Newswise — New medical evidence shows improved mechanical fixation with a novel inset shoulder implant that minimizes surgical complications and may increase the longevity of artificial shoulder replacements. 

The shoulder is a ball and socket joint. The humeral head is shaped like a ball, and the glenoid is shaped like a golf tee. The ball sits on the tee in a stable fashion if the entire surface of the tee remains intact. However, any partial destruction of the tee may cause the ball to fall off the tee. The glenoid surface functions in a similar fashion, stabilizing the shoulder joint through conformity with the humeral head. Since the glenoid platform surface turns sideways like a vertical wall every time we sit or stand up, a concavity compression system from the rotator cuff and important ligaments are necessary to stabilize the joint.

When the shoulder joint loses the protective cartilage lining of the surface bone, patients develop bone on bone degenerative arthritis which causes pain, stiffness, and loss of function. Joint replacement surgery has the potential to both restore motion and function as well as eliminating pain. However, replacing the shoulder joint surface with artificial implants, especially in the setting of bone loss like a broken golf tee, can be very technically challenging. The surgeon has to correct deformity, remove scar, balance the soft tissue constraints, stabilize the joint, rigidly fix the artificial implants, and repair the rotator cuff.  If any of these steps fail, then the patient may experience loosening of the implants or instability of the shoulder joint. For all of these reasons, shoulder implants still have a higher loosening rate than hip and knee implants, and this limits their long-term success.

However, a new implant and surgical technique, invented by Dr. Stephen Gunther from Charlottesville, Virginia, is designed to increase the fixation strength, minimize complications of the surgery, and increase the longevity of the implants. His technique involves imprinting the artificial glenoid implant partially in to the glenoid bone socket like a puzzle piece, and thus providing significantly increased immediate mechanical fixation strength. Typical glenoid implants sit on the surface of the bone without any initial stability, relying completely on backside fixation with pegs, like a painting hanging on the wall. Unfortunately, typical onlay glenoid implants sitting on the surface of the glenoid bone, like a painting hanging on a vertical wall, tend to loosen fairly often and may require complicated, expensive revision surgery. Some studies show loosening rates up to 40% five years after surgery. However, scientific studies of inset fixation show substantial improvements with this new technique. Mechanical testing studies published in the Journal of Shoulder and Elbow Surgery in 2012 & 2017 show up to 40 times increase in fixation strength with this novel inset fixation technique when compared to classic onlay implants. Also, finite element computer analyses of this inset fixation technique show less stress on the bone-cement bonding interface holding the implant to the bone and less stress on the edges of the polyethylene implant edges.  Thus, there is potential to decrease glenoid implant loosening rates, decrease polyethylene implant surface wear, and eliminate the generation of particulate joint debris causing an inflammatory autoimmune response.

US surgeons perform more than 50,000 shoulder replacements per year, and this volume is expected to double before 2030. The volume of revision surgeries performed each year is expanding, and more patients are seeking surgery at a younger age. Since patients may live another 30 to 40 years after their joint replacement surgery, implant longevity is an extremely important factor for these patients. The US total shoulder replacement market is expected to expand by more than 500 million dollars over the next 10 years. Therefore, designing better implants with a longer lifespan offers great benefits to patients and could save millions of dollars for our healthcare system.

Dr. Bryce Rutter, Founder and CEO of Metaphase Design Group, Inc., thinks that developing a user-centered design strategy can help orthopedic implant companies develop a competitive advantage. “The DNA of all product and brand experiences lives in the look, feel, sound, intuitive usability, and performance of instruments, devices, packaging, apps, and services. In the DNA of all highly successful medical products is the unrelenting pursuit of user proficiency, efficiency, comfort, and safety.” Market analysis demonstrates that innovative healthcare companies in the shoulder market have expanded their market share substantially over the past 10 years by solving common problems for surgeons and patients. On the other hand, some of the biggest orthopedic device companies who developed successful shoulder platforms 20 to 30 years ago have lost substantial market share over the past 10 years due to lack of customer centric design and innovation in their shoulder portfolio.

