A force-sensing device for assistance in soft-tissue balancing during knee arthroplasty

Nowadays, the large majority of the instrumentation for orthopaedic surgery consists of mechanical tools with varying degrees of complexity. To increase the accuracy and the safety of orthopaedic interventions, sensors and computers were recently introduced in the operating room. Computer Assisted Orthopaedic Surgery (CAOS) uses a navigation system that tracks the movements of surgical instruments in real-time and displays their exact location in relation to the operative area. Such technology improved the quality of orthopaedic arthroplasties, but it is still limited to the measurement of kinematic parameters such as axial alignments, position and angle measurements. In Total Knee Arthroplasty (TKA), the ligament balance, which is crucial for the stability and lifetime of implants, is currently only qualitatively assessed. The goal of this thesis was therefore to demonstrate the importance of intraoperative measurement of musculoskeletal forces through the development of a force-sensing device designed to improve the ligament balancing procedure during TKA. Three possible device designs were proposed and evaluated using finite element analysis in an iterative optimization process. The final design consists of two sensitive plates, one for each condyle, a tibial base plate and a set of spacers to adapt the device thickness to the patient-specific tibiofemoral gaps. Each sensitive plate is equipped with three deformable bridges instrumented with thick-film piezoresistive sensors, which allow accurate measurements of the amplitude and location of the tibiofemoral contact forces. The net varus-valgus moment is then computed to characterize the ligament imbalance. Laboratory experiments showed that the device has appropriate accuracy and dynamic range for the intended application. The first experimental trials on a plastic knee joint model and on a cadaver specimen demonstrated the proper in-situ functioning of the device. The performance and surgical advantages of the device were then evaluated in an in-vitro study including four different experiments: 1) Six knee joints were axially loaded. Comparing applied and measured compressive forces demonstrated the accuracy and reliability of in-situ measurements. 2) To estimate the importance of keeping the patella in its anatomical place during imbalance assessment, the effect of patellar eversion on the mediolateral distribution of tibiofemoral contact forces was measured. One fourth of the patellar load was shifted to the lateral compartment. 3) Assessment of knee stability based on condyle contact forces or varus-valgus moments were compared to the current surgical method (difference of varus-valgus loads causing condyle lift-off). The force-based assessment found to be equivalent to the surgical method while the moment-based technique, which is considered optimal, showed a tendency of lateral imbalance. 4) Finally the effect of minor and major medial collateral ligament releases was biomechanically quantified. Large variation among specimens reflected the difficulty of ligament release and the need for intraoperative force monitoring. Two clinical trials were carried out to evaluate the device performance in a surgical environment. After the tibial cut, the medial and lateral tibiofemoral gaps ensuring the knee stability were determined from the device measurements and compared to the femoral cuts performed on the basis of standard instrumentation. The agreement between the two approaches was generally good. The only significant difference was measured on the first patient at 90° flexion. At this point, the surgeon also estimated that the knee was not optimally balanced, thus demonstrating the consistency between his perception and the device measurements. In conclusion, the proposed force-sensing device for assistance in ligament balancing during TKA provides accurate, reliable and useful measurements. In addition to the precise imbalance assessment based on the measurement of forces and moments, important clinical advantages, such as the possibility to keep the patella in its anatomical place during the measurement or the real-time force monitoring during the delicate phase of ligament release, were demonstrated. The encouraging results of the in-vivo trial proved the usability of the device in a surgical environment and opens the way for larger clinical studies. The developed device has thus potential to improve the ligament balancing procedure, the consistency of surgery and the lifetime of TKA, illustrating thereby the clinical benefit of measuring forces during orthopaedic surgeries.

