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The effect of triangular fibrocartilage complex injury on extensor carpi ulnaris function and friction
Clinical Biomechanics 24 (2009) 807–811
Contents lists available at ScienceDirect
Clinical Biomechanics journal homepage: www.elsevier.com/locate/clinbiomech
The effect of triangular fibrocartilage complex injury on extensor carpi ulnaris function and friction Zachary J. Domire a,*, Furkan E. Karabekmez a, Ahmet Duymaz a, Timothy S. Rutar b, Peter C. Amadio c, Steven L. Moran d a
Biomechanics Laboratory, Division of Orthopedic Research, Mayo Clinic, Rochester, MN, USA Washington State University, College of Veterinary Medicine, Pullman, WA, USA Departments of Orthopedics and Biomedical Engineering, Mayo Clinic, Rochester, MN, USA d Departments of Orthopedics and Plastic Surgery, Mayo Clinic, Rochester, MN, USA b c
a r t i c l e
i n f o
Article history: Received 23 February 2009 Accepted 3 August 2009
Keywords: Triangular fibrocartilage complex Extensor carpi ulnaris Friction Biomechanics Simulation
a b s t r a c t Background: It has been previously shown that injury to the triangular fibrocartilage complex increases the moment arm of the extensor carpi ulnaris. This will reduce the force producing capacity of the muscle in some situations, but will also increase its mechanical advantage. It is also possible that the change in the tendon path may increase tendon friction, predisposing the patient to future repetitive motion injury. It is the purpose of this study to determine the effects of triangular fibrocartilage complex injury on extensor carpi ulnaris moment producing capacity and tendon friction. Methods: A simple simulation was used to examine muscle moment producing capacity throughout the range of motion, at varying speeds and in both injured and healthy states. Six fresh frozen human cadaveric wrists were used to determine the effect of injury on tendon friction. A custom made device was used to move the wrists through a range of motion, while a constant force was applied to the proximal tendon and force was recorded at the distal tendon. Friction was measured before and after the creation of injury. Findings: The decreases in muscle force following injury were small, even in the worse case. The moment producing capacity of the muscle was increased following injury. Tendon friction during flexion–extension was decreased following injury. The friction during radial-ulnar deviation was unchanged. Interpretation: When making surgical decisions about triangular fibrocartilage complex repair, it is not necessary to consider extensor carpi ulnaris moment producing capacity or tendon friction. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Injury to the triangular fibrocartilage complex (TFCC) is a frequently occurring wrist injury (Palmer et al., 1983). It can occur in isolation, but more commonly occurs in conjunction with other structures in the distal radioulnar joint. Treatment options following injury vary widely. It has been proposed that extensor carpi ulnaris (ECU) function is an important consideration when making surgical decisions about TFCC repair (Tang et al., 1998). Tang and associates (1998) showed that the TFCC acts as a retinacular component for the ECU tendon. They examined the moment arm of the ECU throughout a range of wrist extensions in eight cadaver forearms. They then simulated an injury to the TFCC and examined the changes in moment arm. Following injury to the TFCC bowstringing of the ECU occurs during extension and as a result the moment arm of the ECU is increased by nearly 30%. * Corresponding author. Address: 200 First Street, Mayo Clinic College of Medicine, SW, Rochester, MN 55905, USA. E-mail address: [email protected] (Z.J. Domire). 0268-0033/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.clinbiomech.2009.08.001
