Keeping It Together
By Steve Hill, CO

Many orthotic and prosthetic devices involve disparate materials that have to be bonded together. Foam and plastic can be bonded together with heat because they are made of the same material. Crepe and plastic are bonded together with glue, partly because one material is pliable and the other is stiff. 

But often we need to bond metal uprights with plastic or carbon fiber. These are very different materials and can’t, therefore, be coupled together with heat. Gluing these two stiff materials together won’t work, especially not under load. For this type of attachment we use a mechanical bond instead. 

Rivets or screws?
It seems to me that most people like to bond the two using copper rivets. Rivets are usually lying around because of shoe work, and they’re easy to use. In fact, if you use moleskin to cover the heads, you might not even have to countersink the inside of the cuff. But the two things that rivets do not provide are strength and adjustability. 

Screws are a stronger connection for two reasons. First, screws are stronger than rivets because they are made of steel instead of copper. The shear strength of copper isn’t even in the same ballpark as steel. 

Second, screws are stronger simply because of the way they’re constructed. Rivets fasten two things together by squeezing them and keeping them together through a head at each end. If both parts aren’t pressed tightly together, the rivet will fail. It’s just a matter of time. 

But a screw captures the metal upright by way of circular threads that run the length of its shaft. The thread winds its way around the screw and connects along the thread in the upright. So the screw isn’t attached for just the depth of the hole, it’s attached along the whole length of the thread. 

Depending on the thread pitch, the area of attachment can be many times greater than the hole’s depth. Consequently, the strength of the attachment is many times greater than that of non-threaded fasteners. 

The second reason that screws are stronger is that screws make it easy to have a tight connection between the two surfaces. Rivets are strong only if the surfaces between them are pressed together tightly prior to setting. In many situations, doing this can be tricky. When using screws, you can simply crank down and remove any play between the upright and cuff. 

How strong does the connection between upright and cuff need to be? Where I come from it needs to be as strong as possible. Copper rivets may be strong enough for your applications. 

However, increased strength, in my opinion, is secondary to the ability to remove an upright easily. Copper rivets are more difficult to remove than screws should the device ever need adjustment or repair. With screws, a few turns of the driver and the upright comes right off for adjustment or replacement. Sure, the inside of the hole in the cuff will need to be countersunk, but that’s easy. 

Countersinking holes 
Countersinking holes is important because one goal of any orthotic or prosthetic device is comfort. Rivets or screw heads that protrude past the cuff and dig into the patient mean he or she probably won’t wear it long. And if the patient doesn’t wear the appliance, it’s not going to help much. 

Rivet heads are much thinner than screw heads since they have no need for a tool slot. For this reason they are easier to manage. A very shallow countersink or just some moleskin covering the head will be sufficient to provide a relatively smooth inner surface. 

Since screws have a much larger head, they will need some special attention to countersink properly. 

Two ways 
There are two ways to countersink a hole in a plastic cuff. The first way is to use a countersinking bit. Countersinking bits come in different types, but they all work on the same principle as a drill bit. They have a cutting edge with either flutes or holes that removes material at the selected engagement angle. 

The trouble is that this type of tool bit cuts plastic very quickly. It’s easy to lose control and drill straight through the hole. Another downside is that the engagement angle of the tool may not match the angle of the screw head. The best way to get around this problem is by using the actual screw to match the countersink angle. 

The second way to countersink the hole is by using friction heat to create the proper angle. When purchasing the screws you’ll be using to attach the uprights (typically 10-32 or similar thread pitch), buy some extras of different shaft lengths. You’ll want some suitable for attaching 3/16” and 1/4” bar stock, but you’ll also want a few with shaft lengths of 1” or more to use as countersinking bits. 

Drill a hole in the plastic cuff to the correct size for your screw shaft. Take the one-inch screw and put it into the hole, with the head inside the cuff and the shaft sticking out toward where the bar will attach. Clamp the end of the shaft into a drill press that has been set to its fastest speed setting. 

Hold the cuff so that the screw head is level against the hole, turn the drill press on and apply pressure downward against the back of the screw head. The rotating screw will produce heat and gradually sink into the plastic. 

As the plastic melts, wipe away any excess with your thumb, checking the depth as you go. You must be careful not to go too deep and ruin the hole. Just go the depth of the screw head. Do it a little at a time and remember that the screw will compress into the plastic a tad once you tighten the screw into the bar. Any plastic burrs can be removed with a razor blade. 

Not only does this method save you from buying another tool bit, but the angle of the countersunk hole matches the angle and depth of the screw perfectly. There’s no guessing involved, and it’s safer than a rotating metal bit. 

The bottom line
Whether you use copper rivets or stainless steel screws, the attachment is only as good as the method used. Be sure to follow proper fabrication guidelines and keep patient comfort and device strength in mind. 

Steve Hill is secretary of OPTA and CEO of Delphi Ortho, located in Asheville, N.C. and on the Web at www.delphiortho.com.