Hip contact forces and gait patterns from routine activities.

In vivo loads acting at the hip joint have so far only been measured in few patients and without detailed documentation of gait data. Such information is required to test and improve wear, strength and fixation stability of hip implants. Measurements of hip contact forces with instrumented implants and synchronous analyses of gait patterns and ground reaction forces were performed in four patients during the most frequent activities of daily living. From the individual data sets an average was calculated. The paper focuses on the loading of the femoral implant component but complete data are additionally stored on an associated compact disc. It contains complete gait and hip contact force data as well as calculated muscle activities during walking and stair climbing and the frequencies of daily activities observed in hip patients. The mechanical loading and function of the hip joint and proximal femur is thereby completely documented. The average patient loaded his hip joint with 238% BW (percent of body weight) when walking at about 4 km/h and with slightly less when standing on one leg. This is below the levels previously reported for two other patients (Bergmann et al., Clinical Biomechanics 26 (1993) 969-990). When climbing upstairs the joint contact force is 251% BW which is less than 260% BW when going downstairs. Inwards torsion of the implant is probably critical for the stem fixation. On average it is 23% larger when going upstairs than during normal level walking. The inter- and intra-individual variations during stair climbing are large and the highest torque values are 83% larger than during normal walking. Because the hip joint loading during all other common activities of most hip patients are comparably small (except during stumbling), implants should mainly be tested with loading conditions that mimic walking and stair climbing.     

Strength requirements for internal and external prostheses

Throughout the history of development of joint replacement implants and external prostheses there have been mechanical failures due to a discrepancy between material strength, cross-sectional characteristics and the loads developed in normal or abnormal function by the patient utilising the device. Particularly for internal prostheses attention is being paid at the present time to wear characteristics and the requirements for the articulating surfaces and the volume of wear particles produced during tests simulating the use of the device within the patient. The particular importance of the wear particles is that they seem to be associated with accelerated resorption of bone at areas essential for successful fixation of the implant within it. This article will consider joint replacements at the knee and hip and external prostheses for the leg. If failure due to external trauma is ignored the loads to be considered in testing standards correspond in implants to the muscular and ligamentous forces related to the forces developed between ground and foot and to the bending moments in the structure of leg prostheses. Generally it can be assumed that the treatment of the patient following trauma is more easily accomplished and more likely to be successful if the prosthesis has failed and not the bony structure of the patient. However, the author is unaware that these devices have ever been designed to have lower intrinsic strength than the anatomical structures to which they are connected; indeed in many cases particularly for implants they are much stronger than the bone to which they are connected. The major difficulty in rational design of prosthetic devices has been uncertainty about the importance of occasionally applied loads of a high value relative to those on a frequent basis and also to the frequency of application of these overloads. In this paper consideration is given to methods of determination of load systems relevant to the mechanical performance of implanted joint replacements at the hip and the knee and external prostheses for leg amputees. New data are presented relating to walking, other daily activities and the corresponding frequency of occurrence of these. Loading data on implants obtained by various biomechanical models is compared and related to the loads actually measured by implanted transducers. The philosophy of the standardised test load systems and the performance requirements is reviewed.     

Hip endoprosthesis for in vivo measurement of joint force and temperature.

Friction between the prosthetic head and acetabular cup increases the temperature in hip implants during activities like walking. A hip endoprosthesis was instrumented with sensors to measure the joint contact forces and the temperature distribution along the entire length of the titanium implant. Sensors and two inductively powered telemetry units are placed inside the hip implant and hermetically sealed against body fluids. Each telemetry unit contains an integrated 8-channel telemetry chip and a radio frequency transmitter. Force, temperature and power supply data are transmitted at different frequencies by two antennas to an external twin receiver. The inductive power supply is controlled by a personal computer. Force and temperature are monitored in real time and all data are stored on a video tape together with the patient's images. This paper describes the design and accuracy of the instrumented implant and the principal function of the external system components.     

