Comparison of in vivo measured loads in knee, hip and spinal implants during level walking

The effect of a collar on primary stability of standard and undersized cementless hip stems: a biomechanical study

INTRODUCTION

Aseptic loosening and periprosthetic fractures are main reasons for revision after THA. Quite different from most other stem systems, Corail cementless hip stems show better survival rates than their cemented counterpart, which can possibly be explained by the use of a collar. The study aimed to investigate primary stability with standard and undersized hip stems both collared and collarless.

MATERIALS AND METHODS

Primary stability of cementless, collared and collarless, femoral stems was measured in artificial bones using both undersized and standard size. After preconditioning, 3D micromotion was measured under cyclic loading at the bone-implant interface.

RESULTS

The use of a collar resulted in higher micromotion within the same stem size but showed no statistically significant difference for both standard and undersized hip stems. The collared and collarless undersized stems showed no significant differences in 3D micromotion at the upper measuring positions compared to the standard stem size. Micromotion was significantly higher in the distal measuring positions, with and without collar, for the undersized stems (vs. standard collarless stem size).

CONCLUSION

The key finding is that the collarless and collared Corail hip stems, within one stem size, showed no significant differences in primary stability. Undersized stems showed significantly higher micromotion in the distal area both with and without collar.

Ultra-processed food consumption, genetic susceptibility, and the risk of hip/knee osteoarthritis

BACKGROUND

The associations between ultra-processed food (UPF) consumption, genetic susceptibility, and the risk of osteoarthritis (OA) remain unknown. This study was to examine the effect of UPF consumption, genetic susceptibility, and their interactions on hip/knee OA.

METHODS

Cohort analyses included 163,987 participants from the UK Biobank. Participants' UPF consumption was derived from their 24-h dietary recall using a questionnaire. Genetic risk scores (GRSs) of 70 and 83 single nucleotide polymorphisms (SNPs) for hip and knee OA were constructed.

FINDINGS

After 1,461,447 person-years of follow-up, 11,540 patients developed OA. After adjustments, compared to participants in the low quartile of UPF consumption, those in the high quartile had a 10 % (hazard ratio [HR], 1.10; 95% confidence interval [CI], 1.03-1.18) increased risk of knee OA. No significant association was found between UPF consumption and hip OA. Replacing 20% of UPF diet weight with an equivalent proportion of unprocessed or minimally processed food caused a 6% (HR, 0.94; 95% CI, 0.89-0.98) decreased risk of knee OA, respectively. A significant interaction was found between UPF consumption, genetic predisposition, and the risk of knee OA (P = 0.01). Participants with lower OA-GRS scores experienced higher knee OA risks due to UPF consumption.

INTERPRETATION

UPF consumption was associated with a higher risk of knee OA but not hip OA, particularly in those with lower genetic susceptibility. These results highlight the importance of reducing UPF consumption to prevent knee OA.

A framework based on subject-specific musculoskeletal models and Monte Carlo simulations to personalize muscle coordination retraining

Excessive loads at lower limb joints can lead to pain and degenerative diseases. Altering joint loads with muscle coordination retraining might help to treat or prevent clinical symptoms in a non-invasive way. Knowing how much muscle coordination retraining can reduce joint loads and which muscles have the biggest impact on joint loads is crucial for personalized gait retraining. We introduced a simulation framework to quantify the potential of muscle coordination retraining to reduce joint loads for an individuum. Furthermore, the proposed framework enables to pinpoint muscles, which alterations have the highest likelihood to reduce joint loads. Simulations were performed based on three-dimensional motion capture data of five healthy adolescents (femoral torsion 10°-29°, tibial torsion 19°-38°) and five patients with idiopathic torsional deformities at the femur and/or tibia (femoral torsion 18°-52°, tibial torsion 3°-50°). For each participant, a musculoskeletal model was modified to match the femoral and tibial geometry obtained from magnetic resonance images. Each participant's model and the corresponding motion capture data were used as input for a Monte Carlo analysis to investigate how different muscle coordination strategies influence joint loads. OpenSim was used to run 10,000 simulations for each participant. Root-mean-square of muscle forces and peak joint contact forces were compared between simulations. Depending on the participant, altering muscle coordination led to a maximum reduction in hip, knee, patellofemoral and ankle joint loads between 5 and 18%, 4% and 45%, 16% and 36%, and 2% and 6%, respectively. In some but not all participants reducing joint loads at one joint increased joint loads at other joints. The required alteration in muscle forces to achieve a reduction in joint loads showed a large variability between participants. The potential of muscle coordination retraining to reduce joint loads depends on the person's musculoskeletal geometry and gait pattern and therefore showed a large variability between participants, which highlights the usefulness and importance of the proposed framework to personalize gait retraining.

Loading of the Hip and Knee During Swimming: An in Vivo Load Study

BACKGROUND

Swimming is commonly recommended as postoperative rehabilitation following total hip arthroplasty (THA) and total knee arthroplasty (TKA). So far, in vivo hip and knee joint loads during swimming remain undescribed.

METHODS

In vivo hip and knee joint loads were measured in 6 patients who underwent THA and 5 patients who underwent TKA with instrumented joint implants. Joint loads, including the resultant joint contact force (F Res ), torsional moment around the femoral shaft axis or the tibial axis (M Tors ), bending moment at the middle of the femoral neck (M Bend ), torsional moment around the femoral neck axis (M Tne ), and medial force ratio (MFR) in the knee, were measured during breaststroke swimming at 0.5, 0.6, and 0.7 m/s and the breaststroke and crawl kicks at 0.5 and 1.0 m/s.

RESULTS

The ranges of the median maximal F Res were 157% to 193% of body weight for the hip and 93% to 145% of body weight for the knee during breaststroke swimming. Greater maxima of F Res (hip and knee), M Tors (hip and knee), M Bend (hip), and M Tne (hip) were observed with higher breaststroke swimming velocities, but significance was only identified between 0.5 and 0.6 m/s in F Res (p = 0.028), M Tors (p = 0.028), and M Bend (p = 0.028) and between 0.5 and 0.7 m/s in F Res (p = 0.045) in hips. No difference was found in maximal MFR between different breaststroke swimming velocities. The maximal F Res was significantly positively correlated with the breaststroke swimming velocity (hip: r = 0.541; p < 0.05; and knee: r = 0.414; p < 0.001). The maximal F Res (hip and knee) and moments (hip) were higher in the crawl kick than in the breaststroke kick, and a significant difference was recognized in F Res Max for the hip: median, 179% versus 118% of body weight (p = 0.028) for 0.5 m/s and 166% versus 133% of body weight (p = 0.028) for 1.0 m/s.

CONCLUSIONS

Swimming is a safe and low-impact activity, particularly recommended for patients who undergo THA or TKA. Hip and knee joint loads are greater with higher swimming velocities and can be influenced by swimming styles. Nevertheless, concrete suggestions to patients who undergo arthroplasty on swimming should involve individual considerations.

LEVEL OF EVIDENCE

Therapeutic Level IV . See Instructions for Authors for a complete description of levels of evidence.

