Mind the Gap - MAKO Surgical Corp.

Mind the Gap – Achieving aNaturally Balanced andAligned Knee Following UKAwith the MAKOplasty ®ProcedureMichael Conditt, Ph.D.Director of Clinical ResearchMAKO Surgical Corp.Robert Van Vorhis, Ph.D.Principal ScientistMAKO Surgical Corp.

Mind the Gap – Achieving aNaturally Balanced andAligned Knee Following UKAwith the MAKOplasty ®ProcedureThere are distinct differences in thephilosophies of post-operative limbalignment between total knee arthroplastyand medial unicompartmentalarthroplasty. The goal of TKA is typicallyto align the post-operative knee inneutral, thus loading both compartmentsevenly or in slight valgus. 1-3 It has beengenerally accepted that this is the mostimportant variable predicting survival ofthe implant. 4 This long-standing goal ofTKA has actually been questionedrecently, perhaps because the normalknee loads the medial side more thanthe lateral side. 5Figure 1: Medial compartment osteoarthritis results in narrowing of the medial joint space due to theloss of articular cartilage.femoral deficitslack medial collateralcollapsed medial compartmentMLtibial deficitWith medial UKA, the desired postoperativelimb alignment is still unclear.Overcorrection has been implicated as acause of progression of the disease tothe opposite compartment, 6,7 howeverundercorrection has also been identifiedas leading to accelerated wear of thepolyethylene. 8 With the minimallyinvasive nature of both the MAKOplasty ®procedure and the all polyethylene tibialinlay component, the alignment goal ofeach component is to match the naturalanatomy, thus simply replacing theworn joint surface and restoring themechanical axis to its position beforethe onset of degenerative changes. 9-13MAKOplasty ® provides a method fordetermining where this relative positionwas before the onset of osteoarthritis.Then, three-dimensional planning ofthe implant position to achieve thisalignment and precise preparation ofthe bones enables achievement of thisalignment post-operatively.A preliminary plan for the position andorientation of the components is createdfrom a three-dimensional reconstructionof a pre-operative CT scan of the patient’sleg and CAD models of the implantedcomponents. This preliminary plan isbased primarily on the geometry of theindividual bones. During the surgicalprocedure, standard navigation markersare placed in the femur and the tibia andare also mounted on a robotic arm.A registration procedure consisting ofkinematically determining the center ofrotation of the hip and digitization ofvarious bony landmarks allows the correlationof the previously reconstructedmodels of the bones along with theircorresponding component plans to thecurrent three-dimensional space of theoperating room. Motions of the bonesand the robot are continuously monitoredin real time.Once the registration has been completed,osteophytes interfering with medialcollateral ligament function are removedand capsular adhesions interfering withknee function are relieved (Figure 1).As two of the indications for a UKA area correctable deformity and functioningACL, removal of these impedimentsmakes it possible to achieve correct legkinematics and tissue tension duringpassive manipulation throughout the fullrange of motion with an applied valgusFigure 2: Manual application of a valgus stress to theknee opens the medial compartment and allows theplanning of the placement of the components to fillthe articular gap left by the loss of articular cartilage.(force via pelvis)taught medialcollateralopen medialcompartmentexternallyinducedvalgusmoment

Figure 3: Graph used in planning implant placement at different flexion angles during which the desired limb alignment was captured. The graph displaysgap/overlap of the femoral and tibial components. Each component can then be fine-tuned in all 6 degrees of freedom, upon which the graph is immediatelyupdated. This allows the three-dimensional adjustment of implant placement to optimize the desired joint laxity throughout flexion.0ºloose4.0Gap Between Articular Surfaces (mm)3.02.01.00.0-1.0tight-2.00 45 90ExtensionFlexion90º4 poses captured 4 “gap” values at posesstress (Figure 2). Correct leg kinematicsand soft tissue tension should representthe relative positioning of the femur andthe tibia before the loss of cartilage. Thecamera system captures the positions ofthe femur and the tibia during manipulationof the knee through a range ofmotion with the disease induced varusdeformity corrected with an externallyapplied valgus stress. It should be notedthat applying a pure valgus stressbecomes more difficult with increasingknee flexion as there is tendency forrotation at the hip. These capturedthree-dimensional positions representthe appropriate spacing within the jointwith the medial collateral ligamentproperly tensioned. While variability willexist in the manual application of thevalgus moment, the high stiffness of theMCL attenuates the manifestation ofthis variability in the medial joint spaceopening provided the force is largeenough to create a taught collateral.The planned positions of the femoraland tibial components can then befine-tuned such that the joint articularsurfaces are placed to fill the gap leftby the disease process throughout theflexion arc (Figure 3). The philosophybeing that after resection and componentimplantation, knee mechanics will beproperly restored throughout the rangeof motion.This final planned position then definesthe volume of bone to be resected bythe high speed burr attached to therobotic arm. While inside the volume ofbone to be resected, the robotic armoffers no resistance. As the burrapproaches the boundary, the roboticarm resists that motion and keeps theburr only within the accepted volume.The effectiveness of this process isdifficult to quantify. A surrogatevariable indicative of success is thenumber of soft tissue releases performedand the number of times theplanned insert thickness is not changedintra-operatively. Eighty-nine percent ofMAKOplasty ® procedures result in theuse of the planned insert thickness*,*Data on file.

