Mechanical performance in axial compression of a titanium polyaxial locking plate system in a fracture gap model

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.

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.

Smart fracture plate for quantifying fracture healing: Preliminary efficacy in a biomechanical model

The diagnosis of fracture nonunion following plate osteosynthesis is subjective and frequently ambiguous. Initially following osteosynthesis, loads applied to the bone are primarily transmitted through the plate. However, as callus stiffness increases, the callus is able to bear load proportional to its stiffness while forces through the plate decrease. The purpose of this study was to use a "smart" fracture plate to distinguish between phases of fracture healing by measuring forces transmitted through the plate. A wireless force sensor and small adapter were placed on the outside of a distal femoral locking plate. The adapter converts the slight bending of the plate under axial load into a transverse force which is measurable by the sensor. An osteotomy was created and then plated in the distal femur of biomechanical Sawbones. Specimens were loaded to simulate single-leg stance first with the osteotomy defect empty (acute healing), then sequentially filled with silicone (early callus) and then polymethyl methacrylate (hard callus). There was a strong correlation between applied axial load and force measured by the "smart" plate. Data demonstrate statistically significant differences between each phase of healing with as little as 150 N of axial load applied to the femur. Forces measured in the plate were significantly different between acute (100%), early callus (66.4%), and hard callus (29.5%). This study demonstrates the potential of a "smart" fracture plate to distinguish between phases of healing. These objective data may enable early diagnosis of nonunion and enhance outcomes for patients.

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.

Biomechanical Comparison of Use of Two Screws versus Three Screws Per Fragment with Locking Plate Constructs under Cyclic Loading in Compression in a Fracture Gap Model

OBJECTIVES

 The aim of this study was to measure and compare the stiffness and cyclic fatigue of two plate-bone model constructs, with either two or three locking screws per fragment, under cyclic compression.

METHODS

 A 10-hole 3.5 mm stainless steel locking compression plate (LCP) was fixed 1 mm from a synthetic bone model in which the fracture gap was 47 mm. Two groups of 10 constructs, prepared with either two or three bicortical locking screws placed at the extremities of each fragment, were tested in a load-controlled compression test until failure.

RESULTS

 The three-screw constructs were stiffer than the two-screw constructs (196.75 ± 50.48 N/mm and 102.43 ± 22.93 N/mm, respectively) and the actuator displacements of the two-screw constructs were higher (18.02 ± 1.07 mm) than those of the three-screw constructs (14.48 ± 2.25 mm). The number of cycles to failure of the two-screw constructs was significantly lower (38,337.50 ± 2,196.98) than the that of the three-screw constructs (44,224.00 ± 1,515.24). Load at irreversible deformation was significantly lower in the two-screw constructs (140.93 ± 13.39 N) than in the three-screw constructs (184.27 ± 13.17 N). All constructs failed by plate bending at the gap between the two cylinders.

CLINICAL SIGNIFICANCE

 Omission of the third innermost locking screw during bridging osteosynthesis subjected to compression forces led to a 13.3% reduction in the number of cycles to failure and a 23.5% reduction of the load withstood by the plate before plastic deformation occurred.

Stiffness of a type II external skeletal fixator and locking compression plate in a fracture gap model

OBJECTIVE

To compare the stiffness of constructs fixed with a type II external skeletal fixator (ESF) or a 3.5-mm locking compression plate (LCP) in axial compression and bending with a fracture gap model.

STUDY DESIGN

Quasi-static four-point bending and axial compression tests.

SAMPLE POPULATION

Ten LCP and 10 ESF immobilizing epoxy cylinders with a 40-mm fracture gap.

METHODS

Five constructs of each type were tested in nondestructive mediolateral (ML) four-point bending and then rotated and tested in nondestructive craniocaudal (CC) four-point bending. Five additional constructs of each type were tested in nondestructive axial compression. Stiffness was compared between loading modes by construct type and between construct types by loading mode.

RESULTS

Type II ESF were stiffer than LCP in ML bending (difference, 1474 N/mm, P < .0001) and in axial compression (difference, 458 N/mm, P = .008) but not in CC bending (P = .1673). Type II ESF were stiffer in ML bending than in CC bending (difference, 999 N/m, P < .0001), while LCP were stiffer in CC bending than in ML bending (difference, 634 N/mm, P < .0001).

