Quantitative Assessment of Polyethylene Component Position in a Mobile-Bearing Prosthetic Knee, Using a Single Radiograph

Rationale and Objectives. To reduce tibio-femoral misalignment, the polyethylene bearing-component of a new knee prosthesis was allowed limited motion on the underlying metallic component. The object of the work presented here was to develop a suitable radiographic technique for quantifying the in-vivo position of the bearing. By collecting these data at discrete flexion angles, the functional operation of the prosthesis could be determined. Methods. The known geometries between landmarks on the two components were used to produce algorithms for reconstructing their spatial positions from a single radiograph. A custom-designed computer program utilized these algorithms to determine the relative translation and rotation of the polyethylene component. Results. This technique produced typical errors of 0.54 mm translation and 0.56 degrees rotation between the polyethylene component and the underlying metallic component. Conclusions. A practical method has been developed for assessing mobile-bearing motion, in vivo. This method can be applied to other prosthetic devices, or combinations of components, once the requirement for identifiable landmarks has been addressed. Clinical Relevance. Skeletal and soft-tissue changes in the pathological knee may produce abnormal rotations and translations in the transverse tibial plane. This technique is intended both to validate the design philosophy of a mobile-bearing prosthesis and to provide additional data on any pathological motions, which will have implications for future prosthetic designs.     

Limited rotation of the mobile-bearing in a rotating platform total knee prosthesis

The hypothesis of this study was that the polyethylene bearing in a rotating platform total knee prosthesis shows axial rotation during a step-up motion, thereby facilitating the theoretical advantages of mobile-bearing knee prostheses. We examined 10 patients with rheumatoid arthritis who had a rotating platform total knee arthroplasty (NexGen LPS mobile, Zimmer Inc. Warsaw, USA). Fluoroscopic data was collected during a step-up motion six months postoperatively. A 3D-2D model fitting technique was used to reconstruct the in vivo 3D kinematics. The femoral component showed more axial rotation than the polyethylene mobile-bearing insert compared to the tibia during extension. In eight knees, the femoral component rotated internally with respect to the tibia during extension. In the other two knees the femoral component rotated externally with respect to the tibia. In all 10 patients, the femur showed more axial rotation than the mobile-bearing insert indicating the femoral component was sliding on the polyethylene of the rotating platform during the step-up motion. Possible explanations are a too limited conformity between femoral component and insert, the anterior located pivot location of the investigated rotating platform design, polyethylene on metal impingement and fibrous tissue formation between the mobile-bearing insert and the tibial plateau.     

Kinematic evaluation of cruciate-retaining total knee replacement patients during level walking: A comparison with the displacement-controlled ISO standard

Differences between wear-scar features of simulator-tested and retrieved tibial total knee replacement (TKR) liners have been reported. This disagreement may result from differences between in vivo kinematic profiles and those defined by the International Organization for Standardization (ISO). The purpose of this study was to determine the knee kinematics of a TKR subject group during level walking and compare them with the motion profiles defined by the ISO standard for a displacement-controlled knee wear testing simulator. Twenty-nine patients with a posterior cruciate ligament-retaining TKR design were gait tested using the point cluster technique to obtain flexion–extension (FE) rotation, anterior–posterior (AP) translation and internal–external (IE) rotation knee motions during a complete cycle of level walking. Relative ranges of motion and timing of key points within the in vivo motion data were compared against the same ranges and same key points from the input profiles of the displacement-controlled wear testing standard ISO14243-3. The subjects exhibited a FE pattern similar to ISO, with an insignificant difference in range of FE rotation from midstance to terminal stance. However, the subjects had a significantly higher range of knee flexion from terminal stance into swing. The subjects also exhibited a phase delay for the entire gait cycle. For AP translation, the standard profile had statistically significant lower magnitudes than seen in vivo. Opposite pattern of AP motion was also apparent from midstance and swing. Similarly, ISO specified a smaller IE total range of rotation with a motion pattern in complete opposition to that seen in vivo. In conclusion, significant differences were found in both the magnitudes and pattern of in vivo motion compared with ISO.     

