Screw configuration in proximal humerus plating has a significant impact on fixation failure risk predicted by finite element models

Enhancing fixation stability in proximal humerus fractures: screw orientation optimization in PHILOS plates through finite element analysis and biomechanical testing

The optimal treatment strategy for proximal humerus fractures (PHFs) is debatable owing to the relatively high failure rate of locking plates. Optimizing implants may enhance the fixation stability of PHFs and reduce the rate of mechanical failures. We developed a finite element (FE) model to simulate the treatment of PHFs with Proximal Humerus Internal Locking System (PHILOS) plates. The model evaluated the average bone strain around the screw tips under vertical loading (as an alternative to the risk of cyclic screw cutout failure verified through biomechanical testing) to minimize this strain and maximize predicted fixation stability. After determining the optimal screw configuration, further FE analysis and in vitro biomechanical testing were conducted on both standard and optimized PHILOS screw orientation to assess whether the optimized plates have biomechanical advantages over the standard plates. The FE-based optimized configuration exhibited significantly lower bone strain around the implant than the standard PHILOS screw orientation (- 17.24%, p < 0.001). In both FE analysis and in vitro biomechanical testing, the optimized PHILOS plates achieved significantly lower average bone strain around the screws (p < 0.05), more uniform stress distribution, and greater structural stiffness (p < 0.05) than the standard PHILOS screw orientation. Our results show that biomechanical performance of the PHILOS plates can be improved by altering the orientation of the locking screws. This approach may be useful for future patient-specific design optimization of implants for other fractures.

Biomechanical design of a new proximal humerus fracture plate using alternative materials

Comminuted proximal humerus fractures are often repaired by metal plates, but potentially still experience bone refracture, bone "stress shielding," screw perforation, delayed healing, and so forth. This "proof of principle" investigation is the initial step towards the design of a new plate using alternative materials to address some of these problems. Finite element modeling was used to create design graphs for bone stress, plate stress, screw stress, and interfragmentary motion via three different fixations (no, 1, or 2 "kickstand" [KS] screws across the fracture) using a wide range of plate elastic moduli (EP = 5-200 GPa). Well-known design optimization criteria were used that could minimize bone, plate, and screw failure (i.e., peak stress < ultimate tensile strength), reduce bone "stress shielding" (i.e., bone stress under the new plate ≥ bone stress for an intact humerus, titanium plate, and/or steel plate "control"), and encourage callus growth leading to early healing (i.e., 0.2 mm ≤ axial interfragmentary motion ≤ 1 mm; shear/axial interfragmentary motion ratio <1.6). The findings suggest that a potentially optimal configuration involves the new plate being manufactured from a material with an EP of 5-41.5 GPa with 1 KS screw; but, using no KS screws would cause immediate bone fracture and 2 KS screws would almost certainly lead to delayed healing. A prototype plate might be fabricated using alternative materials suggested for orthopedics and other industries, like fiber-metal laminates, fiber-reinforced polymers, metal foams, pure polymers, shape memory alloys, or 3D-printed porous metals.

Lateral Wall Integrity of the Greater Tuberosity Is Important for the Stability of Osteoporotic Proximal Humeral Fractures After Plate Fixation

BACKGROUND

Previous studies assessing surgical fixation of osteoporotic proximal humeral fractures have primarily focused on medial calcar support. In this study, we utilized a specific model for 2-part surgical neck fracture of the osteoporotic proximal humerus to investigate how severe comminution of the greater tuberosity (GT) lateral wall affects biomechanical stability after fixation with a plate.

METHODS

Ten matched pairs of cadaveric humeri (right and left) were assigned to either a surgical neck fracture alone (the SN group) or a surgical neck fracture with GT lateral wall comminution (the LW group) with use of block randomization. We removed 5 mm of the lateral wall of the GT to simulate severe comminution of the lateral wall. Axial compression stiffness, torsional stiffness, varus bending stiffness, and the single load to failure in varus bending were measured for all plate-bone constructs.

RESULTS

Compared with the SN group, the LW group showed a significant decrease in all measures, including torsional stiffness (internal, p = 0.007; external, p = 0.007), axial compression stiffness (p = 0.002), and varus bending stiffness (p = 0.007). In addition, the mean single load to failure in varus bending for the LW group was 62% lower than that for the SN group (p = 0.005).

CONCLUSIONS

Severe comminution of the GT lateral wall significantly compromised the biomechanical stability of osteoporotic, comminuted humeral surgical neck fractures.

CLINICAL RELEVANCE

Although the generalizability of this cadaveric model may be limited to the extreme clinical scenario, the model showed that severe comminution of the GT lateral wall significantly compromised the stability of osteoporotic humeral surgical neck fractures fixed with a plate and screws alone.

