The Right Fit

Custom designed implants and instruments provide patients with greater comfort and fewer complications.

Michael Barbella
Managing Editor

The battle for equality between men and women is as old as time itself. In biblical times, men blamed women for their fall from God’s grace (it was, after all, Eve who shared the forbidden fruit with Adam).

During China’s Zhou dynasty, a married aristocrat who became infatuated with another woman could bring that woman into his family as a concubine, where she would rank beneath the wife.

Women in ancient Greece were under the authority of their fathers or husbands, though Spartan women were given an education and enjoyed considerably more rights and equality to men than anywhere else in the classical world. Still, women were not allowed to compete in the Olympics (it took them literally thousands of years to overcome that barrier).

Ancient Irish women enjoyed nearly as many freedoms and opportunities as men. Considered queens in their own right, Celtic women even led troops into battle. Ironically though, their rights were ended by one of their own—Queen Elizabeth I of England.

In the United States, the equal rights struggle between the sexes can be traced back to July 1848, when the first convention advocating women’s rights was held in Seneca Falls, N.Y. The convention produced a Declaration of Sentiments that echoed the Declaration of Independence: “We hold these truths to be self-evident, that all men and women are created equal.”

While women’s rights advocates would like to think there is truth to such an edict, the fact is that men and women are not created equally. Not biologically, anyway.

From the size of their brains to the angle of their femurs and the length of their metatarsals (foot bones), men and women are distinctly different. Besides the obvious distinctions in size and strength, men have larger brains (it is both heavier and contains more brain cells than the female organ), more red blood cells (an additional 400,000 to 1 million), and longer bones in their feet.

Women, on the other hand, have a wider pelvis than men, rounder eye cavities and thicker skulls. They also tend to have more natural motion in their knees, though the degree of flexibility varies among ethnic groups (Japanese women, for instance, have a greater range of motion than their Caucasian counterparts).

Such differences between men and women have fueled both growth and interest in custom designed implants and instruments. Industry experts who spoke with Orthopedic Design & Technology said these implants can improve the overall fit of an artificial joint and help manufacturers overcome one of their greatest challenges—designing a device that can be used universally but also addresses a patient’s individual anatomy.

“Patient-specific technologies, in general, provide surgeons the ability to tailor more personalized solutions for their patients and may also improve the efficiency, accuracy and reproducibility of the surgery itself,” said Jeffery A. McCaulley, President of Zimmer Reconstructive, based in Warsaw, Ind. “In the end, what surgeons and their patients want most is optimal fit, function and feel, and that is the promise of patient-specific technologies. These technologies also benefit hospitals, because they enable us to deliver a more streamlined surgical kit that can reduce turnover time and sterilization costs. There’s a lot in the value stream for all stakeholders that we are addressing positively with patient-specific technologies.”

Zimmer has been adding to the value stream with its patient specific instruments and Gender Solutions Knee. The company uses magnetic resonance imaging (MRI) to capture an accurate picture of the knee; computer software allows surgeons to customize positioning guides based on patients’ individual anatomies.

Zimmer based its Gender knee on the anatomical differences between men and women, though surgeons and industry experts disagree whether the disparities truly are gender-based. Wright Medical Technologies Inc. claims on its website that studies which have shown anatomical differences in the knees of men and women did not account for the physical stature of patients. Studies that have examined other factors besides gender, Wright noted, have concluded that the size of patients results in greater differences in knee anatomies than gender.

But Robert E. Booth, M.D., chief of orthopedic surgery at Pennsylvania Hospital in Philadelphia, insists that gender differences do indeed exist. “There are several aspects to the gender knee,” noted Booth, the first surgeon in the United States to implant Zimmer’s Gender Solutions Knee (in 2006). “One is accommodating the fact that the female knee is more trapezoidal, more delicate and definitely has a different shape than a man’s knee. John Insall knew this 20 years ago when he only had four knee sizes to use and he asked Zimmer to make him a special knee because he knew he wasn’t fitting females well.”

Finding the Felicitous Fit

Zimmer touts its Gender Solutions knee as “the first and only” knee replacement designed specifically for the female anatomy. The implant’s thinner profile, contoured shape and more natural movement addresses three main differences between male and female knees, resulting in a better fit for patients, the company claims. The thinner profile of the Gender Solutions knee is based on research showing that men’s kneecaps are thicker and more prominent than women’s. Implants that are too bulky can cause pain and affect a patient’s range of motion.

