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Stress Shielding

ABR as the Method for Bypassing Stress Shilding

More recent medical observations and research have come to recognize a phenomenon that has been termed:  “Stress Shielding Effect” or osteopenia.

In healthy individual loads or stresses are continuously being passed through the body in the form of fluctuations of pressure and tension.  The visco-elastic properties of the bodies entire connective tissue system allows for these stress fluctuations to pass through layers of tissues in a similar manner as waves pass through water.

In certain pathological conditions however or in the case of implants or prosthesis, a harder or denser element made of metal or hard plastic, will have the tendency to absorb the stresses being transmitted to the specific bodily area.  When the bodily tissues surrounding such an implant are thus “shielded” from stress impact by the harder material, they will begin to disintegrate or to be reabsorbed.

Initial awareness of this effect was registered in cases of hip-implant surgeries.  Often, when a hip-prosthesis is implanted into an otherwise healthy bone tissue, the harder and denser object absorbs stresses normally being submitted to the femur through daily movement and weight bearing.  If the surrounding bone tissue is not receiving enough stress or loads, bone material is reabsorbed by the body and the density of the bone itself begins to deteriorate.  When this bone material becomes overly porous, the implant will no longer hold and slips out.

In such cases, the implant or prosthesis works as a “stress shielder”  -shielding the bone from the pressures that are necessary for maintenance of normal bone structure.  When the pressure fluctuations are no longer evenly distributed throughout the bone tissue, the bone disintegrates.

Similar effects have been observed and described in arthritis patients, whereby hardened and calcified joints no longer work as proper hydraulic stress transfers and bone marrow density reduces.

On hand of such negative examples, the medical community has become aware of the importance of continuous loads or stress for tissue health.

The stress shielding effect is equally applicable for patients with limitations in their muscular skeletal functioning.  In patients with cerebral palsy or related disabilities, it is typical that one group of muscles become exceedingly short and strong – typically known as spastic.  The reciprocal muscles however are overly long and weak. 

When the child attempts to move or is brought into a weight bearing position, the shortened and tightened muscles will act as stress shielders to the surrounding tissues and to joints and muscles along the usual transfer chains.

To give a concrete example:

In the C.P. patient where compressional weakness is present, one would usually find tightly packed pectoral muscle groups and dense and shortened trapezius layers.  Any attempt to bring the child in a weight bearing position upon the elbows or the full arms will necessarily result in a reaction of these tight muscle groups.  Not only will these overly strong muscles fire first, leaving the weaker groups unaffected, but at the same time these groups, being over-proportionally dense and compact, will have the tendency to absorb most of the loads and stresses submitted to the body through weight bearing. 

If muscular skeletal imbalance were not present, the loads would distribute evenly and would be distributed further throughout surrounding fascial layers and to the joints.  In cases similar to that described above, what is overly strong and overly dense becomes even more so; other bodily structures that are in dire need of a balanced stress input – such as the shoulder joint in this example - are effectively “shielded” from such an input.

The same principle is true in the legs.  In cases of compressional weakness, where muscular skeletal imbalance is present, weight bearing on the legs results in excessive loads being transmitted to all of the adductor groups.  These groups however are already being noted and targeted by today’s medical community as the ones responsible for spasticity in the legs!

This is also the reason that standing frames, casting and splints have so little effect in strengthening the hip joint or preventing hip luxation.

In a child where compressional weaknesses are present – whether in a severe or in a more moderate degree – the stress shielding effect works to prevent strengthening of internal soft tissue under normal conditions or through normal physical training methods. 

The ABR Technique – in all of its manifestations - is designed to successfully bypass the stress shielding effect.  How ABR achieves this goal is described in the chapters about the ABR Technique.