Crackless high-aspect-ratio processing of a silica glass with a temporally shaped ultrafast laser.

In this Letter, we propose a crackless high-aspect-ratio processing method based on a temporally shaped ultrafast laser. The laser pulse is temporally split into two sub pulses: one with smaller energy is used to excite electrons but without ablation so that the applied pressure to the sample is weak, and the other one is used to heat the electrons and achieve material removal after it is temporally stretched by a chirped volume Bragg grating (CVBG). Compared with the conventional ultrafast laser processing, the crack generation is almost suppressed by using this proposed method. The hole depth increases more than 3.3 times, and the aspect ratio is improved at least 2.2 times. Moreover, processing dynamics and parameter dependence are further experimentally studied. It shows that the processing highly depends on the density of electrons excited by the first pulse (P1) and the energy of the second pulse (P2). This novel, to the best of our knowledge, method provides a new route for the precise processing of wide-bandgap materials.

Mechanism and performance evaluation of transient and selective laser processing of glass based on optical monitoring.

Femtosecond laser processing has been widely applied in glass processing owing to its ability to fabricate microscale components. To improve processing efficiency, a transient and selective laser (TSL) processing technique was previously developed, in which electron excitation was induced inside a transparent medium by a single pulse of femtosecond (fs) laser, and a single pulse of microsecond (µs) laser can be selectively absorbed in this excited region to heat and remove the material. However, because of its high speed removal process, the unclear mechanism and inefficient evaluation of its processing performance limit its further application. This study analyzes the transient spatiotemporal evolution of the induced plasma and the related material removal mechanism of the TSL processing using a side high-speed monitoring method. To achieve a rapid performance evaluation, a quantitative analysis of the optical plasma signals (on a microsecond timescale) generated in TSL processing was performed by employing a developed coaxial high-speed monitoring method using a photodetector. The variations in the shapes, intensity distribution, and dimensions of the plasma were quantitatively investigated. In addition, the relation between the plasma signal and drilling performance under different laser parameters, including hole depth, hole types, and cracks, was explored and quantitatively analyzed. The revealed mechanism is expected to contribute to the broadening of the application of TSL processing in microfabrication. Furthermore, the developed high-speed and precision monitoring technology can be utilized for high-speed evaluation and precision control of machining quality in real time during ultrahigh-speed laser machining, without time-consuming camera observations.

Importance of posterior tibial slope in joint kinematics with an anterior cruciate ligament-deficient knee.

AIMS: To fully quantify the effect of posterior tibial slope (PTS) angles on joint kinematics and contact mechanics of intact and anterior cruciate ligament-deficient (ACLD) knees during the gait cycle. METHODS: In this controlled laboratory study, we developed an original multiscale subject-specific finite element musculoskeletal framework model and integrated it with the tibiofemoral and patellofemoral joints with high-fidelity joint motion representations, to investigate the effects of 2.5° increases in PTS angles on joint dynamics and contact mechanics during the gait cycle. RESULTS: The ACL tensile force in the intact knee was significantly affected with increasing PTS angle. Considerable differences were observed in kinematics and initial posterior femoral translation between the intact and ACLD joints as the PTS angles increased by more than 2.5° (beyond 11.4°). Additionally, a higher contact stress was detected in the peripheral posterior horn areas of the menisci with increasing PTS angle during the gait cycle. The maximum tensile force on the horn of the medial meniscus increased from 73.9 N to 172.4 N in the ACLD joint with increasing PTS angles. CONCLUSION: Knee joint instability and larger loading on the medial meniscus were found on the ACLD knee even at a 2.5° increase in PTS angle (larger than 11.4°). Our biomechanical findings support recent clinical evidence of a high risk of failure of ACL reconstruction with steeper PTS and the necessity of ACL reconstruction, which would prevent meniscus tear and thus the development or progression of osteoarthritis.Cite this article: Bone Joint Res 2022;11(10):708-719.

High-speed microgroove processing of glass by expanding the high-temperature region formed by transient and selective laser absorption.

Microgroove processing of glass is important in many fields, however, it is difficult to achieve the processing with a high speed. In this study, we developed a novel method for the high-speed microgroove processing of glass using two types of lasers, namely a femtosecond laser and a near-infrared continuous-wave (CW) laser. A single femtosecond laser pulse was initially focused on the surface of the material, enabling the area to absorb the CW laser, which is otherwise not absorbed by the glass. The CW laser was then scanned along the material surface, expanding the machined hole to form a groove. The resulting grooves, with a width of approximately 10 µm and depths of up to 350 µm, can be machined with a scanning speed of up to 200 mm/s, 25 times faster than conventional methods. This method exhibits the potential to improve the industrial application of fast laser microprocessing of glass.

