title: Biomechanical evaluation of an integrated fixation cage during fatigue loading: a human cadaver study.
abstract: OBJECTIVE Lumbar cages with integrated fixation screws offer a low-profile alternative to a standard cage with anterior supplemental fixation. However, the mechanical stability of integrated fixation cages (IFCs) compared with a cage with anterior plate fixation under fatigue loading has not been investigated. The purpose of this study was to compare the biomechanical stability of a screw-based IFC with a standard cage coupled with that of an anterior plate under fatigue loading. METHODS Eighteen functional spinal units were implanted with either a 4-screw IFC or an anterior plate and cage (AP+C) without integrated fixation. Flexibility testing was conducted in flexion-extension (FE), lateral bending (LB), and axial rotation (AR) on intact spines, immediately after device implantation, and post-fatigue up to 20,000 cycles of FE loading. Stability parameters such as range of motion (ROM) and lax zone (LZ) for each loading mode were compared between the 2 constructs at multiple stages of testing. In addition, construct loosening was quantified by subtracting post-instrumentation ROM from post-fatigue ROM. RESULTS IFC and AP+C configurations exhibited similar stability (ROM and LZ) at every stage of testing in FE (p ≥ 0.33) and LB (p ≥ 0.23) motions. In AR, however, IFCs had decreased ROM compared with AP+C constructs at pre-fatigue (p = 0.07) and at all post-fatigue time points (p ≤ 0.05). LZ followed a trend similar to that of ROM in AR. ROM increased toward intact motion during fatigue cycling for AP+C and IFC implants. IFC specimens remained significantly (p < 0.01) more rigid than specimens in the intact condition during fatigue for each loading mode, whereas AP+C construct motion did not differ significantly (p ≥ 0.37) in FE and LB and was significantly greater (p < 0.01) in AR motion compared with intact specimens after fatigue. Weak to moderate correlations (R2≤ 56%) were observed between T-scores and construct loosening, with lower T-scores leading to decreased stability after fatigue testing. CONCLUSIONS These data indicate that a 4-screw IFC design provides fixation similar to that provided by an AP+C construct in FE and LB during fatigue testing and better stability in AR motion.
authors.affiliation: Division of Applied Mechanics, Office of Science and Engineering Laboratories, and.
authors.forename: Vivek
authors.initials: V
authors.lastname: Palepu
authors.affiliation: Division of Orthopedic Devices, Office of Device Evaluation, Center for Devices and Radiological Health, US Food and Drug Administration, Silver Spring; and.
authors.forename: Jonathan H
authors.initials: JH
authors.lastname: Peck
authors.affiliation: Division of Applied Mechanics, Office of Science and Engineering Laboratories, and.
authors.forename: David D
authors.initials: DD
authors.lastname: Simon
authors.affiliation: Department of Orthopaedics, Walter Reed National Military Medical Center, Bethesda, Maryland.
authors.forename: Melvin D
authors.initials: MD
authors.lastname: Helgeson
authors.affiliation: Division of Applied Mechanics, Office of Science and Engineering Laboratories, and.
authors.forename: Srinidhi
authors.initials: S
authors.lastname: Nagaraja
mesh_terms: D000368:Aged; D001696:Biomechanical Phenomena; D002102:Cadaver; D005260:Female; D006801:Humans; D016268:Internal Fixators; D008159:Lumbar Vertebrae / Q000503:physiopathology / Q000601:surgery*; D008297:Male; D018583:Pliability; D011475:Prosthesis Failure; D016059:Range of Motion, Articular
keywords: AP = anterior plate; AP+C = anterior plate and cage; AR = axial rotation; DXA = dual-energy x-ray absorptiometry; FE = flexion-extension; FSU = functional spinal unit; IFC = integrated fixation cage; LB = lateral bending; LZ = lax zone; ROM = range of motion; anterior plate; cadaver; fatigue; functional spinal unit; integrated fixation cage; range of motion