Novel technique for evaluation of knee function continuously through the range of flexion

A digital twin framework for robust control of robotic-biological systems

Medical device regulatory standards are increasingly incorporating computational modelling and simulation to accommodate advanced manufacturing and device personalization. We present a method for robust testing of engineered soft tissue products involving a digital twin paradigm in combination with robotic systems. We developed and validated a digital twin framework for calibrating and controlling robotic-biological systems. A forward dynamics model of the robotic manipulator was developed, calibrated, and validated. After calibration, the accuracy of the digital twin in reproducing the experimental data improved in the time domain for all fourteen tested configurations and improved in frequency domain for nine configurations. We then demonstrated displacement control of a spring in lieu of a soft tissue element in a biological specimen. The simulated experiment matched the physical experiment with 0.09 mm (0.001%) root-mean-square error for a 2.9 mm (5.1%) length change. Finally, we demonstrated kinematic control of a digital twin of the knee through 70-degree passive flexion kinematics. The root-mean-square error was 2.00°, 0.57°, and 1.75° degrees for flexion, adduction, and internal rotations, respectively. The system well controlled novel mechanical elements and generated accurate kinematics in silico for a complex knee model. This calibration method could be applied to other situations where the specimen is poorly represented in the model environment (e.g., human or animal tissues), and the control system could be extended to track internal parameters such as tissue strain (e.g., control knee ligament strain). Further development of this framework can facilitate medical device testing and innovative biomechanics research.

The lateral meniscus extrudes with and without root tear evaluated using ultrasound

PURPOSE

The purpose of the current study was to measure extrusion of the intact lateral meniscus as a function of knee flexion angle and loading condition and to compare the changes in extrusion with a posterior root tear using a robotic testing system and ultrasound.

STUDY DESIGN

Controlled laboratory study.

METHODS

Eight fresh-frozen cadaveric knees were subjected to external loading conditions (passive path position (no external load), 200 axial compression, 5-N-m internal tibial torque, 5-N-m valgus torque) at full extension, 30°, 60° and 90° of flexion using a robotic testing system. A linear array transducer was placed in the longitudinal orientation. Extrusion and kinematics data were recorded for two meniscus states: intact and posterior lateral root deficiency. Therefore, a complete radial root tear in the lateral meniscus at 10 mm from the tibial insertion was made in all 8 cadaveric knees using arthroscopy. The resultant forces in the lateral meniscus were also quantified by reproducing recorded paths after the removal of the lateral meniscus.

RESULTS

A lateral meniscus root tear resulted in a statistically significant increase (up to 250%) of extrusion for the lateral meniscus (p < 0.05) in comparison to the intact lateral meniscus for all externally applied loads. Without external load (passive path position), significant differences were also found between the intact and posterior lateral root deficient meniscus except at full extension (1.0 ± 0.7 mm vs. 1.9 ± 0.4 mm) and 30° of flexion (1.4 ± 0.5 mm vs. 1.8 ± 0.5 mm). Overall, with increasing flexion angle, lateral meniscus extrusion decreased for the intact as well as for the posterior lateral root deficient meniscus, with the lowest measurements in response to internal tibial torque at 90° of flexion (-3.3 ± 1.1 mm). Knee kinematics were similar whether intact or posterior lateral root tear (n.s.). Ultrasound measurement of lateral meniscus extrusion showed good inter-rater (0.65 [0.30-0.97]-0.71 [0.34-0.94]) and excellent intra-rater reliability (0.81 [0.43-0.99]).

CONCLUSION

Dynamic Ultrasound is a reliable diagnostic modality to measure the lateral meniscus extrusion which can be helpful in the diagnosis and quantification of lateral meniscal root tears.

LEVEL OF EVIDENCE

III.

