Summary Statement. In this study, excellent positioning of custom-made glenoid components was achieved using patient-specific guides. Achieving the preoperatively planned orientation of the component improved significantly and more screws were located inside the scapular bone compared to implantations without such guide. Introduction. Today's techniques for total or reverse shoulder arthroplasty are limited when dealing with severe glenoid defects. The available procedures, for instance the use of bone allografts in combination with available standard implants, are technically difficult and tend to give uncertain outcomes (Hill et al. 2001; Elhassan et al. 2008; Sears et al. 2012). A durable fixation between bone and implant with optimal fit and implant positioning needs to be achieved. Custom-made defect-filling glenoid components are a new treatment option for severe glenoid defects. Despite that the patient-specific implants are uniquely designed to fit the patient's bone, it can be difficult to achieve the preoperatively planned position of the component, resulting in less optimal screw fixation. We hypothesised that the use of a patient-specific guide would improve implant and screw positioning. The aim of this study was to evaluate the added value of a newly developed patient-specific guide for implant and screw positioning, by comparing glenoid implantations with and without such guide. Patients & Methods. Large glenoid defects, representative for the defects encountered in clinical practice, were created in ten cadaveric shoulders. A CT scan of each cadaver was taken to evaluate the defects and to generate three-dimensional models of the scapular bones. Based on these models, custom glenoid components were designed. Furthermore, a newly developed custom guide was designed for five randomly selected shoulders. New CT scans were taken after implantation to generate 3D models of the bone and the implanted component and screws. This enabled to compare the experimentally achieved and preoperatively planned reconstruction. The location and orientation of the glenoid component and screw positioning were determined and differences with the optimal preoperative planning were calculated. Results. An excellent component positioning (difference in location: 1.4±0, 7mm; difference in orientation: 2, 5±1, 2°) was achieved when using the guide compared to implantations without guidance (respectively 1, 7±0, 5mm; 5, 1±2, 3°). The guide improved component orientation significantly (P<0.1). After using the guide, all screws were positioned inside the scapular bone whereas 25% of the screws placed without guidance were positioned outside the scapular bone. Discussion/Conclusion. In this study, excellent positioning of custom-made glenoid components was achieved using patient-specific guides. Achieving the preoperatively planned orientation of the component improved significantly and more screws were located inside the scapular bone compared to implantations without such guide

Introduction. Glenoid loosening, still a main complication for shoulder arthroplasty, was suggested to be related implant design, surgical aspects, and also bone quality. However, typical studies of fixation do not account for heterogeneity in bone morphology and density which were suggested to affect fixation failure. In this study, a combination of cyclic rocking horse tests on cadaver specimens and microCT-based finite element (microFE) analysis of specimens of a wide range of bone density were used to evaluate the effects of periprosthetic bone quality on the risks of loosening of anatomical keeled or pegged glenoid implants. Methods. Six pairs of cadaveric scapulae, scanned with a quantitative computer tomography (QCT) scanner to calculate bone mineral density (BMD), were implanted with either cemented anatomical pegged or keeled glenoid components and tested under constant glenohumeral load while a humeral head component was moved cyclically in the inferior and superior directions. Edge displacements were measured after 1000, 4000 and 23000 test cycles, and tested for statistical differences with regards to changes or implant design. Relationships were established between edge displacements and QCT-based BMD below the implant. Four other specimens were scanned with high-resolution peripheral QCT (82µm) and implanted with the same 2 implants to generate virtual models. These were loaded with constant glenohumeral force, varying glenohumeral conformity and superior or inferior load shifts while internal stresses at the cement-bone and implant-cement interfaces were calculated and related to apparent bone density in the periprosthetic zone. Results. Mean displacements at the inferior and superior edges showed no statistical difference between keeled and pegged designs (p>0.05). Compression and distraction were however statistically different from the initial reference measurement at even 1000 and 4000 cycles for both implant designs (p<0.05). For both implant designs, superior and inferior distractions were generally highest at each measurement time in specimens where BMD below the lifting edge was lower, showing a trend of increased distraction with decreased BMD. Moreover, the microFE models predicted higher bone and cement stresses for specimens of lower apparent bone density. Finally, highest peak stresses were located at the cement-bone interface, which seemed the weaker part of the fixation. Discussion. With this combined experimental and numerical study, it was shown that implant distraction and stresses in the cement layer are greater in glenoids of lower bone density for both implant designs. This indicates that fixation failure will most likely occur in bone of lower density, and that fixation design itself may play a secondary role. These results have important impact for understanding the mechanisms of glenoid component failure, a common complication of total shoulder arthroplasty