title: Biomechanical Comparison of External Fixation and Compression Screws for Transverse Tarsal Joint Arthrodesis.
abstract: BACKGROUND
  Transverse tarsal joint arthrodesis is commonly performed in the operative treatment of hindfoot arthritis and acquired flatfoot deformity. While fixation is typically achieved using screws, failure to obtain and maintain joint compression sometimes occurs, potentially leading to nonunion. External fixation is an alternate method of achieving arthrodesis site compression and has the advantage of allowing postoperative compression adjustment when necessary. However, its performance relative to standard screw fixation has not been quantified in this application. We hypothesized that external fixation could provide transverse tarsal joint compression exceeding that possible with screw fixation.
  
  
  METHODS
  Transverse tarsal joint fixation was performed sequentially, first with a circular external fixator and then with compression screws, on 9 fresh-frozen cadaveric legs. The external fixator was attached in abutting rings fixed to the tibia and the hindfoot and a third anterior ring parallel to the hindfoot ring using transverse wires and half-pins in the tibial diaphysis, calcaneus, and metatarsals. Screw fixation comprised two 4.3 mm headless compression screws traversing the talonavicular joint and 1 across the calcaneocuboid joint. Compressive forces generated during incremental fixator foot ring displacement to 20 mm and incremental screw tightening were measured using a custom-fabricated instrumented miniature external fixator spanning the transverse tarsal joint.
  
  
  RESULTS
  The maximum compressive force generated by the external fixator averaged 186% of that produced by the screws (range, 104%-391%). Fixator compression surpassed that obtainable with screws at 12 mm of ring displacement and decreased when the tibial ring was detached. No correlation was found between bone density and the compressive force achievable by either fusion method.
  
  
  CONCLUSIONS
  The compression across the transverse tarsal joint that can be obtained with a circular external fixator including a tibial ring exceeds that which can be obtained with 3 headless compression screws. Screw and external fixator performance did not correlate with bone mineral density. This study supports the use of external fixation as an alternative method of generating compression to help stimulate fusion across the transverse tarsal joints.
  
  
  CONCLUSIONS
  The findings provide biomechanical evidence to support the use of external fixation as a viable option in transverse tarsal joint fusion cases in which screw fixation has failed or is anticipated to be inadequate due to suboptimal bone quality.
authors.affiliation: University of Arizona, Tucson, AZ, USA dlatt@email.arizona.edu.
authors.forename: L Daniel
authors.initials: LD
authors.lastname: Latt
authors.affiliation: Duke University Medical Center, Durham, NC, USA.
authors.forename: Richard R
authors.initials: RR
authors.lastname: Glisson
authors.affiliation: Duke University Medical Center, Durham, NC, USA.
authors.forename: Samuel B
authors.initials: SB
authors.lastname: Adams
authors.affiliation: Medical University of Vienna, Vienna, Austria.
authors.forename: Reinhard
authors.initials: R
authors.lastname: Schuh
authors.affiliation: Penn State Medical Center, Hershey, PA, USA.
authors.forename: John A
authors.initials: JA
authors.lastname: Narron
authors.affiliation: Duke University Medical Center, Durham, NC, USA.
authors.forename: Mark E
authors.initials: ME
authors.lastname: Easley
mesh_terms: D000328:Adult; D000368:Aged; D001174:Arthrodesis / Q000295:instrumentation* / Q000379:methods; D001696:Biomechanical Phenomena; D015519:Bone Density; D001863:Bone Screws; D002102:Cadaver; D019245:Compressive Strength / Q000502:physiology*; D016267:External Fixators; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D012680:Sensitivity and Specificity; D013640:Tarsal Joints / Q000601:surgery*
keywords: arthrodesis; biomechanics; bone mineral density; compression screw; external fixator; fusion; hindfoot; spatial frame; talonavicular; transverse tarsal joint