Intramedullary versus locking plate fixation for proximal humerus fractures: indications and technical considerations

Background

The incidence of proximal humerus fractures (PHFs) continues to increase with an aging population, and intramedullary nailing (IMN) and locking plate fixation are two commonly employed techniques for the surgical management of PHF. However, the optimal fixation method can be a source of ongoing controversy. Some influencing factors include the extent of humeral head involvement, fracture complexity, patient age, and surgeon preference. There are many studies that provide a mix of data either when comparing the two techniques or analyzing them in isolation. The aim of this review is to further elucidate the indications and technical considerations involved specifically in IMN vs. locking plate fixation for PHF to further aid orthopedic surgeons when choosing surgical management.

Methods

A narrative approach was chosen for this review allowing for a comprehensive review of literature, including recent findings pertaining to the comparison of management options for PHF. A comprehensive literature search was conducted using the PubMed, Embase, and Cochrane Library databases. The inclusion criteria involved studies that discussed "proximal humerus fracture" and either "intramedullary nail" or "locking plate fixation."

Results

Complications such as avascular necrosis, hardware failure, additional surgical interventions, infection, fracture redisplacement, rotator cuff rupture, and nonunion did not show significant differences between the two groups. Newer generation humeral nails have minimized early complications. As both techniques undergo further refinement and utilization when specifically indicated, functional outcomes, potential complications, and postoperative pain continue to be improved.

Conclusion

The available evidence suggests that both intramedullary nails and locking plates can effectively restore shoulder function in the treatment of displaced proximal humeral fractures, with unclear superiority of either method. The choice of technique should be tailored to patient factors such as fracture type, age, bone quality, and functional expectations. Surgeon experience also plays a role. While certain presentations may exhibit trends that favor one fixation, no specific technique can be universally recommended. Both IMN and LP have shown comparable and satisfactory outcomes, and the final fixation method chosen should take into account the unique characteristics of each patient.

Assessing screw length impact on bone strain in proximal humerus fracture fixation via surrogate modelling

A high failure rate is associated with fracture plates in proximal humerus fractures. The causes of failure remain unclear due to the complexity of the problem including the number and position of the screws, their length and orientation in the space. Finite element (FE) analysis has been used for the analysis of plating of proximal humeral fractures, but due to computational costs is unable to fully explore all potential screw combinations. Surrogate modelling is a viable solution, having the potential to significantly reduce the computational cost whilst requiring a moderate number of training sets. This study aimed to develop adaptive neural network (ANN)-based surrogate models to predict the strain in the humeral bone as a result of changing the length of the screws. The ANN models were trained using data from FE simulations of a single humerus, and after defining the best training sample size, multiple and single-output models were developed. The best performing ANN model was used to predict all the possible screw length configurations. The ANN predictions were compared with the FE results of unseen data, showing a good correlation (R2 = 0.99) and low levels of error (RMSE = 0.51%-1.83% strain). The ANN predictions of all possible screw length configurations showed that the screw that provided the medial support was the most influential on the predicted strain. Overall, the ANN-based surrogate model accurately captured bone strains and has the potential to be used for more complex problems with a larger number of variables.

Impact of blade direction on postoperative femoral head varus in PFNA fixed patients: a clinical review and biomechanical research

Intertrochanteric femur fracture is a common type of osteoporotic fracture in elderly patients, and postoperative femoral head varus following proximal femoral nail anti-rotation (PFNA) fixation is a crucial factor contributing to the deterioration of clinical outcomes. The cross-angle between the implant and bone might influence fixation stability. Although there is a wide range of adjustment in the direction of anti-rotation blades within the femoral neck, the impact of this direct variation on the risk of femoral head varus and its biomechanical mechanisms remain unexplored. In this study, we conducted a retrospective analysis of clinical data from 69 patients with PFNA fixation in our institution. We judge the direction of blade on the femoral neck in on the immediate postoperative lateral X-rays or intraoperative C-arm fluoroscopy, investigating its influence on the early postoperative risk of femoral head varus. p < 0.05 indicates significant results in both correlation and regression analyses. Simultaneously, a three-dimensional finite element model was constructed based on the Syn-Bone standard proximal femur outline, exploring the biomechanical mechanisms of the femoral neck-anti-rotation blade direction variation on the risk of this complication. The results indicated that ventral direction insertion of the anti-rotation blade is an independent risk factor for increased femoral head varus. Complementary biomechanical studies further confirmed that ventral angulation leads to loss of fixation stability and a decrease in fixation failure strength. Therefore, based on this study, it is recommended to avoid ventral directional insertion of the anti-rotation blade in PFNA operation or to adjust it in order to reduce the risk of femoral head varus biomechanically, especially in unstable fractures. This adjustment will help enhance clinical outcomes for patients.

