Basic science for FRCS (Tr & Orth)

This tests the area moment of inertia. For a solid cylinder the area moment of inertia is πr⁴/4, so bending rigidity (and, through the polar moment, torsional rigidity) rises with the fourth power of the radius: moving from a 10 mm to a 12 mm nail roughly doubles rigidity. The third power describes how the bending stiffness of a plate changes with its thickness, which is the usual source of confusion. Linear and squared relationships describe axial stiffness, which depends on cross-sectional area. Hollow nails follow the same principle using outer and inner radii. Reference: Ramachandran, Basic Orthopaedic Sciences 2e.

Bioabsorbable implants are made of alpha-hydroxy acid polyesters such as polyglycolic acid and poly-L-lactic acid. Their ester bonds are split by hydrolysis, without the need for enzymes or cells, into lactic or glycolic acid, which enters the citric acid cycle and is excreted as carbon dioxide and water; macrophages then clear the remaining fragments. Polyglycolic acid degrades within months and is associated with sterile effusions and sinus formation, whereas poly-L-lactic acid takes years. Osteoclasts resorb calcium phosphate ceramics, not polymers. Proteolysis breaks down collagen-based materials. Oxidation degrades polyethylene, and ionic dissolution describes calcium sulphate and metals. Reference: Miller's Review of Orthopaedics 8e.

This tests the RANK, RANKL and osteoprotegerin axis. Osteoblasts and osteocytes express RANKL, which binds RANK on osteoclast precursors and drives their differentiation, activation and survival. Osteoprotegerin is a soluble decoy receptor, also made by osteoblasts, that binds RANKL and prevents it from reaching RANK; denosumab is a monoclonal antibody to RANKL and so does the same job. Macrophage colony-stimulating factor is the other signal needed for osteoclast formation and promotes resorption. Sclerostin and Dickkopf-1 inhibit Wnt signalling in osteoblasts and restrain formation, not resorption. Osteocalcin is a matrix protein and a marker of bone formation. Reference: Miller's Review of Orthopaedics 8e.

The choice of test depends on the type of data, its distribution, the number of groups and whether they are paired. Here an ordinal, skewed outcome is compared between two independent groups, which calls for the Mann–Whitney U test, the non-parametric counterpart of the unpaired t-test. The unpaired t-test assumes a normally distributed continuous variable and is unreliable with skewed data in small samples. The Wilcoxon signed-rank test is the closest distractor but is for paired observations, such as scores before and after surgery in the same patients. Kruskal–Wallis compares three or more independent groups, and chi-squared compares proportions of categorical data. Reference: Ramachandran, Basic Orthopaedic Sciences 2e.