Aims. The surgical target for optimal implant positioning in robotic-assisted total knee arthroplasty remains the subject of ongoing discussion. One of the proposed targets is to recreate the knee’s functional behaviour as per its pre-diseased state. The aim of this study was to optimize implant positioning, starting from mechanical alignment (MA), toward restoring the pre-diseased status, including ligament strain and kinematic patterns, in a patient population. Methods. We used an active appearance model-based approach to segment the preoperative CT of 21 osteoarthritic patients, which identified the osteophyte-free surfaces and estimated cartilage from the segmented bones; these geometries were used to construct patient-specific musculoskeletal models of the pre-diseased knee. Subsequently, implantations were simulated using the MA method, and a previously developed optimization technique was employed to find the optimal implant position that minimized the root mean square deviation between pre-diseased and postoperative ligament strains and kinematics. Results. There were evident biomechanical differences between the simulated patient models, but also trends that appeared reproducible at the population level. Optimizing the implant position significantly reduced the maximum observed strain root mean square deviations within the cohort from 36.5% to below 5.3% for all but the anterolateral ligament; and concomitantly reduced the kinematic deviations from 3.8 mm (SD 1.7) and 4.7° (SD 1.9°) with MA to 2.7 mm (SD 1.4) and 3.7° (SD 1.9°) relative to the pre-diseased state. To achieve this, the femoral component consistently required translational adjustments in the anterior, lateral, and proximal directions, while the tibial component required a more posterior slope and varus rotation in most cases. Conclusion. These findings confirm that MA-induced biomechanical alterations relative to the pre-diseased state can be reduced by optimizing the implant position, and may have implications to further advance pre-planning in robotic-assisted surgery in order to restore pre-diseased knee function. Cite this article: Bone Joint J 2024;106-B(11):1231–1239

Aims

Focal knee arthroplasty is an attractive alternative to knee arthroplasty for young patients because it allows preservation of a large amount of bone for potential revisions. However, the mechanical behaviour of cartilage has not yet been investigated because it is challenging to evaluate in vivo contact areas, pressure, and deformations from metal implants. Therefore, this study aimed to determine the contact pressure in the tibiofemoral joint with a focal knee arthroplasty using a finite element model.

Mechanical alignment (MA) in total knee arthroplasty (TKA), although considered the gold standard, reportedly has up to 25% of patients expressing post-operative dissatisfaction. Biomechanical outcomes following kinematic alignment (KA) in TKA, developed to restore native joint alignment, remain unclear. Without a clear consensus for the optimal alignment strategy during TKA, the purpose of this study was to conduct a paired biomechanical comparison of MA and KA in TKA by experimentally quantifying joint laxity and medial collateral ligament (MCL) strain. 14 bilateral native fresh-frozen cadaveric lower limbs underwent medially-stabilised TKA (GMK Sphere, Medacta, Switzerland) using computed CT-based subject-specific guides, with KA and MA performed on left and right legs, respectively. Each specimen was subjected to sensor-controlled mediolateral laxity tests. A handheld force sensor (Mark-10, USA) was used to generate an abduction-adduction moment of 10Nm at the knee at fixed flexion angles (0°, 30°, 60°, 90°). A digital image correlation system was used to compute the strain on the superficial medial collateral ligament. A six-camera optical motion capture system (Vicon MX+, UK) was used to acquire kinematics using a pre-defined CT-based anatomical coordinate system. A linear mixed model and Tukey's posthoc test were performed to compare native, KA and MA conditions (p<0.05). Unlike MA, medial joint laxity in KA was similar to the native condition; however, no significant difference was found at any flexion angle (p>0.08). Likewise, KA was comparable with the native condition for lateral joint laxity, except at 30°, and no statistical difference was observed. Although joint laxity in MA seemed lower than the native condition, this difference was significant only for 30° flexion (p=0.01). Both KA and MA exhibited smaller MCL strain at 0° and 30°; however, all conditions were similar at 60° and 90°. Medial and lateral joint laxity seemed to have been restored better following KA than MA; however, KA did not outperform MA in MCL strain, especially after mid-flexion. Although this study provides only preliminary indications regarding the optimal alignment strategy to restore native kinematics following TKA, further research in postoperative joint biomechanics for load bearing conditions is warranted

Aims

The aim of this study was to determine the risk of tibial eminence avulsion intraoperatively for bi-unicondylar knee arthroplasty (Bi-UKA), with consideration of the effect of implant positioning, overstuffing, and sex, compared to the risk for isolated medial unicondylar knee arthroplasty (UKA-M) and bicruciate-retaining total knee arthroplasty (BCR-TKA).

