In VitroBiomechanical Comparison of 3.5 String of Pearl Plate Fixation to 3.5 Locking Compression Plate Fixation in a Canine Fracture Gap Model

Biomechanical comparison of titanium alloy additively manufactured and conventionally manufactured plate-screw constructs

AIM

To biomechanically compare the bending stiffness, strength, and cyclic fatigue of titanium additively manufactured (AM) and conventionally manufactured (CM) limited contact plates (LCP) of equivalent dimensions using plate-screw constructs.

METHODS

Twenty-four 1.5/2.0-mm plate constructs (CM: n = 12; AM: n = 12) were placed under 4-point bending conditions. Data were collected during quasi-static single cycle to failure and cyclic fatigue testing until implants plastically deformed or failed. Bending stiffness, bending structural stiffness, and bending strength were determined from load-displacement curves. Fatigue life was determined as number of cycles to failure. Median test variables for each method were compared using the Wilcoxon rank sum test within each group. Fatigue data was also analysed by the Kaplan-Meier estimator of survival function.

RESULTS

There was no evidence for a difference in bending stiffness and bending structural stiffness between AM and CM constructs. However, AM constructs exhibited greater bending strength (median 3.07 (min 3.0, max 3.4) Nm) under quasi-static 4-point bending than the CM constructs (median 2.57 (min 2.5, max 2.6) Nm, p = 0.006). Number of cycles to failure under dynamic 4-point bending was higher for the CM constructs (median 164,272 (min 73,557, max 250,000) cycles) than the AM constructs (median 18,704 (min 14,427, max 33,228) cycles; p = 0.02). Survival analysis showed that 50% of AM plates failed by 18,842 cycles, while 50% CM plates failed by 78,543 cycles.

CONCLUSION AND CLINICAL RELEVANCE

Additively manufactured titanium implants, printed to replicate a conventional titanium orthopaedic plate, were more prone to failure in a shorter fatigue period despite being stronger in single cycle to failure. Patient-specific implants made using this process may be brittle and therefore not comparable to CM orthopaedic implants. Careful selection of their use on a case/patient-specific basis is recommended.

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.

Do locking plugs improve implant strength? Biomechanical comparison of polyaxial locking constructs with and without locking plugs in a fracture gap model

BACKGROUND

The objective of this study was to investigate the effects of locking plugs and the biomechanical properties of a 3.5 mm 8-hole polyaxial locking plate in a fracture gap model. Our hypothesis was that locking plugs would increase the strength and stiffness of the construct. Twelve 3.5 mm 8-hole plates were used to evaluate two different construct designs (with locking plugs vs. without locking plugs) with validated bone substitutes in a 25 mm bridging osteosynthesis gap model. Each construct was subjected to a single cycle four-point bending load to failure using a servo-hydraulic testing machine. Bending stiffness, bending strength, and bending structural stiffness were calculated and compared using an unpaired Student´s t-test.

RESULTS

The plating construct with locking plugs did not show any significant increase in terms of bending stiffness, bending strength, and bending structural stiffness compared to plating construct without locking plugs in a 25 mm gap fracture model during a single cycle four-point bending.

CONCLUSIONS

Under the conditions tested, filling empty plate holes with locking plugs in bridging osteosynthesis does not increase stiffness or strength of the plate-bone construct.

The effect of pearl spacing on single-cycle load-to-failure and cyclic loading parameters of 2.0 mm pearl locking plates

AIMS

To compare the mechanical performance and mode of failure in four-point bending of two different 2.0 mm "string of pearls" locking plates that differ in dimensions.

METHODS

Ten *2.0 mm, 82 mm long, 10-hole (Plate A) and ten 2.0 mm, 69 mm long, 12-hole (Plate B) Cortical Pearl Systems were secured to plate extenders and centred beneath an Instron tensile tester in four-point bending. In all constructs, a simulated fracture gap was maintained at 33 mm. Due to differences in plate dimensions, 33 mm corresponded to four pearls (Plate A) and six pearls (Plate B). Following an initial preload of 10 N, ramped single-cycle load-to-failure at 0.1 mm/second was performed in five Plate A and five Plate B constructs. Load and displacement were recorded. Constant frequency sinusoidal cyclic loading (33 N) at 20 mm/minute was performed on five Plate A and five Plate B constructs following 10 N of preload. Maximum moment and cycle count were recorded. Testing and data analysis were completed in accordance with the American Society for Testing and Materials F382-14 guidelines. Differences in performance and mode of failure were compared.

