title: Embossing of a screw thread and TCP granules enhances the fixation strength of compressed ACL grafts with interference screws.
abstract: OBJECTIVE
  Fixation of soft tissue grafts with interference screws relies on the friction of the graft between the screw and the bone tunnel. The goal of this study was to precondition such grafts by mechanical compression in order to reduce anticipated and undesired viscoelastic adaptation of the graft to screw pressure. Further, the otherwise slippery graft surface was modified with impressed tricalcium phosphate granules (TCP) to improve friction and mechanical hold.
  
  
  METHODS
  Fresh flexor digitorum tendons from young bovines were used to create bundles with a diameter of 8-9 mm and were divided into 10 groups to compare the pullout strength and bone damage in a variety of construct scenarios. Specifically, the effects of graft precompression to reduce preimplantation graft diameter were investigated. Further the effects of impressing TCP granules and/or a screw thread into the tendon surface during the compression process were studied.
  
  
  RESULTS
  In sawbone tests, radial graft compression allowed for a smaller bone tunnel (7 mm), but resulted in a significantly lower pullout strength of 174 N (95% CI: 97, 250), compared with controls [315 N (204, 426)]. In contrast, TCP coated [402 N (243, 561)], screw embossed grafts [458 N (302, 614)], and the combination of TCP and embossing [409 N (274, 543)] achieved higher pullout strengths when compared to the standard technique. In porcine bone, untreated grafts using an 8 mm screw pulled out at 694 ± 93 N, significantly higher loads were required to pullout compressed grafts with or without TCP coating (870 ± 74 and 878 ± 131 N), yet fixed with a 7 mm screw.
  
  
  CONCLUSIONS
  Modification of the tendon graft surface has a large influence on the biomechanical performance of interference screw fixation and results in less bone damage inflicted during insertion to a smaller tunnel diameter, while simultaneously achieving superior pullout strength.
authors.affiliation: Balgrist University Hospital, University of Zürich, Forchstrasse 340, 8008 Zurich, Switzerland. mazda.farshad@balgrist.ch
authors.forename: Mazda
authors.initials: M
authors.lastname: Farshad
authors.forename: Robert A
authors.initials: RA
authors.lastname: Weinert-Aplin
authors.forename: Michael
authors.initials: M
authors.lastname: Stalder
authors.forename: Peter P
authors.initials: PP
authors.lastname: Koch
authors.forename: Jess G
authors.initials: JG
authors.lastname: Snedeker
authors.forename: Dominik C
authors.initials: DC
authors.lastname: Meyer
mesh_terms: D000818:Animals; D059549:Anterior Cruciate Ligament Reconstruction / Q000295:instrumentation / Q000379:methods*; D001696:Biomechanical Phenomena; D001863:Bone Screws / Q000009:adverse effects; D002130:Calcium Phosphates; D002417:Cattle; D066298:In Vitro Techniques; D011312:Pressure / Q000009:adverse effects; D013552:Swine; D013710:Tendons / Q000502:physiology / Q000637:transplantation*; D053656:Tenodesis / Q000295:instrumentation / Q000379:methods*; D013718:Tensile Strength
chemical_list: D002130:Calcium Phosphates; C018392:tricalcium phosphate