Abstract
Background and Aims
The Angle Bisector Method uses the cortices of the tibia and fibula as reference points and suggests a syndesmotic fixation trajectory through the bisector of the angle formed by two lines tangent to the anterior and posterior aspects of the tibia and fibula. This study aimed to assess whether the Angle Bisector Method can provide a patient- and level-specific syndesmotic fixation angle that is reproducible, safe, and independent of the surgeon in a cadaveric setting.
Methods
Twelve matched above-knee leg specimens from six cadavers, underwent syndesmotic fixation (using either screws or suture-buttons) at two levels—2 cm and 3.5 cm proximal to the tibial plafond. The fixations were carried out using the angle bisector method by two surgeons employing an open lateral approach. Subsequently, CT images of the cadavers were obtained. The angle between the true centroidal axis and the axis of the syndesmotic implant trajectory was measured using radiological software. Additionally, distances between fibular entry points of the centroidal axis and the implant trajectory were measured. Distances between the positioned K-wires and major neurovascular structures were measured on cadaver dissections, and any resultant damage was documented.
Results
The average angle between the centroidal axis and screw trajectory was 2.7 ± 2.9 degrees at the 2 cm level and 1.8 ± 2.5 degrees at the 3.5 cm level. The average distance between the fibular entry points of the centroidal axis and the screw was 1.7 ± 1.2 mm at the 2 cm level and 1.2 ± 1.0 mm at the 3.5 cm level. The results demonstrated low inter-surgeon variability and high intra- and inter-observer reliability (ICC>0.80). The distance between the placed K-wires and major neurovascular structures was always higher than 5 mm, affirming the safety of the technique.
Conclusion
Our data suggests that the angle bisector method can provide a reliable trajectory for syndesmotic fixation and may be safely used in surgical procedures. This approach can be applied with K-wires or a specially designed jig to facilitate syndesmotic fixation.
Graphical Abstract
1
Introduction
Syndesmotic fixation is a commonly required orthopaedic procedure where acheiving precise anatomical reduction and stability is critical to restoring ankle biomechanics and preventing unfavorable clinical outcomes. Despite this, malreduction remains a common issue in practice, with reported rates reaching up to 52 % . Research has identified accurate syndesmotic reduction as the most significant predictor of functional outcomes following ankle injuries . Various aspects of syndesmotic fixation—such as implant selection, fixation level, ankle positioning, number of cortices, and the use of clamps—have been extensively discussed in the literature to address the risk of malreduction . However, the fixation angle has received comparatively less attention. Some studies suggest that improper implant alignment during fixation can result in iatrogenic malreduction, as non-perpendicular placement relative to the tibiofibular joint may cause unintended fibular displacement during the procedure , .
The AO guidelines recommend placing syndesmotic fixation 2–3.5 cm proximal to the tibial plafond, with an angular trajectory of 20–30 degrees to the coronal plane . However, recent studies suggest that the optimal alignment follows a line connecting the centroids of the tibia and fibula, which can vary between 8 and 38 degrees , . This variability indicates that the conventional approach may not be suitable for all patients . Determining the ideal fixation angle intraoperatively remains uncertain, as the recommended angles are not tailored to individual anatomical differences. Consequently, the choice of angular direction in the axial plane is often left to the surgeon’s discretion, making it challenging to achieve consistent, patient-specific alignment.
Previous studies using lower extremity CTs and 3D-printed models have demonstrated that the angle bisector method, guided by two K-wires, can accurately target the true syndesmotic axis , . However, no cadaveric studies have been conducted to validate its accuracy or safety for clinical use. This study therefore aims to assess whether the angle bisector method can reliably provide a patient- and level-specific syndesmotic fixation angle that is reproducible and independent of surgeon variability in a cadaveric model. Additionally, we seek to evaluate its safety by analyzing the proximity to major anterior and posterior neurovascular structures, ensuring safety for potential surgical application. We hypothesized that the Angle Bisector Method provides an accurate estimate of the true syndesmotic axis and that its application is both safe and reproducible when used by different surgeons.
2
Materials and methods
2.1
Cadavers and surgical procedure
Ethical approval for the project was obtained from the institutional review board, and fresh-frozen cadaveric specimens were acquired. Twelve matched above-knee leg specimens from six cadavers (4 male, 2 female; average age 84.2 ± 7.9 years), without any evident or known foot and ankle pathology, underwent syndesmotic fixation (using either screws or suture-buttons) at two levels—2 cm and 3.5 cm proximal to the tibial plafond, aligned parallel to it. ( Fig. 1 )
After marking the joint line, and the levels 2 cm and 3.5 cm proximal to joint line (left), the angle bisector method was applied to find the trajectories of syndesmotic implants, and the drill and implants were applied to all cadavers at two levels (right).
The fixations were performed using the angle bisector method by two fellowship trained foot and ankle surgeons, with cadaver specimens randomly assigned between them. The angle bisector method involved placing a drill and syndesmotic implant, cortical screw or suture button implant [TightRope (Arthrex, Naples, FL, USA)] along the bisector of the angle formed by two 1.8 mm K-wires, which were inserted percutaneously at the fixation level. These K-wires were positioned tangentially to the anterior and posterior surfaces of the fibula and tibia by manually advancing them to contact the cortices, without the use of a power drill. To ensure stability and prevent displacement during dissection—although not required for the technique itself—we advanced the K-wires until they pierced the skin on the opposite side, as shown in Fig. 3 C. This provided greater stability by increasing the length of their path through the soft tissue.
