Introduction Undersizing of an uncemented femoral stem is a post-operative description of a radiograph with unclear clinical importance. It may not always translate into a suboptimal clinical outcome. Aim To describe a novel classification system of undersizing in a fully hydroxyapatite coated femoral implant, assess if it could be reliably reported between observers, and describe a simple way to assess the degree of undersizing on an AP radiograph. Methods A retrospective review of 1000 consecutive total hip arthroplasties was conducted. Two investigators reviewed post-operative radiographs independently for stem undersizing. The data was tested for inter and intra-observer agreement using the Kappa method. Results Undersizing was classified as: Varus – Stem in contact proximally with medial calcar, and distally with lateral cortex. Valgus – Stem in contact proximally with lateral cortex, and distally with medial cortex. Uniform – Continuous margin around the stem of at least one-half tip width of the stem throughout. Champagne flute – Distal stem fills the narrow diaphysis with insufficient bony contact at the wide metaphysis. If varus or valgus undersizing was present we described the formation of two triangles of excess cancellous space around the implant, each having overlapping apices signifying undersizing by at least two sizes. 32% of the stems were undersized with Kappa demonstrating very good (89%) to good (86%) inter and intra observer agreement respectively. 50% of undersized stems were uniformly undersized, 36% were varus, 6% valgus, and 8% were in a champagne fluted shaped femur. A higher percentage of the lateralised (KLA) stems were undersized compared to other neck configurations (40%). 54% of these were varus undersized and this increased the offset up to 2cm. Conclusion We describe a classification system for undersizing in hip arthroplasty. KLA stems have an increased tendency to be undersized in varus, thereby increasing the overall offset

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