Total knees designed for normal kinematics evaluated in an up-and-down crouching machine

A Robotic Test Rig for Performance Assessment of Prosthetic Joints

Movement within the human body is made possible by joints connecting two or more elements of the musculoskeletal system. Losing one or more of these connections can seriously limit mobility, which in turn can lead to depression and other mental issues. This is particularly pertinent due to a dramatic increase in the number of lower limb amputations resulting from trauma and diseases such as diabetes. The ideal prostheses should re-establish the functions and movement of the missing body part of the patient. As a result, the prosthetic solution has to be tested stringently to ensure effective and reliable usage. This paper elaborates on the development, features, and suitability of a testing rig that can evaluate the performance of prosthetic and robotic joints via cyclic dynamic loading on their complex movements. To establish the rig’s validity, the knee joint was chosen as it provides both compound support and movement, making it one of the major joints within the human body, and an excellent subject to ensure the quality of the prosthesis. Within the rig system, a motorised lead-screw simulates the actuation provided by the hamstring-quadricep antagonist muscle pair and the flexion experienced by the joint. Loads and position are monitored by a load cell and proximity sensors respectively, ensuring the dynamics conform with the geometric model and gait analysis.

Background: Robotics, Prosthetics, Mechatronics, Assisted Living.

Methods: Gait Analysis, Computer Aided Design, Geometry Models.

Conclusion: Modular Device, Streamlining Rehabilitation.

The effect of total knee geometries on kinematics: An experimental study using a crouching machine

Obtaining anatomic knee kinematics after a total knee is likely to improve outcomes. We used a crouching machine to compare the kinematics of standard condylar designs with guided motion designs. The standard condylars included femoral sagittal radii with constant radius, J-curve and G-curve; the tibial surfaces were of low and high constraint. The guided motion designs were a medial pivot and a design with asymmetric condylar shapes and guiding surfaces. The machine had a flexion range from 0° to 125°, applied quadriceps and hamstring loading, and simulated the collateral soft tissues. The kinematics of all standard condylar knees were similar, showing only small anterior-posterior displacements and internal-external rotations. The two asymmetric designs showed posterior displacements during flexion, but less axial rotations than anatomic knees. The quadriceps forces throughout flexion were very similar between all designs, reflecting similar lever arms. It was concluded that standard condylar designs, even with variations in sagittal radii, are unlikely to reproduce anatomic kinematics. On the other hand, designs with asymmetric constraint between medial and lateral sides, and other guiding features, are likely to be the way forward. The mechanical testing method could be further improved by superimposing shear forces and torques during the flexion-extension motion, to include more stressful in vivo functional conditions.

Femorotibial kinematics and load patterns after total knee arthroplasty: An in vitro comparison of posterior-stabilized versus medial-stabilized design

BACKGROUND

Femorotibial kinematics and contact patterns vary greatly with different total knee arthroplasty (TKA) designs. Therefore, guided motion knee systems were developed to restore natural knee kinematics and make them more predictable. The medial stabilized TKA design is supposed to replicate physiological kinematics more than the posterior-stabilized TKA system. We conducted this study to compare a newly developed medial stabilized design with a conventional posterior-stabilized design in terms of femorotibial kinematics and contact patterns in vitro.

METHODS

Twelve fresh-frozen knee specimens were tested in a weight-bearing knee rig after implantation of a posterior stabilized and medial-stabilized total knee arthroplasty under a loaded squat from 20° to 120° of flexion. Femorotibial joint contact pressures in the medial and lateral compartments were measured by pressure sensitive films and knee kinematics were recorded by an ultrasonic 3-dimensional motion analysis system.

FINDINGS

The medial stabilized design showed a reduction of medial femorotibial translation compared to posterior-stabilized design (mean 3.5mm compared to 15.7 mm, P<0.01). In the lateral compartment, both designs showed a posterior translation of the femur with flexion, but less in the medial stabilized design (mean 14.7 mm compared to 19.0mm, P<0.01). In the medial femorotibial compartment of medial stabilized design, we observed an enlarged contact area and lower peak pressure, in contrast in the lateral compartment there was a reduced contact area and an increased peak pressure.

INTERPRETATION

While posterior-stabilized design enforces a medio-lateral posterior translation, the medial stabilized arthroplasty system enables a combination of a lateral translation with a medial pivot, which restores the physiological knee kinematics better.