Correlation of compartment pressure data from an intraoperative sensing device with postoperative fluoroscopic kinematic results in TKA patients

Fluoroscopy has recently been used to analyze postoperative kinematics in total knee arthroplasty (TKA). These analyses have reported varying results even in patients with similar implant design. In addition, patterns of wear in retrieved tibial polyethylene inserts of similar design have been found to vary substantially. These findings suggest that surgical technique, especially soft tissue balancing, may play a role in postoperative kinematics and implant failure. Accurate soft-tissue balancing is hypothesized to result in similar pressures within the medial and lateral compartments of the knee. However, a method of easily measuring these pressures at TKA has not been developed. In the present study, 32 patients were implanted with a mobile-bearing LCS TKA utilizing the balanced gap technique. An electronic pressure sensor, developed specifically to record pressure magnitude and distribution in the medial and lateral compartments, was incorporated into the implant trials. The knee was then passively taken through a range of motion while pressure data was recorded via computer. Postoperatively, 16 patients underwent active fluoroscopic kinematic analysis to assess for condylar liftoff and femorotibial translation. We found that abnormal compartment pressures and distributions as recorded by the intraoperative pressure sensor were correlated with inappropriate or paradoxical postoperative kinematics. In addition, subjects having similar pressures in both compartments throughout a range of motion did not experience condylar liftoff values greater than 1.0 mm. These data suggest that surgical technique influences the magnitude and distribution of forces at the articulation, postoperative kinematics, and likely, implant longevity.     

Mandibular kinematics represented by a non-orthogonal floating axis joint coordinate system

There are many methods used to represent joint kinematics (e.g., roll, pitch, and yaw angles; instantaneous center of rotation; kinematic center; helical axis). Often in biomechanics internal landmarks are inferred from external landmarks. This study represents mandibular kinematics using a non-orthogonal floating axis joint coordinate system based on 3-D geometric models with parameters that are "clinician friendly" and mathematically rigorous. Kinematics data for two controls were acquired from passive fiducial markers attached to a custom dental clutch. The geometric models were constructed from MRI data. The superior point along the arc of the long axis of the condyle was used to define the coordinate axes. The kinematic data and geometric models were registered through fiducial markers visible during both protocols. The mean absolute maxima across the subjects for sagittal rotation, coronal rotation, axial rotation, medial-lateral translation, anterior-posterior translation, and inferior-superior translation were 34.10 degrees, 1.82 degrees, 1.14 degrees, 2.31, 21.07, and 6.95 mm, respectively. All the parameters, except for one subject's axial rotation, were reproducible across two motion recording sessions. There was a linear correlation between sagittal rotation and translation, the dominant motion plane, with approximately 1.5 degrees of rotation per millimeter of translation. The novel approach of combining the floating axis system with geometric models succinctly described mandibular kinematics with reproducible and clinician friendly parameters.     

The influence of design, materials and kinematics on the in vitro wear of total knee replacements

Debris-induced osteolysis due to surface wear of ultra high molecular weight polyethylene (UHMWPE) bearings is a potential long-term failure mechanism of total knee replacements (TKR). This study investigated the effect of prosthesis design, kinematics and bearing material on the wear of UHMWPE bearings using a physiological knee simulator. The use of a curved fixed bearing design with stabilised polyethylene bearings reduced wear in comparison to more flat-on-flat components which were sterilised by gamma irradiation in air. Medium levels of crosslinking further improved the wear resistance of fixed bearing TKR due to resistance to strain softening when subjected to multidirectional motion at the femoral-insert articulating interface. Backside motion was shown to be a contributing factor to the overall rate of UHMWPE wear in fixed bearing components. Wear of fixed bearing prostheses was reduced significantly when anterior-posterior displacement and internal-external rotation kinematics were reduced due to decreased cross shear on the articulating surface and a reduction in AP displacement. Rotating platform mobile bearing prostheses exhibited reduced wear rates in comparison to fixed bearing components in these simulator studies due to redistribution of knee motion to two articulating interfaces with more linear motions at each interface. This was observed in two rotating platform designs with different UHMWPE bearing materials. In knee simulator studies, wear of TKR bearings was dependent on kinematics at the articulating surfaces and the prosthesis design, as well as the type of material.     

