This study was designed to test the hypothesis
that the sensory innervation of bone might play an important role
in sensing and responding to low-intensity pulsed ultrasound and
explain its effect in promoting fracture healing. In 112 rats a
standardised mid-shaft tibial fracture was created, supported with
an intramedullary needle and divided into four groups of 28. These
either had a sciatic neurectomy or a patellar tendon resection as
control, and received the ultrasound or not as a sham treatment.
Fracture union, callus mineralisation and remodelling were assessed using
plain radiography, peripheral quantitative computed tomography and
histomorphology. Daily ultrasound treatment significantly increased the rate of
union and the volumetric bone mineral density in the fracture callus
in the neurally intact rats (p = 0.025), but this stimulating effect
was absent in the rats with sciatic neurectomy. Histomorphology
demonstrated faster maturation of the callus in the group treated
with ultrasound when compared with the control group. The results
supported the hypothesis that intact innervation plays an important
role in allowing low-intensity pulsed ultrasound to promote fracture
healing.
Using the transverse processes of fresh porcine lumbar spines as an experimental model we evaluated the heat generated by a rotating burr of a high-speed drill in cutting the bone. The temperature at the drilled site reached 174°C with a diamond burr and 77°C with a steel burr. With water irrigation at a flow rate of 540 ml/hr an effective reduction in the temperature was achieved whereas irrigation with water at 180 ml/hr was much less effective. There was a significant negative correlation between the thickness of the residual bone and the temperature measured at its undersurface adjacent to the drilling site (p <
0.001). Our data suggest that tissues neighbouring the drilled bone, especially nerve roots, can be damaged by the heat generated from the tip of a high-speed drill. Nerve-root palsy, one of the most common complications of cervical spinal surgery, may be caused by thermal damage to nerve roots arising in this manner.