Effect of gait types and external weight carrying strategies on the femoral neck strains during stair descent

Gait and weight carrying method may change the femoral neck load during stair descent. Applying specific gait and weight carrying methods may reduce the femoral neck load during stair descent, which may reduce hip pain, hip pain related falls and fall related fractures for the older population. The purpose of this study was to test the effect of different gait types (step-over-step v.s. step-by-step) and external weight carrying strategies (ipsilateral v.s. contralateral side) on the femoral neck load, discover which method could reduce the femoral neck load effectively. Seventeen healthy adults from 50 to 70 yrs old were recruited. The kinematic and kinetic analysis, musculoskeletal modelling method were used to estimate the joint and muscle loads for the lower extremities. Finite element analysis was used with the femur model to calculate the femoral neck strains during stair descent with different gait types and weight carrying strategies. The compressive strains were reduced for step-by-step gait method than step-over-step (p<0.015, 12.3-17.4% decrease of strains), the tensile strains were significantly increased for the trailing leg of step-by-step than the leading leg (p<0.001, 24.7% increase of strains). Contralateral weight carrying increased compressive and tensile strains than ipsilateral (p<0.001, 9.9-24.5% increase of strains) in most conditions. Applying step-by-step method and avoiding contralateral side weight carrying could be effective to reduce femoral neck strains. These outcomes could be helpful for the older population to reduce the risks of hip pain, femoral neck pain or pain related falls and fractures.

Lower-limb internal loading and potential consequences for fracture healing

Introduction: Mechanical loading is known to determine the course of bone fracture healing. We hypothesise that lower limb long bone loading differs with knee flexion angle during walking and frontal knee alignment, which affects fracture healing success. Materials and methods: Using our musculoskeletal in silico modelling constrained against in vivo data from patients with instrumented knee implants allowed us to assess internal loads in femur and tibia. These internal forces were associated with the clinical outcome of fracture healing in a relevant cohort of 178 extra-articular femur and tibia fractures in patients using a retrospective approach. Results: Mean peak forces differed with femoral compression (1,330-1,936 N at mid-shaft) amounting to about half of tibial compression (2,299-5,224 N). Mean peak bending moments in the frontal plane were greater in the femur (71-130 Nm) than in the tibia (from 26 to 43 Nm), each increasing proximally. Bending in the sagittal plane showed smaller mean peak bending moments in the femur (-38 to 43 Nm) reaching substantially higher values in the tibia (-63 to -175 Nm) with a peak proximally. Peak torsional moments had opposite directions for the femur (-13 to -40 Nm) versus tibia (15-48 Nm) with an increase towards the proximal end in both. Femoral fractures showed significantly lower scores in the modified Radiological Union Scale for Tibia (mRUST) at last follow-up (p < 0.001) compared to tibial fractures. Specifically, compression (r = 0.304), sagittal bending (r = 0.259), and frontal bending (r = -0.318) showed strong associations (p < 0.001) to mRUST at last follow-up. This was not the case for age, body weight, or localisation alone. Discussion: This study showed that moments in femur and tibia tend to decrease towards their distal ends. Tibial load components were influenced by knee flexion angle, especially at push-off, while static frontal alignment played a smaller role. Our results indicate that femur and tibia are loaded differently and thus require adapted fracture fixation considering load components rather than just overall load level.

Influence of femoral anteversion angle and neck-shaft angle on muscle forces and joint loading during walking

Femoral deformities, e.g. increased or decreased femoral anteversion (AVA) and neck-shaft angle (NSA), can lead to pathological gait patterns, altered joint loads, and degenerative joint diseases. The mechanism how femoral geometry influences muscle forces and joint load during walking is still not fully understood. The objective of our study was to investigate the influence of femoral AVA and NSA on muscle forces and joint loads during walking. We conducted a comprehensive musculoskeletal modelling study based on three-dimensional motion capture data of a healthy person with a typical gait pattern. We created 25 musculoskeletal models with a variety of NSA (93°-153°) and AVA (-12°-48°). For each model we calculated moment arms, muscle forces, muscle moments, co-contraction indices and joint loads using OpenSim. Multiple regression analyses were used to predict muscle activations, muscle moments, co-contraction indices, and joint contact forces based on the femoral geometry. We found a significant increase in co-contraction of hip and knee joint spanning muscles in models with increasing AVA and NSA, which led to a substantial increase in hip and knee joint contact forces. Decreased AVA and NSA had a minor impact on muscle and joint contact forces. Large AVA lead to increases in both knee and hip contact forces. Large NSA (153°) combined with large AVA (48°) led to increases in hip joint contact forces by five times body weight. Low NSA (108° and 93°) combined with large AVA (48°) led to two-fold increases in the second peak of the knee contact forces. Increased joint contact forces in models with increased AVA and NSA were linked to changes in hip muscle moment arms and compensatory increases in hip and knee muscle forces. Knowing the influence of femoral geometry on muscle forces and joint loads can help clinicians to improve treatment strategies in patients with femoral deformities.

Walking with a Posterior Cruciate Ligament Injury: A Musculoskeletal Model Study

The understanding of the changes induced in the knee's kinematics by a Posterior Cruciate Ligament (PCL) injury is still rather incomplete. This computational study aimed to analyze how the internal loads are redistributed among the remaining ligaments when the PCL is lesioned at different degrees and to understand if there is a possibility to compensate for a PCL lesion by changing the hamstring's contraction in the second half of the swing phase. A musculoskeletal model of the knee joint was used for simulating a progressive PCL injury by gradually reducing the ligament stiffness. Then, in the model with a PCL residual stiffness at 15%, further dynamic simulations of walking were performed by progressively reducing the hamstring's force. In each condition, the ligaments tension, contact force and knee kinematics were analyzed. In the simulated PCL-injured knee, the Medial Collateral Ligament (MCL) became the main passive stabilizer of the tibial posterior translation, with synergistic recruitment of the Lateral Collateral Ligament. This resulted in an enhancement of the tibial-femoral contact force with respect to the intact knee. The reduction in the hamstring's force limited the tibial posterior sliding and, consequently, the tension of the ligaments compensating for PCL injury decreased, as did the tibiofemoral contact force. This study does not pretend to represent any specific population, since our musculoskeletal model represents a single subject. However, the implemented model could allow the non-invasive estimation of load redistribution in cases of PCL injury. Understanding the changes in the knee joint biomechanics could help clinicians to restore patients' joint stability and prevent joint degeneration.

Patient reported outcomes do not correlate to functional knee recovery and range of motion in total knee arthroplasty

Background

Many total knee arthroplasty (TKA) patients exhibit continued pain and limited function following surgery. Determining TKA outcomes is typically reliant on post-operative evaluations and completing patient-reported outcomes (PROMs). Due to low compliance rates, it is essential to identify new strategies for monitoring patients. The purpose of this analysis was to assess the correlations between gait kinematics, PROMs, and knee range of motion (ROM).

Methods

130 patients (75 female) received Persona IQ TKA (Zimmer Biomet, Warsaw, IN, USA) which includes a stem extension with embedded accelerometer and gyroscope. PROM scores were compared at baseline and 6 weeks post-TKA using a paired t-test. Gait kinematics were recorded daily via the Persona IQ stem extension. Pearson's correlation coefficients were derived between PROMs and average gait kinematics.

Results

Knee Injury and Osteoarthritis Outcome Score (KOOS Jr.) and Veterans RAND 12 (VR-12) physical scores improved following surgery (p ≤ 0.001, p = 0.003, respectively). Weak statistically significant correlations were found between PROMS and gait kinematics.

Conclusion

Weak correlations between PROMs and gait kinematics indicate patient perception of improvement and objectively measured functional status may not be interchangeable. Further, compliance with Persona IQ data reached 95.4-97.7% (depending on the parameter) at 6 weeks following surgery, a 20% higher compliance rate over PROMs. Daily functional measurements provide insight into the patient's progression and may be useful in detecting poor outcomes.