Introduction. Although total knee arthroplasty (TKA) is generally considered successful, 16–30% of patients are dissatisfied. There are multiple reasons for this, but some of the most frequent reasons for revision are instability and joint stiffness. A possible explanation for this is that the implant alignment is not optimized to ensure joint stability in the individual patient. In this work, we used an artificial neural network (ANN) to learn the relation between a given standard cruciate-retaining (CR) implant position and model-predicted post-operative knee kinematics. The final aim was to find a patient-specific implant alignment that will result in the estimated post-operative knee kinematics closest to the native knee. Methods. We developed subject-specific musculoskeletal models (MSM) based on magnetic resonance images (MRI) of four ex vivo left legs. The MSM allowed for the estimation of secondary knee kinematics (e.g. varus-valgus rotation) as a function of contact, ligament, and muscle forces in a native and post-TKA knee. We then used this model to train an ANN with 1800 simulations of knee flexion with random implant position variations in the ±3 mm and ±3° range from mechanical alignment. The trained ANN was used to find the implant alignment that resulted in the smallest mean-square-error (MSE) between native and post-TKA tibiofemoral kinematics, which we term the dynamic alignment. Results. Dynamic alignment average MSE kinematic differences to the native knees were 1.47 mm (± 0.89 mm) for translations and 2.89° (± 2.83°) for rotations. The implant variations required were in the range of ±3 mm and ±3° from the starting mechanical alignment. Discussion. In this study we showed that the developed tool has the potential to find an implant position that will restore native tibiofemoral kinematics in TKA. The proposed method might also be used with other alignment strategies, such as to optimize implant position towards native ligament strains. If native knee kinematics are restored, a more normal gait pattern can be achieved, which might result in improved patient satisfaction. The small changes required to achieve the dynamic alignment do not represent large modifications that might compromise implant survivorship. Conclusion. Patient-specific implant position predicted with MSM and ANN can restore native knee function in a post-TKA knee with a standard CR implant

Poor soft tissue balance in total knee arthroplasty (TKA) is one of the most primary causes of dissatisfaction and reduced joint longevity, which are associated with postoperative instability and early implant failure. 1. Therefore, surgical techniques, including mechanical instruments and 3-D guided navigation systems, in TKA aim to achieve optimum soft tissue balancing in the knee to improve postoperative outcome. 2. Patella-in-Place balancing (PIPB) is a novel technique which aims to restore native collateral ligament behaviour by preserving the original state without any release. Moreover, reduction of the joint laxity compensates for the loss of the visco-elastic properties of the cartilage and meniscus. Following its clinical success, we aimed to evaluate the impact of the PIPB technique on collateral ligament strain and laxity behaviour, with the hypothesis that PIPB would restore strains in the collateral ligaments. 3. . Eight fresh-frozen cadaveric legs were obtained (KU Leuven, Belgium, H019 2015-11-04) and CT images were acquired while rigid marker frames were affixed into the femur, and tibia for testing. After carefully removing the soft tissues around the knee joint, while preserving the joint capsule, ligaments, and tendons, digital extensometers (MTS, Minnesota, USA) were attached along the length of the superficial medial collateral ligament (MCL) and lateral collateral ligament (LCL). A handheld digital dynamometer (Mark-10, Copiague, USA) was used to apply an abduction or adduction moment of 10 Nm at fixed knee flexion angles of 0°, 30°, 60° and 90°. A motion capture system (Vicon Motion Systems, UK) was used to record the trajectories of the rigid marker frames while synchronized strain data was collected for MCL/LCL. All motion protocols were applied following TKA was performed using PIPB with a cruciate retaining implant (Stryker Triathlon, MI, USA). Furthermore, tibiofemoral kinematics were calculated. 4. and combined with the strain data. Postoperative tibial varus/valgus stresses and collateral ligament strains were compared to the native condition using the Wilcoxon Signed-Rank Test (p<0.05). Postoperative tibial valgus laxity was lower than the native condition for all flexion angles. Moreover, tibial valgus of TKA was significantly different than the native condition, except for 0° (p=0.32). Although, tibial varus laxity of TKA was lower than the native at all angles, significant difference was only found at 0° (p=0.03) and 90° (p=0.02). No significant differences were observed in postoperative collateral ligament strains, as compared to the native condition, for all flexion angles, except for MCL strain at 30° (p=0.02) and 60° (p=0.01). Results from this experimental study supported our hypotheses, barring MCL strain in mid-flexion, which might be associated with the implant design. Restored collateral ligament strains with reduced joint laxity, demonstrated by the PIPB technique in TKA in vitro, could potentially restore natural joint kinematics, thereby improving patient outcomes. In conclusion, to further prove the success of PIPB, further biomechanical studies are required to evaluate the success rate of PIPB technique in different implant designs

