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Journal Club

July 2010

Far Cortical Locking Can Improve Healing of Fractures Stabilized with Locking Plates

Author: M Bottlang, M Lesser, J Koerber, J Doornink, B Von Rechenberg, P Augat, D Fitzpatrick, S Madey, J Marsh

Institution: Legacy Biomechanics Lab Portland Oregon, University of Zurich Switzerland, Institute of Biomechanics Murnau Germany

Journal: JBJS Volume 92-A, Number 7, July 7 2010

Funding: Direct grant Zimmer National Health Institutes of Health, Author/s received > $10000 Zimmer < $10000 Stryker

Introduction

• Locking plates
  • Locking plates rely on secondary fracture healing by callus formation
  • Improved fixation in osteoporotic bone
  • Biological friendly fixation
  • Locking plates are reported to be as stiff as conventional plating
  • Can suppress fracture motion and secondary bone healing
• Far-cortical locking
  • is capable of reducing stiffness
  • Maintain strength
  • Act as elastic cantilever beams
  • Allows increased motion at fracture site
• Hypothesis
  • That a stiffness-reduced far cortical locking construct can improve fracture healing compared with a standard locked plate construct
  • Far-locking screw - parallel interfragmentary motion under axial load
    • Fracture model in sheep
      • 3mm osteotomy gap of tibia stabilized with a standard locking plate and a far-cortical locking plate construct

Methodology

• 12 sheep randomly assigned to fixation method
  • Calculated on detecting 20% change in torsional strength with 80% confidence
• Monitored radiographically
  • Weekly xrays from week 3
• Animals killed at week 9
  • Callus assessed with CT and histologically
  • Mechanical torsional strength tested
• Plating Construct
  • 4.5mm titanium plate
    • Six staggered holes
    • 2 types of screws
      • Fully threaded locking screws
      • Partially threaded locking screws
  • Construct stiffness tested
    • Locked plates mean 3922 +/- 474N/mm
      • Far-locking construct
      • < 400N 628 +/- 81N/mm (84% lower stiffness)
      • > 400N 2672 +/- 594N/mm
      • Parallel motion at the osteotomy gap
• Animal and Surgical
  • Established sheep model approved and conforming to the ethics and animal care committees
  • Sheep
    • Mean age 2.5 +/-0.8
    • Mean weight 61 +/-9kg
    • Medial incision over tibia
    • Predrilled screw holes with template
    • Osteotomy - 3mm gap after plate application
    • Plates applied 1mm elevation
    • Limb protected with cast and harness
    • Allowed full weight-bearing
    • Protected animals during sleep to prevent peak loads during standing or bolting to standing position
• Radiography
  • Xrays immediately and weekly from week 3
  • Quantified callus formation at lateral, anterior and posterior aspects
  • After death and implant removal, bone mineral content were qualified by CT
• Mechanical Testing
  • Tested strength, stiffness, torsional displacement and energy to failure
  • Embedded both ends in cement
  • Rotational force around tibia
• Histological
  • One longitudinal slice was harvested from midsagittal plane
  • Toluidine blue stain to see callus differentiation
• Statistics
  • All data reported as mean and standard deviation
  • Differences calculated using student T test, repeated measures analysis of variance and paired testing

Results

• Sheep
  • Nil complication, able to walk on post-operative day 1
  • No difference in osteotomy gap after immediate first gap
• Assessment of Callus
  • Periosteal callus increased until week 7
  • Far-cortical locking had 58% more callus at week 7 and 37% at week 9 (p<0.05)
  • CT at week 9 found far cortical locking group had
    • 36% higher total callus
      • 90% higher for medial cortex between the group
    • 44% greater bone mineral density
      • Due to medial cortex having same density as lateral cortex (117% of locked group)
      • Medial cortex was 49% lower than lateral cortex in locked plating construct
• Mechanical Testing and Histology
  • Far locking group had higher
    • Failure to torsion (54%)
    • Absorbed 156% more energy
  • 3/6 locked plating specimens had deficient medial cortex bridging callus
  • 6/6 far cortical plating specimens had complete cortical bridging

Discussion

• Some concern that locked bridge plating constructs too stiff and impair fracture motion
• Recommendations exist that need to increase bridging span by omitting screw holes adjacent to fracture site
• Contrasting published data on the effectiveness
• Evidence that less rigid fixation improved fracture healing

• Far-cortical locking had factors to encourage secondary healing
  • Reduced stiffness - 6 fold compared to locked plates
  • Parallel interfragmentary motion
  • Biphasic stiffness - allowed increased stiffness at increasing loads

Pros of Study

• Well designed animal experimental study
• Used validated animal model, callus measurement tools, bone mineral density tests
• Conducted a power analysis
• Clear and well documented surgical and post-operative technique
• Histological analysis of fracture site
• Documented limitations of own study

Cons of Study

• Funded by Zimmer, with authors receiving funding from Stryker
• Used a titanium locking plate
• Specific designed fracture - only 3mm gap
• Only tested torsional strength
• Animal model - need to be careful of clinical application
• Did not look into periarticular plating model