Effects of Load-Bearing in Cerebral Palsy: Bones

The intent of this post is two-fold: 1) to discuss and explore a very relevant, critical, yet controversial subject of dicussion within Cerebral Palsy (CP) (and other disorders of movement and posture)...2) to link previous posts together with this one to shape a formulate the overall narrative. As always, my overall intention is not to discredit any particular school of thought, strategy, or central philosophy...rather to bring the realities of the mechanical challenegs (and consequences) that accompany CP to the forefront so that specific, goal-oriented strategies can be formulated. It should be clear that these strategies are case specific and therefore need to be evaluated with it´s own set of standards.

This first installment will focus on the direct effect of load-bearing (weight-bearing) on the bones. However, before any in-depth discussion begins, I should define what load-bearing means within the context of this post. To be very specific, load bearing is: exposing an architectural system to structural load or stress. Therefore, in the human organism this is very clearly achived in either a seated or standing position.

The benefits of load bearing or weight bearing in the healthy person are quite obvious and straightforward...the ground force reaction promotes to development of growing bones and contributes to the development of healthy bone density. Although this is very true, the fundamental mistake that is quite often made is that this statement is imported to the CP community. Why is this a mistake? This statement assumes that the transmission of ground force reaction is efficient and therefore travels to the relevant load bearing structures throughout the body...in disorders of movement and posture (CP, for example), force transmission is extremely disrupted and inefficient. This disruption leads to improper stress distribution, accessory and compensatory activation of skeletal muscle, overuse and fatigue syndromes, as well as structural deterioration. To be very clear: mechanical forces in the CP individual DO NOT travel in a manner that facilitates healthy growth and development.

I doubt any responsible professional would debate this fact...however, a fundamental question is therefore raised: if this is an understandable fact, why is there such a rush to implement "standing" protocols? This is a glaring contradiction that generally goes unseen, but certainly exists...and therefore needs to be addressed.

In order to address this delicate topic intelligently and responsibly, we need to dissect the effects of load bearing into some fundamental component parts: the effect on the bony skeleton itself and the effect on the soft tissue. The effects on the system as a whole are actually greater than the sum of the component parts, but far more complex...therefore a basic understanding of the component parts should give some clarity on the potential "biomechanical tax" that is paid when weight bearing is aggressively promoted.

MECHANOTRANSDUCTION IN BONES: If the term Mechanotransduction is new to you, i would refer you to my previous post titled "Mechanotransduction: Response to Manual Therapy". However, to summarize the concept, it is the cellular response to mechanical forces or loads. The precise mechanism in which bones sense mechanical stimulii is still largely being researched, however the concept of cellular mechanotransduction in the bones is widely accepted as a likely mechanism. With respect to the skeleton, there is an integration of the understanding of the connective tissue memebrane of the bone (periosteum), cellular tensegrity, and the effects of mechanotransduction on the nucleus of the osteoblast. Effectively, mechanical stress and deformation that is load-induced activates a change in the genetic program of the bone (gene transcription). Mechanical information is relayed from the bone to the gene by a succession of long-term deformation and changes in conformation. If the stresses applied to the bone are not distributed properly, skeletal growth and absorption are disrupted...which leads to eventual bony deformation and therefore impaired function.

Although this is a common understanding in the field of cellular biochemistry, it doesnt seem to reach the professional "masses" to be applied responsibly in rehabilitative protocols. Given this knowledge, it becomes clear that consistent, intensive, and aggressive strategies geared towards standing protocols (standers, for example) is potentially costly when you consider the "biomechanical tax" to be paid in the long-term. Although bony misalignment is a reality in the CP individual, load bearing and exposing an altered structural "scaffold" to loads exacerbates the deterioration of structural integrity.

Although significant, this is only part of the overall perspective. The effects of load bearing with respect to the soft-tissue will be the subject of the second load-bearing post. Hopefully both or either will stimulate some analytical thought on the subject and consequenty result in more informed and effective strategies.

Cheers.