Thousands of patients who have osteoarthritis on one side of the knee end up having both sides of their femur bone drilled out and replaced with metal implants.

It's not that patients want to have more bone removed than they need to, or that a total-knee replacement pays better than a partial knee job. Rather it's often because properly balancing a knee after surgery on just one side is more challenging for a surgeon than doing both sides at once.

Now a Plymouth company called Blue Belt Technologies is using advanced robotics and 3-D modeling to improve orthopedic surgeons' precision with single-side knee resurfacing, known as unicompartmental knee replacement, or a "uni." The goal is to offer patients quicker and less-painful knee surgery and drive down hospital costs.

"If you look at orthopedics in general, there is a need for technology. There is a need to make things more consistent, more reproduceable and more efficient," said company CEO Eric Timko, a longtime medical device executive who took the reins at Blue Belt in 2011.

Unis serve as proof-of-concept for the Navio Surgical System, Blue Belt's $400,000 robotically assisted freehand bone drill and imaging system. Later this year, the company hopes to file an application with the Food and Drug Administration to expand its permitted uses to treat hip pain from femoroacetabular impingement. In 2016, the company may seek FDA clearance for a total-knee replacement indication.

Replacing lower-body joints is Medicare's most common procedure. The 446,148 cases of surgical replacement of hips, knees and ankles in 2013 cost Medicare $6.6 billion, more than any other single service, according to data released in June. Those figures don't include Medicare outpatient cases or procedures covered by private insurers.

"When we look at this huge baby boom population that is entering their 70s and 80s, the need for joint replacement is almost unfathomable," said Dr. Jay Kruse, an orthopedic surgeon who uses a Navio Surgical System at Twin Cities Orthopedics in Coon Rapids. "When we look at the impact that will have on health care dollars, it's unsustainable."

Although the Navio increases upfront costs, Blue Belt salespeople tell providers that they can make their money back by treating about two patients with partial-knee replacements per month.

Hospitals profit because they can discharge knee-surgery inpatients more quickly while increasing the number of same-day surgeries, which are cheaper and potentially more profitable. Company executives say public interest in robotics tends to attract new patients.

Robotics in health care is not new. Silicon Valley's Intuitive Surgical sells the Da Vinci Surgical System, which has multiple robot arms and a free-standing surgical console to treat soft tissues.

In orthopedics, Blue Belt competes directly with Florida-based Mako Surgical Corp., which sells a system for partial-knee resurfacing that has a single robotic arm to make sure the surgeon only drills out enough bone to accommodate the implant. The system reportedly sells for between $1.2 million and $1.5 million, though the company declined to comment on pricing.

Blue Belt's Navio system has no arm. Rather, the surgeon uses a handheld drill with a robotically controlled tip. If the surgeon's hand moves the tool outside the patient's 3-D surgical plan, the computer automatically stops the drill bit with a reaction time of about 100 milliseconds.

"With the robot, by putting a couple antennas here and here, it tells the computer where these bones are in three-dimensional space at all times, with incredible precision," Kruse said, pointing to gray antenna lobes on a Navio drill.

In its second full year of commercialization, Blue Belt has sold about 60 of the systems worldwide, including two in Minnesota — one at Twin Cities Orthopedics and another at the St. Cloud Surgical Center. The company counts as many as 150 surgical cases per month these days.

"We've hit the ground running, fast," Timko said.