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To determine the screw starting point and trajectory for the dorsal approach to scaphoid fractures that provides a combination of length and compression at the fracture site.
Methods
Computed tomography scans were obtained of 10 scaphoid fractures for 3 common fracture types. A computerized model was generated for each scaphoid. Screw starting point, length, and angle to the fracture plane were analyzed for starting points and trajectories within a safe zone that protected against cortical penetration. A novel analysis was developed to assess a combination of screw length and angle to fracture plane, termed “effective compression length” (ECL). ECL assessed the screw working distance perpendicular to the fracture. Results were analyzed to determine optimal screw starting point and trajectory.
Results
For proximal pole fractures, a screw perpendicular to the fracture was 9.7 mm from the longitudinal axis starting point (LASP). The screw with the largest ECL was 6.8 mm from the LASP, crossing the fracture at a 67° obliquity. For waist fractures, a perpendicular screw was 7.8 mm from the LASP. The screw with the largest ECL was 6.0 mm away, crossing the fracture at 74°. For distal oblique fractures, a perpendicular screw was 10.2 mm from the LASP. The screw with the largest ECL was 6.4 mm away, crossing the fracture at 70°. A screw with the classic starting point and trajectory crossed the fracture at obliquities of 48°, 51°, and 45° for proximal, waist, and distal fractures, respectively.
Conclusions
Scaphoid screws placed with the classic starting point and trajectory cross the fracture at an obliquity. By altering the screw starting point and trajectory, screws with adequate length will be more perpendicular to the fracture plane.
Clinical relevance
Screw starting point and trajectory for scaphoid fractures may be altered based on fracture type to obtain a long screw that is closer to perpendicular to the fracture.
properly treating these fractures is paramount to getting patients back to work and preventing wrist arthrosis. Although there is currently no consensus regarding the best treatment for scaphoid fractures, certain studies have shown that advantages of surgical treatment may include a higher union rate, quicker return to work, and better cost effectiveness than nonsurgical treatment with casting.
Multiple cadaveric studies have shown that longer screws with central screw placement for waist fractures had increased stiffness, less fracture motion, and greater stability than shorter and eccentrically placed screws.
Rather than treating all scaphoid fractures alike, there has been an argument to treat scaphoid fractures similar to that of other fractures, in which the screw is placed perpendicular to the fracture site. Recent studies have shown that there is less motion and equal, or higher, stability of scaphoid fractures treated with perpendicular screws rather than screws down the center of the longitudinal axis.
showed that screws placed more perpendicular to the scaphoid fracture plane had a shorter time to union as well.
Scaphoid fractures are classified based on the Herbert classification, with B1 fractures referring to distal oblique fractures, B2 as waist fractures, and B3 as proximal pole fractures.
Our study examines screw placement using the dorsal approach. Classically, this approach begins with a screw placed down the center of the proximal pole, approximately 1–2 mm radial to the scapholunate ligament.
and can be used for all 3 types of fractures. Mini-open fixation of the scaphoid from the dorsal approach has been found to be both safe and effective in treating scaphoid fractures with minimal dissection of the soft tissues and important vasculature of the scaphoid.
there has been little in terms of establishing a starting point and trajectory of the screw to be as perpendicular as possible to the fracture plane while maintaining a screw with adequate length and purchase. Using a similar computed tomography (CT) model as Leventhal et al,
we used CT scans of actual scaphoid fractures to determine screw starting points and trajectory for screws with the maximum length and screws that were perpendicular to the fracture plane. In addition, we determined the starting point and trajectory for screws with the highest “effective compression length” (ECL), a screw that had the longest perpendicular working distance to the fracture plane in the smaller of the fracture fragments, as shown in Figure 1. We defined the working distance of the screw as the length of the screw from the fracture site to its starting or ending location within the scaphoid bone. This gave us 2 different working distances for each screw with one on either side of the fracture. When determining the screw with the largest ECL, we looked at the smaller of the 2 working distances of each screw between the 2 fracture fragments. Finally, only the length of the screw that was perpendicular to the fracture plane was assessed as this is the aspect of the screw that places compression across the fracture.
