X-Ray Jig Technique

 

 X-Ray Jig Approach Technique for Entry Portal Selection

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SPINAL PROBING & DISCOGRAPHY TECHNIQUE

Patient Position

Patient position is crucial to the successful performance of spinal probing, discography and endoscopic lumbar surgery. The use of a humpback radiolucent table extension fitted to a conventional operating table has proven valuable. This should have a polyurethane moulded table cushion with adjustment for width, abdominal aperture and leg length. It should allow variation of the angulation between the body and the legs so that the intersegmental vertebral alignment can be flexed and the foramen opened for improved posterolateral access.

Figure 1 The table extension with variable angulation, leg length adjustment and saddle width correction.

The lateral position, in which the patient lies on one side, limits the access to one side and is uncomfortable for the patient. The prone position is more versatile allowing double sided interventions when required and easy antero-posterior and lateral x-ray access. The use of a hump backed profile table is optimal for image intensifier access but requires that the patient be accurately placed on the table. Head down tilt can be increased easily to limit caudal migration of the patient down the table.

The prone position can also be used to advantage in facilitating the “Push-up” test and the “Straight Leg Flexion” test and “Straight Leg Extension” tests These tests are effected on the operating table before surgery commences and the symptoms in the back, buttock or leg are recorded. Subsequent diminution after foraminal clearance and undercutting usually indicates sufficient treatment of the pathology and undercutting has usually been achieved in uni-segmental pathology.

ENTRY PORTAL SELECTION

Short pedicles, retrolisthesis, intradiscal tilting, hyperextension or flat back alignment, obesity, wide build or a narrow pelvis compromises the posterolateral approach to the lumbar spine. To ensure the optimal approach to the foramen an X-ray alignment jig has been designed to select the optimal skin portal and approach line to the foramen

NAVIGATION CONCEPT

Figure 2 Triangular concept underlying the navigational system

The jig and an AP line taken in the sagittal plane form the two sides of a right angle. The approach line forms the base of a triangle where the pendular line has been transposed to the midline. The base makes a 450 angle to the other lines. If the sagittal or pendular line is extended anteriorly, the corresponding measurement extends the transverse line laterally. The target, the Foramen lies more posteriorly. When approached from this entry portal based upon the anterior vertebral margin, the approach will more nearly approximate to 350 in keeping with the needs of ELF.

The system is based upon a right angle triangle providing a 450 approach to the anterior vertebral margin and consequently a 300 - 350 approach to the foramen. It is tailored to the idiosyncrasies of skeletal structure, variations in alignment, patient habitus and optimises the approach.

X-RAY GUIDANCE JIG APPLICATION TECHNIQUE

The L3/4 and L4/5 approach

The approach to levels L3/4 and L4/5 uses the standard technique. The point of skin entry should be at right angles to the point of entry in to the disc in the lateral plane. Entry in to any lumbar disc is ideally effected at the midpedicular line in the antero-posterior (AP) projection and on the posterior annulus. Ideal alignment may be confounded by intradiscal tilting, asymmetrical vertebrae or scoliosis. The use of the jig will assist in minimising these distorting effects.

Figure 3 Radiological anatomical landmarks in the antero-posterior (AP) view

Misleading features can be accommodated by setting the C-arm parallel to the endplates in the AP projection while ensuring that the dorsal spines are equidistant between the pedicles and that overlap of the endplate margins is avoided at the outset by adjusting the tilt of the C-arm in the AP plane.

Figure 4 X-Ray jig applied. This shows a caudally directed view of the jig with the C-arm in AP alignment with the patient lying prone

Figure 5 X-ray jig Cephalic directed view.

The lower margin of the jig is then aligned with the midpoint of the disc space in the AP projection and a line is drawn on the skin along the lower border of the jig. This line is drawn out to the flank and represents the “Transverse Line” (T).

The C-arm is rotated in to the lateral position. In the lateral projection, the C-arm is swung about the vertical axis until the vertical free hanging pendulae overlap and are therefore parallel.
Figure 6 AP view of Jig aligned to lower border of L3

The wheels of the C-arm are locked so that the C-arm base can ONLY be moved parallel to the sagittal axis of the patient thus retaining the ideal transverse alignment of the C-arm.

The pendulum “Bob” on the emitter side is raised or lowered until it overlies the “midpoint” of the disc for a 450 approach to the interstices of the disc and foramen. However, it is recommended that the “Bob” be set for length to the “anterior” margin of the disc to produce an angle of approach of approximately 350 to the foramen for endoscopic procedures. These chosen angles are used for specific but separate applications.