Optische Augenlängenmessung für klinische Anwendungen

Beate Möller

Cataract is the leading cause of blindness worldwide. The clouding of the human eye lens is called "Grauer Star" in German. In the industrialized countries this process of increased blindness is corrected by the surgical removal of the clouded lens and its replacement by a plastic lens (intraocular lens, IOL). Cataract surgery is the most frequent surgery worldwide. In Europe more than two million operations are performed annually. For the selection of an optimum replacement the refractive power of the lens is calculated from the radius of curvature of the cornea, the length of the eye and the anterior chamber depth. Therefore these data have to be measured before every surgery. Until now the length of the eye was measured exclusively by ultrasound in the clinical praxis. For this procedure the sound transducer has to be placed directly or via a water funnel onto the cornea. It was the aim of the work presented in this thesis to lay the framework for introducing into the clinical praxis a contact free optical length measurement of the eye. The basic parameters for the measurement were determined. Starting from the state of the art new solutions were found, implemented in a laboratory device, and tested clinically. A two-beam interferometer with light of short coherence length that had been developed at the University Vienna was analyzed and compared with the one-beam technique. Neglecting practical limitations due to the optical arrangement and assuming shot-noise limited detection the one-beam technique allows the measurement of reflected light that is a factor of two lower compared to the two-beam technique. From the point of view of detection both techniques are therefore equivalent. The advantage of the two-beam technique is its independence from the eye movement. Until now the two-beam technique utilizes for detection only 1 over 400 of the light reflected from the retina and 1 over 2500 to 1 over 4000 of the light reflected from the cornea. This causes the dynamic range to drop by 26 dB. Furthermore the noise of the electronics (excess noise) comes into play. In this work a novel concept for the optical configuration in the two-beam interferometer is presented. By using a diffractive lens cornea and retina are imaged simultaneously onto the detector and the losses of the light from the retina are minimized. The cornea is imaged with an efficiency of 5.6 per cent. In order to minimize the requirements on the patient the measuring time was shortened to 1 s. Therefore the scan speed was increased by a factor of four compared to the state of the art. In the laboratory device it fluctuated by 20 per cent. To keep the detection narrow band it was necessary to implement a signal processing that is largely independent of the scan speed. By using a synchronous demodulation of two 90 degrees phase-shifted channels that are added as vector quantities a filter with self-adjusting center frequency was created. Its bandwidth is 4 kHz with signal frequencies between 30 kHz and 90 kHz. This concept was realized as a compact slit-lamp head and tested clinically. A shot-noise limited detection with a dynamic range of 105 dB was achieved (input power 200 µW). On the healthy eye the signal-to-noise ratio was limited by the digitization range of 8 bit. On the eye with cataract the measuring was not limited by the degree of clouding any more. The optical lengths were determined along the line of sight. They agreed with the post operative results. In some cases refractive errors of more than 2 dpt would have been avoidable by determining the IOL from the measurement of the optical length of the eye. This technique allows a comfortable, contact free optical eye length measurement that works correctly in 95 per cent of all eyes with cataract. Additionally smaller post operative refractive errors can be achieved as compared with traditional techniques. These results were the basis for the development of the IOL-Master – a novel device for the cataract treatment – at the Carl Zeiss Jena GmbH. With this device all biometric data can be measured in a non-contact fashion and intraocular lenses can be calculated and selected.

EPFL2001

Design of Dexterous Mechanical Systems for Intuitive Telemanipulation in Minimally Invasive Surgery

