The influence of uncemented femoral stem length and design on its primary stability: a finite element analysis

Abstract

One of the crucial factors for short- and long-term clinical success of total hip arthroplasty cementless implants is primary stability. Indeed, motion at the bone-implant interface above 40 μm leads to partial bone ingrowth, while motion exceeding 150 μm completely inhibits bone ingrowth. The aim of this study was to investigate the effect of two cementless femoral stem designs with different lengths on the primary stability. A finite element model of a composite Sawbones(®) fourth generation, implanted with five lengths of the straight prosthesis design and four lengths of the curved prosthesis design, was loaded with hip joint and abductor forces representing two physiological activities: fast walking and stair climbing. We found that reducing the straight stem length from 146 to 54 mm increased the average micromotion from 17 to 52 μm during fast walking, while the peak value increased from 42 to 104 μm. With the curved stem, reducing length from 105 to 54 mm increased the average micromotion from 10 to 29 μm, while the peak value increased from 37 to 101 μm. Similar findings are obtained for stair climbing for both stems. Although the present study showed that femoral stem length as well as stem design directly influences its primary stability, for the two femoral stems tested, length could be reduced substantially without compromising the primary stability. With the aim of minimising surgical invasiveness, newer femoral stem design and currently well performing stems might be used with a reduced length without compromising primary stability and hence, long-term survivorship.

Inter-subject variability effects on the primary stability of a short cementless femoral stem

“…the limit values of metrics such as in-out aspect ratio (0.02), edge length ratio (20), angular/volume skew (1/0.99), shape factor (0.01%), minimum dihedral angle (2), maximum dihedral angle (177), jacobian (0.019) or minimum edge length (0.01) were not violated. The mesh settings resulted in stem average micromotions comparable to those obtained by Reimeringer et al (2012). Node-to-node correspondence at the bone-implant interface was achieved and contact surface pairs and node sets for application of boundary conditions and loading were automatically created.…”

“…Excessive bone-implant relative micromovements can compromise this stability (Pillar et al, 1986;Engh et al 1992;Soballe et al, 1993). These micromovements depend on implant design and positioning (Howard et al, 2004;Paratte et al, 2007;Andreaus et al, 2008;Park et al, 2009;Dopico-González et al, 2010;Reggiani et al, 2008;Bah et al, 2011;Reimeringer et al, 2012), possible interfacial gaps and the magnitude of forces acting on the proximal femur and patient anatomy (Pancanti et al, 2003). Therefore, when introducing new stem designs, it is essential that rigorous preclinical testing is conducted, both computationally and physically, since clinical problems associated with new designs may not be evident for some time.…”

“…by modifying loading, bone shape and quality. With faster computers and more advanced image processing softwares, more representative CT or MRI-based musculoskeletal models (Sigal et al, 2009;Baldwin et al, 2010;Vahdati et al, 2014;Martelli et al, 2014;Simpleware Ltd., 2014) can be now readily generated for analysis and micromotions along the entire implant surface can be predicted (Pancanti et al, 2003;Andreaus et al, 2009;Park et al, 2009;Pettersen et al, 2009;Bah et al, 2011;Reimeringer et al, 2012;Fitzpatrick et al, 2014). In this respect, novel pre-clinical evaluation tools have been developed to: (a) enable the variability in patient geometry and bone quality using statistical shape modelling (Bryan et al, 2010;Blanc et al, 2012;Bah et al, 2013;Blanc et al, 2012;Rao et al, 2013;Fitzpatrick et al, 2014); (b) automatically assess the effects of implant positioning, loading, or bone-implant interface conditions for a specific patient (Abdul-Kadir et al, 2008;Bah et al, 2009;Dopico-González et al, 2010) or (c) evaluate and compare the robustness of existing implant designs (Sakai et al, 2008;Reimeringer et al, 2012;Fitzpatrick et al, 2014).…”

Short-stem prostheses in primary total hip arthroplasty

“…These SSs focus on metaphyseal fixation. Reimeringer et al, [6] in an analysis of a finite element model, demonstrated that reducing stem length to <105 mm was not associated with the stability of implants. SS designs depend upon a stable metaphyseal fit and require optimal proximal load transfer, and are beneficial to conserve bone mass and reduce thigh pain.…”

Structural health monitoring: An orthopaedic perspective

“…Titanium alloy (Ti-6Al-4V) and aluminium (Aluminium_2014) properties in the NX material library were assigned to the fixation and the constraining rig, respectively. The use of homogeneous material properties for the fourth-generation sawbone material is consistent with FE work reported by Elenes et al, 16 Reimeringer et al 17 To simulate the curing of epoxy at the cut, the material properties of the mesh strip which represents the fracture were varied such that its density and Young's modulus were at fractions of the original values. The Poisson's ratio remained unaltered.…”