Effect of strain rate and bone quality on the bending behaviour of whole bone

Abstract

Forty ovine femurs were harvested and allocated into four testing groups; Fast-Normal, Fast- Decalcified, Slow-Normal, Slow-Decalcified. Contralateral pairings were used within these groups for closer comparison.

Dynamic testing apparatus was designed and built allowing rates of strain similar to road traffic accidents to be investigated. These strain rates were achieved by using a pneumatic actuator to apply the load. Slow rate loading was achieved by testing with a commercially available mechanical testing machine at a rate of strain similar to that created by walking.

Bone quality was altered by ultrasonically assisted decalcification in EDTA. Levels of mineral dissolution equivalent to the loss of bone mineral density (BMD) of a 75 year old woman were targeted.

Whole bone was used for these experiments to facilitate comparison with real fracture radiographs obtained from NHS database. Fracture patterns and degree of comminution were similar between experimental and patient data.

Bone is often analysed as a simple beam (engineers bending theory).

This method of stress analysis was compared with a method that recognised the change in cross section over the length of the bone. Accounting for this had a highly significant effect on the calculated flexural modulus (p<0.0005).The length to depth ratio of whole bone indicates that shear forces cannot be ignored. The effect of the contribution from shear force on the deflection was investigated. After accounting for deflections due to shear, calculated normal strains agreed with literature values. Deflection due to shear was found to make a significant contribution to the deflection

The effect of storage (freezing) on the mechanical properties at high strain rate was evaluated: no significant differences were found for force and deflection at failure.

The main body of testing gave the following results: Normal quality bone, rate compared showed significant differences for Ultimate Stress, Ultimate Strain, Yield Strain, Flexural Modulus and Toughness.

Demineralising bone resulted in no statistically significant differences between the loading rates for the Stress at failure.

Yield Strain, Ultimate Strain, Flexural Modulus and Toughness did show significant differences.The fast loading tests showed significant differences when comparing quality for Stress at failure but not at Yield. Significant differences were found when comparing toughness. Slow loading tests showed significant differences between bone qualities for Stress at failure in contralateral pairs. No significant differences were found for strain or toughness.

These results indicate that bone of normal quality can withstand higher than normal stresses for short durations. This ability is lost in demineralised bone.

The high loading rate tests revealed closely matched strains at failure for both bone qualities, lending support to the strain based failure theory for bone at traumatic strain rates.