JBJS | The Effectiveness of Isolated Tibial Insert Exchange in Revision Total Knee Arthroplasty

The Journal of Bone & Joint Surgery, Volume 84, Issue 1

Scientific Article | January 01, 2002

The Effectiveness of Isolated Tibial Insert Exchange in Revision Total Knee Arthroplasty

George C. Babis, MD; Robert T. Trousdale, MD; Bernard F. Morrey, MD

Investigation performed at the Mayo Clinic and Mayo Foundation, Rochester, Minnesota

George C. Babis, MD
Department of Orthopaedic Surgery, University of Athens, "KAT" Accident Hospital, 2 Nikis Str., GR 145 61 Kifissia, Athens, Greece

Robert T. Trousdale, MD
Bernard F. Morrey, MD
Department of Orthopedics, Mayo Clinic and Mayo Foundation, 200 First Street S.W., Rochester, MN 55905

No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. No funds were received in support of this study.

The Journal of Bone & Joint Surgery. 2002; 84:64-68

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Abstract

Background: Despite improvements in the design and manufacturing of the components used in total knee arthroplasty, wear of the polyethylene bearing remains a potential source of failure. One theoretical advantage of modular tibial implants is that, when the components are well fixed, patients with wear or instability of the tibial insert can be treated with isolated polyethylene exchange. The aim of this study was to assess the results of isolated tibial insert exchange during revision surgery in a relatively large, consecutive group of patients.

Methods: From 1985 through 1997, we performed fifty-six isolated tibial insert exchanges in fifty-five patients (twenty-nine men [one man had bilateral revision] and twenty-six women; mean age, sixty-six years) primarily because of wear or instability. Patients with loosening of any of the components, a history of infection, severe stiffness of the knee, recognized malposition of any component, or problems with the extensor mechanism were excluded. Twelve knees had had one, two, or three prior revisions. The duration of follow-up averaged 8.3 years (range, 1.6 to 16.2 years) after the index arthroplasty and 4.6 years (range, two to fourteen years) after the revision.

Results: The mean Knee Society knee and function scores improved from 56 and 50.9 points prior to the revision to 76 and 59 points at the time of final follopcow-up. Fourteen (25%) of the fifty-six knees subsequently required rerevision at a mean of only three years (range, 0.5 to 6.8 years) after the tibial insert exchange. The cumulative survival rate at 5.5 years was 63.5% (95% confidence interval, 14.4%, with nineteen patients remaining at risk). Of the twenty-seven knees with preoperative instability, eight were rerevised and another four were considered failures because of severe pain. Of the twenty-four knees that were treated with the index revision because of wear of the insert, five were rerevised. In addition, one extremity in this group was amputated above the knee as a result of chronic osteomyelitis of the ankle concomitant with chronic pain at the site of the total knee arthroplasty and another two inserts were considered failures because of severe pain.

Conclusions: Isolated tibial insert exchange led to a surprisingly high rate of early failure. Tibial insert exchange as an isolated method of total knee revision should therefore be undertaken with caution even in circumstances for which the modular insert was designed and believed to be of greatest value.

Figures in this Article

Introduction

Introduction | Materials and Methods | Results | Discussion | References

The ultra-high molecular weight polyethylene insert of a total knee prosthesis^1-7 remains a weak link of the implanted components^8,9. Apart from loosening, various modes of polyethylene damage are often the cause of failure after total knee arthroplasty^10,11. Instability, early or late, is another cause of pain and functional impairment that may necessitate revision surgery¹². Modularity of the tibial insert was developed with the expectation of providing an easy solution to isolated wear-related problems necessitating revision. Modularity is an attractive solution because it offers the benefits of flexibility; allows hospitals to reduce their inventory of implants; and permits a quick, simple, safe, and bone-stock-preserving isolated tibial insert exchange for selected patients. A reasonable prerequisite is that the components be securely fixed in an acceptable position without signs of impending failure. There is little information documenting the outcomes of isolated tibial insert exchange. We are aware of only one previous report, which showed unfavorable results and included an analysis of the causes of failure¹³. In the present study, for which we used strict inclusion parameters, we evaluated the effectiveness of isolated tibial insert exchange in the salvage of malfunctioning total knee replacements in which all components were well fixed and well aligned.