Increasing the angle between caudal screw and the transverse plane may aggravate the risk of femoral head necrosis by deteriorating the fixation stability in patients with femoral neck fracture

Necrosis of the femoral head is the main complication in femoral neck fracture patients with triangle cannulated screw fixation. Instant postoperative fixation instability is a main reason for the higher risk of femoral head necrosis. Biomechanical studies have shown that cross screw fixation can effectively optimize fixation stability in patients with proximal humerus fractures and pedicle screw fixation, but whether this method can also effectively optimize the fixation stability of femoral neck fractures and reduce the corresponding risk of femoral head necrosis has yet to be identified. In this study, a retrospective review of imaging data in femoral neck fracture patients was performed. The cross angle between the femoral neck and the caudal cannulated screw was reported; if the angle between the screw and the transverse plane increased, it was recorded as positive; otherwise, it was recorded as negative. Angle values and their corresponding absolute values were compared in patients with and without femoral head necrosis. Regression analysis identified potential risk factors for femoral head necrosis. Moreover, the biomechanical effect of the screw-femoral neck angle on fixation stability was also verified by numerical mechanical simulations. Clinical review presented significantly larger positive angle values in patients with femoral head necrosis, which was also proven to be an independent risk factor for this complication. Moreover, fixation stability progressively deteriorated with increasing angle between the caudal screw and the transverse plane. Therefore, increasing the angle between the caudal screw and the transverse plane may aggravate the risk of femoral head necrosis by deteriorating the fixation stability in patients with femoral neck fracture.

Preventing secondary screw perforation following proximal humerus fracture after locking plate fixation: a new clinical prognostic risk stratification model

INTRODUCTION

After locking plate (LP) fixation, secondary screw perforation (SSP) is the most common complication in proximal humerus fracture (PHF). SSP is the main cause of glenoid destruction and always leads to reoperation. This study aimed to identify independent risk parameters for SSP and establish an individualized risk prognostic model to facilitate its clinical management.

METHODS

We retrospectively reviewed the medical information of patients with PHF who underwent open reduction and internal LP fixation at one medical center (n = 289) between June 2013 and June 2021. Uni- and multivariate regression analyses identified the independent risk factors. A novel nomogram was formulated based on the final independent risk factors for predicting the risk of SSP. We performed internal validation through concordance indices (C-index) and calibration curves. To implement the clinical use of the model, we performed decision curve analyses (DCA) and risk stratification according to the optimal cutoff value.

RESULTS

A total of 232 patients who met the inclusion criteria were enrolled. The incidence of SSP was 21.98% at the last follow-up. We found that fracture type (odds ratio [OR], 3.111; 95% confidence interval [CI], 1.223-7.914; P = 0.017), postoperative neck-shaft angle (OR, 4.270; 95% CI 1.622-11.239; P = 0.003), the absence of calcar screws (OR, 3.962; 95% CI 1.753-8.955; P = 0.003), and non-medial metaphyseal support (OR,7.066; 95% CI 2.747-18.174; P = 0.000) were independent predictors of SSP. Based on these variables, we developed a nomogram that showed good discrimination (C-index = 0.815). The predicted values of the new model were in good agreement with the actual values demonstrated by the calibration curve. Furthermore, the model's DCA and risk stratification (cutoff = 140 points) showed significantly higher clinical benefits.

CONCLUSIONS

We developed and validated a visual and personalized nomogram that could predict the individual risk of SSP and provide a decision basis for surgeons to create the most optional management plan. However, future prospective and externally validated design studies are warranted to verify our model's efficacy.

Biomechanical design optimization of proximal humerus locked plates: A review

BACKGROUND

Proximal humerus locked plates (PHLPs) are widely used for fracture surgery. Yet, non-union, malunion, infection, avascular necrosis, screw cut-out (i.e., perforation), fixation failure, and re-operation occur. Most biomechanical investigators compare a specific PHLP configuration to other implants like non-locked plates, nails, wires, and arthroplasties. However, it is unknown whether the PHLP configuration is biomechanically optimal according to some well-known biomechanical criteria. Therefore, this is the first review of the systematic optimization of plate and/or screw design variables for improved PHLP biomechanical performance.

METHODS

The PubMed website was searched for papers using the terms "proximal humerus" or "shoulder" plus "biomechanics/biomechanical" plus "locked/locking plates". PHLP papers were included if they were (a) optimization studies that systematically varied plate and screw variables to determine their influence on PHLP's biomechanical performance; (b) focused on plate and screw variables rather than augmentation techniques (i.e., extra implants, bone struts, or cement); (c) published after the year 2000 signaling the commercial availability of locked plate technology; and (d) written in English.

