Copyright®2016NGD.AllrightsreservedNeilEmbleton,B.Sc.,DVMandVeronicaBarkowski,DVMHelivetMobileSurgicalServices,Sundre,AB,Canada

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July2016

SIMITRISTABLEINSTRIDE®PREOPERATIVEPLANNING

Aswithallprocedures,properpatientselectionisanimportantfirststepinasuccessfulprocedure.TounderstandpreoperativeplanningforSimitriStableinStride®wewillexplain:

• Patientassessmentandselection• Preoperativeradiographs• Radiographicmeasurements• Implantselectionandpositioning

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A. PatientAssessmentandSelection

Priortosurgeryallpatientsmustbeassessedbybothphysicalexaminationandstifleradiography.Thefollowingshouldbekeptinmindwhenassessingapatientpriortosurgery:

1) Patientsmustbebelowthemaximumallowableweightforthesizeofimplantchosen.(seeTable2-1)

2) Patientsmustbefreeofclinicalsignsconsistentwithinfection.

3) Patientsmustbefreeofanysignificantdentaldisease.

4) Patientsmustnotbeonimmunosuppressivemedicationtotheextentthatitcouldadverselyaffecttheoutcomeofthesurgicalprocedure.

5) Patientsmustbefreeoffemoralandortibialvalgusorvarusdeformitiestotheextentthattheycouldadverselyaffecttheoutcomeofthesurgicalprocedure.

6) Patientsshouldbefreeofexcessivetibialtorsion.

7) Ownersshouldbefullyawareofallaspectsoftheprocedureandbepreparedtofollowthepostoperativecarehandouttotheletter.

8) Ownersshouldbeinformedofnecessitytomaintaindentalhygienefortheirpet,priortoandintheyearsfollowingtheprocedure.

9) ReviewindicationsandcontraindicationsforthisprocedureinSection1-C.

2.7 mm standard

3.5 mm standard

3.5 mm broad

≤ 12 kg (25 lb)

≤ 34 kg (75 lb)

≤55 kg (120 lb)

Table 2-1: Maximum weight for each implant size

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B. PreoperativeRadiographsTworadiographicviewscenteredontheinjuredstiflearerequired:

1. Extendedlateralwithcondylessuperimposed(Figure2-1;Itiscriticalthatfemoralcondylesaresuperimposed)

2. Cranialcaudalviewwithhalfofthefemurandhalfofthetibiaonlyincludedinview(Figure2-2).

Figure 2-1 This is an example of a correctly positioned diagnostic extended lateral stifle radiograph. Condyles are superimposed, the radiograph is collimated to include half of the femur and tibia and a marker of known dimensions (R marker) is included in the image and has been raised to the level of the joint (in this case with a stack of gauze squares).

Figure 2-2 This is an example of a correctly positioned cranial caudal radiograph. The patella is centered over the femur, half of the femur and tibia are in view, the radiograph is centered over the stifle joint and the tail has been moved out of the way of the stifle joint. The calibration marker (R marker) has been raised to the level of the stifle joint with a stack of gauze squares.

Note:Asecondcranialcaudalviewcenteredoverthetibiashaftandincludingthedistalfemur(patella)andhockjointmaybeusefultoassessfortibialtorsion(Figure2-3).Thepositionofthecalcaneusrelativetothedistaltibiaservesasanindicationofthedegreeoftibialtorsion.Markedtibialtorsionmaycausesignificantinternalrotationoftheproximaltibiaduringflexionofthestifle.ThiscanbeextremelydifficulttomanagesurgicallyandmayinterferewiththecorrectpositioningandfunctionoftheSimitriStableinStride®implant.Asradiographsmayfailtodiagnosesomecasesoftibialtorsion,thepatientshouldalwaysbeassessedintraoperativelyforexcessiverotation

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whichischaracterizedbythetibialplaterotatingmorethanisallowedbytheflangesofthetibialinsert(i.e.10degrees).ThisisintraoperativeassessmentisdescribedinModule3–SurgicalProcedure.

Figure 2-3 Cropped image of distal tibia with tibial torsion. Note that the calcaneus is not centered in this image but appears to be in a lateral position, this is due to torsion of the tibial shaft.

C. RadiographicMeasurementsMeasurementsobtainedfromthepreoperativeradiographsareusedtoselecttheappropriatesizeofimplantforyourpatientandtoaidinpositioningtheimplantduringsurgery.Toobtainaccuratemeasurementsthecalibrationmarkerisusedtoadjustforthemagnificationoftheradiographicimage.Asdiscussedabove,amarkerofknowndimension(inmm)mustbeoneveryradiographicimage(atthelevelofthestiflejoint)andwillbeusedtocalibratethesoftwareusedtomeasuretheradiographorusedtoadjustthemeasurementsobtainedontheradiograph.

