Direct comparison of calculated hip joint contact forces with those measured using instrumented implants. An evaluation of a three-dimensional mathematical model of the lower limb

Characterisation of hip joint contact forces is essential for the definition of hip joint prosthesis design requirements. In vivo hip joint contact force measurements have been made using instrumented hip joint prostheses. However, to allow determination of the range of values of joint contact force and their directions relative to anatomical structures in a range of subject groups sufficient to form an agreed data base it is necessary to adopt a different approach without the use of an implanted transducer. The use of mathematical models of the lower limb to examine the forces in soft tissues and at the joints has provided valuable insight into internal loading conditions. Several authors have proposed mathematical musculo-skeletal models. However, there have been only limited attempts at validation of these models. It is possible to use the results of in vivo force measurements from instrumented prostheses to validate the results calculated using the mathematical models. In this study two subjects with instrumented hip joint prostheses were studied. Forces at the hip joints were calculated using a three-dimensional model of the leg. Walking at slow, normal and fast speeds (0.97-2.01m/s), weight transfer from two to one leg and back again, and sit to stand were studied. Direct comparisons were made between the 'gold standard' measured hip joint contact forces and the calculated forces. There was general agreement between the calculated and measured forces in both pattern and magnitude. There were, however, discrepancies. Reasons for these differences in results are discussed and possible model developments suggested.     

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.     

The mechanism of generation and transmission of forces in leg extension.

This paper deals with the mechanical and electromyographic evaluation of the mechanism generating and transmitting the resultant leg extension force by maximal isometric contraction in two directions, the knee and hip joint being kept at 90 degrees. The two directions were a) from the center of gravity of the body to the ankle joint and b) from a point near the knee to the ankle. Six male subjects in a supine position exerted a maximal leg extension force of 47-112 kg for a) and 51-73 kg for b). These values were close to the smaller values of two forces estimated at the knee and at the hip from maximal isometric forces at the corresponding joint of the same joint angle. It was thus suggested that the joint limiting the resultant leg extension force was the knee for a) and the hip for b). The single joint muscles exhibited almost maximal activities when they concerned the joint which limited the resultant leg extension force. The double joint muscles were often contracted only moderately during the maximal isometric leg extension, indicating a different role of double joint muscles even at the maximal force production at a particular joint.     

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.     

Lower extremity control during turns initiated with and without hip external rotation

The pirouette turn is often initiated in neutral and externally rotated hip positions by dancers. This provides an opportunity to investigate how dancers satisfy the same mechanical objectives at the whole-body level when using different leg kinematics. The purpose of this study was to compare lower extremity control strategies during the turn initiation phase of pirouettes performed with and without hip external rotation. Skilled dancers (n=5) performed pirouette turns with and without hip external rotation. Joint kinetics during turn initiation were determined for both legs using ground reaction forces (GRFs) and segment kinematics. Hip muscle activations were monitored using electromyography. Using probability-based statistical methods, variables were compared across turn conditions as a group and within-dancer. Despite differences in GRFs and impulse generation between turn conditions, at least 90% of each GRF was aligned with the respective leg plane. A majority of the net joint moments at the ankle, knee, and hip acted about an axis perpendicular to the leg plane. However, differences in shank alignment relative to the leg plane affected the distribution of the knee net joint moment when represented with respect to the shank versus the thigh. During the initiation of both turns, most participants used ankle plantar flexor moments, knee extensor moments, flexor and abductor moments at the push leg׳s hip, and extensor and abductor moments at the turn leg׳s hip. Representation of joint kinetics using multiple reference systems assisted in understanding control priorities.     

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.     

Reduction of knee range of motion during continuous passive motion due to misaligned hip joint centre

When using continuous passive motion (CPM) devices, appropriate setting of the device and positioning of the patient are necessary to obtain maximum range of motion (ROM). In this study, the ROMs in both the knee joint and CPM device during CPM treatment were measured using a motion analysis system for three different CPM devices. Additionally, the trajectories of the angles at the knee for hip joint misalignments were evaluated using kinematic models of the three CPM devices. The results showed that discrepancies in ROM between the knee joints and the CPM device settings during CPM treatment were revealed regardless of the CPM device and that the effect of misalignment is dependent on the design of the CPM device. The present technology could be applied for the development of a better design configuration for the CPM device to reduce the discrepancy in ROM at the knee joint.     

Reduction of knee range of motion during continuous passive motion due to misaligned hip joint centre

When using continuous passive motion (CPM) devices, appropriate setting of the device and positioning of the patient are necessary to obtain maximum range of motion (ROM). In this study, the ROMs in both the knee joint and CPM device during CPM treatment were measured using a motion analysis system for three different CPM devices. Additionally, the trajectories of the angles at the knee for hip joint misalignments were evaluated using kinematic models of the three CPM devices. The results showed that discrepancies in ROM between the knee joints and the CPM device settings during CPM treatment were revealed regardless of the CPM device and that the effect of misalignment is dependent on the design of the CPM device. The present technology could be applied for the development of a better design configuration for the CPM device to reduce the discrepancy in ROM at the knee joint.     

