This is the second installment in the series on internal fixation. In this episode we will discuss fracture biomechanics - why hardware works or fails. We will discuss concept such as load bearing and load sharing, working length of fixation, plate span ratio, stress shielding and fatigue failure, and how real construct planning happens in the operating room.
A fracture heals biologically. But biology is governed by mechanics.
Bone cells are mechanosensitive. They respond to motion, strain, compression, shear, and tension.
And that means one central truth: The surgeon is not simply fixing bone. The surgeon is engineering the conditions under which biology can succeed.
Strain in terms of bone healing describes the amount of motion relative to the fracture gap. Too much strain, and the local biology will favor granulation tissue formation. Small amounts of strain will lead to cartilage or callous formation, and very little strain sets up the environment for bone formation. The surgeon is tasked with determining the fracture needs - sometimes absolute stability (low strain) is required and sometimes relative stability. For example, in deformity correction and reconstructive surgery, the desire is to create absolute stability. In certain fracture scenarios, such as comminution, relative stability is preferred to allow the multiple fragments to unite without excessive compression and without further devitalizing bone.
The choice of fixation- the construct, the materials, and the placement - are all variables a surgeon puts together in the operating room depending on the specific circumstances.