Are there any differences in various polyaxial locking systems? A mechanical study of different locking screws in multidirectional angular stable distal radius plates

Effect of screw angulation and multiple insertions on load-to-failure of polyaxial locking system

PURPOSE

Polyaxial locking plates rely on the alignment between the thread-to-thread connections of the screw head and the plate hole. These implants have provided substantial support for surgeons. In particular, extended screw positioning have proven to be beneficial in the fixation of challenging fractures. This study aimed to investigate the mechanical properties of ChM 5.0 ChLP polyaxial screws inserted in off-axis trajectories, including multiple insertions and to correlate these parameters with the screw head and the plate hole thread-to-thread engagement.

METHODS

Polyaxial locking screws were inserted into the plates at various angles (0°,10°,15°, -15° off-axis). Multiple time inserted screws were placed firstly at 15°, then 0° and finally -15° off-axis in the same plate hole. A microCT scan of the plate-hole and screw-head interface was conducted before destructive tests. Representative screws from each group were also examined by Scanning Electron Microscope.

RESULTS

The standard insertion at 0° sustained the greatest maximum bending strength without relocation in the screw hole. Screws inserted at 10° and 15° (one time) showed a significant reduction in load-to-failure of up to 36% and 55%, (p = 0.001) (p = 0.001) respectively. Screws inserted at -15° after a maximum of three multiple insertions with angle shift, showed a total reduction in force of up to 70% (p = 0.001). A microCT analysis of thread engagement showed significant correlations. However, the results obtained for multiple insertions were highly variable.

CONCLUSIONS

ChM 5.0 ChLP polyaxial locking system has valuable properties that foster fracture fixation, providing various surgical options. Nevertheless, the freedom of off-axis placement and multiple insertions of the screws comes at the price of reduced force. When possible surgeons should minimize the angles of insertions.

Effect of screw angulation on the bending performance of polyaxial locking interfaces: a micro-CT evaluation

Polyaxial locking plates rely on a specific thread-to-thread interface of the screw head and the plate hole. The objective of this study was to evaluate the mechanical performance of single screw interfaces when inserted off-axis and to establish correlations between those parameters and the engagement of the screw head and the plate hole thread. Three polyaxial locking screw systems were inserted into the corresponding plates at various angles (0°, 5°, 10°, and 15° off-axis). The screws were tested until failure. A micro-CT was performed to examine the interface between the plate hole and the screw head. The standard insertion at 0° sustained the greatest maximum bending strength without relocation in the screw hole. Screws inserted at 15° showed a significant reduction in force of up to 44%, 55% and 57%, respectively. Micro-CT analysis of the interface showed a significant loss of thread engagement for off-axis insertion. Polyaxial plates offer additional advantages for off-axis placement of screws. However, this flexibility is related to a significant decrease in both thread engagement and bending strength compared to monoaxial insertion. Regardless the insertion angle, the loss of stability is comparable when screws are placed off-axis. Surgeons are advised to consider off-axis insertion as a salvage option, providing access to better bone stock.

Outcome after polyaxial locking plate osteosynthesis in proximal tibia fractures: a prospective clinical trial

Background

The aim of this study was to evaluate the clinical and/or radiologic outcome using different polyaxial locking plates for the treatment of proximal tibia fractures, the Non-Contact-Briding plate (NCB-PT®) by Zimmer or the Variable Angle Locking Compression Plate (VA-LCP®) by Synthes.

Methods

This study enrolled 28 patients with proximal tibia fractures (AO/ OTA 41 B-C) and indication for locking plate osteosynthesis. All patients were treated with a polyaxial locking plate system. Depending on the fracture morphology, patients were either treated with a NCB-PT® or VA-LCP®. The implant was chosen according to the surgeon’s experience and preference, in case of a higher degree of comminution the tendency was observed to use the NCB-PT® plate. After a time interval of 12 months postoperative we conducted clinical (e.g. range of motion, the Rasmussen score) and radiological (e.g. primary/secondary loss of reduction) follow-ups.

