Finding the Right Fit: Studying the Effect of Tap Design on Screw Pullout Strength

Presented at SMISS Annual Forum 2014
By Ehsan Jazani MD
With Steven Ludwig MD, Brandon Bucklen PhD, Joseph OBrien , Carmen Petraglia MD, Comron Saifi MD, Mark Moldavsky MS, Omar Elkassabany , Yiwei Cai ,

Disclosures: Ehsan Jazani MD None Steven Ludwig MD A; Globus Medical Inc. B; DePuy Synthes, Globus Medical Inc., K2Medical. C; DePuy Synthes. D; Globus Medical Inc., Alphatec Spine, ASIP, Spinicity, ISD. F; DePuy Synthes, Globus Medical Inc., Thieme, , Brandon Bucklen PhD E; Globus Medical Inc., Joseph OBrien A; NSF (National Science Foundation); NuVasive, Inc.; RTI Surgical. B; Globus Medical, Inc.; RTI Surgical; DePuy Synthes; 4WEB Medical. D; RTI Surgical; Alphatec Spine; 4WEB Medical. F; Globus Medical, Carmen Petraglia MD None, Comron Saifi MD None, Mark Moldavsky MS E; Globus Medical Inc., Omar Elkassabany E; Globus Medical Inc., Yiwei Cai E; Globus Medical Inc.,

Compromise of pedicle screw purchase and subsequent screw pullout is a major concern in maintaining rigid fixation in spinal surgery. Various tapping techniques are employed by surgeons with little consistency between manufacturers. We questioned whether a mismatch between tapping dimensions (pitch, depth) and the intended screw dimensions affected the biomechanics.

The current study, used an osteoporotic bone model to compare the pullout strength of pedicle screws when the path is prepared with taps of varying pitch (P) and depth (D).

Foam blocks were used to model osteoporotic bone. Each block was perforated with an awl and probed to a depth of 40mm. The blocks were tapped using custom made taps of varying thread depth and pitch. The blocks were sorted into 7 groups: 1) probe only, 2) [0.5mm D, 1.5mm P], 3) [0.5mm D, 2mm P], 4) [0.75mm D, 2mm P], 5) [0.75mm D, 2.5mm P], 6) [0.75mm D, 3mm P] and 7) [1mm D, 2.5mm P]. Finally a pedicle screw, 6.5 mm in diameter 40 mm in length (Globus Medical Inc.; Audubon, PA) [0.75mm D, 2.5mm P] was inserted to a depth of 40 mm. Axial pullout testing was performed using an MTS 858 Mini Bionix test machine at a rate of 5mm/min on ten blocks from each group. The means were analyzed using ANOVA statistical analysis (p≤0.05).

In axial pullout tests, the probed group had the highest average maximum load 761±25 N. The other six groups’ results were as follows: [0.5mm D, 1.5mm P = 713 ± 53 N], [0.5mm D, 2mm P = 726 ± 55 N], [0.75mm D, 2mm P = 698 ± 57 N], [0.75mm D, 2.5mm P = 758 ± 48 N], 0.75mm D, 3mm P = 755 ± 48 N], [1mm D, 2.5mm P = 717 ± 45 N]. There was no significant difference between the groups. The mode of failure was screw pullout in all the groups except in group 1 (probe only) where 50% of the bone blocks fractured before the screw pulled out.

In an osteoporotic bone model, changing the pitch of the tap has no statistical effect on axial pullout or cantilever bend strength. Osteoporotic bone may render tap design features (pitch and depth) marginal. The high rate of fracture in the probe group was a surprising result which may suggest a potential benefit of tapping in order to prevent catastrophic failure of bone.