Wrist kinetics after luno-triquetral dissociation: the changes in moment arms of the flexor carpi ulnaris tendon

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1 ELSEVIER Journal of Orthopaedic Research 20 (2002) Journal of Ort hopaed ic Research hres Wrist kinetics after luno-triquetral dissociation: the changes in moment arms of the flexor carpi ulnaris tendon Jin Bo Tang *, Ren Gou Xie, Xiao Wei Yu, Feng Chen Hand Surgery Research Center and Biomechanics LUhOrLltOIy, Dt~partment of Orthopaedics, Affiliated Hospitul of Nantong Mctlicul College. 20 West Temple Rond, Nuntong , Jiangsu, China Received 30 November 2001; accepted 1 April 2002 Abstract Wrist biomechanics after luno-triquetral (LT) dissociation is important for understanding the clinical sequelae of the disease and for determining its treatment options. The LT interosseous ligament plays an important role in stabilizing the joint and damage to the ligament would be expected to significantly increase moment arms of tendon of the flexor carpi ulnaris (FCU), the principal ulnar wrist flexor. We investigated the changes in moment arms of FCU tendon after various amounts of sectioning of the ligaments proven to be associated with LT dissociation. In six fresh frozen cadaveric upper extremities, excursions of the FCU tendon were recorded simultaneously with wrist joint angulation during wrist flexion-extension and radioulnar deviation. Tendon excursions were measured in intact wrists, in wrists with sectioning of the dorsal portion of the LT interosseous ligament, in wrists with sectioning of the entire LT interosseous ligament, and finally in wrists with further sectioning of the dorsal radiotriquetral and intercarpal ligaments. Moment arms of the tendon were calculated from tendon excursions and joint motion angulations and expressed as percentage changes from those in the intact wrist. During wrist flexion-extension, moment arms of the FCU tendon after sectioning of the entire LT interosseous ligament and after sectioning of the two capsular ligaments were 112 i 7% and 114 8'%,, respectively; these values were significantly greater than those in the intact wrist. During radioulnar deviation, the moment arms were 114?C 11% after sectioning of the dorsal portion of the LT interosseous ligament, 134 * 15'% after sectioning of the entire ligament, and 153 i 18% after sectioning of the capsular ligaments, again being significantly greater than the normal wrist. Increase in moment arms of the FCU tendon after loss of integrity of the LT interosseous ligament and dorsal capsular ligaments may contribute to clinical sequelae of LT dissociation and difficulty in treating this disorder Orthopaedic Research Society. Published by Elsevier Science Ltd. All rights reserved. Keywords: Joint; Wrist; Ligament; Tendon; Moment; Kinetics Introduction The wrist joint is a complex joint in both its structure and its function. Wrist disorders cause serious symptoms and functional loss and are difficult to diagnosis and treat. Disorders relating to symptoms in the ulnar carpus include ulnar styloid fracture, triangular fibrocartilage tear, dislocation of the extensor carpi ulnaris tendon, luno-triquetral (LT) dissociation, and arthrosis of the distal radioulnar or ulnocarpal joints. LT dissociation describes a spectrum of instability caused by injury to the ligamentous constraints between or around the lunate and the triquetrum [7,9,12]. The disease pre- *Corresponding author. Tel.: ; fax: address: hrsc~~ub.iit.jsinfo.net (J.B. Tang). sents as LT joint diastasis initially and may progress to a volar intercalated segmental instability (VISI). Pathological findings range from disruption of a part of the LT interosseous ligament to injuries to multiple ligaments, including the radiotriquetral and the dorsal intercarpal ligaments [7,9,12,28-30]. The LT interosseous ligament plays an important role in stabilizing the joint, but severe forms of LT dissociation cannot be produced without loss of the two dorsal capsular ligament restraints [7,13]. Sectioning of these ligaments produced kinematic changes that lead to static VISI of the wrist in experimental models [7,13,25]. Wrist biomechanics after LT dissociation is important for understanding the clinical sequelae of the disease and for determining its treatment options. Considerable work has been performed in ligament anatomy [3,4,16,26,27], carpal kinematic changes [5-7,13,25], /02/$ - see front matter Orthopaedic Research Society. Published by Elsevier Science Ltd. All rights reserved PII: SO (0 2)OOO 67-0

