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.     

Mechanisms of synovial joint and articular cartilage formation: recent advances, but many lingering mysteries

Synovial joints are elegant, critically important, and deceptively simple biomechanical structures. They are comprised of articular cartilage that covers each end of the opposing skeletal elements, synovial fluid that lubricates and nourishes the tissues, ligaments that hold the skeletal elements in check, and a fibrous capsule that insulates the joints from surrounding tissues. Joints also exhibit an exquisite arrays of shapes and sizes, best exemplified by the nearly spherical convex femoral head articulating into a nearly spherical concave hip acetabulum, or a phalangeal joint with two condyles on the distal side articulating in reciprocally-shaped sockets on the opposing proximal side. Though few in number, joint tissues are highly specialized in structure and function. This is illustrated by articular cartilage with its unique extracellular matrix, unique biomechanical resilience, its largely avascular nature, and its ability to persist through life with minimal turnover of its cells and components. The fact that interest in synovial joints has remained unabated for decades is a reflection of their fundamental importance for organism function and quality of life, and for their susceptibility to a variety of acquired and congenital conditions, most importantly arthritis. This has led to many advances in this field that encompass molecular genetics to biomechanics to medicine. Regrettably, what continues to be poorly understood are the mechanisms by which synovial joints actually form in the developing embryo. If available, this information would be not only of indisputable biological interest, but would also have significant biomedical ramifications, particularly in terms of designing novel tissue regeneration or reconstruction therapies. This review focuses on recent advances in understanding the mechanisms of synovial joint formation in the limbs, and places and discusses the information within the context of classic studies and the many mysteries and questions that remain unanswered.     

Postoperative Changes in In Vivo Measured Friction in Total Hip Joint Prosthesis during Walking

Loosening of the artificial cup and inlay is the most common reasons for total hip replacement failures. Polyethylene wear and aseptic loosening are frequent reasons. Furthermore, over the past few decades, the population of patients receiving total hip replacements has become younger and more active. Hence, a higher level of activity may include an increased risk of implant loosening as a result of friction-induced wear. In this study, an instrumented hip implant was used to measure the contact forces and friction moments in vivo during walking. Subsequently, the three-dimensional coefficient of friction in vivo was calculated over the whole gait cycle. Measurements were collected from ten subjects at several time points between three and twelve months postoperative. No significant change in the average resultant contact force was observed between three and twelve months postoperative. In contrast, a significant decrease of up to 47% was observed in the friction moment. The coefficient of friction also decreased over postoperative time on average. These changes may be caused by ‘running-in’ effects of the gliding components or by the improved lubricating properties of the synovia. Because the walking velocity and contact forces were found to be nearly constant during the observed period, the decrease in friction moment suggests an increase in fluid viscosity. The peak values of the contact force individually varied by 32%-44%. The friction moment individually differed much more, by 110%-129% at three and up to 451% at twelve months postoperative. The maximum coefficient of friction showed the highest individual variability, about 100% at three and up to 914% at twelve months after surgery. These individual variations in the friction parameters were most likely due to different ‘running-in’ effects that were influenced by the individual activity levels and synovia properties.     

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.     

"In vivo" determination of hip joint separation and the forces generated due to impact loading conditions

Numerous supporting structures assist in the retention of the femoral head within the acetabulum of the normal hip joint including the capsule, labrum, and ligament of the femoral head (LHF). During total hip arthroplasty (THA), the LHF is often disrupted or degenerative and is surgically removed. In addition, a portion of the remaining supporting structures is transected or resected to facilitate surgical exposure. The present study analyzes the effects of LHF absence and surgical dissection in THA patients. Twenty subjects (5 normal hip joints, 10 nonconstrained THA, and 5 constrained THA) were evaluated using fluoroscopy while performing active hip abduction. All THA subjects were considered clinically successful. Fluoroscopic videos of the normal hips were analyzed using digitization, while those with THA were assessed using a computerized interactive model-fitting technique. The distance between the femoral head and acetabulum was measured to determine if femoral head separation occurred. Error analysis revealed measurements to be accurate within 0.75mm. No separation was observed in normal hips or those subjects implanted with constrained THA, while all 10 (100%) with unconstrained THA demonstrated femoral head separation, averaging 3.3mm (range 1.9-5.2mm). This study has shown that separation of the prosthetic femoral head from the acetabular component can occur. The normal hip joint has surrounding capsuloligamentous structures and a ligament attaching the femoral head to the acetabulum. We hypothesize that these soft tissue supports create a passive, resistant force at the hip, preventing femoral head separation. The absence of these supporting structures after THA may allow increased hip joint forces, which may play a role in premature polyethylene wear or prosthetic loosening.     

