The first-generation anatomical medial meniscus prosthesis led to unsatisfactory results: a first-in-human study.

PURPOSE: The purpose of this first-in-human study was to evaluate the effect of a polycarbonate anatomical meniscus prosthesis system, including the surgical procedure, on knee pain and describe potential adverse events in patients with post-meniscectomy pain syndrome. METHODS: Eleven patients with post-meniscectomy pain syndrome and limited underlying cartilage damage were enrolled in the study. Five received a medial polycarbonate urethane meniscus prosthesis which was clicked onto 2 titanium screws fixated at the native horn attachments on the tibia. The KOOS score was planned to be collected at baseline and at 3, 6, 12 and 24 months following the intervention including radiographs at 6, 12 and 24 months. MRI scans were repeated after 12 and 24 months. RESULTS: The surgical technique to select an appropriately sized implant and correct positioning of the fixation screws and meniscus prosthesis onto the tibia was demonstrated to be feasible and reproducible. Inclusion stopped after 5 patients because of serious adverse device-related events. All patients reported knee joint stiffness and slight effusion in their knee at 6 months follow-up. In 3 patients the implant was removed because of implant failure and in 1 patient the implant was removed because of persistent pain and extension limitation. In none of the patients did the KOOS score improve in the first 6 months after surgery. However, in the patient who still has the implant in situ, PROMs started to improve 1 year after surgery and this improvement continued through 2 years of follow-up. The KOOS Pain, symptoms and ADL were close to the maximal 100 points. KOOS QoL and sport did improve but remained suboptimal. CONCLUSION: This first version of the meniscus prosthesis led to impaired knee function and failed in four out of five patients. The patients where the prosthesis was removed were salvable and the PROMs returned to pre-study levels. The results in the patient where the device is still in place are promising. LEVEL OF EVIDENCE: Level II.

The sensitivity of an anatomical coordinate system to anatomical variation and its effect on the description of knee kinematics as obtained from dynamic CT imaging.

Accurate assessment of knee kinematics is important to investigate knee pathology and the effect of orthopaedic interventions. Anatomical coordinate systems are used to describe knee kinematics but inherently show interpersonal differences. The purpose of this study was to determine the sensitivity of an anatomical coordinate system of the knee to anatomical variation, and to establish its effect on the description of knee kinematics. A statistical shape model of the knee was made based on a CT dataset. The statistical shape model was used to generate shapes with a specific variation. A coordinate system was calculated and the rotations relative to a mean coordinate system were calculated. From a dynamic CT dataset, knee kinematics were calculated for a flexion-extension movement. The largest rotational changes of the coordinate systems were then applied to the knee kinematics. The femoral and tibial coordinate system were relatively insensitive to anatomical variation, while the patellar coordinate system showed a larger sensitivity. Hence, tibiofemoral kinematics could be calculated with an accuracy of <5.01°, while patellofemoral kinematics showed a noticeably larger range of uncertainty (<13.48°). The findings from this study can be used to investigate whether differences in knee kinematics are due to anatomy or pathology.

The importance of continuous remnant preservation in anterior cruciate ligament reconstruction.

