Image-free CAS getting better for hip arthroplasty

Adding pelvic tilt measurements led to greater accuracy in component placement.

Issue: July 2005

aaos-washington [icon] A study of image-free computer assisted methods for total hip arthroplasty found the technique to be effective and precise, especially after pelvic tilt measurements were added to the software calculations.

“We have already seen a natural evolution to improved alignment tools, from freehand positioners to the more traditional mechanical alignment devices. Computer navigation represents a major potential advance because it is a more sophisticated measuring tool. It gives real-time feedback and three-dimensional qualitative and quantitative information,” said Andrew Yun, MD, of the Arthritis Institute in Inglewood, Calif.

Image-free computer assisted surgery (CAS) represents an improvement in many ways over systems that use imaging modalities. The costs and radiation hazards associated with preoperative CT scans or intraoperative fluoroscopy are eliminated, and image-free systems increase precision in component positioning and eliminate outliers.

218 hips

Yun and colleagues studied 218 consecutive primary total hip replacements that used image-free CAS. The system, Yun said, works off an infrared tower that tracks both the patient and the implant in real time. Surgeons perform the operation through a posterior mini-incision. The navigation system allows the surgeon to map out the circumference and the anterior and posterior diameters of the acetabulum, which also provides the starting size for the reamer. “Reaming is then done under a combination of direct vision and image guidance,” Yun said at the American Academy of Orthopaedic Surgeons 72nd Annual Meeting.

Throughout the operation, the computer system provides detailed information regarding implant and patient positioning.

Initially, their research offered disappointing results, Yun said. “In this series of 108 hips, we saw very poor precision with large standard deviations, poor correlation between X-ray and computer, three postoperative dislocations and complete frustration where the surgeon abandoned the computer in 20 cases.”

Incorporating pelvic tilt into the calculations improved the results dramatically. Pelvic tilt is the relative position of the pelvis to the longitudinal axis of the body. Yun noted that the pelvis can move significantly during surgery and that most patients did have at least some degree of pelvic tilt.

Improved precision

“When we began to incorporate [pelvic tilt] into our software calculations, we noted an increase in precision, outcomes and a reduction in clinical complications,” Yun said. “We saw a reduction in standard deviation as well as a close correlation between the X-ray and the computer; there was now a statistically significant correlation between the two measurements and a mean difference of 2° [for abduction].”

Surgeons found greater improvement in anteversion, with standard deviation down to 4° and a mean difference of 0° between X-ray and computer. “Using this we now have a way to appreciate and account for the inherent variability of pelvic motion,” Yun said.

He said that since the paper was submitted, surgeons have used the technology on another 41 hips with even greater precision. “Using an uncoupled pointer, we track the acetabulum after preparation is complete.” This helps explain slight changes occurring with liner insertion and screw insertion. With this addition, researchers have had no outliers greater than 5° in the computer-to-X-ray comparison, and an increase in precision down to 2°.

“The system isn’t perfect,” Yun said. “It’s still costly, it won’t replace the technical skills of the surgeon in terms of implant stability and it doesn’t provide the surgeon with help regarding issues of soft tissue tension and impingement. But it has provided a way to track the cup and pelvis [in real time], provides intraoperative feedback and has improved our overall precision.”

For more information:
Yun A, Dorr LD. Image-free computer assisted total hip arthroplasty. #145. Presented at the American Academy of Orthopaedic Surgeons 72nd Annual Meeting. Feb 23-27, 2005. Washington.