Cages in ACDF are associated with a higher non-union rate than allograft

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Investigators found a higher rate of non-union associated with intervertebral cages than with allograft in a recent retrospective analysis, which led to the conclusion that allograft may be superior to cages in anterior cervical discectomy and fusion (ACDF). This research, by Sean Pirkle (Department of Orthopaedic Surgery and Rehabilitation Medicine, University of Chicago Medical Center, Chicago, USA) and colleagues, was recently published in Spine.

The investigators note that in recent years, the use of an intervertebral cage in anterior discectomy and arthrodesis has gained popularity. Concurrently, attention has been given to the use of either bone graft or cages during this procedure, as well as the occurrence of non-union, which the authors mention can lead to “poor clinical outcome and the need for revision surgery, whether posterior or anterior, presenting an additional risk to the patient”.

Pirkle and colleagues remark that the increased prevalence of cage use, alongside the negative effects of non-union for the patient, make this a necessary and timely study.

The authors describe the research as a retrospective database review of 6,130 patients registered in the PearlDiver national database through Humana Insurance from 2007–2016. All ACDF patients with anterior plating who were active in the database for at least one year were included in the study. Pirkle and colleagues excluded patients with a fracture history within one year of intervention, past arthrosis of hand, foot, or ankle, or a planned posterior approach.

Patients were stratified by number of levels treated, tobacco use, and diabetic condition. Non-union rates of structural allograft and intervertebral cage groups after one year were then compared using Chi-squared analyses.

The investigators report that 4,063 patients were included in an allograft treatment group, while 2,067 were included in a cage treatment group. Overall, they found that non-union rates were “significantly higher” in the cage group (5.32%) than in the allograft group (1.97%) and when controlling for confounders, they observed increased rates of non-union in the cage group, achieving statistical significance in 25 of the 26 analyses.

Pirkle and colleagues comment: “Although the non-resorbable nature of the cage may allow for stability as the fusion mass forms, it may also represent a literal mechanical block for fusion formation. Fusion mass cannot form in the space occupied by the synthetic cage.” With less endplate surface area and less intervertebral volume available for arthrodesis, the authors therefore hypothesised that the use of a cage in ACDF would be “significantly associated” with the development of a non-union.

According to the authors, existing literature consists primarily of single-centre studies with inconsistent findings. The purpose of this study was therefore to analyse the rate of non-union in patients treated with structural allograft and intervertebral cages in ACDF in a larger patient cohort.

The investigators also note that their finding is in contrast with prior literature. To date, there have been five studies that compared union rates in ACDF using cages versus bone graft. However, “None of them were able to demonstrate a difference between the two techniques.” Pirkle and colleagues mention in particular that the sample size of these studies were low. “Even when the data from these five studies are pooled, there are only 122 patients in the bone graft ACDF group and 147 in the cage group. This low combined sample size precludes sufficient analysis and control for confounding variables, whereas our study allows for a larger stratified analysis.”

The investigators also mention some weaknesses of the present study. For example, they were unable to obtain radiographic evidence of non-union for individual patients and instead relied on the diagnosis codes for non-union, which they describe as “an important assumption we have made in this study”.

Furthermore, as this was an observational database study, the authors were unable to determine the constitution of each cage placed, whether that be polyetheretherketone (PEEK), titanium, mesh, or porous material.

Going forward, Pirkle and colleagues hope that future studies utilising other data sources with sufficient sample size may be of value in further investigation. However, they also highlight the strength of their study, noting that the PearlDiver data have been widely utilised in peer-reviewed publications and also that, to date, this is the largest comparative study examining the fusion rates of ACDF using cages and structural bone graft.

They comment: “Our practice, like the majority of spine surgeons in North America, is to utilise structural bone graft in ACDF. These data suggest that allograft, when available, may be a superior option than the use of a cage in achieving arthrodesis in the cervical spine.”

However, they add: “ACDF cages may continue to have a role in cervical spine arthrodesis. In situations where structural allograft may not be readily available, cervical cages represent a reasonable alternative with a well-documented fusion rate, though perhaps not as high as allograft.”

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