人工膝关节置换限制性的选择

Constraint in Total Knee ArthroplastyWhen and What?James P.McAuley,MD,FRCSC,and Gerard A.Engh,MD Abstract:Tibiofemoral instability is increasingly recognized as a common mode of failure in total knee arthroplasty.Despite this,there are few published guidelines concerning treatment options,component selection,and expected results.In the treatment of the unstable total knee arthroplasty,cruciate-retaining designs can be considered only in the presence of well-balanced ligaments,including the posterior cruciate ligament.More predictable results are obtained with the use of cruciate-substituting components,but they provide no varus-valgus stability and cannot compensate for severeflexion laxity.Varus-valgus constrained designs cannot pro-vide long-term stability in cases of varus-valgus instability or severeflexion laxity. Such cases are most reliably treated with a linked implant.Key words:total knee arthroplasty(TKA),instability,revision.©2003Elsevier Inc.All rights reserved.The goals of any revision knee procedure must include reconstruction of bone deficiency and stable fixation of the components to the underlying bone. Also important for a functional and durable revision is restoration of knee stability.With many choices of component designs and levels of constraint,it can be a very difficult process to select the optimum implant for a given patient.The purpose of this paper is to provide simple guidelines to facilitate component selection for knee instability,an area that is still incompletely understood.Preoperative Planning Templating of preoperative radiographs is a fa-miliar process to most arthroplasty surgeons.It is important to plan the reconstruction of bone defi-ciency and determine the appropriate augments or stems needed to obtain stable,durablefixation. Easily forgotten is the importance of a careful eval-uation of knee stability.The need to restore stability during knee revision cannot be overstated.The preoperative plan,therefore,must also provide for restoration of stability using the required degree of component-to-component constraint.During the revision procedure,systematic evaluation of the adequacy and balance of the soft tissue envelope greatly simplifies the selection of the appropriate implant.Cruciate-Retaining ImplantsCruciate-retaining designs represent the least amount of component constraint,and their success depends on a sound biomechanical environment. This translates to the presence of good-quality bone with minimal defects,intact soft tissues,and a pos-terior cruciate ligament(PCL)that remains func-tional and balanced.This represents a relatively rare situation,such as a previous PCL-retaining proce-dure with minimal bone loss.This can occur inFrom the Anderson Orthopaedic Research Institute,Alexandria,Virginia.No benefits or funds were received in support of this study.Reprint requests:James P.McAuley,MD,FRCSC,c/o RebeccaWolf,Anderson Orthopaedic Research Institute,PO Box7088,Alexandria,VA22307.©2003Elsevier Inc.All rights reserved.0883-5403/03/1803-1014$30.00/0doi:10.1054/arth.2003.50103The Journal of Arthroplasty Vol.18No.3Suppl.1200351cases of polyethylene insert wear with intact com-ponents,early failure offixation,or in revision of failed unicompartmental arthroplasty.For example,in the Anderson Clinic series of revi-sions of failed unicompartmental knees,cruciate-re-taining femoral components were used in80%of cases[1].Conversely,in most revision situations,cru-ciate-retaining implants are not indicated.For exam-ple,the presence of a looseflexion space and a defi-cient PCL is not appropriate for retaining designs. Such a combination often fails because of recurrent posterior instability(Fig.1)[2,3].Cruciate-Substituting ImplantsThe next level in constraint is cruciate substitu-tion.What is gained?Obviously this design me-chanically substitutes for PCL function.Because all the technical and judgment issues of balancing the PCL are eliminated,many peoplefind this option easier and more forgiving.Equally important,however,is recognizing what is not gained with PCL-substituting designs.There is no gain in varus-valgus stability(Fig.2),and real-istically speaking,minimal rotational stability.Thus,for a posterior-stabilized implant to succeed,a functional soft tissue envelope is needed to provide varus-valgus stability.However,the need for good flexion-extension balancing is also important.With a residually looseflexion space,the femoral com-ponent can ride up on the tibial post(Fig.3)and result in posterior tibiofemoral dislocation[4].Another reported option for providing posterior stability in the absence of a functioning PCL is the use of an“ultracongruent”tibial insert[5].With this concept,posterior stability is provided by a 12.5-mm