The Gender knee’s contoured shape is designed to prevent the implant from overhanging the bone and potentially pressing on or damaging surrounding ligaments and tendons. In addition, the contouring provides patients with a more precise fit, according to Zimmer.

One of the most significant differences between male and female knees is the angle of patellar track (also known as Q-angle), according to various studies. The angle between the hip and knee affects the way the kneecap moves over the thighbone when the knee is in motion. Because of their wider pelvis, women often have a slightly different angle between the hip and knee when compared to men, though orthopedic surgeons, research associates and engineers refute this claim. Nevertheless, Zimmer’s Gender knee accounts for this difference, allowing for more natural movement.

“I think there is significant growth potential for patient-specific instruments and implant systems that enable more personalized treatments,” McCaulley said. “Zimmer’s Gender Solutions knee products are a successful example of this trend.More recently, Zimmer’s Kinectiv technology, which enables independent leg length and offset optimization, is a growth area for us.We’ve also introduced products, such as our Continuum Cup system, that provide options for different bearing and fixation surfaces that give surgeons the flexibility to tailor implants to their patient’s unique lifestyles and expectations. We have been pioneering these personalized medicine solutions for years and expect these technologies will continue to grow, because, in the long run, they will be better for patients and the system.”

Stryker’s Triathlon Knee System features a single-radius designed to promote stability and a rapid return to function. Photo courtesy of Stryker Corp.

Finding a technology that is best for patients, however, is likely to grow more arduous as orthopedic companies refine their custom designed implants and increase their offerings. Most of the major manufacturers—and several smaller firms— have a custom designed artificial joint or patient specific implant (the names vary by company) on the market, and the selection is only expected to grow as patients demand higher-quality hip and knee replacements that allow them to retain their active lifestyles. (Editor’s note: For a profile of the 21st century joint replacement patient, please visit ajay.odt.rodmanadmin.com).

Wright Medical Group Inc. has developed several implants that are tailored to patients’ individual anatomies. The company’s Advance Stature femoral component features distinct sizing advantages for patients who need a knee implant that is more narrow than the standard size. The components of this device use the same patellar track angle that replicates the patellar track structure of both males and females, according to literature about the product. In addition, the component’s medial-lateral dimension was reduced by five millimeters from Wright’s standard Advance Knee system, and the front flange was contoured to ensure the patella could track easily.

Wright also offers a total hip system with femoral heads ranging in size from 36-54 mm in diameter and cups ranging from 46-64 mm. The Conserve Total Hip System with BFH Technology is designed to eliminate dislocation, according to product literature. The largest femoral head (54mm) coupled with a -3.5mm head produces a range of motion of 167 degrees. The system also has a theoretical jumping distance of up to 24 mm, meaning the femoral head must travel 24 mm before it can dislocate.

In early August, Wright launched the Evolution Medial-Pivot Knee system, a device that the Arlington, Tenn.-based firm dubbed “the next generation in medial-pivot knees.” While it is not considered a custom designed or patient specific implant, the Evolution system does offer patients and orthopedic surgeons more sizing options and a posterior stabilized option.

“The Evolution knee system is more specific to a patient’s kinematics than others that are currently on the market, but it’s not patient specific and it’s not a custom implant,” explained C. Lowry Barnes, M.D., a surgeon at St. Vincent Orthopaedic Center of Excellence in Little Rock, Ark., who helped develop the implant for Wright. “In the past, we had five [knee] implant sizes for patients and they didn’t always fit perfectly, requiring surgeons to modify techniques. With this knee design it fits more precisely, and we have a better chance of reproducing a patient’s normal anatomy. If we do that we have a good chance of reproducing a patient’s normal kinematics also. Any patient can be treated with the Evolution knee and the surgeon can choose among PCL retaining, PCL sacrificing and PCL substituting designs. I currently use the Evolution knee 100 percent of the time and I have been very pleased.”

The Evolution system builds upon the company’s Advance Medial-Pivot Knee, launched in 1998. Like its newest family member, the Advance knee system was designed to mimic the rotation, twists, bends, flexion and stability of the natural knee. Wright claims that its Advance knee system is superior to its competitors’ devices because the implant is stable when the knee is in motion as opposed to sliding slightly forward.

