Radiographic Quantification of Safe Zones for Screw Placement in the Femoral Neck: A Computed Tomography-Based Analysis

Orientated-quantitative computed tomography study on individualized axial safety target area of femoral neck screw channel and establishment of a stable spatial coordinate system based on anterior cortex of femoral neck basilar

BACKGROUND

Frequent in-out-in femoral neck screws were reported potential huge iatrogenic-injury risks, related to axial safe target area (ASTA) of femoral neck screws channel. However, orientated-quantitative ASTA based on stable coordinate system was unreported before.

METHODS

Three-dimensional reconstruction was performed on computed tomography (CT) images of 139 intact normal hips, and the intersection area, defined as ASTA, was obtained by superimposing the axial CT images of each femoral neck. Taking anterior cortex of femoral neck basilar (AC-FNB) as landmark, a coordinate system was established to measure the anterior-posterior diameter (D-AP), the superior-inferior diameter (D-SI) and the oblique angle respectively. Each intersection was overlaid up to the axial CT images to determine the coronal location of the ASTA boundaries.

RESULTS

Each ASTA presented an inclined rounded triangle with a flat anterior base coincided with AC-FNB. There were significant sex differences in D-SI (male: 33.6±2.3 vs. female: 29.4±1.9 mm) and D-AP (male: 25.3±2.1 vs. 21.9±1.9 mm), P <0.001. D-SI was found to be positively correlated with D-AP ( R2 =0.6). All fluoroscopic visible border isthmus completely matched the corresponding ASTA boundaries. The oblique angle was 5-53° (male: 28.1±10.3°, female: 27.1±8.2°) without significant difference between sexes.

CONCLUSION

The intersection method was employed to conveniently acquire orientated-quantitative individualized ASTA. Under this coordinate system, x-ray data of screws could be converted to axial coordinates in CT ASTA, which could help surgeons design combined screws configuration preoperatively and evaluate quantitatively their axial position intraoperatively.

A new posterosuperior screw placement strategy to avoid in-out-in screws in femoral neck ractures

Objective

The inverted triangle configuration of the three cannulated screws is the classic fixation method most commonly performed for undisplaced femoral neck fractures in young and geriatric patients. However, the posterosuperior screw has a high incidence of cortical breach, known as an in-out-in (IOI) screw. In this study, we present a novel posterosuperior screw placement strategy to prevent the screw from becoming IOI.

Methods

Using computed tomography data and image-processing software, 91 undisplaced femoral neck fractures were reconstructed. The anteroposterior (AP), lateral, and axial radiographs were simulated. To simulate the intraoperative screw placement process, participants used three screw insertion angles (0°, 10°, and 20°) to place the screw on the AP and lateral views of the radiograph according to the three established strategies. On the AP radiograph, a screw was placed abutting (strategy 1), 3.25 mm away from (strategy 2), or 6.5 mm away from (strategy 3) the superior border of the femoral neck. On the lateral radiograph, all the screws were placed abutting the posterior border of the femoral neck. Axial radiographs were used to evaluate the screw position.

Results

In strategy 1, all the placed screws were IOI regardless of the screw insertion angle. In strategy 2, 48.3% (44/91) of IOI screws occurred at a 0° screw insertion angle, 41.7% (38/91) of IOI screws occurred at a 10° screw insertion angle, and 42.9% (39/91) of IOI screws occurred at a 20° screw insertion angle situation. In strategy 3, no IOI screw occurred, and the screw insertion angles did not affect the safety and accuracy of screw placement.

Conclusions

Screws placed according to strategy 3 are safe. The reliability of this screw placement strategy is unaffected by a screw insertion angle of less than 20 degrees.

A Novel Fluoroscopic View for Improved Assessment of the Safety of the Posterosuperior Screw in Femoral Neck Fracture Fixation

BACKGROUND

The purpose of the present study was to determine specific fluoroscopic views of the femoral neck to accurately identify partially extraosseous ("in-out-in"; IOI) placement of the posterosuperior screw for fixation of femoral neck fractures.

METHODS

A 3.2-mm guide pin was placed in the posterosuperior aspect of 2 synthetic femur models: 1 entirely intraosseous and 1 IOI. Sequential fluoroscopic images were made at 5° intervals in order to identify which fluoroscopic projections identified IOI guide pin placement. These images were utilized to inform screw placement and assessment in the second phase of the study, which involved the use of cadaveric specimens. In Phase II, the posterosuperior screw of the inverted triangle was placed in 10 cadaveric specimens with use of a standard posteroanterior fluoroscopic view and 1 of 2 lateral views, either (1) neck in line with the shaft, i.e., 0° lateral; or (2) a -15° rollunder view. The final fluoroscopic views (i.e., the posteroanterior and multiple lateral and oblique views) were randomized and blinded for review by 10 orthopaedic residents and 5 attending orthopaedic traumatologists. Specimens were stripped of soft tissue and inspected for screw perforation.

RESULTS

Overall accuracy of respondents was 68.8%, with no difference between the attending traumatologists (71.8%) and resident surgeons (67.4%; p = 0.173). Interobserver reliability was moderate (κ = 0.496). Dissection identified that 4 (40%) of 10 screws were extraosseous. All of the extraosseous screws were placed with use of the 0° lateral view. The -15° rollunder lateral view was the most sensitive (81.7%) and specific (92.2%) view for identifying IOI screw placement.

CONCLUSIONS

Surgeons often utilize the standard posteroanterior and 0° lateral fluoroscopic views to safely place screws; however, many of these screws are IOI. The addition of a -15° rollunder lateral view significantly improved identification of IOI screws in the posterosuperior femoral neck. Unidentified IOI screw placement may result in damage to the blood supply of the femoral head.