The effect of repetitive pilot-hole use on the insertion torque and pullout strength of vertebral system screws

Value of different preoperative bone evaluation methods in predicting intraoperative screw insertion torque-a prospective clinical comparative trial

Background Context Establishing good screw-bone structural stability is conducive to reducing the risk of postoperative screw loosening. Screw insertion torque is an objective index for evaluating screw-bone structural stability. Therefore, accurate prediction of screw insertion torque can improve the preoperative evaluation of patients, optimize the surgical plan, and improve the surgical effect. At present, the correlation between different bone assessment methods and screw insertion torque is unclear. Purpose The aim of this study was to evaluate the correlation between different bone assessment methods and screw insertion torque and to optimize the predictive performance of screw insertion torque through mathematical modeling combined with different radiology methods. Design Prospective cross-sectional study Patient samples Seventy-seven patients with preoperatively available DXA, CT and MRI data who underwent spinal fixation surgeries between October 2022 and September 2023 and 357 sets of screw data were included in this analysis. Outcome Measures Spinal, vertebrae-specific and screw trajectory's BMD were measured preoperatively by different imaging modalities. Intraoperative screw insertion torque was measured using an electronic torque wrench. Methods Pearson linear correlation, scatter plots and univariate linear regression were used to evaluate the correlation between different bone evaluation methods and screw insertion torque. Different bone evaluation methods were fitted into the prediction model of screw torque and the related equations were obtained. Results Screw insertion torque had the strongest positive correlation with the volumetric bone mineral density (vBMD) of the screw trajectory (Pedicle screw insertion torque (PSIT): R = 0.618, p < 0.001; Terminal screw insertion torque (TSIT): R = 0.735, p < 0.001). A weak negative correlation was found between the screw insertion torque and level specific vertebral bone quality (VBQ) (PSIT: R = -0.178, p = 0.001; TSIT: R = -0.147, p = 0.006). We also found that the PSIT was strongly correlated with the TSIT (R = 0.812, p < 0.001). Conclusions Compared to other bone quality assessment methods, screw trajectory vBMD may be better predict the magnitude of screw insertion torque. In addition, we further optimized preoperative assessments by constructing a mathematical model to better predict screw insertion torque. In conclusion, clinicians should select appropriate preoperative bone quality assessment methods, identify potential low-torque patients, optimize surgical plans, and ultimately improve screw insertion accuracy and reduce postoperative screw loosening rate.

Biomechanical Comparison of Facet Versus Laminar C2 Screws

BACKGROUND

Transpedicular or transisthmic screws for C2 instrumentation represent the gold standard; however, the anatomy is not always compatible (hypoplastic pedicles, procidentia of the vertebral artery). Laminar screws (LS) have been proposed as a rescue technique and recently, bicortical facet screws (FS). To date, the biomechanical property of FS remains unknown.

OBJECTIVE

To compare the pull-out resistance of bicortical facet (FS) vs laminar (LS) C2 screws.

METHODS

Thirty-two human cadaveric C2 vertebrae were screened by CT scan imaging and dual x-ray absorptiometry before receiving both techniques and were randomized according to side and sequence (FS or LS first). Screw positioning was validated using 2-dimensional x-rays. Sixty-four mechanical tests were performed using pure tensile loading along the axis of the screws until pull-out. Mean pull-out strengths were compared using paired tests, multivariate and survival analysis (Kaplan-Meier curves).

RESULTS

The morphometric data were consistent with previous studies. Over 64 tests, the mean pull-out strength of LS (707 ± 467 N) was significantly higher than that of FS (390 ± 230 N) ( P = .0004). Bone mineral density was weakly correlated with pull-out strength (r = 0.42 for FS and r = 0.3 for LS). Both techniques were mechanically equivalent for vertebrae in which intralaminar cortical grip was not achievable for LS. The mean pull-out strength for LS with laminar cortical grip (1071 ± 395 N) was significantly higher than that of LS without (423 ± 291 N) ( P < .0001).

CONCLUSION

Our results suggest that bicortical FS of C2 offer less mechanical resistance than LS.

Evaluation of the efficacy of modal analysis in predicting the pullout strength of fixation bone screws

Background

Pilot hole preparation has been shown to have an impact on the short and long-term stability of the screw fixation constructs.

Purpose

Investigation and comparison of two nondestructive modal analysis methods with conventional insertion torque (IT) and pullout tests in optimum pilot hole diameter detection.