2 clinical diagnosis, and treatment [I 1,13,28,30] of this disease. However. thc moment arms of the wrist motor tendons after LT dissociation have not been studied. We know of no information about how LT dissociation affects the moment arms of the wrist motor muscles and, consequently, how these changes contribute to the mechanism responsible for condition. The flexor carpi ulnaris (FCU) tendon attaches to the triquetrum through the pisiform and has the closest functional relation with the triquetrum, which can directly affect the position of the triquetrum and motion of the LT joint. In this study, we determined changes in the excursions and average moment arms of the FCU tendon following injuries to the LT ligament (including dorsal, proximal, and palmar parts) and dorsal capsular ligaments in a cadaveric model. We hypothesized that sectioning of the LT interosseous ligament and the dorsal capsular ligaments would significantly increase moment arms of the FCU tendon. Materials and methods Six fresh frozen cadaver upper extremities (three right and three left) were harvested through a disarticulation at the elbow from four men and two women. The ages of the donors ranged from 40 to 72 years. with an average of hl years. Approval of the Institutional Revieu Board was obtained for the use of human cadaveric material. The donors had died of diseases unrelated to their hands. Radiographs excluded specimens with skeletal abnormalities. No ligamentous laxity of the wrist or restriction in wrist motion was noted. These specimens were thawed overnight at room temperature before testing. The soft tissues were stripped from thc upper arms to 4 cm above the wrist, preserving the forearm interosseous membrane and ligaments. All tendons and ligaments were preserved within 6.0 cin proximal to the wrist. Care was taken to protect the extensor and flexor retiiiacula of the wrist. Dacron braided Ethilon sutures (Ethicon Inc.. Somerville, NJ, LISA) were secured to the proximal ends of the cut tendons. The specimens were mounted to a custom jig with two Steiniiiann pins through the middle and distal parts of the radius and ulna. The forearm was kept in neutral rotation, and the pins tirmly fixed the radius and ulna to the jig to prevent forearm rotation and translation. Motions of the metacarpo-phalangeal (MP) and inter- phalangeal (IP) joints of the thumbs and fingers were prevented by insertion of Kirschiier wires into the phalanx and metacarpal bones. Motion of the wrist joint was not restricted. Eupi~rinzmr~d srtup 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 of the jig. Each of the tendons was preloaded to 250 g to prevent slack during wrist motion. The proximal end of the suture line connected to the FCU tendon wab routed around the pulley of a rotational potentiometer (Series P2500, Novotechnik US, Inc.. Southborough, MA, USA). which recorded the amplitude of voltage produced by the tendon excursions during wrist movement. The voltage was converted to tendon excursion by computer software. A dual-axis electrogoniometer (Model SG65, Biometrics Ltd., Cwmfelinfach, Gwent, UK) mounted onto the distal ulna and third metacarpal shaft monitored global wrist motion. Tendon excursion and joint angulation were simultaneously collected by a data acquisition system to a personal computer. Joint angles were calibrated to an accuracy of 0.1'. and values of tendon excursions were rounded to the nrarest 0.1 nim. System accuracy was measured for joint angulation and tendon excursion. The gonionieter was moved on a gauge block through YO" 10 times. The recorded angulation was ". The accuracy of thc excursion measurement was tested by measuring 10 mm lengths in a micrometer caliper 10 times. The measured excursion was mm. A Steinmann pin was inserted into the phalanges of the middle finger in each hand to serve as the guide for wrist motion. The pins beyond the fingertips were moved along metal bars placed in the sagittal and frontal planes of the wrist. The wrists were passively moved through full range of flexion to extension and full radial to ulnar deviation, respectively. Excursion of the FCU tendon was recorded for a 10 s period during each passive motion with a sampling rate of 10 Hz. The data were obtained in each of the following sequence (Fig. I). First, excursions of the wrist motor tendons in the normal wrists