Dr Rajesh PurushothamanAssociate ProfessorGovernment Medical College, KozhikodeGraft Fixation Options in ACL Reconstruction
GoalFunctional and anatomicalrestoration of ACL insymptomatic ACL deficiency
Most Important FactorsUnder Surgeons Control• Graft selection• Graft positioning• Graft fixation• Rehabilitation
Graft HealingBTB graft heal by bone to bonehealing by 6 weeksSoft tissue grafts incorporate bySharpey fibers by 12weeksAllografts take longerTill that time, fixation device shouldsecure the graft
Current RehabAccelerated rehab protocol•Early weight bearing•Early return to full ROM•Neuromuscular coordination•Strengthening
Biomechanics of Rehab• Reconstructed ACL subjected to150-500N forces by activities ofdaily living and rehab• Within the first 6 weeks, the graftis subjected to 2,20,000 suchloading cyclesNoyes FR, Butler DL, Grood ES, Zernicke RF, Hefzy MS. Biomechanical analysis of human ligamentgrafts used in knee-ligament repairs and reconstructions. J Bone Joint Surg (Am) 1984;66:344–352.
Biomechanics•Ultimate load to failure is about 3000N for BPTBand 4000N for QSTGG•This far exceeds the usual forces of 150-500N•Fixation has load to failure of about 500NFixation is the weakest link in the earlypostoperative period
3 Types of Graft Motion• Longitudinal motion calledBungee Cord Effect• Horizontal motion called Wind-wiper effect• Creep of graft tissue leading toelongation
What Does Graft TunnelMotion Do?• >3mm motion interferes with graftincorporation• May cause tunnel widening
Tunnel Widening• Due to biological and mechanical causes• More with non-aperture fixation
Biomechanics of Fixation• Strength - Ultimate load to failure• Stiffness- Resistance to displacement underload• Slippage of graft- Change in initial positionunder specific number of submaximal cycles
Ideal Fixation• Strong enough to avoidfailure• Stiff enough to restoreknee stability• Secure enough to avoidslippage
Types of FixationAperture FixationAt the joint level-Interference screwsSuspensory Fixation• Cortical- Endobutton,staples, screw posts• Cancellous- Transfixionpins
Interference ScrewsInterference is definedas the amount by whichdiameter of the screwexceeds the gapbetween graft and thetunnel
Advantages ofAperture Fixation• Minimises Graft-Tunnel motion• Less femoral canalwidening• Creep is less
Factors Influencing Interference Screw FixationLengthSize and geometry ofscrewDivergence of screw• Torque of screwinsertion• BMD• Screw Material
Length• Longer screw providebetter fixation• In BPTB engage onlythe bone plugStadelmaier DM, Lowe WR, Elah OA, Noble PC, Kohl HW 3rd. Cyclic pull-out strengthof hamstring tendon graft ﬁxation with soft tissue interference screws: inﬂuence ofscrew length. Am J Sports Med 1999;27:778-83.
SizeScrew diameter should be 1mm more than tunneldiameter for soft tissue grafts and same for boneplug graftKohn D, Rose C. Primary stability of interference screw ﬁxation:inﬂuence of screw diameter and insertion torque. Am J SportsMed 1994;22:334-8.
Geometry• Use soft threads screw for softtissue fixation• Use reverse threaded screw for leftsideWeiler A, Hoffmann RF, Siepe CJ, Kolbeck SF, Sudkamp NP. Theinﬂuence of screw geometry on hamstring tendon interference ﬁtﬁxation. Am J Sports Med 2000; 28:356-9.
Insertion Torque• More the resistance better thefixation• More with metal screws• Torque higher if screw diameter ismore• Torque better if tunnel is preparedby dilatation method than byextraction drillingKohn D, Rose C. Primary stability of interference screw ﬁxation: inﬂuence of screwdiameter and insertion torque. Am J Sports Med 1994;22:334-8.
Tunnel Dilatation• Under ream by 2mm• Next 2mm increase doneusing dilators• Compacts the bone thanremoving it
Divergence• Difference between theangle of tunnel andscrew direction• More with transtibialtechnique of femoraltunnel preparation• >20 degreecompromises stabilitySchroeder FJ. Reduction of femoral interference screw divergence during endoscopic anteriorcruciate ligament reconstruction. Arthroscopy 1999;15:41-8.
BMD• <0.6 gm2 associatedwith less pull outstrength• Use hybrid techniqueBrand JC Jr, Pienkowski D, Steenlage E, et al. Interference screw ﬁxation strength of a quadrupledhamstring tendon graft is directly related to bone mineral density and insertion torque. Am J Sports Med2000;28:705-10.
Bioabsorbable screws• Polyglycolide-absorbs early, henceloses fixation early• Crystallinepolylactides takeyears to be absorbed
Attractions• No need for implantremoval• Does not interfere withMRI• Revision easier
But…….• May break during insertion. Usewith special screw driver only• Tissue reaction in some• Fixation lost after partialdegradation
And…•By 6 weeks, 80% loss of strength and60% loss of stiffness•Incidence of effusion is more•Tunnel widening more
Titanium Or Steel•Titanium gets covered by bone•Steel gets enveloped by fibroustissue
Suspensory Fixation- CorticalEndobuttonTightropeGraft tunnelmotion moreMay lead to tunnelwideningClinical outcomestudies show nodifference
Endobutton• First generation suspensoryfixation• Femoral tunnel has 2 parts-Insertion & Connection parts• Insertion part drilled to thediameter of graft• Connection part is of 4.5mmdiameter
Maths of EndobuttonFemoral tunnel length –Desired graft insertionlength = Loop lengthInsertion tunnel lengthshould be 10mm morethan desired graftinsertion length
If the tunnel length is 60mm, desiredinsertion length is 40mm then the looplength should be 20mm and the insertiontunnel should be 50mm long
TightropeSecond generationsuspensory fixationLoop length reducedafter flipping bytightening the ropeAllows full length fillingof graft part of thefemoral tunnel
Bone Plug FixationTibia• Staples with or withoutsutures• Screws as screwposts• Interference screws
Soft Tissue FixationFemur• Interference screws• Suspension type likeEndobutton, Tightrope• Transfixation typeInterference screw fixationespecially with bioscrewsneeds slower rehabilitation
Soft Tissue FixationTibia• Staples singly or with two staplesusing belt buckle technique• Screws as posts or with spikedwashers• Interference screwsInterference screw fixation especially withbioscrews needs slower rehabilitation.Hybrid fixation preferrable
Tibial Vs FemoralFixation• Tibial fixation is lesssecure• Reduced bone density• Angle of forces in linewith the graft• Bone strength fallsrapidly away from thejoint line
Conclusions• In the early postoperative period fixation is theweakest link• Tibial fixation is at greater risk of failure• Clinical results of various methods arecomparable• Tunnel widening is a growing concern
Conclusions• Aperture fixation theoretically better• Interference screws are the gold standard• Tunnel dilation improves fixation• Hybrid fixation is becoming more popular
Thank YouDr Rajesh PurushothamanAssociate Professor of OrthopaedicsGovernment Medical College,Kozhikode, Kerala, Indiadrrajp@gmail.com