Knee mechanics: a review of past and present techniques to determine in vivo loads

This review article evaluates various techniques that have been used to determine in vivo loads in the human knee. Two main techniques that have been used are telemetry, which is an experimental approach, and mathematical modeling, which is a theoretical approach. Telemetric analyses have previously been used to determine the in vivo loading of the human hip and more recently evaluated in the determination of in vivo knee loads. Mathematical modeling approaches can be categorized two ways; those that use optimization techniques to solve an indeterminate system and those that utilize a reduction method that minimizes the number of unknowns, keeping the system solvable as the number of equations of motion are equal to the number of unknown quantities. More recently, we have developed an approach that relies fully on the use of in vivo data from fluoroscopy, CT scanning, magnetic resonant imaging and a revised motion analysis technique that involves only two markers on each rigid body. A review of all techniques revealed a wide range of forces at the human knee, ranging from 1.9 to 7.2 times body weight during level walking.     

Frictional heating of total hip implants. Part 1: measurements in patients

Hip implants heat up due to friction during long lasting, high loading activities like walking. Thermal damage in the surrounding soft and hard tissues and deteriorated lubrication of synovial fluid could contribute to implant loosening. The goal of this study was to determine the implant temperatures in vivo under varying conditions. Temperatures and contact forces in the joints were measured in seven joints of five patients using instrumented prostheses with alumina ceramic heads and telemetry data transmission. The peak temperature in implants with polyethylene cups rose up to 43.1 degrees C after an hour of walking but varied considerably individually. Even higher temperatures at the joints are probable for patients with higher body weight or while jogging. The peak temperature was lower with a ceramic cup, showing the influence of friction in the joint. During cycling the peak temperatures were lower than during walking, proving the effect of force magnitudes on the produced heat. However, no positive correlation was found between force magnitude and maximum temperature during walking. Other individual parameters than just the joint force influence the implant temperatures. Based on the obtained data and the available literature about thermal damage of biological tissues a detrimental effect of friction induced heat on the stability of hip implants cannot be excluded. Because the potential risk for an individual patient cannot be foreseen, the use and improvement of low friction implant materials is important.     

High-tech hip implant for wireless temperature measurements in vivo

When walking long distances, hip prostheses heat up due to friction. The influence of articulating materials and lubricating properties of synovia on the final temperatures, as well as any potential biological consequences, are unknown. Such knowledge is essential for optimizing implant materials, identifying patients who are possibly at risk of implant loosening, and proving the concepts of current joint simulators. An instrumented hip implant with telemetric data transfer was developed to measure the implant temperatures in vivo. A clinical study with 100 patients is planned to measure the implant temperatures for different combinations of head and cup materials during walking. This study will answer the question of whether patients with synovia with poor lubricating properties may be at risk for thermally induced bone necrosis and subsequent implant failure. The study will also deliver the different friction properties of various implant materials and prove the significance of wear simulator tests. A clinically successful titanium hip endoprosthesis was modified to house the electronics inside its hollow neck. The electronics are powered by an external induction coil fixed around the joint. A temperature sensor inside the implant triggers a timer circuit, which produces an inductive pulse train with temperature-dependent intervals. This signal is detected by a giant magnetoresistive sensor fixed near the external energy coil. The implant temperature is measured with an accuracy of 0.1°C in a range between 20°C and 58°C and at a sampling rate of 2-10 Hz. This rate could be considerably increased for measuring other data, such as implant strain or vibration. The employed technique of transmitting data from inside of a closed titanium implant by low frequency magnetic pulses eliminates the need to use an electrical feedthrough and an antenna outside of the implant. It enables the design of mechanically safe and simple instrumented implants.     

Four-channel telemetry system for in vivo measurement of hip joint forces

The long-term loosening of artificial hip joints remains a serious clinical problem. Optimization of implant design and material will improve the fixation, but it requires a detailed knowledge of the forces which act on the implant. A four-channel telemetric transmitter was developed and arranged completely inside a hermetically closed artificial hip joint. This permits long term in vivo measurements of the three-dimensional forces without endangering the patient. The external telemetry system consists of an inductive power supply, an RF receiver, a microcomputer with hardware extension and a VHS video system. The personal computer offers real-time data processing of three orthogonal force components as well as slow motion analysis of recorded measurements. After several years of animal tests, two instrumented prostheses were implanted in the first patient (male) in May and August of 1988. In March 1990 a third prosthesis was implanted in a second patient (female). Joint force measurements have regularly been performed from the first post-operative day until now for several kinds of activity.     

