Kathleen Denis

Influence of bone milling parameters on the temperature rise, milling forces and surface flatness in view of robot-assisted total knee arthroplasty

Abstract The authors have developed a robot-assisted procedure to prepare the tibial surface in total knee arthroplasty (TKA). Using hybrid force/velocity control, the surgeon machines the bone by means of an end mill attached to the robot-arm. The milling forces can give information on the local bone quality intra-operatively. This paper describes experiments on human tibial bone to analyse the influence of the feed rate and the milling speed on the temperature rise, the milling forces and the surface flatness.

How correctly does an intramedullary rod represent the longitudinal tibial axes

In a robot-assisted procedure for preparing the tibia in total knee arthroplasty, developed in the authors’ laboratory, an intramedullary rod is used to register the tibia. In 18 formalin-fixed tibias, the difference in orientation was calculated between the intramedullary rod and several longitudinal tibial axes used in clinical practice. This was done using roentgenstereophotogrammetric analysis. Three tibial axes and two insertion techniques were considered. In three-dimensional space, small differences between the axes are observed. The results showed a high standard deviation, indicating the importance of anatomic differences. In the frontal plane, the difference in orientation between rod and tibial axes never exceeded ± 2°. In the sagittal plane, the observed differences were larger. Significant differences between the considered axes appeared. The results of the two insertion techniques were not significantly different. Because an intramedullary rod frequently is used for alignment of the tibia in conventional surgery, these results also are valuable for conventional surgery. In the current study, the accuracy of the intramedullary alignment is examined, without influences of the sawing procedure. Moreover, the study is not limited to the frontal plane; the total accuracy in three-dimensional space, and the accuracy in the frontal and the sagittal planes were studied.

A semi-active milling procedure in view of preparing implantation beds in robot-assisted orthopaedic surgery.

Abstract Bone cutting in total joint reconstructions requires a high accuracy to obtain a well-functioning and long-lasting prosthesis. Hence robot assistance can be useful to increase the precision of the surgical actions. A drawback of current robot systems is that they autonomously machine the bone, in that way ignoring the surgeons experience and introducing a safety risk. This paper presents a semi-active milling procedure to overcome that drawback. In this procedure the surgeon controls robot motion by exerting forces on a force-controlled lever that is attached to the robot end effector. Meanwhile the robot constrains tool motion to the planned motion and generates a tool feed determined by the feed force that the surgeon executes. As a case study the presented milling procedure has been implemented on a laboratory set-up for robot-assisted preparation of the acetabulum in total hip arthroplasty. Two machining methods have been considered. In the first method the surgeon determines both milling trajectory and feed by the forces that he/she executes on the force-controlled lever. In the second method the cavity is machined contour by contour, and the surgeon only provides the feed. Machining experiments have shown that the first method results in large surface irregularities and is not useful. The second method, however, results in accurate cavity preparation and has therefore potential to be implemented in future robot systems.

Comparison of Registration Procedures of the Tibia in Robot-Assisted Total Knee Arthroplasty

The authors have implemented registration procedures by means of an intramedullary rod and by means of surface matching in their robot-assisted procedure for milling the tibia in TKA. Registration by means of an intramedullary rod is a simple and fast procedure that does not need CT-images. The accuracy is better than 2° in the frontal plane. In case of a very deformed tibia or when the tibia is equipped with intramedullary nails from a previous operation however, it is not suitable. Registration using surface matching is an accurate and generally usable procedure. The difference between the predicted and the real orientation is less than 1.1° in the frontal plane and less than 0.5° in the sagittal plane. These registration procedures need immobilization of the tibia. Rigid immobilization is not necessary in case the registration is performed dynamically.

Development of an acoustic measurement protocol to monitor acetabular implant fixation in cementless total hip Arthroplasty: A preliminary study

In cementless total hip arthroplasty (THA), the initial stability is obtained by press-fitting the implant in the bone to allow osseointegration for a long term secondary stability. However, finding the insertion endpoint that corresponds to a proper initial stability is currently based on the tactile and auditory experiences of the orthopedic surgeon, which can be challenging. This study presents a novel real-time method based on acoustic signals to monitor the acetabular implant fixation in cementless total hip arthroplasty. Twelve acoustic in vitro experiments were performed on three types of bone models; a simple bone block model, an artificial pelvic model and a cadaveric model. A custom made beam was screwed onto the implant which functioned as a sound enhancer and insertor. At each insertion step an acoustic measurement was performed. A significant acoustic resonance frequency shift was observed during the insertion process for the different bone models; 250 Hz (35%, second bending mode) to 180 Hz (13%, fourth bending mode) for the artificial bone block models and 120 Hz (11%, eighth bending mode) for the artificial pelvis model. No significant frequency shift was observed during the cadaveric experiment due to a lack of implant fixation in this model. This novel diagnostic method shows the potential of using acoustic signals to monitor the implant seating during insertion.