Investigation of multi-timescale processing phenomena in femtosecond laser drilling of zirconia ceramics.

Femtosecond lasers have been applied to machining of zirconia (ZrO2) ceramics because of their ultrashort pulse duration and high peak power. However, an unclear understanding of the ultrafast laser-material interaction mechanisms limits the achievement of precision processing. In this study, a pump-probe imaging method comprising a focusing probe beam integrated with a high-speed camera was developed to directly observe and quantitatively evaluate the multi-timescale transient processing phenomena, including electron excitation, shockwave propagation, plasma evolution, and hole formation, occurring on the picosecond to second timescales, inside a ZrO2 sample. The variation mechanism in the shapes, lifetimes, and dimensions of these phenomena and their impacts on the drilling performance under different laser parameters were explored. The clear imaging and investigation of the above phenomena contribute to revealing the ultrafast laser-material interaction mechanisms and precision processing in the laser-drilling of zirconia ceramics.

Temporal-spatial characteristics of filament induced by a femtosecond laser pulse in transparent dielectrics.

The evolution mechanism of femtosecond laser-induced filaments has been widely investigated owing to its application prospects in microprocessing. However, the material dependence of the excitation, stability, and decay of filaments is not well understood despite the importance of their precise utilization. In this study, the spatiotemporal evolution of filaments induced by a single femtosecond laser pulse in sapphire and silica glass was investigated using time-resolved pump-probe shadowgraphy on femtosecond and picosecond timescales. The results revealed that the evolution was significantly different in the two typically transparent dielectrics in terms of the electronic plasma dynamics and filament lifetimes. This difference can be attributed to the self-trapped excitons (STEs) in silica glass. Furthermore, the filament dependence on pump energy and focal position was experimentally analyzed. Divergent filaments were observed when the focal position was near the surface because of the effect of the excited plasma on beam propagation. Moreover, the evolution of filament length in the two materials was discussed. This study contributes to the applications of filaments in precise processing.

Multiscale finite element musculoskeletal model for intact knee dynamics.

BACKGROUND AND OBJECTIVE: The dynamic characteristics of the intact knee joint are valuable for treating knee osteoarthritis and designing knee prostheses. However, it remains a challenge to elucidate the detailed dynamics of the knee due to its complexity of anatomical structure and complex interaction with body dynamics. METHODS: In this study, a unique subject-specific musculoskeletal model with a concurrent high-accuracy intact finite element knee model was created and used to simultaneously evaluate the kinematics and mechanics of an intact knee joint during the gait cycle. RESULTS: A medial pivot motion with external rotation, and a large parallel anterior translation were observed in the stance and swing phases, respectively, which is consistent with the in vivo fluoroscopy measurements. The maximum axial contact force on the knee joint, observed at 45% of the gait cycle, is approximately 2.89 times the body weight. The medial cartilage bears 65.7% of the total axial contact force. The results demonstrate that the cartilage-cartilage contact bears most of the joint load (62.5%) compared to the cartilage-meniscus-cartilage contact (37.5%). Regarding contact mechanics, the maximum contact pressure on both sides of the tibial cartilage (8.2 MPa) is almost similar to the first axial loading peak (14%) of the gait cycle. Additionally, the maximum contact pressure (6.01 MPa) was observed during the stance phase of the gait cycle on the patellofemoral joint. CONCLUSIONS: The predicted results on the tibiofemoral and patellofemoral joints provide a theoretical basis for the treatment of knee joint diseases and knee prosthesis design. Moreover, this approach presents a comprehensive tool to evaluate the mechanics at both the body and tissue levels. Therefore, it has a high potential for application in human biomechanics.

Design of a self-centring drill bit for orthopaedic surgery: A systematic comparison of the drilling performance.

Bone drilling is an indispensable and demanding operation among many orthopaedic operations. A dedicated drill bit that can achieve low-trauma and self-centring drilling is in urgent need. In this study, a three-step orthopaedic low-traumatic drill bit design was proposed. In order to evaluate the drilling performance of the proposed drill, comprehensive comparison tests were carried out with various commercial medical drills in terms of skiving force, thrust force, temperature rise, and surface quality. The experimental results show that the proposed three-step drill design with the optimal point angle, a small chisel edge, transition arc and web thinning can obtain lower and more stable thrust force, slighter bending force, smaller temperature rise, and higher hole quality compared with the commercial drill bits. The proposed drill shows satisfactory drilling performance and has great application potential in clinical surgery.

