Bioabsorbable Screws

A unique type of bone screw made of a most unlikely material is making waves in equine fracture repair at the Equine Research Centre in Guelph, Ontario, Canada. The results indicate that the screws also could have important applications in human bone surgery.

Surgeon John Field, BVSc, heads a team that is exploring the use of the 4.5 mm-wide screws, made of almost pure lactic acid, in certain types of fracture repair. Thus far, the results are promising, with two Thoroughbred racehorses which had suffered dorsal cortical (saucer) fractures of their cannon bones returning to work six months after the screws were inserted. One, a gelding named Count Your Shekels, has gone on to race--and win!--in the aftermath of his surgery.

"Scientifically, of course," says Field, "two horses don't constitute a statistically significant study--but we have to take what we can get. These screws are still experimental, so we're delighted to have had these two horses to try them out on--and we're very pleased that Count Your Shekels has been able to provide such positive results!"

The other Thoroughbred which had fracture surgery with the bioabsorbable screws also recovered to soundness, but for unrelated reasons has not raced again. He is likely to become a pleasure mount.

Many types of bone fractures in horses can be treated with the insertion of screws, which help hold the bone fragments in place until healing can cement them together
permanently. Traditionally, screws made of stainless steel are used in equine surgeries of this type, because few other materials have demonstrated the strength needed to help support the weight of a thousand-pound animal--especially one which might not be especially easy on himself during recovery.

The basic procedure of a leg fracture repair surgery goes like this:

Once a treatable fracture has been diagnosed (through the observation of pain and swelling, radiographs, and possibly nuclear scintigraphy if the fracture is not obvious on X-ray plates), the horse is anesthetized, and the area of the fracture clipped and scrubbed. The surgeon makes a single small incision (usually about 3 cm) over the top of the fracture, then drills a hole through the detached bone fragment into the middle of the bone.

With a tap (in some ways similar to the tap used by eventers and showjumpers to clean out caulk holes in a shoe), he or she cuts "threads" in the hole to give the screw something to grip.

The screw is inserted, the wound is sutured and bandaged, and the horse is left in a recovery stall to recover from the anesthetic (a crucial point in the surgery, since many horses panic and thrash and do further injury to themselves, as well as potentially injure the hospital staff).

"All in all, it's a quick procedure, which is good for the horse," said Field. "The average time in surgery is about an hour." More complex surgeries, of course, can be performed if two or more screws need to be inserted.

Stainless steel screws are used in most surgeries, but they can be problematic. They are extremely strong, but also very inflexible--and thus quite incompatible with the natural properties of the bone in which they are inserted. This mismatch between the strength of the screw and the strength of the bone sometimes makes the screw loosen and back out of the hole in which it was inserted. Often, horses to whom this happens end up back in surgery to have the screw removed prematurely--a risk because of the additional surgery, and because the resulting hole can become a weak spot where the horse can re-fracture the bone. Even when everything goes smoothly, a horse which has had a stainless steel screw inserted in his leg must eventually have it removed--and that means another general anesthetic, and another chance that he might recover badly, panic, and either aggravate the current injury or cause new injury.

Start of Equine Research

In 1992, Field was approached by a Finnish company that manufactures bioabsorbable, lactic acid screws, and was asked to explore their possible applications in equine surgery. The screws, developed in Scandinavia, have been used extensively in human fracture repair for about a decade there, but they were practically unknown in North America--and had never been used on horses. Naturally, Field was intrigued.

"I thought it was a wonderful opportunity," he said. "On the whole, I think biocompatible orthopedic devices are the future."

The technology used to produce rigid screws out of almost pure lactic acid is complex (and patented); however, Field says that as he understands it, lactic acid (a naturally-occurring by-product of anaerobic metabolism in mammals) is converted into a polymer (repeating chain of molecules) and injection-molded into the screw shape.

"In some ways," notes Field, "it's similar to absorbable suture material."

The end result looks like translucent plastic, and it is very slightly malleable--making it far more like natural bone than stainless steel could ever hope to be.

Because lactic acid is a product the horse's body breaks down and eliminates naturally (through the Krebs cycle), the screws are deemed bioabsorbable. Over the course of a few years (four to five total), the composition of the screw breaks down, and it is gradually replaced by new bone, which fills in the hole. The release of lactic acid into the system is so gradual that the body can easily handle it without suffering any toxic side-effects. In humans, the only downside is the occasional appearance of a small, fluid-filled swelling (a result of the lactic acid breakdown, according to Scandinavian studies) over the site of the screws. The fluid generally is drawn off with a needle and creates no further complications. Although neither of the two horses which received lactic acid screws in the preliminary study suffered fluid swellings, Field expects that it is possible, and that it probably would be treated in the same manner.

