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Permeability of the subsynovial connective tissue in the human carpal tunnel: A cadaver study
Clinical Biomechanics 22 (2007) 524–528 www.elsevier.com/locate/clinbiomech
Permeability of the subsynovial connective tissue in the human carpal tunnel: A cadaver study Naoki Osamura, Chunfeng Zhao, Mark E. Zobitz, Kai-Nan An, Peter C. Amadio
*
Biomechanics Laboratory, Division of Orthopedic Research, Mayo Clinic College of Medicine, 200 First Street SW, Rochester, MN 55905, USA Received 7 August 2006; accepted 3 January 2007
Abstract Background. The purpose of this study was to determine the permeability of the normal carpal tunnel subsynovial connective tissue. Methods. Subsynovial connective tissue samples (10 mm2) were obtained from 10 fresh frozen human cadavers without a history of carpal tunnel syndrome. The thickness of the sample was measured using a charge-coupled device laser displacement system. Each specimen was tested for permeability in a closed pressure chamber at 13.8, 41.3, 68.9 and 96.5 kPa. Findings. Since permeated flow was very low in all specimens, the permeability could be calculated only for eight specimens at 96.5 kPa pressure and for three specimens at 68.9 kPa. The mean permeability at 96.5 kPa was mean 0.89 (SD 0.93) · 1014 m4/Ns and at 68.9 kPa was mean 1.04 (SD 1.54) · 1014 m4/Ns. Interpretation. The subsynovial connective tissue is the most characteristic tissue in the carpal tunnel; it is found in no other location in such abundance. It is well known that carpal tunnel syndrome is the result of increased pressure within the carpal tunnel. This lack of permeability in the subsynovial connective tissue may explain the predisposition of this region for pressure buildup and subsequent neuropathy. 2007 Elsevier Ltd. All rights reserved. Keywords: Carpal tunnel syndrome; Permeability; Subsynovial connective tissue
1. Introduction Gelberman et al. (1992) have identified two main classes of tendon, intrasynovial and extrasynovial, which differ significantly in their response to injury and loading. The gliding mechanism of the flexor tendons in the carpal tunnel is a hybrid of the intrasynovial and extrasynovial mechanisms (Ettema et al., 2004). The difference between the typical intra- or extrasynovial tendon and the mixed pattern in the carpal tunnel is the presence of the subsynovial connective tissue (SSCT), which separates the flexor tendons from the visceral synovium of the ulnar bursa. (Ettema et al., 2004) The SSCT contains multiple microvacuoles bordered by collagenous fibers, blood and lymphatic vessels (Guimberteau, 2001). *
Corresponding author. E-mail address: [email protected] (P.C. Amadio).
0268-0033/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.clinbiomech.2007.01.004
Carpal tunnel syndrome (CTS) is the most common entrapment peripheral neuropathy. Although various etiologies, including anatomic anomalies (Leslie and Ruby, 1985; Amadio, 1987), autoimmune or hematologic disorders, arthritis, trauma, and neoplasms (Sidiq et al., 1972; Barfred and Ipsen, 1985; Bardin and Kuntz, 1987; Amadio et al., 1988; Imran and Bainbridge, 1999) have been reported, the most often assigned etiology is idiopathic. The most common physiological alteration associated with CTS is elevated pressure within the carpal tunnel (Gelberman et al., 1981; Luchetti et al., 1989; Okutsu et al., 1989). The root cause of this pressure elevation is unknown. Histologically, non-inflammatory fibrosis of the SSCT has been most commonly observed, and it is believed that SSCT fibrous or edema could increase the volume of the contents in patients with idiopathic CTS (Phalen, 1966; Neal et al., 1987; Lluch, 1992; Tucci et al., 1997; Nakamichi and Tachibana, 1998).