A method for in vivo measurements of implants used in the reconstruction of spinal fractures]

Little is known about the forces and moments acting on the spinal column during various movements and activities, and thus about the loading of implants used in the treatment of spinal fractures. A measuring method using an external fixation device was developed to determine the in vivo loading of spinal implants. On the basis of these data the forces acting on the spine can be calculated. The present paper describes this measuring method as applied to the fixation device, and initial experience with this device in a first application is presented.     

In vivo hip joint loads and pedal forces during ergometer cycling

The rising prevalence of osteoarthritis and an increase in total hip replacements calls for attention to potential therapeutic activities. Cycling is considered as a low impact exercise for the hip joint and hence recommended. However, there are limited data about hip joint loading to support this claim. The aim of this study was to measure synchronously the in vivo hip joint loads and pedal forces during cycling. The in vivo hip joint loads were measured in 5 patients with instrumented hip implants. Data were collected at several combinations of power and cadence, at two saddle heights.Joint loads and pedal forces showed strong linear correlation with power. So the relationship between the external pedal forces and internal joint forces was shown. While cycling at different cadences the minimum joint loads were acquired at 60 RPM. The lower saddle height configuration results in an approximately 15% increase compared to normal saddle height.The results offered new insights into the actual effects of cycling on the hip joint and can serve as useful tools while developing an optimum cycling regimen for individuals with coxarthrosis or following total hip arthroplasty. Due to the relatively low contact forces, cycling at a moderate power level of 90 W at a normal saddle height is suitable for patients.     

Musculo-skeletal loading conditions at the hip during walking and stair climbing.

Musculo-skeletal loading plays an important role in the primary stability of joint replacements and in the biological processes involved in fracture healing. However, current knowledge of musculo-skeletal loading is still limited. In the past, a number of musculo-skeletal models have been developed to estimate loading conditions at the hip. So far, a cycle-to-cycle validation of predicted musculo-skeletal loading by in vivo measurements has not been possible. The aim of this study was to determine the musculo-skeletal loading conditions during walking and climbing stairs for a number of patients and compare these findings to in vivo data.Following total hip arthroplasty, four patients underwent gait analysis during walking and stair climbing. An instrumented femoral prosthesis enabled simultaneous measurement of in vivo hip contact forces. On the basis of CT and X-ray data, individual musculo-skeletal models of the lower extremity were developed for each patient. Muscle and joint contact forces were calculated using an optimization algorithm. The calculated peak hip contact forces both over- and under-estimated the measured forces. They differed by a mean of 12% during walking and 14% during stair climbing.For the first time, a cycle-to-cycle validation of predicted musculo-skeletal loading was possible for walking and climbing stairs in several patients. In all cases, the comparison of in vivo measured and calculated hip contact forces showed good agreement.Thus, the authors consider the presented approach as a useful means to determine valid conditions for the analysis of prosthesis loading, bone modeling or remodeling processes around implants and fracture stability following internal fixation.     

Instrumented hip implants: Electric supply systems

Instrumented hip implants were proposed as a method to monitor and predict the biomechanical and thermal environment surrounding such implants. Nowadays, they are being developed as active implants with the ability to prevent failures by loosening. The generation of electric energy to power active mechanisms of instrumented hip implants remains a question. Instrumented implants cannot be implemented without effective electric power systems. This paper surveys the power supply systems of seventeen implant architectures already implanted in-vivo, namely from instrumented hip joint replacements and instrumented fracture stabilizers. Only inductive power links and batteries were used in-vivo to power the implants. The energy harvesting systems, which were already designed to power instrumented hip implants, were also analyzed focusing their potential to overcome the disadvantages of both inductive-based and battery-based power supply systems. From comparative and critical analyses of the methods to power instrumented implants, one can conclude that: inductive powering and batteries constrain the full operation of instrumented implants; motion-driven electromagnetic energy harvesting is a promising method to power instrumented passive and active hip implants.     

