Fixing Tough Hip Fractures: The Off-Axis Screw Breakthrough

Hey There, Let’s Talk Bones!

So, imagine you’ve got a really tricky break in the top part of your thigh bone, right near the hip joint. We’re talking about what folks in the medical world call a vertical femoral neck fracture. Now, these aren’t your average breaks; they’re notoriously unstable. Think of it like trying to fix a fence post that’s leaning way over – gravity and sideways forces are just working against you constantly. According to the experts (they use something called the Pauwels classification), these vertical ones, especially Type III, get hit with massive shearing, bending, and pulling forces. Ouch!

Naturally, the goal is to fix it up nicely and save the femoral head (that’s the ball part of the hip joint). The go-to method is usually surgery to put things back together and stabilize them internally. But because of those nasty shear and rotational forces, getting a stable fix on these vertical breaks is a real headache. Honestly, the success rates haven’t been stellar, with failure rates sometimes hitting over 40% and issues like avascular necrosis (where the bone loses blood supply) popping up in 16-21% of cases. Finding the *best* way to fix these is still a hot topic among orthopedic surgeons.

The Usual Suspects and a Clever Idea

For years, we’ve relied on a couple of main types of hardware: fixed angle plates or, more commonly, percutaneous cannulated cancellous screws. Cannulated screws are popular because they mean smaller cuts, better rotational stability, and they mess with the bone less – which is always a plus in my book.

To try and tackle the instability of vertical breaks, a smart technique called the off-axis screw was developed. The basic idea? Add a third screw that goes in *perpendicular* to the fracture line. This is supposed to be the superhero that neutralizes those pesky shear forces, while two other screws run parallel to the femoral neck axis, doing their usual job. This off-axis approach has been studied quite a bit, but whether it’s truly the magic bullet is still debated.

Now, these off-axis screws come in a couple of flavors based on their size and how they’re threaded:

• 4.5 mm fully threaded off-axis screws (4.5-FTOS): These usually anchor into a dense part of the bone called the calcar cortex.
• 6.5 mm partially threaded off-axis screws (6.5-PTOS): These sit in a different spot, the inferior-medial quadrant of the femoral head.

Some studies found that the 4.5-FTOS was stiffer and could handle more load than the 6.5-PTOS. But here’s a catch: the 4.5-FTOS might not work if the medial cortex (inner bone layer) is all broken up, which, turns out, happens in a huge number of these vertical fracture cases (like, 84-94%!). So, 4.5-FTOS isn’t a one-size-fits-all solution.

Introducing the New Contender: 7.3-FTOS

Recently, fully threaded cannulated screws have gained traction because they offer better stability and resist pulling out more effectively. So, we thought, why not combine the benefits of a fully threaded screw with the off-axis technique? That’s where the 7.3 mm fully threaded off-axis screw (7.3-FTOS) comes in. We designed this new configuration with the 7.3 mm fully threaded screw placed in the inferior-medial part of the femoral head.

But the big question remained: Is this 7.3-FTOS configuration actually *better* than the others, especially the 4.5-FTOS or the standard three-screw setup (ICCS)? That’s exactly what we set out to discover in this study. We wanted to put the 7.3-FTOS head-to-head against the traditional inverted cannulated compression screws (ICCS), the 7.3 mm partially threaded off-axis screw (7.3-PTOS), and the 4.5 mm fully threaded off-axis screw (4.5-FTOS) using some serious computer modeling.

Getting Technical: Finite Element Analysis

To do this, we didn’t break any actual bones (phew!). Instead, we used a technique called Finite Element Analysis (FEA). Think of it like building a super detailed virtual model of the bone and the screws and then simulating exactly how they behave under stress, like when someone is standing or walking. We started with a CT scan of a healthy 35-year-old guy’s femur to get a realistic 3D model. Then, we digitally created a vertical fracture with a steep 80° Pauwels angle – a really unstable one.

Next, we virtually inserted the four different screw configurations into this fractured bone model:

• ICCS: The classic three screws in an inverted triangle.
• 7.3-PTOS: Two parallel 7.3 mm partially threaded screws plus one 7.3 mm partially threaded off-axis screw.
• 4.5-FTOS: Two parallel 7.3 mm partially threaded screws plus one 4.5 mm fully threaded off-axis screw anchored in the calcar.
• 7.3-FTOS: Two parallel 7.3 mm partially threaded screws plus one 7.3 mm fully threaded off-axis screw.

We used fancy software (Geomagic, SolidWorks, and Ansys) to build these models, make sure everything was connected properly (screws to bone, bone to bone), and apply a realistic load simulating standing (about 300% of body weight!). We then measured things like how much the bone and screws moved (displacement) and where the stress concentrated (von Mises stress).

The Results Are In!

