Abstract
Background:
TDetection of distal radioulnar joint (DRUJ) instability has proven inconsistent despite numerous examination maneuvers. Computed tomography (CT) has been suggested as a modality for evaluating DRUJ instability; however, without stress across the DRUJ, it fails to reliably identify this. No study has simultaneously assessed stress CT with clinical stress maneuvers. As such, the purpose of this study was to compare both methods in stable and unstable wrists.
Methods:
An arthrometer was developed to evaluate a clinical stress test of the DRUJ in various degrees of forearm rotation. In each forearm position, specimens were subjected to standardized volar and dorsal loads to simulate clinical stress to the DRUJ. Computed tomography images were acquired in each position with additional unstressed images. The triangular fibrocartilage complex (TFCC) was then sectioned to simulate DRUJ instability.
Results:
Nine upper extremities were used. The arthrometer could detect a significant difference between sectioned and intact TFCCs in the supinated forearm position. A large proportion of the translation measured by the arthrometer was due to rotation within the clamps. In the unstressed state, CT analysis using the radioulnar ratio failed to show significant differences in DRUJ stability for any forearm orientation. By applying stress across the DRUJ, CT analysis of ulnar translation along the sigmoid notch showed significant differences between TFCC intact and sectioned wrists.
Conclusions:
Clinical examination maneuvers and arthrometers are observing a rotational component that occurs due to the inability to directly clamp the underlying bone, whereas stress CT is more accurate for quantifying underlying bony translation. Unstressed CT analysis did not identify instability.
Keywords: distal radioulnar joint (DRUJ), instability, stress CT, arthrometer
Introduction
The distal radioulnar joint (DRUJ) comprises osseous and soft tissue components that function synergistically to maintain stability during forearm rotation. 1 Its articulation is composed of the concave sigmoid notch of the distal radius and the convex ulnar head. It has been demonstrated through anatomical studies that the radius of curvature between these 2 surfaces is asymmetric.1,2 Kinematic studies have shown that under normal conditions the radius both rotates and translates relative to the ulna.3,4 Subsequent work has quantified that the bony morphology of the DRUJ only contributes to approximately 20% of its stability.1,5 Of these structures, the triangular fibrocartilage complex (TFCC) has been shown to be the most important stabilizer, with its distal radioulnar ligaments providing the greatest contribution to stability. Other components of the TFCC (including the ulnar collateral ligament, meniscus homologue, extensor carpi ulnaris [ECU] subsheath, ulnolunate, and ulnotriquetral ligaments) as well as the joint capsule play comparatively minor roles in DRUJ stability. 5 As such, disruption of the TFCC can result in DRUJ instability, which may be associated with chronic pain and functional limitations. 6
Despite our knowledge regarding the anatomical stabilizers of the DRUJ, clinical detection of DRUJ instability has proven inconsistent. Numerous physical examination maneuvers have been evaluated without a clear gold standard. The DRUJ ballottement test is a commonly used test that involves stabilizing the radius while placing the ulna under supraphysiological loads in different wrist positions. It typically is performed bilaterally, and differences in laxity are compared side to side.7,8 Comparison of the DRUJ ballottement test with 2 other clinical examination techniques (the ulno-carpal stress test and piano-key tests) showed that the ballottement test was the only test to demonstrate a statistically significant degree of accuracy in testing for DRUJ instability. 9 Other studies have shown the ballottement test had a high interobserver and intraobserver reliability, particularly when the carpus was stabilized during its performance. 4 Conversely, others have criticized its subjectivity and questioned its diagnostic ability.10 -12
Computed tomography (CT) has been suggested as a modality for evaluating the DRUJ with numerous parameters proposed to-date.13 -15 Thus far, CT analysis is not well correlated with stress test findings and, though objective, primarily shows static alterations in DRUJ alignment. 16 Currently, no study has simultaneously assessed stress CT with clinical stress maneuvers such as the ballottement test. As such, the purpose of this study was to compare the ballottement test with stress CT analysis of stable and unstable wrists.
