A morphological study of the scaphoid using a mathematical technique and comparative study of the three-dimensional measurements of the scaphoid

3D computational anatomy of the scaphoid and its waist for use in fracture treatment

BACKGROUND

A detailed understanding of scaphoid anatomy helps anatomic fracture reduction, and optimal screw position. Therefore, we analysed (1) the size and shape variations of the cartilage and osseous surface, (2) the distribution of volumetric bone mineral density (vBMD) and (3) if the vBMD values differ between a peripheral and a central screw pathway?

METHODS

Forty-three fresh frozen hand specimens (17 females, 26 males) were analysed with high-resolution peripheral quantitative computed tomography (HR-pQCT) and dissected to compute a 3D-statistical osseous and cartilage surface model and a 3D-averaged vBMD model of the scaphoid. 3D patterns were analysed using principal component analysis (PCA). vBMD was analysed via averaging HR-pQCT grey values and virtual bone probing along a central and peripheral pathway.

RESULTS

(1) PCA displayed most notable variation in length ranging from 1.7 cm (- 2SD) to 2.6 cm (mean) and 3.7 cm (+ 2SD) associated with differences of the width and configuration of the dorsal surface (curved and narrow (4 mm) to a wider width (9 mm)). (2) High vBMD was located in the peripheral zone. Lowest vBMD was observed in the centre and waist. (3) Virtual probing along a peripheral pathway near to the cartilage surfaces for the capitate and lunate allowed the center region to be bypassed, resulting in increased vBMD compared to a central pathway.

CONCLUSION

High anatomical variations regarding the osseous and cartilage surfaces were associated with three distinct concentrically arranged zones with notable different vBMD. The complex scaphoid anatomy with its waist might alter the strategy of fracture fixation, education and research.

Estimating Scaphoid Lengths Using Anatomical Measurements in the Wrist

PURPOSE

In reconstructive surgery of scaphoid nonunions with humpback deformity, some surgeons recommend restoration of the normal scaphoid length whereas others overexpand the normal length to ensure carpal realignment and prevent late collapse. To be able to define overexpansion and investigate which levels of overexpansion yield optimal clinical results, a precise method for estimating the original scaphoid length is required. The purpose of this anatomic study was to investigate the precision of estimating normal scaphoid lengths based on intact adjacent bone dimensions, compared with using the contralateral scaphoid length.

METHODS

From bilateral computed tomographic scans of 28 healthy wrist pairs, 3-dimensional virtual bone models were created. The left and right scaphoid lengths were determined at the central axis. The capitate length at the central axis and the distal radius width served to derive an ipsilateral scaphoid length estimate. Estimation precision for individual cases was based on the 95% range (±1.96 × SD) of the observed differences between the actual and estimated lengths.

RESULTS

On average, the capitate length was 10% smaller than the scaphoid length; the radius width was 9% larger. Consequently, we averaged the capitate length and radius width for ipsilateral estimations. The average difference between the scaphoid length and the latter ipsilateral estimate was 0.1 mm. The average contralateral scaphoid length difference was also 0.1 mm. Estimation precisions, however, were ±2.2 and ±1.4 mm, respectively.

CONCLUSIONS

Scaphoid length estimation based on the contralateral scaphoid is more precise than the estimating scaphoid length using the ipsilateral radius and capitate.

CLINICAL RELEVANCE

Scaphoid overexpansion can be ensured if the restored length is at least 1.4 mm longer than the contralateral length. This may be valuable information when establishing a target length for reconstruction and investigating the consequences of scaphoid overexpansion on clinical function, such as range of motion, which are currently unknown.

Three-Dimensional Assessment of Bilateral Symmetry of the Scaphoid: An Anatomic Study

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.

Determination of the Central Axis of the Scaphoid

Purpose Determine the central axis of the scaphoid and its relation to surrounding anatomic landmarks to facilitate internal fixation of the scaphoid. Methods Seventeen cadaveric dissections of the wrist were performed. Measurements of the height and width of the proximal pole, waist, and distal pole were made. The midpoint of the height and width of each measurement were plotted on a scatter plot graph and a forecast line was developed. The formula of the resultant line was used to calculate the position of the central axis at the proximal pole, waist, and distal pole. The inverse tangent of the slope of the line was then used to determine the angle of the line from proximal to distal. Results The average central axis fell along a line measuring at points from the ulnar to the radial border and from the dorsal to the volar border of the proximal pole, waist, and distal pole at 7.86 mm, 7.61 mm, and 7.31 mm respectively; an angle of 13.78 degrees from ulnar to radial and dorsal to volar. The proximal point can be determined by measuring ∼44 mm radially from the ulnar styloid along the watershed line of the radius and 14 mm volar from the dorsal tip of the Lister tubercle. The distal point can be determined by measuring ∼4 mm ulnar from a line extending distally from the volar radial corner, and 7 mm volar from the most dorsal point of the combined surface of the trapezium and triquetrum. No significant difference existed between male and female specimens. Conclusions The central axis of the scaphoid can be described to exist along a line extending from the relative central point of the proximal pole, measured 7.86 mm radial from the scapholunate ligament and 8.31 mm volar of the most dorsal point; through the waist, and extending to the relative central point of the distal pole measured 3.77 mm ulnar of the volar radial corner and 7.36 mm volar of the most dorsal point at an angle directed radially and volarly at 13.78 degrees. Level of Evidence Level III Type of Study Diagnostic/ therapeutic.

