The key outcome of this study was the validation of a 7 DOF robotic manipulator for the in vitro replication of manually-induced humerus motions on a native whole shoulder girdle. The present results demonstrated that the opportunities offered by robotic manipulators can be extended to complex kinematic chains in intra-corporal conditions while ensuring a high level of reliability, validity and fidelity. This feature opens new avenues in native and pathological joint function exploration as well as in medical device testing.
On average, the robotic manipulator was able to perform requested end-effector motions with a reliability of 0.28 ± 0.57 mm and 0.15 ± 0.25°, and a fidelity of 0.27 ± 0.56 mm and 0.22 ± 0.28°. These results are generally within the targeted reliability and fidelity thresholds (i.e. < 1 mm and < 1°) and may thus ensure sufficient accuracy in future studies. These results are also generally similar to those obtained from studies performed on single diarthrodial joints in extra-corporal conditions. Concerning hip and knee joints, Smith et al. 24 reported a reliability of 0.2 mm and 0.2° while Darcy et al. 25 reported a reliability of 0.3 mm and 0.1°, respectively. Concerning shoulder joint, mean absolute errors reported in our study in terms of position and orientation were always below 5% of the explored joint range of motion. These values are in line with those of Aliaj et al. study 5. The values reported by Lee et al. 6 are lower, with a reliability of 0.03 mm and an accuracy of 0.11 mm and 0.13°. However, the experimentations were performed in a smaller range of motion than in the present study (i.e. translations ranging between 1.3 mm and 17.4 mm). In our results, it appeared that horizontal flexion-extension was more subject to errors than other humerus motions. Practically, this motion was often close to the manipulator workspace boundaries. Resulting errors could thus be reduced by decreasing the requested range of motion. In particular the maximal humerus horizontal flexion which tended to put the manipulator arm in full extension.
According to the manufacturer specifications, the KUKA LBR IIWA 14 R820 has an instrumental error of 0.1 mm. Regarding the present results, another source of error may have decreased our protocol accuracy. As pointed out by several authors 5,26, the relationship between the manipulator end-effector coordinate system and the attached segment coordinate system (humerus coordinate system in our case) may be subject to errors. The rigid transformation between these two coordinate systems has been estimated in the literature by the use of custom fixtures 26 or by an identification procedure using an optical tracking system 5. In this study, a custom 3D-printed cylinder was rigidly secured on the transected humerus and mounted via a custom fixture to the manipulator end-effector. The motions observed on the cylinder were expressed in the end-effector coordinate system under the assumption of fully known rigid transformations (based on the geometry of the different parts) and flush mount joints. While they were not assessed in this study, some errors may have been introduced on these assumptions and should thus be estimated in the future.
Except in several studies investigating foot/ankle dynamics 8,9, most of the previously published joint cadaveric simulators based on a robotic manipulator focused on a single diarthrodial joint. By proposing an intra-corporal condition protocol, our study made the full kinematic chain of the shoulder girdle available for analysis. As proposed by Oki et al. 1 such a condition opens new opportunities, e.g. by allowing the individual contribution of acromioclavicular and coracoclavicular ligaments to the shoulder kinematics under various humerus motions (i.e. a closer to physiological condition than mobilising the clavicle while keeping fixed the scapula). This was made possible as robotic manipulators, instead of UTM, are open chain mechanisms that do not limit the explored kinematic chain length. Still, the manipulator has a limited workspace that restrains the potential positions of the exploring bones and joints in the manipulator coordinate system. In robotics, it may be more common to adapt the position of the robot (e.g. mobile manipulator, humanoid robot) than to move the targeted object 27. In the present study, this issue was managed by optimising the spatial organisation of the specimen with respect to the manipulator to allow requested motions. This procedure, repeated for each shoulder, allowed for the personalisation of specimen position and orientation depending on humerus length and humerus range of motion.
Another feature of this study was that our protocol allowed to reproduce specimen-specific humerus motions induced by an operator. This procedure simulates intra-operative shoulder passive mobilisation performed by the surgeon to assess joint reconstruction or joint arthroplasty efficiency 16. As observed by Goldsmith et al. 17 using a similar protocol to explore the hip joint, the use of a robotic manipulator allows for reliability in induced motions. However, while these authors used predefined rotation axes to approximately replicate manually-induced motions, our protocol directly uses the recorded manually-induced motions (i.e. the intra-operative shoulder passive mobilisation performed by the surgeon) for the robotic motion planning. To the best of our knowledge, this is the first time that specimen-specific motions, recorded during manual passive mobilisation, are used for robotic motion planning. Instead, several studies used various motions obtained from open-source datasets compiling records made on healthy participants 5,15,28−31. The use of specimen-specific motions better allows to respect related joint kinematic constraints (e.g. bony or soft tissue constraints that may limit further joint motion) and thus to better respect physiologic boundaries. Still, the application of specimen-specific motions recorded during the native condition of a joint may not be applicable in injured or repaired joint conditions, where related joint kinematic constraints may have been modified 17. Thus, the replication of native humerus motions may not be applied under injured or repaired joint conditions without the monitoring of the resulting passive moment 17 to avoid tissue degradation or joint dislocation. The replication of native humerus motions, though, allows assessing whether the injured or repaired joint still permits the requested motion.
This study remains subject to some limitations. First, all specimens used were over 60 years old. While the shoulders were inspected prior to inclusion in terms of degenerative joint disease or previous ligamentous injury, resulting range of motion may be lower than in younger subjects. Still, the humerus elevation amplitudes reported in this study are similar to the ones reported during in vivo studies 32 or cadaveric studies 1. Second, humerus motions did not include muscle contraction. Consequently, bones kinematics, and in particular scapula kinematics, might not be comparable to the in vivo kinematics observed in healthy subjects. However, acromioclavicular joint kinematics observed on cadaveric specimens during passive humerus motions is known to be similar to the joint kinematics measured on healthy participants during active humerus motions 1,33. Furthermore, the present protocol can be compared to intraoperative joint assessment performed by the surgeon, during full muscle relaxation, as suggested by Goldsmith et al. 17. Third, as specimen-specific motions were defined and applied for each shoulder, induced motions may not be perfectly similar between shoulders. Furthermore, without a rigorous humerus mobilisation protocol, the resulting motions may not have been perfectly performed around anatomical axes. This issue can be corrected by defining precisely anatomical axes, for example by applying the recommendations of the ISB 4,12. However, in our case, the goal was more to reproduce intraoperative humerus mobilisations (i.e. not necessarily fully aligned with anatomical axes) than to produce pure rotations around a single axis. Last, the present protocol does not allow for specimen repositioning. The optimised position and orientation applied on the specimen in the manipulator coordinate system remained strictly the same between the manually-induced humerus motions and the following motion replications using the robotic manipulator. Consequently, if the specimen has to be removed and then replaced (e.g. to perform a surgery), the validity of the resulting humerus motion replications can not be ensured. The literature has already proposed some procedures to cover this issue. However, to the best of our knowledge, they were applied on a unique bony segment, during single diarthrodial joint analysis 26. Still, these procedures could be applied on the thorax of the specimens to ensure the correct repositioning of the end of the kinematic chain of the shoulder girdle.