECRB tendon after the SLIL section was 15 rt 3% of that in the intact wrist. The increase in the moment arms of the ECRL and ECRB tendons were statistically significant after SLIL sectioning compared with those in the intact wrist (P <.1) (Fig. 2). Excursion of the ECU tendon decreased by an average of 1.4 mm, and the corresponding decrease in the moment arm was.6 mm, with a large deviation in the values. The FCR and FCU tendons had little changes in their excursions and moment arms after the SLIL section. The changes in the ECU, FCR, and FCU tendons were not statistically significant (Fig. 2). During radioulnar deviation, the moment arms of the ECRL and ECRB tendons after the SLIL section were not significantly different from those in the intact wrist (Fig. 3). The moment arms of the ECU and FCU tendons after the SLIL section were 93 * 8% and 84 * 12(%1 of that of the intact wrist, respectively. Changes in the ECU and FCU tendons were statistically significant (P <.5 and P <.1, respectively). The moment arms of the FCR tendon decreased, but the deviation was Table 1 Tendon excursions during wrist flexion-extension of 12" and during wrist radioulnar deviation of 5" (Mean-* SD, mm) Wrist motors Experimental stages Intact wrist SLIL cutting Moderate SL gap Severe SL gap Flexion-extension Extensor carpi radialis longus Extensor carpi radialis brevis Extensor carpi ulnaris Flexor carpi radialis Flexor carpi ulnaris Radiodnar deoiuticm Extensor carpi radialis longus Extensor carpi radialis brevis Extensor carpi ulnaris Flexor carpi radialis Flexor carpi ulnaris 19.9f i i f f f f f f f f ?c f i f I f f ?c f i ?c f % f i i f i f i f f i f 2.7

4 '18 J. B. Tung et al. I Journul qf' Orthopuedic Research 2 (22) 21s-221 Table 2 Moment arms of the wrist movers during wrist flexionxxtension and wrist radioulnar deviation in each experimental stage (Mean 1 SD, mm) Wrist motor tendons Experimental stages Intact wrist SLlL cutting Moderate SL gap Severe SL gap Rc.~i~)iz-cxtensiot? Extensor carpi radialis longus 9.4* & 2.2 Extensor carpi radialis brevis i 7.6 Extensor carpi ulnaris 3.7% i i Flexor carpi radialis 14.2 & i I f 1.7 Flexor carpi ulnaris 17. & i & h 2. Rutlioulnur. de~1iutioii Extensor carpi radialis longus 18.8 & 2.1 Extensor carpi radialis brevis 12.5 It 1.7 Extensor carpi ulnaris Flexor carpi radialis 5.7 f 2.5 Flexor carpi ulnaris 12.3 h3.l 18.3 i i f & h h & h & % 3.1 Ul Normal El SLlL Section 15 Moderate SL Gap w W Severe SL Gap W I" " z 2, I I a9 5 - *- Normal SLlL Section I3 Moderate SL Gap W Severe SL Gap (A) ECRL ECRB ECU (B) (I FCR FCU Fig. 2. Percentage changes of moment arms of principal wrist tendons in the columns with significant difference from the intact wrists. sagittal plane. (A) Wrist extensors; (3) wrist flexors. Asterisks indicate 2 15 W LI z 5 Severe SL Gap W z Ill Normal SLlL Section Moderate SL Gap W Severe SL Gap (A) ECRL ECRB FCR (B) ECU FCU Fig. 3. Percentage changes of moment arms of the wrist motor tendons in frontal plane. (A) Radial deviators; (B) ulnar deviators. Asterisks indicate columns with significant difference from the intact wrists. large, and the difference was not statistically significant compared with the intact wrist. Efkt qf inodrrutr n9icioiiing qf the SL joint Excursions of the ECRL, ECRB, and ECU tendons decreased by 1.7,, and 1.5 mm during wrist flexion- extension of 12", and thus the moment arm decreased by.9,, and.7 mm, respectively. The moment arms of the ECRL, ECRB, and ECU tendons were 9 i 15/;,, 1 31 IO'i/O, and 73 Ilt 28% of those in the intact wrist, respectively. Of these, only the changes in the ECU tendon were of statistical significance (P <.5). Excursions of the FCR and FCU tendons increased by 3.8

5 J. B. Tang et al. I Journal of Orthopuehc Rewarch 2 ( and 1.8 mm compared with those in the intact wrist, respectively. These correspond to an increase in moment arms of 1.8 mm in the FCR and.8 mm in the FCU tendon. The moment arms of the two tendons were significantly greater after moderate widening of the SL joint than in the intact wrist (P <.1 and P <.1, respectively) (Fig. 3). During wrist radioulnar deviation, excursions of the ECRL, ECRB, ECU, and FCU tendons decreased, while excursions of the FCR tendon increased. The moment arms of the ECRL, ECRB, ECU, and FCU tendons decreased by 1.7 to 2.5 mm. The moment arm of the FCR tendon increased by 2.1 mm. The moment arms were 91 f 7% for the ECRL, 82 18% for the ECRB, 133 f 38% for the FCR, 87 f 23% for the ECU, and 84 f l3yo for the FCU tendons compared with those in the intact wrist. Except for