This study was designed to determine how static distraction affects the three dimensional conformation of the carpus in live subjects. We found that the overall length of the carpus (radius to third metacarpal) increased by 3.3±3.1mm with a load of 98 N, primarily due to separation of the radiocarpal and midcarpal joints. Distraction at the third carpometacarpal joint was small (0.2±0.5mm) and not statistically significant. Almost all of the bones in the carpus also shifted dorsally with distraction, with the distal row moving more than the proximal row. In the proximal row, the scaphoid moved distally and extended slightly, as its long axis became more aligned with the long axis of the radius. Radial translation of the lunate and triquetrum towards the laterally fixed scaphoid effectively narrowed the proximal row.
In contrast to our findings regarding translation, the average rotation of the individual bones in the carpus was relatively small (less than 2° in any given direction: extension, radial deviation or pronosupination). This was more due to the lack of consistent pattern in rotation than universally low magnitude of rotation, as the total rotation for selected bones after loading was almost 10°.
We did not expect to see much if any dorsovolar or lateral compaction of the wrist during distraction, as the bones are quite congruent and tightly packed. We did, however, measure a substantial and significant decrease in centroid spacing between the radius and lunate bones. This occurred in spite of the fact that the surface of the lunate actually separated from the surface of the radius during distraction. The decrease in centroid spacing was due to radial translation of the lunate as it moved distally during distraction. This closed the gap between its centroid and the (alternate) centroid on the radius, which was located on the radiocarpal surface of the radius.
The relative distraction of the different carpal bones is consistent with ligament anatomy. In particular, the scaphocapitate and the scaphotrapezial ligament complexes are substantial structures that tightly bind the scaphoid to the capitate and trapezium 23,24
. These structures and the similarly robust carpometacarpal ligaments with their primarily axial orientation explain the limited distraction at the STT and carpometacarpal joints 25
. In contrast, the radiocarpal ligaments (volar radioscaphocapitate, radiolunate, ulnocarpal and dorsal radiotriquetral) are orientated oblique to the longitudinal axis of the wrist. This orientation accommodates significant mobility of the scaphoid, lunate and capitate during normal wrist motion26,27
but makes them less effective at resisting axial traction. Similarly, the four primary volar ligaments (radiolunate, ulnolunate, radioscaphoid and arcuate) are arranged to form two inverted “V”'s that reduce the resistance to axial separation. The smaller “V” is formed by the radiolunate and ulnolunate ligaments, which originate on the radius and TFCC and insert on the lunate. The ligaments of the larger “V”, the radioscaphocapitate and arcuate ligaments, follow similar paths, but originate further laterally on the radius and ulna and insert onto the capitate 28-31
. The longer, more peripheral and distal “V” ligaments elongate more than the shorter and more central “V” ligaments, resulting in greater distraction at the midcarpal and radioscaphoid articulations than the radiolunate joint. Similar arguments can be made for the intrinsic ligaments. Distraction between the centroids of the lunate and triquetrum and the lunate and scaphoid were very small (on the order of 0.1mm), which makes sense given the taut connections and short lengths of the interosseous ligaments 32-36
It remains unclear why most of the carpal bones translated dorsally and radially with distraction. The “V” shaped volar ligaments are reported to be stronger than the dorsal ligaments 29
, and cadaver studies have suggested that distraction is arrested by the volar ligaments before the dorsal ligaments reach their maximum length 7,37
. Assuming that the volar ligaments do restrain distraction, the bones would be expected to translate volarly, towards their attachments. Therefore the mechanism by which the carpal bones translated dorsally and radially remains to be identified, but likely includes a complex interaction between ligament morphology, ligament stiffness and the geometry of the articulations.