Methods

Twenty conical core titanium screws were embedded in high-density polyethylene blocks with different pilot hole diameters. The maximum IT was recorded for each screw during implantation. Then, two modal analysis methods including accelerometer (classical modal analysis [CMA]) and acoustic modal analysis (AMA) were carried out to measure the natural frequency (NF) of the bone-screw structure. Finally, stiffness (S), pullout force (F_ult), displacement at F_ult (d_ult) and energy dissipation (ED) were obtained from the destructive pullout test.

Results

The IT increased, as the pilot hole diameter decreased. The maximum value of IT was observed in the smallest pilot hole diameter. The same trend was found for the F_ult and the first NF derived from both modal methods except for 5.5 mm pilot hole diameter. The natural NFs derived from CMA and AMA showed high correlations in different groups (R² = 0.94) and did not deviate from y = x hypothesis in linear regression analysis. The F_ult, d_ult, and ED were measured 4800 ± 172 N, 3.10 ± 0.08 mm and 14.23 ± 1.10 N.mm, respectively.

Discussion

No significant change was observed in "S" between the groups. The highest F_ult and first NF were obtained for the 5.5 mm pilot hole diameter. Both CMA and AMA were found to be reliable methods and can promote the undesirable contradiction between F_ult and IT.

A influência da incompatibilidade do macho de rosca na resistência à extração do parafuso pedicular

Objective  We aimed to study the “in vitro” pullout strength of SpineGuard/Zavation Dynamic Surgical Guidance Z-Direct Screw (DSG Screw, SpineGuard Inc, Boulder, Colorado, USA), a screw designed to be inserted using a direct insertion technique.

Methods  Dynamic Surgical Guidance Screws of 5.5 and 6.5 mm were introduced into polyurethane blocks with a density of 10 PCF (0,16g/cm ³ ). According to the experimental group, screws were inserted without pilot hole, with pilot without tapping, undertapping and line-to-line tapping. Screw pullout tests were performed using a universal test machine after screw insertion into polyurethane blocks.

Results  Screws inserted directly into the polyurethane blocks without pilot hole and tapping showed a statistically higher pullout strength. Insertion of the screw without tapping or with undertapping increases the pullout screw strength compared with line-to-line tapping.

Conclusion  Dynamic Surgical Guidance Screw showed the highest pullout strength after its insertion without pilot hole and tapping.

DIRECT PEDICLE SCREW INSERTION PULLOUT STRENGTH

Objective:

Study the in vitro pullout strength of SpineGuard/Zavation Dynamic Surgical Guidance Z-Direct Screw (DSG Screw), a screw pedicle designed to be inserted using a direct insertion technique.

Methods:

DSG Screws of 5.5 mm and 6.5 mm were introduced into polyurethane blocks with a density of 10 PCF (0,16 g/cm³). According to the experimental group, screws were inserted without pilot hole, with pilot without tapping, undertapping and line-to-line tapping. Screw pullout tests were performed using a universal test machine after screw insertion into polyurethane blocks.

Results:

Screws inserted directly into the polyurethane blocks without pilot hole and tapping showed a statistically higher pullout strength. Insertion of the screw without tapping or with undertapping increases the pullout screw strength compared to line-to-line tapping.

Conclusion:

DSG Screw showed the highest pullout strength after its insertion without pilot hole and tapping. Level of Evidence V, Expert Opinion.

Biomechanical comparison of sacral and transarticular sacroiliac screw fixation

STUDY DESIGN

A detailed finite element analysis of screw fixation in the sacrum and pelvis.

OBJECTIVE

To biomechanically assess and compare the fixation performance of sacral and transarticular sacroiliac screws. Instrumentation constructs are used to achieve fixation and stabilization for the treatment of spinopelvic pathologies. The optimal screw trajectory and type of bone engagement to caudally anchor long fusion constructs are not yet known.

METHODS

A detailed finite element model of the sacroiliac articulation with two different bone densities was developed. Two sacral and one transarticular sacroiliac screw trajectories were modeled with different diameters (5.5 and 6.5 mm) and lengths (uni-cortical, bi-cortical and quad-cortical purchase). Axial pullout and flexion/extension toggle forces were applied on the screws representing intra and post-operative loads. The force-displacement results and von Mises stresses were used to characterize the failure pattern.

