a Departamento de Ortopedia, Santa Casa de São Paulo, São Paulo, SP, Brasil
b Serviço de Ortopedia e Traumatologia, Hospital Israelita Albert Einstein, Universidade Federal de São Paulo (Unifesp), São Paulo, SP,Brasil
c Instituto de Ortopedia, Hospital das Clínicas, Faculdade de Medicina, Universidade de São Paulo (USP), São Paulo, SP, Brasil
d Escola Paulista de Medicina, Universidade Federal de São Paulo (Unifesp), São Paulo, SP, Brasil
Introduction
Total knee arthroplasty has been reported to be one of the mostsuccessful orthopedic procedures, with implant longevitygreater than 15 years in around 90% of the cases.1,2Among thecomplications most commonly reported, problems relating tohealing of the operative wound, infection, thromboembolicdisorders, postoperative stiffness, periprosthetic fractures,alterations to patellar tracking, osteolysis, aseptic loosen-ing, instability and polyethylene wear can be highlighted.3,4Although the polyethylene component is subjected to cyclicalstress with the possibility of failure secondary to fatigue, dislo-cation of this component is a very rare event.4,5Here, we reporta case of this, which we believe to be the first one described inthe Brazilian literature.
Case report
The patient was a 68-year-old woman with a history of arthro-plasty of the left knee performed in 2002. Eight months afterthe first procedure, she started to present a condition of localpin and persistent effusion. After six months of follow-up,with persistently positive inflammation tests and indicativescintigraphy tests with technetium and gallium, but withthree arthrocentesis samples that did not produce bacterialgrowth in cultures, the prosthesis was removed and a cementspacer was inserted. The germ Streptococcus viridians was iso-lated and antibiotic therapy was instituted, guided by deeptissue that had been collected during the surgical procedure.
In October 2003, the arthroplasty was revised using nailsand wedges, cement containing antibiotic and a posterosta-bilized polyethylene component. There was no recurrence ofinfection after this operation.
In 2012, the patient sought the emergency service of ourhospital, with a complaint of sudden pain and instability inthe knee that had been operated, after abrupt physical effort.
On examination, coarse instability of the knee was observed,with joint effusion and diffuse pain. Although the initial diag-nostic hypothesis suggested the possibility of periprostheticfracturing, radiographs demonstrated displacement of the tib-ial polyethylene (Fig. 1).
The patient therefore underwent a new revision procedure.During the operation, an area of osteolysis in the proximalmedial tibial metaphysis was shown, in addition to dislocationof the polyethylene (Fig. 2). Nevertheless, the tibial nail pre-sented adequate stability. Thus, the polyethylene piece wasexchanged and a larger medial metal wedge was inserted(Figs. 3 and 4).
Currently, one year after this operation, the patientpresents excellent postoperative evolution: no pain, no effu-sion, complete extension, flexion of 115?, varus and valgusstability and adequate patellar tracking.
Discussion
The real incidence of dislocation of the polyethylene piecefrom its tibial base is unknown. Only a few cases have beenreported in the literature. Up to 2007, only four papers pub-lished in English described this phenomenon.5Most of thereports related to cases of implants in which the cruciate lig-aments were preserved.
The causes have not been well determined. Technicalerrors in inserting the polyethylene during the surgery, suchas incomplete seating, and also occurrences of trauma, maycause damage and failure of the locking system.4Forcedflexion movements starting from extension, which generategreater posterior load concentration, may contribute towardanterior loosening. Polyethylene dislocation may also becaused by the implant design, in situations of a shallow tibialbase or an excessively narrow track for the polyethylene pieceto be fitted into.
8In relation to the treatment, surgery for suspected casesis recommended in the literature. There are reports of goodresults achieved by exchanging only the polyethylene piece.However, an in vitro trial demonstrated that the force neededto displace the polyethylene became progressively lower withincreasing numbers of exchanges, which suggests that thelocking system becomes weakened.4Therefore, during revi-sion surgery, in cases in which there is movement betweenthe preexisting tibial base and the new polyethylene piece,revision of the tibial component will also be indicated.
Conflicts of interest
The authors declare no conflicts of interest.
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