Joel Gillard

Biomechanical testing of a novel, minimally invasive rib fracture plating system.

BACKGROUND A novel rib fracture repair plating system was developed to provide durable fixation with a shorter length than standard systems and thus facilitate minimally invasive repair. We hypothesized that U-plate fixation would be at least equivalent in durability to standard anterior fixation. STUDY Twenty fresh frozen ribs (10 pairs) from two human cadavers were first tested for intact stiffness (force or deformation). A gap of 5 mm was then created in the middle of each rib with a saw. Each rib was reconstructed with either the U-plate (4.6 cm length, Acute Innovations, LLC, Hillsboro, OR) with four screws or a 2.4-mm anterior locking plate (9.5 cm length, Synthes, Paoli, PA) with six screws. The U-plates were placed on one rib and the anterior plates on the contralateral rib of the paired levels. The reconstructed ribs were cycled 50,000 times with a load of +/-2N at 1 Hz in a simulation of the repetitive loading of deep breathing. The stiffness of the construct was measured throughout the test. RESULTS Stiffness decreased from the intact rib to the transected/plated rib for both types of fixation; however, a significant decrease in stiffness was observed only with the anterior repair (p = 0.03). After 50,000 cycles, the U-plated ribs lost 0.12 +/- 0.03 N/mm (1.9%) stiffness, whereas the anterior-plated ribs lost 0.72 +/- 0.13 N/mm (9.9%) stiffness (p = 0.001). CONCLUSIONS In this simulation of an unstable rib fracture with a small bony gap, U-plate fixation was more durable than standard anterior fixation. The greatly diminished size of the U-plate compared with the standard may facilitate minimally invasive rib fracture repair.

Comparison of stiffness and failure load of two cervical spine fixation techniques in an in vitro human model

Objective: Recently, an unpaired threaded cage has been introduced as a fusion device for the cervical spine. No biomechanical comparison of a stand-alone single interbody threaded cage to a standard plated Smith-Robinson construct has been reported. Accordingly, an in vitro biomechanical comparison of a single threaded cylindrical interbody fusion cage versus a plated Smith-Robinson cervical discectomy and fusion construct was conducted to establish whether a single cylindrical interbody cage in the cervical spine would perform mechanically as well as a plated structural interbody graft. Methods: Six fresh-frozen human cadaveric cervical spines were used for biomechanical testing. Flexion-extension and load-to-failure testing were performed on two single-level discectomy and interbody fusion constructs from each specimen. Results: Initial range of motion (ROM) was significantly greater for the specimens implanted with a cage than specimens implanted with a structural graft and plate (9.1° ± 3.7° vs 5.8° ± 2.4°; P = 0.040). Initial stiffness in flexion in caged specimens was significantly less than in plated specimens (0.7 ± 0.3 vs 0.9 ± 0.3 Nm/°; P = 0.028). Cage specimens also failed at a significantly lower load than plated specimens (9.8 ± 3.5 vs 15.8 ± 4.1 Nm; P = 0.0104). Conclusions: This study demonstrates that a plated Smith-Robinson cervical discectomy and fusion construct provides greater stiffness and failure load and reduced ROM across operated levels than a single interbody cage construct. Although clinical success may not directly correlate with biomechanical data, these results raise concern regarding the use of a single threaded interbody cage as a stand-alone device for cervical interbody fusion.