Anterior Vertebral Body Screw Pullout Testing With The Hollow Modular Anchorage System - A Comparative in vitro Study. Hohltonnenschrauben als neues Verankerungskonzept an der Wirbelsäule - Lastauszugsversuche als biomechanische Vergleichsstudie

In vitro comparison of the pullout strength of 3 anterior double-screw fixation techniques with different screw angulations

Object

The pullout resistance of double-screw fixation systems in anterior spine surgery has been shown to be dependent on screw length as well as on screw angulation. The objective of the study was to evaluate the pullout strength of anterior double-screw systems with different angulations.

Methods

The authors conducted a comparative pullout test of converging, parallel, and diverging angulations of double-screw systems in human cadavers. Twenty-four human vertebral bodies from T-11 to L-1 were harvested from 8 donors, dissected from surrounding tissue, and matched to 3 different fixation groups. Three systems were tested: VentroFix, with near parallel screw direction; the Hopf Anterior Fixation System (HAFS), with converging screw angulation; and the ART anterior system, with diverging screw angulation.

Results

The mean (± SD) pullout strength of the VentroFix system was 699 ± 214 N, whereas the HAFS resisted to 591 ± 372 N. The ART anterior system with diverging screws demonstrated a pullout resistance of 810 ± 273 N. There was no significant difference amongst the pullout forces of the 3 groups (p > 0.05). In the HAFS and the ART anterior group, a weak correlation of pullout strength and bone mineral density measured by quantitative CT was found (r = 0.59 and r = 0.62, respectively), whereas the pullout force of the VentroFix system was not correlated with bone mineral density (r = 0.33).

Conclusions

The in vitro pullout resistance of anterior double-screw systems does not appear to depend on screw angulation.

Transsacral screw fixation for high-grade spondylolisthesis

BACKGROUND CONTEXT

Symptomatic high-grade spondylolisthesis (Meyerding III-V) is usually treated by surgery. Recent literature shows that in situ fusion is better than reduction of the slip and fusion in high-grade spondylolisthesis. Furthermore, the outcome is improved if circumferential fusion is performed in severe spondylolisthesis. We have performed a new technique of circumferential fusion in high-grade spondylolisthesis using two transsacral hollow modular anchorage (HMA) screws supplemented with pedicle screw fixation and posterolateral fusion.

PURPOSE

The aim of the study is to analyze the results of circumferential fusion using transsacral HMA screws supplemented with posterolateral fusion and pedicle screw fixation.

STUDY DESIGN

Retrospective study.

PATIENT SAMPLE

Twelve patients with high-grade spondylolisthesis were reviewed.

OUTCOME MEASURES

Outcome was measured using short form 36 (SF-36) and the ability to return to work at the most recent follow-up.

METHODS

All patients had interbody fusion using transsacral HMA screws filled with cancellous bone graft and supplemented with pedicle screw instrumentation and posterolateral fusion.

RESULTS

The male to female ratio was 2:1 with a mean age of 31 years (range 13-54 years). Eleven of 12 patients had disappearance of leg pain. There were no neurological complications in any of them. Circumferential fusion was achieved in all of them at a mean follow-up of 21 months. The average physical function score improved from 22.50+/-10.34 to 57.50+/-17.39 (p=.001, 95% confidence interval [CI] -44.48 to -25.52), whereas the average pain score improved from 22.22+/-13.40 to 61.11+/-15.35 (p=.001, 95% CI -51.12 to -26.66).

CONCLUSIONS

HMA screws avoid the complications associated with autologous cortical fibular strut graft and also are useful to promote interbody fusion, as the hollowness in the screw can be filled with cancellous bone graft that helps in better fusion. Supplementary pedicle screw fixation is necessary to protect the HMA screws, and they together give a stable construct that can achieve a circumferential fusion in high-grade spondylolisthesis.

Comparing the fixation of a novel hollow screw versus a conventional solid screw in human sacra under cyclic loading

STUDY DESIGN

The loosening rates of two monocortical pedicle screw designs (hollow and solid) were compared in a cadaveric sacrum model subjected to cyclic loading.

OBJECTIVE

To determine if a hollow screw would be more resistant to loosening than a solid pedicle screw when placed into the pedicles of S1 and tested under stair-cased cyclic loading.

SUMMARY OF BACKGROUND DATA

Screw loosening is a clinical problem for lumbosacral fusions. No previous literature has evaluated the use of a monocortical hollow screw within the sacrum; however, in other vertebral bodies, results of using this screw have been varied.

