The aim of this investigation was to screen and diagnose asymptomatic ankle SI in fencers by using CT images and 3D reconstructions. Finite element models were generated to estimate the influence of each injury on the elite fencers’ loading conditions and to provide evidence as to whether the asymptomatic injury would increase the risk of serious injuries. An investigation of this nature therefore adds to the current literature base in fencing by providing important clinical information regarding injury evaluation and prediction of fencers.
Research in young athletes has shown that asymptomatic foot radionuclide bone imaging abnormalities and diffuse tibia abnormalities were common, and that asymptomatic lower limb abnormalities accounted for about 34% of injuries . MRI observations of the tibia in 21 elite college long-distance runners shows signs of 43% of tibial stress reaction . MRI findings showed a 77% (i.e., 30 out of 39) asymptomatic hockey players with pathological abnormalities in the hip or groin . Imaging diagnosis revealed that the frequency of foot and ankle injuries in fencers is much higher than expected as almost all participants showed different levels of SI (Table 3). Caution should be paid to interpret the association between these findings and the future severe sports injuries .
Higher ankle joint angles in lunges make the ankle experience high-energy axial, rotational, and horizontal stress, which are not evenly distributed , leading to a high frequency of foot and ankle SI. Importantly, the anterior tibiofibular and interosseous tibiofibular ligament are part of the distal tibiofibular syndesmosis  and avulsion fracture decreases the stability of the distal tibia and fibula. The lateral process of the talus is not only the attachment point for the posterior talofibular ligament, but also a partial attachment point for the support belt of the flexor hallucis longus. The posterior talofibular and the talofibular ligament jointly stabilize the ankle joint . Ligamentous laxity may further lead to ankle joint instability and increase the risk of sports injuries , such as tarsal fractures. Indeed, Table 3 and Fig. 1 showed that participants had 5 other tarsal fractures in addition to talus fractures. Although unable to determine the sequence of fractures, our observations confirm that decreased ankle joint stability increases the risk of fractures in other tarsi. The dorsiflexion of the trail foot and strike pattern of the trail foot in lunges  were shown to increase the risk of fractures at the lateral process of the talus. Fractures at the lateral process of talus were observed in both feet in 3 participants. In addition, 2 participants had bilateral avulsion fractures at the calcaneonavicular ligament and calcaneocuboid ligament attachment points. Fractures at the spring ligament complex, plantar cuneocuboid ligament, and plantar calcaneocentral ligament attachment points in the trail foot may be related to the dorsiflexion movement of the trail foot in lunges, while fractures at the lead foot may be related to the retraction movement in lunges .
Figures 5 and 8 as well as Table 4 show that structural fractures were present at the sesamoid of the left leg in 6 participants and of the lead foot in 2 participants. Among 20 feet, 9 sesamoid fractures and 7 flat lateral SGs were observed, resulting in SG and ridge ablation. The sesamoid and first metatarsal bone are the framework parts of the MTPJ. The morphology of the medial and lateral sesamoids and the SGs determines the contraction results of the flexor hallucis longus and flexor hallucis brevis muscles . Therefore, sesamoid fractures, SG and ridge ablation weaken the windlass efficiency of the first MTPJ . Sesamoid fractures at the trail foot may be associated with dorsiflexion movements of the trail foot whilst sesamoid fractures at the lead foot may be related to the retraction movements during the lunge . The medial and lateral sesamoids are components of the first MTPJ, and thus essential to the function of MTPJ . We found that since sesamoid had one of the highest injury incidences for fencers, it should be included as an occupational injury for the elite fencers.
By combining with imaging diagnosis results, P2 was found to have a flat lateral SG (Table 4). A flat SG lacks geometric constraints to the sesamoid, impairs the contraction of the flexor hallucis brevis muscles, reduces the efficiency of the propulsive force, and thus increases the risk of injury or lesion of the first MTFJ (e.g., hallux valgus). Therefore, the high principal stress of the medial SG caused a medial sesamoid fracture in P2 (Table 3). Typically, the risk of medial sesamoid fracture is higher, due to the strong impact on the MTPJ when the trail foot lunges. On the other hand, the increase of ankle joint angle in the sagittal plane enlarges TS difference between the medial and lateral SGs. These asymmetric changes to the Max-PS and TS increase the risk of avulsion fractures , so multiple ankle injuries are observed in the left foot of P2 (Table 3). The medial sesamoid fracture in P2 brings greater TS to the lateral SG, and the Max-PS and Min-PS of the medial and lateral sesamoids also becomes greater, generating great tension to the ligaments and muscles of the first MTPJ . Tension generated by the dorsiflexed position of the foot and the limitation of ankle joint rotation increases the risk of foot varus, affecting the tibia and fibula by a torsional moment while pulling to the meniscus. Because the meniscus inside the knee joint capsule is thin in the middle and thick in the edge, when the knee joint extends backward, the thicker edge of the meniscus is in the knee joint cavity, reducing the range of motion in it, leading to unavoidable friction. So, foot varus can trigger injury to multiple ligaments (e.g., anterior talofibular ligament) and tendons (e.g., Achilles tendon) or even to the meniscus.
P3’s Max-PS of the medial and lateral sesamoids are very high, maybe resulting from his smallest curvature diameter of the medial and lateral sesamoids among the selected three participants. Also, his medial sesamoid groove leans to the lateral side (Table 4, − 1.5°). The medial sesamoid groove does not parallel much with the lateral one, considerably affecting the MTPJ angle to load force. During the propulsion phase of the lunge, power is generated by the ankle plantar flexors and knee/hip extensors of the trail leg.
Lunge can be successfully performed by stepping the fore foot and extending the rear foot, generating more hip flexion force . Once the trail foot passes in front of the lead foot, the thrust-absorption cycle is repeated with a reversal of the power and of absorption of the lower limbs . The overly high CV of P3 indicates a high risk of the above-mentioned injury in future training and competition, especially when using a lunge technique with a larger ankle angle. P7’s mean TS difference and CV between the medial and lateral sesamoids are kept at a stable level − 41.78 N and 3.69%, respectively (Table 5). When the ankle joint angle changes, the principal stress peak at the medial SG is usually higher than that at the lateral SG, while the TS value gives the opposite results, suggesting that the risk of stress fractures on the medial sesamoid increases when the rotation angle of the ankle joint is too wide in lunges.
Different types of MTPJ structures have different stress distribution characteristics, which were all affected by an increased ankle joint angle, leading to instability at the ankle itself . These observations should attract attention from fencers because foot and ankle SI reduce the stability of the lower limb and might cause more injuries to the foot and ankle or even the knee and waist. There are many tactical and technical decisions that affect how the lunge is executed , but an excessively large trail foot angle will not optimally generate force or speed. Therefore, fencers should seek to reduce the rotation angle of the ankle joint at the trail foot in lunges as long as the quality of performance can be maintained.
A limitation of this study is that it is a cross-sectional analysis. Future longitudinal follow-up investigations concerning symptom development of these fencers should be conducted to prove the predictive function of the asymptomatic foot and ankle SI. Identifying the kinematic and kinetic characteristics of different asymptomatic ankle SI of participants may have potential in distinguishing asymptomatic ankle SI, so the symptom development of athletes can be monitored by routine biomechanical analyses.
This study utilized a simplified FEA whereby the performance of soft tissues was not considered, which may reduce the applicability of this model. For instance, we cannot estimate plantar pressure, nor the stress variation caused by different metatarsal bones’ geometric shape to the soft tissues. Also, our finite element model was designed based on single subject, even though the geometric model was normalized to assume bone structure to be the only difference, the results could not represent target population of different age, weight and bone mass characteristics.