Some of the "smarter" versions of the big explosive devices nowadays are guided by seeing-eye satellite technology that can allow a bomb released miles away to arrive through the front door of a structure that warrants a good blasting.

Contrary to the destructive mission of a smart bomb, this sort of computer-generated guidance is being made available in a much more miniaturized and fine-line form to replace rickety old human hip joints.

Instead of satellites circling the globe, a Stryker navigation system uses infrared sensors placed in an operating room around a total hip replacement patient. Those sensors react to markers and operating instruments to guide the surgery and placement of prosthesis with previously unattainable, or at least unlikely, precision.

In the same way that a global positioning system can deliver a destructive payload from miles away into a washtub-sized receiver, a similarly computerized but tremendously downsized guidance system can pinpoint work to reconstruct a faulty joint and provide blessed relief from pain and dysfunction.

"Without a computer guiding a hip replacement, a surgeon has to rely on jigs and guides on instruments to put in the hip socket or cup," said Dr. Burton Stodghill of Paducah. "Effectively, you have to eyeball the cup to install it. Like throwing darts at a bull's-eye, part of the time you'll miss by a little."

Stodghill, a partner in Purchase Orthopedic Associates, is thought to be one of just two Kentucky surgeons who now are using the new computerized alignment system for total hip replacement.

"It's become a craze to use computer guidance to do knee replacements, but some studies are showing no real advantages over our normal jigs and guides for knee replacements," Stodghill said. "The results aren't really better, and it just makes for longer operation times and it increases costs for patients."

Now, hips, that's a different story, Stodghill said. Computer-guided replacement for hips, particularly the placement of the prosthetic hip cup, allows much more precision in the achieved angle, which in turn reduces the potential of later hip dislocation (one of the surgery's most significant drawbacks), speeds recovery time and provided an eventual better range of movement and performance of the man-made joint, he said.

"When we do a total hip replacement, the natural socket is removed and the damaged cartilage is reamed off," he said. "The cup is replaced by impacting it -- it's pressed fit onto the hip -- and held in place by at least one and maybe two screws.

"The cup is attached in such a way that it induces the bone to grow and help hold it," Stodghill said.

The artificial ball placed atop the leg bone fits into the cup that's anchored onto the hip bone.

"There are certain positions that you can get the cup into that raise the possibility that the ball can pop out of the socket if the patient moves the leg at a bad angle," Stodghill said. "It can leave you with a high chance of dislocation, and that's fairly uncomfortable for the patient. We typically have to do surgery, to put you to sleep to replace the ball back in the cup."

Computerized replacement of the cup dramatically reduces those odds of dislocation by getting the angle of this receiving part of the joint replaced correctly in the first place, he said. In this manner, the guidance system overcomes the greatest potential failing of the surgery, according to Stodghill.

There are other gains seen, however. Stodghill said the extra stability that is gained from the precise alignment of hips replaced with computer guidance paves the way for longer-lasting prosthetic joints.

Most common among replacement hip joints now are those with a combination of metal against a surgical grade polyethylene plastic, which work well but only may have a good working span of about 15 years. On the other hand, these serve well with a large contact surface, so they are rather stable, even when fitted to a patient with slightly less than perfect alignment, Stodghill said.

Hard-on-hard replacement joints of all metal or all ceramic surface generally feature smaller surface areas and thus are not quite as stable (and are more prone to dislocation) when installed in a less-than-perfect angle.

The prosthetics that are all metal or ceramic surfaces do last longer, however, and the use of computerized guidance when installing them negates the disadvantages of the smaller contact surfaces. The upshot of this, Stodghill said, is that younger people with joint problems can receive hip replacements -- hard-on-hard parts serving them longer and lessening the need for secondary surgeries, while the precision of computer-assisted fitting reduces the odds for dislocations.

"For somebody who is in his or her 70s, a metal and plastic hip is still a good option," Stodghill said. "You can put in one with a larger head and it will be more stable, and the joint will last them the rest of their life.

"But for somebody who's younger and still has that bad hip, we can give him back some quality of life by installing a hard-on-hard hip with a somewhat smaller head, but it will be stable because we can get it aligned precisely - and the metal or ceramic surfaces will give him years more service," Stodghill said.