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To assess carpal kinematics in various ranges of motion in 3 dimensions with respect to lunate morphology.
Methods
Eight cadaveric wrists (4 type I lunates, 4 type II lunates) were mounted into a customized platform that allowed controlled motion with 6 degrees of freedom. The wrists were moved through flexion-extension (15°–15°) and radioulnar deviation (RUD; 20°–20°). The relative motion of the radius, carpus, and third metacarpal were recorded using optical motion capture methods.
Results
Clear patterns of carpal motion were identified. Significantly greater motion occurred at the radiocarpal joint during flexion-extension of type I wrist than a type II wrist. The relative contributions of the midcarpal and radiocarpal articulations to movement of the wrist differed between the radial, the central, and the ulnar columns. During wrist flexion and extension, these contributions were determined by the lunate morphology, whereas during RUD, they were determined by the direction of wrist motion. The midcarpal articulations were relatively restricted during flexion and extension of a type II wrist. However, during RUD, the midcarpal joint of the central column became the dominant articulation.
Conclusions
This study describes the effect of lunate morphology on 3-dimensional carpal kinematics during wrist flexion and extension. Despite the limited size of the motion arcs tested, the results represent an advance on the current understanding of this topic.
Clinical relevance
Differences in carpal kinematics may explain the effect of lunate morphology on pathological changes within the carpus. Differences in carpal kinematics due to lunate morphology may have implications for the management of certain wrist conditions.
described 2 morphological variants of the lunate, based on the shape of the distal articular surface. Type I lunates have a single facet articulating with the capitate, and type II lunates have an additional medial facet articulating with the hamate. Type II lunates have a prevalence of 63% to 73%.
Lunate type is also associated with carpal pathology. Type I lunate wrists have a higher incidence of dorsal intercalated segment instability deformity in the setting of scaphoid nonunion,
These studies examined wrist motion only in the radioulnar deviation (RUD) plane. Type II lunate wrists demonstrate greater scaphoid flexion during radial deviation,
compared with type I lunate wrists. These studies used static images (fluoroscopy, radiographs, and magnetic resonance imaging [MRI]) and inferred the motion of the carpals from the relative orientation of the bones to one another, rather than measuring the actual range and direction of each bone’s movement. We have previously reported a method for using a Stewart platform and optical motion capture methods to re-create motion in a cadaveric wrist.
This method allows accurate 3-dimensional quantification of carpal kinematics.
The aim of this study was to use the Stewart platform setup to investigate whether lunate morphology had an effect on carpal motion during wrist motion in multiple planes, using an ex vivo model.
We hypothesized that wrists with type I and type II lunates would have different intercarpal motions during wrist motion. Furthermore, we hypothesized that the motion at the midcarpal and radiocarpal articulations would not be equal in either wrist type during wrist motion.
Materials and Methods
The Stewart platform is a mechanical testing device that has the ability to move in 6 degrees of freedom with high accuracy. The system we used was developed specifically for biomechanical testing and has the ability to move specimens through physiologically valid movements. Details on the accuracy and more technical aspects of the modelling methods and Stewart platform have previously been published.
Ding B, Cazzolato BS, Stanley RM, Grainger S, Costi JJ. Stiffness analysis and control of a Stewart platform-based manipulator with decoupled sensor-actuator locations for application in biomechanical testing. J Dyn Sys Meas Control. In press.
Eight fresh frozen cadaveric wrists were assessed. The mean age at time of death was 88 years (range, 74–97 y). An MRI scan was performed and reviewed by a fellowship-trained hand surgeon (K.E., G.I.B.) to assess the lunate morphology and ensure there were no previous fractures or degenerative process present. This was confirmed by comparing to the helical computed tomography scan performed to confirm the wire placement (see later section). Lunate morphology was defined using the capitate-triquetrum distance as described by Nakamura et al.
A capitate-triquetrum distance of less than 2 mm was defined as a type I lunate wrist (type I wrist) and greater than 4 mm was defined as a type II lunate wrist (type II wrist). There were 4 type I wrists and 4 type II wrists. Coronal MRI has been reported to be as accurate as dissection for determining lunate type.
Kirschner wires (1.6 mm) were inserted into the distal radius, distal ulna, third metacarpal, scaphoid, lunate, triquetrum, and capitate. Three reflective markers were fitted to each wire. The correct positioning of the wires was confirmed using helical computed tomography scans. Each specimen was thawed for 12 to 16 hours prior to the experiment. Phalanges were disarticulated and the radius and ulna were transected at their proximal third, so that the distance between proximal and distal ends of the specimen was 280 mm. Skin, muscle, and fat were removed from the proximal 50 mm of the forearm and on the distal half of the metacarpals.
