JORT Journal

Introduction

Besides pain relief, achieving appropriate range of motion after total knee arthroplasty (TKA) is an important component of a patient’s overall functional outcome [1]. However, patients rarely flex their knees beyond 120° following TKA [1-4]. Thus, the NexGen LPS-Flex total knee system (Zimmer, Warsaw, IN, USA) was introduced to enhance knee flexion beyond 120° after TKA. The NexGen LPS-Flex system includes a 2-mm extension of the posterior condyle of the femoral component that replaces an extra 2 mm of bone that is removed from the posterior femoral condyles; this approach increases posterior femoral translation and the range of flexion.

The Freedom Total Knee System (MAXX Orthopedics, Inc., Norristown, PA, USA) was introduced to conserve bone while achieving a condylar profile that allows for high knee flexion. The Freedom knee system was designed to enhance knee flexion by accommodating changes in the femoral condylar radii of curvature based on the degree of flexion change and the amount of rollback needed through knee range of motion while minimizing bone resection from the posterior femoral condyles [5].

Several authors [6-8] reported that the high-flexion knee prosthesis had a higher risk of loosening at high flexion angles. The motivation of this study was to compare the results of the NexGen LPSFlex TKA (removal of the extra bone from the posterior femoral condyles) and Freedom TKA (bone conserving posterior femoral condyles).

We examined 5 hypotheses: (1) the amount of bone resected from the distal femur, posterior femoral condyles, and intercondylar notch is less for a Freedom TKA compared with a NexGen LPS TKA; (2) the clinical and radiographic results are at least as good in patients who receive a NexGen LPS-Flex prosthesis compared with those who receive a Freedom prosthesis (Figure. 1); (3) the knee range of motion achieved with a NexGen LPS-Flex prosthesis is better than the knee range of motion achieved with a Freedom prosthesis; (4) patient satisfaction is the same for NexGen LPSFlex and Freedom TKAs; and (5) the complication rate is lower with a Freedom TKA compared with a NexGen LPS-Flex TKA. Photographs showing the design features of NexGen LPS-Flex and Freedom knee prostheses (Figures 1A-B).

Figure 1A: Frontal views of the NexGen LPS-Flex (left) and Freedom (right) total knee prostheses. The Freedom component is narrower in the intercondylar notch (box cut).

Figure 1B: Lateral views of the NexGen LPS-Flex (left) and Freedom (right) total knee prostheses. In the Freedom prosthesis, the anterior and posterior femoral condyles are thinner.

Materials and Methods

Demographics

We identified 296 patients (488 knees) who received a NexGen LPS-Flex knee prosthesis. These 296 patients were matched individually with 295 patients who received a Freedom knee prosthesis, for age, sex, body mass index (BMI), preoperative active range of motion, and preoperative Knee Society knee and function scores. We performed the matching process blinded to the eventual outcomes of NexGen LPS-Flex and Freedom TKAs. We matched preoperative variables by using a spreadsheet program in the following order: age, BMI, preoperative knee score, preoperative function score, preoperative active range of motion, and sex. There were no differences in the preoperative characteristics of the patients in the NexGen LPS-Flex and Freedom TKA groups.

All patients who underwent a TKA performed by a senior author at one institute were included in the study. The indication for TKA in both groups was osteoarthritis that was severe enough to warrant a TKA after an adequate trial of nonoperative therapy. Among the 296 patients who received a NexGen LPS-Flex prosthesis, 3 died and 8 were lost to follow-up, leaving 285 patients available with a minimum follow-up duration of 2 years (mean, 3.5 years; range, 2 to 5 years). Among the 295 patients with a Freedom knee prosthesis, 4 died and 11 were lost to follow-up, leaving 280 patients available with a minimum duration of follow-up of 2 years (mean, 3.5 years; range, 2 to 5 years). We obtained approval from our institutional review board before beginning this study (Table 1).

Table 1: Demographic Data of 950 Total Knee Arthroplasties*.

Surgical Technique

A pneumatic tourniquet at a pressure of 250 mmHg was used in all patients after exsanguination with an Esmarch bandage. In all knees, an anterior midline skin incision (12 to 15 cm in length) was used, followed by a medial parapatellar capsular incision. The femur was prepared first, followed by the tibia. All patellae were resurfaced routinely with a polyethylene patellar prosthesis. Femoral implant sizing was done using an anterior referencing. All implants were cemented using modern techniques, including pulsed lavage, drying, and pressurization of the cement. Intraoperatively, the amount of resected bone from the medial and lateral condyles of the distal femur and those of the posterior femoral condyles was measured the greatest dimension of width, length and depth using a caliper in. The rationale for measuring the resected bone volume was to determine whether there is a correlation between the amount of bone resection and clinical results, particularly knee range of motion.

Rehabilitation

Starting on postoperative day 1, the patients started active knee range of motion exercise and began standing at the bedside and transitioning to weight bearing as soon as it was comfortable.

