Calcium Phosphate Cement Cranioplasty: Clinical and Radiographic Follow-up

摘要

The use of hydroxyapatite cements to replace bone deficits has entered widespread practice in craniofacial surgery. Although these substances have been shown to be at least partially replaced by bone in some experimental models, their long-term outcome in human cranial replacement or augmentation is not known. This study evaluated eight patients who had undergone cranioplasty using a hydroxyapatite cement. The cement was used in both inlay and onlay fashion. The mean follow-up interval was nineteen months and ranged from eight months to twenty-seven months. All patients underwent repeat CT scanning at the time of latest follow-up. Only one significant complication was noted, consisting of a postoperative persistent seroma that was resistant to repeat needle aspiration and required open drainage and removal of the material. CT scans revealed variable resorption of the cement in three cases. In the two cases in which the material was used primarily as an inlay, significant fragmentation of the material was seen. The appearance of the junction of the cement and the native bone was variable. While in some cases this interface could not be distinguished, in other areas it was seen as a distinct lucency. Although clinically minor asymmetries were noted in the appearance of the cranial vault and forehead, no patient was felt to be in need of revisional surgery. Even in areas of fragmentation, the reconstructions felt solid without evidence of mobility. No patient developed an infection or extruded the material. This study, though not providing histologic evidence, does seem to indicate that bone cements are well tolerated but are not always successfully firmly incorporated into the host bone. Introduction Hydroxyapatite is the primary mineral component of bone, and is composed of interconnected calcium phosphate molecules (Ca10(PO4)6(OH2) (1,2,3). In the past, hydroxyapatite implants were primarily in the ceramic form (2). These ceramic implants are created in a process known as sintering, in which the calcium phosphate is heated at very high temperatures (3). Although such ceramic implants can be manufactured in any of a variety of shapes, intraoperative shaping is limited to reducing the size of the implant by burring. Consequently, nonceramic forms of hydroxyapatite have been developed. Rather than require heating for the crystallization process, these products crystallize when mixed with an aqueous solution, most commonly containing phosphate (2,3). As the implant crystallizes, it takes on a putty-like consistency and can be contoured to any defect. Typically, within ten to fifteen minutes, the implant completes the process and hardens, securing its position within the defect. An issue with these implants is their porosity. Pores of 100 micron diameter are thought to be necessary for bone ingrowth (3). These nonceramic forms of hydroxyapatite are typically microporous, limiting bone ingrowth. Although porous forms of hydroxyapatite have demonstrated significant evidence of bone ingrowth when placed in inlay fashion (10), there has been little evidence for this with the ceramic forms (3,5). There has however been little study regarding the differences in this property when the cement is used as an onlay versus an inlay. Bone ingrowth would intuitively be optimized in inlay situations when the cement was surrounded by vascularized bone, as opposed to onlay cases when the implant may be positioned on the surface of relatively sclerotic bone. Another issue not addressed in the literature is the behavior of these substances in the growing cranial vault. As these cements are frequently used as an onlay in children with residual contour defects following cranial vault remodeling, the interaction of the bone cement and host bone over time is quite relevant. This study evaluated these issues in a group of children augmented with onlay and inlay bone cement for residual contour deformities or defects following previous surgery for craniosy