Effects On Subtalar Joint Stress Distribution After Cannulated Screw Insertion At Different Positions And Clinical Efficacy Of Subtalar Fusion

Subtalar fusion is the gold standard therapy for severe subtalar arthritis. Its influentialoperation factors include operation approach, cartilage removal, bone graft use, andselection of fixation method. Internal fixation is critical for the postoperatively clinicalefficacy of subtalar fusion. Among numerous fixation methods, screw fixation is thedominant one currently. However, a screw insertion approach has not been establishedconclusively which is capable to compress subtalar facet uniformly and has powerfulanti-rotary strength and anti-inversion/eversion strength.Referring to the progress made by Chinese and overseas researchers, we designed anew screw insertion approach considering the anatomy of subtalar joint and the feasibilityand safety of clinical application. The procedures were described below:1) talar neck andposterior calcaneal tubercle were divided in triplicate transversally;2) after the guide pinwas inserted percutaneously from talar neck toward posterior calcaneal tubercle, one screwwas inserted from the lateral equal division point of talar neck to the corresponding equaldivision point of posterior calcaneal tubercle, and the other screw was inserted from themedial equal division point of posterior calcaneal tubercle to the corresponding equaldivision point of talar neck. However, none of strong biomechanical evidence and clinicalefficacy report is available yet to prove the effects of the designed approach on subtalarfacet stress distribution, anti-rotary strength and anti-inversion/eversion strength.The powerful3D finite element modeling method can be used to statically anddynamically simulate the objects with complex geometric shapes and material parametersunder different force conditions. Generally the foot skeleton varies in shape and has acomplicated structure and numerous tissues; after the foot skeleton is loaded a force, thecalculation of stress and strain often involves non-linear computations. With3D finite element analysis, not only the stress state of the whole3D foot structure can be clarified,but also the contact stress can be studied reproducibly to show the facet stress distribution.In order to establish a screw insertion approach with the most uniform subtalar facetstress distribution, we independently designed novel lateral-medial parallel screw insertionby3D finite element analysis and then compared with classic anterior-posteriorlongitudinally parallel screw insertion in the aspects of stress distribution, anti-rotarystrength and anti-inversion/eversion strength. Furthermore, through analysis on clinicalefficacy, we validated that the independently designed screw insertion approach was safe,effective and reliable.Part1: Effects of cannulated screw insertion on subtalar joint stress distributionIn this part, we investigated the effects of cannulated screw insertion on subtalar facetstress distribution by3D finite element analysis to establish a cannulated screw insertionapproach with the most uniform subtalar joint stress distribution, and further compared theindependently designed lateral-medial parallel screw insertion and classic anterior-posteriorlongitudinally parallel screw insertion in the aspects of stress distribution, anti-rotarystrength and anti-inversion/eversion strength.2.1Experiment1: To establish a cannulated screw insertion approach with the mostuniform subtalar joint stress distribution2.1.1Methods2.1.1.13D finite element modeling of subtalar jointA3D geometric model of talus, calcaneus and screw was established with the spiralCT scan data of healthy male ankles using a3D modeling software, and then subjected tofinite element mesh division.2.1.1.2In the calcaneal-talar3D finite element model, the basic conditions for screwinsertion were set as follows:1. two screws (diameter=7.3mm) penetrated subtalar facet;2.bony areas for screw entry: bone surface of talar neck (between anterior ankle joint line andtalonavicular joint line), and areas of posterior and posterosuperior calcaneal tubercles;3.the subtalar facet stress distribution was the most uniform. The exploration of an idealscrew insertion approach included distance between two screws, optimal insertion point,and angle between screw and subtalar facet.2.1.2Results The screw insertion approach allowing for the most uniform subtalar facet stressdistribution was lateral screw insertion nearby the border of talar neck plus medial screwinsertion close to ankle joint facet in the talus neck，lateral screw insertion nearby the nodalbrim medial screw insertion close to the Achilles tendon check point in calcaneus nodalbrim.the distance between two screws was1.2cm, the angle between screw and subtalarfacet was45°, the uniform stress on subtalar facet was7.510MPa, and the maximumuniform stress loading area was79.56mm2.2.2Experiment2: To compare the stress distribution, anti-rotary strength andanti-inversion/eversion strength of three types of screw