Purpose: When we treat burst fractures, we try to preserve the movable vertebra as much as possible and see to it that the instrument can be extracted finally.

We have performed short-segment posterior spinal instrumentation and fusion (PSIF) for cases with no neurological symptoms, and combined short-segment posterior spinal instrumentation and fusion with anterior decompression and fusion (PSIF with AF) for cases with obvious neurological symptoms.

In this report, we review the postoperative results of our methods.

Methods: We have operated on 18 cases of burst fracture in the past seven years, eleven of them, who had been treated with PSIF (attachment of one level above the fracture to one level below the fracture), were selected for the subjects of the investigation. They consisted of 7 males and 4 females. The average age was 42 years and the mean follow-up of the postoperative image findings was one year and six months (range, eight months to two years and ten months). The number of the cases by traumatic ascensus was T11: 1, T12: 2, L1: 5, L2: 2. Among them, seven cases underwent PSIF. All the cases were operated on with a pedicle screw in combination with a hook. Four cases underwent PSIF with AF. In these cases, only a pedicle screw was used for the posterior, and only the bone transplantation after decompression was done for the anterior. For all cases, the angulation, alignment, and compliance were measured and examined before and after the operation using lateral radiographs, in addition to degree of improvement in the neurological symptoms.

Result: Preoperatively, the results of these cases showed that PSIF and PSIF with AF tend to cause larger damage to all of angulation, alignment, and compliance. Postoperatively, the difference in values between PSIF and PSIF with AF was small, and both groups maintained their respective values even with time. Based on the preoperative Frankel classification, the numbers of the cases undergoing PSIF were C: 1, D: 3, and E: 3. The numbers of the cases undergoing PSIF with AF were B: 1, C, 2, and E: 1. Improvement of one stage was seen in three cases undergoing PSIF. Improvement of two stages was seen in two cases undergoing PSIF with AF. No case showed postoperative deterioration of the neurological symptoms.

Conclusion: We perform PSIF with no neurological symptoms. In these cases, a pedicle screw and a hook are installed in the same vertebral body and arch to reduce the load on the pedicle screw and prevent the pedicle screw damage. AF is performed together with PSIF, and decompression is done surely for cases with obvious neurological symptoms. In these cases, a pedicle screw is used solely and no hook is used because there is a transplant bone as the prop in the anterior. At the moment, we cannot refer to the occurrence of kyphosis transformation in the future for lack of sufficient cases and length of the observation period. However, it was suggested that cases with no neurological symptoms could be treated with PSIF solely.

We have examined the process of fusion of the intertransverse processes and bone graft in the rabbit by in situ hybridisation and evaluated the spatial and temporal expression of genes encoding pro-α1 (I) collagen (COL1A1), pro-α1 (II) collagen (COL2A1) and pro-α1 (X) collagen (COL10A1).

Beginning at two weeks after operation, osteogenesis and chondrogenesis occurred around the transverse process and the grafted bone at the central portion of the area of the fusion mass. Osteoblasts and osteocytes at the newly-formed woven bone expressed COL1A1. At the cartilage, most chondrocytes expressed COL2A1 and some hypertrophic chondrocytes COL10A1. In some regions, co-expression of COL1A1 and COL2A1 was observed. At four weeks, such expressions for COL1A1, COL2A1 and COL10A1 became prominent at the area of the fusion mass. From four to six weeks, bone remodelling progressed from the area of the transverse processes towards the central zone. Osteoblasts lining the trabeculae expressed a strong signal for COL1A1. At the central portion of the area of the fusion mass, endochondral ossification progressed and chondrocytes expressed COL2A1 and COL10A1.

Our findings show that the fusion process begins with the synthesis of collagens around the transverse processes and around the grafted bone independently. Various spatial and temporal osteogenic and chondrogenic responses, including intramembranous, endochondral and transchondroid bone formation, progress after bone grafting at the intertransverse processes. Bone formation through cartilage may play an important role in posterolateral spinal fusion.