By Robert Lee and John Schmidt
One of the great misconceptions in spinal surgery concerns the type, amount and quality of the bone that is available for anchoring a bone screw. Furthermore, there is the view that a pedicle screw is just a fancy bone screw with a head to capture a rod. Long bones like the femur have a well-defined cortical shell that is typically 1–2mm thick. This allows an orthopaedic surgeon, fixing a fracture, the ability to achieve solid uni- or bicortical fixation. But in the spine your options are much more limited.
The average thickness of the cortical shell of the vertebral body (from C3 to L5) is only 0.32mm thick.1 In addition many spinal patients suffer from osteoporosis, which is a double curse for the spinal surgeon. Not only does the cortical shell get thinner but it also decreases in bone mineral density. Any damage done during placement of the pedicle screw increases the risk of revision surgery.
Spinal surgeons are typically taught a basic method for pedicle screw insertion during their training—make a starting hole, use a standard probe to make the screw path and then tap 1mm below the pedicle screw diameter. A study we presented at IMAST challenges that technique in the following ways: changing the pilot hole changes the pullout strength of the pedicle screw. However the true effects were unexpected. The curve of pullout strength vs. pilot hole size is parabolic, with the parabola opening down. That indicates that there is an optimum size pilot hole for the screw size tested. And as our study explained, the recommended size pilot hole is half the diameter of the screw.
Some surgeons insert a tapered curved probe and then rotate it in place. This has the effect of reaming the cortical bone and at the same time expanding the size of the hole. Also, a standard Lenke probe is 4mm —too large for a standard 6.5mm pedicle screw.
The method of inserting a screw, removing it and re-probing the pilot hole is well known. This has the effect of tapping the pilot hole at the same diameter as the screw. The tap size section of our paper shows that, in this case, the pullout strength of the screw is diminished by one half. In fact even undertapping by 1mm reduces pull-out strength by 200N. Surgeons should be tapping two sizes smaller than the screw for optimum strength.
The intent of spinal surgery is to restore sagittal balance. If balance is not attained, the loading on the screws can cause toggling. Testing showed that 80% of the damage occurs on the first toggling cycle and, since bone takes six weeks to remodel, revision surgery is inevitable. Toggling should be prevented at all costs. Surgeons need to get the construct right first time round or it will fail the moment the patient begins to ambulate.
One of the criticisms of the paper has been that the testing was not performed in cadavers, so it is not clinically relevant. Testing was performed under highly controlled conditions in reference grade foam. A look at the test results shows that our data have a standard deviation that is typically 5–8% of the mean. In contrast, a review of the literature using cadaveric testing shows standard deviations that are 40–70% of the mean. When the standard deviation is that high all that can be said is, while the data may be clinically relevant it is neither clinically, nor statistically significant.
References
1. Ritzel et al. J Bone and Mineral Res 1997; 12: 89–95
Robert Lee, consultant spinal surgeon, Royal National Orthopaedic Hospital, Stanmore, UK
John Schmidt, K2M, Leesburg, USA
Vishal Prasad, Medway Maritime Hospital, UK, Addisu Mesfin, University of Rochester, USA, and Julie Reigrut, K2M, Leesburg, USA, also contributed to this article.