were recorded to serve as the control for the analysis of changes after ligamentous section. Next, the dorsal portion of the LT interosseous ligament was sharply transected through a dorsal approach distal to the fourth extensor compartment using a No. 15 surgical blade. Care was taken to section only the dorsal part of the ligament. Then, the proximal and the palinar parts of the LT ligament were sectioned. Finally, the dorsal radiotriquetral and intercarpal ligaments were sectioned. D~itti tmulni.r No apparent changes were found in the range of global wrist motion after ligament sectioning. Tendon excursions used for analysis 1. Intact Wrist 2. Section of Dorsal LT 3. Section of Dorsal, Proximal Ligament and Palmar LT ligament 4. Further Section of Dorsa Capsular Ligaments Fig. 1. The four stages of the experiment on LT dissociation

3 J. B. Tang et al. I Journd c?f'ort/iopuri& Rewurch 20 (2002) were those from 60" of flexion to 60" of extension and from 20" of radial deviation to 30" of ulnar deviation. The moment arm was calculated according to the relationship that excursion along an arc equals the radius of a circle multiplied by the subtended angle (in radians) [1,2, The moment arms were the average moment arms in the analyzed motion ranges. The individual specimens presented a certain extent of variation in normal tendon excursions and moment arms. Therefore, tendon excursions and moment arms in the wrist with ligament sectioning were adjusted by the values in the same wrist with intact ligaments to obtain percentage changes. The percentage changes in tendon excursions and moment arms were analyzed by one-way analysis of variance (ANOVA) with significance level at p < Tukey-Kramer Honest Significant Difference tests were used to determine the significance in percentage changes of tendon excursions and moment arms after ligament section when the F-test of the ANOVA was significant. Results Efect o j sectioning dorsal LT interosseous ligarvieni The excursion of the FCU tendon over 120" of flexion-extension increased by an average of 1.9 mm (Table 1); the moment arms increased by 0.8 mm. The moment arm of the FCU tendon after the dorsal LT interosseous ligament was sectioned was 108 k 7% (mean & S.D.) of that in the intact wrist. The increase in the FCU tendon moment arm was not statistically significant after sectioning of the dorsal LT interosseous ligament compared with those in the intact wrist (Fig. 3). During radioulnar deviation, the FCU tendon excursions and moment arms after sectioning were 1 14 i 11% of that of the intact wrist, a significant difference. Eflect of sectioning of the all parts of the LT interosseoics ligaments FCU tendon excursions increased by 2.8 inm, and thus the moment arm increased by 1.3 mm during wrist flexion-extension of 120'. The FCU tendon moment arm was 112 i 7% of that in the intact wrist. These changes were statistically significant (Fig. 2). During wrist radioulnar deviation, FCU tendon excursions increased by 2.8 mm and moment arms increased by 3.7 mm. The moment arms of the FCU tendon were 134 f 15'%1 compared with those in the in- 140 /' P < r- P< r Iutaet - ;* DO~S~I Lr Entire LT LT, RT and ST Ligameut Cut Ligament Cut Ligament Cut Experimental Stages Fig. 2. Changes in FCU tendon moment arms in the sagittal plane after sectioning of the ligaments around the LT joint. Asterisks indicate the columns with a significant difference compared with those in the intact wrists v in f 120 u s D +pi ' P I** r- -- Intact Dona1 1,T Ligament Cut Ligament Cut Ligament Cut Experimental Stages Fig. 3. Changes in FCU tendon moment arms in the frontal plane after sectioning of the ligaments around the LT joint. Asterisks indicate the columns with a significant difference compared with those in the intact wrists. tact wrist. The changes in the moment arms of the FCU tendon were statistically significant (Fig. 3). Effect of sectioning dorsal rudiotriqzietral and intercarpal ligunien ts After further sectioning of the dorsal radiotriquetral and intercarpal ligaments, the FCU tendon excursions Table 1 Excursions and moment arms (MA) of the FCU tendon during carpal motions Stages Flexion-extension Radioulnar deviation Excursions MA MA (flexion) Excursions MA MA (ulnar deviation) i f * i I 26.7 f i d 8.6 & i i f L i i f f i zt =k i 3.0 Stages are as defined in Fig. 1. Data are presented as mean f S.D. a Denotes values were identical to the values shown in the preceding columns.

