A. Rohlmann

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.

Is staircase walking a risk for the fixation of hip implants

Considerable forces and moments act at hip prostheses during most kinds of physical activities. High torque around the stem axis may contribute to implant loosening. With instrumented hip prostheses the joint force and its direction, the bending moment in the frontal plane and the torque were measured in two patients during upstairs, downstairs and level walking. The data give information on whether or not stairclimbing causes a more severe loading situation for the implants than walking. While going upstairs at normal speed the joint force is 10% higher than during walking at 3 km h-1. Downstairs it increases by 20%. The bending moments change by nearly the same amounts. Upstairs the torsional moment is about twice as high as during slow walking. But walking at 5 km h-1 or slow jogging causes forces and moments of similar magnitudes. Even higher loads were observed when the patients stumbled without falling. Although torque during staircase walking is high, extreme values exclusively during stairclimbing are not confirmed by our data. The torsional moments now observed in vivo are close to or even exceed the experimentally determined limits of the torsional strength of implant fixations, found in the literature. Obviously, torsional moments play an important role for the potential loosening of hip prostheses.

Loads on an internal spinal fixation device during walking

Only little knowledge exists concerning the loads on internal spinal fixation devices during walking. In this study, forces and moments were measured in two patients using telemeterized spinal fixators. Although implant loads differed strongly before and after anterior fusion as well as between the two patients, some results were consistent. In every test series, implant loads were higher in walking than in lying, sitting or standing. Walking speed had little influence on implant loads. Staircase walking put slightly higher loads on the implants than normal level walking. Normal use of two crutches reduced implant loads only slightly, whereas a wheeled invalid walker reduced them by about 25%.

Estimation of muscle forces in the lumbar spine during upper-body inclination

OBJECTIVE To estimate the muscle forces during upper-body inclination and to determine their influence on stress distribution in the annulus fibrosus of the lumbar spine discs. DESIGN The muscle forces and stresses were calculated using a non-linear finite element model of the lumbar spine. BACKGROUND Little is known about the influence of muscle forces on the deformation of, and stresses in, the lumbar spine. In most studies, muscle forces are neglected. METHODS Three-dimensional non-linear finite element models of the ligamentous lumbar spine, with and without internal spinal fixators, were created. They were validated by use of experimental data from in vitro measurements on cadaver specimens. In a second step, the influence of muscle forces on stresses in the annulus fibrosus of the lumbar spine discs was investigated in a parameter study. This was done for different inclination angles of the upper-body. RESULTS Good agreement between analytical and experimental results proved achievable when loading with pure moments in the three main planes of the lumbar spine. For inclination of the upper-body, the flexion angle clearly has a strong influence on the stresses in the lumbar spine while the influence of local muscles was small. The stress distribution in the discs differed considerably when the muscle forces are neglected and only a pure moment is applied. CONCLUSIONS This study confirmed earlier ones that have shown that muscle forces should not be neglected when studying the stresses in the lumbar spine. The local dorsal muscles, however, have only a small influence on the stresses in the discs. RELEVANCE For investigations of the biomechanical effects of spinal implants and surgical procedures, experimental or analytical methods are used. Due to the complexity involved, as well as to a lack of information, muscle forces are often neglected. Our study showed that muscles do in fact have a major influence on the mechanical behaviour of the spine and should always be taken into account.

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.

Finite-element-analysis and experimental investigation in a femur with hip endoprosthesis

The stress distribution in a human femur with an endoprosthesis was determined. The finite element method (FEM) was used for a three-dimensional model with more than 15000 degrees of freedom. Geometrical and material data had been taken for this model from a left femur with endoprosthesis. On the contralateral bone a strain gauge investigation was performed to validate the calculations. Reasonable agreement was achieved. Various modes of loading were investigated. A perfect bond at the interface between materials of different elastic moduli was assumed. The results are valid for endoprosthesis with such structured stem surfaces as allow transfer of tensile and shear stresses.

A comparison of hip joint forces in sheep, dog and man

The hip joint forces of sheep and dogs were measured with instrumented endoprostheses and the results were compared with reported data concerning these forces in man. In all animals load directions with 0 to 30 degrees inclinations relative to the femoral axis predominated. The transverse components mostly acted from medio-ventral directions. While the force orientations varied little during each single stance phase, they changed rapidly during the swing phase. Strong inter- and intra-individual differences of load directions were found in all animals. Irregular forces, acting upwards or transverse to the femur, were frequently observed. Maximum joint forces were up to 110% of body weight and depended more on the postoperative time than on the walking speed. Load orientations in the animals were similar to those reported for man. In this regard sheep and dogs appear equally well suited for tests of hip endoprostheses for man.

Multichannel strain gauge telemetry for orthopaedic implants

In vivo measurements of the loads and deformations occurring in orthopaedic implants will allow future improvements to be made. This paper describes an extremely small telemetry for long term measurements with three strain gauges and methods for an absolutely safe implant design. Developed for measuring the load at hip prostheses, the telemetry can also be used for other implants. Its size makes feasible instrumentation of devices with only slight mechanical modifications. In addition to the description of our own measuring system, the paper gives a survey on the problems of telemetrized implants, on methods for measuring spatial loads, and on the investigations of other authors. Future publications will present in vivo measurements with this telemetry, among others on hip endoprostheses.

2000 Volvo Award winner in biomechanical studies: Monitoring in vivo implant loads with a telemeterized internal spinal fixation device.

Study Design. Implant loads were measured in 10 patients using telemeterized internal spinal fixation devices. Objective. To determine the postoperative temporal course of implant loads. Summary of Background Data. Little information exists regarding the temporal course of loads on internal spinal fixation devices. Methods. The telemeterized internal spinal fixator allows the measurement of three force components and three moments acting in the fixator. Implant loads were determined in up to 20 measuring sessions for different activities, including walking, standing, sitting, lying in the supine position, and lifting an extended leg while in the supine position. Results. Implant loads often increased shortly after anterior interbody fusion was performed. Several patients retained the same high level even after fusion had taken place. This explains the reason why screw breakage sometimes occurs more than half a year after implantation. The time of fusion could not be pinpointed from the loading curves. Conclusions. The results show that fixators may be highly loaded even after fusion has occurred. A flexion bending moment acts on the implant even with the body in a relaxed lying position. This means that already shortly after the anterior procedure, the shape of the spine is not neutral and unloaded, but slightly deformed, which loads the fixators. Pedicle screw breakage more than half a year after insertion does not prove that anterior interbody fusion has not occurred.