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The Effect of a High-flex Implant on Postoperative Flexion After Primary Total Knee Arthroplasty

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Abstract

Compared to a statistically similar cohort, patients implanted with a high-flex poster-stabilized prosthesis achieved greater return of flexion than that with a standard posterior stabilized implant. In addition, patients with a high flexion arc preoperatively retained this high arc of flexion after surgery. Finally, there was an increase in the range of flexion seen after surgery in both groups of patients, but larger in the group in which the modified high-flexion implant was used.

Postoperative flexion is one of the parameters that orthopedic surgeons use to evaluate the success of total knee arthroplasty (TKA). Twenty-five percent of the Knee Society score is attributable to the patient’s flexion. Postoperatively, flexion can be affected by the amount of pain the patient experiences, the rehabilitation program, and the accuracy of the surgery. This study was performed to help determine what effect the implant itself has on the flexion the patient develops after primary TKA.

Materials and Methods

During 7 months, 40 primary unilateral TKAs were performed in 40 consecutive patients. Each of these patients had osteoarthritis, and each had failed a regimen of nonoperative care. A cemented Genesis II posterior stabilized (Smith & Nephew; Memphis, Tenn; Figure 1) was implanted in these patients (group 1). In the subsequent 7 months, 40 additional primary TKAs were performed in 40 other patients with osteoarthritis (group 2) using the cemented Genesis II high-flex posterior-stabilized prosthesis (Smith & Nephew; Figure 2).

All surgeries were performed using a combined spinal epidural anesthetic, supplemented by a femoral nerve block. A mini midvastus capsular incision was made, and bone resections and ligament balancing were performed in a standard manner described previously.¹

Postoperatively, the epidural catheter was used to administer a patient-controlled analgesic of a local anesthesia and fentanyl. Drains were used in all cases and were removed on postoperative day 1. The patients began performing flexion exercises using a controlled passive motion machine in the postanesthesia care unit, set initially at 70° and then increased by the patient as tolerated. This was supplemented by sitting flexion exercises supervised by a physical therapist. All patients were placed on parenteral antibiotics for 24 hours after surgery. Coumadin (Bristol-Myers Squibb; Princeton, NJ) was used as an antithromboembolic medication for 4 weeks after surgery.

No statistical difference in age, body mass index (BMI), sex, deformity, or preoperative flexion existed between the two groups.

The mean preoperative flexion in group 1 was 116°, with a range of 90° to 145°. Twenty one of these patients, preoperatively flexed >125° (mean = 130°).

The mean preoperative flexion in Group 2 was 117°, with a range of 95° to 140°. Nineteen of thes patients, preoperatively flexed >125° (mean = 132°).

Knee Society scores were calculated preoperatively and then at 4 weeks, 3 months, 1 year, and 2 years after surgery. Passive knee flexion was measured with the patient supine with the ipsilateral hip flexed.

Results

At 2 years’ postoperatively, 1 patient from group 1 was lost to follow-up and none from group 2. No patient underwent revision during the 2-year follow-up period.

The mean Knee Society score was 90 for group 1 and 92 for group 2 (no significant difference). There were no statistical differences in the pain, alignment, or stability scores. Mean flexion was 118° for group 1 and 133° for group 2. The difference was statistically significant ( P < .01). Two patients from group 1 and one patient from group 2 reported anterior knee pain.

The mean postoperative femoral tibial angle was 5.2° for group 1 and 5.0° for group 2 (P>.1).

By the second postoperative year, 8 patients from group 1 had a range of flexion that was at least 20° greater then their preoperative flexion. In group 2, 35% of the patients achieved at least 20° increase of flexion.

Of the 21 patients in group 1 who preoperatively flexed >125°, 20 were available for evaluation at 2 years’ follow-up. Mean flexion in this group was 123° (7° less than they had preoperatively). Mean flexion of those in group 2 who preoperatively flexed >125° was 137° (5° more than they had preoperatively).

As shown by lateral radiography at 2 years’ postoperatively, 80% of the patients who flexed >130° at 2 years had a positive blend sign between the posterior femoral condyles and the distal tibia (Figure 3).

Discussion

The amount of flexion a patient with a TKA can obtain depends on a number of factors including preoperative range of motion (ROM),² pain control, and patient motivation. The implant itself can likewise affect the potential for flexion after surgery.

The Total Condylar Prosthesis (Howmedica; Rutherford, NJ) used worldwide during the late 1970s and early 1980s had a conforming surface with an elevated posterior lip. Although this geometry resulted in good polyethylene wear characteristics, it limited flexion. Most studies using this implant reported flexion arcs of approximately 90°.^3,4 This amount of flexion was sufficient for walking on level surfaces; however, it was often insufficient for ascending and descending stairs. In one study,³ <50% of patients with this implant were able to ascend and descend stairs in a reciprocal manner. Furthermore, because flexion of approximately 115° to 120° is needed to stand easily from a seated position,⁵ these patients often had to use chair rails for assistance.

