Radiographic Lucency around Pedicle Screws with Cobalt Chromium Rod Constructs in the Setting of Lumbar Fusion

Abstract

Background Context: Hybrid spinal implants that combine cobalt-chrome rods with titanium screws have been introduced with the goal of reducing the profile of implants used for lumbar fusion. 4.5mm and 4.75mm cobalt-chrome rods have been reported by the manufacturers as having similar results in biomechanical testing to conventional 5.5mm titanium rods, however little is known about the long-term clinical performance of these implants. This report discusses an interesting radiographic finding routinely encountered in our practice following use of these implants. Purpose: To describe the occurrence of radiographic lucency surrounding pedicle screws following the use of low-profile hybrid spinal implants for instrumented posterior lumbar fusion. Study Design: Case report. Methods: This is an observational case series of 14 patients undergoing lumbar fusion instrumented with hybrid cobalt/chrome – titanium implants. All cases involved instrumented posterior lumbar fusion for the treatment of degenerative spondylolisthesis. Patients were followed for one year with radiographic evaluation of both screw lucency and bony fusion as well as subjective outcomes including visual analog scale (VAS) pain score and Oswestry disability index (ODI). Results: At one year post-operative, all 14 patients had radiographic lucency around at least two pedicle screws in the construct despite radiographic bony fusion. All patients reported subjective improvement in VAS score and ODI, and all patients returned to work within 3 months post-operative. Conclusion: We note a 100% rate of pedicle screw lucency in our series of patients undergoing posterior spinal fusion instrumented with hybrid low-profile cobalt-chrome implants. Lucency around pedicle screws has been previously associated in the literature with screw loosening, micro motion, and pseudo arthrosis. Despite this observation, the patients in our series subjectively report favorable outcomes. The significance and cause of lucency around pedicle screws with these low-profile implants is unknown, but may be due to decreased construct stiffness related to smaller rod diameter. Long-term experience with these implants is lacking and further studies are warranted.