Single Versus Multiple-Radii Cruciate-Retaining Total Knee Arthroplasty: An In Vivo Mobile Fluoroscopy Study

BACKGROUND

Previous fluoroscopic studies, using static C-arm systems, have shown nonnormal kinematic patterns in cruciate-retaining (CR) total knee arthroplasty (TKA). This study compares in vivo the kinematic differences in subjects implanted with single sagittal radius (SR) vs multiradii (MR) CR TKA for various activities using a novel mobile fluoroscopic system.

METHODS

Using mobile fluoroscopy and 3D to 2D registration, tibiofemoral kinematics were analyzed for 25 subjects with an SR, symmetrical condylar CR TKA and 25 subjects with an MR, asymmetric condylar CR TKA for three dynamic weight-bearing activities: (1) deep knee bend (DKB), (2) walking up a ramp, and (3) walking down a ramp.

RESULTS

During DKB, from full extension to maximum knee flexion, the SR (-0.43 ± 3.43 mm) and MR (-1.00 ± 3.23 mm) groups experienced statistically similar anterior/posterior (AP) motion in the lateral condyle. The SR (3.51 ± 2.68 mm) group had significant anterior movement compared to the MR (-0.42 ± 2.20 mm) group in the medial condyle. This resulted in a significantly larger amount of normal axial rotation experienced by the SR (5.20 ± 3.93°) group compared to the MR (0.75 ± 5.12°) group. During ramp activities, the SR TKA consistently exhibited a significantly more posterior position of both condyles compared to the MR TKA.

CONCLUSION

Although the SR TKA exhibited larger amounts of axial rotation compared to the MR TKA in DKB, neither design exhibited weight-bearing kinematics as previously reported for the normal knee. Additional research on the normal knee for ramp activities is required to understand the importance of condylar position during these activities.

Evaluation of total knee mechanics using a crouching simulator with a synthetic knee substitute

Mechanical evaluation of total knees is frequently required for aspects such as wear, strength, kinematics, contact areas, and force transmission. In order to carry out such tests, we developed a crouching simulator, based on the Oxford-type machine, with novel features including a synthetic knee including ligaments. The instrumentation and data processing methods enabled the determination of contact area locations and interface forces and moments, for a full flexion-extension cycle. To demonstrate the use of the simulator, we carried out a comparison of two different total knee designs, cruciate retaining and substituting. The first part of the study describes the simulator design and the methodology for testing the knees without requiring cadaveric knee specimens. The degrees of freedom of the anatomic hip and ankle joints were reproduced. Flexion-extension was obtained by changing quadriceps length, while variable hamstring forces were applied using springs. The knee joint was represented by three-dimensional printed blocks on to which the total knee components were fixed. Pretensioned elastomeric bands of realistic stiffnesses passed through holes in the block at anatomical locations to represent ligaments. Motion capture of the knees during flexion, together with laser scanning and computer modeling, was used to reconstruct contact areas on the bearing surfaces. A method was also developed for measuring tibial component interface forces and moments as a comparative assessment of fixation. The method involved interposing Tekscan pads at locations on the interface. Overall, the crouching machine and the methodology could be used for many different mechanical measurements of total knee designs, adapted especially for comparative or parametric studies.

In vitro method for assessing the biomechanics of the patellofemoral joint following total knee arthroplasty

The patellofemoral joint is a common site of pain and failure following total knee arthroplasty. A contributory factor may be adverse patellofemoral biomechanics. Cadaveric investigations are commonly used to assess the biomechanics of the joint, but are associated with high inter-specimen variability and often cannot be carried out at physiological levels of loading. This study aimed to evaluate the suitability of a novel knee simulator for investigating patellofemoral joint biomechanics. This simulator specifically facilitated the extended assessment of patellofemoral joint biomechanics under physiological levels of loading. The simulator allowed the knee to move in 6 degrees of freedom under quadriceps actuation and included a simulation of the action of the hamstrings. Prostheses were implanted on synthetic bones and key soft tissues were modelled with a synthetic analogue. In order to evaluate the physiological relevance and repeatability of the simulator, measurements were made of the quadriceps force and the force, contact area and pressure within the patellofemoral joint using load cells, pressure-sensitive film, and a flexible pressure sensor. The results were in agreement with those previously reported in the literature, confirming that the simulator is able to provide a realistic physiological loading situation. Under physiological loading, average standard deviations of force and area measurements were substantially lower and comparable to those reported in previous cadaveric studies, respectively. The simulator replicates the physiological environment and has been demonstrated to allow the initial investigation of factors affecting patellofemoral biomechanics following total knee arthroplasty.

