Data Source

Ten different male cadavers (tibia plateau to toe-tip; seven left, three right) were included (mean age 63 ± 9 [range 47–70] years; mean weight 83.1 ± 13.5 [range 63.5–104.8] kg; mean BMI 26.3 ± 3.8 [range 20.1–32.3] kg/m²). The cadavers have been provided by United Tissue Network (Norman, OK, USA), and Science Care (Phoenix, AZ, USA). Inclusion criteria were 20–70 years of age and a body mass index (BMI) of less than 35 kg/m². To ensure a homogeneous cohort, only male cadavers were included. Exclusion criteria were a history of any foot and ankle injuries or a history of surgery of the foot and ankle.

Experiments

Before any experiments were performed, each specimen was thawed for at least 24 hours at room temperature [17]. A radiolucent frame consisting of a base plate and four pillars located on the lateral sides of the foot was created to hold the specimens in a plantigrade position (Fig. 14.1). Each cadaver was fixed with an Ilizarov apparatus (using four 1.5 mm Kirschner wires [K-wires] drilled through the tibia) that fit into the frame. Tightening of the K-wires was done using a dynamometric wire tensioner (Smith & Nephew). The hindfoot was fixed using two 1.5 mm K-wires drilled through the calcaneus. In addition, a two-part resin (Bondo®, 3 M) was used to stabilize the hindfoot.

Intact ankles (native) were scanned first. Then, each ankle underwent AITFL transection (Condition 1). The deltoid ligament was transected next (Condition 2), followed by the IOM (Condition 3), and the PITFL (Condition 4). Transection of the deltoid ligament included the tibio-navicular and tibio-spring ligament. Then, the tibiocalcaneal ligament was transected. Finally, the anterior and posterior parts of the deep deltoid ligament were transected under visual control. While Conditions 1 and 2 mimic incomplete injuries, Conditions 3 and 4 mimic more complete injuries to the distal tibiofibular syndesmosis.

Weight bearing (85 kg; determined from the average specimen weight) CT scans with and without application of 10 Newton meter (Nm) internal rotation torque applied at the Ilizarov apparatus (corresponding to external torque of the foot and ankle) were performed (PedCAT, CurveBeam LLC, Warrington, USA, medium view, 0.3 mm slice thickness, 0.3 mm slice interval, kVp 120, mAs 22.62) [13]. Before the experiments were performed, preconditioning of the specimens was done by loading the frame with 42.5 kg and 85 kg for 2 minutes each.

Imaging and Measurements

Digitally reconstructed radiographs (DRRs) were reconstructed automatically using the CT scan dataset (CurveBeam LLC, Warrington, USA, Version 3.2.1.0). Previous research showed that measurements on DRRs were comparable to conventional radiographs [16]. As conventional radiographs are still the standard imaging modality to assess syndesmotic injuries, this approach provides additional information regarding the impact of torque on measurements when using conventional radiographs. The anteroposterior (AP) view of the ankle joint was generated perpendicular to a longitudinal axis of the foot [18, 19]. The mortise view was generated by virtually internally rotating the foot until a symmetrical widening of the medial and lateral gutters were visible [20]. As torque impacts talar rotation, it is difficult to reconstruct a mortise view under torque conditions [21]. Therefore, the same degree of internal rotation in native conditions was used for reconstruction of a mortise view from an AP view in the same specimens while under torque loading. The TFCS and the TFO were measured 1 cm above and the MCS 1 cm below the medial edge of the distal tibial plafond on both AP and mortise views (Fig. 14.2) [22, 23].

CT scans 1 cm above the medial edge of the distal tibial plafond (axial images) were reconstructed at the highest point of the distal tibial plafond on the sagittal view and used for measurements of the TFO and the TFCS (Fig. 14.2) [24]. CT scans 1 cm below the medial edge of the distal tibial plafond (axial images) were additionally reconstructed. Those images were used for measurement of the MCS (Fig. 14.2). The MCS was defined as the distance between the most anterior articular surface of the medial malleolus to the talus.

Statistical Analysis

Intraclass correlation coefficients (ICC) were used to quantify the agreement of measurements between and within observers. Estimates and 95% confidence intervals (CI) were calculated for each type of measurement within each view. Interobserver agreement was modeled with a two-way random effect model of absolute agreement with a single measurement per observation, defined as ICC. Intraobserver agreement was modeled with a two-way mixed effect model of consistency with a single measurement per observation, defined as ICC. Agreement was rated as very good (ICC > 0.80), good (ICC = 0.61–0.80), moderate (ICC = 0.41–0.60), fair (ICC = 0.21–0.40), and poor (ICC < 0.20) [25]. Measurements for interobserver agreement calculation were done by a physician and a research analyst. For calculation of the intraobserver agreement, measurements were performed two times with an interval of 3 weeks by a physician.

