Module: #15 Lumbar Spine Fusion. Author(s): Jenni Buckley, PhD. Date Created: March 27 th, Last Updated:

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1 Module: #15 Lumbar Spine Fusion Author(s): Jenni Buckley, PhD Date Created: March 27 th, 2011 Last Updated: Summary: Students will perform a single level lumbar spine fusion to treat lumbar spinal stenosis. The instructor will first give a brief overview of lumbar spinal anatomy and pathology and then present the students with a model of the procedure, which involves discectomy, anterior placement of a femoral ring allograft, and bilateral posterior screw-rod fusion. Students will use power and manual equipment to cut bone and install hardware. After finishing the procedure, students will manually check the rigidity of the fusion and also determine whether their procedure maintains the natural lordotic curvature of the spine. Learning Objectives Understand basic anatomy of the lumbar spine Understand symptoms and treatment for lumbar spinal stenosis Apply surgical planning and basic measurement techniques to create a solid anterior/posterior fusion Apply manual skills working with oscillating saw, hand and/or power drills, and several types of drivers Teaching Aids 8.5 x 11 laminated posters Demonstration Models 1. Surgical procedure Supplemental video on lumbar spinal stenosis ( Detailed Instructions 1. Overview from Instructor 1.1. (Model 1, Slides #1-3) Identify/define the following: Three regions of the spine (# vertebrae): Cervical (7), Thoracic (12), Lumbar (5) Kyphotic & lordotic curvature Anatomic directions: Anterior, posterior, medial, lateral, ventral, dorsal Joints of the spine: intervertebral disc (anterior), facet joints (posterior) Major ligaments: Anterior longitudinal, posterior longitudinal, ligamentum flavum Anatomy of a lumbar vertebra: superior & inferior facets, lamina, pedicle, spinous process, transverse process 2011, The Perry Initiative Page 1

2 Intervertebral Disc: Annulus Fibrosus, Nucleus Pulposus Nerves: Dorsal nerve roots, cauda equina Q. What is unique about the L5 vertebra? A. The L5 vertebra is wedge shaped, and this wedge contributes substantially to the natural lordotic curvature of the lumbar spine. Because of this wedging, the L4/L5 disc is prone to slipping anteriorly, causing back pain Lumbar spinal stenosis A common degenerative condition that occurs with aging Stenosis derives from the Greek word for choking, and it refers to the choking of the nerve roots and cauda equina (base of the spinal cord in the lumbar region) by bony or soft tissue protrusions in the lumbar spine Symptoms of stenosis include radiating leg pain (sciatica) and a painful tingling sensation in the lower back, buttocks, and legs. These symptoms are alleviated by laying down, sitting, or bending forward Stenosis may have many causes, including: (1) bone spurs on the underside of the lamina or the back surface (posterior) of the vertebral body; (2) bulging discs; or (3) bone spurs on the lateral margin of the vertebra that impinge on the exiting nerve roots Surgical treatment of spinal stenosis involves removing these impingements so that the nerve roots are no longer irritated. 2. Case Presentation & Surgical Treatment 2.1. (Slide 3) Patient has severe central spinal stenosis at L3 with bone spurs protruding from the underside of the lamina into the spinal canal and a painful L3/L4 disc (Model #2) Recommended treatment is a 2-stage anterior/posterior fusion Stage 1: Decompression with anterior placement of femoral ring allograft Stage 2: Posterior approach. L3 laminectomy and single level posterior screw-rod fusion (Model #2) Key observations from surgery The only thing holding the femoral ring allograft in place is the posterior screwrod fusion. For this reason, it must be very rigid Laminectomy exposes the spinal cord, but there are overlying muscles that still offer some protection to the cord The fusion maintains the natural lordotic curvature of the lumbar spine. 3. Logistics 3.1. Students are divided into small groups of 3-4 students and given an intact model of the lumbar spine At least one fully trained volunteer should be present at each station due to heavy use of power equipment Students are guided through Steps 5-9 in small groups. 2011, The Perry Initiative Page 2

