Maximizing the fixation strength of modular components by impaction without tissue damage

Abstract

ObjectivesTaper junctions between modular hip arthroplasty femoral heads and stems fail by wear or corrosion which can be caused by relative motion at their interface. Increasing the assembly force can reduce relative motion and corrosion but may also damage surrounding tissues. The purpose of this study was to determine the effects of increasing the impaction energy and the stiffness of the impactor tool on the stability of the taper junction and on the forces transmitted through the patient’s surrounding tissues.MethodsA commercially available impaction tool was modified to assemble components in the laboratory using impactor tips with varying stiffness at different applied energy levels. Springs were mounted below the modular components to represent the patient. The pull-off force of the head from the stem was measured to assess stability, and the displacement of the springs was measured to assess the force transmitted to the patient’s tissues.ResultsThe pull-off force of the head increased as the stiffness of the impactor tip increased but without increasing the force transmitted through the springs (patient). Increasing the impaction energy increased the pull-off force but also increased the force transmitted through the springs.ConclusionsTo limit wear and corrosion, manufacturers should maximize the stiffness of the impactor tool but without damaging the surface of the head. This strategy will maximize the stability of the head on the stem for a given applied energy, without influencing the force transmitted through the patient’s tissues. Current impactor designs already appear to approach this limit. Increasing the applied energy (which is dependent on the mass of the hammer and square of the contact speed) increases the stability of the modular connection but proportionally increases the force transmitted through the patient’s tissues, as well as to the surface of the head, and should be restricted to safe levels.Cite this article: A. Krull, M. M. Morlock, N. E. Bishop. Maximizing the fixation strength of modular components by impaction without tissue damage. Bone Joint Res 2018;7:196–204. DOI: 10.1302/2046-3758.72.BJR-2017-0078.R2.

Quantification of assembly forces during creation of head-neck taper junction considering soft tissue bearing: a biomechanical study

“…However, the difference accounted for only a few percent. Krull et al [27] also carried out experiments on an in vitro model in the laboratory. They showed that both the stiffness of the tip of impactor tool and the bearing of the taper have major influence on the resulting forces.…”

Investigation of Assembly Forces During Creation of Head-Neck Taper Junction Following Total Hip Arthroplasty: A Biomechanical Study

“…However, the difference accounted for only a few percent. Krull et al [22] also carried out experiments on an in vitro model in the laboratory. They showed that both the stiffness of the tip of impactor tool and the damping of the taper bearing have major in uence on the resulting forces.…”

What the Surgeon Can Do to Reduce the Risk of Trunnionosis in Hip Arthroplasty: Recommendations from the Literature

“…The characteristics of the impactor greatly influence the force transmitted from the hammer blow to the taper connection. Hard plastic tips found on commercially available impactors avoid damage to the head, but they reduce the assembly force by approximately 20% compared to a metallic tip [77]. Older impactors (with questionable stiffness) and impactors with rubber tips should be abandoned [77].…”

“…Impaction with a hammer has its advantages, in that a short impulse causes a lower transmission of energy to the tissues distal to the taper than a slower application of the force [44,55,77]. It might be expected that the impaction of the taper with a hammer is associated with a low risk of femoral fracture, even if high forces are applied.…”