Friedmar Graichen

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.

Hip joint loading during walking and running, measured in two patients.

The resultant hip joint force, its orientation and the moments were measured in two patients during walking and running using telemetering total hip prostheses. One patient underwent bilateral joint replacement and a second patient, additionally suffering from a neuropathic disease and atactic gait patterns, received one instrumented hip implant. The joint loading was observed over the first 30 and 18 months, respectively, following implantation. In the first patient the median peak forces increased with the walking speed from about 280% of the patients body weight (BW) at 1 km h-1 to approximately 480% BW at 5 km h-1. Jogging and very fast walking both raised the forces to about 550% BW; stumbling on one occasion caused magnitudes of 720% BW. In the second patient median forces at 3 km h-1 were about 410% BW and a force of 870% BW was observed during stumbling. During all types of activities, the direction of the peak force in the frontal plane changed only slightly when the force magnitude was high. Perpendicular to the long femoral axis, the peak force acted predominantly from medial to lateral. The component from ventral to dorsal increased at higher force magnitudes. In one hip in the first patient and in the second patient the direction of large forces approximated the average anteversion of the natural femur. The torsional moments around the stem of the implant were 40.3 N m in the first patient and 24 N m in the second.

Loading of the knee joint during activities of daily living measured in vivo in five subjects

Detailed knowledge about loading of the knee joint is essential for preclinical testing of implants, validation of musculoskeletal models and biomechanical understanding of the knee joint. The contact forces and moments acting on the tibial component were therefore measured in 5 subjects in vivo by an instrumented knee implant during various activities of daily living. Average peak resultant forces, in percent of body weight, were highest during stair descending (346% BW), followed by stair ascending (316% BW), level walking (261% BW), one legged stance (259% BW), knee bending (253% BW), standing up (246% BW), sitting down (225% BW) and two legged stance (107% BW). Peak shear forces were about 10-20 times smaller than the axial force. Resultant forces acted almost vertically on the tibial plateau even during high flexion. Highest moments acted in the frontal plane with a typical peak to peak range -2.91% BWm (adduction moment) to 1.61% BWm (abduction moment) throughout all activities. Peak flexion/extension moments ranged between -0.44% BWm (extension moment) and 3.16% BWm (flexion moment). Peak external/internal torques lay between -1.1% BWm (internal torque) and 0.53% BWm (external torque). The knee joint is highly loaded during daily life. In general, resultant contact forces during dynamic activities were lower than the ones predicted by many mathematical models, but lay in a similar range as measured in vivo by others. Some of the observed load components were much higher than those currently applied when testing knee implants.

Hip joint contact forces during stumbling.

AimTo determine whether load directions for stumbling are similar to those for common activities and whether stumbling can be realistically simulated under laboratory conditions without endangering the patients.MethodThe magnitudes and directions of hip contact forces were measured during real and simulated stumbling and compared with those found during various other everyday activities. Measurements were obtained by use of hip implants with built-in load sensors and telemetry.ResultsPeak forces are approximately twice as high during real stumbling as during any other activity and may range higher than eight-times the body weight. Simulated stumbling leads to much lower contact forces, especially if this happens after a warning. Accidental stumbling in everyday situations should, therefore, be avoided, especially in patients with hip replacements or arthrosis.ConclusionsThe directions of peak hip contact forces relative to the femoral bone are nearly constant for any activity, including real stumbling. This observation supports the assumption that muscle and bone anatomy plus muscle function are optimized in order to minimize stresses in bone and muscles. Any impairment of such a mechanically balanced system will increase the musculoskeletal loads. Malposition of total hip implants or muscle deficits caused by the surgical approach must, therefore, be avoided or minimized.

ESB clinical biomechanics award 2008: Complete data of total knee replacement loading for level walking and stair climbing measured in vivo with a follow-up of 6-10 months