PURPOSE: Selective anteromedial or posterolateral bundle reconstruction is recognized as a treatment modality in partial anterior cruciate ligament (ACL) reconstruction (ACLR) with a biomechanically sufficient ACL remnant. However, there is paucity in literature investigating clinical outcomes of standard ACLR with preservation of residual continuous but biomechanically insufficient ACL tissue. The aim of this study was to investigate the influence of preservation of residual continuous but biomechanical insufficient ACL tissue in standard ACLR on complication and repeat surgery rate, and patient reported and clinical outcome. METHODS: The retrospective cohort comprised 134 patients (age 23 ± 7 years; Tegner 6 ± 3) with an isolated acute ACL tear. In 67 patients, residual continuous but biomechanically insufficient ACL tissue was present and preserved based on visual inspection, probing of the ACL tissue and Lachman test under arthroscopic view (standard reconstruction with tissue preservation; SRTP). These patients were matched to 67 patients that underwent ACLR where no residual ACL tissue could be preserved (standard reconstruction; SR) based on gender, age and chondral and/or meniscal status. Clinical failure (recurrent instability, pathological ACL graft laxity and/or ACL graft discontinuity), other complication and repeat-surgery rate within index surgery and 1-year and within index surgery and 2-year follow-up, and patient reported and clinical outcomes at 1-year and at 2-year follow-up were compared. RESULTS: A statistically significant lower clinical failure rate within index surgery and 1-year (SRTP, 3%; SR, 13%; P = 0.028) and within index surgery and 2-year follow-up (SRTP, 3%; SR, 23%; P = 0.001), and revision ACL surgery rate within index surgery and 1-year (SRTP, 2%; ST, 10%; P = 0.029) and within index surgery and 2-year follow-up (SRTP, 2%; SR, 18%; P = 0.001) was found in the SRTP group. No statistically significant differences were found for other investigated outcomes in patients that were without clinical failure. CONCLUSION: This study shows that in ACLR surgery, preservation of residual continuous but biomechanical insufficient ACL tissue might lead to lower clinical failure rate and ACL revision surgery rate within index surgery and 1-year, and within index surgery and 2-year follow-up compared to standard ACLR where no residual continuous ACL tissue could be preserved. LEVEL OF EVIDENCE: III.

Biomechanical loading of the porcine femorotibial joint during maximal movements: An exploratory, ex vivo study.

Thus far, there is a lack of scientific investigation regarding the hypothesis that biomechanical factors contribute to the cross-species pathogenesis of osteochondrosis (OC). Therefore, the aim of this pilot study was to investigate whether high (peak) pressures occur in the porcine femorotibial (FT) joint. In this experimental, ex vivo study, the right hind limbs of seven weaned piglets were subjected to maximal joint excursions, as a priori radiologically estimated. Subsequently, the intra-articular pressures were measured using sensors placed in both the medial and the lateral compartments of the FT joint. The overall highest individual peak pressure was found in the lateral FT joint during maximal extension (2611 kPa; group mean ± standard deviation (SD) 982.3 ± 988.2 kPa). In the medial FT joint, the highest individual peak pressure was found during maximal adduction (1481 kPa; group mean ± SD 664.9 ± 393.2 kPa). Moreover, nearly 30% of the ex vivo peak pressures were above published thresholds for cartilage catabolism (>500 kPa/0.5 MPa), but not for interfering with cell viability (>5 MPa). In conclusion, this ex vivo study on FT joint pressures in weaned piglets showed that FT joint movements at maximal excursions are related to concomitant internal peak joint pressures. More studies should be performed to evaluate the possible biomechanical relation of these observations with osteochondrosis, which would allow the design of preventive measures in the pig industry, to avoid extreme limb movements and concomitant joint peak pressures in vivo.

Axial cortical involvement of metastatic lesions to identify impending femoral fractures; a clinical validation study.

BACKGROUND AND PURPOSE: Patients with advanced cancer may develop painful bone metastases, potentially resulting in pathological fractures. Adequate fracture risk assessment is of key importance to prevent fracturing and maintain mobility. This study aims to validate the clinical reliability of axial cortical involvement with a 30 mm threshold on conventional radiographs to assess fracture risk in femoral bone metastases. MATERIALS AND METHODS: All patients with bone metastases who received radiotherapy for pain included in two multicentre prospective studies were selected. Conventional radiographs obtained at a maximum of two months prior to radiotherapy were collected. Three experts independently measured lesions and scored radiographic characteristics. Sensitivity, specificity, positive (PPV) and negative predictive value (NPV) were calculated. RESULTS: Hundred patients were included with a median follow-up of 23.0 months (95%CI: 10.6-35.5). Two fractures occurred in lesions with axial cortical involvement <30 mm, and 12 in lesions ≥30 mm. Sensitivity, specificity, PPV and NPV of axial cortical involvement for predicting femoral fractures were 86%, 50%, 20% and 96%, respectively. Patients with lesions ≥30 mm had a 5.3 times higher fracture risk than patients with smaller lesions. CONCLUSION: Our validation study confirmed the use of 30 mm axial cortical involvement to assess fracture risk in femoral bone metastases. Until a more accurate and practically feasible method has been developed, this clinical parameter remains an easy method to assess femoral fracture risk to aid patients and clinicians to choose the optimal individual treatment modality.

