Musculoskeletal Imaging Original Research

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1 Musculoskeletal Imaging Original Research Chiavaras et al. Ultrasound of the Trapeziometacarpal Joint Musculoskeletal Imaging Original Research Mary M. Chiavaras 1 Srinivasan Harish 1,2 Glen Oomen 3 Terry Popowich 2 ruce Wainman 3 James R. ain 4,5 Chiavaras MM, Harish S, Oomen G, Popowich T, Wainman, ain JR Keywords: anterior oblique ligament, thumb, ultrasound DOI: /JR Received February 4, 2010; accepted after revision May 5, This article was presented as an abstract for scientific oral presentation at the 2010 merican Roentgen Ray Society meeting in San Diego, C. 1 Department of Radiology, McMaster University Medical Centre, Hamilton, ON, Canada. 2 Department of Diagnostic Imaging, St. Joseph s Healthcare, 50 Charlton ve. E, Hamilton, ON L8N 46, Canada. ddress correspondence to S. Harish (sriniharish@gmail.com). 3 Education Program in natomy, McMaster University Medical Centre, Hamilton, ON, Canada. 4 Department of Surgery, McMaster University Medical Centre, Hamilton, ON, Canada. 5 Department of Plastic Surgery, McMaster University Medical Centre, Hamilton, ON, Canada. WE This is a Web exclusive article. JR 2010; 195:W428 W X/10/1956 W428 merican Roentgen Ray Society Sonography of the nterior Oblique Ligament of the Trapeziometacarpal Joint: Study of Cadavers and symptomatic Volunteers OJECTIVE. The purpose of this study was to evaluate the ability of ultrasound to identify and characterize the anterior oblique ligament of the thumb in cadavers and asymptomatic volunteers. SUJECTS ND METHODS. The anterior oblique ligaments of four cadaveric hands were imaged with a high-resolution transducer. The ligaments were then injected with 0.1% methylene blue using ultrasound guidance. To confirm identification of the ligament, the base of the thumb was immediately dissected, revealing the exact location of the dye. The bilateral ligaments in 40 asymptomatic adult volunteers were imaged. RESULTS. Surgical dissection confirmed injection of methylene blue into all cadaveric ligaments. The proximal attachment of the anterior oblique ligament was well defined in all the hands, and the distal attachment was well defined in 94% of the hands. The mean thickness of the anterior oblique ligament at the metacarpal attachment (0.7 mm), midportion (0.98 mm), and trapezial attachment (0.65 mm) did not differ significantly with respect to sex, right and left side, or hand dominance and was weakly correlated with weight, height, body mass index, and age. The length of the ligament was statistically significantly different between the dominant (10.6 mm) and nondominant (9.6 mm) hands. The volar metacarpal translation with palmar abduction stress did not differ significantly between the dominant (0.7 mm) and nondominant (0.8 mm) hands. There was no association between the degree of translation and the biologic characteristics (weight, height, body mass index, and age). CONCLUSION. High-resolution ultrasound can be used to identify and measure the thickness of the anterior oblique ligament. Dynamic ultrasound imaging can depict volar translation of the metacarpal, which may facilitate diagnosis of ligamentous injury. T he anterior oblique ligament is considered to be one of the principal stabilizing ligaments of the trapeziometacarpal joint of the thumb [1 6]. The anterior oblique ligament consists of the superficial anterior oblique ligament and the deep anterior oblique ligament [7, 8] (Figs. 1 1C). The superficial anterior oblique ligament originates from the volar aspect of the trapezium proximal to the articular surface and inserts broadly on the volar tubercle of the first metacarpal [1, 7] (Fig. 1). The deep anterior oblique ligament is seen deep in relation to the superficial anterior oblique ligament and inserts at the articular margins of the trapezium and first metacarpal [7, 8] (Figs. 1 and 1C). Insufficiency of the anterior oblique ligament has been noted as a consistent imaging and operative finding in traumatic instability of the trapeziometacarpal joint [3, 6]. It is recognized that trapeziometacarpal osteoarthritis (O) is accompanied by anterior oblique ligament insufficiency [1, 9]. Radiography has traditionally been the mainstay for imaging assessment of trapeziometacarpal disorders of the thumb [1, 4, 6, 10]. MRI features of anterior oblique ligament in cadavers and in patients with traumatic anterior oblique ligament tears have been described [3, 11] (Fig. 2). To our knowledge, there are no reports in the English-language literature about the ultrasound assessment of the anterior oblique ligament. We hypothesized that ultrasound can also be used to evaluate the anterior oblique ligament. Reliable identification and knowledge of the normal ultrasound appearance of the anterior oblique ligament is essential for accurate assessment of the abnormal ligament. The purpose of this study was to assess the ability of high-resolution ultrasound to delineate the thickness and echogenicity of the W428 JR:195, December 2010

