'Higher the grade-smaller the pedicle': a study of pedicle dimensional variations in 100 cases of high grade lytic spondylolisthesis.

Purpose: To measure the pedicle dimensions in high grade lytic spondylolisthesis (HGL) and to classify them, which helps the spine surgeon in proper selection of pedicle screws.

Methods: A study of CT scans in 100 consecutive patients between Jan 2017 and April 2021 diagnosed as single-level HGL on standing radiographs. Pedicle height (PH), Pedicle width (PW), Differential pedicle height (DPH), Screw length (SL) and Transverse pedicle angle (TPA) were measured and analyzed. PH and PW were classified into four grades as-grade A less than 5.0 mm, grade B between 5.0 and 6.0 mm, grade C between 6.0 and 7.0 mm, and grade D above 7.0 mm.

Results: 5 males and 95 females with mean age of 49.1 years. PH in 44% lytic vertebra were grade A, B (less than 6 mm) and the rest 56% had grade C, D (greater than 6 mm). PH averaged 6.6 mm in grade 3 HGL, 5.61 mm in grade 4 HGL. Change in PW, SL and TPA was not statistically significant with regards to grade or level of listhesis. A total of 37 cases were noted to have DPH (25 cases had a difference < 2 mm and 12 had a difference > 2 mm).

Conclusion: 44% of the PH in lytic vertebra was grade A and B (less than 6 mm) that stresses the importance of pre-op CT assessment and planning the appropriate screw dimensions. Change in PH was statistically significant with regards to the grade of listhesis (P-value < 0.01). Differential pedicle height also need to be looked for.