The plus side of using lactic acid screws is simple--their bioabsorbable properties mean that they do not require removal. This, in turn, means one less surgery and one less time for the injured horse
to undergo the always-risky general

"If you can put something in and not have to re-anesthetize the horse (to take it out), you're ahead not only in terms of stress on the animal, but also in terms of the money saved on the second (and possibly third) surgeries," says Field. "The screws themselves are a little more expensive (than stainless steel screws, which cost only pennies), but overall, it's a negligible difference."

Further, the gradual breakdown of the screw material allows new bone to remodel itself at a natural pace, with less risk of re-fracture during the healing process. The screw's strength lasts about a year, which is ample time for the fracture to heal and knit, and for the horse to return to work; meanwhile, the screw is still giving some residual support of the injured area. (When metallic screws are used, the horse must rest for another six to eight weeks after their removal before work can begin.) And because the lactic acid screws are more analogous to the structure of natural bone, they don't seem to work themselves loose as readily as do metal screws.

But, of course, no new technology is without its own set of problems.

The Downside

"The chief drawback of lactic acid screws," says Field, "is that they don't have the strength of metallic screws."

He discovered this during the initial testing process, when he created fractures in equine cadaver bones, repaired them with the lactic acid screws, then forcibly tried to separate the bone fragments. In certain types of injuries, such as distal condylar fractures (a "wedge" type of break that runs diagonally from the back of the fetlock up across the cannon bone), the lactic acid screws were unable to withstand the stress.

"On the really nasty fractures, the bioabsorbable screws wouldn't do the job," says Field. "We came to the conclusion that this technology wouldn't be useful for major load-bearing fractures."

The same is true in human fracture repair; the lactic acid screws are most often employed in bones where direct weight-bearing can be avoided (for example, in ankle fractures where the patient can get around on crutches while healing).

While this ruled out many types of bone breaks in horses, there still were some valuable applications for the lactic acid screws. In addition to the dorsal cortical fractures (an aggravation of the common racing condition called bucked shins) treated in the two "guinea pig" Thoroughbreds, Field speculates that the screws could be employed in fractures of the third carpal bone in the knee and in sesamoid fractures, to name only two. He also suspects that they could prove useful in cases where several screws need to be inserted with a metal plate, as in a long fracture of the cannon bone. When a plate is inserted with metal screws, several surgeries are often required to remove all the appliances, and the sudden lack of support for the bone when screws and plates are removed makes the healing bone vulnerable to re-injury. But if lactic acid screws were to be alternated with metallic screws, says Field, the horse would require less subsequent surgery, and enjoy better support thanks to the lingering structure of the lactic acid screws.

"It's something for the future," he says, "but I like the idea of trying it."

Whichever type of screw is used, the recovery time for a fracture surgery is about the same, approximately three months of stall rest, with hand walking after one month if all seems well and a gradual re-introduction to work after about six months (and the removal of the screws, if stainless steel). Bone scans, performed every two weeks up to the three-month mark, will give the veterinarian an excellent idea of how healing is progressing. If there are no observable problems, Field suggests that past the three-month milestone, bone scans be taken again only at six and 12 months.

In the case of the two Thoroughbreds treated with lactic acid screws, Field says they both were followed very closely with nuclear scintigraphy throughout their recovery phases in 1995 and 1996, and that their recoveries were very similar to what is normally expected.

"Metallic screws are still the way to go with many types of fractures," confirms Field, "and of course the success rate (of bioabsorable screws) depends largely on the fracture site. It's always problematic, and you know that going in--even if the surgery goes well, there's still the possibility that the horse will 'lose it' in the recovery stall and undo all of your work in a split second. But I think the lactic acid screws are very interesting, as long as you're selective as to where you use them."

As a result of this work (which Field notes was generously supported by the Mohill Orthopedic Research Initiative and the E. P. Taylor Equine Research Fund), a great deal of interest in bioabsorbable screws has recently been generated in human orthopedic surgery in Canada. The Sunnybrook Health Sciences Centre in Toronto is now collaborating with Field in a number of fracture repair projects. Like many veterinarians seeking innovative solutions to badly pared-down research funding, "I have purposely gone out for research that crosses species barriers," he says. "Comparative research gets better funding than purely veterinary research."

In fact, while Field is eager to try out the bioabsorbable screws on other types of equine fractures should the opportunity present itself, he notes that currently equine research has taken a back seat to a huge new research effort in bone plate design, which will have important applications for both humans and horses. More on that work next month, when we take a look at a number of different methods of fracture repair.

About the Author

Karen Briggs

Karen Briggs is the author of six books, including the recently updated Understanding Equine Nutrition as well as Understanding The Pony, both published by Eclipse Press. She's written a few thousand articles on subjects ranging from guttural pouch infections to how to compost your manure. She is also a Canadian certified riding coach, an equine nutritionist, and works in media relations for the harness racing industry. She lives with her band of off-the-track Thoroughbreds on a farm near Guelph, Ontario, and dabbles in eventing.

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