Results

Patients provided with the NCB-PT® (9 patients) showed longer operation time, use of longer implants, longer interval from injury to surgery and lower clinical scores after the 12 months follow-up compared with the VA-LCP® group (19 patients). Interestingly, the results showed no significant differences regarding the clinical and radiologic outcome.

Conclusions

The small number of patients as well as the heterogeneity of fractures constitute a limitation of this study. Nevertheless, the differentiated use of implants is associated with comparable clinical and radiological outcomes. This trial emphasizes the need for further prospective randomised trials with higher patient numbers.

Trial registration

Retrospectively registered 21.12.2020. Registration number NCT04680247.

Is early full weight bearing safe following locking plate ORIF of distal fibula fractures?

Background

In the modern western world appraisal of economical points such as treatment and disability after trauma present a financial burden. In this context open reduction internal fixation techniques allowing for early full weight bearing might not only improve the clinical outcome but also shorten the period of disability in working life. The aim of the study was to analyze whether ORIF of ankle fractures using either a standard semitubular plate or a new polyaxial locking plate system result in a better clinical outcome.

Methods

In this prospective study, all patients with distal fibula fractures (AO 44 B1.1, B1.2, B1.3), with indication for surgery were included. Patients were randomized to either the DePuy Synthes® one-third semitubular plate (Group I) or NEWCLIP TECHNICS, Active Ankle® polyaxial locking plate (Group II). Primary outcome parameter was function of the ankle joint, assessed by the Olerud and Molander ankle score, Foot and Ankle outcome score and Karlsson and Peterson Scoring System for Ankle function. Secondary outcome parameter were postoperative complications. Superficial wound infection, delayed wound healing, mechanically prominent implant, skin irritations were considered as minor and deep wound infection, material loosening, loss of reduction were regarded as major complications requiring revision surgery. Clinical and radiological follow-up were performed 6 and 12 weeks, 6 months and 1 year postoperatively.

Results

Fifty-two patients (31 W/21 M) with a mean age of 43 yrs. (range 22–64 yrs.) were enrolled. Seven patients (13.5%) were excluded, so that 45 patients were available for follow up. Twenty-five patients were treated with DePuy Synthes® one-third semitubular plate (55.6%; group I) while 20 patients received an anatomically preformed polyaxial locking plate (44.4%, group II). Four minor complications occurred in Group I (16%) compared to two minor complications in group II (10%). Significant better clinical results regarding OMAS (p < 0.02, < 0.04), KPSS (p < 0.04) and FAOS (p < 0.02, < 0.03) were observed 6 and 12 weeks after surgery in group II.

Conclusions

The results of the presented study demonstrate a significant better clinical functional outcome in the early postoperative follow-up in patients treated with a polyaxial locking plate. Furthermore, our data show that ORIF using polyaxial locking plates in combination with an early postoperative weight bearing presents a safe, stable treatment option for ankle fractures so that patients benefit especially in the early stages of recovery.

Trial registration

Registered 20 April 2020, retrospectively on ClinicalTrails.gov (NCT04370561).

Excellent results and low complication rate for anatomic polyaxial locking plates in comminuted proximal ulna fractures

BACKGROUND

Comminuted fractures of the proximal ulna remain a great challenge in orthopedic surgery. The purpose of this study was to report the outcomes of comminuted proximal ulna fractures treated with a new construct type-anatomically preformed polyaxial locking compression plates.

METHODS

Between 2013 and 2016, 46 patients with isolated comminuted fractures were treated with a 2.7-/3.5-mm VA-LCP Olecranon Plate (Synthes, Umkirch, Germany). Of these, 44 were available for an assessment after a mean follow-up period of 2.5 years (range, 12-50 months). We evaluated range of motion, time to recovery, revision rate, and indications, as well as functional scores. Radiographs were analyzed.