Knee joint secondary motion accuracy improved by quaternion-based optimizer with bony landmark constraints

Skin marker-based motion analysis has been widely used in biomechanical studies and clinical applications. Unfortunately, the accuracy of knee joint secondary motions is largely limited by the nonrigidity nature of human body segments. Numerous studies have investigated the characteristics of soft tissue movement. Utilizing these characteristics, we may improve the accuracy of knee joint motion measurement. An optimizer was developed by incorporating the soft tissue movement patterns at special bony landmarks into constraint functions. Bony landmark constraints were assigned to the skin markers at femur epicondyles, tibial plateau edges, and tibial tuberosity in a motion analysis algorithm by limiting their allowed position space relative to the underlying bone. The rotation matrix was represented by quaternion, and the constrained optimization problem was solved by Fletcher's version of the Levenberg-Marquardt optimization technique. The algorithm was validated by using motion data from both skin-based markers and bone-mounted markers attached to fresh cadavers. By comparing the results with the ground truth bone motion generated from the bone-mounted markers, the new algorithm had a significantly higher accuracy (root-mean-square (RMS) error: 0.7 ± 0.1 deg in axial rotation and 0.4 ± 0.1 deg in varus-valgus) in estimating the knee joint secondary rotations than algorithms without bony landmark constraints (RMS error: 1.7 ± 0.4 deg in axial rotation and 0.7 ± 0.1 deg in varus-valgus). Also, it predicts a more accurate medial-lateral translation (RMS error: 0.4 ± 0.1 mm) than the conventional techniques (RMS error: 1.2 ± 0.2 mm). The new algorithm, using bony landmark constrains, estimates more accurate secondary rotations and medial-lateral translation of the underlying bone.     

Analysis of a new polycentric above-knee prosthesis with a pneumatic swing phase control

The four-bar linkage mechanism used by many designers for the knee joint of an above-knee prosthesis can provide knee rotation suitable for walking only. In Afro-Asian countries people are habituated to the squatting posture in their daily activities. A six-bar linkage knee-ankle mechanism above-knee prosthesis with a pneumatic swing phase control is developed and its analysis shows its suitability for producing a motion closely resembling that of a normal subject during walking and squatting.     

An automatic 2D–3D image matching method for reproducing spatial knee joint positions using single or dual fluoroscopic images

Fluoroscopic image technique, using either a single image or dual images, has been widely applied to measure in vivo human knee joint kinematics. However, few studies have compared the advantages of using single and dual fluoroscopic images. Furthermore, due to the size limitation of the image intensifiers, it is possible that only a portion of the knee joint could be captured by the fluoroscopy during dynamic knee joint motion. In this paper, we presented a systematic evaluation of an automatic 2D–3D image matching method in reproducing spatial knee joint positions using either single or dual fluoroscopic image techniques. The data indicated that for the femur and tibia, their spatial positions could be determined with an accuracy and precision less than 0.2 mm in translation and less than 0.4° in orientation when dual fluoroscopic images were used. Using single fluoroscopic images, the method could produce satisfactory accuracy in joint positions in the imaging plane (in average up to 0.5 mm in translation and 1.3° in rotation), but large variations along the out-plane direction (in average up to 4.0 mm in translation and 2.2° in rotation). The precision of using single fluoroscopic images to determine the actual knee positions was worse than its accuracy obtained. The data also indicated that when using dual fluoroscopic image technique, if the knee joint outlines in one image were incomplete by 80%, the algorithm could still reproduce the joint positions with high precisions.     