Forces acting on the clavicle during shoulder abduction, forward humeral flexion and activities of daily living.

BACKGROUND: The forces acting on the human clavicle in vivo are difficult if not impossible to measure. The goal of this study is to quantify the forces acting on the human clavicle during shoulder abduction, forward humeral elevation and three activities of daily living using the Delft Shoulder and Elbow Model. METHODS: The Delft Shoulder and Elbow Model and a computed tomography scan of a clavicle were used to calculate the forces and moments acting on the entire clavicle and on three planes within the middle third of the clavicle during the simulated movements. FINDINGS: The largest resultant force simulated across the clavicle was 126 N during abduction. Maximum resultant moments of 2.4 Nm were identified during both abduction and forward humeral elevation. The highest forces in the middle third of the clavicle were of a compressive nature along the longitudinal axis of the clavicle, increasing to 97 N during forward humeral elevation and 91 N during abduction. Forces in opposite direction along the y-axis were identified on either side of the conoid ligament. The three simulated activities of daily living had similar ranges of forces and moments irrespective of the sagittal plane in which these activities were performed. INTERPRETATION: Peak forces occurred at different locations on the middle third of the clavicle during different movements. The results create an understanding of the forces and their distribution across the clavicle during activities of daily living. These data may be helpful in the development of clavicular fixation devices. LEVEL OF EVIDENCE: Biomechanical study.

Can patient-specific finite element models better predict fractures in metastatic bone disease than experienced clinicians?: Towards computational modelling in daily clinical practice.

OBJECTIVES: In this prospective cohort study, we investigated whether patient-specific finite element (FE) models can identify patients at risk of a pathological femoral fracture resulting from metastatic bone disease, and compared these FE predictions with clinical assessments by experienced clinicians. METHODS: A total of 39 patients with non-fractured femoral metastatic lesions who were irradiated for pain were included from three radiotherapy institutes. During follow-up, nine pathological fractures occurred in seven patients. Quantitative CT-based FE models were generated for all patients. Femoral failure load was calculated and compared between the fractured and non-fractured femurs. Due to inter-scanner differences, patients were analyzed separately for the three institutes. In addition, the FE-based predictions were compared with fracture risk assessments by experienced clinicians. RESULTS: In institute 1, median failure load was significantly lower for patients who sustained a fracture than for patients with no fractures. In institutes 2 and 3, the number of patients with a fracture was too low to make a clear distinction. Fracture locations were well predicted by the FE model when compared with post-fracture radiographs. The FE model was more accurate in identifying patients with a high fracture risk compared with experienced clinicians, with a sensitivity of 89% versus 0% to 33% for clinical assessments. Specificity was 79% for the FE models versus 84% to 95% for clinical assessments. CONCLUSION: FE models can be a valuable tool to improve clinical fracture risk predictions in metastatic bone disease. Future work in a larger patient population should confirm the higher predictive power of FE models compared with current clinical guidelines.Cite this article: F. Eggermont, L. C. Derikx, N. Verdonschot, I. C. M. van der Geest, M. A. A. de Jong, A. Snyers, Y. M. van der Linden, E. Tanck. Can patient-specific finite element models better predict fractures in metastatic bone disease than experienced clinicians? Towards computational modelling in daily clinical practice. Bone Joint Res 2018;7:430-439. DOI: 10.1302/2046-3758.76.BJR-2017-0325.R2.

Low-field magnetic resonance imaging offers potential for measuring tibial component migration.

BACKGROUND: Roentgen stereophotogrammetric analysis (RSA) is used to measure early prosthetic migration and to predict future implant failure. RSA has several disadvantages, such as the need for perioperatively inserted tantalum markers. Therefore, this study evaluates low-field MRI as an alternative to RSA. The use of traditional MRI with prostheses induces disturbing metal artifacts which are reduced by low-field MRI. The purpose of this study is to assess the feasibility to use low-field (0.25 Tesla) MRI for measuring the precision of zero motion. This was assessed by calculating the virtual prosthetic motion of a zero-motion prosthetic reconstruction in multiple scanning sessions. Furthermore, the effects of different registration methods on these virtual motions were tested. RESULTS: The precision of zero motion for low-field MRI was between 0.584 mm and 1.974 mm for translation and 0.884° and 3.774° for rotation. The manual registration method seemed most accurate, with µ ≤ 0.13 mm (σ ≤ 0.931 mm) for translation and µ ≤ 0.15° (σ ≤ 1.63°) for rotation. CONCLUSION: Low-field MRI is not yet as precise as today's golden standard (marker based RSA) as reported in the literature. However, low-field MRI is feasible of measuring the relative position of bone and implant with comparable precision as obtained with marker-free RSA techniques. Of the three registration methods tested, manual registration was most accurate. Before starting clinical validation further research is necessary and should focus on improving scan sequences and registration algorithms.

