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Jungle Orthopaedics

ORTHOPAEDIC PROBLEMS IN A PRIMITIVE SETTING


John E. Bullock, D.O., M.D., F.A.C.S.
Diplomate American Board of Orthopaedic Surgery
Date of production in this revised format: February 15, 2001

Monograph #2, October 31, 1994

Screws and Plates

We have considered two very important basic tools of the orthopaedic surgeon: Kirschner wires and Steinmann pins. Now, let's look at some other very useful hardware.

Screws and plates are important in the armamentarium of the orthopaedic surgeon, and they come in a great variety of shapes, styles, and metals. The most commonly used screws and plates are stainless steel, and this is usually an alloy called "type 316", consisting of iron, chromium and nickel. The exact formulation may differ somewhat from one manufacturer to another, and even from one batch to another. Another common alloy is called Vitallium. Other alloys are also in use.

IT IS IMPORTANT TO AVOID PUTTING DISSIMILAR METALS TOGETHER, TO AVOID "BATTERY EFFECT", WITH RESULTANT CORROSION, TISSUE NECROSIS AND LOOSENING OFTHE IMPLANT.

Vitallium usually has a dull finish, whereas stainless steel is usually shiny. Whenever you get a new batch of screws and plates, it is very important to examine them and separate out the vitallium plates and screws from the stainless steel ones. Put them in a separate place, to prevent the CSR crew from mixing them up. Don't mix vitallium and stainless. This battery effect can often cause pain to the patient, as well as the other effects of the corrosion.

Screws come in a great variety of styles:

Common head styles:

The Woodruff head has the advantage that the slots are concave, deeper in the center than at the edges. This allows the special Woodruff screwdriver bit to engage the head even when held at an angle. This screw type can also be driven with a straight screwdriver.

Other head styles include Straight slot, Phillips and hexagonal.

Threads: Standard screws have 20 threads per inch, and are called "coarse thread". Fine-threaded screws have 32 threads per inch. In general it is probably best to use the coarse threaded screws as they have better holding power in the bone.

Thickness, or diameter, of screws: The most commonly used thickness is 0.138" (3.5 mm. ), and this requires a 7/64" drill hole. Sometimes it is necessary to use a thicker screw, which is 0.164" (4.2 mm). This screw requires a 9/64" drill hole.

PITFALL! When you are using screws, inspect each one as it is handed to you to see that it is the thickness you want to use, and whether it is coarse-threaded or fine-threaded. Also, be sure you are using the correct size drill for the pilot hole. Try to use the same type head on all the screws -- this makes it more likely that they will be compatible metals, and makes the removal easier later on.

Points: In general, points may be either self-tapping or non-self-tapping. Self tapping points have a cutout at the end, which allows the screw to cut its own threads in the bone:

If the screws do not have this "cutting tip", you must use a correct-size tap. Purists will tell you to use a tap with all screws, and this is fine if you have the tap and the time. It does make a better contact between the threads and the thread-channel in the bone, with greater holding strength. If you do not have the correct tap, you can make an improvised tap by cutting a slot with a file in one side of the tip of one screw and then use it to tap the holes for the other screws.

Length: It is best to use a length that will allow the screw to just protrude from the distant cortex (so that perhaps one thread is visible). If the screw protrudes further than this, there is the danger of damage to the soft-tissues. If shorter than this, there will be inadequate holding power in the far-cortex. Screws should penetrate both cortices of a long-bone. Length is measured like this:

Cancellous bone screws: These have much coarser and deeper threads, to give greater holding power in soft bone. There are only 11 threads per inch. Diameters vary, and required drill sizes vary, so be sure to check these carefully before using them. The partially threaded shaft or shank is useful in applying compression techniques, which will be discussed later.

There are many other types of screws for special purposes. Some of these will be discussed later on, as we deal with specific fractures.

Use of Screws: Screws can be used as inter-fragmentary holding devices, or for applying plates or other metal devices to bone. If screws are used to hold two fragments of bone together, the proximal hole should be over-drilled, so that the threads will have purchase only in the distal fragment. Tightening the screw will compress the fragments together. Commonly the pilot hole is first drilled with a 7/64" bit, through both fragments. Then, a I /8" bit is used to enlarge the hole through the proximal fragment only. When the screw is inserted and tightened it will "slip through" the proximal fragment but firmly engage the distal fragment, and when it is tightened it will draw the proximal fragment down against the distal one.

Compression plating: Compression of fracture fragments together will result in much faster healing. There are basically two ways compression can be applied: (1) Use of a compression plating device, and (2) use of a self-compressing plate.

(1) Compression-plating devices involve attaching a plate to one side of the facture and then drawing it toward the other side of the fracture by pulling against an "anchor" screw in the opposite side. One type of such device is the Meuller device. After the compression has been applied, one screw is placed in the distal fragment, the compression device removed, and the final screw or screws applied.

Compression can be improvised by partially inserting a screw on each side of the fracture, encircling both screws with a strong wire, and then twisting the wire to draw the two screws toward each other, thereby compressing the fracture. A plate can then be applied beside the wire, and the wire and temporary screws removed.

(2) Self-compressing plates: These plates have elongated screw holes. The plate is fastened to one side of the fracture with screws. Then, on the other side of the fracture, screws are driven in the holes at the "far-end" (away from the fracture line) of each elliptical hole. When the screw is tightened, the shoulder of the screw will force the bone to migrate toward the fracture, thereby compressing the fracture site. There are various types of these plates, but the principle is basically the same.

PITFALL: When drilling the pilot holes for screws AVOID THE WOBBLE! Wobbling the handle of the drill while drilling makes an hourglass shaped hole, and the holding power of the screw is decreased. Hold the drill as steadily as possible tile drilling.

Double plating: Sometimes it is necessary to apply two plates to a long bone, 90 degrees apart. This gives increased stability and lessens the likelihood of plate failure. This may be considered in an uncooperative patient, who is likely to use the limb against orders.

Removal of plates: Removal of plates is optional. The patient may request removal. People sometimes don't like the thought of that "thing" that is inside their bodies, and that they can see on the radiograph. Stress-shielding by the plate prevents normal bone remodelling and the bone does not get as strong as it would without the plate. It may be desirable to remove plates for this reason, e. g. in the case of an athlete.

It must be remembered that drilling holes in bone causes stress points and weakens the bone. After removal of the plate and screws the extremity should be protected by a cast or brace until x-rays show restoration of the medullary canal and obliteration of the screw holes. In case of double plating, only one plate should be removed at a time, with a six-month interval before removal of the second one.

Here are guidelines for timing of metal removal:

PRINCIPLE: AVOID USING OR SCREWSAND PLATES IN OPEN FRACTURES!! There is a high incidence of infection in such cases, and the fixation device will loosen, losing its effectiveness. Alternatively, see INTRAMEDULLARY FIXATION, USE IN OPEN FRACTURES.

PRINCIPLE: REMEMBER THAT PLATES AND SCREWSARE ONLY INTERNAL SPLINTS, AND SHOULD NOT BE EXPECTED TO BEAR THE STRESS OF WEIGHT BEARING OR OTHER STRESSESAND STRAINS. PROTECTTHE EXTREMITY UNTILADEQUATE CALLUS IS VISIBLE ON X-RAY

These are some general principles regarding screws and plates. We will discuss more details as we consider specific uses.