Optimizing Range of Motion in Cruciate-retaining Mobile-bearing TKA: Experience with 2000 Cases

Abstract

We describe experience with 2000 cruciate-retaining Sigma RP total knee arthroplasties performed between September 2000 to January 2006. All procedures were performed with a midvastus arthrotomy, medioposterior release, and posterior condylar clean-out technique. This implant design uses a congruent polyethylene bearing to diminish contact stress while allowing rotation between the polyethylene bearing surface and tibial tray. Of the 2000 knees, 1596 had follow-up data at one year. Seven hundred-twenty-seven had an average flexion of 123°, 692 had an average Knee Society Score of 94, and 672 had an average function score of 86.

Optimal range of motion (ROM) after total knee arthroplasty (TKA) is determined by numerous factors. It can be attributed to implant design, choice of implant for the particular patient, and implantation technique. The host patient’s distinct biological environment also contributes to the resulting ROM. The surgeon’s objective is to optimize these factors and achieve as a result a functional knee that provides patients with the desired function, ROM, and pain relief.

Mobile-bearing TKA was introduced over two decades ago in an effort to reduce polyethylene contact stress while minimizing strain on the implant/bone interface. Early fixed-bearing posterior cruciate-retaining (cruciate-retaining) designs had so-called “flat-on-flat” articulations, which led to high-contact stresses in the polyethylene and higher rates of failure. These designs were necessary because satisfactory knee function required tibiofemoral rotation, which, in more constrained articulations, transfers stress to the fixation interface and can lead to loosening. The rotating-platform mobile-bearing design addresses this “kinematic conflict” by using a congruent polyethylene bearing to diminish contact stress while allowing rotation to occur between the polyethylene bearing surface and tibial tray. The original mobile-bearing implant, the Low Contact Stress knee (P.F.C. Sigma LCS, DePuy Orthopaedics Inc, Warsaw, Ind), has shown excellent clinical success. In an effort to maintain the proven success of rotating-platform knee implants while improving implant design and instrumentation, DePuy Orthopaedics introduced the P.F.C. Sigma RP knee in 2000. This design is currently available in posterior cruciate-retaining or cruciate-substituting designs. This article describes experience with 2000 cruciate-retaining rotating-platform knees and the surgical techniques used to optimize ROM.

Materials and Methods

From September 2000 through January 2006, the senior author (W.M.G.) performed 2000 TKAs using the P.F.C. Sigma cruciate-retaining implant. The mean patient age was 66 years; 99.85% of the knees received a cruciate-retaining design, and 98.5% had the patella resurfaced.

Surgical Technique

The knee was exposed using a medial midvastus arthrotomy. The authors (W.M.G. and A.C.G.) used a measured resection technique without flexion/extension blocks or tensioners to achieve ligament balance. The femoral preparation was identical to that used in the fixed-bearing technique using intramedullary alignment for the distal cut and posterior referencing for implant sizing. Cruciate-retaining is preferred by the authors (W.M.G. and A.C.G.), because it eliminated the danger of over-resection of the posterior condyles, which could occur with an anterior-referenced design. Over-resection of the posterior condyles may cause limited postoperative flexion due to loss of offset between the condyles and the posterior surface of the femoral metaphysis.

Tibial exposure was enhanced with a posteromedial release, which created a medial soft-tissue sleeve that helped balance the knee (Figure 1).¹

Extramedullary tibial alignment was used, and the cutting block included a built-in 3° posterior slope. Adequate tibial bone resection was a key to success during this procedure. The thinnest polyethylene/tibial tray composite available in this system had a measured thickness of approximately 11.7 mm, so inadequate bone resection could lead to tight flexion and extension gaps. With a varus knee, the authors (W.M.G. and A.C.G.) found that placing the stylus below the level of the intact lateral cartilage usually provided satisfactory resection.

After achieving acceptable balance with the trial components, the authors (W.M.G. and A.C.G.) performed a “posterior condylar clean-out” technique.² This maneuver maintained the normal posterior condylar recess to allow greater flexion of the knee (Figures 2-4).

Figure 1: Posterior medial release.	Figure 2: Use of curved osteotome and mallet to clean posterior condyles.
Figure 3: Using ronguer to remove debris.	Figure 4: Using ronguer to scan and clean posterior condyles.