Methods
Arthrometer Design
An arthrometer was developed to evaluate a stress test of the DRUJ in various degrees of forearm rotation. It was designed to simulate and objectively quantify the examiners’ visual perspective of ulnar head movement during the ballottement test. Briefly, the system consisted of 2 independent clamps for the distal aspects of the radius and ulna. The clamps were situated on a circular track that could be used to manipulate forearm orientation. The system was completely modular to accommodate specimens of various sizes and handedness. Specimens were secured at the wrist using 2 independent c-shaped clamps for the radius and ulna. The radial clamp was secured at the distal aspect, and the ulna was secured 2 cm proximal. After securing the specimen at the wrist, the specimen was secured proximally at the medial and lateral epicondyles with the elbow fixed at 90° of flexion. The mid-shaft of the humerus was clamped to fully stabilize the arm within the arthrometer. To load the wrist, the radial clamp was fixed, and the ulna was loaded in the volar and dorsal directions along a linear bearing surface using a standardized force. Fifty newtons was determined during pilot testing as the force required to achieve clinically relevant translation of the ulna. The force was applied and maintained using a calibrated spring (Figure 1).
Figure 1.
Custom-built arthrometer for measuring distal radioulnar joint stability.
Note. The red arrow indicates the static radial clamp. The yellow arrow indicated the sliding ulnar clamp. The laser for measuring translation is fixed to the radial clamp and denoted by the blue arrow.
The arthrometer measured translation using an OptoNCDTI130 laser extensometer (Micro-Epsilon, Ortenburg, Germany) mounted on the static radial clamp to isolate relative motion, and its position was aimed at the ulnar head, along the palpable radial border of the ECU groove. An adhesive radiopaque bead was placed on the skin at the laser strike site to allow the localization of this point on CT.
Specimen Preparation
Nine fresh-frozen cadaveric upper extremities were obtained through standard institutional protocols (REB #20150553-01H). All specimens were amputated above the elbow and thawed at room temperature. The specimens were then denuded of skin, muscles, and tendons proximal to the elbow joint, ensuring to spare the elbow joint capsule, collateral ligaments, and interosseous membrane. Only specimens without signs of previous surgeries, obvious trauma, and those with no clinical signs of DRUJ instability were included.
CT Acquisition Protocol
Once mounted, the forearm was placed in 3 separate positions: neutral, maximum supination, and maximum pronation. Computed tomography scans were performed both in the unloaded condition and with each arm subjected to a standardized 50 N volar and dorsal force (stress CT). This reproduced the translation that would be achieved when stressing the DRUJ clinically. Computed tomography images were acquired in each position using a General Electric Discovery HD 750 64 row all detector CT scanner (GE Healthcare Waukesha, Wisconsin). Once completed, the radioulnar ligaments of the TFCC were sectioned in each specimen to simulate DRUJ instability. This was achieved through a longitudinal incision over the ulnar styloid, with complete release of the ligamentous insertions from the ulnar styloid and fovea. The radioulnar joint surfaces themselves were not altered. The incision was then closed with simple interrupted sutures. The previous forearm positioning and loading sequence was then repeated (Supplementary Figure 1).
Unstressed CT
The radioulnar ratio was used to compare an unstressed DRUJ in various forearm orientations for stable and unstable wrists. The method for calculating the radioulnar ratio 14 was as follows and is depicted in Figure 2a: A circle of best fit was made to define the center of the ulnar head, and a line perpendicular to the chord of the sigmoid notch (AB) through the center of the ulnar head was drawn. The AC/AB is the radioulnar ratio (Figure 2a).
Figure 2.
Bony translation versus “pseudotranslation” (a) Bony translation. Translation from point D to E is the distance the ulnar head travels along the sigmoid notch when subject to volar and dorsal loads and represents bony translation. A circle of best first was used to locate the center of the ulnar head and these points were projected to the sigmoid notch. The radioulnar ratio is AC/AB. (b) Pseudotranslation. Translation from point A to B based on the rotation of the radius. The angle of the radius was defined by a line perpendicular to the sigmoid notch midway between the dorsal and palmar borders.
Stress CT
Two outcomes, bony translation and the change in radioulnar ratio from dorsal to volar, were used to assess the DRUJ under stress in various forearm orientations for intact and TFCC-sectioned wrists. Based on the parameters defined by the radioulnar ratio above, bony translation was calculated by measuring linear distance traveled along the sigmoid notch from dorsal to volar loading configurations (DE, Figure 2a). The change in the radioulnar ratio was the difference between the radioulnar ratio under dorsal stress and volar stress.