N. Osamura et al. / Clinical Biomechanics 22 (2007) 524–528
Sud et al. (2002) have shown that the SSCT fibrosis, which is typical of CTS, in turn influences the absorptive properties of the SSCT, suggesting that the permeability of the SSCT might be affected in idiopathic CTS. Although many authors have reported the permeability of bone and various soft tissues (Maroudas et al., 1968; Muir et al., 1970; Mansour and Mow, 1976; Li et al., 1987; Mow et al., 1989; Proctor et al., 1989; Joshi et al., 1995), the permeability of the SSCT has not been measured. In this study, we measured the hydrostatic permeability of fresh human cadaver SSCT. 2. Methods 2.1. Specimen preparation For this study, 10 SSCT samples were obtained from 10 fresh frozen cadavers (mean age; 75.3 years, age range; 57– 87 years). The medical records of these individuals were available for review; none mentioned a history of carpal tunnel syndrome. A full thickness, 10 mm2 piece of SSCT adjacent to the volar surface of the flexor digitorum superficialis tendons of the index, long and ring fingers was taken from the intra-carpal tunnel region of each cadaver. 2.2. Testing device Each sample was tested in a custom permeability testing device (Fig. 1), based on a previously described method (Vaughn et al., 2002). This device consists of a pressure regulated chamber connected by plastic tubing to a collecting reservoir. This tubing was interrupted with a mounting
525
clamp that held the SSCT sample and maintained a closed pressurized system. The mounting clamp consisted of two metal parts with a 5 mm diameter circular hole at the center for a perfusion fluid flow and with three screw holes circumferentially. The contact area on both sides of the specimen was covered with rubber seal to prevent the permeating fluid from leaking through the mounting clump. Specimens were mounted on a flat, thin metal mesh (thickness; 100 lm, pore size; 100 lm), keeping their original size in order to prevent the specimens from sagging or breaking under the fluid pressure. Each specimen was mounted on the mounting clamp with the visceral synovial surface of the bursa facing toward the pressure regulated chamber. The mounting-clamp was locked tightly with the three screws to prevent any leaks. The pressure chamber was filled with 90 ml of phosphate buffered saline. The phosphate buffered saline that was pushed through the sample was collected in the collecting reservoir. The weight of the fluid in the collecting reservoir was measured with a load transducer which is accurate to 0.025 g (GSO-50, Transducer Techniques, Temecula, CA, USA) at 1 min intervals for 1 h. During testing, the temperature of the room and the perfusion fluid were kept at 25 C. The volumetric flow rate was determined by calculating the regression coefficient of the weight of the collected fluid versus time. The tests were performed on each specimen at four different pressures, 13.8, 41.3, 68.9 and 96.5 kPa, for each specimen. 2.3. SSCT thickness measurement The thickness of the specimen was measured before the trials, with a charge-coupled device (CCD) laser displace-
Fig. 1. Schematic representation of the experimental system for pressurization and measuring the volumetric flow rate (left). Details of the mounting clamp and set-up for the specimen (right). Each specimen is mounted on the metal mesh and then sandwiched with the mounting clamp. The mounting clamp is locked with three screws. PC, personal computer; PBS, phosphate buffered saline.
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N. Osamura et al. / Clinical Biomechanics 22 (2007) 524–528
ment sensor (LK-081, Keyence Corporation, Osaka, Japan). In this system, the CCD detects the peak intensity of the laser beam spot and identifies this as the target position. Each sample was mounted on a flat table, keeping its original shape. Because the SSCT is translucent, a uniform, 5 mm diameter circular filter paper with a thickness of 100 lm was placed on the surface of the moistened specimen to be measured. The moistened paper was placed carefully in order to avoid air bubbles between the paper and the surface of the specimen. The surface of the mounting table was set to the 0 position and then the laser beam was emitted perpendicular to the sample surface (Fig. 2a). The thickness was measured at five different points on the sample and the averaged data were considered as a thickness of each sample after subtracting the paper thickness (Fig. 2b). 2.4. Calculation of permeability The intrinsic permeability of the SSCT was obtained using the following equation:
k ¼ Qh=pA; where k is the intrinsic permeability coefficient (m4/Ns), Q is the volumetric flow rate (m3/s), A represents the surface area of the sample (m2), Dp is the pressure gradient across the membrane (Pa) and h represents the thickness of the sample (m). The volumetric flow rate was obtained by measuring the weight of the fluid permeating through the sample over time, as described above. The pressure gradient across the specimen was controlled using the pressure regulator and was recorded continuously with a pressure sensor system during testing. The thickness of the sample was measured using the CCD laser displacement system described above. 3. Results The permeability was obtained with a 96.5 kPa pressure gradient for eight specimens. For the remaining two specimens, no volumetric flow permeating through the specimen was detected under any pressure gradient up to 96.5 kPa. For three of these specimens, permeability was also measured at a 68.9 kPa pressure gradient. At 13.8 and 41.3 kPa pressure gradient, the permeability could not be calculated for any the specimens because there was no measurable fluid flow. The intrinsic permeability coefficient was mean 0.89 (SD 0.93) · 1014 m4/Ns (n = 8) at 96.5 kPa and mean 1.04 (SD 1.54) · 1014m4/Ns (n = 2) at 68.9 kPa. The thickness of specimens was mean 0.33 (SD 0.14) mm (n = 10). A summary of these results is given in Table 1. 4. Discussion In carpal tunnel syndrome, the main pathology is due to an elevation in pressure within the carpal tunnel (Gelberman et al., 1981; Luchetti et al., 1989; Okutsu et al., 1989). The cause of this elevation is unclear, but some investigators have suspected abnormal fluid flows in the SSCT (Lluch, 1992; Tucci et al., 1997; Freeland et al., Table 1 Summary of results Sample number
Fig. 2. (a) Schema of the CCD laser displacement system used to measure the thickness of specimen. The specimen is mounted on the flat table which is position-adjustable and the uniform paper (100 lm) is put on the specimen to prevent the laser beam from passing though it. The laser beam is emitted to the surface of specimen perpendicularly and the reflected light is collected by the CCD receiver lens. (b) Schema of the measuring points for the specimen. Five points on the surface being permeated through the fluid are measured with the CCD laser displacement system for calculating the thickness of specimen.