RESULTS

Overall, sacroiliac screws provided forces to failure 2.75 times higher than sacral fixation screws. On the contrary, the initial stiffness was approximately half as much for sacroiliac screws. High stresses were located at screw tips for the sacral trajectories and near the cortical bone screw entry points for the sacroiliac trajectory. Overall, the diameter and length of the screws had significant effects on the screw fixation (33% increase in force to failure; 5% increase in initial stiffness). A 20% drop in bone mineral density (lower bone quality) decreased the initial stiffness by 25% and the force to failure by 5-10%. High stresses and failure occurred at the screw tip for uni- and tri-cortical screws and were close to trabecular-cortical bone interface for bi-cortical and quad-cortical screws.

CONCLUSIONS

Sacroiliac fixation provided better anchorage than sacral fixation. The transarticular purchase of the sacroiliac trajectory resulted in differences in failure pattern and fixation performance.

Pullout Strength After Multiple Reinsertions in Radial Bone Fixation

Background: Due to bone cutting loss from self-tapping screws (STS), progressive destruction of bone can occur with each reinsertion during surgery. When considering the use of jigs that utilize multiple insertions such as those seen in ulnar and radial shortening osteotomy systems, or scenarios where a screw needs to be removed and reinserted due to some technical issue, this can be concerning, as multiple studies examining the effects of multiple reinsertions and the relationship between insertional torque and pullout strength have had mixed results. Methods: Insertional torque and pullout strength were experimentally measured following multiple reinsertions of STS for up to 5 total insertions for various densities and locations along radial sawbone shafts. Results: Torque and pullout strength were significantly greater in middle segments of the radial shaft. Our trials corroborate previous literature regarding a significant reduction in fixation between 1 and 2 insertions; beyond this, there was no significant difference between pullout strength across all segment locations as well as bone densities for 3 to 5 insertions. There was a moderate to high correlation of insertional torque to pullout strength noted across all bone densities and segments (Pearson r = 0.663, P < .001). Conclusion: While reinsertion of STS between 1 and 2 insertions has been shown to significantly differ in pullout strength, beyond this, there does not appear to be a significant difference in up to 5 insertions at any specific region of radial bone across a range of sawbone densities. Further insertions may be considered with caution.

Helical blade compression failure occurred during PFNA implantation: A rare case and ingenious solution

RATIONALE

Femoral intertrochanteric fracture happens easily in the elderly, especially those with osteoporosis. As a standard intramedullary fixation implant, Proximal Femoral Nail Anti-rotation (PFNA) is applied to various types of stable or unstable intertrochanteric fractures of femur. Due to blade-related factors, such as cutting-through into the hip joint, cutting out or back out, there are endless postoperative failure cases of internal fixation, but reports about perioperative failure that the helical blade cannot be tightened are lacking. In this case, we firstly report an intraoperative blade compression failure and an effective technique to help surgeons out of the dilemma by using cannulated hexagonal screwdriver which was already included in the orthopedic instrument box.

PATIENT CONCERNS

An 81-year-old lady suffered left hip pain, swelling and limitation of activity, after slipping and falling when she was in the toilet.

DIAGNOSES

X-ray and computed tomography (CT)-scan showed comminuted left intratrochanteric fracture with a Jensen classification of IIa type.

INTERVENTIONS

The patient was treated by closed reduction and internal fixation with PFNA. We suffered an unprecedented problem that helical blade cannot be tightened by the blade impactor as usual. For fear of helical blade disintegration during removal and a significant decrease in pullout strength after reinsertions, we eliminated the dilemma by using a cannulated hexagonal screwdriver to tighten the "problem" helical blade.

OUTCOMES

The "problem" helical blade was finally locked by using the SW4.0 mm cannulated hexagonal screwdriver which was already included in the instrument box. The operation ended successfully after completing subsequent steps.

LESSONS

The cannulated hexagonal screwdriver is an effective instrument that can help surgeons out of the dilemma when the blade impactor fails to tighten the helical blade in PFNA implantation.

BIOMECHANICS OF BONE BLOCK GRAFT MODELS OF DIFFERENT GEOMETRY

OBJECTIVE

To evaluate the mechanical behavior of different geometry bone block grafts in wooden models.