Our measured distractions are similar to those previously reported. Bartosh and Saldana37
found an average of 3mm of overall wrist distraction in 19 cadaver wrists, regardless of the amount of load used. Loebig et al.6
performed distractions on twelve cadaveric wrists using a standard servohydraulic mechanical testing device. They measured almost 2.5 times as much distraction at 80N of load than we did at 98 N(almost 8 mm vs. 3.3±3.1 mm). However, it is difficult to make direct comparisons between the two data sets because their testing was done in vitro
and their specimens were stripped of all skin and soft tissue, with the exception of the wrist capsule and ligaments. That said, it is possible to compare relative
bone motions to a maximum of 3.3mm of distraction, which was the average for our subjects. At 3.3mm of distraction, Loebig et al.6
found that lunocapitate distraction was 2.4 times as much as radiolunate distraction, which is consistent with our findings. Similarly, they measured 2.2 times more distraction at the radioscaphoid joint than the radiolunate joint, which is consistent with our findings6
In light of the large distractions reported in the study by Loebig et al.6
, we must acknowledge the possibility that our data under-represents carpal bone motion during wrist distraction. Although we asked our volunteers to relax during loading, and we made an effort to confirm relaxation via palpation of the forearm muscles, it is possible that some may have resisted the loading via active forearm muscle contraction. We did have subjects whose wrists distracted only nominally, and others whose joints distracted substantially more, as evidenced by our large sample variances. At this point it is impossible to determine whether the differences were volitional or simply a reflection of normal variability in joint laxity, as generalized joint laxity was not quantified during the screening examination prior to enrollment.
Our analysis of the influence of gender on wrist compliance revealed no statistically significant difference in overall wrist distraction between our male and female volunteers, despite nearly a twofold difference in the axial component of the radius-to-third metacarpal centroid spacing (2.3±2.3mm vs. 4.3±3.7mm, respectively). The lack of significance persisted even after we normalized our data by wrist size (i.e. third metacarpal length), which is known to differ with gender 12
. There are two interpretations for our finding: Either there is no gender-related difference in wrist compliance, or there is a difference and our study was not sufficiently powered to detect it. Of the two, we feel the later is more likely, given the large difference in the means and large sample variances.
Clinically, carpal distraction is used to help align the fragments in comminuted distal radial fractures1
. Previous studies have suggested that distraction of the wrist is a potential source of complications and adverse outcomes8,9,38
. To be maximally effective for fracture reduction, the effect of distraction should be localized to the radiocarpal joint39
. However, we found greater separation at the midcarpal joint than at the radiocarpal joint (e.g. twofold more separation at the lunocapitate joint than at the radiolunate joint). Accordingly, it must be recognized that distractive changes at the midcarpal joint are significantly greater than at the radiocarpal joint and there is a risk of damaging soft tissues, if too much force is applied37
Wrist distraction is also used clinically as an aid in the diagnosis of scapholunate ligament injury. For the “carpal stress tests” the examiner places a 5kg (49N) distractive load across the wrist and evaluates the step-off between the scaphoid and lunate 4
or scapholunate diastasis5
on posteroanterior radiographs or fluoroscopy. Our findings suggest caution when interpreting the results of the step-off technique. We found the scaphoid displaces an average of 2.4 times further than the lunate in healthy subjects, a result that might be interpreted as pathological. Admittedly, our loading protocol differed from that used clinically (i.e. we used 10kg applied to all five digits with the arm horizontal, as opposed to 5kg (plus the weight of the arm) applied to the thumb and/or thumb and first finger with the forearm arm vertical, and we measured centroid displacements as opposed to “edge” step-off or scapholunate diastasis). However, our results suggest that the uninjured scapholunate joint might be more compliant than previously appreciated. The scapholunate diastasis test5
may be more conservative, as we found only a nominal (<0.1mm) change in scapholunate centroid spacing with loading.
In summary, this study provides further insight into the mechanical behavior of the ligamentous carpus under distraction loads. These data may be useful clinically when considering the conformational changes that occur during wrist arthroscopy, the effect of traction on the reduction of distal radius fractures, and the diagnosis of scapholunate ligament tears. It also provides in vivo data for the development and validation of computer models of the normal wrist 40,41