Translation Due to Rotation—“Pseudotranslation”
Movement measured by the arthrometer when stressing the DRUJ was broken into 2 components on CT: translation of the ulnar head within the sigmoid notch and bony rotation within the clamps. Rotation within the clamps occurs because the arthrometer (and an examiner) are not clamping directly to the underlying bone. The interposed soft tissue allows the radius to rotate as well as translate during the ballottement maneuver and it is impossible to exert a force that isolates pure translation of the ulnar head at the sigmoid notch. The proportion of translation observed by the arthrometer due to in-clamp rotation was determined as follows (Figure 2b): A line was defined perpendicular to the sigmoid notch at its midpoint for each loaded position. The laser line-of-sight was identified and re-created on the CT images using a radiopaque marker, allowing for the observed ulnar head translation due to rotation to be calculated.
Statistical Analysis
Differences between TFCC intact and TFCC-sectioned groups were compared using a 1-tailed paired t test as it is expected that TFCC sectioning would only lead to an increased displacement. All other comparisons were more calculated using a 2-tailed paired t test. Data are presented using group means ± 1 standard deviation.
Results
Nine upper extremities were used for this study from 6 donors (5 men, 1 woman). The mean donor age was 73.2 ± 12.2 years.
Arthrometer-Measured Translation
The total translation measured by the arthrometer for TFCC intact and sectioned specimens for each wrist orientation is shown in Figure 3a. The average translation detected by the arthrometer was 12.0 ± 2.9 mm and 14.1 ± 4.0 mm for intact and TFCC-sectioned wrists, respectively (P < .05). There was a statistically significant increase between stable and simulated unstable wrists observed with a supinated forearm orientation, 11.9 ± 1.7 versus 14.4 ± 3.4 mm (Figure 3a). The arthrometer did not measure a statistically significant change after sectioning the TFCC in neutral or pronated forearm orientations.
Figure 3.
Arthrometer-measured translation (a) and pseudotranslation (b) in each forearm orientation for stable and unstable wrists.
*P < .05.
Translation Due to Rotation—“Pseudotranslation”
A large proportion of the translation measured by the arthrometer was due to rotation within the clamps (Figure 4). This accounted for approximately 73% of arthrometer-measured translation in neutral, 62% in pronation, and 74% in supination forearm orientations. The summation of pseudotranslation and bony translation compared with arthrometer-measured translation is depicted in Figure 4.
Figure 4.
Comparison of arthrometer-measured translation and the summation of pseudotranslation and bony translation.
Unstressed CT
There was no statistically significant difference between the intact and sectioned states for any position of forearm rotation when unstressed (Figure 5).
Figure 5.
Computed tomography assessment of unstressed wrists using the radioulnar ratio in each forearm orientation for stable and unstable wrists.
Stress CT
The average translation of the ulnar head along the sigmoid notch was 5.8 ± 2.3 mm and 7.5 ± 2.6 mm for intact and TFCC-sectioned wrists, respectively (P < .005). When evaluating for total bony translation along the sigmoid notch, a significant difference between stable and unstable wrists was observed in supinated and neutral forearm orientations (Figure 6a). In supination, the mean translation was 4.8 ± 2.4 mm for stable wrists and 7.1 ± 1.8 mm for simulated unstable wrists (P < .005). In a neutral orientation, the translation increased from 7.2 ± 2.2 mm to 9.3 ± 3.0 mm (P = .01) after sectioning the TFCC. The difference in pronation did not reach statistical significance (5.5 ± 1.9 mm vs 6.2 ± 2.1 mm, P = .10). The mean change in radioulnar ratio under load for the showed a statistically significant difference between intact and TFCC-sectioned wrists in neutral (0.44 ± 0.15 vs 0.44 ± 0.15, P = .003, Figure 6b) and in supination (0.31 ± 0.16 vs 0.46 ± 0.11, P = .002, Figure 6b).
Figure 6.
Computed tomography assessment of stressed wrists using bony translation (a) and the change in radioulnar ratio (b) in each forearm orientation for stable and unstable wrists*P < .05.