METHODS

Constructs with rectangular (G1) and trapezoidal (G2) profile "grafts", fixed with 3.5 mm 8-hole dynamic compression plates were submitted to non-destructive bending, with the load applied alternately on the same surface as that of the plate fixation (upper) and on the opposite surface (lower), and torsion tests. A 50 N maximum load for bending and a 5° maximum deformation for torsion were considered. Rigidity (N/mm) was recorded for the former and torque (N.m) was recorded for the latter.

RESULTS

Rigidity was consistently higher in G2 than in G1, but not significantly so for all comparisons. The exception was for the load applied on the same surface of plate fixation, significantly higher in G1 than in G2. Torque was higher in G1, but not significantly so.

CONCLUSION

The two different-profile "grafts" present a similar mechanical behavior and can be indistinctly used in clinical practice. Level of evidence V, specialist's opinion based on basic studies.

Useful Intraoperative Technique for Percutaneous Stabilization of Bilateral Posterior Pelvic Ring Injuries

Treating patients with bilateral posterior pelvic ring injuries can be challenging. Placement of transiliac-transsacral style screws in available S1 or S2 osseous fixation pathways is becoming an increasingly common fixation method for these unstable injuries. We propose a percutaneous technique that sequences reduction and stabilization of 1 hemipelvis with at least 1 transiliac-transsacral screw and then uses the existing transiliac-transsacral screw and accompanying guide wires to assist in temporary stabilization and definitive fixation of the second hemipelvis.

Screw Stripping After Repeated Cortical Screw Insertion-Can We Trust the Cancellous "Bailout" Screw?

BACKGROUND

During osteosynthesis standard nonlocking cortical screws often require reinsertion, raising concern over possible decrease in their effectiveness. This study aims to quantify that potential loss of fixation with reinsertions as well as examine the ability of a cancellous "bailout screw" to regain insertion torque in a previously stripped screw hole.

METHODS

Four different types of bone surrogates were chosen to represent normal cortical bone, osteoporotic cortical bone, high-density (normal) cancellous bone, and low-density (osteoporotic) cancellous bone; nonlocked 3.5-mm cortical screws were inserted into the predrilled holes 1, 2, 3, 4, or 5 times before being torqued maximally to the point of stripping. A 4.0-mm cancellous "bailout" screw was then placed into the same hole and torqued until stripping. Torque was measured continuously using a torque-measuring screwdriver and maximal insertion torque (MIT) of 3.5 and 4.0 screws before stripping was recorded.

RESULTS

MIT decreased with reinsertion of nonlocked cortical screws. By the third reinsertion in all but the normal bone surrogates, the screws lost approximately one third to one half of their original MIT (50%-71% of original torque). The bailout screw succeeded in restoring the original MIT in the osteoporotic cancellous bone surrogate and the normal cortical bone surrogate. In the normal cancellous and osteoporotic cortical bone surrogates, the bailout screw was only able to restore an average of 50% (range 31%-63%) of the original MIT.

CONCLUSIONS

Screw reinsertion may significantly reduce the MIT of 3.5-mm nonlocked cortical screws. Use of the bailout cancellous screw for a stripped cortical screw should be expected to restore MIT only in normal cortical bone and osteoporotic cancellous bone. In other scenarios, the bailout screw should not be expected to uniformly restore full insertion torque.

Short isthmic versus long trans-isthmic C2 screw: anatomical and biomechanical evaluation

INTRODUCTION

The Harms technique is now considered as the gold standard to stabilize C1-C2 cervical spine. It has been reported to decrease the risk of vertebral artery injury. However, the risk of vascular injury does not totally disappear, particularly due to the proximity of the trans-isthmic C2 screw with the foramen transversarium of C2. In order to decrease this risk of vertebral artery injury, it has been proposed to use a shorter screw which stops before the foramen transversarium.

OBJECT

The main objective was to compare the pull-out strength of long trans-isthmic screw (LS) versus short isthmic screw (SS) C2 screw. An additional morphological study was also performed.

METHOD

Thirteen fresh-frozen human cadaveric cervical spines were included in the study. Orientation, width and height of the isthmus of C2 were measured on CT scan. Then, 3.5-mm titanium screws were inserted in C2 isthmus according to the Harms technique. Each specimen received a LS and a SS. The side and the order of placement were determined with a randomization table. Pull-out strengths and stiffness were evaluated with a testing machine, and paired samples were compared using Wilcoxon signed-rank test and also the Kaplan-Meier method.

