Understanding the Basics of Computational Models in Orthopaedics: A Nonnumeric Review for Surgeons

Computational modeling of revision total hip arthroplasty involving acetabular defects: A systematic review

Revision total hip arthroplasty (rTHA) involving acetabular defects is a complex procedure associated with lower rates of success than primary THA. Computational modeling has played a key role in surgical planning and prediction of postoperative outcomes following primary THA, but modeling applications in rTHA for acetabular defects remain poorly understood. This study aimed to systematically review the use of computational modeling in acetabular defect classification, implant selection and placement, implant design, and postoperative joint functional performance evaluation following rTHA involving acetabular defects. The databases of Web of Science, Scopus, Medline, Embase, Global Health and Central were searched. Fifty-three relevant articles met the inclusion criteria, and their quality were evaluated using a modified Downs and Black evaluation criteria framework. Manual image segmentation from computed tomography scans, which is time consuming, remains the primary method used to generate 3D models of hip bone; however, statistical shape models, once developed, can be used to estimate pre-defect anatomy rapidly. Finite element modeling, which has been used to estimate bone stresses and strains, and implant micromotion postoperatively, has played a key role in custom and off-the-shelf implant design, mitigation of stress shielding, and prediction of bone remodeling and implant stability. However, model validation is challenging and requires rigorous evaluation and comparison with respect to mid- to long-term clinical outcomes. Development of fast, accurate methods to model acetabular defects, including statistical shape models and artificial neural networks, may ultimately improve uptake of and expand applications in modeling and simulation of rTHA for the research setting and clinic.

Comparison of four different screw configurations for the fixation of Fulkerson osteotomy: a finite element analysis

BACKGROUND

Conventionally, two 4.5 mm cortical screws inserted toward the posterior tibial cortex are usually advocated for the fixation of Fulkerson osteotomy. This finite element analysis aimed to compare the biomechanical behavior of four different screw configurations to fix the Fulkerson osteotomy.

MATERIALS AND METHODS

Fulkerson osteotomy was modeled using computerized tomography (CT) data of a patient with patellofemoral instability and fixed with four different screw configurations using two 4.5 mm cortical screws in the axial plane. The configurations were as follows: (1) two screws perpendicular to the osteotomy plane, (2) two screws perpendicular to the posterior cortex of the tibia, (3) the upper screw perpendicular to the osteotomy plane, but the lower screw is perpendicular to the posterior cortex of the tibia, and (4) the reverse position of the screw configuration in the third scenario. Gap formation, sliding, displacement, frictional stress, and deformation of the components were calculated and reported.

RESULTS

The osteotomy fragment moved superiorly after loading the models with 1654 N patellar tendon traction force. Since the proximal cut is sloped (bevel-cut osteotomy), the osteotomy fragment slid and rested on the upper tibial surface. Afterward, the upper surface of the osteotomy fragment acted as a fulcrum, and the distal part of the fragment began to separate from the tibia while the screws resisted the displacement. The resultant total displacement was 0.319 mm, 0.307 mm, 0.333 mm, and 0.245 mm from the first scenario to the fourth scenario, respectively. The minimum displacement was detected in the fourth scenario (upper screw perpendicular to the osteotomy plane and lower screw perpendicular to the posterior tibial cortex). Maximum frictional stress and maximum pressure between components on both surfaces were highest in the first scenario (both screws perpendicular to the osteotomy plane).

CONCLUSIONS

A divergent screw configuration in which the upper screw is inserted perpendicular to the osteotomy plane and the lower screw is inserted perpendicular to the posterior tibial cortex might be a better option for the fixation of Fulkerson osteotomy. Level of evidence Level V, mechanism-based reasoning.

Biomechanical Modeling of Connecting Intermetacarpal K-Wires in the Treatment of Metacarpal Shaft Fractures

BACKGROUND

Clinical series have been published using the configuration of 2 intercarpal Kirschner wires (K-wires) adjacent to the fracture being connected, but biomechanical analysis is lacking. The objective of this pilot biomechanical study was to model and compare the effects of externally connecting 2 intermetacarpal K-wires for the stabilization of transverse metacarpal shaft fractures. Our research hypothesis was that the connected constructs would be stiffer than the unconnected K-wires.

METHODS

A 3-dimensional computer-based model of small finger transverse metacarpal fracture stabilization was designed with 3 transverse 1.1 mm K-wires being anchored to the adjacent metacarpal. Three arrangements were tested: all 3 K-wires in parallel, the middle K-wire angled toward the proximal wire, and the middle angled K-wire being rigidly fixed to the proximal K-wire. The proximal wire was proximal to the fracture. A finite element analysis was performed by applying a cantilever force of 100 N at the head of the metacarpal. The metacarpal was considered to be uniform in composition with parameters typical for human bone. Kirschner wire parameters for stainless steel were used. Force (N) versus displacement was measured.

RESULTS

The configuration with the middle angled K-wire being rigidly fixed to the proximal K-wire showed greater stiffness (12 N/mm) than nonattached constructs. The connected construct was 2.3 times more stiff than the unattached parallel construct and 2.5 times more stiff than angling the middle K-wire without attachment.

CONCLUSIONS

In a computer model simulation, our results show that attaching 2 K-wires adjacent to the fracture provides more than twice the stiffness of unconnected K-wires.

