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Manufacturing Implants with Residual Compression

Using finite element modeling of the applied stresses and a patented fatigue design protocol,8 a custom residual stress field was created for the application of LPB to hip implants. Exhibit 1 shows the real life loading conditions that were simulated on a model to obtain the stresses applied in service and determine the required depth of compression. Several factors needed to be taken into account: equilibrating residual tensile stresses, dimensional changes and the cost of processing. Exactech had to be sure that the surface enhancement treatment chosen would not simply move the location of maximum stress, weakening another section. LPB prevents this by evenly dispersing the equilibrating tensile stresses. Component dimensions had to be maintained because of the tight tolerances of the tapered joint of modular hip implants. LPB processing was developed so that the critical dimensions in treated parts were maintained. The complete LPB stress field can be seen in Exhibit 2.

 

Exhibit 1: FEA showing applied forces in normal use

 

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Exhibit 2: FEA showing LPB treatment

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Life Improvement
High Cycle Fatigue (HCF) testing was performed on untreated (baseline) and LPB-treated hip stems to demonstrate the performance improvement. Tests were run in accordance with the appropriate ISO 7206 standards.

Baseline specimens were tested within a load range of 1,050 to 1,400 lbs. All fatigue failures in baseline specimens occurred in the fretted tapered neck region of the hip segment. Baseline results indicate a fatigue strength at 107 cycles of 1,050 lbs. LPB processed neck segments were tested in a higher load range of 1,400 to 1,600 lbs. to produce any failures. Full results are detailed in Exhibit 3. LPB improved the fatigue strength of the hip stem more than 40% and increased the life by 15x, as shown in Exhibit 4. No failures initiated in the fretted LPB processed neck taper region.

Exhibit 3: High Cycle Fatigue Data

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HCF results indicating LPB produces a 40% increase in fatigue strength at 107 cycles. Arrows indicate run-outs.

Exhibit 4: Life increase from LPB Treatment

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