The specimen was mounted in a custom-built alignment device to ensure repeatable mounting. Each end of the specimen was placed into an aluminium pot in a neutral pronation-supination position and fixed using polymethylmethacrylate. The specimen was then mounted in the Stewart platform. Because of the technical limitations of positioning the wrists in the Stewart platform, wrist motion could be performed only through a 30° arc in the flexion-extension (FE) plane and 40° arc in the RUD plane.
Data collection
The Stewart platform motion was computer controlled from signals generated from either in vivo experiments or sinusoid signals in the plane of interest. A small axial load (5–10 N) was placed across the wrist. The wrist was moved about a fixed center of rotation located at the midpoint of the capitate lunate articulation.
Movement occurred through a cycle beginning in neutral alignment, moving to maximum flexion (or ulnar deviation), then to maximum extension (or radial deviation), and then back to neutral. Each cycle was performed over 10 seconds, repeated 3 times, and recorded at 100 Hz. We analyzed 2 planes of wrist motion, FE and RUD. The FE was performed from 15° flexion to 15° extension with the RUD angle kept constant at 0°. RUD was performed from 20° ulnar deviation to 20° radial deviation and the FE angle kept at 0°.
A 12-camera Vicon MX-F20 motion capture system (Vicon, Oxford, UK) was used to record the trajectories of each marker pin during wrist movement. The system has been reported to have a linear accuracy of less than 1 mm.
The marker trajectories were reconstructed and labelled using Vicon Nexus software version 1.8.2 (Vicon, Oxford, UK). The raw kinematic data were filtered using Visual3D software (Version 5, C-Motion Inc). The filtered data were then analyzed using Matlab software (R2012a, The Mathworks Inc) to obtain carpal and wrist rotations. Technical details of the setup and registration processes have been published previously.
The total measurement error of the system (Stewart platform, motion capture system, and CT scan) was 2°. Therefore, recorded motions of < 2° were considered to be negligible.
Data processing
We assessed the movements of third metacarpal relative to radius (global wrist motion); the scaphoid, lunate, and triquetrum relative to radius (radiocarpal motions); and the capitate relative to scaphoid, lunate, and triquetrum (midcarpal motions).
Because there is relatively little motion within the distal carpal row, this method allowed assessment of the radial, central, and ulnar columns of the wrist. Radial column motion occurs at the radioscaphoid and scaphocapitate articulations. Central column motion occurs at the radiolunate and lunocapitate articulations. Ulnar column motion occurs at the radiotriquetrum and triquetrocapitate articulations.
Movement of all articulations refers to the direction of movement of the distal bone relative to the proximal bone.
Statistical analysis
The data were assessed for normality using a Shapiro-Wilks test. It was determined the data were sampled from a nonnormal distribution and as such nonparametric tests were used, and nonparametric measures for the effect size. A Mann-Whitney U test compared the difference between the carpal rotations. Statistical significance was defined as P of .05 or less. Effect sizes were also calculated using the r as required for independent groups and nonparametric tests.
When assessing the data, an effect size greater than 0.8 was considered a large change between the 2 groups, and 0.5 to 0.79 was considered a moderate change. An effect size less than 0.3 represented a difference in rotation that was very small and likely to be within the boundary of the measurement error (2°).
Interpretation of data
The rotations are described as being in plane or out of plane motion. In plane motion was carpal motion in the same plane as the wrist movement (eg, wrist flexion producing scaphoid flexion) and out of plane motion was in a different plane (eg, wrist flexion producing scaphoid ulnar deviation or pronation).
For the in plane motions, we identified dominant, nondominant, and restricted articulations in each carpal column. The dominant articulation was defined as having 50% or more of the sum of in plane motions occurring across both articulations in the column. Nondominant articulations had less than 50% of this motion. A restricted articulation was defined as having in plane motion of less than 4°.
For the out of plane measurements, articulations that had statistically significantly different motion amplitudes between the type I and the type II wrists were identified. Dominant, nondominant, and restricted articulations were not identified for these secondary movements, because the amplitudes were relatively small.
Results
Appendices A and B with the complete sets of movements for each carpal articulation during both FE and RUD of the wrist are available on the Journal’s Web site at www.jhandsurg.org.
Wrist flexion-extension
In plane motion: type I versus type II lunate wrists
During wrist flexion, type I wrists had significantly greater in plane motion at the radioscaphoid and capitolunate articulations, but significantly less at the radiolunate articulation. During wrist extension, type I wrists had significantly greater motion at the radioscaphoid articulation.
In plane motion: dominant and restricted articulations
During flexion and extension, the dominant and restricted articulations of all 3 columns were determined by the morphology of the lunate.
The radiocarpal articulation was the dominant articulation in the radial and ulnar columns of both wrist types during flexion and extension. In the central column, the dominant articulation was midcarpal in type I wrists and radiocarpal in type II wrists (Figs. 1, 2).
