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

Morphometric analysis of the proximal thoracic pedicles in Lenke II and IV adolescent idiopathic scoliosis: an evaluation of the feasibility for pedicle screw insertion

PURPOSE

Pedicles on the concave side of the proximal thoracic (PT) curve in adolescent idiopathic scoliosis (AIS) patients with Lenke II and IV deformities tend to be narrow and dysplastic, making pedicle screw (PS) insertion challenging. The aim of this study was to evaluate the feasibility for PS placement in these patients using pedicle chord length, diameter, and channel morphology.

METHODS

In this retrospective study, 56 consecutive AIS patients with Lenke II or IV curves who underwent instrumented posterior spinal fusion (PSF) were studied. The mean age at surgery was 14.8 years and the mean PT curve measured 45°. Two independent investigators evaluated all visible pedicles from T₁ to T₆ vertebral levels using axial images from intraoperative computed tomography-guided navigation recording the pedicle: (1) maximum transverse diameter 'd' at the isthmus, (2) maximum chord length 'l', and (3) qualitative assessment of the channel morphology (types A-D).

RESULTS

Two hundred and sixty-eight concave and 264 convex pedicles were measured. The mean 'd' of the concave pedicles at T₃ and T₄ was < 3.0 mm, compared to > 5.0 mm for the convex counterparts (p < 0.001). Of all concave pedicle channels, 48% had morphology characteristics that were riskier for PS cannulation (type C or D) compared to 2% of all convex pedicle channels (type A or B) (p < 0.001).

CONCLUSION

Almost half of all concave pedicles have morphologic characteristics that make them too small to accommodate a PS. Though PSs could be inserted using an in-out-in technique in these patients, alternative fixation anchors may improve strength and safety.

A biomechanical investigation of different screw head designs for vertebral derotation in scoliosis surgery

BACKGROUND CONTEXT

The posterior pedicle screw-rod system, which is widely used to correct spinal deformities, achieves a good correction rate in the frontal and coronal planes but not in the axial plane. Direct vertebral derotation (DVD) was developed to correct axial plane deformities. However, the design of screw head and body connection, in terms of monoaxial, polyaxial, and uniplanar screw, may influence the efficiency of DVD.

PURPOSE

This study compared the efficiency of a newly designed uniplanar screw with that of monoaxial and polyaxial screws in the DVD maneuver.

STUDY DESIGN

A porcine spine model and monoaxial, polyaxial, and uniplanar screws were used to examine the biomechanics of the DVD maneuver.

METHODS

Six T7-T13 porcine thoracic spine segments were used as test specimens in this study. Pedicle screws were inserted in the left pedicles of the T9-T11 spinal segments and then connected with a rod. Three types of pedicle screws with different screw head designs (monoaxial, polyaxial, and uniplanar) were employed in this study. The material testing system (MTS) machine generated a rotational moment through the derotational tube on the T10 (apical body) pedicle screw, which simulated the motion applied during the surgical vertebral derotational procedure. The pedicle strain and the kinematics of the vertebral body and derotational tube were recorded to evaluate the derotational efficiency of different pedicle screw head designs.

RESULTS

The variances of the derotation for the monoaxial, polyaxial, and uniplanar screws were 2.22°±1.43°, 32.23°±2.26°, and 4.75°±1.60°, respectively; the derotation efficiency was 0.65, 0.51, and 0.12, respectively, when the torques of the spinal constructs reached 3 Nm. The rotational variance of the polyaxial screw was statistically greater than that of the monoaxial and uniplanar screws (p<.05). The maximum micro-strains of the pedicles for the monoaxial, polyaxial, and uniplanar screws were 1,067.45±550.35, 747.68±393.56, and 663.55±271.04, respectively, with no statistically significant differences (p>.05).

CONCLUSIONS

The screw head design played an important role in the efficiency and variance of the derotation during the DVD maneuver. The derotational efficiency of the newly designed uniplanar screw was closer to that of the monoaxial screw group than to that of the polyaxial screw group. The polyaxial screw was inferior to DVD owing to a derotational variance between the derotational tube and the apical body that was correlated with the range of motion of the screw head. In the present study, the pedicle strain was similar in all groups. However, the pedicle strain of the uniplanar screw group was lower than that of the monoaxial screw group and was similar to that of the polyaxial screw group when the angle of rotation of the apical body increased.

Intra-operative measurement of applied forces during anterior scoliosis correction

BACKGROUND

Spinal instrumentation and fusion for the treatment of scoliosis is primarily a mechanical intervention to correct the deformity and halt further progression. While implant-related complications remain a concern, little is known about the magnitudes of the forces applied to the spine during surgery, which may affect post-surgical outcomes. In this study, the compressive forces applied to each spinal segment during anterior instrumentation were measured in a series of patients with Adolescent Idiopathic Scoliosis.