1 2 3 4 5 6 7 8 9 10 Average SD
Sex
F F F M M M M F M F
Age (y)
87 75 57 80 83 78 83 66 74 70 75.3 8.6
Thickness (mm)
0.22 0.50 0.16 0.31 0.29 0.34 0.18 0.51 0.56 0.24 0.33 0.14
Permeability (·1014 m4/Ns) pressure gradient (kPa) 13.8
41.3
68.9
96.5
– – – – – – – – – – NA
– – – – – – – – – – NA
– – – – 2.82 – 0.13 – 0.17 – 1.04 1.54 (n = 3)
0.73 – 1.95 – 2.47 1.83 0.18 0.28 0.43 0.13 0.89 0.93 (n = 8)
NA: not applicable. SD; standard deviation.
N. Osamura et al. / Clinical Biomechanics 22 (2007) 524–528
2002; Sud et al., 2002; Ettema et al., 2004; Hirata et al., 2004; Oh et al., 2004). This study reports an important physical property, the hydrostatic permeability, of normal human carpal tunnel SSCT. The permeability of the SSCT was quite low, even though the normal human SSCT is a thin structure, mean 0.33 (SD 0.14) mm thickness. In other studies, the value of permeability has been reported to be 5.6–35 · 1014 m4/Ns for cortical bone (Li et al., 1987), 0.14–2.0 · 1015 m4/Ns for articular cartilage (Maroudas et al., 1968; Muir et al., 1970; Mansour and Mow, 1976) and 0.81–1.99 · 1015 m4/Ns for meniscus (Proctor et al., 1989; Joshi et al., 1995). We found that the permeability of human SSCT was slightly lower than that of cortical bone and higher than that of cartilage or meniscus. This seems clinically reasonable given the histology of these tissues, and the known low wettability of articular cartilage (Sander and Nauman, 2003). In this study, the pressure gradient added to the specimens was set to 13.8, 41.3, 68.9 and 96.5 kPa. However, the permeability could not be measured for any of the specimens at the lower pressure gradients, 13.8 and 41.3 kPa, because no flow permeated through the specimens over the 1 h testing time. If the testing time were much longer, we might have been able to measure a permeability value for these lower pressure gradients, but then we could not neglect the effect of evaporation of the permeated fluid. Additionally, we did not measure permeability for two specimens even at the highest pressure, 96.5 kPa. While higher pressures could have been pursued, a higher pressure gradient could deform the specimen and decrease its permeability, as described in cartilage permeability studies (Mansour and Mow, 1976; Mow et al., 1989), although this phenomenon was not observed for cortical bone (Li et al., 1987), a much stiffer material. The hydrostatic pressure effect on permeability was not determined in our study. If the pressure effect could be neglected for the SSCT, the permeability could be measured under a much higher pressure gradient. However, above 96.5 kPa the pressure caused the thin and soft tissue to burst. Therefore, there are some limitations in our system to measure soft tissue specimens with lower permeability; especially less than 1 · 1016m4/Ns. A large range of SSCT permeability (0.13–2.47 · 1014 m4/Ns) was observed at the 96.5 kPa pressure gradient, indicating that our series might have included cases with conditions predisposing to idiopathic CTS. How this variability relates to a predisposition to carpal tunnel syndrome is unknown. Factors which might affect this variability are also unknown, but may include differences in SSCT stiffness, age or gender. In our small series, we observed no gender-related difference. However, such a difference might not have been found due to our small sample size. Future studies into factors affecting permeability, and the permeability of SSCT synovium, would be worthwhile. The SSCT contains multiple microvacuoles, collagen, blood and lymph vessels (Guimberteau, 2001). A previous study showed that collagen type III fibers were more abundant in the SSCT of idiopathic CTS patients than in normal
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SSCT, suggesting that the SSCT might be injured in idiopathic CTS (Ettema et al., 2004). Once the SSCT injury occurs, blood and lymph vessels inside of SSCT could also be affected and the vascular permeability in the SSCT could change. Many investigators have reported the pathological vascular changes of the SSCT in idiopathic CTS, and believe the affected vascular change could increase the vascular permeability, cause SSCT edema and eventually increase the intra-carpal tunnel pressure (Neal et al., 1987; Lluch, 1992; Nakamichi and Tachibana, 1998; Freeland et al., 2002; Hirata et al., 2004; Oh et al., 2004). In this study, we found that the SSCT permeability is basically very low, suggesting that the SSCT has a propensity to retain any interstitial fluid which might leak from the abundant blood and lymph vessels within it. Thus, increased vascular permeability within the SSCT could cause SSCT edema and elevated carpal tunnel pressure. Sud et al. (2002) described a change in the absorptive properties of the flexor tenosynovium collected from CTS patients, compared to controls. Although