How to Cut Bone in Orthopedic Surgery without Destroying Bone Cells and Vascularity
Kambiz Behzadi
February 20, 2024
Hybrid Ultrasonic Assisted Saw
Eliminate the Silent Killer of Bone Cells – Heat
1. The sagittal saw used for orthopedic surgery today is very similar to the original version described in 1890. The contemporary saw oscillates at 10,000 to 20,000 cpm , at a relatively small angle, which generates significant heat with temperatures of up to 100C. This is significant because cell death (necrosis) occurs at ~ 50C, and studies have shown a direct correlation between osteonecrosis, lack of bone growth, and implant loosening.
2. There have been two innovations in bone cutting, however, neither has found utility in large bone surgery.
- These include ultrasonic saws which are precise and safe but are slow and not effective for large bones.
- Lasers, which have the disadvantage of loss of haptic and depth control, as well as difficulty controlling safe guidance of laser beam, cost, and thermal damage.
3. Some innovations that have found utility in large bone surgery include specialty blades and robotics, which essentially provide either a stiffer blade or a stiffer arm along with haptic safe planes. Both of which are supposed to prevent inaccurate cuts and skiving.
4. However, the effects of these innovations on surgeon behavior and ultimate effect on bone health have not been studied.
A thick blade and/or a stiff arm with haptic protection give the surgeon a false sense of safely, which typically leads to the surgeon pushing harder to complete the cut faster.
How does pushing harder on the saw affect bone health?
5. Most of the science of cutting comes from industrial machining, where cutting efficiency is defined by a formula where material removal rate (MRR) is the product of feed, speed and depth
6. Applying these concepts to sawing, feed rate translates to the surgeon’s push, while drill speed translates to blade speed.
7. So, increasing Feed and Speed both increase cutting efficiency of the saw, but you get a rougher cut with higher temperatures when you increase feed and smoother cut and lower temperatures when you increase speed.
8. Therefore, there is concern that current innovations in cutting bone, which use thicker blades and/or stiffer arms with haptic safe planes, can lead to surgeons unwittingly increasing feed, which increases temperature and produces rougher cuts. This phenomenon may then decrease vascularity and cellularity in bone, leading to poor bone ingrowth for the press fit prosthesis.
9. So how we you increase cutting efficiency without the negative attributes of increasing feed? We know increasing feed is bad and we can’t increase speed, because we are restricted by the physical limitations of a handheld motor.
10. Superimposition of axial ultrasonic vibrations on traditional machining has existed since 1964, and has been associated with reduced cutting force, lower temperatures, increased cutting efficiency and superior surface finish.
11. Anticipated theoretical benefits of Hybrid Ultrasonic Saw include lower cutting force, smoother finish, lower temperatures, preservation of blood supply, increased accuracy, shorter operative time and ultimately decrease damage to osteocytes and bone vascularity, which may lead to decrease in incidence of implant loosening.
