Structural stability of different reconstruction techniques following total sacrectomy: A biomechanical study

Abstract

Background

The biomechanical stability of spino-pelvis structure after varying reconstruction methods following total sacrectomy remains poorly defined. The objective of this study was to compare the structural stability of different reconstruction techniques.

Methods

Six fresh human cadavers (L2-pelvis-femora) were used to compare biomechanical stability after reconstruction using four different techniques: (1) sacral rod reconstruction; (2) bilateral fibular flap reconstruction; (3) four-rod reconstruction; and (4) improved compound reconstruction. After total sacrectomy, the construction was carried out using each method once in each cadaver. Structural stiffness was evaluated by linear and angular ranges of motion. L5 relative shift-down displacement, abduction angle on the coronal plane and rotation angle on the sagittal plane, were calculated based on displacement of the identification point under 500 N axial loading. Overall stiffness was estimated using load displacement curve.

Findings

Improved compound reconstruction resulted in significantly higher stiffness than all three other techniques. The structural stability following bilateral fibular flap reconstruction was superior to that after sacral rod reconstruction. Four-rod reconstruction achieved worst stability due to the lack of anterior bracing applied in three other methods.

Interpretation

Improved compound reconstruction produces optimal structural stability after total sacrectomy. This finding suggests that both anterior bracing and alternation of screw trajectory are important in achieving optimal structural stability.

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.