Medical DHS Lag Screw 12.7mm?
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    Default Medical DHS Lag Screw 12.7mm?

    We are currently looking to create a custom lag screw for a company, that is used to relieve pressure to a hip fracture. They sent a sample part and from what i can tell its a 12.7mm x 3mm thread. The root of the thread has a flat into a radius, into the angle of the thread. Does anyone have any experience with medical grade lag screws?

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    I have a bag of them they removed from my back after my spinal fusion took. I have never ran any though. I would almost venture a guess they come in different sizes and pitches due to application.

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    The ones I have seen all have proprietary thread forms, I guess to differentiate themselves from the competition.

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    It will be a custom threadform; you'll likely need to have custom thread whirling inserts made. The customer's print should specify the threadform. If they don't have a print they'll never get it past the FDA.

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    I made thousands of them and yes, they are proprietary. Modified Acme and by modified I mean really modified. I think they were pulling my leg somewhat. I grabbed this off the web since I don't work for Synthes anymore.

    synthes-bone-screw.jpg

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    Quote Originally Posted by Booze Daily View Post
    The ones I have seen all have proprietary thread forms, I guess to differentiate themselves from the competition.

    That and there really are advantages to some. Fine threads for the thick outer bone layer and the ones I posted above for the inner, honeycomb layer. We even made ones that had threads on the head also so that the head could be threaded into a plate for a more secure hold. Spherical head screws with a spherical thread so you could get at least one full thread engagement regardless of the angle of entry relative to the plate.

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    Its one of those "how about we send you a sample part and you figure it out" type deals. Its not a traditional cortical thread, here is a picture.

    280.50_113295086214f3eb10d98266.jpg

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    What exactly is your question?

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    I'm trying to figure out what type it is, if there is a standard that would be close.

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    "Close" isn't good enough for medical

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    Hi FilandFran:
    That looks an awful lot like part of a hip fixation device I prototyped a few years ago and posted about the threading challenge on Practical Machinist.

    If my guess is correct and if they didn't modify the thread further after what I did to it, you sadly will not be able to whirl it in one go because the root of the thread is not a constant shape...it varies as you travel along the length of the thread.

    It was a bitch to program and cut then (I turned the prototypes by single pointing the threads with a custom wire EDM cut round nosed tool that had the correct flank profiles).
    I walked that round nosed tool along the floor of the part a bit like you'd do 3D surfacing with a ball cutter on a CNC mill.
    My stepovers were 0.005" or so as I recall.
    Something like a thousand lines of code all programmed longhand to rough it and finish it (for those who don't usually program for turning that's a LOT of code for a lathe program).

    You're NEVER going to find something close...bite the bitter pill now and expect to have to commission custom inserts, then find a really smart lathe programmer who's good with hand coding and loops and macros and all that other exotica, so you don't have to experience the pain I did.

    Cheers

    Marcus
    Implant Mechanix • Design & Innovation > HOME
    Vancouver Wire EDM -- Wire EDM Machining
    Attached Thumbnails Attached Thumbnails dscn3513.jpg  

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    implmex,

    That is exactly what it is, is a Dynamic hip screw that is used to take the pressure off a hip fracture. We have the ability to make custom tooling in house. Im trying to nail down the geometry, looks like the front angle is 39° and the back is angled a few degrees. Its such a unique thread, but looks to be standard among the different manufactures of the DHS system.

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    Hi again FilandFran:
    Is it OK if you take the sample and butcher it on the wire EDM?
    I ask because often times the easiest and most accurate way to interrogate something like this is to split it in half lengthwise on the wire EDM and then just pop it on the shadowgraph or the microscope to measure it.

    It violates a few rules about not hacking up the customer's samples, but if replacements are readily available and the customer is cool with it...??

    Cheers

    Marcus
    Implant Mechanix • Design & Innovation > HOME
    Vancouver Wire EDM -- Wire EDM Machining

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    Quote Originally Posted by FilandFran View Post
    I'm trying to figure out what type it is, if there is a standard that would be close.

    There's no real standard... Same company can make 15 similar screws and have 11 different thread forms. Different flat widths, different leading/flank angles (often by only 1 degree), different allowable crest widths, etc.

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    Hi TeachMePlease:
    I certainly hear you and agree fully.
    It's almost as bad as the dental implant game; a gazillion different threadforms for a glorified wood screw.
    I was heavily involved in development work in that game a couple of decades ago, and what a shitshow it was!

    Frickin' wanker marketing types!!
    I remember all the bullshit very well.

    Cheers
    Marcus
    Implant Mechanix • Design & Innovation > HOME
    Vancouver Wire EDM -- Wire EDM Machining

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    I've thread whirled miles of implants screws and not a chance in hell would I except an order from a custom that wanted me to reverse engineer the pattern. Mirroring a widget is one thing but a medical implant? Walk away.....

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    Check this out:

    https://www.iso.org/obp/ui/#iso:std:iso:5835:ed-1:v1:en

    But the threadform should be very well documented on the designer's end; if it isn't, they're pretty fly-by-night.

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    Quote Originally Posted by mhajicek View Post
    Check this out:

    https://www.iso.org/obp/ui/#iso:std:iso:5835:ed-1:v1:en

    But the threadform should be very well documented on the designer's end; if it isn't, they're pretty fly-by-night.
    Sounds to me like he's got a customer trying to rip off someone else's design, or why would they not have a drawing?

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