Fall Safe Floors for Prevention of Hip Fractures

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Fall Safe Floors for Prevention of Hip Fractures

Designers: Maile Kruse, Jared Haden, Jonathan Quick
Client Coordinator: Uday Vaidya, Department of Materials Science and Engineering

 

A flooring system to serve as passive prevention of hip fracture was the goal of the present work. The constraints for the floor material were that the floor must withstand normal walking i.e. the 385 N peak force caused by walking barefoot.  Under impact of a 35 kg load at 2.6 impact velocity, the floor must attenuate 2.23 kN from the hip surrogate in order to prevent hip fracture of the fifth percentile woman.  Hip padding systems were capable of lowering the femoral impact force well below 4 kN N, the mean force required to fracture the elderly femur in vitro in a side fall loading configuration at realistic loading rates (Courtney et al, 1994).  Robinovitch et al. (1991) revealed that for the average hip under impact, the effective stiffness, k was 70 kN/m and the effective damping constant, b was 60 kN*s/m for a 35 kg female, descent height of 0.35 m to provide an impact velocity of 2.6 m/s.  The design constraints for a hip surrogate to test the floor were a peak force of 6.4 kN in 20-30 msec after impact of a 35 kg load at 2.6 m/sec impact velocity.  Furthermore, all components of the surrogate must be durable enough to withstand multiple impacts.

TECHNICAL DESCRIPTION

Floor

A layered sandwich composite structure was developed.  A core subsystem is the primary energy absorber. Options for the core included honeycomb, prismatic, and foam / laponite mixture.  Honeycomb core made of aramid paper was chosen for its low weight and cost efficiency.  The HexWeb® HRH-10-1/8(in)-3.0(lb/ft3) absorbs approximately 60 J upon buckling and therefore was considered adequate for the flooring purposes.  A series of E-glass fiber and Coremat were chosen for their high affinity to bond to each other.  In order to impart rigidity to the face sheets, the layers were bonded together using Freeman’s FMSC 690 epoxy resin followed by vacuum bagging and 24-hour cure.  The honeycomb core was sandwiched between the two face sheets with 3M’s Scotchweld DP – 125 Grey epoxy adhesive bonding.  The floor was put into an oven at 160oF (71oC) for 2 hours to attain full cure.  The finished floor tile was then cut into 8.9” x 11.4” tiles for testing (Fig. 20).

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Figure 20a.  (left) Sandwich composite floor; (right) hip surrogate with four springs and one adjustable shock absorber

Surrogate

A hip surrogate was constructed using wood, springs and an adjustable shock absorber (Fig. 4), extending upon results from the earlier EGR 100 students designs.  The femoral head of a SawbonesR 3rd generation composite femur was crazy-glued onto the center of the top plate’s superior face.  A layer of SorbothaneR .75 cm thick was attached on top the femoral head by spray adhesive.  A hole of 3.8 cm diameter was drilled into the center of the 25.5 cm x 25.5 cm bottom plate to house the 25 cm tall damper.  The damper was placed through this hole and fastened to the plate with two nuts, one on either side.  Four 8.5 cm x 8.5 cm x 12.5 cm blocks of wood were attached to the bottom plate to provide clearance for the height of the damper.  Then a hole of 1.5 cm diameter (the diameter of the springs) was drilled into the center of eight 8.5 cm x 7.5 cm x 3.7 cm blocks of wood.  Four of these blocks were screwed into the corners of the superior face of the bottom wooden plate.  The other four blocks were screwed into the corners of the inferior face of the top plate.  Furthermore, a block of wood with a 3 cm diameter hole in the center was screwed into the center of the inferior face of the top plate, to enclose the head of the damper.  The springs were placed into each hole of the bottom plate.  The top plate was placed on top of the springs and damper to unite all three subsystems.  Two screws were tightened through the housing of the damper head to ensure that the three subsystems act as one system under impact.

Floor testing

After construction of the composite flooring, compression and point load tests were performed.  The floor stiffness was found to be independent of the loading rate.  Point load testing was conducted.  As expected, the floor failed at a lower force with the small indenters versus the large indenters, but none at levels below 400 N.  The surrogate was the impacted with 35 kg raised 32 cm from the impact surface to generate a 2.6 m/s impact velocity.  The data was decomposed by a fast fourier transform and filtered using a low pass filter with a cut-off frequency of 140 Hz.   The surrogate underwent a peak force of 6.4 kN within a response duration of 60 msec.

In floor evaluations, the floor tile was attached to the 7.62 cm x 10.16 cm impact plate by spray adhesive and dropped onto the hip surrogate under the same mass and velocity conditions.  The data was filtered as described previously, and the floor was found to decrease the peak force experienced by the hip surrogate by 1.4 kN, which did not decrease the peak force to the target 4.1 kN.  The flooring system, therefore, was not successful in the prevention of hip fracture for the fifth percentile woman.

Total cost for materials = $820
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Figure 20b.  Impact force data for hip surrogate revealing a 6.3 kN peak force (dark line), and with the sandwich composite flooring, which reduced the peak force to roughly 5 kN.