1.1. Overview
Degenerative disc disease (DDD) in the lumbar spine can be associated with displacement of the vertebral body. DDD with concomitant spinal stenosis is among the most frequent conditions in the aging adults. Treatment options for spinal stenosis continue to be discussed among spine professionals, but several studies have shown that surgical procedures provided better improvement in pain and function compared to usual nonoperative care [1, 2]. Various surgical options have been studied to evaluate safety and optimal radiological and clinical options. A new minimally invasive, stand-alone alternative different to conservative and standard surgical implants has been developed. The new Latero (Latero; A-Spine Asia, Taipei, Taiwan) device uses the vertebrae as the fulcrums. The lateral vertebral plate utilizes a stabilizing mechanism which is dissimilar from those of other devices. This technique is novel in that it can be used to gain access to the lumbar spine via a lateral approach. Hence, the potential complications with an anterior approach to the lumbar spine can be avoided.
1.2. Motivation and objectives
Interbody cages have been certified to restore disc height and to increase stability of the spinal segment, and thereby enhance fusion in the surgical treatment of low back pain, spondylolisthesis and degenerative lumbar disc disease. Since 1991, Obenchain described the first laparoscopic lumbar discectomy [3], the field of minimally invasive spine surgery has continued to evolve. Surgeon and patient alike have been attracted by the advantages of minimally invasive surgery, including less tissue trauma during the surgical approach, less postoperative pain, shorter hospital stays, and faster return to activities of daily living. These reported advantages led to the laparoscopic anterior lumbar approach and mini-open anterior
2
lumbar interbody fusion becoming commonly performed procedures [4, 5]. Therefore, surgical options turn to minimize collateral muscle/bone damage while achieving excellent clinical results, with minimal risk and complication rates.
In this study, a novel stand-alone implant Latero is investigated. As a minimally invasive option, the lateral approach to interbody fusion avoids related complications from posterior-approach and anterior-approach, achieves spinal stabilization and provides indirect decompression [6-11]. Additionally, the lateral approach preserves the inherent biomechanical integrity of the motion segment through maintenance of all the ligamentous structures, including the anterior longitudinal ligament (ALL) [6, 12, 13], which is considered to be one of the major stabilizing components of the lumbar spine.
The loading conditions for the spinal motions are highly complex, therefore, not been fully characterized. Various simplifications have been made in experimental and numerical studies.
It is not clear which loading condition deliver more realistic results. An experimental technique, called follower load, developed by Patwardhan et al. [14], applied compressive preload along a path following the lordotic curve of the lumbar spine, and allowed the in vitro spinal models to support higher physiologic loads without damage or instability. The follower load was applied on the lumbar spine to permit a certain amount of shear force in the three-dimensional FE models. It was shown that the load carrying capacity of the spine was significantly increased with little change in the shape of the spine at all vertebrae in comparison with the vertical direction loading. In this study, the follower load setting was investigated using the finite element (FE) method.
This study focused on the analysis of Latero design. The study was separated into two parts. In the part-1 of the study, it was focused on comparison of Latero implant with respect to other stand-alone implants. The finite element models inserting of conventional fusion devices were created for the evaluation the new implant. These finite element models were used to compare different stand-alone implants in all physiological motions including flexion,
3
extension, lateral bending and axial rotation. In the part-2 of the study, in order to improve the design of Latero implant, two improvement parameters were proposed: position of implant and bending angle of lateral plate. The evaluation results derived from finite element models data were focused on ROM for stability assessment.
1.3. Outline
This dissertation is divided into six chapters:
Introduction:
This chapter introduces the overview, objectives, and outline of this dissertation.
Background:
This chapter reviews the spine anatomy and biomechanics, spinal pathology and treatments, and follower load settings.
Materials and methods:
The first subject of the chapter includes FE modeling of the five-segment intact lumbar spine and the validation of follower load setting in FE analysis.
The second subject of the chapter includes the four implant models: Latero, SynFix (SynFix; Synthes Spine Inc., PA, USA), Stabilis (Stabilis; Stryker, Michigan, USA), and SynCage-Open (Synthes Spine, Inc., PA, USA) with pedicle screws fixation.
The third subject of the chapter includes the three implant models: Latero inserted in anterior position, Latero inserted in posterior position and Latero with modified lateral plate.
The fourth subject includes the boundary and loading conditions.
Results:
The first subject includes data of Latero implantation, SynFix implantation, Stabilis implantation, and SynCage-Open with pedicle Screws Fixation models.
The second subject includes data of Latero implantations with different implant
4
modifications.
Discussion:
The first subject discusses biomechanical effect of Latero implantation compared with traditional implantations.
The second subject discusses biomechanical effect of Latero implantation with different implant modifications.
The third subject includes the limitations.
Conclusion and future work:
The concluding remarks and several topics that can be extended from this research are summarized in this chapter.
5