METHODS

Seven fresh-frozen cadaveric sacra were thawed and stripped of soft tissues. Solid and hollow screws were inserted contralaterally into the pedicles of S1. A materials testing machine applied alternating flexion and extension bending moments at 1 Hz, to each screw independently, via a standard connecting rod. Flexion moments were applied starting at 0.5 Nm and increased by 0.5 Nm after every 1000 cycles until the screw had visibly failed. Extension moments were maintained at 0.5 Nm. Screw rotation (flexion) relative to the sacrum was recorded using a custom optical tracking system, and analyzed using 2-way repeated measures analyses of variances and post hoc Student-Newman-Keuls tests (alpha = 0.05).

RESULTS

Screw rotation tended to gradually increase to six degrees, after which the screw was grossly loose. Overall, the hollow screw required fewer loading cycles (P = 0.004) and less applied moment (P = 0.003) to achieve the same magnitude of screw rotation as the solid screw. To achieve 6 degrees of screw rotation, the number of loading cycles were 6301 +/- 2161 and 11151 +/- 4221 for hollow and solid screws, respectively. The corresponding applied moments were 3.5 +/- 1.0 Nm and 5.8 +/- 2.0 Nm. CONCLUSION.: The novel hollow screw was less resistant to loosening when compared with a conventional solid pedicle screw in this sacral model under cyclic loading.

Pullout strength of anterior spinal instrumentation: a product comparison of seven screws in calf vertebral bodies

A lot of new implant devices for spine surgery are coming onto the market, in which vertebral screws play a fundamental role. The new screws developed for surgery of spine deformities have to be compared to established systems. A biomechanical in vitro study was designed to assess the bone-screw interface fixation strength of seven different screws used for correction of scoliosis in spine surgery. The objectives of the current study were twofold: (1) to evaluate the initial strength at the bone-screw interface of newly developed vertebral screws (Universal Spine System II) compared to established systems (product comparison) and (2) to evaluate the influence of screw design, screw diameter, screw length and bone mineral density on pullout strength. Fifty-six calf vertebral bodies were instrumented with seven different screws (USS II anterior 8.0 mm, USS II posterior 6.2 mm, KASS 6.25 mm, USS II anterior 6.2 mm, USS II posterior 5.2 mm, USS 6.0 mm, USS 5.0 mm). Bone mineral density (BMD) was determined by quantitative computed tomography (QCT). Failure in axial pullout was tested using a displacement-controlled universal test machine. USS II anterior 8.0 mm showed higher pullout strength than all other screws. The difference constituted a tendency (P = 0.108) when compared to USS II posterior 6.2 mm (+19%) and was significant in comparison to the other screws (+30 to +55%, P < 0.002). USS II posterior 6.2 mm showed significantly higher pullout strength than USS 5.0 mm (+30%, P = 0.014). The other screws did not differ significantly in pullout strength. Pullout strength correlated significantly with BMD (P = 0.0015) and vertebral body width/screw length (P < 0.001). The newly developed screws for spine surgery (USS II) show higher pullout strength when compared to established systems. Screw design had no significant influence on pullout force in vertebral body screws, but outer diameter of the screw, screw length and BMD are good predictors of pullout resistance.

Biomechanical comparison of the screw-bone interface: optimization of 1 and 2 screw constructs by varying screw diameter

STUDY DESIGN

In vitro biomechanical investigation of 1 and 2-screw anterior scoliosis constructs with varying screw diameters.

OBJECTIVE

To determine a possible optimal configuration of screw number and diameter at varying levels within the thoracic spine for anterior vertebral body fixation.

SUMMARY OF BACKGROUND DATA

Single-rod systems are typical in anterior thoracic and thoracolumbar correction of adolescent idiopathic scoliosis; although dual rod systems may offer more flexural stability. Loss of fixation remains problematic, particularly in the proximal thoracic vertebrae, and it remains unclear how screw diameter or the number of screws within the vertebrae affect fixation.

METHODS

Individual vertebral levels from 10 cadaveric thoracic spines were randomly assigned to either 1 or 2 screws of 5, 6, or 7-mm diameter. Bone-screw interface failures were created in coronal plane cantilever plow, and failure loads were compared across vertebral levels for each instrumentation method.

RESULTS

Two-screw constructs had significantly higher failure loads than single-screw constructs, while increasing screw diameter also produced significant changes in fixation strength. Two-screws had improved performance in the mid and lower thoracic spine, while a single screw was more stable in the upper thoracic spine.

CONCLUSIONS

Failure modes for 1-screw constructs almost entirely (89%) showed gradual plowing through the bone, whereas acute fracture through the vertebral body or pedicles were common forms of failure (85%) for 2-screw constructs.