Increasing the posterior tibial slope lowers in situ forces in the native ACL primarily at deep flexion angles

High tibial osteotomy is becoming increasingly popular but can be associated with unintentional posterior tibial slope (PTS) increase and subsequent anterior cruciate ligament (ACL) degeneration. This study quantified the effect of increasing PTS on knee kinematics and in situ forces in the native ACL. A robotic testing system was used to apply external loads from full extension to 90° flexion to seven human cadaveric knees: (1) 200 N axial compressive load, (2) 5 Nm internal tibial + 10 Nm valgus torque, and (3) 5 Nm external tibial + 10 Nm varus torque. Kinematics and in situ forces in the ACL were acquired for the native and increased PTS state. Increasing PTS resulted in increased anterior tibial translation at 30° (1.8 mm), 60° (1.7 mm), and 90° (0.9 mm) flexion and reduced in situ force in the ACL at 30° (57.6%), 60° (69.8%), and 90° (75.0%) flexion in response to 200 N axial compressive load. In response to 5 Nm internal tibial + 10 Nm valgus torque, there was significantly less (39.0%) in situ force in the ACL at 90° flexion in the increased compared with the native PTS state. Significantly less in situ force in the ACL at 60° (62.8%) and 90° (67.0%) flexion was observed in the increased compared with the native PTS state in response to 5 Nm external tibial + 10 Nm varus torque. Increasing PTS affects knee kinematics and results in a reduction of in situ forces in the native ACL during compressive and rotatory loads at flexion angles exceeding 30°. In a controlled laboratory setting PTS increase unloads the ACL, affecting its natural function.

Tibiofemoral bony morphology impacts the knee kinematics after anterolateral capsule injury and lateral extraarticular tenodesis differently than intact state

Anterolateral capsule injury, often concomitant with anterior cruciate ligament (ACL) injuries, may result in high-grade rotatory instability. Lateral extraarticular tenodesis (LET) is sometimes added to ACL reconstruction to address this instability. However, LET is a non-anatomic procedure and concerns regarding increased tibiofemoral contact pressure and reduced internal rotation exist for some individuals which may be due to their tibiofemoral bony morphology. Therefore, the objective of this study was to analyze the effect of bony morphology on knee kinematic and contact pressure before and after anterolateral capsule injury and LET. A (1) 134-N anterior tibial load with 200-N axial compression and (2) a 7-Nm internal torque with a 200-N axial compression were applied to cadaveric knees (n = 8) using a 6 degree-of-freedom robotic testing system. Tibiofemoral bony morphology was captured with computed tomography scans and analyzed using 3D statistical shape modeling. Kinematics at each state were correlated with the results from the statistical shape model. Two femoral and three tibial modes of variation correlated with kinematic and contact pressure data before and after anterolateral capsule injury and LET. A decreased lateral tibial plateau elevation correlated with greater internal rotation and anterior tibial translation after anterolateral capsule deficiency and LET. Decreased notch width correlated with decreased contact area after anterolateral capsule deficiency and LET demonstrating it as a risk factor for ACL injury. The results of this study demonstrate that bony morphology if properly understood, could help improve the efficacy of LET procedures and that bony morphology has different effects after injury and repair.

Small lateral meniscus tears propagate over time in ACL intact and deficient knees

PURPOSE

To quantify propagation of small longitudinal tears in the lateral meniscus in ACL intact and deficient knees.

METHODS

Using a robotic testing system, 5-Nm of external tibial torque + 5-Nm of valgus torque + 250-N of axial compression was applied to 14 fresh-frozen cadaveric knees while the knees were flexed from 30° to 90°. Knees were divided into two groups: intact (N = 8) and ACL deficient (N = 6). Kinematic data was recorded for four knee states: intact or ACL deficient knee, after posterior arthrotomy, meniscus tear at baseline, and after 500 cycles of the applied loading condition.

RESULTS

Lateral meniscus tear length increased throughout the 500 cycles regardless of the ACL integrity (p < 0.001). Overall, an increase of 28.7% and 26.1% was observed in intact and ACL deficient knees, respectively. In intact knees, external tibial rotation increased with meniscus tear propagation at all flexion angles by up to 45.5% (p = 0.019). In contrast, knee kinematics in ACL deficient knees were not affected by meniscus tear propagation (n.s.). In ACL deficient knees, resultant forces in the lateral meniscus increased at all flexion angles by up to 116.5% (p = 0.012). No differences in forces were observed in the intact knees (n.s.).

CONCLUSION

The data of this study show that small longitudinal tears in the lateral meniscus propagate significantly regardless of the integrity of the ACL and even after only 100 cycles of knee loading. The propagation of such tears altered kinematics and forces in the knee. Therefore, small, longitudinal lateral meniscus tears that are untreated in current clinical practices may propagate when loaded.