2 Ultrasound of the Trapeziometacarpal Joint normal anterior oblique ligament in the hands of cadavers and asymptomatic volunteers. Subjects and Methods Research ethics board approval was obtained before the start of the study. Fig. 1 Schematic diagrams of volar aspect of hand., Diagram shows superficial anterior oblique ligament (blue arrows) coursing from volar aspect of trapezium to volar aspect of first metacarpal base., Diagram shows anterior oblique ligament. lack arrows indicate deep anterior oblique ligament, and blue arrows indicate superficial anterior oblique ligament. Superficial anterior oblique ligament has been cut away in middle to show relationship to deep anterior oblique ligament. C, Schematic sagittal diagram of trapeziometacarpal articulation depicts course of superficial anterior oblique ligament (blue arrows) and deep anterior oblique ligament (black arrows) deep in relation to thenar musculature (white arrow). Cadaveric Study Four cadaveric hands were obtained from our institution s anatomy department for the purposes of this study. The cadaveric donors were anonymous to the researchers. Informed consent had been obtained before death to use tissues for approved research and educational purposes. Four hands, disarticulated at the level of the midhumerus from embalmed skeletally mature cadavers, were assessed in our study. ll sonograms were obtained in real time with the use of a scanner (ntares, Siemens Healthcare) with a high-resolution linear array transducer (5 13 MHz, hockey stick probe). ll cadaveric hands were scanned by a radiologist with 5 years of experience in musculoskeletal ultrasound. Sonography was performed over the expected location of the anterior oblique ligament, which courses from its proximal attachment on the volar tubercle of the trapezium to its distal attachment on the volar tubercle of the first metacarpal (Fig. 3). To determine the correct localization of the ligament, one small drop of 0.1% methylene blue dye was injected through a 27-gauge needle that was positioned with its tip in the structure identified as the anterior oblique ligament, according to ultrasound guidance (Fig. 3). Immediately after the injection of dye, a plastic surgeon with over 20 years of experience dissected each hand using an anterior approach with a curvilinear incision along the lateral aspect of the first metacarpal, extending proximally and in a medial direction, to identify the anterior oblique ligament and to determine the location of the dye (Fig. 4). During dissection, every layer of soft tissue overlying the anterior oblique ligament was carefully examined for evidence of the dye before further dissection into deeper tissues. symptomatic Volunteer Study Forty asymptomatic adult volunteers agreed to participate in the study and gave informed consent. Exclusion criteria were as follows: having seen a physician or other health care professional for hand problems, current hand pain, previous surgery on the hand, and trauma to the hand in the past 5 years. ll sonographic images of the volunteers were obtained by the radiologist and an ultrasound technologist with over 10 years of experience in musculoskeletal ultrasound. ll sonographic images of each Fig. 2 Sagittal oblique fast spin-echo proton density image through base of thumb in volunteer shows anterior oblique ligament (arrows) deep in relation to thenar muscles. 1st MC = first metacarpal, T = trapezium. C JR:195, December 2010 W429

3 Chiavaras et al. Fig. 3 nterior oblique ligament in cadaver., Longitudinal sonogram shows anterior oblique ligament at its metacarpal attachment (blue arrow), midportion (black arrow), and trapezial attachment (white arrow)., Longitudinal sonogram shows needle (yellow arrows) injecting methylene blue dye under ultrasound guidance into anterior oblique ligament. nterior oblique ligament at its metacarpal attachment (blue arrow) and midportion (black arrow) is seen. volunteer were interpreted in consensus by the radiologist and the technologist. The sonographic images were obtained using a scanner (Logiq E9, GE Healthcare) with a high-frequency matrix transducer (ML6 15, GE Healthcare). oth hands of each volunteer were scanned. Scanning technique Sonography was performed with the volunteer positioned comfortably in a chair and the hand placed on the examination table in a supine position. The anterior oblique ligament was localized by scanning the volar aspect of the hand at the thenar eminence (Fig. 5). The insertions of the anterior oblique ligament at the first metacarpal base and the trapezium were discerned. Then it was determined whether the deep anterior oblique ligament could be discerned as separate Fig. 4 nterior oblique ligament in cadaver. Photograph obtained during dissection of gross