Mind the Gap - Achieving a Naturally Balanced and Aligned Knee Following UKA with the MAKOplasty ® Procedure6mm, which is the thinnest possibleinsert. This three-dimensional planningmethod, which is a combination ofpre- and intra-operative planning,empowers the robotic arm to effectivelyact as a virtual instrument set thatprecisely assists the surgeon to executethe plan to create an appropriately,naturally balanced and aligned knee.References1. Insall, J. N.; Binazzi, R.; Soudry, M.; and Mestriner,L. A.: Total knee arthroplasty. ClinicalOrthopaedics And Related Research, (192):13-22, 1985.2. Ritter, M. A.; Faris, P. M.; Keating, E. M.; andMeding, J. B.: Postoperative alignment of totalknee replacement. Its effect on survival. ClinicalOrthopaedics And Related Research, (299):153-6, 1994.3. Hsu, H. P.; Garg, A.; Walker, P. S.; Spector, M.; andEwald, F. C.: Effect of knee component alignmenton tibial load distribution with clinical correlation.Clinical Orthopaedics And Related Research,(248): 135-44, 1989.4. Moreland, J. R.: Mechanisms of failure in totalknee arthroplasty. Clinical Orthopaedics AndRelated Research, (226): 49-64, 1988.5. Parratte, S.; Pagnano, M. W.; Trousdale, R. T.; andBerry, D. J.: The mechanical axis may be thewrong target in computer assisted TKA In 75thAnnual Meeting of the American Academy ofOrthopaedic Surgeons. Edited, San Francisco, CA,2008.6. Laskin, R. S.: Unicompartmental tibiofemoralresurfacing arthroplasty. The Journal of Bone andJoint Surgery, American, 60(2): 182-5, 1978.7. Scott, R. D.; Cobb, A. G.; McQueary, F. G.; andThornhill, T. S.: Unicompartmental kneearthroplasty. Eight- to 12-year follow-upevaluation with survivorship analysis. ClinicalOrthopaedics And Related Research, (271):96-100, 1991.8. Barrett, W. P., and Scott, R. D.: Revision of failedunicondylar unicompartmental knee arthroplasty.The Journal of Bone and Joint Surgery, American,69(9): 1328-35, 1987.9. Cartier, P., and Deschamps, G.: Surgical principlesof unicompartmental knee replacement. InUnicompartmental Knee Arthroplasty. Edited byCartier, P.; Epinette, J. A.; Deschamps, G.; andHernigou, P., Paris, Expansion ScientifiqueFrancaise, 1977.10.Keene, G.; Simpson, D.; and Kalairajah, Y.: Limbalignment in computer-assisted minimally-invasiveunicompartmental knee replacement. The Journalof Bone and Joint Surgery, British, 88(1): 44-8,2006.11.Repicci, J. A.: Mini-invasive knee unicompartmentalarthroplasty: bone-sparing technique.Surgical Technology International, 11: 282-6,2003.12.Repicci, J. A., and Eberle, R. W.: Minimallyinvasive surgical technique for unicondylar kneearthroplasty. Journal of the Southern OrthopaedicAssociation, 8(1): 20-7; discussion 27, 1999.13.Emerson, R. H., Jr.; Head, W. C.; and Peters, P. C.,Jr.: Soft-tissue balance and alignment in medialunicompartmental knee arthroplasty. The Journalof Bone and Joint Surgery, British, 74(6): 807-10,1992.© MAKO Surgical Corp. 203027 r00 09/09 www.makosurgical.com