Application of a novel design method for knee replacements to achieve normal mechanics

PURPOSE

The purpose of this study was to utilize a novel method for the design of total knee replacements for use in the absence of the cruciate ligaments, with the design criteria of reproducing the medial stability and lateral mobility characteristics of the normal anatomic knee.

SCOPE

The starting point was a femoral component with surfaces approximating anatomic. This surface was moved into multiple positions describing a neutral path of motion and laxity about the neutral path. The distal part of the femoral composite was then used to define the tibial surface. By varying the femoral design, different tibial surfaces were produced. The reference design featured a dished medial tibial surface and a shallow lateral tibial surface, but this provided limited motion guidance. To provide further guidance, two types of design were generated, one using intercondylar guide surfaces, the other providing guidance from the condylar surfaces themselves.

CONCLUSIONS

The design method was capable of generating a range of total knee surfaces which could potentially return the arthritic knee to more normal function.

The effect of geometric variations in posterior-stabilized knee designs on motion characteristics measured in a knee loading machine

BACKGROUND

In different posterior-stabilized (PS) total knees, there are considerable variations in condylar surface radii and cam-post geometry. To what extent these variations affect kinematics is not known. Furthermore, there are no clearly defined ideal kinematics for a total knee.

QUESTIONS/PURPOSES

The purposes of this study were to determine (1) what the kinematic differences are caused by geometrical variations between PS total knee designs in use today; and (2) what design characteristics will produce kinematics that closely resemble that of the normal anatomic knee.

METHODS

Four current PS designs with different geometries and one experimental asymmetric PS design, with a relatively conforming medial side, were tested in a purpose-built machine. The machine applied combinations of compressive, shear, and torque forces at a sequence of flexion angles to represent a range of everyday activities, consistent with the ASTM standard test for measuring constraint. The femorotibial contact points, the neutral path of motion, and the AP and internal-external laxities were used as the kinematic indicators.

RESULTS

The PS designs showed major differences in motion characteristics among themselves and with motion data from anatomic knees determined in a previous study. Abnormalities in the current designs included symmetric mediolateral motion, susceptibility to excessive AP medial laxity, and reduced laxity in high flexion. The asymmetric-guided motion design alleviated some but not all of the abnormalities.

CONCLUSIONS

Current PS designs showed kinematic abnormalities to a greater or lesser extent. An asymmetric design may provide a path to achieving a closer match to anatomic kinematics.

CLINICAL RELEVANCE

One criterion for the evaluation of PS total knees is how closely the kinematics of the prosthesis resemble that of the anatomic knee, because this is likely to affect the quality of function.

Differences in knee joint kinematics and forces after posterior cruciate retaining and stabilized total knee arthroplasty

BACKGROUND

Posterior cruciate ligament (PCL) retaining (CR) and -sacrificing (PS) total knee arthroplasties (TKA) are widely-used to treat osteoarthritis of the knee joint. The PS design substitutes the function of the PCL with a cam-spine mechanism which may produce adverse changes to joint kinematics and kinetics.

METHODS

CR- and PS-TKA were performed on 11 human knee specimens. Joint kinematics were measured with a dynamic knee simulator and motion tracking equipment. In-situ loads of the PCL and cam-spine were measured with a robotic force sensor system. Partial weight bearing flexions were simulated and external forces were applied.

RESULTS

The PS-TKA rotated significantly less throughout the whole flexion range compared to the CR-TKA. Femoral roll back was greater in the PS-TKA; however, this was not correlated with lower quadriceps forces. Application of external loads produced significantly different in-situ force profiles between the TKA systems.

CONCLUSIONS

Our data demonstrate that the PS-design significantly alters kinematics of the knee joint. Our data also suggest the cam-spine mechanism may have little influence on high flexion kinematics (such as femoral rollback) with most of the load burden shared by supporting implant and soft-tissue structures.

Patellofemoral contact patterns before and after total knee arthroplasty: an in vitro measurement

Background

Patellofemoral complications are one of the main problems after Total Knee Arthroplasty (TKA). Retropatellar pressure distribution after TKA can contribute to these symptoms. Therefore we evaluated retropatellar pressure distribution subdivided on the ridge, medial and lateral surface on non-resurfaced patella before and after TKA. Additionally, we analyzed axial femorotibial rotation and quadriceps load before and after TKA.

