Augmentation of pedicle screw fixation with bone cement has been used for many years and experimental studies have demonstrated that various cements such as PMMA, hydroxyapatite, and calcium sulfate are effective for augmentation of the pedicle screw
2,3,4,5,7,8,9,11,22). Among them, PMMA has been becoming the most popular material used due to its simplicity, availability, and cost effectiveness. Moreover, some studies have demonstrated that PMMA has a stronger augmentation power than calcium phosphate and calcium sulfate cement for primary screw augmentation
12,13,17). PMMA has been shown to increase the pull out strength from 96% to 262% and transverse bending stiffness up to 153%
20). Indeed, we made the bulk of the screw with bone cement by injecting bone cement to the spinal cancellous bone; this prevented the "teeter-totter" motion of screws in the spinal pedicles until bone fusion
17). However, bone cement can leak into the epidural space, which can cause an associated myelopathy or radiculopathy. The intravertebral space initially created by the tap screw allows placement of more viscous, partially cured, bone cement. Viscous bone cement can be injected using low pressure with this method. When the cement is injected, the only way for cement to flow is from the middle to posterior and/ or anterior vertebral body. In that circumstance, most of the injected cement will stay in the vertebral body and will not easily flow back from the space between needle and pedicle tract. There-fore, cement leakage from the unrecognized breached pedicle into the spinal canal or intervertebral foramen can be avoided. In addition, the inserted depth of the biopsy needle should be in the middle third of the vertebral body. It is presumed that the injected cement will flow from the middle to anterior or posterior part of the vertebral body, and then the pedicle. Finally, the viscosity of bone cement must be considered. If it is too thick, the injection of bone cement can cause problems. If it is too thin, it may leak into the blood vessels or the neural foramen. Furthermore, bone cement might spread too widely on the vertebral body, and as a result, the fixation strength would be decreased
7,8). In addition, systemic complications including fever, infection, hypoxia and pulmonary embolism have been reported with the use of PMMA
6). There was one case of methicillin-resistant staphylococcus aureus wound infection in Group I that was well controlled. No surgical death, myocardial infarction, catastrophic pulmonary embolism or fat embolism occurred. MacAfee et al
10). reported that PMMA augmentation increased the physical interaction of titanium implants with the contact area of cancellous bone. Ultimately, repetitive cyclic loading in the absence of an effective interbody or posterolateral fusion will result in implant and PMMA loosening and progressive bony erosion, a phenomenon familiar to orthopedic surgeons that apply bone cement in hip replacement surgery. Thus, careful attention to a bony arthrodesis is still necessary until a bioactive or osteoinductive bone cement becomes available. Theoretically, bone fusion is almost always necessary for most patients that have spinal instrumentation; except for patients that have tumor metastasis and have a short life expectancy. Once the fusion is achieved, the fixation force of the spinal instrumentation would not be necessary. In that situation, whether the cement is biodegradable, osteoconductive, or has some negative effect on the bone remodeling is no longer important. By contrast, if nonunion of the spinal fusion occurred, the screw would eventually have problems, whatever the initial rigidity of the fixation force
1). In order to obtain a solid fusion, there must be adequate fixation to immobilize the fusion area while the bone graft heals to the vertebral segments. In this study, none of the patients in Group I had screw loosening, back-out or breakage after a mean of 12 months of follow-up in contrast to Group II. This result suggests that the PMMA augmentation force can adequately withstand and serve to immobilize while bone fusion has been achieved. The limitations of this study include the following. First, one of the concerns in interpreting the results of this procedure was the difficulty in determining fusion success radiographically due to the short follow-up period. It is generally accepted that a solid fusion cannot be confirmed radiographically until 6 to 9 months after surgery. Pseudoarthrosis may be defined in the lumbar spine as a failure of a solid fusion 1 year after surgery
16). It is for that reason that the follow-up period was from the shortest duration of 10 months and the mean time period was 1 year after surgery. However although the clinical results were satisfactory based on the fusion criteria for the follw up period in our study, further investigations including more rigorous selection of patients and long term follow-up are needed. Second, in spite of the successful results, other factors that might depend on screw fixation strength such as screw diameter or insertion techniques were not considered in this study. Moreover, bone cement augmentation may cause serious complications related to cement leakage such as motor weakness and hypoesthesia similar to the patients in this study. Hence, the use of PMMA must be monitored carefully in spite of its advantages.