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.

Management of Canine Y-T Humeral Fractures Using Titanium Polyaxial Locking Plates in 17 Dogs

The objective of this study was to document the short- and medium-to-long-term outcomes and complication rates of Y-T humeral condylar fractures fixed using titanium polyaxial locking plate (T-PLP). A retrospective review was performed of the medical records and radiographs of dogs with a Y-T humeral condylar fracture treated with T-PLP at a single veterinary referral center (2012–2018). Seventeen cases met the inclusion criteria. Medium- to long-term follow-up (.6 mo) information was derived using the Liverpool Osteoarthritis in Dogs (LOAD) questionnaire. Recorded complications were catastrophic (1/17) and minor (2/17). Gait at 10–12 wk following surgery was subjectively assessed as good or excellent for 13 cases. Radiographic bone union was achieved in 7/12 cases at 4–6 wk. LOAD scores obtained a mean of 15 mo (range 6–29 mo) following surgery and indicated no or mild impairment in 15/16 and moderate functional impairment in 1. The application of T-PLP for the treatment of Y-T humeral condylar fractures resulted in adequate stabilization allowing successful fracture healing and medium- to long-term outcomes comparable to previous reports. According to results of LOAD testing, the medium- to long-term follow-up suggests that clients were aware of mild to moderate functional impairment in all cases.

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.