RESULTS

: Plate A constructs produced higher mean values for bending stiffness (19.8 (SD 2.0) N/mm vs. 10.1 (SD 0.6) N/mm; p < 0.001), bending structural stiffness (0.77 (SD 0.08) Nm² vs. 0.39 (SD 0.02) Nm²; p < 0.001), yield point (64.1 (SD 4.2) N vs. 54.6 (SD 3.9) N; p = 0.01), proof load (65.4 (SD 3.2) N vs. 55.6 (SD 4.0) N; p = 0.005), and bending strength (1.3 (SD 0.1) Nm vs. 1.1 (SD 0.08) Nm; p = 0.005) when compared to Plate B constructs in single cycle load-to-failure. Plate A constructs had a greater (p = 0.001) mean cycle count to failure (26,178 (SD 4,061) cycles) when compared with Plate B constructs (15,550 (SD 1,291) cycles). All plates failed by non-catastrophic plastic deformation.

CONCLUSIONS

Plate A, which is wider, thicker and has a greater spacing between pearls, was mechanically superior to Plate B in four-point bending under single-cycle load-to-failure and sinusoidal cyclic loading.

CLINICAL RELEVANCE

Although mechanical differences were identified in four-point bending, in vivo biomechanical performance remains undetermined. By selecting Plate B, the clinician may gain bone purchase through a greater number of pearls and thus screws per unit length, however, the inferior mechanical characteristics, as evaluated in four-point bending, should also be considered. Further research into the mechanical and biomechanical performance of these plating systems is warranted.

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.

Locking Plate Fixation for Canine Acetabular Fractures

OBJECTIVE

 The aim of this study was to document application of locking plates for stabilization of acetabular fractures along with complications and clinical outcomes. We hypothesized that complications and clinical outcomes would be comparable to historical reports using non-locking plate and screw constructs.

STUDY DESIGN

 Medical records of dogs presented with acetabular fractures at a single referral centre between 2010 and 2018 were reviewed. Cases surgically managed by open reduction and internal fixation, involving placement of at least one locking plate and screws, were included. Signalment, fracture classification, type of implant used, additional methods of stabilization, along with complications and neurological deficits, were collected. Long-term follow-up was based on Liverpool Osteoarthritis in Dogs questionnaire.

RESULTS

 Eighteen acetabula were repaired in 17 dogs. Locking implants were VetLOX (4/18), string-of-pearls (7/18) and locking compression plates (7/18). Locking plates and screws were used as the sole method of fixation in 10/18 acetabula, and as adjunctive fixation with other implants in 8/18 acetabula. Two minor complications and one catastrophic complication were reported. Sixteen of seventeen dogs returned to function with radiographic documentation of fracture healing. Complication rate was comparable to historic reports using non-locking implants. Long-term outcomes assessed by Liverpool Osteoarthritis in Dogs questionnaire were positive in most cases.

CONCLUSION

 Locking plates are a viable means of stabilizing acetabular fractures in dogs and can provide positive clinical outcomes. Surprisingly, problems including inaccurate fracture reduction and implant loosening were not fully avoided by use of locking implants but incidence was low.

Ex vivo biomechanical comparison of 2.7 mm string-of-pearl plate versus screw/wire/Polymethylmethacrylate composite fixation and 2.7 mm veterinary acetabular plate for repair of simulated canine acetabular fractures

Background

Acetabular fractures comprise 12–30% of canine pelvic fractures and require accurate anatomic reduction and rigid stability to ensure proper healing and minimize future osteoarthritis. Many techniques have been used to repair these fractures, with common techniques including veterinary acetabular plates or use of screw/wire/polymethylmethacrylate constructs. String-of-Pearl™ plating systems have also been used clinically but there is a lack of research supporting their use for these fractures. The purpose of this study was to compare fracture reduction accuracy, biomechanical characteristics, and mode of failure between String-of-Pearls™, veterinary acetabular plates, screw/wire/polymethylmethacrylate constructs in a simulated, ex-vivo acetabular fracture model. We hypothesized that the String-of-Pearls™ constructs would have equivalent or greater mechanical properties and reduction compared to the other constructs.

Results

The mean craniocaudal acetabular diameter before fixation (mean 25.2 mm; range 20 mm – 30.1 mm) was not significantly different from after fixation (mean 23.9 mm; range 20 mm – 28.3 mm) for any fixation method. Comparison of reduction scores between groups revealed no significant differences. No significant differences were noted for cyclical displacement or stiffness. There was significant difference with superior failure load of String-of-Pearls™ compared to screw/wire/polymethylmethacrylate in the 75th percentile of animal weight (P = 0.0021), and superior failure load of String-of-Pearls™ compared to veterinary acetabular plates in the 50th (P = 0.0232) and 75th percentiles (P = 0.0058). Stiffness of the String-of-Pearls™ construct was significantly greater than the veterinary acetabular plate construct (P = 0.0417). For ultimate load, String-of-Pearls™ constructs were significantly greater than screw/wire/polymethylmethacrylate (P = 0.0331) and veterinary acetabular plates (P = 0.0218).

