Effects of Progressive Deltoid Ligament Sectioning on Weber B Ankle Fracture Stability

Abstract

Background: Conventionally, transsyndesmotic fibula fractures with concomitant signs of deltoid ligament injury have been considered unstable and thus treated operatively. Recent studies have indicated that partial deltoid ligament rupture is common and may allow for nonoperative treatment of stress-unstable ankles if normal tibiotalar alignment is obtained in the weightbearing position. Biomechanical support for this principle is scarce. The purpose of this study was to evaluate the biomechanical effects of gradually increasing deltoid ligament injury in transsyndesmotic fibula fractures. Methods: Fifteen cadaveric ankle specimens were tested using an industrial robot. All specimens were tested in 4 states: native, SER2, SER4a, and SER4b models. Ankle stability was measured in lateral translation, valgus, and internal and external rotation stress in 3 talocrural joint positions: 20 degrees plantarflexion, neutral, and 10 degrees dorsiflexion. Talar shift and talar valgus tilt in the talocrural joint was measured using fluoroscopy. Results: In most tests, SER2 and SER4a models resulted in a small instability increase compared to native joints and thus were deemed stable according to our predefined margins. However, SER4a models were unstable when tested in the plantarflexed position and for external rotation in all positions. In contrast, SER4b models had large-magnitude instability in all directions and all tested positions and were thus deemed unstable. Conclusion: This study demonstrated substantial increases in instability between the SER4a and SER4b states. This controlled cadaveric simulation suggests a significant ankle-stabilizing role of the deep posterior deltoid after oblique transsyndesmotic fibular fracture and transection of the superficial and anterior deep deltoid ligaments. Clinical Relevance: The study provides new insights into how the heterogenicity of deltoid ligament injuries can affect the natural stability of the ankle after Weber B fractures. These findings may be useful in developing more targeted and better treatment strategies.

Effects of Fibular Plate Fixation on Ankle Stability in a Weber B Fracture Model With Partial Deltoid Ligament Sectioning

“… 4 However, recent biomechanical evidence suggests that Weber B fractures with a partial deltoid ligament tear but with an intact dPTTL are stable. 2 , 9 These findings are coherent with clinical studies demonstrating acceptable patient-reported and radiographic outcomes in the short and midterm after nonoperatively treating SER4a fractures, 4 - 6 , 13 and thus challenges the traditional practice of operative treatment of SER4a fractures because of potential instability. The results of the present study provide evidence that plate fixation of the fibula only improves external rotational stability, in SER4a injury models.…”

“…A previous study demonstrated that external rotation instability already occurs to some degree in cadaver models with no medial side injury (Weber B/SER2 injury models). 2 …”

“…The test setup, specimen preparation and fixation have previously been described in detail. 2 The tests were performed using a floor-mounted industrial robot (KR6 R900 Sixx; KUKA, Augsburg. Germany) fitted with a force/torque sensor (Gamma; ATI Industrial Automation, Apex, NC) measuring all 6 components of force and torque independently.…”

“…Two recent biomechanical studies support the nonoperative treatment of SER4a fractures. 2,9 These studies demonstrated that ankles upheld stability with oblique transsyndesmotic fibular fracture and transection of the superficial and deep anterior deltoid ligaments. Still, they showed substantial increases in instability between the SER4a and SER4b states, suggesting a significant ankle-stabilizing role of the dPTTL, indicating that rupture of the dPTTL (SER4b) may represent the threshold for when the ankle becomes unstable.…”

“… 4 - 6 The traditional binary understanding of stability, either as stable or unstable, has been challenged by clinical and biomechanical findings indicating that partial deltoid ligament injuries, where the deep posterior tibiotalar ligament (dPTTL) remains intact, are common. 2 , 4 - 6 , 9 It has been proposed to classify such injuries as SER4a. 4 In clinical practice, this injury is identified by an unstable radiographic stress test but a stable weightbearing radiograph.…”