In-situ assessment of collateral ligaments strain could be key to improving total knee arthroplasty outcomes by improving the ability of surgeons to properly balance the knee intraoperatively. Ultrasound (US) speckle tracking methods have shown promise in their capability to measure in-situ soft tissue strain in large tendons but prior work has also highlighted the challenges that arise when attempting to translate these approaches to the in-situ assessment of collateral ligaments strain. Therefore, the aim of this project was to develop and validate an US speckle tracking method to specifically assess in-situ strains of both the MCL and LCL. We hypothesize that coefficients of determination (R. 2. ) would be above 0.90 with absolute differences below 0.50% strain for the comparison between US-based and the reference strain, with better results expected for the LCL compared with the MCL. Five cadaveric legs with total knee implants (NH019 2017-02-03) were submitted to a varus (LCL) and valgus (MCL) ramped loading (0 – 40N). Ultrasound radiofrequency (rf) data and reference surface strains data, obtained with 3D digital image correlation (DIC), were collected synchronously. Prior to processing, US data were qualitatively assessed and specimens displaying substantial imaging artefacts were discarded, leaving five LCL and three MCL specimens in the analysis. Ultrasound rf data were processed in Matlab (The MathWorks, Inc., Natick, MA) with a custom-built speckle tracking approach adapted from a method validated on larger tendons and based on normalized cross-correlation. Digital image correlation data were processed with commercial software VIC3D (Correlated Solutions, Inc., Columbia, SC). To optimize speckle tracking, several tracking parameters were tested: kernel and search window size, minimal correlation coefficient and simulated frame rate. Parameters were ranked according to three comparative measures between US- and DIC-based strains: R. 2. , mean absolute error and strains differences at 40N. Parameters with best average rank were considered as optimal. To quantify the agreement between US- and DIC-based strain of each specimen, the considered metrics were: R. 2. , mean absolute error and strain differences at 40N. The LCL showed a good agreement with a high average R. 2. (0.97), small average mean absolute difference (0.37%) and similar strains at 40N (DIC = 2.92 ± 0.10%; US = 2.99 ± 1.16%). The US-based speckle tracking method showed worse performance for the MCL with a lower average correlation (0.55). Such an effect has been observed previously and may relate to the difficulty in acquiring sufficient image quality for tracking the MCL compared to the LCL, which likely arises due to structural or mechanical differences; notably MCL is larger, thinner, more wrapped around the bone and stretches less. However, despite these challenges, the MCL tracking still showed small average mean absolute differences (0.44%) and similar strains at 40N (DIC = 1.48 ± 0.06%; US = 1.44 ± 1.89%). We conclude that the ultrasound speckle tracking method developed is ready to be used as a tool to assess in-situ strains of LCL. Concerning the MCL strain assessment, despite some promising results in terms of strain differences, further work on acquisition could be beneficial to reach similar performance

Aims

Commonly performed unicompartmental knee arthroplasty (UKA) is not designed for the lateral compartment. Additionally, the anatomical medial and lateral tibial plateaus have asymmetrical geometries, with a slightly dished medial plateau and a convex lateral plateau. Therefore, this study aims to investigate the native knee kinematics with respect to the tibial insert design corresponding to the lateral femoral component.