the ECU tendon, changes in the tendons were statistically significant (P <.5 or P <.1) (Fig. 3). Efect of severe widening of the SL joint The effects of severe widening of the SL joint on excursions and moment arms of the wrist motor tendons were similar to those of moderate joint widening. However, the changes in tendon moment arms were more pronounced when severe SL joint widening presented. During wrist flexion-extension, the excursion and moment arm of the ECU tendon were 67 f 29% of those in the intact wrist, a significant decrease (P <.5). Excursions and moment arms of the FCR and FCU tendons increased significantly (P <.1 and P <.1, respectively). Moment arms of the FCR tendon after severe widening of the SL joint were 117f 1% of those in the intact wrist (Fig. 3). Excursions and moment arms of the tendons decreased except for the FCR tendon during radioulnar deviation. Moment arms in this stage were 9 * 6% of those in the intact wrist for the ECRL, 77 f 16% for the ECRB, 138 f 24% for the FCR, 84 f 24% for the ECU, and 84 f 12% for the FCU tendons. Changes in moment arms of all the tendons except the ECU were statistically significant (P <.5 and P <.1) (Fig. 3). Discussion This study focused on the impact of the SLIL disruption and persistent widening of the SL joint on changes in the moment arms of the principal wrist motor tendons. The results support our original hypothesis that both sectioning of the SLIL and persistent widening of the SL joint significantly affect the kinetics of the principal wrist motor tendons. Changes in tendon moment arms were more remarkable after persistent widening of the SL joint than after SLIL sectioning. The results showed that moment arms of the ECRL and ECRB tendons after the SLIL section were 11% and 15% of those in the intact wrist, respectively. In the wrist with moderate or severe widening of the SL joint, moment arms of the flexors increased significantly. Moment arms of the ECRL, ECRB, ECU, and FCU tendons decreased after the SLIL sectioning or the SL joint widening during wrist radioulnar deviation. However, moment arms of the FCR tendon significantly increased to 122% after the SLIL section, 133% after the moderate SL joint widening, and 138% after the severe SL joint widening compared with those of the intact wrists. An important finding of this study was that moment arms of the ECRL and ECRB tendons increased significantly during wrist flexion-extension after the SLIL section. The proximal pole of the scaphoid has been noted to serve as an effective shaft pulley for gliding of the ECRL and ECRB tendons [1,18]. Armstrong [2], Linscheid [lo], and Ruby et al. [I71 reported palmarflexion of the scaphoid and dorsal subluxation of the proximal pole after SLIL section. The subluxated proximal scaphoid may protrude the ECRL and ECRB tendons dorsally and increase their moment arms. This would increase the longitudinal compression force of the ECRL and ECRB tendons to the scaphoid and favor palmarflexion of the scaphoid. On the other hand, persistent SL joint widening increased the moment arms of the flexors in the sagittal plane. Increase in moment arms of the flexor tendons is in agreement with known kinematic changes of the scaphoid and triquetrum. The scaphoid was found to flex palmarly, and the triquetrum was found to shift palmarward after SL diastasis [13,15,17]. We speculate that the distal pole of the flexed scaphoid and shifted triquetrum may protrude on the flexor tendons anteriorly and increase their moment arms. An increase in the moment arms of either the radial extensors or the flexor tendons may result in increased moment and compression force through the radial carpus, which favors palmarflexion of the distal carpal row and formation of the dorsal intercalated segmental instability (DISI). In addition, disturbance in moment of these tendons does not favor the powerful flexion with combination of wrist ulnar deviation [ 1,181, which help explain why patients with this disorder experience difficulty in daily living. A prominent kinetic change potentially relevant to clinical sequelae of the SL dissociation was the increase in the moment arm of the FCR tendon during wrist radioulnar deviation, in contrast to the decrease in those of the other tendons. Sectioning of the SLIL and SL joint widening may lead to twisting of the carpal link with reduction in the carpal height [ 1 1,12,19], which may be a pathological basis for the changes in the moment arms noted in this study. It is difficult to perceive why the moment arms of the FCR tendon increased.

