Validation of a six degree-of-freedom robotic system for hip in vitro biomechanical testing

Contribution of the Medial Iliofemoral Ligament to Hip Stability After Total Hip Arthroplasty Through the Direct Anterior Approach

BACKGROUND

Dislocation after total hip arthroplasty (THA) is a primary reason for THA revision. During THA through the direct anterior approach (DAA), the iliofemoral ligament, which provides the main resistance to external rotation (ER) of the hip, is commonly partially transected. We asked: (1) what is the contribution of the medial iliofemoral ligament to resisting ER after DAA THA? and (2) how much resistance to ER can be restored by repairing the ligament?

METHODS

A fellowship-trained surgeon performed DAA THA on 9 cadaveric specimens. The specimens were computed tomography scanned before and after implantation. Prior to testing, the ER range of motion of each specimen to impingement in neutral and 10° of extension was computationally predicted. Each specimen was tested on a 6-degrees-of-freedom robotic manipulator. The pelvis was placed in neutral and 10° of extension. The femur was externally rotated until it reached the specimen's impingement target. Total ER torque was recorded with the medial iliofemoral ligament intact, after transecting the ligament, and after repair. Torque at extremes of motion was calculated for each condition. To isolate the contribution of the native ligament, the torque for the transected state was subtracted from both the native and repaired conditions.

RESULTS

The medial iliofemoral ligament contributed an average of 68% (range, 34 to 87) of the total torque at the extreme of motion in neutral and 80% (58 to 97) in 10⁰ of extension. The repaired ligament contributed 17% (1 to 54) of the total torque at the extreme of motion in neutral and 14% (5 to 38) in 10⁰ of extension, restoring on average 18 to 25% of the native resistance against ER.

CONCLUSIONS

The medial iliofemoral ligament was an important contributor to the hip torque at the extreme of motion during ER. Repairing the ligament restored a fraction of its ability to generate torque to resist ER.

Biomechanical and clinical evaluation of minimal invasive plate osteosynthesis for two-part clavicle shaft fractures

BACKGROUND

Many surgical treatment methods exist for clavicle shaft fractures. A locking compression plate (LCP) fixation with three screws per fracture side is commonly used. For certain fractures a stabilization with 2 screws per side is potentially suitable, offering the advantage of reduced soft tissue approach, while avoiding the disadvantages of minimally-invasive nailing at the same time. This hypothesis was evaluated biomechanically and clinically.

METHODS

Four treatment procedures were investigated biomechanically using composite human clavicle specimens. A load-to-failure test was performed using a three-point cantilever test. In group 1, a simple shaft fracture was simulated and stabilized with 2 screws per fracture side (5-hole LCP). In the second group 3 screws per side (7-hole LCP) were used. In group 3, a non-reduced fracture zone was simulated and treated with 3 screws per side (7-hole LCP). In group 4, an anatomically reduced fracture zone was simulated and treated with 3 screws per side (7-hole LCP). Furthermore 27 patients treated with a short plate and 2 screws per side (similar to group 1) were assessed after a minimum follow-up of 12 months (Constant and DASH Score).

RESULTS

The maximum load-to-failure of group 1 was 367N. We observed the highest load-to-failure in group 2 with 497N and the lowest in group 3 with 90N. In group 4 a maximum load-to-failure of 298N could be evaluated. There was no significant difference in load-to-failure between the treatment of a simple clavicle fracture using 5- or 7-hole LCP (p = 0.121). However, we found a significant difference of load-to-failure between the simple and anatomically reduced fracture using a 7-hole plate (p = 0.014). The mean constant score of the surgically treated patients was 95 and the DASH score 3.0. Fracture consolidation was observed in 96.3%.

CONCLUSIONS

For certain non-fragmented and well interlocking 2-part fractures, a plate osteosynthesis fixed with only 2 screws per fracture side might offer sufficient biomechanical stability, better soft tissue preservation and comparable fusion rates compared to the operative treatment with 3 screws per side. However, the maximum load-to-failure of the 7-hole LCP was higher than of the 5-hole LCP, but this difference was not statistically significant.

TRIAL REGISTRATION

Approval from the ethics committee of the Technical University of Dresden was retrospectively obtained (EK 588122019).

