Cement-implant interface gaps explain the poor results of CMW3 for femoral stem fixation

Background The Norwegian Arthroplasty Register reported that CMW3 cement performed poorly for femoral stem fixation.

Methods We implanted collared, satin-finished stems (Ra = 0.35 µm) into cadaver femora using CMW3 and with Simplex as control. Cement mantle function was quantified by stem migration after 300,000 cycles of “stair climbing”. Cement cracks and interface gaps were quantified in transverse sections.

Results The variances of the CMW3 migrations were substantially higher than for the control (p < 0.001): subsidence for CMW3: –32 (SD 42) µm, and for Simplex: –7 (SD 9) µm (p = 0.2); retroversion for CMW3: 0.60° (SD 0.25), and for Simplex: 0.37° (SD 0.04) (p = 0.08). Crack length-densities were similar. CMW3 had significantly more non-apposed stem/cement interface: 52% (SD 17) versus 33% (SD 8) (p = 0.04).

Migrations could be predicted by the fraction of non-apposed stem/cement interface (retroversion: R²=0.80, p < 0.001; subsidence: R² = 0.46, p = 0.02) but not by cement cracks or non-apposed cement-bone interface.

Interpretation We found that increased stem/cement non-apposition resulted in increased stem migration. Early migration is known to correlate with risk of revision. Thus, the higher stem-revision risk for CMW3 cement reported by the Norwegian Arthroplasty Register may have been due to inferior and variable stem/cement apposition.