Estimating severity of sideways fall using a generic multi linear regression model based on kinematic input variables.

Many research groups have studied fall impact mechanics to understand how fall severity can be reduced to prevent hip fractures. Yet, direct impact force measurements with force plates are restricted to a very limited repertoire of experimental falls. The purpose of this study was to develop a generic model for estimating hip impact forces (i.e. fall severity) in in vivo sideways falls without the use of force plates. Twelve experienced judokas performed sideways Martial Arts (MA) and Block ('natural') falls on a force plate, both with and without a mat on top. Data were analyzed to determine the hip impact force and to derive 11 selected (subject-specific and kinematic) variables. Falls from kneeling height were used to perform a stepwise regression procedure to assess the effects of these input variables and build the model. The final model includes four input variables, involving one subject-specific measure and three kinematic variables: maximum upper body deceleration, body mass, shoulder angle at the instant of 'maximum impact' and maximum hip deceleration. The results showed that estimated and measured hip impact forces were linearly related (explained variances ranging from 46 to 63%). Hip impact forces of MA falls onto the mat from a standing position (3650±916N) estimated by the final model were comparable with measured values (3698±689N), even though these data were not used for training the model. In conclusion, a generic linear regression model was developed that enables the assessment of fall severity through kinematic measures of sideways falls, without using force plates.

Relaxation of the MCL after an Open-Wedge High Tibial Osteotomy results in decreasing contact pressures of the knee over time.

PURPOSE: The objective of this study was to investigate the effect of a medial open-wedge osteotomy (OWO) and the release of the superficial medial collateral ligament (MCL) on the tibiofemoral cartilage pressure, the MCL tension and the valgus laxity of the knee. METHODS: Seven fresh-frozen, human cadaveric knees were used. Medial and lateral mean contact pressure (CP), peak contact pressure (peakCP), and contact area (CA) were measured using a pressure-sensitive film (I-Scan; Tekscan, Boston, MA). The MCL tension was measured using a custom-made device. These measurements were continuously recorded for 5 min after an OWO of 10°. After the osteotomy, the valgus laxity was measured with a handheld Newtonmeter. For one knee, the measurements were continued for 24 h. At the end, a complete release of the superficial MCL was performed and the measurements were repeated at 10°. RESULTS: There was relaxation of the MCL after the osteotomy; the tension dropped in 5 min with 10.7% (mean difference 20.5 N (95% CI 16.1-24.9)), and in 24 h, the tension decreased by 24.2% (absolute difference 38.8 N) (one knee). After the osteotomy, the mean CP, peakCP and CA increased in the medial compartment (absolute difference 0.17 MPa (95% CI 0.14-0.20), 0.27 MPa (95% CI 0.24-0.30), 132.9mm2 (95% CI 67.7-198.2), respectively), and decreased in the lateral compartment (absolute difference 0.02 MPa (95% CI 0.03 -0.01), 0.08 MPa (95% CI 0.11 - 0.04), 47.0 mm2 (95% CI -105.8 to 11.8), respectively). Only after a release of the superficial MCL, the mean CP, peak CP and CA significantly decreased in the medial compartment (absolute difference 0.17, 0.27 MPa, 119.8 mm2, respectively), and increased in the lateral compartment (absolute difference 0.02, 0.11 MPa, 52.4 mm2, respectively). After the release of the superficial MCL, a mean increase of 7.9° [mean difference - 0.1° (95% CI -1.9 to 1.6)] of the valgus laxity was found. CONCLUSIONS: A release of the superficial MCL helps achieve the goal of reducing medial cartilage pressure in an OWO. There was considerable relaxation of the MCL after an OWO that resulted in a decrease of the mean CP in the medial and lateral compartments of the knee over time. However, cartilage pressure shifted from the medial to the lateral compartment only after release of the superficial MCL. The release of the superficial MCL caused a significant increase in the valgus laxity, which could influence stability after an OWO. LEVEL OF EVIDENCE: I.

