Measurement of lumbar isthmus parameters for novel starting points for cortical bone trajectory screws using computed radiography.

The current study aims to measure distance parameters in lumbar isthmus to develop new references for lumbar pedicle screw insertion. Using computed radiography, the distance between the median pedicle tangent and the isthmus lateral tangent (D1) and the isometric distance between the isthmus lateral tangent and the inferior border of transverse process (D2) were measured on 120 spine X-ray images. A derived distance D3 (equal to D2 minus 1 mm) was calculated. These parameters were used to define the starting points for pedicle screws. Cortical bone trajectory (CBT) fixations were carried out on six wet (including 3 male and 3 female) and 4 dry lumbar specimens using the new system, and were evaluated for accuracy and safety of screw insertion. Measurements showed that D1 (mm) was 2.1±0.13 (L1), 2.3±0.13 (L2), 3.6±0.33 (L3), 4.4±0.36 (L4), 5.7±0.36 (L1); D2 was 5±0.83 (L1), 6.19±0.84 (L2), 5.38±0.86 (L3), 3.66±0.42 (L4) and 2.30±0.37 (L5). A total of 100 CBTs were evaluated. Among them, 7 out of the 60 screws in the 6 wet specimens penetrated into the lateral pedicle bone, the successful rate was 88.7%; 2 out of the 40 screws in the 4 dry specimens penetrated through the lateral or median pedicle bone, the successful rate was of 95%. With our new system, CBT screws can be placed based on these parameters, which can be obtained less invasively, and irrespective of articular degeneration in lumbar facet and morphological variation in the transverse process. Our data show that the technique is safer, less invasive, and easier to operate. It would help reduce bleeding, intraoperative X-ray exposure and surgical time.