Many factors influence the range of flexion after total knee arthroplasty, including preoperative knee motion, surgical technique, prosthetic design, and rehabilitation issues. However, in many clinical studies even patients with good preoperative range of motion often lose deep flexion (defined as flexion of >120°) after total knee arthroplasty1-9. New components, including posterior cruciate ligament-retaining and posterior stabilized designs, have been created in an effort to improve flexion. These new designs have attracted considerable attention. However, the benefit of so-called high-flexion total knee replacement remains a subject of debate10-13. Several studies have found no significant difference in terms of clinical outcomes and range of motion between the high-flexion posterior stabilized and the standard posterior stabilized total knee arthroplasty12,13. With regard to a cruciate-retaining total knee arthroplasty, we are aware of no report comparing the range of motion and clinical outcomes of standard and high-flexion designs.
A recently developed high-flexion cruciate-retaining design (NexGen CR-Flex Fixed Bearing Knee; Zimmer, Warsaw, Indiana) includes a posterior femoral condyle that extends 2 mm beyond that of the standard cruciate-retaining design, which allows a larger contact area between the femoral and the tibial polyethylene components at high flexion angles (Fig. 1). Most et al.14 concluded in their in vitro study that the tibiofemoral contact behavior of the high-flexion cruciate-retaining design is better than that of the standard cruciate-retaining design. Sharma et al.15 recently studied the contact mechanics of cruciate-retaining high-flexion designs compared with high-flexion posterior stabilized designs and found that both designs maintain acceptable contact area and contact stresses at the high flexion angle.
In the present study, we compared the range of motion after a minimum follow-up of two years in patients who had implantation of high-flexion fixed-bearing cruciate-retaining designs and patients managed with standard fixed-bearing cruciate-retaining designs. We hypothesized that patients with a high-flexion cruciate-retaining knee replacement would have better postoperative ranges of motion than would those with the standard cruciate-retaining knee replacement.
Materials and Methods
One hundred and four consecutive patients awaiting unilateral primary total knee arthroplasty were enrolled. During preoperative assessments (performed seven to fourteen days before surgery), the patients were randomized into a high-flexion group (implanted with a NexGen CR-Flex Fixed Bearing Knee) or a standard group (implanted with NexGen CR; Zimmer) (Figs. 2-A and 2-B). Patients were allocated, with use of sealed envelopes, into the high-flexion or the standard group. The study protocol had been approved by our institutional review board of Chonnam National University Hwasun Hospital (No: 07-La), and written informed consent was obtained from all patients. Patients who had a history of open knee surgery that required placement of metallic implants, a revision total knee arthroplasty, a severe deformity (>20° of varus alignment or a flexion contracture of >30°), or a diagnosis other than osteoarthritis were excluded. Patients who had a prior open meniscectomy or cruciate ligament reconstruction were not excluded. Four patients were lost to follow-up (three because of a change of residency and one because of death). Their data were excluded, leaving 100 knees (fifty in each group) available for the study.
The high-flexion group consisted of six men and forty-four women with an average age of 69.2 years (range, fifty to eighty-five years) at the time of surgery, and the standard group consisted of ten men and forty women with an average age of 67.5 years (range, fifty-four to eighty-two years) at the time of surgery (Table I). The groups were similar with respect to age at the time of the operation (p = 0.191) and sex (p = 0.243). The average body mass index was 25.7 kg/m2 (range, 18.6 to 36.9 kg/m2) in the high-flexion group and 26.6 kg/m2 (range, 18.3 to 36.9 kg/m2) in the standard group; the difference was not significant (p = 0.356). All knees had a varus deformity ranging from 2° to 19°, with no difference between the groups (p = 0.550). The groups were similar with respect to the mean preoperative non-weight-bearing range of motion, Hospital for Special Surgery (HSS) scores, and Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) scores (Table I).
