Preoperative 3D CT imaging techniques provide displacement analysis of the distal scaphoid fragment in 3D space, using the matched opposite scaphoid as reference. Its accuracy depends on the presence of anatomical bilateral symmetry, which has not been investigated yet using similar techniques. Our purpose was to investigate symmetry by comparing the relative positions of distal and proximal poles between sides. We used bilateral CT scans of 19 adult healthy volunteers to obtain 3D scaphoid models. Left proximal and distal poles were matched to corresponding mirrored right sides. The left-to-right positional differences between poles were quantified in terms of three translational and three rotational parameters. The mean (SD) of ulnar, dorsal, and distal translational differences of distal poles relative to proximal poles was 0.1 (0.6); 0.4 (1.2); 0.2 (0.6) mm and that of palmar rotation, ulnar deviation, and pronation differences was −1.1 (4.9); −1.5 (3.3); 1.0 (3.7)°, respectively. These differences did not significantly differ from zero and thus were not biased to left or right side. We proved that, on average, the articular surfaces of scaphoid poles were symmetrically aligned in 3D space. This suggests that the contralateral scaphoid can serve as reference in corrective surgery. No level of evidence is available.
A scaphoid waist fracture with displacement in which the proximal and distal poles are malaligned is seen as an indication for surgery [
In current clinical practice, assessment of displacement is based on measurements using two-dimensional (2D) images (i.e., radiographs and single CT slices) which is subjective due to manual measurements, position of the wrist during imaging, and/or slice selection [
Quantitative 3D CT imaging techniques can be applied to assess the level of scaphoid fracture displacement in 3D space, demonstrated by several recently published studies [
Scheme of 3D model of scaphoid fragments before virtual reduction (a) and after reduction (b). The mirrored opposite scaphoid (dotted outline) serves as guide to virtually reduce the nonunion fragments. This method enables quantifying the amount of displacement of the distal fragment in 3D space.
A prerequisite for a reduction technique that uses the opposite scaphoid as reference is the presence of normal bilateral symmetry [
The purpose of this anatomic 3D CT study was to investigate the symmetry of healthy scaphoid pairs. To this end, we quantified side-to-side differences of the positions of distal poles within healthy scaphoid pairs in terms of the three translational and three rotational parameters. We hypothesize that there is no bias to the left or right side in each of these parameters, showing average difference values not significantly different from zero.
Nineteen healthy right-handed volunteers participated in this study (13 women and six men; average age: 26 y; range: 22–56 y). The subjects had no history of wrist injury or other musculoskeletal disorders. A high-resolution CT scan (Philips Brilliance 64 CT scanner, Cleveland, OH) was made of both wrists (i.e., bilateral CT scan) of each individual using standardized methods (voxel size 0.45 × 0.45 × 0.45 mm., 120 kV, 150 mAs, pitch 0.6, and slice thickness 0.67 mm.). The CT scans were used for subsequent 3D image analyses. To determine the methodological accuracy and reproducibility of our method, one cadaver arm was scanned multiple times (10x), using the same scan protocol. This study was approved by our Human Research Committee. Informed consent of each individual was obtained prior to participation.
First, from each scaphoid pair, the left scaphoid is segmented from a CT scan, based on custom made software [
Virtual model of a left scaphoid with anatomical coordinate system, defining translational and rotational differences. After matching the proximal (blue; 25%) poles, side-to-side differences are shown as the degree in which the positions of the distal poles (green; 25%) differ between the left and right sides.
Next, we selected a proximal and a distal pole of 25% of the total length of the left scaphoid (Figure
For this registration process, first, a 3D double-contour polygon is automatically created based on the initial 3D polygon mesh of the left scaphoid by sampling the image intensity 0.3 mm toward the inside (high CT value) and outside (low CT value) of the bone, along the surface normal vector. The points of the double-contour polygon of the left proximal pole are registered with the reference image of the mirrored right scaphoid in a rigid point-to-image registration procedure [
Then, side-to-side differences are expressed as the degree in which the positions of the distal poles differ, relative to the proximal pole, between left and right scaphoids. The three translational differences (ulnar, dorsal, and distal translational) and three rotational differences (palmar rotation, ulnar deviation, and pronation) are derived from the 4 × 4 transformation matrices that resulted from image registration [
Statistical analyses of the measurements included the Shapiro Wilks W test as normality test, determining the mean and standard deviation (SD) for normally distributed data. A one-sample
We assessed the accuracy and reproducibility of our method by investigating the influence of the segmentation and matching procedure on translational and rotational side-to-side differences of the distal poles. To this end, we used ten CT scans of a single cadaveric arm. For each scan, the arm was scanned at a slightly different position inside the scanner to include possible variations in the reconstructed 3D image due to different positions of the wrist. Hereafter, a single 3D model of the scaphoid was obtained from the first CT scan. The proximal pole of this scaphoid model was selected and subsequently matched to the remaining nine scans of the same cadaver arm (Figure
Scheme of the cadaveric experiment assessing the accuracy and reproducibility of the matching procedure. After scanning one cadaveric arm tenfold, the scaphoid from one CT scan was segmented (left model). The proximal (blue) poles were matched to the remaining 9 scans enabling displacement analysis of the distal (green) pole.