Validation of a video-based motion analysis technique in 3-D dynamic scapular kinematic measurements

BackgroundCurrent non-invasive 3-D scapular kinematic measurement techniques such as electromagnetic tracking are subjected to restrictions of wired sensors and limited capture space. Video-based motion analysis provides greater freedom with relatively less movement restriction. However, video-based motion analysis was rarely used in and not validated for scapular kinematics.MethodsScapular kinematics of five subjects performing abduction, scaption, and internal/external rotation was captured simultaneously with video-based motion analysis and dynamic stereo X-ray, a gold standard for tracking scapular movements. The data from video-based motion analysis was correlated with the data from dynamic stereo X-ray for validity evaluation.FindingsStrong and significant correlations were identified in scapular protraction/retraction and medial/lateral rotation during abduction and scaption, and scapular medial/lateral rotation and anterior/posterior tilt during internal/external rotation.InterpretationVideo-based motion analysis is valid for evaluating a single subject's scapular movement pattern in protraction/retraction during abduction and scaption, and medial/lateral-rotation during internal/external rotation. Anterior/posterior-tilt during abduction and scaption should be investigated with caution. Video motion analysis is also valid for evaluating group average of scapular kinematics except for protraction/retraction during internal/external rotation. While acknowledging the inherent limitations, video-based motion analysis is an appropriate technique for tracking scapular kinematics.     

Patient-Specific Design and Biomechanical Evaluation of a Novel Bipolar Femoral Hemi-Knee Prosthesis

While total knee replacement is successful, hemiarthroplasty is necessary for some young, obese and active patients who are especially not suitable for unicompartmental or total knee prostheses. Hemiarthroplasty also provides an opportunity for children with bone tumors. The design ofhemiarthroplasty should be patient-specific to reduce contact stress and friction as well as instability, compared to conventional hemi-knee prosthesis. A novel bipolar hemi-knee prosthesis with two flexion stages was developed according to a healthy male＇s knee morphological profile. The motion mode of the bipolar hemi-knee prosthesis was observed through roentgenoscopy in vitro experiment. The biomechanical properties in one gait cycle were evaluated though finite element simulation. The bipolar hemi-knee prosthesis was found to produce knee flexion at two stages through X-ray images. The first stage is the motion from upright posture to a specified 60~ flexion, followed by the second stage of motion subsequently to deep flexion. The finite element simulation results also show that the designed hemi-knee prosthesis has the ability to reduce stresses on the joint contact surfaces. Therefore, it is possible for the bipolar hemi-knee prosthesis to provide better biotribological performances because it can reduce stresses and potentially wear on the opposing contacting surface during a gait cycle, orovidin~ a t~romisin~ treatment strate~v in future Joint renair znd renlneement     

A subject-specific finite element musculoskeletal framework for mechanics analysis of a total knee replacement

Concurrent use of finite element (FE) and musculoskeletal (MS) modeling techniques is capable of considering the interactions between prosthetic mechanics and subject dynamics after a total knee replacement (TKR) surgery is performed. However, it still has not been performed in terms of favorable prediction accuracy and systematic experimental validation. In this study, we presented a methodology to develop a subject-specific FE-MS model of a human right lower extremity including the interactions among the subject-specific MS model, the knee joint model with ligament bundles, and the deformable FE prosthesis model. In order to evaluate its accuracy, the FE-MS model was compared with a traditional hinge-constraint MS model and experimentally verified over a gait cycle. Both models achieved good temporal agreement between the predicted muscle force and the electromyography results, though the magnitude on models is different. A higher predicted accuracy, quantified by the root-mean-square error (RMSE) and the squared Pearson correlation coefficient (r²), was found in the FE-MS model (RMSE = 177.2 N, r² = 0.90) when compared with the MS model (RMSE = 224.1 N, r² = 0.81) on the total tibiofemoral contact force. The contact mechanics, including the contact area, pressure, and stress were synchronously simulated, and the maximum contact pressure, 22.06 MPa, occurred on the medial side of the tibial insert without exceeding the yield strength of the ultra-high-molecular-weight polyethylene, 24.79 MPa. The approach outlines an accurate knee joint biomechanics analysis and provides an effective method of applying individualized prosthesis design and verification in TKR.     

