In Vitro Monitoring of Hoop Strains during Impaction Grafting of the Femur

A test was designed to monitor hoop strains around the proximal femur during impaction grafting. Four electrical resistance strain gauges were attached to Sawbone femurs just below the level of the lesser trochanter. Experimentation with these femoral models included the use of a device called an “impactometer”, which delivered controllable impaction energies. Test material used was ovine morsellised bone graft prepared from sheep femoral condyles. Instruments used were “X-change” revision instruments from the Exeter hip system. Maximum strains occurred at the medial and lateral cortices and there were lesser strains on anterior and posterior cortices. These findings raise questions about the quality of impaction anterior and posterior to the femoral stem. At the medial and lateral gauges significant differences in levels of strain were recorded at the three different levels of impaction. An individual hoop strain was found to have three components: peak, rapid drop and slow drop. The strain level did not drop back to the starting point at the end of the drop phase but dropped only halfway back to the starting level. This resulted in a rise in strain levels in a stepladder pattern. Strains continued to rise significantly when the impaction process was continued beyond the endpoint of impaction. Early recognition of the endpoint of impaction can save the weak femoral bone from unnecessary high levels of hoop strain.