Minimally Invasive Lumbar Fusion: Technique and Candidates

Minimally invasive lumbar fusion stabilises the lower spine with screws, rods and a cage, and frees the compressed nerve roots, through small incisions guided by three-dimensional imaging and navigation. Dr. SK Rajan, Chief - Neuro Spine Surgery & Additional Director - Neurosurgery, Artemis Hospitals, Gurugram, India, performs it this way, and his patients typically get up the day after surgery and stay in hospital for about two days.

Lumbar fusion is one of the most commonly performed operations in spine surgery, used for degeneration, spondylolisthesis (a forward slip of one vertebra), disc disease and instability. It is also the operation that has changed most visibly with minimally invasive technique. This article follows the operation step by step, then looks at the patients who were once turned away from it: those with osteoporotic bone, the morbidly obese, and those with slipped vertebrae or stenosis at several levels. The imaging and navigation technology behind it is explained in the guide to recent advances in spine surgery.

How minimally invasive lumbar fusion is planned with navigation

In a navigated minimally invasive lumbar fusion, every screw position, incision and cage size is planned on a three-dimensional screen before the skin is cut. A reference frame is fixed to the patient, a camera tracks it, and a screen displays the patient's anatomy in three dimensions. By holding instruments over the back and watching their projection on the screen, the surgeon works out where each screw should enter the bone and marks the incisions to match. Instruments placed on the skin in the same way indicate the size of the interbody cage, the spacer placed in the disc space, required and the angle it needs to sit at. Everything is decided in advance, which is what makes the operation smooth, pre-planned and accurate.

Minimally invasive lumbar fusion step by step

Only then is the first small cut made. The sequence that follows is consistent:

1. Guide wire. A guide wire is passed through the small incision into the pedicle, the bony column that connects the back of the vertebra to its body. Its position is confirmed with an intraoperative scan.
2. Dilators. Sequential dilators are slid over the wire to open a channel through the muscle rather than cutting it away.
3. Navigated tap. A navigated tap prepares the screw track and tells the surgeon what size, direction and diameter of screw the bone will take, so that there is no error in selection.
4. Screw placement. The screw is inserted while a three-dimensional rendering of it moves live on the screen, showing exactly where it is going inside the bone.
5. Decompression through the port. On the other side, a port is placed. Through it the surgeon drills the bone, reaches the disc, removes it and frees the nerve roots. The port expands once inside, giving a much wider working space at depth than the incision suggests.
6. Cage and rods. A cage is placed in the disc space to support it, and the screws are connected.

Throughout, the navigation is re-registered against the reference frame at intervals to confirm it remains accurate.

Recovery and hospital stay after minimally invasive lumbar fusion

Because the muscles are not stripped from the bone, the pain that comes from the surgical wound itself is small. Dr. Rajan's patients get up the very next day, and both back pain and leg pain often improve dramatically on the first day. The usual hospital stay is about two days.

Lumbar fusion in osteoporosis: cement-augmented screws

Cement-augmented screws have made spinal fusion possible in osteoporotic patients, who once could not undergo it at all. Screws placed in weak bone do not grip, and patients who were bed-bound with back pain were judged too unsafe to operate on. The screws used are fenestrated, meaning they have side openings through which bone cement can flow into the surrounding vertebra once the screw is in place. In Dr. Rajan's technique, adapters are attached to the screws, the cement is mixed and loaded into small syringes, and it is injected gradually through each screw. Its consistency is critical: it must be thick enough not to enter the venous system and travel to the lungs. Once set, the cement surrounds the screw and gives a much stronger hold in weak bone.

The result is fixation strong enough for patients previously considered inoperable to get up and walk the next day. In one case of cement-augmented fixation Dr. Rajan presented, a spondylolisthesis was reduced, a cage was placed for support and the screws were cement-augmented, all through small incisions, where an open operation would once have been needed. He also values the longevity and durability of the construct.

Minimally invasive spine surgery for obese patients

Modern intraoperative imaging now allows minimally invasive spine surgery in morbidly obese patients. Obesity once ruled out minimally invasive surgery for a simple technical reason: X-ray images taken through a very large body showed soft tissue but no bone, so the surgeon could not see where to go through a small opening. Current imaging penetrates that tissue, and Dr. Rajan now operates through small cuts on patients weighing 140 to 150 kg.

He explains the logic to his residents with an analogy: open surgery is like digging a pit to reach underground water, while minimally invasive surgery is like sinking a tube well. The heavier the patient, the more they benefit from minimally invasive surgery, because the open alternative means a very long incision, extensive muscle dissection, more blood loss and great difficulty for the surgeon in retracting the tissue. With strong fixation and cement-augmented screws, these patients also get up and walk the next day.

Minimally invasive surgery for spondylolisthesis

Spondylolisthesis, in which one vertebra slips forward over the one below, can now be corrected through keyhole surgery with navigation, although it was once considered unsuitable for it. Dr. Rajan recalls that if he said he would operate on it this way, even colleagues would not believe him and would advise the patient to have open surgery instead.

Navigation has changed that. Through a port, the screws and rods are placed and the slipped vertebra is reduced back into position. The disc space can be elevated and the curvature of the spine improved. A school of thought held that a slip of this kind required at least six, eight or ten screws, with additional levels fixed above and below. With navigated keyhole technique, the same correction is achieved without extending the construct (the set of screws and rods), through the same small incisions and with much less morbidity.

Lumbar canal stenosis and multi-level instability

Lumbar canal stenosis, a narrowing of the spinal canal in the lower back that squeezes the nerves, is one of the conditions doctors meet most often. The classic presentation is neurogenic claudication: back pain radiating into the legs, an inability to walk or stand for long, tingling, burning or pins and needles, and the need to stop every 5, 15 or 20 minutes, wait a few minutes and then walk again.

Dr. Rajan treats it through the same minimally invasive route. In one patient with stenosis at multiple levels and marked tightness around the nerves, the compressing bone was removed and cages and screws were placed, a decompression and fixation done entirely through small incisions. In another, an international patient with severe back pain and a spine so unstable that the vertebrae were moving relative to one another, the team avoided an open multi-level operation; navigation guided the alignment and fixation through small cuts, and because no muscle was dissected the patient was up the next day, or within a day more.

Multi-level minimally invasive lumbar fusion: how many levels

Navigation and three-dimensional imaging now allow minimally invasive surgery not only at one, two or three levels, but at four and five. The decision is individual. Some patients cannot tolerate the longer operation; in others, poor bone quality or hypertrophy (enlargement) of the facet joints, the small paired joints at the back of the spine, makes the anatomy unfavourable. In those cases the surgeon converts to open surgery, or plans it from the start.

Dr. Rajan's ballpark from his own practice, given in the masterclass live discussion:

• One or two lumbar levels: minimally invasive, without question.
• Three or four levels: roughly half of patients have minimally invasive surgery and half have open surgery.

He is clear that the split at three and four levels reflects anatomical constraints, not any inferiority of the minimally invasive approach, which he regards as still the better operation when it can be done. Other limitations he mentions include the higher cost of the implants, but he sees few beyond that. As experience grows, his unit finds that a wider range of patients can be treated this way.

The same principles extend to the thoracic and cervical spine, to tumours and to the craniovertebral junction, covered in minimally invasive complex spine surgery.