The importance of quadriceps and hamstring muscle loading on knee kinematics and in-situ forces in the ACL

This study investigated the effect of hamstring co-contraction with quadriceps on the kinematics of the human knee joint and the in-situ forces in the anterior cruciate ligament (ACL) during a simulated isometric extension motion of the knee. Cadaveric human knee specimens (n = 10) were tested using the robotic universal force moment sensor (UFS) system and measurements of knee kinematics and in-situ forces in the ACL were based on reference positions on the path of passive flexion/extension motion of the knee. With an isolated 200 N quadriceps load, the knee underwent anterior and lateral tibial translation as well as internal tibial rotation with respect to the femur. Both translation and rotation increased when the knee was flexed from full extension to 30 of flexion; with further flexion, these motion decreased. The addition of 80 N antagonistic hamstrings load significantly reduced both anterior and lateral tibial translation as well as internal tibial rotation at knee flexion angles tested except at full extension. At 30 of flexion, the anterior tibial translation, lateral tibial translation, and internal tibial rotation were significantly reduced by 18, 46, and 30%, respectively (p<0.05). The in-situ forces in the ACL under the quadriceps load were found to increase from 27.8+/-9.3 N at full extension to a maximum of 44.9+/-13.8 N at 15 of flexion and then decrease to 10 N beyond 60 of flexion. The in-situ force at 15 was significantly higher than that at other flexion angles (p<0.05). The addition of the hamstring load of 80 N significantly reduced the in-situ forces in the ACL at 15, 30 and 60 of flexion by 30, 43, and 44%, respectively (p<0.05). These data demonstrate that maximum knee motion may not necessarily correspond to the highest in-situ forces in the ACL. The data also suggest that hamstring co-contraction with quadriceps is effective in reducing excessive forces in the ACL particularly between 15 and 60 of knee flexion.     

Using relative velocity vectors to reveal axial rotation about the medial and lateral compartment of the knee

A new technique is presented that utilizes relative velocity vectors between articulating surfaces to characterize internal/external rotation of the tibio-femoral joint during dynamic loading. Precise tibio-femoral motion was determined by tracking the movement of implanted tantalum beads in high-speed biplane X-rays. Three-dimensional, subject-specific CT reconstructions of the femur and tibia, consisting of triangular mesh elements, were positioned in each analyzed frame. The minimum distance between subchondral bone surfaces was recorded for each mesh element comprising each bone surface, and the relative velocity between these opposing closest surface elements was determined in each frame. Internal/external rotation was visualized by superimposing tangential relative velocity vectors onto bone surfaces at each instant. Rotation about medial and lateral compartments was quantified by calculating the angle between these tangential relative vectors within each compartment. Results acquired from 68 test sessions involving 23 dogs indicated a consistent pattern of sequential rotation about the lateral condyle (approximately 60 ms after paw strike) followed by rotation about the medial condyle (approximately 100 ms after paw strike). These results imply that axial knee rotation follows a repeatable pattern within and among subjects. This pattern involves rotation about both the lateral and medial compartments. The technique described can be easily applied to study human knee internal/external rotation during a variety of activities. This information may be useful to define normal and pathologic conditions, to confirm post-surgical restoration of knee mechanics, and to design more realistic prosthetic devices. Furthermore, analysis of joint arthrokinematics, such as those described, may identify changes in joint mechanics associated with joint degeneration.     

