In this study we have established that the normal tibio-fibular clear space is 3.78mm and that the mean widening of the tibio-fibular clear space is 0.3mm for ER-N and 0.44mm for ER-IR on dynamic ultrasound examination. In neutral position a measurement of 5.06 mm (Std. Dev. 0.64, 3.78+2SD=5.06) would be 2 standard deviations above the mean. This suggests a measurement greater than this should be suspected as abnormal. Females had increased clear space opening with forced external rotation, with a significantly greater “widening ratio” of the AITFL to applied stress. This agrees with the literature as female subjects tend to be more flexible than males [7]. There was no correlation between clear space opening and activity, height or leg length. Those who participate in gymnastics and dance tend to have increased joint laxity, [8] however in this study sports activity was not shown to be a significant factor, although a trend was evident.
Younger subjects showed higher absolute clear space opening measurements and stress opening than older subjects, a result which was evident in both genders. This finding is expected given the general increase in stiffness that occurs with aging [9, 10]. We also found that activity, height or leg length did not correlate significantly with opening of the joint. That means that tall athletes, for an example, would not have a larger opening than younger non-active females.
The results of a previously published study indicate that a dynamic ultrasound examination can accurately diagnose a syndesmotic injury in cases of latent (not evidenced on radiograph films) grade 3 AITFL sprains [4]. Similar to other ligament injuries, injuries to the AITFL are graded based on stretch (grade1), partial tear (grade 2), or complete rupture (grade 3) [11]. Complete rupture of the AITFL is described as a stage one syndesmotic injury, which with additional force may cause the talus to spread the tibia and fibula apart. This can subsequently damage the posterior aspect of the syndesmosis (PITFL and transverse ligament) and cause a stage three injury [12] or a frank fracture.
One of the strengths of this study is that it provides information regarding the width of the anterior tibio-fibular clear space in a neutral unloaded position and also provides dynamic assessment of the opening of this space with the application of a deforming force. We believe this is the first study to record syndesmotic opening dynamically in normal subjects.
One of the features of this presented method is reproducibility. Ultrasound examination is operator dependent and requires regular practice to be familiar with localized imaging and normal values. We have used a published standardized technique to measure the width of the anterior tibio-fibular clear space [4]. In this series one experienced radiologist (XX) performed the US examinations accompanied by a muscloskeletal ultrasound trained sports physician (YY) in an attempt to minimize intra-observer error. Prior to the initiation of the study, reproducibility was verified by two other musculoskeletal radiologists in our institute. The technique of ultrasound measurement of the anterior clear space was quick to learn and was considered to be reproducible, comparable with MRI and shown to have an accuracy of 89-100% for AITFL injury [4]. By recording the values of syndesmotic opening in normal subjects we have provided normal reference values, which can assist less experienced musculoskeletal ultrasonographers. It is important that all measurements are taken at the same level above the ankle joint in order to standardize the relation between N, ER and IR positions. Even if the probe is placed a few millimeters higher, the absolute clear space opening measurement in neutral position might be increased but dynamic evaluation will keep the same relationships.
Another potential weakness is the lack of a standardized deforming force to recreate the opening of the syndesmosis. As this was a clinical study on live subjects the syndesmosis was not tested to failure but merely the width at maximal opening was determined. External rotation was performed until a firm recognizable end point was reached. Error was minimized by the same examiner applying a similar rotational force, and appreciating the same firm end point from neutral dorsiflexion/plantarflexion in each case.
Age groups compared in this study were under and over 40 years of age. We have chosen 40years as a cutoff age between young and older group as most of our study subjects fell into these two main groups. We had one cluster around 20–30 years of age, which usually stand for the population who tend to suffer from syndesmosis injuries, and another cluster around the 50 years of age. We had very few subjects between 35 to 45 years of age, and no one at that exact age.
The kinematic biomechanics of the distal tibio-fibular syndesmosis are subtle and are coupled with the ankle joint. With ankle dorsiflexion the talus rotates about 5 degrees externally about it’s axis in dorsiflexion and internally with plantarflexion [13, 14]. Correspondingly the fibula also rotates 3–5 degrees externally about its axis in the fibular groove of the tibia with a full range of ankle motion [13, 15]. Norkus and Floyd [15] established that with fibula rotation the syndesmosis widens by 1–2 mm when the foot is moved from plantar to dorsiflexion [15].
Given the subtle movements of the intact syndesmosis it is not surprising that clinical detection of syndesmotic instability is difficult. The calf compression test (squeeze test) [2], the External Rotation test [1], and the direct eversion maneuver have all been described to clinically detect syndesmotic instability, with sensitivity ranging from 33-92% [3]. Imaging therefore plays a key role in the detection of syndesmotic injury.
AP and Mortise plain static radiographs have shown sensitivity, specificity, and accuracy of 44.1%, 100%, and 63.5% for the AP view and 58.3%, 100%, and 71.2% for the mortise view when compared to arthroscopic examination [16]. Nielson et al. [17] found no association between the tibio-fibular clear space and overlap measurements in patients with syndesmotic injuries on plain radiographs when compared to MRI. Since the width of the tibio-fibular clear space does not change with rotation and the tibio-fibular overlap, width of the tibia and fibula, and medial clear space all do change with rotation comparison views may warranted to account for biologic variation [5, 18, 19].
CT scanning is sensitive for bony alignment and can identify 2–3 mm diastases not seen on a plain radiograph [20]. Spiral CT scan analysis reveals that a 1 mm diastasis increases the syndesmosis volume by approximately 43% with an additional 20% for each subsequent millimeter [21].
MRI is considered the optimal static imaging modality for AITFL integrity, but cost and availability do not warrant use for all suspected syndesmotic injuries [6, 22, 23]. There is potential for the development of dynamic MR imaging to visualize ligament laxity and joint instability in the future.
Direct arthroscopic visualization of clear space opening and syndesmotic instability may be considered to be the gold standard but is invasive and not without risks. Milz et al. [6] compared static ultrasound examination and MRI studies for the examination of the lateral ligaments complex and tibio-fibular syndesmosis. Ultrasound was found to have a sensitivity of 66% and specificity of 91% for AITFL injury.
In a previous study assessing ultrasound imaging of the syndemsosis in AITFL injured patients [4], the syndesmosis was found to widen as much as 1.1mm from neutral to external rotation and 1.85mm moving from external to internal rotation, 11 and 9 times greater than in normal joints which has movement of this 0.1 mm and 0.2 mm in respectively., In comparing these two studies it is evident that healthy syndesmoses widen significantly less than traumatized syndesmoses with AITFL injury. This demonstrates the potential of ultrasound examination in determining the integrity of the syndesmosis following injury.
The control healthy patients of the previous published study showed a normal small opening between the anterior fibula and tibia at the syndesmosis level with mean (ER-IR) of 0.2 mm. These values are similar to the findings of this series.
In the previous study a minimal value of 0.4mm for increased opening of the joint clear space with ER-N had a sensitivity and specificity of 89% for AITFL injury. This cut off is reinforced by this current study where a maximal opening of the normal clear space was found to be 0.3mm.
Using dynamic ultrasound examination we have determined the width and opening of the anterior tibio-fibular clear space in neutral and with forced external and internal rotation positions, Allowing radiologists and sports physicians to determine abnormal syndesmotic laxity and AITFL injury in a readily accessible, reproducible and radiation free imaging modality.