“Our customers are always telling us that our knee feels more natural than the others out there,” said Alex Winber, director of Wright’s OrthoRecon-Knees. “We’re constantly looking for ways to make our products better, and one of the things our customers asked for was better sizing. They wanted a better fit. We use CT [computed tomography] scans to help design our implants, so we traversed the globe, collected a bunch of CT scans and performed virtual surgeries using these scans. We were able to figure out what sizes the implants needed to be so they would fit well. The Evolution is not a patient specific device. It’s a knee system with the most modern sizing options available.”

Advanced sizing options also are available from Kalamazoo, Mich.-based Stryker Corp. The company’s Triathlon Knee System is manufactured in a broad range of sizes based on an anthropometric measurement study for improved interplay between implant geometry and anatomic structure for men and women. The design incorporates a variable aspect ratio to adequately fit the female anatomy while still accommodating the male patients. The seven-degree anterior flange design of the Triathlon Knee system is designed to provide the flexibility to downsize the femoral component while avoiding the incidence of notching, which can provide patients with a more customized fit.

To get a customized fit though, surgeons often must cut part of the femur or tibia. Though cutting off bone is considered a necessary step in joint replacement procedures, experts note that it could lead to complications in future surgeries. Each time a surgeon cuts the bone to better fit an implant to a patient, one expert told ODT, “that’s bone you’ve lost for future surgeries. You’re burning bridges.”

Zimmer Patient Specific Instruments streamline total knee replacement surgery by ensuring accurate and reproducible guide fixation. Photo courtesy of Zimmer Holdings, Inc.

ConforMIS keeps the bridge-burning to a minimum with image-to-implant technology that creates an authentic patient-specific device. Rather than sawing through bone to fit the patient to the implant, the Burlington, Mass.-based firm fits the implant to the patient by creating a 3-D model of the knee from pre-surgical MRI or CT scans and designing a device that conforms to the precise area in need of repair (hence the first part of the company’s name—the second part stands for minimally invasive surgery). The company uses computer-driven machining or prototyping tools to fabricate the artificial joint, a process that takes four to six weeks, as well as custom surgical tools for implanting it.

ConforMIS claims its iFit technology and iJig instrumentation help preserve bone and cartilage, minimize surgical trauma and scarring, and simplify surgical techniques. Such a made-to-order approach, however, comes with a business advantage as well: It eliminates the need for the firm to keep an inventory of implants, which enables ConforMIS to sell its devices at prices comparable to its competitors.

An Uphill Climb

Certainly, custom-built and patient-specific implants will help drive future growth in the orthopedic device industry as companies look to improve upon current designs and patients place greater demands on their replacement joints. Experts, however, are at odds over the role these custom-built joints will play in the industry’s future.

“We have seen an increase in demand for patient-specific instruments and implants over the last few years,” noted Dave Bonvenuto, vice president and general manager of Oberg Medical, a division of Oberg Industries, a Freeport, Pa.-based medical device contract manufacturer. “We see this as a great opportunity to have our engineers work closely with the design engineers of our customers in understanding the design intent of the patient-specific item as well as engaging our manufacturing managers to ensure the item can be made in the most economical manner.”

Economics, in fact, is one of the major obstacles companies must overcome in creating patient-specific implants. With an anticipated explosion in demand for hip and knee implants over the next 20 years (six-fold and 17-fold, respectively), customizing implants for individual patients could be economically overwhelming, experts noted.

Zimmer Patient Specific Instruments custom pin guides are designed with position stabilizing features and anatomical reference lines to verify positioning and eliminate variability of placement. Photo courtesy of Zimmer Holdings, Inc.

There’s also the time factor. While 3-D modeling, rapid prototyping and rapid manufacturing techniques enable orthopedic manufacturers to create replacement joints in a matter of days, patient-specific implants take longer (ConforMIS’ devices take four to six weeks to create).
Perhaps the most important obstacle to growth, however, is the lack of long-term data on patient-specific implants and proof that they are scientifically better for patients than traditional devices.

“A patient-specific implant is a wonderful concept. We all want to feel like an implant is made specifically for us,” Wright’s Winber said. “The articulation between the implants is what makes them last. The articulation has to have a very high tolerance and I don’t know if patient-specific implants can always guarantee that you’re going to have the most beneficial articulation.”

