Biomechanical evaluation of a simulated T-9 burst fracture of the thoracic spine with an intact rib cage

Abstract

<jats:sec> <jats:title>Object</jats:title> <jats:p>Classic biomechanical models have used thoracic spines disarticulated from the rib cage, but the biomechanical influence of the rib cage on fracture biomechanics has not been investigated. The well-accepted construct for stabilizing midthoracic fractures is posterior instrumentation 3 levels above and 2 levels below the injury. Short-segment fixation failure in thoracolumbar burst fractures has led to kyphosis and implant failure when anterior column support is lacking. Whether shorter constructs are viable in the midthoracic spine is a point of controversy. The objective of this study was the biomechanical evaluation of a burst fracture at T-9 with an intact rib cage using different fixation constructs for stabilizing the spine.</jats:p></jats:sec> <jats:sec> <jats:title>Methods</jats:title> <jats:p>A total of 8 human cadaveric spines (C7–L1) with intact rib cages were used in this study. The range of motion (ROM) between T-8 and T-10 was the outcome measure. A robotic spine testing system was programmed to apply pure moment loads (± 5 Nm) in lateral bending, flexion-extension, and axial rotation to whole thoracic specimens. Intersegmental rotations were measured using an optoelectronic system. Flexibility tests were conducted on intact specimens, then sequentially after surgically induced fracture at T-9, and after each of 4 fixation construct patterns. The 4 construct patterns were sequentially tested in a nondestructive protocol, as follows: 1) 3 above/2 below (3A/2B); 2) 1 above/1 below (1A/1B); 3) 1 above/1 below with vertebral body augmentation (1A/1B w/VA); and 4) vertebral body augmentation with no posterior instrumentation (VA). A repeated-measures ANOVA was used to compare the segmental motion between T-8 and T-10 vertebrae.</jats:p></jats:sec> <jats:sec> <jats:title>Results</jats:title> <jats:p>Mean ROM increased by 86%, 151%, and 31% after fracture in lateral bending, flexion-extension, and axial rotation, respectively. In lateral bending, there was significant reduction compared with intact controls for all 3 instrumented constructs: 3A/2B (−92%, p = 0.0004), 1A/1B (−63%, p = 0.0132), and 1A/1B w/VA (−66%, p = 0.0150). In flexion-extension, only the 3A/2B pattern showed a significant reduction (−90%, p = 0.011). In axial rotation, motion was significantly reduced for the 3 instrumented constructs: 3A/2B (−66%, p = 0.0001), 1A/1B (−53%, p = 0.0001), and 1A/1B w/VA (−51%, p = 0.0002). Between the 4 construct patterns, the 3 instrumented constructs (3A/2B, 1A/1B, and 1A/1B w/VA) showed comparable stability in all 3 motion planes.</jats:p></jats:sec> <jats:sec> <jats:title>Conclusions</jats:title> <jats:p>This study showed no significant difference in the stability of the 3 instrumented constructs tested when the rib cage is intact. Fractures that might appear more grossly unstable when tested in the disarticulated spine may be bolstered by the ribs. This may affect the extent of segmental spinal instrumentation needed to restore stability in some spine injuries. While these initial findings suggest that shorter constructs may adequately stabilize the spine in this fracture model, further study is needed before these results can be extrapolated to clinical application.</jats:p></jats:sec>

Effects of follower load and rib cage on intervertebral disc pressure and sagittal plane curvature in static tests of cadaveric thoracic spines

“…Because of the importance of the rib cage, several recent studies examining basic and clinical biomechanics in the human cadaveric thoracic spine have left the rib cage intact during testing (Healy et al, 2015; Mannen et al, 2015a; Perry et al, 2014). However, these studies did not include a physiological level of spinal compressive loading.…”

Traumatic Fractures of the Thoracic Spine

“…From a biomechanical view, long-segment instrumentations extending at least two levels above and below the fractured vertebra are significantly more effective in immobilising the involved segment than short-segment constructs [26,27]. Thus, long-segment instrumentations of the thoracic spine achieve a 90 % reduction of extension/flexion [27]. However, more recent finite element analysis shows that short-segment stabilisation may well be sufficient for physiological loading [28].…”

“…On the whole, however, there is little available literature dealing with the question of whether short-segment management using index screws for the middle and upper thoracic spine is adequate enough. Given the proven reduction of biomechanical stability, short-segment management has no place in the treatment of highly unstable type A4 fractures and type B and C fractures, especially those associated with injury to the bony ribcage [26,27]. In addition, it should be kept in mind that, on the one hand, clinical studies have shown an increased tendency for recurrent kyphotic deformity following short-segment management and, on the other hand, no functional difference has been demonstrated between long-segment and short-segment management [20,29].…”

Biomechanics and clinical outcome after posterior stabilization of mid-thoracic vertebral body fractures: a systematic literature review

“…In general, the structural instability of thoracic fractures is treated with posterior instrumentation 2 levels above and below the fracture site, but in case of intact rib cage a short segment fixation with 1 level above and below the fractured vertebra could be an alternative. Therefore, Perry et al [30] created a burst fracture at T9 in eight human thoracic spines (C7-L1) with intact rib cages and tested a long segment instrumentation (3 above, 2 below), a short segment instrumentation (1 above/1 below) with and without vertebral augmentation and vertebral augmentation without instrumentation. In their study, the long segment instrumentation showed a significant reduction of ROM during flexion-extension (− 90%), whereas the other instrumentations only tended to reduce motion.…”

“…In their study, the long segment instrumentation showed a significant reduction of ROM during flexion-extension (− 90%), whereas the other instrumentations only tended to reduce motion. However, Perry et al [30] suggested that in case of intact rib cages short segment instrumentation might adequately stabilize the spine. A common strategy to increase stability of short segment fixation is the addition of cross-links or screws at fracture site (index screws).…”