Medical Device Daily National Editor

PALM DESERT, California — The trilogy of pacemakers/implantable defibrillators, heart valves and stents has the cardiovascular sector atop the heap when it comes to spending on medical implants. But orthopedic implants perhaps are top-of-the-mind in terms of public knowledge and a increasing athletic and aging population.

That seems to be the case at this week’s Materials and Processes for Medical Devices Conference at the Marriott Desert Springs Resort, with the largest portion of the focus being directed to orthopedic implants than any other product area.

(Editor’s note: The legal news of yesterday highlighting this interest in a very different way, according to MDD’s two lead stories, above.)

As its name implies, the annual conference sponsored by ASM International (Materials Park, Ohio) has a materials engineering and process engineering slant, with various breakout sessions dealing with the latest in research in those areas.

In one such session — part of the Fatigue Life I and II program — Laura Borgstede, of orthopedic implant maker Zimmer (Warsaw, Indiana), discussed durability test methods for patella implants.

“The biomechanical function of the patella is as a pulley, aiding the quadriceps muscle in effecting knee extension,” she said. “Although it is not common, failure can occur in all manufacturers’ patellas.”

With the goal of developing a durability test to determine longevity of patella implants, Borgstede and other Zimmer engineers came up with two testing protocols. One, featuring 219,000 cycles, represents 30 deep squats per day for 20 years — the so-called “Muslim prayer ritual” protocol. The other, at 438,000 cycles, amounts to two “prayer rituals” a day for 20 years.

Testing both fibiofemoral extension and lateral shear load, the protocols test “worst-case conditions,” she said.

The tests were done on six samples each of the existing — or predicate — patellar implant design and a proposed new design. The materials involved included a conventional all-polyethelyne (PE) patella and a highly-cross-linked all-PE patella.

Of the predicate-design patella samples, five survived to about 35,000 cycles while five of the new-design samples registered 51,000 cycles before failing.

Borgstede said the data gathered in the testing showed “clinically relevant” failure results.

In a presentation on a different group’s efforts at Zimmer, Devendra Gorhe, an engineer in the metals research area, discussed “MRI interactions with Knee and Hip Orthopedic Implants.”

He gave a quick primer on MRI technology, citing the three types of magnets used — static, gradient or radio frequency — and the resulting “image artifacts.”

What the engineers were measuring was the specific absorption rate (SAR) of magnetic output by both knee and hip implants.

In discussing image artifacts, “Implants,” said Gohre, “have different magnetic susceptibility than tissue.”

While noting that MRI “can cause local disturbance of the static field near an implant,” he said testing indicated that in a typical 1.5 Tesla MRI unit, static field displacement was “minimal.”

Gohre said that testing of a knee implant system indicated “much less heating” with the RF system.

Materials selection, particularly insofar as coronary stents are concerned, was another topic of considerable interest at the conference.

In a session on “Materials Selection for Stents and Cardiovascular Devices,” Arthur Fairfull of Granta Design (Cambridge, UK), was the presenter of a paper authored by Neil Morgan of AdvaNiTi Consulting (Wilts, UK).

In perhaps the most interesting single graphic shown over the two and one-half days of sessions, he noted that the materials used in the first angioplasty procedure, performed in 1964 by Charles Dotter, MD, included a VW speedometer cable, insulation from a cable found in the trash – and guitar strings.

So much for the high-tech in early high-tech.

Balloon-expandable stents came along in 1985, and in 1994 the FDA approved the first broadly commercialized coronary stent, the Palmaz-Schatz.

Morgan’s paper noted that as stenting has progressed through design innovation and processing techniques, the choice of structural materials also has progressed.

He cited nitinol as “having a stronghold on the self-expanding stent segment,” but added that adoption of new platform materials for future iterations of stents is being widely studied.

That research, according to Morgan, includes “exploitation of current alloys, as well as study of absorbable stent materials and coatings.”

In another materials-selection session, Rameesh Marrey of the Cordis (Warren, New Jersey) unit of Johnson & Johnson (New Brunswick, New Jersey), discussed “Probabilistic Methods for Medical Device Design.”

“You want to manage design risk in a realistic fashion,” he said, adding that probabilistic analysis methods such as “Monte Carlo” simulations provide a way of understanding risk by generating mathematical distributions for device safety.

“These distributions allow predictions of device safety at desired levels of product reliability,” Marrey said in the session abstract.

He promoted the probabilistic method to the more commonly used “deterministic,” or “worst-case,” analysis, which he said can be “excessively conservative,” as well as not quantifying the degree of conservatism.

“Probabilistic techniques can be used to quantify and manage design risk,” Marrey said.

He added that they are “theoretically applicable to any experiment where the input variables are characterized and transfer functions known” and are “consistent with the largest number of input variables.”

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