Effects of implant design parameters on fluid convection, potentiating third-body debris ingress into the bearing surface during THA impingement/subluxation

Aseptic loosening from polyethylene wear debris is the leading cause of failure for metal-on-polyethylene total hip implants. Third-body debris ingress to the bearing space results in femoral head roughening and acceleration of polyethylene wear. How third-body particles manage to enter the bearing space between the closely conforming articulating surfaces of the joint is not well understood. We hypothesize that one such mechanism is from convective fluid transport during subluxation of the total hip joint. To test this hypothesis, a three-dimensional (3D) computational fluid dynamics (CFD) model was developed and validated, to quantify fluid ingress into the bearing space during a leg-cross subluxation event. The results indicated that extra-articular joint fluid could be drawn nearly to the pole of the cup with even very small separations of the femoral head (<0.60mm). Debris suspended near the equator of the cup at the site of maximum fluid velocity just before the subluxation began could be transported to within 11 degrees from the cup pole. Larger head diameters resulted in increased fluid velocity at all sites around the entrance to the gap compared to smaller head sizes, with fluid velocity being greatest along the anterosuperolateral cup edge, for all head sizes. Fluid pathlines indicated that suspended debris would reach similar angular positions in the bearing space regardless of head size. Increased inset of the femoral head into the acetabular cup resulted both in higher fluid velocity and in transport of third-body debris further into the bearing space.     

Friction in Total Hip Joint Prosthesis Measured In Vivo during Walking

Friction-induced moments and subsequent cup loosening can be the reason for total hip joint replacement failure. The aim of this study was to measure the in vivo contact forces and friction moments during walking. Instrumented hip implants with Al₂O₃ ceramic head and an XPE inlay were used. In vivo measurements were taken 3 months post operatively in 8 subjects. The coefficient of friction was calculated in 3D throughout the whole gait cycle, and average values of the friction-induced power dissipation in the joint were determined. On average, peak contact forces of 248% of the bodyweight and peak friction moments of 0.26% bodyweight times meter were determined. However, contact forces and friction moments varied greatly between individuals. The friction moment increased during the extension phase of the joint. The average coefficient of friction also increased during this period, from 0.04 (0.03 to 0.06) at contralateral toe off to 0.06 (0.04 to 0.08) at contralateral heel strike. During the flexion phase, the coefficient of friction increased further to 0.14 (0.09 to 0.23) at toe off. The average friction-induced power throughout the whole gait cycle was 2.3 W (1.4 W to 3.8 W). Although more parameters than only the synovia determine the friction, the wide ranges of friction coefficients and power dissipation indicate that the lubricating properties of synovia are individually very different. However, such differences may also exist in natural joints and may influence the progression of arthrosis. Furthermore, subjects with very high power dissipation may be at risk of thermally induced implant loosening. The large increase of the friction coefficient during each step could be caused by the synovia being squeezed out under load.     

Lubrication of the human ankle joint in walking with the synovial fluid filtrated by the cartilage with the surface zone worn out: steady pure sliding motion.

A mixture model of synovial fluid filtration by cartilage in the human ankle joint during walking is presented for steady sliding motion of the articular surfaces. In the paper the cartilage surface zone is assumed worn out. The same model has been recently applied to the squeeze-film problem for the human hip joint loaded by the body weight during standing (Hlavácek, Journal of Biomechanics 26, 1145-1150, 1151-1160, 1993; Hlavácek and Novák, Journal of Biomechanics 28, 1193-1198, 1199-1205, 1995). The linear biphasic model for cartilage (elastic porous matrix + ideal fluid) due to Prof. V. C. Mow and his co-workers and the biphasic model for synovial fluid (viscous fluid + ideal fluid), as used in the above-mentioned squeeze-film problem, are applied. For the physiologic parameters of the ankle joint during walking, a continuous synovial fluid film about 1 microm thick is maintained under steady entraining motion according to the classical model without the fluid transport across the articular surface. This is not the case in the filtration model with the cartilage surface zones worn out. On the contrary, this filtration model indicates that synovial fluid is intensively filtrated by such cartilage, so that no continuous fluid film is maintained and a synovial gel layer, about 10(-8) m thick, develops over the majority of the contact. Thus, if the cartilage surface zones are worn out, boundary lubrication should prevail in the ankle joint under steady sliding motion for the mean values of loading and the sliding velocity encountered in walking cycle.     