4 and momcnt arms had the most pronounced increase, especially during wrist radiouliiar deviation. During flexion-extension, the cxcursions and moment arms of the tendons were 114 & 8'%, of those in the intact wrists, a statistically significant difference. However, excursions and moment arms of the FCU tendon were not significantly different from those of the wrists with only the LT interosseous ligament sectioned (Fig. 2). Excursions and moment arms of the tendons increased most remarkably during radioulnar deviation. FCU tendon moment arms increased by 5.4 mm and were 153 & 18% of those in the intact wrist. Changes in FCU tendon moment arms were statistically significant compared with those in the intact wrist or with sectioning of the LT interosseous ligament (Fig. 3). Discussion In this study, we investigated changes in moment arms of a principal wrist motor that has the closest anatomic relation with the triquetrum after injuries to the ligaments around the LT joint. The study focused on the effect of disruption of the LT interosseous ligament and dorsal radiotriquetral and intercarpal ligaments on excursions and moment arms of the FCU tendons. The moment arms of the principal ulnar flexor tendon changed significantly after sectioning of all three parts of the LT interosseous ligament, and the changes were more remarkable after sectioning of the dorsal capsular ligaments. The moment arm of the FCU tendon was about 110% to 135% that of the intact wrist after sectioning of the LT interosseous ligament and I15'%) to 155% that of the intact wrist after sectioning of the dorsal radiotriquetral and dorsal intercarpal ligaments. These findings indicate that both LT interosseous ligaments and the dorsal capsular ligaments are important in maintaining normal FCU function. The capsular ligaments prevent further mechanical disturbance in the wrist after disruption of the interosseous ligaments. LOSS of the stabilizing role of the two capsular ligaments of the wrist with the LT interosseous ligament results in more pronounced changes in ulnocarpal biomechanics than does disruption of the LT interosseous ligament alone. Changes in moment arms of a joint motor tendon reflect alteration in the instantaneous centers of rotation of the tendon during a part or all of the arc of motion [1,2,5,17,18,20]. Although wrist motion is nonlinear and the instantaneous centers vary in different carpal bones, the approximate center of carpal rotation is about the proximal head of the capitate [8,27]. An increase in moment arm indicates a widening of the distance between the tendon and the center of rotation of the wrist. Kinematic studies revealed an increase in ulnar devia- tion, supination, and flexion of the triquetrum with respect to the distal radius and hypermobility of the triquetruni after sectioning of the LT interosseous ligament and the dorsal capsular ligaments [6,13,25]. The changes in kinematics may be a result of an increase in the FCU tendon moment arms. We speculate that the increase in moment arms increases the torque for flexion and ulnar deviation of the triquetrum. Palmar flexion and ulnar deviation of the triquetrum after ligamentous disruption may also result in an increase in FCU tendon moment arms. Because the FCU tendon inserts to the pisiform attaching to the volar aspect of triquetrum [3 11, kinematic changes of the triquetrum, and alteration in the distance of wrist motors from the rotation axes of the carpus are closely related. Consistent with our findings, the kinematic changes found after disruption of the two capsular ligaments were greater than after sectioning of only the LT interosseous ligament [6,13]. Ulnocarpal instability is a difficult entity for which a clear understanding of etiology and pathological changes is lacking. Wrist kinematics is based on structural characteristics, bony anatomy, and the torque produced on the bones by muscles across the joint (kinetics). The wrist motor examined in the present study has close anatomical and functional relation with the triquetrum. Changes in FCU tendon moment arms can provide insight about kinetics that is important to ulnocarpal instability. LT joint dissociation clinically presents as dynamic or static VlSI and tends to