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TABLE I: Results of Isolated Insert Exchange According to the Indication for the Revision

*Above-the-knee amputation was performed because of chronic osteomyelitis of the ankle and chronic pain at the site of the total knee arthroplasty. †At 5.8 years (range, two to 14.2 years) after insert exchange. ‡At 4.6 years (range, two to 8.8 years) after insert exchange.

Indication	Total No. of Knees (N = 56)	No. of Failed Knees (N = 21)		No. of Surviving Knees (N = 35)	Knee Society Knee Score for Surviving Knees (points)
Rerevised	Severe Pain
Instability	27	8	4	15†	?80 (50-100)
Wear	24	5 (+1*)	2	16‡	87 (33-99)
Other	5	1	—	4	89 (67-97)

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TABLE I: Results of Isolated Insert Exchange According to the Indication for the Revision

Indication	Total No. of Knees (N = 56)	No. of Failed Knees (N = 21)		No. of Surviving Knees (N = 35)	Knee Society Knee Score for Surviving Knees (points)
Rerevised	Severe Pain
Instability	27	8	4	15†	?80 (50-100)
Wear	24	5 (+1*)	2	16‡	87 (33-99)
Other	5	1	—	4	89 (67-97)

Materials and Methods

Introduction | Materials and Methods | Results | Discussion | References

Fifty-six consecutive isolated modular tibial insert exchanges were performed from 1985 through 1997 at our institution. Patients who had a worn insert or instability of a nonhinged total knee replacement were included in the study, whereas patients who had loosening of any of the components, a history of infection, severe stiffness of the knee, recognized malposition of any component, or problems with the extensor mechanism were excluded. Patients with an incidental or a coincidental problem with a patellar prosthesis were excluded as well.

All knees were considered to be adequately aligned on the basis of a preoperative evaluation that included assessment of radiographs. Acceptable placement of the implant was defined as a femoral component in 0Â° to 10Â° of valgus, a tibial component in neutral to 4Â° of varus or valgus, and an overall alignment of neutral to 10Â° of anatomic valgus on the anteroposterior radiograph. On the lateral radiograph, the implant positions were deemed acceptable when the femoral component was parallel ( ± 5Â°) to the femoral axis and the tibial component had 0Â° to 10Â° of posterior slope.

The study included twenty-nine men (one man had bilateral replacement) and twenty-six women with a mean age of sixty-six years (range, thirty-five to eighty-three years) at the time of the insert exchange. The underlying diagnosis leading to the primary total knee arthroplasty was osteoarthrosis in forty-five knees (80%); rheumatoid arthritis in one knee; post-traumatic arthritis in seven knees; and avascular necrosis, osteochondritis dissecans, and a benign tumor in one knee each. Twelve knees had had one, two, or three revisions before the index tibial component exchange. The duration of follow-up averaged 8.3 years (range, 1.6 to 16.2 years) after the index arthroplasty and 4.6 years (range, two to fourteen years) after the revision.

Twenty-seven inserts were replaced with a thicker device in an effort to manage painful ligamentous instability. Polyethylene wear was not noted on radiographs or during the operation. Varus-valgus instability of grade 2¹⁴ or more was detected in fourteen of the twenty-seven unstable knees, whereas anteroposterior instability was seen in two knees, flexion instability was seen in two knees, and global instability was seen in nine knees. Twenty-four knees were revised because of polyethylene failure from wear seen radiographically and confirmed intraoperatively; this group included seven knees with catastrophic wear through the modular tibial insert and two knees with dislodgment of the polyethylene from its tibial base-plate. Twenty-two of the twenty-four patients who underwent revision because of wear were experiencing pain before the revision. Five additional knees had the insert exchanged for other reasons, including an intra-articular foreign body requiring removal (one knee), impingement of the insert post on the intercondylar notch (one knee), painful synovitis without macroscopic evidence of wear or instability (two knees), and pain (after a cementless knee arthroplasty) due to a tibial component screw that was transfixing the head of the fibula (one knee).

