Fatigue debonding of the roughened stem–cement interface: Effects of surface roughness and stem heating conditions

Abstract

The aim of this study was to determine the effects of cyclic loading on the debond process of a roughened stem-cement interface used in total hip arthroplasty. The specific goals were to assess the effects of two surgeon-controlled variables (stem heating and degree of stem surface roughness) and to determine if an independent finite element-based fracture mechanics model could be used to predict the debond response. A clamped cantilever beam geometry was used to determine the fatigue debond response of the stem-cement interface and was created using an experimental mold that simulated in vivo cementing conditions. A second experiment was performed using a torsion-loading model representative of the stem-cement-bone composite. For both experiments, two stem heating (room temperature and 50°C) and surface roughness conditions (grit blasted: Ra = 2.3 and 5.1 μm) were used. Finally, a finite element model of the torsion experiment with provision for crack growth was developed and compared with the experimental results. Results from both experiments revealed that neither stem preheating nor use of a stem with a greater surface roughness had a marked effect on the fatigue debond response. There was substantial variability in the debond response for all cases; this may be due to microscopic gaps at the interface for all interface conditions. The debond rate from the finite element simulation (10 −7.31 m/cycle) had a magnitude similar to the experimental torsion model (10 − (6.77 ± 1.25) m/cycle). This suggests that within the context of the experimental conditions studied here that the debond response could be assessed using a linear elastic fracture mechanics-type approach.

The effect of vacuum mixing and pre-heating the femoral component on the mechanical properties of the cement mantle

“…21 The reversed curing from the stem towards the bone eliminates the porosities induced by shrinkage at the stem-cement interface. 17,24 Our study confirms that the highest reduction in porosity at the stem-cement interface can be obtained when the stem is pre-heated and that vacuum mixing used alone is less effective.…”

“…Great efforts have been made to optimise cement formulation, [11][12] to improve mixing and/or injection, [13][14][15] and to select the best texture for the surface of the implant. 16,17 Despite these efforts, some degree of porosity can still be observed in bone cement mantles. [18][19][20] It has been suggested that porosity can be reduced by thermally influencing the rate of polymerisation of cement by preheating the stem before its insertion into the medullary canal.…”

“…21,22 This generates an increase in the static strength of bone cement 23 and the strength of the implant-cement interface. 17,24 A recent paper investigated the influence of pre-heating the stem on the fatigue behaviour of bone cement, 25 which is the most important mechanical characteristic for its long-term performance. 3,8 In that preliminary study it was found that pre-heating appeared to improve the fatigue strength of cement, although neither a control group nor vacuum mixing were considered.…”

Measurement of transient and residual stresses during polymerization of bone cement for cemented hip implants

“…During this complex phenomenon, porosity and stresses at both interfaces and inside bulk cement are observed (Ahmed et al, 1982a;Bishop et al, 1996;Gilbert et al, 2000;Mann et al, 2007;Nuñ o and Amabili, 2002). Numerous studies (Bishop et al, 1996;Damron et al, 2006;Iesaka et al, 2003;Wang et al, 2003) have shown significant reduction of stem-cement interfacial porosity by stem pre-heating. Mann et al (2007) have shown that porosity at the stem-cement interface directly affects stem migration, and is thought to induce implant loosening.…”

“…Damron et al (2006) use the aluminum mold with surface asperities to represent trabecular bone, which allows interlocking between the cement and cortical wall. In the present study, the aluminum mold is polished to avoid interlocking between cement and bone and refers to no adhesion at this interface.…”