A novel application of velocity-based force control for use in robotic biomechanical testing.

This paper presents a novel application of a velocity-based force control routine used for robotic biomechanical testing. The routine employs a jog function, available from the robot's motion commands, that permits easy adjustment of velocity on each axis. Force and moment targets are achieved by adjusting jog velocities in proportion to force or moment errors while limiting the maximum velocity of the system. The force control jog routine does not require specimen stiffness values and is inherently stable. The performance of the method was shown to be suitable for unconstrained in vitro spine testing in a rabbit model where extremely small motions are necessary to maintain the target force values. The jogging feature on which this work is based is a feature available on most robots and is equally applicable to a serial robot. The simplicity, stability, and performance of this method warrant its consideration for other robotic biomechanical testing applications where force control is required.

Neutral zone and range of motion in the spine are greater with stepwise loading than with a continuous loading protocol. An in vitro porcine investigation.

Biomechanical testing of the spine has traditionally been performed to help understand the normal function of the spine as well as to evaluate the effects of injury and surgical procedures on spinal behaviour. The overall objective of this investigation was to compare traditional stepwise loading with the recently introduced continuous loading protocol, determining the effect of loading protocol on the mechanical behaviour of the spine. For all tests, a custom spine testing machine was used to apply pure moments of flexion extension, axial rotation, and lateral bending to a maximum of 2 Nm, using six porcine cervical spine specimens (C2-C4). Motions of C2 with respect to C4 were measured with an optoelectronic camera system. Motion parameters calculated were range of motion (ROM), neutral zone (NZ), and the ratio of NZ and ROM. The continuous loading protocol had smaller values for all motion parameters in each loading direction (p<0.05). ROM for the continuous test ranged between 88% and 93% of that of stepwise for the three loading directions. The continuous protocol NZ was 56-75% of that of the stepwise test. The findings of the study demonstrate that the two loading protocols provide differing spinal behaviours.

Anterior occiput to axis screw fixation: part II: a biomechanical comparison with posterior fixation techniques.

STUDY DESIGN: This biomechanical study used flexibility testing on fresh-frozen human cadaveric specimens (occiput to C3) and compared the range of motion and neutral zone for three occipitocervical fixation techniques. OBJECTIVES: To contrast the stabilization provided by a new technique of anterior occipitocervical screw fixation with two other commonly used posterior occipitocervical fixation techniques. SUMMARY OF BACKGROUND DATA: There are no published reports describing this novel technique of anterior occipitocervical screw fixation. METHODS: Six human occipitocervical spine specimens were mounted in a custom-designed, spine-testing machine that applied a pure moment in flexion-extension, lateral bending, and axial rotation. The specimens were tested intact, after an odontoid osteotomy with capsular injury, and after each of three fixation methods: posterior wiring, posterior plate fixation with C1-C2 transarticular screws, and finally with anterior occipitocervical screws. Intervertebral motion was measured with an optoelectronic measurement system, and the range of motion and neutral zone were the kinematic variables measured and used for analysis. RESULTS: In flexion and extension testing, the posterior plate with transarticular screws provided greater stabilization than posterior wiring or anterior occipitocervical screws. In lateral bending and rotation, the anterior screws were similarly effective to the posterior plate, both of which were more effective than posterior wiring. CONCLUSION: The anterior screw fixation technique was as effective as a posterior plate with transarticular screws in stabilizing between the occiput and C2 in axial rotation and lateral bending. In extension and flexion, the anterior screw technique was not as effective as a posterior plate with transarticular screws in providing stability.

Biomechanical evaluation of proximal humeral fracture fixation supplemented with calcium phosphate cement.

BACKGROUND: Proximal humeral fractures are common injuries, and numerous surgical methods have been described for their treatment. The biomechanical characteristics of various internal fixation devices that are used to treat these fractures have not been extensively studied, nor has the potential beneficial effect of calcium phosphate cement supplementation. METHODS: We used a cadaveric three-part proximal humeral osteotomy model to perform a biomechanical evaluation of three types of internal fixation devices: a cloverleaf plate, an angled blade-plate, and Kirschner wires. The effect of supplementing the fixation with SRS (Skeletal Repair System) calcium phosphate cement was evaluated as well. Eighteen pairs of fresh-frozen humeri were obtained, and the bone-mineral density of each specimen was measured. In each pair, one specimen was secured with internal fixation alone and the contralateral specimen was secured with internal fixation combined with calcium phosphate cement. The specimens were tested cyclically in abduction and in external rotation for 250 cycles to evaluate interfragmentary motion. The specimens were then loaded to failure in external rotation to measure torsional load to failure and torsional stiffness. RESULTS: Overall, there were no significant differences between the specimens treated with the blade and cloverleaf plates, whereas the specimens treated with Kirschner wires demonstrated more interfragmentary motion, less stiffness, and lower torque to failure. In general, supplementation with calcium phosphate cement led to significant improvements in the mechanical performance of all three forms of internal fixation as demonstrated by a significant decrease in interfragmentary motion, a significant increase in torque to failure, and a significant increase in torsional stiffness. The addition of calcium phosphate cement increased the stiffness of even the most osteoporotic specimens to levels that were higher than those of the most osteodense specimens that had been treated with internal fixation alone. CONCLUSION: The initial biomechanical properties of internal fixation as measured with use of a proximal humeral osteotomy model and three methods of fixation were significantly improved by the addition of calcium phosphate cement.