Comparison of Kinematics and Contact Mechanics in Normal Knee and Total Knee Replacements: A Computational Investigation.

An objective of total knee replacement (TKR) is to restore the mechanical function of a normal knee. Joint kinematics and contact mechanics performance are two of the primary indices that indicate the success of TKR devices. The aim of this study was to compare the kinematics and contact mechanics of TKR and normal knee joints. An experimentally evaluated finite-element (FE) knee model was developed and used to investigate the performance of four TKR designs (fixed cruciate-retaining (CR), mobile CR, posterior-stabilized (PS), medial pivot design (MP)) and the normal knee joint during a gait cycle. The predicted kinematic results showed that the MP design presented similar kinematics to those of the normal knee joint and did not demonstrate paradoxical motion of the femur. A considerably larger contact area and lower contact pressure were found on the normal knee joint (1315 mm2, and 14.8 MPa, respectively) than on the TKRs, which was consistent with the previous in-vivo fluoroscopic investigation. The mobile CR and PS designs exhibited the smallest and greatest contact pressures of the four TKR designs, respectively. The results of the present study help to understand the kinematics and contact mechanics in the TKR during the gait cycle, and provide comprehensive information about the performance of the normal knee joint.

In vivo kinematical validated knee model for preclinical testing of total knee replacement.

BACKGROUND AND OBJECTIVE: A computational knee model facilitates efficient component design evaluations and preclinical testing under various dynamic loadings. However, the development of a highly mimicked dynamic whole knee model with specified ligament constraints that provides high predictive accuracy with in-vivo experiments remains a challenge. METHODS: In the present study, a musculoskeletal integrated force-driven explicit finite-element knee model with tibiofemoral and patellofemoral joints constrained with detailed soft tissue was developed. A proportional-integral-derivative controller was concurrently added to the knee model to track the boundary conditions. The actuations of the quadriceps and hamstrings were predicted via a subject-specific musculoskeletal model and matched with electromyography results. RESULTS: Compared to in-vivo fluoroscopic results in a gait cycle, the predicted results of the kinematics of the tibiofemoral joint exhibited an agreement in terms of tendency and magnitude (anterior-posterior translation: RMSE = 1.1 mm, r2 = 0.87; inferior-superior translation: RMSE = 0.83 mm, r2 = 0.84; medial-lateral translation: RMSE = 0.82 mm, r2 = 0.05; flexion-extension rotation: RMSE = 0.23°, r2 = 1; internal-external rotation: RMSE = 1.85°, r2 = 0.65; varus-valgus rotation: RMSE = 1.39°, r2 = 0.08). Contact mechanics, including the contact area, pressure, and stress, were synchronously simulated on the tibiofemoral and patellofemoral joints. CONCLUSIONS: The study provides a calibrated knee model and a kinematical validation approach that can be widely used in preclinical testing and knee prosthesis design.

Enhanced In-Silico Polyethylene Wear Simulation of Total Knee Replacements During Daily Activities.

A computational wear simulator is an efficient tool for evaluating the wear of artificial knee joints. The classical Archard's wear law-based simulator has questionable accuracy and is focused on walking. In this study, an in silico polyethylene wear simulation of total knee replacements was developed considering the various highly demanding daily activities. A good predicted accuracy (error = 8.1%) was found through comparison of the experimental results. A relatively larger averaged wear loss was found under the loading condition (1.53 mg/mc) of daily activities compared with the walking condition (1.32 mg/mc). The squatting movement (2.57 mg/mc) produces the highest overall wear rate. In addition, a relatively larger amount of wear was found on the medial side knee prosthesis than that on the lateral side. The enhanced in silico polyethylene wear simulator provides an accurate and comprehensive tool for wear prediction in preclinical wear testing.

Ultrafast internal modification of glass by selective absorption of a continuous-wave laser into excited electrons.

The internal modification of glass using ultrashort pulse lasers has been attracting attention in a wide range of applications. However, the remarkably low processing speed has impeded its use in the industry. In this study, we achieved ultrafast internal modification of glass by coaxially focusing a single-pulse femtosecond laser and continuous-wave (CW) laser with the wavelength that is transparent to the glass. Compared with the conventional method, the processing speed increased by a factor of 500. The observation of high-speed phenomena revealed that the CW laser was absorbed by the seed electrons that were generated by the femtosecond laser pulse. This technique may help expand the applications of femtosecond lasers in the industry.