Calibration procedure and biomechanical validation of an universal six degree-of-freedom robotic system for hip joint testing

PURPOSE

Among various test methods for different human joints, the use of robot systems has attracted major interest and inherits the potential to become a gold standard in biomechanical testing in the future. A key issue associated with those robot-based platforms is the accurate definition of parameters, e.g., tool center point (TCP), length of tool or anatomical trajectories of movements. These must be precisely correlated to the physiological parameters of the examined joint and its corresponding bones. Exemplified for the human hip joint, we are creating an accurate calibration procedure for a universal testing platform by using a six degree-of-freedom (6 DOF) robot and optical tracking system for recognition of anatomical movements of the bone samples.

METHODS

A six degree-of-freedom robot (TX 200, Stäubli) has been installed and configured. The physiological range of motion of the hip joint composed of a femur and a hemipelvis was recorded with an optical 3D movement and deformation analysis system (ARAMIS, GOM GmbH). The recorded measurements were processed by automatic transformation procedure (created in Delphi software) and evaluated in 3D CAD system.

RESULTS

The physiological ranges of motion were reproduced for all degrees of freedom with the six degree-of-freedom robot in adequate accuracy. With the establishment of a special calibration procedure by using a combination of different coordinate systems, we were able to achieve a standard deviation of the TCP depending of the axis between 0.3 and 0.9 mm and for the length of tool between + 0.67 and - 0.40 mm (3D CAD processing) resp. + 0.72 mm to - 0.13 mm (Delphi transformation). The accuracy between the manual and robotic movement of the hip shows an average deviation between - 0.36 and + 3.44 mm for the points on the movement trajectories.

CONCLUSION

A six degree-of-freedom robot is appropriate to reproduce the physiological range of motion of the hip joint. The described calibration procedure is universal and can be used for hip joint biomechanical tests allowing to apply clinically relevant forces and investigate testing stability of reconstructive osteosynthesis implant/endoprosthetic fixations, regardless of the length of the femur, size of the femoral head and acetabulum or whether the entire pelvis or only the hemipelvis will be used.

Larger Medial Contact Area and More Anterior Contact Position in Medial-Pivot than Posterior-Stabilized Total Knee Arthroplasty during In-Vivo Lunge Activity

This study aimed to compare the in-vivo kinematics and articular contact status between medial-pivot total knee arthroplasty (MP-TKA) and posterior stabilized (PS) TKA during weight-bearing single-leg lunge. 16 MP-TKA and 12 PS-TKA patients performed bilateral single-leg lunges under dual fluoroscopy surveillance to determine the in-vivo six degrees-of-freedom knee kinematics. The closest point between the surface models of the femoral condyle and the polyethylene insert was used to determine the contact position and area. The nonparametric statistics analysis was performed to test the symmetry of the kinematics between MP-TKA and PS-TKA. PS-TKA demonstrated a significantly greater range of AP translation than MP-TKA during high flexion (p = 0.0002). Both groups showed a significantly greater range of lateral compartment posterior translation with medial pivot rotation. The contact points of PS-TKA were located significantly more posterior than MP-TKA in both medial (10°–100°) and lateral (5°–40°, 55°–100°) compartments (p < 0.0500). MP-TKA had a significantly larger contact area in the medial compartment than in the lateral compartment. In contrast, no significant differences were observed in PS-TKA. The present study revealed no significant differences in clinical outcomes between the MP and PS groups. The PS-TKA demonstrated significantly more posterior translations than MP-TKA at high flexion. The contact points are located more posteriorly in PS-TKA compared with MP-TKA. A larger contact area and medial pivot pattern during high flexion in MP-TKA indicated that MP-TKA provides enhanced medial pivot rotation.

Reconstruction of massive bone defects after femoral tumor resection using two new-designed 3D-printed intercalary prostheses: a clinical analytic study with the cooperative utilization of multiple technologies

BACKGROUND

To reconstruct massive bone defects of the femoral diaphysis and proximal end with limited bilateral cortical bone after joint-preserving musculoskeletal tumor resections, two novel 3D-printed customized intercalary femoral prostheses were applied.

METHODS

A series of nine patients with malignancies who received these novel 3D-printed prostheses were retrospectively studied between July 2018 and November 2021. The proximal and diaphyseal femur was divided into three regions of interest (ROIs) according to anatomic landmarks, and anatomic measurements were conducted on 50 computed tomography images showing normal femurs. Based on the individual implant-involved ROIs, osteotomy level, and anatomical and biomechanical features, two alternative 3D-printed prostheses were designed. In each patient, Hounsfield Unit (HU) value thresholding and finite element analysis were conducted to identify the bone trabecula and calcar femorale and to determine the stress distribution, respectively. We described the characteristics of each prosthesis and surgical procedure and recorded the intraoperative data. All patients underwent regular postoperative follow-up, in which the clinical, functional and radiographical outcomes were evaluated.

RESULTS

With the ROI division and radiographic measurements, insufficient bilateral cortical bones for anchoring the traditional stem were verified in the normal proximal femur. Therefore, two 3D-printed intercalary endoprostheses, a Type A prosthesis with a proximal curved stem and a Type B prosthesis with a proximal anchorage-slot and corresponding locking screws, were designed. Based on HU value thresholding and finite element analysis, the 3D-printed proximal stems in all prostheses maximally preserved the trabecular bone and calcar femorale and optimized the biomechanical distribution, as did the proximal screws. With the 3D-printed osteotomy guide plates and reaming guide plates, all patients underwent the operation uneventfully with a satisfactory duration (325.00 ± 62.60 min) and bleeding volume (922.22 ± 222.36 ml). In the follow-up, Harris Hip and Musculoskeletal Tumor Society scores were ameliorated after surgery (P < 0.001 and P < 0.001, respectively), reliable bone ingrowth was observed, and no major complications occurred.

CONCLUSIONS

Two novel 3D-printed femoral intercalary prostheses, which achieved acceptable overall postoperative outcomes, were used as appropriate alternatives for oncologic patients with massive bone defects and limited residual bone and increased the opportunities for joint-preserving tumor resection. Several scientific methodologies utilized in this study may promote the clinical design proposals of 3D-printed implants.

The Impact of Coronal Configuration of the Proximal Femur on its Mechanical Properties and the Validation of a New Theoretical Model: Finite Element Analysis and Biomechanical Examination

OBJECTIVE

This study aims to establish the coronal configuration of the proximal femur as an independent factor for its mechanical properties and provide validation for the theoretical model "fulcrum-balance-reconstruction."

METHODS

The digital 3D femur model constructed with the lower extremity high-resolution computed tomography of a senior subject was applied with the axial compression of 2100N under 5 different α angles of 10°, 5°, 0°, -5°, -10°. The equivalent stress distribution of the femoral geometric model under each angle were calculated. Under the same five α angles, fatigue test was performed on 15 composite artificial left femurs (three specimens in each angle group) to obtain the failure cycle and fracture site. The statistical analysis was accomplished using One-Way ANOVA.