Introduction. Unicompartmental knee arthroplasty (UKA) currently experiences increased popularity. It is usually assumed that UKA shows kinematic features closer to the natural knee than total knee arthroplasty (TKA). Especially in younger patients more natural knee function and faster recovery have helped to increase the popularity of UKA. Another leading reason for the popularity of UKA is the ability to preserve the remaining healthy tissues in the knee, which is not always possible in TKA. Many biomechanical questions remain, however, with respect to this type of replacement. 25% of knees with medial compartment osteoarthritis also have a deficient anterior cruciate ligament [1]. In current clinical practice, medial UKA would be contraindicated in these patients. Our hypothesis is that kinematics after UKA in combination with ACL reconstruction should allow to restore joint function close to the native knee joint. This is clinically relevant, because functional benefits for medial UKA should especially be attractive to the young and active patient. Materials and Methods. Six fresh frozen full leg cadaver specimens were prepared to be mounted in a kinematic rig (Figure 1) with six degrees of freedom for the knee joint. Three motion patterns were applied: passive flexion-extension, open chain extension, and squatting. These motion patterns were performed in four situations for each specimen: with the native knee; after implantation of a medial UKA (Figure 2); next after cutting the ACL and finally after reconstruction of the ACL. During the loaded motions, quadriceps and hamstrings muscle forces were applied. Infrared cameras continuously recorded the trajectories of marker frames rigidly attached to femur, tibia and patella. Prior computer tomography allowed identification of coordinate frames of the bones and calculations of anatomical rotations and translations. Strains in the collateral ligaments were calculated from insertion site distances. Results. Knee kinematics and collateral ligament strains were quite close to the native situation after both UKA and ACL reconstruction for all motor tasks. Nevertheless, some statistically significant differences were detected, which may be relevant clinically and biomechanically. In general, insertion of a UKA led to a knee joint which was somewhat less adducted (Figure 3), with a medial femoral condyle located slightly higher, confirming previously published findings [2]. These effects were slightly reduced both after cutting as well as after reconstructing the ACL. The joint became somewhat less stable in the AP direction after insertion of a UKA and this instability persisted not only after cutting but even after reconstructing the ACL

Aims

The purpose of this multicentre observational study was to investigate the association between intraoperative component positioning and soft-tissue balancing on short-term clinical outcomes in patients undergoing robotic-arm assisted unicompartmental knee arthroplasty (UKA).

Introduction. There are over ½ million total knee replacement (TKR) procedures performed each year in the United States and is projected to increase to over 3.48 million by 2030. Concurrent with the increase in TKR procedures is a trend of younger patients receiving knee implants (under the age of 65). These younger patients are known to have a 5% lower implant survival rate at 8 years post-op compared to older patients (65+ years), and they are also known to live more active lifestyles that place higher demands on the durability and functional performance of the TKR device. Conventional TKR designs increase articular conformity to increase stability, but these articular constraints decrease patient range of knee motion, often limiting key measures of femoral rollback, A/P motion, and deep knee flexion. Without this articular constraint however, many patients report TKR “instability” during activities such as walking and stair descent, which can significantly impede confidence of movement. Therefore, there is a need for a TKR system that can offer enhanced stability while also maintaining active ranges of motion. Materials and Methods. A novel knee arthroplasty system has been designed that uses synthetic ligament systems that can be surgically replaced, to provide ligamentous stability and natural motion to increase the functional performance of the implant. A computational anatomical model (AnyBody) was developed that incorporated ligaments into an existing Journey II TKR. Ligaments were modeled and given biomechanical properties from literature. Simulated A/P drawer tests and knee flexion were analyzed for 2,916 possible cruciate ligament location and length combinations to determine the effects on the A/P stability of the TKR. A physical model was then constructed, and the design was verified by performing 110 N A/P drawer tests under 710 N of simulated body weight. Results and Discussion. As ACL insertion location moved posteriorly on the femur, it was found to decrease ACL ligament strain, enabling a higher range of flexion. In general, as ACL and PCL length increased, the A/P laxity of the TKR system increased linearly. Range of motion was found to be more dependent on ligament attachment location, and laxity was more dependent on ligament length. In this work, TKR stability was clearly affected by changes in synthetic ligament length and location. When comparing the laxity between a TKR with and without ligaments, the TKR with synthetic ligaments experienced significantly less displacement than a TKR without synthetic ligaments. Conclusions. The stability of a TKR can be increased while maintaining range of motion by incorporating synthetic ligaments into its design. The effectiveness of the ligaments was clearly dependent on two factors: length and location. It is imperative to the success of the implant to obtain the correct lengths and locations because improper placement or length can impact the outcome significantly. These results emphasize the need for a knee replacement that incorporates synthetic ligaments, with calibrated location and lengths, to significantly influence stability and possible kinematic performance of the TKR system, and potentially influencing long-term functional outcomes

Objectives

Unicompartmental knee arthroplasty (UKA) is one surgical option for treating symptomatic medial osteoarthritis. Clinical studies have shown the functional benefits of UKA; however, the optimal alignment of the tibial component is still debated. The purpose of this study was to evaluate the effects of tibial coronal and sagittal plane alignment in UKA on knee kinematics and cruciate ligament tension, using a musculoskeletal computer simulation.

