Background
Total knee arthroplasty (TKA) is widely performed in order to reduce the pain that is caused by a joint destruction due to joint problems that are accompanied by functional disorders and, in addition, to promote a stable joint movement by correcting deformities [
1]. With the advancements that have been made in surgical techniques and equipment design, the long-term survival rate of the prosthetic joint has increased when compared to that when the procedure was first introduced [
1,
2].
Since Freeman et al. [
3] introduced the procedure of posterior cruciate ligament (PCL) resection in 1977, the decision whether to retain the ligament during a TKA procedure has been a point of controversy from various perspectives. If the posterior cruciate ligament is retained during a TKA operation, the flexion range of motion (ROM) of the knee can increase because a femoral rollback occurs during the flexion like a normal knee. The posterior cruciate ligament is the strongest ligament in the knee, and therefore, if it is retained during a TKA operation, the knee’s original stability can be preserved post procedure [
4]. Additional advantages of a posterior cruciate ligament retention are that patients are more functional when walking and climbing stairs because of a better proprioception [
1]. The loosening of the implant is also less likely because the ligament reduces the friction between the implant and the bone [
4,
5].
The patella in the patellofemoral joint bears up to seven times the body weight during joint exercises. Hence, when considering the various functions and symptoms involving the patella, whether to retain it during a TKA surgical process remains controversial [
6]. Waters and Bentley [
7] argued that patellar resurfacing had a better outcome in terms of pain reduction, with a patient’s satisfaction and less complications, whereas Burnett et al. [
8] argued for the retention of the patellar, and Jung et al. [
9] advocated for selective patellar resurfacing.
Therefore, in the current study, we aimed at comparing the clinical and the radiological outcomes of patellar resurfacing against patellar retention by evaluating the long-term follow-up results over at least 14 years of post cruciate retention when using the NexGen®-CR system for a TKA operation.
Discussion
The goals of a TKA procedure are focused on pain relief, stabilization of the joint, and correcting any deformities. With the recent advancements in scientific tools for prosthetic knee joints and surgical techniques, the 15- to 20-year long-term survival rate of the prosthesis is as high as 90–98% in both young and elderly patients [
16].
The NexGen®-CR (Zimmer, Warsaw, Indiana, USA) system, a prosthetic knee joint that was developed in the mid 1990s, has an incurvate shape, with a small radius that assures a wide contact with and a good conformity to the femur condyle. Additional characteristics of the system are the combination of a diverse array of implants; an increment of the contact area and conformity to the joint by an anatomically designed femoral implant with improved knee tracking; compatibility between the different sizes of implants; and the selection of a variety of surgical instruments in order to perform the procedure. In addition, a change can be made from PCL retention to a PCL substitution, according to the need during the procedure [
17].
Surgeons who prefer to retain the posterior cruciate ligament have argued that the ligament is the strongest part in the knee and that posterior tibial subluxation can be prevented by retaining it in order to preserve the original stability. In addition, they argue that proprioception is better with posterior cruciate ligament retention, and therefore, there is a better function when walking and climbing stairs [
18]. The balance in the muscles around the knee is also better through an appropriate perceptual feedback [
19]. In contrast, a posterior cruciate ligament resection has been reported to have the advantages of an easier surgical operation, a minimization of the tibial resection margin, an avoidance of tension in the ligament, a wide contact with the joint in order to reduce the likelihood of polyethylene wear, and an easier deformity correction [
20,
21].
Lowry and Sledge [
22] have argued that a posterior cruciate ligament resection is preferred for those patients with a varus deformity of more than 25°, or a knee flexion contracture of more than 30°, if the ligament is deformed. Accordingly, in the present study, we have used these conditions as the surgical criteria in determining whether to retain or resect the posterior cruciate ligament. So far, there is no clinical index available in order to determine the condition of the posterior cruciate ligament. Hence, we have tried to minimize the errors in a case selection, by having a single surgeon making consistent decisions across all of the cases with regard to the function of the posterior cruciate ligament and on the basis of a visual inspection and a tension test. In our clinical practice, we have experienced satisfying outcomes with posterior cruciate ligament retention. Because of the lack of a histological index to help a surgeon determine the condition of the posterior cruciate ligament and the difficulty in making such a decision, we decided that a visual inspection with a test for the tension of the ligament could serve as a clinical index when determining whether to retain the ligament in a TKA procedure.
