The operative procedure was performed through a standard medial parapatellar approach and included débridement of the knee. Femoral and tibial cuts were performed with the instrumentation, with the so-called spacer block, available for the total condylar system. The use of a special drill-guide permitted the drilling of holes for impaction of the three tibial pegs; the holes were slightly undersized to permit initial press-fit fixation. Additional fixation could be obtained by the insertion of a cancellous-bone screw through a central-anterior hole in the component. All femoral components were fixed with cement. A special drill-guide also was used to permit the drilling of two undersized holes for press-fit fixation of the pegs of the patellar components that were inserted without cement; this method was not used for the eight patellar components that were inserted with cement.
The operating room was equipped with vertical laminar air-flow. Prophylactic antibiotics were administered to the patient for forty-eight hours postoperatively, and Hemovac drains were used in all knees for twenty-four hours postoperatively. All patients continued partial weight-bearing for six weeks and progressed to full weight-bearing by three months.
The patients were followed prospectively, and function of the knee was monitored yearly with use of a questionnaire. The result was graded, according to the modified knee-rating system of The Hospital for Special Surgery11, as excellent (85 to 100 points), good (70 to 84 points), fair (60 to 69 points), or poor (less than 60 points).
Routine anteroposterior, lateral, and patellar radiographs were made, with the patient bearing weight, at six weeks, three months, six months, one year, and then yearly thereafter. The radiographs were evaluated for subsidence of the component and for radiolucent lines at the prosthesis-bone interface. Areas of reactive sclerosis and radiolucent lines were divided into thirteen zones on the anteroposterior radiograph and into nine zones on the lateral tibial, lateral femoral, and skyline patellar radiographs.
Failure was defined as removal of the implant (revision total knee arthroplasty). Kaplan-Meier survivorship curves15 with 95 per cent confidence intervals were used to estimate the probability of failure of the implant as a function of time since the operation.
None of the thirty-six patients (thirty-seven knees) were lost to follow-up. However, eight patients (eight knees; 22 per cent) died at an average of seventy-one months (range, six to 108 months) after the procedure. In the group of patients who died, the average knee score11 at the latest examination had been 91 points (range, 81 to 98 points), and none of the implants had failed. In addition to the eight patients who died, eleven patients (eleven knees; 30 per cent) had a revision at an average of sixty-five months (range, four to ninety-five months) postoperatively, leaving a total of eighteen knees in seventeen patients available for a final evaluation at an average of eleven years (range, nine to twelve years).
Thirty-four (92 per cent) of the thirty-seven tibial components were well fixed, both clinically and radiographically, at the time of the latest follow-up. The other three tibial components were loose because of a late hematogenous infection (one knee; 3 per cent) or failure to achieve fixation by ingrowth (two knees; 5 per cent).
Of the eleven knees that had a revision, three had it because of failure of the metal-backed patellar component; one, because of excessive wear of the polyethylene of the tibial component; four, because of a hematogenous infection; two, because of aseptic loosening of the tibial component; and one, because of chronic synovitis.
In all three knees (8 per cent) that were revised because of failure of the metal-backed patellar component, severe wear of the polyethylene of the tibial component secondary to intra-articular metallic debris necessitated revision of the tibial component as well. In one additional knee, the patellar component was removed but the tibial and femoral components were not revised. In all of these knees, the tibial component was stable at the time of the revision.
The one implant (3 per cent) that failed because of excessive wear of the polyethylene of the tibial component was revised with a condylar tibial component, inserted with cement, from a total condylar knee system. The original tibial component was solidly fixed at the time of the revision.