RESULTS

The study included 8 Mayo IB, 29 Mayo IIB, and 7 Mayo IIIB fractures. All fractures showed bony union, and only 2 revision surgical procedures were performed because of a new trauma. No other complications occurred. Mean range of motion was 139° (range, 105°-150°), and the mean pain level was less than 1 on a visual analog scale. Regarding elbow function, we found a mean Mayo Elbow Performance Score of 98; a mean Oxford Elbow Score of 44; and a mean Disabilities of the Arm, Shoulder and Hand score of 6. Of the plates, 41% were removed after an average time of 16 months, whereby a significant improvement in extension deficit was detected (P < .01). Ninety-five percent of all patients returned to their pretrauma sports level.

CONCLUSION

The 2.7-/3.5-mm VA-LCP Olecranon Plate represents an effective option with excellent fracture reduction, sufficient stability for early postoperative functional rehabilitation, and a minimum of complications.

Evolution of plate design and material composition

The evolution of plate fixation of fracture was accompanied by advances in metallurgy and improvement in understanding of the requirements for successful fracture healing. Locked internal fixation minimizes biologic damage and when used in conjunction with minimally invasive approaches may optimize fracture healing. Some current metal locked plate constructs may actually be too stiff, and various methods including screw modification, plate hole modification, and changes in plate material composition may provide a solution to optimize fracture healing. This paper reviews the evolution of plate design and describes the early clinical experience with the use of carbon fibre reinforced reinforced polyetheretherketone composite plates.

Not All Polyaxial Locking Screw Technologies Are Created Equal: A Systematic Review of the Literature

BACKGROUND

Locking plate fixation strength relies on axial alignment of the screw axis and plate hole, with small deviations in alignment substantially decreasing the load to failure. In an effort to overcome this technical deficiency, polyaxial locking plates were designed to provide increased flexibility of screw positioning with the intent of not sacrificing fixation strength. The purpose of this article is to review the variety of polyaxial locking mechanisms currently available, to compare the biomechanical performance of these designs, and to highlight their differences, which may have clinical implications.

METHODS

A systematic review using the search terms "polyaxial locking," "variable angle locking," "polyaxial screws," and "variable angle screws" was conducted to identify all English-language articles assessing variable-angle locking screw technology. All articles directly comparing the biomechanical performance of polyaxial locking technologies were included.

RESULTS

Polyaxial locking is achieved by 5 described mechanisms: point-loading thread-in, cut-in, locking cap, expansion bushing, and screw-head expansion. With increasing insertion angulation, point-loading thread-in and cut-in designs demonstrate reduced failure strength. However, locking-cap fixation maintains consistent failure strength with increasing off-axis insertion angles.

CONCLUSIONS

Reports comparing polyaxial locking technologies are limited. The current biomechanical literature raises concerns that these mechanisms have various strengths and performance characteristics. Based on the results of the few studies that exist, it appears that locking-cap fixation provides superior biomechanical strength when compared with point-loading and cut-in designs. Additional studies are needed to assess variable-angle locking mechanisms more completely.

Polyaxial locking plates in treating distal humeral fractures: a comparative randomized trial for clinical outcome

Background

Management of distal humeral fractures remains to be one of the most challenging aspects in trauma surgery. Low profile plating systems with variable angle screw fixation represent a crucial advancement to the established angular stable locking plates with considerable attention in current research. The aim of the prospective randomized trial was to review the preliminary results and patients’ outcome following treatment with these newly developed implants and to rule out potential differences in fracture treatment of two different plating systems.

Methods

Twenty patients with distal humeral fractures (AO 13-A1 – AO 13-C3) were included in the current study since 2014. After completing the randomization plan, patients were distributed into two groups for different variable angle locking plates (DePuy Synthes® VA-LCP vs. Medartis® Aptus Elbow). Functional elbow scoring (ROM, MEPS, QuickDASH) served as primary outcome parameter, while radiological fracture consolidation served as secondary outcome parameter. Follow-ups were conducted 6 weeks, 12 weeks, 6 months and 12 months after the operation.