the interstitial fluid movement of human SSCT in vivo has not been specifically studied, their results suggested that the SSCT would be prone to edema in idiopathic CTS. Although we did not measure the SSCT permeability of idiopathic CTS patients, we speculate the SSCT fibrosis noted in CTS may also affect the permeability of the SSCT, and decrease permeability even beyond that noted in our study. Combining all this evidence, it may be reasonable to hypothesize that the movement of the interstitial fluid in the SSCT is impeded by the increased thickness of the collagen fibrils in idiopathic CTS (Oh et al., 2006), which in turn then results in pressure elevation and the characteristic pressure induced neuropathy of CTS. Several authors have reported that the intra-carpal tunnel pressure increases in idiopathic CTS patients (Gelberman et al., 1981; Luchetti et al., 1989; Okutsu et al., 1989). The intra-carpal pressure at the resting position is 13–14 mmHg (1.7–1.8 kPa) in healthy people; however, it is increased up to 26–43 mmHg (3.5–5.7 kPa) in idiopathic CTS patients (Gelberman et al., 1981; Luchetti et al., 1989; Okutsu et al., 1989). Okutsu et al. (1989) reported that the intra-carpal tunnel pressure averaged 206.2 mmHg (27.5 kPa) with a range of 48–250 mmHg (6.4–33.3 kPa) with active grip motion in patients with idiopathic CTS. Based on these reports, it is assumed that the range of the intra-carpal tunnel pressure would be below 33.3 kPa physiologically. We did not detect a permeability for the SSCT at 13.8 kPa or 41.3 kPa pressure gradients, even though these pressure gradients are at or exceed the pathological maximum intra-carpal tunnel pressure. Thus our in vitro study suggests that the magnitude of mechanical pressure experienced within even the pathological carpal tunnel may not be enough to pump interstitial fluid out of the SSCT. The fibrotic alteration in SSCT microarchitecture, combined with interstitial edema, could very easily explain how carpal tunnel pressure rises normally, and why it rises more and stays elevated longer in patients with carpal tunnel syndrome.
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In conclusion, we measured the permeability of human carpal tunnel subsynovial connective tissue (SSCT) in fresh human cadavers. The intrinsic permeability of SSCT was quite low (0.13–2.47 · 1014 m4/Ns at 96.5 kPa pressure gradient), suggesting that the SSCT intrinsically may be prone to edema. This lack of permeability in the SSCT may explain the predisposition of this tissue for pressure buildup and subsequent neuropathy. Acknowledgements This study was funded by grants from NIH (NIAMS AR49823) and Mayo Foundation. References Amadio, P.C., 1987. Bifid median nerve with a double compartment within the transverse carpal canal. J. Hand Surg. 12A, 366–368. Amadio, P.C., Reiman, H.M., Dobyns, J.H., 1988. Lipofibromatous hamartoma of nerve. J. Hand Surg. 13A, 67–75. Bardin, T., Kuntz, D., 1987. The arthropathy of chronic haemodialysis. Clin. Exp. Rheumatol. 5, 379–386. Barfred, T., Ipsen, T., 1985. Congenital carpal tunnel syndrome. J. Hand Surg. 10A, 246–248. Ettema, A.M., Amadio, P.C., Zhao, C., Wold, L.E., An, K.N., 2004. A histological and immunohistochemical study of the subsynovial connective tissue in idiopathic carpal tunnel syndrome. J. Bone Joint Surg. 86A, 1458–1466. Freeland, A.E., Tucci, M.A., Barbieri, R.A., Angel, M.F., Nick, T.G., 2002. Biochemical evaluation of serum and flexor tenosynovium in carpal tunnel syndrome. Microsurgery 22, 378–385. Gelberman, R.H., Hergenroeder, P.T., Hargens, A.R., Lundborg, G.N., Akeson, W.H., 1981. The carpal tunnel syndrome. A study of carpal canal pressures. J. Bone Joint Surg. 63A, 380–383. Gelberman, R.H., Seiler 3rd, J.G., Rosenberg, A.E., Heyman, P., Amiel, D., 1992. Intercalary flexor tendon grafts. A morphological study of intrasynovial and extrasynovial donor tendons.. Scand. J. Plast. Reconstr. Surg. Hand Surg. 26, 257–264. Guimberteau, J.C., 2001. The sliding system. Vascularized flexor tendon transfers, new ideas in hand flexor tendon surgery. Aquitaine Domain Forestier, Bordeaux, France. Hirata, H., Nagakura, T., Tsujii, M., Morita, A., Fujisawa, K., Uchida, A., 2004. The relationship of VEGF and PGE2 expression to extracellular matrix remodelling of the tenosynovium in the carpal tunnel syndrome. J. Pathol. 204, 605–612. Imran, D., Bainbridge, L.C., 1999. Carpal tunnel syndrome after distal release of the flexor digitorum profundus and subsequent retraction of the lumbrical muscle into the carpal tunnel. J. Hand Surg. 24B, 303– 304. Joshi, M.D., Suh, J.K., Marui, T., Woo, S.L., 1995. Interspecies variation of compressive biomechanical properties of the meniscus. J. Biomed. Mater. Res. 29, 823–828. Leslie, B.M., Ruby, L.K., 1985. Congenital carpal tunnel syndrome. A case report. Orthopedics 8, 1165–1167.