MostcommerciallyavailableDICOMviewersoftwarehavemeasuringtoolsthatallowyoutomakemeasurementsondigitalradiographs.Ideallyyourmeasuringtoolshouldbesettomeasureinmillimeterstoonedecimalplace.Dependingonthesoftwareuseditmaybepossibletosetthescaleofthemeasuringtoolpriortomakingmeasurementsofyourpatient’sradiograph.Thiswillbedonebymeasuringtheimageofyourcalibrationmarkerontheradiographandthenenteringtheactualmeasurementofthemarkerinmmintotheprogramwhichwillthencorrectforthemagnificationonallmeasurementsubsequentlymadeonthatradiograph(Figure2-4).Itisimportanttorecalibrateforeverynewimagetaken.Ifthesoftwarecannotbecalibratedthemeasurementsobtaincanbeadjustedtotruetoscalemanuallybymultiplyingthemeasurementsbythemagnificationfactor(actualmeasurementofmarker/softwaremeasurementofmarker).

ImageJisaprogramdevelopedbytheNationalInstituteofHealth,USAavailableathttp://imagej.nih.gov/ijthatallowstruetoscalemeasurementstobemadeonavarietyofdigitalimagesincludingJPEG.Thisisalsoconvenientformeasurementofdigitalradiographicimagesfromreferringpractices.

Forradiographicfilms,measurementscaneitherbemadewitharulerontheprocessedfilmsandthenmanuallyadjustedforthemagnificationasdescribedaboveor,adigital

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photoofthefilmcanbetakenandsoftwaresuchasImageJcanbeusedtomeasurethedigitalimageofthefilm.

Figure 2-4 This is an example of how the magnification is corrected using ImageJ software. A line has been drawn across the R marker (arrow) using the ImageJ measuring tool. The marker was previously measured with a ruler and found to be 20 mm in width. By entering the “Known distance” (found in the “Set Scale” feature under “Analyze), the program will automatically provide actual true to scale measurements of any line subsequently drawn on this radiograph. Note: The scale must be reset for every radiograph measured.

Onceamethodforobtainingtruetoscalemeasurementshasbeenestablishedtheextendedlateralradiographcanbeusedtoobtainthenecessarypreoperativemeasurementsdescribedstepbystepinthefollowingpages(Figures2-5toFigure2-12).

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STEP1:FindA-widthoffemoraldiaphysis

Figure 2-5 The scale of this image was set using the width of the R marker as previously described. A sagittal line is drawn across the femoral diaphysis proximal to the supracondylar tuberosity (between arrows). Measure this line and mark the midpoint. The length of this line is A.

A

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STEP2:FindYaxis

Figure 2-6 A second sagittal line is drawn distal to the supracondylar tuberosity and the midpoint is found. A longitudinal line is drawn bisecting the midpoints of both sagittal lines – this represents the Y axis of the distal femur.

XYaxis

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STEP3:FindmidpointofBlumensaat’sline

Figure 2-7 Find the midpoint of Blumensaat’s line (prominent radiopaque line that represents the top of the intercondylar fossa).

MidpointofBlumensaat’sline

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STEP4:FindB

Figure 2-8 Draw a line from the midpoint of Blumensaat’s line to the distal edge of the medial condyle (parallel to the Y axis). The length of this line is B.

B

Yaxis

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STEP5:FindC-widthofmedialcondyle

Figure 2-9 Draw a line the intersects the midpoint of Blumensaat’s line that is perpendicular to the Y axis. This represents the X axis of the medial femoral condyle and the length of this line is C.

C(Xaxis)

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STEP6:FindD

Figure 2-10 Measure the distance from the Y axis to the caudal edge of the medial condyle along C (between arrows). This is measurement D.

Thesefourmeasurementsareusedtoselecttheappropriatesizeoffemoralplateforyourpatientandtopositionthefemoralplateduringsurgery.

D

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Figure 2-11 This figure summarizes the measurements made on the preoperative extended lateral radiograph. A, B, C and D aid in selection of the appropriate size of implant and B and D aid in surgical positioning of the implant.

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STEP7:FindPatientTibialOffset

Figure 2-12 Starting approximately 50 mm distal to the proximal end of the tibia (red arrow), draw a line parallel to the medial tibial cortex. This line continues proximally and will intersect the wider proximal tibia and medial femoral condyle. A second line is drawn parallel to the first, touching the medial edge of the medial femoral condyle at the location of the white arrow as shown. The distance measured between these two lines is the patient offset in mm and will be used to select the tibial plate offset size that best fits your patient.