3 4. Safety 4.1. Model can be held by spinous processes in universal bone holder Demo use of oscillating saw ( multi-tool ). Safety glasses must be worn by all participants when saw is on Students should be aware of location of Exacto knives at all times. 5. Surgical planning 5.1. Label vertebrae L1-L5 and sacrum Stage 1 Surgery: Anterior decompression with placement of femoral ring allograft Measure disc height and note wedge angle (if any) Plan cuts for femoral ring allograft from provided femur 5.3. Stage 2 Surgery: Laminectomy and Single-Level Pedicle Screw-Rod Fusion Mark cuts for laminectomy at L Mark start locations for L3 and L4 pedicle screws 6. Stage 1 Surgery: Anterior decompression with placement of femoral ring allograft 6.1. Cut the ALL with the safety scalpel Clean out the disc space with the rongeurs, preserving the PLL and as much of the lateral annulus as possible Using the oscillating saw, cut a section of the femur for the interbody spacer. Use the rasp and sandpaper to shape the ends to best fit the disc space Make sure that the interbody spacer is secure as possible. If not, the graft may fall out during Stage 2 Surgery. Students may manually replace, but it should be pointed out that surgeons cannot do this in actual practice because they will only have access to the posterior (dorsal) side of the spine during Stage 2 Surgery. 7. Stage 2 Surgery: Laminectomy and Single-Level Pedicle Screw-Rod Fusion 7.1. Perform laminectomy with oscillating saw (multi-tool). Students should be careful not to penetrate too deeply into the spinal canal Drill holes for pedicle screws, making sure not to penetrate pedicle cortex. Use sequentially larger drill bits up to final drill diameter of 1/ Students will not tap drill holes for this module, but normally surgeons will tap in real practice. Q. What is the advantage of tapping the pedicle screw holes? A. With tapping, you can use a smaller diameter drill hole and not risk fracturing the pedicle when you insert the screw. In order to prevent fracture without tapping (like the students did in this exercise), you must use a larger diameter drill hole. The larger the drill hole, the less screw purchase, and the easier it is to pull-out the screw Install pedicle screws bilaterally at L3 and L Connect screws with rods and secure rods with cap screws. 2011, The Perry Initiative Page 3

4 Q. The provided pedicle screws have some very sophisticated engineering design components, namely: (1) a polyaxial head design; and (2) a conical shape with different depth of threading. Both of these design components should be discussed, with students being asked to hypothesize on their purpose. A. (1) The polyaxial head can swivel and tilt to accommodate the rod coming in from different angles. (2) The conical shape and different threading depth increases screw purchase. The drill hole and tapping for the screw is sized for the threading at the screw tip, which means that the larger core diameter, thicker threads at the near end (closer to the screw head) will press fit into the threads cut by the screw tip. Q. What is the mechanism that locks the polyaxial screw head in place? What about the mechanism that locks the rod to the screw? A. The cap screw serves both purposes. It pushes the rod into the tulip of the polyaxial head. The base of the screw head is tapered, so that as the rod pushes against it, it locks in its current orientation Students will use household drivers and wrenches to install the pedicle screws and rods. In actual practice, surgeons use custom tools, a single set of which are provided for the instructor to demonstrate. Q. What are the advantages of using these custom tools for inserting pedicle screws? A. There are several custom tools, each with a specific purpose. (1) Polyaxial screw driver: Allows surgeon to insert the screw while holding the polyaxial head steady (it will otherwise toggle, as the students may have observed); (2) Polyaxial head stabilizer & nested cap screw driver: Allows surgeon to stabilize the polyaxial screw head while applying the cap screw. This prevents cross-threading of the cap screw. Students may have observed that it is difficult to install the cap screw without cross-threading. 8. Self-assessment 8.1. Students should manually bend (flexion, extension, right/left lateral, right/left axial rotation) to test the rigidity of their fusion Students should assess whether their fusion maintains the natural lordotic curvature of the lumbar spine Lumbar lordotic angle (LLA) is degrees for 95% of healthy adults (Lin et al, 1992) Reference points for this measurement are shown in Slide K-wires and goniometer are provided to make direct measurement of LLA. Students should be encouraged to problem-solve to construct a method to make this measurement. One solution is to insert k-wires parallel to inferior L5 and superior L1 endplates, extending anteriorly, and measure included angle using goniometer Time permitting, students may adjust the hardware for better spinal alignment and fusion. 2011, The Perry Initiative Page 4

5 9. Break-Down 9.1. Students should remove all hardware and place into appropriate bins Students may not keep the lumbar spine models, but they may keep the femoral ring allograft. 2011, The Perry Initiative Page 5

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