Direct evidence of "damage accumulation" in cement mantles surrounding femoral hip stems retrieved at autopsy: cement damage correlates with duration of use and BMI.

The "damage accumulation" phenomenon has not been quantitatively demonstrated in clinical cement mantles surrounding femoral hip stems. We stained transverse sections of 11 postmortem retrieved femoral hip components fixed with cement using fluorescent dye-penetrant and quantified cement damage, voids, and cement-bone interface gaps in epifluorescence and white light micrographs. Crack density (Cr.Dn), crack length-density (Cr.Ln.Dn), porosity, and cement-bone interface gap fraction (c/b-gap%) were calculated, normalized by mantle area. Multiple regression tests showed that cement damage (Cr.Ln.Dn. & Cr.Dn.) was significantly positively correlated (r(2)=0.98, p<0.001) with "duration of use" and body mass index ("BMI") but not cement mantle "porosity". There were significant interactions: "duration of use"*"BMI" was strongly predictive (p<0.005) of Cr.Dn.; and "duration of use"*"porosity" was predictive (p=0.04) of Cr.Ln.Dn. Stem related cracks accounted for approximately one fifth of Cr.Dn and one third of Cr.Ln.Dn. The mean c/b-gap% was 13.8% but it did not correlate (r(2)=0.01, p=0.8) with duration of use. We concluded that duration-dependent fatigue damage accumulation occurred during in vivo use. BMI strongly influenced cement crack length and the rate of new crack formation over time. Voids did not increase the rate of crack initiation but appeared to have promoted crack growth over time. Although not progressive, substantial bone resorption at the cement-bone interface appeared to be common.