specimen shows anterior oblique ligament lifted by forceps. lack arrow indicates trapezial end, white arrow indicates metacarpal end, and yellow arrow indicates base of first metacarpal. structure deep in relation to the superficial anterior oblique ligament. The examination included imaging of the anterior oblique ligament in the longitudinal plane with respect to size and echotexture. Dynamic evaluation was performed to assess the amount of volar translation of the first metacarpal base with respect to the anterior aspect of the trapezium with palmar abduction stress. This was done by applying volarly directed stress using the index finger of the operator while the base of the first metacarpal was held between the index finger and thumb of the operator (Fig. 5). During this maneuver, the anterior cortex of the trapezium was maintained parallel to the probe. Qualitative analyses The qualitative features evaluated included perceptibility, echogenicity (graded as hyperechoic, isoechoic, or hypoechoic relative to surrounding muscle), and the ability to discern the midportion of the ligament, the attachment of the ligament to the first metacarpal and the trapezium, which was graded as well defined, not well defined, or not visualized. The ability to discern the deep anterior oblique ligament as a separate structure was graded as possible or not possible. Quantitative analyses The longitudinal shortaxis dimension of the anterior oblique ligament was obtained, overlying the first metacarpal, trapezium, and the midportion (Fig. 6). The length of the ligament from the trapezial to the metacarpal attachment was also measured. For each volunteer, age, sex, height, weight, body mass index, and hand Fig. 5 Probe positions for examination of anterior oblique ligament., Image shows probe position for static examination of anterior oblique ligament. Volunteer is positioned and sonographic transducer is placed in longitudinal plane of anterior oblique ligament during examination., Image shows position for dynamic examination of anterior oblique ligament. Volunteer is positioned and sonographic transducer is placed in longitudinal plane of anterior oblique ligament during examination. Note position of index finger and thumb of operator exerting volarly directed palmar abduction stress on base of first metacarpal. W430 JR:195, December 2010

4 Ultrasound of the Trapeziometacarpal Joint dominance were recorded. The dimensions of the ligament were obtained to the nearest 0.1 mm. Dynamic evaluation is illustrated in Fig. 6. In the neutral position, a horizontal line was drawn tangential to the volar cortex of the trapezium, and another line was drawn tangential to the volar cortex of the metacarpal base, paralleling the first line. The vertical distance between the two lines was measured in millimeters (measurement 1). This measurement was recorded as a positive value if the volar cortex of the metacarpal was anterior relative to the cortex of the trapezium, and it was recorded as a negative value if the metacarpal cortex was posterior relative to the trapezial cortex. If they were at the same level, the measurement was recorded as 0 mm. Next, the measurements were repeated with palmar abduction stress applied, and the vertical distance between the trapezial and metacarpal lines was measured in millimeters (measurement 2). The degree of volar metacarpal translation with palmar abduction stress (i.e., the distance that the base of the first metacarpal base shifted volarly) was calculated as the difference between measurement 2 and measurement 1. Statistical Methods Descriptive statistics and box plots were used to confirm that the data for the thickness of the anterior oblique ligament conformed to a normal distribution. Extreme outliers were rare and did not significantly affect the assumption of a normal distribution. nonparametric binomial test was used to confirm that the sample was nonbiased with respect to hand dominance. Subsequent Student s t tests (equality of variances assumed) were performed for sex (unpaired t test), right and left sides (paired t test), and dominant versus nondominant hands (paired t test). Pearson s bivariate correlation coefficients were calculated to assess the association between biologic characteristics (height, weight, body mass index, and age) and each of the primary outcomes (thickness at the metacarpal attachment, midportion, and trapezial attachment as well as the degree of translation with palmar abduction stress). onferroni correction was used to adjust for multiple testing and to derive a more stringent critical test for p, which was set at for all statistical analyses. Statistical analysis was performed with a software package (SPSS version 17.0, SPSS). Results Cadaveric Study Surgical dissections of the cadaveric hands were performed immediately after ultrasound-guided injection of methylene blue into the anterior oblique ligament. Methylene