Thermographic assessment of heat-induced cellular damage during orthopedic surgery.

The heat generated during orthopedic surgery can cause thermal damage to bone cells, leading to cell necrosis, death, and bone resorption. In this study, the drill-exit surface in cortical bone drilling was firstly investigated by infrared thermography to understand the thermal characteristics of bone cutting. In order to mimic the short-term thermal condition of high temperature during surgical cutting, the osteoblasts were exposed to heat shock for short periods of time to investigate the effect of cutting heat on the bone. Necrosis and apoptosis were investigated immediately after heat shock for 2 s, 5 s, and 15 s at 50 °C, 60 °C, 70 °C, and 80 °C, respectively. The cells were then incubated for 4 days at 37 °C and analyzed by fluorescein annexin V-FITC/PI double staining. The temperature and heat-duration were precisely controlled by a novel heating approach. In comparison to the control group (37 °C), immediate necrotic and apoptotic response to heat shock was found in cells exposed to 50 °C for 5 s (11.8%, p<0.05); however, the response was negligible in cells exposed to 50 °C for 2 s. In addition, recovery was found in the group exposed to 50 °C and 60 °C for 2 s (p ≤ 0.05) after incubation for 4 days. Cell damage depends on the magnitude and duration of heat exposure. These findings provide fundamental knowledge for future developments of surgical tool design and cutting methods.

Simulation of ultrashort pulse laser drilling of glass considering heat accumulation.

In accordance with the increasing demand for high-speed processing, the repetition rate of ultrashort pulse lasers has continued to increase. With the development of these lasers, there is a growing demand for the prediction of shapes processed at high repetition rates. However, the prediction of these shapes is a major challenge, because of the difficulty associated with the estimation of heat accumulation. In this study, we developed a simulation of ultrashort laser drilling in glass including heat accumulation calculation between pulses. In this simulation model, temperature is considered as an additional criterion of material removal, thus, the dependency of the repetition rate can be estimated. Two model parameters of laser absorption at high temperatures are investigated and determined by experiments under high environmental temperatures. Using the simulation model, high shape-prediction accuracy at high repetition rates was achieved and validated by comparison with experiments. This study may contribute to broadening the applications of high-repetition-rate ultrashort pulse lasers.

Improvements of material removal in cortical bone via impact cutting method.

Bone cutting with high efficiency as well as low levels of forces and damage has a great significance for orthopaedic surgeries. Due to the brittleness and anisotropy of cortical bone, a conventional cutting process can cause irregular crack propagation and fractured bone chip, affecting the tissue removal process and postoperative recovery. In this paper, a high-frequency impact cutting method is investigated, and its effect on fracture propagation, chip formation and cutting forces is studied for orthogonal cutting. Experimental results show that cracks are deflected by cement lines in conventional cutting, forming fractured blocks or split chips. In impact cutting, the cutting-induced fractures expand along a main shear direction, generating small pieces of triangular segmented chips. Cutting forces are significantly reduced with vibration-induced impacts; especially, the main cutting force is nearly 70% lower than that in the conventional cutting. The main reason for this is much higher strain rates in high-frequency impact cutting than in a conventional process, and direct penetration of fractures across the osteonal matrix without deflections along the cement lines. This results in a straighter path along the main shear plane and totally different chip morphology; so, a lower consumption of cutting energy in the main shear direction reduces the macroscopic cutting force. The results of this study have an important theoretical and practical value for revealing the mechanism of impact cutting, improving the efficiency of osteotomy and supporting the innovation in bone surgical instruments.

Simultaneous measurement of the surface shape and thickness for an optical flat with a wavelength-tuning Fizeau interferometer with suppression of drift error.

Two types of phase-shifting algorithms were developed for simultaneous measurement of the surface and thickness variation of an optical flat. During wavelength tuning, phase-shift nonlinearity can cause a spatially nonuniform error and spatially uniform DC drift error. A 19-sample algorithm was developed that eliminates the effect of the spatially uniform error by expanding the 17-sample algorithm with characteristic polynomial theory. The 19-sample algorithm was then altered to measure the surface shape of the optical flat by rotation of the characteristic diagram. The surface shape and thickness variation were measured with these two algorithms and a wavelength-tuning Fizeau interferometer.