Figure 7 Radiological anatomical landmarks in the lateral view

The anterior vertebral margin as a marker approximates to a 450 approach to the disc and allows access to the Safe Working Zone (SWZ) for small (2 mm) diameter instrumentation in procedures such as LDD. For endoscopic approaches, this angle will cause the dilator tube to impinge on the lateral aspect of the facet and cause pressure on the exiting nerve root. Excessive verticality may force the surgeon to excessively displace the exiting nerve or mistakenly to operate lateral to the nerve. This will preclude access to the epidural space through the foramen more especially in cases where there is disc settlement or facet joint hypertrophy. The anterior border of the disc is preferred as the marker for ELF because it facilitates an approach passing medial and posterior to the exiting nerve root and facilitates the exploration of the epidural space and the SWZ. The selected length is measured as the “Bob length” (B). This length (B) is then marked from the midline along the “Transverse Line” (T) and marks the lateral distance from the midline for the desired entry portal.

The L5/S1 approach

When addressing the L5/S1 level, the surgeon should make especial effort to choose the optimal skin incision because the L5/S1 approach is more likely to be confounded by anatomical constraints than that at L3/4 or L4/5.

Figure 8 The Jig aligned on L4 (AP view)

AP X-ray view of lower margin of jig aligned on the lower border of L4 with the C-arm rotated in to the axis of the disc

However, the portal can often be used to approach both L5/S1 and L4/5 levels at the same intervention. For the L5/S1 approach, the jig is set up as described above and aligned on the L4/L5 disc in the AP and lateral planes (Figure 64 above & Figure 65 below).

Figure 9 The Jig aligned on L4 (Lateral view)

AP X-ray view of lower margin of jig aligned on the lower border of L4 with the C-arm rotated in to the axis of the disc

The overlapping pendulae and L4 vertebra are positioned at the junction of the cephalad-middle third of the X-ray frame. The L5/S1 disc space should lie at or just above the caudal-middle third junction of the screen. The C-arm is rotated until the vertically hanging pendulae overlap.

Figure 10 The Jig pendulae rotated to L5/S1 (lateral prior to distal replacement of the C-arm)

Lateral x-ray view of pendulae rotated to align with the L5/S1 disc. The pendulae may lie above or below the S1 endplate in this preliminary position because of parallax. The base of the C-arm is then moved caudally, along the sagittal axis of the patient until the pendulae overlap.

Figure 11 L5 Pendular distal convergence

The pendulae overlap as the C-arm is moved en masse distally

The position of the jig should be adjusted superiorly or inferiorly until the pendulae clear the lower margin of the transverse process of L5. Adjustment must be associated with alteration in the position of the body of the C-arm to maintain the overlap of the pendulae and retention of parallel alignment with the lower margin of the L5 endplate.

The bobs are aligned with promontory of S1 and the selected length is measured as the “Bob length” (B). This length is measured along the pendulum rather than vertically. This distance is marked from the midline along the “Transverse Line” (T). This length marks the distance from the midline for the entry portal, but does not account for the idiosyncratic obliquity of the approach enforced by the iliac crests and an additional compensatory step is required.

The C-arm is returned to the AP projection and the vertical angle adjusted to match the L5 endplate plane. The “Bob” length is marked from the midline along the lower margin of the jig. The sidearm is rotated caudally crossing the jig margin at this mark (Figure 68 below). The L5/S1 level approach may be anatomically compromised by factors such as the obstructive presence of the iliac crests, higher in males, the narrowness of the android pelvis, the presence of L5 vertebral retroversion, disc settlement, posterior and lateral osteophytes, hypertrophic facet joints and pelvic anteversion, large transverse processes and pseudarthroses (Figure 69 below).

Figure 12 X-Ray Jig with sidearm extended

The jig seeks to optimise the approach by addressing these problems by the use of the “Side arm line” (S)

Figure 13 Selecting the entry portal

The compensatory step consists of a line is drawn along the inferior border of the sidearm and projected until it coincides with the distance from the midline established by the “Bob” length. This optimises the approach even with an android pelvis.

The side arm is swung out from the lower border of the jig until the lower free margin of the side arm ruler passes below the pedicle of L5 and overlaps the midpedicular point on the L5/S1 annulus. The C-arm is moved medio-laterally until the hinge lies at the midpoint of the outer third of the screen. The sidearm is deployed and swung out to “graze” the iliac crest initially guided by palpation and projected towards the L5/S1 pedicles (Figure 70 below). It is subsequently checked fluoroscopically.