RESULTS

The mean isthmus transversal orientation was 20° ± 6°. The mean width of C2 isthmus was less than 3.5 mm in 35 % of the cases. The mean pull-out strength for LS was 340 ± 85 versus 213 ± 104 N for SS (p = 0.004). The mean stiffness for the LS was 144 ± 40 and 97 ± 54 N/mm for the SS (p = 0.02).

DISCUSSION

The pull-out strength of trans-isthmic C2 screws was significantly higher (60 % additional pull-out resistance) than SSs. Although associated with an inferior resistance, SSs may be used in case of narrow isthmus which contraindicates 3.5-mm screw insertion but does not represent the first option for C2 instrumentation.

LEVEL OF EVIDENCE

Level V.

In vitro biomechanical study of pedicle screw pull-out strength based on different screw path preparation techniques

BACKGROUND

Poor screw-to-bone fixation is a clinical problem that can lead to screw loosening. Under-tapping (UT) the pedicle screw has been evaluated biomechanically in the past. The objective of the study was to determine if pedicle preparation with a sequential tapping technique will alter the screw-to-bone fixation strength using a stress relaxation testing loading protocol.

MATERIALS AND METHODS

Three thoracolumbar calf spines were instrumented with pedicle screws that were either probed, UT, standard-tapped (ST), or sequential tapped to prepare the pedicle screw track and a stress relaxation protocol was used to determine pull-out strength. The maximum torque required for pedicle screw insertion and pull-out strength was reported. A one-way ANOVA and Tukeys post-hoc test were used to determine statistical significance.

RESULTS

The pedicle screw insertion torques for the probed, UT, ST and sequentially tapped (SQT) techniques were 5.09 (±1.08) Nm, 5.39 (±1.61) Nm, 2.93 (±0.43) Nm, and 3.54 (±0.67) Nm, respectively. There is a significant difference between probed compared to ST (P ≤ 0.05), as well as UT compared to both ST and SQT (P ≤ 0.05). The pull-out strength for pedicle screws for the probed, UT, ST and SQT techniques was 2443 (±782) N, 2353(±918) N, 2474 (±521) N, and 2146 (±582) N, respectively, with no significant difference (P ≥ 0.05) between techniques.

CONCLUSIONS

The ST technique resulted in the highest pull-out strength while the SQT technique resulted in the lowest. However, there was no significant difference in the pull-out strength for the various preparation techniques and there was no correlation between insertion torque and pull-out strength. This suggests that other factors such as bone density may have a greater influence on pull-out strength.

Effects of rod reduction on pedicle screw fixation strength in the setting of Ponte osteotomies

BACKGROUND CONTEXT

The use of a rod reduction device can have deleterious consequences on pedicle screw pullout strength (POS) in the thoracic spine. However, posterior-only osteotomies in the thoracic spine are often performed to improve flexibility of the spine and offset forces of deformity correction maneuvers.

PURPOSE

To investigate the effect on pedicle screw POS caused by the rod reduction technique in the presence of facet osteotomies in the thoracic spine.

STUDY DESIGN/SETTING

The study is a biomechanical study using human cadaveric spine specimens.

METHODS

Thoracic Ponte osteotomies were performed on 3 thoracic levels in 15 cadaveric specimens. The right rod was contoured with a 5-mm residual gap at the middle level and was reduced using a rod reduction device. On the left side (paired control), a rod with no mismatch was placed. Biomechanical testing was performed with tensile load to failure "in line" with the screw axis and POS measured in Newtons (N).

RESULTS

After rod reduction, thoracic pedicle screw POS was significantly decreased (40%) compared with the control (419±426 N vs. 708±462 N, p=.002) and remained statistically significant after adjusting for bone mineral density (BMD) (p=.05). Eleven (73%) of the pedicle screws had visible pullout/failure during the reduction attempt and occurred irrespective of BMD.

CONCLUSIONS

Despite thoracic Ponte osteotomies and increased flexibility of the spinal segments, the rod reduction device still significantly decreased pedicle screw POS, typically resulting in outright failure of the screw-bone interface. Therefore, rod reduction technique of any kind should be performed with caution as it frequently results in suboptimal pedicle screw fixation.

Pedicle screw reinsertion using previous pilot hole and trajectory does not reduce fixation strength

STUDY DESIGN

Fresh-frozen human cadaveric biomechanical study.