Figure 1Mean FE at the radiocarpal and midcarpal joints during flexion of type I and type II wrists. Positive values indicate flexion; negative values indicate extension. SDs are omitted for easier reading; refer to Appendix A (available on the Journal’s Web site at www.jhandsurg.org) for details. All differences were statistically significant (P < .001). See text for definitions of columns.
Figure 2Mean FE at the radiocarpal and midcarpal joints during extension of type I and type II wrists. Positive values for flexion plane motion indicate flexion; negative values indicate extension. SDs are omitted for easier reading; refer to Appendix A (available on the Journal’s Web site at www.jhandsurg.org) for details. All differences were statistically significant (P < .001) except for the middle and ulnar column movements of type II lunate wrists (P = .223 and P = .122, respectively). See text for definitions of columns.
In type I wrists, all 3 of the nondominant articulations were classified as being restricted during wrist flexion, but during wrist extension, only the midcarpal articulation of the radial column was restricted. In type II wrists, all 3 of the nondominant (midcarpal) articulations were restricted during both wrist flexion and extension (Fig. 3).
During flexion of type I wrists, there was ulnar deviation of the radiolunate articulation, with compensatory radial deviation occurring at the midcarpal articulation. There were no carpal pronation/supination movements greater than 2° in either wrist type during wrist flexion or extension.
Radioulnar deviation
In plane motion: type I versus type II lunate wrists
During RUD, there was no significant difference in motion of the carpus between type I and II wrists. However, during ulnar deviation, type II wrists tended to have greater ulnar deviation of the capitate relative to the scaphoid. During radial deviation, type II wrists tended to have greater radial deviation of the capitate relative to the triquetrum.
In plane motion: dominant and restricted articulations
In both wrist types, the dominant articulation in the radial column was dependent upon the direction of wrist motion; in ulnar deviation, the radiocarpal was dominant, and in radial deviation, the midcarpal was dominant. In the central column of both wrist types, the midcarpal articulation was dominant during both radial and ulnar deviation, although the effect was more pronounced in the latter. In the ulnar column, the radiocarpal articulation was dominant in both wrist types during ulnar deviation. During radial deviation, the radiocarpal was dominant in type I wrists, but in type II wrist, the movement at the radiocarpal and midcarpal articulations was equal (Figs. 4, 5).
Figure 4Mean RUD at the radiocarpal and midcarpal joints during ulnar deviation of type I and type II wrists. Positive values indicate ulnar deviation; negative values indicate radial deviation. SDs are omitted for easier reading; refer to Appendix B (available on the Journal’s Web site at www.jhandsurg.org) for details. All differences were statistically significant (P ≤ .004). See text for definitions of columns.
Figure 5Mean RUD at the radiocarpal and midcarpal joints during radial deviation of type I and type II wrists. Positive values indicate ulnar deviation; negative values indicate radial deviation. SDs are omitted for easier reading; refer to Appendix B (available on the Journal’s Web site at www.jhandsurg.org) for details. All differences were statistically significant (P < .004) except for the ulnar column motion of type I wrists (P = .337). See text for definitions of columns.
In type I wrists, the nondominant articulations in the radial column were also restricted during both wrist movements. In the central column of both wrists, the radiocarpal articulation was restricted during radial deviation only (Fig. 6).
Figure 6In plane motion with wrist radial (20°) and ulnar deviation (20°). Dashed line indicates dominant articulation (≥ 50%); solid bar indicates restricted articulation (< 4°). The dominant articulations in the radial and central columns of both wrist types are influenced by the direction of wrist motion. In the ulnar column of type I wrists, the radiocarpal articulation was dominant and the midcarpal was markedly restricted (2°). In contrast, there was equal motion at both articulations in the type II wrists.
There were no significant differences in out of plane motion between type I and type II wrists.
Discussion
This ex vivo study aimed to identify the differences in carpal motion between wrists with differing lunate morphologies. The lunate morphology affected the range of motion at the radiocarpal and midcarpal articulations during wrist flexion and extension. Second, each carpal column moved in a different way, with the relative contributions of the radiocarpal and midcarpal joints depending on either the lunate morphology (during wrist FE) or the direction of wrist motion (during RUD). In type II wrists, there were marked restrictions of some of the carpal articulations, particularly in the central column, such that they represented less than 10% of the carpal column motion.
There are some possible explanations for the restriction of midcarpal motion in type II wrists. If the additional lunate facet is not in the plane of wrist motion, it is likely to act as a locking mechanism and have a restricting effect on in-plane motion. The ligamentous anatomy may also be a factor. Nakamura et al
described that the triquetrocapitate ligament occurred almost exclusively in type II lunate wrists, suggesting that differing ligamentous anatomy may affect the carpal mechanics.
