Forty-five asymptomatic wrists (of forty-five individuals) without a history of injury, deformity, or other dysfunction were examined. There were thirty men and fifteen women, and their ages ranged from twenty to seventy-three years (average, thirty-eight years). Thirty-eight of these wrists were in individuals who had had a fracture of the distal part of one radius. Their injured and uninjured wrists had been examined with computed tomography scans after the original fracture had healed to evaluate the instability of the distal radioulnar joint following the distal radial fracture. Only the computed tomography scans of the normal side were included in this study. (Our institutional review board approved the study without requiring informed consent in view of its diagnostic purpose.) The remaining seven individuals had two normal wrists, and the right or left wrist, chosen at random, was scanned. Standard radiographs of the wrist were made for all subjects to exclude any individual with a suspected previous injury, osseous lesion, or abnormal alignment.
The computed tomography scans were made at 3-mm intervals with use of a GE Hi-Speed Advantage helical computed tomography scanner (General Electric Medical Systems, Milwaukee, Wisconsin). Axial sections of the wrist at the distal radioulnar joint were obtained with the hand in three positions: 70° of supination, neutral, and 70° of pronation. To minimize exposure to radiation, four sections across the distal radioulnar joint were obtained in each position. The forearm was placed on a custom-designed positioning device, which included a handgrip bar to allow reproducible positioning of the hand at 70° of pronation and supination. The individual was asked to hold the handgrip bar firmly while undergoing the scans to minimize the passive motion occurring at the radiocarpal joint (Fig. 1).
Computed tomography of the distal radioulnar joint is usually performed with the patient prone with the hand over the head, the shoulder in maximum abduction, and the elbow in extension because of the design of the conventional scanning table. Rotation of the forearm with the elbow extended should induce shoulder rotation. In order to minimize the error resulting from positioning of the forearm in rotation, the subjects in this study stood with the lumbar spine bent laterally so that the shoulder joint could be maintained in an adducted position and the elbow joint could be flexed to almost 90° (Fig. 1). All of the scans were assessed to determine if adequate forearm rotation had been achieved on the scanning table, and the subjects whose scans showed inadequate rotation were excluded from the study.
At each forearm position, the scans that projected the largest areas of the sigmoid notch and the ulnar head, including the ulnar styloid and the Lister tubercle, were selected. The images were printed at 1.5 times magnification for the manual measurement.
Four current methods for diagnosing subluxation of the distal radioulnar joint with computed tomography were used to measure the translation of the ulnar head with respect to the radius. These included the radioulnar line method suggested by Mino et al.1,8, the epicenter method suggested by Wechsler et al.9, the radioulnar ratio method suggested by Lo et al.10, and the subluxation ratio method, which is similar to the method described by Mino et al. except that a line perpendicular to the sigmoid notch was used instead of the radioulnar line.
Radioulnar Line Method1,8 (Fig. 2-A): A line was drawn through the dorsal ulnar and radial borders of the radius, and a second line was drawn through the volar ulnar and radial borders of the radius. If the ulnar head was located outside of these two lines, the amount of the ulnar head that was either volar or dorsal to these lines was measured and the ratio of this distance to the length of the sigmoid notch was calculated. Dorsal translation of the ulnar head was recorded as being positive, and volar translation was recorded as being negative. If the ulnar head lay between these two lines, the articulation of the ulna with the radius was considered to be normal and the value was recorded as 0.
Subluxation Ratio Method (Fig. 2-B): Two lines perpendicular to a line connecting the volar and dorsal margins of the sigmoid notch were drawn from the volar and dorsal margins of the sigmoid notch. The amount of the ulnar head that was either volar or dorsal to these lines was measured, and the ratio of this distance to the length of the sigmoid notch was calculated. Dorsal translation of the ulnar head was recorded as being positive, and volar translation was recorded as being negative.
Epicenter Method9 (Fig. 2-C): The center of rotation of the distal radioulnar joint was marked at a point halfway between the center of the ulnar styloid and the center of the ulnar head. A perpendicular line was then drawn from the center of rotation to a line connecting the dorsal and volar margins of the sigmoid notch. The distance between the perpendicular line and the midpoint of the sigmoid notch was measured, and the ratio of that distance to the length of the sigmoid notch was calculated. Dorsal displacement from the midpoint of the sigmoid notch was recorded as being positive, and volar displacement was recorded as being negative.