OBJECTIVE

To evaluate the biomechanical consequence of pedicle screw reinsertion in the thoracic spine.

SUMMARY OF BACKGROUND DATA

During pedicle screw instrumentation, abnormal appearance on fluoroscopic imaging or low current reading with intraoperatively evoked electromyographic stimulation of a pedicle screw warrants complete removal to reassess for pedicle wall violation or screw malposition. However, screw fixation strength has never been evaluated biomechanically after reinsertion using a previous pilot hole and trajectory.

METHODS

Thirty-one thoracic individual fresh-frozen human cadaveric vertebral levels were instrumented bilaterally with 5.5-mm titanium polyaxial pedicle screws, and insertional torque (IT) was measured with each revolution. A paired comparison was performed for each level. Screw reinsertion was performed by completely removing the pedicle screw, palpating the tract, and then reinserting along the same trajectory. Screws were tensile loaded to failure "in-line" with the screw axis.

RESULTS

There was no significant difference for pedicle screw pullout strength (POS) between reinserted and control screws (732 ± 307 N vs. 742 ± 320 N, respectively; P = 0.78). There was no significant difference in IT between initial insertion for the test group (INI) (0.82 ± 0.40 N·m) and control (0.87 ± 0.50 N·m) (P = 0.33). IT for reinserted screws (0.58 ± 0.47 N·m) had significantly decreased compared with INI and control screws (29% decrease, P = 0.00; 33% decrease, P = 0.00, respectively). The test group screws in the thoracic spine had significant correlations between initial IT and POS (r = 0.79, P = 0.00), and moderate correlations between reinsertion IT and POS in the thoracic spine (r = 0.56, P = 0.00).

CONCLUSION

Despite a significant reduction in pedicle screw IT, there was no significant difference in pedicle screw POS with reinsertion. Therefore, when surgeons must completely remove a pedicle screw for tract inspection, reinsertion along the same trajectory may be performed without significantly compromising fixation strength.

LEVEL OF EVIDENCE

N/A.

The biomechanical consequences of rod reduction on pedicle screws: should it be avoided?

BACKGROUND CONTEXT

Rod contouring is frequently required to allow for appropriate alignment of pedicle screw-rod constructs. When residual mismatch is still present, a rod persuasion device is often used to achieve further rod reduction. Despite its popularity and widespread use, the biomechanical consequences of this technique have not been evaluated.

PURPOSE

To evaluate the biomechanical fixation strength of pedicle screws after attempted reduction of a rod-pedicle screw mismatch using a rod persuasion device.

METHODS

Fifteen 3-level, human cadaveric thoracic specimens were prepared and scanned for bone mineral density. Osteoporotic (n=6) and normal (n=9) specimens were instrumented with 5.0-mm-diameter pedicle screws; for each pair of comparison level tested, the bilateral screws were equal in length, and the screw length was determined by the thoracic level and size of the vertebra (35 to 45 mm). Titanium 5.5-mm rods were contoured and secured to the pedicle screws at the proximal and distal levels. For the middle segment, the rod on the right side was intentionally contoured to create a 5-mm residual gap between the inner bushing of the pedicle screw and the rod. A rod persuasion device was then used to engage the setscrew. The left side served as a control with perfect screw/rod alignment. After 30 minutes, constructs were disassembled and vertebrae individually potted. The implants were pulled in-line with the screw axis with peak pullout strength (POS) measured in Newton (N). For the proximal and distal segments, pedicle screws on the right side were taken out and reinserted through the same trajectory to simulate screw depth adjustment as an alternative to rod reduction.

RESULTS

Pedicle screws reduced to the rod generated a 48% lower mean POS (495±379 N) relative to the controls (954±237 N) (p<.05) and significantly decreased work energy to failure (p<.05). Nearly half (n=7) of the pedicle screws had failed during the reduction attempt with visible pullout of the screw. After reduction, decreased POS was observed in both normal (p<.05) and osteoporotic (p<.05) bone. Back out and reinsertion of the screw resulted in no significant difference in mean POS, stiffness, and work energy to failure (p>.05).

CONCLUSIONS

In circumstances where a rod is not fully seated within the pedicle screw, the use of a rod persuasion device decreases the overall POS and work energy to failure of the screw or results in outright failure. Further rod contouring or correction of pedicle screw depth of insertion may be warranted to allow for appropriate alignment of the longitudinal rods.

