Lower lumbar vertebra size and anatomic variation: An Anatomo-Radiologic Study

In routine practice, it is often necessary to use shorter screws in L5 than L4. The present study measured L5 versus L4 vertebral pedicles, to guide surgical strategy.

CT or MRI scans for 95 patients were analyzed. Radiographic measurements (anteroposterior diameter (APD), pedicle length (PL) and pedicle width (PW)) were taken by a spine surgeon. Statistical analysis used R 3.4.3 software.

Ninety-five patients were included: 48 female (50.53%), 47 male (49.47%); mean age, 57 years (range, 19-85 years). Univariate analysis found a strong correlation between right and left PL values in L4 and L5. Right and left values were pooled, obtaining a mean L4 PL of 55.34mm (range, 54.23-56.45mm) and L5 PL of 51.80mm (44.81-58.80) and L4 PW of 10.48mm (10.06-10.91) and L5 PW of 9.90mm (7.43-12.39). Multivariate analysis disclosed significant effects of age and gender, with greater age and male gender associated with greater anteroposterior vertebral diameter. Mean anteroposterior vertebral length was significantly shorter in L5 than L4 by 3.57mm (range, 4.08-3.06mm).

Anteroposterior pedicle length was shorter in L5 than L4, in line with the literature. This answers the surgeon's question: "Should pedicle screws be shorter in L5 than L4?". From these results, it seems logical to use an L5 screw that is 5mm shorter than in L4, to secure good intra-body screw fixation.