This technique was essential, considering that a 6-mm overhang of bony osteophyte at the edge of the posterior femoral component could limit knee flexion to 102° (Figure 5).²

With use of a curved tibial polyethylene insert in the P.F.C. Sigma rotating-platform component, there is a limited amount of posterior roll-back on flexion. The tibial component will externally rotate between 10° and 20°. In the event of excessive tightness of the posterior cruciate ligament (PCL) during placement of trial components, the posterior lateral condyle may sublux over the posterior edge of the tibial polyethylene insert. This occurs when external rotation of the polyethylene insert has reached its limit, the posterior lateral flexion space from a tight PCL is too tight, and further posterior roll-back subluxes the lateral femoral condyle over the lateral lip of the implant. It is corrected with a PCL recession where either part or most of the PCL is cut from the femur, tibia, or the main substance of the ligament. The knee will subsequently articulate in a normal manner with the rotating-platform knee.

The final components were then cemented into place, the wound was closed in layers, and the tourniquet deflated. No drains or continuous passive motion therapies were ordered. An indwelling epidural catheter was used for pain control postoperatively for 24 hours, and fascia iliaca blocks were used thereafter.³ Supervised physical therapy (PT) was started on postoperative day 1; patients were usually discharged home or to an extended care facility on day 3. Patients remained on warfarin for 3 weeks for thromboembolic prophylaxis.

Figure 5: Comparison of ROM with and without 6-mm overhang of osteophyte in a size 5 press-fit condylar P.F.C. Sigma rotating-platform knee.2

Results

The authors (W.M.G. and A.C.G.) began using the P.F.C. Sigma rotating-platform tibial components in all patients younger than 75 years of age in the first year they began to use these implants. This was due to the fact that the early results with ROM in the first year were superior, and the easier rehabilitation was important. In the first 500 knees having a minimum follow-up period of 1 year, the average flexion was 123°, average Knee Society Score was 95 (preoperative average was 57), and average knee function score was 84. Complications in this group included one spin-out in a 6’5”-man who needed an implant larger than the largest size available at surgery and who had no posterior condylar space. (This implant size is currently available.) One bilateral patient experienced subluxation in one knee and was successfully revised with a thicker insert and posterior-cruciate recession. There were seven deep postoperative infections that required surgical intervention.

There were 1596 knees with follow-up data at the 1-year postoperative period. Of this group, 727 knees had average flexion of 123°, 692 knees had an average Knee Society score of 94, and 672 knees had an average function score of 86.

Discussion

Much has been written suggesting that preoperative ROM predicts postoperative ROM. The authors suggest that this may be the influence of quadriceps excursion prior to TKA. When patients lose ROM over an extended period, there are likely changes in the cellular structure of the muscle and tendon due to the lack of movement of the quadriceps through the normal physiologic range. Snyder-Mackler et al⁴ suggested that the quadriceps may have persistent weakness after reconstruction of the anterior cruciate ligament due to irreversible muscle atrophy or an alteration in muscle cells. The lack of muscle excursion and inhibition of the muscle due to pain, swelling, mechanical blocking, impingement of osteophytes, fluid, and hypertrophied synovium can result in irreversible atrophy and changes. The atrophy and disappearance of the muscle cell with the structure of the sarcomere remove the inherent ability for length change. As time passes with further muscle atrophy, muscle and soft tissue can be replaced by extensive, fibrous scar tissues. In addition, there is frequently growth of osteophytes that might impinge and mechanically block motion. Soft-tissue adhesions between the quadriceps and the femur can create additional limitations as adhesions in the intrasynovial lining have a tethering effect.

The space-occupying effect of inflammatory, scar, fibrous tissues, and osteophytes will limit the excursion within the joint as well. The TKA procedure will remove osteophytes, release tight adhesions, and remove space-occupying structures. The tension that creates the flexion and extension space will be also be equalized. There is an unpredictable contribution of scar-tissue regrowth as a direct result of the surgical trauma during TKA. This will often respond to PT and/or manipulation. The implantation of a total knee in a less-than-perfect position can limit flexion due to incongruence between the surface of the tibial tray and the femur with femoral malrotation. Difficulties with exposure can cause an incorrect rotation of the tibial tray, which, particularly in a fixed-bearing knee, can decrease ROM and cause premature wear. The incorrect femoral component rotation can have a deleterious effect in the patellofemoral joint, which can cause excessive tightness of the extensor mechanism and limit flexion. It has been stated that overstuffing of the patella will also limit flexion due to increasing the tightness within the quadriceps and removing the inherent laxity in the muscle fiber. The malrotation of the femoral component can increase tightness of the flexion space medially or laterally.

