M ODIFIED D ESIGN T ESTING

To reduce any potential shear loading, a custom XY stage was designed and built to add translational freedom to the caudal end of the test specimen (for details on XY

Figure 6.1: Bending Moment Efficiency

The concept of bending moment efficiency is calculated as the maximum positive or negative recorded Caudal Moment (output) divided by the maximum positive or negative Applied Moment (input) multiplied by 100%. In the sample graph above, the positive bending moment efficiency is nearly 100%, where the negative moment efficiency is less than 100%.

stage design, see Appendix G).9 Briefly, this was a biaxial bearing design that had four linear bearings running along two round shafts in both the X and Y directions (Figure 6.2), providing approximately 57mm of travel along each shaft. This XY stage was designed to fit between the testing platform and the caudal six-DOF load cell. Further, locking collars were added to each shaft to allow for the option of fixing the specimen in place.

A single C4-C5 motion segment was used to evaluate the effect of adding this biaxial bearing system, placed between the testing platform and the caudal end of the specimen (Figure 6.3). The motion segment was isolated to leave the ligaments and disc intact, and potted using the technique described in Section 3.2. To examine the impact of “aggressive”, specimen-specific tuning aimed at minimizing error in the desired 0N axial load, tuning for this specimen was achieved using a target input wave switched to a tensile-compression sine wave set to cycle the axial force between ±30N.

Two specimen states (intact and SIM) were tested using the same loading protocol described in Section 3.2 (i.e., target load of ±1.5Nm) for all three simulated motions. For each testing state, the biaxial bearing system was tested in both the “free” state and in the “fixed” state, with the locking collars in place. The axial load was set to hold 0N throughout testing. Data analysis examined the bending moment efficiency and measured caudal forces from the final loading cycle for each motion in both the intact and SIM and for the fixed versus free stage conditions.

6.3

R

6.3.1

C

D

T

Data analysis of the previously collected Chapter 5 specimens found that, in axial rotation, there was an average bending moment efficiency of 100% (Table 6.1), with no difference between the intact and injured states (p>0.05). In flexion-extension and lateral

9

The design and development of this testing stage was undertaken by a group of undergraduate students for their 4th year design project. A number of potential options were explored, with the final design chosen as the biaxial bearing system.

Figure 6.2: Biaxial Bearing System

A rendering of the biaxial bearing XY stage design is shown. There are two shafts running in perpendicular directions. Along each shaft are two linear bearings (only one shown in current figure). These bearings should allow for “free” XY translation of the caudal end of the specimen.

Figure 6.3: Testing of a C4-C5 with Biaxial Bearing System

A single C4-C5 motion segment was tested in the modified simulator design, with the addition of the biaxial bearing system and more aggressive, specimen-specific axial actuator tuning. Output loads at the caudal end were measured with a six-DOF load cell.

Table 6.1: Bending Moment Efficiency of Chapter 5 Load Data

The average ± standard deviation (SD) positive and negative bending moment efficiencies (%) based on the maximum measured applied (input) and caudal (output) bending loads (n=7). Motion State Average ± S.D. Positive Moment Efficiency (%) Average ± S.D. Negative Moment Efficiency (%) Axial Rotation Intact 100 ± 2 101 ± 3 SIM 100 ± 2 101 ± 3 Flexion- Extension Intact 112 ± 19 105 ± 23 SIM 101 ± 16* 41 ± 34* Lateral Bending Intact 138 ± 18 154 ± 15 SIM 110 ± 23* 125 ± 20*

Note: Since cyclic testing was completed to ±1.5Nm, both a positive and negative moment efficiency calculation were considered. Positive efficiency represents loading inducing flexion, ipsilateral axial rotation, and ipsilateral lateral bending. The asterisk symbol represents significant differences between the intact and SIM states for a given motion.

bending, the average bending moment efficiency was generally larger than 100% (Table 6.1). Furthermore, in comparing the intact and injured states, moment efficiency was found to decrease with injury for both flexion-extension (p<0.05) and lateral bending (p<0.05).

The calculated shear forces in the X and Y directions at the caudal end in these specimens were consistently less than ±6N (Table 6.2). Axial forces were larger, with the highest loads seen in compression (~10-30 N). There were no differences between intact and SIM states for axial and shear forces (p>0.05).

6.3.2

M

D

T

The new loop tuning protocol determined specimen-specific PID settings of 12.7, 1.7, and 0.4. With these new settings, the average axial force during the final loading cycle was approximately 0N for all testing states, with a peak difference of less than 20N (Table 6.3). Average shear forces were approximately 1N or less for all testing states, and did not exceed 4N (Table 6.3). The largest changes in force were measured immediately following a change in loading direction (Figure 6.4). Between the free and fixed XY stage conditions, there were no changes in the average measured forces, and no visible stage movements observed. Bending moment efficiency for axial rotation was again approximately 100%, with smaller and larger efficiencies measured for flexion- extension and lateral bending, respectively (Table 6.4). With injury, both flexion- extension and lateral bending efficiencies decreased, whereas axial rotation efficiency did not change.

6.4

D

The pure bending moment technique has become the standard protocol for use in custom spinal loading simulators, but its implementation is rarely validated or reported (Panjabi, 1988; Wilke et al., 1998). To improve the transparency of this loading technique, this study examined two measures to describe how well the simulator was creating a pure bending moment: the concept of “bending moment efficiency” and the measurement of forces at the caudal end of the test specimen. These measures were quantified while the degree of caudal end constraint was varied and the actuator control

Table 6.2: Caudal Forces Measured in Chapter 5 Load Data

The average ± standard deviation (largest value) maximum and minimum caudal forces measured during the final loading cycle (n=7).

Shear X Force (N) Shear ‘Y’ Force (N) Axial ‘Z’ Force (N)