Radioulnar Ratio Method10 (Fig. 2-D): The center of the ulnar head was identified with use of a transparent template marked with concentric circles. From this point, a perpendicular line was drawn to the line connecting the volar and dorsal margins of the sigmoid notch. The distance from the intersection of these two lines to the volar margin of the sigmoid notch was then measured. The ratio of this distance to the length of the sigmoid notch was calculated.
Three observers (one orthopaedic hand surgeon, one hand surgery fellow, and one senior resident) measured all of the scans independently in a blinded and randomized manner, and their findings were used to determine the interobserver reliability of each method. The measurements were repeated by the same observers three months later, and those measurements were used to determine intraobserver reliability. The three observers' measurements for each subject were averaged and considered to be the final value for that subject. The normal value of each technique in each position was defined as the mean value and the 95% confidence interval.
Although patients with instability of the distal radioulnar joint were not included in the reliability study, it was assumed that the scans demonstrating dorsal or volar translation of the ulnar head in the position of pronation or supination represented examples of subluxation. Intraclass correlation coefficients were used to determine the interobserver and intraobserver reliability of each method. The analysis of interobserver reliability was based on the first round of observations to prevent training bias. The two-way random-effects intraclass correlation coefficient model was used to calculate the interobserver agreement, and the one-way random-effects intraclass correlation coefficient model was used to calculate the intraobserver agreement. The interpretation of the intraclass correlation coefficient values was based on the criteria proposed by Landis and Koch13. Intraclass correlation coefficient values of 0.00 to 0.20 represented slight agreement, 0.21 to 0.40 represented fair agreement, 0.41 to 0.60 represented moderate agreement, 0.61 to 0.80 represented substantial agreement, and >0.81 represented almost perfect agreement.
The number of subjects required for the reliability study was calculated14 to ensure that significant agreement would be seen if the correlation were at least 0.75 with an alpha of 0.05 and a beta of 0.2. The minimum sample size was found to be forty-three scans for each position.
Reliability Studies
Table I shows the intraclass correlation coefficients for each method for quantifying translation of the distal radioulnar joint. The highest intraclass correlation coefficient for interobserver reliability, which was substantial to almost perfect, was obtained with the subluxation ratio method. The intraclass correlation coefficient values showed substantial to almost perfect reliability for the radioulnar line method, substantial reliability for the radioulnar ratio method, and moderate to substantial reliability for the epicenter method. The intraobserver reliability of the three observers was almost perfect (intraclass correlation coefficient = 0.80 to 0.97) with use of all four methods.
Normal Values
The normal values for each method in each position are summarized in Table II. The radioulnar line method showed the ulnar head to be outside of the dorsal radioulnar line in pronation and outside of the volar radioulnar line in supination in all of the subjects (Figs. 3-A, 3-B, and 3-C). The radioulnar line method indicated a mean of 16% dorsal displacement of the ulnar head with respect to the sigmoid notch in pronation and 11% volar displacement in supination. The normal ranges derived with the radioulnar line method, based on the 95% confidence interval, included 35% dorsal displacement of the ulnar head in pronation and 27% volar displacement of the ulnar head in supination. In neutral rotation, the normal values ranged from 26% dorsal displacement to 22% volar displacement of the ulnar head outside of the radioulnar lines. The subluxation ratio method showed means and confidence intervals that were very similar to those demonstrated by the radioulnar line method.
The epicenter values on all of the scans were <0.25 either dorsally or volarly. This means that the line drawn perpendicular from the center of rotation of the distal radioulnar joint, which was defined as the midpoint between the ulnar styloid process and the center of the ulnar head, fell in the middle half of the sigmoid notch in all positions. In pronation, the normal locations of the rotation center were distributed within the middle 40% of the sigmoid notch. In supination, the center was found to be located entirely dorsal to the middle of the sigmoid notch. The tendency for dorsal location of the center of rotation of the distal radioulnar joint was also observed in the neutral position.