Figure 1Demonstration of ECL, which refers to the working distance of the screw perpendicular to the fracture in the smaller of the 2 fracture fragments (red line). The smaller fracture fragment was chosen to ensure adequate purchase in the smaller fragment.
Under institutional review board approval at the Hospital for Special Surgery, a retrospective review of all wrist CTs previously performed at our institution was conducted to identify those with radiology reports indicating a scaphoid fracture. The most recent 500 were reviewed to select 10 patients exhibiting each of the 3 Herbert fracture types: B1 (distal oblique), B2 (waist), and B3 (proximal pole). Patients whose scans exhibited a severely displaced fracture, a large amount of sclerosis or cystic degeneration consistent with nonunion, screw fixation before CT acquisition, or those with subsequent imaging indicating the fracture had been repaired using a volar approach, were excluded.
Multioblique image reformatting was used to identify the fracture plane for all 30 patients by reorienting the imaging plane to coincide with the fracture plane (AW Server 3.2; GE Healthcare, Waukesha, WI). Fracture plane orientation was determined as that which resulted in the lowest signal (low x-ray attenuation) within the interior of the scaphoid. The fracture plane image metadata were exported for later use.
To create 3-dimensional scaphoid surface models, a region-growing segmentation approach was applied to the CT image sets using a seed point selected within the cortex of the scaphoid. If signal intensity was insufficient between the proximal and distal fracture segments to facilitate region-growing across both fracture segments, additional seeds were selected. Region-growing required a threshold to isolate bone from background soft tissue (approximately 450 HU), which was varied as necessary to facilitate segmentation of images acquired at different resolutions, from different scanners, or that were reconstructed using different algorithms and/or kernels. As shown in Figure 2, a 3D surface mesh was generated from each of the segmented CT scans using the Neighboring Cells algorithm. Bone segmentation and meshing were conducted using the custom MeVisLab software (MeVis Medical Solutions, AG, Bremen, Germany). A “safe zone” was established to constrain possible screw paths such that the screw would lie entirely within the scaphoid cortex and ensure that it was entirely in bone. The safe zone was offset by half a screw diameter (D = 3.6 mm) plus 0.35 mm to account for cortical thickness and an additional 0.25 mm margin of safety, after Leventhal et al.
A new mesh bounding this safe zone was generated using the custom Matlab code (Matlab; The MathWorks, Natick, MA) and MeshLab scripts (3D-COFORM Consortium, EU). A screw diameter of 3.6 mm was used as it was inclusive of commonly used screws for scaphoid fracture fixation.
Figure 2Surface reconstructions of a scaphoid (left), interior offset “safe zone” (center), and an overlay of the bone surface mesh and the underlying “safe zone” (right).
The previously identified fracture plane was used to divide the safe zone mesh into proximal and distal segment submeshes using the voxel-to-world transformation information in the DICOM header of the identified fracture plane image. A brute-force search of possible screw paths was performed by establishing all possible pairings of each mesh vertex in the proximal submesh to every vertex in the distal submesh. The set of all possible screw paths was then evaluated to determine screw placements based on 3 different sets of criteria. These criteria were: (1) longest overall screw, (2) longest screw placed perpendicular to the fracture, and (3) longest ECL. ECL was defined as the longest screw path that, when projected onto the normal vector of the fracture plane, yielded the greatest length in the smaller of the 2 fracture segments (Fig. 1). We chose the smaller of the 2 fracture segments because it allowed us to ensure an appropriate length of the screw within the bone on both sides of the fracture, and make sure that our screw working distance and length was appropriate in both segments of the scaphoid. Screws oblique to the fracture plane provide compression (perpendicular to the fracture) but also induce shear along the fracture plane. ECL quantifies the portion of screw length that acts perpendicular to the fracture, contributing to compression.