RESULTS

The 41 eligible papers involved experimental testing and/or finite element modeling. Plate variables investigated by these papers were geometry, material, and/or position, while screw variables studied were number, distribution, angle, size, and/or threads. Numerical outcomes given by these papers included stiffness, strength, fracture motion, bone and implant stress, and/or the number of loading cycles to failure. But, no paper fully optimized any plate or screw variable for a PHLP by simultaneously applying four well-established biomechanical criteria: (a) allow controlled fracture motion for early callus generation; (b) reduce bone and implant stress below the material's ultimate stress to prevent failure; (c) maintain sufficient bone-plate interface stress to reduce bone resorption (i.e., stress shielding); and (d) increase the number of loading cycles before failure for a clinically beneficial lifespan (i.e., fatigue life). Finally, this review made suggestions for future work, identified clinical implications, and assessed the quality of the papers reviewed.

CONCLUSIONS

Applying biomechanical optimization criteria can assist biomedical engineers in designing or evaluating PHLPs, so orthopaedic surgeons can have superior PHLP constructs for clinical use.

Treatment of Metaphyseal Defects in Plated Proximal Humerus Fractures with a New Augmentation Technique-A Biomechanical Cadaveric Study

Background and Objectives: Unstable proximal humerus fractures (PHFs) with metaphyseal defects-weakening the osteosynthesis construct-are challenging to treat. A new augmentation technique of plated complex PHFs with metaphyseal defects was recently introduced in the clinical practice. This biomechanical study aimed to analyze the stability of plated unstable PHFs augmented via implementation of this technique versus no augmentation. Materials and Methods: Three-part AO/OTA 11-B1.1 unstable PHFs with metaphyseal defects were created in sixteen paired human cadaveric humeri (average donor age 76 years, range 66-92 years), pairwise assigned to two groups for locked plate fixation with identical implant configuration. In one of the groups, six-milliliter polymethylmethacrylate bone cement with medium viscosity (seven minutes after mixing) was placed manually through the lateral window in the defect of the humerus head after its anatomical reduction to the shaft and prior to the anatomical reduction of the greater tuberosity fragment. All specimens were tested biomechanically in a 25° adduction, applying progressively increasing cyclic loading at 2 Hz until failure. Interfragmentary movements were monitored by motion tracking and X-ray imaging. Results: Initial stiffness was not significantly different between the groups, p = 0.467. Varus deformation of the humerus head fragment, fracture displacement at the medial humerus head aspect, and proximal screw migration and cut-out were significantly smaller in the augmented group after 2000, 4000, 6000, 8000 and 10,000 cycles, p ≤ 0.019. Cycles to 5° varus deformation of the humerus head fragment-set as a clinically relevant failure criterion-and failure load were significantly higher in the augmented group, p = 0.018. Conclusions: From a biomechanical standpoint, augmentation with polymethylmethacrylate bone cement placed in the metaphyseal humerus head defect of plated unstable PHFs considerably enhances fixation stability and can reduce the risk of postoperative complications.

Effect of Calcar Screw in Locking Compression Plate System for Osteoporotic Proximal Humerus Fracture: A Finite Element Analysis Study

This study proposes a finite element analysis (FEA) model for complex fractures at the osteoporotic proximal humerus and investigates the relevance of using a calcar screw in surgical treatments using this model. Two types of three-dimensional (3D) fracture models of patients with osteoporotic humerus were constructed reflecting the mechanical properties of the osteoporotic humerus, such as the Young’s modulus and Poisson’s ratio, and two load conditions mimicking the clinical environment were applied for simulation. Using the 3D models and the conditions, the FEA software calculated the concentration and distribution of stresses developing in the humerus, locking compression plate (LCP), and screws. Then, we evaluated and predicted the fixed state of a LCP system depending on whether the maximum stress value exceeded tensile strength. When axial force was applied, insertion of the calcar screw led to significant reduction of stress applied on screws in the fracture model having a medial gap by approximately 61%, from 913.20 MPa to 351.84 MPa. Based on the results, it was clearly confirmed that using of calcar screws improved the stability of a three-part fractures and simultaneously reinforced medial support.

Predictive Indicators for Complications of Proximal Humerus Fractures Treated with Locking Plate or Intramedullary Nail Fixation

Objective

The purpose of this study was to evaluate the best placement of calcar screws in proximal humerus fracture surgeries.

Methods

This retrospective cohort study included clinical and radiographic outcomes of 98 patients treated with proximal humerus fracture surgeries between January 2017 and June 2020. Demographic data of patients were obtained from medical records. The surgical and radiographic results were also collected: operation time, blood loss, time to surgery, fibular allograft, disruption of medial region hinge, Neer classification, and recovery of medial support. Patients were allocated into two groups: the locking plate group (n = 65) and the intramedullary nail group (n = 33). In this study, we proposed new predictive indicators, named horizontal ratio (HR) and vertical ratio (VR), to quantify the placement of calcar screws in these two groups. A receiver operating characteristic (ROC) analysis was conducted to display the accuracy of these indicators. Shoulder activity, visual scale analog (VAS) score, and Constant score were performed to evaluate postoperative clinical outcomes at 1 year follow‐up.

