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LAS VEGAS -- Younger people with stubborn osteoarthritis (OA) of the knee that is not sufficiently advanced to warrant arthroplasty may soon have a new treatment option, results of a 2-year study suggested.

A subcutaneously implanted shock absorber, bolted to the lower femur and upper tibia, helped 86% of participants in a prospective trial achieve meaningful symptom relief lasting 2 years, reported David Flanigan, MD, of Ohio State University in Columbus.

Patients who received high tibial osteotomy (HTO) realignment in a separate study met the same endpoint at a rate of 66% (P=0.01), Flanigan told attendees at the American Academy of Orthopaedic Surgeons annual meeting here. (Results were also in Cartilage.)

The FDA is now considering market authorization for the so-called under its de novo classification program. MISHA is the product's latest iteration; previous names included Atlas and Calypso.

HTO is now the main treatment for mild-moderate knee OA that doesn't respond to physical and medical therapy. It's considered effective, but like any invasive joint surgery, recovery can take weeks. "For a working-age population, [this] is an important factor," Flanigan said, and often leads to patient refusal.

The new device, made by California-based Moximed, is intended to be a less invasive solution with shorter recovery time. It consists of a capsule containing polycarbonate urethane, a springy biocompatible plastic, with titanium brackets at either end that screw into the sides of the knee bones. Consequently, the surgeon does not need to open the joint capsule and the device doesn't "distract" the joint. Lab studies have indicated that the implant absorbs 30% of impact energy, a load reduction similar to that with HTO.

For the , 81 patients were recruited in the U.S. and Europe to receive the MISHA device. Outcomes were compared to 81 similar patients undergoing HTO in an earlier European study also sponsored by Moximed. Importantly, enrollment was limited to patients whose disease and pain originates in the medial joint; those with significant lateral or patellofemoral involvement were excluded.

Specific inclusion criteria were ages 25-65, radiographic confirmation of medial knee OA, at least 6 months of nonsurgical treatment without success, body mass index less than 35, and pain scores of at least 40 on the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC). Patients were excluded if they had large marginal osteophytes (which might interfere with the device), more than 10° of varus malalignment, or flexion contracture greater than 10°.

The primary efficacy endpoint was achievement of all of the following:

• At least 20% and 10-point improvement in WOMAC pain score
• At least 20% and 10-point improvement in WOMAC function score
• No conversion to more invasive treatment
• No infection, neurological or vascular injury, or (for HTO) non-union
• No implant breakage (for MISHA)

Flanigan said that, although the study was negotiated with FDA as a non-inferiority trial, the device outperformed HTO on the two WOMAC efficacy measures. They were met by 95.8% and 91.7% of patients, respectively, compared with 87.9% and 81.3% in the HTO group. It also matched HTO for the other three measures. Flanigan felt comfortable in saying the device proved to be superior to HTO.

Additionally, secondary endpoints also favored the device versus HTO (all P<0.02):

• Mean time to full weight-bearing: 13 days vs 58 days
• WOMAC pain improvement at 2 years: 76.0% vs 64.7%
• WOMAC function improvement at 2 years: 73.9% vs 58.8%

These benefits were already apparent within a few weeks, Flanigan said. With mean pain scores of 60 at baseline, the average fell to 25 after 3 months. Improvement in function was similarly rapid.

Another finding was that the device helped participants return to favored activities. At enrollment, participants were asked to list three activities that they formerly liked but could no longer do comfortably. About half selected running/hiking and one-quarter picked squats and other weightlifting types of activity.

By month 6, 50% of the MISHA group indicated that they could perform their chosen activities "with minimal or no limitation," reaching almost 65% at the end of 2 years.

The principal unanswered questions have to do with longer-term efficacy and safety. However, HTO is rarely a permanent solution; the Hospital for Special Surgery in New York City, for example, that the procedure's benefits "typically fade after 8 to 10 years." Thus, a need for eventual revision or replacement for MISHA wouldn't necessarily put it at a disadvantage.

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