Original Article Proc IMechE Part H: J Engineering in Medicine 1–11 Ó IMechE 2019 Article reuse guidelines: sagepub.com/journals-permissions DOI: 10.1177/0954411919827984 journals.sagepub.com/home/pih Biomechanical comparison of screw-based zones of a spatial subchondral support plate for proximal humerus fractures Ali Jabran 1 , Chris Peach 1,2 , Zhenmin Zou 1 and Lei Ren 1 Abstract Stabilisation of proximal humerus fractures remains a surgical challenge. Spatial subchondral support (S3) plate promises to overcome common complications associated with conventional proximal humerus plates. This study compared the biomechanical performance of S3 plate with a fixed-angle hybrid blade (Equinoxe Fx) plate and a conventional fixed-angle locking plate (PHILOS). The effects of removal of different S3 plate screws on the humeral stability were also investi- gated. A total of 20 synthetic left humeri were osteotomised transversely at the surgical neck to simulate a two-part fracture and were each treated with an S3 plate. Head screws were divided into three zones based on their distance from the fracture site. Specimens were divided into four equal groups where one group acted as a control with all screws and three groups had one of the screw zones missing. With humeral head fixed, humeral shaft was first displaced 5mm in extension, flexion, valgus and varus direction (elastic testing) and then until 30mm varus displacement (plastic testing). Load–displacement data were recorded to determine construct stiffness in elastic tests and assess specimens’ varus stability under plastic testing. Removal of the screw nearest to the fracture site led to a 20.71% drop in mean elas- tic varus bending stiffness. Removal of the two inferomedial screw above it resulted in a larger drop. The proximal screw pair had the largest contribution to extension and flexion bending stiffness. Varus stiffness of S3 plate constructs was higher than PHILOS and Fx plate constructs. Stability of humeri treated with S3 plate depends on screws’ number, orien- tation and location. Varus stiffness of S3 plate construct (10.54N/mm) was higher than that of PHILOS (6.61N/mm) and Fx (7.59N/mm) plate constructs. We attribute this to S3 plates’ thicker cross section, the 135° inclination of its screws with respect to the humeral shaft and the availability of pegs for subchondral support. Keywords Biomechanical testing, proximal humerus fractures, plate fixation, spatial subchondral support, locking plate Date received: 29 July 2018; accepted: 12 December 2018 Background Fractures of the proximal humerus are the third most common fractures in the over-65 patient population, and their stable fixation remains a key challenge in orthopaedic and trauma surgery. 1 With the ageing pop- ulation and increasing cases of osteoporosis, the inci- dence of proximal humerus fracture is predicted to triple over the next 30 years. 2 While the majority of fracture cases are minimally displaced and can be man- aged non-operatively to achieve satisfactory outcomes, complex cases require surgical intervention. 3,4 Open reduction internal fixation using plate and screw has become a well-known treatment modality in the past few decades, especially since the advent of locking tech- nology. PHILOS (Proximal Humerus Internal Locking System; Synthes, Paoli, PA, USA) is one of the widely available proximal humerus fixation plates. It offers insertion of fixed-angle locking screws along multiple directions to enhance structural stability. Biomechanical studies on locking plates report superior torsion, bending and axial compression stiff- ness as compared to non-locking plates. 5–7 Clinical 1 School of Mechanical, Aerospace and Civil Engineering, The University of Manchester, Manchester, UK 2 Department of Shoulder and Elbow Surgery, University Hospital of South Manchester, Manchester, UK Corresponding author: Lei Ren, School of Mechanical, Aerospace and Civil Engineering, The University of Manchester, Sackville Street, Manchester M13 9PL, UK. Email: lei.ren@manchester.ac.uk