Influence of trabecular bone quality and implantation direction on press‐fit mechanics

Abstract

Achieving primary stability of uncemented press-fit prostheses in patients with poor quality bone can involve axial implantation forces large enough to cause bone fracture. Radial implantation eliminates intraoperative impaction forces and could prevent this damage. Platens of two commercial implant surfaces ("Beaded" and "Flaked") were implanted onto trabecular bone specimens of varying quality in a press-fit simulator. Samples were implanted with varying interference, either axially (shear) or radially (normal). Push-in and pull-out forces were measured to assess stability. Microstructural changes in the bone were determined from μCT analysis. For force-defined implantation analysis, push-in and pull-out forces both increased proportionally with increasing radial force, independent of implantation direction, bone quality or implant surface. For position-defined implantation analysis, pull-out forces were generally found to increase with interference and to be greater for radial than axial implantation direction, and to be lower for poor quality bone. Bone density increased locally at the tested interface due to implantation, in particular for the Beaded surface under axial implantation. If a safe radial stress can be determined for cortical bone in a particular patient, the associated implantation force, and pull-out force which represents primary stability, can be directly derived, regardless of implantation direction, bone quality or implant surface. Radial implantation delivers primary stability that is no worse than that for axial implantation and may eliminate potentially damaging impaction forces. Development of implant designs based on this principal might improve implant fixation. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:224-233, 2017.

Effect of cavity preparation and bone mineral density on bone‐interface densification and bone‐implant contact during press‐fit implantation of hip stems

“…The results of this study support previous work suggesting that bone densification is design‐ and BMD‐dependent . This is evident in the observed reduced densification with sharp extraction broaching as well as the positive association between bone densification and initial BMD with compaction broaching.…”

“…Another limitation involves the method to calculate the contact area. Superimposition of the cavity and implant CT scans may have underestimated the amount of contact area since axial stem insertion during implantation causes additional bone deformation and densification and may distribute bone mass further . Furthermore, the contact area analysis was rather conservative since surface triangles in the transition zone were not regarded as contact which excluded areas of “line‐to‐line” contact without overlap.…”

“…The ideal primary stability is achieved by optimizing the degree of press‐fit versus the risk of periprosthetic fracture. This can be a challenge for surgeons, due to the heterogeneous bone geometry and morphology among patients . As the age range of patients undergoing total hip arthroplasty (THA) becomes broader, bone quality differs among patients substantially.…”

“…This effect might originate from the spring‐back effect of compacted trabecular bone, and could be dependent on the BMD of the patient. While bone densification around implant components for a range of BMD has not been evaluated to date, for small sections of trabecular bone of human femoral heads, Damm et al showed a trend for increased bone densification during insertion of platens with different implant surface finishes with higher initial BMD. Results on bone compaction by preparation broaches in metaphyseal human femora of different bone mineral densities have not been presented to date.…”

Biomechanical behaviours of the bone–implant interface: a review

“…Another testing configuration was developed by Bishop and colleagues [104][105][106] in a series of papers modelling the press-fit configuration (figure 2d) and taking into account the effect of the interference fit. They considered two parallelepiped specimens for the bone sample and for the implant.…”