Increasing the moment arm of the ECU will adversely effect force production in two ways. First, the excursion of the muscle will be larger over the same range of motion. This will result in the muscle operating over a larger portion of the force length curve and decrease force production primarily near full extension of the wrist. Second, for a given angular velocity of the wrist the muscle velocity will be larger. This will result in reduced muscle force whenever the muscle is shortening. Although increasing the moment arm of the ECU will adversely effect its force production, muscle function is determined by muscle moment not muscle force. Increasing the moment arm of the ECU will increase the mechanical advantage of the muscle and therefore increase the maximum moment the muscle can produce. It is also possible that TFCC injury may increase friction acting on the ECU tendon. This increased friction may cause additional wear on the surface of the tendon, which would further increase friction. The resulting vicious cycle of tendon damage could lead to tendonitis, tendon injury and pain. The relationship between increased moment arm and muscle function is clearly a complex one. The following study will examine
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Z.J. Domire et al. / Clinical Biomechanics 24 (2009) 807–811
the effects of increasing the moment arm of the ECU on moment producing capacity of the muscle throughout the range of motion and at a range of different joint velocities. It is hypothesized that ECU function will be impaired only near maximum wrist extension and at high joint velocities. The following study will also examine the effects of TFCC injury on ECU tendon friction. It is hypothesized that ECU friction will be increased following TFCC injury and therefore may contribute to ulnar sided wrist pain. 2. Methods 2.1. Function Isometric and isokinetic joint flexion–extension strength curves were simulated for the ECU in both injured and non-injured states. Strength curves were simulated for a range of motion from neutral to 60° extension. Based upon data from Tang and colleagues (1998) the moment arm for the ECU was 4.8 mm when the TFCC was intact and 6.2 mm when the TFCC was injured. Radial-ulnar deviation was not simulated as Tang and colleagues showed no change in the ulnar deviation moment arm as a result of TFCC injury. Muscle model parameters (Table 1) were taken from Loren and Lieber (1995) and Challis (2004). The ECU was modeled as a Hill type, muscle like actuator. The muscle had force–length and force–velocity properties and was connected in series with an elastic element representing the tendon. The muscle model can be represented by the following equation:
F m ¼ q F max F l F v where Fm is the force produced by the muscle, q is the normalized active state of the muscle fibers, Fmax is the maximum isometric force the muscle can produce, Fl is the fraction of maximum isometric force the muscle can produce at a given length, Fv is the fraction of maximum isometric force the muscle can produce at a given velocity. For complete details on the muscle model used see Domire and Challis (2007). It was assumed that active state was maximal during all simulations; therefore q was equal to 1. This results in the largest possible difference between injured and healthy for a given position and velocity. Isokinetic simulations were performed at 60°/s and 300°/s. Simulation procedure was similar to Bobbert and Harlaar (1993). Joint angular velocities were constant throughout each of the simulations. 2.2. Friction Six fresh frozen human cadaveric upper extremities (all males, mean age 86.3 ± 8.7 years), amputated 15 cm proximal to the wrist joint were used for this study. Specimens were stored in a freezer at 20 °C, thawed at room temperature immediately prior to test-
Table 1 Summary of the muscle model parameters. Fmax Lfopt w Ltr c Vmax K
51.5 N 58.8 mm 0.56 215.1 mm 0.04 mm/mm 6 Lfopt/sec 4
Note: Fmax – maximum isometric force, Lfopt – optimal fiber length, w – spread of the force–length curve, Ltr – resting length of tendon, c – tendon strain under maximum isometric force, Vmax – maximum unloaded shortening velocity, K – force–velocity curvature constant.
