Passage Bone growth and remodeling during fracture healing are often slow and have potentially poor outcomes, including malunion or nonunion at the fracture site. Magnesium bone implants can aid in bone formation and safely biodegrade in the body according to the following reaction:Mg(s) + 2H 2 O(l) → Mg(OH) 2 (s) + H 2 (g)Reaction 1 Corrosion of magnesium metal in aqueous mediumHowever, magnesium metal tends to degrade too quickly and forms hydrogen gas pockets in the body. Corrosion is slowed as the metal naturally forms a protective layer of magnesium hydroxide in a process called passivation. Magnesium hydroxide is less soluble than magnesium metal, and its dissolution is described by the endothermic reaction: Reaction 2 Solubility equilibrium of Mg(OH) 2 in water at 25°CAlthough passivation slows down the corrosion rate, it must be slowed even further to adequately facilitate bone reformation.Researchers are interested in the differences in the corrosion rates of magnesium metal protected by different biocompatible coatings. They compared 10-mm magnesium disks coated with compounds of various cation and anion combinations, with solubility results shown in Table 1. The coated disks were immersed in simulated body fluid (SBF) at room temperature for 50 hours.Table 1 Solubility for Biocompatible Inorganic Salts Given in mmol/100mL of Pure Water at 25°C Adapted from Huan ZG, Leeflang MA, Zhou J, Fratila-apachitei LE, Duszczyk J. In vitro degradation behavior and cytocompatibility of Mg-Zn-Zr alloys. J Mater Sci Mater Med. 2010;21(9):2623-35. -Magnesium corrodes quickly in the body because Mg²⁺ combines with anions other than OH⁻ to form soluble salts. Based on the solubility data given in Table 1, which of the following salts, if present, will cause the magnesium implant to corrode most quickly?

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11ecba2d_11a8_4dde_a3bf_b5a9cd7f5426_MD0008_00 Metals such as magnesium, in the presence of anions with which they form soluble salts, will corrode more quickly than they will in pure water. Based on the anions in the answer choices, the possible magnesium salts are MgF₂, Mg₃(PO₄)₂, Mg(OH)₂, and MgCl₂. Solubility tables provide information on the relative solubilities of various combinations of cations and anions. According to Table 1, MgCl₂ is substantially more soluble than magnesium with any other available anion. Therefore, magnesium will corrode much more readily in the presence of chloride anions, provided by NaCl, than it will in the presence of any other anion. In fact, the NaCl present in blood plasma is a major factor contributing to the high corrosion rate of magnesium implants in the body. (Choice A) The anion in calcium phosphate is PO₄³⁻ and, based on Table 1, Mg₃(PO₄)₂ has very low solubility. In addition, because calcium phosphate does not readily dissolve, very few PO₄³⁻ ions would be available in solution to interact with Mg²⁺. (Choice B) Calcium hydroxide would produce hydroxide ions, which are already part of magnesium hydroxide. No new salt would form. (Choice D) Magnesium fluoride, which would form in the presence of NaF, is several orders of magnitude less soluble than magnesium chloride. Educational objective: The rate of metal corrosion increases in the presence of anions with which the metal forms soluble salts. Solubility tables provide information on the relative solubilities of potential salts to be formed. Greater solubility corresponds to a faster corrosion rate.

Passage Bone Growth and Remodeling During Fracture Healing Are Often Slow