Correlations
The correlation between arthrometer-measured translation and bony translation was weak (R2 = .268, Figure 7a). A similar correlation was observed between arthrometer translation and change in radioulnar ratio (R2 = .286, Figure 7b). The correlation between arthrometer-measured translation and pseudotranslation was higher (R2 = .540, Figure 7c).
Figure 7.
Correlation between the arthrometer and bony translation (a), pseudotranslation (b), and change in radioulnar ratio (c).
Discussion
Injury to the TFCC can cause DRUJ instability and is associated with chronic pain and functional limitations. 6 A reliable, accurate clinical examination is therefore critical in the diagnosis of DRUJ instability. The DRUJ ballottement test is a common physical examination maneuver to test DRUJ instability, but its accuracy and subjectivity have made its overall diagnostic value questionable.10 -12 By recreating the DRUJ ballottement test in a biomechanical model, we were able to objectively compare the ballottement test with stress CT analysis of stable and unstable wrists.
The arthrometer output was derived from a proxy for visual translation of the ulnar head during this clinical examination maneuver. Instability created by sectioning the TFCC could only be detected by the arthrometer if the wrist was stressed and supinated. We observed a compelling difference between arthrometer-measured translation and actual bony translation. A large amount of rotation within the clamps was contributing to the perceived translation at the DRUJ—thus termed “pseudotranslation.” This was due to the inability to clamp directly to the underlying bone and was upward of 74% of total arthrometer-measured translation. The summation of pseudotranslation and bony translation often overestimated the arthrometer output and is likely due to the oblique angle at which bony translation is measured relative to the laser line of sight. Logically, clamp tightness and amount of soft tissue influence observed pseudotranslation. As such, the arthrometer output was well correlated with pseudotranslation, and only weakly correlated with bony translation. Clinicians should be aware of this, so they do not overcall the magnitude of translation occurring at the DRUJ and make a false-positive conclusion about instability.
Interestingly, in the TFCC-sectioned state, the magnitude of loaded translation detected by the laser arthrometer increased from pronation to supination to neutral. We generally expect the pronated forearm position to be the most unstable because of interosseous membrane laxity. We did not observe this experimentally, which may imply that bony impingement between the radial and ulnar shaft in full pronation restricts the absolute translation which can occur at the DRUJ, even when the TFCC is disrupted.
The TFCC is composed of numerous structures; however, its primary stability is derived from the dorsal and volar radioulnar ligament (RUL). Disruption to either of these structures, such as in distal radius fractures,17,18 can lead to instability. In this study, the distal radioulnar ligaments were not evaluated in isolation. Further biomechanical studies examining isolated dorsal versus volar RUL disruption would be valuable to clarify these effects.
Several models have been developed to assess the utility of the ballottement test. Pickering et al 19 designed a similar arthrometer to the one used in this study for in vivo assessment of DRUJ translation in normal adults and those with instability. Their normal wrists had significantly less translation than the wrists tested in this study, 6.5 versus 12.0 mm, respectively, which may be due to the age difference of the experimental groups and/or the cadaveric nature of our study. Interestingly, the difference between unstable wrists in their study and TFCC-sectioned wrists in our study was similar, 14.1 versus 16.3 mm, respectively. Onishi et al. used a cadaveric model and found magnitudes of DRUJ movement to be 10.8 and 12.4 mm for intact and TFCC-sectioned wrists, respectively. These results are within range of our study although their method for measuring translation was slightly different. 20 It is worth noting that the radius clamp did not include the carpus, leaving the wrist and hand free during testing. Prior studies, such as Onishi et al., 20 performed the DRUJ ballottement test with the carpus fixed to the radius (“holding” technique). Fixation of the carpus has been shown to reduce the influence of the wrist capsule and traversing tendons, potentially improving test reliability. Our design therefore differs from these prior methods and should be considered when comparing results across studies. Although many of these models have found significant differences between stable and unstable wrists, it remains unclear whether this difference could be detected reliably by an examiner. A study of 30 consultants demonstrated that most clinicians were unable to reliably detect DRUJ instability. 21 Recent studies have shown that using a holding technique when performing the ballottement test has a higher reliability and relatively high diagnostic accuracy compared with the traditional examination technique, although the stated positive predictive value was still only 65%. 22
Computed tomography has been proposed as a method for assessing DRUJ stability although it only has moderate or fair agreement with physical examination testing. 16 In addition, CT analysis is typically in an unstressed state, meaning the analysis is more an assessment of alignment rather than dynamic instability, which is the goal of a clinical ballottement maneuver. In our study, unstressed CT analysis was unable to detect a statistically significant difference between stable and simulated unstable wrists regardless of forearm rotation. Currently, only one other study has used stress CT to assess the DRUJ. This study, however, only examined healthy volunteers without clinical instability. They demonstrated bony translation in a neutral forearm orientation to be 3.2 mm, which is significantly lower than the 7.2 mm in this study. 23 This may be due to the cadaveric nature of our study and lack of pain inhibition. Stress CT detected statistically significant differences in DRUJ stability by either the radioulnar ratio or bony translation when the TFCC was sectioned for both the neutral and supinated forearm orientations. These findings challenge the current gold standard of unstressed CT in its ability to accurately diagnose DRUJ instability. It also supports stress CT as a possible solution for diagnosing DRUJ instability from peripheral TFCC lesions.