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Conventional knee and hip replacements have been used successfully for more than 30 years and are the two most common implant surgeries performed worldwide. But changing patient demographics has prompted orthopedic companies to develop new technologies and procedures for replacing aging or diseased joints. Custom designed implants increasingly are becoming a viable alternative to traditional “off-the-shelf” artificial joints, as they address the subtle musculoskeletal differences between men and women as well as the unique anatomy of individual patients. Though these devices show promise, several obstacles stand in the way of their future growth, most notably manufacturing costs, turnaround times and the lack of long-term clinical data. Experts believe that future growth of these devices will be tied to the ability to deliver value to all industry stakeholders. Patrick Treacy, vice president and general manager of Knee Reconstruction, Biomaterials and OtisMed at Stryker, volunteered the following recipe for growth: “Not only will implants and instruments need to be designed to streamline surgical workflow, orthopedic delivery models of the future will need to transcend the operating room and deliver technological, operational, business, educational and clinical value to all stakeholders in the industry.”

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Changing Patient Demographics Creating New Priorities

The question is so common now, it almost has become routine.

“How long will it be before I can play golf?” patients usually ask C. Lowry Barnes, M.D., an orthopedic surgeon at St. Vincent Orthopaedic Center of Excellence in Little Rock, Ark.

Golf?

Welcome to the world of the millennium joint replacement patient, where golf games, tennis matches, gym routines and running take precedence over such basic concerns as implant longevity, the fit or surgical risks.

“The question today of what is important to patients is becoming more and more critical. We’re seeing patients that put an absolute priority on the ability to perform certain activities, often many activities,” noted Philip Noble, Ph.D., a professor of orthopedic surgery and director of orthopedic research in the Barnhart Department at Baylor College of Medicine in Houston, Texas. “To many of these patients, these activities are more important than any talk about [implant] longevity or the implant itself.”

Activities certainly have become more important to patients over the last 20 years, as aging baby boomers attempt to retain their youth and vitality as long as they can. Unlike their parents (or grandparents), today’s patients want to do more than simply walk without pain. They want to master the plié, bike up a hill and increase the weight on the leg press machine. They also are demanding quicker recovery times and an earlier discharge from hospitals, forcing surgeons to focus more on patient activity wish lists and less on the traditional goals of infection prevention, radiolucencies and overall survivorship.

Some of these demands—particularly speedy recovery times, improved aesthetics and implant functionality, and cutting-edge technology—have been fueled by the media rather than the literature associated with artificial joints, industry experts claim. Relying solely on media reports for information, however, can lead to unrealistic patient expectations, surgeons told Orthopedic Design & Technology.

“A decade ago people were more understanding of the fact that an artificial knee or a synthetic knee with plastic and metal was not going to last forever. It was sort of a stopgap solution,” noted Jon Hyman, M.D., an orthopedic surgeon at Emery Adventist Hospital in Smyrna, Ga. “Now, the baby boomer generation has matured and as technology or the announcements of technology continue to get more attention, people’s expectations are becoming somewhat unrealistic. Patients will say things like ‘surely you can replace a knee now that will last 50 years.’ The answer is no. The longevity and survivorship hasn’t appreciably changed in the last decade. [Patients’] expectations are that [implants] will last much longer and that they can do more on them—they can run and play tennis and maybe ski. They can be more active and despite the increased activity, the [implant]—whether they are artificial, synthetic or biological—will last longer. That’s not everybody’s expectation but it’s an increasing trend that we’re observing.”

Such expectations are likely to continue as demand for joint implants grows among younger patient populations. Next year, more than half of the demand for total hip arthroplasty (THA) and total knee arthroplasty (TKA) will come from patients younger than 65, according to research conducted by Steven M. Kurtz, Ph.D., Michael P. Kelly, M.D., and Kevin J. Bozic, M.D., and presented at a meeting of the American Academy of Orthopaedic Surgeons.

By 2016, demand for TKA is expected to grow the fastest among patients aged 45-54, the data showed. Researchers expect a 17-fold increase in the number of total knee replacements in this age group over the next 20 years, rising to 994,104 procedures in 2030 from 59,077 procedures in 2006. The demand for total hip replacements in the same age category (45-54) is projected to grow nearly six-fold in 2030.

Obviously, younger patients will drive future growth in the orthopedic market. Perhaps more importantly, though, these patients will help shape the development of future joint replacements and challenge surgeons to adapt to yet another set of demands and expectations.

“In the years to come demographic and generational factors are going to change the quality of our experience with patients because the expectations and satisfaction of our patients will drift,” Noble said. “They are going to expect to do the normal things as they find them. This is rather different from what we are accustomed to. Discussions with patients about what they want to do will become critically important if we’re going to have a group of satisfied surgeons.” — M.B.