The mean values for the radioulnar ratio (0.66 in pronation, 0.51 in neutral, and 0.42 in supination) reflected the normal translation of the ulnar head with respect to the sigmoid notch during rotational motion of the distal radioulnar joint. The changes in the radioulnar ratio with the position of rotation showed a consistent pattern in each subject, but the 95% confidence interval showed a large normal variation ranging from a mean of -0.2 to a mean of +0.2.
It has been suggested that computed tomography is the imaging modality of choice for evaluation of the distal radioulnar joint because it best delineates the cross-sectional anatomy of the joint without overlapping of the adjacent structures and it can be performed with varying positions of forearm rotation5-7. Various computed tomography parameters for measuring translation of the distal radioulnar joint have been proposed for diagnosis of subluxation8-10. Guidelines have been proposed on the basis of measurement techniques for the evaluation of patients with suspected instability of the distal radioulnar joint, but the value of these measurements as objective criteria for diagnosing abnormal subluxation remains controversial.
It is well known that rotation of the radius around the ulna is accompanied by translation as a result of the different radii of curvature of the two articular surfaces. The radius translates volarly during pronation and dorsally during supination, so that the ulna is volar in supination and dorsal in pronation relative to the radius11. Computed tomography scans of wrists with subluxation of the distal radioulnar joint need to be carried out with the forearm in both supination and pronation to detect dynamic subluxation because the distal radioulnar joint might be reduced in one position and subluxated in another. The purpose of this study was to obtain normal reference data with the forearm in different positions of rotation. In this study, all of the wrists were scanned with the forearm in supination, neutral, and pronation. In order to standardize the amount of rotation between subjects, a custom-designed positioning device with a handgrip bar was used to reproducibly position the forearm in 70° of pronation and supination. The active motion was induced by having the subjects hold the handgrip bar firmly while undergoing the scans. This was necessary to minimize the radiocarpal laxity, which can cause insufficient forearm rotation.
To our knowledge, Mino et al.1 were the first to describe computed tomography criteria for making a diagnosis of subluxation of the distal radioulnar joint. They found that the ulnar head normally falls between lines drawn through the dorsal and volar borders of the distal part of the radius and interpreted computed tomography scans that showed the ulnar head volar or dorsal to the radioulnar lines as demonstrating abnormality in all positions of rotation. The results of the current study show that normal ulnar head displacement ranged from 35% dorsal to the radioulnar line in pronation to 27% volar in pronation. Even in the neutral position, the 95% confidence interval showed a relatively wide variation in displacement of the ulnar head, which ranged from 26% dorsal to 22% volar. These results suggested that the radioulnar line method is unreliable for the diagnosis of subluxation of the distal radioulnar joint because of the unacceptably high rate of false-positive results.
Nakamura et al.4 suggested modified criteria based on the computed tomography findings in normal wrists; they considered the findings to be within normal limits when dorsal displacement of the subluxated ulna was less than one-quarter of the diameter of the sigmoid notch in the pronation arc and volar displacement was less than one-quarter of the diameter of the sigmoid notch in the supination arc. In our study, seven of the forty-five normal subjects had >25% dorsal displacement in pronation and three of the forty-five subjects had >25% volar displacement in supination. The modified criteria appear to be more reliable than the original criteria of the radioulnar line method for diagnosing abnormal subluxation, but there is still the risk of false-positive results, particularly in the assessment of scans made with the forearm in pronation.
The greatest difficulty with drawing a radioulnar line might be the selection of the radial border of the line because the lateral aspect of the radius at the level of the sigmoid notch is commonly rounded. We measured translation of the distal radioulnar joint with the subluxation ratio method, which involves drawing a line perpendicular to the sigmoid notch instead of along the somewhat subjective dorsal and volar borders of the distal part of the radius. The subluxation ratio method was more reliable than the radioulnar line method because fewer reference points are required. The normal values derived with the subluxation ratio showed a distribution similar to those derived with the radioulnar line method, indicating that the same criteria regarding the amount of the ulnar head outside of the radioulnar lines can be applied. The subluxation ratio is considered to be very useful for evaluation of instability of the distal radioulnar joint, particularly in patients with a history of a fracture of the distal part of the radius.