An Approach to Robotic Testing of the Wrist Using Three-Dimensional Imaging and a Hybrid Testing Methodology

Robotic technology is increasingly used for sophisticated in vitro testing designed to understand the subtleties of joint biomechanics. Typically, the joint coordinate systems in these studies are established via palpation and digitization of anatomic landmarks. We are interested in wrist mechanics in which overlying soft tissues and indistinct bony features can introduce considerable variation in landmark localization, leading to descriptions of kinematics and kinetics that may not appropriately align with the bony anatomy. In the wrist, testing is often performed using either load or displacement control with standard material testers. However, these control modes either do not consider all six degrees-of-freedom (DOF) or reflect the nonlinear mechanical properties of the wrist joint. The development of an appropriate protocol to investigate complexities of wrist mechanics would potentially advance our understanding of normal, pathological, and artificial wrist function. In this study, we report a novel methodology for using CT imaging to generate anatomically aligned coordinate systems and a new methodology for robotic testing of wrist. The methodology is demonstrated with the testing of 9 intact cadaver specimens in 24 unique directions of wrist motion to a resultant torque of 2.0 N·m. The mean orientation of the major principal axis of range of motion (ROM) envelope was oriented 12.1 ± 2.7 deg toward ulnar flexion, which was significantly different (p < 0.001) from the anatomical flexion/extension axis. The largest wrist ROM was 98 ± 9.3 deg in the direction of ulnar flexion, 15 deg ulnar from pure flexion, consistent with previous studies [1,2]. Interestingly, the radial and ulnar components of the resultant torque were the most dominant across all directions of wrist motion. The results of this study showed that we can efficiently register anatomical coordinate systems from CT imaging space to robotic test space adaptable to any cadaveric joint experiments and demonstrated a combined load-position strategy for robotic testing of wrist.

Does Lateral Extra-articular Tenodesis of the Knee Affect Anterior Cruciate Ligament Graft In Situ Forces and Tibiofemoral Contact Pressures?

PURPOSE

To quantify the effects of lateral extra-articular tenodesis (LET) on tibiofemoral compartment contact area and pressures, knee kinematics, and forces.

METHODS

Nine cadaveric knees were tested using a robotic testing system. Two loading conditions, (1) anterior tibial translational load coupled with axial compression and (2) internal tibial torque coupled with axial compression, were applied for each knee state at full extension and 30°, 60°, and 90° of knee flexion. Kinematic data was recorded for 3 knee states: anterolateral capsule (ALC) competent, ALC deficient, and post-LET using a 6-mm semitendinosus graft. In situ force in the anterior cruciate ligament (ACL) was quantified using the principle of superposition by comparing the change in force measured before and after the removal of the ALC. Contact area and pressures in each tibiofemoral compartment were measured by replaying kinematics after soft tissues were removed and pressure sensors were inserted.

RESULTS

In response to an anterior tibial translational load, mean contact area in the medial compartment decreased by 33.1% from the ALC-competent to post-LET knee states at 90° of knee flexion (P = .042). No significant differences in lateral compartment contact pressure were found between knee states. In situ force in the ACL in response to an anterior tibial translational load decreased by 43.4% and 50% from the ALC-deficient to post-LET knee states at 60° (P = .02) and 90° (P = .006). No significant difference in kinematics was observed between the ALC-competent and post-LET knee states in each of the loading conditions at all knee flexion angles (P > .05).

CONCLUSIONS

In this in vitro model, LET with a semitendinosus graft did not significantly overconstrain the knee or increase pressure in the lateral compartment. Additionally, LET reduced the in situ force in the ACL in the setting of ALC injury.

CLINICAL RELEVANCE

The lack of knee overconstraint without significant increases in lateral compartment pressures indicates that if an LET with semitendinosus graft is not overtensioned, accelerated degenerative changes in the lateral compartment may not be expected after this procedure.

Lateral Meniscal Allograft Transplantation With Bone Block and Suture-Only Techniques Partially Restores Knee Kinematics and Forces

BACKGROUND

The ability of lateral meniscal allograft transplantation (MAT) to improve knee stability and the meniscal load-bearing function in patients after meniscectomy is critical for surgical success.