In the current study, we found that the ROM significantly improved from a mean of 109.2° to a mean of 123.4° post procedure in those patients who underwent a cruciate-retaining TKA that was performed with the NexGen® system and with a prosthetic knee joint. The prosthetic survival rate after a mean follow-up period of 14 years and 8 months was 95.7%. These outcomes are satisfactory when in comparison with those of other prosthetic knee replacement surgeries.
The importance of an axial alignment between the femur and the tibia after a TKA operation has been stressed in the literature. For example, Lotke and Ecker [
23] have shown that a position of between 3° and 7° valgus was desirable. We observed a satisfactory level of deformity correction from the preoperative alignment of 7.6° varus to the postoperative alignment of 4.9° valgus.
Kraay et al. [
24] reported that radiolucent lines were most frequently seen in the most proximal area of an anterior flange. They speculated that this might be due to nonconformity of the femoral implant to the anterior resection surface of the femur and the insertion of a flexed implant. King and Scott [
25] reported that 15 out of approximately 1600 cases of a TKA procedure experienced a loosening of the femoral implant. An implant loosening occurred in zone 4 in 13 cases, and radiolucent lines were observed immediately after the procedure in 8 cases.
They argued that the early loosening of the femoral implants that were fixated with cement was caused by a lack of support in the posterior condyle area of the femoral implant. This was because of an incomplete resection of the posterior femoral condyle, a poor cementing technique, and an insufficient structure of the condyle. However, when analyzing the influence of radiolucent lines on patients, we observed radiolucent lines in zones 1 and 2 (the lateral side of the tibia on the anteroposterior surface), finding different results than the study of King and Scott. We believe that this was because we used a full cement technique to apply the cement onto the femur, the tibia, the patella, and the implant. No cases in our study showed aseptic loosening.
Patellar resurfacing is a subject of much controversy [
4,
26]. However, it has been reported that patellar resurfacing is preferred in those cases who suffer from severe patellofemoral arthritis, rheumatoid arthritis, a poor patellofemoral alignment, a patellofemoral pain prior to surgery, and an abnormal alignment and height of the patella. Shih et al. [
27] reported that they observed degenerative changes in the patellofemoral joint and with a valgus displacement in those cases with a preoperative patellar maltracking. They argued on the basis of their findings that patella maltracking could be an indication for a patellar resurfacing. However, they did not find an association between the radiological findings and the patellar scores. Other researchers have also reported a lack of significant differences in the clinical outcomes between patellar resurfacing and patellar retention [
1,
19]. In the present study, we did not find significant differences, clinically or radiologically, between the patellar resurfacing group (68 cases) and the patellar retention group (48 cases).
In patella resurfacing group, there were three (4.4%) patients who showed patellar subluxation. There were several reasons which influenced on patellar subluxation after patellar resurfacing surgery. One of the reasons was the consequent tension of the lateral side. Resection of the lateral facet or the distal pole leads to tightness of the lateral retinaculum and a tendency to subluxation [
28,
29]. However, there was no symptomatic subluxation which led to anterior knee pain or functional problem. Therefore, we did not perform an additional surgical procedure. Conservative methods as quadriceps exercises, braces, or avoiding activities that aggravate instability were applied in subluxations and with time scarring of the retinacular tissues lead to resolutions of the symptoms.
There are a few limitations in the present study. First, it was retrospectively conducted with only one type of implant. Second, the decisions on performing either patellar resurfacing or patellar retention were made by the surgeon’s subjective judgment. However, this study is of significance in that it reported the long-term follow-up results regarding the NexGen®-CR system. This particular procedure was used for posterior cruciate ligament retention. In addition, the clinical outcomes of patellar resurfacing and patellar retention were compared in all of the patients when using the same implant system.