Biomechanical optimization of different fixation modes for a proximal femoral L-osteotomy

Simulated effects of surgical corrections on bone-implant micromotion and implant stresses in paediatric proximal femoral osteotomy

BACKGROUND AND OBJECTIVE

Proximal femoral osteotomy (PFO) is a surgical intervention, typically performed on paediatric population, that aims to correct femoral deformities caused by different pathologies (e.g., slipped capital femoral epiphysis). A PFO involves introduction of an implant to fix the proximal and distal sections of femur following the surgical corrections. The femoral neck-shaft angle (NSA) and anteversion angle (AVA) are key geometric parameters that influence PFO outcomes. To date, the effects of NSA and AVA on bone-implant system mechanics in paediatric populations have not been examined.

METHODS

This study used an established neuromusculoskeletal modelling process paired with finite element analysis to determine the sensitivity of the implanted femur's mechanics to variations in NSA and AVA during the stance phase of walking. Three male patients aged 9-12 years with different pathology (Spastic diplegia, Perthes disease and Slipped Capital Femoral Epiphysis), weight (377, 747, 842 N), height (1.39, 1.55, 1.71 m) and femur lengths (34.1, 39.4, 43.7 cm) and geometries (NSA: 143, 102, 111 deg; AVA: 29, 17, -22 deg) were examined. For each patient, a three-dimensional bone model was created from computed tomography imaging and digital surgical corrections were applied to systematically vary the NSA and AVA. Personalized motion and loading conditions driven from a neuromusculoskeletal modelling process were applied to each model and its associated permutations of NSA and AVA.

RESULTS

Results indicated significant intra-participant variability in post-PFO bone-implant micromotion and peak von Mises stress on implant. For models with a post-surgery NSA of 135° and AVA of 12°, the averaged micromotion increased by 87 % and the peak von Mises stress decreased by 63% between patient 1 and 2. Between patient 2 and 3, the averaged micromotion decreased by 55% while the peak von Mises stress increased by 84%.

CONCLUSIONS

Furthermore, post-PFO bone-implant mechanics were sensitive to variation in NSA and AVA in a subject-specific manner. Optimization of PFO planning is recommended based on patient-specific characteristics.

Identification of screw spacing on pediatric hip locking plate in proximal femoral osteotomy

This study describes a computational analysis technique for evaluating the effect of screw spacing and angle on the pediatric hip locking plate system in proximal femoral osteotomy in pediatric patients having DDH with an aberrant femoral head and femoral angle. Under static compressive load conditions, the stresses of the screw and bone were examined as the screw spacing and angle changed. The spacing and angle of various screws were specifically considered as variables in this study based on the pile mechanism studied in civil engineering. As with the group pile mechanism, the tighter the screw spacing under static compressive loads, the more the overlapping effect between the bone stresses and the screws develops, increasing the risk of injuring the patient's bone. Therefore, a series of simulations was performed to determine the optimal screw spacing and angles to minimize the overlapping effect of bone stress. In addition, a formula for determining the minimum screw spacing was proposed based on the computational simulation results. Finally, if the outcomes of this study are applied to pediatric patients with DDH in the pre-proximal femoral osteotomy stage, post-operative load-induced femur damage will be reduced.

Biomechanical analysis of internal fixation system stability for tibial plateau fractures

Background: Complex bone plateau fractures have been treated with bilateral plate fixation, but previous research has overemphasized evaluating the effects of internal fixation design, plate position, and screw orientation on fracture fixation stability, neglecting the internal fixation system's biomechanical properties in postoperative rehabilitation exercises. This study aimed to investigate the mechanical properties of tibial plateau fractures after internal fixation, explore the biomechanical mechanism of the interaction between internal fixation and bone, and make suggestions for early postoperative rehabilitation and postoperative weight-bearing rehabilitation. Methods: By establishing the postoperative tibia model, the standing, walking and running conditions were simulated under three axial loads of 500 N, 1000 N, and 1500 N. Accordingly, finite element analysis (FEA) was performed to analyze the model stiffness, displacement of fractured bone fragments, titanium alloy plate, screw stress distribution, and fatigue properties of the tibia and the internal fixation system under various conditions. Results: The stiffness of the model increased significantly after internal fixation. The anteromedial plate was the most stressed, followed by the posteromedial plate. The screws at the distal end of the lateral plate, the screws at the anteromedial plate platform and the screws at the distal end of the posteromedial plate are under greater stress, but at a safe stress level. The relative displacement of the two medial condylar fracture fragments varied from 0.002-0.072 mm. Fatigue damage does not occur in the internal fixation system. Fatigue injuries develop in the tibia when subjected to cyclic loading, especially when running. Conclusion: The results of this study indicate that the internal fixation system tolerates some of the body's typical actions and may sustain all or part of the weight early in the postoperative period. In other words, early rehabilitative exercise is recommended, but avoid strenuous exercise such as running.