Method for the evaluation of factors influencing the dislocation stability of total hip endoprotheses]

Dislocation of the artificial joint is a serious complication of total hip replacement. Various factors with an influence on dislocation stability were determined clinically. Our goal was to develop a method for evaluating experimentally the parameters implant design, position and the load situation for their influence on joint stability. With the newly developed testing device the range of motion to impingement and to dislocation can be determined at different implant positions. In addition, the rotational moments on subluxation, i.e. the "levering out" of the femoral head, can be determined. By way of example several hip implants were examined during movements associated with dislocation, e.g. (internal-)rotation in 90 degrees flexion and 0 degrees adduction as well as with (external-)rotation in combination with 10 degrees extension and 15 degrees adduction. Irrespective of implant design and position, the following movement phases can be differentiated: undisturbed motion, impingement, subluxation and, finally, complete dislocation of the head. On the basis of the range of motion of the specific phases, the moments occurring and the direction of dislocation, different implant systems can be compared. In this study the influence of the head diameter on the dislocation stability of the hip endoprosthesis is shown. With the aid of the model presented herein, a data set showing the most favourable and/or most dislocation stable implant position can be acquired for different combinations of the implant components. Additionally, useful information for implant design can be deduced and applied to new developments and/or modifications of existing implant components.     

Use of an absorbable membrane to position biologically inductive materials in the periprosthetic space of cemented joints

A device is presented that positions ultrahigh molecular weight polyethylene (UHMWPE) debris against periprosthetic bone surfaces. This can facilitate the study of aseptic loosening associated with cemented joint prostheses by speeding the appearance of this debris within the periprosthetic space. The device, composed of a 100 microm thick bioabsorbable membrane impregnated with 1.4 x 10(9) sub-micron particles of UHMWPE debris, is positioned on the endosteum of the bone prior to the insertion of the cemented orthopedic implant. An in vitro pullout study and an in vivo canine pilot study were performed to investigate its potential to accelerate "time to aseptic loosening" of cemented prosthetic joints. Pullout studies characterized the influence of the membrane on initial implant fixation. The tensile stresses (mean+/-std.dev.) required to withdraw a prosthesis cemented into canine femurs with and without the membrane were 1.15+/-0.3 and 1.54+/-0.01 MPa, respectively; these findings were not significantly different (p > 0.4). The in vivo pilot study, involving five dogs, was performed to evaluate the efficacy of the debris to accelerate loosening in a canine cemented hip arthroplasty. Aseptic loosening and lameness occurred within 12 months, quicker than the 30 months reported in a retrospective clinical review of canine hip arthroplasty.     

More than one way to be a giant: Convergence and disparity in the hip joints of saurischian dinosaurs

Saurischian dinosaurs evolved seven orders of magnitude in body mass, as well as a wide diversity of hip joint morphology and locomotor postures. The very largest saurischians possess incongruent bony hip joints, suggesting that large volumes of soft tissues mediated hip articulation. To understand the evolutionary trends and functional relationships between body size and hip anatomy of saurischians, we tested the relationships among discrete and continuous morphological characters using phylogenetically corrected regression. Giant theropods and sauropods convergently evolved highly cartilaginous hip joints by reducing supraacetabular ossifications, a condition unlike that in early dinosauromorphs. However, transitions in femoral and acetabular soft tissues indicate that large sauropods and theropods built their hip joints in fundamentally different ways. In sauropods, the femoral head possesses irregularly rugose subchondral surfaces for thick hyaline cartilage. Hip articulation was achieved primarily using the highly cartilaginous femoral head and the supraacetabular labrum on the acetabular ceiling. In contrast, theropods covered their femoral head and neck with thinner hyaline cartilage and maintained extensive articulation between the fibrocartilaginous femoral neck and the antitrochanter. These findings suggest that the hip joints of giant sauropods were built to sustain large compressive loads, whereas those of giant theropods experienced compression and shear forces.     

Relationship between T1rho magnetic resonance imaging,synovial fluid biomarkers,and the biochemical and biomechanical properties of cartilage

