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

Advancements in the Management of Fragility Fractures in Orthopaedic Patients

Osteoporosis is a major risk factor for fragility fractures. The British Orthopaedics Association Standards for Trauma and Orthopaedics (BOAST) and Getting it Right First Time (GIRFT) guidelines on fragility fracture management highlight the need to initiate prompt, coordinated multidisciplinary care with a focus on early mobilisation to improve patient outcomes. Medical management of fragility fractures focuses on the prevention of progressive frailty. Advancements in medical therapy include romosozumab, recommended by the National Institute for Health and Care Excellence guidance in patients with imminent fracture risk, which improves overall bone mineral density. Regional nerve blocks are an increasingly common form of perioperative anaesthesia with fewer side effects than opioids and rates of postoperative delirium. Surgical management of osteoporotic fractures poses unique challenges, such as complex fracture patterns and increased risk of implant failure. The surgical approach to fragility fractures has undergone major advancements over the past 20 years, with developments such as polyaxial locking and far cortical locking systems that achieve secondary bone healing, as well as cement augmented screw fixation to provide stable fixation in osteoporotic bone. The development of minimally invasive surgical approaches has led to improved periosteal blood flow around a fracture site, as well as reduced operating time, hospital stay, and time to pain-free weight-bearing. In the future, we are likely to see a focus on minimally invasive surgical techniques for vertebral and pelvic fragility fractures to improve patients' mobility and independence before discharge, subsequently improving quality of life and preventing progressive frailty.

Influence of Screw Angulation on the Mechanical Properties on a Polyaxial Locking Plate Fixation

Polyaxial locking systems are widely used for strategic surgical placement, particularly in cases of osteoporotic bones, comminuted fractures, or when avoiding pre-existing prosthetics. However, studies suggest that polyaxiality negatively impacts system stiffness. We hypothesize that a new plate design, combining a narrow plate with asymmetric holes and polyaxial capabilities, could outperform narrow plates with symmetric holes. Three configurations were tested: Group 1 with six orthogonal screws, and Groups 2 and 3 with polyaxiality in the longitudinal and transverse axes, respectively. A biomechanical model assessed the bone/plate/screw interface under cyclic compression (5000 cycles) and torsion loads until failure. Screws were inserted up to 10° angle. None of the groups showed a significant loss of stiffness during compression (p > 0.05). Group 1 exhibited the highest initial stiffness, followed by Group 3 (<29%) and Group 2 (<35%). In torsional testing, Group 1 achieved the most load cycles (29.096 ± 1.342), while Groups 2 and 3 showed significantly fewer cycles to failure (6.657 ± 3.551 and 4.085 ± 1.934). These results confirm that polyaxiality, while beneficial for surgical placement, reduces biomechanical performance under torsion. Despite this, no group experienced complete decoupling of the screw-plate interface, indicating the robustness of the locking mechanism even under high stress.

Functionally graded 3D printed plates for rib fracture fixation

BACKGROUND

Design freedom offered by additive manufacturing allows for the implementation of functional gradients - where mechanical stiffness is decreased along the length of the implant. It is unclear if such changes will influence failure mechanisms in the context of rib fracture repair. We hypothesized that our novel functionally graded rib implants would be less stiff than controls and decrease occurrence of secondary fracture at implant ends.

METHODS

Five novel additively manufactured rib implants were tested along with a clinically used Control implant. Fracture reconstructions were modeled with custom synthetic rib bones with a transverse B1 fracture. Ribs were compressed in a cyclic two-point bend test for 360,000 cycles followed by a ramp to failure test. Differences in cyclic stiffness, 3D interfragmentary motions, ramp-to-failure stiffness, maximum load, and work to failure were determined.

FINDINGS

The Control group had lower construct stiffness (0.76 ± 0.28 N/mm), compared to all novel implant designs (means: 1.35-1.61 N/mm, p < 0.05) and rotated significantly more about the bending axis (2.7° ± 1.3°) than the additively manufactured groups (means between 1.2° - 1.6°, p < 0.05). All constructs failed via bone fracture at the most posterior screw hole. Experimental implants were stiffer than Controls, and there were few significant differences between functional gradient groups.

INTERPRETATION

Additively manufactured, functionally graded designs have the potential to change the form and function of trauma implants. Here, the impact of functional gradients was limited because implants had small cross-sectional areas.

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.

Failure analysis of a locking compression plate with asymmetric holes and polyaxial screws

Locking compression plates (LCP) with asymmetrical holes and polyaxial screws are effective for treating mid-femoral fractures, but are prone to failure in cases of bone nonunion. To understand the failure mechanism of the LCP, this study assessed the material composition, microhardness, metallography, fractography and biomechanical performance of a retrieved LCP used for treating a bone fracture of AO type 32-A1. For the biomechanical assessment, a finite element surgical model implanted with the intact fixation-plate system was constructed to understand the stresses and structural stiffness on the construct. Also, to avoid positioning screws around the bone fracture, different working lengths of the plate (the distance between the two innermost screws) and screw inclinations (±5°, ±10° and ±15°) were investigated. The fracture site of the retrieved LCP was divided into a narrow side and broad side due to the asymmetrical distribution of holes on the plate. The results indicated that the chemical composition and microhardness of the LCP complied with ASTM standards. The fatigue failure was found to originate on the narrow side of the hole, while the broad side showed overloading characteristics of crack growth. When the screws were inserted away from the region of the bone fracture by increasing the working length, the stress of the fixation-plate system decreased. Regardless of the screw insertion angle, the maximum stress on the LCP always appeared on the narrow side, and there was little change in the structural stiffness. However, angling the screws at -10° resulted in the most even stress distribution on the fixation-plate system. In conclusion, the LCP assessed in this study failed by fatigue fracture due to bone nonunion and stress concentration. The narrow side of the LCP was vulnerable to failure and needs to be strengthened. When treating an AO type 32-A1 fracture using an LCP with asymmetrical holes and polyaxial screws, inserting the screws at -10° may reduce the risk of implant failure and positing screws around the fractured area of the bone should be avoided.