The same surgical procedure was used in both groups. Following an anterior midline skin incision and medial parapatellar arthrotomy, a medial soft-tissue release was performed by elevating a subperiosteal sleeve from the anteromedial aspect of the tibia to the posteromedial aspect. Intramedullary instrumentation was used for femoral alignment, and a 6° valgus cut was selected for all knees. The distal and posterior femoral condylar resections were done with an attempt to remove an amount of bone equal to the thickness of the femoral component to be inserted. The amount of the bone resected from the posterior femoral condyle was 2 mm greater in the high-flexion group than in the standard group. The tibial cut was performed with use of extramedullary instrumentation aligned perpendicular to the tibial shaft in the coronal plane, while carefully protecting the posterior cruciate ligament by preserving an island of bone around it. The tibial slope was usually set to 7° of posterior slope. In both groups, the posterior cruciate ligament was retained and the patella was not resurfaced. The aponeurosis around the patella was released by electrocautery, and osteophytes were removed in all patients. All procedures were performed by the same surgeon, and all prostheses were fixed with tobramycin-laden cement (Simplex P; Stryker, Mahwah, New Jersey).
The intraoperative and postoperative pain and rehabilitation protocols were identical for both groups. A tourniquet was used for all procedures and was not deflated until after the wound closure. A drain was placed in all knees, and it was removed on the second postoperative day. Postoperatively, all patients received standardized physical therapy that began with full weight-bearing on the first postoperative day.
Clinical evaluations were performed at the time of the final follow-up. To compare the groups with respect to range of motion, we evaluated the arcs of maximal non-weight-bearing passive flexion and weight-bearing flexion as well as the numbers of knees that allowed comfortable kneeling and cross-legged sitting. Non-weight-bearing flexion was measured with the patient in the supine position, and weight-bearing flexion was measured during a lunge activity with use of a goniometer. One arm of the goniometer was placed parallel to the shaft of the femur (which was estimated from the location of the greater trochanter and the lateral femoral condyle), and the other arm was placed parallel to the shaft of the tibia (which was estimated from the fibular head and the lateral malleolus)7. Functional outcomes, assessed on the basis of the HSS and WOMAC scores at the time of final follow-up, were also compared. All range-of-motion and clinical data obtained at the time of the final follow-up visits were evaluated and recorded by a physician assistant who was not a part of the operative team. Average follow-up periods were 26.1 months (range, twenty-four to twenty-nine months) in the high-flexion group and 25.4 months (range, twenty-four to thirty-one months) in the standard group (Table I).
Radiographic indices included the femorotibial angle on a standing anteroposterior radiograph of the knee made at the time of the final follow-up. We also compared the difference in the posterior femoral condylar offset preoperatively and at the time of the final follow-up. The posterior femoral condylar offset was evaluated by measuring the maximal thickness of the posterior femoral condyle projected posteriorly to the tangent of the posterior cortex of the femoral shaft on the lateral radiograph of the knee (Fig. 3). In order to obtain an accurate lateral view, the x-ray beam was adjusted under fluoroscopic control. We also evaluated the posterior slope of the tibia on the lateral radiographs of the knee made at the time of the final follow-up. All measurements were made by an orthopaedic surgery resident who was unaware of the implant used and the clinical outcome.
To detect a clinically relevant improvement of 7° in the range of motion between the groups, forty patients were required to achieve sufficient statistical power (power = 0.8 and p < 0.05)7,12. The Student t test was used to analyze the continuous data, and the chi-square test was used to analyze the categorical data. The level of significance was set at p < 0.05.
Source of Funding
There was no external funding for this study.
Range of Motion
The preoperative maximal flexion and flexion contracture in the high-flexion group (mean and standard deviation, 132.5° ± 10.5° and 8.4° ± 7.1°, respectively) and the standard group (mean, 133.3° ± 12.1° and 7.7° ± 7.6°) were similar (p = 0.638 and p = 0.724, respectively). At the time of the final follow-up, the mean non-weight-bearing maximal flexion in the high-flexion group and the standard group (135.3° ± 8.2° and 134.3° ± 8.4°, respectively) were similar (p = 0.563) (Table II). In addition, no significant difference was found between the groups with regard to the average amount of weight-bearing maximal flexion at the time of the final follow-up (p = 0.592) (Table II).
