iMAS Technique Combines Open and Minimally Invasive Approaches for Treatment of Degenerative Spine Disorders

Lead author Robert L. Masson, MD, FACS and SpineUniverse Editorial Board member, Joshua M. Ammerman, MD comment

Peer Reviewed

The intervertebral micro access surgery (iMAS) technique for transforaminal lumbar interbody fusion (TLIF) offers a novel approach for treatment of select degenerative lumbar disease processes, according to a paper in the September issue of Operative Neurosurgery. This hybrid of both open and minimally invasive techniques is believed to result in the same benefits of minimally invasive approaches, including faster recovery, reduced blood loss, and reduced length of stay.
Illustration of a skeleton on a touch interface tabletThe researchers are collecting prospective data on clinical and radiographic outcomes to provide data on long-term efficacy. Photo Source: and efficacy data for the iMAS technique are currently based on case studies and case series. The researchers are collecting prospective data on clinical and radiographic outcomes to provide data on long-term efficacy.

“The iMAS Procedure is mini-open microsurgical 360º segmental spinal reconstructive platform,” explained senior author Robert L. Masson, MD, FACS, spine surgeon and founder of NeuroSpine Institute of Orlando. “It was developed to create a segmental, sequential, reproducible algorithm whereby the surgical execution of a decompression, reconstruction, and stabilization of a segment is optimized and sequentially organized in order to build a progressive growth curve within the surgeon’s execution journey,” said Dr. Masson, who also is President of NSI Surgery Centers.

The 10 key steps in the iMAS procedure as described in the paper are 1) preoperative planning, 2) incision and fascial release, 3) pedicle screw insertion, 4) facetectomy for interbody, 5) transforaminal interbody implant placement, 6) direct decompression, 7) rod/screw cap assembly and compression, 8) decortication, 9) onlay fusion, and 10) closure.

The working zone of this procedure is defined as the interpedicular space, which the researchers describe as the “3-dimensional space bounded by the superior pedicle, the inferior pedicle, and the midline of a standard intervertebral segment.”

Differences Between iMAS and Traditional TLIF

One of the major changes in the iMAS procedure over traditional TLIF is that the former technique involves changing the sequence, Dr. Masson said. “In iMAS procedures we use headless pedicle screws placed fluoroscopically early in the case, without retraction, enabling an in situ navigational structure. Using lateral fluoroscopy and microsurgery, the surgeon has complete anatomical awareness of the anatomy that she/he is dissecting relative to the matrix of the placed pedicle screws.”

“In iMAS procedures, we also perform indirect decompression, via the iMAS specific transforaminal lumbar interbody fusion (TLIF) approach, before direct decompression,” Dr. Masson said. “This inherently decreases the amount of work and risk associated with decompression of a severely stenotic workspace and adds considerably to the efficiency and minimalist requirements of the verified decompression.”

Benefits of the iMAS Approach

Among the benefits of the iMAS procedure over traditional approaches to TLIF are shortened length of stay, ability to perform the procedure in an outpatient setting in healthy patients, substantial reduction in blood loss, and improved efficiency of procedure execution and time, according to Dr. Mason.

Dr. Masson said he is able to perform a one-level TLIF in less than 40 minutes, using a 14-25 mm midline incision.

iMAS Is Suitable for Select Degenerative Conditions

The iMAS procedure is believed to be suitable for degenerative conditions, including grade I spondylolisthesis, unilateral synovial cysts with instability, unilateral disc herniations with instability, and recurrent disc herniations, the researchers wrote.

“The entry level iMAS Procedure learning curve is optimal for one- and two-level degenerative spondylosis cases with less than grade 1 spondylolisthesis, a dominant one-sided nerve symptom syndrome with a well-defined nerve root target in a relatively normal or thin patient,” Dr. Masson told SpineUniverse. “With experience, we have used iMAS for every thoracolumbosacral case that we do, including revisions, scoliosis, grade 4 spondylolisthesis, tumors, trauma, and infection.”

Risks Are Similar to Open Approach in Skilled SurgeonsHands

“For a skilled microsurgeon, the risks are no different than for an open approach; however, because of the sequence and repetitiveness, we have found that over many cases the risks actually decrease with increased surgical team performance,” Dr. Masson told SpineUniverse.

The researchers will launch the iMAS Study Group in 2017, which will investigate the long-terms outcomes of the procedure.


Joshua M. Ammerman, MD
Chief of the Neurosurgery Section
Chair of the Department of Surgery
Sibley Memorial Hospital
Neurosurgeon, Washington Neurosurgical Associates
Washington, DC

The iMAS procedure represents a hybrid procedure in the middle ground between the true percutaneous minimally invasive TLIF and the midline lumbar interbody fusion (MidLIF) style procedure with cortical screw fixation. It is based on solid principles of rigid pedicle based fixation, interbody support, and minimizing muscle trauma. 

In the hands of an experienced surgeon, the iMAS procedure appears to offer extensile access to 1 or 2 lumbar spinal levels through a limited midline incision in association with good clinical/radiological outcomes. The procedure does require a fundamental shift in workflow, which may be difficult for some surgeons to adopt, and the exposure may be disorienting for surgeons who traditionally access the dorsal spine via a midline subperiosteal approach.

In addition, this technique utilizes pedicle screws with detachable multiaxial heads to facilitate the procedure. I, and I'm sure other surgeons have unpleasant memories of past systems that permitted delayed head attachment and the attendant risks of delayed hardware failure. The iMAS approach lends further support to the notion that surgical techniques that preserve the dorsal spinal musculature are robust, effective, and here to stay.

Updated on: 10/28/19
Continue Reading
Instrument Assisted Soft Tissue Mobilization
Joshua M. Ammerman, MD
Washington Neurosurgical Associates
Washington, DC

Get new patient cases delivered to your inbox

Sign up for our healthcare professional eNewsletter, SpineMonitor.
Sign Up!