Results

In the multivariate logistic regression analysis, only time to surgery and effective medial support were considered statistically significant factors of postoperative complications (p < 0.05). Significant differences were observed between medial support and postoperative complications both in the locking plate group and the intramedullary nail group (p < 0.05). Only the vertical ratio of locking plate (VRLP) was a statistically significant predictor of postoperative complications (p < 0.05). The area under curve was calculated to assess the predictive value of VRLP, which came to 0.84. In addition, a ROC analysis found quantifiable thresholds of the VR was 0.1713 as measures to avoid postoperative complications in the locking plate fixation.

Conclusion

In locking plate fixation, the incidence of postoperative complications increased significantly when the VR of calcar screws greater than 0.1713, which was beneficial to surgeons to place calcar screws.

Locking Plates With Computationally Enhanced Screw Trajectories Provide Superior Biomechanical Fixation Stability of Complex Proximal Humerus Fractures

Joint-preserving surgical treatment of complex unstable proximal humerus fractures remains challenging, with high failure rates even following state-of-the-art locked plating. Enhancement of implants could help improve outcomes. By overcoming limitations of conventional biomechanical testing, finite element (FE) analysis enables design optimization but requires stringent validation. This study aimed to computationally enhance the design of an existing locking plate to provide superior fixation stability and evaluate the benefit experimentally in a matched-pair fashion. Further aims were the evaluation of instrumentation accuracy and its potential influence on the specimen-specific predictive ability of FE. Screw trajectories of an existing commercial plate were adjusted to reduce the predicted cyclic cut-out failure risk and define the enhanced (EH) implant design based on results of a previous parametric FE study using 19 left proximal humerus models (Set A). Superiority of EH versus the original (OG) design was tested using nine pairs of human proximal humeri (N = 18, Set B). Specimen-specific CT-based virtual preoperative planning defined osteotomies replicating a complex 3-part fracture and fixation with a locking plate using six screws. Bone specimens were prepared, osteotomized and instrumented according to the preoperative plan via a standardized procedure utilizing 3D-printed guides. Cut-out failure of OG and EH implant designs was compared in paired groups with both FE analysis and cyclic biomechanical testing. The computationally enhanced implant configuration achieved significantly more cycles to cut-out failure compared to the standard OG design (p &lt; 0.01), confirming the significantly lower peri-implant bone strain predicted by FE for the EH versus OG groups (p &lt; 0.001). The magnitude of instrumentation inaccuracies was small but had a significant effect on the predicted failure risk (p &lt; 0.01). The sample-specific FE predictions strongly correlated with the experimental results (R2 = 0.70) when incorporating instrumentation inaccuracies. These findings demonstrate the power and validity of FE simulations in improving implant designs towards superior fixation stability of proximal humerus fractures. Computational optimization could be performed involving further implant features and help decrease failure rates. The results underline the importance of accurate surgical execution of implant fixations and the need for high consistency in validation studies.

Optimization of Locking Plate Screw Angle Used to Treat Two-Part Proximal Humerus Fractures to Maintain Fracture Stability

Proximal humerus fractures increase with the aging of the population. Due to the high failure rates of surgical treatments such as open reduction and internal fixation (ORIF), biomechanical studies seek to optimize the treatments and intervening factors to improve the quality of life of people undergoing these treatments. The aim of the present study was to determine the optimal insertion angle configuration of screws used in a two-part proximal humerus fracture-locking plate osteosynthesis treatment based on finite element analysis (FEA). A series of 3D models of PHILOS locking plates with different screw insertion angle configurations were designed using a matrix system for screw angulation. The locking plate models were evaluated in a two-part proximal humerus fracture with surgical neck fracture under bending and compressive loading conditions using FEA and statistically analyzed using a design of experiments (DOE). The optimal screw insertion angle setting showed an improvement in relation to the interfragmentary strain value of the fracture. Moreover, calcar screws were the most significant feature in fracture stability throughout the tests, followed by the divergence of the most proximal screws and the proximal–distal alignment of the locking plate.

[Cement augmentation and bone graft substitutes-Materials and biomechanics]

BACKGROUND

Materials with different characteristics are used for cement augmentation and as bone graft substitutes.

OBJECTIVE

Cement augmentation and bone graft substitutes are the subject of current research. The evaluation of new knowledge allows its specific application.

MATERIAL AND METHODS

Selective literature search and outline of experimental research results on cement augmentation and bone graft substitutes.

RESULTS

Augmentation and bone graft substitutes are essential components of current trauma surgical procedures. Despite intensive research all materials have specific disadvantages. Cement augmentation of implants enhances not only the anchorage but also influences the failure mode.