Dual perfluorocarbon method to noninvasively monitor dissolved oxygen concentration in tissue engineered constructs in vitro and in vivo

Noninvasive in vivo monitoring of tissue implants provides important correlations between construct function and the observed physiologic effects. As oxygen is a key parameter affecting cell and tissue function, we established a monitoring method that utilizes ¹⁹F nuclear magnetic resonance (NMR) spectroscopy, with perfluorocarbons (PFCs) as oxygen concentration markers, to noninvasively monitor dissolved oxygen concentration (DO) in tissue engineered implants. Specifically, we developed a dual PFC method capable of simultaneously measuring DO within a tissue construct and its surrounding environment, as the latter varies among animals and with physiologic conditions. In vitro studies using an NMR‐compatible bioreactor demonstrated the feasibility of this method to monitor the DO within alginate beads containing metabolically active murine insulinoma βTC‐tet cells, relative to the DO in the culture medium, under perfusion and static conditions. The DO profiles obtained under static conditions were supported by mathematical simulations of the system. In vivo, the dual PFC method was successful in tracking the oxygenation state of entrapped βTC‐tet cells and the surrounding peritoneal DO over 16 days in normal mice. DO measurements correlated well with the extent of cell growth and host cell attachment examined postexplantation. The peritoneal oxygen environment was found to be variable and hypoxic, and significantly lower in the presence of metabolically active cells. The significance of the dual PFC system in providing critical DO measurements for entrapped cells and other tissue constructs, in vitro and in vivo, is discussed. © 2011 American Institute of Chemical Engineers Biotechnol. Prog., 2011     

Developing simulations to reproduce in vivo fluoroscopy kinematics in total knee replacement patients

For clinically predictive testing and design-phase evaluation of prospective total knee replacement (TKR) implants, devices should ideally be evaluated under physiological loading conditions which incorporate population-level variability. A challenge exists for experimental and computational researchers in determining appropriate loading conditions for wear and kinematic knee simulators which reflect in vivo joint loading conditions. There is a great deal of kinematic data available from fluoroscopy studies. The purpose of this work was to develop computational methods to derive anterior–posterior (A–P) and internal–external (I–E) tibiofemoral (TF) joint loading conditions from in vivo kinematic data. Two computational models were developed, a simple TF model, and a more complex lower limb model. These models were driven through external loads applied to the tibia and femur in the TF model, and applied to the hip, ankle and muscles in the lower limb model. A custom feedback controller was integrated with the finite element environment and used to determine the external loads required to reproduce target kinematics at the TF joint. The computational platform was evaluated using in vivo kinematic data from four fluoroscopy patients, and reproduced in vivo A–P and I–E motions and compressive force with a root-mean-square (RMS) accuracy of less than 1 mm, 0.1°, and 40 N in the TF model and in vivo A–P and I–E motions, TF flexion, and compressive loads with a RMS accuracy of less than 1 mm, 0.1°, 1.4°, and 48 N in the lower limb model. The external loading conditions derived from these models can ultimately be used to establish population variability in loading conditions, for eventual use in computational as well as experimental activity simulations.     

Standardized Loads Acting in Knee Implants

The loads acting in knee joints must be known for improving joint replacement, surgical procedures, physiotherapy, biomechanical computer simulations, and to advise patients with osteoarthritis or fractures about what activities to avoid. Such data would also allow verification of test standards for knee implants. This work analyzes data from 8 subjects with instrumented knee implants, which allowed measuring the contact forces and moments acting in the joint. The implants were powered inductively and the loads transmitted at radio frequency. The time courses of forces and moments during walking, stair climbing, and 6 more activities were averaged for subjects with I) average body weight and average load levels and II) high body weight and high load levels. During all investigated activities except jogging, the high force levels reached 3,372–4,218N. During slow jogging, they were up to 5,165N. The peak torque around the implant stem during walking was 10.5 Nm, which was higher than during all other activities including jogging. The transverse forces and the moments varied greatly between the subjects, especially during non-cyclic activities. The high load levels measured were mostly above those defined in the wear test ISO 14243. The loads defined in the ISO test standard should be adapted to the levels reported here. The new data will allow realistic investigations and improvements of joint replacement, surgical procedures for tendon repair, treatment of fractures, and others. Computer models of the load conditions in the lower extremities will become more realistic if the new data is used as a gold standard. However, due to the extreme individual variations of some load components, even the reported average load profiles can most likely not explain every failure of an implant or a surgical procedure.     

