Background:

Although pedicle screw sizes may affect the rate of bone union after lumbar fusion surgery, there is currently no supportive clinical evidence.

Methods:

Eighty-five patients older than 50 years who underwent single-level L4/5 TLIF with posterior pedicle screw (PS) fixation were analyzed. Patients with factors that potentially inhibit bone fusion, such as Parkinson’s disease, were excluded. Bone union was assessed using computed tomography (CT) and dynamic radiographs 1 year after surgery. Explanatory factors considered included sex, age, smoking, bone density, material of the cage, PS diameter (PSD), relative PS length, theoretical maximum PSD (PSD_max), which was defined as the maximum diameter of the screw that may be inserted without breaking cortical bone around the pedicle, and the filling index (FI), which was defined as the difference between the cross-sectional area of maximum PS and actual PS (PSD_max ˆ 2 – PSD ˆ 2). Japanese Orthopaedic Association (JOA) scores before and 1 year after surgery were evaluated as a clinical outcome.

Results:

Nineteen levels were diagnosed as pseudoarthrosis. A multivariate logistic regression analysis identified a larger FI (P = 0.016) and older age (P = 0.047) as risk factors for pseudoarthrosis. The JOA score 1 year after surgery and its recovery rate were significantly worse in patients with pseudoarthrosis than in those with fusion.

Conclusions:

The selection of an appropriately sized screw is important for achieving rigid fusion after TLIF. Preoperative planning using multiplanar reconstruction CT is an important approach for ensuring good clinical results.