Can high-friction intraannular material increase screw pullout strength in osteoporotic bone?

Biomechanics and finite element analysis comparing posterior T-plates with LCP for fixation of posterolateral tibial plate fractures

Objective: The treatment for posterolateral tibial plateau fractures (PTPF) have been subjects of controversy. We conducted a study to improve the fixation of PTPF through a lateral approach. Methods: We utilized 40 synthetic tibias and categorized the fracture models into five groups based on the locking compression plate (LCP) and T-distal radius plate (TPP) via various forms of fixation with screws through the posterolateral (PL) fracture fragments. I: Two-screw fixation using two locking screws (LPTL). Ⅱ: Two-screw fixation with both variable angle locking screws (LPTV). Ⅲ: One-screw fixation with one locking screw (LPOL). Ⅳ: One-screw fixation with one locking screw and two anteroposterior lag screws (LPOLTL). Ⅴ: a distal radius plate with three locking screws (TPP). Biomechanical tests were conducted to observe the axial compression displacement of the PL fracture fragments at force levels of 250 N, 500 N, and 750 N, as well as to determine the failure load and the axial stiffness for each respective group. Results: Under a 750 N load condition, the displacements within the five experimental groups exhibited the following trend: Ⅴ < Ⅱ < Ⅰ< Ⅳ < Ⅲ. However, there were no significant differences between Group V and Group II, Group I and Group IV (p > 0.05), and only Group Ⅲ demonstrated a displacement exceeding 3 mm. The failure load and the axial stiffness exhibited the same trend. Conversely, statistical significance was identified among the remaining group compared with Group Ⅲ (p < 0.05). Regarding the finite element analysis, the maximum displacements for the five models under the load of 750 N exhibited the following trend: Ⅴ < Ⅱ < Ⅰ< Ⅳ < Ⅲ. The following trends were observed in maximum von Mises stresses for these models under the load of 750 N: Ⅴ < Ⅱ < Ⅳ< Ⅰ < Ⅲ. Conclusion: It is crucial to address the inadequate mechanical strength associated with single screw fixation of LCP for fixing PL fractures in a clinical setting. The biomechanical strength of two-screw fixation surpasses that of single-screw fixation. Introducing variable-angle screws can further enhance the fixation range. Furthermore, the addition of two lag screws threaded from anterior to posterior can compensate the mechanical stability, when PL fracture is fixed with single screw in clinic.

Biomechanics of PHILOS plates in Vancouver B1 periprosthetic femoral fracture

Objective: To investigate the clinical efficacy of PHILOS plates in the treatment of Vancouver B1 periprosthetic femoral fracture (PFF) and to validate its biomechanical reliability via finite element analysis and mechanical testing on the Synbone femoral models. Methods: Ten males and eight females with Vancouver B1 PFF who underwent PHILOS plate fixation between September 2017 and January 2022 were selected. The average age was 72.61 ± 8.19 years, with a range of 57-86 years old. X-ray films were taken to assess the fracture healing situation around the femoral prosthesis as well as the position of the PHILOS plates and femoral prosthesis. Two different plates (the PHILOS plate and the Cable GTR plate) were used for fixation, and the differences in biomechanical stability of the two fixation methods were compared using finite element analysis and mechanical testing on the Synbone femoral models to validate the biomechanical dependability of the PHILOS plate. Results: All 18 cases were followed for at least 1 year, as a result. The average period of follow-up was 17 months, ranging from 12 to 36 months. At the most recent follow-up, Harris scores for the hip joints of patients ranged from 82 to 89, with an average score of 86. The X-rays revealed that all fractures surrounding the femoral prosthesis had healed and that there was no looseness in the femoral prosthesis. None of the PHILOS license plates had expired. All patients were able to perform full-load walking, and pain and claudication in affected limbs were significantly reduced. Finite element analysis and mechanical testing of the Synbone femoral model revealed that the fixation effect of the PHILOS group was superior to that of the Cable group; consequently, PHILOS plates can be used to effectively fix fractures around the proximal femoral prosthesis. Conclusion: PHILOS plates are initially used in the treatment of Vancouver B1 PFF, which may be a good choice due to their simpler operation, lower medical costs, and satisfactory clinical efficacy.

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.