Okay, drumroll please! The FEA results gave us some clear answers. We looked at several key metrics:

• Maximum Femoral Displacement (how much the whole bone moved):
  • ICCS: 10.40 mm (Yikes!)
  • 7.3-PTOS: 8.08 mm
  • 4.5-FTOS: 7.71 mm
  • 7.3-FTOS: 7.15 mm (Best!)
• Maximum Implant Displacement (how much the screws moved):
  • ICCS: 9.76 mm
  • 7.3-PTOS: 7.55 mm
  • 4.5-FTOS: 7.25 mm
  • 7.3-FTOS: 6.78 mm (Best!)
• Maximum Gap Displacement (how much the fracture surfaces moved apart): This is super important for healing!
  • ICCS: 2.73 mm (Big gap!)
  • 7.3-PTOS: 1.19 mm
  • 4.5-FTOS: 0.94 mm
  • 7.3-FTOS: 0.55 mm (Tiny gap! Awesome!)

  The 7.3-FTOS showed the least movement at the fracture line, which is fantastic for resisting that sideways “varus” collapse.

• Maximum Z-axis Displacement (representing shear force movement): Remember those nasty shear forces?
  • ICCS: 2.93 mm
  • 7.3-PTOS: 2.69 mm
  • 4.5-FTOS: 2.66 mm
  • 7.3-FTOS: 2.63 mm (Least shear movement!)

  Again, the 7.3-FTOS was the champion at resisting shear.

• Peak von Mises Stress (where the stress is highest): We looked at stress on the bone (distal femur) and on the implants themselves. High stress on the bone near the screws can mean the screw might cut out. High stress on the implants means they might break.
  • Stress on Distal Femur: ICCS (218.04 MPa) was highest, followed by 4.5-FTOS (123.42 MPa), 7.3-PTOS (121.99 MPa), and 7.3-FTOS (113.86 MPa – Lowest!). Lower stress on the bone is better!
  • Stress on Implants: ICCS (397.55 MPa) was *way* higher than the others. 4.5-FTOS (216.61 MPa) and 7.3-PTOS (202.35 MPa) were next. 7.3-FTOS (164.69 MPa – Lowest!). Lower stress on the screws means they’re less likely to fail.

Putting it all together, the 7.3-FTOS configuration consistently showed superior biomechanical performance. It resisted movement (both overall and at the fracture gap) and shear forces better than the others, and it put less stress on both the bone and the implants. The traditional ICCS, on the other hand, performed the worst across the board, which isn’t surprising given how unstable these fractures are.

Why Does 7.3-FTOS Work So Well?

There are a few reasons why we think the 7.3-FTOS configuration shines. First, the fully threaded design of the off-axis screw is key. While the parallel screws (partially threaded) help compress the fracture, the fully threaded off-axis screw grabs onto the bone more firmly along its entire length, providing better anchorage and support against those high shear forces. It’s like having a stronger anchor point right where you need it most.

Second, the placement matters. The off-axis screw is positioned in the inferior part of the femoral neck. This area often has denser bone and is closer to the calcar, which provides extra cortical support. This strategic placement, combined with the robust fully threaded design, creates a powerful combination that counteracts the forces trying to displace the fracture.

Compared to the parallel screws, which can sometimes allow a “sliding effect” that contributes to displacement, the off-axis screw, especially the 7.3-FTOS, seems to generate more of a compressive “dragging force” perpendicular to the fracture line, actively fighting against shear.

We also saw that the 4.5-FTOS performed better than the 7.3-PTOS, likely because the 4.5 mm screw was anchored bicortically (through two layers of bone) in the calcar, offering more stability than the unicortical (through one layer) placement of the 7.3 mm partially threaded screw in the 7.3-PTOS model. However, as we mentioned, the 4.5-FTOS isn’t always an option due to bone comminution.

Connecting to the Real World (and What’s Next)

Now, I know what you’re thinking: this is a computer simulation. And you’re right! While FEA is a super powerful tool for understanding biomechanics, it has its limitations. Our study was static (didn’t simulate movement like walking), didn’t include statistical analysis (though that’s common in these types of simulations), and wasn’t directly validated with physical experiments on real bones (though we did check our model’s validity internally). Plus, real-world bone quality varies a lot!

The clinical picture for off-axis screws is also a bit mixed right now. Some studies show they reduce complications like femoral neck shortening, while others have reported high failure rates compared to different methods. Our findings suggest that maybe the *type* and *placement* of the off-axis screw, as highlighted by the difference between 7.3-PTOS, 4.5-FTOS, and 7.3-FTOS, could be a big part of why clinical results vary. Standardizing the technique based on biomechanical evidence like this could be really important.

The Big Takeaway

Based on our finite element analysis, the 7.3-FTOS configuration looks incredibly promising for treating those tough vertical femoral neck fractures. It does a fantastic job of reducing unwanted movement and stress at the fracture site, creating a much better environment for healing. For these specific, unstable breaks, our results strongly suggest that the 7.3-FTOS configuration is the preferred choice among the ones we tested, followed by the 4.5-FTOS and then the 7.3-PTOS. And the traditional three-screw ICCS? Probably best to avoid it for these vertical fracture patterns.

This study provides a solid foundation, but the next steps are crucial: real-world biomechanical testing and, eventually, large-scale clinical studies to see how this holds up in patients. But for now, it’s exciting to see how a smart screw design and placement can make such a big difference!

Source: Springer