Twenty-three knees (46%) in the high-flexion group and twenty-two knees (44%) in the standard group allowed the patients to kneel comfortably, and thirty-eight knees (76%) in the high-flexion group and thirty-six knees (72%) in the standard group allowed comfortable cross-legged sitting. However, no significant differenctrun -1e between the groups was evident with regard to kneeling (p = 0.418) or cross-legged sitting (p = 0.318) (Table II).
The groups had similar results with respect to the HSS (p = 0.130) and WOMAC total knee scores (p = 0.498) preoperatively and at the time of the final follow-up (p = 0.116 and p = 0.154, respectively) (Tables I and II). At the time of the final follow-up, the mean total HSS and WOMAC knee scores were 94.4 points (range, 73 to 100 points) and 32 points (range, 24 to 52 points), respectively, in the high-flexion group and 92.4 points (range, 77 to 100 points) and 34.3 points (range, 22 to 54 points) in the standard group (Table II).
The pain scores for the groups as determined with the HSS and WOMAC systems at the time of final follow-up were also similar. The mean scores were 25.0 ± 3.5 points and 6.3 ± 1.5 points, respectively, for the high-flexion group and 30.0 ± 4.0 points and 6.7 ± 2.7 points for the standard group (Table II).
No clinical complications, including wound-healing, infection, instability, or symptoms of patellar clunk syndrome, were observed. There was no evidence of component loosening or osteolysis on any of the radiographs.
The mean femorotibial angle at the final follow-up evaluation was 5.7° (range, 1.5° to 10.8°) of valgus alignment in the high-flexion group and 5.6° (range, 1.8° to 10.2°) of valgus alignment in the standard group; the difference was not significant (p = 0.931). The mean tibial posterior slope in the groups at the time of the final follow-up was also similar (5.4° ± 2.5° in the high-flexion group and 4.9° ± 2.4° in the standard group; p = 0.771). The mean difference in the posterior femoral condylar offset at the preoperative and final follow-up evaluations was approximately the same (0.6 ± 3.0 mm in the high-flexion group and 0.3 ± 2.4 mm in the standard group; p = 0.270).
High-flexion components were introduced to posterior cruciate ligament-substituting total knee arthroplasty designs in the expectation that this would allow a greater functional range of knee movement. However, clinical studies on the effectiveness of these designs to provide high flexion following total knee replacement have produced conflicting results (Table III). Recently, a high-flexion cruciate-retaining total knee arthroplasty implant (NexGen CR-Flex Fixed Bearing Knee; Zimmer) was developed to improve range of motion. This design differs from the standard cruciate-retaining total knee arthroplasty design (NexGen CR Fixed Bearing Knee; Zimmer) in two major aspects. First, the CR-Flex knee replacement features increased posterior condylar offset with the potential for increased posterior conformity and improved contact area in flexion. Second, there is a deeper patellar tendon recess in the tibial component. In an in vitro study, Most et al.14 reported that the CR-Flex total knee arthroplasty design reaches the posterior edge of the polyethylene in high flexion, which suggests that this design has an advantage over the standard design in terms of tibiofemoral contact stresses. This has been recently supported by an in vivo kinematic study by Sharma et al.15, which compared the high-flexion cruciate-retaining and posterior stabilized total knee arthroplasty designs. The authors found that both knee types maintain sufficient contact area, and, thus, the contact stresses in both cases are below the yield strength of cross-linked polyethylene. However, we are aware of no in vivo study that has established the superiority of the CR-Flex design over the standard cruciate-retaining design in terms of range of motion and functional outcomes. Therefore, we undertook the present study to compare knee flexion and the functional outcomes of patients managed with a CR-Flex and a standard cruciate-retaining design at a minimum follow-up of two years. Our findings indicate that the two designs are similar with respect to the amount of flexion as well as clinical outcome. The average maximal flexion for the high-flexion (135°) and standard (134°) groups was similar under passive non-weight-bearing conditions. These results are consistent with previous reports of the high-flexion posterior stabilized total knee arthroplasty design10-12. Several factors, such as good preoperative ranges of motion and low body-mass indices, may have contributed to the high levels of flexion achieved in the two study groups. Patient motivation and the need to achieve deep knee flexion in the study group also may have contributed to the high flexion achieved postoperatively. Deep flexion is often necessary to perform key routine activities in the Asian culture, such as kneeling, squatting, and sitting with both legs crossed, and in Western leisure activities, such as gardening and bathing16,17.