Li, G.P., Bronk, J.T., An, K.N., Kelly, P.J., 1987. Permeability of cortical bone of canine tibiae. Microvasc. Res. 34, 302–310. Lluch, A.L., 1992. Thickening of the synovium of the digital flexor tendons: cause or consequence of the carpal tunnel syndrome? J. Hand Surg. 17B, 209–212. Luchetti, R., Schoenhuber, R., De Cicco, G., Alfarano, M., Deluca, S., Landi, A., 1989. Carpal-tunnel pressure. Acta Orthop. Scand. 60, 397– 399. Mansour, J.M., Mow, V.C., 1976. The permeability of articular cartilage under compressive strain and at high pressures. J. Bone Joint Surg. 58A, 509–516. Maroudas, A., Bullough, P., Swanson, S.A., Freeman, M.A., 1968. The permeability of articular cartilage. J. Bone Joint Surg. 50B, 166–177. Mow, V.C., Gibbs, M.C., Lai, W.M., Zhu, W.B., Athanasiou, K.A., 1989. Biphasic indentation of articular cartilage – II. A numerical algorithm and an experimental study. J. Biomech. 22, 853–861. Muir, H., Bullough, P., Maroudas, A., 1970. The distribution of collagen in human articular cartilage with some of its physiological implications. J. Bone Joint Surg. 52B, 554–563. Nakamichi, K., Tachibana, S., 1998. Histology of the transverse carpal ligament and flexor tenosynovium in idiopathic carpal tunnel syndrome. J. Hand Surg. 23A, 1015–1024. Neal, N.C., McManners, J., Stirling, G.A., 1987. Pathology of the flexor tendon sheath in the spontaneous carpal tunnel syndrome. J. Hand Surg. 12B, 229–232. Oh, J., Zhao, C., Amadio, P.C., An, K.N., Zobitz, M.E., Wold, L.E., 2004. Vascular pathologic changes in the flexor tenosynovium (subsynovial connective tissue) in idiopathic carpal tunnel syndrome. J. Orthop. Res. 22, 1310–1315. Oh, J., Zhao, C., Zobitz, M.E., Wold, L.E., An, K.N., Amadio, P.C., 2006. Morphological changes of collagen fibrils in the subsynovial connective tissue in carpal tunnel syndrome. J. Bone Joint Surg. 88A, 824–831. Okutsu, I., Ninomiya, S., Hamanaka, I., Kuroshima, N., Inanami, H., 1989. Measurement of pressure in the carpal canal before and after endoscopic management of carpal tunnel syndrome. J. Bone Joint Surg. 71A, 679–683. Phalen, G.S., 1966. The carpal-tunnel syndrome. Seventeen years’ experience in diagnosis and treatment of six hundred fifty-four hands. J. Bone Joint Surg. 48A, 211–228. Proctor, C.S., Schmidt, M.B., Whipple, R.R., Kelly, M.A., Mow, V.C., 1989. Material properties of the normal medial bovine meniscus. J. Orthop. Res. 7, 771–782. Sander, E.A., Nauman, E.A., 2003. Permeability of musculoskeletal tissues and scaffolding materials: experimental results and theoretical predictions. Crit. Rev. Biomed. Eng. 31, 1–26. Sidiq, M., Kirsner, A.B., Sheon, R.P., 1972. Carpal tunnel syndrome. First manifestation systemic lupus erythematosus. JAMA 222, 1416– 1417. Sud, V., Tucci, M.A., Freeland, A.E., Smith, W.T., Grinspun, K., 2002. Absorptive properties of synovium harvested from the carpal tunnel. Microsurgery 22, 316–319. Tucci, M.A., Barbieri, R.A., Freeland, A.E., 1997. Biochemical and histological analysis of the flexor tenosynovium in patients with carpal tunnel syndrome. Biomed. Sci. Instrum. 33, 246–251. Vaughn, J., Czipura, A., Humphrey, J.D., 2002. Measurement of finitestrain dependent permeability of biomembranes. J. Biomech. 35, 287– 291.
Permeability of the subsynovial connective tissue in the human carpal tunnel: A cadaver study Naoki Osamura, Chunfeng Zhao, Mark E. Zobitz, Kai-Nan An, Peter C. Amadio
*
Biomechanics Laboratory, Division of Orthopedic Research, Mayo Clinic College of Medicine, 200 First Street SW, Rochester, MN 55905, USA Received 7 August 2006; accepted 3 January 2007
Abstract Background. The purpose of this study was to determine the permeability of the normal carpal tunnel subsynovial connective tissue. Methods. Subsynovial connective tissue samples (10 mm2) were obtained from 10 fresh frozen human cadavers without a history of carpal tunnel syndrome. The thickness of the sample was measured using a charge-coupled device laser displacement system. Each specimen was tested for permeability in a closed pressure chamber at 13.8, 41.3, 68.9 and 96.5 kPa. Findings. Since permeated flow was very low in all specimens, the permeability could be calculated only for eight specimens at 96.5 kPa pressure and for three specimens at 68.9 kPa. The mean permeability at 96.5 kPa was mean 0.89 (SD 0.93) · 1014 m4/Ns and at 68.9 kPa was mean 1.04 (SD 1.54) · 1014 m4/Ns. Interpretation. The subsynovial connective tissue is the most characteristic tissue in the carpal tunnel; it is found in no other location in such abundance. It is well known that carpal tunnel syndrome is the result of increased pressure within the carpal tunnel. This lack of permeability in the subsynovial connective tissue may explain the predisposition of this region for pressure buildup and subsequent neuropathy. 2007 Elsevier Ltd. All rights reserved. Keywords: Carpal tunnel syndrome; Permeability; Subsynovial connective tissue
1. Introduction Gelberman et al. (1992) have identified two main classes of tendon, intrasynovial and extrasynovial, which differ significantly in their response to injury and loading. The gliding mechanism of the flexor tendons in the carpal tunnel is a hybrid of the intrasynovial and extrasynovial mechanisms (Ettema et al., 2004). The difference between the typical intra- or extrasynovial tendon and the mixed pattern in the carpal tunnel is the presence of the subsynovial connective tissue (SSCT), which separates the flexor tendons from the visceral synovium of the ulnar bursa. (Ettema et al., 2004) The SSCT contains multiple microvacuoles bordered by collagenous fibers, blood and lymphatic vessels (Guimberteau, 2001). *
Corresponding author. E-mail address: [email protected] (P.C. Amadio).