There are many limitations associated with this study. We used elderly specimens that may have had degeneration or prior injury to the ligamentous and cartilaginous structures affecting stability. The cadaveric nature of the study and lack of concomitant pain inhibition may have also affected the results. Our methodology was developed in a cadaveric setting to directly compare clinical stress testing with stress CT analysis of the DRUJ. Although this setup cannot be directly applied to patients, the arthrometer could be redesigned for in vivo use, provided that patient positioning and tolerance to applied loads are considered.
Conclusions
In summary, this in vitro study provides a direct comparison between the DRUJ ballottement test and stress CT analysis in simulated stable and unstable wrists. Clinical examination maneuvers and arthrometers are likely observing a significant degree of ulnar head “pseudotranslation” due to radial rotation beneath a compliant soft tissue envelope. As such, this study further calls into question the clinical validity of the DRUJ ballottement test as a stand-alone test in detecting DRUJ instability. Stress CT can be used to measure the absolute bony translation of the ulnar head at the sigmoid notch. Our study also suggests that unstressed CT fails to identify DRUJ instability when the TFCC is sectioned. Stress CT demonstrated significant differences in DRUJ translation in neutral and supination. Further research is required in the clinical role of stress CT for the diagnosis of DRUJ instability.
Supplemental Material
Supplemental material, sj-docx-1-han-10.1177_15589447261416972 for A Biomechanical Analysis of the Distal Radioulnar Joint Ballottement Test Using Stress CT by Kathryn Culliton, Kendrick Au, Sebastian Undurraga, Hakim Louati, Heathcliff D’Sa and Braden Gammon in HAND
Footnotes
Author Contributions: KC—study design, data acquisition/analysis/interpretation, and manuscript preparation. AU—study design, data acquisition/analysis/interpretation, and manuscript preparation. SU—study design, data acquisition /interpretation, and critical review of manuscript. HL—study conception and design, data interpretation, and critical review of manuscript. HD—study conception and design, and critical review of manuscript. BG—study conception and design, data acquisition/interpretation, and critical review of manuscript. All authors approved the final version of the article for publication and agreed to be accountable for all aspects of the work and resolved any issues related to its accuracy or integrity.
Ethical Approval: This study was approved by our institutional review board.
Statement of Human and Animal Rights: All procedures followed were in accordance with the ethical standards of the responsible committee on human experimentation (institutional and national) and with the Helsinki Declaration of 1975, as revised in 2008 (5).
Statement of Informed Consent: Cadaveric upper extremities were obtained through standard institutional protocols (REB #20150553-01H). Donors provided informed consent for use of their bodies in research through the University of Ottawa Body Bequeathal Program prior to death.
The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding: The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: BG received startup funds from the Department of Surgery which were used toward the completion of this study.
ORCID iDs: Kathryn Culliton 
https://orcid.org/0000-0002-1899-7588
Braden Gammon
https://orcid.org/0000-0003-0621-3384
Supplemental material is available in the online version of the article.
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Supplementary Materials
Supplemental material, sj-docx-1-han-10.1177_15589447261416972 for A Biomechanical Analysis of the Distal Radioulnar Joint Ballottement Test Using Stress CT by Kathryn Culliton, Kendrick Au, Sebastian Undurraga, Hakim Louati, Heathcliff D’Sa and Braden Gammon in HAND