Conclusion

Although the ease of application for the String-of-Pearls™ implant was subjectively better than other implants, no significant differences were found in fracture reduction scores. The String-of-Pearls™ constructs were stiffer than veterinary acetabular plates and exhibited greater failure and ultimate loads compared to veterinary acetabular plates and screw/wire/polymethylmethacrylate fixations. The String-of-Pearls™ implant appears to be a suitable fixation choice for simple canine acetabular fractures.

Retrospective evaluation of perioperative and short term clinical outcomes in appendicular long bone skeleton fractures repaired via the string of pearls (SOP) locking plate system

BACKGROUND

Internal plate fixation and, more recently, locking plate fixation are commonly used in the repair of fractures in small animal surgery. This retrospective study reviewed the use of the String of Pearls locking plate system in the fixation/repair of appendicular long bone skeleton fractures in 31 small animal veterinary patients (33 fractures).

RESULTS

Major complications necessitating revision surgery occurred in 7/33 (21%), with implant failure as the inciting cause in all cases. Variables corresponding to an unsuccessful outcome were evaluated, and a correlation was found with plates placed in a bridging manner (placed without rigid anatomic reconstruction, p = 0.02) and length of follow-up (p = 0.01).

CONCLUSIONS

The SOP plating system can be used in the repair of appendicular longbone skeletal fractures, however, the authors propose that adjunct fixation, such as intramedullary pin, double plating, or external coaptation would likely improve results and should be considered imperative in cases in which anatomic reconstruction is either not desirable or achievable.

Comparison of double locking plate constructs with single non-locking plate constructs in single cycle to failure in bending and torsion

Objective: To evaluate the biomechanical properties of single 3.5 mm broad dynamic compression plate (DCP) and double 3.5 mm String-of-Pearls (SOP) plate constructs in single-cycle bending and torsion. We hypothesized that the double SOP construct would outperform the broad DCP in both bending and torsional testing.

Methods: Broad DCP plates and double 3.5 mm SOP plates were secured to a previously validated bone model in an effort to simulate bridging osteosynthesis. Constructs were tested in both four-point bending and torsional testing.

Results: The double SOP constructs had significantly greater bending stiffness, bending strength, bending structural stiffness, and torsional stiffness when compared to the broad DCP constructs. The single broad DCP constructs had significantly higher yield torque and yield angles during torsional testing.

Clinical relevance: Although the in vitro mechanical performance of the double SOP construct was significantly greater than the single broad DCP constructs under bending loads, the actual differences were small. Various patient, fracture, and implant factors must be considered when choosing an appropriate implant for fracture fixation.

Biomechanical comparison of a 3.5-mm conical coupling plating system and a 3.5-mm locking compression plate applied as plate-rod constructs to an experimentally created fracture gap in femurs of canine cadavers

OBJECTIVE To compare stiffness and resistance to cyclic fatigue of two 3.5-mm locking system plate-rod constructs applied to an experimentally created fracture gap in femurs of canine cadavers. SAMPLE 20 femurs from cadavers of 10 mixed-breed adult dogs. PROCEDURES 1 femur from each cadaver was stabilized with a conical coupling plating system-rod construct, and the contralateral femur was stabilized with a locking compression plate (LCP)-rod construct. An intramedullary Steinmann pin was inserted in each femur. A 40-mm gap then was created; the gap was centered beneath the central portion of each plate. Cyclic axial loading with increasing loads was performed. Specimens that did not fail during cyclic loading were subjected to an acute load to failure. RESULTS During cyclic loading, significantly more LCP constructs failed (6/10), compared with the number of conical coupling plating system constructs that failed (1/10). Mode of failure of the constructs included fracture of the medial or caudal aspect of the cortex of the proximal segment with bending of the plate and pin, bending of the plate and pin without fracture, and screw pullout. Mean stiffness, yield load, and load to failure were not significantly different between the 2 methods of stabilization. CONCLUSIONS AND CLINICAL RELEVANCE Both constructs had similar biomechanical properties, but the conical coupling plating system was less likely to fail than was the LCP system when subjected to cyclic loading. These results should be interpreted with caution because testing was limited to a single loading mode.