Accuracy and reproducibility of radioulnar, palmodorsal, and proximodistal translation of the distal pole relative to the proximal pole were (mean (SD)) −0.1 (0.1), 0.1 (0.1), and 0.0 (0.1) mm, respectively. Accuracy and reproducibility of dorsopalmar rotation, radioulnar deviation, and supination and pronation deviation of the distal pole relative to the proximal pole were (mean (SD)) equal to −0.1 (0.5), 0.1 (0.4), and −0.1 (0.3) degrees, respectively. All means did not deviate more than a tenth of a millimeter or degree from zero indicating a high level of methodological accuracy. All standard deviations were lower than a tenth of a millimeter or half a degree, indicating a relatively high reproducibility.
Values of all translational and rotational differences of the distal poles of the 19 scaphoid pairs were normally distributed. Corresponding means and standard deviations are listed in Table
Results of the left-to-right alignment differences of the distal poles represented by the six side-to-side differences based on the anatomical coordinate system. Negative displacement values represent opposite directions.
Displacement | Mean | Compared to 0 ( |
SD |
---|---|---|---|
Translational | |||
Ulnar (mm) | 0.1 | 0.50 | 0.6 |
Dorsal (mm) | 0.4 | 0.11 | 1.2 |
Distal (mm) | 0.2 | 0.17 | 0.6 |
Rotational | |||
Palmar rotation (deg.) | −1.1 | 0.32 | 4.9 |
Ulnar deviation (deg.) | −1.5 | 0.07 | 3.3 |
Pronation (deg.) | −1.0 | 0.27 | 3.7 |
Scatterplot showing the left-to-right alignment differences of the distal poles of the 19 uninjured scaphoid pairs. Each dot represents a side-to-side difference for an individual healthy subject expressed in terms of an anatomical coordinate system (Figure
We used a quantitative 3D CT method to investigate the degree of positional differences of distal poles between healthy scaphoids sides. The proposed method of evaluation includes determination of an anatomical coordinate system that permits objectively comparing side-to-side differences of different individuals. The applied technique has proven to be accurate and highly reproducible. Overall, the translation and rotation differences between sides did not significantly differ from zero. This implied that there was no bias to the left or right side, indicating anatomical bilateral symmetry of the scaphoid poles in 3D space.
A limitation of our study is that all participants were right handed, which does not provide information about the side-to-side differences in left-handed individuals. Despite being not proven in this study, we expect similar results for left-handed individuals.
In upper extremity surgery, the opposite healthy bone can be used to plan and guide reconstruction of complex fractures [
Regarding the scaphoid, the level of bilateral symmetry has previously been investigated in several studies. Smith investigated left-to-right differences based on length, height, and intrascaphoid angle in 2D reconstructed sagittal and coronal sections from 30 healthy scaphoid pairs [
Although, on average, we found no left or right bias, in some individual cases, side-to-side differences were as large as 2 mm or 5–10°. These differences are small compared to values reported in clinical 3D CT studies investigating scaphoid nonunion deformity [
The best reference is obviously the native, pretraumatic scaphoid itself, but scaphoid images before injury are only rarely available by coincidence. When using alternative references such as the contralateral side, one should question what difference between the postreduction and desired pretraumatic alignment can be considered clinically acceptable. This question is difficult to answer since recent articles focusing on the consequences of scaphoid malalignment are sparse. Some relatively old clinical articles suggested an association of malunion with pain, loss of motion and weakness after fracture healing [
In conclusion, we proved that, on average, the articular surfaces of left and right scaphoid poles were symmetrically aligned. This suggests that the contralateral side is a useful reference in preoperative planning in reconstruction surgery of scaphoid fractures. Three-dimensional fracture displacement analysis provides objective information which may help the surgeon in characterizing complex fractures and surgical decision making.
All named authors hereby declare that they have no conflict of interests to disclose.
Paul W. L. ten Berg received a Ph.D. grant (2014) from the Academic Medical Center (Amsterdam, Netherlands) supporting this research.