Results of identifying a source of bone with “necrotic” bone properties

In this study, samples of bone from a number of different locations on porcine and bovine femurs were tested in uniaxial compression. The aim was to identify a source of bone that would provide a 41% reduction in UCS and a 59% reduction in elastic modulus when compared to bone from the central portion of porcine femoral heads (Section 2.2). A source of bone was identified that gave a 42%-51% lower mean stiffness and a 44%-63% lower mean UCS.

The locations from which bone samples were taken with properties closest to the target values for maximum elastic modulus and UCS were samples from the bovine lateral epicondyle:

central on the lateral epicondyle and immediately superior to the central point. Henceforth these samples are referred to as donor bone A and B. The results for all locations are summarised in Table 8.

Box-whisker plots of the results are shown in Figure 15 and Figure 16. Although the modulus for both donor locations closely matched the target, the ultimate compressive strength was approximately 30%-40% lower. Comparing the properties of the donor bone to the properties of bone taken from the central portion of porcine femoral heads, samples from the donor locations had a 42%-51% lower stiffness and a 44%-63% lower UCS. The box-whisker plots indicated that there was large variation and that, for maximum elastic modulus especially; the properties of the donor bone chosen may be significantly closer to the host value than the target relationship.

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Table 8. Summary of mechanical properties for bone samples taken from various locations on porcine and bovine femurs (Figures 13 and 14). The number of samples column indicates the total number of samples taken, with the number of samples used to generate the final statistics in parentheses. The target properties for the donor bone were a maximum elastic modulus of 130MPa (95% C.I. 89-172) and an ultimate compressive strength of 9MPa (95%C.I. 8-10).

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Figure 15. Box-whisker plot of maximum elastic modulus. The mean of the data set is represented by a diamond, the median by a bar. Mean maximum elastic modulus for donor bone (A) (Central on the lateral epicondyle, n=8) and B (Immediately superior to location H, n=10), both from the bovine lateral epicondyle. The error bars indicate 95%

confidence interval. The target maximum and minimum are based on a nominal maximum elastic modulus that represents a 59% reduction in modulus compared to samples taken from porcine femoral heads with the same confidence interval as the original data.

Figure 16. Box-whisker plot of ultimate compressive strength. The mean of the data set is represented by a diamond, the median by a bar. Ultimate compressive strength for donor bone (A) and (B), both from the Bovine lateral epicondyle. The error bars indicate 95% confidence interval. The target maximum and minimum values represent a nominal strength that represents a 41% reduction in UCS compared to samples taken from porcine femoral heads with the range based on the same confidence interval as the original data.

50 100 150 200 250 300 350 400 450 500

Porcine femoral head Donor bone (A) Donor bone (B)

Elastic modulus (MPa)

Page 82 of 273 Several samples were removed:

• Porcine femoral head samples: two were removed for reasons described in Section 2.2.3

• Donor bone A: Four samples (2, 3, 4 and 5) were removed: Two failed at the growth plate rather than through material failure; the experimental setup was incorrect for the other two.

• Donor bone B: One sample (sample 2) was removed for the same reason as previously described in Section 2.2.3. The stress-strain plot for this sample is shown in Figure 17.

Again, the plot is characterised by a short region of linear-elastic behaviour followed by a gradual decrease in modulus and large stress plateau that is atypical of acute compressive failure. The UCS was approximately 50% lower than the other samples

Figure 17. Stress-strain plot for bovine sample J2. This sample was removed because of its atypical yield profile: a very short linear elastic region with a gradually reducing gradiant (modulus) followed by a large stress plateau. The location of automatically detected maximum elastic modulus is indicated by the diagonal cross and the location of ultimate compressive strength is shown by the horizontal cross.

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2.3.4 Discussion

The purpose of this investigation was to find an appropriate source of bone that could be used to simulate necrotic bone in a femoral head based disease model. Overall, the inter-quartile ranges for the bone properties were consistent between bone taken from porcine femoral heads and that taken from the bovine lateral epicondyle. This suggests that properties could be matched to give a consistent ratio. However, given the lack of data regarding the variation in necrotic bone properties, a better approach is to randomise the allocation of “necrotic” bone samples and include this as a variable within the analysis of the disease model.

Mechanical testing of biological specimens is subject to a number of sources of variation.

Differences in activity level and maturity between individual animals can lead to variation in trabecular structure and bone mineral density, which have a direct impact on the mechanical properties of the sample.

Sample preparation can also have a significant impact. Where possible the magnitude of this impact was reduced by using a consistent plug diameter and standardised protocol for sample preparation. However, samples were harvested from locations identified relative to anatomic landmarks such as the Greater Trochanter, insertion of ligaments and the articular surfaces.

These vary from sample to sample, so slight variation in the exact sample location is expected.

The test methodology potentially introduced additional variability. To reduce this potential, care was taken to remove a consistent thickness of superficial bone to maintain as much homogeneity in the samples as possible. A constant strain rate of 0.01s^-1 was used throughout to reduce the influence of viscoelasticity (Carter and Hayes, 1976) and all samples were stored in a way that had minimal effect on their material properties of the calcified portion of bone (Pelker et al., 1984; Linde et al., 1993).

Combining results from both locations, the donor bone had a mean modulus of 157 MPa (Range 105-228 MPa) and a mean UCS of 6.3 MPa (Range 4.9-7.6 MPa). The mean elastic modulus was comparable to that reported for necrotic human bone. The ultimate compressive strength for the simulated necrotic bone was lower than that reported for pre-collapse necrotic bone but was comparable to the strength of both osteoporotic and osteoarthritic femoral heads. (Table 9).

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Table 9. Comparison between material properties derived experimentally for porcine femoral heads (healthy bone) and plugs taken from the lateral epicondyle of a bovine femur (necrotic bone) and those reported for human bone in the literature.

Author Healthy bone Diseased bone

Elastic

results 318±119 15.2±2.85 Simulated

AVN 157±52 6.3±1.4

Median 310 Not reported Osteoarthritis Median 356 Median 4.3 Osteoporosis Median 247 Median 2.5 Haba et al.,

2012 (Femoral head)

Not reported Not reported Osteoarthritis 232±130 6.1±3.0 Steinhauser et

Variation in the region of 50% of the mean was found for ultimate compressive strength. The definition of ultimate compressive strength was less ambiguous than that of maximum stiffness because there was always an apogee: a single data point that signified the transition from a positive (increasing compressive stress with strain) to negative (decreasing stress with strain) modulus, whereas the maximum slope could occur over a relatively large section of the data.

Variation in the region of 100% of the mean was found in the modulus derived for these samples. A qualitative comparison of the stress-strain plots in Figure 12 suggests that this variation was a true representation of the spread within the data.