Results

Seventeen of 20 patients (85%) concluded all follow-up examinations. Postoperative elbow extension deficiencies showed significant differences between the two groups in all follow-up examinations with a mean of Ø 18 +/− 7.4 degrees in the DePuy Synthes® VA-LCP group compared to a mean of Ø 6.5 +/− 7.5 degrees in the Medartis® Aptus Elbow group (p = 0.002) 12 months postoperatively. Functional scoring showed a disparate pattern. The Medartis® Aptus Elbow group achieved significantly better MEP scores during follow-up. However, the analysis of the QuickDash revealed better results of the DePuy Synthes® VA-LCP group in the first half and better results of the Medartis® Aptus Elbow group in the second half of the follow-up examination instead.

Conclusions

Considering the complexity of distal humeral fractures, the usage of anatomically preshaped low profile variable angle locking plates for operative treatment leads to good clinical results. Even though there might be some advances of the Medartis® Aptus Elbow plating system concerning postoperative ROM and elbow function, a consistent difference in the overall clinical outcome between the two plating systems could not be detected.

Trial registration

https://clinicaltrials.gov/ct2/show/NCT03272490 Retrospectively Registered 1. September 2017.

Locking Cap Designs Improve Fatigue Properties of Polyaxial Screws in Upper Extremity Applications

OBJECTIVES

This study sought to examine fatigue characteristics of 2 polyaxial locking screw designs: locking cap (LC) and cross-threaded (CT). The goal was to compare LC and CT implants at 0, 10, and 15 degrees of angulation to determine the effect of locking mechanism on screw-plate interface failure. The hypothesis was that LC implants would have superior fatigue properties in comparison to CT designs and that increased angulation of the screw would have a negative impact on the fatigue life of CT implants, but would not have any effect on LC implants.

METHODS

A total of 72 screws were tested in 4 upper extremity implants. Implants were subjected to cyclic shear loads and subsequent ramp to failure. Performance characteristics were statistically compared using nonparametric statistical methods.

RESULTS

Fatigue testing demonstrated that LC designs were consistently able to sustain a significantly higher number of cyclic loads than CT designs. There were no significant differences in the number of cycles sustained by LC designs because of changes in screw angle, but CT implants exhibited decreases in screw stability with increasing angulation.

CONCLUSIONS

Likely because of the spherical screw head geometry, LC fatigue characteristics are not influenced by the orientation of the screw relative to the plate. Application of an LC in the operating room requires additional time, but provides significantly more robust fixation of the screw, especially at oblique angles to the plate and provides a more predictable and consistent biomechanical result.

Concept of variable angle locking--evolution and mechanical evaluation of a recent technology

Applications for fracture-adapted screw positioning offered by variable angle locking screws are increasing. The locking strength of the variable angle locking mechanism at different insertion angles was compared to conventional fixed angle locking screws. Stainless steel (S) and titanium (Ti) variable and fixed angle 2.4 mm locking screws, inserted at different inclinations (0°-15°), and locked at 0.8 Nm were subjected to a load-to-failure test. Ultimate failure moment at the screw-head interface and failure mode of the screws were determined. Significant differences were detected by one-way ANOVA (p < 0.05). Stainless steel and titanium variable angle locking screws inserted at 2°-10° inclination exhibited a failure moment comparable to the 0° position (S 1.67± 0.04 Nm; Ti 1.67 ± 0.14 Nm) and failed predominantly at the screw neck, with the head remaining fixed to the plate. Inserted at 15° inclination, screws revealed a lower failure moment (S 1.33 ± 0.06 Nm, Ti 1.58 ± 0.05Nm), and failed predominantly by breakout of the head thread. Fixed angle locking screws inserted at an inclination >2° did not lock properly in the plate hole, providing insufficient locking strength. Variable angle locking screws offer a stable head-locking mechanism at different inclinations, comparable to the locking strength of orthogonal inserted fixed angle locking screws. Marginal inclinations >15° should be used with care.