ing and examined to exclude gross pathological evidence of injuries or major degenerative changes around the wrist. Friction was tested during flexion/extension and radial-ulnar deviation movements with the TFCC intact and then damaged. Frictional testing was performed by applying a constant load to the proximal tendon and measuring the load at the distal tendon while moving through a range of motion. To measure the load at the distal tendon, the ECU tendon was detached from its insertion and 15 mm of the fifth metacarpal was excised, after first ensuring stability of the remaining bone by driving a K-wire into distal heads of the 2–5th metacarpals. A load cell (custom made, full bridge, simple cantilever design) was attached to the remaining bone and the tendon was reattached to this load cell. Care was taken to maintain the direction of force application. The load to the proximal tendon was applied by a 200 g deadweight. A low friction pulley was used to apply the deadweight force in a horizontal direction. A custom made motorized apparatus was used to move the specimens (Fig. 1). The proximal ends of the radius and ulna were clamped to the stationary component of the device with the forearm in a supinated position. For testing radial-ulnar deviation, the hand was fixed to the mobile component of the device by clamping a K-wire driven into distal heads of the 2–5th metacarpals to the device. For testing flexion/extension the specimen was turned 90° and the proximal ends of the radius and ulna were re-clamped to the stationary component of the device with the forearm in a supinated position. The hand was fixed to the mobile component of the device by clamping a K-wire passed through the distal head of the third metacarpal perpendicular to the prior K-wire to the device. For each direction of movement, care was taken in the placement of the specimen to ensure the axis of rotation of the wrist joint aligned with the axis of rotation for the testing device. Three cycles of motion were completed for both radial-ulnar deviation and flexion/extension. The direction of motion tested first was randomized. The range of motion was from 35° extension to 40° flexion for flexion/extension and 15° radial to 35° ulnar deviation for radial-unlar deviation. The speed was 5°/s for each test. Force at the distal tendon was measured throughout the motion at 45 Hz. To create a TFCC injury, a 2 cm longitudinal incision was made over the volar side of the ulnocarpal joint. A volar approach was chosen to avoid any inadvertent damage to soft tissues around the ECU, specifically the ECU subsheath. The palmar fascia was incised and dissection was then carried down to the joint capsule, which was incised in line with the skin incision. Triangular fibrocartilage (TFC) was explored to verify that there was no evidence of a pre-existing peripheral tear. Following TFCC examination the attachment to the TFCC to the fovea and peripheral margin of ulna was separated with a scalpel. The ulnolunate ligament and the ulnotriquetral ligament, the palmar and dorsal radial-ulnar ligaments were also incised. The horizontal portion of the TFC was divided to the ulnar styloid but ulnar styloid was left intact. To complete the injury the TFCC’s dorsal edge, which is adherent to the ECU subsheath, was divided carefully to avoid any damage to the ECU tendon. Following the creation of the injury, friction was tested repeating the same procedure as above. Care was taken throughout the testing to ensure the specimens were kept moist. Before calculation of friction the force data were smoothed using a fourth order bi-directional Butterworth filter with a cutoff of 4 Hz. From the filtered force data, friction was calculated at each joint angular position. This was done by calculating the difference between the recorded force and the dead weight force for each direction of movement and then averaging the absolute value of this difference for each direction of movement. This procedure allows for cancelation of any artifacts such as friction in the pulley.
Z.J. Domire et al. / Clinical Biomechanics 24 (2009) 807–811
809
Fig. 1. Sketch of the experimental setup for testing friction during flexion/extension.
Fig. 2. ECU force plotted against joint angle. Top – isometric. Middle – isokinetic at 60°/s. Bottom – isokinetic at 300°/s.
The mean friction was calculated by averaging across all three cycles of motion. Friction was compared between the intact and the injured condition using a paired t-test. The level of significance was set at 0.05.
The ECU moment producing capacity was increased following injury (Fig. 3). The difference decreased with the amount of wrist extension and with increasing velocity. The smallest increase in moment was seen at 60° of extension at 300°/s. The difference in moment here was nearly 30 Nmm.
3. Results 3.1. Function
3.2. Friction
The ECU force was reduced following injury (Fig. 2). The difference increased with the amount of wrist extension and with increasing velocity. The largest decrease in force was seen at 60° of extension at 300°/s. The difference in force here was less than 4 N.
Friction during flexion/extension was slightly decreased following TFCC injury. The difference was small in absolute terms, but was statistically significant and was seen in all six specimens. Friction was unchanged following TFCC injury for radial-ulnar deviation (Table 2).
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Z.J. Domire et al. / Clinical Biomechanics 24 (2009) 807–811
Fig. 3. ECU moment plotted against joint angle. Top – isometric. Middle – isokinetic at 60°/s. Bottom – isokinetic at 300°/s.