Introduction. There are over one-half million total knee replacement (TKR) procedures performed each year in the United States and is projected to increase to over 3.48 million by 2030. Concurrent with the increase in TKR procedures is a trend of younger patients receiving knee implants (under the age of 65). These younger patients are known to have a 5% lower implant survival rate at 8 years post-op compared to older patients (65+ years), and they are also known to live more active lifestyles that place higher demands on the durability and functional performance of the TKR device. Conventional TKR designs increase articular conformity to increase stability, but these articular constraints decrease patient range of knee motion, often limiting key measures of femoral rollback, A/P motion, and deep knee flexion. Without this articular constraint however, many patients report TKR “instability” during activities such as walking and stair descent, which can significantly impede confidence of movement. Therefore there is a need for a TKR system that can offer enhanced stability while also maintaining active ranges of motion. Materials and Methods. A novel knee arthroplasty system was designed that uses synthetic ligament systems that can be surgically replaced, to provide ligamentous stability and natural motion to increase the functional performance of the implant. Using an anatomical knee model from the AnyBody software, a computational model that incorporated ligaments into an existing Journey II TKR was developed. Using the software ligaments were modeled and given biomechanical properties developed from equations from literature. Simulated A/P drawer tests and knee flexion test were analyzed for 2,916 possible cruciate ligament location and length combinations to determine the effects on the A/P stability of the TKR. A physical model was constructed, and the design was verified by performing 110 N A/P drawer tests under 710 N of simulated body weight. Results and Discussion. As ACL insertion location moved posteriorly on the femur, it was found to decrease ACL ligament strain, enabling a higher range of flexion. In general, as ACL and PCL length increased, the A/P laxity of the TKR system increased linearly. Range of motion was found to be more dependent on ligament attachment location, and laxity was more dependent on ligament length. In this work, TKR stability was clearly affected by changes in synthetic ligament length and location. When comparing the laxity between a TKR with and without ligaments, the TKR with synthetic ligaments experienced significantly less displacement than a TKR without synthetic ligaments as seen in Figure 1. Conclusions. This study shows that the stability of a TKR can be increased while maintaining range of motion by incorporating synthetic ligaments into this design. The effectiveness of the ligaments was clearly dependent on two factors: length and location, with incorrect lengths and locations significantly impairing ranges of motion. These results verify that a knee replacement can incorporate synthetic ligaments, and that with calibrated location and lengths, they can significantly influence stability and possible kinematic performance of the TKR system, and potentially influencing long-term functional outcomes. For any figures or tables, please contact the authors directly

Objectives. Loss of motion following spine segment fusion results in increased strain in the adjacent motion segments. However, to date, studies on the biomechanics of the cervical spine have not assessed the role of coupled motions in the lumbar spine. Accordingly, we investigated the biomechanics of the cervical spine following cervical fusion and lumbar fusion during simulated whiplash using a whole-human finite element (FE) model to simulate coupled motions of the spine. Methods. A previously validated FE model of the human body in the driver-occupant position was used to investigate cervical hyperextension injury. The cervical spine was subjected to simulated whiplash exposure in accordance with Euro NCAP (the European New Car Assessment Programme) testing using the whole human FE model. The coupled motions between the cervical spine and lumbar spine were assessed by evaluating the biomechanical effects of simulated cervical fusion and lumbar fusion. Results. Peak anterior longitudinal ligament (ALL) strain ranged from 0.106 to 0.382 in a normal spine, and from 0.116 to 0.399 in a fused cervical spine. Strain increased from cranial to caudal levels. The mean strain increase in the motion segment immediately adjacent to the site of fusion from C2-C3 through C5-C6 was 26.1% and 50.8% following single- and two-level cervical fusion, respectively (p = 0.03, unpaired two-way t-test). Peak cervical strains following various lumbar-fusion procedures were 1.0% less than those seen in a healthy spine (p = 0.61, two-way ANOVA). Conclusion. Cervical arthrodesis increases peak ALL strain in the adjacent motion segments. C3-4 experiences greater changes in strain than C6-7. Lumbar fusion did not have a significant effect on cervical spine strain. Cite this article: H. Huang, R. W. Nightingale, A. B. C. Dang. Biomechanics of coupled motion in the cervical spine during simulated whiplash in patients with pre-existing cervical or lumbar spinal fusion: A Finite Element Study. Bone Joint Res 2018;7:28–35. DOI: 10.1302/2046-3758.71.BJR-2017-0100.R1

