Structural stability of different reconstruction techniques following total sacrectomy: A biomechanical study
Introduction
Total sacrectomy is a treatment option for highly aggressive diseases such as sacral vertebral carcinoma and tuberculosis. Following total sacrectomy, the continuity between the spine and pelvis must be resored. To achieve structural stability necessary for ambulation, it is critical to maintain the spino-pelvic physical bearing bow and overall frame motion stiffness on the coronal and sagittal planes.
Pedicle screw-rod construct, represented by the Galveston L-rod technique, initially developed by Allen in 1978, is the most frequently used technique for re-establishing the connectivity between the spine and pelvis. Based on the use of two rods with pedicle screw fixation from L3 to L5, various techniques have been developed. These techniques could be grouped into two main categories: the triangular frame reconstruction (TFR) (Murakami et al., 2002) and modified Galveston reconstruction (MGR) (Gokaslan et al., 1997). Murakami et al. (2002) compared the two methods in a mechanical study, and concluded that with TFR, there is excessive stress on the iliac bone around the sacral rod, which in turn could cause loosening of the sacral rod. With MGR, excessive stress concentrates at the spinal rod between the spine and pelvis could result in failure of the spinal rod. In recent years, various new reconstruction techniques have been developed. These techniques include sacral rod reconstruction (SRR) (Kawahara et al., 2003), bilateral fibular flap reconstruction (BFFR) (Dickey et al., 2005), and four-rod reconstruction (FRR) (Shen et al., 2006). These techniques provide the destabilized spino-pelvis with solid construction, but the rate of fixation failure (e.g., fixation breakage and loosening) still remains at a high level. Kawahara et al. (2003) compared the efficacy of SRR with TFR and MGR following total sacrectomy in theoretical biomechanical study, but not in human cadaveric study. To date, biomechanical stability of spino-pelvis after treatment with these different techniques has not been studied in human cadaver.
We recently developed an improved compound reconstruction (ICR) technique. A custom-designed experimental rig was employed to imitate the human physiological standing state. The impact of ICR on structural stiffness following total sacrectomy was compared with three classical techniques (SRR, BFFR and FRR). The results from the current study indicated that ICR is a better option after total sacrectomy.
Section snippets
Specimen preparation
Six fresh human cadaveric L2-pelvis-femora specimens (3 men, 3 women; age: 28–76 years) were used in the current study. All cadavers were radiographically examined and the specimens with anatomical abnormalities and/or gross osteopenia were excluded. Finally there was no unqualified specimen upon radiographic examination. Soft bone tissues were carefully removed. The ligaments, bones and discs were preserved.
To avoid force deflection during compression loading, a round polyester resin platform
Axial compressive stiffness
The mean axial compressive stiffness in the intact condition was 301.1 (242.4) N/mm. Significant differences among five different conditions were noticed under 500 N compressive loading (P = 0.02). The axial compressive stiffness values of the reconstructed specimens following total sacrectomy by SRR, BFFR, FRR and ICR techniques were 46.7%, 52.9%, 18.0%, and 74.9%, respectively, of the intact condition. The stiffness under SRR and FRR was significantly lower than that under the intact condition (P
Discussion
Since the classical Galveston L-rod technique was applied in reconstruction following total sacrectomy, controversy has remained with regards to hollows around screws and rods (King et al., 2000, Moshirfar et al., 2005), rod fracture (Jackson and Gokaslan, 2000), and loosening (Spiegel et al., 1999). A variety of techniques were proposed, but optimal methods have not been established, partly due to lack of biomechanical evidence. Except that, the contrast test was once used to compare the SRR
Conclusions
Following total sacrectomy, ICR produces optimal structural stability in axial stiffness, anti-abduction in the coronal plane and anti-rotation in the sagittal plane, and may be associated with low risk of instrumentation failure. FRR produces least structural stability among the four techniques included in this study.
Acknowledgments
This study was supported by the Program for New Century Excellent Talents at the University of China (NCET-06-0375).
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