Introduction:

What is overlooked in clinical studies are the possibilities of manufacturing and design aspects of the instrumentation that could initiate rod fracture. Although revision because of hardware fracture is a small fraction of the overall revision rates (12.1% to 13.7%), there are sufficient numbers of revision cases where hardware removed can undergo a thorough metallurgic analysis. This study is unique in that rod characteristics, such as alloy, surface markings, and fracture type, seen at fracture surfaces are considered in the analysis.

Methods:

This work was conducted under both a retrospective and prospective IRB. Patients considered for this study were between the ages of 18 and 85 years who underwent or were undergoing revision spine surgery with previous instrumentation in the cervical, thoracic, or lumbar region and evidence of at least one of the following: catastrophic hardware failure, pseudarthrosis, implant loosening, or nonfusion. Inclusion criteria were determined through radiographic and medical records review.

Results:

Fifty-six patients who had revision procedures because of different indications were included; 101 rods were removed, tested for fracture, and included in the analysis. Laser marking is significantly (P < 0.0001) associated with rod fracture. Detailed analysis showed notable surface and subsurface changes as the result of the marking, such as surface melting, cracking, and notching, creating locations to initiate a fracture. The three most informative variables to clinical rod fracture using multiple regression modeling were body mass index, presence or absence of laser mark (yes/no), and length of posterior fusion (≤2 spinal levels/>2 spinal levels). It was found that the relative risk of rod fracture is 23 times higher during 20 postoperative years than in cases with this index <0.4.

Discussion:

For a patient with a given body mass index, if they require a multilevel fixation greater than two levels and rods with laser marks are used, the risk of early rod fracture increases by 40%.