Biomechanical comparison of bone-screw-fasteners versus traditional locked screws in plating female geriatric bone

Biomechanical comparison of a new undercut thread design vs the V-shape thread design for pedicle screws

BACKGROUND CONTEXT

Thread shape is regarded as an important factor influencing the fixation strength and osseointegration of bone screws. However, commercial pedicle screws with a V-shaped thread are prone to generating stress concentration at the bone-screw interface, thereby increasing the risk of screw loosening. Thus, modification of the pedicle-screw thread is imperative.

PURPOSE

This study aimed to investigate the fixation stability of pedicle screws with the new undercut thread design in comparison to pedicle screws with a V-shaped thread.

STUDY DESIGN

In vitro cadaveric biomechanical test and finite element analysis (FEA).

METHODS

Pedicle screws with the undercut thread (characterized by a flat crest feature and a tip-facing undercut feature) were custom-manufactured, whereas those with the V-shaped thread were procured from a commercial supplier. Fixation stability was assessed by the cyclic nonpullout compressive biomechanical testing on cadaveric female osteoporotic vertebrae. The vertical displacement and rotation angle of the 2 types of pedicle screws were calculated every 100 cycles to evaluate their resistance to migration and rotation. FEA was conducted to investigate the stress distribution and bone damage at the bone-screw interface for both types of pedicle screws.

RESULTS

Biomechanical testing revealed that the pedicle screws with the undercut thread exhibited significantly lower vertical displacement and rotation angles than the pedicle screws with the V-shape thread (p<0.05). FEA results demonstrated a more uniform stress distribution in the bone surrounding the thread in the undercut design than in the V-shape design. Additionally, bone damage resulting from the pedicle screw was lower in the undercut design than in the V-shape design.

CONCLUSIONS

Pedicle screws with an undercut thread are less prone to migration and rotation and thus more stable in the bone than those with a V-shape thread.

CLINICAL SIGNIFICANCE

The undercut thread design may reduce the incidence of pedicle-screw loosening.

A Novel Bone-Screw-Fastener Demonstrates Greater Maximum Compression Force Before Failure Compared With a Traditional Buttress Screw

OBJECTIVES

This study compared the maximal compression force before thread stripping of the novel bone-screw-fastener (BSF) with the traditional buttress screw (TBS) in synthetic osteoporotic and cadaveric bone models.

METHODS

The maximum compression force of the plate-bone interface before loss of screw purchase during screw tightening was measured between self-tapping 3.5-mm BSF and 3.5-mm TBS using calibrated load cells. Three synthetic biomechanical models were used: a synthetic osteoporotic diaphysis (model 1), a 3-layer biomechanical polyurethane foam with 50-10-50 pounds-per-cubic-foot layering (model 2), and a 3-layer polyurethane foam with 50-15-50 pounds-per-cubic-foot layering (model 3). For the cadaveric metaphyseal model, 3 sets of cadaveric tibial plafonds and 3 sets of cadaveric tibial plateaus were used. A plate with sensors between the bone and plate interface was used to measure compression force during screw tightening in the synthetic bone models, while an annular load cell that measured screw compression as it slid through a guide was used to measure compression in the cadaver models.

RESULTS

Across all synthetic osteoporotic bone models, the BSF demonstrated greater maximal compression force before stripping compared with the TBS [model 1, 155.51 N (SD = 7.77 N) versus 138.78 N (SD = 12.74 N), P = 0.036; model 2, 218.14 N (SD = 14.15 N) versus 110.23 N (SD = 8.00 N), P < 0.001; model 3, 382.72 N (SD = 20.15) versus 341.09 N (SD = 15.57 N), P = 0.003]. The BSF had greater maximal compression force for the overall cadaver trials, the tibial plafond trials, and the tibial plateau trials [overall, 111.27 N vs. 97.54 N (SD 32.32 N), P = 0.002; plafond, 149.6 N versus 132.92 N (SD 31.32 N), P = 0.006; plateau, 81.33 N versus 69.89 N (SD 33.38 N), P = 0.03].

CONCLUSIONS

The novel bone-screw-fastener generated 11%-65% greater maximal compression force than the TBS in synthetic osteoporotic and cadaveric metaphyseal bone models. A greater compression force may increase construct stability, facilitate early weight-bearing, and reduce construct failure.

Biomechanical comparison of compact versus standard flute drill bits, and interlocking versus buttress thread self-tapping cortical bone screws in cadaveric equine third metacarpal condyle

OBJECTIVES

To compare (1) performance of compact versus standard flute drill bits, (2) screw insertion properties and (3) pullout variables between interlocking thread (ITS) and buttress thread (BTS) self-tapping screws in third metacarpi.

STUDY DESIGN

In vitro experimental study.

SAMPLE POPULATION

Paired third metacarpi from 11 Thoroughbreds aged 2-4 years.

METHODS

Screws were inserted into the lateral condylar fossae following bone preparation using the respective drill bit for each screw type. Screw pullout was achieved using a mechanical testing system. Density and porosity of bone surrounding screw holes was measured with microcomputed tomography following each pullout test. Drilling, screw insertion and pullout variables were compared between drill bit and screw types using repeated measures ANOVA. Linear regression analyses were used to characterize relationships between bone tissue properties and drill bit and screw outcomes.

RESULTS

Maximum torque power spectral density (PSD) was lower for compact flute drill bits. Insertion torque was 50% higher for ITS. BTS had 33% greater preyield stiffness and 7% greater mean yield force. Bone tissue properties affected measured variables similarly for both screw and drill bit types.