PURPOSE

To compare the effects of 2 lateral MAT fixation techniques-bone block and suture only-on knee kinematics and forces.

STUDY DESIGN

Controlled laboratory study.

METHODS

With a robotic testing system, loads were applied during flexion on 10 fresh-frozen cadaveric knees: 134-N anterior tibial load + 200-N axial compression, 5-N·m internal tibial + 5-N·m valgus torques, and 5-N·m external tibial + 5-N·m valgus torques. Kinematic data were recorded for 4 knee states: intact, total lateral meniscectomy, lateral MAT bone block, and lateral MAT suture-only fixation. In situ force in the anterior cruciate ligament and resultant forces in the lateral meniscus and in the meniscal allograft were quantified via the principle of superposition. A repeated measures analysis of variance was used to analyze variations in kinematics and forces at 0°, 30°, 60°, and 90° of knee flexion. Significance was set at P < .05.

RESULTS

When anterior loads were applied, a decrease in medial translation of the tibia that was increased after total lateral meniscectomy was observed at 30°, 60°, and 90° of knee flexion for both the lateral MAT bone block (54.2%, 48.0%, and 50.0%) and the MAT suture-only (50.0%, 40.0%, and 34.6%) fixation techniques (P < .05). Yet, most of the increases in knee kinematics after lateral meniscectomy were not significantly reduced by either lateral MAT technique (P > .05 for each MAT technique vs the total lateral meniscectomy state). Resultant forces in the meniscal allograft were 50% to 60% of the resultant forces in the intact lateral meniscus in response to all loading conditions at all flexion angles (P < .05). Overall, no significant differences between lateral MAT techniques were observed regarding kinematics and forces (P > .05).

CONCLUSION

Lateral MAT partially restored medial translation of the tibia, and the resultant forces in the meniscal allograft were only 50% to 60% of the intact lateral meniscus forces in the cadaver model. In the majority of testing conditions, no significant changes of the in situ force in the anterior cruciate ligament were observed. Surgeons should consider the potential benefits of lateral MAT when deciding the appropriate treatment for symptomatic patients after lateral meniscectomies. Both lateral MAT techniques functioned similarly.

CLINICAL RELEVANCE

The load-bearing function of the meniscal allograft observed in this study may be beneficial in ameliorating the short- and long-term disability associated with lateral meniscal deficiency.

In situ force in the anterior cruciate ligament, the lateral collateral ligament, and the anterolateral capsule complex during a simulated pivot shift test

The role of the anterolateral capsule complex in knee rotatory stability remains controversial. Therefore, the objective of this study was to determine the in situ forces in the anterior cruciate ligament (ACL), the anterolateral capsule, the lateral collateral ligament (LCL), and the forces transmitted between each region of the anterolateral capsule in response to a simulated pivot shift test. A robotic testing system applied a simulated pivot shift test continuously from full extension to 90° of flexion to intact cadaveric knees (n = 7). To determine the magnitude of the in situ forces, kinematics of the intact knee were replayed in position control mode after the following procedures were performed: (i) ACL transection; (ii) capsule separation; (iii) anterolateral capsule transection; and (iii) LCL transection. A repeated measures ANOVA was performed to compare in situ forces between each knee state (*p < 0.05). The in situ force in the ACL was significantly greater than the forces transmitted between each region of the anterolateral capsule at 5° and 15° of flexion but significantly lower at 60°, 75°, and 90° of flexion. This study demonstrated that the ACL is the primary rotatory stabilizer at low flexion angles during a simulated pivot shift test in the intact knee, but the anterolateral capsule plays an important secondary role at flexion angles greater than 60°. Furthermore, the contribution of the "anterolateral ligament" to rotatory knee stability in this study was negligible during a simulated pivot shift test. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:847-853, 2018.

The Role of Extra-Articular Tenodesis in Combined ACL and Anterolateral Capsular Injury

BACKGROUND

The "gold standard" treatment of anterolateral capsular injuries in anterior cruciate ligament (ACL)-deficient knees has not been determined. The purpose of this study was to determine the effects of ACL reconstruction and extra-articular reconstruction on joint motion in the ACL-deficient knee and in the combined ACL and anterolateral capsule-deficient knee.