CONCLUSION

Cement augmentation has large potential especially in osteoporotic bone. In load-bearing regions acrylic-based cements remain the standard of choice. Ceramic cements are preferred in non-load-bearing areas. Their combination with resorbable metals offers still largely unexplored potential. Virtual biomechanics can help improve the targeted application of cement augmentation.

One size may not fit all: patient-specific computational optimization of locking plates for improved proximal humerus fracture fixation

BACKGROUND

Optimal treatment options for proximal humerus fractures (PHFs) are still debated because of persisting high fixation failure rates experienced with locking plates. Optimization of the implants and development of patient-specific designs may help improve the primary fixation stability of PHFs and reduce the rate of mechanical failures. Optimizing the screw orientations in locking plates has shown promising results; however, the potential benefit of subject-specific designs has not been explored yet. The purpose of this study was to evaluate by means of finite element (FE) analyses whether subject-specific optimization of the screw orientations in a fixed-angle locking plate can reduce the predicted cutout failure risk in unstable 3-part fractures.

METHODS

FE models of 19 low-density proximal humeri were generated from high-resolution computed tomographic images using a previously developed and validated computational osteosynthesis framework. The specimens were virtually osteotomized to simulate unstable malreduced 3-part fractures and fixed with the PHILOS plates using 6 proximal locking screws. The average principal compressive strain in cylindrical bone regions around the screw tips-a biomechanically validated surrogate for the risk of cyclic screw cutout failure-was defined as the main outcome measure. The angles of the 6 proximal locking screws were optimized via parametric analysis for each humerus individually, resulting in subject-specific screw orientations (SSO). The average peri-implant strains of the SSO were statistically compared with the previously reported cohort-specific (CSO) and original PHILOS screw orientations (PSO) for females vs. males.

RESULTS

The optimized SSO significantly reduced the peri-screw bone strain vs. CSO (6.8% ± 4.0%, P = .006) and PSO (25.24% ± 7.93%, P < .001), indicating lower cutout risk for subject-specific configurations. The benefits of SSO vs. PSO were significantly higher for women than men.

CONCLUSION

The findings of this study suggest that subject-specific optimization of the locking screw orientations could lead to lower cutout risk and improved PHF fixation. These computer simulation results require biomechanical and clinical corroboration. Further studies are needed to evaluate whether the potential benefit in stability could justify the increased efforts related to implementation of individualized implants. Nevertheless, computational exploration of the biomechanical factors influencing the outcome of fracture fixations could help better understand the fixation failures and reduce their incidence.

Finite Element Analysis of Fracture Fixation

PURPOSE OF REVIEW

Fracture fixation aims to provide stability and promote healing, but remains challenging in unstable and osteoporotic fractures with increased risk of construct failure and nonunion. The first part of this article reviews the clinical motivation behind finite element analysis of fracture fixation, its strengths and weaknesses, how models are developed and validated, and how outputs are typically interpreted. The second part reviews recent modeling studies of the femur and proximal humerus, areas with particular relevance to fragility fractures.

RECENT FINDINGS

There is some consensus in the literature around how certain modeling aspects are pragmatically formulated, including bone and implant geometries, meshing, material properties, interactions, and loads and boundary conditions. Studies most often focus on predicted implant stress, bone strain surrounding screws, or interfragmentary displacements. However, most models are not rigorously validated. With refined modeling methods, improved validation efforts, and large-scale systematic analyses, finite element analysis is poised to advance the understanding of fracture fixation failure, enable optimization of implant designs, and improve surgical guidance.

Additively manufactured patient-specific prosthesis for tumor reconstruction: Design, process, and properties

Design and processing capabilities of additive manufacturing (AM) to fabricate complex geometries continues to drive the adoption of AM for biomedical applications. In this study, a validated design methodology is presented to evaluate AM as an effective fabrication technique for reconstruction of large bone defects after tumor resection in pediatric oncology patients. Implanting off-the-shelf components in pediatric patients is especially challenging because most standard components are sized and shaped for more common adult cases. While currently reported efforts on AM implants are focused on maxillofacial, hip and knee reconstructions, there have been no reported studies on reconstruction of proximal humerus tumors. A case study of a 9-year-old diagnosed with proximal humerus osteosarcoma was used to develop a patient-specific AM prosthesis for the humerus following tumor resection. Commonly used body-centered cubic (BCC) structures were incorporated at the surgical neck and distal interface in order to increase the effective surface area, promote osseointegration, and reduce the implant weight. A patient-specific prosthesis was fabricated using electron beam melting method from biocompatible Ti-6Al-4V. Both computational and biomechanical tests were performed on the prosthesis to evaluate its biomechanical behavior under varying loading conditions. Morphological analysis of the construct using micro-computed tomography was used to compare the as-designed and as-built prosthesis. It was found that the patient-specific prosthesis could withstand physiologically-relevant loading conditions with minimal permanent deformation (82 μm after 105 cycles) at the medial aspect of the porous surgical neck. These outcomes support potential translation of the patient-specific AM prostheses to reconstruct large bone defects following tumor resection.