Clinical Evaluation

Clinical evaluations were done at the following times after the operation: 3 months, 1 year, and then 2-3 years thereafter. The mean follow-up duration was 3.1 and 3.5 years (range, 2 to 5 years), in each group. All clinical data at the time of each followup were recorded and compiled by a research associate who was not part of the operative team. The Knee Society score [9] and the Western Ontario and McMaster Universities Osteoarthritis (WOMAC) index [10] were determined. Active knee range of motion of the patient in the supine position was determined by 2 observers with the use of a standard (60-cm) clinical goniometer before the operation and at each follow-up [11]. When the measured knee motion differed by > 5° between the 2 observers, the values were averaged, and that number was reported. Intraobserver agreement regarding knee range of motion was 0.95 to 0.99. Patient satisfaction was assessed with a visual analog scale (VAS) that ranged from 0 to 10. The answers were categorized as ≤ 2 (fully dissatisfied); 3, 4, or 5 (somewhat dissatisfied); 6, 7, or 8 (somewhat satisfied); or 9 or 10 (fully satisfied). The level of activity was assessed by using the University of California, Los Angeles (UCLA) activity score [12]. 

Radiographic Evaluation

Anteroposterior radiographs of knees (taken with the patient standing as well as lying supine), lateral radiographs, and skyline patellar radiographs were taken preoperatively and at each follow-up. They were assessed for the alignment of the limb (tibiofemoral angle), the position of the components, the posterior slope of the tibial components, the level of the joint line, and the presence and location of radiolucent lines at the bone-cement interface in accordance with the recommendations of the Knee Society [9]. Anteroposterior standing radiographs were used to determine the sequential change in the alignment of the limb as a result of polyethylene wear and/or loosening of the implant. Supine anteroposterior radiographs were used to determine the presence of a radiolucent line more precisely. Skyline patellar radiographs were examined to determine the presence of patellar tilt, subluxation, or dislocation. All radiographs were taken under fluoroscopic guidance to control rotation of the knee. Radiographic data at the time of each follow-up were analyzed and recorded by a research associate who was not part of the operative team.

Data Analyses

We performed an a priori power analysis to minimize the chance of a type-2 error and to increase the power of our study. We assumed the difference in the Knee Society knee score to be 5 points with a power of 0.99, which revealed that a total of 252 patients (468 knees) would be needed in each group. We recruited approximately 10% more patients to account for possible dropouts. We calculated descriptive statistics (mean, standard deviation and proportions) for the continuous study variables [13-15]. We compared the amount of resected bone, the Knee Society score, the WOMAC index, the knee range of motion, and the radiological data between the groups with a paired t-test. We compared the postoperative Knee Society pain score between the groups using the Mantel-Haenszel chi-square test. We compared the complication rate between the groups with a chi-squared test. Moreover, we performed survivorship analysis to determine the cumulative rate of survival of the implant during the study period [16].

Results

The resection thickness from the distal femur (averaged 8.7 mm from the medial and lateral sides for the NexGen LPS-Flex TKA group and averaged 7.8 mm from the medial and lateral sides for the Freedom TKA group). The resection thickness from the posterior femoral condyles (averaged 11.5 mm from the medial and lateral sides for the NexGen LPS-Flex TKA group and averaged 5.7 mm from the medial and lateral sides for the Freedom TKA group). And the resection volume of from the intercondylar notch was 11 cm³ for the NexGen LPS-Flex TKA group and 8.1 cm³ for the Freedom TKA group. (Table 2).

Table 2: Resected Bone Volume.

Clinical Results

In this short-term follow-up study, there were no differences in clinical function between the NexGen LPS-Flex TKA and Freedom TKA groups. Specifically, the mean preoperative Knee society score was 31 points for each group, while the mean postoperative Knee Society score was 91 points for the NexGen LPS-Flex TKA group and 93 points for the Freedom TKA groups, and the mean postoperative functional score was 81 points for the NexGen LPS-Flex TKA group and 83 points for the Freedom TKA group. The knee range of motion was similar between the groups preoperatively (126° for the NexGen LPS-Flex TKA group and 125° for the Freedom TKA group) and postoperatively (125° for the NexGen LPS-Flex TKA group and 128° for Freedom TKA). Overall, at a mean follow-up of 3.5 years after the operation, the clinical data were not significantly different between the NexGen LPS-Flex TKA and Freedom TKA groups (Table 3).

Table 3: Comparison of Clinical Results

Radiographic Results

The radiographic results were similar in the NexGen LS-Flex and Freedom TKA groups with regard to the alignment of the knee and the position of the femoral and tibial components in the coronal and sagittal planes. If one assumes a tolerance level of 3°, then the prevalence of outliers ranged from 2 to 4% for all parameters in the NexGen LPS-Flex TKA group and from 1 to 3% in the Freedom TKA group. These differences between the groups were not significant (P > 0.05; Table 4). No knee in either group had osteolysis around the components (Figures 2 and 3).

Table 4: Radiographic Results.

Radiographs of the knees of a 67-year-old woman who had osteoarthritis in both knees (Figure 2A-B).

Figure 2A: Standing anteroposterior radiograph of both knees taken 4 years after surgery, showing that the NexGen LPS-Flex prostheses are embedded solidly in a satisfactory position. There are no radiolucent lines and there is no osteolysis around the components in either knee.

Figure 2B: Lateral radiograph showing that NexGen LPS-Flex prostheses are fixed satisfactorily. There are no radiolucent lines and there is no osteolysis around the components in either knee.

Radiographs of the knees of a 65-year-old woman who had osteoarthritis in both knees (Figure 3).