Method for application of locator in pedicle approach
The method (pedicle approach) was applicable to percutaneous pedicle screw surgery, including spine fracture, spine fusion) and percutaneous vertebroplasty (PVP) or percutanouskyphoplasty (PKP). This type of operation requires the establishment of a "bone channel" through the pedicle to enter the vertebral body. A "bony channel" which enters the vertebral body through the pedicle has to be established parallelly to the upper and lower endplates of vertebral body. It is not interfered by the factors such as changes of body posture, patient size, spinal degeneration. It only depends on the individual differences of each vertebral body and the purpose of operation. Therefore, the same reference plane must be selected for different vertebral bodies in order to accurately establish a bony channel. Usually, the patient is in the prone position in spine surgery, so that the horizontal plane was selected as the reference plane. Applying the locator, it was very important to make sure the base of locator parallel to the ground (A level instrument was used to ensure that the locator base was level in our surgery). Then the vertical line of the locator base plane was considered as the reference to set the head and tail tilting angles.
1. Determination of parameters based on X-ray and CT images before operation The method of 622-1 means that one point (The percutaneous puncture point) was determined by 6 lines, 2 angles and 2 distances.
There were 4 lines based on images, including the linea mediana posterior (Ml) and the pedicle channel line (PCl) in cross-sectional CT image through pedicles (Fig. 2A), the line through the needle entry point on pedicle (PUl) which paralleled with the upper edge of vertebral body (terminal plate of vertebral body) and a horizontal line that passed through the needle entry point on pedicle (Hl). Both PUl and Hl were in lateral X-ray image (Fig. 2B).
The other 2 lines were marked on the body surface of the patient, including the linea mediana posterior (ML, both Ml and ML are in the sagittal aspect of the body) and the horizontal line (HL, it was same as Hl in lateral X-ray image) through the projections of bilateral pedicles on the body surface (It was marked with the assistance of X-ray fluoroscopy) (Fig. 2C).
The 2 angles included the extroversion angle of a pedicle (named by Angle α) which was the included angle between Ml and PCl through the pedicle in CT image (Fig. 2A) and the head or tail tilting angle (named by Angle β) which was the included angle between PUl and Hl in lateral X-ray image (Fig. 2B).
The 2 distances were side opening distance (SD) and the head or tail tilting distance (H/TD). SD in CT image was the distance from the point where the line Ml passed through skin to the point where the line PCl passed through skin. While on the patient’s body surface, SD was the distance from the percutaneous puncture point to the line ML (Fig. 2A). H/TD in X-ray image was the distance from the point where the line PUl passed through skin to the point where the line Hl passed through skin While on the patient’s body surface, H/TD was the distance from the percutaneous puncture point to the line HL, Fig. 2B).
The percutaneous puncture point “P” on the patient’s body surface can be determined by the above-mentioned "lines" and "distances" (Fig. 2D). The Angle α and β were
2. The steps of using locator
After measurement of the above-mentioned "lines", "angles" and "distances", the steps of using locator in a surgery were as the followings.
Ⅰ. Posture Prone position was adopted. Lateral position could be adopted for special needs.
Ⅱ. Mark the lines ML and HL on the patient’s body surface The line HL was marked with the aid of X-ray fluoroscopy.
Ⅲ. Mark the percutaneous puncture point “P” on the patient’s body surface according to SD (the side opening distance) and H/TD (the head or tail tilting distance) Mark a dashed line(L) that was parallel to ML and SD cm away from ML, then mark a point (The percutaneous puncture point “P”) above HL on the line L to make it HD cm away from HL(If the pedicle was tilting to head) or mark it below HL on the line L to make it TD cm away from HL (If the pedicle was tilting to tail, Fig. 2D)
Graph a showing Line Ml and PCl, distance SD and angle α in cross–section CT image; Graph b showing Line PUl and Hl, distance H/TD and angle β in lateral X-ray image. Graph c showing Line ML and HL on body surface showed by a X-ray image. Graph d showing Line ML and HL, distance SD and H/TD and point P on body surface. Graph e showing Angle α and β showed on the two scale dials of locator. Graph f showing the locator in use. Ml and ML = linea mediana posterior; PCl = pedicle channel line; PUl = needle entry point on pedicle; Hl and HL = horizontal line; SD = side opening distance; H/TD = head or tail tilting distance; Angle α = extroversion angle; Angle β = head or tail tilting angle; P = percutaneous puncture point.
Ⅳ. Set the angles on the locator Adjust the hands position of the dials to Angle α and Angle β respectively (Fig. 2E), and place the locator horizontally on the back of patient by adjusting the screws at the bottom of locator with the aid of a level instrument and make the longitudinal axis of locator overlap or parallel to ML.
Ⅴ. Placement of pedicle guide needle Place a guide sleeve into the locator, and align the distal end of sleeve to the percutaneous puncture point “P” on the body surface of patient. Then put a guide needle into the sleeve, and insert percutaneously the guide needle into the dorsal bony cortex of the pedicle (Fig. 2F).
Ⅵ. The steps from Ⅲ to Ⅴ were repeated until all pedicle guide needles were placed.
Ⅶ. Confirm the accuracy of pedicle guide needles placement Take a positive and lateral of X-ray fluoroscopy to check whether all pedicle guide needles were placed accurately. If some guide needles are found to be unsatisfactory, correct them.
Ⅷ. Continue to perform the remaining operation.
Precautions in use of locator
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1. ML and HL must be marked accurately on the body surface of patient.
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2. The longitudinal axis of locator must overlap or be parallel to ML.
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3. The locator can’t be tilted left or right in use.
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4. If the patient has a scoliosis or rotation, the camber angle can be adjusted appropriately, so that the guide needles of different segments are kept in a longitudinal line.