No abstract

Fixation failure of glenoid components is the main cause of unsuccessful total shoulder arthroplasties. The characteristics of these failures are still not well understood, hence, attempts at improving the implant fixation are somewhat blind and the failure rate remains high. This lack of understanding is largely due to the fundamental problem that direct observations of failure are impossible as the fixation is inherently embedded within the bone.Twenty custom made implants, reflecting various common fixation designs, and a specimen set-up was prepared to enable direct observation of failure when the specimens were exposed to cyclic superior loads during laboratory experiments. Finite element analyses of the laboratory tests were also carried out to explain the observed failure scenarios.All implants, irrespective of the particular fixation design, failed at the implant–cement interface and failure initiated at the inferior part of the component fixation. Finite element analyses indicated that this failure scenario was caused by a weak and brittle implant–cement interface and tensile stresses in the inferior region possibly worsened by a stress raiser effect at the inferior rim.The results of this study indicate that glenoid failure can be delayed or prevented by improving the implant/cement interface strength. Also any design features that reduce the geometrical stress raiser and the inferior tensile stresses in general should delay implant loosening.

Background: The glenoid component in reverse total shoulder arthroplasty is recommended to be positioned inferiorly or with a downward tilt with the intention of reducing scapular notching. However, it is still unclear whether modifying the position of the glenoid prosthesis affects implant stability. The aim of this study was to determine the association between implant positioning and glenoid prosthesis fixation using Grammont reverse total shoulder arthroplasty. Methods: Four positions for the glenoid prosthesis were studied using the finite element method. The glenosphere was positioned as follows: 1) in the middle of the glenoid fossa, 2) flush with the inferior glenoid rim, 3) with an inferior overhang, 4) with a 15° inferior inclination. Bone-prosthesis micromotions and strain-induced bone adaptations were quantified during five daily activities. Findings: When the glenoid component was tilted inferiorly, the activities producing anteriorposterior shear forces (e.g. standing up from an armchair) caused an increase in peak micromotions. In the lateral-middle glenoid, inferior positioning caused a 64.6% reduction in bone apparent density. In the lateral-inferior glenoid, central positioning led to the most severe bone resorption, reaching 43.9%. Interpretation: Reducing activities which generate anterior-posterior shear forces on the shoulder joint will increase bone formation and may improve the primary stability of the implant when fixed in the position with an inferior tilt. Postoperative bone resorption is highly dependent on implant positioning. Understanding the relationship between bone resorption and implant positioning will help surgeons improve the long-term stability of reverse total shoulder arthroplasty.

Background: The high incidence of scapular notching in reverse total shoulder arthroplasty (RTSA) has spurred several methods to minimize the bone loss. However, up to 93 % of RTSA with accompanying scapular notching have been reported to maintain good implant stability for over 10 years. The purpose of this study is to investigate the correlation between scapular notching and glenoid fixation in RTSA.Methods: An in-vitro setup was used to measure the notch-induced variations of the strain on the scapular surface and the micromotion at the bone-prosthesis interface during arm abductions of 30°, 60° and 90°. Finite element analysis (FEA) was used to study the bone and screw stresses as well as the bone-prosthesis micromotion in cases of a grade 4 notch during complicated arm motions.Results: The notch resulted in an apparent increase of inferior screw stress in the root of the screw cap and the notch-screw conjunction. However, the maximal stress (172 MPa) along the screw after notch is still much less than the fatigue strength of the titanium screw (600 MPa) under cyclic loading. The bone-prosthesis micromotion results did not present significant notch-induced variations.Conclusions: Scapular notching will not lead to significant effects on the initial stability of glenoid component in RTSA. This finding may explain the long-term longevity of RTSA in cases of severe scapular notching. The relationship between scapular notching and weak regions along the inferior screw may explain why fractures of the inferior screw are sometimes reported in patients with RTSA clinically.