The next generation of implants included modifications in the tibial articular surface, including a flattening of the posterior lip and/or the use of a cam and post to encourage femoral rollback. With these changes, additional flexion was reported in patients who underwent TKA.^3,6,7

As the average age of patients who undergo TKA continues to decrease, surgeons have attempted to modify their prostheses to allow more flexion as would be required for many low-impact sporting activities. Likewise, as TKA has become more widely performed worldwide, there has been an attempt to obtain higher degrees of flexion for those patients whose culture requires sitting or kneeling on low surfaces.⁸

The results of using high-flex prostheses have varied. Huang et al⁹ reported that using the LPS High Flex (Zimmer; Warsaw, Ind) showed no difference in eventual Knee Society scores. However, they found that patients with the high-flex prosthesis had an average flexion approximately 10° greater than a similar cohort in whom a standard posterior stabilized implant was used. To the contrary, in a randomized prospective study using the same set of prostheses during bilateral TKA, Kim et al¹⁰ found no difference in eventual flexion. Yamazaki et al,¹¹ using the Hy-Flex II implant (Johnson and Johnson; Rutherford, NJ), demonstrated that almost 75% of the patients were able to flex beyond 120° after surgery.

The implant used for patients in both groups 1 and 2 had a femoral component with a tight posterior radius of curvature. This allowed blending of the posterior femoral runners with the distal femur without a need for increased bony resection from the posterior femur. Blending present on lateral radiography was a good prognostic sign that the patient would develop higher degrees of flexion.

The posterior stabilized tibial component had several modifications. The anterior portion of the polyethylene was chamfered to prevent impingement with the patellar tendon in deep flexion (Figure 2). The tibial post was likewise chamfered anteriorly to prevent possible impingement with the patellar component in deep flexion (Figure 2). Finite element analysis of this modified high-flex implant revealed no potential impingement until 155° of flexion was obtained (Figure 4). Finally, the posterior corner of the prosthesis was contoured to minimize edge loading and decrease contact stresses while maintaining ligament tension in high flexion (Figure 5).

The implant was tested in a knee joint simulator at 155° of flexion, with a 1.4× bodyweight superimposed load. After 500,000 cycles, there was no component failure, and no failure of the peripheral capture or of the locking mechanism of the tibial component.

Although Kotani et al² reported that postoperative range of flexion was essentially equal to that which was present preoperatively, the present series has not shown this result. How flexion is measured is important (ie, with the patient recumbent or sitting, with the patient weight bearing or not). As long as the measurements are made in an analogous manner preoperatively and postoperatively, this variable should not influence the results. However, preoperative flexion is determined not only by the compliance in the patient’s capsular sleeve and quadriceps musculature (which would not be expected to change after surgery), but also by the presence of posterior osteophytes. Removal of these should be expected to have a salutary effect on flexion, regardless of type of implant used. Furthermore, in most studies, the determination of preoperative flexion is made with the patient conscious and, often, in pain. The mere elimination of most of the knee pain, as routinely occurs after surgery, could likewise be expected to remove the inhibitory effect on flexion that occurred preoperatively.

In this series, implant modification affected postoperative flexion. The present investigators do not believe that a high-flex implant gives the patient more flexion. Rather, it is felt that the modification in the prosthesis enables the patient to obtain the maximum amount of flexion possible within his or her capsular sleeve limitations.

One limitation of this study was that the patients were not randomized prospectively, but rather a preexisting cohort study was used for comparison. Although no statistical difference was noted in the demographics between the two groups in this study, nor were conscious changes made in any of the anesthetic or postoperative rehabilitation protocols during the study period for the two groups, the possibility exists that some subjective parameter may have affected the results.

References

1. Laskin RS, Phongkunakorn A, Becsac B, Davis J, et al. Minimally invasive total knee replacement: an outcome study. Clin Orthop Relat Res. 2004; 428:74-82.
2. Kotani A, Yonekura A, Bourne RB. Factors influencing range of motion after contemporary total knee arthroplasty. J Arthroplasty. 2005; 20:850-856.
3. Laskin RS. Total condylar knee replacement in patients who have rheumatoid arthritis: A ten year follow-up study. J Bone Joint Surg Am. 1990; 72:529-535.
4. Maloney WJ, Schurman DJ. The effects of implant design on range of motion after total knee arthroplasty: total condylar versus posterior stabilized condylar designs. Clin Orthop Relat Res. 1992; 278:147-152.
5. Laubenthal HN, Smidt GL, Kettelkamp DB. Quantitative analysis of knee motion during activities of daily living. Phys Ther. 1972; 52:34-43.
6. Schai PA, Thornhill TS, Scott RD. Total knee arthroplasty with the PFC system: results at a minimum of ten years and survivorship analysis. J Bone Joint Surg Am. 1998; 80:850-858.
7. Laskin RS. Genesis II total knee arthroplasty: a five year follow up of 100 consecutive cases. Knee. 2005; 12:163-167.
8. Koshino T, Saitop T, Orito K, et al. Increase in range of knee motion to obtain floor sitting after high tibial osteotomy for osteoarthritis. Knee. 2002; 9:189-196.
9. Huang HT, Su JY, Wang GJ. The early results of high flex total knee arthroplasty: a minimum of two years of follow up. J Arthroplasty. 2005; 20:674-678.
10. Kim YH, Sohn KS, Kim JS. Range of motion of standard and high-flexion posterior stabilized total knee prostheses: a prospective randomized study. J Bone Joint Surg Am. 2005; 87:1470-1475.
11. Yamazaki J, Ishigami S, Nagashima M, Yoshino S. Hy-Flex II total knee system and range of motion. Arch Orthop Trauma Surg. 2002; 122:156-160.

Author

Dr Laskin is Chief of the Division of Arthroplasty of the Hospital for Special Surgery, and Professor of Orthopedic Surgery at Weill Medical College of Cornell University, New York, NY.

Dr Laskin is a consultant for and has received compensation from Smith & Nephew.