METHODS

An anterior tibial load of 134 N and internal tibial torque of 7 Nm were applied to 7 fresh-frozen cadaveric knees using a robotic testing system continuously throughout the range of flexion. The resulting joint motion was recorded for 6 knee states: intact, ACL-deficient, ACL-reconstructed, combined ACL and anterolateral capsule-deficient, ACL-reconstructed + anterolateral capsule-deficient, and ACL-reconstructed + extra-articular tenodesis.

RESULTS

Anterior tibial translation of the ACL-reconstructed + anterolateral capsule-deficient knee in response to an anterior tibial load was restored to that of the intact knee at all knee-flexion angles (p > 0.05). However, for this knee state, internal tibial rotation in response to internal tibial torque was not restored to that of the intact knee at 60° or 90° of knee flexion (p < 0.05). For the knee state of ACL-reconstructed + extra-articular tenodesis, internal rotation in response to internal tibial torque was restored to the motion of the intact knee at each of the tested knee-flexion angles (p > 0.05). Compared with the intact knee, 2 of 7 specimens showed decreased internal tibial rotation with ACL reconstruction + extra-articular tenodesis.

CONCLUSIONS

In this study, an extra-articular tenodesis was necessary to restore rotatory knee stability in response to internal tibial torque in a combined ACL and anterolateral capsule-deficient knee. The amount of rotatory knee instability should be carefully assessed to avoid over-constraint of the knee in these combined ligament-reconstruction procedures.

CLINICAL RELEVANCE

On the basis of our findings, the surgical procedure needs to be personalized depending on the amount of rotatory knee instability in the injured knee and the amount of rotation in the contralateral knee.

Biomechanical evaluation of knee endpoint during anterior tibial loading: Implication for physical exams

BACKGROUND

Physical exams that apply anterior tibial loads are typically used to evaluate knees with anterior cruciate ligament (ACL) injuries. The amount of anterior tibial translation that occurs during these exams can be difficult to assess due to a "soft" endpoint. Therefore, the objective of this study is to determine the biomechanical characteristics of the endpoint for the intact and ACL deficient knee using quantitative criteria.

METHODS

Eight porcine knees were tested using a robotic testing system. An 89N anterior tibial load was applied to the intact and ACL deficient knee at 30°, 45°, 60° and 75° of flexion. The stiffness of the toe and linear regions was determined from the load-translation curve. The width of the transition region was defined by the distance between the points where the best-fit lines used to define the stiffness of the toe and linear regions diverged from the load-translation curve.

RESULTS

Stiffness of the toe and linear regions significantly decreased after transecting the ACL at all flexion angles (71-85% and 38-62%, respectively). Width of the transition region was significantly increased in the ACL deficient knee at all flexion angles (approximately four to five times and four to nine times, respectively).

CONCLUSIONS

The novel quantitative criteria developed in this study have the potential to be deployed in clinical practice by coupling them with data from knee arthrometers that are commonly used in clinical practice. Thus, additional information from the load-translation curve can be provided to improve the diagnosis of ACL injury.

The Anterolateral Capsule of the Knee Behaves Like a Sheet of Fibrous Tissue

BACKGROUND

The function of the anterolateral capsule of the knee has not been clearly defined. However, the contribution of this region of the capsule to knee stability in comparison with other anterolateral structures can be determined by the relative force that each structure carries during loading of the knee. Purpose/Hypothesis: The purpose of this study was to determine the forces in the anterolateral structures of the intact and anterior cruciate ligament (ACL)-deficient knee in response to an anterior tibial load and internal tibial torque. It was hypothesized that the anterolateral capsule would not function like a traditional ligament (ie, transmitting forces only along its longitudinal axis).

STUDY DESIGN

Controlled laboratory study.

METHODS

Loads (134-N anterior tibial load and 7-N·m internal tibial torque) were applied continuously during flexion to 7 fresh-frozen cadaveric knees in the intact and ACL-deficient state using a robotic testing system. The lateral collateral ligament (LCL) and the anterolateral capsule were separated from the surrounding tissue and from each other. This was done by performing 3 vertical incisions: lateral to the LCL, medial to the LCL, and lateral to the Gerdy tubercle. Attachments of the LCL and anterolateral capsule were detached from the underlying tissue (ie, meniscus), leaving the insertions and origins intact. The force distribution in the anterolateral capsule, ACL, and LCL was then determined at 30°, 60°, and 90° of knee flexion using the principle of superposition.