No significant intergroup difference was found with respect to maximal weight-bearing flexion. Both groups demonstrated smaller flexion differences (<10°) during weight-bearing compared with non-weight-bearing conditions, and this was comparable in both groups. The smaller flexion under the weight-bearing condition compared with non-weight-bearing in the knees implanted with cruciate-retaining total knee arthroplasty designs is believed to occur because of abnormal knee kinematics caused by decreased femoral rollback. It has been shown that, in the cruciate-retaining total knee arthroplasty, the femorotibial contact is translated anteriorly with increasing knee flexion18,19.
No significant difference was found with regard to the number of knees in each group that allowed the patient to kneel (46% in the high-flexion group and 44% in the standard group) or to sit cross-legged (76% in the high-flexion group and 72% in the standard group). These results are similar to those in the study by Cho et al.20, who reported 30% for kneeling and 84% for cross-legged sitting in patients who had high-flexion knees. Furthermore, no difference in functional outcomes, as determined by HSS scores, was observed between the groups at the two-year follow-up evaluation.
Massin and Gournay21 investigated the potential effects of posterior femoral condylar offset and tibial slope on the range of knee flexion and showed that a 3-mm decrease in the posterior condylar offset reduced knee flexion by 10° before tibiofemoral impingement occurred. Furthermore, a simultaneous decrease of 5° in the tibial slope reduced the range of flexion by an additional 5°. Our study showed that posterior femoral condylar offsets were well restored to the preoperative values in both groups. We did not find any significant intergroup difference because, although an additional 2 mm of the posterior femoral condyle was resected in the CR-Flex design, this additional resection was compensated for by the 2-mm increase in the thickness of the posterior condyle of the femoral component. Also, there was no significant difference between the groups with regard to the postoperative tibial posterior slope.
Our study has several limitations that should be noted. First, the range of motion after total knee arthroplasty is influenced by preoperative, intraoperative, and postoperative factors in addition to implant design. However, in the present study, these factors were minimized by recruiting only patients with osteoarthritis. Moreover, the groups were similar with respect to preoperative factors, such as range of motion, degree of deformity, body mass index, and function (Table I). Second, range of motion was measured in all patients by one individual using a goniometer rather than using radiographic techniques. Radiographic measurement of the range of motion is considered to be the most accurate technique7, but, in clinical practice, most surgeons measure range of motion with a goniometer. Several studies have described the reproducibility of range-of-motion measurement with use of a goniometer and have shown high intraobserver and interobserver correlations22-24. Therefore, the flexion data were deemed reliable and suitable for the purpose of this comparative study. Third, we did not compare the groups with respect to in vivo kinematic variables, such as contact area, which are likely to influence polyethylene wear and long-term survival.
In conclusion, although short-term clinical outcomes were satisfactory, the high-flexion cruciate-retaining knees did not have greater knee flexion compared with those that had the standard cruciate-retaining design. We conclude that maximal flexion after total knee arthroplasty is probably dictated more by patient characteristics and surgical technique than by differences between the designs of the implant used.
Disclosure: The authors did not receive any outside funding or grants in support of their research for or preparation of this work. Neither they nor a member of their immediate families received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, division, center, clinical practice, or other charitable or nonprofit organization with which the authors, or a member of their immediate families, are affiliated or associated.
Investigation performed at the Department of Orthopedics, Chonnam National University Hwasun Hospital, Jeonnam, South Korea
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