blue dye was injected directly into the TLE 1: Longitudinal Short-xis Thickness of nterior Oblique Ligament on Ultrasound Images Location Metacarpal attachment anterior oblique ligament in all cases (Fig. 4). Sonography revealed that all four cadaveric hands had an anterior oblique ligament with fibrillar echotexture (Fig. 3). Study With Healthy Volunteers total of 40 asymptomatic volunteers (21 women and 19 men; mean age, 36 years; range, years) were examined in this study. Qualitative analysis The anterior oblique ligament was visualized along the volar aspect of the trapeziometacarpal joint and exhibited a fibrillar pattern in all hands (Fig. 6). The anterior oblique ligament was well visualized in 98% (39/40) of the volunteers. The proximal attachment of the anterior oblique ligament to the trapezium was well defined bilaterally in 100% (40/40) of the volunteers and in 100% (80/80) of the hands scanned. The distal attachment of the anterior oblique ligament to the base of the first metacarpal was well defined in 88% (35/40) of volunteers and in 94% (75/80) of the hands scanned. In four hands, the metacarpal attachment was only partly visualized, ranging in thickness from 0.5 to 0.6 mm. In one hand, the metacarpal attachment could not be visualized. The midportion of the anterior oblique ligament was well defined bilaterally in 100% (40/40) of the volunteers and in 100% of the hands scanned. The overall echogenicity of the anterior oblique ligament was hyperechoic in 99% (79/80) of the hands examined and isoechoic in 1% (1/80). It was not possible to TLE 3: Degree of Translation of Metacarpal ase With Respect to Trapezium With Palmar bduction Stress Location Thickness (mm) Mean SD 95% CI Right side Left side Dominant side Nondominant side Midportion Right side Left side Dominant side Nondominant side Trapezial attachment Right side Left side Dominant side Nondominant side TLE 2: Length of nterior Oblique Ligament on Ultrasound Images Location Length (mm) Mean SD 95% CI Dominant side Nondominant side Degree of Translation (mm) Mean SD 95% CI Dominant side Nondominant side JR:195, December 2010 W431

5 Chiavaras et al. confidently distinguish the superficial anterior oblique ligament from the deep anterior oblique ligament in any of the volunteers. Quantitative analysis Thirty-three (82.5%) of the 40 volunteers were right-hand dominant, and seven (17.5%) were left-hand dominant. The mean thickness of the anterior oblique ligament at the metacarpal attachment (0.7 mm), midportion (0.98 mm), and trapezial attachment (0.65 mm) did not significantly differ with respect to sex (p = 0.56, p = 0.71, and p = 0.05, respectively), right and left side (p = 0.85, p = 0.70, and p = 0.40, respectively), or hand dominance (p = 0.90, p = 0.54, and p = 0.52, respectively) (Table 1). The thickness of the ligament at the metacarpal attachment, midportion, and trapezial attachment did not significantly correlate with weight (r = 0.096, r = 0.076, and r = 0.237, respectively), height (r = 0.116, r = 0.199, and r = 0.375, respectively), body mass index (r = 0.190, r = 0.034, and r = 0.074, respectively), and age (r = 0.247, r = 0.073, and r = 0.145, respectively). The mean lengths of the ligaments showed a statistically significant difference between the dominant (10.6 mm) and nondominant (9.6 mm) hands (p = 0.008) (Table 2). The mean degree of translation with palmar abduction stress did not significantly differ between the dominant (0.7 mm) and nondominant (0.8 mm) hands (p = 0.57) (Table 3). In addition, there was no association between the degree of translation and the biologic characteristics (weight, height, body mass index, and age). Discussion On the basis of the dissection of the cadaveric hands after ultrasound-guided injection of methylene blue into the anterior oblique ligament, we have shown that ultrasound can be used to identify the anterior oblique ligament. On the basis of the scanning of the asymp tomatic volunteers, we have shown that ultrasound can be consistently used to show the anterior oblique ligament, which was seen Fig year-old volunteer with healthy anterior oblique ligament., Longitudinal sonogram shows anterior oblique ligament between measurement calipers. M = metacarpal, T = trapezium., Split longitudinal sonographic images in neutral position and palmar abduction stress show anterior oblique ligament. lue arrow indicates metacarpal attachment, black arrow indicates midportion of anterior oblique ligament, and white arrow indicates trapezial attachment. Red line is horizontal line tangential to volar cortex of trapezium in this example, in same plane as line drawn tangential to volar cortex of metacarpal base (measurement 1 is 0). Yellow line is horizontal line tangential to volar cortex of trapezium with stress. lue line is horizontal line tangential to volar cortex of first metacarpal base with stress. Double-ended white arrow indicates