Humans are bipedal animals and usually use one lower limb alternately in walking. During a gait, the body’s center of gravity will move to the contralateral side and introduces huge bending and rotational moments . An ideal implant used for stabilizing the fractured or dislocated lower limb must tolerate huge loads stereoscopically.
Anatomically, the structures of the ankle are unfavorable for multi-directional movements. A large disparity of sizes between the distal tibia and the distal fibula introduces load transfer markedly unsymmetrically [5,6]. Additionally, the talus is simply allowed for plantar-dorsiflexion action . As long as various directional loads are applied on the ankle, various complex ankle injuries will occur. Therefore, until now a simple, complete, and convincing classification for ankle injuries has yet been established. In the literature, combined Weber and Lauge-Hansen classifications are supported by many orthopedic surgeons [9-11]. They believe that the combined classifications can comprise the majority of ankle injuries related to the malleoli and syndesmosis.
Because reported articles with large sample sizes in treating the combined injuries are few, the optimal screw use in stabilizing the diastatic syndesmosis has yet achieved consensus [24,25]. In the present study, the favored screw location is enthusiastically pursued from clinical and theoretical considerations. Although the clinical results cannot support either superiority, TR screw insertion may be better based on 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), the less loaded is the screw. Therefore, TR insertion of a syndesmotic screw should be safer than SU insertion. Because the majority of stabilizing screws have been removed at an average of 2.2 months in the present study, screw breakage is few (4.2%, 2 / 48). Statistical comparison cannot therefore make a definite conclusion. If the syndesmotic screw is removed late or let it alone without removal, the majority of screws will fail. In 2005, Kukreti et al. reported treatment of 36 patients with SD . Nineteen cases were treated with SU screws and 17, TR screws. Syndesmotic screws were removed at 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 mainly gain the vascularity from the perforating branches of 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 will very possibly injure the perforating branches and hinder the ligament healing. Once the stabilizing screw is removed at 2.2 months for prevention of screw breakage, SD may recur. In the present study, SU insertion is found to increase the possibility of re-diastasis of the syndesmosis despite statistical insignificance (35.3% versus 12.9%; p= 0.068). The delayed healing of the four surrounding ligaments may be contributing factor.
In the literature, an unstable ankle caused by a diastatic syndesmosis will introduce a poor functional outcome [30,31]. Although SU insertion may 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 is still similar. The causes may be imputed to insufficient 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 should be closely related to residual instability of the ankle [31,32]. In normal gait, the talus will be pushed by 20-60 external rotation and 1.5 mm later shift at the stance phase . If SD recurs, wear of articular cartilage will be speeded and osteoarthritis will be progressive. Clinically, this complication is critical but it is normally preventable as long as the joint stability is restored well . The present study may therefore provide an important knack.
Situations of a syndesmotic screw affect the treatment outcomes are multiple. Screw diameter, location, numbers, and purchased cortices are considered without affecting the treatment outcomes [3,4,13]. However, the sample sizes reported in the literature are generally insufficient. All results are often contradicted without consensus. Purely making a conclusion from clinical studies seems to be less possible. The present study integrating clinical and basic concepts may speculate a relatively reasonable technique to improve the treatment outcome.
Well stabilized syndesmosis by a cortical screw may affect ankle function . Therefore, the screw is advised to be removed after ligament injuries are healed. In the literature, the time of screw removal is reversely correlated to the incidence of re-diastasis of the syndesmosis . However, late removal of the screw will increase the incidence of screw breakage. In the present study, two cases with screw breakage had no screw removal early. In the literature, screw breakage may have a better ankle function despite that it is not removed [21,36,37]. Accordingly, too early removal of the syndesmotic screw is unreasonable. To prevent screw breakage and syndesmotic re-diastasis, TR insertion of the screw with late removal (> 3 months) may be the better choice. The tibiofibular synostosis is generally minimal and TR screw insertion will not hinder the ankle function [27,38].
In 2014, Verim et al. performed a study of finite elemental analysis . The syndesmotic screw was placed at six different altitudes above the plafond. The loads were applied from the tibial plateau and found that the proximal screw insertion had the least syndesmosis widening. They then concluded the best placement of syndesmotic screw to be 30-40 mm above the plafond. This conclusion is completely contradicted 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 it is the talus pushing the lateral malleolus postero-laterally at the stance phase . In other words, the lower screw nearing 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 point of force application. Clinically, our interpretation should be more reasonable. Although various favored screw locations had been reported in the literature, none had explained the reasons clearly (Table 2) [21,24,36,39-44].
Favored screw location for stabilization of syndesmotic diastasis
Reported studies Level proximal to plafond (cm) Study modes
Stiehl40 1990 2 Review article
Xenos41 1995 2.5-4 Cadaveric test
Hovis42 2002 1-2 Case series
Beumer43 2005 2 Cadaveric test
Nousiainen44 2008 2.5 Cadaveric test
Dattani36 2008 2-5 Review article
Hsu21 2011 2-3 Case series
Verim39 2014 3-4 FEA study
Schepers24 2014 < 4 Case series
Present study 2020 1-2 Case series
FEA, finite element analysis
The limitations of our study may include followings: (1) Insufficient sample size, short follow-up, and early screw removal let the comparison be insignificant statistically. Clinically, patients who have malleolar fractures with SD and accept syndesmotic screw stabilization are not many. In our study, only 63 cases within the period of 10 years. In other words, two months are necessary to collect one case. The sample sizes needed in the present study are 56 for each group. Therefore, the statistical significance is difficult to achieve. (2) Based on the literature, modes of a stabilizing screw can affect the treatment outcomes but which is the leading role is still inconclusive. Our study chooses screw location for investigation and other parameters (screw diameter, numbers or purchased cortices) may confound the comparison. Finally, no difference in clinical comparison is obtained. Theoretical superiority becomes the cornerstone of treatment recommendation in the present study.