Dorsal Median Band aka plica mediana dorsalis – Cause of the elusive midline placement

plicaOne of the most common questions I get in training physicians in SCS is how to deal with the difficult midline placement.  We’ve all seen case where the lead or paddle flows nicely into the epidural space but then suddenly darts left or right.

The cause is an elusive band of tissue called the Dorsal Median Band aka plica mediana dorsalis between the dorsal dura and the underside of the lamina.  This band of tissue has been implicated in the administration of epidural anesthesia whereby patients get unilateral coverage specifically.  This has been described in early radiology literature.  Y. Eugene Mironer, MD describes use of this band as a midline anchor for percutaneous electrodes.

Many times this band is removed when the laminectomy is performed.  However, when the paddle is advanced beyond the laminectomy site, midline placement can be very difficult.  The lateralization of the paddle can be further exacerbated by the triangulated shape of the thoracic lamina.

Image

I encountered this several times in my own practice and often made modifications to paddles as illustrated here.

Image

While this is a helpful modification, modern day paddles with their tight contact spacing prohibit this type of alteration.

The three primary vendors Boston Scientific, Medtronic and St. Jude Medical offer dissectors to help in paddle placement.  The first two manufacture blunt peel-pack hockey stick style elevators, while St. Jude reps typically carry 1cm Aesculap malleable brain retractor blades.  All three are effective at clearing the epidural space, however I recommend sharp dissection of this band to avoid cord compression.

When I encounter the Dorsal Median Band, my preferred technique is to perform a second laminectomy above the initial one.  I then undercut the two sites and occassionally dissect with a Penfield #3 to dissect the epidural space.  Inevitably this technique is much quicker than repeated attempts at blunt dissection and multiple passes which are a potential source of cord trauma.

Retrograde cervical open trial

This is a 68 year old woman with history of C2-C7 laminectomy and fusion.  She had painful C2-3 radicular pain but an anterograde trial was unsuccessful as the lead could not be advanced through the scar.  With a complete laminectomy there is such extensive scar tissue it is impossible to advance a lead in standard fashion.

retrograde open trial

The red line demonstates the scar tissue from previous laminectomy.  You can also see the paraspinous muscle atrophy.  The inferior portion of the skull is also visible as it the existing hardware.  I performed a suboccipital exposure under local MAC anesthesia and fed two percutaneous leads in via a retrograde fashion.

Lateral paddle migration – salvaged with percutaneous lead

This 59 year old woman with midthoracic dextroscoliosis underwent a paddle placement by outside physician at the target level after a successful trial.  Several weeks post-op the patient experienced loss of coverage on the left side (inside of curve).  This is a common complication in placement of the paddle.  Leads tend to migrate to the outside of the curve so implantation should be performed on the medial side.  Furthermore there is significant rotatory deformity as you can see from the spinous process position.

lateralmigration

While I love the Artisan paddle, this lateral migration is more common given its narrow width.

latermig2

As you can see, this paddle placement violates my preference for anchoring immediately below the paddle at the crotch of the tails.

latmigrpost

I was able to salvage this implant with a percutaneous lead placed on the inside of the curve in about 15 minutes (much easier than a paddle revision).

Axial neck pain – retrograde paddle

This 47 year old man had previous thoracic implant for low back and leg pain with great success.  He presented with worsening neck pain, persistent after an anterior cervical fusion by outside physician.  He had a trial by his pain physician with good relief at top of the midline electrode.

cervical

To optimize cervical coverage and reduce migration I placed dual Exclaim leads by St. Jude in a retrograde fashion.  The leads tunnel from the epidural space at the Occipital-cervical membrane under the ring of C1 and docked halfway under C2.  This lead contains six exclamation point style contacts for a total of 12 individual contacts.  However, each row of dots is wired together to function as a single contact for a total of eight functional electrodes.

retrogradedual

My experience with this technique is that it provides excellent side to side coverage and can often cover the C2 nerve root as it emerges from the cord.  This can give occipital nerve coverage as well as axial neck coverage.  However, I have had one case of a lead fracturing at the site of entry into the paddle, presumably from too much stress at this point despite placing a strain relief.

Omega Plate – ideal bone anchor

I use a cranial fixation dogbone bent like an Omega sign as an screw-on anchor for rigid fixation.  While midline spinous process straddling is my preferred paddle anchor there are cases when that is not possible.  For example, most cervical cases, especially retrograde do not have an adequate bony keel to anchor to.  Likewise if I use a unilateral laminotomy to place a paddle I have not exposed enough bone.  In these cases I anchor the Omega directly to the bone with 4mm screws.

Omega plate

See the Omega anchor technique illustrated here.

Lateralized Central Point of Stimulation

SpectraCP

The ability to lateralize the central point of stimulation is made possible through Anode Intensification.  This can direct current off midline which is useful for cases where patient demonstrate a non-anatomic midline.  It is also useful in patients with severe scoliosis.