Patientoffset

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D. ImplantSelectionandPositioningThemeasurementsobtainedfromthepreoperativeradiographsareusedtofindtheidealfemoralballpositionforeachpatient.Bydeterminingthisposition,itispossibletochoosetheimplantthatwillbestfityourpatient.Therearefourkeypointsfordeterminingwhichimplantfitsapatientbest(Figure2-13).

1. Thepatientcannotexceedthemaximumweightfortheimplant.2. ThefemoraldiaphysiswidthAmustbelargeenoughtofitthescrew

segment.3. ThefemoralcondylewidthCmustbelargeenoughtofittheballand

stemsegment4. Withthefemoralballintheidealpositionoverthemedialfemoral

condyle,thescrewsegmentoftheimplantmustfitwithintheconfinesofthefemoraldiaphysis.

Thefirst3determinewhetheryourpatientislikelytofita2.7mmora3.5mmfemoralplate(Table2-2).ThefourthultimatelydetermineswhichcomponentbestfitsyourpatientandhowthescrewsegmentwillbepositionedrelativetotheYaxisofthefemoraldiaphysis.

Figure 2-13 This diagram demonstrates the relationship between the patient measurements and the dimensions of the implants. D – B is related to the distance from the center of the femoral ball to the center of the screw holes. The dimensions of each implant are different and are given in the Table below:

Table 2-2 Femoral plate dimensions shown as they relate to patient measurements. The minimum patient measurements for A, B and C are shown and the patient must not exceed maximum weight. D – B is related to ideal ball position and is explained below.

Femoral Plate dimensions

2.7 mm 3.5 mm (with 10 mm tibial plate)

3.5 mm (with 13 and 16 mm tibial plates)

3.5 mm (broad)

A

9 mm 10 mm 10 mm TBA

B

6 mm 7.5 mm 7.5 mm TBA

C

23 mm 25 mm 30 mm TBA

D

18.5 mm 21.5 mm 26.5 mm TBA

D minus B

12.5 mm 14 mm 19 mm TBA

Weight

≤ 12 kg ≤ 34 kg ≤ 34 kg ≤ 55 kg

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IdealBallPositionThefemoralballisideallypositionedwhenitisequidistantfromthecaudalanddistaledgesofthemedialfemoralcondyleontheXaxis.Therefore,theidealballposition(midpointofball)isBmmfromthecaudaledgeofthecondyle(Figure2-14).MeasurementBwillbeusedduringsurgerytofindtheidealballpositionoverthemedialcollateralligament.

Figure 2-14 The ideal ball position is represented by the red circle and is B mm from the caudal and distal edges of the medial femoral condyle.

D – B represents the distance form the center of the femoral ball to the Y axis and is used to determine which femoral plate size best fits your patient.

B will be used during surgery to find the ideal ball location on the patient (Figure 2-15).

Thedistancebetweenthefemoralball(midpoint)andthecenterofthescrewholesisuniquetoeachsizeoffemoralplate(Table2-2).Forafemoralplatetofityourpatientthescrewsegmentmustfitwithintheconfinesofthefemoraldiaphysiswiththeballinitsidealpositionoverthemedialfemoralcondyle.Thefollowingformulawilldeterminethedistancebetweenthecenteroftheidealballpositionandthecenterofthefemoraldiaphysis(Y-axis)foryourpatient.

D–B=distancefromcenterofballtoYaxis

Thefemoralplatethatmostcloselymatchesyourpatient’sD-Bwillfityourpatientbest(Table2-2).Itmaybenecessarytomovethescrewsegment(screwholes)cranialorcaudaloftheYaxisifD–Bdoesnotexactlymatchoneoftheplatesizes.TheamountofmovementcanbedeterminedbysubtractingD–BfromtheplatedimensionshowninTable2-2.

Theamountofcranialorcaudalmovementavailableislimitedbythewidthofthefemoraldiaphysisastheentirescrewsegmentmustremainwiththeconfinesofthefemoraldiaphysistoensuregoodscrewplacementthereforepatientswithsmaller

D

B

Yaxis

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femoraldiaphysiswidthshavelessroomforadjustment.TheroomforadjustmentisroughlyA–widthofscrewsegmentoffemoralplate.