The four patients (four knees; 11 per cent) who had a hematogenous infection had a two-stage revision at an average of forty-seven months (range, four to eighty-eight months) postoperatively. The first patient had had a previous revision total knee arthroplasty on the contralateral side. Six years after the index primary total knee arthroplasty, the patient had coronary-artery bypass grafting. An acute urinary-tract infection and idiopathic thrombocytopenic purpura developed after that procedure, and the patient was managed with high-dose steroids. Two months later, an acute infection in the knee that had had the index procedure necessitated a revision arthroplasty. Eight months later, an infection in the contralateral knee necessitated a second revision arthroplasty on that side. In the second patient, who had rheumatoid arthritis and who previously had had a revision total knee arthroplasty on the contralateral side as well as a primary total hip arthroplasty on the ipsilateral side, an infection developed six months after the index primary total knee arthroplasty. The third patient, who weighed 122 kilograms, had a revision total hip arthroplasty on the ipsilateral side one year after the primary total knee arthroplasty; seven months later, an infection developed in the knee. The three knees in these patients were functioning well and the components were well fixed at the time of removal. In the fourth knee in which an infection developed, only the tibial pegs appeared to be well fixed at the time of removal at eighteen months postoperatively. This patient had been doing well clinically and had had little pain in the knee until the onset of the hematogenous infection.
The two knees (5 per cent) that had aseptic loosening of the tibial component were revised at an average of nine months postoperatively.
Finally, in the knee that was revised because of chronic synovitis, the tibial component was stable at the time of the revision, five months postoperatively.
The average knee score11 of the eighteen knees in the seventeen living patients who had not had a revision was 47 points (range, 26 to 65 points) preoperatively and 87 points (range, 55 to 100 points) at the time of the latest follow-up. Eleven knees were rated as excellent; five, as good; none, as fair; and two, as poor.
Fourteen of the eighteen knees were not painful; two, slightly painful; and two, severely so. One of the two patients who had severe pain in the knee also had pain at the site of an ipsilateral total hip replacement as well as severe spinal stenosis. The other patient had a history of multiple operations on the knee for post-traumatic osteoarthrosis, including a previous total knee arthroplasty with cement. Aseptic loosening had necessitated a revision total knee arthroplasty with use of the fiber-metal tibial component that was used in the present study. This patient continued to have diffuse pain with no objective findings after the revision.
Fifteen of the eighteen knees were in patients who had no limp; two, in patients who had a slight limp; and one, in a patient who had a moderate limp. Thirteen knees were in patients who did not use any supports; three, in patients who used a cane for walking long distances; and two, in patients who used a cane full-time because of problems unrelated to the knee.
The average flexion of the eighteen knees was 99 degrees (range, 65 to 120 degrees) preoperatively and 98 degrees (range, 62 to 120 degrees) at the time of the latest follow-up.
Radiographs were available for the eighteen knees in which the prosthesis had been retained, the eight knees in the patients who had died, and ten of the eleven knees in which the prosthesis had failed. The average duration of radiographic follow-up for the eighteen knees in which the prosthesis had been retained and the eight knees in the patients who had died was ten years (range, six months to twelve years). In these two groups of knees, no complete radiolucent lines were noted around any component and there was no evidence of subsidence or periprosthetic osteolysis. Partial radiolucent lines at the tibial prosthesis-bone interface were seen on the anteroposterior radiograph of five knees (19 per cent) and on the lateral radiograph of four knees (15 per cent). These lines were seen only on the medial and lateral peripheries of the proximal part of the tibia, and none of the lines were more than two millimeters thick. In one knee (4 per cent), however, a radiolucent line that was less than two millimeters thick was seen under the tibial plate in multiple zones on the anteroposterior radiograph. No radiolucent lines were noted on the anteroposterior radiograph of sixteen knees (62 per cent) and on the lateral radiograph of fifteen knees (58 per cent). No radiolucent lines were noted about the pegs of the tibial component in any knee. The anteroposterior radiograph of four knees (15 per cent) and the lateral radiograph of seven knees (27 per cent) did not adequately demonstrate the tibial prosthesis-bone interface.
A partial radiolucent line was noted about the femoral component in two knees (8 per cent). No radiolucent lines were seen around the femoral component in nineteen knees (73 per cent), and the radiographs of five knees (19 per cent) did not adequately demonstrate the femoral prosthesis-bone interface. No radiolucent lines were noted about the patellar component on the radiographs of twelve of the eighteen knees in which that component had been inserted without cement, and the patellar component in one other knee had been removed previously because it had failed. The skyline radiographs of the other five knees did not adequately demonstrate the patellar prosthesis-bone interface. No radiolucent lines were noted about any of the all-polyethylene patellar components that had been fixed with cement.