The inserts were exchanged at a mean of four years (range, three months to 12.3 years) after the previous total knee replacement. A wide variety of implants were in place at the time of the insert exchanges.

The inserts that were being replaced had a mean thickness of 11.1 mm (range, 6 to 30 mm), whereas those that replaced them had a mean thickness of 15.4 mm (range, 8 to 45 mm). In four cases, an insert of the same height was used. Lateral retinacular release was performed in seventeen knees. No collateral ligament reconstructions were performed. In two knees with osteolysis, curettage and filling of the osseous cysts with acrylic cement or allograft bone was performed. The total tourniquet time averaged fifty-four minutes (range, twelve to 132 minutes).

Preoperative and postoperative assessments included determination of clinical Knee Society knee and function scores¹⁵ and performance of the Knee Society roentgenographic evaluation¹⁴. Preoperative and postoperative radiographs were available for all patients. Inserts were defined as failures when patients underwent rerevision or graded the pain as severe.

Data analysis was performed with JMP statistical software (version 3.2.6; SAS Institute, Cary, North Carolina). Survival analysis was done according to the Kaplan-Meier method¹⁶. The 95% confidence intervals were calculated with use of the formula: 95% confidence interval = ± 1.96 standard error of the mean. The log-rank test was used to compare the survival among groups with different indications for insert exchange. The Fisher exact probability two-tailed test was used to evaluate whether gender, alignment of the prosthesis, use of a stemmed or cemented prosthesis, or use of a revision or primary prosthesis affected the rate of failure. Logistic regression was used to assess the rate of failure associated with the different indications for exchange and types of total knee arthroplasty as well as the gender, age, weight, and height of the patients. One-way analysis of variance was used to confirm differences among the various groups with respect to the final clinical score. Differences in data occurring with <0.05 likelihood of chance were considered significant.

Results

Introduction | Materials and Methods | Results | Discussion | References

Fourteen (25%) of the fifty-six implants were rerevised after a mean of three years (range, 0.5 to 6.8 years). One additional patient had an above-the-knee amputation. Four patients died during the follow-up period of causes unrelated to the insert exchange. The forty-one knees not requiring an additional revision were followed for an average of 5.2 years (range, two to fourteen years). The Knee Society knee and function scores for these forty-one knees improved from a mean of 56 points (range, 6 to 96 points) and 50.9 points (range, —5 to 100 points) prior to the revision to 76 points (range, 21 to 100 points) and 59 points (range, 0 to 100 points) at the time of final follow-up. Patients subjectively graded the results as much better for twenty-three knees, somewhat better for eight, the same for five, and worse for five compared with the status before the exchange. The cumulative overall survival rate (free of rerevision and severe pain) was found to be 63.5% at 5.5 years (95% confidence interval, ± 14.4%, with nineteen patients remaining at risk). On the basis of the numbers available, the gender, age, height, and weight of the patient had no significant impact on failure. Also, no significant difference in terms of failure or final knee score was associated with the indication for the insert exchange, the type of total knee arthroplasty, the type of fixation, or whether the knee had been previously revised. How the time interval from the previous operation to the insert exchange operation influenced the final result could not be detected with the numbers available, but the trend suggested that early initial failure portends a poorer outcome of an isolated insert exchange (p < 0.09).

Instability: Twelve (44%) of the twenty-seven knees that had had the insert exchange because of instability failed at a mean of three years (range, 0.6 to 6.4 years). Eight of the twelve failed knees were rerevised: five because of persisting instability, one because of loosening of the tibial component, one because of painful stiffness, and one because of deep infection. Four of the twelve failed knees were severely painful and had a mean knee score of 30.8 points (range, 21 to 42 points) at the time of the most recent follow-up. Fifteen of the twenty-seven knees had survived at a mean of 5.8 years (range, two to 14.2 years) and had an average knee score of 80 points (range, 50 to 100 points) (Table I). The cumulative survival rate at 5.5 years was 54.4%, with a 95% confidence interval of ± 20.1% and seven knees remaining at risk. The nine knees with preoperative global instability had a worse survival rate than the fourteen knees with isolated varus-valgus instability (p < 0.035, log-rank test).