CONCLUSIONS

Lower torque PSD may increase durability of the compact flute drill bit. ITS had greater insertional torque, which may reflect greater bone engagement. BTS had greater resistance to axial pullout forces.

CLINICAL SIGNIFICANCE

Metacarpal bone provides a simple model for comparison of drill bit and screw designs. Use of ITS to repair equine fractures subject to predominantly tensile forces is not justified based on the results of this study.

Ex vivo biomechanical evaluation of a bone-screw-fastener for tibial plateau leveling osteotomy

Introduction

The objective of this study was to investigate the effect of a novel screw type on stiffness and failure characteristics of a tibial plateau leveling osteotomy (TPLO) construct under cyclic loading conditions. The authors hypothesized that bone-screw-fasteners (BSF) would result in superior biomechanical stability compared with locking buttress screws (LBS).

Materials and Methods

Twelve pairs of canine cadaveric pelvic limbs were included in this ex vivo biomechanical study. A TPLO was performed using a 3.5mm locking TPLO plate and stabilized using either LBS or BSF. Cyclic loading was performed for 30,000 cycles at 4Hz with a peak-load of 1000N (50N valley). The cyclic test was then continued by stepwise incremental increase of peak-load at a rate of 75N per 500 cycles until failure.

Results

Cycles to failure for LBS (44,260 ± 5,770) and BSF (41,540 ± 7,686) were not significantly different (p = 0.36). Maximum force for LBS (3,134 ± 797N) and BSF (2,940 ± 831N) was not significantly different either (p = 0.58). Dynamic stiffness for LBS (1,778 ± 932 N/mm) and BSF (1,574 ± 677 N/mm) was not significantly different (p = 0.58).

Discussion

Stabilization of the TPLO with BSF provided similar biomechanical stability under cyclic axial loading conditions as the LBS. BSF may be an acceptable alternative to traditional locking screws for TPLO.

Surgical treatment of sacral nonunions

Sacral fractures are complex injuries that follow a bimodal distribution, typically involving acute high energy trauma in young adults and low energy trauma in older adults (> 65 years old). Nonunion is a rare but debilitating potential complication of undiagnosed or improperly managed sacral fractures. Various surgical techniques, including open reduction and internal fixation, sacroplasty, and percutaneous screw fixation, have been used to manage these fracture nonunions. In addition to reviewing the initial management of sacral fractures and the risk factors for fracture nonunion, this article describes techniques, specific cases and outcomes of these treatment strategies.

Fixation stability comparison of bone screws based on thread design: buttress thread, triangle thread, and square thread

Background

The influence of thread profile on the fixation stability of bone screws remains unclear. This study aimed to compare the fixation stability of screws with different thread profiles under several loading conditions.

Methods

Bone screws that differed in thread profile (buttress, triangle, and square thread) only were made of stainless steel. Their fixation stabilities were evaluated individually by the axial pullout test and lateral migration test, besides, they were also evaluated in pairs together with a dynamic compression plate and a locking plate in polyurethane foam blocks under cyclic craniocaudal and torsional loadings.

Results

The triangle-threaded and square-threaded screws had the highest pullout forces and lateral migration resistance. When being applied to a dynamic compression plate, higher forces and more cycles were required for both triangle- and square-threaded screws to reach the same displacement under cyclic craniocaudal loading. On the other hand, the triangle-threaded screws required a higher torque and more cycles to reach the same angular displacement under cyclic torsional loading. When being applied to a locking plate, the square-threaded screws needed higher load, torque, and more cycles to reach the same displacement under both cyclic craniocaudal and torsion loadings.

Conclusions

The triangle-threaded screws had superior pullout strength, while square-threaded screws demonstrated the highest lateral migration resistance. Moreover, dynamic compression plate fixation with triangle- and square-threaded screws achieved more favorable fixation stability under craniocaudal loading, while triangle-threaded screws demonstrated superior fixation stability under torsional loading. Locking plate fixation with a square-threaded screw achieved better fixation stability under both loading types.

Development and initial validation of a novel undercut thread design for locking screws

BACKGROUND

Locking screws with a typical buttress thread have high levels of failure in patients with osteoporotic bones. This study aims to develop a novel thread design for the locking screw and compare its fixation stability with the typical buttress thread.

METHODS

Locking screws with a novel thread design that possess an undercut feature and locking screws with a typical buttress thread were manufactured from stainless steel. Their fixation stabilities were then evaluated individually under a lateral migration test and evaluated in pairs together with a locking plate (LP) in an osteoporotic bone substitute under cyclic craniocaudal and torsional loadings. A finite element analysis (FEA) model was constructed to analyze the stress distributions present in the bone tissue adjacent to the novel thread versus the buttress thread.

RESULTS

The biomechanical test revealed that the novel thread had a significantly higher lateral migration strength than the buttress thread. When applied to a LP, the locking screw with the novel thread requires more cycles and higher forces or torque to resist migration up to 5 mm or 10° than the buttress thread. The FEA simulation showed that the novel thread can make the stress distribute more evenly at the adjacent bone tissue when compared with the buttress thread.

CONCLUSIONS

The locking screw with the novel undercut thread had superior lateral migration resistance during both initial and continued migration and superior fixation stability when applied to a LP under both cyclic craniocaudal loading and torsional loading than the locking screw with a typical buttress thread.