Biomechanical Studies of Stability of Lumbar Spinal Dynamic Stabilizer
Date Issued
2011
Date
2011
Author(s)
Liu, Chun-Hsien
Abstract
Summary of Background Data: Fusion surgery is often used to treat unstable spinal diseases. Fusion surgery usually decreases the motion of the implanted levels and induces compensation behaviors at the adjacent levels. It is wildly believed that the excess motions at the adjacent levels cause disc degeneration. Some dynamic devices, ex. Dynesys system, have been developed to solve the problems by preserving motion at the implanted levels. However, the flexibility of these products varies and their performances on reducing adjacent disc degeneration are still absurd.
Objective: The purpose of this study is to find the proper flexibility of posterior lumbar dynamic stabilizers by evaluating the neutral zone, range of motion, intradiscal pressure and intervertebral foramen area of the implanted and adjacent motion segments.
Materials and methods: Eight 4-level (L2-L5) lumbar spine were dissected from 6-month old pigs. All soft tissues except the surrounding ligaments and facet capsule were carefully removed. Specimens were wrapped in saline-soaked gauze and stored in the freezer until the experiment. The flexion / extension angular displacement of the specimen in the intact status were measured under 8 Nm of pure moment. Then the specimen was injured at L3-L4 level by damaging the facet joints and surrounding ligaments. The angular flexion/extension displacement obtained from the specimen in the intact status under 8 Nm of pure moment was applied to the specimens in the injured status and consecutively in 5 levels of constrained status which was controlled by a self-designed adjustable dynamic stabilizer implanted at L3-L4. The flexion / extension motion of L3-4 was constrained to be 0%, 20%, 40%, 60%, and 80 % of the flexion / extension motion of the injured status. The intersegmental neutral zone (NZ), range of motion (ROM), changes of intradiscal pressure (det IDP) and changes of intervertebral foramen area (det IFA) of the implanted and adjacent cranial/caudal motion segments were calculated. The IDP was measured by in-house 20 G needle pressure transducers inserted into the disc. The intervertebral foramen area was calculated based on lateral radiographys. The “det IDP” and “det IFA” was defined as the difference of IDP / IFA before and after the angular displacement loading.
Results: (1) During flexion. The NZ, ROM and the det IDP of the implanted and the adjacent cranial / caudal motion segments decreased with the increase of motion at the implanted level. The ROMs of the implanted and adjacent cranial/caudal motion segments in the status of 60% constrained motion at the implanted level were the same as those in the intact status. The det IDPs of the implanted and adjacent cranial/caudal motion segments in the status of 40%, 60% and 80% constrained motions were less than those in the intact status. The det IFAs of all motion segments in the injured and 5 constrained status were similar to those in the intact status. (2) During extension. The NZ, ROM and det IFA of the adjacent cranial and caudal motion segments decreased with the increase of motion at the implanted segment. The ROMs of the implanted and adjacent cranial/caudal motion segments in the status of 40% constrained motion at the implanted segment were the same as those in the intact status. The det IFAs of the implanted and adjacent cranial/caudal motion segments in the status of 60% and 80% constrained motions of the implanted segments were less than those in the intact status. The det IDPs of all motion segments in the injured and 5 constrained status were similar to those in the intact status.
Conclusions: It is found that 60% constrained flexion motion and 40% constrained extension motion at the implanted level induce least compensation ROM, IDP change or IFA change at the adjacent cranial and caudal levels without violating the stability at the implanted level. The result of this study is expected to be helpful for the development of new dynamic stabilization systems.
Subjects
Non-Fusion Surgery
Range of Motion
Intradiscal Pressure
Foramen Size
Neutral Zone
Biomechanics
Type
thesis
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