Tibial component with and without stem extension in a trabecular metal cone construct.

PURPOSE: The purpose of this study was to investigate stability and strain distribution of a tibial plateau reconstruction with a trabecular metal cone while the tibial component is implanted with and without a stem, and whether prosthetic stability was influenced by bone mineral density. Trabecular metal cones are designed to fill up major bone defects in total knee arthroplasty. Tibial components can be implanted in combination with a stem, but it is unclear whether this is necessary after reconstruction with a trabecular metal cone. Implanting a stem can give extra stability, but may have negative side effects. METHODS: Tibial revision arthroplasties with trabecular metal cones were performed after reconstruction of a 2B bone defect according to the Anderson Orthopedic Research Institute classification. Components were implanted in seven pairs of cadaveric tibiae; one tibia of each pair was implanted with stem and the other without. All specimens were loaded to one bodyweight alternating between the medial and lateral tibial component. Implant-bone micro-motions, bone strains, bone mineral density and correlations were measured and/or calculated. RESULTS: Tibial components without a stem showed only more varus tilt [difference in median 0.14° (P < 0.05)], but this was not considered clinically relevant. Strain distribution did not differ. Bone mineral density only had an effect on the anterior/posterior tilt [ρ: -0.72 (P < 0.01)]. CONCLUSION: Tibial components, with or without a stem, which are implanted after reconstruction of major bone defects using trabecular metal cones produce very similar biomechanical conditions in terms of stability and strain distribution. If in vivo studies confirm that a stem extension is not mandatory, orthopaedic surgeons can decide not to implant a stem. LEVEL OF EVIDENCE: II.

Subject-specific musculoskeletal loading of the tibia: Computational load estimation.

The systematic development of subject-specific computer models for the analysis of personalized treatments is currently a reality. In fact, many advances have recently been developed for creating virtual finite element-based models. These models accurately recreate subject-specific geometries and material properties from recent techniques based on quantitative image analysis. However, to determine the subject-specific forces, we need a full gait analysis, typically in combination with an inverse dynamics simulation study. In this work, we aim to determine the subject-specific forces from the computer tomography images used to evaluate bone density. In fact, we propose a methodology that combines these images with bone remodelling simulations and artificial neural networks. To test the capability of this novel technique, we quantify the personalized forces for five subject-specific tibias using our technique and a gait analysis. We compare both results, finding that similar vertical loads are estimated by both methods and that the dominant part of the load can be reliably computed. Therefore, we can conclude that the numerical-based technique proposed in this work has great potential for estimating the main forces that define the mechanical behaviour of subject-specific bone.

Three-dimensional ultrasound strain imaging of skeletal muscles.

In this study, a multi-dimensional strain estimation method is presented to assess local relative deformation in three orthogonal directions in 3D space of skeletal muscles during voluntary contractions. A rigid translation and compressive deformation of a block phantom, that mimics muscle contraction, is used as experimental validation of the 3D technique and to compare its performance with respect to a 2D based technique. Axial, lateral and (in case of 3D) elevational displacements are estimated using a cross-correlation based displacement estimation algorithm. After transformation of the displacements to a Cartesian coordinate system, strain is derived using a least-squares strain estimator. The performance of both methods is compared by calculating the root-mean-squared error of the estimated displacements with the calculated theoretical displacements of the phantom experiments. We observe that the 3D technique delivers more accurate displacement estimations compared to the 2D technique, especially in the translation experiment where out-of-plane motion hampers the 2D technique. In vivo application of the 3D technique in the musculus vastus intermedius shows good resemblance between measured strain and the force pattern. Similarity of the strain curves of repetitive measurements indicates the reproducibility of voluntary contractions. These results indicate that 3D ultrasound is a valuable imaging tool to quantify complex tissue motion, especially when there is motion in three directions, which results in out-of-plane errors for 2D techniques.

