Deviating from the Recommended Torque on Set Screws Can Reduce the Stability and Fatigue Life of Pedicle Screw Fixation Devices

Impact of Rod Placement and Tulip Design on Screw-Rod Gripping Capacity in Spinopelvic Fixation: Evaluation Across a Spectrum of Recessed to Extended Lengths

BACKGROUND CONTEXT

High rates of pelvic instrumentation failure (4.5-38%) have been reported, often attributed to issues within the screw-tulip-rod connection. While previous research has explored various aspects of this connection, the influence of tulip design and relative rod placement on mechanical failure remains unclear.

PURPOSE

This study aims to investigate how screw-tulip design and variations in rod placement relative to the tulip affect the integrity of the screw-tulip-rod connection, utilizing axial and torsional gripping capacity tests to evaluate mechanical stability.

STUDY DESIGN/SETTING

Biomechanical METHODS: Mechanical testing was conducted following ASTM F1798-21 to assess the interconnection mechanisms in pelvic fixation constructs. Using 5.5mm Cobalt Chromium rods with porous fusion/fixation (PFFS) screws, axial gripping capacity (AGC) tests measured the axial load before translatory slippage of the rod, while torsional gripping capacity (TGC) tests assessed the torque required to induce rotational slippage. Variations in rod placement at the tulip head were tested in recessed (-2mm, -1mm), flush (0mm), and extended positions (+1mm, +10mm), simulating failure during flexion, extension, and rotation for both open and closed tulip-head designs. ANOVA was used to evaluate the effects of rod placement on connection failure, with significance set at p<0.05.

RESULTS

AGC and TGC tests revealed significant reductions for recessed rod placements, indicating suboptimal placement. At -1mm and -2mm, AGC for simulated flexion decreased by 28.8% (p<0.010) and 45.6% (p<0.001) for the open-head design and 30.5% (p<0.018) and 57.5% (p<0.001) for the closed-head design, respectively, compared to the non-recessed rod placement. TGC also showed a significant decline at -2mm, with a 25.4% reduction compared to the +1mm extended length (p<0.001) and a 20.3% reduction compared to the -1mm recessed length (p=0.005), irrespective of head design. The open and closed-head designs exhibited similar trends; however, the closed-head design was shown to better resist structural failure at recessed lengths. At -2mm simulating extension, the closed-head design was 54.8% greater than the open-head design for AGC (p<.001) and 28.3% greater for TGC.

CONCLUSION

Our findings underscore that both flush (0mm) and extended (+1, +10mm) rod placements relative to the screw-tulip offer sufficient gripping capacity whereas recessed placements (-1, -2mm) have substantial reductions. The closed-head design was shown to better resist structural failure at recessed placements.

CLINICAL SIGNIFICANCE

Rod placement relative to the most distal pelvic screw during spinopelvic fixation varries in surgical practice - whether flush to, extended past, or recessed into the screw-head. Biomechanical evaluating of the axial and torsion gripping capacities at these positions provies a foundation for clinical decision-making.

Forcefully engaging rods into tulips with gap discrepancy leading to pedicle screw loosening-a biomechanical analysis using long porcine spine segments

BACKGROUND CONTEXT

Long-segment pedicle screw instrumentation is widely used to treat complex spinal disorders. Rods are routinely precontoured to maximize assistance on the correcting side of the deformity, but there often exists a residual gap discrepancy between the precontoured rods and screw tulips. No previous research has investigated the diminished pullout strength of the most proximal or distal pedicle screw resulting from a mismatched rod in long-segment pedicle screw instrumentation.

PURPOSE

The present study aimed to investigate the decreased pullout force of pedicle screws affected by the gap discrepancy when forcefully engaging a mismatched rod into a tulip in a normal-density porcine spine.

STUDY DESIGN

The pedicle screw fixation strength under axial pullout force was compared among three different gap discrepancies between rods and tulips using long porcine spine segments.

METHODS

Twelve porcine lumbar vertebrae (L3-L6) were implanted with pedicle screws and rods. Screws on one side had no gap between the tulip and rod (0-mm group), while the most proximal screw on the other side had an intentional gap of 3 mm (3-mm group) or 6 mm (6-mm group). Three hours after forcefully engaging the rod into the tulips at room temperature, the set screws in all specimens were loosened, and each specimen was dissected into individual vertebrae for subsequent pullout testing.

RESULTS

The control group exhibited significantly greater pullout strength (1987.68 ± 126.80 N) than the groups from different rod-tulip configurations (p<.05), with significantly greater strength in the 3-mm group (945.62 ± 97.43 N) than the 6-mm group (655.30 ± 194.49 N) (p<.05). Only 47.6% and 33.0% of the pullout strength was retained in the 3-mm and 6-mm groups, respectively, compared to the control group.

CONCLUSIONS

Gap discrepancies between rods and tulips can significantly reduce pedicle screw pullout strength, with a correlation between decreased strength and increased gaps. Surgeons should avoid forcefully engaging mismatched rods and consider well-fitted contoured rods in spinal surgery to minimize the risk of screw loosening.

CLINICAL SIGNIFICANCE

The gap discrepancy between rod and tulip significantly affected pullout strength, with greater gaps leading to reduced strength. Forcefully engaging mismatched rods into tulips in degenerative spinal surgery should be avoided to minimize the risk of early screw pullout.

Use of the "Two-Three Click" Protocol in Screw Stabilization of a Patient With Loosened Nuts and Dislocation of Rods - A Case Report

Vertebral fixation, utilizing titanium screws, is a highly prevalent technique employed to address spinal instability. Screw stabilization malfunction due to pedicle screw nuts loosening is rare. Under tightening the internal nut in the pedicle screw head may increase the likelihood of rod movement within the system resulting in severe pain when moving. Our goal is to raise the attention of surgeons when tightening the screws nuts of the screw stabilization because the consequences for the patient can be subsequent additional operations and complications.  This report describes a clinical case of a 40-year-old man who underwent three surgeries at different clinics several years ago for disc herniation at the L4-L5 level and screw stabilization at the same level. The patient presents to the neurosurgery clinic of Saint Marina University Hospital with a clinical manifestation of low back pain escalating with movement, with a pain intensity rating of six on the Visual Analogue Scale (VAS). From the CT scan, it was revealed a malfunction in the screw stabilization with loosening of the screw nuts and dislodgement of the rods. Screw stabilization was restored using intraoperative X-ray guidance and following the "two-three click" protocol. The patient was mobilized on the first day after surgery and discharged on the fifth day with neurological improvement (VAS=1). The patient was followed up for a period of six months, and no further complications were observed. Surgeons must use caution while tightening the screw nuts, as not doing so may result in additional surgeries and complications for the patient in the future. The "two-three click" protocol for screw stabilization is an effective method for minimizing the issues associated with inner loosening and rod migration.