Analysis of fracture, force, and temperature in orthogonal elliptical vibration-assisted bone cutting.

Bone is a natural composite and its cutting is a common procedure in orthopedic surgery. The processing damage, cutting force, and cutting heat strongly influence postoperative recovery. In this study, a orthogonal elliptical vibration-assisted (EVA) bone cutting system is developed based on semi-brittle behaviors of bone to experimentally investigate fracture, cutting force, roughness and temperature rise. To prevent large-scale fractures during bone cutting, an extended finite element method model incorporating detailed microstructure and material properties of bone is created to understand the crack-propagation mechanism. Both the simulation and the experiments demonstrate that the elliptical vibration could effectively control the direction of crack propagation. The experimental results also demonstrate that the cutting force and surface roughness decreases with an increase in the vibration frequency or amplitude, whereas temperature rise increases with the vibration frequency. These findings prove that the EVA could allow for low-trauma bone cutting in orthopedic surgery.

Experimental study of temperature rise during bone drilling process.

An excessive temperature rise during bone drilling processes can result in osteonecrosis or impairment of the osteogenic potential. However, the effect of geometric parameters of the surgical drill bit, drilling process parameters, and the bone type on the temperature rise have not been fully investigated. In this study, thermocouples are introduced to measure the temperature rise, and three experimental designs are utilized separately to investigate the temperature rise with respect to each parameter, identify the effect of important drill geometric parameters and their interaction on the temperature rise, and develop a quadratic model of the temperature rise with respect to process parameters. The results show that the temperature rise can be significantly affected by geometric parameters of the surgical drill bit, drilling process parameters, and the bone type. The effects of the point angle and the interaction between the web thickness and the helix angle on the temperature rise are very significant. The quadratic regression equation obtained using response surface methodology can provide accurate predictions under a wide range of drilling process conditions, and the optimized drilling process parameters are in good agreement with the experimental results.

Experimental and Finite Element Analysis of Force and Temperature in Ultrasonic Vibration Assisted Bone Cutting.

Bone cutting is an essential procedure of orthopedic surgery, while irreversible bone damage would be inevitably caused using the conventional cutting (CC) method. In this study, an ultrasonic vibration-assisted cutting (UVAC) method was applied in bone cutting to investigate the cutting performance, considering the cutting force and temperature rise, in comparison with CC. In addition, a finite element (FE) model was developed to investigate the cutting mechanism and the influence of a wide range of processing parameters on the performance of cutting bone. The results indicate that the proposed FE model shows good correlation with the experimental results for both cutting force and temperature. UVAC can significantly reduce the cutting force and increase the temperature in comparison with CC from the experimental and predicted results. The cutting force tends to decrease with the increasing vibrational parameters and decreasing cutting speed, while the temperature increases. The verified FE bone cutting model provides an efficient way to assist the optimization of the processing conditions in bone cutting operations.

Mechanism of material removal in orthogonal cutting of cortical bone.

ANALYSIS: of a mechanism of bone cutting has an important theoretical and practical significance for orthopaedic surgeries. In this study, the mechanism of material removal in orthogonal cutting of cortical bone is investigated. Chip morphology and crack propagation in cortical bone for various cutting directions and depth-of-cut (DOC) levels are analysed, with consideration of microstructural and sub-microstructural features and material anisotropy. Effects of different material properties of osteons, interstitial matrix and cement lines on chip morphology and crack propagation are elucidated for different cutting directions. This study revealed that differences in chip morphology for various DOCs were due to comparable sizes of the osteons, lamellae and DOC. Acquired force signals and recorded high-speed videos revealed the reasons of fluctuations of dynamic components in tests. Meanwhile, a frequency-domain analysis of force signals showed a frequency difference between formation of a bulk fractured chip and small debris for different cutting directions. In addition, SEM images of the top and side surfaces of the machined bone were obtained. Thus, the analysis of the cutting force and surface damage validated the character of chip formation and explained the material-removal mechanism. This study reveals the mechanism of chip formation in the orthogonal cutting of the cortical bone, demonstrating importance of the correlation between the chip morphologies, the depth of cut and the microstructure and sub-microstructure of the cortical bone. For the first time, it assessed the fluctuations of cutting forces, accompanying chip formation, in time and frequency domains. These findings provide fundamental information important for analysis of cutting-induced damage of the bone tissue, optimization of the cutting process and clinical applications of orthopaedic instruments.