Linear mixed effect models were fit for responses. Within DRR and CT measurements, separate models were fit for each measurement (MCS, TFO, and TFCS) and, for DRR measurements, within each view. Cadaver, a random effect, and foot (left or right), a fixed effect, were included in all models in addition to the variables presented. Models were fit for subsets of the data and estimates, and 95% CI were given for differences in measurements in different levels of a specific variable. For each of the models, only the differences in response that were associated with either different torques or different conditions were calculated; the data was subset by the other two variables, and they remained constant within each model. The first set of models compared the differences in response for 10 Nm versus 0 Nm of torque applied with full weight bearing load (condition constant within each model). The second set of models compare the differences in response between Conditions 1 through 4 and the native ankles, with full weight bearing load (torque constant within each model). Coefficients and 95% confidence intervals were reported, and statistical significance (marked by an asterisk in all tables and graphs) was determined based on a P-value of less than 0.05.

Pearson’s correlation was used to assess the relationship between DRR—AP view only—and CT scan. Correlation was graded as very strong (>0.80), strong (0.60–0.80), moderate (0.40–0.60), weak (0.20–0.40), and very weak (<0.20). For each measurement, values were paired by cadaver, foot, condition, and torque, and the correlation estimated between the pairs was computed. Confidence intervals were constructed using the Fisher transformation. All calculations were done in R 3.4.1, specifically using packages psych and lmerTest.

Digitally Reconstructed Radiographs

The average internal rotation to reconstruct a mortise view out of an AP view was 9.9 ± 1.4 (range 8.0–13.9) degrees. Inter- and intraobserver agreement for measurements made on DRRs were rated as very good/good for the MCS and TFO (Table 14.1). TFCS measurements were rated as moderate for interobserver and very good (AP view)/good (mortise view) for intraobserver reliability. Torque significantly impacted MCS measurements if any of the four ligaments was transected, but not on native ankles (Table 14.2). The TFCS significantly decreased for native ankles (AP and mortise view) and additionally for Condition 1 (mortise view) if torque was applied. A significant increase of the TFCS was evident for Condition 4 (mortise view, including torque). TFO measurements on native ankles and on each condition significantly decreased if torque was applied. The AITFL and deltoid ligament had to be transected to detect a significant difference for the MCS on the AP and mortise view compared to native ankles (including torque, Fig. 14.3). For the TFCS, only fully dissected ankles (Condition 4) significantly differed from native ankles (both views, including torque). Using the mortise view, Condition 3 additionally differed significantly from native ankles when the TFCS was measured (including torque, Fig. 14.4). For the TFO, isolated AITFL release significantly differed from native ankles if torque was applied (Fig. 14.5).

Table 14.1

Reliability of digitally reconstructed radiographs (DRR) and computed tomography (CT) measurements assessed by intraclass correlation coefficient (ICC) [1]

		Interobserver: ICC(2,1) Estimate (95% CI)	Intraobserver: ICC(3,1) Estimate (95% CI)
AP (DRR)	MCS	0.75	0.93
		(0.51, 0.87)	(0.87, 0.96)
	TFCS	0.59∗	0.88
		(0.35, 0.76)	(0.79, 0.94)
	TFO	0.95	0.99
		(0.88, 0.98)	(0.97, 0.99)
Mortise (DRR)	MCS	0.87	0.95
		(0.77, 0.93)	(0.91, 0.98)
	TFCS	0.55∗	0.70
		(0.29, 0.73)	(0.50, 0.83)
	TFO	0.92	0.97
		(0.77, 0.97)	(0.94, 0.98)
CT scan	MCS	0.94	0.95
		(0.89, 0.97)	(0.92, 0.98)
	TFCS	0.93	0.94
		(0.74, 0.97)	(0.89, 0.97)
	TFO	0.92	0.99
		(0.53, 0.98)	(0.97, 0.99)

DRR digitally reconstructed radiograph, AP anteroposterior, CT computed tomography, MCS medial clear space, TFCS tibiofibular clear space, TFO tibiofibular overlap, CI confidence interval, ICC intraclass correlation coefficient

∗Indicates ICC < 0.61

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