What Is the Cost of Off-Axis Insertion of Locking Screws? A Biomechanical Comparison of a 3.5 mm Fixed-Angle and 3.5 mm Variable-Angle Stainless Steel Locking Plate Systems

OBJECTIVE

 The aim of this study was to evaluate the effect of screw insertion angle and insertion torque on the mechanical properties of a 3.5 fixed-angle locking plate locking compression plate (LCP) and 3.5 variable-angle locking plate polyaxial locking system (PLS).

METHODS

 In the LCP group, screws were placed abaxially at 0, 5 and 10 degrees. In the PLS group, screws were placed at 0, 5, 10, 15 and 20 degrees abaxially. The insertion torque was set to 1.5 and 2.5 Nm in the LCP and PLS groups respectively. A load was applied parallel to the screw axis, and the screw push-out force was measured until the locking mechanism was loosened.

RESULTS

 The 3.5 LCP showed higher push-out strength than the 3.5 PLS when the screws were placed at 0 degree regardless of the insertion torque. The off-axis insertion of 3.5 LCP locking screws resulted in a significant decrease in push-out strength (p < 0.05). A higher insertion torque value significantly increased the screw holding strength for the 3.5 LCP (p < 0.05). The 3.5 PLS system had a significantly higher push-out force when the screws are at 0 degree than at 5, 10 and 15 degrees, and 20 degrees (p < 0.05) at any given insertion torque. An increase in the insertion torque did not have a significant effect on the push-out strength of the 3.5 PLS locking system.

CONCLUSION

 The 3.5 PLS is more sensitive to the screw insertion angle than to the insertion torque, whereas the 3.5 LCP is affected by both factors. Placing 3.5 LCP locking screws off-axis significantly reduces the screw holding strength; therefore, this approach has to be avoided. The findings of our research indicate that a 1.5 Nm torque can be used for a 3.5 PLS.

Biomechanics of Osteoporotic Fracture Care: Advances in Locking Plate and Intramedullary Nail Technology

Osteoporotic fractures are extremely common and will continue to increase. Methods of internal fixation must address challenges presented by architectural changes of weakened bone. The goals of surgery are to provide mechanically stable internal fixation with minimal biologic insult that provides rapid rehabilitation and early mobilization. Novel techniques and technology that reinforce preservation of periosteal blood supply and utilization of biomechanically stable constructs diminish failure rates. Advents in locking plate technology, intramedullary nail designs, bone augmentation, and multiple implant constructs maximize strength while mitigating axial, torsional, and bending failure modes to provide optimal patient outcomes.

A cadaveric comparison of two methods for isolated talonavicular arthrodesis: Two-screws versus plate with integrated compression screw

BACKGROUND

This study compared stiffness between two constructs for talonavicular arthrodesis: a dorsomedial plating system and two partially threaded cannulated cancellous screws. We hypothesized that the plate would exhibit greater stiffness and resistance to deformation during cyclic loading.

METHODS

The constructs were implanted in eight matched pairs of cadaveric feet and subjected to axial torsion, cantilever bending in two directions, and cyclic loading to failure.

RESULTS

The two-screw constructs were significantly stiffer in plantar-dorsal bending (p = .025) and trended towards a higher number of cycles before failure than the plate group (p = .087). No significant differences were observed in internal torsion (p = .620), external torsion (p = .165), or medial-lateral bending (p = .686).

CONCLUSIONS

This study provided the first biomechanical assessment of a plating system with an integrated compression screw, which was significantly less stiff than a two-screw construct when loaded from plantar to dorsal.

Treatment of Distal Femur Fractures With the DePuy-Synthes Variable Angle Locking Compression Plate

OBJECTIVES

To determine the failure rate of the DePuy-Synthes variable angle locking compression curved condylar plate (VA-LCP) and quantify failure modes.

DESIGN

Retrospective review.

SETTING

Level I Trauma Center.

PATIENTS/PARTICIPANTS

One hundred thirteen patients with 118 OTA/AO classification 33A and 33C distal femoral fractures were included in the study.

INTERVENTION

Internal fixation using only the DePuy-Synthes VA-LCP plate.

MAIN OUTCOME MEASUREMENTS

Primary outcomes included mechanical failure rate of the DePuy-Synthes VA-LCP plate in open and closed fractures. Secondary outcomes included overall failure rate of treatment, risk factors for mechanical failure, and the specific location of failure: loss of fixation in the proximal segment, implant failure over the working length, or failure of locking screw fixation distally.

RESULTS

There were 11 total failures (9.3%) in 118 fractures. Failure rates for the closed and open fracture groups were 5.4% and 15.9%, respectively. Twenty patients (16.9%) required reoperation to promote union. Open fractures (P = 0.00475), the presence of medial metaphyseal comminution (P = 0.037), the length of the zone of comminution (P = 0.037), and plate length (P = 0.0096) were significantly higher in those with implant failure. Most failures (63.6%) were in the working length of the implant.

CONCLUSIONS

The use of the Synthes VA-LCP is a viable option in distal femoral fractures and has an acceptable failure rate and reoperation to promote union rate.

LEVEL OF EVIDENCE

Therapeutic Level IV. See Instructions for Authors for a complete description of levels of evidence.