BACKGROUNDnDetailed information about the loading of the knee joint is required for various investigations in total knee replacement. Up to now, gait analysis plus analytical musculo-skeletal models were used to calculate the forces and moments acting in the knee joint. Currently, all experimental and numerical pre-clinical tests rely on these indirect measurements which have limitations. The validation of these methods requires in vivo data; therefore, the purpose of this study was to provide in vivo loading data of the knee joint.nnnMETHODSnA custom-made telemetric tibial tray was used to measure the three forces and three moments acting in the implant. This prosthesis was implanted into two subjects and measurements were obtained for a follow-up of 6 and 10 months, respectively. Subjects performed level walking and going up and down stairs using a self-selected comfortable speed. The subjects activities were captured simultaneously with the load data on a digital video tape. Customized software enabled the display of all information in one video sequence.nnnFINDINGSnThe highest mean values of the peak load components from the two subjects were as follows: during level walking the forces were 276%BW (percent body weight) in axial direction, 21%BW (medio-lateral), and 29%BW (antero-posterior). The moments were 1.8%BW*m in the sagittal plane, 4.3%BW*m (frontal plane) and 1.0%BW*m (transversal plane). During stair climbing the axial force increased to 306%BW, while the shear forces changed only slightly. The sagittal plane moment increased to 2.4%BW*m, while the frontal and transversal plane moments decreased slightly. Stair descending produced the highest forces of 352%BW (axial), 35%BW (medio-lateral), and 36%BW (antero-posterior). The sagittal and frontal plane moments increased to 2.8%BW*m and 4.6%BW*m, respectively, while the transversal plane moment changed only slightly.nnnINTERPRETATIONnUsing the data obtained, mechanical simulators can be programmed according to realistic load profiles. Furthermore, musculo-skeletal models can be validated, which until now often lacked the ability to predict properly the non-sagittal load values, e.g. varus-valgus and internal-external moments.

Implantable 9-Channel Telemetry System for In Vivo Load Measurements With Orthopedic Implants

Knowledge of the loads to which orthopedic implants are subjected is a fundamental prerequisite for their optimal biomechanical design, long-term success, and improved rehabilitation outcomes. In vivo load measurements are more accurate than those obtained using mathematical musculoskeletal models. An inductively powered integrated circuit inside the implant measures six load components as well as the temperature and supplied voltage. This low-power circuit includes a 9-channel multiplexer, a programmable memory, a pulse interval modulator, and a radio-frequency transmitter. Together with a few passive components, the integrated circuit is mounted on a ceramic substrate with thick-film hybrid technology. The sensor signals are multiplexed, modulated, and transmitted to an external device. The microcontroller of the external device regulates the alternating magnetic field produced by a power oscillator and synchronizes the pulse interval modulated data stream. A personal computer displays forces, moments, and temperatures in real time. The new telemetry transmitter has, thus far, been used for in vivo load measurements in three patients with shoulder endoprostheses. Eight instrumented vertebral body replacements are ready for implantation, and an instrumented tibial tray is being submitted to laboratory tests

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.

Realistic loads for testing hip implants

The aim here was to define realistic load conditions for hip implants, based on in vivo contact force measurements, and to see whether current ISO standards indeed simulate real loads. The load scenarios obtained are based on in vivo hip contact forces measured in 4 patients during different activities and on activity records from 31 patients. The load scenarios can be adapted to various test purposes by applying average or high peak loads, high-impact activities or additional low-impact activities, and by simulating normal or very active patients. The most strenuous activities are walking (average peak forces 1800 N, high peak forces 3900 N), going up stairs (average peak forces 1900 N, high peak forces 4200 N) and stumbling (high peak forces 11,000 N). Torsional moments are 50% higher for going up stairs than for walking. Ten million loading cycles simulate an implantation time of 3.9 years in active patients. The in vitro fatigue properties of cementless implant fixations are exceeded during stumbling. At least for heavyweight and very active subjects, the real load conditions are more critical than those defined by the ISO standards for fatigue tests.

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.

In vivo measurement of shoulder joint loads during activities of daily living

Until recently the contact loads acting in the glenohumeral joint have been calculated using musculoskeletal models or measured in vitro. Now, contact forces and moments are measured in vivo using telemeterized shoulder implants. Mean total contact forces from four patients during eight activities of daily living are reported here. Lifting a coffee pot (1.5kg) with straight arm caused an average force of 105.0%BW (%body weight) (range: 90-124.6%BW), while setting down the coffee pot in the same position led to higher forces of 122.9%BW on the average (105.3-153.4%BW). The highest joint contact forces were measured when the straight arm was abducted or elevated by 90 degrees or more, with a weight in the hand. Lifting up 2kg from a board up to head height caused a contact force of 98.3%BW (93-103.6%BW); again, setting it down on the board led to higher forces of 131.5%BW (118.8-144.1%BW). In contrast to previously calculated high loads, the contact force during passive holding of a 10kg weight laterally was only 12.3%BW (9.2-17.9%BW), but when lifting it up to belt height it increased to 91.5%BW (87-95%BW). The moments transferred inside the joint at our patients varied much more than did the forces both inter and intra-individually. Our data suggest that patients with shoulder problems or during the first post-operative weeks after shoulder fractures or joint replacements should avoid certain activities encountered during daily living e.g. lifting or holding a weight with an outstretched arm. Some energy-related optimization criteria used in the literature for analytical musculoskeletal shoulder models must now be reconsidered.