Design, calibration and pre-clinical testing of an instrumented tibial tray

An instrumented tibial tray was developed that enables the measurement of six load components in a total knee arthroplasty (TKA). The design is fully compatible with a commonly available knee arthroplasty product since it uses the original tibial insert and femoral component. Two plates with hollow stems made from titanium alloy are separated by a small gap. Six semiconductor strain gages are used for measuring the load-dependent deformation of the inner hollow stem. A 9-channel telemetry unit with a radio-frequency transmitter is encapsulated hermetically in the cavity of the prosthesis. The telemetry is powered inductively and strain gage signals are transmitted via a small antenna at the tip of the implant. The mean sampling rate is 125Hz. The calibration of the prosthesis resulted in an accuracy better than 2% mean measuring error. Fatigue testing of the implant was performed up to 10 million loading cycles and showed no failure. The pending in vivo application will give further insight into the kinetics of TKA. The measured values will enhance the quality of future pre-clinical testing, numerical modeling in knee biomechanics and the patients' physiotherapy and rehabilitation.     

Design and calibration of load sensing orthopaedic implants

Contact forces and moments act on orthopaedic implants such as joint replacements. The three forces and three moment components can be measured by six internal strain gauges and wireless telemetric data transmission. The accuracy of instrumented implants is restricted by their small size, varying modes of load transfer, and the accuracy of calibration. Aims of this study were to test with finite element studies design features to improve the accuracy, to develop simple but accurate calibration arrangements, and to select the best mathematical method for calculating the calibration constants. Several instrumented implants, and commercial and test transducers were calibrated using different loading setups and mathematical methods. It was found that the arrangement of flexible elements such as bellows or notches between the areas of load transfer and the central sensor locations is most effective to improve the accuracy. Increasing the rigidity of the implant areas, which are fixed in bones or articulate against joint surfaces, is less effective. Simple but accurate calibration of the six force and moment components can be achieved by applying eccentric forces instead of central forces and pure moments. Three different methods for calculating the measuring constants proved to be equally well suited. Employing these improvements makes it possible to keep the average measuring errors of many instrumented implants below 1-2% of the calibration ranges, including cross talk. Additional errors caused by noise of the transmitted signals can be reduced by filtering if this is permitted by the sampling rate and the required frequency content of the loads.     

Determination of trace cobalt concentrations in human serum by adsorptive stripping voltammetry

The goal of our study was to develop an accurate and reliable method for determining trace cobalt concentrations in human serum. The method was used to determine cobalt in the sera of healthy persons and patients with orthopaedic implants containing cobalt - a possible source of systemic release of cobalt into the human body. This goal is of vital interest since cobalt and its compounds are classified by IARC as potentially carcinogenic to humans. We used an electrochemical method, adsorptive stripping voltammetry (AdSV), which made possible the low detection limit and high sensitivity needed for measurements in human serum. The serum was acid digested by a combination of H2SO4, HNO3 and H2O2 in a 10 mL Kjeldhal flask. The digested sample was then dissolved in 0.1 mol/L ammonia buffer, pH 9.0 +/- 0.2. The determination is based on the adsorptive collection of the complex of cobalt (II) with dimethylglyoxime on a hanging mercury drop electrode (HMDE). The optimum values of adsorption potential and time were determined to be -0.8 V and 60 s. The optimisation of the sample digestion protocol and measurement procedures ensured the reliable assessment of low cobalt concentrations, down to 0.03 microg/L. The mean concentration of serum cobalt in four healthy persons was 0.11 +/- 0.06 microg/L, and in four patients with total hip replacements 0.34 +/- 0.07 microg/L. This method will be used routinely for measuring serum cobalt levels in patients with total hip replacements.     

