A quantitative analysis of strain at adjacent segments after segmental immobilization of the cervical spine.

STUDY DESIGN: A biomechanical study on human cadaveric cervical spines with segmental fixation. OBJECTIVES: To quantify the strains across all segments of the spine after simulated fusion. SUMMARY OF BACKGROUND DATA: Clinical evidence suggests that degenerative changes occur at adjacent levels after cervical fusion. This may, in part be due to increased stress and motion at the adjacent segments. MATERIALS AND METHODS: Seven fresh frozen human cervical cadaveric spines were used. The spines were mounted onto frames at C2 and C7. Biomechanical testing was performed on a modified MTS tester. The specimens were tested in rotation control. To simulate fusion, a block was used to replace the disc. Fixation was enhanced using an anterior plate and stainless steel wire through the spinous processes. Testing was then performed with the same displacement magnitudes used for the intact spine. Displacement across 5 disc spaces was recorded using extensometers. The same preparation and testing was done for 1, 2, and 3-level simulated fusions. All data were normalized to the individual intact specimen. RESULTS: After 1-level simulated fusion at C5-6, flexion-extension rotation increased by 60% at the superior adjacent level (C4-5) and by 15% at the adjacent inferior level (C6-7). Lateral bending increased by 51% at C4-5 and by 16% at C6-7. Axial rotation increased by 25% at C4-5 and by 200% at C6-7. Flexion-extension, lateral bending and axial rotation increased at all other segments, not only at adjacent segments, after 1, 2 and 3-level fixation. CONCLUSIONS: Cervical fusion results in increased strains at adjacent levels, and to all other levels, inferiorly and superiorly. This study represents the first to quantify the increased strain at all adjacent levels.

Fixation technique influences the monotonic properties of equine mandibular fracture constructs.

OBJECTIVE: To determine the optimal fixation technique for equine interdental space fractures by evaluating the biomechanical characteristics of 4 fixation techniques. STUDY DESIGN: In vitro randomized block design. SAMPLE POPULATION: Twenty-seven adult equine mandibles. METHODS: Mandibles with interdental osteotomies were randomly divided into 4 fixation groups (n = 6/group). Fixation techniques were the following: (1) dynamic compression plates (DCP), (2) external fixator (EF), (3) external fixator with interdental wires (EFW), and (4) intraoral splint with interdental wires (ISW). Three intact (nonosteotomized) mandibles were tested as controls. Mandibles were subjected to monotonic cantilever bending until failure. Angular displacement data (radians) were derived from continuously recorded gap width measurements provided by extensometers placed across the osteotomy site. Osteotomy gap width data (mm) at 50 and 100 Nm were selected for standardized comparison of gap width before the yield point and failure point, respectively of all constructs tested. Stiffness (Nm/radian), yield strength (Nm), and failure strength (Nm) were determined from bending moment-angular displacement curves and were compared using ANOVA with appropriate post hoc testing when indicated. Radiographs were obtained prefixation, postfixation, and posttesting. RESULTS: Bending stiffness, yield, and ultimate failure loads were greatest for intact mandibles. Among osteotomized mandibles, stiffness was greatest for DCP constructs (P <.05) and was not significantly different among EF, EFW, and ISW constructs. Yield load was greatest for ISW constructs (P <.05) and was not significantly different among DCP and EFW constructs. Yield and ultimate failure loads were lowest (P <.05) and osteotomy gap width at 50 and 100 Nm were greatest for EF constructs (P =.09 and P <.05, respectively). There was no significant difference in failure loads and osteotomy gap widths among DCP, EFW, and ISW constructs (P <.05). Failure occurred through the screw-bone interface (DCP), acrylic splint (ISW), acrylic connecting bar and/or pin-bone interface (EF, EFW), and wire loosening (EFW). All 3 intact mandibles fractured through the vertical ramus at its attachment to the testing apparatus. CONCLUSIONS: Among osteotomized mandibles, DCP fixation had the greatest stiffness under monotonic bending to failure; however, the relatively low yield value may predispose it to earlier failure in fatigue testing without supplemental fixation. Techniques using tension-band wiring (EFW and ISW) were similar to DCP constructs in yield, failure, and osteotomy displacement, whereas EF constructs were biomechanically inferior to all other constructs. CLINICAL RELEVANCE: DCP fixation is most likely the most stable form of fixation for comminuted interdental space fractures. However, for simple interdental space fractures, ISW fixation may provide adequate stability with minimal invasiveness and decreased expense. Tension-band wiring significantly enhances the strength of type II external skeletal fixators and should be used to augment mandibular fracture repairs.

Nonlinear viscoelasticity in rabbit medial collateral ligament.

The goal of this study was to characterize the viscoelastic behavior of the rabbit medial collateral ligament (MCL) at multiple levels of strain (between 0% and approximately 5%) and their corresponding stresses (between 0 and approximately 55 MPa) for stress relaxation and creep, respectively. We hypothesized that in the rabbit MCL the rate of stress relaxation would be strain dependent and the rate of creep would be stress dependent. Thirty MCLs from 15 rabbits were tested ex vivo for this study. Results show that within the physiologically relevant region of ligament behavior, the rate of stress relaxation is strain dependent in the rabbit MCL, with the rate of relaxation decreasing with increasing tissue strain. The rate of creep is stress dependent in the rabbit MCL, with the rate of creep decreasing with increasing stress. These results support our hypothesis, with the greatest nonlinearities in a physiologically relevant region of loading. As such, these nonlinearities should be considered when quantifying ligament viscoelastic behavior with a rabbit model.