FRCS (Tr & Orth) Oral Examination:IM Nail Biomechanics
IM Nail Biomechanics
E X A M I N E R: This is a radiograph of a broken femoral nail that was used to fix a distal femoral sha fracture (Figure 9.1). How can you prevent nail breakage?
I would insert the largest diameter solid nail that is available for use.
Not a good start. The answer isn't particularly well thought out and is not scoring the candidate any marks.
E X A M I N E R: Why would you use a solid nail? Most nails used in orthopaedics are hollow.
Because a solid nail will be stronger than a hollow nail.
This is continuing with a poor choice of imprecise terms and more importantly the candidate is missing scoring opportunities.
E X A M I N E R: What do you mean by the term ‘stronger’?
Strong is the ability of a material to resist deformation.
This is an incorrect definition and the viv a is going nowhere.
E X A M I N E R: How does this relate to ultimate tensile strength (UTS)?
UTS is the highest stress observed on the stress versus strain diagram while the failure strength is the stress value at which the material eventually fails. Strong is an imprecise biomechanical term. A strong material has a high ultimate tensile strength.
[Silence …] Can I retract my last few statements and say that the torsional and bending rigidity of a solid nail will be greater than that of a hollow nail?
The candidate is not getting past the opening questions. Better to discuss factors predisposing to nail breakage. This will then lead onto discussions about area and polar moments of inertia, the biomechanical benefits of using larger-diameter nails for long bone fractures, the solid versus hollow nail dialogue, benefits of IM reaming, etc. Another road to journey down is patient factors that could predispose to non-union and eventual nail breakage(smoking, alcohol, malnutrition etc) (Score 4.)
C A N D I D A T E 2: An unstable fracture pattern (segmental or comminuted) or the use of a small unreamed diameter nail increases the risk of nail breakage.
Distal femoral fractures have a high incidence of intramedullary nail breakage, especially if the fracture has been produced by high-energy trauma and the patient encouraged to weight bear early. Early weight bearing with delayed fracture healing increases the time over which cyclic stress may act to cause fatigue failure and nail breakage.
Technical errors in nail insertion, such as scoring the nail during locking, may weaken the nail or create stress risers. Excessive impact during nail insertion due to under-reaming of the medullary canal may also weaken the nail.
Intramedullary nails rarely break when no locking screws are used. Statically locked nails can produce high concentrations of stress at the proximal or distal end of the nail, predisposing it to breakage. Nail design changes such as increased material thickness around screw holes, cold forming which increases material strength reduce risk of nail breakage.
Distal femoral fractures have a high incidence of intramedullary nail breakage, especially if the fracture has been produced by high-energy trauma and the patient encouraged to weight bear early. Early weight bearing with delayed fracture healing increases the time over which cyclic stress may act to cause fatigue failure and nail breakage.
Technical errors in nail insertion, such as scoring the nail during locking, may weaken the nail or create stress risers. Excessive impact during nail insertion due to under-reaming of the medullary canal may also weaken the nail.
Intramedullary nails rarely break when no locking screws are used. Statically locked nails can produce high concentrations of stress at the proximal or distal end of the nail, predisposing it to breakage. Nail design changes such as increased material thickness around screw holes, cold forming which increases material strength reduce risk of nail breakage.
(Score 6.)
E X A M I N E R: What do you mean by fatigue failure?
Failure of a material with repetitive loading below the ultimate tensile strength.
E X A M I N E R: What do we mean by S-N curves?
As mentioned, cyclic loading of an object can result in fatigue failure. An S-N curve gives information about the number of cycles a material can endure for a given stress level.
(Figure 9.2)
E X A M I N E R: What makes fatigue failure more likely?
In the presence of stress risers such as a hole, a sharp edge, an indentation, notch or scratch, the loads can go beyond the normal limit that is localized in that area. This can lead to crack propagation.
(Score 6.)
Figure 9.1: Lateral radiograph of left distal femur demonstrating a broken femoral nail.
Figure 9.2: S-N curve.