METHODS

A force transducer was designed and retrofit to a routinely used surgical tool, and compressive forces applied to each segment during surgery were measured for 15 scoliosis patients. Cobb angle correction achieved by each force was measured on intra-operative fluoroscope images. Relative changes in orientation of the screw within the vertebra were also measured to detect intra-operative screw plough.

FINDINGS

Intra-operative forces were measured for a total of 95 spinal segments. The mean applied compressive force was 540N (SD 230N, range 88N-1019N). There was a clear trend for higher forces to be applied at segments toward the apex of the scoliosis. Fluoroscopic evidence of screw plough was detected at 10 segments (10.5%).

INTERPRETATION

The magnitude of forces applied during anterior scoliosis correction vary over a broad range. These forces do reach magnitudes capable of causing intra-operative vertebral body screw plough. Surgeons should be aware there is a risk for tissue overload during correction, however the clinical implications of intra-operative screw plough remain unclear. The dataset presented here is valuable for providing realistic input parameters for in silico surgical simulations.

A BIOMECHANICAL STUDY OF SHEAR LOAD ON BONE–SCREW INTERFACE OF THORACOLUMBAR VERTEBRAE

Vertebral screw failure by cutting through or pulling out of the vertebral body is common in intraoperative and postoperative stages of anterior thoracolumbar surgery, especially in osteoporotic patients. This biomechanical study was conducted to investigate the maximum shear force thoracolumbar vertebrae can withstand and the corresponding displacement of screws and analyze their correlation with vertebral bone mineral density. Forty individual vertebra specimens (T11-L3) were obtained from eight fresh adult cadaveric thoracolumbar spine specimens and randomly divided into an experimental group and a control group. Screws were placed in the center point of vertebrae and penetrated the contralateral cortex. Shear loading and axial pullout tests were conducted on the experimental group while only axial pullout test was conducted on the control group. The maximum shear forces and maximum axial pullout forces were recorded. The conditions of vertebral body destruction and screw channel were observed and the maximum axial pullout forces were recorded and analyzed. A large amount of thread bone debris was observed in the control group. In the experimental group, however, only a small amount of thread bone debris was observed; the widths of screw paths were larger than those in the control group and gradually increased from screw tips in the direction of loading. The vertebral bodies had an average shear strength of [Formula: see text][Formula: see text]N, and the corresponding average screw displacement was [Formula: see text][Formula: see text]mm. Linear regression analysis showed that the shear strength had a significant positive correlation with vertebral bone mineral density (BMD) (r[Formula: see text][Formula: see text][Formula: see text]0.958, P[Formula: see text][Formula: see text][Formula: see text]0.01), while the screw displacement had a significant negative correlation with vertebral BMD (r[Formula: see text][Formula: see text][Formula: see text]−0.933, P[Formula: see text][Formula: see text][Formula: see text]0.01). No significant difference in bone density was found between the destruction and the control groups (P[Formula: see text][Formula: see text][Formula: see text]0.05); the difference in maximum axial pullout strength between the destruction and the control groups was significant (P[Formula: see text][Formula: see text][Formula: see text]0.01). These results indicated that vertebral BMD positively correlated with the maximum shear force and negatively with the screw displacement. It is important that the corrective strength for spinal deformity may affect bone–screw interface in anterior thoracolumbar surgery.

Biomechanical comparison of endplate forces generated by uniaxial screws and monoaxial pedicle screws

Current surgical treatment of idiopathic scoliosis involves the use of various segmental instrumentation. Various pedicle screws have allowed for improved correction. Although monoaxial screws have improved rotational control compared with polyaxial screws, their use may increase screw-bone interface or vertebral endplate forces if not inserted in an exactly straight trajectory. Uniaxial screws potentially decrease these forces while retaining the advantages of monoaxial screws with respect to better rotational control. The purpose of this study was to compare the vertebral endplate forces with monoaxial or uniaxial screws when being reduced to a rod. Thirty-two plastic, surrogate T11 vertebrae were prepared with monoaxial (n=16) or uniaxial (n=16) screws. Screw angles relative to inferior vertebral endplates were assessed with lateral radiographs. The vertebrae were fixed to a load cell, and loads were measured as the screw was reduced to a rod. Monoaxial screws demonstrated a linear progression of endplate force with increasing screw angle. Uniaxial screws demonstrated minimal endplate force until approximately 20°, coinciding with screwhead excursion angle. As this maximum excursion angle was passed, uniaxial screws demonstrated a force slope similar to the monoaxial screws.The measured endplate forces should be equivalent to forces at the screw-bone interface. The reduced force with uniaxial screws is expected to have less cranial-caudal plow potential as the screw is coupled to a rod for deformity correction. This could have potential implications for screw failure, especially in less dense bone.