Influence of muscle activity on the forces in the femur: An in vivo study

Experiments were performed on two patients with custom-made instrumented massive proximal femoral prostheses implanted after tumour resection. In vivo axial forces transmitted along the prostheses were telemetered during level walking, single- and double-leg stance, and isometric exercises of the hip muscles. These activities varied the lever arms available to the external loads: minimum for double-leg stance and maximum for hip isometric exercises. Kinematic, force plate, EMG and telemetered force data were recorded simultaneously. The force magnification ration (FMR; the ratio of the telemetered axial force to the external force) was calculated. The FMRs ranged from 1.3 (during double-leg stance) to 29.8 (during abductors test), indicating that a major part of the axial force in the long bones is a response to muscle activity, the strength of which depends on the lever arms available to the external loads. From these results, it was shown that the bulk of the bending moment along limbs is transmitted by a combination of tensile forces in muscles and compressive forces in bones, so moments transmitted by the bones are smaller than the limb moments. It was concluded that appropriate simulation of muscle forces is important in experimental or theoretical studies of load transmission along bones.     

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.     

Gait alterations in rats following attachment of a device and application of altered knee loading

Animal models are widely used to study cartilage degeneration. Experimental interventions to alter contact mechanics in articular joints may also affect the loads borne by the leg during gait and consequently affect the overall loading experienced in the joint. In this study, force plate analyses were utilized to measure parameters of gait in the rear legs of adult rats following application of a varus loading device that altered loading in the knee. Adult rats were assigned to Control, Sham, or Loaded groups (n≥4/each). Varus loading devices were surgically attached to rats in the Sham and Loaded groups. In the Loaded group, this device applied a controlled compressive overload to the medial compartment of the knee during periods of engagement. Peak ground reaction forces during walking were recorded for each rear leg of each group. Analyses of variance were used to compare outcomes across groups (Control, Sham, and Loaded), leg (contralateral, experimental) and device status (disengaged, engaged) to determine the effects of surgically attaching the device and applying a compressive overload to the joint with the device. The mean peak vertical force in the experimental leg was reduced to 30% in the Sham group in comparison to the contralateral leg and the Control group, indicating an effect of attaching the device to the leg (p<0.01). No differences were found in ground reaction forces between the Sham and Loaded groups with application of compressive overloads with the device. The significant reduction in vertical force due to the surgical attachment of the varus loading device must be considered and accounted for in future studies.     

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 multiaxial force-sensing implantable tibial prosthesis

Accurate in vivo measurement of tibiofemoral forces is important in total knee arthroplasty. These forces determine polyethylene stresses and cold-flow, stress distribution in the implant, and stress transfer to the underlying implant bone interface. Theoretic estimates of tibiofemoral forces have varied widely depending on the mathematical models used. The six degrees of freedom of motion, complex articular surface topography, changing joint-contact position, intra- and extra-articular ligaments, number of muscles crossing the knee joint, and the presence of the patellofemoral joint contribute to the difficulty in developing reliable models of the knee. A prototype instrumented total knee replacement tibial prosthesis was designed, manufactured, and tested. This prosthesis accurately measured all six components of tibial forces (R2>0.997). The prosthesis was also instrumented with an internal microtransmitter for wireless data transmission. Remote powering of the sealed implanted electronics was achieved using magnetic coil induction. This device can be used to validate existing models of the knee that estimate these forces or to develop more accurate models. In conjunction with kinematic data, accurate tibiofemoral force data may be used to design more effective knee-testing rigs and wear simulators. Additional uses are intraoperative measurement of forces to determine soft-tissue balancing and to evaluate the effects of rehabilitation, external bracing, and athletic activities, and activities of daily living.     

Influence of weak hip abductor muscles on joint contact forces during normal walking: probabilistic modeling analysis

The weakness of hip abductor muscles is related to lower-limb joint osteoarthritis, and joint overloading may increase the risk for disease progression. The relationship between muscle strength, structural joint deterioration and joint loading makes the latter an important parameter in the study of onset and follow-up of the disease. Since the relationship between hip abductor weakness and joint loading still remains an open question, the purpose of this study was to adopt a probabilistic modeling approach to give insights into how the weakness of hip abductor muscles, in the extent to which normal gait could be unaltered, affects ipsilateral joint contact forces. A generic musculoskeletal model was scaled to each healthy subject included in the study, and the maximum force-generating capacity of each hip abductor muscle in the model was perturbed to evaluate how all physiologically possible configurations of hip abductor weakness affected the joint contact forces during walking. In general, the muscular system was able to compensate for abductor weakness. The reduced force-generating capacity of the abductor muscles affected joint contact forces to a mild extent, with 50th percentile mean differences up to 0.5 BW (maximum 1.7 BW). There were greater increases in the peak knee joint loads than in loads at the hip or ankle. Gluteus medius, particularly the anterior compartment, was the abductor muscle with the most influence on hip and knee loads. Further studies should assess if these increases in joint loading may affect initiation and progression of osteoarthritis.     