insertion technique.2.2.1Methods2.2.1.13D finite element modeling of subtalar joint2.2.1.2Testing of subtalar facet stress distribution, anti-rotary strength and anti-inversion/eversion strength after the independently designed lateral-medial parallel screwinsertion2.2.1.3Testing of subtalar facet stress distribution, anti-rotary strength and anti-inversion/eversion strength after the classic anterior-posterior longitudinally parallel screwinsertion2.2.1.4Testing of anti-rotary strength and anti-inversion/eversion strength after thescrew insertion with the most uniform stress distribution2.2.1.5Comparison of subtalar joint stress distribution among three screw insertions2.2.1.6Comparison of subtalar joint anti-rotary strength and anti-inversion/eversionstrength among three screw insertions2.2.2Results2.2.2.1The independently designed lateral-medial parallel screw insertion wassuperior to the classic anterior-posterior longitudinally parallel screw insertion in respect ofstress distribution uniformity, anti-rotary strength and anti-inversion/eversion strength.2.2.2.2The independently designed lateral-medial parallel screw insertion was worsebut slightly better than the screw insertion with the most uniform stress distributionseparately in stress distribution uniformity and in anti-rotary strength and anti-inversion/eversion strength. 2.3Conclusions2.3.1We have successfully established a screw insertion approach allowing for themost uniform subtalar joint stress distribution,lateral screw insertion nearby the border oftalar neck plus medial screw insertion close to ankle joint facet.2.3.2In respect of stress distribution uniformity, the independently designedlateral-medial parallel screw insertion is slightly inferior to the screw insertion with themost uniform stress distribution but superior to the classic anterior-posterior longitudinallyparallel screw insertion.2.3.3In respect of anti-rotary strength and anti-inversion/eversion strength, theindependently designed lateral-medial parallel screw insertion is superior to both the screwinsertion with the most uniform stress distribution and the classic anterior-posteriorlongitudinally parallel screw insertion.2.3.4The independently designed screw insertion is an approach with simplelocalization, convenient operation and good safetyPart2: Efficacy analysis of subtalar fusion by independently designedlateral-medial parallel screw insertionIn this part, we further confirmed the efficacy, feasibility and safety of subtalar fusionby independently designed lateral-medial parallel screw insertion in the aspect of clinicalefficacy.3.1Materials and Methods3.1.1Collection of casesFrom April2008to April2012,88patients (90feet) with severe subtalar arthritis, whohad no response to conservative treatment for over6months, were selected for subtalarfusion. There were56males and32females with an average age of43.7years (15-74years).3.1.2Subtalar fusion by independently designed lateral-medial parallel screw insertion3.1.3Clinical evaluationThe following parameters were evaluated: general data, function data and image data.The general data included symptom improvement, complications, satisfaction and woundhealing. The function data included AOFAS score and VAS score. The preoperative imagedata included calcaneal orthophoric, lateral and axial X-ray films and ankle joint MAIimages, or ankle joint CT images if there was severe metapodium deformity and calcaneal fracture; the postoperative image data included calcaneal orthophoric, lateral and axialX-ray films, and/or ankle joint CT images acquired to determine the subtalar fusion statewhich was infeasible with X-ray films.3.2ResultsThe follow-up was successfully performed in76patients, with a follow-up rate of89.4%;61patients provided complete clinical data according to the above criteria, and thefollow-up time was18.5months (6-40months). The postoperative X-ray or CTexaminations showed60patients had bone fusion and1patient had bone disunion. Thefusion rate was98.3%, and the mean fusion time was11.8weeks (8-16weeks). The VASscore was markedly decreased to1.03at the last visit from6.00before operation (p<0.05),and the AOFAS score was increased to80.6from57.0, significantly in statistics (p<0.05).Preoperative scoring: there were10cases of good,49cases of moderate and9cases of bad,and the excellent/good rate was16%; postoperative scoring: there were20cases ofexcellent,35cases of good,11cases of moderate and2cases of bad, and the excellent/goodrate was90.8%. Fifty-eight patients felt satisfied about the operation outcome,2patientsdeveloped anterolateral foot numbness within6months after operation,2patientsdeveloped metapedal pain within5months after operation,1patient developed middle footpain within1.5years after operation, and1patient developed contracture of Achillestendon.3.3ConclusionSubtalar fusion by independently designed lateral-medial parallel screw insertion forfixation has many advantages, such as a high fusion rate, simple localization duringoperation, rapid recovery, few complications, and so on.