Treatment for bone fractures typically involves restoring fractured parts to their initial position and immobilizing the parts until they heal. Restoration and immobilization of fractured bones usually requires drilling of bone using orthopedic drills, a battery drill, surgical drill, or medical drill to produce holes for screw insertion. Drilling of bone during the surgical process causes excessive frictional heat and an increase in bone temperature that can lead to local bone necrosis; this thermal osteonecrosis can compromise the strength and stability of the fixation.
Parameters of the Drill and Bone Drilling that Affect Bone Heating
The parameters of the drill and the parameters of the bone drilling itself can significantly influence bone heating during drilling.
Parameters of the drill bit include:
Drill design – grooves in the body of the drill, known as flutes, provide cutting lips that facilitate the removal of chips and allow cutting fluids to reach the flutes; because feed-rate is proportional to cutting efficiency, it would seem that the higher efficiency of a 3-fluted drill would provide a greater reduction in heat, but research shows otherwise
Drill diameter – drill diameter correlates exponentially with bone temperature, even though larger drills have bigger flutes that eliminate bone chips and facilitate efficient drilling
Drill wear and tear – overuse causes drills to become blunt and ineffective, as blunt drills typically require the application of extra force that contributes to frictional heat. One study reported that no severe deformations or blunting was detected even after 50 uses.
Parameters of bone drilling include:
Drill speed – there is a lack of consensus on the ideal drill speed to reduce bone heating in that early research found that bone temperature was proportional to drilling speed, while later studies show that was true only in speeds up to 10,000 rpm; still another study found no temperature changes from speeds of 345 rpm up to 2900 rpm
Feed rate – the rate at which the drill penetrates the bone; along with drilling speed, feed rate exhibits a significant influence on drilling force, temperature, bone debris and hole morphology
Drilling depth – bone temperature increases with drilling depth because drilling a deeper hole increases contact time between the drill bit and the bone, which increases the amount of temperature-raising friction generated
Bone thickness – the duration of bone drilling depends largely on the thickness of the cortex, which is the hardest part of bone; the higher the bone mineral density (BMD), the more heat generated when all other parameters are equal
Irrigation – facilitates the effective removal of chips and debris to prevent clogging of the flutes to reduce heat generation; research shows irrigation can keep temperatures of bone tissue below the critical value of 47°C
Research will undoubtedly shed more light on the effects these parameters can have on bone drilling. A greater understanding of the association between temperature and bone drilling can help reduce the incidence and effects of thermal osteonecrosis.