Methods

Seven fresh frozen cadaver knees were tested in a force controlled knee rig before and after TKA (Aesculap, Tuttlingen, Germany, Columbus CR) while isokinetic flexing the knee from 20° to 120° under weight bearing. Ridge, medial and lateral retropatellar surface were defined and pressure distribution was dynamically measured while quadriceps muscles and hamstring forces were applied. Aside axial femorotibial rotation and quadriceps load was recorded.

Results

There was a significant change of patella pressure distribution before and after TKA (p = 0.004). In physiological knees pressure distribution on medial and lateral retropatellar surface was similar. After TKA the ridge of the patella was especially in higher flexion grades strongly loaded (6.09 +/−1.31 MPa) compared to the natural knee (2.92 +/−1.15 MPa, p < 0.0001). Axial femorotibial rotation showed typical internal rotation with increasing flexion both before and after TKA, but postoperatively it was significantly lower. The average amount of axial rotation was 3.5° before and after TKA 1.3° (p = 0.001). Mean quadriceps loading after implantation of knee prosthesis did not change significantly (575 N ±60 N in natural knee and after TKA 607 N ±96 N; p = 0.28).

Conclusions

The increased retropatellar pressure especially on the ridge may be one important reason for anterior knee pain after TKA. The trochlea of the femoral component might highly influence the pressure distribution of the non-resurfaced retropatellar surface. Additionally, lower axial femorotibial rotation after TKA might lead to patella maltracking. Changing the design of the prosthesis or a special way of patella shaping might increase the conformity of the patella to trochlea to maintain natural contact patterns.

Assessment of a medial pivot total knee arthroplasty design in a cadaveric knee extension test model

A total knee has been designed to mimic less-compliant medial and more-compliant lateral behavior. In vivo testing compared open-kinematic chain behaviors of cadaver knees in their normal state and after implantation of the knee prosthesis. Specimen's limbs were computed tomography scanned, and infrared arrays on tibia and femur were registered to bone markers. Motion of the joint and quadriceps force were reported from 90° flexion to full extension. Less medial and more lateral anterior-posterior motion was seen in both the intact and the implanted knees. Tibiofemoral rotation and translation were similar in direction but were reduced in magnitude for the prosthetic knees. Quadriceps force, defined as that applied force required to extend the knee, required after implantation was variable between specimens but not statistically different from the intact condition. The prosthesis tested exhibits kinematic behavior similar to that in their normal state, with no difference in quadriceps force required for extension.

Conceptual Design for Condylar Guiding Features of a Total Knee Replacement

This study investigates the design requirements for guiding features that can be incorporated into the shapes of the femoral condyles and the tibial component geometry of a knee replacement system without occupying the intercondylar space of the joint so that the cruciates can be spared and still produce more physiological motions. A conceptual design for a surface-guided knee is introduced to induce effective guiding both in flexion and extension by novel features incorporated in the shape of the lateral condyle. This design can accommodate preservation of either of the cruciates while deficiencies in the functions of the other are compensated by contributions of the articular geometry in guiding the motion and stabilizing the joint. The preliminary kinematic tests on a prototype demonstrated viability of the features in guiding motion under compression.

Future directions in knee replacement

The use of artificial joints for the treatment of osteoarthritis is expected to expand considerably over the next decade. While newer technologies can offer yet further improvements in total knee systems, implementation will be strongly affected by the need to satisfy apparently competing requirements. Patients expect quicker rehabilitation, improved performance, and lifelong durability; on the other hand, economic constraints require a reduction in cost for each procedure, as well as early intervention and preventative measures, while there is increased pressure from health care systems to use evidence-based medicine as the standard of choice for implants and techniques. The success of a knee replacement depends on the design itself, the surgical technique, the rehabilitation, and, not least, the patient. The major goal of the implant design can be redefined as a restoration of normal knee mechanics, whether by maximum preservation of tissues, or by guiding surfaces that replace their function. Surgical technique needs to be less invasive but achieve optimal patient-specific alignment and soft tissue balancing. Rehabilitation procedures must achieve the expectations of realistic patients. Testing and evaluation methods need to be upgraded for enhanced predictability. This paper discusses current trends and future possibilities to address this expansive scope of design criteria.