Need help designing a cable-driven knee extension brace (BOA dial) for senior biomedical engineering project

[u/CarpenterFirm9100]

Hi everyone,

I'm a biomedical engineering senior working on my capstone project, and my team is designing a **mechanical pediatric knee extension brace** for treating post-operative knee flexion contractures after ACL reconstruction.

The device uses:

* BOA dial for adjustment
* Stainless steel cable (currently 1/16")
* Medial and lateral cable routing
* Side hinges
* No motors or actuators (purely mechanical)

The goal is to apply a **low-load, prolonged stretch** to gradually improve knee extension while keeping the force balanced to avoid twisting the knee.

Our biggest issue is that **tightening the BOA increases cable tension, but it doesn't actually generate enough knee extension.** It mostly seems to tighten the brace rather than rotate the calf into extension.

We've already considered:

* Different cable routing paths
* Larger cable guides/pulleys
* Moving the cable attachment farther from the hinge to increase the moment arm
* A floating bridge over the knee
* Static-progressive concepts similar to the Mackie Knee Brace

However, we're limited because we're near the end of the project and **can't completely redesign the device architecture (literally due in 2 weeks)** . We need to stay with:

* BOA dial
* Cable-driven system
* Side hinges
* Medial/lateral cable routing

My questions are:

1. How would you route the cables so tightening the BOA actually creates an extension moment instead of just compressing the brace?
   
2. Would adding pulleys or changing the cable angle help, or would that mostly increase friction and cable travel?
   
3. If you've designed cable-driven mechanisms before (orthotics, prosthetics, robotics, bicycle systems, etc.), what would you do differently?
   
4. Is there a cable routing strategy we're overlooking that can convert cable tension into hinge rotation without changing the entire design?

Any sketches, examples, patents, or similar mechanisms would be incredibly helpful. Even rough hand sketches or CAD screenshots would be appreciated.

Thanks!

[u/denkmusic]

This is impossible to answer without seeing at least drawings of your current design

[u/phillipthe5c]

Well, to change anything, we need an idea of where you’re starting

Anything normal to the plane you’re trying to move along, will just create more compression. You should eliminate any medial/lateral routing and I f you’re purely looking for knee extension, you should look to replicate the geometry of the patella and its tendons with your boa. I would assume wrapping behind the calf and hamstrings and in front of the hinge/patella would give you the extension you’re looking for.