Postoperative stability following a triple pelvic osteotomy is affected by implant configuration: a finite element analysis

BACKGROUND

The triple pelvic osteotomy is an established surgical method with multiple modifications regarding surgical technique and choice of implant. The stability of the osteotomy is affected by numerous factors, and among these, the three-dimensional implant configuration is a scientifically less explored aspect.

METHODS

We used a finite element model of a hemi-pelvis with a standardized triple osteotomy to calculate relative flexibility for loads in all translational degrees of freedom for five different implant configurations. Two of the configurations used entry points only feasible when implant removal was not necessary.

RESULTS

The stability of the osteotomy improved with an increased distance between the implants in the plane of the osteotomy as well as for a more perpendicular angle relative to the osteotomy plane. The implant configurations with more entry points available made this easier to adhere to.

CONCLUSION

The use of bioabsorbable implants may provide better opportunities for optimal implant constructs which can, to a certain degree, compensate for the lesser mechanical stiffness of bioabsorbable polymers as compared to metal implants.

Locking compression plate fixation of femoral intertrochanteric fractures in patients with preexisting proximal femoral deformity: a retrospective study

Background

To investigate the clinical efficacy of locking compression plate fixation for the treatment of femoral intertrochanteric fractures in patients with preexisting proximal femoral deformity.

Methods

A retrospective analysis was conducted on 37 patients with femoral intertrochanteric fractures combined with preexisting proximal femoral deformity between January 2013 and July 2019. The patients included 24 males and 13 females aged from 23 to 69 years old, with an average age of 47.5 years. The preexisting proximal femoral deformities resulted from poliomyelitis sequela, proximal femoral fibrous dysplasia, malunion and implant failure combined with coxa vara after intramedullary nailing fixation. There were 6 cases of 31-A2.1, 6 cases of 31-A2.2, 20 cases of 31-A3.1, and 5 cases of 31-A3.2, determined based on the AO classification of intertrochanteric fractures. All fractures were managed through open reduction and locking plate fixation. The hip disability and osteoarthritis outcome score (HOOS) was used to assess hip function before injury and at the last postoperative follow-up. The short form 36 (SF-36) Health Survey Questionnaire was used to assess quality of life.

Results

Thirty-seven patients were followed up for 12 to 27 months (average, 20.7 months). All patients achieved bone healing within 5.1 months on average (range, 3 to 6 months). Postoperative complications included deep vein thrombosis in three patients, bedsores in one and delayed union in one patient. No other complications, such as surgical site infection, fat embolism, nonunion and re-fracture, were presented. There was no significant difference in the HOOS scores and the SF-36 Health Questionnaire outcomes at pre-injury and at the last postoperative follow-up (p > 0.05).

Conclusions

It is difficult to perform intramedullary fixation of femoral intertrochanteric fractures in patients with preexisting proximal femoral deformity, while locking compression plate fixation is a simple and effective method of treatment.