Non-invasive techniques for quantifying early biochemical and biomechanical changes in articular cartilage may provide a means of more precisely assessing osteoarthritis (OA) progression. The goals of this study were to determine the relationship between T1rho magnetic resonance (MR) imaging relaxation times and changes in cartilage composition, cartilage mechanical properties, and synovial fluid biomarker levels and to demonstrate the application of T1rho imaging to evaluate cartilage composition in human subjects in vivo. Femoral condyles and synovial fluid were harvested from healthy and OA porcine knee joints. Sagittal T1rho relaxation MR images of the condyles were acquired. OA regions of OA joints exhibited an increase in T1rho relaxation times as compared to non-OA regions. Furthermore in these regions, cartilage sGAG content and aggregate modulus decreased, while percent degraded collagen and water content increased. In OA joints, synovial fluid concentrations of sGAG decreased and C2C concentrations increased compared to healthy joints. T1rho relaxation times were negatively correlated with cartilage and synovial fluid sGAG concentrations and aggregate modulus and positively correlated with water content and permeability. Additionally, we demonstrated the application of these in vitro findings to the study of human subjects. Specifically, we demonstrated that walking results in decreased T1rho relaxation times, consistent with water exudation and an increase in proteoglycan concentration with in vivo loading. Together, these findings demonstrate that cartilage MR imaging and synovial fluid biomarkers provide powerful non-invasive tools for characterizing changes in the biochemical and biomechanical environments of the joint.     

运用偏振光观测聚乙烯磨屑颗粒诱导人工关节假体无菌性松动

目的:探讨运用偏振光显微镜来观测无菌性松动人工关节假体周围的聚乙烯颗粒分布,评估其在研究磨屑颗粒诱导假体无菌性松动机制及防治措施等实验研究中的可行性。方法:我们用雌性新西兰大白兔建立动物模型,在左侧胫骨髓腔内植入羟基磷灰石(hydroxyapatite,HA)涂层假体。并分别于假体表面和膝关节腔内植入0.5×107超高分子量聚乙烯(Ultra-high molecular weight polyethylene,UHMWPE)颗粒。术后行四环素荧光双标记。膝关节滑膜组织苏木精-伊红(hematoxylin-eosin,HE)染色、骨组织改良丽春红染色后分别用普通光镜和偏振光镜观察,未染色的骨组织行荧光显微镜和偏振光镜观察。结果:在聚乙烯颗粒刺激下,膝关节滑膜组织增生明显,骨-假体结合差,假体周围骨小梁稀疏,偏振光显微镜可清晰显示双折光性的聚乙烯颗粒在膝关节分布于滑膜及其深层结缔组织中,在骨-假体间隙间大量充填,阻碍骨-假体整合。结论:运用偏振光显微镜可以清晰而简便地观察滑膜和假体周围的聚乙烯颗粒分布,与传统实验方法相比,更加直观、简便和经济。     

The thixotropic effect of the synovial fluid in squeeze-film lubrication of the human hip joint.

The thixotropic (shear-thinning) effect of the synovial fluid in squeeze-film lubrication of the human hip joint is evaluated, taking into account filtration of the squeezed synovial film by biphasic articular cartilage. A porous, homogeneous, elastic cartilage matrix filled with the interstitial ideal fluid, with the intact superficial zone (of lower permeability and stiffness in compression) already disrupted or worn away, models an early stage of arthritis. Due to a high viscosity of the normal synovial fluid at very low shear rates, the squeezed synovial film at a fixed time after the application of a steady load is found to be much thicker in a small central part of the lubricated contact area. In the remaining part, the film is thin as it corresponds to the Newtonian fluid with the same high-shear-rate viscosity. Filtration is lower for the normal cartilage with the intact superficial zone due to its lower permeability and compression stiffness. But even in the fictitious case of zero filtration, calculations show that the effect of thixotropy on the increase of the minimum synovial film thickness would manifest itself as late as after several tens of seconds since the physiologic load application. At that time, this thickness would be as low as about 0.3 microm. It follows that thixotropy of the normal synovial fluid (and so much more of the inflammatory fluid) is irrelevant in squeeze-film lubrication of both the normal and arthritic human hip joints.     

The transmission of load through the human hip joint

This paper describes the results of loading experiments carried out on human hip joints. The unloaded surfaces of the femoral head and the acetabulum are slightly incongruous. The location and magnitude of the contact areas between the surfaces therefore depend on the magnitude and direction of the applied load. The contact areas were determined experimentally for a variety of loads typical of normal walking. Two distinct contact areas were found on the anterior and posterior aspects of the acetabulum at light loads, gradually merging with increasing load until, at a certain transition load, the dome of the acetabulum comes into contact and contact is then complete. The value of the transition load depends on the rate of loading, due to creep of the cartilage, and was found to vary from 50 per cent of body weight in young specimens to 25 per cent of body weight for elderly specimens for rates of loading typical of normal walking. Thus, the dome of the acetabulum is out of contact for a substantial portion of the swing phase of normal walking.

