Adaptation of a clinical fixation device for biomechanical testing of the lumbar spine.

In-vitro biomechanical testing is widely performed for characterizing the load-displacement characteristics of intact, injured, degenerated, and surgically repaired osteoligamentous spine specimens. Traditional specimen fixture devices offer an unspecified rigidity of fixation, while varying in the associated amounts and reversibility of damage to and "coverage" of a specimen - factors that can limit surgical access to structures of interest during testing as well as preclude the possibility of testing certain segments of a specimen. Therefore, the objective of this study was to develop a specimen fixture system for spine biomechanical testing that uses components of clinically available spinal fixation hardware and determine whether the new system provides sufficient rigidity for spine biomechanical testing. Custom testing blocks were mounted into a robotic testing system and the angular deflection of the upper fixture was measured indirectly using linear variable differential transformers. The fixture system had an overall stiffness 37.0, 16.7 and 13.3 times greater than a typical human functional spine unit for the flexion/extension, axial rotation and lateral bending directions respectively - sufficient rigidity for biomechanical testing. Fixture motion when mounted to a lumbar spine specimen revealed average motion of 0.6, 0.6, and 1.5° in each direction. This specimen fixture method causes only minimal damage to a specimen, permits testing of all levels of a specimen, and provides for surgical access during testing.

Unique model evokes the supination/pronation deficits found after Mason II fractures.

A rapid prototyping model of Mason II fracture was used to investigate baseline recommendations for surgical intervention founded on kinematic forearm rotational blockage. Exact replicas of the radial heads in nine cadaveric specimens were produced and specimens were tested in a physiologic elbow simulator. After testing supination/pronation, the rotations were repeated with native replicas and with replicas modeling 3 mm depressed Mason II fractures with and without a gap of 1 mm between the body and fragment. The fragments were located circumferentially around the radial head at 10, 2 and 6 o'clock positions. There was no statistical difference between the range of motion of the native case and the native replica without fracture. After inclusion of the fracture, seven of the nine specimens showed rotational blockages. A two-way ANOVA found no statistical difference due to type of Mason II fracture (p > 0.87) or fracture location (p > 0.27). A χ-square analysis showed that presence of a kinematic deficit with a fractured radial head was significant (p < 0.03). The results support continued surgical intervention for a 3 mm depressed fracture and also establish the use of the rapid prototype as a model for kinematic investigation of fractures in a cadaveric model when ligamentous attachments are preserved.

Femoral bone strains during antegrade nailing: a comparison of two entry points with identical nails using finite element analysis.

BACKGROUND: Antegrade femoral nailing has become the standard treatment for diaphyseal femoral shaft fractures. Concerns linger that improper location of the nail entry point may lead to iatrogenic fracture and further complications. This study used finite element analysis to compare the strain magnitude and distribution resulting from each of two entry points in the proximal femur during antegrade nailing. METHODS: A finite element model was created from a CT scan of a 37 year old male femur and of a standard antegrade nail. Using implicit time-stepping, the nail was inserted through piriformis and trochanteric entry points and strain was computed at 9 anatomic locations. FINDINGS: The strain levels were higher overall when inserting a nail through the trochanteric starting point. The highest strain occurred immediately medial and lateral to the trochanteric entry point. The posterior greater trochanter also showed very high strain levels during nail insertion. All strain values for nail insertion through the piriformis entry point were less than 2000 µm/m. INTERPRETATION: The trochanteric entry will have a much greater potential of iatrogenic fracture of the proximal femur during insertion of a nail. Strains with this entry point exceed the yield level of bone and the repeated loading with the progression of the nail could cause fissures or fractures. Caution should be taken during insertion of an antegrade nail when utilizing a lateral trochanteric starting point secondary to an increased risk of trochanteric fracture and lateral cortex fracture.