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

Intraoperative assessment of the dorso-ulnar fragment stability with stress fluoroscopy: An observational study

This study aims to investigate the effectiveness of intraoperative stress radiographs in evaluating the stability and fixation adequacy of the dorso-ulnar fragment (DUF) after volar plate application. Sixty-four patients who underwent open reduction and internal fixation due to comminuted distal radius fracture accompanied by DUF between May 2020 and February 2022 were reviewed retrospectively. Two groups were compared, with and without stress radiographs used in addition to routine fluoroscopic imaging during the surgical treatment of distal radius fractures. DUF sizes and fracture classifications were made according to preoperative computed tomography. Displacement of the DUF, dorsal cortex screw penetration, and the number of screws inserted into the DUF were evaluated on immediate postoperative CT scans and direct radiographs. DUF displacement at the patients' last follow-up was significantly higher in the control group (1.62 mm) than in the additional stress fluoroscopy applied group (0.53 mm). It was observed that the amount of displacement increased as the dorso-volar size of the DUF decreased. No significant difference was observed in dorsal cortex screw penetrations between the 2 groups. In the additional stress fluoroscopy applied group, stabilization rates with at least 1 screw over volar-locking plate for DUF were significantly higher (P < .001). Compared to the stress fluoroscopy group, the change in ulnar variance (P < .001) and volar tilt (P < .001) was significantly higher in the control group in the last follow-up radiography. No significant difference was observed between the implant removal rates of both groups. Evaluation of the stability of the DUF with stress radiographs after fixation is an effective method to reveal the need for additional fixation. Dorsal stress radiographs allow dynamic evaluation of fixation strength.

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.

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.

Distal radius fractures

The management of distal radius fractures has evolved considerably in the last two decades. Techniques and hardware have improved so much that the surgeon can usually assure good results in these debilitating fractures. Yet no one method can be used for all injuries, as the personality of each fracture demands a customized approach. Furthermore, what works in one age group may not work in another. For this reason, every available treatment has its own space in our options. The need for surgeons to be well versed in all techniques is critical. We offer a concise update of important evolutionary and current treatment guidelines for this common fracture.

Development and first biomechanical validation of a score to predict bone implant interface stability based on clinical qCT scans

Sufficient implant anchoring in osteoporotic bone is one major challenge in trauma and orthopedic surgery. In these cases, preoperative planning of osteosynthesis is becoming increasingly important. This study presents the development and first biomechanical validation of a bone-implant-anchorage score based on clinical routine quantitative computer tomography (qCT) scans. 10 pairs of fresh frozen femora (mean age 77.4 years) underwent clinical qCT scans after placing 3 referential screws (for matching with the second scan). Afterwards, three 4.5 mm cortical screws (DePuy Synthes, Zuchwil, Switzerland) were placed in each distal femur in the dia-metaphyseal transition followed by the second CT scan. The femur was segmented using thresholding and its outer shape was visualized as a surface model. A 3D model of the cortex screw in STL format was used to model the screw surface precisely. For each femur, the 3 cortex screw models were exactly positioned at the locations previously determined using the second CT scan. The BMD value was calculated at the center of each triangle as an interpolation from the measured values at the three vertices (triangle corners) in the CT. Scores are based on the sum of all the triangles’ areas multiplied by their BMD values. Four different scores were calculated. A screw pull-out test was performed until loss of resistance. A quadratic model adequately describes the relation between all the scores and pull-out values. The square of the best score explains just fewer than 70% of the total variance of the pull-out values and the standardized residual which were approximately normally distributed. In addition, there was a significant correlation between this score and the peak pull-out force (p < 0.001). The coefficient of determination was 0.82. The presented score has the potential to improve preoperative planning by adding the mechanical to the anatomical dimension when planning screw placement.

