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BIOMECHANICS OF SPINAL INSTRUMENTATION

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  1. BIOMECHANICS OF SPINAL INSTRUMENTATION

  2. SPINAL INSTRUMENTATION • Goal: • To maintain anatomic alignment of injured spinal segments by sharing the loads acting on the spine until a solid biologic fusion takes place • Prerequisite Understandings: • Pathology • Spine Biomechanics • Biomechanics of implant constructs

  3. SPINAL INSTRUMENTATION • ADVANTAGES- Promote fusion - Enhance early mobilization • DISADVANTAGES- Adverse effects on the adjacent segments - Stress-shielding effects on the stabilized segment - Hardware Failure

  4. HARDWARE FAILURE • Breakage of Rods or Screws • Hook Dislodgement • Loosening • Screw Pull-out in cases of extreme Osteoporosis

  5. Biomechanical Consideration Factors • Biomechanical Strength (Static and Dynamic) • Impact strength • Surgical construct strength • Metal-metal interface strength • Bone-metal interface strength • Stability • Segmental stiffness/flexibility • Load Sharing • Implant survival • Stress shielding • Fusion rate and quality

  6. Implant Design Factors • Mechanical strength: • Profile: • Low profile preferred particularly in anterior instrumentation • User Friendliness: • Top loading • Poly-axial screw insertion • Lateral-medial ajustment • Easy-to-use surgical insruments • Versatility:

  7. Biomechanical Consideration Factors • Fretting Corrosion: • Any damage to the material that takes place at the edge of contacting parts or within the local contact area • Associated with wear, surface damage and accumulated debris • The rate of fretting corrosion is mostly governed by micromotion between component surfaces within an interconnection • Cause of late infection and loosening of interconnections • Greater tendency for: • designs with lower strength interconnections • Improper assembly such as misalignment and incomplete seating • Higher applied loads

  8. PULL-OUT AND CYCLIC FAILURES OF THE ANTERIOR VERTEBRAL SCREW FIXATION IN RELATION TO BONE MINERAL DENSITY Howard S. An, M.D. Tae-Hong Lim, Ph.D. Christopher Evanich, M.D. Toru Hasegawa, M.D. Kaya Y. Hasanoglu, M.D. Linda McGrady, B.S.

  9. Anterior Spinal Instrumentation • Fractures • Tumors • Correction of Deformities

  10. Hardware Failure • Breakage of rods or screws • Screw loosening • Screw pullout in cases of extreme Osteoporosis

  11. PREVIOUS STUDIES • Screw Pullout Strength in relation to Design Variables and Insertion Methods of Pedicle Screw • - Zindrick et al., 1986; Krag et al., 1986; Skinner -et al., 1990; Daftari et al., 1992 • Screw Pullout Strength in relation to Bone Mineral Density (BMD) • - Coe et al., 1990; Soshi et al., 1991; Yamagata et al., 1992; An et al., 1993

  12. PREVIOUS STUDIES • Loosening of the Pedicles Screw in relation to Design Variables and Insertion Methods of Pedicle Screw • - Zindrick et al., 1986 • No BMD Measurement

  13. Correlation between Anterior Vertebral Screw Fixation Failure And Bone Mineral Density (BMD) Of the Verebral Body

  14. PURPOSE OF THE STUDY • Relationships between BMD and: • - Pull-out Strength • - Cyclic Screw Loosening

  15. PART I: Pull-out Test (6 fresh-frozen lumbar spines) • PART II: Cyclic Loosening Test (5 fresh-frozen lumbar spines)

  16. MEASUREMENT OFBMD (g/cm2) • Dual Energy X-ray Absorptiometry (DEXA) - Scan Speed: 60 mm/sec - Resolution: 1 x 1 mm • Lateral view of each lumbar spine

  17. Kaneda Screw (6.5 mm diameter; 55 mm long) • Measurement of Torque (Nm) and Vertebral Width (mm) for screw insertion

  18. Pull-out Strength Test • Screw was pulled out along its long axis. • Loading Rate: 10 mm/min (Displacement Control)

  19. DATA ANALYSIS(Pull-out Strength Test) • Pull-out Strength • Maximum pull-out force in the load displacement curve • Regression Analyses

  20. Cyclic Loading Test to Induce Screw Loosening Cyclic loading was applied to the screw in the cephalad-caudal direction using an MTS machine. • Loading Frequnecy: 0.5 Hz • Loading Amplitude: 200 N  100 N 

  21. DATA ANALYSIS(Cyclic Loosening Test) • Number of Loading Cycles (NLC) to Induce the Screw Loosening: • when the displacement > the displacement at the first peak load + 1 mm. • Regression analysis • NLCs for • specimens with BMD < 0.45 vs. BMD > 0.45.

