Factors influencing taper failure of modular revision hip stems

Abstract

Stem modularity of revision hip implant systems offers the advantage of the restoration of individual patient geometry but introduces additional interfaces, which are subjected to repetitive bending loading and have a propensity for fretting corrosion. The male stem taper is the weakest part of the modular junction due to its reduced cross section compared to the outside diameter of the stem. Taper fractures can be the consequence of overloading in combination with corrosion. The purpose of this study was to assess the influence of implant design factors, patient factors, and surgical factors on the risk of taper failure of the modular junction of revision stems. An analytical bending model was used to estimate the strength of the taper connection for pristine, fatigued and corroded conditions. Additionally, a finite element contact model of the taper connection was developed to assess the relative motion and potential for surface damage at the taper interface under physiological loading for varyied assembly and design parameters. Increasing the male taper diameter was shown to be the most effective means for increasing taper strength but would require a concurrent increase in the outer implant diameter to limit a greater risk of total surface damage for a thinner female taper wall. Increasing the assembly force decreases the total surface damage but not local magnitudes, which are probably responsible for crack initiation. It is suggested that in unfavourable loading conditions a monobloc implant system will reduce the risk of failure.

Short proximal components in modular revision stems carry a higher risk for stem fractures

“…Besides better bone support, a second argument for the employment of a long proximal component is that, as a locus minoris resistentiae , the taper connection should be positioned in the area where it intersects with the force vector of the hip joint. 25 For modern modular revision stems, however, the taper lies above this position, which increases the maximum bending force exerted on the taper, with the bending force being proportional to the distance of the taper to the intersection (Figure 3). 25 Implantation of a larger proximal component minimises this distance between the taper and the intersection, and, for this reason, is recommended.…”

“…25 For modern modular revision stems, however, the taper lies above this position, which increases the maximum bending force exerted on the taper, with the bending force being proportional to the distance of the taper to the intersection (Figure 3). 25 Implantation of a larger proximal component minimises this distance between the taper and the intersection, and, for this reason, is recommended.…”

“…Representation of the bending stresses exerted on the taper due to the joint force and the offset from the taper. Redrawn from Krull et al 25 …”

Implant Breakage Risk for Modular Fluted Tapered Stems in Revision Total Hip Arthroplasty at 5-Year Follow-Up

“…The relative fragility of the metaphyseal-stem junction of revision implants has been well-examined and is due to several factors [22][23][24]. 1/ The design of the implant: the biomechanical study by Krull et al [25] highlights the importance of the balance between the diameter of the "male taper" and therefore the diameter of the stem and that of the female metaphyseal part. This stress distribution prevents overloading of the taper and therefore metal fatigue.…”

Author’s response: Letter “Short proximal components in modular revision stems carry a higher risk for stem fractures”

“…2 The second and third comments both concern the analytical model that we used to estimate the bending stresses at the taper junction. Firstly, Krull et al 3 developed this model at the Hamburg University of Technology. They published this separately and comments should primarily be directed to them.…”