vertical distance between yellow and blue lines (measurement 2). in the vast majority of patients at the attachment sites and the midportion. The distinction between the superficial and deep components of the anterior oblique ligament could not be made on ultrasound in this study. This study is important because it describes an inexpensive technique to evaluate a ligament, the injury of which is often missed or detected late after the injury is entering a chronic instability phase. If it is identified and investigated early, injury to the anterior oblique ligament can be managed conservatively with splinting, if it is an incomplete injury. O of the hand usually involves the base of the thumb, affecting normal pincer grasp and impairing activities of daily living [1]. Repetitive use or a single episode of trauma can result in tearing of anterior oblique ligament. This tearing places the patient at risk for joint instability and contributes to future disability. Early recognition of anterior oblique ligament injury and surgical reconstruction in the appropriate context is essential to preserve trapeziometacarpal stability and prevent O [3, 6, 12, 13]. The trapeziometacarpal joint is a biconcave convex saddle joint that is inherently dependent on strong ligamentous support for providing stability. Up to 16 ligaments have been described around the trapeziometacarpal joint on cadaveric hands [7]. etween five and seven of these ligaments are thought to be direct contributors to joint stability [11, 14]. In reality, most hand surgeons are concerned with only two of these ligaments the anterior oblique ligament and the dorsoradial ligament [15]. The anterior oblique ligament has long been considered the most important for ligamentous stability of the trapeziometacarpal joint, including by the surgeon involved in this study [1 6, 12]. More recent reports have highlighted the contribution of the dorsoradial ligament as an important stabilizer of the trapeziometacarpal joint [15 17]. The consensus among the surgeons might be that the dorsoradial ligament is at least as important as the anterior oblique ligament, with the anterior oblique ligament playing an important part in stopping volar metacarpal subluxation and the dorsoradial ligament playing a role in preventing dorsal metacarpal subluxation [11, 14, 18]. When surgical reconstruction is performed for trapeziometacarpal instability, the anterior oblique ligament pathway is the part that is usually reconstructed using the flexor carpi radialis tendon [6, 12]. Radiographs are commonly used to assess the trapeziometacarpal joint in cases of post- W432 JR:195, December 2010

6 Ultrasound of the Trapeziometacarpal Joint traumatic instability. However, in traumatic cases without fracture or a documented dislocation, radiography may not be sensitive enough to detect significant ligamentous laxity [6]. Detection of anterior oblique ligament insufficiency can be difficult in the office setting because of factors such as tenderness, swelling, and apprehension. nterior oblique ligament tear is suspected when a patient has thenar pain when pushing up from a flat surface, such as climbing out of a bath [6]. Symptoms of trapeziometacarpal instability could be mimicked by entities such as De Quervain disease, flexor carpi radialis tenosynovitis, and trigger thumb [14]. Imaging could play a central role in such instances in documenting or excluding anterior oblique ligament tears. In the only series (to our knowledge) to date on MRI of trapeziometacarpal trauma, Connell et al. [3] showed anterior oblique ligament tears at the metacarpal attachment in 10 of 11 patients with acute injuries and anterior oblique ligament thickening and laxity in three patients with chronic injuries. MR arthrography has been shown to help visualize both components of the anterior oblique ligament [11]. Technical limitations need to be considered with MRI, including the use of dedicated coils, obtaining robust fat suppression, and avoidance of movement artifact [3]. Ultrasound will have its advantages here with a relatively quick scan time. In cases where the clinical examination is inconclusive, a quick ultrasound to check the integrity of the anterior oblique ligament would be helpful to the surgeon in the decision-making process. This study was in fact prompted by the surgeon in our group, who strongly encouraged us to see whether ultrasound can be used to determine anterior oblique ligament integrity. lthough we used different probes for the two parts of the study for logistical reasons, the standard linear high-frequency probe is preferred for use in routine practice. The anterior oblique ligament has been shown in studies to consist of the superficial portion and a deep portion, also called the beak ligament [2, 8, 9, 11]. The deep anterior oblique ligament is an intraarticular ligament, which is shorter than the superficial component and becomes taut