Central versus eccentric internal fixation of acute scaphoid fractures

PURPOSE

To accurately calculate the cross-sectional area of typical scaphoid fracture patterns and compare the amount of fracture surface area available for healing when a screw was positioned in the center of the scaphoid (central) versus perpendicular (eccentric) to the fracture plane.

METHODS

We employed a laser scanning technique to create 3-dimensional models of the scaphoid and permit the precise calculation of area along any cross-sectional cut of the bone. We computed approximate bone apposition areas for typical acute Herbert and Fisher fractures for 10 dry bone specimens. Next, we modeled internal fixation for each of these fracture geometries with screws placed either along the central axis of the scaphoid or eccentrically, perpendicular to the fracture plane. We calculated the proportional areas occupied by the screw and remaining area available for fracture healing.

RESULTS

The mean surface area of the simulated distal oblique, complete waist, and proximal pole fractures was 131, 86, and 58 mm(2), respectively. There was little difference in available area for complete waist and proximal pole fractures, but eccentric screw fixation perpendicular to the plane in distal oblique fractures consumed significantly less area than the centrally placed position.

CONCLUSIONS

The area available for apposition is widely variable and depends on each bone's unique morphology, the orientation of the fracture plane, and the design of the screw. Of the 3 fracture patterns studied, the obliquity of the screw with respect to the long axis was greatest for distal oblique fractures where perpendicular (eccentric) placement is preferable to maximize surface area available for healing.

CLINICAL RELEVANCE

Given the poor vascular supply of the scaphoid and morbidity associated with scaphoid fracture nonunions, this study examined a key determinant of bone healing by characterizing the area of these fractures and amount of bone apposition available for blood flow and healing when internally fixed with a compression screw.

Three-dimensional computed tomographic analysis of 11 scaphoid waist nonunions

PURPOSE

To virtually assess nonunions of the scaphoid waist using 3-dimensional computed tomography (CT) reconstruction for the amount of displacement of the distal fragment and the postfracture reduction position using the intact opposite scaphoid for reference.

METHODS

We generated 3-dimensional reconstructions for 11 nonunions of the scaphoid waist and the contralateral intact scaphoids based on CT. The mean age of the patients was 25 years and the time from injury to the CT scan was 2.4 years. We used the mirrored 3-dimensional model of the healthy scaphoid to guide virtual reduction of the nonunion and calculated the amount of displacement of the distal pole fragment from prereduction to postreduction. We compared the results with the intrascaphoid angles calculated using single CT slices.

RESULTS

The scaphoid nonunions showed a mean flexion deformity of 23°, an ulnar deviation of 5°, and a pronation deformity of 10°. Mean translation was 0.9 mm volarward, 0.2 mm radialward, and 3.3 mm distalward. After reduction, all scaphoids showed a bony overlap on the dorsoradial side; the mean volume of this region was 3% of total bone volume. There was no correlation between the degree of displacement and the intrascaphoid angle measurements.

CONCLUSIONS

Preoperative planning for scaphoid reconstruction is usually performed using conventional radiographs and single CT slices. However, by synthesizing the information from the CT into a 3-dimensional reconstruction, an exact analysis is possible. This method also allows quantification of prosupination displacement. The postreduction area of dorsal bone overlap may be due to appositional callus formation.

CLINICAL RELEVANCE

Simple volar opening of the scaphoid allows correction of angulation deformities but results in lengthening of the scaphoid. Correct reduction of the scaphoid fragments is often only possible if the dorsal appositional callus is resected.

The length and position of the long axis of the scaphoid measured by analysis of three-dimensional reconstructions of computed tomography images

We established the maximum length and the position of the long axis of the scaphoid from three-dimensional reconstructions of spiral computed tomography in 30 pairs of wrists. The distance between two points on the three-dimensional scaphoid surface model were calculated using commercially available software and corresponding coordinates of the two points were documented. The mean length was 29.3 (SD 1.6) mm for men and 26.6 (SD 1.8) mm for women. The location of the distal point was at the centre of the scaphoid tuberosity, with the proximal point of the long axis located at the dorsal ridge of the scapholunate facet.