Automatic Bone Surface Restoration for Markerless Computer-assisted Orthopaedic Surgery

An automatic markerless knee tracking and registration algorithm has been proposed in the literature to avoid the marker insertion required by conventional computer-assisted knee surgery, resulting in a shorter and less invasive surgical workflow. However, such an algorithm considers intact femur geometry only. The bone surface modification is inevitable due to intra-operative intervention. The mismatched correspondences will degrade the reliability of registered target pose. To solve this problem, this work proposed a supervised deep neural network to automatically restore the surface of processed bone. The network was trained on a synthetic dataset that consists of real depth captures of a model leg and simulated realistic femur cutting. According to the evaluation on both synthetic data and real-time captures, the registration quality can be effectively improved by surface reconstruction. The improvement in tracking accuracy is only evident over test data, indicating the need for future enhancement of the dataset and network.

Biomechanical Consequences of Nail Insertion Point and Anterior Cortical Perforation for Antegrade Femoral Nailing

This biomechanical study assessed the influence of changing antegrade cephalomedullary nail insertion point from anterior to neutral to posterior locations relative to the tip of the greater trochanter with or without anterior cortical perforation in the distal femur. Artificial osteoporotic femurs and cephalomedullary nails were used to create 5 test groups each with 8 specimens: intact femur without a nail or perforation, anterior nail insertion point without perforation, neutral nail insertion point without perforation, posterior nail insertion point without perforation, and posterior nail insertion point with perforation. Nondestructive biomechanical tests were done at 250 N in axial, coronal 3-point bending, sagittal 3-point bending, and torsional loading in order to measure overall stiffness and bone stress. The intact femur group vs. all femur/nail groups had lower stiffness in all loading modes ( p ≤ 0.018 ), as well as higher bone stress in the proximal femur ( p ≤ 0.027 ) but not in the distal femur above the perforation ( p = 0.096 ). Compared to each other, femur/nail groups only showed differences in sagittal 3-point bending stiffness for anterior and neutral vs. posterior nail insertion points without ( p ≤ 0.025 ) and with perforation ( p ≤ 0.047 ). Although it did not achieve statistical significance ( p ≥ 0.096 ), moving the nail insertion point from anterior to neutral to posterior to posterior with perforation did gradually increase bone stress by 45% (proximal femur) and 46% (distal femur). No femur or hardware failures occurred. Moving the nail insertion point and the presence of a perforation had little effect on stiffness, but the increased bone stress may be important as a predictor of fracture. Based on current bone stress results, surgeons should use anterior or neutral nail insertion points to reduce the risk of anterior cortical perforation.

Influence of a metaphyseal sleeve on the stress-strain state of a bone-tumor implant system in the distal femur: an experimental and finite element analysis

Background Aseptic loosening of distal femoral tumor implants significantly correlates with the resection length. We designed a new “sleeve” that is specially engaged in the metaphysis at least 5 cm proximal to the knee joint line to preserve as much bone stock as possible. This study investigates the influence of a metaphyseal sleeve on the stress-strain state of a bone tumor implant system in the distal femur. Methods Cortex strains in intact and implanted femurs were predicted with finite element (FE) models. Moreover strains were experimentally measured in a cadaveric femur with and without a sleeve and stem under an axial compressive load of 1000 N. The FE models, which were validated by linear regression, were used to investigate the maximal von Mises stress and the implanted-to-intact (ITI) ratios of strain in the femur with single-legged stance loading under immediate postoperative and osseointegration conditions. Results Good agreement was noted between the experimental measurements and numerical predictions of the femoral strains (coefficient of determination (R2) ≥ 0.95; root-mean-square error (RMSE%) ≈ 10%). The ITI ratios for the metaphysis were between 13 and 28% and between 10 and 21% under the immediate postoperative and osseointegration conditions, respectively, while the ITI ratios for the posterior and lateral cortices around the tip of the stem were 110% and 119% under the immediate-postoperative condition, respectively, and 114% and 101% under the osseointegration condition, respectively. The maximal von Mises stresses for the implanted femur were 113.8 MPa and 43.41 MPa under the immediate postoperative and osseointegration conditions, which were 284% and 47% higher than those in the intact femur (29.6 MPa), respectively. Conclusions This study reveals that a metaphyseal sleeve may cause stress shielding relative to the intact femur, especially in the distal metaphysis. Stress concentrations might mainly occur in the posterior cortex around the tip of the stem. However, stress concentrations may not be accompanied by periprosthetic fracture under the single-legged stance condition.

