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The Effect of Computed Tomography Scans Oriented Along the Longitudinal Scaphoid Axis on Measurements of Deformity and Displacement in Scaphoid Fractures
Reformatting computed tomography (CT) scans along the scaphoid longitudinal axis improves the ability to detect scaphoid fractures compared with reformats along the wrist axis. However, it remains unclear whether scaphoid axis reformats affect measurements of displacement or deformity, which are factors that drive the clinical decision to perform open reduction internal fixation. Our null hypothesis was that reformatting CT scans along the scaphoid axis does not affect measurements of fracture displacement and deformity.
Methods
Thirty patients with CT scans demonstrating scaphoid fractures (4 proximal pole, 17 midwaist fractures, and 9 distal) were identified and reformatted along 2 axes: the longitudinal axis of the scaphoid and the longitudinal axis of the wrist. The reformatted scans were sent to 2 musculoskeletal radiologists and 2 orthopedic hand surgeons who made the following measurements: (1) fracture gap, (2) displacement of the articular surface, (3) intrascaphoid angle, and (4) height to length (H:L) ratio.
Results
The reliability of each of the measurements cited above was compared for all raters between the 2 axes using intraclass correlation coefficients. Measurement of fracture gap and articular displacement trended toward more reliability in the wrist axis, whereas measurement of H:L ratio and intrascaphoid angle trended toward more reliability in the scaphoid axis. However, no differences in measurements between the 2 axes were statistically significant.
Conclusions
This study demonstrates that reformatting CT scans in line with the axis of the scaphoid does not result in more reliable measurements of displacement or deformity.
Clinical relevance
Measurements of displacement and deformity in scaphoid fractures can be made in the wrist axis with comparative reliability to those in the longitudinal scaphoid axis.
CT scans have a sensitivity of 93% and specificity of 99% for diagnosing scaphoid fractures compared with magnetic resonance imaging (MRI) studies that have a sensitivity of 96% and specificity of 99%.
Identifying the correct patient population to whom surgery should be offered is paramount, especially because some scaphoid fractures can be managed nonsurgically—studies report union rates varying from 94% to 98% for nondisplaced fractures.
Surgical versus nonsurgical treatment of acute minimally displaced and undisplaced scaphoid waist fractures: Pairwise and network meta-analyses of randomized controlled trials.
Cast immobilization with and without immobilization of the thumb for nondisplaced and minimally displaced scaphoid waist fractures: a multicenter, randomized, controlled trial.
CT scans can be useful because they allow the surgeon to make precise measurements to aid in determining whether surgery is indicated based on the presence of greater than 1 mm of displacement.
Measurement of the scaphoid humpback deformity using longitudinal computed tomography: intra- and interobserver variability using various measurement techniques.
CT scans are more accurate than plain radiographs in determining the degree of fracture displacement, identifying fracture patterns more prone to nonunions, and measuring the degree of articular displacement.
They can also be used to make more specialized measurements such as height to length (H:L) ratios and intrascaphoid angles, all of which may aid in decision making.
Measurement of the scaphoid humpback deformity using longitudinal computed tomography: intra- and interobserver variability using various measurement techniques.
No standard protocol for scaphoid CT imaging exists with regard to formatting the cuts: some centers routinely provide sagittal cuts in line with the wrist, others in line with the long axis of the scaphoid, and still others in line with the CT table regardless of patient hand positioning. Reformatting CT scans along the long axis of the scaphoid has been shown to improve the detection of scaphoid fractures, with 1 study demonstrating an increase in sensitivity from 33% to 67%, and increase in specificity from 89% to 96%.
However, it remains unclear whether scaphoid axis reformats affect measurements of displacement or deformity, which are factors that may drive the clinical decision to perform open reduction and internal fixation. Our null hypothesis was that reformatting CT scans along the scaphoid axis does not affect measurements of fracture displacement and deformity.
Methods
This institutional review board–approved (University of Pennsylvania, Philadelphia, PA) retrospective study identified 30 patients with acute scaphoid fractures using an online database (Montage Healthcare Solutions; Nuance Communications Inc.) of radiology reports. We searched for patients 18 years and older with scaphoid fractures who underwent a CT scan of the wrist. We excluded anyone who had any previous instrumentation of the wrist or carpus in place. In patients with multiple wrist CT scans, we chose the earliest study available in the database, choosing only 1 scan if multiple scans were available. We identified patients in a reverse chronological order beginning from August 2016 to April 2011. All patients were identified from 1 institution (University of Pennsylvania Health System).