advance to arthrosis of the LT and ulnocarpal joints [12,14,28,29]. In a wrist with LT dissociation, increases in moment arms of the FCU tendon increase the force that pulls the triquetrum to displace from the other bones. The FCU tendon, the only tendon inserting on the carpal bones, is a powerful wrist flexor and an ulnar deviator. The dominant patterns of wrist motion are dorsiflexion with radial deviation and palmar flexion with ulnar deviation [15]. The actions of picking up, throwing, lifting, and writing all involve these two principal patterns of wrist motion. Changes in moment arms of the FCU tendon would interfere with normal motion pattern. The results of this study may help explain disability in daily life associated with LT dissociation. The rate of success in LT arthrodesis is usually low [12,14,30]. The powerful pull of the FCU tendon on the relatively small triquetrum might subject the arthrodesis to a greater shearing force, which could substantially hamper bony healing. This study revealed that the changes in FCU tendon moment arms are greater in the plane of wrist radioulnar deviation than in the plane of flexion-extension. This might relate to the fact that the triquetrum inherently tends to migrate ulnarly because of its ligamentous connection with the hamate and the ulna head [8,9,16]. Sectioning the LT interosseous ligament and dorsal

5 J.B. Tang ef al. I Journal of Orthopedic Research 20 (-7002) capsular ligaments eliminates the restraints that hold the triquetrum close to the lunate. Ulnar translation of the triquetrum-pisiform complex increased FCU tendon moment arms. However, relatively tense capsular ligaments located volar or dorsal to the triquetrum restricted pronounced translation of the triquetrum and pisiform in the sagittal plane. Values of tendon moment arms in the ranges of wrist flexion and ulnar deviation indicate that increases in the moment arms may be more predominant in wrist flexion and ulnar deviation. As the wrist does not subtend a perfect circle during radioulnar deviation and flexion-extension [lo], the average moment arm during an arc of motion does not represent an instantaneous moment arm at a specific angle of joint motion. Rather, it is an approximation that reflects the tendency of changes during tested motion arcs. The instantaneous moment arms can be obtained by mathematical analysis as shown in our previous studies on scaphoid waist fracture [17] and trapeziometacarpal joint instability [ 113. Average tendon moment arms have been measured in our studies of distal radial fracture [19,20,24], flexor pulley injuries [33,23], and scapholunate dissociation [21]. The moment arms in intact wrists are rather consistent in this study with those in our previous tests [17,19,20]. Loading each tendon to 250 g eliminated slack in the tendons and avoided discriminatory tendon elongation; such elongation could interfere with the tendon excursion data. We did not attempt to simulate active joint motion; thus physiological loading was not applied to the tendons. This study was limited in that the model of LT dissociation cannot completely duplicate the clinical pathology of LT dissociation, and contributions of gradual attenuation and laxity of the carpal ligaments cannot be assessed in a cadaveric model. To our knowledge, this study is the first study to investigate biomechanical changes of the wrist motors after ulnar carpal instability. Although carpal kinematics after LT dissociation was delineated years ago, kinetic changes involving this disorder have not been previously clarified. This study demonstrated kinetic changes of the principal ulnar wrist motor after disruption of the ligaments (including the LT interosseous, dorsal radiotriquetral and dorsal intercarpal ligaments) that may contribute to the formation and clinical sequelae of LT dissociation. The results indicate that integrity of these ligaments is of great importance to biomechanics and function of the ulnar wrist flexor tendon. Acknowledgements The funding sources of this study were grants from Jiangsu Provience Health Bureau, Ministry of Communication, and Ministry of Education of China. References [I] Amstrong TJ, Chaffin DB. 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