Use of the dynamic locking screw in the thoraco-lumbal spine plate in order to prevent a delayed bone healing on the spinal column

Status:

completed 05/2013

Aims:

A biomechanical cadaver study with ten cadaver spines is carried out to evaluate:

1. Vertical micro movement at the cadaver spine over two segments
2. Primary stability of the DLS in the vertebral body
3. Torsion stability of the DLS at the cadaver spine
4. Flexibility of the DLS/plate construct at the cadaver spine

Activities/Methods:

In this study, the use of the dynamic locking screw (DLS) 5.5 mm in the thoraco-lumbar spine plate (TLSP) will be tested. Therefore, ten cadaver spines will be measured at the University of Innsbruck´s test bench for spinal cords with the spine testing machine disposing of six degrees of fineness. With a stepper motor, a preload will be applied and the load can be regulated by a feedback system. Next, a measurement with an optical 3D measurement system will be carried out. This system enables an exact three-dimensional movement analysis. The goal of this second measurement consists in receiving information about the three-dimensional displacement of movements, in order to be able to compare the rigidity of different spines among each other.

In all ten cadaver spines, the segments of movement Th10-L2 (thoracolumbar spine) are dissected. In order to eliminate pathological changes, like for example bone cysts, tumors and fractures, a CT scan will be carried out. The bone mineral density of the different spine specimens will be determined through a quantitative computed tomography (qCT). After that, the spines will be subdivided into two groups of five according to their bone mineral density and the complete surface will be removed while preserving the attaching ligaments and joint capsules. For fixation, the anatomical preparations will be embedded up to 50 % in Poly(Methyl methacrylate) (PMMA) cement. During the embedding, the horizontal alignment of the median vertebral body Th12 has to be controlled with the help of a pair of cross scales. After placing the screws for the fixation of the 3D movement analysis systems laterally at the vertebral bodies Th11 and L1, orthogonal systems of coordinates that make a digitalization of the movement analysis system possibly can be defined.

Results:

According goal 1: Applying the dynamic system lead to a wider motion capability in all three main directions of movement in the bridged segments between L2 and L4 (rigid area). The additional motion capability was situated in the normal scope for bone healing and resulted in a significant reduction of rigidity compared to conventional angular stabile plate osteosynthesis.

According goal 2: The comparison with the intact spine showed, that the fixation with the dynamic locking screw was more in line with the physiology with exception of the rotational movements. That led to a reduction of occurring forces in the "healthy" adjacent spinal segments.

According goal 3: The failure load of the dynamic locking screw lay in all tested series over 1000 N und hence way above all possible physiological forces that occur at the spine.

According goal 4: With a cycle test series a decreased failure behavior of the dynamic screws could be verified compared to the angular stable screws. This was shown to an even greater extend for osteoporotic bones.

Additional results: The 3D optical surface analysis with PONTOS by GOM for the first time enabled to provide a matched CAD-data analysis of force vectors under movement of the spine. Additionally micro movements of the implants and the spine could be measured allowing a detailed analysis of the relative movements.

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