2.2
Cadaveric CT evaluation
CT images of the cadaver specimens were obtained, and the angle between the true centroidal axis and the axis of the syndesmotic implant tunnel created using the angle bisector method was measured by specialized software (Mimics Innovation Suite, Materialise, Leuven, Belgium). Two blinded observers performed the measurements twice, with a two-week interval between sessions. Additionally, the distance (in mm) between the most lateral fibular intersections of the centroidal axis and the implant tunnel was measured by the same blinded observers in a similar manner ( Fig. 2 ).
The CT analysis of the cadavers demonstrated the angular differences between the centroidal axis and the syndesmotic implant trajectories, as well as the distances between their entry points at 2 cm and 3.5 cm proximal to the ankle joint space. For these cadavers, the angular difference was 1.17 degrees at 2 cm (left) and 0.88 degrees at 3.5 cm (right). The distance between the entry points was 0 mm at 2 cm (left) and 0.9 mm at 3.5 cm (right).
2.3
Cadaveric safety analysis
The distances (in mm) between the positioned K-wires and major neurovascular structures—including the anterior and posterior neurovascular bundles, superficial peroneal nerve, and sural nerve—were measured during cadaver dissections. Any instances of neurovascular damage were carefully documented ( Fig. 3 ).
Photos from dissections to evaluate safety of the technique. A) Superficial peroneal nerve (SPN), B) Anterior neurovascular bundle (ANVB) including Anterior tibial artery, Anterior tibial vein, Deep peroneal nerve, C) Posterior neurovascular bundle (PNVB) including Tibial nerve, Posterior tibial artery, D) Sural nerve.
2.4
Statistical analysis
Descriptive statistics (mean, standard deviation, and range) were calculated for the measurements. The angular differences relative to the true syndesmotic axis and the distances to the actual lateral fibular entry point were analyzed to assess the reliability of the angle bisector method. For the method to be considered reliable, both measurements needed to be equal or close to zero with minimal variability.
Intra- and inter-surgeon variability in the cadaveric measurements, as well as intra- and inter-observer consistency in the radiological assessments, were evaluated using the intraclass correlation coefficient (ICC). A two-way mixed-effects model was employed to analyze the absolute agreement of the exact measurements.
3
Results
3.1
Cadaveric CT measurements after syndesmotic fixations
The measurements showed that the angle bisector method provides an accurate trajectory for syndesmotic fixation, as indicated in Table 1 , demonstrating low inter-surgeon variability and high intra- and inter-observer reliability (ICC > 0.80). The average angle between the centroidal axis and screw trajectory was 2.7 ± 2.9 degrees at the 2 cm level and 1.8 ± 2.5 degrees at the 3.5 cm level. The average distance between the fibular entry points of the centroidal axis and the screw was 1.7 ± 1.2 mm at the 2 cm level and 1.2 ± 1.0 mm at the 3.5 cm level.
Table 1
CT measurements for the angle between the centroidal axis and the implant tunnel trajectory (ACT), as well as the distance between the fibular entrance points of the centroidal axis and the implant tunnel trajectory (DCT).
| Mean | Standard deviation | Minimum-Maximum | Inter-surgeon ICC (95 % CI) |
Inter-observer ICC
(95 % CI) |
Intra-observer ICC (95 % CI) | |
|---|---|---|---|---|---|---|
| ACT at 2 cm (Pooled) | 2.7 | 2.9 | 0–9.2 | 0.811 (0.758–0.841) | 0.882 (0.848–0.924) | 0.911 (0.855–0.942) |
| Surgeon 1 | 2.3 | 3.6 | 0–9.2 | |||
| Surgeon 2 | 3.1 | 2.2 | 0–6.7 | |||
| ACT at 3.5 cm (Pooled) | 1.8 | 2.5 | 0–7.8 | 0.832 (0.769–0.871) | 0.891 (0.858–0.943) | 0.899 (0.847–0.931) |
| Surgeon 1 | 1.3 | 2.4 | 0–3.8 | |||
| Surgeon 2 | 2.3 | 2.2 | 0–7.8 | |||
| DCT at 2 cm (Pooled) | 1.7 | 1.2 | 0–3.8 | 0.826 (0.759–0.883) | 0.865 (0.812–0.892) | 0.875 (0.829–0.923) |
| Surgeon 1 | 1.3 | 1.6 | 0–3.8 | |||
| Surgeon 2 | 2.0 | 1.0 | 0–3.2 | |||
| DCT at 3.5 cm (Pooled) | 1.2 | 1.0 | 0–2.5 | 0.868 (0.824–0.926) | 0.872 (0.832–0.917) | 0.912 (0.869–0.943) |
| Surgeon 1 | 1.3 | 1.2 | 0–2.5 | |||
| Surgeon 2 | 1.2 | 1.0 | 0–2.3 |
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