Force distribution across wrist joint: Application of pressure-sensitive conductive rubber A new pressure-sensitive conductive rubber sensor was used for investigation of the pressure distribution through the radio-ulno-carpal joint. Twelve of these transducers were placed in the radio-ulno-carpal joint. Pressure was measured in seven different wrist positions under loads incrementally increasing from 0 to 12 kg. Half of the sensors showed less than 0.5 MPa, even at maximum load, while a high-pressure area was located palmarly in each fossa. The peak pressure measured in the wrist neutral position was 2.4 MPa on the scaphoid fossa, 1.5 MPa on the lunate fossa, and 1.1 MPa on the triangular fibrocartilage with a 10 kg load. The peak pressure ratio between the scaphoid and the lunate was 1.7 in the neutral wrist position. This increased in radial deviation to 2.9 and decreased in ulnar deviation to 0.8. The force-transmission ratio was 50% through the scaphoid fossa, 35% through the lunate fossa, and 15% through the triangular fibrocartilage in the neutral position. The advantage of this sensor is that it is thin and flexible and provides reliable reproducible quasi-instantaneous measurements. (J HANDSURG 1992;17A:339-47.)
Toshiaki Hara, PhD, Emiko Horii, MD, Kai-Nan An, PhD, William P. Cooney, MD, Ronald L. Linscheid, MD, and Edmund Y. S. Chao, PhD, Rochester, Minn.
A
n understanding of normal joint mechanics is necessary if one is to comprehend the mechanisms of injury, the pathogenesis of degenerative arthritis, the implication of malangulated fractures, and the design of joint prostheses. In the wrist joint, analysis of force transmission is especially important when one is considering malunions of the distal radius, Kienbock’s disease, ulnar impingement problems, carpal instability, and the results of various operative procedures. A review of the literature reveals several studies that have analyzed force distribution across the wrist joint. ‘-x Of current techniques used to measure force
From the Biomechanics Laboratory and the Department of Orthopedics. Mayo Clinic/Mayo Foundation, Rochester, Minn. Received for publication July 18. 1991.
Feb. 22, 1991; accepted
in revised form
Although none of the authors have received or will receive benetits for personal or professional use from a commercial party related directly or indirectly to the subject of this article, benefits have been or will be received but are directed solely to a research fund, foundation. educational institution. or other nonprofit organization with which one or more of the authors are associated. Reprint requests: Kai-Nan An, PhD, Biomechanics Laboratory, Clinic/Mayo Foundation, Rochester, MN 55905. 311133283
Mayo
across the wrist joint, one is experimental’-’ and the other theoretical.* Experimentally, Fuji filrn,le5 strain gauge, 6*’ and small load cells’ have been used for force and pressure measurements. Each technique has several limitations with respect to accurate measurement of intercarpal pressure, in large part because of the complexity of the joint. Problems in direct pressure measurement have led to the second approach-theoretical analysis of wrist forces with the use of a “rigid body spring model.“’ Theoretical methods require experimental and clinical validation, limiting their direct clinical application, Recently, a new pressure-sensitive conductive rubber sensor (CS57-7RSC. Yokohama Rubber Co., Ltd., Tokyo) (PSR) has been developed and evaluated at Niigata University in Japan (Hara T, Sasagawa K, Koga Y. First World Congress of Biomechanics, San Diego, Calif., September 1990). This transducer has several advantages: it is thin, it is flexible, and it is capable of quasi-instantaneous measurements. These measurements can be performed without changing the loading configuration or replacing the transducer, both of which are necessary with Fuji film. This sensor, therefore, has the potential of providing accurate direct joint pressure measurements under a variety of loading conditions as well as simulated wrist disease. The purpose of the present study is to evaluate the
THE
JOURNAL
OF HAND
SURGERY
339
340
The Journal of HAND SURGERY
Hara et al.
Calibration Voltage
Voltage
10.0
10.0
8.0
/-
6.0
l .
/
4.0 /
8.5 2.0
0.2
0.4
0.6
a. Pressure,
0.0
1.0
MPa
1.0
f.5
2.0
2.5
b. Pressure,
3.0
3.5
4.0
MPa
Fig. 1. Calibration curves obtained in oil chamber. a, Calibration curves with range of 0 to 1.2 MPa of applied pressure. b, Calibration with range of 1.0 to 4.0 MPa.
applicability of this new technique in the investigation of force distribution in the normal wrist joint and to compare the results with those achieved with other methods.
Materials and methods Pressure-sensitive conductive rubber sensor (PSR). The mechanism of differential electrical conductivity of PSR was based on the distributed volume of carbon black within the rubber. Such characteristics as creep, durability, and load application rate of the rubber had already been thoroughly investigated. Because of its flexibility, this sensor could be applied onto concave surfaces such as the articular surface of the distal radius. The PSR with the leading wire soldered was sealed with plastic film for waterproofing. The entire device was 0.9 mm thick. The size of PSR used for this experiment varied from 3 X 4 mm2 to 4 X 5 mm’. The sensors were subjected to pressures of 0 to 4.0 MPa (1 MPa = 1 newton/mm’ = 0.102 kg/mm*), and calibration curves of high reliability were obtained. Typical characteristic loading curves of this transducer calibrated at two different pressure ranges are shown in Fig. 1.