The imbalance of the flexion and extension space can also limit flexion. Implantation of a thicker tibial insert to balance extensor space is required if the joint line is raised by over-resecting the distal femur. If the femur is not also downsized, the flexion space will be too tight and limit flexion. In a more subtle finding, the PCL will have excessive tightness and, thus, limit flexion.

Implant design has recently been a large contributor to ROM and flexibility. Knee kinematics is a complex interaction of sliding and rotation movements. The design of the mobile-bearing knee was one of the earliest to address this issue. Early total knee designs removed a large amount of bone, including the posterior femoral condyle. This resulted in a small amount of condylar offset, or distance between the apex of the femoral condyle and the posterior surface of the femoral metaphysis. As a result, when the patient flexed, there was impingement of the tibial tray and bone. An improvement in knee design added a central post and cam mechanism to allow roll-back and creation of a space for implant flexion. Subsequently, improvement came in the availability and use of instruments for porous-coated implants, which retained the PCL. They were based on measured resection and bone preparation using precise jigs. They removed less bone and retained a larger posterior condylar offset. Later femoral implants also had a similar anatomic design, with the addition of a box to accommodate a posterior-stabilizing post or the original implant that accommodated the PCL.

The influence of posterior condylar off-set was described by Bellemans et al,⁵ who performed fluoroscopic analysis of deep flexion kinematics after TKA. Some have found this implant to provide greater flexion due to more predictable roll-back. Studies of cruciate-retaining and cruciate-substituting TKAs have reported similar ROM.⁶ Fluoroscopic studies by Dennis et al⁷ demonstrated a paradoxical anterior slide of the femur in patients with cruciate-retaining implants, which would eliminate roll-back and allow impingement of the tray and soft tissues. Others have not found this to occur and have demonstrated similar ROM with the cruciate-retaining implant.²

Yoshiya et al⁸ describe their prospective study on bilateral TKA in which 1 patient received cruciate-retaining TKA in one knee and cruciate-substituting TKA in the opposite knee. The cruciate-substituting TKA (posterior-stabilized) had superior ROM. Under weight-bearing conditions, posterior-stabilized implants have roll-back that is more predictable and prevents anterofemoral translation seen in cruciate-retaining TKA that may limit flexion.⁷ If there is a lack of off-set or if the off-set space is blocked by osteophytes, the rollback caused by the cruciate-substituting post may create a posterior space to allow deeper flexion as the articulation is shifted to the post in deep flexion. However, if the presence of an osteophyte leads to impingement prior to roll-back, the limits to flexion will be similar in both designs. The presence of osteophytes or scarring in the off-set space can also tether the posterior structures and limit extension.

Johal et al⁹ studied weight-bearing and nonweight-bearing knee kinematics using “interventional” magnetic resonance imaging and found that during flexion to 120°, the femur rotated externally through an angle of 20°. However, on flexion beyond 120°, both femoral condyles moved posteriorly to a similar degree.

In a previous study by the senior author,¹⁰ the difference of ROM at 6, 12, and 52 weeks between 131 P.F.C. Sigma and 113 P.F.C. Sigma rotating-platform knees performed between September 2000 and October 2001 was demonstrated. The fixed-bearing P.F.C. Sigma cruciate-retaining knees had an average ROM of 112.2° preoperatively, 105.2° at 6 weeks, 111.8° at 12 weeks, and 116.1° at 52 weeks. The P.F.C. Sigma rotating-platform knees had an average ROM of 110.7° preoperatively, 105.2° at 6 weeks, 115.7° at 12 weeks, and 124.0° at 52 weeks.¹⁰

Finite element analysis (ABAQUS Inc, Providence, RI) demonstrated that to achieve maximal flexion, a cruciate-substituting posterior-stabilized implant was needed (Figure 6).²

Figure 6: The effect on flexion with cruciate-substituting femur with soft tissues removed.2

