49Deep Knee Flexion in the Asian Population 49 49

49 Deep Knee Flexion

in the Asian Population

M. Akagi

Summary

The design of a high-flexion prosthesis should allow deep knee flexion after total knee arthroplasty “without affect- ing the durability and stability of the prosthesis.” Al- though it is not impossible to design a knee prosthesis that provides full flexion, the task is difficult. There is al- ways a trade-off between the range of flexion possible with the prosthetic design, the intrinsic stability provid- ed by the articular geometry, and the durability of the prosthesis. The high-flexion knee has to balance these three factors to a fine degree. Until now this has been a new frontier and a great challenge in the world of TKA.

Surgeons must be meticulous in their selection of pa- tients and prostheses. Furthermore, appropriate educa- tion and careful long-term follow-up are necessary for the patients.

Introduction

Total knee arthroplasty (TKA) is a common procedure with established outcomes [1]. In general, modern TKA designs produce satisfactory pain relief and improvement in walking ability. However, one major problem that has not been fully resolved is that patients usually do not re- gain deep flexion over 120° after TKA [2].

In the traditional Japanese life style, as well in some other countries such as Korea, India, China, and Muslim countries, deep flexion of the knee is necessary for daily activities such as eating, socializing, and religious or tra- ditional ceremonies. Even in non-traditional Japanese houses many people continue to sit down on a tatami mat and adopt a posture of either cross-legged sitting or full squatting, and sometimes sit with one knee down and one up (

) [3].

When people practice flower arranging at school or in public, or participate in the tea ceremony, they are sup- posed to sit in a full squatting position (

). Even patients with a good preoperative range of motion (ROM) often lose deep flexion after TKA.Gross loss of flexion can result in patient dissatisfaction with the operation, and therefore the surgical indications for TKA have tended to

be limited to patients with poor preoperative ROM.When TKA is indicated for a patient with good ROM but severe disability due to knee pain, the surgeons must inform the patient of the probable postoperative loss of deep flexion – with the possible consequence that the patient will adopt a Western lifestyle using a bed, a Western-style toi- let,tables,and chairs – and must obtain informed consent on this point. Many elderly Japanese patients follow a tra- ditional lifestyle in a Japanese-style house, which they have to modify to accommodate a Western lifestyle suit- able for their knee function after surgery. In this chapter, the author discusses the challenges in Japan to achieving high flexion in total knee design and a worldwide trend towards developing posterior-stabilized prostheses that can achieve deep flexion.

Although the task is difficult, it is not impossible to design a knee prosthesis that provides full flexion reach- ing 155°. An enlarged posterior femoral condyle with smaller radii of curvature can reduce contact stress in the polyethylene of the tibial insert [4].A number of knee im- plant manufacturers in the US and Europe have devel- oped, or are in the process of developing, total knee pros- theses aimed at full flexion reaching 155°. However, it is important to recognize that full flexion of the knee is achievable even using some conventional knee prosthe- ses in patients with full preoperative ROM if the surgery is technically appropriate, if there is a high degree of mo- tivation to regain deep flexion, and if the patients coop- erate fully with postoperative rehabilitation [5]. If this is

⊡ Fig. 49-1.Sitting postures on a tatami mat. (a) Cross-legged position;

(b) sitting in full squatting position, called seiza in Japanese, which means

"formal sitting" in English; (c) sitting with one knee down and one up

a b c

the case,how does the design of a high-flexion knee pros- thesis differ from that of a conventional prosthesis?

A high-flexion prosthesis should not interfere with deep knee flexion after total knee arthroplasty “without affecting the durability and stability of the prosthesis”

[6]. There is always a trade-off between the range of flex- ion allowed by the prosthetic design, the intrinsic stabil- ity provided by the articular geometry, and the durabili- ty of the prosthesis. The high-flexion knee has to balance these three factors to a fine degree.Furthermore,the knee with an ordinary condylar-type prosthesis has never managed to restore normal knee kinematics of deep flex- ion. The author, in collaboration with Dr. Taiyo Asano, is investigating the three-dimensional relationship between the femur and tibia of the normal knee in the full squat- ting position using the biplanar image-matching tech- nique [7]. It has been demonstrated that the femur can be subdislocated on the tibia posteriorly in full flexion be- cause of a large roll-back and axial rotation (

).