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

PURPOSE

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

METHODS

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

RESULTS

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

CONCLUSION

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

Dynamic Analysis and Trajectory Tracking Control for a Parallel Manipulator with Joint Friction

To overcome the bearing capacity deficiencies of traditional serial hip joint simulators, complex trajectory simulation, among others, as well as a parallel manipulator with two pairs of artificial hip joints and two moving platforms are proposed. The movements and driving forces of the parallel manipulator under the required motion and loading are studied to provide a basis for further research. In this study, the modeling and analysis of inverse kinematics and dynamics for a parallel manipulator with joint friction are derived. In the inverse kinematic model, kinematic relationships between the linear module slider and the moving platform are established, and expressions for the slider are deduced. Subsequently, by analyzing the frictional forces of the artificial hip joint and thrust ball bearing, a rigid body dynamics model of the parallel manipulator with joint friction is established, which is subsequently decomposed into four driving torques associated with the moving platform, joint lever, slider, and screw. Finally, the difference in the kinematic performance between the two moving platforms is analyzed using numerical simulations and experiments, and the accuracy of the established model is verified.

Replicating dynamic humerus motion using an industrial robot

Transhumeral percutaneous osseointegrated prostheses provide upper-extremity amputees with increased range of motion, more natural movement patterns, and enhanced proprioception. However, direct skeletal attachment of the endoprosthesis elevates the risk of bone fracture, which could necessitate revision surgery or result in loss of the residual limb. Bone fracture loads are direction dependent, strain rate dependent, and load rate dependent. Furthermore, in vivo, bone experiences multiaxial loading. Yet, mechanical characterization of the bone-implant interface is still performed with simple uni- or bi-axial loading scenarios that do not replicate the dynamic multiaxial loading environment inherent in human motion. The objective of this investigation was to reproduce the dynamic multiaxial loading conditions that the humerus experiences in vivo by robotically replicating humeral kinematics of advanced activities of daily living typical of an active amputee population. Specifically, 115 jumping jack, 105 jogging, 15 jug lift, and 15 internal rotation trials-previously recorded via skin-marker motion capture-were replicated on an industrial robot and the resulting humeral trajectories were verified using an optical tracking system. To achieve this goal, a computational pipeline that accepts a motion capture trajectory as input and outputs a motion program for an industrial robot was implemented, validated, and made accessible via public code repositories. The industrial manipulator utilized in this study was able to robotically replicate over 95% of the aforementioned trials to within the characteristic error present in skin-marker derived motion capture datasets. This investigation demonstrates the ability to robotically replicate human motion that recapitulates the inertial forces and moments of high-speed, multiaxial activities for biomechanical and orthopaedic investigations. It also establishes a library of robotically replicated motions that can be utilized in future studies to characterize the interaction of prosthetic devices with the skeletal system, and introduces a computational pipeline for expanding this motion library.

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.

Hip Joint Capsular Anatomy, Mechanics, and Surgical Management

➤Hip joint capsular ligaments (iliofemoral, ischiofemoral, and pubofemoral) play a predominant role in functional mobility and joint stability. ➤The zona orbicularis resists joint distraction (during neutral positions), and its aperture mechanism stabilizes the hip from adverse edge-loading (during extreme hip flexion-extension). ➤To preserve joint function and stability, it is important to minimize capsulotomy size and avoid disrupting the zona orbicularis, preserve the femoral head size and neck length, and only repair when or as necessary without altering capsular tensions. ➤It is not fully understood what the role of capsular tightness is in patients who have cam femoroacetabular impingement and if partial capsular release could be beneficial and/or therapeutic. ➤During arthroplasty surgery, a femoral head implant that is nearly equivalent to the native head size with an optimal neck-length offset can optimize capsular tension and decrease dislocation risk where an intact posterior hip capsule plays a critical role in maintaining hip stability.

Hip Joint Torsional Loading Before and After Cam Femoroacetabular Impingement Surgery

BACKGROUND

Surgical management of cam femoroacetabular impingement (FAI) aims to preserve the native hip and restore joint function, although it is unclear how the capsulotomy, cam deformity, and capsular repair influence joint mechanics to balance functional mobility.