Tapping insertional torque allows prediction for better pedicle screw fixation and optimal screw size selection

BACKGROUND CONTEXT

There is currently no reliable technique for intraoperative assessment of pedicle screw fixation strength and optimal screw size. Several studies have evaluated pedicle screw insertional torque (IT) and its direct correlation with pullout strength. However, there is limited clinical application with pedicle screw IT as it must be measured during screw placement and rarely causes the spine surgeon to change screw size. To date, no study has evaluated tapping IT, which precedes screw insertion, and its ability to predict pedicle screw pullout strength.

PURPOSE

The objective of this study was to investigate tapping IT and its ability to predict pedicle screw pullout strength and optimal screw size.

STUDY DESIGN

In vitro human cadaveric biomechanical analysis.

METHODS

Twenty fresh-frozen human cadaveric thoracic vertebral levels were prepared and dual-energy radiographic absorptiometry scanned for bone mineral density (BMD). All specimens were osteoporotic with a mean BMD of 0.60 ± 0.07 g/cm(2). Five specimens (n=10) were used to perform a pilot study, as there were no previously established values for optimal tapping IT. Each pedicle during the pilot study was measured using a digital caliper as well as computed tomography measurements, and the optimal screw size was determined to be equal to or the first size smaller than the pedicle diameter. The optimal tap size was then selected as the tap diameter 1 mm smaller than the optimal screw size. During optimal tap size insertion, all peak tapping IT values were found to be between 2 in-lbs and 3 in-lbs. Therefore, the threshold tapping IT value for optimal pedicle screw and tap size was determined to be 2.5 in-lbs, and a comparison tapping IT value of 1.5 in-lbs was selected. Next, 15 test specimens (n=30) were measured with digital calipers, probed, tapped, and instrumented using a paired comparison between the two threshold tapping IT values (Group 1: 1.5 in-lbs; Group 2: 2.5 in-lbs), randomly assigned to the left or right pedicle on each specimen. Each pedicle was incrementally tapped to increasing size (3.75, 4.00, 4.50, and 5.50 mm) until the threshold value was reached based on the assigned group. Pedicle screw size was determined by adding 1 mm to the tap size that crossed the threshold torque value. Torque measurements were recorded with each revolution during tap and pedicle screw insertion. Each specimen was then individually potted and pedicle screws pulled out "in-line" with the screw axis at a rate of 0.25 mm/sec. Peak pullout strength (POS) was measured in Newtons (N).

RESULTS

The peak tapping IT was significantly increased (50%) in Group 2 (3.23 ± 0.65 in-lbs) compared with Group 1 (2.15 ± 0.56 in-lbs) (p=.0005). The peak screw IT was also significantly increased (19%) in Group 2 (8.99 ± 2.27 in-lbs) compared with Group 1 (7.52 ± 2.96 in-lbs) (p=.02). The pedicle screw pullout strength was also significantly increased (23%) in Group 2 (877.9 ± 235.2 N) compared with Group 1 (712.3 ± 223.1 N) (p=.017). The mean pedicle screw diameter was significantly increased in Group 2 (5.70 ± 1.05 mm) compared with Group 1 (5.00 ± 0.80 mm) (p=.0002). There was also an increased rate of optimal pedicle screw size selection in Group 2 with 9 of 15 (60%) pedicle screws compared with Group 1 with 4 of 15 (26.7%) pedicle screws within 1 mm of the measured pedicle width. There was a moderate correlation for tapping IT with both screw IT (r=0.54; p=.002) and pedicle screw POS (r=0.55; p=.002).

CONCLUSIONS

Our findings suggest that tapping IT directly correlates with pedicle screw IT, pedicle screw pullout strength, and optimal pedicle screw size. Therefore, tapping IT may be used during thoracic pedicle screw instrumentation as an adjunct to preoperative imaging and clinical experience to maximize fixation strength and optimize pedicle "fit and fill" with the largest screw possible. However, further prospective, in vivo studies are necessary to evaluate the intraoperative use of tapping IT to predict screw loosening/complications.

Effect of pilot hole on biomechanical and in vivo pedicle screw-bone interface

PURPOSE

To experimentally study the influence of pilot hole diameter (smaller than or equal to the internal (core) diameter of the screw) on biomechanical (insertion torque and pullout strength) and histomorphometric parameters of screw-bone interface in the acute phase and 8 weeks after pedicle screw insertion.

