SPINAL FUSION AND INSTRUMENTATION Tae-Hong Lim, Ph.D. Department of Biomedical Engineering The University of Iowa Iowa City, Iowa

Normal Function of the Spine Protect spinal cord and nerves Support the body weight and external load Stability Allow motion of the body for various activities Flexibility

Spinal Disorders Trauma Tumor Infection & Inflammatory Disease Fractures, Whiplash injury, etc. Tumor Infection & Inflammatory Disease Deformity Scoliosis, spondylolisthesis, degenerative lumbar kyphosis, etc. Cervical & Low-back Pain Degenerative disease, such as disc herniation, stenosis, spondylolisthesis, etc.

Treatment of Spinal Disorders Conservative Treatment Degenerative disease Stable fracture Mild deformity Surgical Treatment Failed conservative treatment Unstable fracture (dislocation) Progressive deformity

Goals of Spine Surgery Relieve pain by eliminating the source of problems (decompression) Stabilize the spinal segments after decompression Restore the structural integrity of the spine (almost normal mechanical function of the spine) Maintain the correction Prevent the progression of deformity of the spine

Spinal Fusion Elimination of segmental movement across an intervertebral segment by bone union One of the most commonly performed, yet incompletely understood procedures in spine surgery Non-union rate: 5 to 35 %

Types of Fusion

Factors for Consideration in Spine Fusion Biologic Factors Local Factors: Soft tissue bed, Graft recipient site preparation, Radiation, Tumor and bone disease, Growth factors, Electrical or ultrasonic stimulation Systematic Factors: Osteoporosis, Hormones, Nutrition, Drugs, Smoking Graft Factors Material, Mechanical strength, Size, Location Biomechanical Factors Stability, Loading

Properties of Graft Materials Graft Osteogenic Oseto- Osteo- Materials Potential induction conduction Autogenous bone o o o Bone marrow cells o ? x Allograft Bone x ? o Xenograft bone x x o DBM x o o BMPs x o x Ceramics x x o DBM = Demineralized bone matrix; BMP = Bone morphogenetic proteins

Spinal Instrumentation Goals of Spinal Instrumentation: Correction of deformities or misaligned segments; Enhancement of solid fusion; Maintain anatomic alignment until a solid fusion takes place; and Allow early mobilization of patients by providing an immediate stability

Spinal Instrumentation Types Implantation Method: Wiring, Hooks, Screws Rods vs. Plates Spinal Level: Cervical, Thoracolumbar Position: Anterior vs. Posterior Instrumentation Vertebra Graft Vertebra Pedicle screw instrumentation

Cervical Spine Instrumentation

Cervical Spine Instrumentation

Thoracolumbar Spine Instrumentation Z-plate (Danek) Kaneda (AcroMed)

Thoracolumbar Spine Instrumentation

Thoracolumbar Spine Instrumentation

Operative Techniques Patient Positioning: The intra-abdominal pressure must be minimized to avoid venous congestion and excess intraoperative bleeding, while allowing adequate ventilation of the anesthetized patient. Surgical exposure of the lumbar spine: Midline incision extended to an additional level

Screw Hole Preparation GSFS Implantation Procedure Screw Hole Preparation Exposure of the junction between the pars interarticularis and transeverse processes Pedicle entrance point is at the crossing of two lines Vertical line: 2-3 mm lateral from the pars and slants slightly from L4 to S1. Horizontal line passes through the middle of the insertion of the transverse processes or 1-2 mm below the joint line. 1-2 mm lateral from the center of the pedicle to insert the screw without disturbing the facet joint above and to medialize the screw for better fixation.

Screw Hole Preparation GSFS Implantation Procedure Screw Hole Preparation Angle and depth of the screw holes?

Direction and Depth of the Screw

Preparation of Fusion Bed and Grafting GSFS Implantation Procedure Preparation of Fusion Bed and Grafting Decortication Marking screw holes Grafting

Screw Selection and Insertion GSFS Implantation Procedure Screw Selection and Insertion Screw Diameter: approx. 80% of the medial diameter of the pedicle Perforation of the pedicle into the medial or inferior side has higher chance of nerve root injury. Screw Length: Long enough to pass the half of the vertebral body but Short enough not to penetrate the anterior cortex Screw Length For GSFS

Rod-Connector-Screw Assembly GSFS Implantation Procedure Rod-Connector-Screw Assembly Rod Length: - Rod length must not be too long so that the proximal tip of the rod do not touch the inferior facet of the upper vertebra. Rod Bending Connector Selection Rod-Connector Assembly Screw-Connector-Rod Assembly Tightening the nuts and set screws

Rod-Screw Assembly

Rod-Screw Assembly

Rod-Screw Assembly Medial-lateral adjustability can eliminate: The use of additional components; and Application of force in medial-lateral directions or additional rod bending In order to make the rod-screw connection

Rod-Connector-Screw Assembly GSFS Implantation Procedure Rod-Connector-Screw Assembly GSFS: - Screw-Connector: Polyaxial - Connector Length: M-L Adjustment *No precise rod-bending is required. *Screw alignment is not as critical.

