RCT finds equivalent percentage coronal curve correction between low- and high-implant density AIS treatment


For Lenke 1A curves between 45 and 65 degrees treated with low- versus high-implant density in adult idiopathic scoliosis (AIS) patients, a prospective, multicentre, randomised controlled study has shown equivalent percentage coronal curve correction. The Minimise implants maximise outcomes (MIMO) study was conducted by A Noelle Larson (Mayo Clinic, Rochester, USA), David Polly (University of Minnesota, Minneapolis, USA), and their colleagues in the MIMO Study Group. This result was presented by Larson at the 54th Annual Meeting of the Scoliosis Research Society in Montréal, Canada (18–21 September).

The investigators conclude, “Given potential cost savings, surgeons should consider a low implant density construct [<1.4 screws per level fused] for Lenke 1A curves between 45 and 65 degrees treated with posterior spinal fusion.”

Larson and colleagues found no difference in age, gender, preoperative curve magnitude, or race between patients treated with either low- or high-implant density. Mean percentage of Cobb correction at three months was 74% in the high density group, compared to 70% in the low density group. The results at one-year follow-up were 69% and 66%, and at two-years, 67% and 64%, respectively.

In addition, there was no difference in operative time, blood loss, or length of stay amongst the two groups. There was no difference in pre- or postoperative thoracic kyphosis between the high and low density cohorts. There were six reoperations in each group. Age was lower for patients who completed two-year follow-up, but there was no difference in race or preoperative curve magnitude.

According to the authors, the role of implant density, defined as anchors per level fused, has not been prospectively studied for a large cohort of AIS patients. Larson and colleagues hypothesised that the use of more screws versus fewer screws would result in similar coronal correction for Lenke 1A curve patterns treated with posterior spinal instrumented fusion.

In the Minimise implants maximise outcomes (MIMO) clinical trial of equivalence, the investigators randomised patients with Lenke 1A curves to more versus fewer screws (high or low density) to determine if there was a difference in the percentage of coronal correction.

There were 14 sites included in the MIMO trial. Between 2013 and 2017, patients with Lenke 1A curves between 45 and 65 degrees were enrolled in the study and randomised to a high (>1.8 screws per level fused) versus low (<1.4 screws per level fused) implant density. Patients were followed prospectively for two years. Power analysis showed 174 patients needed to prove equivalence within six degrees. Follow-up was available at three months for 189 patients, one year for 156 patients, and two years for 124 patients. Data were collected and X-rays measured by trained reviewers.

The investigators summarise that for 206 patients with Lenke 1A curves who underwent posterior spinal instrumented fusion randomised to more screws (implant density >1.8 screws per level fused) versus fewer screws (implant density <1.4) at three-months, one-year, and two-years follow-up, there was no statistically significant difference in percentage curve coronal correction.


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