RESULTS

The maximum stress of the entire femur in physiological angle (α = 10°) occurred below femoral neck with a value of 63.91 MPa. When the proximal femur is in extreme abducted angle (α = -10°), the maximum stress shift to the lower medial cortex of femoral shaft with a value of 105.2 MPa. As the α angle changed from 10° to -10°, the greater trochanteric region had the largest increment in maximum stress (2.78 times for cortex and 1.67 times for cancellous bone) locally at the proximal femur. The failure cycles of the artificial femurs with a variety of abduction angle were averagely 9126 ± 2453.87 (α = -10°), 58,112.33 ± 1293.84 (α = -5°), 92,879.67 ± 2398.54 (α = 0°), 172,045.3 ± 11011.11 (α = 5°), and 264,949.3 ± 35,067.26 (α = 10°), and the statistical analysis revealed that the α angle of the group of concern is proportional to the P value of the corresponding group compared to the 10° group(α = 5° & α = 10°, P = 0.01; α = 0 & α = 10°, P = 0.001; α = -5°, -10° & α = 10°, P < 0.001). In fatigue test, the fracture appeared on femoral neck for the α angles of 10° (three subcapital), 5° (two basal; one transcervical), and 0° (one transcervical). Fracture sites located at trochanteric region were observed with the more abducted angles including 0° (two subtrochanteric) and -5° (two intertrochanteric; one subtrochanteric). The fracture line was only found on femoral shaft in the -10° group.

CONCLUSION

With increasing hip abduction, the proximal femur shows declining mechanical properties, which suggests higher risk of hip fracture and increasement in the fraction of trochanteric fracture subtype. Furthermore, the hypothesis of "fulcrum-balance-reconstruction" was validated by our study to a certain extent.

Estimation of Tibiofemoral Joint Contact Forces Using Foot Loads during Continuous Passive Motions

Continuous passive motion (CPM) machines are commonly used after various knee surgeries, but information on tibiofemoral forces (TFFs) during CPM cycles is limited. This study aimed to explore the changing trend of TFFs during CPM cycles under various ranges of motion (ROM) and body weights (BW) by establishing a two-dimensional mathematical model. TFFs were estimated by using joint angles, foot load, and leg−foot weight. Eleven healthy male participants were tested with ROM ranging from 0° to 120°. The values of the peak TFFs during knee flexion were higher than those during knee extension, varying nonlinearly with ROM. BW had a significant main effect on the peak TFFs and tibiofemoral shear forces, while ROM had a limited effect on the peak TFFs. No significant interaction effects were observed between BW and ROM for each peak TFF, whereas a strong linear correlation existed between the peak tibiofemoral compressive forces (TFCFs) and the peak resultant TFFs (R2 = 0.971, p < 0.01). The proposed method showed promise in serving as an input for optimizing rehabilitation devices.

A Cadaveric Study Addressing the Feasibility of Remote Patient Monitoring Prosthesis for Total Knee Arthroplasty

BACKGROUND

Since the COVID-19 pandemic of 2020, there has been a marked rise in the use of telemedicine to evaluate patients after total knee arthroplasty (TKA). The purpose of our study was to assess a novel stem with an embedded sensor that can remotely and objectively monitor a patient's mobility after TKA.

METHODS

A single anatomically designed knee system was implanted in concert with an interconnected tibial stem extension containing 3D accelerometers, 3D gyroscopes, a power source, and a telemetry transmission capability in 3 cadaveric pelvis to toe specimens. The legs were moved by hand to preset tibial positions at full knee extension, midflexion, flexion, and back to midflexion and extension for a total of 16 trials across 6 knees.

RESULTS

Sensor data were successfully transmitted with good quality of signal to an external base station. Good correlation to the range of motion of the tibia was found (mean error 0.1 degrees; root mean square error 3.8 degrees). The signal from the heel drop tests suggests the sensor could detect heel strike during activities of daily living in vivo and the potential for additional signal processing to analyze vibratory and motion patterns detected by the sensors. A frequency domain analysis of a properly cemented and poorly cemented implant during the heel drop test suggests a difference in accelerometer signal in these implant states.

CONCLUSION

The results confirm signals generated from an embedded TKA sensor can transmit through bone and cement, providing accurate range of motion data and may be capable of detecting changes in prosthesis fixation remotely.

Is Training With Gym Machines Safe After Hip Arthroplasty?—An In Vivo Load Investigation

Background: Training with gym machines is one of the most popular physical activities after total hip arthroplasty (THA). However, to date, there are no evidence-based recommendations for physical activity after THA, worldwide. The aim of the study is to evaluate the in vivo hip joint loads during exercises on four widely used gym machines in order to provide a source for an evidence-based patient counselling for arthroplasty surgeons.

Methods: The in vivo hip joint loads in seven patients (59.6 ± 6.4 years, 28.6 ± 2.1 kg/m²) with instrumented hip implants were assessed. The resulting force (F_res), bending moment (M_bend), and torsional moment (M_tors) were evaluated during the training on leg curl/leg extension machines (loads: 20, 30, and 40 kg), leg press machine [backrest: 10°, 30°, and 60°; load: 50, 75, and 100%BW (bodyweight)], and a rope pull machine (abduction/adduction/flexion/extension; each ipsi- and contralateral; load 10 kg). These loads were compared with the loads during walking on treadmill at 4 km/h (median peak values: F_res 303%BW, M_bend 4.25%BWm, and M_tors 2.70%BWm).

Results: In each of the four performed exercises with a total of 23 different load conditions or exercise modes analyzed, a significantly lower or not different load was detected with respect to F_res, M_bend, and M_tors measured while walking with 4 km/h. Nevertheless, F_res and M_bend demonstrated a trend to increased loading during the ipsilateral monopod standing rope pull exercises hip flexion, extension, and abduction.

Conclusion: Based on our investigation, we assume that the investigated gym machines and external loads can be considered mainly as low-impact sports (with some exceptions) and thus as safe physical activity after THA. Due to the fact that the examinations were conducted in the mean 17.4 months after THA, the applicability of the results to the immediate postoperative period is limited.

'SMART' implantable devices for spinal implants: a systematic review on current and future trends

Background

'SMART' implants refer to modified orthopedic implants that combine the biomechanical safety and efficacy of traditional devices with the intelligence of data-logging sensors. This review aims to systematically assess the available literature on SMART spinal implants and present these findings in a clinically relevant manner.

Methods

A search of PubMed, Scopus, and Google Scholar databases was conducted by two separate reviewers. Information including sensor type, intended application, and sample size, was extracted from included studies. Risk of bias assessment was conducted using the Office of Health Assessment and Translation (OHAT) risk of bias tool.

Results

Eighteen studies were included for analysis. Eight studies involved SMART rods and ten studies used SMART vertebral body replacements (VBR). No more than 20 patients are reported to have received a SMART spinal implant. Including non-primary evidence, seven unique designs for SMART spinal implants were found. The majority of these used strain gauges with recent designs including thermometers and accelerometers.

Discussion

At present, SMART spinal implants have primarily focused on utilising strain gauges to report loading on the implant itself. This is a logical first step as it allows quantification of real-world requirements of an implant, detection of catastrophic failure, while also allowing researchers and clinicians to estimate changes in load sharing between newly forming bone and the implant itself, providing real-time information on the progression of healing and fusion. Future work includes documenting the correlation between data provided by these SMART implants and clinical findings, including complications such as pedicle screw loosening and interbody cage subsidence.

Fixation of intraoperative proximal femoral fractures during THA using two versus three cerclage wires - a biomechanical study

Background

Intraoperative proximal femoral fractures (IPFF) are relevant complications during total hip arthroplasty. Fixation using cerclage wires (CW) represents a minimally-invasive technique to address these fractures through the same surgical approach. The goal of treatment is to mobilise the patient as early as possible, which requires high primary stability. This study aimed to compare different cerclage wire configurations fixing IPFF with regard to biomechanical primary stability.