buildup of the anterior tibial insert,rather than the post and cam mechanism of traditional posterior-stabilized implants.This has been used successfully in both primary and revision knee ar-throplasty[5].Clearly,the same requirements for varus-valgus stability and adequateflexion-exten-sion balancing apply to this concept as well. Varus-Valgus–Constrained ImplantsThe next level of constraint is varus-valgus con-straint.Such components provide a significant de-gree of rotational control,and more significantly,a great deal of constraint to varus-valgus angulation. The trade-off is the theoretical disadvantage of in-creased stress transmission to the component–bone interfaces.Despite this,at least in the short term, the results seem comparable to less constrained designs[6–10].However,in the longer term,theresults may not be quite as encouraging[11].One must not forget that severeflexion instabilityis still a limitation for varus-valgus constrained im-plants.Despite the taller tibial post in these designs,the implant can still jump posteriorly in the pres-ence of severeflexion space laxity,resulting inposterior dislocation of the knee(Fig.4).teral radiograph shows posterior subluxation ofthe tibia.Fig.2.Photograph of a cruciate-substituting knee,whichdoes not provide varus-valgus stability.52The Journal of Arthroplasty Vol.18No.3Suppl.1April2003Because these implants limit varus-valgus angu-lation between the femoral and tibial components,it would seem intuitive that they could be used in cases of severe medial or lateral instability.Unfor-tunately,our experiences and those of others have shown poor results in treating both medial and lateral instability with varus-valgus constrained im-plants.In the Anderson Clinic experience,attempts at revision for severe varus or valgus instability were successful in only 2of 10cases.The mode of failure was late recurrence of the instability.Simply stated,the post of a varus-valgus con-strained knee cannot be expected in isolation to provide stability in the long term.However,the post can provide short-term support for healing collateral structures or in association with collateral reconstruction.Linked (Hinged)DesignsFor the reasons stated earlier,less constrained components have severe limitations in the absence of collateral soft tissue support or in the presence of gross flexion extension instability.Unfortunately,going to the highest degree of constraint (a hingedimplant)has historically produced disappointing re-sults,predominantly because of implant loosen-ing [12,13].Despite less rotational constraint with rotating hinges,results with early series of these hinges have not shown much better results,with disappointing clinical outcomes and high complication rates [14].However,newer designs of rotating hinges have produced more encouraging clinical and radio-graphic results [15,16].Therefore,the indications for a hinged compo-nent continue to evolve,the strongest indications being the absence of medial collateral soft tissue support and severe flexion instability.Relative in-dications include neuropathic joints and elderly,low-demand patients who have a poor soft tissue envelope.With major structural revisions,such as allograft prosthetic composites,one would predict the need for constrained devices.However,the required degree of constraint depends on the soft tissue envelope,not on the pre-existing bone de fi-ciency.When the capsule and collateral ligaments are maintained,they can provide extension stabil-ity.Flexion stability can be restored with reattach-ment of the collaterals,so surprisingly little compo-nent constraint may be necessary.In summary,in cases of instability or severe de-formity,constrained components must be available.In the presence of severe flexion-extension mis-match or de ficient medial stabilizing soft tissues,a rotating hinge design should be considered.Instability is increasingly recognized as a major factor in failures of primary and revision knee ar-teral radiograph of posterior dislocation of a varus-valgus constrained knee.Fig.3.Diagram of the mechanism of posterior dislocation of the tibial post under the femoral cam,which can occur with flexion instability.Constraint in Total Knee Arthroplasty •McAuley and Engh 53throplasty.Fig.5provides a simpli fied algorithm for choosing implant constraint.In seeking the opti-mum long-term result,physicians should be aware that careful preoperative and intraoperative assess-ment will greatly simplify decision making,assuring the best match of 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rotating hinge for com-plex total knee arthroplasty.Clin Orthop 392:292,2001Fig.5.Algorithm for choosing the degree of constraint.54The Journal of Arthroplasty Vol.18No.3Suppl.1April 2003。