Total ankle replacement (TAR) is the main surgical option in case of severe joint osteoarthritis. The high failure rate of current TAR is often associated to inappropriate prosthetic articulating surfaces designed according to old biomechanical concepts such the fixed axis of rotation, thus resulting in non-physiological joint motion. A recent image-based 3D morphological study of the normal ankle (Siegler et al. 2014) has demonstrated that the ankle joint surfaces can be approximated by a saddle-shaped cone with its apex located laterally (SSCL). We aimed at comparing the kinematic effects of this original solution both with the intact joint and with the traditional prosthetic articulating surfaces via in-silico models and in-vitro measurements. Native 3D morphology of ten normal cadaver ankle specimens was reconstructed via MRI and CT images. Three custom-fit ankle joint models were then developed, according to the most common TAR designs: cylindrical, symmetrically-truncated medial apex cone (as in Inman's pioneering measures), and the novel lateral apex cone, i.e. SSCL. Bone-to-bone motion, surface-to-surface distance maps, and ligament forces and deformations were evaluated via computer simulation. Prototypes of corresponding prosthesis components were designed and manufactured via 3D-printing, both in polymer-like-carbon and in cobalt-chromium-molybdenum powders, for in-vitro tests on the cadaver specimens. A custom testing rig was used for application of external moments to the ankle joint in the three anatomical planes; a motion tracking system with trackers pinned into the bone was used to measure tibial, talar and calcaneal motion (Franci et al. 2009), represented then as tibiotalar, subtalar and ankle complex 3D joint rotations. Each ankle specimen was tested in the intact joint configuration and after replacement of the articulating surfaces according with the three joint models: cylindrical, medial apex cone and SSCL. Results. Small intra-specimen data variability in cycle-to-cycle joint kinematics was found in all cadaver ankles, the maximum standard deviation of all rotation patterns being smaller than 2.0 deg. In-silico ligament strain/stress analysis and in-vitro joint kinematic and load transfer measurements revealed that the novel SSCL surfaces reproduce more natural joint patterns than those with the most common surfaces used in current TAR. TAR based on a saddle-shaped skewed truncated cone with lateral apex is expected to restore more normal joint function. Additional tests are undergoing for further biomechanical validation. The present study has also demonstrated the feasibility and the quality of the full process of custom TAR design and production for any specific subject. This implies a thorough procedure, from medical imaging to the production of artificial surfaces via 3D printing, which is allowing for personalised implants to become the future standard in total joint replacement

Introduction. Achieving proper ligament tension in knee flexion within cruciate retaining (CR) total knee arthroplasty (TKA) has long been associated with clinical success. The distal femoral joint line (DFJL) is routinely used as a variable to assist in achieving proper flexion-extension gap balancing. No prior study has observed the possible effects of properly restoring the DFJL may have on ligament tension in flexion. The purpose of this computational analysis was to determine what effect the DFJL may have on ligament strains and tibiofemoral kinematics of CR knee designs in flexion. Methods. A computational analysis was performed utilizing a musculoskeletal modeling system with ligaments modeled as non-linear elastic. Tibiofemoral kinematics, contact points estimated from the femoral condyle low points, and ligament strain, change in length relative to the unloaded length, were measured at 90° knee flexion during a deep knee bend activity. Two different knee implants, a High Flexion CR (HFCR) and a Guided Motion CR (GMCR) design were used. Simulations were completed for changes in superior-inferior (SI) positioning of the femoral implant relative to the femur bone, in 2mm increments to simulate over and under resection of the DFJL. Results. The medial condyle of the femoral implant was 0.67mm and 0.47mm more posterior relative to the tibia per 1mm elevation of the DFJL for the HFCR and GMCR designs respectively. The lateral condyle was 0.80mm and 1.06mm more posterior relative to the tibia per 1mm elevation of the DFJL for the HFCR and GMCR designs, respectively. The strain in the LCL and MCL changed less than 0.0005mm/mm per 1mm change in DFJL indicating that those structures were not affected. The PCL bundles and the ITB were affected by changes in DFJL with strain increasing 0.005 and 0.004mm/mm in the AL PCL bundle respectively for HFCR and GMCR, strain increasing 0.006mm/mm in the PM PCL for both HFCR and GMCR, and ITB strain decreasing 0.006 and 0.004mm/mm respectively for the HFCR and GMCR per 1mm elevation of the DFJL. Discussion. Our findings suggest that DFJL affects ligament tension at 90° knee flexion and therefore flexion balance for cruciate retaining implants. The effect on ligament tension results from changes in the position of the femur bone and its ligament attachments with respect to the tibia, which is dependent on the implant geometry. DFJL places greater strain on the PCL because the conformity of the medial condyle prevents the femoral implant from sitting more posterior by the full amount of the DFJL elevation, which would be necessary to maintain the same AP position of the of the femur bone relative to the tibia and avoid increasing PCL strain. These results indicate that elevating the DFJL to address a tight extension space in a CR knee while the flexion space is well balanced could result in increased flexion tension especially when the flexion-extension mismatch is large, so to achieve balanced flexion and extension the amount of DFJL elevation may need to be reduced and the tibial resection may also need to be increased

