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:

    Kinematics is the study of motion in terms of displacement, velocity and acceleration without reference to the cause of the motion.

  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.