in abduction [9]. The deep anterior oblique ligament shares the function with the superficial anterior oblique ligament of preventing volar subluxation of the first metacarpal [9]. However, some authors do not necessarily make the distinction between the two components [1, 6]. During operative intervention, most of the ligaments around the trapezium are not typically seen separately from one another [9]. These ligaments are appreciated in cadaveric studies after careful sectioning of the superficial ligaments. MRI in clinical cases of traumatic anterior oblique ligament tears could not make the distinction between the superficial and deep anterior oblique ligaments [3]. We could not confidently identify a deep anterior oblique ligament separately from the superficial component in our study. The cause for this is uncertain. However, the differentiation of the superficial and deep portions of the anterior oblique ligament is more of an academic interest, because this correlates with the original anatomic descriptions. s far as our surgeon is concerned, if clinically there is a significant injury of either segment with instability, treatment would be warranted. The mean thickness of the anterior oblique ligament at the midportion (1 mm) was thicker than at the attachment sites (0.7 mm). This compares well to the thickness of the anterior oblique ligament in other studies where it measured 1 mm and 1.3 mm [8, 9]. The mean lengths of the anterior oblique ligament in our study (10.6 mm in the dominant side and 9.6 mm in the nondominant side) correlates reasonably well with the findings of other anatomic studies, in which the anterior oblique ligament measured approximately 9.45 mm and 10.3 mm in mean length [8, 9]. To dynamically test the trapeziometacarpal joint and anterior oblique ligament, we held the joint in palmar abduction and applied maximal volarly directed stress on the base of the first metacarpal. This somewhat replicates a clinical test described to detect anterior oblique ligament insufficiency [6]. In the abducted position, anteriorly directed pressure will cause volar translation of the metacarpal in relation to the trapezium with no end point seen if anterior oblique ligament is deficient or torn. This maneuver reproduces the symptoms of instability in a patient [6]. This is the primary function of the anterior oblique ligament, particularly the superficial component (i.e., to prevent volar metacarpal subluxation) [9, 14]. The mean volar translation of around 0.7 mm in our study compares well with those from other studies, where measures of mean laxity have been observed to be 1 mm or less [17]. If ultrasound is able to identify an incomplete anterior oblique ligament injury, this may support conservative management, but if instability is noted on ultrasound and if the injury is entering the chronic phase, the surgeon may elect to perform a ligament reconstruction. Restoration of stability improves patients symptoms (pain, weak pinch, and grip) and may decrease the probability of patients proceeding to O. We recognize the limitations of our study. Musculoskeletal ultrasound is operator dependent, and this property is applicable to assessment of the anterior oblique ligament. We did not assess other ligaments of the trapeziometacarpal joint, because the surgeon involved in this study is primarily interested in the integrity of the anterior oblique ligament. We did not perform anatomic dissection of the anterior oblique ligament because the purpose of the study was to determine whether ultrasound imaging can depict a normal anterior oblique ligament complex. There were no pathologic examples with which to compare. Dynamic imaging simulating stress testing for anterior oblique ligament instability, as well as thickness measurements of the anterior oblique ligament, were not subjected to interobserver reliability testing. In conclusion, the results of our study show that ultrasound can be used to recognize and measure the thickness of the anterior oblique ligament. The superficial and deep anterior oblique ligament could not be discerned as separate structures on ultrasound. Further studies with surgical correlation are needed to assess the effectiveness of ultrasound, including dynamic evaluation, in the detection of an abnormal anterior oblique ligament. cknowledgments We extend special thanks to Susan Kinnear (Ultrasound Department, McMaster University Medical Centre) and Janet urr (MRI Department, St. Joseph s Healthcare) for their help with this project. We also thank Mary Lou Schmuck (McMaster University) for her help with the statistical evaluation. References 1. Imaeda T, n KN, Cooney WP 3rd, Linscheid R. natomy of trapeziometacarpal ligaments. J Hand Surg m 1993; 18: Pellegrini VD Jr. Osteoarthritis of the trapeziometacarpal joint: the pathophysiology of articular cartilage degeneration. 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