Musculoskeletal modeling of human lower limb during normal walking, one-legged forward hopping and side jumping: Comparison of measured EMG and predicted muscle activity patterns.

This study focused on comparing muscle activities predicted by the Musculoskeletal Modeling System with EMG from ten healthy subjects who performed normal walking, one-legged forward hopping and side jumping. Eight EMG electrodes measured the activity of eight right leg muscles. Specific thresholds per muscle were applied on the EMG prior comparison. These thresholds were determined by equalizing the duration of EMG to AMS muscle activity. Three graph variables, number of onsets, offsets and hills were used to quantify the level of agreement by using Cohen׳s kappa analysis. The Pearson correlation coefficient was also calculated as a result comparison. Overall, visual inspection showed comparable activity patterns. However, when quantifying them some differences became apparent. The mean level of agreement of all tests was <0.20, meaning poor agreement. Pearson correlation showed better agreement compared to kappa analysis. In general, a more prescribed motion like FH and SJ showed a better agreement than NW. This explorative study shows that there are distinct differences between the model and EMG pattern. Those differences can be attributed to inevitable modeling limitation within the AMS framework like miscalculating the knee net moment, absence of co-contraction, simplified knee joint. Moreover, the delay between EMG and AMS has a clear effect on the comparison and this delay is obviously missing in the model. Despite those differences, this study can serve as a baseline measurement allowing progress in scientific work in order to reduce uncertainties with the aim to generate more reliable and robust musculoskeletal models in a valid manner.

Curved Beam Computed Tomography based Structural Rigidity Analysis of Bones with Simulated Lytic Defect: A Comparative Study with Finite Element Analysis.

In this paper, a CT based structural rigidity analysis (CTRA) method that incorporates bone intrinsic local curvature is introduced to assess the compressive failure load of human femur with simulated lytic defects. The proposed CTRA is based on a three dimensional curved beam theory to obtain critical stresses within the human femur model. To test the proposed method, ten human cadaveric femurs with and without simulated defects were mechanically tested under axial compression to failure. Quantitative computed tomography images were acquired from the samples, and CTRA and finite element analysis were performed to obtain the failure load as well as rigidities in both straight and curved cross sections. Experimental results were compared to the results obtained from FEA and CTRA. The failure loads predicated by curved beam CTRA and FEA are in agreement with experimental results. The results also show that the proposed method is an efficient and reliable method to find both the location and magnitude of failure load. Moreover, the results show that the proposed curved CTRA outperforms the regular straight beam CTRA, which ignores the bone intrinsic curvature and can be used as a useful tool in clinical practices.

Short-term clinical results of revision elbow arthroplasty using the Latitude total elbow arthroplasty.

AIMS: Revision total elbow arthroplasty (TEA) is often challenging. The aim of this study was to report on the clinical and radiological results of revision arthroplasty of the elbow with the Latitude TEA. PATIENTS AND METHODS: Between 2006 and 2010 we used the Latitude TEA for revision in 18 consecutive elbows (17 patients); mean age 53 years (28 to 80); 14 women. A Kudo TEA was revised in 15 elbows and a Souter-Strathclyde TEA in three. Stability, range of movement (ROM), visual analogue score (VAS) for pain and functional scores, Elbow Functional Assessment Scale (EFAS), the Functional Rating Index of Broberg and Morrey (FRIBM) and the Modified Andrews' Elbow Scoring System (MAESS) were assessed pre-operatively and at each post-operative follow-up visit (six, 12 months and biennially thereafter). Radiographs were analysed for loosening, fractures and dislocation. The mean follow-up was 59 months (26 to 89). RESULTS: The ROM of the elbow did not improve significantly. The mean EFAS and MAESS scores improved significantly six months post-operatively (18.6 points, standard deviation (sd) 7.7; p = 0.03 and 28.8 points, sd 8.6; p = 0.006, respectively) and continued to improve slightly or reached a plateau. The mean pain scores at rest (Z = -3.2, p = 0.001) and during activity (Z = -3.2, p = 0.001), and stability (Z = -3.0, p = 0.003) improved significantly six months post-operatively. Thereafter scores continued to improve slightly or a plateau was reached. There were no signs of loosening. CONCLUSION: Revision surgery using the Latitude TEA results in improvement of functionality, reduced pain and better stability of the elbow. Improvement of ROM of the elbow should not be expected. Cite this article: Bone Joint J 2016;98-B:1086-92.