Introduction. Achieving proper ligament tension in knee flexion within posterior cruciate retaining (CR) total knee arthroplasty (TKA) has long been associated with clinical success. Ligament balance has been achieved through specific surgical technique steps. No prior study evaluated the possible effects of varying levels of posterior cruciate ligament (PCL) release on femorotibial contact location and PCL ligament strain. The purpose of this computational analysis was to determine what effect-varying levels of PCL release may have on the tibiofemoral kinematics and PCL strain. Methods. A computational analysis was performed utilizing a musculoskeletal modeling system with ligaments modeled as non-linear elastic structures and ligament insertions. A single CR knee system with two different tibial insert designs was tested, a Guided Motion (GM) and an ultra-congruent, Deep Dished (DD) design. Varying levels of PCL release were simulated by setting the stiffness of both bundles of the PCL to a percentage, ranging from 0–100% in 25% increments. Tibiofemoral kinematics was evaluated by measuring the contact points estimated from the femoral condyle low points, and ligament strain of the anterior-lateral (AL) and posterior-medial (PM) bundles. The maximum PCL strain was determined for each bundle to evaluate the risk of PCL rupture based on the PCL failure strain. Results. The femoral AP position of both medial and lateral condyles became more anterior as the PCL stiffness was reduced to simulate greater release in both GM and DD inserts. The effect of reduction in PCL stiffness on femoral AP position increased as the PCL stiffness became a smaller percentage of the intact stiffness. The DD insert had smaller changes in femoral AP position resulting from reduced PCL stiffness than the GM insert. PCL strain in both bundles increased as PCL stiffness was reduced. The effect of reduction in PCL stiffness on PCL strain increased as the PCL stiffness became smaller. The DD insert had smaller changes in PCL strain resulting from reduced PCL stiffness than the GM insert. The model predicts that the AL bundle should not rupture for a 75% release of the PCL. The maximum PM bundle strain data indicates that the risk of PM bundle rupture is greater than AL bundle. Discussion. Our findings suggest that a partial PCL release does have an impact on tibiofemoral kinematics and ligament tension throughout the knee flexion range of motion for varying implant designs. The effects of increased PCL release were: more anterior femoral position on the tibia and increased strain in both bundles of the PCL. The maximum strain data indicates that the AL bundle of the PCL should be able to safely withstand a 75% release, but the PM bundle of the PCL may be at risk of rupture after as little as a 25% release because its stiffness is lower than the AL bundle. Our findings indicate that though partial PCL release can be correlated with both knee kinematics and PCL strain, the relationship is rather dynamic and care should be taken when seeking to find optimal balance intra-operatively

Introduction. The role of soft tissue balancing in optimizing functional outcome and patient satisfaction after total knee arthroplasty surgery is gaining interest. This is due in part to the inability of pure alignment to demonstrate excellent functional outcomes 6. Consistent soft tissue balancing has been aided by novel technologies that can quantify loads across the joint at the time of surgery 4. In theory, compressive load equilibrium should be correlated with ligamentous equilibrium between the medial and lateral collateral ligaments. The authors propose to use the Collateral Ligaments Strain Ratio (CLSR) as a functional tool to quantify and track surgical changes in laxity of the collateral ligaments and correlate this ratio to validated functional scores and patient reported outcomes. The relationship with intra-operative balancing of compartmental loads can then be scrutinized. The benefits of varus-valgus balancing within 2o include increased range of motion 7, whereas pressure imbalance between the medial and lateral joint compartments has been linked to condylar liftoff and abnormal kinematics post-TKA 8. Methods. The study is a prospective IRB approved clinical study with three cohorts of 50 patients each: (1) a surgical prospective study group (2) a matched control group of non-operated high function patients; (3) a matched control group of high function knee arthroplasty recipients. Standard statistical analysis method is applied. The testing of the CLSR is performed using a validated Smart Knee Brace developed by the authors and previously reported 1. The output variables consist of the maximum angular change of the knee in the coronal plane at 10 degrees of flexion produced by a controlled torque application in the varus and valgus (VV) directions. This creates measureable strain on the lateral and medial collateral ligaments, which is reported as a ratio (CLSR). The New Knee Society Score is used to track outcomes. The intra-operative balance is achieved by means of an instrumented tibial tray (OrthoSensor, Inc). Results. Pre-operative scatter graphs (Fig 1) demonstrate a wide distribution of absolute VV values, reflecting the spectrum of pathological states. The relative distribution of strain after surgery trends towards consolidation. The median CLSR is 0.55 with a SD of 0.20 at 4 weeks post-operative. This asymmetrical value indicates a shift toward a tighter medial side as noted in the non-operated cohort. Scatter graphs demonstrate post-operative clustering similar to that reported by the authors for kinetic loads after soft tissue balancing (Fig 2)3. The overall displacement values range from 0 −4 degrees. Discussion. The angular changes under standard torque appear to correlate with previously reported linear displacement values 3. Past studies do indicate a shift toward a tighter medial side in healthy older individuals, with an average CLSR in extension and flexion of 0.55 5. Success in achieving soft tissue balancing of the knee at the time of arthroplasty surgery may be predicted by a defined collateral ligament strain ratio under controlled VV testing. This study demonstrates clustering of the strain ratio in slight medial tightness with a range of absolute angular displacements of 0–4 degrees