Lateral migration resistance of screw is essential in evaluating bone screw stability of plate fixation

Conventional evaluation of the stability of bone screws focuses on pullout strength, while neglecting lateral migration resistance. We measured pullout strength and lateral migration resistance of bone screws and determined how these characteristics relate to screw stability of locking plate (LP) and dynamic compression plate (DCP) fixation. Pullout strength and lateral migration resistance of individual bone screws with buttress, square, and triangular thread designs were evaluated in polyurethane foam blocks. The screw types with superior performance in each of these characteristics were selected. LP and DCP fixations were constructed using the selected screws and tested under cyclic craniocaudal and torsional loadings. Subsequently, the association between individual screws’ biomechanical characteristics and fixation stability when applied to plates was established. Screws with triangular threads had superior pullout strength, while screws with square threads demonstrated the highest lateral migration resistance; they were selected for LP and DCP fixations. LPs with square-threaded screws required a larger force and more cycles to trigger the same amount of displacement under both craniocaudal and torsional loadings. Screws with triangular and square threads showed no difference in DCP fixation stability under craniocaudal loading. However, under torsional loading, DCP fixation with triangular-threaded screws demonstrated superior fixation stability. Lateral migration resistance is the primary contributor to locking screw fixation stability when applied to an LP in resisting both craniocaudal and torsional loading. For compression screws applied to a DCP, lateral migration resistance and pullout strength work together to resist craniocaudal loading, while pullout strength is the primary contributor to the ability to resist torsional loading.

Cross-elements to enhance fixation in osteoporotic bone with application to proximal humeral locking plates: a biomechanical study

BACKGROUND

Proximal humeral fractures occur predominantly in elderly, osteoporotic individuals, especially women, with surgery performed in one-fifth. Proximal humeral locking plates are the gold standard operative treatment; however, complications are frequent, partially because of poor screw purchase in osteoporotic bone. A new method uses threaded posts through which threaded cross-elements orthogonally pass to create a 3-dimensional scaffold for bone engagement. We examined the pullout characteristics of the posts with (1 or 2) or without the cross-elements and tested 2 types of 3.5-mm cortical locking screws for comparison.

METHODS

Low-density closed-cell polyurethane foam served as a model osteoporotic bone substrate. Following implantation in the substrate, the devices were axially loaded by a mechanical test system. Quantities of interest included failure mode, peak load, displacement to peak load, initial stiffness, and work expended.

RESULTS

The post groups outperformed the 3.5-mm screw groups, as expected. Relative to posts with no cross-elements, 1 and 2 cross-elements increased the peak load by 29% and 87% and increased the work to peak load by 126% and 343%, respectively. After reaching peak load, 1 and 2 cross-elements increased the work-resistance to further displacement by 158% and 330%, respectively.

CONCLUSION

Cross-elements significantly increased the ability of the threaded posts to resist axial displacement from a model osteoporotic bone substrate. This suggests that posts, used in conjunction with cross-elements, have the potential to enhance the stability of proximal humeral locking plates in osteoporotic bone.

Biomechanical and histologic assessment of a novel screw retention technology in an ovine lumbar fusion model

BACKGROUND CONTEXT

Screw loosening is a prevalent failure mode in orthopedic hardware, particularly in osteoporotic bone or revision procedures where the screw-bone engagement is limited.

PURPOSE

The objective of this study was to evaluate the efficacy of a novel screw retention technology (SRT) in an ovine lumbar fusion model.

STUDY DESIGN/SETTING

This was a biomechanical, radiographic, and histologic study utilizing an ovine lumbar spine model.

METHODS

In total, 54 (n=54) sheep lumbar spines (L₂-L₃) underwent posterior lumbar fusion (PLF) via pedicle screw fixation, connecting rod, and bone graft. Following three experimental variants were investigated: positive control (ideal clinical scenario), negative control (simulation of compromised screw holes), and SRT treatments. Biomechanical and histologic analyses of the functional spinal unit (FSU) were determined as a function of healing time (0, 3, and 12 months postoperative).

RESULTS

Screw pull-out, screw break-out, and FSU stability of the SRT treatments were generally equivalent to the positive control group and considerably better than the negative control group. Histomorphology of the SRT treatment screw region of interest (ROI) observed an increase in bone percentage and decrease in void space during healing, consistent with ingrowth at the implant interface. The PLF ROI observed similar bone percentage throughout healing between the SRT treatment and positive control. Less bone formation was observed for the negative control.

CONCLUSIONS

The results of this study demonstrate that the SRT improved screw retention and afforded effective FSU stabilization to achieve solid fusion in an otherwise compromised fixation scenario in a large animal model.