CEMENTED FEMORAL STEM PERFORMANCE - EFFECTS OF PROXIMAL BONDING, GEOMETRY, AND NECK LENGTH

The effects of proximal bonding, distal stem geometry, and femoral nec k length on cement and interface stresses were determined to understan d better their role in clinical performance. The effects of stem desig n were compared with the effects of environmental variables, patient w eight, and patient activity, Finite element models were used to determ ine peak cement and interface stresses, and an experimental layout was used to separate design and environmental effects, Bonding reduced ce ment mantle stresses by 35% to 60%, to levels below the cement fatigue strength. A flat sided implant provided more torsional resistance, re ducing shear stresses at the proximal cement-prosthesis interface by 2 2% to 73% with respect to a distal round implant. Neck length had mini mal effects on stresses compared with bonding or implant geometry, Cem ent-bone interface stresses were more sensitive to patient activity th an to the design variables. Therefore, claims that a strong cement and prosthesis bond may be harmful to the bone-cement interface are unjus tified based on these results. The best combination of design variable s was a proximally bonded, flat sided implant with neck length left to the surgeon's discretion. This combination was most effective at prot ecting the cement mantle and prosthesis interface and perhaps the ceme nt-bone interface by minimizing stresses associated with cement debris generation.