It is nowadays widely recognized that patient satisfaction following knee arthroplasty strongly depends on ligament balancing. To obtain this balancing, the occurring ligament strain is assumed to play a crucial role. To measure this strain, a method is described in this paper that allows full field 3D evaluation of the strains. The latter is preferred over traditional measurement techniques, e.g. displacement transducers or strain gauges, as human soft tissue is not expected to deform uniformly due to its highly inhomogeneous and anisotropic properties. To facilitate full field strain measurements, the 3D digital image correlation (DIC) technique was adopted. This technique was previously validated by our research group on human tissue. First, a high contrast speckle pattern was applied on the sMCL. Therefore, the specimens are first coated with a small layer of methylene blue. Following, a random white speckle pattern is applied. During knee flexion, two cameras simultaneously take pictures of the deforming region at predefined flexion angles. Using dedicated software, the captured images are eventually combined and result in 3D full field strains and displacements. Using this method, the strain distribution was studied in six cadaveric knees during flexion extension movement. Therefore, the femur was rigidly fixed in a custom test rig. The tibia was left unconstrained, allowing the six degrees of freedom in the knee. A load was applied to all major muscles in physiological directions of each muscle by attaching a series of calibrated weights (Farahmand et al., J Orthop Res., 1998;16(1)). The direction of the pulling cables was controlled using a digital inclinometer for each specimen. As a result, a statically balanced muscle loading of the knee was obtained. From these cadaveric experiments, it is observed that on average the sMCL behaves isometrically between 0° and 90° of flexion. However, high regional differences in strain distribution are observed from the full field measurements. The proximal region of the sMCL experiences relatively high strains upon flexion. These strains are positive (tension) in the anterior part and negative (compression) in the posterior region. In contrast, the distal region remains approximately isometric upon knee flexion (see Figure 1). It is accordingly concluded that the sMCL behaves isometric, though large regional differences are observed. The proximal region experiences higher strains. Furthermore, the DIC technique provided valuable insights in the deformation of the sMCL. This technique will therefore be applied to study the impact of knee arthroplasty in the near future. Caption with figure 1: Full field strain distribution in the sMCL's longitudinal direction for specimen in 45° (a) and 90° (b) of knee flexion

Today controversy exists whether restoration of neutral mechanical alignment should be attempted in all patients undergoing TKA. The restoration of constitutional rather than neutral mechanical alignment may in theory lead to a more physiological strain pattern in the collateral ligaments, and could therefore potentially be beneficial to patients. It was therefore our purpose to measure collateral ligament strains during three motor tasks in the native knee and compare them with the strains noted after TKA in different postoperative alignment conditions. Six cadaver specimens were examined using a validated knee kinematics rig under physiological loading conditions. The effect of coronal malalignment was evaluated by using custom made tibial implant inserts in order to induce different alignment conditions. The results indicated that after TKA insertion the strains in the collateral ligaments resembled best the preoperative pattern of the native knee specimens when constitutional alignment was restored. Restoration to neutral mechanical alignment was associated with greater collateral strain deviations from the native knee. Based upon this study, we conclude that restoration of constitutional alignment during TKA leads to more physiological periarticular soft tissue strains during loaded as well as unloaded motor tasks