Slightly more errors found using standard vs. MIS TKA instruments

Both types of instruments performed poorly in sclerotic bone and led toerrors in the sagittal plane, with component flexion to 3°.

Issue: June 2005

--- S. David Stulberg

Researchers studying the use of computer assistance to measure the performance of conventional and minimally invasive total knee arthroplasty have identified key technical factors that may affect the accuracy of minimally invasive procedures.

Total knee arthroplasty (TKA) surgeons need to be aware of the key factors, such as block positioning, in order to better control for them and avoid additional intraoperative and postoperative problems, according to S. David Stulberg, MD, of Chicago.

Based on the results of his study, Stulberg recommended avoiding the use of most cutting blocks in patients with osteopenic bone, for example, because blades can sometimes move more than expected. He also suggested making sure the cutting surface is stable for every bone-cutting step during TKA.

“Minimally invasive surgery (MIS) offers … a great promise for improvement. You can’t compromise the results of the arthroplasty. Computer-assisted techniques I think are valuable as measurement tools and I suspect will have an increasing use in the surgeon’s armamentarium in the future,” he said.

Comparative study

Researchers studied standard TKA surgeries done with either conventional or MIS instrumentation and compared outcomes. “The goal was to measure the impact of the instruments, not the surgical exposure itself,” Stulberg said in presenting these data at the American Academy of Orthopaedic Surgeons 72nd Annual Meeting.

To determine the extent of any loss in surgical accuracy arising from MIS instruments, particularly when combined with reduced exposure, researchers used computer-assisted techniques to measure each step of the TKA surgery using each type of instrumentation. They also sought to determine any impact on bone resection accuracy resulting from rotation of the cutting blocks.

The researchers analyzed 25 TKA surgeries done using conventional instrumentation and standard surgical techniques, and similar standard TKA surgeries done using MIS instruments. In each case, they placed the cutting block, measured its position and cut the bone, noting whether the block moved. They next measured the resection, checking whether the resection, saw block and cutting block remained in the same position, Stulberg explained.

Next, the surgeons placed the tibial jig, measured its position, changed its posterior slope to 0°, 5° and 10°, and then rotated the device 45°, after which they again changed the slope to 0°, 5° and 10°, hoping to see how that impacted varus/valgus and flexion/extension positions.

“Basically, we were getting at this concept that is evolving of doing off-line resections to see what the impact is.”

Researchers found the performance of MIS blocks differed very little from that of the conventional ones. They only moved a little bit during the maneuvers, even though they were smaller, which means “you can design minimally invasive blocks which are smaller and can be stable.” Stulberg noted.

All of the cutting blades tended to deflect, which for the conventional block was up to 3° on the femur in hyperextension, even using stiff, well-fitting blades. Deflection was slightly better with MIS blocks, which Stulberg attributed to their newer design, and which may have allowed for this occurrence. In the tibia, similar deflection problems occurred.

Alignment errors caused by both types of instruments were more pronounced in the sagittal plane (average component flexion 3°; range, 4° of hyperextension to 5° of flexion).

To avoid some of these TKA errors, Stulberg offered the following guidelines:

Osteoporotic bone is vulnerable when hammering implants into the medullary canal.
Avoid placing blocks on uneven surfaces, which is easy to do when approaching from the side.
Sclerotic bone is extremely prone to blade deflection.
Insert pins with power tools, not by hand.
Use headless pins; those with heads obscure their true orientation.
Use saw blades that fit precisely and retractors that offer optimal exposure.

Some of this movement is hard to see but this research proves it occurs, noted Stulberg, who added that it might be even more difficult to see movement during MIS TKA — especially during procedures where the surgeon moves the block and then continues cutting.

“Rotating the cutting block from the frontal plane changes the orientation. If you were off in your original orientation and then you rotate it, your errors are going to be even greater.”

If rotating the block is necessary, it helps to have the cutting surfaces at 0°. “Be careful in the sagittal plane,” Stulberg added.

For more information:

Stulberg SD, Brander VA, Adams A, et al. Factors affecting the accuracy of MIS-TKA. #69. Presented at the American Academy of Orthopaedic Surgeons 72nd Annual Meeting. Feb. 23-27, 2005. Washington.