Thermal Generation in Spinal Surgery: Does Rate of Irrigation Matter During Anterior Cervical Discectomy?

Discussion

The most used burrs are cutting serrated carbide steel burrs and diamond-coated burrs. The latter is preferred for delicate decompression during uncoforaminal decompression because they will not snag soft tissue. This minimizes the risk of injury to the underlying dura, spinal cord, and nerve roots.6

C5 palsy has a distinctive time delay of up to 3 days until its clinical manifestation. The proposed mechanisms of C5 palsy include perioperative nerve root injury, the “tethering theory” (whereby nerve root traction occurs secondary to the posterior shift of the spinal cord), ischemia, and reperfusion injury following decompression.7 Recent studies have proposed that the frictional heat generated from high-speed drills causes histological nerve root injury.1,7,8

Hoogeveen et al reported that ultrastructural changes in the rat sciatic nerve, such as immediate edema, blood vessel occlusion, and severe endothelial cell injury, were observed after exposure at 45°C. They also noted ischemia and accumulation of cell organelles, concluding that hyperthermia is linked to direct endothelial and myelin sheath injury, while axon degeneration is a likely consequence of ischemia.9,10 Xu and Pollock reported that the delayed ischemic lesions were secondary to extensive thromboses of the vasa nervorum.11

C5 palsy due to thermal injury is not easy to prove in vivo. Takenaka et al used muscle strength to investigate the effect of chilled saline irrigation on the incidence of C5 palsy, of which at least a 2-fold decrease of deltoid weakness was observed in chilled vs room temperature saline.8 This highlights the importance of adequate irrigation during cervical decompression surgery to minimize the incidence of upper limb palsy.

Our focus was on the potential thermal injury to the nerve roots, given the proximity of the site of burring to the cervical nerve roots. Delivery of a coolant solution eliminates bone chips and lubricates the drills, both of which reduce friction-generated heat. To the best of our knowledge, this is the first study investigating the significance of the rate of small-volume automated irrigation in anterior cervical decompression.

At 0 cc/min, the steel burr’s mean temperature was 72.8℃, with a maximum of 141℃. The diamond burrs, on the other hand, were hotter with a mean temperature of 90.66℃ and a maximum of 177℃. Hosono et al found similar results in 2009.1

Any amount of irrigation at a rate of 0.5 cc/min or more maintains the maximum burr temperatures below 45°C. There is a statistically significant difference (P < 0.001; Kruskal-Wallis test) at 0.5 cc/min (30 mL/h); this is a much smaller volume of irrigation than earlier studies. The exact reasons are unknown; however, the targeted delivery of irrigation and the high specific heat capacity of water are likely to contribute to the above findings.

Hosono et al concluded that water irrigation at a rate of 540 mL/h (9 cc/min) was required to reduce the bone temperature of porcine lumbar transverse processes effectively. The authors used the Stryker Total Performance System high-speed drill, which prohibits direct comparison with our results, given the differences in equipment designs. At 540 mL/h (9 cc/min), the maximum temperature with the diamond burr was 52°C, and the maximum temperature with the steel burr was 44°C.1 High flow rates would cause significant obscuration of view, especially in the tight intervertebral spaces during an ACDF, as compared with a posterior approach lumbar spine laminectomy. Our experience with irrigation rates of 0.5 cc/min and above recorded maximum burr temperatures of 40.6℃ in steel burrs and 38.9℃ in diamond burrs. As such, the hypothesis that an increased irrigation rate leads to lower burr temperatures does not hold true for irrigation rates at 0.5 cc/min and above.

Matthes et al also compared burr temperatures during lumbar decompression using automatic or manual irrigation. They demonstrated that the average burr temperature was 82°C (maximum 119.5°C) for high-speed drills with 1200 mL/h (20 cc/min) automatic irrigation and 100°C (maximum 148.2°C) for manual irrigation at an average rate of 1665 mL/h (27.8 cc/min) (P < 0.001), concluding that automated irrigation was superior to manual irrigation.2 Again, a much larger amount of irrigation was used compared with the 0.5 cc/min (30 mL/h) used in our study.

There are several limitations to this study. First, a larger sample size would make this a more powerful study. However, the study was conducted at an elective spinal orthopedic theater, availability was severely affected by coronavirus disease 2019, and the data collection time would have become protracted. Second, we used infrared thermography as an indirect measure of temperature, where certain values must be extrapolated12 and can be dependent on working conditions, such as ambient temperature and humidity.13 However, its noninvasive, noncontact properties of temperature measurement make it safe and practical, which have been used in several similar studies.1,2,4 Due to the limited intervertebral space and the depth at which uncoforaminal decompression is performed, the burr tip temperatures were measured as the closest representation of the friction-generated heat at the bone-burr interface. It also must be noted that our findings may not apply to all high-speed drill systems. Lastly, measuring the burr tip with infrared thermography introduced a time interval of approximately 5 seconds. A real-time measurement would increase the accuracy of this study because some heat will have dissipated between the end of friction and the start of measurement.