Locking plate constructs in subtrochanteric fixation: a biomechanical comparison of LCP screws and AO-nuts

OBJECTIVES

Various studies have reported the use of the 95-degree condylar blade plate in the treatment of a subtrochanteric fracture or non-union. However, the holding power of standard screws in the metaphyseal and diaphyseal area is often diminished due to osteopenia. The alternative in this area is the use of locking plates, Schühlis or AO-nuts. With the latter two, non-locking screws in the blade plate can be converted to a fixed angle fixation. The objective of this study was to compare the stiffness and strength of the AO-nut augmented 95-degree condylar blade plate construct with that of a locking plate construct. In addition, a clinical series of eight patients treated with the AO-nut augmented 95-degree condylar blade plate construct is presented.

METHODS

Single screw-plate constructs of a 5.0 mm locking screw/locking compression plate (LCP) and a 4.5 mm non-locking screw/4.5 mm dynamic compression plate (DCP), converted to a fixed-angle screw construct using AO-nuts, were tested by cantilever bending. During loading, force and displacement were recorded, from which the bending stiffness (N/mm) and the yield strength (N) were determined. Secondarily, all patients that underwent surgical treatment for subtrochanteric fracture, malunion or non-union by the senior author using this technique, underwent chart review.

RESULTS

The stiffness of the locking screws was about four times higher compared to the AO-nut augmented construct. The yield strength was 2.3 times higher for the locking screw construct. In none of the eight patients treated with the fixed-angle blade plate, failure of the AO-nut augmented construct occurred.

CONCLUSIONS

Although the stiffness and strength of the AO-nut augmented construct is less than of the locking screw, excellent clinical outcomes can be achieved utilizing this construct.

Half-threaded holes markedly increase the fatigue life of locking plates without compromising screw stability

Aims

To determine whether half-threaded screw holes in a new titanium locking plate design can substantially decrease the notch effects of the threads and increase the plate fatigue life.

Methods

Three types (I to III) of titanium locking plates were fabricated to simulate plates used in the femur, tibia, and forearm. Two copies of each were fabricated using full- and half-threaded screw holes (called A and B, respectively). The mechanical strengths of the plates were evaluated according to the American Society for Testing and Materials (ASTM) F382-14, and the screw stability was assessed by measuring the screw removal torque and bending strength.

Results

The B plates had fatigue lives 11- to 16-times higher than those of the A plates. Before cyclic loading, the screw removal torques were all higher than the insertion torques. However, after cyclic loading, the removal torques were similar to or slightly lower than the insertion torques (0% to 17.3%), although those of the B plates were higher than those of the A plates for all except the type III plates (101%, 109.8%, and 93.8% for types I, II, and III, respectively). The bending strengths of the screws were not significantly different between the A and B plates for any of the types.

Conclusion

Removing half of the threads from the screw holes markedly increased the fatigue life of the locking plates while preserving the tightness of the screw heads and the bending strength of the locking screws. However, future work is necessary to determine the relationship between the notch sensitivity properties and titanium plate design.Cite this article: Bone Joint Res 2020;9(10):645-652.

How Do the Locking Screws Lock? A Micro-CT Study of 3.5-mm Locking Screw Mechanism

OBJECTIVE

 To quantify the amount of the screw head thread and the plate hole thread connection in two 3.5 mm locking plates: Locking Compression Plate (LCP) and Polyaxial Locking System (PLS).

MATERIALS AND METHODS

 A micro - CT scan of a screw head - plate hole connection was performed pre- and post destructive tests. Tests were performed on bone surrogates in a fracture gap model. The 3.5 LCP and 3.5 PLS plates, with 3 perpendicular screws per segment were used in a destructive static test. The 3.5 PLS plates with mono- and polyaxial screws were compared in a cyclic fatigue tests in two orthogonal directions. Pre - and post - test scan datasets were compared. Each dataset was converted into serial images depicting sections cut orthogonally to locking screw axis. The amount of engagement was detected through automated image postprocessing.

