Minimal Access Spinal Surgery

Contents

• Introduction
• Benefits of minimal access spinal surgery
• What are the basic requirements?
• Different types of spinal devices
• How is MASS performed and what are the various indications for its use?
• Spinal Fusion
• Specialised surgeons

Introduction

The aim of minimal access spinal surgery (MASS) is the reduction of 'collateral damage' to muscles, ligaments and soft tissue associated with traditional spinal surgery whilst obtaining the same clinical outcomes of traditional open procedures. The technique has been developed to complement minimal invasive spinal surgery (MISS) as the latter does not allow for direct visualisation of the spine. In that regard these two techniques DO NOT alter the indications or goals of surgery.

Benefits of minimal access spinal surgery

There are a number of real patient benefits associated with MASS which include:

• Reduced blood loss
• Reduced post-operative incisional pain
• Minimised respiratory difficulties
• Improved mobilisation
• Early discharge from hospital
• Enhanced rehabilitation and early return to activities and work.

There is also a clear reduction in direct and indirect health care costs when employing MISS or MASS techniques.

What are the basic requirements?

Essential equipment includes an image guidance device, modified instruments, a light source (direct light or endoscopic assisted) and an optional access portal.

The two main methods of image guidance used are fluoroscopy and computer assisted operative surgery (CAOS). The advantage of fluoroscopy is that it is relatively inexpensive, widely available, simple to use and provides immediate imaging feedback which may be in multiple planes. Its main disadvantage is the increased risk of radiation exposure.

CAOS remains an expensive technology because of costly hardware and software, but the principal benefit is the limitation of radiation exposure. CAOS requires the use of a special device to register the position of the spine (this is called a dynamic reference array) that is typically attached to both the spine and the base of modified instruments (Fig 3). Recent significant advances in computer software technology, modification of specialised instruments and enhanced metallurgy of implants have greatly improved the precision of the screws that are inserted and fixed into the spine (pedicle and facet screws) to within sub-millimeter accuracy. Additionally, CAOS has the unique ability to educate surgeons regarding the quality of their technique and therefore allow for the improvement of accuracy and reproducibility of the surgical procedure.

Different types of spinal devices

When the front of the spine (anterior spine) is approached and exposed, a complete discectomy and/or vertebral body excision (or corpectomy) is performed followed by the placement of an implant (interbody device) used to stabilise the disc. In patients with disabling low back pain secondary to disc degeneration, FDA approved motion preserving devices or total disc replacements, (e.g. the Charite III from Depuy, Johnson and Johnson, and the Prodisc-L from Synthes- Stratec Switzerland), are being routinely used to successfully treat this condition.

How is MASS performed and what are the various indications for its use?

An interbody fusion is inserted into the defect followed by the placing of a locking metal plate or rod. The rod spans and therefore neutralises the compressive forces across the interbody device allowing fusion to occur. The development of cannulated systems and image guidance has allowed for the ease of insertion of locking screws to hold the metal plate or rod rigidly against the spine. Cannulated screws are inserted over carefully placed K-wires. This requires meticulous planning and the surgeon needs to possess superior three-dimensional spatial awareness. Supplementary fixation from behind (posterior) may be required for those patients where improved biomechanics is essential to restore damaged posterior structures such as ligaments or joints. This procedure can be performed using MASS or MISS techniques.

Spinal Fusion

The two main techniques for posterior percutaneous spinal fixation are pedicle screws and facet screws. When treating lumbar degenerative disc disease, both techniques have been developed to compliment anterior lumbar interbody fusion (ALIF) in order to perform a circumferential or 360 degree fusion with minimal patient morbidity. Currently the most widely used percutaneous pedicle screw system is the SextantTM (Medtronic Inc, Minneapolis, MN), but many other systems are now commercially available for screw insertion either percutaneously or using MASS techniques.

MASS techniques are also being increasingly practiced when performing posterior lumbar interbody fusions (PLIF) and transforaminal interbody fusions (TLIF). An access portal consisting of a tubular retractor (e.g. QuadrantTM, Medtronic Inc, Minneapolis MN) is used in a minimal access muscle splitting approach (Fig 5). Direct visualisation can be accomplished using a surgical microscope, endoscope or loupes. The access channel created then allows for an effortless insertion of pedicle screws and interbody cages with minimal disruption to the posterior lumbar tissues. Decompression of bony and ligament blockage of the spinal canal that results in back pain and sciatica (lumbar spinal stenosis) and disc herniations can also be easily performed using tubular retractors, (e.g. METRxTM, Medtronic Inc, Minneapolis MN). The blunting of modified sharp instruments allows the surgeon to slip past the nerve root whilst minimising soft tissue retraction.

Specialised surgeons

Only surgeons competent in MASS should perform these techniques because catastrophic complications can occur from injury to the abdominal viscera, blood vessels and neurological structures.

An aspiring spinal surgeon must be able to master the conventional open technique before embarking on MASS techniques. Akin to MISS, MASS also has a relatively steep learning curve and the potential for complications remains identical to that of conventional open approaches. In addition to the need to possess inherent three-dimensional spatial awareness, the surgeon must undergo mandatory training and certification in order to master the hand-eye co-ordination tasks required. As indicated, meticulous planning and thorough knowledge of the surgical anatomy and equipment is essential for achieving success in MASS. In carefully selected patients, a more experienced surgeon will realise and appreciate the intricacies of a well executed MASS, often being rewarded by patients who will enjoy a marked reduction in approach-related postoperative pain, hospital stay and post-operative rehabilitation.