Duringsurgeryastaysutureisplacedoverthemedialcollateralligament(MCL)toaidinpositioningthedistaledgeofthefemoralplateonthepatient.A22gaugehypodermicneedleisusedtolocatetheproximaltibiaonthecranialborderofthemedialcollateralligament.UsingasterilecaliperorruleradistanceofB–6mmismeasuredproximaltotheneedleandastaysutureisplacedcenteredovertheMCL.

Figure 2-15 A stay suture is used to marked the edge of the femoral plate distal to the femoral ball. The hypodermic needle marks the proximal tibia, the distance from the needle to the stay suture is B – 6 mm.

The femoral ball is 7.5 mm from the edge of the plate however we need to add back 1.5 mm to account for the articular cartilage and joint space between the tibia and the distal edge of the medial femoral condyle as seen on the radiograph.

needle

Staysuture

MCL

needle

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IdealTibialPlateOffsetThetwoprecontouredbendsinthetibialplatecreatetheoffsetbetweenthearticulatingportionofthetibialplateandthescrewsegment(Figure2-16).Thesebendsallowthetibialplatetoengagethemedialsideofthefemoralplateatthearticulationwhilecloselyengagingthetibialdiaphysisthroughthescrewsegment.Tibialplatesareofferedin10mm,13mmand16mmoffsetsfor3.5mmplatesand10mmoffsetsfor2.7mmplates.

Measurementsobtainedfromthecranialcaudalstifleradiographcanbeusedwiththefollowingformulatodeterminethebesttibialplateoffsetsizeforyourpatient.Tobeaccuratethemarkerusedtocalibratethescalemustbeatthelevelofthestiflejointabovetheplate.

Tochoosethetibialplatesizethatbestfitsyourpatientusethefollowingformula(seeFigure2-16):

Patientoffset(mm)+7mm=tibialplateoffset

Figure 2-16 On the left are two tibial plates with different offsets. A is the articulating segment of the tibial plate, shown with the articular insert removed and B is the screw segment. The offset of each plate is the distance between the two arrows.

On the right is the expanded view of articulation of an implant demonstrating that the 7 mm accounts for the thickness of (from left to right) the soft tissues, the femoral plate and the articular insert flange which separate the medial side of the femoral condyle and the inside of the articulating segment of the tibial plate.

7mm

Femoralcondyle

A

B

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Choosethetibialplateoffsetsizethatbestfitsyourpatientwithminimalcontouring.Itisimportanttonotethat3.5mmtibialplatescomein10,13and16mmoffsetsandthatthesewillfitboththesmallandlargesizesof3.5mmfemoralplates(notthebroad),thereforeitispossibletochooseboththebestfittingfemoralandtibialcomponentforyourpatient.

Itisalsopossibletocontourboththeplatestofityourpatienthowevergreatcaremustbetakentonotdamagethearticularinsertorthescrewholes(eg.uselockingscrewplugspriortoanycontouring).

SelectingFemoralandTibialPlateSizeThemeasurementsmadeonthepreoperativeradiographsareusedtoselectthebestfittingimplantforyourpatient(Table2-2).

Example:

Fortheradiographsshownintheprevioussectionthepatientmeasurementswereasfollows:

Weight26.7kg

A=13.4mm

B=7.6mm

C=25.7mm

D=18.4mm

D–B=10.8mm

Offset=3.1mm+7

=10.1mm

Itisimportanttonotethatsmall3.5mmfemoralplatescomeprepackagedwiththe10mmtibialplate,andthelarge3.5mmplatescomeprepackagedwiththe13or16mmtibialplates,howeverallofthesefemoralandtibialplatesareinterchangeable.Therefore,itispossibletochooseboththebestfittingfemoralandtibialcomponentforyourpatienthoweveritmaybenecessarytoopenmorethanonepackage.

Based on weight, this patient will need a 3.5 mm implant.

Femoral Plate:

Based on C it is likely that the small 3.5 femoral plate (packaged with the 10 mm tibial plate) will fit this patient.

Based on A = 13.4 mm – 10 mm (width of screw segment) there is roughly 3.4 mm of movement from the Y axis available on the diaphysis.

By subtracting D – B from the plate sizes we can confirm which 3.5 plate is the best fit and determine the location of the screw holes.

19 mm – 10.8 mm = 8.2 mm (screw holes will not fit within confines of femoral diaphysis)

14 mm – 10.8 mm = 3.2 mm (screw holes of small plate will be positioned 3.2 mm cranial of Y axis) = BEST FIT

Tibial Plate:

3.5 mm tibial plates are available in 10, 13 and 16 mm offsets.

Based on tibial offset of 10.1 mm the best fit for this patient is the 10 mm offset tibial plate.