Table 2 Mean friction.
Flexion/extension* Radial-ulnar deviation *
Intact
Injury
Mean 0.18 N (SD 0.11 N) Mean 0.21 N (SD 0.10 N)
Mean 0.13 N (SD 0.09 N) Mean 0.21 N (SD 0.13 N)
Indicates significant difference between intact and injured conditions.
Individual differences would result in minor differences to the output of the model. However, it is unlikely that any individuals would see a decrease in ECU function as a result of TFCC injury. The frictional testing performed did not account for increased motion of the ulnar head as a result of distal radioulnar joint (DRUJ) instability, which has been previously shown to increase friction acting on the tendon (Tanaka et al., 2006). However, in cases where significant DRUJ instability is present this alone should warrant TFCC repair.
4. Discussion 5. Conclusions The reduction in ECU force, as a result of TFCC injury, was small even near full extension and at relatively high velocities. Function, however, is determined by moment producing capacity not force production. The increased moment arm as a result of injury was more than enough to overcome the small loss in force. ECU moment producing capacity was actually increased following injury to the TFCC. As a result of TFCC injury friction, a small decrease in friction on the ECU tendon was seen during flexion/ extension. TFCC injury had no effect on friction during radial-ulnar deviation. The reason behind the small force decrease following TFCC injury is related to the ratio of the optimum fiber length to the moment arm of the muscle. This ratio has been previously identified as an important factor to consider when performing tendon transfer (Lieber et al., 1993; Zajac, 1992), as it is highly important for muscle function. Assuming an optimal fiber length of 58.8 mm (Loren and Lieber, 1995) and a moment arm of 4.8 mm (Tang et al., 1998) this ratio is found to be very large (12.25) for the ECU. Consequences of a high optimum fiber length to the moment arm ratio are very flat relationships between muscle and joint angle as well as muscle force and joint angular velocity. While injury to the TFCC does decrease this ratio, it is still large (9.48) following injury. The model used here is relatively simple and the parameters for the model are reflective of an average person not every individual.
While injury to the TFCC does result in decreased ECU force producing capacity, moment producing capacity is increased. Additionally, as a result of TFCC injury, the friction acting on the ECU tendon was decreased during flexion/extension movements and unchanged during radial-ulnar deviation. There are many valid reasons for deciding to perform a TFCC repair, including pain and instability; however, neither ECU function nor friction should be a consideration when deciding on TFCC repair. Conflict of interest None of the authors have a professional or financial conflict of interest to disclose. Acknowledgments This study was supported by a grant from the Mayo Foundation. We would like to thank Larry Berglund and Chunfeng Zhao for their technical assistance. One author (FK) was supported TUBITAK (The Scientific and Technological Research Council of Turkey).
Z.J. Domire et al. / Clinical Biomechanics 24 (2009) 807–811
References Bobbert, M.F., Harlaar, J., 1993. Evaluation of moment-angle curves in isokinetic knee extension. Med. Sci. Sports Exerc. 25, 251–259. Challis, J.H., 2004. Examination of the scaling of human jumping. J. Strength Cond. Res. 18, 803–809. Domire, Z.J., Challis, J.H., 2007. The influence of squat depth on maximal vertical jump performance. J. Sports Sci. 25, 193–200. Lieber, R.L., 1993. Skeletal muscle architecture: implications for muscle function and surgical tendon transfer. J. Hand Ther. 6, 105–113. Loren, G.J., Lieber, R.L., 1995. Tendon biomechanical properties enhance human wrist muscle specialization. J. Biomech. 28, 791–799.
