Biomechanics of humeral locking plate augmented with fibular strut allograft and intramedullary strut plate using finite element analysis

 Source

·         Lee CH et al. Sci Rep. 2025 Aug 9;15(1):29211.
doi: 10.1038/s41598-025-09848-5.

 

Source

·         Locking plates have been widely used internal fixation method for proximal humeral fracture, with its risks including implant failure and associated malunion.

·         Augmentation of locking plates with fibular strut allografts increases success chance and reduces failure rate and complication rate, as it adds further construct stability, however, at the cost of technical and resource requirements to harvest the fibular graft.

·         To bypass these drawbacks, an alternative was made in the form of intramedullary strut plate (IMP), that was modeled and tested in FEA ANSYS to compare its efficacy in distributing stresses and withstanding forces, compared to LP + FA (locking plates + fibular strut allograft), LP only, and intact humerus (IH).

·         It was found that LP + IMP and LP + FA has markedly higher resistance to deformation and lower stress compared to LP alone, with LP + FA being slightly lower in material stress experienced.

 

Backgrounds

·         Proximal humeral fractures are common fractures of the elderly.

o   Preferred management is through open reduction and internal fixation with locking plate.

o   Osteoporosis or comminution increases unfavorable outcome (loss of reduction, or implant failure).

o   Medial column support is the cornerstone of proximal humeral fracture fixation, however, medial column is more accessible in 2-part fractures, but not for osteoporotic, comminuted, 3-part, and 4-part fractures.

o   Augment combinations are necessary to restore medial column support, such as using bone void fillers (fibular strut graft, cancellous allograft or autograft, bone cement), inferomedial screws or calcar screws, and medial buttress plate.

·         Locking plate + fibular strut allograft vs Locking plate alone for proximal humeral fractures.

o   Better clinical and radiological outcome.

o   During humeri cyclical loads: lesser displacements, markedly higher maximum failure load (around 1.72 to 2.00+ times higher) and stiffness (around 3.82 times higher).

o   Absence of varus collapses and plate bending, unlike in locked plate alone cases, but no broken screws in both groups.

o   Reduced relative movements at the interface under bending loads.

o   More favorable in terms of change in humeral head height (HHH) and neck-shaft angle (NSA), Constant-Murley score, VAS score, varus malunion rate, screw penetration rate, final American Shoulder and Elbow Surgeons (ASES) score, and lower risk of complications.

o   No difference in humeral head osteonecrosis rate.

·         Drawbacks of locking plate + fibular strut allograft:

o   Difficult placement and difficult procurement.

o   May have higher risk of infection.

·         An alternative to nullify the drawbacks of fibular strut allograft is the focus of this study, by employing an implant termed Intramedullary strut plate (IMP), tested with Finite Element Analysis (FEA, with ANSYS Workbench software), a widely used testing protocol across orthopedics that reduces variability factor between in vivo and vitro testing of various fixation methods.

 

Methods

·         Four computer models of humerus was made, based on Visible Human Project from NIH, including an intact humerus, fractured proximal humerus with locking plate alone, locking plate + fibular strut allograft (FA) and locking plate + IMP.

o   Humerus length of the model is 135 mm.

o   Locking plate length is 16 – 35 mm, width of 3 mm, height of 120 mm.

o   Screws used are 35 mm, diameter of 3 mm.

o   Fibular strut allograft height of 105 mm, and diameter of 10 – 15 mm (also modeled after Visible Human Project)

o   Intramedullary strut plate length is 16 mm, width of 2.5 mm, height of 105 mm.

o   The IMP has non-locking (unthreaded) screw holes, but with a diameter equals to the outermost diameter of the screw, allowing the screw to fit snugly and engage with the plate upon insertion.

o   For the fracture models, a 5 mm bone gap was introduced to simulate comminuted fracture’s instasbility, in which there is absence of direct structural support, allowing assessment of internal augmentation efficacy.

·         The simulated force include:

o   Axial, 500 N downward toward top region of the humeral head.

o   Oblique, 500 N downward the top region of the humeral head with the humerus tilted 20 degree, simulating patient’s daily walking with crutches.

o   Torsion, 10 Nm to the top of the humeral head along the long axis of the humeral head.

·         FEA materials: cortical bone, cancellous bone, plate, screw.

o   Assumed homogenous, isotropic, and linear elastic.

 

Discussion

·         The main outcomes of this study are the von Mises stresses values and displacements of the structures.

o   LP structure experienced higher von Mises stresses and displacements regardless of force applied, while FA and IMP-augmented groups have comparably markedly lower numbers (FA has lower von Misses stress values than IMP).

o   Contrary to intuitive expectations, the IH group does not have the least displacement, this is because the formula derived to determine displacement Young’s modulus inversely, yet this is a multi-material model, requiring multiple Young’s modulus, causing the IH to have relatively smaller average Young’s modulus than FA and IMP group, causing IH to not have the least displacement.

·         Limitations:

o   The model does not incorporate scenarios such as:

§  Osteoporosis

§  Different plate placement, as generally LP’s are placed 5-8 mm below the topmost level of greater tuberosity of humerus.

§  Different screw sizes.

§  Multiple plates.

§  Cancellous bone fillings in the bone gap, soft tissue integrity and other factors that influence additional stability into the fracture as opposed to true instability introduced by the fracture gap in the model.