Principles of Locking Plate Fixation of Proximal Humerus Fractures

Proximal humerus fractures are common, particularly in elderly patients and those with osteopenia or osteoporosis. Although nonsurgical management results in satisfactory outcomes for most patients, surgical treatment is indicated in select cases. Despite an increasing trend toward arthroplasty, open reduction and internal fixation of proximal humerus fractures can still provide excellent clinical outcomes. Proper technique for internal fixation of the proximal humerus requires an understanding of osseous and neurovascular anatomy. In particular, understanding reliable regions of biomechanically superior bone can help prevent failure of fixation. Biomechanical studies have shown that locked plating of proximal humerus fractures provides stable fixation. Cadaveric and finite element models underscore the importance of screw placement in the posteromedial metaphysis. When medial column support is challenging to obtain, or when bone quality is poor, augmentation with bone autograft, allograft, and/or synthetic composites can improve the biomechanics of internal fixation constructs. The purpose of this review is to outline the anatomic, biologic, and biomechanical principles of plate fixation for proximal humerus fractures to provide evidence-based recommendations for optimizing fixation and preventing fixation failure.

Implant Selection for Proximal Humerus Fractures

Proximal humerus fractures (PHF) are a common orthopedic injury; however, their treatment remains largely controversial with evidence supporting a wide array of treatments. Although many injuries can be treated nonoperatively, there has been much debate about surgical management of PHF. A detailed review of the literature was performed relative to operative management options specifically related to implant choices. Although no definitive answers are available regarding best practice, there is literature to guide operative decision-making and implant selection based on both patient- and surgeon-specific factors.

How Many Proximal Screws Are Needed for a Stable Proximal Humerus Fracture Fixation?

Purpose

This biomechanical study investigates the optimal number of proximal screws for stable fixation of a 2-part proximal humerus fracture model with a locking plate.

Methods

Twenty-four proximal humerus fracture models were included in the study. An unstable 2-part fracture was created and fixed by a locking plate. Cyclic loading and load-to-failure tests were used for the following 4 groups based on the number of screws used: 4-screw, 6-screw, 7-screw, and 9-screw groups. Interfragmentary gaps were measured following cyclic loading and compared. Consequently, the load to failure, maximum displacement, stiffness, and mode of failure at failure point were compared.

Results

The interfragmentary gaps for the 4-screw, 6-screw, 7-screw, and 9-screw groups were significantly reduced by 0.24 ± 0.09 mm, 0.08 ± 0.06 mm, 0.05 ± 0.01 mm, and 0.03 ± 0.01 mm following 1000 cyclic loading, respectively. The loads to failure were significantly different between the groups with the 7-screw group showing the highest load to failure. The stiffness of the 7-screw group was superior compared with the 6-screw, 9-screw, and 4-screw groups. The maximum displacement before failure showed a significant difference between the comparative groups with the 4-screw group having the lowest value. The 7-screw group had the least structural failure rate (33.3%).

Conclusion

At least 7 screws would be optimal for proximal fragment fixation of proximal humerus fractures with medial comminution to minimize secondary varus collapse or fixation failure.

Level of Evidence

Basic science study.

Role of Additional Inferomedial Supporting Screws in Osteoporotic 3-Part Proximal Humerus Fracture: Finite Element Analysis

INTRODUCTION

Importance of inferomedial supporting screws in preventing varus collapse has been investigated for the proximal humerus fracture. However, few studies reported the results of osteoporotic complex fracture. This study aimed to demonstrate the stress distribution pattern, particularly in osteoporotic 3-part proximal humerus fractures involving greater tuberosity (GT) with different screw configurations.

MATERIALS AND METHODS

Using the computed tomography (CT) images of 2 patients, who had osteoporosis and the other had normal bone density, 3-part fractures involving the GT, without medial support were reconstructed. To reflect the osteoporosis or real bone density, Hounsfield unit of CT scans were utilized. A force of 200 N was applied in 30° varus direction. The proximal screws were set in 2 ways: 6 screws without inferomedial supporting screws and 9 screws with inferomedial supporting screws. Qualitative and quantitative analysis of internal stress distribution were performed.

RESULTS

The most proximal part area near humeral head vertex and near the 1st screw's passage and tip had more stress concentrated in osteoporotic 3-part fractures. The stress distribution around the proximal screws was found near the GT fracture line and its lateral side, where the local max values located. Inferomedial supporting screws decreased these effects by changing the points to medial side from the GT. The ratio in osteoporotic bone model decreased to that in normal bone model when inferomedial supporting screws were applied (normal bone, 2.97%-1.30%; osteoporosis bone, 4.76%-1.71%).