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Palmer, A.K., Levinsohn, E.M., Kuzma, G.R., 1983. Arthrography of the wrist. J. Hand Surg. [Am] 8, 15–23. Tanaka, T., Amadio, P.C., Zhao, C., Zobitz, M.E., An, K.N., 2006. Effect of wrist and ulna head position on gliding resistance of the extensor digitorum minimi and extensor digitorum communis III tendons: a cadaver study. J. Orthop. Res. 24, 757–762. Tang, J.B., Ryu, J., Kish, V., 1998. The triangular fibrocartilage complex: an important component of the pulley for the ulnar wrist extensor. J. Hand Surg. [Am] 23, 986–991. Zajac, F.E., 1992. How musculotendon architecture and joint geometry affect the capacity of muscles to move and exert force on objects: a review with application to arm and forearm tendon transfer design. J. Hand Surg. [Am] 17, 799–804.
Contents lists available at ScienceDirect
Clinical Biomechanics journal homepage: www.elsevier.com/locate/clinbiomech
The effect of triangular fibrocartilage complex injury on extensor carpi ulnaris function and friction Zachary J. Domire a,*, Furkan E. Karabekmez a, Ahmet Duymaz a, Timothy S. Rutar b, Peter C. Amadio c, Steven L. Moran d a
Biomechanics Laboratory, Division of Orthopedic Research, Mayo Clinic, Rochester, MN, USA Washington State University, College of Veterinary Medicine, Pullman, WA, USA Departments of Orthopedics and Biomedical Engineering, Mayo Clinic, Rochester, MN, USA d Departments of Orthopedics and Plastic Surgery, Mayo Clinic, Rochester, MN, USA b c
a r t i c l e
i n f o
Article history: Received 23 February 2009 Accepted 3 August 2009
Keywords: Triangular fibrocartilage complex Extensor carpi ulnaris Friction Biomechanics Simulation
a b s t r a c t Background: It has been previously shown that injury to the triangular fibrocartilage complex increases the moment arm of the extensor carpi ulnaris. This will reduce the force producing capacity of the muscle in some situations, but will also increase its mechanical advantage. It is also possible that the change in the tendon path may increase tendon friction, predisposing the patient to future repetitive motion injury. It is the purpose of this study to determine the effects of triangular fibrocartilage complex injury on extensor carpi ulnaris moment producing capacity and tendon friction. Methods: A simple simulation was used to examine muscle moment producing capacity throughout the range of motion, at varying speeds and in both injured and healthy states. Six fresh frozen human cadaveric wrists were used to determine the effect of injury on tendon friction. A custom made device was used to move the wrists through a range of motion, while a constant force was applied to the proximal tendon and force was recorded at the distal tendon. Friction was measured before and after the creation of injury. Findings: The decreases in muscle force following injury were small, even in the worse case. The moment producing capacity of the muscle was increased following injury. Tendon friction during flexion–extension was decreased following injury. The friction during radial-ulnar deviation was unchanged. Interpretation: When making surgical decisions about triangular fibrocartilage complex repair, it is not necessary to consider extensor carpi ulnaris moment producing capacity or tendon friction. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Injury to the triangular fibrocartilage complex (TFCC) is a frequently occurring wrist injury (Palmer et al., 1983). It can occur in isolation, but more commonly occurs in conjunction with other structures in the distal radioulnar joint. Treatment options following injury vary widely. It has been proposed that extensor carpi ulnaris (ECU) function is an important consideration when making surgical decisions about TFCC repair (Tang et al., 1998). Tang and associates (1998) showed that the TFCC acts as a retinacular component for the ECU tendon. They examined the moment arm of the ECU throughout a range of wrist extensions in eight cadaver forearms. They then simulated an injury to the TFCC and examined the changes in moment arm. Following injury to the TFCC bowstringing of the ECU occurs during extension and as a result the moment arm of the ECU is increased by nearly 30%. * Corresponding author. Address: 200 First Street, Mayo Clinic College of Medicine, SW, Rochester, MN 55905, USA. E-mail address: [email protected] (Z.J. Domire). 0268-0033/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.clinbiomech.2009.08.001