CONCLUSION

Type II ESF generated stiffer constructs compared with LCP in ML bending and in axial compression without a difference in CC bending. External skeletal fixator and LCP bending stiffness varied by loading direction.

CLINICAL SIGNIFICANCE

A type II ESF should be considered in a comminuted fracture requiring increased stability in ML and axial directions.

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.

Clinical outcomes and complications of pancarpal arthrodesis stabilised with 3.5 mm/2.7 mm locking compression plates with internal additional fixation in 12 dogs

AIMS

To report the clinical outcomes and complications after the use of 3.5 mm/2.7 mm locking compression plates (LCP) with additional internal fixation for pancarpal arthrodesis (PCA) in dogs.

METHODS

This was a retrospective study using medical records from a single orthopaedic referral hospital between December 2015 and April 2018. The inclusion criteria were the use of a dorsally applied LCP for PCA in dogs with a minimum follow-up period of 12 months. Additional crossed 2.7 or 3.5 mm cortical screws or Kirschner wires were placed in all limbs to further stabilise the joints. A light dressing without external coaptation was applied postoperatively to all limbs. Postoperative lameness assessment was recorded at the last clinical evaluation.

RESULTS

Twelve dogs with 13 arthrodesed limbs were included, with carpal hyperextension injury being the most common indication for surgery (4/13; 31%). One dog was recorded with a minor complication, which was a metacarpal fracture distal to the bone plate. Major complications were observed in 4/13 (31%) limbs, with surgical site infection being recorded in all four limbs and screw loosening in one limb. No implant failure was reported. At the final clinical evaluation (43-437 days after surgery), none or mechanical lameness was recorded in 9/13 (69%) limbs, mild lameness in 3/13 (23%) limbs, and moderate lameness in one 1/13 (8%) limb.

CONCLUSIONS AND CLINICAL RELEVANCE

Locking plate and screw fixation with additional internal fixation resulted in comparable complication and infection rates in canine PCA to previous published studies using hybrid dynamic compression plates. No implant failure was reported for any of the limbs despite the use of a light dressing without external coaptation.

Single cycle to failure in bending of three titanium polyaxial locking plates

Objective: Evaluation of the bending properties in one direction of three titanium polyaxial locking plate systems.

Materials and methods: The Polyaxial Advanced Locking System (PAX®) straight plate (PAX SP), the PAX® reconstruction plate (PAX RP), and the VetLOX reconstruction plates (VetLOX) were evaluated individually and as constructs applied to a bone model simulating a fracture gap and compared using a two-way analysis of variance and Tukey posthoc analysis.

Results: The PAX SP had the highest values of bending stiffness, bending structural stiffness and bending strength. When tested as plates alone, the PAX RP and VetLOX showed no differences with regard to bending stiffness and bending structural stiffness, whilst the PAX RP had significantly higher strength. The PAX RP construct had significantly higher bending stiffness, bending structural stiffness and bending strength than the VetLOX construct.

Clinical relevance: The PAX RP and VetLOX reconstruction plates are much more likely to fail when used as bridging implants, thus adjunct support is needed. The lower bending strength of the VetLOX reconstruction plates suggests it should not be used in fractures under high loads.

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.

Biodegradable Materials for Bone Repair and Tissue Engineering Applications

This review discusses and summarizes the recent developments and advances in the use of biodegradable materials for bone repair purposes. The choice between using degradable and non-degradable devices for orthopedic and maxillofacial applications must be carefully weighed. Traditional biodegradable devices for osteosynthesis have been successful in low or mild load bearing applications. However, continuing research and recent developments in the field of material science has resulted in development of biomaterials with improved strength and mechanical properties. For this purpose, biodegradable materials, including polymers, ceramics and magnesium alloys have attracted much attention for osteologic repair and applications. The next generation of biodegradable materials would benefit from recent knowledge gained regarding cell material interactions, with better control of interfacing between the material and the surrounding bone tissue. The next generations of biodegradable materials for bone repair and regeneration applications require better control of interfacing between the material and the surrounding bone tissue. Also, the mechanical properties and degradation/resorption profiles of these materials require further improvement to broaden their use and achieve better clinical results.