PURPOSE

To examine the contributions of the capsule and cam deformity to hip joint mechanics. Using in vitro, cadaveric methods, we examined the individual effects of the surgical capsulotomy, cam resection, and capsular repair on passive range of motion and resistance of applied torque.

STUDY DESIGN

Descriptive laboratory study.

METHODS

Twelve cadaveric hips with cam deformities were skeletonized to the capsule and mounted onto a robotic testing platform. The robot positioned each intact hip in multiple testing positions: (1) extension, (2) neutral 0°, (3) flexion 30°, (4) flexion 90°, (5) flexion-adduction and internal rotation (FADIR), and (6) flexion-abduction and external rotation. Then the robot performed applicable internal and external rotations, recording the neutral path of motion until a 5-N·m of torque was reached in each rotational direction. Each hip then underwent a series of surgical stages (T-capsulotomy, cam resection, capsular repair) and was retested to reach 5 N·m of internal and external torque again after each stage. During the capsulotomy and cam resection stages, the initial intact hip's recorded path of motion was replayed to measure changes in resisted torque.

RESULTS

Regarding changes in motion, external rotation increased substantially after capsulotomies, but internal rotation only further increased at flexion 90° (change +32%, P = .001, d = 0.58) and FADIR (change +33%, P < .001, d = 0.51) after cam resections. Capsular repair provided marginal restraint for internal rotation but restrained the external rotation compared with the capsulotomy stage. Regarding changes in torque, both internal and external torque resistance decreased after capsulotomy. Compared with the capsulotomy stage, cam resection further reduced internal torque resistance during flexion 90° (change -45%, P < .001, d = 0.98) and FADIR (change -37%, P = .003, d = 1.0), where the cam deformity accounted for 21% of the intact hip's torsional resistance in flexion 90° and 27% in FADIR.

CONCLUSION

Although the capsule played a predominant role in joint constraint, the cam deformity provided 21% to 27% of the intact hip's resistance to torsional load in flexion and internal rotation. Resecting the cam deformity would remove this loading on the chondrolabral junction.

CLINICAL RELEVANCE

These findings are the first to quantify the contribution of the cam deformity to resisting hip joint torsional loads and thus quantify the reduced loading on the chondrolabral complex that can be achieved after cam resection.

Editorial Commentary: Hip Arthroscopy Capsular Management-Cut Your Cloth to Measure!

Microinstability of the hip is a relatively recent concept but one that is gaining increased acceptance. As our understanding of the factors that contribute to microinstability has increased, so too has our ability to identify "at-risk" patients, in whom a capsular repair should be considered after hip arthroscopy to achieve optimal results and avoid iatrogenic instability (dislocation or microinstability). However, each of our patients is different, and as such, we must be able to tailor our capsulotomies and repairs accordingly based on the bony morphology, capsular volume, and properties of the tissue.

Robotic hip joint testing: Development and experimental protocols

The use of robotic systems combined with force sensing is emerging as the gold standard for in vitro biomechanical joint testing, due to the advantage of controlling all six degrees of freedom independently of one another. This paper describes a novel robotic platform and the experimental protocol used for hip joint testing. An experimental protocol implemented optical tracking and registration techniques in order to define the position of the hip joint centre of rotation (COR) in the coordinate system of the robot's end effector. The COR coordinates defined the origin of the task-related coordinate system used to control the robot, with a hybrid force/position law to simulate standard clinical tests. The axes of this frame were defined using the International Society of Biomechanics (ISB) anatomical coordinate system. Experiments were carried out on two cadaveric hip joint specimens using the robotic testing platform and a mechanical testing rig previously developed and described by our group. Simulated internal-external and adduction/abduction laxity tests were carried out with both systems and the resulting peak range of motion (ROM) was measured. Similarities and differences were observed in these experiments, which were used to highlight some of the limitations of conventional systems and the corresponding advantages of robotics, further emphasising their added value in vitro testing.

