Synthesis of an Orthopedic Cement by the Addition of Anhydrous Calcium Hydrogen Phosphate to Calcium Sulfate Hemihydrate

Abstract

Commercially available calcium sulfate hemihydrate (CSH) powders were mixed with anhydrous calcium hydrogen phosphate (monetite) in order to develop a novel resorbable orthopedic cement. The setting solution, distilled water, converted the CSH into gypsum as proven by an X-ray diffraction. This conversion into gypsum is what gives the cement its high compressive strength. The addition of monetite gives the cement a high bioresorbablity and remodeling ability that is not present in the pure CSH cements on the market today. Instead of merely dissolving into powder when placed in body fluids like CSH cements, the phosphate-enhanced cement in vivo may convert into apatitic bone-like structures.

Introduction

Thanks to the considerable advances in orthopedic surgery in the past twenty years, surgeons today are faced with many different techniques and substances with which to fill and correct bone defects. The most common treatment of choice for these corrections, however, despite new technologies, is autologous bone grafting. This is a process in which the surgeon scrapes bone cells from the patient’s hipbone to place in the defect site. This treatment displays three main characteristics associated with the bone healing process: osteoconduction (the ability of materials to serve as a scaffold for new bone growth), osteoinduction (the ability of materials to promote and/or stimulate the formation of new bone), and osteogeneration (the ability to directly create bone tissue) (Smith & Smith, 2000). However, there are also disadvantages to the bone grafting method. Pain, infection, or nerve damage is reported by 10-25% of all patients and there may not be a sufficient amount of material available at the harvesting site, as often is the case in children. For the surgeon, disadvantages include a much longer operation time (Bloemers, Blokhuis, Patka, Bakker, Wippermann, & Haarman, 2003). Using an artificial bone substitute in place of autologous bone grafting has helped to circumvent these difficulties and various artificial bone substitutes have been developed for this purpose.

Calcium sulfate hemihydrate (CSH) is used as an artificial bone substitute and is the most prevalent orthopedic cement in the United States market today, since its FDA approval in 2001. With the addition of a setting solution of water or saline, the CSH converts into a calcium sulfate dihydrate, more commonly known as gypsum. Gypsum shows good compressive strength results, however it does not maintain that strength for long after it is placed in body fluid. Gypsum in vivo does not withstand the effects of body fluid and dissolves into powder within the first few months after the surgery, before strong new bone can replace it. The optimal cement would uphold the mechanical strength during the stages of cellular resorption in the first three to four months but would be gradually replaced by new bone at the defect site within a year of the operation (Tas, 2004).

When synthesizing a novel orthopedic substitute, one must consider many various factors including toxicity, pH, and body temperature. The elements