Methods

Standardised IPFF (type II, Modified Mallory Classification) were created in human fresh frozen femora and were fixed either by two or three CW (1.6 mm, stainless steel). All cadaveric specimens (n = 42) were randomised to different groups (quasi-static, dynamic) or subgroups (2 CW, 3 CW) stratified by bone mineral density determined by Dual Energy X-ray Absorptiometry. Using a biomechanical testing setup, quasi-static and dynamic cyclic failure tests were carried out. Cyclic loading started from 200 N to 500 N at 1 Hz with increasing peak load by 250 N every 100 cycles until failure occurred or maximum load (5250 N) reached. The change of fracture gap size was optically captured.

Results

No significant differences in failure load after quasi-static (p = 0.701) or dynamic cyclic loading (p = 0.132) were found between the experimental groups. In the quasi-static load testing, all constructs resisted 250% of the body weight (BW) of their corresponding body donor. In the dynamic cyclic load testing, all but one construct (treated by 3 CW) resisted 250% BW.

Conclusions

Based on this in vitro data, both two and three CW provided sufficient primary stability according to the predefined minimum failure load (250% BW) to resist. The authors recommend the treatment using two CW because it reduces the risk of vascular injury and shortens procedure time.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12891-021-04956-5.

In vivo analysis of hip joint loading on Nordic walking novices

Objective

To evaluate the influence of Nordic walking (NW) on hip joint loads in order to determine whether it can be safely performed during postoperative physiotherapy in patients after orthopeadic surgery of the hip.

Methods

Internal hip joint loads were directly measured in vivo in 6 patients using instrumented hip prostheses during NW and ordinary walking (OW). All patients received training in two different NW techniques (double-poling and the diagonal technique) by a certified NW instructor. Measurements were conducted on a treadmill at a speed of 4 km/h on level ground, at 10% inclination and at 10% slope as well as on a level lawn at a self chosen comfortable speed. Resultant contact force (F_res), bending moment (M_bend) and torsional torque (M_tors) were compared between NW and OW as well as between both NW techniques.

Results

Joint loads showed a double peak pattern during all setups. Neither NW technique significantly influenced hip joint loads at the time of the first load peak during contralateral toe-off (CTO), which was also the absolute load peak, in comparison to OW. Compared to OW, double-poling significantly reduced F_res and M_bend at the time of the second load peak during the contralateral heel strike (CHS) on level ground both on the treadmill (− 6% and − 7%, respectively) and on the lawn (− 7% and − 9%). At 10% inclination, the diagonal technique increased F_res and M_bend at CHS (by + 6% and + 7%), but did not increase the absolute load peak at CTO.

Conclusion

Joint loads during NW are comparable to those of OW. Therefore, NW can be considered a low-impact activity and seems to be safe for patients that are allowed full weight bearing, e.g. during postoperative rehabilitation after THA.

Trans-obturator cable fixation of open book pelvic injuries

Operative treatment of ruptured pubic symphysis by plating is often accompanied by complications. Trans-obturator cable fixation might be a more reliable technique; however, have not yet been tested for stabilization of ruptured pubic symphysis. This study compares symphyseal trans-obturator cable fixation versus plating through biomechanical testing and evaluates safety in a cadaver experiment. APC type II injuries were generated in synthetic pelvic models and subsequently separated into three different groups. The anterior pelvic ring was fixed using a four-hole steel plate in Group A, a stainless steel cable in Group B, and a titan band in Group C. Biomechanical testing was conducted by a single-leg-stance model using a material testing machine under physiological load levels. A cadaver study was carried out to analyze the trans-obturator surgical approach. Peak-to-peak displacement, total displacement, plastic deformation and stiffness revealed a tendency for higher stability for trans-obturator cable/band fixation but no statistical difference to plating was detected. The cadaver study revealed a safe zone for cable passage with sufficient distance to the obturator canal. Trans-obturator cable fixation has the potential to become an alternative for symphyseal fixation with less complications.

In vivo loading on the hip joint in patients with total hip replacement performing gymnastics and aerobics exercises

A further increase in the number of total hip arthroplasty (THA) is predicted, in particular the number of young THA patients has raised and with it their demands. There is no standardized evidence-based rehabilitation program and no reliable guidelines for sports activities after THA. Stretching and strengthening gymnastics are routinely performed in rehabilitation and aerobics as a sport after THA. The aim of the investigation was to determine the in vivo force and moments acting on the hip prosthesis during gymnastics and aerobic exercises to provide a source for evidence-based recommendations. Hip joint loads were measured in six patients with instrumented hip implants. The resulting force F_Res, bending moment M_Bend at the neck and torsional moment M_Tors at the stem were examined during seven strengthening (with two different resistance bands) and four stretching gymnastic exercises and seven aerobic exercises with and without an aerobic step board compared to the loads during the reference activity walking. The stretching and strengthening gymnastics exercises and the aerobic exercises with and without a board demonstrated in their median peak force and moments mostly lower or similar values compared to walking. Significantly increased loads were recorded for the flexor stretching exercise in monopod stand (F_res and M_Bend), the strengthening abduction exercise on the chair (M_Tors) and the strengthening flexion exercise with the stronger resistance band (M_Tors). We also found a significant increase in median peak values in aerobic exercises with a board for the "Basic Step" (ipsilateral started F_res and M_Tors; contralateral started M_Tors), "Kickstep ipsilateral started" (F_res and M_Tors) and "Over the Top contralateral started" (F_res). The in vivo loads in THA patients during frequently performed stretching, strengthening and aerobic exercises were demonstrated for the first time. It was proved that stretching gymnastic exercises are safe in terms of resulting force, bending and torque moments for THA patients, although an external assistance for stabilization may be considered. Strengthening gymnastics exercises are reliable in terms of F_res, M_Bend and M_Tors, but, based on our data, we recommend to adhere to the communicated specific postoperative restrictions and select the resistance bands with lower tension. Aerobic exercises without an aerobic board can be considered as reliable activity in terms of force and moments for THA patients. Aerobic exercises with a board are not recommended for the early postoperative period and in our opinion need to be adapted to the individual muscular and coordinative resources.

Towards planning of osteotomy around the knee with quantitative inclusion of the adduction moment: a biomechanical approach

PURPOSE

Despite practised for decades, the planning of osteotomy around the knee, commonly using the Mikulicz-Line, is only empirically based, clinical outcome inconsistent and the target angle still controversial. A better target than the angle of frontal-plane static leg alignment might be the external frontal-plane lever arm (EFL) of the knee adduction moment. Hypothetically assessable from frontal-plane-radiograph skeleton dimensions, it might depend on the leg-alignment angle, the hip-centre-to-hip-centre distance, the femur- and tibia-length.

METHODS

The target EFL to achieve a medial compartment force ratio of 50% during level-walking was identified by relating in-vivo-measurement data of knee-internal loads from nine subjects with instrumented prostheses to the same subjects' EFLs computed from frontal-plane skeleton dimensions. Adduction moments derived from these calculated EFLs were compared to the subjects' adduction moments measured during gait analysis.