术前心理沟通疏导对人工膝关节置换术患者应激反应、心理状态及术后疼痛的影响

目的:分析术前心理沟通疏导对人工膝关节置换术患者应激反应、心理状态以及术后疼痛的影响。方法:选取2018年1月～2019年1月汕头大学医学院第一附属医院和南方医科大学附属东莞市人民医院收治的拟行人工膝关节置换术的患者89例,以随机数字表法分为术前干预组和常规手术组,常规手术组按常规行术前患者教育,术前干预组行术前心理沟通疏导,比较两组术后康复情况、应激反应,采用抑郁自评量表(SDS)评分、焦虑自评量表(SAS)评分、视觉模拟评分(VAS)以及简化的McGill问卷(SF-MPQ)评价患者的抑郁、焦虑、疼痛情况。结果:术前干预组术后2周膝关节活动度(ROM)、膝关节特种外科医院(HSS)评分高于常规手术组(P0.05);术前干预组术后即刻醛固酮(ALD)、血管紧张素Ⅱ(AngⅡ)、去甲肾上腺素(NE)水平低于常规手术组(P0.05);术前干预组出院时SDS评分、SAS评分低于常规手术组(P0.05);术前干预组术后VAS评分、SF-MPQ评分低于常规手术组(P0.05)。结论:术前心理沟通疏导能够明显减轻接受人工膝关节置换术治疗的患者的应激反应,改善不良情绪,减轻术后疼痛感,促进患者康复。     

Design of Adjustable Frame-Type Prosthetic Socket for Lower Limb

ObjectivesProsthetic socket is the contact interface between the stump and the prosthesis, and also the interface component that transmits forces from the stump to the prosthesis distal. The current prosthetic socket fit is a major factor affecting rehabilitation, especially with the stump volume fluctuations. The main goal of this article is to design an adjustable frame-type prosthetic socket with constant force to adapt to the stump volume fluctuations.Materials and methodsIn this paper, an adjustable frame-type prosthetic socket with constant force is designed. The constant force device is designed based on the superelasticity of the shape memory alloy for maintaining constant stump-socket interface stress and automatically adapting to certain volume fluctuations. The constant force extrusion performance of this prosthetic socket was verified and optimized by finite element analysis.ResultsThe results suggest that the constant force unit may maintain constant interface stress. According to the optimization results, the shape memory alloy dimensional parameters could be selected according to different requirements.ConclusionThe adjustable frame-type prosthetic socket allows the user to adjust the socket volume through the cable system and has a large heat dissipation area. The constant force unit maintains constant interface stress and automatically adapts to stump volume fluctuations.     

Tibiofemoral kinematics and condylar motion during the stance phase of gait

Accurate knowledge of the dynamic knee motion in-vivo is instrumental for understanding normal and pathological function of the knee joint. However, interpreting motion of the knee joint during gait in other than the sagittal plane remains controversial. In this study, we utilized the dual fluoroscopic imaging technique to investigate the six-degree-of-freedom kinematics and condylar motion of the knee during the stance phase of treadmill gait in eight healthy volunteers at a speed of 0.67 m/s. We hypothesized that the 6DOF knee kinematics measured during gait will be different from those reported for non-weightbearing activities, especially with regards to the phenomenon of femoral rollback. In addition, we hypothesized that motion of the medial femoral condyle in the transverse plane is greater than that of the lateral femoral condyle during the stance phase of treadmill gait. The rotational motion and the anterior–posterior translation of the femur with respect to the tibia showed a clear relationship with the flexion–extension path of the knee during the stance phase. Additionally, we observed that the phenomenon of femoral rollback was reversed, with the femur noted to move posteriorly with extension and anteriorly with flexion. Furthermore, we noted that motion of the medial femoral condyle in the transverse plane was greater than that of the lateral femoral condyle during the stance phase of gait (17.4±2.0 mm vs. 7.4±6.1 mm, respectively; p<0.01). The trend was opposite to what has been observed during non-weightbearing flexion or single-leg lunge in previous studies. These data provide baseline knowledge for the understanding of normal physiology and for the analysis of pathological function of the knee joint during walking. These findings further demonstrate that knee kinematics is activity-dependent and motion patterns of one activity (non-weightbearing flexion or lunge) cannot be generalized to interpret a different one (gait).     

