Orthotics/Prosthetics

Ring locks, also known as drop locks due to the gravity-operated nature of the locking mechanism, are easily one of the most popular style of knee joint used on KAFOs in the United States. They are so popular because they are simple, easy to use, inexpensive and versatile.

	Steve Hill

One of the things about ring locks is that, theoretically, they always fall to the locking point when in a vertical, or standing, position. This is part of the design, but if you require that the rings stay in the unlocked position until a locked knee is desired, then you have a decision to make. You can either order the knee joints with ball bearing lock retainers already installed (Becker Orthopedic offers this service for a small upcharge) or you can install them yourself.

For the uninitiated, ball bearing lock retainers are a method for keeping the drop locks temporarily suspended above the point where the ring lock prevents flexion of the hinge. A spring loaded ball bearing is implanted into the joint head so that it partly protrudes above the surface, preventing the lock from progressing over the point where the hinge folds. Because the ball bearing is spring loaded, it is easily overcome be pushing it downward with just a little force. Many patients find them useful and convenient.

Installing them yourself can be easier said than done. I’ve seen quite a few perfectly good knee joints destroyed by inattentive or overzealous technicians who drilled a little too deeply. Armed with bit of foreknowledge, however, you will find that installing a lock retainer into a ring lock can actually be fun. Okay, not fun per se, but at least you won’t waste time or knee joints during the undertaking.

The right tools

To install ball bearing lock retainers you will need a few basic tools. You will need the obvious ones, such as a hammer, drill press, and drill bits (#11 drill bit for 3/4" wide uprights and a #30 drill bit for 5/8" and 1/2" uprights) and you will also need a corking tool. Not the kind used to install corks in wine bottles; this one is simply a stainless steel rod about 4" to 6" long with an appropriately sized indentation at one end. I say appropriately sized because these tools come in two sizes, one to accept the 3/16" ball bearings and another to accept the 1/8" ball bearings.

	Drill through the outside layer and upright only. Don’t drill through the inside layer.
	Image: Steve Hill

In addition to these tools you will also need the hardware, which will actually be installed in the joints. This consists of the ball bearings and springs. When installing a 3/16" ball bearing you will use a 3/16" diameter spring and a 1/8" spring for a 1/8" ball bearing. Simple enough.

Now comes the hard part

The hard part is drilling the hole. It must be located in the correct area and it must be of the correct depth. Failure on either of these details will result in failure of the entire project. Although these details are critical they are really not all that difficult to figure out. Lock retainers are installed on the proximal joint head at a level that will prevent the ring from covering the knee axis. For its placement you will need to avoid the rivets that hold the uprights to the joint head. This area will differ on various size joint heads but it’s not hard to find. There’s a very small area from which to choose.

Be sure that the hole you drill is above the point at which the ring intersects the distal section, and as far away from the rivets and side of the joint head as you can make it.

With a center punch, mark where you are going to drill your hole and select the proper size drill bit for the ball bearing you’re going to use. Chuck the bit into your drill press and set the depth stop. This is the other critical part. The idea here is to drill through the outward side of the stainless steel joint head and the upright which is right behind it but not through the inward side of the joint head.

You can measure this dimension by simply looking at the side of the joint head. The depth can be measured as the thickness of the upright plus the thickness of one of the side plates. Drill through the joint head to this depth. If you drill through to the other side, you’ve gone too far. Hang up your apron, clock out and go home. Call in sick the next day and hopefully by the time you return someone else will have fixed your blunder in your absence.

Set the ball bearing

Assuming you were successful, clean the hole and remove any debris. Add a drop of oil in the hole to ensure the spring will work freely and insert the correct length spring for the hole’s depth. The spring should be a coil to a coil and a half above the surface of the joint head. Now it’s time to put in the ball bearing and set it with the corking tool. You must be very careful during this procedure as the ball will be under spring tension. Wear eye protection; doing this carelessly will almost certainly result in the ball bearing, and probably the spring as well, flying across the room at velocities unthought-of from a mere spring-powered weapon.

The idea behind getting the ball bearing set correctly is to create a slight burr at the top of the hole which will prevent the ball from exiting. Place the corking tool over the ball bearing in the hole and apply a few short, sharp shots of the hammer. That should create enough of a burr to keep the ball in place. Don’t overdo it or the ball bearing won’t move freely. Carefully check the fit after each hammer blow, making sure the ball bearing and spring don’t fly across the room as in the aforementioned dangerous scenario.

Check the fit by first pushing down on the ball with a screwdriver or similar object to assure it is set properly and the spring moves the ball freely. Next, apply the ring lock and slide it over the lock retainer a few times to make sure that it will do two things. First, it must hold the ring up and allow free motion at the knee. Second, the ring must be able to slide down over the retainer into the locked position. If it does those two things and the ball bearing stays put, you will have successfully installed a ball bearing lock retainer.

Find a spare

Here’s the most valuable piece of advice I can give anyone attempting any new procedure such as this one. When possible, try it out on a spare or broken knee joint. Better to hone your skills on something that isn’t going on a patient today. Nothing can replace experience so if you can gain that experience without ruining someone’s orthosis, all the better.

Ball bearing lock retainers are an easy way to add value and functionality to your patient’s KAFO. They’re also an easy way increase your value as a technician. The more things you can learn to do the more you will be worth to your employer. Seek out articles like this one and attend local and national O&P shows to gain knowledge and learn new skills. It will pay big dividends in the end.

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Steve Hill, CO is secretary and co-founder of the Orthotic Prosthetic Technological Association and president of Delphi Ortho, an orthotic consulting firm based in Asheville, N.C. He serves on the American Orthotic and Prosthetic Association Assembly Planning Committee and is a Board of Certification/Accreditation, International Facility Accreditation Inspector.