All CT scans were then made anonymous by removing all patient identifiers. Using the TeraRecon software system (Aquarius iNtuition, Foster City, CA), each original CT scan was then reformatted along the longitudinal axis of the scaphoid as determined on the sagittal and coronal views of the scaphoid (Fig. 1). This set of reformats is referred to as the scaphoid axis CT scans. When analyzing the scans, we realized that the orientation of the original CT scans was not standardized, be it in the axis of the wrist or the scaphoid. Rather, the orientation was typically more closely aligned with the longitudinal axis of the table rather than a conventional wrist axis. In order to create a standardized control group, we created another set of CT reformats aligned with the conventional wrist axis oriented along the longitudinal axis of the radius. This set of reformats was our control group, referred to as the wrist axis CT scans (Fig. 2).
Figure 1Determination of the longitudinal axis of the scaphoid. A Coronal view. B Sagittal view.
Figure 2Determination of the wrist axis (control group). A Sagittal view shows the axis aligned with the longitudinal axis of the radius. B Axial view demonstrates the determination of the coronal plane for the wrist axis reformats. Note that the center of the axis is parallel to the longitudinal axis of the radius previously determined on the A sagittal and coronal (not shown) views.
The scaphoid axis and wrist axis CT scans were sent to 2 musculoskeletal radiologists (P.J.N. and B.K.) and 2 board-certified orthopedic hand surgeons (D.S. and D.B.). All the studies from both groups were randomly sorted and all the reformats were independently created prior to sending to evaluators. All primary studies and reformats were loaded into an image-viewing software (TeraRecon) that was calibrated equally for measurement and resolution in all users. The image series were delivered to each evaluator via a portable hard drive. Prior to making any measurements, 2 separate instructional meetings were held where all 4 evaluators agreed upon a uniform method of making measurements with real-time practice images. Each evaluator then independently read the randomly sorted CT scans and recorded their findings in a blinded fashion. On each CT scan, the following measurements were made: location of the fracture, fracture gap, displacement of articular surface, intrascaphoid angle, and H:L ratio.
An example of 1 of these measurements is demonstrated in Figure 3.
Figure 3Comparison of measurement of the intrascaphoid angle in the wrist and scaphoid axes. A Intrascaphoid angle being measured in the sagittal cut of the wrist axis. B Intrascaphoid angle being measured in the sagittal cut of the scaphoid axis. Images shown are the same numbered cut within each axis to allow for accurate comparison.
The location of the fractures was recorded as 1 of the following: proximal pole fracture, midwaist fracture, or fracture distal to the waist not including the scaphoid tuberosity. The fracture gap was identified as the largest displacement in millimeters seen on either the coronal, the axial, or the sagittal planes. The displacement of the articular surface was identified as the largest measurement in millimeters on either the coronal, the axial, or the sagittal planes.
For the H:L ratio, each orthopedic surgeon and radiologist identified the sagittal cut with the largest flexion or humpback deformity.
The reliability of each of these measurements was compared for all raters between the 2 axes using intraclass correlation coefficients (ICCs) with 95% confidence intervals in a 2-way random fashion assessing single measures for absolute agreement. Measurements of reliability such as ICCs are not amenable to comparison by routine statistical analyses such as t tests. Instead, more specialized methods of measuring variance must be used. One such test is the F test, which assesses variance between 2 measures of reliability—ICC being that measure in this case. The advantage of an F test is that it can be used to compare one group of ICCs with another group of ICCs, rather than each individual ICC separately. For example, we compared all the ICCs of fracture gap in the wrist axis from all evaluators with all of the ICCs of fracture gap in the scaphoid axis for all evaluators collectively. An F value was calculated for the ICCs for each of the measurements of displacement and deformity described previously. The critical F value was then identified based on the degrees of freedom allowed from the study’s sample size and an alpha of 0.05.
The calculated F values were then compared with the critical F value to determine if the degree of variance of the ICCs between the 2 axes was significant.
Finally, the sample size needed to power the ICCs at a beta of 0.2 was determined. The lowest ICC from all the axes was used to determine the sample size because lower ICCs require larger sample sizes to be adequately powered for differences to be observed.
Archives of Orofacial Sciences: A simplified guide to determination of sample size requirements for estimating the value of intraclass correlation coefficient: a review.