Development and validation of a robotic system for ankle joint testing

Ankle sprains are the most common sports injury. Gaining a better understanding of ankle mechanics will help improve current treatments, enabling a better quality of life for patients following surgery. In this paper, the development of a robotic system for ankle joint testing is presented. It is composed of an industrial robot, a universal force/torque sensor and bespoke holders allowing high repositioning of specimens. A specimen preparation protocol that uses optical tracking to register the ankle specimens is used. A registration technique is applied to define and calibrate the task related coordinate system needed to control the joint's degrees of freedom and to simulate standardised, clinical ankle laxity tests. Experiments were carried out at different flexion angles using the robotic platform. Optical tracking was used to record the resulting motion of the tibia for every simulated test. The measurements from the optical tracker and the robot were compared and used to validate the system. These findings showed that the optical tracking measurements validate those from the robot for ankle joint testing with interclass coefficients equal to 0.991, 0.996 and 0.999 for the anterior-posterior translations, internal-external and inversion-eversion rotations.

Biomechanical Assessment of Hip Capsular Repair and Reconstruction Procedures Using a 6 Degrees of Freedom Robotic System

BACKGROUND

Although acetabular labral repair has been biomechanically validated to improve stability, capsular management of the hip remains a topic of growing interest and controversy.

PURPOSE

To biomechanically evaluate the effects of several arthroscopically relevant conditions of the capsule through a robotic, sequential sectioning study.

STUDY DESIGN

Controlled laboratory study.

METHODS

Ten human cadaveric unilateral hip specimens (mean age, 51.3 years [range, 38-65 years]) from full pelvises were used to test range of motion (ROM) for the intact capsule and for multiple capsular conditions including portal incisions, interportal capsulotomy, interportal capsulotomy repair, T-capsulotomy, T-capsulotomy repair, a large capsular defect, and capsular reconstruction. Hips were biomechanically tested using a 6 degrees of freedom robotic system to assess ROM with applied 5-N·m internal, external, abduction, and adduction rotation torques throughout hip flexion and extension.

RESULTS

All capsulotomy procedures (portals, interportal capsulotomy, and T-capsulotomy) created increases in external, internal, adduction, and abduction rotations compared with the intact state throughout the full tested ROM (-10° to 90° of flexion). Reconstruction significantly reduced rotation compared with the large capsular defect state for external rotation at 15° (difference, 1.4°) and 90° (difference, 1.3°) of flexion; internal rotation at -10° (difference, 0.4°), 60° (difference, 0.9°), and 90° (difference, 1.4°) of flexion; abduction rotation at -10° (difference, 0.5°), 15° (difference, 1.1°), 30° (difference, 1.2°), 60° (difference, 0.9°), and 90° (difference, 1.0°) of flexion; and adduction rotation at 0° (difference, 0.7°), 15° (difference, 0.8°), 30° (difference, 0.3°), and 90° (difference, 0.6°) of flexion. Repair of T-capsulotomy resulted in significant reductions in rotation compared with the T-capsulotomy condition for abduction rotation at -10° (difference, 0.3°), 15° (difference, 0.9°), 30° (difference, 1.3°), 60° (difference, 1.7°), and 90° (difference, 1.5°) of flexion and for internal rotation at -10° (difference, 0.9°), 60° (difference, 1.5°), and 90° (difference, 2.6°) of flexion. Similarly, repair of interportal capsulotomy resulted in significant reductions in abduction (difference, 0.9°) and internal (difference, 1.4°) rotations compared with interportal capsulotomy at 90° of flexion. In most cases, however, after the repair procedures, ROM was still increased in comparison with the intact state.

CONCLUSION

The results of this study suggest that common hip arthroscopic capsulotomy procedures can result in increases in external, internal, abduction, and adduction rotations throughout a full range (-10° to 90°) of hip flexion. However, capsular repair and reconstruction succeeded in partially reducing the increased rotational ROM caused by common capsulotomy procedures. Thus, consideration should be allotted toward capsular repair or reconstruction in cases with an increased risk of residual instability.

CLINICAL RELEVANCE

Although complete restoration of joint stability may not be fully achieved at time zero, capsular repair and reconstruction may lead to improved patient outcomes by bringing hip rotational movements nearer to normal values in the immediate postoperative period, especially in cases in which extensive capsulotomy is performed.