RESULTS

Highly significant relationships (0.88 ≤ R2 ≤ 0.90) were found for both the peak adduction moment measured during gait analysis and the medial compartment force ratio measured in vivo to EFL calculated from frontal-plane skeleton dimensions. Both correlations exceed the respective correlations with the leg alignment angle, EFL even predicts the adduction moment's first peak. The guideline EFL for planning osteotomy was identified to 0.349 times the epicondyle distance, hence deducing formulas for individualized target angles and Mikulicz-Line positions based on full-leg radiograph skeleton dimensions. Applied to realistic skeleton geometries, widespread results explain the inconsistency regarding correction recommendations, whereas results for average geometries exactly meet the most-consented "Fujisawa-Point".

CONCLUSION

Osteotomy outcome might be improved by planning re-alignment based on the provided formulas exploiting full-leg-radiograph skeleton dimensions.

Analysis of hip joint loading during walking with different shoe types using instrumented total hip prostheses

Hip joint loads need careful consideration during postoperative physiotherapy after joint replacement. One factor influencing joint loads is the choice of footwear, but it remains unclear which footwear is favorable. The objective of the present study was to investigate the influence of footwear on hip joint loads in vivo. Instrumented hip endoprostheses were used for in vivo load measurements. The parameters resultant contact force (F_res), bending moment (M_bend) and torsional moment (M_tors) were evaluated during treadmill walking at 4 km/h with different shoe types. In general, footwear tended to increase hip joint loading, with the barefoot shoe having the least influence. F_res and M_bend were significantly increased during heel strike for all shoe types in comparison to barefoot walking, with everyday shoe (34.6%; p = 0.028 and 47%; p = 0.028, respectively) and men’s shoe (33.2%; p = 0.043 and 41.1%; p = 0.043, respectively) resulting in the highest changes. M_tors at AbsMax was increased by all shoes except for the barefoot shoe, with the highest changes for men’s shoe (+ 17.6%, p = 0.043) and the shoe with stiffened sole (+ 17.5%, p = 0.08). Shoes, especially those with stiff soles or elaborate cuishing and guiding elements, increase hip joint loads during walking. The influence on peak loads is higher for M_tors than for F_res and M_bend. For patients in which a reduction of hip joints loads is desired, e.g. during physiotherapy after recent surgery or to alleviate symptoms of osteoarthritis, low profile shoes with a flexible sole may be preferred over shoes with a stiff sole or elaborate cushioning elements.

Finite element analysis of femoral neck strains during stair ascent and descent

For older population, a better understanding of the hip joint loading environment is needed for the prevention of hip pain, and the reduction of the stress fractures and fall risks. Using the motion analysis and inverse dynamics methods, combined with musculoskeletal modelling, static optimization, and finite element (FE) femur model, the difference of femoral neck strains between stair ascent vs. descent, young vs. older populations was compared. A two-way repeated-measures MANOVA was applied to test the effect of age and stair direction on the femoral neck strains. The strains at the femoral neck cross-section were greater for stair descent than ascent for both age groups (mostly P = 0.001 to 0006) but there was no difference between age groups. In this study, femoral neck strains represented bone loading environment in more direct ways than joint reaction forces/moments or joint contact forces, the risk of hip pain, falls and stress fractures might be greater during stair descent than ascent. Possible preventative methods to reduce these risks should be developed in the future studies.

Recommendations for Patients with High Return to Sports Expectations after TKA Remain Controversial

(1) Background: Improved surgical techniques and implants in total knee arthroplasty (TKA) have led to broader indications for surgical interventions of osteoarthritis of the knee. There is a growing young and active patient subgroup with high return to sports (RTS) expectations after TKA. The current lack of evidence regarding RTS capacity in this patient cohort, requires the consolidation of experts' opinions and experiences to address the special needs among these patients. The aim of this study was to assess current expert opinions in regard to preoperative patient assessment, surgical technique and decision-making and patient counseling for these patients. (2) Methods: We performed a survey among surgeons specialized in arthroplasty with a questionnaire designed to assess current recommendations, surgical techniques, and implant preferences as well as patient counseling in patients with high expectations for RTS after TKA. (3) Results: The majority of surgeons are in favor of return to low-impact sports after TKA within 3 to 6 months. Some even recommend return to high-impact sports. Despite improvement of surgical techniques and implants, we observed no clear preference for a single surgical technique or implant specification in active patients. (4) Conclusions: Current evidence for sports-associated complications after TKA is scarce. Despite a growing array of surgical techniques and implants, the available literature is still controversial with no single surgical technique or TKA design distinguishing itself clearly from others. Surgeons' recommendations are mostly based on their experience and training. Nonetheless, we observed growing faith in modern implants with some surgeons even recommending high-impact sports after TKA.

A novel design, analysis and 3D printing of Ti-6Al-4V alloy bio-inspired porous femoral stem

The current study is proposing a design envelope for porous Ti-6Al-4V alloy femoral stems to survive under fatigue loads. Numerical computational analysis of these stems with a body-centered-cube (BCC) structure is conducted in ABAQUS. Femoral stems without shell and with various outer dense shell thicknesses (0.5, 1.0, 1.5, and 2 mm) and inner cores (porosities of 90, 77, 63, 47, 30, and 18%) are analyzed. A design space (envelope) is derived by using stem stiffnesses close to that of the femur bone, maximum fatigue stresses of 0.3σ_ys in the porous part, and endurance limits of the dense part of the stems. The Soderberg approach is successfully employed to compute the factor of safety N_f > 1.1. Fully porous stems without dense shells are concluded to fail under fatigue load. It is thus safe to use the porous stems with a shell thickness of 1.5 and 2 mm for all porosities (18-90%), 1 mm shell with 18 and 30% porosities, and 0.5 mm shell with 18% porosity. The reduction in stress shielding was achieved by 28%. Porous stems incorporated BCC structures with dense shells and beads were successfully printed.

Gender differences in the knee joint loadings during gait

BACKGROUND

The differences in anatomical structure between men and women are widely known. Unfortunately, the influence of gender on the biomechanics of a healthy knee joint during gait is still poorly understood.

RESEARCH QUESTION

The aim of the presented study was to determine loads acting in the knee joint during gait, based on the observation of a large group of healthy young adults, in particular to determine the influence of gender on values of forces and moments and their time characteristics during gait cycle.

METHODS

Time-spatial gait parameters and ground reaction force were registered for 86 persons (43 females and 43 males) using a motion capture system and force plates. The numerical simulation with the AnyBody system was used to estimate loadings acting in the knee joint. Differences between women and men were tested using the unpaired Student's t-test with a Bonferroni correction.

RESULTS

The maximum values of loadings acting in the knee joint were: 411.1 %BW (body weight) for resultant force, 390.6 %BW for proximo-distal force, 110.8 %BW for antero-posterior force, 77.0 %BW for medio-lateral force, 2.63 %BWh (body weight times height) for flexion/extension moment, 0.97 %BWh for internal/external rotation moment and 5.7 %BWh for abduction/adduction moment. In general, the normalised forces were greater in the male group, while the normalised external moments acting on the knee were greater in the female group. Local extrema of forces during the stance phase were observed earlier for women.

SIGNIFICANCE

Knowledge about gender differences in loadings acting in the knee joint can be of great importance in the case of detecting the early stages of gait abnormalities and treatment planning.

The Anterior Subcutaneous Pelvic Ring Fixator: No Biomechanical Advantages Compared with External Fixation

BACKGROUND

Subcutaneous fixation of the anterior pelvic ring is an emerging surgical technique for trauma patients. The aim of this study was to biomechanically evaluate 2 internal fixation devices for stabilizing a disrupted pelvic ring and compare them with traditional external fixation.

