Abstract
Total disk replacement (TDR) has met considerable reluctance from the medical community, mainly in the United States, because the insurance companies have refused to reimburse surgeons for fear of delayed complications, revisions and unknown secondary costs. Typical long-term vertebrae-implant related structural complications include subsidence, migration, implant displacement, endplate fracture, wear and loosening. Intervertebral disc implant size, shape, position, endplate removal and compressive strength of trabecular bone further affect the risk of implant subsidence and loosening. The aim of the present study is to understand the combined effects of the different depth positioning of the ProDisc-L implant and endplate removal during surgery on the vertebral bone strain behaviour. Manufactured synthetic spinal L3-L4 segments were used to experimentally predict vertebrae cortex strain behaviour for different depth implant positioning and endplate thickness removal. In addition, validated finite element models were developed to assess the structural behaviour of cancellous-bone. Measured cortex strains showed significant differences relative to the intact vertebra for the most extreme depth implant positioning. The endplate thickness reduction tends to decrease significantly compressive cortex strains for all strain gauges. A two- to three-fold cancellous-bone strain increase occurs when more than 50% of the endplate thickness is removed, independently of the depth implant position. It is concluded that the implanted strain distribution that better fits intact vertebra strain behaviour is achieved when the depth centred implant position is combined with a partial endplate thickness removal.
Highlights
Total disk replacement (TDR) has met considerable resistance from the medical community, mainly in the United States, because insurers have refused to reimburse surgeons for TDRs due to concerns with delayed complications, revisions, and unknown secondary costs [1,2]
The implanted models were constructed from the intact models, in order to test three different depth implant positioning; anterior, centered and posterior which were combined with three L4 vertebra upper endplate thicknesses (Figure 2)
The test procedure for each implanted spine specimen begins with the intact L4 vertebra endplate (≈0,5mm thickness, measured by CT-scan), the upper vertebra endplate thickness was reduced by milling to ≈0,25mm and the vertebral endplate is completely removed (≈0mm thickness); the three depth implant positioning were tested for each one endplate thickness
Summary
Total disk replacement (TDR) has met considerable resistance from the medical community, mainly in the United States, because insurers have refused to reimburse surgeons for TDRs due to concerns with delayed complications, revisions, and unknown secondary costs [1,2]. The proportion of current TDR patients who will develop subsidence and loosening will increase, as the predominantly younger TDR population ages and experiences progressive bone loss and osteoporosis [11] Parameters, such as implant positioning combined with cortical endplate preparation (removal), are related to the patient’s anatomy and condition, as well as, on the level of experience of the surgeon. The aim of the present study is to understand the combined effects of different depth positioning of the ProDisc-L implant and endplate removal (thickness) on the vertebral bone strain behaviour. The aim of the present study is to understand the combined effects of the different depth positioning of the ProDisc-L implant and endplate removal during surgery on the vertebral bone strain behaviour. It is concluded that the impla nted strain distribution that better fits intact vertebra strain behaviour is achieved when the depth centred implant position is combined with a partial endplate thickness removal
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