A three-dimensional musculoskeletal model of the dog

The domestic dog is interesting to investigate because of the wide range of body size, body mass, and physique in the many breeds. In the last several years, the number of clinical and biomechanical studies on dog locomotion has increased. However, the relationship between body structure and joint load during locomotion, as well as between joint load and degenerative diseases of the locomotor system (e.g. dysplasia), are not sufficiently understood. Collecting this data through in vivo measurements/records of joint forces and loads on deep/small muscles is complex, invasive, and sometimes unethical. The use of detailed musculoskeletal models may help fill the knowledge gap. We describe here the methods we used to create a detailed musculoskeletal model with 84 degrees of freedom and 134 muscles. Our model has three key-features: three-dimensionality, scalability, and modularity. We tested the validity of the model by identifying forelimb muscle synergies of a walking Beagle. We used inverse dynamics and static optimization to estimate muscle activations based on experimental data. We identified three muscle synergy groups by using hierarchical clustering. The activation patterns predicted from the model exhibit good agreement with experimental data for most of the forelimb muscles. We expect that our model will speed up the analysis of how body size, physique, agility, and disease influence neuronal control and joint loading in dog locomotion.

Intertrochanteric fracture with distal extension: When is the short proximal femoral nail antirotation too short?

INTRODUCTION

The lesser trochanter (LT) fragment in the multifragmentary intertrochanteric femur fracture (AO 31A2.2) may extend distally. If the fragment extends too distally, fixation with a short proximal femoral nail antirotation (PFNA-II) device may not be sufficient. The exact length of distal extension that can be tolerated by the short PFNA-II is not known, therefore it is our objective to determine it.

MATERIALS AND METHODS

A finite element analysis was performed on AO 31A2.2 fracture fixed with a 200mm length size 10 PFNA-II. The construct was loaded vertically to clinical failure of 10mm displacement. This was repeated with the size of the LT fragment increasing distally at intervals, up to 120mm from the base of the LT. The process was also repeated with the bone properties substituted with osteoporotic properties. The stiffness, maximum vertical reaction force, and the plastic deformation area were investigated.

RESULTS

In both non-osteoporotic and osteoporotic model, the stiffness and the maximum vertical reaction force of the construct dropped significantly when the LT fragment is larger than 40mm. Beyond 40mm of LT fragment size, there was a rapid increase in the area of plastic deformation of the cortical bone distal to the intertrochanteric fracture, signifying structural failure of the construct.

CONCLUSION

A long PFNA-II should be considered when fixing a multifragmentary intertrochanteric fracture if the LT fragment extends 40mm distal to the distal base of the LT as the construct fails rapidly upon uniaxial load to failure. Clinically, this threshold may be smaller to account for the multi-axial and dynamic stresses.

Mechanical Strength of the Proximal Femur After Arthroscopic Osteochondroplasty for Femoroacetabular Impingement: Finite Element Analysis and 3-Dimensional Image Analysis

PURPOSE

To examine the influence of femoral neck resection on the mechanical strength of the proximal femur in actual surgery.

METHODS

Eighteen subjects who received arthroscopic cam resection for cam-type femoroacetabular impingement (FAI) were included. Finite element analyses (FEAs) were performed to calculate changes in simulative fracture load between pre- and postoperative femur models. The finite element femur models were constructed from computed tomographic images; thus, the models represented the shape of the original femur, including the bone resection site. Three-dimensional image analysis of the bone resection site was performed to identify morphometric factors that affect strength in the postoperative femur model. Four oblique sagittal planes running perpendicular to the femoral neck axis were used as reference planes to measure the bone resection site.

RESULTS

At the transcervical reference plane, both the bone resection depth and the cross-sectional area at the resection site correlated strongly with postoperative changes in the simulated fracture load (R² = 0.6, P = .0001). However, only resection depth was significantly correlated with the simulated fracture load at the reference plane for the head-neck junction. The resected bone volume did not correlate with the postoperative changes in the simulated fracture load.

CONCLUSIONS

The results of our FEA suggest that the bone resection depth measured at the head-neck junction and transcervical reference plane correlates with fracture risk after osteochondroplasty. By contrast, bone resection at more proximal areas did not have a significant effect on the postoperative femur model strength in our FEA. The total volume of resected bone was also not significantly correlated with postoperative changes in femur model strength.

CLINICAL RELEVANCE

This biomechanical study using FEA suggest that there is a risk of femoral neck fracture after arthroscopic cam resection, particularly when the resected lesion is located distally.

The effect of distal femoral extension osteotomy on muscle lengths after surgery

PURPOSE

The distal femoral extension osteotomy (DFEO) is often used in the treatment of crouch gait to help compensate for knee flexion contractures. The effects of DFEO on skeletal and muscle lengths are incompletely understood, but are important to consider in planning concomitant surgeries such as patellar tendon advancement (PTA). Therefore, the purpose of this study was to quantify the changes in femur, quadriceps, and hamstring lengths with DFEO, and to determine the sensitivity to surgical factors such as wedge location and magnitude.

METHODS

A musculoskeletal model with six degrees of freedom tibiofemoral and patellofemoral joints was used for analysis. A wedge was removed from the distal femur and the remaining bone segments were plated together to simulate the DFEO. After simulating the knee's post-operative equilibrium, the surgically-induced changes in muscle and bone lengths were analysed.

RESULTS

Relative to the pre-operative state, DFEO stretches the hamstrings while shortening the femur and quadriceps. A more posterior wedge apex location (i.e. creation of a cuneiform wedge) diminished the stretch of the hamstrings, but induced greater shortening of the femur and quadriceps. More proximal wedge locations necessitated greater translation of the distal fragment to maintain the knee joint axis.

CONCLUSION

Reduced quadriceps length after DFEO shown in this study is consistent with the need for simultaneous PTA. The induced hamstring stretch also may represent a potential mechanism for post-operative nerve palsies. Overall, the numerical results provide a firmer basis for planning the specifics of DFEO such that desired muscle lengths and joint alignment are achieved.