0268-0033/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.clinbiomech.2007.01.004
Carpal tunnel syndrome (CTS) is the most common entrapment peripheral neuropathy. Although various etiologies, including anatomic anomalies (Leslie and Ruby, 1985; Amadio, 1987), autoimmune or hematologic disorders, arthritis, trauma, and neoplasms (Sidiq et al., 1972; Barfred and Ipsen, 1985; Bardin and Kuntz, 1987; Amadio et al., 1988; Imran and Bainbridge, 1999) have been reported, the most often assigned etiology is idiopathic. The most common physiological alteration associated with CTS is elevated pressure within the carpal tunnel (Gelberman et al., 1981; Luchetti et al., 1989; Okutsu et al., 1989). The root cause of this pressure elevation is unknown. Histologically, non-inflammatory fibrosis of the SSCT has been most commonly observed, and it is believed that SSCT fibrous or edema could increase the volume of the contents in patients with idiopathic CTS (Phalen, 1966; Neal et al., 1987; Lluch, 1992; Tucci et al., 1997; Nakamichi and Tachibana, 1998).
N. Osamura et al. / Clinical Biomechanics 22 (2007) 524–528
Sud et al. (2002) have shown that the SSCT fibrosis, which is typical of CTS, in turn influences the absorptive properties of the SSCT, suggesting that the permeability of the SSCT might be affected in idiopathic CTS. Although many authors have reported the permeability of bone and various soft tissues (Maroudas et al., 1968; Muir et al., 1970; Mansour and Mow, 1976; Li et al., 1987; Mow et al., 1989; Proctor et al., 1989; Joshi et al., 1995), the permeability of the SSCT has not been measured. In this study, we measured the hydrostatic permeability of fresh human cadaver SSCT. 2. Methods 2.1. Specimen preparation For this study, 10 SSCT samples were obtained from 10 fresh frozen cadavers (mean age; 75.3 years, age range; 57– 87 years). The medical records of these individuals were available for review; none mentioned a history of carpal tunnel syndrome. A full thickness, 10 mm2 piece of SSCT adjacent to the volar surface of the flexor digitorum superficialis tendons of the index, long and ring fingers was taken from the intra-carpal tunnel region of each cadaver. 2.2. Testing device Each sample was tested in a custom permeability testing device (Fig. 1), based on a previously described method (Vaughn et al., 2002). This device consists of a pressure regulated chamber connected by plastic tubing to a collecting reservoir. This tubing was interrupted with a mounting
525
clamp that held the SSCT sample and maintained a closed pressurized system. The mounting clamp consisted of two metal parts with a 5 mm diameter circular hole at the center for a perfusion fluid flow and with three screw holes circumferentially. The contact area on both sides of the specimen was covered with rubber seal to prevent the permeating fluid from leaking through the mounting clump. Specimens were mounted on a flat, thin metal mesh (thickness; 100 lm, pore size; 100 lm), keeping their original size in order to prevent the specimens from sagging or breaking under the fluid pressure. Each specimen was mounted on the mounting clamp with the visceral synovial surface of the bursa facing toward the pressure regulated chamber. The mounting-clamp was locked tightly with the three screws to prevent any leaks. The pressure chamber was filled with 90 ml of phosphate buffered saline. The phosphate buffered saline that was pushed through the sample was collected in the collecting reservoir. The weight of the fluid in the collecting reservoir was measured with a load transducer which is accurate to 0.025 g (GSO-50, Transducer Techniques, Temecula, CA, USA) at 1 min intervals for 1 h. During testing, the temperature of the room and the perfusion fluid were kept at 25 C. The volumetric flow rate was determined by calculating the regression coefficient of the weight of the collected fluid versus time. The tests were performed on each specimen at four different pressures, 13.8, 41.3, 68.9 and 96.5 kPa, for each specimen. 2.3. SSCT thickness measurement The thickness of the specimen was measured before the trials, with a charge-coupled device (CCD) laser displace-
Fig. 1. Schematic representation of the experimental system for pressurization and measuring the volumetric flow rate (left). Details of the mounting clamp and set-up for the specimen (right). Each specimen is mounted on the metal mesh and then sandwiched with the mounting clamp. The mounting clamp is locked with three screws. PC, personal computer; PBS, phosphate buffered saline.