Finite Element Analysis of Necessity of Reduction and Selection of Internal Fixation for Valgus-impacted Femoral Neck Fracture

BackgroundAlthough valgus-impacted fractures don’t displace obviously, a larger valgus angle and posterior tilt of the femoral head can cause hip joint dysfunction and femoral head necrosis. The optimal surgical approach for valgus-impacted femoral neck fractures remains controversial. This study compared the biomechanical characteristics of different treatment strategies based on finite element analysis.MethodsTen valgus-impacted femoral neck fractures were analyzed in terms of posterior tilt and valgus angle measured on X-ray radiographs. Using these data, we generated 7 finite element models that were compared in terms of von Mises stress distribution and displacement.ResultsIn the intact femur, von Mises stress was concentrated at the medial and inferior sides of the femoral neck. In valgus-impacted femoral neck fractures, von Mises stress was at the same locations but was 5.66 times higher than that in the intact femur. When 3 cannulated screws were used for internal fixation, anatomic reduction diminished the stress at the fracture end from 140.6 to 59.14 MPa, although displacement increased from 0.228 to 0.450 mm. When the fracture was fixed with a sliding hip screw (SHS) + cannulated screw, there was less stress at the fracture end and greater displacement with anatomic reduction than that without reduction (stress: 15.9 vs 37.9 MPa; displacement: 0.329 vs 0.168 mm).ConclusionsThe SHS + cannulated screw has superior biomechanical stability than 3 cannulated screws, and is recommended following anatomic reduction to treat valgus-compacted femoral neck fractures.

Femoral Stress Changes after Total Hip Arthroplasty with the Ribbed Prosthesis: A Finite Element Analysis

Background. A total hip reconstruction is related to the stress distribution throughout the prosthesis, cement, and femur. Researches on reducing the stress in all components to minimize the risk of failure are of great significance. The objective of our study was to determine the biomechanical variation in overall femoral stress and periprosthetic femoral stress distribution after implantation with the Ribbed anatomic prosthesis. Methods. Three-dimensional finite element models of intact femur and Ribbed prosthesis were developed according to the morphology, while the hip joint loading and the strength of related muscles were applied in the models. The overall stress changes of the intact femur before and after the implantation were analyzed, and the periprosthetic stress distribution especially in the proximal region of the femur was quantified. Results. As a result, the overall stress pattern of the femur did not change after the implantation compared with the intact femur. The region of peak stress value was located in the middle and lower segments of the full length femur, but the stress value level decreased. The final prosthesis resulted in a significant decrease in the equivalent stress level of the periprosthetic bone tissue, and the most severe area appeared at the endmost posterior quadrant. The stress shielding ratio of the Ribbed prosthesis was 71.6%. The stress value level gradually increased towards the distal part of the prosthesis and recovered to physiological level at the end of the prosthesis. Conclusions. The Ribbed prosthesis can cause significant stress shielding effect in the proximal femur. These results may help optimize prosthetic design to reduce stress shielding effect and improve clinical outcomes.

Effects of Prosthesis Stem Materials on Stress Distribution of Total Hip Replacement

Bone loss and bone thickening phenomenon occurred due to different stiffness of the implant and femur. Implant with stiffer materials than the bone carries majority of the load and it transferred down along the implant till the distal tip of the stem. Both phenomenons contribute to stress shielding and loosening of the prosthesis stem. In this study, the stress distributions in intact femur and THR femur are established using finite element method. The THR femur model consists of cemented hip Ti6Al4V and CoCrMo prosthesis stem implanted inside the femur bone. Effects of different material properties of the prosthesis stem on the resulting stress distributions are investigated. Results shows that the largest discrepancy in stress values between intact and THR femur is predicted along the middle region at both lateral and medial planes. The distal region shows that the calculated stress for both THR femur experienced higher stress magnitude than that of intact femur. The higher stress in THR femur leads to bone thickening at the particular region. The corresponding stress magnitude saturates at 25 MPa for THR femur while intact femur is slightly lower at 22 MPa.

The effect of cortex thickness on intact femur biomechanics: A comparison of finite element analysis with synthetic femurs

Biomechanical studies on femur fracture fixation with orthopaedic implants are numerous in the literature. However, few studies have compared the mechanical stability of these repair constructs in osteoporotic versus normal bone. The present aim was to examine how changes in cortical wall thickness of intact femurs affect biomechanical characteristics. A three-dimensional, linear, isotropic finite element (FE) model of an intact femur was developed in order to predict the effect of bicortical wall thickness, t, relative to the femur's mid-diaphyseal outer diameter, D, over a cortex thickness ratio range of 0 ≤ t/ D ≤ 1. The FE model was subjected to loads to obtain axial, lateral, and torsional stiffness. Ten commercially available synthetic femurs were then used to mimic ‘osteoporotic’ bone with t/ D = 0.33, while ten synthetic left femurs were used to simulate ‘normal’ bone with t/ D = 0.66. Axial, lateral, and torsional stiffness were measured for all femurs. There was excellent agreement between FE analysis and experimental stiffness data for all loading modes with an aggregate average percentage difference of 8 per cent. The FE results for mechanical stiffness versus cortical thickness ratio (0 ≤ t/ D ≤ 1) demonstrated exponential trends with the following stiffness ranges: axial stiffness (0 to 2343 N/mm), lateral stiffness (0 to 62 N/mm), and torsional stiffness (0 to 198 N/mm). This is the first study to characterize mechanical stiffness over a wide range of cortical thickness values. These results may have some clinical implications with respect to appropriately differentiating between older and younger human long bones from a mechanical standpoint.