Differences in out of plane motion between the 2 wrist types were identified. The out of plane motions tended to be greater for the type II wrists, and the in-plane motions were greater for the type I wrists. We hypothesized that differences in the ligamentous anatomy could be responsible for this observation: as the wrist proceeds through its arc of motion, the wrist ligaments become taut, producing obligate out of plane motion in the carpus.
Other authors have reported results from similar experiments. Werner et al
reported carpal kinematics during wrist flexion and extension in a cadaveric model using motion sensors. However, their study did not discriminate between different lunate morphologies. During wrist flexion and extension, the movements of the scaphoid, lunate, and triquetrum were reported to represent 90%, 50%, and 65% of the total wrist movement, respectively. Capitate movement on the lunate thus represented 50% of wrist motion.
This result is comparable with ours when averaging type I and II wrists together. However, when considering them separately, types I and II wrists behaved differently. This highlights the importance of analyzing types I and II wrists separately.
In the current study, lunate morphology was associated with statistically significant differences in the motion of the scaphoid, lunate, and capitate. The range of motion previously reported lies between the different values for type I and type II wrists found in the current study. We believe the differences between our results and those of Werner et al
are a result of a heterogeneous group of lunate morphologies in the wrists used in the previous study, producing results that averaged out the differences associated with each lunate type.
There was greater amount of scaphoid flexion during radial deviation in a type II lunate wrist, which only demonstrated a trend to significance. This may be because the arc of radioulnar movement (20°–20°) employed in the current study was not large enough to demonstrate a difference in the carpal kinematics during ulnar deviation. Nakamura et al
reported that kinematic differences between the 2 wrist types were apparent earlier in radial deviation compared with ulnar deviation. Ulnar deviation was taken to maximum (> 30° in each case) and the differences were apparent only toward the end range of movement.
A surgeon can assess the type of lunate using standard radiological investigations. Based on the results of this study, a surgeon can therefore use a coronal MRI scan to determine the kinematic aspects of the individual wrist. This is potentially important in a number of applications, such as improving the success of ligament reconstruction or determining ideal rehabilitation regimens for the patient.
Lunate morphology has been associated with certain pathological processes in the wrist. This study provides insight into the kinematics of the carpus during various movements of the wrist. These differences in kinematics, such as the relative restriction of the midcarpal motion during flexion of type II lunate wrists, can potentially improve our understanding of the factors that predispose the carpus to pathology. For example, type II lunates have been associated with a lower incidence of dorsal intercalated segment instability deformity in the setting of scaphoid nonunion. This may be due to the relative stiffness of the midcarpal joint in a type II lunate wrist.
The restricted FE at a type II lunocapitate joint identified in the current study supports this theory.
In the type II wrists, there is RUD, but almost no midcarpal flexion. This is likely to produce shear of the articular surface and may explain the increased incidence of hamate arthrosis in this group.
Limitations
The measurement error in this study was 2°, which represents 7% of the 30° total arc of motion. The accuracy of the motion capture system was typical of that reported in other studies,
and this represented the vast majority of the total measurement error within the system. This level of accuracy compares favorably with other studies on this topic, which employed manual measurements on radiological images. Robertson et al
reported the accuracy of computer software measurement of distal radius fracture angulation and reported an intraobserver difference of 0° to 2° when subjects used computer software to measure the angle between 2 predrawn lines on a radiograph. This was before other sources of error such as image quality, image exposure, and limb positioning (ie, the accuracy of placement of these predrawn lines) were taken into account.
Given the relatively large interspecimen variability in carpal rotations (Appendices A and B, available on the Journal’s Web site at www.jhandsurg.org), the sample size in this study was relatively small. This most likely affected the number of measurements that reached statistical significance, yet significant differences were still identified. The setup of the experimental equipment allowed only a 30° arc FE movement and a 40° arc of RUD. Therefore, the results of this study do not include the extremes of motion of the wrist, at which carpal kinematics can exhibit large changes. A larger series with a greater arc of motion would have produced a better understanding of the out of plane motion.
Appendix
Appendix AMean Carpal Motion During Wrist Flexion and Extension
Ding B, Cazzolato BS, Stanley RM, Grainger S, Costi JJ. Stiffness analysis and control of a Stewart platform-based manipulator with decoupled sensor-actuator locations for application in biomechanical testing. J Dyn Sys Meas Control. In press.
The authors wish to acknowledge the following people and organizations: Dr. Boyin Ding, School of Mechanical Engineering, University of Adelaide, for technical support; Mr. Richard Stanley, School of Computer Science, Engineering and Mathematics, Flinders University, for technical support; and The Ray Last Anatomy Laboratory, University of Adelaide, for providing the specimens.
The study received institutional approval from the University of South Australia and the Southern Adelaide Human Research Ethics Committees.
No benefits in any form have been received or will be received related directly or indirectly to the subject of this article.
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