Relationship Between the Quantity of Oral Candida and Systemic Condition/Diseases of the Host: Oral Candida Increases with Advancing Age and Anemia

Background

The impact of host systemic conditions/diseases on the prosperity of oral Candida colonies remains unclear. The aim of the present study was to investigate whether a relationship exists between the quantity of oral Candida and the systemic condition/diseases of the host.

Patients and Methods

The cross-sectional relationship between Candida mannan concentrations and health check-up results was analyzed in consideration of local conditions that influence the prevalence of oral Candida.

Results

Candida mannan concentrations correlated with age, the number of untreated decayed teeth, number of prosthetic teeth, salivary pH, HbA1c, and the red blood cell count in a univariate analysis. In a multivariate analysis, Candida mannan concentrations correlated with age, the number of untreated decayed teeth, number of prosthetic teeth, salivary pH, and the red blood cell count. Candida mannan concentrations were higher in subjects older than 80 years, with a higher number of either untreated or prosthetic teeth, with a lower salivary pH, and with a decreased red blood cell count. Mannan concentrations were slightly higher in subjects with elevated HbA1c.

Conclusions

The present results suggest a close relationship between the quantity of oral Candida and the systemic condition/diseases of the host. Oral Candida may increase in immunocompromised hosts.

     

Accurate internal-external rotation measurement in total knee prostheses: A magnetic solution

In this work we tackled the problem of accurate measurement of internal-external (IE) rotations in the prosthetic knee. We presented a magnetic measurement system to be implanted in the knee prostheses in order to measure IE without soft tissue artifacts. The measurement system consisted of a permanent magnet attached under the tibial plate of the prosthesis and a combination of magnetic sensors in the polyethylene insert. Two different sensor configurations were designed, and five different angle estimators for measurement of IE angles were defined and tested based on several static and dynamic measurements toward a stereophotogrammetry motion capture system. Also a noise analysis was done to see which estimators are less sensitive to measurement noise. One-sensor configuration provided lower power budget with dynamic RMS error of 0.49° and a noise range of ±0.53°. Two-sensor configuration doubles the power consumption but provided slightly lower dynamic RMS error (0.37°) and a noise range of ±0.42°, and offers the possibility of having redundancy in case of damaged sensor.     

Functionally interpretable local coordinate systems for the upper extremity using inertial & magnetic measurement systems

BackgroundInertial Magnetic Measurement Systems (IMMS) are becoming increasingly popular by allowing for measurements outside the motion laboratory. The latest models enable long term, accurate measurement of segment motion in terms of joint angles, if initial segment orientations can accurately be determined. The standard procedure for definition of segmental orientation is based on the measurement of positions of bony landmarks (BLM). However, IMMS do not deliver position information, so an alternative method to establish IMMS based, anatomically understandable segment orientations is proposed.MethodsFor five subjects, IMMS recordings were collected in a standard anatomical position for definition of static axes, and during a series of standardized motions for the estimation of kinematic axes of rotation. For all axes, the intra- and inter-individual dispersion was estimated. Subsequently, local coordinate systems (LCS) were constructed on the basis of the combination of IMMS axes with the lowest dispersion and compared with BLM based LCS.FindingsThe repeatability of the method appeared to be high; for every segment at least two axes could be determined with a dispersion of at most 3.8°. Comparison of IMMS based with BLM based LCS yielded compatible results for the thorax, but less compatible results for the humerus, forearm and hand, where differences in orientation rose to 17.2°.InterpretationAlthough different from the ‘gold standard’ BLM based LCS, IMMS based LCS can be constructed repeatable, enabling the estimation of segment orientations outside the laboratory.ConclusionsA procedure for the definition of local reference frames using IMMS is proposed.