The Geometry of Longevity: Why Anatomy Must Dictate Design
How we are civil engineers of the human body
There is a pervasive misconception in orthopedics that we are high-level carpenters. We saw, we drill, we hammer. But if you approach joint replacement with the mindset of a carpenter, you are failing your patients.
We aren’t just carpenters; we are civil engineers of the human body.
When I look at a femoral stem, I am not just looking at a piece of titanium to “fill a hole.” I am calculating load transfer, interference fit, and hoop stresses in a dynamic, biological environment. I see a structure that must withstand millions of cycles of axial load without shifting a millimeter.
If the geometry of the metal doesn’t respect the geometry of the biology, the construct fails. Physics does not negotiate.
I have spent my career at the intersection of engineering and surgery—designing implants for Stryker and Enovis and implanting them in thousands of patients. And the data we have generated proves one thing conclusively: Precision is not a luxury; it is a requirement.
The Myth of “Forgiving” Biology
For years, a dangerous philosophy permeated the hip replacement world: the idea that “biologic fixation” was a safety net.
The theory was simple (and seductive): If you put a hydroxyapatite-coated tapered wedge stem into the femoral canal, the bone would eventually “find it,” grow into it, and stabilize it. This led many surgeons to adopt a “passive” approach to broaching—undersizing the implant slightly to avoid the risk of fracture, trusting the biology to do the rest.
As an engineer, I knew this was flawed. Biology requires mechanical stability to initiate. You cannot have osseointegration without an initial, rigid interference fit. If the implant moves (subsides) before the bone grows in, fibrous tissue forms. If fibrous tissue forms, the implant loosens.
We decided to prove this. In a study we published in The Journal of Arthroplasty, we compared two distinct surgical techniques using the exact same modern taper-wedge stem in 250 consecutive hips.
Surgeon A (The Engineer): Used a meticulously aggressive broaching technique to maximize mediolateral fill and achieve “3-point fixation.”
Surgeon B (The Passive Operator): Relied on preoperative templating and a less aggressive “potting” of the stem, trusting the coating to confer stability.
The results were not subtle. They were a wake-up call.
The High Cost of Poor Geometry
The data revealed that geometry—specifically Distal Canal Fill—is the single greatest predictor of failure.
When we compared the outcomes between the aggressive engineering approach and the passive approach, the difference was stark:
Subsidence: The passive group saw 4X the subsidence at one month compared to the aggressive group (1.3mm vs 0.3mm).
Progression: By one year, 51.6% of the passive group saw additional subsidence. In the aggressive group? Only 0.8%.
Failure: The passive group suffered a 2% aseptic loosening rate requiring revision surgery within 3 years. The aggressive group had zero.
Why did this happen? It wasn’t the metal. It wasn’t the patient’s age or BMI. It was the fill.
We found a critical threshold: when the stem achieved <85% fill at 60mm below the lesser trochanter, subsidence skyrocketed. Conversely, achieving maximum fill locked the implant into a stable construct, allowing biology to take over.
Engineering the “Perfect” Fit
This is where the engineering background becomes a surgical instrument. A taper-wedge stem is designed to convert axial load (walking) into hoop stresses that compress the bone. But this conversion only works if the wedge actually engages the cortex.
In our study, the aggressive technique utilized a lateral-directed pressure during broaching. This created a subtle valgus alignment—mechanically locking the stem at three critical points:
Medially at the neck cut.
Laterally at the stem shoulder.
Medially at the distal tip.
This is 3-point fixation. It is a fundamental engineering principle for stability. If you simply “pot” the stem in the diaphysis without this metaphyseal engagement, you are relying on luck, not physics.
Innovation is Discipline
In my work designing the EMPOWR systems and previously with Triathlon, we obsess over the geometry of the implant. We agonize over millimeters of offset and degrees of taper. But an implant is only as good as the surgical technique that implants it.
True innovation isn’t always about a new material, a new robot, or a shiny object. Sometimes, innovation is the discipline to ignore the urge to be “gentle” or “fast” and instead be precise.
It is the discipline to broach aggressively. It is the discipline to listen to the sound of the mallet changing pitch as the hoop stresses engage. It is the discipline to ensure that the geometry of the metal perfectly opposes the geometry of the bone.
The Bottom Line
As we move toward outpatient arthroplasty and value-based care, we cannot afford a 2% failure rate due to aseptic loosening. That is a devastating complication for a patient and an unsustainable cost for the healthcare system.
We must stop treating surgery as an art form where “good enough” is acceptable. We must treat it as an engineering challenge where tolerances matter.
Distal canal fill matters. Technique matters. Geometry matters.
R. Michael Meneghini, MD
If you liked this, share it with a colleague who still thinks surgery is just carpentry. Next, I’m tackling the “Stability Paradox”—why our race to eliminate hip dislocations might have engineered a new, silent epidemic.
Love this quote from you: "We must stop treating surgery as an art form where “good enough” is acceptable. We must treat it as an engineering challenge where tolerances matter.
Good enough isn't good enough in medicine anymore. It's a science afterall. Love how you're changing the landscape.
This connects closely to the Peace Index for me.
When load is shared and structure is sound, tension drops.
Longevity often begins where pressure is allowed to redistribute.