CONCLUSIONS

In osteoporotic 3-part proximal humerus fracture, the stress distribution was concentrated on the area near the humeral vertex, 1st row screw tips, and lateral side region from the GT fracture line. Moreover, inferomedial supporting screws ensured that the stress distribution is similar to that in normal bone setting, particularly in osteoporotic condition.

Trends in the Characterization of the Proximal Humerus in Biomechanical Studies: A Review

Proximal humerus fractures are becoming more common due to the aging of the population, and more related scientific research is also emerging. Biomechanical studies attempt to optimize treatments, taking into consideration the factors involved, to obtain the best possible treatment scenario. To achieve this, the use of finite element analysis (FEA) is necessary, to experiment with situations that are difficult to replicate, and which are sometimes unethical. Furthermore, low costs and time requirements make FEA the perfect choice for biomechanical studies. Part of the complete process of an FEA involves three-dimensional (3D) bone modeling, mechanical properties assignment, and meshing the bone model to be analyzed. Due to the lack of standardization for bone modeling, properties assignment, and the meshing processes, this article aims to review the most widely used techniques to model the proximal humerus bone, according to its anatomy, for FEA. This study also seeks to understand the knowledge and bias behind mechanical properties assignment for bone, and the similarities/differences in mesh properties used in previous FEA studies of the proximal humerus. The best ways to achieve these processes, according to the evidence, will be analyzed and discussed, seeking to obtain the most accurate results for FEA simulations.

Cement augmentation of calcar screws may provide the greatest reduction in predicted screw cut-out risk for proximal humerus plating based on validated parametric computational modelling: Augmenting proximal humerus fracture plating

Aims

Fixation of osteoporotic proximal humerus fractures remains challenging even with state-of-the-art locking plates. Despite the demonstrated biomechanical benefit of screw tip augmentation with bone cement, the clinical findings have remained unclear, potentially as the optimal augmentation combinations are unknown. The aim of this study was to systematically evaluate the biomechanical benefits of the augmentation options in a humeral locking plate using finite element analysis (FEA).

Methods

A total of 64 cement augmentation configurations were analyzed using six screws of a locking plate to virtually fix unstable three-part fractures in 24 low-density proximal humerus models under three physiological loading cases (4,608 simulations). The biomechanical benefit of augmentation was evaluated through an established FEA methodology using the average peri-screw bone strain as a validated predictor of cyclic cut-out failure.

Results

The biomechanical benefit was already significant with a single cemented screw and increased with the number of augmented screws, but the configuration was highly influential. The best two-screw (mean 23%, SD 3% reduction) and the worst four-screw (mean 22%, SD 5%) combinations performed similarly. The largest benefits were achieved with augmenting screws purchasing into the calcar and having posteriorly located tips. Local bone mineral density was not directly related to the improvement.

Conclusion

The number and configuration of cemented screws strongly determined how augmentation can alleviate the predicted risk of cut-out failure. Screws purchasing in the calcar and posterior humeral head regions may be prioritized. Although requiring clinical corroborations, these findings may explain the controversial results of previous clinical studies not controlling the choices of screw augmentation.

Comparison of optimal screw configurations in two locking plate systems for proximal humerus fixation - a finite element analysis study

BACKGROUND

Management of proximal humerus fractures is challenging, especially in elderly. Locking plating is a common surgical treatment option. The Proximal Humerus Internal Locking System (plate-A) has shown to lower complication rates compared to conventional plates, but is associated with impingement risk, which could be avoided using Peri-articular Proximal Humerus Plate (plate-B). Nevertheless, biomechanical performance and optimal screw configuration of plate-B is unknown. The aim of this study was to evaluate different screw configurations of plate-B and compare with plate-A using finite element analyses.

METHODS

Twenty-six proximal humerus models were osteotomised to create unstable three-part fractures, fixed with either of the two plates, and tested under three anatomical loading conditions using a previous established and validated finite element simulation framework. Various clinically relevant screw configurations were investigated for both plates and compared based on the predicted peri-implant bone strain, being a validated surrogate of cyclic cut-out failure.

FINDINGS

Besides increasing the number of screws, the placement of the posterior screws in combination with the calcar screw in the plate-B significantly decreased the predicted failure risk. Generally, plate-A had a lower predicted failure risk than plate-B.

INTERPRETATION

The posterior and calcar screws may be prioritized in plate-B. Compared to plate-A, the more distal positioning, less purchase in the posterior aspect and a smaller screw spread due to not fitting of the most distal calcar screw in most investigated subjects led to a significantly higher predicted failure risk for most plate-B configurations. The findings of the simulations study require clinical corroboration.