  22. RESULTS

  23. Means (SD) ofMeasured Parameters(Pull-out Test) BMD: 0.58 g/cm2 (STD 0.14) Torque: 0.86 Nm (STD 0.2) Depth: 41.0 mm (STD 3.28) Pull-out Strength: 211 N (STD 124)

  24. CORRELATION COEFFICIENTS(Pull-out Test) Pull-out Strength BMD Torque (T) Width (W) Pull-out Strength 1.0 0.85 0.47 No relation BMD - 1.0 0.58 No relation - - 1.0 No relation Torque (T) - - - 1.0 Width (W)

  25. CORRELATION Pull-out Strength vs. BMD R = 0.83 (p < 0.0002) Pull-out Strength = -226 + 774 x BMD

  26. Pull-out Strength vs. BMD Pull-out Force (N) BMD (g/cm2)

  27. Means (SD) ofMeasured Parameters(Cyclic Loosening Test) BMD: 0.32 g/cm2 (STD 0.10) Torque: 6.9 Kg-cm (STD 2.8) NLC: 149 (STD 234) (5 to 960)

  28. CORRELATION(Cyclic Loosening Test) NLC vs. BMD Second Order Polynomial Relationship NLC = -1190 BMD + 3168 BMD2 R = 0.80 (p < 0.01)

  29. NLC vs. BMD

  30. CORRELATIONS(Cyclic Loosening Test) • NLC vs. Screw Insertion Torque: Second Order Polynomial R = 0.68 (p < 0.01) • BMD vs. Screw Insertion Torque: Linear R = 0.51 (p < 0.02)

  31. MEAN NLCs (SD) • 12 Specimens with BMD<0.45: 18.0 (20.1) 11 Specimens with BMD>0.45: 270.8 (278.8) • These were significantly different. (p=0.003)

  32. BMD: • correlation with pull-out strength as well as cyclic loosening failure of the anterior vertebral screw • a useful means to evaluate the severity of osteoporosis • DEXA: measurement of BMD - Low-radiation dose - Short scanning time - Increased image resolution - Improved precision

  33. CONCLUSION • Quantitative assessment of BMD using a DEXA unit may be a good predictor of anterior vertebral screw fixation failure. • BMD < 0.45 g/cm2 may be the critical value for osteoporosis in screw loosening.

  34. PREDICTION OF FATIGUE LOOSENING FAILURE OF THE PEDICLE SCREW FIXATION Tae-Hong Lim, Ph.D. Lee H. Riley, III, M.D. Howard S. An, M.D. Linda M. McGrady, B.S. John Klein, M.S.

  35. Pullout Strength of the Pedicle Screw • In relation to Pedicle Screw Design Variables • - Zindrick et al., 1986; Krag et al., 1986; Skinner -et al., 1990; Daftari et al., 1992 • Effects of screw insertion torque, screw hole preparation method, and screw angulation • - Skinner et al. 1991; Daftari et al. , 1992; Zdeblick et al. , 1993 • In relation to Bone Mineral Density (BMD) • - Coe et al., 1990; Soshi et al., 1991; Yamagata et al., 1992

  36. Loosening Failure of the Screw Fixation • Pedicles Screw Loosening in relation to Design Variables and Insertion Methods of Pedicle Screw • - Zindrick et al., 1986 • Anterior Vertebral Screw Loosening in relation to the Bone Mineral Density of the Vertebral Body • - Lim et al., 1994

  37. PURPOSE Fatigue Loosening of the Pedicle Screw in relation to Bone Mineral Density of the Vertebral Body

  38. PURPOSE Fatigue Loosening of the Pedicle Screw in relation to Pedicle Size and Screw Insertion Torque

  39. MATERIALS • Three fresh frozen human lumbar spines (L1 - L5) were used in this study. • Anterior-posterior and lateral radiographs were taken to exclude spines with gross pathology.

  40. MEASUREMENT OFBMD (g/cm2) • Dual Energy X-ray Absorptiometry (DEXA) - Scan Speed: 60 mm/sec - Resolution: 1 x 1 mm • Lateral view of each lumbar spine

  41. Pedicle Size Measurement • Pedicle Height (PH): Long Axis (cephalad-caudal direction) • Pedicle Width (PW): Short Axis (medial-lateral direction) • Pedicle Screw Placement • A 3.5 mm drill hole and 6.25 mm tapper • A 6.25 mm Steffee Screw (40 mm long) • Screw Insertion Torque (TQ) • Measured using a Torque Wrench

  42. Cyclic Loading Test to Induce Screw Loosening Cyclic loading was applied to the screw in the cephalad-caudal direction using an MTS machine. • Loading Frequency: 0.5 Hz • Loading Amplitude: 200 N  100 N

  43. Cyclic Test Set-up MTS Load Cell MMA Loading Direction Fixture RAM

  44. DATA ANALYSIS • Number of Loading Cycles (NLC) to Induce the Screw Loosening: • when the displacement > the displacement at the first peak load + 1 mm. • Regression analysis • Relationships between NLC, BMD, PH, PW, and TQ

  45. Means (SDs) of the Measured Parameters BMD: 0.465 g/cm2 (0.187) PH: 15.57 mm (4.97) PW: 12.08 mm (2.44) TQ: 9.25 Kg-cm (3.14) NLC: 300 (377)

  46. Correlation between the Measured Parameters BMD PH PW TQ NLC BMD 1.0 - - - - PH 0.54 1.0 - - - PW *0.07 0.51 1.0 - - TQ *0.44 0.59 *0.11 1.0 - NLC 0.580.72 *0.42 0.50 1.0 * indicates relationship with no statistical significance (p > 0.05).

  47. Linear Relationship between NLC and BMD NLC BMD (g/cm/cm)

  48. Linear Relationship between NLC and PH NLC PH (mm)

  49. Multiple regression analysis demonstrated a significant correlation between NLC and other measured parameters. (R = 0.79, p = 0.02) • NLC = 775.8 x BMD + 22.4 x PH - 44.3 PW + 15.5 TQ - 17.2

  50. The stepwise regression analysis revealed that PH was the most significant predictor of pedicle screw loosening.