There were 4 proximal pole fractures, 17 midwaist fractures, and 9 fractures distal to the waist, but no tuberosity fractures. Measurement of fracture gap and articular displacement trended toward more reliability in the wrist axis, whereas measurement of H:L ratio and intrascaphoid angle trended toward more reliability in the scaphoid axis. To test if the ICCs between the 2 axes were statistically different, right-tailed F values for comparing variance were calculated as follows: 1.314 for fracture gap, 1.322 for displacement of articular surface, 1.295 for intrascaphoid angle, and 1.324 for H:L ratio. The critical F value for an alpha of 0.05 was determined to be 1.3519, based on existing references.
All calculated F values were less than the critical F value. This confirmed that there was no statistically significant difference in the reliability of measurements between the scaphoid and the wrist axes. These results are summarized in Figure 4. The required sample size was determined based on the ICC of the wrist axis for the intrascaphoid angle because it was the least reliable measurement.
Archives of Orofacial Sciences: A simplified guide to determination of sample size requirements for estimating the value of intraclass correlation coefficient: a review.
We confirmed the null hypothesis that reformatting CT scans along the scaphoid axis does not increase the reliability of measurements of fracture displacement and deformity because fracture gap, articular displacement, intrascaphoid angle, and H:L ratio between the axes were judged with approximately equal reliability. Despite this, we acknowledge that CT scan reformats in the axis of the scaphoid can still be useful for understanding the alignment of the entire bone, assessing fracture pattern, measuring the size of the fracture fragments, and determining screw sizes.
There are several limitations to this study. For example, it is important to note that we compared measures of reliability, not validity between the 2 axes. Although we can say that neither axis is more reliable for making measurements, we cannot state that one axis is more accurate than the other. That would require comparison of the radiographic measurements to a reference standard, such as intraoperative evaluation of fracture gap, articular displacement, angulation, and so on.
From a technical standpoint, we found that reformatting the original CT scans into the wrist and scaphoid axes using the TeraRecon software resulted in pixilation of the images and some loss of resolution. This is in part because once the raw data from the original CT scan have been used to create the image that the clinician sees, the extraneous data are deleted and not stored in the electronic medical record. As such, future reformats of images are based on an already reformatted image rather than the initial raw data themselves. Furthermore, when we reformat the CT scans from the randomly oriented original scans, we are assuming that the same amount of information can be gleaned from the reformats. In other words, the difference in measurements between the original scan and any reformat thereafter is unknown and needs to be studied further. For instance, the ICCs for all the measurements were generally low, regardless of the axis in which they were measured. This might reflect variability introduced from the reformatting process.
In addition, each reader made the measurements only once. The intrareader reliability of the measurements of deformity and displacement are unknown and may well contribute to the variability in findings. Likewise, H:L ratio measurements were subject to variability based on which cut they were measured. A separate study comparing 2 techniques for measuring H:L ratios found that picking the sagittal cut that the clinician perceived to have the greatest flexion deformity resulted in more precise measurements compared with 3-dimensional measurement.
the evaluators selected the sagittal cut with the worst-appearing flexion deformity. As such, there was inherent variation in the cut used to make the measurement. The original technique by Amadio et al
described measuring the intrascaphoid angle on a lateral tomogram; however, with modern CT scans that generate multiple sagittal images, it is difficult to know which cut would result in the most reliable measurement. To reduce some of this variability, each reviewer measured the intrascaphoid angle and the H:L ratio on the same cut they identified within each axis for every sample. More studies are needed to determine the best technique for measuring intrascaphoid angles in CT scans.
To conclude, although reformatting CT scans in the axis of the scaphoid may be beneficial in diagnosis of occult fractures, they are not any more reliable than wrist axes in making measurements of displacement and deformity.
References
Ty J.M.
Lozano-Calderon S.
Ring D.
Computed tomography for triage of suspected scaphoid fractures.
Surgical versus nonsurgical treatment of acute minimally displaced and undisplaced scaphoid waist fractures: Pairwise and network meta-analyses of randomized controlled trials.
Cast immobilization with and without immobilization of the thumb for nondisplaced and minimally displaced scaphoid waist fractures: a multicenter, randomized, controlled trial.
Measurement of the scaphoid humpback deformity using longitudinal computed tomography: intra- and interobserver variability using various measurement techniques.
Archives of Orofacial Sciences: A simplified guide to determination of sample size requirements for estimating the value of intraclass correlation coefficient: a review.
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