A biomechanical comparison of conventional dynamic compression plates and string-of-pearls™ locking plates using cantilever bending in a canine Ilial fracture model

Background

Fracture of the ilium is common orthopedic injury that often requires surgical stabilization in canine patients. Of the various methods of surgical stabilization available, application of a lateral bone plate to the ilium is the most common method of fixation. Many plating options are available, each having its own advantages and disadvantages. The purpose of this study was to evaluate the biomechanical properties of a 3.5 mm String-of-Pearls™ plate and a 3.5 mm dynamic compression plate in a cadaveric canine ilial fracture model. Hemipelves were tested in cantilever bending to failure and construct stiffness, yield load, displacement at yield, ultimate load, and mode of failure were compared.

Results

The mean stiffness of dynamic compression plate (116 ± 47 N/mm) and String-of-Pearls™ plate (107 ± 18 N/mm) constructs, mean yield load of dynamic compression plate (793 ± 333 N) and String-of-Pearls™ plate (860 ± 207 N) constructs, mean displacement at yield of dynamic compression plate (8.6 ± 3.0 mm) and String-of-Pearls™ plate (10.2 ± 2.8 mm) constructs, and ultimate load at failure of dynamic compression plate (936 ± 320 N) and String-of-Pearls™ plate (939 ± 191 N) constructs were not significantly different. No differences were found between constructs with respect to mode of failure.

Conclusions

No significant biomechanical differences were found between String-of-Pearls™ plate and dynamic compression plate constructs in this simplified cadaveric canine ilial fracture model.

Effect of bending direction on the mechanical behaviour of 3.5 mm String-of-Pearls and Limited Contact Dynamic Compression Plate constructs

OBJECTIVE

To compare the bending properties of String-of-Pearls® (SOP) and Limited Contact Dynamic Compression Plate® (LC-DCP) constructs in orthogonal bending directions.

METHODS

3.5 mm SOP and LC-DCP plates were fixed to a bone model simulating a comminuted tibial fracture. Specimens were non-destructively tested in both mediolateral and craniocaudal bending for 10 cycles. Bending stiffness and total angular deformation were compared using parametric analyses (p <0.05).

RESULTS

For both constructs, stiffness was significantly less when bending moments were applied against the thickness of the plates (mediolateral bending) than against the width (craniocaudal bending). When compared to the mediolateral plane, bending constructs in the craniocaudal plane resulted in a 49% (SOP group) and 370% (LC-DCP group) increase in stiffness (p <0.001). Mediolateral bending stiffness was significantly greater in the SOP than the LC-DCP constructs. Conversely, in craniocaudal bending, SOP constructs stiffness was significantly less than that of the LC-DCP constructs. The differences between the two constructs in total angular deformation had an identical pattern of significance.

CLINICAL SIGNIFICANCE

This study found that SOP showed less variability between the orthogonal bending directions than LC-DCP in a comminuted fracture model, and also described the bi-planar bending behaviour of both constructs. Although not exhibiting identical bending properties in both planes, SOP constructs had a more homogenous bending behaviour in orthogonal loading directions. The difference between the SOP with a circular cross sectional shape compared to the rectangular shape of standard plates is probably responsible for this difference.

Biomechanical comparison of two locking plate constructs under cyclic torsional loading in a fracture gap model. Two screws versus three screws per fragment

OBJECTIVES

The number of locking screws required per fragment during bridging osteosynthesis in the dog has not been determined. The purpose of this study was to assess the survival of two constructs, with either two or three screws per fragment, under cyclic torsion.

METHODS

Ten-hole 3.5 mm stainless steel locking compression plates (LCP) were fixed 1 mm away from bone surrogates with a fracture gap of 47 mm using two bicortical locking screws (10 constructs) or three bicortical locking screws (10 constructs) per fragment, placed at the extremities of each LCP. Constructs were tested in cyclic torsion (range: 0 to +0.218 rad) until failure.

RESULTS

The 3-screws constructs (29.65 ± 1.89 N.m/rad) were stiffer than the 2-screws constructs (23.73 ± 0.87 N.m/rad), and therefore, were subjected to a greater torque during cycling (6.05 ± 1.33 N.m and 4.88 ± 1.14 N.m respectively). The 3-screws constructs sustained a significantly greater number of cycles (20,700 ± 5,735 cycles) than the 2-screws constructs (15,600 ± 5,272 cycles). In most constructs, failure was due to screw damage at the junction of the shaft and head. The remaining constructs failed because of screw head unlocking, sometimes due to incomplete seating of the screw head prior to testing.

CLINICAL SIGNIFICANCE

Omitting the third innermost locking screw during bridging osteosynthesis led to a reduction in fatigue life of 25% and construct stiffness by 20%. Fracture of the screws is believed to occur sequentially, starting with the innermost screw that initially shields the other screws.