A paper from the LSJL scientists with a potential design of a knee loading device

I found another paper that studied the LSJL knee device here.

I happened upon this paper however which was published at a conference and has potential new insights.

FEA analysis of a portable knee rehabilitation device

“The knee loading device examined in this paper only remains effective for small levels of deformation. The intended displacement of the working device is very small, a maximum of only 6.35mm, and therefore the device cannot accommodate large deformations as such deformations will greatly decrease the effective range of motion of the device.”<-So the device mentioned in this paper can only deform the bone by a maximum 6.35mm and likely less than that as just because the device can be displaced by 6.35mm doesn’t mean it will displace the bone by 6.35mm.

According to mechanostat theory, you have to deform the bone by at least 1500microstrain or 0.15% of a bones original length to get into the plastic deformation range to actually stretch the bone out permanently.

The average adult femur is 2.34 cm in diamater and the maximum displacement is the same regardless of femur shape and size.  The diameter of the femur is what matters as the device loads laterally.  The maximum displacement force is well over what is needed to plastically deform the bone.  But remember that the maximum displacement force is not actually how much the bone is deformed and the bone is being loaded laterally and not being stretched.  By loading laterally a moderate stretching force should be applied but not by the entirety of the deformation.

So it is not that the device can induce an increase in deforming the bone but it is possible by changing the microenvironment via degradation of cortical bone, initiation of mesenchymal condensation, and an increase in chondrogenic signaling.

If you click on the link above to see the study and look at figures 2 and 3, you can see that the device looks remarkably like a clamp.

“The described joint loading modality applies lateral loads to synovial joints.”

“To apply such [the needed] load, a device would need to have a means of producing a transverse force directly to the end of a long bone, such as at the knee{My current working theory is the load needs to be applied at the the epiphysis with the intent to maximize bone on bone contact and not on the synovial joint} . A cyclic force applied in such an area would force a slight shift of the fluid within the bone towards the opposite end of the bone in a controlled fashion.  While no such device currently exists for use on humans{I think a clamp device can serve such a role}, a new joint study seeks to develop a portable device designed for human use to be used in future testing.”<-For our purposes we would likely want more fluid flow as to induce cortical bone degradation and mesenchymal condensation.

“a pressure of approximately 6.90KPawas used as the load for each vertical pad [in a potential joint loading device]. The pressure equates to approximately 40 N over the entire surface area which is the desired maximum load for the device.”

An Irwin Quick Grip 12-inch can generate 300lbs of force.

300lbs is 1334N which is well over 40N but it is likely that you are not going to be able to generate that force however it is also likely that you will generate over 40N of force.

“stress is equal to force divided by cross-sectional area and strain is equal to change in length divided by original length. Stress and strain are related by Hooke’s Law, which states that stress is directly related to strain by a factor known as the Modulus of Elasticity, which is unique to every material.”

To effectively measure strain we’d have to be able to measure microchanges in bone length which I do not see as being possible at this juncture.

“Given the duty cycle of 5 minutes of daily operation per patient with a 1
Hz frequency loading function during operation; or 300 cycles per operation, this device is designed to last for 2683 uses. The choice of 1 Hz as one example was linked to daily
human physical activities such as walking. The device is able to induce loads up to 20 Hz, and it is a future task to evaluate appropriate loading frequencies.”<-Given that walking does not traditionally increase bone length we would likely use a different frequency.  But using a clamp it’s very hard to get such frequencies.  20Hz is equal to 20times per second and it would be virtually impossible to rapidly unclamp and reclamp in that amount of time.

What my current LSJL method instead tries to progressively clamp harder and harder(while still being mindful not to clamp to the point of too much pain) to increase the number of “cycles”.

It should be noted that nothing in this study mentions using this device for longitudinal bone growth but the other studies that Yokota et al. have done on the joint loading modality suggests that it could.  If you look at figure 1B(in the study link above), you can see that joint loading puts pressure on the cortical bone from the medullary cavity.  At a sufficient enough pressure, there could be degradation of the cortical bone and there was evidence of this in a diagram in one LSJL study.  The degradation of cortical bone is highly significant as one of the key events of fusion is the joining of the cortical bone of the epiphysis to the diaphysis.  By degrading cortical bone, we can reverse some of the constraining effects of cortical bone on future longitudinal bone growth.  After all, a key event in distraction osteogenesis is the inducement of a cortical bone fracture.