A study published in The Journal of Bone & Joint Surgery (British volume) has found that significantly lower torque is needed to insert pedicle screws into patients with osteoporosis or osteopenia than the torque needed to insert pedicle screws into patients without these conditions.
It is recognised that bone mineral density (BMD) affects the pullout strength of pedicle screws, and that patients with osteoporosis are at increased risk of pedicle screw pullout. Also, in some biomechanical studies, the torque required to insert a pedicle screw has been strongly associated with the pullout strength of the pedicle screw. Therefore, being able to predict the initial fixation strength (via measuring insertional torque) may help to plan post-operative management and reduce the rate of failure.
In the study, Jae Hyup Lee, orthopaedic surgeon, Department of Orthopaedic Surgery, Seoul National University, Korea, and his co-authors used a DEXA scanner (Lunar, GE Healthcare) to measure BMD in 181 patients with degenerative spinal disease in the three months prior to them undergoing pedicle screw instrumentation of the lumbosacral spine. During the implantation of the pedicle screw, a digital measuring device (DTDK-N50EX, Kanon) was used to assess insertional torque. The same type of pedicle screws were used in all patients (Xia, Stryker Spine) and the same surgeon performed all of the operations (Lee).
The mean torque generated for patients with osteoporosis (1.37Nm) and those with osteopenia (1.48Nm) was significantly lower than for patients without either of these conditions (2.08Nm; pet al report, this is the first prospective study of a large group of patients in a clinical setting. However unlike some in vitro studies, Lee et al did find not any significant differences between the required torque in patients with osteoporosis and the required torque in patients with osteopenia. They also did not find, unlike other studies, a relationship between torque and pedicle diameter (diameter of the pedicle has previously been shown to affect torque). They wrote: “We suggest that the trabecular bone inside the pedicle greatly affects the insertional torque, and the relatively larger diameter of the pedicle compared to that of the pedicle screws (6.5mm to 7.5mm) meant there was no correlation between the diameter of the pedicle and the insertional torque.” They added that trabecular bone would also have different mechanical properties to that of cadaveric bone or sawbones model (as used in vitro studies).
In their conclusion, they said that torque-measuring devices were not routinely used in the operating theatre, and that surgeons normally use the amount of force needed to insert the screw to judge the insertional torque. They wrote: “However, this judgement is likely to be inaccurate.
The measurement of insertional torque with a torque gauge in patients with osteoporosis and osteopenia can help to plan the extent of fusion and the duration of post-operative mobilisation.”
Lee told Spinal News International:”If the insertional torque using torque gauge was significantly low, the patient should keep the brace longer than usual cases and the rehabilitation such as range of motion exercise also should be delayed until the stability of the fusion level increased. We developed the regression formula of insertional torque by the preoperative assessment of BMD. This formula would be helpful in estimating the insertional torque of pedicle screw without measuring in the operating field.”