6 77 J.B. Tong et ul. I Journal of Orthopedic Rcwurch ) Changes in scaphoid motions caused by the separation of the SL joint probably had affected excursions of the FCR tendon. In contrast to the decrease in all the other tendons, an increase in moment arm of the FCR tendon increases the ratio of load distribution through the radial side of carpus. We speculate that this kinetic change accelerates the collapse of the scaphoid and proximal migration of the capitate toward the separated SL joint space, an important step in formation of scapholunate advanced collapse (SLAC) [3]. Clinical studies have shown that chronic SL instability presents a specific pattern of joint degeneration [ This process begins at the radioscaphoid interface and progresses to the capitatolunate joint, eventually involving the entire radiocarpal joint. Increases in moment arms of the radial motor tendons may magnify the effect of compression to the radioscaphoid joint and contribute to early osteoarthritis around this joint. Blevens et al. [7] found that scaphoid contact area on the radial articular surface shifted dorsoradially, with an increase in pressure between the scaphoid and radiocarpal articular surface after a complete SLIL section. The increase in moment arms of radial wrist movers observed in this study offers a kinetic explanation for the increase in contact pressure in the radiocarpal joint surface. Measurement of tendon excursions over a fixed joint angulation arc is a simple and accurate way to calculate moment arms of the tendon. We have applied this technique to determine moment arms of the tendons in the models of the scaphoid waist fracture [Z], distal radial fracture [24], shortening of the distal radius [21], and recently the trapeziometacarpal joint instability of the thumb [ 161. With similar experimental settings, excursions of the ECRL, ECRB, and FCR tendons after SLIL sectioning were measured in a previous study [33]. The preliminary study was confined to a study of tendon excursions, which was different from the present study in three respects. First, only radial wrist motor tendons were studied and no ulnar motor tendons were included. Second, persistent SL joint widening was not simulated. Finally, tendon excursions were measured through wrist motions, but the moment arms were not assessed. The tendons were loaded with 25 g each to take up the slack of the tendons and to avoid discriminatory tendon elongation, as elongation could interfere with the excursion values. We did not attempt to simulate active wrist motion; thus physiological loading was not manipulated to tendon stumps in the study. Smooth silicone spacers were inserted to the joint space to produce persistent joint widening. With the spacers between the scaphoid and lunate, carpal motions were smooth during the test. Obviously, this did not completely duplicate chronic SL diastasis with attenuation of the ligaments and malrotation of the scaphoid. Nevertheless, this model provided a reliable way of maintaining a persistent gap in the SL joint space, an important sign of severe chronic SL diastasis. To our knowledge, it has not been possible to reproduce SL dissociation with chronic attenuation of carpal ligaments in a cadaver model. There were several limitations in this study. First, as stated above, the model of SL dissociation in an experimental setting can not completely duplicate clinical pathology of SL dissociation. Second, contribution of gradual attenuation and laxity of the carpal ligaments can not be assessed in this cadaveric model. In addition, the presence of the STLC and DICL was proven to stabilize the distal pole of the scaphoid [8,9,27,29]. Our study did not assess the effect of the STLC, DRCL, and DICL. This study added new information to our understanding of pathomechanics involved in the carpal kinematics and the clinical sequelae of the instability. The SLIL and appropriate SL joint space were shown to be important to the maintenance of normal kinetics of the wrist. Since unfavorable kinetics may contribute to pathomechanics of carpal deformities and osteoarthritis, early correction of the pathological basis for the disturbance should be a goal of treatment. Restoration of proper moment arms of the motor tendons appears to require correction of scaphoid and lunate displacements in sagittal or frontal planes and avoidance of a persistent SL gap. In addition, this study suggests justification of including the kinetic parameters in laboratory criteria for judging appropriate treatments for this spectrum of carpal instability. 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