Ludwig Oberkircher, Christopher Bliemel, Felix Flossdorf, Tim Schwarting, Steffen Ruchholtz and Antonio Krüger
For many Type II fractures of the dens (Anderson and D'Alonzo classification), a double anterior screw fixation is performed. If screw disruption occurs, the location is most often at the anterior caudal endplate and body of the axis and not directly at the fracture line. The authors' objective was to determine the differences in primary mechanical stability at 2 insertion points used in ventral screw fixation of Type II fractures of the C-2 dens.
Screw fixation was performed on 16 formalin-fixed human C-2 dens specimens. The specimens were divided into 2 groups. For Group 1, the screws were inserted directly at the anterior lower endplates; for Group 2, the screws were inserted 2 mm dorsal to the anterior wall of the vertebral body. After a Type II odontoid fracture was created with an oscillating saw, screw fixation was performed using two 3.5-mm partially threaded lag screws with washers. Subsequently, each vertebral body was continuously loaded. The criterion for breakage was reversal of the force vector.
In Group 1, screw disruption occurred at the point of entry; the mean load failure was 290.5 ± 106 N. In Group 2, no screw disruption occurred; the mean load failure was 574.2 ± 170.5 N. These results were significant (p < 0.05).
For double screw fixation of Type II fractures of the dens (Anderson and D'Alonzo classification), placement of the screws as far dorsal to the anterior lower endplate as possible seems to favorably affect primary stability. In actual clinical practice, care should be taken to not damage the anterior wall of the vertebral body of the axis during screw insertion.
Ludwig Oberkircher, Maya Schmuck, Martin Bergmann, Philipp Lechler, Steffen Ruchholtz and Antonio Krüger
The treatment of traumatic burst fractures unaccompanied by neurological impairment remains controversial and ranges from conservative management to 360° fusion. Because of the heterogeneity of fracture types, classification systems, and treatment options, comparative biomechanical studies might help to improve our knowledge. The aim of the current study was to create a standardized fracture model to investigate burst fractures in a multisegmental setting.
A total of 28 thoracolumbar fresh-frozen human cadaveric spines were used. The spines were dissected into segments (T11–L3). The T-11 and L-3 vertebral bodies were embedded in Technovit 3040 (cold-curing resin for surface testing and impressions). To simulate high energy, a metallic drop tower was designed. Stress risers were used to ensure comparable fractures. CT scans were acquired before and after fracture. All fractures were classified using the AO/OTA classification.
The preparation and embedding of the spine segments worked well. No repositioning or second embedding of the specimen, even after fracture, was required. It was possible to create single burst fractures at the L-1 level in all 28 spine segments. Among the 28 fractures there were 16 incomplete burst fractures (Type A3.1), 8 burst-split fractures (Type A3.2), and 4 complete burst fractures (Type A3.3). The differences before and after fracture for stiffness and for anterior, posterior, and central heights were all significant (p < 0.05).
The ability to create reproducible burst fractures of a single vertebral body in a thoracolumbar spine segment may serve as a basis for future biomechanical studies that will provide better understanding of mechanical properties or fixation techniques.
Ludwig Oberkircher, Sebastian Born, Johannes Struewer, Christopher Bliemel, Benjamin Buecking, Christina Wack, Martin Bergmann, Steffen Ruchholtz and Antonio Krüger
Injuries of the subaxial cervical spine including facet joints and posterior ligaments are common. Potential surgical treatments consist of anterior, posterior, or anterior-posterior fixation. Because each approach has its advantages and disadvantages, the best treatment is debated. This biomechanical cadaver study compared the effect of different facet joint injuries on primary stability following anterior plate fixation.
Fractures and plate fixation were performed on 15 fresh-frozen intact cervical spines (C3–T1). To simulate a translation-rotation injury in all groups, complete ligament rupture and facet dislocation were simulated by dissecting the entire posterior and anterior ligament complex between C-4 and C-5. In the first group, the facet joints were left intact. In the second group, one facet joint between C-4 and C-5 was removed and the other side was left intact. In the third group, both facet joints between C-4 and C-5 were removed. The authors next performed single-level anterior discectomy and interbody grafting using bone material from the respective thoracic vertebral bodies. An anterior cervical locking plate was used for fixation. Continuous loading was performed using a servohydraulic test bench at 2 N/sec. The mean load failure was measured when the implant failed.
In the group in which both facet joints were intact, the mean load failure was 174.6 ± 46.93 N. The mean load failure in the second group where only one facet joint was removed was 127.8 ± 22.83 N. In the group in which both facet joints were removed, the mean load failure was 73.42 ± 32.51 N. There was a significant difference between the first group (both facet joints intact) and the third group (both facet joints removed) (p < 0.05, Kruskal-Wallis test).
In this cadaver study, primary stability of anterior plate fixation for dislocation injuries of the subaxial cervical spine was dependent on the presence of the facet joints. If the bone in one or both facet joints is damaged in the clinical setting, anterior plate fixation in combination with bone grafting might not provide sufficient stabilization; additional posterior stabilization may be needed.