Comparative analysis for five fixations of Pauwels-I by the biomechanical finite-element method

Background: Little is known about how biomechanics govern the five fixtures such as DHS, MLS, DHS + LS, LP, and HA are accepted as common therapeutic techniques. Aims and objectives: A series of numerical models for a femoral neck fracture of Pauwels-I will be constructed by innovative approach of finite element in order to determine the most optimized option in comparison with biomechanical performance. Method: Twenty sets of computer tomography scanned femora were imported onto Mimics to extract 3 D models; these specimens were transferred to Geomagic-Studio for a simulative osteotomy and kyrtograph; then, they underwent UG to fit simulative solid models; 5 sorts of fixture were then expressed by Pro-Engineer virtually. After processing with HyperMesh, all compartments (fracture model + internal implant) were assembled onto 5 systems: "Dynamic Hip Screw (DHS), Multiple Lag screw (MLS), DHS + LS, femoral Locking Plate (LP) and HemiArthroplasty (HA)." Eventually, numerical models of the finite-elemental analysis were exported to AnSys to determine the solution. Result: Four models of fixation and a simulation of HA for Pauwels-I were established, validated, and analyzed with the following findings: In term of displacement, these 5 fixtures ranged between 0.3801 and 0.7536 mm have no significant difference; in term of stress, the averages of peaks for integral assemblage are b(MLS) = 43.5766 ≈< d(LP) = 43.6657 ≈< e(Ha) = 43.6657 < c(DHS + LS) = 66.5494 < a(DHS) = 105.617 in MPa indicate that MLS, LP and HA are not significantly different, but less than DHS + LS or DHS in each. Conclusion: A fixture of MLS or LP with optional HA should be recommended to clinically optimize a Pauwels-I facture.

Finite element analysis of the effect of cannulated screw placement and drilling frequency on femoral neck fracture fixation

BACKGROUND

Positioning of the implanted cannulated screw is paramount for stable femoral neck fracture fixation. To avoid overdrilling, the aim of this study is to determine the optimum configuration of three cannulated screws employed in femoral neck fracture fixation.

METHODS

Using a CT scan from a 28 year old healthy male, several models of femoral neck fracture fixation were developed using finite element analysis. After drilling small holes (in either fixed or random patterns) for screw insertion, the mechanical stresses on the screws were compared for three fracture types.

RESULTS

The inverted isosceles triangle was found to be the best screw configuration. Using finite element analysis, the upper limit of drilling frequency and the maximum stress on the screws for 30°, 50°, and 70° drilling were 14, 16, and 19 times and 46.1MPa, 61.9MPa, and 51.0MPa, respectively. The upper limit of drilling frequency and the maximum stress on the screws for subcapital type, transcervical type, and basicervical type were 14, 16, and 40 times and 24.7MPa, 61.9MPa, and 113.5MPa, respectively.

CONCLUSIONS

Results of this study had supported the use of the inverted isosceles triangle as the best screw configuration for femoral neck fracture fixation. Screw position, Pauwels angle, and drilling frequency can all affect the mechanical strength of femoral neck fracture fixation.

Finite element analysis of osteosynthesis screw fixation in the bone stock: an appropriate method for automatic screw modelling

The use of finite element analysis (FEA) has grown to a more and more important method in the field of biomedical engineering and biomechanics. Although increased computational performance allows new ways to generate more complex biomechanical models, in the area of orthopaedic surgery, solid modelling of screws and drill holes represent a limitation of their use for individual cases and an increase of computational costs. To cope with these requirements, different methods for numerical screw modelling have therefore been investigated to improve its application diversity. Exemplarily, fixation was performed for stabilization of a large segmental femoral bone defect by an osteosynthesis plate. Three different numerical modelling techniques for implant fixation were used in this study, i.e. without screw modelling, screws as solid elements as well as screws as structural elements. The latter one offers the possibility to implement automatically generated screws with variable geometry on arbitrary FE models. Structural screws were parametrically generated by a Python script for the automatic generation in the FE-software Abaqus/CAE on both a tetrahedral and a hexahedral meshed femur. Accuracy of the FE models was confirmed by experimental testing using a composite femur with a segmental defect and an identical osteosynthesis plate for primary stabilisation with titanium screws. Both deflection of the femoral head and the gap alteration were measured with an optical measuring system with an accuracy of approximately 3 µm. For both screw modelling techniques a sufficient correlation of approximately 95% between numerical and experimental analysis was found. Furthermore, using structural elements for screw modelling the computational time could be reduced by 85% using hexahedral elements instead of tetrahedral elements for femur meshing. The automatically generated screw modelling offers a realistic simulation of the osteosynthesis fixation with screws in the adjacent bone stock and can be used for further investigations.