The reproducibility was also tested for three sensors. Measurements were repeated 10 times for each sensor at each pressure IeveI. The reproducibility was expressed by subtracting the mean value of 10 repetitions from each absolute value. There was no significant difference in reproducibility at each pressure level, or when pressures among the three sensors were compared. The average voltage difference from the mean value for 330 measurements was 0.002 t 0.001 volt (mean + 1 SD) with a range of 0 to 0.006 volt. Test procedure. Six fresh frozen cadaver limbs were prepared for this study. Before the experiment, anteroposterior and lateral radiographs of the wrist were obtained to confirm freedom from visible deformities and to measure the carpal angles and ulnar variance. There were three right and three left wrists, with a mean ulnar variance’ of 0 mm (range, - 2 to + 2), with 22 degrees of radial incIination. In each specimen, the triangular fibrocartilage (TFC) showed some amount of degenerative change”; two TFCs were intact, three had a slit at the radial attachment, and one had a small central tear. The radiocarpal joint was opened through the dorsal capsule between the dorsal radiotriquetral and scaphoid
Vol. 17A, No. 2 March 1992
Force distribution
ULNAR
RADIAL
B
at’ross wrisr joint
RADIAL
341
ULNAR
DORSAL
Fig. 2. The twelve sensors placed in the radio-&o-carpal joint. A, Sensors in the radio-ulnocarpal joint. S, Scaphoid; L, lunate; Tq, triquetrum. B, Drawing of sensors shows position of sensors related to radio-ulno-carpal joint. The numbers in the figure represent the sensor numbers. C, Typical pressure distribution pattern recorded with wrist in neutral position under 10 kg of axial
loading. < 0.5 MPa;
0.5-l .o;
1.0-1.5;
ligaments. Care was taken to prevent damage to the radiotriquetral ligament. Twelve custom-made PSRs were prepared for each radio-ulno-carpal joint (Fig. 2), so that the combination of smaller sensors would precisely cover the scaphoid and lunate fossae of the distal radius and the TFC. The sensors were firmly sutured through the joint capsules and radiocarpal ligaments. In addition, one PSR was placed into the distal radioulnar joint proximal to the TFC. The position of the PSRs was confirmed by x-ray films (Fig. 3). A Steinmann pin was driven through the third metacarpal to control wrist position. Stay sutures were placed in five wrist motor tendons-extensor carpi radialis longus (ECRL), extensor carpi radialis brevis (ECRB), extensor carpi ulnaris (ECU), flexor carpi radialis (FCR), and flexor carpi ulnaris (FCU)-the finger flexor tendons, and the flexor pollicis longus tendon for controlled force application across the radiocarpal joint. The specimen was fixed to a custom-designed wristloading frame by means of Steinmann pins driven through the humerus and the ulna with the elbow in
triquetral
Toshiaki Hara, PhD, Emiko Horii, MD, Kai-Nan An, PhD, William P. Cooney, MD, Ronald L. Linscheid, MD, and Edmund Y. S. Chao, PhD, Rochester, Minn.
A
n understanding of normal joint mechanics is necessary if one is to comprehend the mechanisms of injury, the pathogenesis of degenerative arthritis, the implication of malangulated fractures, and the design of joint prostheses. In the wrist joint, analysis of force transmission is especially important when one is considering malunions of the distal radius, Kienbock’s disease, ulnar impingement problems, carpal instability, and the results of various operative procedures. A review of the literature reveals several studies that have analyzed force distribution across the wrist joint. ‘-x Of current techniques used to measure force
From the Biomechanics Laboratory and the Department of Orthopedics. Mayo Clinic/Mayo Foundation, Rochester, Minn. Received for publication July 18. 1991.
Feb. 22, 1991; accepted
in revised form
Although none of the authors have received or will receive benetits for personal or professional use from a commercial party related directly or indirectly to the subject of this article, benefits have been or will be received but are directed solely to a research fund, foundation. educational institution. or other nonprofit organization with which one or more of the authors are associated. Reprint requests: Kai-Nan An, PhD, Biomechanics Laboratory, Clinic/Mayo Foundation, Rochester, MN 55905. 311133283
Mayo
across the wrist joint, one is experimental’-’ and the other theoretical.* Experimentally, Fuji filrn,le5 strain gauge, 6*’ and small load cells’ have been used for force and pressure measurements. Each technique has several limitations with respect to accurate measurement of intercarpal pressure, in large part because of the complexity of the joint. Problems in direct pressure measurement have led to the second approach-theoretical analysis of wrist forces with the use of a “rigid body spring model.“’ Theoretical methods require experimental and clinical validation, limiting their direct clinical application, Recently, a new pressure-sensitive conductive rubber sensor (CS57-7RSC. Yokohama Rubber Co., Ltd., Tokyo) (PSR) has been developed and evaluated at Niigata University in Japan (Hara T, Sasagawa K, Koga Y. First World Congress of Biomechanics, San Diego, Calif., September 1990). This transducer has several advantages: it is thin, it is flexible, and it is capable of quasi-instantaneous measurements. These measurements can be performed without changing the loading configuration or replacing the transducer, both of which are necessary with Fuji film. This sensor, therefore, has the potential of providing accurate direct joint pressure measurements under a variety of loading conditions as well as simulated wrist disease. The purpose of the present study is to evaluate the
THE
JOURNAL
OF HAND
SURGERY
339
340
The Journal of HAND SURGERY
Hara et al.