Consequences of reaming with flat and convex reamers for bone volume and surface area of the glenoid; a basic science study

BackgroundThe effect of reaming on bone volume and surface area of the glenoid is not precisely known. We hypothesize that (1) convex reamers create a larger surface area than flat reamers, (2) flat reamers cause less bone loss than convex reamers, and (3) the amount of bone loss increases with the amount of version correction.MethodsReaming procedures with different types of reamers are performed on similar-sized uniconcave and biconcave glenoids created from Sawbones foam blocks. The loss of bone volume, the size of the remaining surface area, and the reaming depth are measured and evaluated.ResultsReaming with convex reamers results in a significantly larger surface area than with flat reamers for both uniconcave and biconcave glenoids (p = 0.013 and p = 0.001). Convex reamers cause more bone loss than flat reamers, but the difference is only significant for uniconcave glenoids (p = 0.007).ConclusionsIn biconcave glenoids, convex reamers remove a similar amount of bone as flat reamers, but offer a larger surface area while maximizing the correction of the retroversion. In pathological uniconcave glenoids, convex reamers are preferred because of the conforming shape.

Body wall force sensor for simulated minimally invasive surgery: Application to fetal surgery

Surgical interventions are increasingly executed minimal invasively. Surgeons insert instruments through tiny incisions in the body and pivot slender instruments to treat organs or tissue below the surface. While a blessing for patients, surgeons need to pay extra attention to overcome the fulcrum effect, reduced haptic feedback and deal with lost hand-eye coordination. The mental load makes it difficult to pay sufficient attention to the forces that are exerted on the body wall. In delicate procedures such as fetal surgery, this might be problematic as irreparable damage could cause premature delivery. As a first attempt to quantify the interaction forces applied on the patients body wall, a novel 6 degrees of freedom force sensor was developed for an ex-vivo set up. The performance of the sensor was characterised. User experiments were conducted by 3 clinicians on a set up simulating a fetal surgical intervention. During these simulated interventions, the interaction forces were recorded and analysed when a normal instrument was employed. These results were compared with a session where a flexible instrument under haptic guidance was used. The conducted experiments resulted in interesting insights in the interaction forces and stresses that develop during such difficult surgical intervention. The results also implicated that haptic guidance schemes and the use of flexible instruments rather than rigid ones could have a significant impact on the stresses that occur at the body wall.

Vibration-based fixation assessment of tibial knee implants: A combined in vitro and in silico feasibility study

The preoperative diagnosis of loosening of cemented tibial knee implants is challenging. This feasibility study explored the basic potential of a vibration-based method as an alternative diagnostic technique to assess the fixation state of a cemented tibia implant and establish the methods sensitivity limits. A combined in vitro and in silico approach was pursued. Several loosening cases were simulated. The largest changes in the vibrational behavior were obtained in the frequency range above 1500 Hz. The vibrational behavior was described with two features; the frequency response function and the power spectral density band power. Using both features, all experimentally simulated loosening cases could clearly be distinguished from the fully cemented cases. By complementing the experimental work with an in silico study, it was shown that loosening of approximately 14% of the implant surface on the lateral and medial side was detectable with a vibration-based method. Proximal lateral and medial locations on the tibia or locations toward the edge of the implant surface measured in the longitudinal direction were the most sensitive measurement and excitation locations to assess implant fixation. These results contribute to the development of vibration-based methods as an alternative follow-up method to detect loosened tibia implants.

Determination of replicate composite bone material properties using modal analysis

Replicate composite bones are used extensively for in vitro testing of new orthopedic devices. Contrary to tests with cadaveric bone material, which inherently exhibits large variability, they offer a standardized alternative with limited variability. Accurate knowledge of the composites material properties is important when interpreting in vitro test results and when using them in FE models of biomechanical constructs. The cortical bone analogue material properties of three different fourth-generation composite bone models were determined by updating FE bone models using experimental and numerical modal analyses results. The influence of the cortical bone analogue material model (isotropic or transversely isotropic) and the inter- and intra-specimen variability were assessed. Isotropic cortical bone analogue material models failed to represent the experimental behavior in a satisfactory way even after updating the elastic material constants. When transversely isotropic material models were used, the updating procedure resulted in a reduction of the longitudinal Youngs modulus from 16.00GPa before updating to an average of 13.96 GPa after updating. The shear modulus was increased from 3.30GPa to an average value of 3.92GPa. The transverse Youngs modulus was lowered from an initial value of 10.00GPa to 9.89GPa. Low inter- and intra-specimen variability was found.

Vibration Analysis of the Biomechanical Stability of Total Hip Replacements

In cases of severe rheumatoid arthritis or osteoarthritis, the hip joint is substituted by an artificial joint composed of a femoral stem fitted with a spherical head that can rotate inside a cup inserted in the acetabulum. This procedure is called total hip replacement (THR) and is one of the most frequently performed orthopaedic surgeries. For a cementless femoral implant, the fixation is achieved by preparing a slightly undersized bone bed, and the implant is forcefully hammered into the bone. The initial stability at the time of surgery is one of the most important factors to establish long-term survival of the implant. With each surgical hammer blow, the fixation of the implant in the bone increases. However, introducing an implant with a diameter wider than the bones inner canal contour (i.e., press fit) introduces stresses in the cortical bone, which can cause femoral fracture. In response to mechanical excitations (i.e., hammer blows or external vibrations), a femur–implant structure will display vibration modes and frequencies, just like any other mechanical structure. Changes in material properties or boundary conditions of a femur–implant structure will change its vibration modes and frequencies, which can be obtained numerically and experimentally. Therefore, this chapter explains how to perform vibration analysis on a THR component (i.e., the femoral implant) in order to assess femur–implant stability, as well as how to analyze, present, and interpret results.