METHODS

Thirty-six synthetic pelvises with a simulated unstable ring fracture (anteroposterior compression type III) were divided into 3 groups. Group A underwent fixation with a supra-acetabular external fixator; group B, with an internal fixator using the USS II polyaxial system; and group C, with an internal fixator using the Click'X polyaxial system. Biomechanical testing included measurement of peak-to-peak displacement at 300, 400, and 500 N; total displacement; plastic deformation; stiffness; and fracture-line displacement.

RESULTS

Statistical analysis of all measured parameters revealed no significant differences among the groups. However, vertical displacement of the preshaped connecting rod within the screw heads occurred as a result of inadequate stability of the internal fixation at the rod-screw interface.

CONCLUSIONS

Although internal fixator devices are placed close to the bone and should therefore maintain greater stiffness, our data did not support the hypothesis of superior stability. Special attention is required when using a curved connecting rod as the rod is easily displaced, placing relevant anatomical structures at risk. These devices require further refinement to avoid potential patient injury.

Effects of a Medial Knee Unloading Implant on Tibiofemoral Joint Mechanics During Walking

The Atlas™ unicompartmental knee system is a second-generation extra-articular unloading implant for patients with mild to moderate medial knee osteoarthritis. The technology acts to reduce a portion of the weight-bearing load exerted on the medial knee during physical activity thereby, reducing the mechanical stress imposed on a degenerative joint. The purpose of the present study was to evaluate the effects of the Atlas™ on tibiofemoral joint mechanics during walking. A computer-aided design assembly of the Atlas™ was virtually implanted on the medial aspect of a previously validated finite element tibiofemoral joint model. Data for knee joint forces and moments from an anthropometrically matched male were applied to the model to quasi-statically simulate the stance phase of gait. Predictions of tibiofemoral joint mechanics were computed pre- and post-virtual implantation of the Atlas™. Compressive force in the medial tibiofemoral compartment was reduced by a mean of 53%, resulting in the decrement of mean cartilage-cartilage and cartilage-meniscus von Mises stress by 31% and 32%, respectively. The Atlas™ was not predicted to transfer net loading to the lateral compartment. The tibiofemoral joint model exhibited less internal-external rotation and anterior-posterior translation post-Atlas™, indicating a change in the kinematic environment of the knee. From a biomechanical perspective, extra-articular joint unloading may serve as a treatment option for patients recalcitrant to conservative care. Evaluation of mechanical changes in the tibiofemoral joint demonstrate the potential treatment mechanism of the Atlas™, in accordance with the available clinical data. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:2149-2156, 2019.

Muscle recruitment strategies can reduce joint loading during level walking

Joint inflammation, with consequent cartilage damage and pain, typically reduces functionality and affects activities of daily life in a variety of musculoskeletal diseases. Since mechanical loading is an important determinant of the disease process, a possible conservative treatment is the unloading of joints. In principle, a neuromuscular rehabilitation program aimed to promote alternative muscle recruitments could reduce the loads on the lower-limb joints during walking. The extent of joint load reduction one could expect from this approach remains unknown. Furthermore, assuming significant reductions of the load on the affected joint can be achieved, it is unclear whether, and to what extent, the other joints will be overloaded. Using subject-specific musculoskeletal models of four different participants, we computed the muscle recruitment strategies that minimised the hip, knee and ankle contact force, and predicted the contact forces such strategies induced at the other joints. Significant reductions of the peak force and impulse at the knee and hip were obtained, while only a minimal effect was found at the ankle joint. Adversely, the peak force and the impulse in non-targeted joints increased when aiming to minimize the load in an adjacent joint. These results confirm the potential of alternative muscle recruitment strategies to reduce the loading at the knee and the hip, but not at the ankle. Therefore, neuromuscular rehabilitation can be targeted to reduce the loading at affected joints but must be considered carefully in patients with multiple joints affected due to the potential adverse effects in non-targeted joints.

A multi-purpose force-controlled loading device for cartilage and meniscus functionality assessment using advanced MRI techniques

Response to loading of soft tissues as assessed by advanced magnetic resonance imaging (MRI) techniques is a promising approach to evaluate tissue functionality beyond (statically obtained) structural and compositional features. As cartilage and meniscus pathologies are closely intertwined in osteoarthritis (OA) and beyond, both tissues should ideally be studied to elucidate further the underlying mechanisms involved in load transmission and its failure leading to OA. Hence, we devised, constructed and validated a dedicated MRI-compatible pneumatic force-controlled loading device to study cartilage and meniscus functionality in a standardized and reproducible manner and in reference to alternative tissue evaluation methods. Mechanical reference measurements using digital force sensors confirmed the reproducible application of forces in the range of 0-76N. To demonstrate the device's utility in a basic research context, MRI measurements of human articular cartilage (obtained from the lateral femoral condyle, n = 5) and meniscus (obtained from lateral meniscus body, n = 5) were performed in the unloaded (δ₀) and loaded configurations (δ₁: [cartilage] 0.75 bar corresponding to 15.1 N, [meniscus] 2 bar corresponding to 37.1 N; δ₂: [cartilage] 1.5 bar corresponding to 28.6 N, [meniscus] 4 bar corresponding to 69.1 N). Cartilage samples were directly indented, while meniscus samples were subject to torque-induced compression using a dedicated lever compression device. Morphological MR Imaging using Proton Density-weighted sequences and quantitative MR Imaging using T2 and T1ρ mapping were performed serially and at high resolution. For reference, samples underwent subsequent biomechanical and histological reference evaluation. In conclusion, the force-controlled loading device has been validated for the non-invasive response-to-loading assessment of human cartilage and meniscus samples by advanced MRI techniques. Hereby, both tissues may be functionally evaluated in combination, beyond mere static analysis and in reference to histological and biomechanical measures.

Dynamic analysis of varus knee using a subject-specific multibody model of the knee before and after osteotomy

Varus misalignment of the hip-knee-ankle angle causes greater loads on the medial compartment of the knee and increases the risk of developing knee osteoarthritis. High tibial osteotomy is a surgical method where the load-bearing axis is shifted laterally. The purpose of this study is to define a subject-specific three-dimensional multibody model of the knee to investigate the effect of osteotomy on cartilages and menisci during the stance phase of gait. It is assumed that osteotomy transfers load-bearing to the lateral parts of the knee. Magnetic resonance images of a patient with varus alignment were used to generate the geometries of the bones, cartilages, and menisci. Then, an experimental approach was used to determine the parameters for the stiffness matrices and compliant contact models of the tibio-menisco-femoral articulations with the use of finite element solutions. As indicated by the research findings, the contact force at the medial cartilage decreased as the load-bearing axis was transferred to the lateral parts. This subject-specific noninvasive analysis of contact force can be considered as a preoperative assessment tool for the surgeon. to predict the effects of high tibial osteotomy and the shifting of the load-bearing axis to the soft tissues of the knee.

Friction between a polyethylene pin and a microtextured CoCrMo disc, and its correlation to polyethylene wear, as a function of sliding velocity and contact pressure, in the context of metal-on-polyethylene prosthetic hip implants

The longevity of metal-on-polyethylene prosthetic hip implant bearings, in which a polished CoCrMo femoral head articulates with a polyethylene liner, is limited by mechanical instability or inflammation resulting from osteolysis caused by polyethylene wear debris. We use pin-on-disc experiments to measure friction and wear of a polyethylene pin that articulates with different microtextured CoCrMo surfaces, covering a wide range of operating conditions including sliding velocity and contact pressure. We determine how the lubrication regime changes as a function of operating conditions, and show that the microtexture accelerates the transition from boundary to elastohydrodynamic lubrication. Additionally, we illustrate that the microtexture could enable tailoring the hip implant to the specific patient needs based on activity level, gender, and age.

