We performed a retrospective cohort comparison, and TR insertion of syndesmotic screws seemed preferable based on the clinical and basic science considerations.
The limitations of our study include (1) Inadequate sample size, short follow-up, and early removal of the screw, which possibly led to no statistically significant difference between the groups. Clinically, there are few patients who have malleolar fractures with SD and undergo stabilization with a syndesmotic screw. In our study, there were only 63 cases over a period of 10 years, which indicates an average of one case every 2 months. The sample sizes required in the present study were 56 for each group. Hence, it is difficult to achieve statistical significance. (2) Based on the literature, several factors related to the stabilizing screw can affect the treatment outcomes; however, there is no consensus about the most important factor. Our study chooses screw location for investigation, and other parameters (screw diameter, numbers, or purchased cortices) may confound the comparison. Finally, no difference was found on clinical comparison. Theoretical superiority forms the basis of treatment recommendation.
Humans are bipedal animals and usually use one lower limb alternately while walking. During a gait, the body’s center of gravity moves to the contralateral side and induces considerable bending and rotational moments [22]. An ideal implant used for stabilizing the fractured or dislocated lower limb must tolerate large loads stereoscopically.
Anatomically, the structures of the ankle are unfavorable for multi-directional movements. A considerable disparity in sizes between the distal tibia and distal fibula leads to significantly unsymmetrical load transfer [5,6]. Additionally, the talus can perform only plantar-dorsiflexion movements [23]. When various directional loads are applied on the ankle, various complex ankle injuries will continue to occur. Therefore, no simple, complete, and convincing classification for ankle injuries has been established until now. In the literature, combined Weber and Lauge-Hansen classifications have been supported by many orthopedic surgeons [9-11]. They believe that the combined classifications include the majority of ankle injuries related to the malleoli and syndesmosis.
Since there are few reported articles about treating the combined injuries including large sample sizes, there is no consensus on the optimal location of the screw for stabilizing the diastatic syndesmosis [24,25]. In the present study, the suggested favorable location of the screw is based on clinical and theoretical considerations. Although the clinical results do not support the superiority of either location, TR screw insertion might be better based on the theoretical comparison.
Biomechanically, the shorter is the lever arm, the less is the load moment [14,26]. When a cortical screw is inserted to maintain the syndesmotic stability, the screw becomes the fulcrum. The nearer is a cortical screw placed to the lateral malleolus (i.e., the point of force application), lesser the load is on the screw. Therefore, TR insertion of a syndesmotic screw seems safer than SU insertion. Since the majority of the stabilizing screws were removed at an average of 2.2 months in the present study, the incidence of screw breakage was low (4.2%, 2 / 48). Therefore, the statistical comparison cannot make a definite conclusion. If the syndesmotic screw is removed late or left unremoved, most of the screws are likely to fail. In 2005, Kukreti et al. reported the treatment outcomes of 36 patients with SD [27]. Nineteen cases were treated with SU screws and 17 with TR screws. Syndesmotic screws were removed at an average of 2 months, and they found no difference in terms of clinical and radiological outcomes. The present study also achieved a comparable result.
Biologically, the four surrounding ligaments of the syndesmosis are mainly supplied by the perforating branches of the peroneal artery in the posterior compartment [28,29]. This site is slightly proximal to the upper border of the syndesmosis. SU insertion of a cortical screw might injure the perforating branches and delay the ligament healing. When the stabilizing screw is removed at 2.2 months later to prevent screw breakage, SD might recur. In the present study, SU insertion was found to increase the possibility of re-diastasis of the syndesmosis despite no statistical significance (35.3% versus 12.9%; p= 0.068). The delayed healing of the four surrounding ligaments might be the contributing factor.
An unstable ankle caused by a diastatic syndesmosis has been reported to achieve a poor functional outcome [30,31]. Although SU insertion might have a higher incidence of syndesmotic re-diastasis (35.3% versus 12.9%, p = 0.068) in the present study, the functional outcome of the ankle was similar between the groups. The causes might be due to inadequate sample sizes (post-hoc power = 0.45) and a short period of follow-up (an average of 1.8 years).
Theoretically, poor ankle function after treatment of the combined injuries might be closely related to the residual instability of the ankle [31,32]. In normal gait, the talus will be pushed by 20-60 of external rotation and 1.5 mm lateral shift during the stance phase [33]. If SD recurs, wear of the articular cartilage will be faster, and osteoarthritis is likely to progress. Clinically, this complication is critical; however, it is usually preventable as long as the joint stability is well restored [34]. The present study may therefore provide an important knack.
Many factors relate to a syndesmotic screw can affect the treatment outcomes. Screw diameter, location, numbers, and purchased cortices are considered not to impact the treatment outcomes [3,4,13]. However, the sample sizes reported in the literature are generally inadequate. All results are often contradicted in the absence of a consensus. Arriving at a conclusion entirely based on clinical studies seems non-feasible. The present study integrating the clinical and basic concepts might present a relatively reasonable technique to improve the treatment outcome.
A well-stabilized syndesmosis using a cortical screw might affect the ankle function [35]. Therefore, it is advisable to remove the screw once the ligament injuries are healed. Previous reports suggest that the time of screw removal is inversely correlated to the incidence of re-diastasis of the syndesmosis [21]. However, late removal of the screw increases the incidence of screw breakage. In the present study, the two cases that had screw breakage did not undergo early screw removal. Some reports suggest that screw breakage might lead to a better ankle function despite non-removal [21,36,37]. Accordingly, very early removal of the syndesmotic screw is unwarranted. To prevent screw breakage and syndesmotic re-diastasis, TR insertion of the screw with late removal (> 3 months) might be a better choice. The tibiofibular synostosis is generally minimal, and TR screw insertion will not impede the ankle function [27,38].
In 2014, Verim et al. performed a study of finite elemental analysis [39]. The syndesmotic screw was placed at six different altitudes above the plafond. The loads were applied from the tibial plateau, and it was found that proximal screw insertion had the least syndesmosis widening. The authors concluded that the best placement of the syndesmotic screw was 30-40 mm above the plafond. This conclusion is completely contrary to our concept. Practically, the study of Verim et al. applied loads on the tibial plateau and observed the screw reaction. In fact, the loads should be applied from the plafond because the talus pushes the lateral malleolus postero-laterally during the stance phase [33]. This indicates that a screw placed near the point of force application (i.e., lateral malleolus) can obtain the least loading moment. The difference between Verim et al. and our studies is the different points of force application. Clinically, our interpretation seems more reasonable. Although various favored screw locations have been reported in the literature, none explained the reasons clearly (Table 2) [21,24,36,39-44].