Radiographs were available for ten of the eleven knees in which the prosthesis had failed, as stated previously. No radiolucent lines had been noted around the tibial or femoral components of the three knees that were revised for failure of the patellar component or the one knee that was revised for polyethylene wear. No radiolucent lines were noted around the femoral, tibial, or patellar components of three of the four knees that were revised because of an infection. The fourth knee in that group had had progressive radiolucent lines that were more than two millimeters thick in four zones on the anteroposterior radiograph of the tibia; the tibial component was found to be loose at the time of the revision. One of the two knees that were revised for aseptic loosening of the tibial component had had radiolucent lines in at least three zones on the anteroposterior radiograph of the tibia; no radiographs of the other knee were available for review. The one knee that was revised for chronic synovitis had had radiolucent lines in two zones on both anteroposterior and lateral radiographs of the tibia. At the time of the revision, the tibial component was found to be stable. All of the femoral components were found to be stable at the time of the revision.
With removal of the implant as the end point, the cumulative rate of survival of the thirty-seven total knee prostheses was 83 per cent (95 per cent confidence interval, 77 to 89 per cent) at fifty-six months and 67 per cent (95 per cent confidence interval, 59 to 75 per cent) at 108 months (Fig. 2). Six of the eleven failures occurred between fifty-six and 108 months: four of these were caused by failure of the patellar component or excessive wear of the polyethylene, and two were secondary to a hematogenous infection.
Concern over fixation of the components of a total knee prosthesis has generated considerable interest, as there have been no long-term follow-up studies, to our knowledge, that have evaluated the prosthesis-bone interface of components inserted without cement. The present study focused specifically on the prosthesis-bone interface of the tibial component as well as the functional status of patients in whom a so-called first-generation total knee prosthesis had been implanted without cement.
The present prospective, non-randomized study included patients who had been managed at the Rush-Presbyterian-St. Luke's Medical Center between 1981 and 1983. Failures were secondary to failure of the metal-backed patellar component, excessive polyethylene wear, a hematogenous infection, aseptic loosening, and chronic synovitis.
Failure of the metal-backed patellar component has been associated with many contemporary designs of total knee prostheses3,26,27 and was the reason for three of the eleven failures in the present series. Two mechanisms of failure have been described. The first, growth of bone into the pegs but not into the base-plate, ultimately can lead to separation between the pegs and the base-plate26. The second, excessive wear of the polyethylene, can lead to delamination of the polyethylene and exposure of the metal backing, which subsequently causes abrasion of the femoral component, accumulation of metal debris, and destruction of the polyethylene of the tibial component3,26. We currently use only all-polyethylene components fixed with cement for resurfacing of the patella. Excessive wear of the bearing surface was the reason for the revision in one patient.
Originally, carbon-fiber-reinforced polyethylene was thought to offer improved resistance to creep, improved wear properties, and increased strength compared with plain (non-reinforced) polyethylene1,35. However, subsequent clinical and laboratory experience failed to show that the reinforced polyethylene had improved resistance to wear. The increase in the modulus of elasticity caused by the addition of carbon fibers was shown to increase contact pressures and other stresses that are associated with surface damage2,18. In addition, carbon-fiber-reinforced material was found to have notably less resistance to propagation of fatigue cracks than plain polyethylene2,4,33. Reports of catastrophic failure of carbon-fiber-reinforced polyethylene as well as the just noted findings have led to the abandonment of the use of this material in total knee arthroplasty34.
Four knees failed because of a late hematogenous infection. Three of the four patients had host factors that are known to increase the risk of infection after total knee arthroplasty, including rheumatoid arthritis and previous revision arthroplasty of the contralateral knee (two patients) and of the ipsilateral hip (one patient). Another possible explanation for this relatively high prevalence (11 per cent) of late infection may be related to the use of carbon-fiber-reinforced polyethylene. We presume that this material generated larger amounts of wear debris than conventional polyethylene would have. This may have led to an inflammatory response and predisposed the patients to hematogenous seeding in the knee. Other authors have noted a similar phenomenon with metal-on-metal articulations14.