Sensitivity of subject-specific models to Hill muscle-tendon model parameters in simulations of gait.

Subject-specific musculoskeletal (MS) models of the lower extremity are essential for applications such as predicting the effects of orthopedic surgery. We performed an extensive sensitivity analysis to assess the effects of potential errors in Hill muscle-tendon (MT) model parameters for each of the 56 MT parts contained in a state-of-the-art MS model. We used two metrics, namely a Local Sensitivity Index (LSI) and an Overall Sensitivity Index (OSI), to distinguish the effect of the perturbation on the predicted force produced by the perturbed MT parts and by all the remaining MT parts, respectively, during a simulated gait cycle. Results indicated that sensitivity of the model depended on the specific role of each MT part during gait, and not merely on its size and length. Tendon slack length was the most sensitive parameter, followed by maximal isometric muscle force and optimal muscle fiber length, while nominal pennation angle showed very low sensitivity. The highest sensitivity values were found for the MT parts that act as prime movers of gait (Soleus: average OSI=5.27%, Rectus Femoris: average OSI=4.47%, Gastrocnemius: average OSI=3.77%, Vastus Lateralis: average OSI=1.36%, Biceps Femoris Caput Longum: average OSI=1.06%) and hip stabilizers (Gluteus Medius: average OSI=3.10%, Obturator Internus: average OSI=1.96%, Gluteus Minimus: average OSI=1.40%, Piriformis: average OSI=0.98%), followed by the Peroneal muscles (average OSI=2.20%) and Tibialis Anterior (average OSI=1.78%) some of which were not included in previous sensitivity studies. Finally, the proposed priority list provides quantitative information to indicate which MT parts and which MT parameters should be estimated most accurately to create detailed and reliable subject-specific MS models.

Experimental Measurement of the Static Coefficient of Friction at the Ti-Ti Taper Connection in Total Hip Arthroplasty.

The modular taper junction in total hip replacements has been implicated as a possible source of wear. The finite-element (FE) method can be used to study the wear potential at the taper junction. For such simulations it is important to implement representative contact parameters, in order to achieve accurate results. One of the main parameters in FE simulations is the coefficient of friction. However, in current literature, there is quite a wide spread in coefficient of friction values (0.15 - 0.8), which has a significant effect on the outcome of the FE simulations. Therefore, to obtain more accurate results, one should use a coefficient of friction that is determined for the specific material couple being analyzed. In this study, the static coefficient of friction was determined for two types of titanium-on-titanium stem-adaptor couples, using actual cut-outs of the final implants, to ensure that the coefficient of friction was determined consistently for the actual implant material and surface finish characteristics. Two types of tapers were examined, Biomet type-1 and 12/14, where type-1 has a polished surface finish and the 12/14 is a microgrooved system. We found static coefficients of friction of 0.19 and 0.29 for the 12/14 and type-1 stem-adaptor couples, respectively.

In-situ mechanical behavior and slackness of the anterior cruciate ligament at multiple knee flexion angles.

In this study the in-situ tensile behavior and slackness of the anterior cruciate ligament (ACL) was evaluated at various knee flexion angles. In four cadaveric knees the ACL was released at the tibial insertion, after which it was re-connected to a tensiometer. After pre-tensioning (10 N) the ACL in full-extension, the knee was flexed from 0° to 150° at 15° increments, during which the ACL tension was measured. At each angle the ACL was subsequently elongated and shortened under displacement control, while measuring the ACL tension. In this manner, the pre-tension or the slackness, and the mechanical response of the ACL were measured. All ACL's displayed a higher tension at low (0°-60°) and high (120°-150°) flexion angles. The ACL slackness depended on flexion angle, with the highest slackness found at 75°-90°. Additionally, the ACL stiffness also varied with flexion angle, with the ACL behaving stiffer at low and high flexion angels. In general, the ACL was stiffest at 150°, and most compliant at 90°. The results of this study contribute to understanding the mechanical behavior of the ACL in-situ, and may help tuning and validating computational knee models studying ACL function.