For each of these 3 screw paths (longest overall screw, longest screw perpendicular to the fracture, and the screw with the largest ECL), we assessed the length of the path within the safe zone, obliquity to the fracture plane, the distance on the scaphoid surface from the “classic” starting point (longitudinal placement), and the ECL. The classic starting point was defined as the most proximal starting point at the center of the longitudinal axis of the scaphoid. Descriptive statistics (mean, standard deviation) were calculated for each of these data points within each fracture type. Statistical analysis was performed using analysis of variance with post hoc Bonferroni correction to compare the screw paths as a group and t test to compare individual screw types.
Results
See Figure 3 for examples of the 3 screw trajectories within the 3 fracture types.
Figure 3Scaphoid fracture models for the 3 different fracture types showing the proposed placement of various screws. Type A shows a proximal pole fracture model. Type B shows a waist fracture model. Type C shows a distal oblique fracture model. The green line demonstrates the longest possible screw. The red line demonstrates the screw with the maximum effective compression length. The yellow line demonstrates the perpendicular screw.
For proximal pole fractures, the fracture plane was an average obliquity of 49.9° from the longitudinal axis. A screw with the longest possible length had a starting point 2.6 mm away from the longitudinal axis starting point (LASP) and crossed the fracture plane at an average obliquity of 48.2°. Its screw length averaged 23.8 mm. A screw perpendicular to the fracture plane was 9.7 mm from the LASP. Its screw length averaged 18.1 mm. The screw with the largest ECL was 6.8 mm from the LASP and crossed the fracture at an obliquity of 66.5°. Its screw length averaged 21.9 mm.
For waist fractures, the fracture plane was an average obliquity of 57.3° from the longitudinal axis. A screw with the longest possible length had a starting point 2.2 mm from the LASP and crossed the fracture plane at an average obliquity of 50.9°. Its screw length averaged 23.3 mm. A screw perpendicular to the fracture plane was 7.8 mm from the LASP. Its screw length averaged 19.3 mm. The screw with the largest ECL was 6.0 mm from the LASP and crossed the fracture at an obliquity of 73.4°. Its screw length averaged 21.6 mm.
For distal oblique fractures, the fracture plane was an average obliquity of 50.8° from the longitudinal axis. A screw with the longest possible length had a starting point 1.7 mm from the LASP and crossed the fracture plane at an average obliquity of 44.7°. Its screw length averaged 24.8 mm. A screw perpendicular to the fracture site was 10.2 mm from the LASP. Its screw length averaged 18.4 mm. The screw with the largest ECL was 6.4 mm away from the LASP and crossed the fracture at an obliquity of 70.0°. Its screw length averaged 21.9°.
For all fracture types studied, there was a difference in starting point location between the 3 different screw paths. In addition, each screw path crossed the fracture plane at a different average obliquity. Finally, the length of each of the different screw paths was different for each proximal pole fracture and each distal oblique fracture, but no difference was found for the length of each screw type in waist fractures. See Table 1 for results grouped by fracture type.
The purpose of this study was to determine the screw starting point and trajectory that provides a combination of length and compression at the fracture site when treating scaphoid fractures. Traditionally, scaphoid fractures have been treated using screws placed down the center of the longitudinal axis of the scaphoid,
and therefore it would make sense that the starting point and trajectory of the screw should vary based on the fracture. This would allow screws to be placed more perpendicular to the fracture site and achieve more purchase on both sides of the fracture. Studies have found decreased fracture site motion, higher stability, and shorter time to union for fractures treated with perpendicular screws rather than screws down the center of the longitudinal axis.
In our study, the goal was to identify the proper starting point for screws that would be perpendicular to the fracture, but also to identify screw placement for a screw with the largest ECL. Although Faucher et al
showed that shorter screws placed perpendicular to a scaphoid fracture had no difference in number of cycles or load to failure than longer screws placed down the longitudinal axis, there is still concern that the shorter screws that are required for placing a screw perpendicular to the fracture plane will have less purchase and less stability. A study by Dodds et al
in 2006 showed that a longer screw provided significantly more stability than a shorter screw in scaphoid fractures. Therefore, we used ECL to find the largest working distance of a screw that was perpendicular to the fracture. If looking at screws of equal length, a screw perpendicular to the fracture site should have a larger ECL than a screw that is oblique, because more of the screw is perpendicular to the fracture. In addition, the screw with the larger ECL should have less shear at the fracture site when compared with an equivalent screw with a smaller ECL because less of the screw is parallel to the fracture. This variable allowed us to combine length of screw and obliquity to the fracture plane to theoretically improve the effectiveness of the screw.