Rod-Connector-Screw Assembly GSFS Implantation Procedure Rod-Connector-Screw Assembly Rod-bending; Insert the rod to the connectors; Temporary tightening of set screws of the proximal and distal most connectors; Place the rod-connector assembly on the screws; Tightening the screw caps and set-screws in the proximal and distal most connectors while holding the rod in a desired shape; and Fix the other screw caps and set-screws in the mid-portion.

Ideal Features The use of connectors: Polyaxial and medial-lateral adjustability; No need for precise rod bending Easy screw-rod connection without a good alignment of screw heads Screw insertion according to the best possible anatomic conditions Rigid connection at rod-connector and screw-cap connection: Strong maintenance of correction Better mechanical environment to enhance bone healing (fusion) Top-tightening: Low Assembly profile:

Consideration Factors in Spinal Instrumentation Materials: Bio-compatibility and Imaging compatibility Stiffness (or elasticity) and strength Corrosion Implant Strength: Component (screw, rod, plate, wire, etc.) strength Metal-metal interface strength Construct strength Bone-metal interface strength: Bone–wire, -hook, and -screws Construct Stability: Segmental stiffness or flexibility Profile: Ease of Use:

Spinal Implant Materials 316L Stainless steel: Biocompatible Strong and stiff Poor imaging compatibility: artifact to CT and MRI Titanium Alloy (Ti6Al4V ELI): No artifacts during CT and MRI Excellent fatigue strength, high strength, high elasticity High resistance to fretting corrosion and wear (surface treatments)

Spinal Implant Strength Static and Fatigue Strength of Components: Depends on the material properties, size and shape of the components Metal-metal Interface Strength: Rod-screw connections GSFS (rod-connector and screw-connector interfaces): excellent Construct Strength: Excellent in GSFS

Bone-Metal Interface Strength Pedicle screws are known to provide the strongest bone purchase compared to wires, hooks, and vertebral screws. Screw Pullout Strength: Affected by major diameter and bone quality (BMD) but not by minor diameter, thread type, and thread size. Insertion depth is not critical. Screw insertion torque was known to have relationship with screw pullout strength. Conical screws showed similar pullout strength to that of the cylindrical screws.

Surgical Construct Stability Construct stability varies depending on the size of the screws and rods (plates). Recommended rod diameter is 6 mm or ¼ inch in adult spine surgery. Preservation of more than 70% of the disc or meticulous anterior grafting is critical to obtain stable construct with no hardware failure (screw or rod breakage). Modern spinal fixation systems, regardless of anterior or posterior fixation, similarly significant stability in flexion, extension, and lateral bending, but not effective in preventing axial rotational (AR) motion. Use of a crosslink (DDT) is recommended to improve the AR stability, particularly in the fixation of long segments (more than 2 levels).

Surgical Construct Stability Ligamentous spines Pure moment in FLX, EXT, LB, and AR Maximum 8.2 Nm 3-D motion analysis system EXT AR LB FLX FLX L2 LB AR EXT L5

Implant Assembly Profile Anterior Instrumentation: Critical in anterior plating of the cervical spine, and the profile must be less than 3 mm. Lower profile is recommended in the anterior fixation of the thoracolumbar spine. Posterior Instrumentation: Assembly profile is not as critical as in anterior fixation, but lower profile is recommended because a high profile may cause a surgery for implant removal due to patients’ uncomfortness.

Ease of System Assembly Screw Insertion: Screw insertion according to the best possible anatomic orientation and location Adjustment in Screw-Rod Assembly: Rod bending Angular adjustment Medial-lateral adjustment Polyaxial screw head vs. Connector Top-tightening All assembly procedures can be made from the top.

BIOMECHANICAL EVALUATION OF DIAGONAL TRANSFIXATION IN PEDICLE SCREW INSTRUMENTATION Tae-Hong Lim, Ph.D. Atsushi Fujiwara, M.D. Jesse Kim, B.S. Timothy T. Yoon Sung-Chul Lee, M.D. Howard S. An, M.D.

