Fig. 1 Intraoperative photo of MIS TKA.
Courtesy of Bryan J. Nestor, MD

AAOS Now

Published 1/1/2010
|
Mary Ann Porucznik

Minimally invasive TKA shows little benefit in outcomes

Postoperative strength, functional recovery similar to standard approach

A prospective, randomized, double-blinded study comparing postoperative strength and functional recovery after minimally invasive or standard total knee arthroplasty (TKA) showed limited benefits for the minimally invasive (MIS) approach—and those benefits had disappeared by 6 weeks after surgery.

“The cited advantages of minimally invasive TKA are well known,” said presenter Bryan J. Nestor, MD. “Most studies to date, however, have been either retrospective or at best prospective comparisons with matched controls and as such fail to control for the influence of patient expectations, placebo effect, or selection bias. Only two studies have measured quantitative differences in quadriceps muscle strength.”

The study involved 27 patients (mean age 66.7 years; 18 females) who were candidates for bilateral TKA. Each patient was randomized to receive a minimally invasive midvastus approach on one knee (Fig. 1) and a standard quad-splitting approach on the other. Skin incisions were of equal length so that both patients and investigators were blinded as to the approach used. All other aspects of the surgery were identical, including the implant design (cemented posterior stabilized design), treatment of the patella (resurfaced), anesthesia (spinal epidural with a femoral nerve block), and postsurgical standardized clinical pathways for physical therapy and pain management.

Quadriceps strength testing
Isometric strength testing was measured with the knee in 30 degrees and 60 degrees of flexion, with peak torques reported for each of three trials. Isokinetic strength was performed at velocities of 60 degrees/second and 180 degrees/second through a motion arc of 0 degrees–90 degrees, and peak torques were reported for each trial.

Quadriceps strength testing was performed before surgery as well as at 3, 6, and 12 weeks after surgery. “At 3 weeks after surgery, we observed a significant increase (p < .05) in isokinetic extensor peak torque and isometric strength in the minimally invasive midvastus group,” reported Dr. Nestor. “Likewise, with the isokinetic data, at 60 degrees/second, the difference was significant at 3 weeks after surgery, again favoring the MIS group.”

The early differences in strength did not last, however; by 3 months after surgery, patients in both groups had returned to presurgery levels of quadriceps strength.

Functional recovery
Researchers used an instrumented walkway to conduct a gait analysis; stride length, single hip stance time, and double limb stance time were also measured and average data from three trials were used for analysis. They found no differences between the standard and minimally invasive knees at any point (3, 6, and 12 weeks after surgery).

“Based on a visual analog scale, patients reported no difference in pain between the two approaches at any point,” said Dr. Nestor. “The only significant difference in range of motion was on postoperative day 3, again favoring the minimally invasive approach. When we asked patients about which knee had less swelling, stiffness, or weakness, we found a patient preference for the MIS knee at 3 weeks, but that difference had disappeared by 6 weeks, which is when most patients return for follow-up. By 12 weeks, most patients reported that both knees felt the same (Fig. 2).”

Radiograph analysis
A radiographic analysis was performed at the 6-week follow-up to assess alignment. Researchers found no significant differences in radiographic outliers between the MIS and standard approach knees, although two knees in the MIS group had tibial malalignment in the coronal plane, compared to no knees in the standard-approach group.

“Unlike previous studies, the cited advantages of MIS TKA—improved early range of motion and less pain—were not observed in this study. Our results do concur with a recent multicenter randomized clinical trial that compared the MIS midvastus and standard approaches and showed no clinical difference. Although we saw modest improvement in quadriceps strength, as well as patient preference for the MIS midvastus approach at 3 weeks, that effect was lost at 6 weeks,” summarized Dr. Nestor.

“At least one study has raised concerns about increased component malalignment,” he continued, “and we did observe tibial malalignment in the MIS group and not in the standard group, although this was not statistically signficiant.

“In conclusion, the MIS mid-vastus approach offers limited, if any, benefits compared to a standard TKA approach, and the potential risk of tibial component malalignment is cause for concern.”

Dr. Nestor’s co-authors for “MIS versus Standard TKA: A prospective, randomized double-blinded study comparing postoperative strength and functional recovery” include Kristin Foote, MEd; Sherry I. Backus, PT, DPT, MA; Stephen Lyman, PhD; Charles E. Toulson, MD; Howard Hillstrom, PhD; and Russell E. Windsor, MD. Dr. Toulson reported ties to DJO Surgical; none of the other authors reported any conflicts.

Mary Ann Porucznik is managing editor of AAOS Now. She can be reached at porucznik@aaos.org