Wear: Eight (33%) of the twenty-four knees with wear of the insert failed at a mean of four years (range, 0.8 to 6.8 years) postoperatively. Five of the eight knees were rerevised: three because of recurrent wear of the polyethylene insert (at 1.8, 2.5, and 5.5 years after the initial revision), one because of wear through a metal-backed patellar component that damaged the femoral component, and one because of painful postoperative stiffness. One patient had an above-the-knee amputation as a result of chronic ankle osteomyelitis concomitant with chronic pain at the site of the total knee arthroplasty. Two of the eight failed knees (one in a patient with Parkinson disease) were considered failures because of severe pain. The knee score was 23 points for each of these two knees. The remaining sixteen knees had survived at a mean of 4.6 years (range, two to 8.8 years) and had a mean knee score of 87 points (range, 33 to 99 points) (Table I). The cumulative survival rate at 5.5 years was 71.6%, with a 95% confidence interval of ± 22.7% and nine patients remaining at risk.

Miscellaneous: Five knees had the isolated insert exchange for reasons other than instability or wear. One knee that had the exchange because of impingement of the polyethylene post or the intercondylar notch was rerevised 2.5 years later as a result of loosening. The remaining four knees had survived at a mean of 7.6 years (range, five to 9.6 years), with an average knee score of 89 points (range, 67 to 97 points).

The knee score was not significantly different between the knees with instability and those with wear (p = 0.25). Although twelve (44%) of the twenty-seven knees that were operated on because of instability failed in contrast to eight (33%) of the twenty-four knees that were operated on because of wear, no difference in the survival curves could be demonstrated (p = 0.38, log-rank test). This finding was also true when only rerevision was set as the end point of the survival analysis (p = 0.7, log-rank test).

Complications

One diabetic patient with an unstable knee and multiple previous revisions had an acute deep infection and was managed with staged revisions. Another patient had persisting osteomyelitis of the ankle along with a chronically painful, poorly functioning knee. This patient was managed with an above-the-knee amputation. One patient, who had Parkinson disease, sustained a patellar fracture, which was treated conservatively. Two years later, the patellar component was revised because of loosening. One patient was operated on again because of so-called patellar clunk syndrome and had a poor final result.

Discussion

Introduction | Materials and Methods | Results | Discussion | References

Wear of the tibial polyethylene of a total knee prosthesis remains a concern and a potential source of failure^8,17,18. One reason why modular implants were developed was to permit the surgeon to perform a straightforward tibial insert exchange as a solution for a malfunctioning total knee replacement. Potential indications for this procedure include isolated wear of the polyethylene insert, instability of the knee, and a so-called tight knee with limited range of motion¹⁹. In addition to our experience with arthrolysis and exchange to a thinner insert¹⁹, we are aware of only one recent report¹³ in the English-language literature that examines whether this design concept has any merit.

Problems related to the extensor mechanism, including severe maltracking, wear, and/or loosening of the patellar component, often coexist in patients requiring consideration of revision of the tibial insert. Inclusion of patients with these problems may lead to biases that will obscure the effect of the exchange of the tibial polyethylene. In one series of forty-eight insert exchanges, twenty-two were performed because of wear of the insert, twenty-one were performed as a part of an operation done because of a failed metal-backed patellar component, two were performed during débridement for the treatment of an acute infection, one was done because of arthrofibrosis, one was done because of rupture of the quadriceps tendon, and one was done because of patellar dislocation¹³. Seven (15%) of the forty-eight knees failed because of wear and required complete rerevision at an average of fifty-four months. Surprisingly, insert exchanges that had been performed in knees with a failure of a metal-backed patellar component resulted in a significantly higher rate of survival than did exchanges performed because of polyethylene wear alone (p < 0.05). However, it is not clear whether the inserts in the former group were exchanged because of coexisting wear or whether the exchange procedure was done coincidentally.

