Abstract
Introduction
The ESP prosthesis is a one-piece deformable but cohesive interbody spacer. it provides 6 full degrees of freedom about the 3 axes including shock absorption (fig1). The prosthesis geometry allows limited rotation and translation with resistance to motion (elastic return property) aimed at avoiding overload of the posterior facets. The rotation center can vary freely during motion in this “silentblock” implant. It thus differs substantially from current prostheses.
Material and methods
Surgeries were performed by 2 senior surgeons in 54 women and 34 men (1level in 72 cases, 2 levels in 3 cases, hybrid construct in 13 cases). Average age was 42 (SD: 7). Average BMI was 24.2kg/m2 (SD: 3,4). Clinical data and X-rays were collected at the preoperative time and at 3, 6, 12, 24, and 60 months post-op. The analysis was performed by a single observer independent from the selection of patients and from the surgical procedure.
The radiological analysis at 60 months follow-up could be realized in only 76 cases because the quality of the dynamic Xrays was not sufficient in 12 patients. We measured the ROM and the location of mean center of rotation (MCR) of the implanted and adjacent levels using the Spineview® software. The MCR is considered to reflect the quality of movement of a segment; it is localized thanks its co-ordinates. X is expressed as a percentage of the length of the vertebral end plate, and Y as a percentage of the height of the posterior wall. The usual location of the MCR is in a circle, whose center is placed between 30 and 50% of the superior vertebral endplate of the vertebra below, and whose diameter is 70% of the vertebral endplate size.
Results
We did not observe device-related complications. All clinical outcomes significantly improved when compared to the pre-operative status (table 1, table 2)
Mean ROM was 6,3° (SD 3,2) at the implanted level, 6,7°(5,2) at the above level, 7°(4,9) at the below level.
Mean x-MCR value at the implanted level was 33.1% ± 20.8 % and mean y-MCR value −12,41% ± 30.1%.
Mean x- MCR value at the above adjacent level was 25,7% ± 30.1% and mean y-MCR value 16%± 39.4%.
Mean x- MCR value at the below adjacent level were 31.1% ± 29.8 % and mean y-MCR value − 0,61% ± 46.8
Conclusion
The concept of the viscoelastic ESP prosthesis is different from that of the “first generation” disc implants. This study reports encouraging clinical results about pain, function, and radiological kinematic behavior especially because the MCR appears similar to those of the natural disc described in the literature. The adaptation ability is one of the main feature of this concept as we need to consider the long term evolution of the global posture and mobility after disc replacement.
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