METHODS

Fifteen sheep were operated upon and pedicle screws inserted in the L1-L3 pedicles bilaterally. The pilot hole was smaller (2.0 mm) than the internal diameter (core) of the screw on the left side pedicle and equal (2.8 mm) to the internal diameter (core) of the screw on the right side pedicle. Ten animals were sacrificed immediately (five animals were assigned to pullout strength tests and five animals were used for histomorphometric bone-screw interface evaluation). Five animals were sacrificed 8 weeks after pedicle screw insertion for histomorphometric bone-screw interface evaluation.

RESULTS

The insertion torque and pullout strength were significantly greater in pedicle screws inserted into pilot holes smaller than internal (core) diameter of the screw. Histomorphometric evaluation of bone-screw interface showed that the percentage of bone-implant contact, the area of bone inside the screw thread and the area of bone outside the screw thread were significantly higher for pilot holes smaller than the internal (core) diameter of the screw immediately after insertion and after 8 weeks.

CONCLUSION

A pilot diameter smaller than the internal (core) diameter of the screw improved the insertion torque and pullout strength immediately after screw insertion as well the pedicle screw-bone interface contact immediately and 8 weeks after screw placement in sheep with good bone mineral density.

Surgical stabilization of the spine in the osteoporotic patient

Osteoporosis affects millions of US citizens, and millions more are at risk for developing the disease. Several operative techniques are available to the spine surgeon to provide care for those affected by osteoporosis. The types of osteoporosis, common surgical complications, medical optimization, and surgical techniques in the osteoporotic spine are reviewed, with an emphasis on preoperative planning.

What is the best way to optimize thoracic kyphosis correction? A micro-CT and biomechanical analysis of pedicle morphology and screw failure

STUDY DESIGN

A human cadaveric biomechanical analysis.

OBJECTIVE

The purpose of this study was to evaluate the bone density/trabecular width of the thoracic pedicle and correlate that with its resistance against compressive loading used during correction maneuvers in the thoracic spine (i.e., cantilever bending).

SUMMARY OF BACKGROUND DATA

As surgeons perform cantilever correction maneuvers in the spine, it is common to have pedicle screws pullout or displace while placing corrective forces on the construct. Currently, surgeons either compress against the cephalad aspect of the pedicle or vice versa. We set out to establish which aspect of the pedicle was the most dense and to determine the optimal direction for screw compression during kyphosis/deformity correction.

METHODS

Fifteen fresh-frozen cadaveric vertebrae (n = 15) were examined by micro-computed tomography to determine percent bone volume/total volume (%BV/TV) within the cephalad and caudad aspects of the pedicle. Specimens were sectioned in the sagittal plane. Pedicles were instrumented according to the straightforward trajectory on both sides. Specimens were then mounted and loading to failure was performed perpendicular to the screw axis (either the cephalad or the caudad aspect of the pedicle).

RESULTS

Mean failure when loading against the caudad aspect of the pedicle was statistically, significantly greater (454.5 ± 241.3 N vs. 334.79 1 ± 158.435 N) than for the cephalad pedicle (P < 0.001). In concordance with failure data, more trabecular and cortical bones were observed within the caudad half of the pedicle compared with the cephalad half (P < 0.001).

CONCLUSION

Our results suggest that the caudad half of the pedicle is denser and withstands higher forces compared with the cephalad aspect. In turn, the incidence of intraoperative screw loosening and/or pedicle fracture may be reduced if the compressive forces (cantilever bending during deformity correction) placed upon the construct are applied against the caudad portion of the pedicle.

The biomechanics of pedicle screw-based instrumentation

There are three basic concepts that are important to the biomechanics of pedicle screw-based instrumentation. First, the outer diameter of the screw determines pullout strength, while the inner diameter determines fatigue strength. Secondly, when inserting a pedicle screw, the dorsal cortex of the spine should not be violated and the screws on each side should converge and be of good length. Thirdly, fixation can be augmented in cases of severe osteoporosis or revision. A trajectory parallel or caudal to the superior endplate can minimise breakage of the screw from repeated axial loading. Straight insertion of the pedicle screw in the mid-sagittal plane provides the strongest stability. Rotational stability can be improved by adding transverse connectors. The indications for their use include anterior column instability, and the correction of rotational deformity.