Biomechanics
Biomechanics
is a subject of immense complexity and it is specially difficult for surgeons
because graduate engineers often fail to appreciate the abysmal ignorance of
mathematics which hampers surgeons in this field. (Sir John Charnley
Despite this sobering thought from Sir John Charnley, all surgeons who
deal with injuries of the musculoskeletal system require a basic understanding
of biomechanics. It is the science of the action of forces, internal or
external, on the living body and dictates how accidents and overloading produce
tissue damage. Effective treatment regimes, surgical or nonsurgical, to restore
the mechanical properties of injured structures must be based on knowledge of
the fundamentals of biomechanics.
Dynamics
Dynamics is the study of the motion of bodies and forces that produce
the motion. There are three subtypes:
• Kinetics relates the
action of forces on bodies to their resulting action.
• Kinesiology studies human
movement and motion.
The
guiding principles by which forces act on bodies are encapsulated in Newton’s
three laws. Statics is the study of the action of forces on bodies at rest and
is governed by Newton’s first law; if a zero net external force acts on a
body, the body will remain at rest or move uniformly. Thus, the sum of the
external forces applied to a body at rest equals zero. Newton’s second law
relates to acceleration. The acceleration (a) of an object of mass (in)
is directly proportional to the force applied to the object (F ma). Newton’s
third law states that for every action there is an equal and opposite reaction.
This allows us to analyse the action of forces on the whole body, or limbs, and
resolve them.
Biomaterials
The effect of externally applied loads on a particular structure, such
as a ligament or tendon, and the resulting internal effects and deformations
induced in these structures is the study of biomaterials. The basic terminology
is given in Table 29.1.
All
materials will eventually fail if sufficient load is placed upon them. If you
clamp the medial collateral ligament including its bony attachments and pull it
apart (a tensile test) then you would be testing a structure whose
properties are defined by a load—deformation curve (Fig.
29.la). If the
experiment were modified to include only the ligament then you would be testing
it as a material resulting in a stress—strain curve (Fig.
29.lb).
In
Fig. 29.lb the first part of the curve (toe-in) occurs as the natural wavy
pattern (crimp) of the fibres is eliminated. As the stress increases the fibres
lengthen proportionally, the slope of this line representing Young’s modulus (E).
As the strain increases beyond physiological loads increasing numbers of
fibres are injured until the highest stress is recorded immediately prior to
failure of the ligament.
It
is therefore apparent that if a single load in excess of the ultimate failure
load of a structure is applied then it will fail. An example of this occurs when
a football player is tackled from the side. The knee is forced into valgus and
the medial collateral ligament is loaded in tension. If the load exceeds the
ultimate failure load it will rupture completely. If the load is below this then
a partial rupture may occur.
If
the ligament is partially ruptured and the player manages to continue playing, a
second injury of similar magnitude results in complete rupture of the ligament.
This brings us to the concept of fatigue failure. Figure 29.2 depicts the
relationship
between load and the number of stress cycles required to produce failure in a
structure. As the applied load diminishes the number of cycles tolerated by the
structure increases. Once the load reaches physiological levels then,
theoretically, the structure will remain intact however many cycles are applied
— the fatigue endurance limit.
The principles outlined in the above example are
applicable to all constituents of the musculoskeletal system: bone, tendon,
ligament and muscle. Whilst the units will vary between tissue types the
patterns will also vary within one tissue depending on the direction of the
applied load, and this property is called anisotropy. For example, bone
has a very high failure load in compression but a relatively low failure load in
tension. So far, the principles described could equally well be applied to an
inert structure. The challenge of biomechanics is apparent when one considers
the multitude of other factors that affect the structural properties of living
tissues. Age has a profound influence on both mode of and loads to failure. In
growing skeletons the junction between ligaments/tendons and bone is weaker than
the ligament itself, and failure typically occurs here. In mature skeletons the
failure usually occurs in the midsubstance of the ligament. The failure load of
all structures peaks in early adult life and declines thereafter. Ligaments,
tendons and muscles are all more compliant (less stiff) at higher temperatures
and circulating hormones, particularly sex hormones, can affect the structural
properties of soft tissues from one day to the next.