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 degrees +/- 3.7 degrees vs 5.8 degrees +/- 2.4 degrees ; 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/ degrees ; 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.

A biomechanical analysis of donor-site ankle instability following free fibular graft harvest.

BACKGROUND: Recent studies concerning the free fibular graft have focused on the high prevalence of donor-site morbidity. The prevalence of ankle pain has been reported to range from 10% to 40%, but its etiology is unclear. The literature is vague with regard to the amount of distal fibular bone that is needed to maintain ankle stability. The aim of the present study was to determine the percentage of the fibula that can be removed while still preserving ankle stability. METHODS: Eleven fresh, paired cadaveric legs were tested. One leg from each pair was tested with the foot mounted in three positions (neutral, 15 degrees of inversion, and 15 degrees of eversion) while an external and internal rotational torque and axial load were imposed. Each specimen was also mounted in a Telos apparatus, and a varus load was applied across the ankle. Each specimen was tested first with an intact fibula to establish baseline stability and then underwent sequential fibular resections, from proximal to distal, until ankle instability was encountered. The contralateral specimen from each pair was then used to evaluate repetitive loading of a stable distal fibular segment over 2000 cycles. RESULTS: Only 10% of the fibula was needed distally to maintain ankle stability. Once the residual fibular length was <10% of the total fibular length, a significant change in motion was seen in the ankle joint (p < 0.05). On visual inspection, a residual fibular length of 10% represented a fibular osteotomy just proximal to the syndesmotic ligaments. The greatest motion occurred with the ankle inverted and in external rotation. No significant change in ankle stability occurred during cyclic testing when the residual fibular length was 10% of the total fibular length. CONCLUSIONS: While previous reports in the literature have suggested that 6 to 8 cm of residual distal fibular length is needed to maintain ankle stability, our data support the possibility that ankle stability can be maintained with even less residual fibular length.