Increasing the moment arm of the ECU will adversely effect force production in two ways. First, the excursion of the muscle will be larger over the same range of motion. This will result in the muscle operating over a larger portion of the force length curve and decrease force production primarily near full extension of the wrist. Second, for a given angular velocity of the wrist the muscle velocity will be larger. This will result in reduced muscle force whenever the muscle is shortening. Although increasing the moment arm of the ECU will adversely effect its force production, muscle function is determined by muscle moment not muscle force. Increasing the moment arm of the ECU will increase the mechanical advantage of the muscle and therefore increase the maximum moment the muscle can produce. It is also possible that TFCC injury may increase friction acting on the ECU tendon. This increased friction may cause additional wear on the surface of the tendon, which would further increase friction. The resulting vicious cycle of tendon damage could lead to tendonitis, tendon injury and pain. The relationship between increased moment arm and muscle function is clearly a complex one. The following study will examine
808
Z.J. Domire et al. / Clinical Biomechanics 24 (2009) 807–811
the effects of increasing the moment arm of the ECU on moment producing capacity of the muscle throughout the range of motion and at a range of different joint velocities. It is hypothesized that ECU function will be impaired only near maximum wrist extension and at high joint velocities. The following study will also examine the effects of TFCC injury on ECU tendon friction. It is hypothesized that ECU friction will be increased following TFCC injury and therefore may contribute to ulnar sided wrist pain. 2. Methods 2.1. Function Isometric and isokinetic joint flexion–extension strength curves were simulated for the ECU in both injured and non-injured states. Strength curves were simulated for a range of motion from neutral to 60° extension. Based upon data from Tang and colleagues (1998) the moment arm for the ECU was 4.8 mm when the TFCC was intact and 6.2 mm when the TFCC was injured. Radial-ulnar deviation was not simulated as Tang and colleagues showed no change in the ulnar deviation moment arm as a result of TFCC injury. Muscle model parameters (Table 1) were taken from Loren and Lieber (1995) and Challis (2004). The ECU was modeled as a Hill type, muscle like actuator. The muscle had force–length and force–velocity properties and was connected in series with an elastic element representing the tendon. The muscle model can be represented by the following equation:
F m ¼ q F max F l F v where Fm is the force produced by the muscle, q is the normalized active state of the muscle fibers, Fmax is the maximum isometric force the muscle can produce, Fl is the fraction of maximum isometric force the muscle can produce at a given length, Fv is the fraction of maximum isometric force the muscle can produce at a given velocity. For complete details on the muscle model used see Domire and Challis (2007). It was assumed that active state was maximal during all simulations; therefore q was equal to 1. This results in the largest possible difference between injured and healthy for a given position and velocity. Isokinetic simulations were performed at 60°/s and 300°/s. Simulation procedure was similar to Bobbert and Harlaar (1993). Joint angular velocities were constant throughout each of the simulations. 2.2. Friction Six fresh frozen human cadaveric upper extremities (all males, mean age 86.3 ± 8.7 years), amputated 15 cm proximal to the wrist joint were used for this study. Specimens were stored in a freezer at 20 °C, thawed at room temperature immediately prior to test-
Table 1 Summary of the muscle model parameters. Fmax Lfopt w Ltr c Vmax K
51.5 N 58.8 mm 0.56 215.1 mm 0.04 mm/mm 6 Lfopt/sec 4
Note: Fmax – maximum isometric force, Lfopt – optimal fiber length, w – spread of the force–length curve, Ltr – resting length of tendon, c – tendon strain under maximum isometric force, Vmax – maximum unloaded shortening velocity, K – force–velocity curvature constant.