Calibration Voltage
Voltage
10.0
10.0
8.0
/-
6.0
l .
/
4.0 /
8.5 2.0
0.2
0.4
0.6
a. Pressure,
0.0
1.0
MPa
1.0
f.5
2.0
2.5
b. Pressure,
3.0
3.5
4.0
MPa
Fig. 1. Calibration curves obtained in oil chamber. a, Calibration curves with range of 0 to 1.2 MPa of applied pressure. b, Calibration with range of 1.0 to 4.0 MPa.
applicability of this new technique in the investigation of force distribution in the normal wrist joint and to compare the results with those achieved with other methods.
Materials and methods Pressure-sensitive conductive rubber sensor (PSR). The mechanism of differential electrical conductivity of PSR was based on the distributed volume of carbon black within the rubber. Such characteristics as creep, durability, and load application rate of the rubber had already been thoroughly investigated. Because of its flexibility, this sensor could be applied onto concave surfaces such as the articular surface of the distal radius. The PSR with the leading wire soldered was sealed with plastic film for waterproofing. The entire device was 0.9 mm thick. The size of PSR used for this experiment varied from 3 X 4 mm2 to 4 X 5 mm’. The sensors were subjected to pressures of 0 to 4.0 MPa (1 MPa = 1 newton/mm’ = 0.102 kg/mm*), and calibration curves of high reliability were obtained. Typical characteristic loading curves of this transducer calibrated at two different pressure ranges are shown in Fig. 1.
The reproducibility was also tested for three sensors. Measurements were repeated 10 times for each sensor at each pressure IeveI. The reproducibility was expressed by subtracting the mean value of 10 repetitions from each absolute value. There was no significant difference in reproducibility at each pressure level, or when pressures among the three sensors were compared. The average voltage difference from the mean value for 330 measurements was 0.002 t 0.001 volt (mean + 1 SD) with a range of 0 to 0.006 volt. Test procedure. Six fresh frozen cadaver limbs were prepared for this study. Before the experiment, anteroposterior and lateral radiographs of the wrist were obtained to confirm freedom from visible deformities and to measure the carpal angles and ulnar variance. There were three right and three left wrists, with a mean ulnar variance’ of 0 mm (range, - 2 to + 2), with 22 degrees of radial incIination. In each specimen, the triangular fibrocartilage (TFC) showed some amount of degenerative change”; two TFCs were intact, three had a slit at the radial attachment, and one had a small central tear. The radiocarpal joint was opened through the dorsal capsule between the dorsal radiotriquetral and scaphoid
Vol. 17A, No. 2 March 1992
Force distribution
ULNAR
RADIAL
B
at’ross wrisr joint
RADIAL
341
ULNAR
DORSAL
Fig. 2. The twelve sensors placed in the radio-&o-carpal joint. A, Sensors in the radio-ulnocarpal joint. S, Scaphoid; L, lunate; Tq, triquetrum. B, Drawing of sensors shows position of sensors related to radio-ulno-carpal joint. The numbers in the figure represent the sensor numbers. C, Typical pressure distribution pattern recorded with wrist in neutral position under 10 kg of axial
loading. < 0.5 MPa;
0.5-l .o;
1.0-1.5;
ligaments. Care was taken to prevent damage to the radiotriquetral ligament. Twelve custom-made PSRs were prepared for each radio-ulno-carpal joint (Fig. 2), so that the combination of smaller sensors would precisely cover the scaphoid and lunate fossae of the distal radius and the TFC. The sensors were firmly sutured through the joint capsules and radiocarpal ligaments. In addition, one PSR was placed into the distal radioulnar joint proximal to the TFC. The position of the PSRs was confirmed by x-ray films (Fig. 3). A Steinmann pin was driven through the third metacarpal to control wrist position. Stay sutures were placed in five wrist motor tendons-extensor carpi radialis longus (ECRL), extensor carpi radialis brevis (ECRB), extensor carpi ulnaris (ECU), flexor carpi radialis (FCR), and flexor carpi ulnaris (FCU)-the finger flexor tendons, and the flexor pollicis longus tendon for controlled force application across the radiocarpal joint. The specimen was fixed to a custom-designed wristloading frame by means of Steinmann pins driven through the humerus and the ulna with the elbow in
triquetral