The analysis of a much simplified model of the hip joint is presented. The dependence of contact area on load is demonstrated, but also a method of determining the transition load for complete contact from the load/deflection relation for the hip is suggested. The values of the transition load quoted above were obtained by this method. The analysis further indicates that the distribution of pressure between the articular surfaces depends critically on the distribution of cartilage thickness throughout the joint. It is suggested that the distribution of cartilage thickness is such as to lead to a state of uniform pressure at the upper end of the physiological load range. Some experimental evidence is presented in support of this suggestion.

It is concluded that the function of joint incongruity is to allow the articular surfaces to come out of contact at light loads so that the cartilage may be exposed to synovial fluid for the purposes of nutrition and lubrication. At large loads, the distribution of cartilage thickness ensures that a state of hydrostatic pressure is achieved in order that cartilage, with a large fluid content, may transmit large pressures without flow and consequent loss of its integrity.     

Analysis of human abnormal walking using zero moment joint: required compensatory actions

This paper presents a multi-body model which can simulate human normal and abnormal walking. The abnormal walking model has a zero moment joint, abbreviated as ZMJ, representing a diseased joint of one leg. The joint can transmit a force to adjacent connected bodies, but cannot generate a moment about the joint to control motions of the bodies. Thus the ZMJ can be considered an extreme case of the diseased joint. Compensatory actions are required to make up for the lost function at the ZMJ in different patterns of variables, such as moments at sound joints, motions of upper torso, and so on. The characteristics of the abnormal walking having the ZMJ at the hip joint became so pronounced that the model could not walk in a realistic manner, not the case in the ZMJ at the knee.     

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.     

A novel sensor concept for optimization of loosening diagnostics in total hip replacement

The main reason for the revision of total hip replacements is aseptic loosening, caused by stress shielding and wear particle induced osteolysis. In order to detect an implant loosening early, the osseointegration of endoprosthetic implants must be measured exactly. Currently applied diagnostic methods, such as standard radiographs and clinical symptomatology, often result in an imprecise diagnosis. A novel radiation-free method to improve the diagnostic investigation of implant loosening is presented. The osseointegration of an implant can be identified using mechanical magnetic sensors (oscillators), which impinge on small membranes inside an implant component, e.g., the femoral hip stem. The maximum velocity after impingement of the oscillator depends on the osseointegration of the implant. Excitation of the oscillator is realized by a coil outside the human body. Another external coil is used to detect the velocity of the oscillator. To demonstrate the principle of the novel loosening sensor, an overdimensioned test device was designed to measure simulated loosening phases in the first experimental tests with different material layers. The overdimensioned test device of the loosening sensor showed significant differences in the various phases of fixation. Analysis of the membrane without any material layer in the case of advanced loosening resulted in a 23% higher maximum velocity compared to an attached artificial bone layer. Based on these preliminary results, the sensor system shows potential for the detection of implant loosening. Moreover, the proposed system could be used in experimental applications to determine the quality of bioactive coatings and new implant materials.     

Recovery of walking speed and symmetrical movement of the pelvis and lower extremity joints after unilateral THA

In 17 patients with unilateral hip disease who underwent total hip arthroplasty (THA), the gait was analyzed preoperatively and 1, 3, 6, and 12 months after unilateral THA using a Vicon system to assess the recovery of walking speed and symmetrical movement of the hip, knee, ankle, and pelvis. The walking speed of these patients reached that of normal Japanese persons by 12 months after surgery. Walking speed was correlated with the range of hip motion on the operated side at 1 month postoperatively, and was correlated with the hip joint extension moment of force on both sides from 3 to 6 months after surgery. Before THA, asymmetry was observed in the range of the hip motion, maximum hip flexion, maximum hip extension, maximum knee flexion, as well as in pelvic obliquity, pelvic tilt, and pelvic rotation. There were no differences of the stride length or step length between both sides throughout the observation period. The preoperative range of hip flexion on the operated side during a gait cycle (21.3+/-7.9 degrees ) was significantly smaller than on the non-operated side (46.7+/-7.1 degrees ), and the difference between sides was still significant at 12 months after surgery (35.1+/-6.2 degrees on the operated side and 43.6+/-5.7 degrees on the non-operated side). The majority (74%) of the difference in hip motion range during this period was due to the difference in maximum extension of the hip. The increase in the range of pelvic tilt and the range of motion of the opposite hip showed an inverse correlation with the range of motion of the operated hip, suggesting a compensatory preoperative role. However, this correlation became insignificant after 6 months postoperatively. Asymmetry of the range of hip motion persisted at 12 months after THA in patients with unilateral coxoarthropathy during free level walking, while the operation normalized the spatial asymmetry of other joints and the walking speed prior to the recovery of hip motion.