Measurement of Lag-Screw Anteversion With an iPhone During Trochanteric Fracture Surgery

Abstract

Introduction A useful way to easily evaluate femoral rotation during surgery for trochanteric fractures is not known. Hence, this pilot study aimed to develop an intraoperative indicator to evaluate anteversion in femoral trochanteric fractures. Material and methods Prospectively, from June 2021 to January 2022, all patients with femoral trochanteric fractures (Orthopaedic Trauma Association classification: 31A1-3) treated using a cephalo-medullary nail with a lag-screw neck-shaft angle of 125° were included in this study. During surgery, lag-screw anteversion (LS-AV) was measured using the goniometer application in an iPhone with the fractured femur table-top-plane level with the traction table floor. Accuracy was analyzed by comparing axial-projected lag-screw anteversion (AxP-LS-AV) and three-dimensional computed tomography lag-screw anteversion (3DCT-LS-AV) measurements after surgery. Results Fifty patients (14 males and 36 females) were included in the study. The mean age was 87 (range; 69-98) years; the Orthopaedic Trauma Association classifications were A1 (28 patients), A2 (18 patients), and A3 (4 patients). The mean LS-AV was 10.7° ± 6.9°, the mean AxP-LS-AV was 12.8° ± 8.3°, and the mean 3DCT-LS-AV was 13.1° ± 8.6°. The median difference between AxP-LS-AV and 3DCT-LS-AV was 3.0° (range: 0°-12°), and 40 (80%) patients had differences of ≤5° (Bland-Altman plot: inside of limit of agreement = 86%, paired t-test p = 0.7, Pearson correlation coefficient r = 0.817, p <0.001). Conclusion Femur malrotation is defined as a deformity of >15° relative to the normal contralateral limb. Intraoperative LS-AV iPhone measurement on table-top-plane standard had sufficient accuracy as an indicator of anteversion in femoral trochanteric fractures.

Keywords: anteversion; femoral rotation; iphone; lag screw; malrotation; smartphone; table-to-plane; trochanteric fracture.

Figure 1. Preparation for setting reference position

In the scissors position on the traction table, the normal femur was positioned above the fractured femur to prevent the overlap of the lateral view of both knees [A/B]. The fractured femur was positioned with the patella facing upward and pulled slightly by traction. The posterior edge of the greater trochanter and lateral condyle of the fractured femur were identified by palpation. These two posterior edges were connected with a line using a permanent marker. The fractured femur was positioned at this line, level with the floor, while checking the iPhone’s goniometer application [A]. Next, the image intensifier was set to the horizontal lateral view [B].

Figure 2. Reference position/image

While setting the image intensifier to the horizontal lateral view, the rotation of the fractured femur was positioned so that posterior condylar outlines are positioned to achieve complete overlap and centered (white arrow) on the fluoroscopic monitor [A]. Then, the reference position is set up. Finally, in the same position, the knee anterior–posterior view image is saved for the reference image, and the tibiofibular joint lines (white dotted lines) are recommended to be used as an indicator for rotation [B].

Figure 3. Reappearance of reference position/image

The fractured femur was positioned to the table-top plane, level with the floor again. The fractured femur was rotated to afford the anterior-posterior view of the fluoroscopic knee on the monitor similar to the “reference image” while comparing on twin monitors.

Figure 4. Measurement of LS-AV

An iPhone’s goniometer application was opened. the iPhone was packaged in a sterilized transparent bag. The iPhone’s goniometer application was zeroed on the instrument table. The lag-screw anteversion (LS-AV) was measured along the supported guide pin [A]. The level line must cross the figure (example: 13°) [B].

Figure 5. Converting LS-AV to AxP-LS-AV

Angle projection [A]: Relationship between the lag-screw anteversion (LS-AV) angle and axial-projected lag-screw anteversion (AxP-LS-AV) angle. Angle-correction graph [B]: Angle-correction graph converting LS-AV to AxP-LS-AV. This graph was specific for implants with a neck-shaft angle of 125°. For example, LS-AV 45° was corrected to AxP-LS-AV 51°. 3DCG femur model [C]: The plot points of the graph were determined from simulation analysis by three-dimensional computer graphics (3DCG). Paint3D software installed in Windows 10 was used to create the 3DCG femur model with a neck-shaft angle of 125°.

Figure 6. Measurement of 3DCT-LS-AV

Three-dimensional computed tomography (3DCT) subjects reconstructed by Synapse Vincent Volume Analyzer: In the cranial view, the distal portion of the nail is initially positioned to zero for abduction/adduction [A]. Next, by vertical rotation around the posterior condyle axis, the posterior edge of the nail end and both posterior condyles are set on a straight line (NE-BPC line), and the three-dimensional computed tomography lag-screw anteversion (3DCT-LS-AV) is defined as the angle between the lag-screw axis and NE-BPC line (dotted line) [B]. The NE-BPC line approximates the table-top-plane line.

Figure 7. Histogram of angle measurements

Histogram of lag-screw anteversion (LS-AV), the mean LS-AV is 10.7° ± 6.9°[A]. Histogram of axial-projected lag-screw anteversion (AxP-LS-AV), the mean AxP-LS-AV is 12.8° ± 8.3° [B]. Histogram of three-dimensional computed tomography lag-screw anteversion (3DCT-LS-AV), the mean 3DCT-LS-AV is 13.1° ± 8.6° [C].

Figure 8. Difference and correlation and agreement, between AxP-LS-AV and 3DCT-LS-AV

[A] Histogram of the difference between the axial-projected lag-screw anteversion (AxP-LS-AV) and three-dimensional computed tomography lag-screw anteversion (3DCT-LS-AV): The median difference between AxP-LS-AV and 3DCT-LS-AV is 3.0° (range: 0°–12°), and 40 (80%) patients had a difference of ≤5°. [B] Correlation shows Pearson correlation coefficient r = 0.817, p < 0.001. [C] Agreement of Bland–Altman plot (SD: standard deviation): 43 (86%) patients were within the limit of agreement. 7(14%) patients who were outside the limit of agreement had no similarity between anterior–posterior view of the knee and reference image when measuring lag screw anteversion.

Figure 9. Axes of hip true lateral view

Before inserting the lag screw, in the reference position, the true hip lateral view probably shows the difference between the neck axis and guide pin axis (example 5°).