How to bore the interior of a femur which is curved to implant a curved prosthesis?

This is the question asked us by Doctor DECKNER, who specializes in the design of revision hip prosthesis.
When the surgeon must change a used hip prosthesis, a revision prosthesis must be implanted, longer than the first one. If the first prostheses are bare/straight, those of revision are curved because they follow the natural curve of the femur.
Usually, before installing the prosthesis, the surgeon calibrates the interior of the femur for the implant size by using a rasp. But in the case of a curved implant, the use of a rectilinear rasp is no longer possible.

Always innovating, Doctor DECKNER had the idea of “a reamer shaped like a cork-screw, sharp but flexible" able to bore the bone, while conforming to the interior geometry of the femur.
When he came to see Sferic Stellite (part of the Deloro Stellite Group of companies), he had already proposed his idea to several companies specializing in the manufacture of instruments and cutting tools.
Facing the complexity of such an idea, each one had declined. Intrigued by the originality of the solution and excited by the technical challenge, we however took up the challenge.

After some analysis, Sferic Stellite proposed to Doctor DECKNER a flexible reamer whose sharp body consists of a conical spiral, identical to the implant’s geometry. At the lower end, a round ogival facilitates the introduction into the bone and the higher end allows the clamping of a driving engine. To guarantee a homogeneous cut and not risk rupture, we defined the geometry of a particular spiral of progressive section and pitch, in order to avoid the zones of stress concentration, and to guarantee a good mechanical resistance.
Thus the reamer bends and cuts, but does not break.

While making it turn, the surgeon inserts it in the femoral canal. Because of its flexibility, the reamer bends while following the interior shape of the bone. It cuts the bone to the exact geometry of the implant. We also optimized the choice of stainless steel to ensure the flexibility, cut and resistance. If the design of the instrument was not simple, its engineering was even more difficult. If the reamer is to be flexible while in use, it has the same properties during machining.

It was necessary to elaborate the machining step of the progressive spiral by limiting the cutting pressures. Thanks to well adapted correction parameters and specific support tools, we managed to develop a machining cycle with 4 cuts only, ground directly in cylindrical blank.

The first samples manufactured permitted Doctor DECKNER to validate his concept, finalize the patent filling, and find a Swiss distributor (PLUS ORTHOPEDICS). Over a hundred surgeons have now adopted this tool, and all are very satisfied Sferic Stellite has now manufactured over a thousand of this spiral reamer.