Ricardo Daniel Rita Beira

Major progress in abdominal surgery has occurred over the last decades with the introduction of laparoscopic and minimally invasive techniques in the operating room. These innovative procedures attracted much attention due to several advantages: the need for smaller abdominal incisions resulting in faster recovery of the patient, improved cosmetics, shorter hospitalization and a significant reduction of costs. However, surgical instrumentation for this type of intervention still remains non-intuitive and much more difficult to use than tools for open surgical procedures. As a consequence, these minimally invasive techniques are limited to fairly simple procedures. Due to the landscape of medical reimbursement, there is a substantial push by insurance companies, health care organizations and hospitals to extend minimally invasive techniques to a wider range of surgical procedures in order to reduce hospital stays and therefore costs. In order to respond to these demands, a strong research effort has been made over the past years on the development of enabling minimally invasive technologies, mainly through the introduction and development of robotic systems. Surgical robots significantly contribute to the improvement of the surgical performance by increasing the dexterity and user-friendliness of surgical procedures through the use of robotic telemanipulation. However, despite years of research, the field of surgical robotics is still only at the beginning of a very promising large scale development. Although a large number of systems have been developed, several issues are not yet addressed, limiting the adoption of surgical robots by a broader range of hospitals. A major limitation is related to the lack of internal dexterity, caused by the mobility constraints imposed by the small entry port. On one hand, it is important to increase the dexterity of the end-effectors inside the body, overcoming the issues of limited manoeuvrability in the abdominal cavity. On the other hand, the system must be introduced through conventional trocars, which are small in diameter. The management of this trade-off is extremely challenging, making the development of dexterous micro-manipulators one of the most important issues in the field of robotic systems for surgery. Another limitation is that the current surgical robots are voluminous, competing for precious space within the operating room and significantly increasing the complexity of operating room logistics. Access to the patient is thus impaired, which raises safety concerns. Furthermore, due to the physical separation from the operating area and lack of force-feedback on the existing surgical systems, surgeons cannot feel the contact forces between instruments and tissue. This limitation may cause long operating times and unintentional damage of tissue and suturing material. Although bringing several technical advantages for surgeons, current robotic systems are extremely expensive in acquisition, maintenance, disposable tools and training, representing much higher direct costs compared with open surgery and laparoscopic instrumentation. For this reason, access to robotic surgery is limited to a minority of hospitals that can afford to purchase such systems and have enough patient volume to justify the acquisition. This tendency towards centralisation of complex minimally invasive surgeries draws patients from hospitals without surgical robots and places a significant burden on the healthcare system. This thesis investigates novel mechanical systems to be used in different surgical telemanipulators, solving the limitations of existing robotic and manual surgical equipment, in three main areas: patient safety, surgical dexterity and health care cost. This can be achieved by providing the surgeon with more compact systems comprising new multi-DOF micromanipulators, and new mechanical telemanipulators able to deliver dexterous manipulation through a more affordable technology. These objectives implied not only an investigation of technical aspects such as the performance requirements of surgical tools, but also the investigation of the different medical procedures and surgical tasks used by doctors during minimally invasive interventions. Although, the solutions studied in this thesis have been applied in the context of surgical systems for minimally invasive surgery, the outcome of this research can be extended to several other application fields. From a general perspective, the goal of this thesis is to propose a document which may be useful and inspiring for machine designers, developers, or scientists who wish to create efficient remotely controlled manipulators for several applications involving multi-DOF manipulation.

EPFL2013

Design and integration of an instrumented knee prosthesis

Matteo Simoncini

Total knee arthroplasty is nowadays one of the most important orthopedic surgery. It consists of a procedure in which parts of the knee are replaced by a prosthesis. The largest indication for total knee arthroplasty is osteoarthritis, a knee disease that can cause the cartilage of the femur and tibia to wear away, so that the bones rub together with use. The major risk factors for osteoarthritis are aging and obesity. Both the life expectancy and the obesity rate are increasing in the developed countries, thus the number of estimated total knee arthroplasties is growing over the years. Although over one million of prosthetic joints are implanted every year in the developed countries, none of them contains sensors to help the orthopedic surgeons in improving the precision of the replacement surgery. The goal of this study is to design an electronic system to be embedded inside a total knee prosthesis, in order to measure the force applied to it and its kinematics. Providing the orthopedic surgeons with quantitative data on the biomechanics of the prosthetic knee can help them in improving the implant precision and, as a consequence, could reduce the risk of an early revision surgery. In the frame of this thesis, we worked with the F.I.R.S.T. prosthesis by Symbios Orthopedie SA, that was instrumented with sensors and electronics to measure, process and transmit force and kinematics data to an external reader. The constraints in the design have been established by the medical doctors and the prosthesis manufacturer and the technical solutions adopted are presented. In order to simplify a future approval for human tests, we decided to keep the shape of the knee artificial joint. To achieve that, we put all the sensors and the electronics inside the middle part of the prosthesis, constituted of a polyethylene insert located between the metallic parts of the artificial joint and whose function is to reduce the rubbing. An original encapsulation was designed to guarantee the bio-compatibility of the instrumented prosthesis and to avoid a potentially dangerous contact between the electronics and the human body. This should be ensured even in case of extreme wearing of the polyethylene insert, that can occur some years after the prosthesis implant and is one of the main indications for a revision surgery. The sensors were tested by using mechanical simulators of the knee joint and validated by means of reference sensors. Different demonstrators have been designed, from the first, with only the sensors located inside the prosthesis and all the electronics fabricated in a large-scale outside of it, to the last miniaturized versions, that can be entirely embedded inside the prosthesis. Moreover, an autonomous sensor for balancing the ligaments tension during the knee replacement surgery was designed, fabricated and tested. Such a device could be an important help for the medical doctors during the surgery to improve the precision of the implant and, being not-implantable, could easily obtain an approval for human clinical trials.

EPFL2014