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ment sensor (LK-081, Keyence Corporation, Osaka, Japan). In this system, the CCD detects the peak intensity of the laser beam spot and identifies this as the target position. Each sample was mounted on a flat table, keeping its original shape. Because the SSCT is translucent, a uniform, 5 mm diameter circular filter paper with a thickness of 100 lm was placed on the surface of the moistened specimen to be measured. The moistened paper was placed carefully in order to avoid air bubbles between the paper and the surface of the specimen. The surface of the mounting table was set to the 0 position and then the laser beam was emitted perpendicular to the sample surface (Fig. 2a). The thickness was measured at five different points on the sample and the averaged data were considered as a thickness of each sample after subtracting the paper thickness (Fig. 2b). 2.4. Calculation of permeability The intrinsic permeability of the SSCT was obtained using the following equation:
k ¼ Qh=pA; where k is the intrinsic permeability coefficient (m4/Ns), Q is the volumetric flow rate (m3/s), A represents the surface area of the sample (m2), Dp is the pressure gradient across the membrane (Pa) and h represents the thickness of the sample (m). The volumetric flow rate was obtained by measuring the weight of the fluid permeating through the sample over time, as described above. The pressure gradient across the specimen was controlled using the pressure regulator and was recorded continuously with a pressure sensor system during testing. The thickness of the sample was measured using the CCD laser displacement system described above. 3. Results The permeability was obtained with a 96.5 kPa pressure gradient for eight specimens. For the remaining two specimens, no volumetric flow permeating through the specimen was detected under any pressure gradient up to 96.5 kPa. For three of these specimens, permeability was also measured at a 68.9 kPa pressure gradient. At 13.8 and 41.3 kPa pressure gradient, the permeability could not be calculated for any the specimens because there was no measurable fluid flow. The intrinsic permeability coefficient was mean 0.89 (SD 0.93) · 1014 m4/Ns (n = 8) at 96.5 kPa and mean 1.04 (SD 1.54) · 1014m4/Ns (n = 2) at 68.9 kPa. The thickness of specimens was mean 0.33 (SD 0.14) mm (n = 10). A summary of these results is given in Table 1. 4. Discussion In carpal tunnel syndrome, the main pathology is due to an elevation in pressure within the carpal tunnel (Gelberman et al., 1981; Luchetti et al., 1989; Okutsu et al., 1989). The cause of this elevation is unclear, but some investigators have suspected abnormal fluid flows in the SSCT (Lluch, 1992; Tucci et al., 1997; Freeland et al., Table 1 Summary of results Sample number
Fig. 2. (a) Schema of the CCD laser displacement system used to measure the thickness of specimen. The specimen is mounted on the flat table which is position-adjustable and the uniform paper (100 lm) is put on the specimen to prevent the laser beam from passing though it. The laser beam is emitted to the surface of specimen perpendicularly and the reflected light is collected by the CCD receiver lens. (b) Schema of the measuring points for the specimen. Five points on the surface being permeated through the fluid are measured with the CCD laser displacement system for calculating the thickness of specimen.
1 2 3 4 5 6 7 8 9 10 Average SD
Sex
F F F M M M M F M F
Age (y)
87 75 57 80 83 78 83 66 74 70 75.3 8.6
Thickness (mm)
0.22 0.50 0.16 0.31 0.29 0.34 0.18 0.51 0.56 0.24 0.33 0.14
Permeability (·1014 m4/Ns) pressure gradient (kPa) 13.8
41.3
68.9
96.5
– – – – – – – – – – NA
– – – – – – – – – – NA
– – – – 2.82 – 0.13 – 0.17 – 1.04 1.54 (n = 3)
0.73 – 1.95 – 2.47 1.83 0.18 0.28 0.43 0.13 0.89 0.93 (n = 8)
NA: not applicable. SD; standard deviation.