ing and examined to exclude gross pathological evidence of injuries or major degenerative changes around the wrist. Friction was tested during flexion/extension and radial-ulnar deviation movements with the TFCC intact and then damaged. Frictional testing was performed by applying a constant load to the proximal tendon and measuring the load at the distal tendon while moving through a range of motion. To measure the load at the distal tendon, the ECU tendon was detached from its insertion and 15 mm of the fifth metacarpal was excised, after first ensuring stability of the remaining bone by driving a K-wire into distal heads of the 2–5th metacarpals. A load cell (custom made, full bridge, simple cantilever design) was attached to the remaining bone and the tendon was reattached to this load cell. Care was taken to maintain the direction of force application. The load to the proximal tendon was applied by a 200 g deadweight. A low friction pulley was used to apply the deadweight force in a horizontal direction. A custom made motorized apparatus was used to move the specimens (Fig. 1). The proximal ends of the radius and ulna were clamped to the stationary component of the device with the forearm in a supinated position. For testing radial-ulnar deviation, the hand was fixed to the mobile component of the device by clamping a K-wire driven into distal heads of the 2–5th metacarpals to the device. For testing flexion/extension the specimen was turned 90° and the proximal ends of the radius and ulna were re-clamped to the stationary component of the device with the forearm in a supinated position. The hand was fixed to the mobile component of the device by clamping a K-wire passed through the distal head of the third metacarpal perpendicular to the prior K-wire to the device. For each direction of movement, care was taken in the placement of the specimen to ensure the axis of rotation of the wrist joint aligned with the axis of rotation for the testing device. Three cycles of motion were completed for both radial-ulnar deviation and flexion/extension. The direction of motion tested first was randomized. The range of motion was from 35° extension to 40° flexion for flexion/extension and 15° radial to 35° ulnar deviation for radial-unlar deviation. The speed was 5°/s for each test. Force at the distal tendon was measured throughout the motion at 45 Hz. To create a TFCC injury, a 2 cm longitudinal incision was made over the volar side of the ulnocarpal joint. A volar approach was chosen to avoid any inadvertent damage to soft tissues around the ECU, specifically the ECU subsheath. The palmar fascia was incised and dissection was then carried down to the joint capsule, which was incised in line with the skin incision. Triangular fibrocartilage (TFC) was explored to verify that there was no evidence of a pre-existing peripheral tear. Following TFCC examination the attachment to the TFCC to the fovea and peripheral margin of ulna was separated with a scalpel. The ulnolunate ligament and the ulnotriquetral ligament, the palmar and dorsal radial-ulnar ligaments were also incised. The horizontal portion of the TFC was divided to the ulnar styloid but ulnar styloid was left intact. To complete the injury the TFCC’s dorsal edge, which is adherent to the ECU subsheath, was divided carefully to avoid any damage to the ECU tendon. Following the creation of the injury, friction was tested repeating the same procedure as above. Care was taken throughout the testing to ensure the specimens were kept moist. Before calculation of friction the force data were smoothed using a fourth order bi-directional Butterworth filter with a cutoff of 4 Hz. From the filtered force data, friction was calculated at each joint angular position. This was done by calculating the difference between the recorded force and the dead weight force for each direction of movement and then averaging the absolute value of this difference for each direction of movement. This procedure allows for cancelation of any artifacts such as friction in the pulley.
Z.J. Domire et al. / Clinical Biomechanics 24 (2009) 807–811
809
Fig. 1. Sketch of the experimental setup for testing friction during flexion/extension.
Fig. 2. ECU force plotted against joint angle. Top – isometric. Middle – isokinetic at 60°/s. Bottom – isokinetic at 300°/s.
The mean friction was calculated by averaging across all three cycles of motion. Friction was compared between the intact and the injured condition using a paired t-test. The level of significance was set at 0.05.
The ECU moment producing capacity was increased following injury (Fig. 3). The difference decreased with the amount of wrist extension and with increasing velocity. The smallest increase in moment was seen at 60° of extension at 300°/s. The difference in moment here was nearly 30 Nmm.
3. Results 3.1. Function
3.2. Friction
The ECU force was reduced following injury (Fig. 2). The difference increased with the amount of wrist extension and with increasing velocity. The largest decrease in force was seen at 60° of extension at 300°/s. The difference in force here was less than 4 N.