Two (5 per cent) of the thirty-seven knees were revised because of aseptic loosening of the tibial component. This rate is slightly higher than the 2 per cent rate of loosening (two of 132 knees) that we have observed with newer types of implants designed to be inserted without cement24,25. Other authors have reported similar rates of aseptic loosening of tibial components implanted without cement, ranging from 0 (of sixty-three knees) to 19 per cent (twenty-one of 108 knees)9,10,19,23. In the present study, the tibial components in the surviving patients who had not had a revision appeared to be well fixed at the time of the latest follow-up on the basis of both the clinical findings and the paucity of radiolucent lines on radiographs (Figs. 3-A and 3-B). Studies of animals have demonstrated that radiolucent lines at the ingrowth interface between the substrate and the bone are found in areas of fibrous tissue ingrowth8,30. We found a complete radiolucent line at the prosthesis-bone interface on the anteroposterior radiograph of only one tibia (Figs. 4-A and 4-B). This patient subsequently had revision of all components because of symptomatic aseptic loosening.
The process of bone ingrowth is highly dependent on adequate initial stability to decrease the potential for micromotion until biological ingrowth has occurred5,8,28,29,30. In the present study, initial stabilization of the tibial base-plate was attempted with a single screw and the so-called press-fit provided by underreaming of the peg-holes. Laboratory testing has demonstrated that the mechanical stability of the tibial component is directly related to adequate fixation with multiple screws31. Despite the apparent mechanical inadequacy of the initial fixation, 92 per cent (thirty-four) of the components appeared to be stable clinically and radiographically.
The tibial component was fabricated by molding the carbon-fiber-reinforced polyethylene directly into the fiber metal. This provided a one-piece non-modular composite device. There are several disadvantages to a non-modular design. The inability to separate the base-plate from the bearing surface prevents the surgeon from placing additional screws in either the medial or the lateral tibial plateau to enhance the stability of the component. Intraoperatively, the surgeon is prevented from changing only the bearing surface during the final stages of ligamentous balancing and from adding various degrees of constraint to the tibial articular surface. During a revision operation, the entire tibial component must be removed to change a worn bearing surface or to correct ligamentous instability, potentially increasing loss of bone stock in the proximal part of the tibia. In addition, with the multitude of sizes, a larger inventory is required than is needed with a modular system. The modularity of updated tibial components that are designed to be inserted without cement has addressed many of these problems but has created new ones; for example, the interface between the metal tray and the insert can be a source of particulate polyethylene debris as a result of micromotion, resulting in accelerated wear13,16. This debris, along with debris from fretting corrosion of the screws into the tibial base-plate, may track down the screw-holes into the proximal part of the tibia, causing osteolysis21.
In the present series of patients who were managed with a porous-coated tibial component without cement, the rate of complications and failures was high for reasons other than aseptic loosening. Some of the failures were secondary to design features that now are considered to be inadequate, including a metal-backed patellar component and carbon-fiber-reinforced polyethylene. The tibial components that were retained remained radiographically stable and appeared to have a secure interface with bone. None of these components were associated with progressive radiolucent lines, and none subsided. Even though the series was small, the present study demonstrates that a porous ingrowth surface can provide a stable interface for biological fixation over the long term.
*One or more of the authors have received or will receive benefits for personal or professional use from a commercial party related directly or indirectly to the subject of this article. In addition, benefits have been or will be directed to a research fund or foundation, educational institution, or other non-profit organization with which one or more of the authors are associated. No funds were received in support of this study.
†Department of Orthopaedic Surgery, Rush-Presbyterian-St. Luke's Medical Center, 1725 West Harrison Street, Suite 1063, Chicago, Illinois 60612. Please address requests for reprints to Dr. Silverton.
Investigation performed at the Rush-Presbyterian-St. Luke's Medical Center, Chicago
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Copyright 1996 by The Journal of Bone and Joint Surgery, Incorporated
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