The innovation needed to elevate artificial shoulder replacement surgery to the next level, according to surgeons, is a patient centered design capable of providing pain relief and improved function for 20-30 years. For this reason, surgeons are adopting this new inset glenoid design. Dr. Jay Keener, chief of the shoulder and elbow service at Washington University in St. Louis, describes this technique as “radically different in its design” as it offers important advantages over other implants. “The inset glenoid concept is completely different than what we are traditionally taught for glenoid resurfacing because you’re not covering the entire glenoid with an onlay design, you’re putting a smaller device in, and the entire device is buried into the surface of the bone. It could give much better fixation biomechanically.” The biomechanical strength improvement over more traditional designs was demonstrated in 2 separate publications in the Journal of Shoulder and Elbow Surgery. In a matched pair cadaver study with mechanical testing of implant fixation, all 8 of the onlay glenoid implants tested loosened at a mean of 1126 cycles while none of the 8 inset glenoids showed any signs of loosening.

The other major advantage of this technique is the flexibility of this system to place stable implants in deficient glenoid bone. Once the humeral head wears in to the small glenoid socket with severe arthritis, it quite often destroys more posterior bone and creates an asymmetric, deficient bone structure similar to a golf tee chopped in half by a golf club. Dr. Gunther actually invented the inset glenoid as a solution to this specific problem. “I was frustrated with difficult revision surgeries that I treated when I was the chief of the shoulder/elbow service at UCSF.  When I saw even more deficient bone in rural northern California where I worked after I left UCSF, I decided that I must create a better solution.” Years before he learned about human centered design at Harvard, Dr. Gunther addressed the surgeon’s conundrum relating to the difficulty of providing stable glenoid implant fixation in a shoulder with both severe arthritis and deformity from bone loss. How do you balance a golf ball on a broken tee? His solution was to utilize the extremely hard, sclerotic surface bone in the deficient glenoid bone by imprinting the glenoid implant partially in the surface bone just a as manhole cover is inserted in to a road or a house foundation is imprinted in to the ground. The result is much stronger peripheral implant fixation, similar to a puzzle piece. Because the peripheral fixation is so much stronger, especially in arthritic, hardened bone, backside fixation is much less important. This allowed Dr. Gunther to create a short central posterior peg which easily fit inside the deficient glenoid vault, avoiding complications such as nerve injuries and vascular injuries caused by a drill or cement broaching through the glenoid vault behind the articular surface. Dr. Gerald Williams, previous President of both the American Shoulder and Elbow Society and then the American Academy of Orthopedic Surgeons, was an early adopter of this technology because of its benefits in deficient bone. “I saw Dr. Steve Gunther present his original cases with his glenoid. I came up to him. I said ‘Listen, I’ve got some patients with deficient glenoids that I think could use this.’” He currently uses the inset glenoid for young patients and patients with deficient bone.

The adoption of this inset technology is increasing progressively as more surgeons learn about the biomechanical advantages, the mechanical testing results, and the flexibility of the solution for multiple types of deficient bone. Dr. Galen Kam, Las Vegas shoulder surgeon, describes his conversion to utilization of the inset glenoid. “I was a cautious adopter at first. I started off with an easy case, saw that I could do this surgery in a way that I don’t think I ever could before and leave that part of the operation with a confidence that I never did before. So now, whenever I do an anatomic total shoulder, that is my glenoid of choice.”

Dr. Gunther partnered with a venture capitalist, Mike DeVries, and engineer, Des O’Farrell, years ago in order to develop the intellectual property rights and FDA approval necessary to make this technology available for patients. They created Shoulder Innovations and then partnered with a medical device incubator, Genesis Innovations Group, lead by a team of the world renowned shoulder industry engineers who left the big orthopedic companies in order to start their own medical device company. These engineers share the founder’s passion for human centered design and patient centered healthcare. Rob Ball, lead engineer on the executive team, describes the company’s vision. "We have a collaborative team of engineers and surgeons who work together to create better implants, prevent surgical complications, streamline the surgical process, and reduce the cost. Our design goal is to improve the process for all stakeholders." The inset glenoid technology, along with a platform of other novel shoulder implants, is now available through Shoulder Innovations. This is a venture backed company with novel intellectual property, inset implants already on the market, and a whole new product line of minimally invasive implants with a streamlined surgical process for both surgeons and nurses.

The short-term success of this technique has been shown to be extremely beneficial for patients with arthritis and glenoid bone loss. Subsequent clinical data, mechanical testing, and FEA computer modeling have also shown multiple advantages of this technique. For further information see



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Journal of Shoulder and Elbow Surgery demonstrates