Friction during flexion/extension was slightly decreased following TFCC injury. The difference was small in absolute terms, but was statistically significant and was seen in all six specimens. Friction was unchanged following TFCC injury for radial-ulnar deviation (Table 2).
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Z.J. Domire et al. / Clinical Biomechanics 24 (2009) 807–811
Fig. 3. ECU moment plotted against joint angle. Top – isometric. Middle – isokinetic at 60°/s. Bottom – isokinetic at 300°/s.
Table 2 Mean friction.
Flexion/extension* Radial-ulnar deviation *
Intact
Injury
Mean 0.18 N (SD 0.11 N) Mean 0.21 N (SD 0.10 N)
Mean 0.13 N (SD 0.09 N) Mean 0.21 N (SD 0.13 N)
Indicates significant difference between intact and injured conditions.
Individual differences would result in minor differences to the output of the model. However, it is unlikely that any individuals would see a decrease in ECU function as a result of TFCC injury. The frictional testing performed did not account for increased motion of the ulnar head as a result of distal radioulnar joint (DRUJ) instability, which has been previously shown to increase friction acting on the tendon (Tanaka et al., 2006). However, in cases where significant DRUJ instability is present this alone should warrant TFCC repair.
4. Discussion 5. Conclusions The reduction in ECU force, as a result of TFCC injury, was small even near full extension and at relatively high velocities. Function, however, is determined by moment producing capacity not force production. The increased moment arm as a result of injury was more than enough to overcome the small loss in force. ECU moment producing capacity was actually increased following injury to the TFCC. As a result of TFCC injury friction, a small decrease in friction on the ECU tendon was seen during flexion/ extension. TFCC injury had no effect on friction during radial-ulnar deviation. The reason behind the small force decrease following TFCC injury is related to the ratio of the optimum fiber length to the moment arm of the muscle. This ratio has been previously identified as an important factor to consider when performing tendon transfer (Lieber et al., 1993; Zajac, 1992), as it is highly important for muscle function. Assuming an optimal fiber length of 58.8 mm (Loren and Lieber, 1995) and a moment arm of 4.8 mm (Tang et al., 1998) this ratio is found to be very large (12.25) for the ECU. Consequences of a high optimum fiber length to the moment arm ratio are very flat relationships between muscle and joint angle as well as muscle force and joint angular velocity. While injury to the TFCC does decrease this ratio, it is still large (9.48) following injury. The model used here is relatively simple and the parameters for the model are reflective of an average person not every individual.
While injury to the TFCC does result in decreased ECU force producing capacity, moment producing capacity is increased. Additionally, as a result of TFCC injury, the friction acting on the ECU tendon was decreased during flexion/extension movements and unchanged during radial-ulnar deviation. There are many valid reasons for deciding to perform a TFCC repair, including pain and instability; however, neither ECU function nor friction should be a consideration when deciding on TFCC repair. Conflict of interest None of the authors have a professional or financial conflict of interest to disclose. Acknowledgments This study was supported by a grant from the Mayo Foundation. We would like to thank Larry Berglund and Chunfeng Zhao for their technical assistance. One author (FK) was supported TUBITAK (The Scientific and Technological Research Council of Turkey).
Z.J. Domire et al. / Clinical Biomechanics 24 (2009) 807–811
References Bobbert, M.F., Harlaar, J., 1993. Evaluation of moment-angle curves in isokinetic knee extension. Med. Sci. Sports Exerc. 25, 251–259. Challis, J.H., 2004. Examination of the scaling of human jumping. J. Strength Cond. Res. 18, 803–809. Domire, Z.J., Challis, J.H., 2007. The influence of squat depth on maximal vertical jump performance. J. Sports Sci. 25, 193–200. Lieber, R.L., 1993. Skeletal muscle architecture: implications for muscle function and surgical tendon transfer. J. Hand Ther. 6, 105–113. Loren, G.J., Lieber, R.L., 1995. Tendon biomechanical properties enhance human wrist muscle specialization. J. Biomech. 28, 791–799.
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