Periprosthetic Fractures around Polished Collarless Cemented Stems: The Effect of Stem Design on Fracture Pattern

BACKGROUND: Predictable patterns of periprosthetic fracture have been observed around polished double tapered stems. Finite element studies have suggested that triple-tapered stems cause less cement strain in torsion compared to double-tapered stems. Hence, we hypothesised that the in vitro behaviour of implanted double- and triple-tapered polished stems, like the CPT (Zimmer, Warsaw, USA) or C-Stem (DePuy, Leeds, U.K.) when subjected to pathological torsional loads may cause different patterns of periprosthetic fractures. METHODS: Ten double-tapered stems (CPT) and ten triple-tapered stems (C-Stem) were cemented into synthetic femur bones. A constant axial compression load of 100 N and a torsional pre-load of 0.1 N.m were applied using a biaxial testing machine. The distal femur was then loaded in external rotation at 45 degrees until failure. RESULTS: Seven of the 10 CPT stems fractured at the level of the stem body while fracturing the cement mantle at the same level. In three of ten of the CPT stems and all ten C-Stems, the synthetic bone fractured at the tip of the prosthesis while the cement mantle remained intact. This was significant for the resulting fracture pattern (P=0.001). There was no significant difference between the groups for either torque (P=0.13) or angle at failure (P=0.49). INTERPRETATION: This biomechanical study indicates that the CPT and C-Stem create a different fracture pattern under the same loading condition. The C-Stem (a triple tapered stem) may produce lower strain in torsion to the cement mantle of a cemented THA. However, fractures that do occur may be more difficult to treat than those produced around a stem like the CPT subjected to comparable loading.