The fracture plane in all 3 fracture types was oblique to the longitudinal axis, ranging from 49.9° for proximal pole fractures, to 57.3° for waist fractures, and 50.8° for distal oblique fractures. These were similar values to those seen in the study by Luria et al.
Therefore, the current screw that allows for its maximal length down the longitudinal axis would lead to increased shear stress across the fracture given the screw’s obliquity to the fracture plane. Alternatively, screws placed perpendicular to the fracture plane that would allow for pure compression were found to be of significantly shorter length for proximal pole and distal oblique fractures. For example, when looking at distal oblique screws, the longest possible screw is 24.8 mm compared with the perpendicular screw, which is only 18.4 mm. The ECL in the smaller, distal fracture piece, however, is only 7.4 for the longest screw compared with 8.0 for the perpendicular screw. Therefore, even though the screw length is 6 mm shorter, it theoretically provides more compression perpendicular to the fracture plane than the longer oblique screw. Alternatively, the screw with the longest ECL has an absolute length of 21.9 mm with an ECL of 9.1, and crosses the fracture at an average obliquity of 20.0° to the fracture. This provides the maximum ECL while minimizing shear forces at the fracture site.
As the screw site shifts from running down the longitudinal axis to providing the highest ECL, the starting point needs to be shifted, as visualized in Figure 3. Although the longest screw possible generally is placed approximately 2–2.5 mm away from the LASP, perpendicular screws are generally placed approximately 10 mm from the LASP and the screw with the largest ECL was placed, on average, between 6.0 and 6.8 mm from the LASP, depending on the fracture type. Classically, the teaching is to start the screw 1–2 mm away from scapholunate ligament on the LASP of the scaphoid.
However, based on the results from this study, a screw started 6–7 mm from the LASP would allow for a screw to be placed more perpendicular to the fracture while maintaining an appropriate working distance.
Based on our results, the trajectory of the screw may also be shifted from the classic trajectory of directly down the center of the longitudinal axis. Based on the fracture plane, the new trajectory can be used to closely approximate a screw perpendicular to the fracture plane and ensure adequate purchase in the smaller fracture fragment piece. For a distal oblique fracture, for example, as shown in Figure 3, the trajectory will often be in the opposite direction of a longitudinal axis screw with the distal portion of the screw ending on the far ulnar side of the distal scaphoid. We routinely obtain preoperative CT scans of scaphoid fractures at our institution to evaluate for humpback deformity and better assess the fracture plane before surgery. This can help with preoperative planning and allows us to template our starting point and trajectory for scaphoid screw placement.
This study has several important weaknesses. First, all of the fracture planes and screw trajectories are done using only computer modeling based on CT scans. Although this allowed for accurate determination of the starting point and trajectory for various screw angles, it would be difficult to precisely match these during an actual surgery. Therefore, the results should only be looked at as guidelines for screw placement. In the future, it would be beneficial to have a simple computer algorithm attached to the imaging software that would allow for screw starting point and trajectory to be calculated quickly using a preoperative CT scan. A second limitation is that the ECL is only a theoretical model for screw compression and purchase. A future biomechanical study in cadavers should be performed, which compares screws with the highest ECL to screws with maximum length and shorter screws placed perpendicular to the fracture plane to determine stability, compression, and shear across the fracture. A third limitation in this study is a lack of a clinical endpoint. In the future, clinical results for scaphoid fractures with screws placed in our proposed starting point and trajectory need to be followed for rates of nonunion and speed of healing to effectively confirm our findings.
References
Van Tassel D.C.
Owens B.D.
Wolf J.M.
Incidence estimates and demographics of scaphoid fracture in the U.S. population.
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