Injury to the distal tibiofibular syndesmosis is common and appears in up to 20% of patients with an ankle sprain or ankle fracture [1–3]. If not treated appropriately, long-lasting disabilities like chronic pain, instability, and ankle joint osteoarthritis may occur [2, 4, 5]. Injury can occur to any of the four main components of the distal tibiofibular syndesmosis: the anterior inferior tibiofibular ligament (AITFL), interosseous membrane (IOM), posterior inferior tibiofibular ligament (PITFL), and transverse tibiofibular ligament (TTFL) [2, 3, 6]. Additionally, a deltoid ligament injury is also frequently present in patients with syndesmotic injury .
Conventional (weight-bearing) radiographs (anteroposterior and mortise view) (Fig. 12.1), CT scans (Fig. 12.2), and magnetic resonance imaging (MRI) (Fig. 12.3) are widely used for assessment of the distal tibiofibular syndesmosis .
While pronounced injuries can be reliably assessed using conventional radiographs, the diagnosis of incomplete injuries, especially in the absence of a fracture (e.g., high ankle sprain), is difficult [8–11]. In addition, measurements on conventional radiographs do not reliably reflect the injury pattern, which limits the general utility of conventional radiographs in assessing the distal tibiofibular syndesmosis . Correlating findings in magnetic resonance imaging (MRI)with patient complaints can prove challenging . Therefore, an accurate imaging modality to assess patients with incomplete injuries to the distal tibiofibular syndesmosis is desirable.
With the introduction of weight-bearing CT scans, detailed assessment of foot and ankle disorders under load-bearing conditions became possible [13–15]. However, the impact of load on two-dimensional (2D) measurements performed on axial CT images to assess the integrity of the distal tibiofibular syndesmosis is debated [16, 17]. The purpose of this cadaver study was to assess the influence of weight on the assessment of incomplete and more complete syndesmotic injuries using 2D measurements on axial CT images. We hypothesized that weight would significantly impact on the assessment of both incomplete and more complete injuries to the distal tibiofibular syndesmosis.
Seven pairs of male cadavers (tibia plateau to toe-tip) were included (mean age 62 ± 7 [range 52–70] years; mean weight 84.9 ± 15.3 [range 65.8–104.8] kg; mean body mass index (BMI) 26.8 ± 5.0 [range 19.7–32.5] kg/m2). Inclusion criteria were 20–70 years of age and a BMI of less than 35 kg/m2. Exclusion criteria were a history of any foot and ankle injuries or a history of surgery of the foot and ankle.
Each specimen was thawed for 24 hours at room temperature before any experiments were performed . A radiolucent frame held the specimens in a plantigrade position (Fig. 12.4). The cadaver was fixed with an Ilizarov apparatus that fit into the frame. Four 1.5 mm Kirschner-wires (K-wires) were drilled through the tibia for fixation to the frame. The K-wires were tightened using a dynamometric wire tensioner (Smith & Nephew). The hindfoot was fixed using two 1.5 mm K-wires drilled through the calcaneus, and two-part resin (Bondo®, 3 M) stabilized the soft tissue envelope below the level of the syndesmotic ligaments. Non-weight-bearing and weight-bearing CT scans were collected (PedCAT, CurveBeam LLC, Warrington, USA, medium view, 0.3 mm slice thickness, 0.3 mm slice interval, kVp 120, mAs 22.62).
First, intact ankles (native) were scanned. Second, one specimen from each pair underwent AITFL transection (Condition 1A), while the contralateral underwent deltoid transection (Condition 1B). Third, the lesions were reversed on the same specimens, and the remaining intact deltoid ligament or AITFL was transected in each ankle (Condition 2). Finally, the interosseous membrane (IOM) was transected in all ankles (Condition 3). Conditions 1A and 1B were considered to mimic incomplete injuries, while Conditions 2 and 3 were considered to mimic more complete injuries. For each condition, non-weight-bearing, half-bodyweight (42.5 kg), and full-bodyweight (85 kg) CT scans were taken. Loading levels were determined from the average of specimen donor anthropometrics. Preconditioning of the specimen was performed by statically loading the frame with 42.5 kg and 85 kg for 2 minutes each before the experiments were performed.
Measurements for interobserver agreement calculation were done by a fellowship-trained orthopedic surgeon and a research analyst. For calculation of the intraobserver agreement, measurements were performed two times with an interval of 3 weeks by a fellowship-trained orthopedic surgeon. Each observer completed a computer-based training before measurements were performed.
Imaging and Measurements
Axial CT images 1 cm above the medial edge of the distal tibial plafond were reconstructed (CurveBeam LLC, Warrington, USA, Version 220.127.116.11) and used for the following measurements: distance between the most anterior point of the tibial incisura and the nearest most anterior point of the fibula (ATFD), distance between the most anterior point of the fibula and a line perpendicular to the most anterior point of the tibial incisura (AFT), distance between the most anterior point of the fibula and a line perpendicular to the connection of the most anterior and posterior point of the tibial incisura (MFT), and distance from the same perpendicular line to the most posterior point of the fibula (PFT, Fig. 12.5) [16, 17, 19]. In addition, the angle between the fibular axis (the line between the most anterior and posterior edge of the fibula) and the line between the anterior and posterior edge of the tibial incisura were measured (Angle 1) . Furthermore, the tibiofibular overlap (TFO, defined as the maximum overlap between the lateral tibia and medial fibula) and the tibiofibular clear space (TFCS, defined as the distance from the lateral border of the posterior tibial tubercle to the medial border of the fibula) were measured on the same axial images . On the level of the talar dome (axial images), the angle between the fibula and medial malleolus (Angle 2) was measured .
Intraclass correlation (ICC) was used to quantify the agreement of measurements between and within observers. Estimates and 95% confidence intervals (CI) were calculated for each type of measurement. Interobserver agreement was modeled with a two-way random effect model of absolute agreement with a single measurement per observation. Intraobserver agreement was modeled with a two-way mixed effect model of consistency with a single measurement per observation. Agreement was rated as excellent with an ICC >0.75; good with an ICC = 0.61–0.75; fair with an ICC = 0.4–0.6; and poor with an ICC <0.4 .
Linear mixed effect models were fit for responses. Cadaver, treated as a random effect, and foot, (left or right) treated as a fixed effect, were included in all models in addition to the variables presented lateral in the tables. Models were fit for subsets of the data (with given weight or condition constant) and estimates and 95% CI are given for differing levels of condition or weight. Confidence intervals were calculated using a Tukey adjustment for multiple comparisons within each model. Significance was determined based on a P-value of less than 0.05 after the Tukey adjustment. All calculations were done in R 3.4.1, specifically using package psych and ImerTest.
Inter- and intraobserver agreement differed between measurements (Table 12.1). Excellent agreement was evident for the TFCS and TFO (intraobserver agreement, 0.79 and 0.94). Poor agreement was evident for Angle 1 (interobserver, 0.39). The agreement of the other measurements (inter- and intraobserver) was either rated as fair or good and ranged from 0.44 to 0.71. Load application had no significant influence on almost every measurement across all conditions (e.g., without subdividing into different conditions; Table 12.2). Divided into the tested conditions, only the ATFD or TFO could identify more complete injuries (Condition 3) from native ankles (Tables 12.3 and 12.4). No significant differences were evident between single AITFL and deltoid ligament transection for the ATFD and TFO. No significant differences were observed within each condition between non-, half-, and full-weight-bearing when using the ATFD or TFO (Tables 12.5 and 12.6).
Agreement of computed tomography scans assessed by intraclass correlation (ICC)