Stemless prostheses are recognized to be an effective solution for anatomic total shoulder arthroplasty (TSA) while providing bone preservation and shortest operating time. Reverse shoulder arthroplasty (RSA) with stemless has not showed the same effectiveness, as clinical and biomechanical performances strongly depend on the design. The main concern is related to stability and bone response due to the changed biomechanical conditions; few studies have analyzed these effects in anatomic designs through Finite Element Analysis (FEA), however there is currently no study analyzing the reverse configuration. Additionally, most of the studies do not consider the effect of changing the neck-shaft angle (NSA) resection of the humerus nor the proper assignment of spatial bone properties to the bone models used in the simulations. The aim of this FEA study is to analyze bone response and primary stability of the SMR Stemless prosthesis in reverse with two different NSA cuts and two different reverse angled liners, in bone models with properties assigned using a quantitative computed tomography (QCT) methodology. Sixteen fresh-frozen cadaveric humeri were modelled using the QCT-based finite element methodology. The humeri were CT-scanned with a hydroxyapatite phantom to allow spatial bone properties assignment [Fig. 1]. Two implanted SMR stemless reverse configurations were considered for each humerus: a 150°-NSA cut with a 0° liner and a 135°-NSA cut with a 7° sloped liner [Fig. 2]. A 105° abduction loading condition was simulated on both the implanted reverse models and the intact (anatomic) humerus; load components were derived from previous dynamic biomechanical simulations on RSA implants for the implanted stemless models and from the OrthoLoad database for the intact humeri. The postoperative bone volume expected to resorb or remodel [Fig. 3a] in the implanted humeri were compared with their intact models in sixteen metaphyseal regions of interest (four 5-mm thick layers parallel to the resection and four anatomical quadrants) by means of a three-way repeated measures ANOVA followed by post hoc tests with Bonferroni correction. In order to evaluate primary stability, micromotions at the bone-Trabecular Titanium interface [Fig. 3b] were compared between the two configurations using a Wilcoxon matched-pairs signed-rank test. The significance level α was set to 0.05.Background
Methods
Total Elbow Arthroplasty (TEA) is recognized as an effective treatment solution for patients with rheumatoid arthritis or for traumatic conditions. Current total elbow devices can be divided into linked or unlinked design. The first design usually presents a linking element (i.e. an axle) to link together the ulnar and humeral components to stabilize the joint; the second one does not present any linkage and the stability is provided by both intrinsic design constraints and the soft tissues. Convertible modular solutions allow for an intraoperative decision to link or unlink the prosthesis; the modular connections introduce however additional risks in terms of both mechanical strength and potential fatigue and fretting phenomena that may arise not only due to low demand activities loads, but also high demand (HD) ones that could be even more detrimental. The aim of this study was to assess the strength of the modular connection between the axle and the ulnar component in a novel convertible elbow prosthesis design under simulated HD and activities of daily living (ADLs) loading. A novel convertible total elbow prosthesis (LimaCorporate, IT) comprising both ulnar and humeral components that can be linked together by means of an axle, was used. Both typical ADLs and HD torques to be applied to the axle were determined based on finite element analysis (FEA); the boundary load conditions for the FEA were determined based on kinematics analysis on real patients in previous studies. The FEA resultant moment acting on the axle junction during typical ADLs (i.e. feeding with 7.2lbs weight in hand) was 3.2Nm while for HD loads (i.e. sit to stand) was 5.7 Nm. In the experimental setup, 5 axle specimens coupled with 5 ulnar bodies through a tapered connection (5 Nm assembly torque) were fixed to a torque actuator (MTS Bionix) and submerged in a saline solution (9g/l). A moment of 3.2 Nm was applied to the axle for 5M cycles through a fixture to test it under ADLs loading. After 5M cycles, the axles were analyzed with regards to fretting behavior and then re-assembled to test them against HD loading by applying 5.7 Nm for 200K cycles (corresponding to 20 years function).Introduction
Methods
The interaction between the mobile components of total elbow replacements (TER) provides additional constraint to the elbow motion. Semi-constrained TER depend on a mechanical linkage to avoid dislocation and have greater constraint than unconstrained TER that rely primarily in soft tissue for joint stability. Greater constraint increases the load transfer to the implant interfaces and the stresses in the polyethylene components. Both of these phenomena are detrimental to the longevity of TER, as they may result in implant loosening and increased damage to the polyethylene components, respectively[1]. The objective of this work was to compare the constraint profile in varus-valgus and internal-external rotation and the polyethylene stresses under loads from a common daily activity between two semi-constrained TER, Coonrad/Morrey (Zimmer-Biomet) and Discovery® (DJO), and an unconstrained TER, TEMA (LimaCorporate). We developed finite element (FE) models of the three TER mechanisms. To reduce computational cost, we did not include the humeral and ulnar stems. Materials were linear-elastic for the metallic components (ETi6Al4V=114.3 GPa, ECoCr=210 GPa, v=0.33) and linear elastic-plastic for the polyethylene components (E=618 MPa, v=0.46; SY=22 MPa; SU=230.6 MPa; εU=1.5 mm/mm). The models were meshed with linear tetrahedral elements of sizes 0.4–0.6 mm. We assumed a friction coefficient of 0.02 between metal and polyethylene. In all simulations, the ulnar component was fixed and the humeral component loaded. We computed the constraint profiles in full extension by simulating each mechanism from 8° varus to 8° valgus and from 8° internal to 8° external rotation. All other degrees-of-freedom except for flexion extension were unconstrained. Then, we identified the instant during feeding that generated the highest moments at the elbow[2], and we applied the joint forces and moments to each TER to evaluate the stresses in the polyethylene. To validate the FE results, we experimentally evaluated the constraint of the design with highest polyethylene stresses in pure internal-external rotation and compared the results against those from a FE model that reproduced the experimental setup (Fig.1-a).Introduction
Methods
Reverse Shoulder Arthroplasty (RSA) is recognized to be an effective solution for rotator cuff deficient arthritic shoulders, but there are still concerns about impingement and range of motion (ROM). Several RSA biomechanical studies have shown that humeral lateralization can increase ROM in planar motions (e.g. abduction). However, there is still a debate whether humeral lateralization should be achieved with a larger sphere diameter or by lateralizing the center of rotation (COR). The latter has shown to decrease the deltoid moment arm and increase shear forces, where the former may pose challenges in implanting the device in small patients. The aim of this study was to evaluate how humeral lateralization achieved by varying COR lateral offset and glenosphere diameter in a reverse implant can affect impingement during activities of daily living (ADLs). Nine shoulder CT scans were obtained from healthy subjects. A reverse SMR implant (LimaCorporate, IT) was virtually implanted on the glenoid and humerus (neck-shaft angle 150°) as per surgical technique using Mimics software (Materialise NV). Implant positioning was assessed and approved by a senior surgeon. The 3D models were imported into a validated shoulder computational model (Newcastle Shoulder Model) to study the effects of humeral lateralization. The main design parameters considered were glenosphere diameter (concentric Ø36mm, Ø40mm, Ø44mm) and COR offset (standard, +2mm, +5mm), for a total of 9 combinations for each subject; −10°, 0° and 10° humeral components versions were analyzed. The model calculated the percentage of impingement (intra-articular, contact of cup with scapula neck and glenoid border; extra-articular, contact of humerus with acromion and coracoid) during 5 ADLs (hand to opposite shoulder, hand to back of head, hand to mouth, drink from mug and place object to head height).Introduction
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