RESULTS

 The mean amount of the thread connection for the LCP was 28.85% before and 18.55% after destructive static test. The mean amount of the connection for the PLS was 16.20% before and 14.55% after destructive static test. When inserted monoaxially, the mean amount of the connection for the PLS screws was 14.4% before and 19.24% after destructive cyclic test. The mean amount of the connection for the polyaxial inserted PLS screws when loaded against plate thickness was 2.99% before and 2.08% after destructive cyclic test. The mean amount of the connection for the polyaxial inserted PLS screws when loaded against plate width was 3.36% before and 3.93% after destructive cyclic test. The 3D visualization of the thread connection showed that the initial interface points between screw head and plate hole are different for both LCP and PLS after the destructive testing. Depending on the type of applied force, there was either loss or increase of the contact.

CLINICAL RELEVANCE

 Micro-CT offers news possibilities in locking implant investigation. It might be helpful in better understanding the nature of locking mechanism and prediction of possible mode of failure in different systems.

Biomechanical Behavior of a Variable Angle Locked Tibiotalocalcaneal Construct

This paper examines the mechanics of the tibiotalocalcaneal construct made with a PHILOS plating system. A failed device consisting of the LCP plate and cortical, locking, and cannulated screws was used to perform the analysis. Visual, microstructure, and fractographic examinations were carried out to characterize the fracture surface topology. These examinations revealed the presence of surface scratching, inclusions, discoloration, corrosion pits, beach marks, and cleavage and striations on the fracture surface. Further examination of the material crystallography and texture revealed an interaction of S, Ni, and Mo-based inclusions that may have raised pitting susceptibility of the device made with Stainless Steel 316L. These features suggest that the device underwent damage by pitting the corrosion-fatigue mechanism and overloading towards the end to fail the plate and screws in two or more components. The screws failed via conjoint bending and torsion fatigue mechanisms. Computer simulations of variable angle locking screws were performed in this paper. The material of construction of the device was governed by ASTM F138-8 or its ISO equivalent 5832 and exhibited inconsistencies in chemistry and hardness requirements. The failure conditions were matched in finite element modeling and those boundary conditions discussed in this paper.

Are Variable-Angle Locking Screws Stable Enough to Prevent Calcaneal Articular Surface Collapse? A Biomechanical Study

PURPOSE

To compare fixed- or variable-angle locking screws in calcaneal fracture plates using a sawbones biomechanical model.

METHODS

Intra-articular calcaneal fractures were created in synthetic bones. Reduction and stabilization was performed with 3 different plates. In group A, a fixed-angle locking plate was used with screws fastened perpendicularly to the plate (3.5 LCP, DePuySynthes). In groups B and C (2.7 VA-LCP, DePuySynthes and 3.5 Aptus, Medartis, respectively), fracture fixation was performed using variable-angle locking plates. Biomechanical testing was conducted. Displacement of the subtalar articular surface, stiffness, maximum displacement, change in the angle of Gissane, and mode of failure under cyclic loading at 200 N, 600 N, and 1000 N (500 cycles each) were determined.

RESULTS

No statistically significant difference of articular surface dislocation under cyclic loading was detected. The stiffness and maximum displacement did not reveal any disparity as well. The change in the angle of Gissane showed only minor displacement of the articular surface.

CONCLUSION

In our study, the resistance of variable-angle locking plates against articular surface displacement was similar to fixed-angle locking screws.

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.

Why and how do locking plates fail?

Locking plates have led to important changes in bone fracture management, allowing flexible biological fracture fixation based on the principle of an internal fixator. The technique has its indications and limitations. Most of the typical failure patterns arise from basic technical errors. Types of locking plates, material properties and the general principles of locking plate applications are reviewed together with their misapplication.

Biomechanical Analysis of the Proximal Femoral Locking Compression Plate: Do Quality of Reduction and Screw Orientation Influence Construct Stability?

OBJECTIVES

To investigate biomechanically in a human cadaveric model the failure modes of the proximal femoral locking compression plate and explore the underlying mechanism.

METHODS

Twenty-four fresh-frozen paired human cadaveric femora with simulated unstable intertrochanteric fractures (AO/OTA 31-A3.3) were assigned to 4 groups with 6 specimens each for plating with proximal femoral locking compression plate. The groups differed in the quality of fracture reduction and plating fashion of the first and second proximal screws as follows: (1) anatomic reduction with on-axis screw placement; (2) anatomic reduction with off-axis screw placement; (3) malreduction with on-axis screw placement; (4) malreduction with off-axis screw placement. The specimens were tested until failure using a protocol with combined axial and torsional loading. Mechanical failure was defined as abrupt change in machine load-displacement data. Clinical failure was defined as 5 degrees varus tilting of the femoral head as captured with optical motion tracking.