Figure 14 The sidearm is aligned on iliac crest by palpation

The hinge of the side arm is moved medio-laterally within the jig beam until the lower margin of the ruler abuts the outline of the iliac crest on the AP X-ray. The medial alignment must be maintained below the pedicle of L5. The lower border of the side arm is marked as the “Side arm line (S).

Figure 15 The sidearm is aligned on

Iliac crest and foramen radiologically

The entry portal is defined as the point of coincidence between the “Side arm line“(S) and the “Transverse Line” (T). Where this crossing point is short of the length established as the “Bob length” (B), then the entry point is arrived at by projecting the “Side arm Line (S)” until the “Bob length” (B) from the midline is obtained.

Figure 16 The convergence of Transverse & Sidearm lines

This technique allows the approach to pass close to the iliac crest and brings the approach probe to the annulus, avoiding impediment from the transverse process of L5. This also allows a direct approach to the L4/L5 disc under normal circumstances thus limiting the number of incisions.

Figure 17 Separate incisions used for L5/S1 & L4/5 are combined by using this technique

The L1/2 & L2/3 Approach

Access to L1/2 & L2/3 is forced by the lower rib margin to be oblique. The procedure starts as that described for the L5/S1 level but modified inversely. The C-arm is set in the AP projection and adjusted until the L2/3 endplates are parallel and free from overlap at the target disc. The jig is placed upon the patient in the “inverted” position with the pendulae cephalad. The jig is applied and aligned in the AP projection to the inferior endplate of L2. The C-arm is rotated into the lateral plane and aligned about the vertical until both pendulae overlap. The “Transverse Line” (T) is marked along the superior margin of the jig.

The pendulae are rotated to lie parallel to the superior margin of the inferior vertebra abutting the target disc. With the C-arm locked about the vertical axis, it is moved en masse cephalad until the pendulae overlap again. In the case of the L1/2 disc, the jig may need to be moved cephalad until the pendulae clear the transverse processed of L1 and the C-arm moved cephalad or caudally until the pendulae overlap again after the body of the jig has been adjusted.

The bobs are adjusted to coincide on the anterior border of the target disc. The selected length is measured as the “Bob length” (B). This length is measured along the pendulum rather than vertically. This distance is marked from the midline along the “Transverse Line” (T) and marks the distance from the midline for the entry portal for the desired procedure but does not account for the idiosyncratic obliquity enforced by the rib margin and the need to circumvent the visceral space and kidneys. The jig seeks to optimise the approach by addressing these problems by the use of the “Side arm line” (S)

The C-arm is restored to the AP projection and the vertical angle adjusted to match the angle of the pendulae locked on the L1 or L2 endplates.

The side arm is swung out from the upper border of the jig until the superior free margin of the side arm ruler passes above the pedicle of L2 or L3 and marks the midpedicular point on the target annulus. The C-arm is moved medio-laterally until the hinge lies at the midpoint of the outer third of the screen.

The axial hinge of the side arm is moved medio-laterally until the upper margin of the ruler abuts the outline of the lowest rib on the AP X-ray. The medial alignment must be maintained above the selected pedicle. The C-arm should be adjusted so that the hinge of the side arm lies between the midpoint of the screen and the lateral third junction.

The upper border of the side arm is marked out as the “Side arm line“(S).

The entry portal is defined as the point of coincidence between the “Sidearm line“(S) and the “Transverse Line” (T). Where this crossing point is short of the length established as the “Bob length” (B), then the entry point is arrived at by projecting along the “Side arm Line (S)” until the “Bob length” (B) is obtained from the midline and measured along the “Transverse Line” (T).

This technique allows the approach to pass close to the lowest rib and brings the approach probe to the annulus, avoiding impediment from the transverse process of L2. It allows direct approach to the L1/L2 or L2/L3 disc under normal circumstances thus limiting the number of incisions.

The approach to L1/2 & L2/3 levels is compromised by factors including the obstructive presence of the lower ribs, long or inferiorly angulated transverse processes, the shallow position of the posterior visceral cavity in some patients and the kidneys and adrenal glands. Analysis of the MRI scan is advised to check these features pre-operatively. When selecting the skin portal the safest point is at the border of the thoracolumbar fascia at the upper border of the jig after displacement to compensate for the lower margin of the rib. When venturing to use the ideal position more laterally the initial approach should initially be shallow deepening after the border of the thoracolumbar facia has been traversed.

Comment

The above technique has facilitated this approach in males and females of widely varying frames over many years of practice and while accommodating for variations in anatomy, habitus, obesity, sex and degeneration, it has avoided damage to perineural or visceral structures to date.