Stability of an underactuated bipedal gait

A self-excited biped walking mechanism consisting of two legs that are connected in series at the hip joint through a servomotor is studied as a cyclic system with collisions. A torque proportional to angle between the shank of the swinging leg and the vertical is seen to sustain a gait. Each leg has a thigh and a shank connected at a passive knee joint that has a knee stopper restricting hyperextension similar to the human knee. A mathematical model for the dynamics of the system including the impact equations is used to analyse the stability of the system through examination of phase plane plots. Attractor lines along which the system approaches stability have been identified. A leg length for optimal stability has been identified. The biological basis for the proposed system has been identified by comparison with human gait.     

Elbow joint biomechanics for preclinical evaluation of total elbow prostheses

Total elbow arthroplasty is a clinically successful procedure, yet long-term implant survival rates have historically lagged behind those reported for total hips and knees. Clinical complications associated with implant wear, osteolysis, stem loosening and device fracture have been implicated as reasons for limited long-term survivorship. Unfortunately, there is little published information on the biomechanics and method(s) for preclinical evaluation of total elbow prostheses that could provide insight into the mechanisms of failure. Additionally, there are no consensus testing standards or summaries of loading profiles of the humero-ulnar joint associated with a range of activities of daily living. Such data would facilitate the standardized preclinical assessment of total elbow devices such is commonplace for other large joints. The objective of the work here is therefore to provide a comprehensive review of elbow joint biomechanics as it relates to preclinical evaluation of total elbow implants. This summary includes a review of elbow joint forces, kinematics, the types and frequency of humero-ulnar joint motions associated with activities of daily living and clinical outcomes, as well as proposing a methodology for deriving humero-ulnar joint reaction force magnitudes and vector orientations as a function of a known mass/force at the hand. From these data, a scalable, bi-axial loading profile is proposed as a foundation for the development of clinically relevant, laboratory simulations for assessment of total elbow prostheses performance.     

An implantable telemetry device to measure intra-articular tibial forces

Tibial forces are important because they determine polyethylene wear, stress distribution in the implant, and stress transfer to underlying bone. Theoretic estimates of tibiofemoral forces have varied between three and six times the body weight depending on the mathematical models used and the type of activity analyzed. An implantable telemetry system was therefore developed to directly measure tibiofemoral compressive forces. This system was tested in a cadaver knee in a dynamic knee rig. A total knee tibial arthroplasty prosthesis was instrumented with four force transducers located at the four corners of the tibial tray. These transducers measured the total compressive forces on the tibial tray and the location of the center of pressure. A microprocessor performed analog-to-digital signal conversion and performed pulse code modulation of a surface acoustic wave radio frequency oscillator. This signal was then transmitted through a single pin hermetic feed-through tantalum wire antenna located at the tip of the stem. The radio frequency signal was received by an external antenna connected to a receiver and to a computer for data acquisition. The prosthesis was powered by external coil induction. The tibial transducer accurately measured both the magnitude and the location of precisely applied external loads. Successful transmission of the radio frequency signal up to a range of 3m was achieved through cadaveric bone, bone cement, and soft tissue. Reasonable accuracy was obtained in measuring loads applied through a polyethylene insert. The implant was also able to detect unicondylar loading with liftoff.     

Experimentally reduced hip abductor function during walking: Implications for knee joint loads

Hip and knee functions are intimately connected and reduced hip abductor function might play a role in development of knee osteoarthritis (OA) by increasing the external knee adduction moment during walking. The purpose of this study was to test the hypothesis that reduced function of the gluteus medius (GM) muscle would lead to increased external knee adduction moment during level walking in healthy subjects. Reduced GM muscle function was induced experimentally, by means of intramuscular injections of hypertonic saline that produced an intense short-term muscle pain and reduced muscle function. Isotonic saline injections were used as non-painful control. Fifteen healthy subjects performed walking trials at their self-selected walking speed before and immediately after injections, and again after 20 min of rest, to ensure pain recovery. Standard gait analyses were used to calculate three-dimensional trunk and lower extremity joint kinematics and kinetics. Surface electromyography (EMG) of the glutei, quadriceps, and hamstring muscles were also measured. The peak GM EMG activity had temporal concurrence with peaks in frontal plane moments at both hip and knee joints. The EMG activity in the GM muscle was significantly reduced by pain (?39.6%). All other muscles were unaffected. Peaks in the frontal plane hip and knee joint moments were significantly reduced during pain (?6.4% and ?4.2%, respectively). Lateral trunk lean angles and midstance hip joint adduction and knee joint extension angles were reduced by ?1°. Thus, the gait changes were primarily caused by reduced GM function. Walking with impaired GM muscle function due to pain significantly reduced the external knee adduction moment. This study challenge the notion that reduced GM function due to pain would lead to increased loads at the knee joint during level walking.