Horizontal Transfixation (HTF) Construct stability No improvement in FLX and EXT Some improvement in LB Significant improvement in AR Increased AR stability when using 2 transfixators Optimum position for TF Proximal 1/4 points for 1 TF Proximal 1/8 and middle points for 2 TF Lim et al. 1995 Transfixator (TF) VB VB Pedicle screw instrumentation

Diagonal Transfixation (DTF) Construct stability No changes in FLX (Texada et al, 1999) Significant improvement in LB and AR (Texada et al., 1999; McLain et al. 1999) Transfixator (TF) VB VB Pedicle screw instrumentation

Diagonal Transfixation (DTF) Clinical application of DTF using 2 TFs may not be practical. Limited space Higher construct profile DTF using 1 TF is feasible, but its effect has not been investigated yet.

PURPOSE To evaluate the effect of diagonal transfixation (DTF) on the construct stability and the corresponding stress changes in the pedicle screw in comparison with the effect of horizontal transfixation (HTF)

MATERIALS and METHODS

Finite element studies Flexibility tests Unstable Calf Spine Model Finite element studies

FLEXIBILITY TESTS 10 Ligamentous calf spines (L2-L5) Pure moment in FLX, EXT, LB, and AR Maximum 8.2 Nm 3-D motion analysis system EXT AR LB FLX FLX L2 LB AR EXT L5

Tested Constructs - Intact - Instrumentation without TF after total discectomy (no TF) - Instrumentation with HTF using 1 TF (HTF) - Instrumentation with DTF using 1 TF (DTF) Diapason Spinale Fixation System (Stryker, Allendale, NJ: 6.5 mm screws and 6 mm rods and TF)

Finite Element Studies To investigate the stress changes in the pedicle screws due to HTF and DTF. Boundary and Loading Conditions: Nodes in lower vertebra were held fixed. FLX, EXT, LB, and AR Moments (8.2 Nm) at the middle point of the vertebra element ADINA Finite Element Analysis S/W

Finite Element Models Moment Moment Vertebrae Transfixators Fixed Nodes (A) Horizontal transfixation (HTF) (B) Diagonal transfixation (DTF)

Data Analysis Rotational motion of L3 with respect to L4 in response to 8.2 Nm Rate of motion change with respect to Intact case No TF case Total load = [Mx2 + My2 + Mz2]1/2 Mx = Torsional moment; My & Mz = Bending moments Stress change  changes in total load

RESULTS

Rotational Motions (deg) responding to Applied Moments of 8.2 Nm INT no TF HTF DTF 9 8 7 6 5 Rotational Angle (deg) 4 3 2 1 Flexion Extension Lateral Bending Axial Rotation Loading Directions

Mean Rate of Motion Change from Intact Case 0.4 no TF HTF 0.2 DTF * * * 0.0 Rate of Motion Change -0.2 -0.4 -0.6 -0.8 -1.0 Flexion Extension Lateral Bending Axial Rotation

Mean Rate of Motion Change from no TF Case 0.2 HTF DTF 0.1 0.0 Rate of Motion Change -0.1 -0.2 * * * -0.3 * -0.4 Flexion Extension Lateral Bending Axial Rotation Loading Modes

Rate of Motion Change with respect to no TF Case (FE Model Predictions)

Rate of Total Load (Stress) Changes in Pedicle Screws (FE Model Predictions)

DISCUSSION

The effect of DTF using 1 crosslinking device on the construct stability and the corresponding stress changes in the pedicle screws was investigated using flexibility tests and finite element techniques. In flexibility tests: Calf spines were used to reduce inter-specimen variability. Most unstable model was made by performing total discectomy to highlight the stabilizing effect of pedicle screw instrumentation. Motion data were normalized by those of the intact and no TF case to emphasize the effect of TF. For FE studies: Beam element was used for modeling for simplification. Predicted motion changes showed a good agreement with measured data. Stress changes were represented by the changes in total load in screws because of linear nature of the model.

Summary of Findings in Comparison with no TF Case DTF Construct stability: Significant improvement in FLX/EXT no improvement in LB and AR Stress in the screws: 12% in left screw & 11% in right screw in FLX/EXT 44% in left screw & 7% in right screw in LB 8% in left screw & 18% in right screw in AR HTF Construct stability: no improvement in FLX/EXT Significant improvement in LB and AR Stress in the screws: No increase in FLX/EXT 28% increase in LB 58% decrease in AR

CONCLUSION DTF provides more rigid fixation in FLX and EXT but less in LB and AR as compared with HTF case. Pedicle screws may experience greater stresses in DTF than in HTF. These limitations of DTF should be considered for clinical application.