A titanium expandable pedicle screw improves initial pullout strength as compared with standard pedicle screws

BACKGROUND CONTEXT

Pedicle screws are now standard for spinal arthrodesis as they provide three-column spinal stabilization. Decreased vertebral body bone density because of aging reduces the stability of the bone-screw interface, potentially increasing screw pullout or pseudarthrosis. Modifications to standard pedicle screw designs to improve screw stabilization may help to compensate for the detrimental effects of decreased vertebral bone density.

PURPOSE

To evaluate differences in initial pullout strength of an expandable titanium pedicle screw as compared with a standard titanium pedicle screw.

STUDY DESIGN

In vitro human cadaveric biomechanical investigation.

METHODS

Fresh thoracolumbar spines from four human cadavers were imaged using quantitative computed tomography to obtain standard lumbar osteoporosis (Dual-energy X-ray absorptiometry [DXA]) T scores. Six bodies were sectioned per spine, and standard titanium 6.5-mm diameter pedicle screws and expandable 6.5-mm diameter titanium screws (maximum expanded diameter=10 mm) were randomized to right and left sides. Screw testing, in axial pullout at 25 mm/min, was randomized to reduce the effects of testing order. Data for stiffness (N/mm), yield load (N), ultimate load (N), and energy (N mm) (area under the load-displacement curve) were analyzed using a one-way analysis of variance (p<.05).

RESULTS

Lumbar DXA scores averaged -3.6. There were no statistical differences between screw types for stiffness. Yield load was not statistically different between groups, although the expandable screw yield load was nearly 25% greater than that of the standard screw. Ultimate load was found to be statistically greater (∼30%) for the expandable screw compared with the standard screw (p<.05). The energy required to cause bone-implant failure was also statistically greater for the expandable screw compared with the standard screw (p<.0001).

CONCLUSIONS

Expandable titanium pedicle screws demonstrated improved screw pullout stability compared with standard titanium screws in osteopenic or osteoporotic bone. Further studies are warranted examining other loading methods to evaluate the stability provided by an expandable pedicle screw.

Anatomical study of position of the rib head for placing anterior vertebral body screws in a chinese population

In this study, the variability of rib head position in a Chinese population in terms of the spinal canal and vertebral body was analyzed using computed tomography (CT). Images from transverse CT scan of the T4 to T12 vertebral bodies of 30 normal individuals were 3-dimensionally reconstructed, and analyzed for measurement of parameters that define the relative anatomic position of the rib head. We have found that the distance between the anterior border of the rib head and the posterior margin of the vertebral body, posterior safe angle, and the distance between the most inferior border of the rib head and inferior end plate in the sagittal plane gradually decrease. However, the distance between the anterior boarder of the rib head and the anterior margin of the vertebral body, transverse dimension, anterior safe angle, and the distance between the most inferior border of the rib head and superior end plate in the sagittal plane gradually increase from T4 to T12. This indicates that the position of the rib head is oriented from a more anterior position to a more posterior position and from a more superior position to a more inferior position as the number of the vertebra increases, which is different from what has been reported from western populations. Our study has identified useful parameters to define the position of the rib head, and provides a comprehensive reference guide for accurate and safe instrumentation of vertebral body screws in treating related spine diseases.

Biomechanical comparison of different anchors (foundations) for the pediatric dual growing rod technique

BACKGROUND CONTEXT

Children with early onset scoliosis benefit from early operative treatment with dual growing rods as they provide an "internal brace" for the spine and allow curve correction and sequential lengthenings to maximize thoracic cage development. The foundations that provide anchor points for the dual growing rods may incorporate hooks, screws, or a hybrid construct. It is unclear how stable different types of foundation constructs are with regard to pullout.

PURPOSE

This study was to determine the differences in stability between four foundation configurations used in the pediatric dual growing rod technique. Eight porcine spines were sectioned and randomly assigned to one of four foundation groups: 1) hook-hook with cross-link; 2) hook-screw with cross-link; 3) screw-screw with cross-link; and 4) screw-screw without cross-link. After instrumentation, biomechanical pullout tests were conducted. Ultimate failure loads, modes of failure, and level of instrumentation were analyzed.

STUDY DESIGN/SETTING

Controlled in vitro laboratory investigation.

RESULTS

Screw-screw with cross-link constructs demonstrated the greatest failure load but this was not statistically significant compared with the screw-screw without cross-link constructs. Both screw-screw constructs were statistically stronger than either construct containing hooks. There was no difference between the hook-screw and hook-hook constructs that was statistically significant. Hook-containing constructs sustained higher failure loads in lumbar versus thoracic vertebrae.

CONCLUSIONS

A foundation composed of four pedicle screws implanted in two adjacent vertebral bodies provides the strongest construct in pullout testing. A cross-link does not seem to enhance fixation. Hook constructs are stronger in lumbar versus thoracic laminae.