In vivo contact kinematics and contact forces of the knee after total knee arthroplasty during dynamic weight-bearing activities

Analysis of polyethylene component wear and implant loosening in total knee arthroplasty (TKA) requires precise knowledge of in vivo articular motion and loading conditions. This study presents a simultaneous in vivo measurement of tibiofemoral articular contact forces and contact kinematics in three TKA patients. These measurements were accomplished via a dual fluoroscopic imaging system and instrumented tibial implants, during dynamic single leg lunge and chair rising-sitting. The measured forces and contact locations were also used to determine mediolateral distribution of axial contact forces. Contact kinematics data showed a medial pivot during flexion of the knee, for all patients in the study. Average axial forces were higher for lunge compared to chair rising-sitting (224% vs. 187% body weight). In this study, we measured peak anteroposterior and mediolateral forces averaging 13.3% BW during lunge and 18.5% BW during chair rising-sitting. Mediolateral distributions of axial contact force were both patient and activity specific. All patients showed equitable medial-lateral loading during lunge but greater loads at the lateral compartment during chair rising-sitting. The results of this study may enable more accurate reproduction of in vivo loads and articular motion patterns in wear simulators and finite element models. This in turn may help advance our understanding of factors limiting longevity of TKA implants, such as aseptic loosening and polyethylene component wear, and enable improved TKA designs.     

Frictional heating of total hip implants. Part 2: finite element study

Due to higher friction artificial hip joints warm up more than natural joints during walking and other continuous activities. This could lead to thermal damage in the surrounding tissues and be a reason for long-term implant loosening, an effect which has not yet been investigated. In vivo measurements with instrumented implants showed temperatures inside the prosthetic head up to 43.1 degrees C (Part 1 of this work). Based on the experimental data a finite element model was developed to calculate the temperatures in the tissues surrounding the hip implant to determine whether these tissues can heat up to critical levels. Various parameters were investigated which could account for the variations in the measured temperatures in the patients, including the perfusion rate in tissues, the volume of synovial fluid, and different implant materials. We found that the synovial fluid is most endangered by thermal damage and consequent deterioration of lubricating properties. Implants with a cobalt-chromium head and a polyethylene cup are unfavourable as they can elevate the temperature in the synovia to more than 46 degrees C. With regard to thermal properties stems made from cobalt-chromium alloys are superior to titanium stems, by better conducting heat to the femur and minimizing the synovial fluid temperature. Factors determining the temperatures during walking are insufficiently known or cannot be determined in the individual patient. Therefore, the risk of a thermally induced implant loosening cannot currently be estimated. Under unfavourable conditions such a risk exists, however. General improvements of implant materials and clinical studies on the possibility of implant loosening due to high temperatures are therefore required.     

Loads acting in an intramedullary nail during fracture healing in the human femur.

The form and function of the musculo-skeletal system is closely related to the forces acting in its components. Significant forces are present in the long bones, but their magnitudes have so far only been estimated from mathematical models. Fracture fixation by means of metal implants provides an opportunity to measure the implant-born forces and to estimate the long bone forces before healing occurs. The load changes during fracture healing may provide additional information. Therefore, a telemetrized, interlocking femoral nail for wireless transmission of forces and moments acting across the fracture site was developed. The design was based on the geometry and material of a 16 mm AO nail with a circular, closed cross-section allowing full protection of the electronic circuits from the body fluids. After careful testing, it was implanted in a 33-year-old patient who had sustained a multifragmentary fracture of the left femur. Measurements at a rate of approx. 0.4 Hz were performed in different patient postures between the 2nd and 26th postoperative week. Significant axial forces and bending moments were measured during several activities such as sitting, unsupported leg elevation and partial weight bearing in a standing position. Forces orthogonal to the nail axis remained small. The reductions of the implant loads due to fracture consolidation were in the order of 50%. Dynamization of the nail did not change the forces. Even though the telemetry system did not allow for dynamic measurements and the results presented here provide data from one subject only, the new information will be useful with respect to implant design, biomechanics of fracture fixation and evaluation of healing progression.