The attempt to isolate cases that had a pure modular tibial insert exchange at our institution was arduous. Incidental or coincidental patellar problems or malalignment of the extensor mechanism that necessitated correction often coexisted. Our series of fifty-six consecutive knees with an isolated insert exchange included twenty-four that were revised because of failure of the insert itself (twenty-two were revised because of wear of the insert and two, because of dislodgment of the tibial insert from the metal tibial tray), twenty-seven revised because of instability, and five revised for other reasons. Overall, fifteen (27%) of the knees subsequently required an additional revision or amputation.

Instability after total knee arthroplasty can lead to early or late failure^20,21 and may present as a sense of instability with or without pain, so-called giving-way of the knee, or a recurrent effusion¹². Treatment options include revision to a thicker, more conforming polyethylene insert, to posterior-cruciate-substituting components, to varus-valgus constrained components, or even to hinged implants with or without ligamentous reconstruction. A thicker insert increases the space between the tibial and femoral components and thus tightens the collateral ligaments, thereby enhancing the stability of the joint. A thicker insert cannot compensate for insufficiency of the collateral ligaments or for unequal flexion and extension spaces. Pagnano et al.¹² reported on patients with flexion instability, three of whom were managed with replacement of the polyethylene liner with one of a thicker size. Two of these inserts failed and were rerevised because of pain and recurrence of instability. Engh²² reported on eight patients with flexion instability who were managed with placement of a thicker tibial insert. Only four knees were stabilized, and one knee was rerevised. In our study, 30% (eight) of the twenty-seven unstable knees failed within three years after the exchange and were rerevised. In addition, four more knees were causing severe pain at the time of final follow-up. These results support the premise that treatment of instability by simple insert exchange yields unpredictable results. The surgeon should have a clear understanding of the type and cause of instability and should use the appropriate constrained implant for each knee²¹.

Polyethylene wear is an important cause of failure and a factor limiting the long-term success of total knee arthroplasty. Metal-backed trays were designed in part to minimize stress at the bone cement-implant interface. These trays occupy 3 to 4 mm of valuable thickness that cannot be occupied by the polyethylene, increasing the contact stress on the polyethylene²³. Micromotion at the interface of the tibial insert and the base-plate, as is found in contemporary implants, can be a source of buildup of particulate debris and subsequent osteolysis^24-26. The results of our study suggest that isolated tibial insert exchange may not be a viable option, bringing into question the value of modularity of the tibial trays. Yet, modular inserts still have value in that they allow a smaller inventory as well as intraoperative adjustments of thickness and constraint.

The malfunctioning of a total knee replacement with well-fixed components is often multifactorial. Severe wear of the polyethylene insert may be the most common cause, but the reasons that lead to wear are often neglected or cannot be determined. Malrotation of the components, chronic ligament attenuation and subsequent instability, underlying loosening, different flexion and extension gaps, or even the presence of a low-grade infection may be important factors. A malfunctioning locking mechanism cannot be fixed at the time of an exchange and is likely to permit unacceptable micromotion of the new insert. The success of an isolated insert exchange is unlikely in these situations. Our data are consistent with the conclusion of Engh et al.¹³ that an isolated insert exchange should not be performed in cases with accelerated wear within ten years after the index surgery.

Note: The authors thank Michael A. Morrey, PhD, for advice on the statistical assessment of the results and Pilar Dechet for conceptual contributions.

References

Introduction | Materials and Methods | Results | Discussion | References

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TABLE I: Results of Isolated Insert Exchange According to the Indication for the Revision

Indication	Total No. of Knees (N = 56)	No. of Failed Knees (N = 21)		No. of Surviving Knees (N = 35)	Knee Society Knee Score for Surviving Knees (points)
Rerevised	Severe Pain
Instability	27	8	4	15†	?80 (50-100)
Wear	24	5 (+1*)	2	16‡	87 (33-99)
Other	5	1	—	4	89 (67-97)

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TABLE I: Results of Isolated Insert Exchange According to the Indication for the Revision

Indication	Total No. of Knees (N = 56)	No. of Failed Knees (N = 21)		No. of Surviving Knees (N = 35)	Knee Society Knee Score for Surviving Knees (points)
Rerevised	Severe Pain
Instability	27	8	4	15†	?80 (50-100)
Wear	24	5 (+1*)	2	16‡	87 (33-99)
Other	5	1	—	4	89 (67-97)