RESULTS

In the intact knee, the force in the ACL in response to an anterior tibial load was greater than that in the other structures ( P < .001). However, in response to an internal tibial torque, no significant differences were found between the ACL, LCL, and forces transmitted between each region of the anterolateral capsule after capsule separation. The anterolateral capsule experienced smaller forces (~50% less) compared with the other structures ( P = .048). For the ACL-deficient knee in response to an anterior tibial load, the force transmitted between each region of the anterolateral capsule was 434% greater than was the force in the anterolateral capsule ( P < .001) and 54% greater than the force in the LCL ( P = .036) at 30° of flexion. In response to an internal tibial torque at 30°, 60°, or 90° of knee flexion, no significant differences were found between the force transmitted between each region of the anterolateral capsule and the LCL. The force in the anterolateral capsule was significantly smaller than that in the other structures at all knee flexion angles for both loading conditions ( P = .004 for anterior tibial load and P = .04 for internal tibial torque).

CONCLUSION

The anterolateral capsule carries negligible forces in the longitudinal direction, and the forces transmitted between regions of the capsule were similar to the forces carried by the other structures at the knee, suggesting that it does not function as a traditional ligament. Thus, the anterolateral capsule should be considered a sheet of tissue.

CLINICAL RELEVANCE

Surgical repair techniques for the anterolateral capsule should restore the ability of the tissue to transmit forces between adjacent regions of the capsule rather than along its longitudinal axis.

Use of Robotic Manipulators to Study Diarthrodial Joint Function

Diarthrodial joint function is mediated by a complex interaction between bones, ligaments, capsules, articular cartilage, and muscles. To gain a better understanding of injury mechanisms and to improve surgical procedures, an improved understanding of the structure and function of diarthrodial joints needs to be obtained. Thus, robotic testing systems have been developed to measure the resulting kinematics of diarthrodial joints as well as the in situ forces in ligaments and their replacement grafts in response to external loading conditions. These six degrees-of-freedom (DOF) testing systems can be controlled in either position or force modes to simulate physiological loading conditions or clinical exams. Recent advances allow kinematic, in situ force, and strain data to be measured continuously throughout the range of joint motion using velocity-impedance control, and in vivo kinematic data to be reproduced on cadaveric specimens to determine in situ forces during physiologic motions. The principle of superposition can also be used to determine the in situ forces carried by capsular tissue in the longitudinal direction after separation from the rest of the capsule as well as the interaction forces with the surrounding tissue. Finally, robotic testing systems can be used to simulate soft tissue injury mechanisms, and computational models can be validated using the kinematic and force data to help predict in vivo stresses and strains present in these tissues. The goal of these analyses is to help improve surgical repair procedures and postoperative rehabilitation protocols. In the future, more information is needed regarding the complex in vivo loads applied to diarthrodial joints during clinical exams and activities of daily living to serve as input to the robotic testing systems. Improving the capability to accurately reproduce in vivo kinematics with robotic testing systems should also be examined.

Basic biomechanic principles of knee instability

Motion at the knee joint is a complex mechanical phenomenon. Stability is provided by a combination of static and dynamic structures that work in concert to prevent excessive movement or instability that is inherent in various knee injuries. The anterior cruciate ligament (ACL) is a main stabilizer of the knee, providing both translational and rotatory constraint. Despite the high volume of research directed at native ACL function, pathogenesis and surgical reconstruction of this structure, a gold standard for objective quantification of injury and subsequent repair, has not been demonstrated. Furthermore, recent studies have suggested that novel anatomic structures may play a significant role in knee stability. The use of biomechanical principles and testing techniques provides essential objective/quantitative information on the function of bone, ligaments, joint capsule, and other contributing soft tissues in response to various loading conditions. This review discusses the principles of biomechanics in relation to knee stability, with a focus on the objective quantification of knee stability, the individual contributions of specific knee structures to stability, and the most recent technological advances in the biomechanical evaluation of the knee joint.