Physical Activities That Cause High Friction Moments at the Cup in Hip Implants

BACKGROUND

High friction moments in hip implants contribute to the aseptic loosening of cementless cups, of which there are approximately 100,000 cases per year; sustained joint loading may cause such high moments. The most "critical" physical activities associated with sustained joint loading were identified in this study.

METHODS

Friction moments in the cup were telemetrically measured about 33,000 times in the endoprostheses of 9 subjects during >1,400 different activities. The highest moments were compared with the cup's fixation stability limit of approximately 4 Nm.

RESULTS

A total of 124 different activities caused friction moments meeting or exceeding the critical limit, with the highest value of 11.5 Nm. Most involved sustained high contact forces before or during the activity. The highest peak moments (6.3 to 11.5 Nm) occurred when moving the contralateral leg during 1-legged stance, during breaststroke swimming, muscle stretching, 2-legged stance with muscle contraction, and during static 1-legged stance. The median moments were highest (3.4 to 3.9 Nm) for unstable 1-legged stance, whole-body vibration training, 2-legged stance with an unexpected push at the upper body, 1-legged stance while exercising the contralateral leg, and running after 2-legged stance.

CONCLUSIONS

Frequent unloading plus simultaneous movement of the joint are required to maintain good joint lubrication and keep the friction moments low. Frequent, sustained high loads before or during an activity may cause or contribute to aseptic cup loosening. During the first months after hip arthroplasty, such activities should be avoided or reduced as much as possible. This especially applies during postoperative physiotherapy. Whether these guidelines also apply for subjects with knee implants or arthrotic hip or knee joints requires additional investigation.

CLINICAL RELEVANCE

The risk of aseptic cup loosening may be reduced by avoiding sustained loading of hip implants without periodic joint movement.

Patient-specific mobility assessment to monitor recovery after total hip arthroplasty

Total hip arthroplasty is a ubiquitously successful orthopedic surgical procedure, whose prevalence is rising worldwide. While many investigations focus on characterizing periprosthetic pathophysiology, the objective of our research is to develop and describe multi-metric assemblies as a first step toward creating a patient-specific mobility index that rehabilitators and orthopedic surgeons can utilize for prescribing their respective procedures. In total, 48 total hip arthroplasty patients (both cemented and uncemented) undergoing unilateral, primary surgery went through computed tomographic scans and gait analysis measurements both before and 1 year following their surgery. Altogether, the reported quantitative metrics include 11 spatial and temporal gait parameters, muscle density, and electromyography signals from the rectus femoris, vastus lateralis, and vastus medialis, and bone mineral density values from bioimage analysis around the implant stem. We found that measured parameters from a subgroup were sensitive to changes observed during patient recovery, implicating the predictive sensitivity of these patient conditions. Most post-operative gait parameters changed significantly, while electromyography data indicated few significant differences. Moreover, results from bioimage analyses indicate a general reduction of periprosthetic bone mineral density after 1 year, in association with increasing density of the quadriceps muscles. Furthermore, this work identifies which quantitative metrics undergo the greatest variation after total hip arthroplasty and demonstrates the clinical feasibility of a multimodal approach to mobility assessment that may ultimately support decision-making for post-surgical rehabilitation protocols.

Towards Subject-Specific Strength Training Design through Predictive Use of Musculoskeletal Models

Lower extremity dysfunction is often associated with hip muscle strength deficiencies. Detailed knowledge of the muscle forces generated in the hip under specific external loading conditions enables specific structures to be trained. The aim of this study was to find the most effective movement type and loading direction to enable the training of specific parts of the hip muscles using a standing posture and a pulley system. In a novel approach to release the predictive power of musculoskeletal modelling techniques based on inverse dynamics, flexion/extension and ab-/adduction movements were virtually created. To demonstrate the effectiveness of this approach, three hip orientations and an external loading force that was systematically rotated around the body were simulated using a state-of-the art OpenSim model in order to establish ideal designs for training of the anterior and posterior parts of the M. gluteus medius (GM). The external force direction as well as the hip orientation greatly influenced the muscle forces in the different parts of the GM. No setting was found for simultaneous training of the anterior and posterior parts with a muscle force higher than 50% of the maximum. Importantly, this study has demonstrated the use of musculoskeletal models as an approach to predict muscle force variations for different strength and rehabilitation exercise variations.

An extended OpenSim knee model for analysis of strains of connective tissues

Background

OpenSim musculoskeletal models provide an accurate simulation environment that eases limitations of in vivo and in vitro studies. In this work, a biomechanical knee model was formulated with femoral articular cartilages and menisci along with 25 connective tissue bundles representing ligaments and capsules. The strain patterns of the connective tissues in the presence of femoral articular cartilage and menisci in the OpenSim knee model was probed in a first of its kind study.

Methods

The effect of knee flexion (0°–120°), knee rotation (− 40° to 30°) and knee adduction (− 15° to 15°) on the anterior cruciate, posterior cruciate, medial collateral, lateral collateral ligaments and other connective tissues were studied by passive simulation. Further, a new parameter for assessment of strain namely, the differential inter-bundle strain of the connective tissues were analyzed to provide new insights for injury kinematics.

Results

ACL, PCL, LCL and PL was observed to follow a parabolic strain pattern during flexion while MCL represented linear strain patterns. All connective tissues showed non-symmetric parabolic strain variation during rotation. During adduction, the strain variation was linear for the knee bundles except for FL, PFL and TL.

Conclusions

Strains higher than 0.1 were observed in most of the bundles during lateral rotation followed by abduction, medial rotation and adduction. In the case of flexion, highest strains were observed in aACL and aPCL. A combination of strains at a flexion of 0° with medial rotation of 30° or a flexion of 80° with rotation of 30° are evaluated as rupture-prone kinematics.

Electronic supplementary material

The online version of this article (10.1186/s12938-018-0474-8) contains supplementary material, which is available to authorized users.

The effect of constitutive representations and structural constituents of ligaments on knee joint mechanics

Ligaments provide stability to the human knee joint and play an essential role in restraining motion during daily activities. Compression-tension nonlinearity is a well-known characteristic of ligaments. Moreover, simpler material representations without this feature might give reasonable results because ligaments are primarily in tension during loading. However, the biomechanical role of different constitutive representations and their fibril-reinforced poroelastic properties is unknown. A numerical knee model which considers geometric and material nonlinearities of meniscus and cartilages was applied. Five different constitutive models for the ligaments (spring, elastic, hyperelastic, porohyperelastic, and fibril-reinforced porohyperelastic (FRPHE)) were implemented. Knee joint forces for the models with elastic, hyperelastic and porohyperelastic properties showed similar behavior throughout the stance, while the model with FRPHE properties exhibited lower joint forces during the last 50% of the stance phase. The model with ligaments as springs produced the lowest joint forces at this same stance phase. The results also showed that the fibril network contributed substantially to the knee joint forces, while the nonfibrillar matrix and fluid had small effects. Our results indicate that simpler material models of ligaments with similar properties in compression and tension can be used when the loading is directed primarily along the ligament axis in tension.