Conclusion

The importance of the natural 20° of lateral condyle rotation posteriorly in the first 120° of flexion is best highlighted by the success of the mobile-bearing P.F.C. Sigma rotating-platform knee. This rotation is intrinsic to the design. The external polyethylene rotation will accommodate the lateral condyle pivot around the medial condyle, yet retain the congruence of the interface to reduce contact stress. This ability to rotate allows the anteroposterior dimension of the mobile-bearing tibial component to be shorter because there is no need to create a surface to accommodate lateral rotation to the posterior femoral condyle (Figures 7 and 8). If the contemporary fixed-bearing designs did not provide this rotation, the two components might move in a similar manner up to 120° and might have symmetric roll-back with a post or a functional PCL.

Recently, a new high-flexion mobile-bearing knee design has moved the post and cam more posteriorly and extended the available posterior condylar surface proximally for greater articulation in flexion (Figure 9). It may show a larger difference in flexion in the future.

Figure 7: Maximal flexion of a mobile-bearing knee.	Figure 8: Radiograph of maximal flexion of a mobile-bearing knee into a cleaned-out posterior condylar space.	Figure 9: Sigma rotating-platform-F.

The perfect TKA will balance the knee and afford roll-back. The patellofemoral joint with normal quadriceps excursion will no longer inhibit maximal flexion. As implant designs improve, they will no longer be a contributing factor in the ultimate flexion of the knee. This will change the limiting factors of flexion to the skin, subcutaneous tissues, and muscle mass that are contracted and compressed as the knee is flexed.

References

1. Goldstein W, Branson J, Berland K. Posterior medial capsular release and external rotation of the tibia to enhance exposure during total knee arthroplasty. Scientific Exhibit presented at: Annual Meeting of the American Academy of Orthopaedic Surgeons; February 13-17, 2002; Dallas, Tex.
2. Goldstein W, Raab D, Gleason T, et al. Why posterior cruciate retaining and substituting total knee arthroplasty have similar range of motion: the importance of posterior condylar offset and cleanout of posterior condylar space. Scientific Exhibit presented at: Annual Meeting of the American Association of Orthopaedic Surgeons; March 21-26, 2006; Chicago, Ill.
3. Blum S, Wu D, Goldstein W, et al. Fascia iliaca blocks for post-operative analgesia in total knee arthroplasty. Scientific Exhibit presented at: Annual Meeting of the American Academy of Orthopaedic Surgeons; March 10-14, 2004; San Francisco, Calif.
4. Snyder-Mackler L, De Luca PF, Williams PR, et al. Reflex inhibition of the quadriceps femoris muscle after injury or reconstruction of the anterior cruciate ligament. J Bone Joint Surg Am. 1994; 76:555-560.
5. Bellemans J, Banks S, Victor J, et al. Fluoroscopic analysis of the kinematics of deep flexion in total knee arthroplasty. Influence of posterior condylar offset. J Bone Joint Surg Br. 2002; 84:50-53.
6. Conditt MA, Noble PC, Bertolusso R, et al. The PCL significantly affects the functional outcome of total knee arthroplasty. J Arthroplasty. 2004; 19:107-112.
7. Dennis DA, Komistek RD, Mahfouz MR, et al. Multicenter determination of in vivo kinematics after total knee arthroplasty. Clin Orthop Relat Res. 2003; 416:37-57.
8. Yoshiya S, Matsui N, Komistek RD, et al. In vivo kinematic comparison of posterior cruciate-retaining and posterior stabilized total knee arthroplasties under passive and weight-bearing conditions. J Arthroplasty. 2005; 20:777-783.
9. Johal P, Williams A, Wragg P, et al. Tibio-femoral movement in the living knee. A study of weight bearing and non-weight bearing knee kinematics using ‘interventional’ MRI. J Biomech. 2005; 38:269-276.
10. Goldstein W, Nasser R, Branson J. The effect of a mobile bearing tibial component on early range of motion following total knee arthroplasty. Scientific Exhibit presented at: Annual Meeting of the American Association of Orthopaedic Surgeons; February 4-9, 2003; New Orleans, La.

Authors

Drs Goldstein and Gordon and Ms Branson, are from the Illinois Bone and Joint Institute, Morton Grove, Ill.