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2002; Sud et al., 2002; Ettema et al., 2004; Hirata et al., 2004; Oh et al., 2004). This study reports an important physical property, the hydrostatic permeability, of normal human carpal tunnel SSCT. The permeability of the SSCT was quite low, even though the normal human SSCT is a thin structure, mean 0.33 (SD 0.14) mm thickness. In other studies, the value of permeability has been reported to be 5.6–35 · 1014 m4/Ns for cortical bone (Li et al., 1987), 0.14–2.0 · 1015 m4/Ns for articular cartilage (Maroudas et al., 1968; Muir et al., 1970; Mansour and Mow, 1976) and 0.81–1.99 · 1015 m4/Ns for meniscus (Proctor et al., 1989; Joshi et al., 1995). We found that the permeability of human SSCT was slightly lower than that of cortical bone and higher than that of cartilage or meniscus. This seems clinically reasonable given the histology of these tissues, and the known low wettability of articular cartilage (Sander and Nauman, 2003). In this study, the pressure gradient added to the specimens was set to 13.8, 41.3, 68.9 and 96.5 kPa. However, the permeability could not be measured for any of the specimens at the lower pressure gradients, 13.8 and 41.3 kPa, because no flow permeated through the specimens over the 1 h testing time. If the testing time were much longer, we might have been able to measure a permeability value for these lower pressure gradients, but then we could not neglect the effect of evaporation of the permeated fluid. Additionally, we did not measure permeability for two specimens even at the highest pressure, 96.5 kPa. While higher pressures could have been pursued, a higher pressure gradient could deform the specimen and decrease its permeability, as described in cartilage permeability studies (Mansour and Mow, 1976; Mow et al., 1989), although this phenomenon was not observed for cortical bone (Li et al., 1987), a much stiffer material. The hydrostatic pressure effect on permeability was not determined in our study. If the pressure effect could be neglected for the SSCT, the permeability could be measured under a much higher pressure gradient. However, above 96.5 kPa the pressure caused the thin and soft tissue to burst. Therefore, there are some limitations in our system to measure soft tissue specimens with lower permeability; especially less than 1 · 1016m4/Ns. A large range of SSCT permeability (0.13–2.47 · 1014 m4/Ns) was observed at the 96.5 kPa pressure gradient, indicating that our series might have included cases with conditions predisposing to idiopathic CTS. How this variability relates to a predisposition to carpal tunnel syndrome is unknown. Factors which might affect this variability are also unknown, but may include differences in SSCT stiffness, age or gender. In our small series, we observed no gender-related difference. However, such a difference might not have been found due to our small sample size. Future studies into factors affecting permeability, and the permeability of SSCT synovium, would be worthwhile. The SSCT contains multiple microvacuoles, collagen, blood and lymph vessels (Guimberteau, 2001). A previous study showed that collagen type III fibers were more abundant in the SSCT of idiopathic CTS patients than in normal
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SSCT, suggesting that the SSCT might be injured in idiopathic CTS (Ettema et al., 2004). Once the SSCT injury occurs, blood and lymph vessels inside of SSCT could also be affected and the vascular permeability in the SSCT could change. Many investigators have reported the pathological vascular changes of the SSCT in idiopathic CTS, and believe the affected vascular change could increase the vascular permeability, cause SSCT edema and eventually increase the intra-carpal tunnel pressure (Neal et al., 1987; Lluch, 1992; Nakamichi and Tachibana, 1998; Freeland et al., 2002; Hirata et al., 2004; Oh et al., 2004). In this study, we found that the SSCT permeability is basically very low, suggesting that the SSCT has a propensity to retain any interstitial fluid which might leak from the abundant blood and lymph vessels within it. Thus, increased vascular permeability within the SSCT could cause SSCT edema and elevated carpal tunnel pressure. Sud et al. (2002) described a change in the absorptive properties of the flexor tenosynovium collected from CTS patients, compared to controls. Although the interstitial fluid movement of human SSCT in vivo has not been specifically studied, their results suggested that the SSCT would be prone to edema in idiopathic CTS. Although we did not measure the SSCT permeability of idiopathic CTS patients, we speculate the SSCT fibrosis noted in CTS may also affect the permeability of the SSCT, and decrease permeability even beyond that noted in our study. Combining all this evidence, it may be reasonable to hypothesize that the movement of the interstitial fluid in the SSCT is impeded by the increased thickness of the collagen fibrils in idiopathic CTS (Oh et al., 2006), which in turn then results in pressure elevation and the characteristic pressure induced neuropathy of CTS. Several authors have reported that the intra-carpal tunnel pressure increases in idiopathic CTS patients (Gelberman et al., 1981; Luchetti et al., 1989; Okutsu et al., 1989). The intra-carpal pressure at the resting position is 13–14 mmHg (1.7–1.8 kPa) in healthy people; however, it is increased up to 26–43 mmHg (3.5–5.7 kPa) in idiopathic CTS patients (Gelberman et al., 1981; Luchetti et al., 1989; Okutsu et al., 1989). Okutsu et al. (1989) reported that the intra-carpal tunnel pressure averaged 206.2 mmHg (27.5 kPa) with a range of 48–250 mmHg (6.4–33.3 kPa) with active grip motion in patients with idiopathic CTS. Based on these reports, it is assumed that the range of the intra-carpal tunnel pressure would be below 33.3 kPa physiologically. We did not detect a permeability for the SSCT at 13.8 kPa or 41.3 kPa pressure gradients, even though these pressure gradients are at or exceed the pathological maximum intra-carpal tunnel pressure. Thus our in vitro study suggests that the magnitude of mechanical pressure experienced within even the pathological carpal tunnel may not be enough to pump interstitial fluid out of the SSCT. The fibrotic alteration in SSCT microarchitecture, combined with interstitial edema, could very easily explain how carpal tunnel pressure rises normally, and why it rises more and stays elevated longer in patients with carpal tunnel syndrome.
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