Wechsler et al.9 proposed the epicenter method as a solution to the limitations of using radioulnar lines. With this method, the axis of rotation of the distal radioulnar joint is identified by determining the midpoint between the center of the ulnar styloid and the center of the ulnar head. Many studies have demonstrated that the center of rotation of the distal radioulnar joint is located around the fovea of the ulnar head, where the distal radioulnar ligaments, the main stabilizer of the distal radioulnar joint, insert11,15. The point of rotation used in the epicenter method appears to coincide well with the fovea of the ulnar head. On the basis of the criteria suggested by Wechsler et al., the position of the radioulnar joint is considered to be normal if the perpendicular line from this point falls in the middle half of the chord drawn across the sigmoid notch.
Use of the center of rotation as a reference point should have the advantage of minimizing the effect of normal translational motion during forearm rotation on the diagnosis of abnormal subluxation. This is supported by the fact that the normal values in this study did not show consistent dorsal or volar patterns according to the positions of rotation. The fact that the normal epicenter values in all subjects were <25% in the volar or dorsal direction from the center of the sigmoid notch, regardless of the rotational position, demonstrates the reliability of this method in predicting abnormal subluxation of the distal radioulnar joint.
The epicenter method was found to have the least interobserver and intraobserver reliability. The main reason for this might be the difficulty in determining the center of rotation. Determining the center of the ulnar styloid is particularly subjective because it is an indiscrete structure rather than a round demarcated one and is often not well visualized at the level of the distal radioulnar joint.
Lo et al.10 proposed the radioulnar ratio method as a new technique for quantifying translation of the distal radioulnar joint on the basis of the location of the center of the ulnar head relative to the sigmoid notch. The radioulnar ratio method eliminates the reliance on both the radial aspect of the radius and the ulnar styloid, which are indiscrete structures that make it difficult to determine the reference points. In a progressive laboratory-induced instability model, Lo et al. demonstrated that the radioulnar ratio could be used to detect subluxation earlier than was possible with either the radioulnar line or the epicenter method, suggesting that the radioulnar ratio method might be a more sensitive tool for detecting subtle instability. They measured the normal values for, and variations in, the radioulnar ratio in thirteen subjects and defined the normal values as the mean and two standard deviations. When the normal values in our study were compared with those reported by Lo et al., the standard deviations at each wrist position were found to be greater in our study whereas the mean values were similar. Because there were more than three times more subjects in our study than in the study by Lo et al., we believe that the 95% confidence intervals obtained in our study should be considered to be the normal radioulnar ratio values in order to avoid incurring a high rate of false-positive results. The interobserver reliability of the radioulnar ratio was substantial, which is less than the intraclass correlation coefficients calculated by Lo et al. We found the radioulnar ratio to be more reliable than the epicenter method but less reliable than the radioulnar line method. The use of a plastic template with concentric circles to identify the center of the ulnar head with the radioulnar ratio method appears to be less consistent than the determination of the dorsal or volar borders of the radius with the radioulnar line method.
The subluxation ratio was found to have the greatest interobserver reliability, and the epicenter method was found to have the least. These results suggest that greater reliability occurs from fewer reference points needed during the measurement. The subluxation ratio is derived with use of lines perpendicular to the sigmoid notch as the main reference, whereas the other methods depend on somewhat subjective reference lines or points. When we take the reliability and simplicity into consideration, it is clear that the subluxation ratio is the most useful of the current computed tomography methods.
Although each method provides criteria for determining whether the distal radioulnar joint is subluxated, there is no gold standard for the diagnosis of instability with use of computed tomography scans. The main difficulty in evaluating the distal radioulnar joint with computed tomography criteria is the large normal variation in the translation of the distal radioulnar joint, as shown in this study. This variation must be related to the difference in the individual laxity of the ligaments and other stabilizing soft-tissue structures (Figs. 3-A, 3-B, and 3-C). For this reason, a comparison with a computed tomography scan of the unaffected, contralateral wrist is quite helpful5. Physical findings should also be considered when diagnosing instability of the distal radioulnar joint. The provocation of symptoms by a stress test is considered an essential step in differentiating an unstable joint from a normal lax joint16. As objective data, the normal ranges of distal radioulnar joint translation measured with the current methods and the reliability of each method provided in this study can aid physicians in interpreting computed tomography findings in patients with suspected instability of the distal radioulnar joint. 