RESULTS

Initial axial stiffness (in N/mm) in groups 1 to 4 was 213.6 ± 65.0, 209.5 ± 134.0, 128.3 ± 16.6, and 106.3 ± 47.4, respectively. Numbers of cycles to clinical and mechanical failure were 16,642 ± 10,468 and 8695 ± 1462 in group 1, 14,076 ± 3032 and 7449 ± 5663 in group 2, 8800 ± 8584 and 4497 ± 2336 in group 3, and 9709 ± 3894 and 5279 ± 4119 in group 4. Significantly higher stiffness and numbers of cycles to both clinical and mechanical failure were detected in group 1 in comparison with group 3, P ≤ 0.044.

CONCLUSIONS

Generally, malreduction led to significantly earlier construct failure. The observed failures were cut-out of the proximal screws in the femoral head, followed by either screw bending, screw loosening, or screw fracture. Proper placement of the proximal screws in anatomically reduced fractures led to significantly higher construct stability. Our data also indicate that once the screws are placed off-axis (>5 degrees), the benefit of an anatomic reduction is lost.

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.

Head-locking durability of fixed and variable angle locking screws under repetitive loading

Polyaxial locking screws are increasingly applied in fracture fixation. To investigate the durability of the head-locking mechanism, the removal torque of variable angle (VA) and fixed angle (FA) stainless steel and titanium locking screws was investigated without and after a cyclic loading test. Stainless steel (St) and titanium (Ti) 2.4 mm orthogonally inserted FA screws and 2.4 mm VA screws inserted in different inclinations (0°-15°) (n = 6 per group) were locked at 0.8 Nm. Removal torque was determined without (W) and after (A) cyclic loading (sinusoidal load, 5 Hz, constant amplitude of 25 N, up to 10'000 cycles, or failure). Significant differences in-between the groups were detected by Student's t-test (p < 0.05). Except VA Ti in 0deg and FA, all groups exhibited a drop in removal torque below the insertion torque without and after cyclic testing. The removal torque was (St: FA W:0.81 Nm ± 0.04 A:0.72Nm ± 0.04; VA0deg W:0.73 Nm ± 0.04 A:0.65 Nm ± 0.05; VA15deg W:0.51 Nm ± 0.05 A:0.50 Nm ± 0.08; Ti: FA W:0.82 Nm ± 0.03 A:0.70 Nm ± 0.04; VA0deg W:0.80 Nm ± 0.02 A:0.72 Nm ± 0.05; VA15deg W:0.55 Nm ± 0.03 A:0.54 Nm ± 0.06). In all groups, the removal torque after cyclic testing did not drop below 16% of the removal torque without cyclic testing. No head loosening was observed after cyclic testing. Stainless steel and titanium 2.4 mm fixed and variable angle locking screws provide a stable and lasting head-locking mechanism. © 2015 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:949-952, 2016.

Periprosthetic fracture fixation in osteoporotic bone

Fixation techniques of periprosthetic fractures are far from ideal although the number of this entity is rising. The presence of an intramedullary implant generates its own fracture characteristics since stiffness is altered along the bone shaft and certain implant combinations affect load resistance of the bone. Influencing factors are cement fixation of the implant, intramedullary locking and extramedullary or intramedullary localization of the implant and the cortical thickness of the surrounding bone. Cerclage wires are ideally suited to fix radially displaced fragments around an intramedullary implant but they are susceptible to axial and torsional load. Screws should be added if these forces have to be neutralized. Stability of the screw fixation itself can be enhanced by embracement configuration around the intramedullary implant. Poor bone stock quality, often being present in metaphyseal areas limits screw fixation. Cement augmentation is an attractive option in this field to enhance screw purchase.