It seems impossible for the artificial knee prosthesis to

Development of a High-flexion Knee in Japan

The background material mentioned above has created strong interest among Japanese knee surgeons in the restoration of deep flexion after total knee arthroplasty since the early days of TKA introduction to Japan.In 1984, Dr. Shinichi Yoshino, in collaboration with Dr. Hiromu Shoji, developed the Y/S total knee system (Biomet Inc., Warsaw, IN, USA), aimed at postoperative high flexion [8].The Y/S knee system,which is a predecessor of the Hy- flex II knee (Depuy International Inc., Leeds, UK) [9], in- corporates several design parameters aimed at high flex- ion following TKA.The specific features of the design and instrumentation system include: (a) gradually reduced radius of curvature in the posterior femoral condyles; (b) flat posterior articular surface of the tibial component to allow a sliding and rolling motion of the femoral condyles and to provide free axial rotation in deep flexion; (c) a 4°

posterior slope for the tibial articular surface; and (d) a ligament tensor for equal medial and lateral ligament bal- ance in both flexion and extension. In one study of 39 pa- tients, this prosthesis was implanted in 50 knees. The av- erage follow-up period was 2.1 years (1.5-2.5 years) and the maximum flexion averaged 124.0° ±19.9° (preoperative motion arc had been 104.7° ±32.9°). In 90% of these pa- tients, the arc of knee motion was better than before surgery. Full flexion necessary to perform full squatting was achieved in 12% of operated knees [8].

In December 1989,the Bisurface knee prosthesis (Ky- ocera Inc., Kyoto, Japan) was developed, with the aim of achieving sufficient improvement in knee flexion after TKA without affecting the durability of the prosthesis.

This prosthesis had been originally used by Dr. Toyoji Ueo,the head doctor of the development team, in the De- partment of Orthopedic Surgery at Kyoto University Hospital, and was released for general use in 1992. The prosthesis incorporates one design-specific feature aimed at improving knee flexion: The knee is a PCL-sub- stituting prosthesis and has a unique ball-and-socket joint in the mid-posterior portion of the femoral and tib- ial component. This secondary joint functions as a pos- terior-stabilizing cam and, as a load-bearing surface in deep flexion, causes femoral roll-back, which prevents posterior tibiofemoral impingement [6].In a new version of the prosthesis (Type 3 Plus and Type 4), the femoral ball comes into contact with the tibial socket at around 60° of flexion. Therefore, the prosthesis has three contact areas on the tibial articular surface at more than 60° of flexion: the medial and lateral condylar surfaces and the ball-and-socket joint [10]. The posterior condylar part of

49

⊡ Fig. 49-2a-c. Biplanar image matching to observe the relationship be- tween the femur and the tibia in a full squatting position. (a) Anteropos- terior and lateral radiographs of the knee taken during full squatting. The bi-directional radiographs were taken almost simultaneously. (b) Projec- tion image of a three-dimensional knee model was matched onto both radiographs. (c) The position of six degrees of freedom was determined and the knee was observed axially (right) and laterally (left). The femur is subdislocated on the tibia posteriorly because of the large roll-back and axial rotation

a

b

c

the tibial articular surface is flattened in the anterior- posterior direction to provide freedom of axial rotation around the ball-and-socket joint.Although this prosthe- sis is classified as a posterior-stabilized knee,the box-cut in the intercondylar part for the cam mechanism is not necessary, and the bone loss for implantation is compa- rable to that with PCL-retaining prostheses. The author reviewed the clinical results of the first 223 arthroplasties performed with this prosthesis.The follow-up period av- eraged 6 years (3.9-9.0 years). The mean postoperative range of flexion was 124°, and there were no failures at- tributable to wear or breakage of the tibial polyethylene insert.Ten percent of the patients achieved full squatting.

However, 20% of these patients felt mild looseness in their knee [6]. These results prompted the author to im- prove the intrinsic stability of the prosthesis by improv- ing the congruity of the ball-and-socket joint. This new prosthesis is called Type 3 Plus and has been used since July 1998. This modification of the ball-and-socket joint associated with the proper ligament balancing technique has drastically reduced the prevalence of knee looseness to less than 1%(

).

Design Requirements in Total Knee Prostheses Aiming at Full Flexion

There are three requirements for the design of a total knee prosthesis. First, it must continue to be serviceable for a sufficient period after implantation. The minimum longevity should be more than 10-15 years if the prosthe- sis has been implanted with appropriate alignment, fixa-

tion, and ligament balance [11]. Second, the knee with the prosthesis should have adequate stability for the activities of daily life,such as walking on flat surfaces,going up and down slopes, negotiating stairs, and standing up from a low chair. In the traditional Japanese lifestyle, patients with knee prostheses have to sit down on, and stand up from, the floor and a 20- to 25-cm-high bathroom chair.

Knee stability after TKA depends not only on the pros- thetic design but also on the surgical technique, includ- ing ligament balance adjustment and alignment of the prosthesis. However, some allowance for technical varia- tions should be built into the prosthetic design.Third,the knee implanted with the prosthesis should simulate nat- ural knee kinematics as far as possible. In particular, in deep flexion over 135°, large roll-back and internal rota- tion of the tibia occur simultaneously, resulting in sub- luxation of the knee, which causes a space to be made in the popliteal region, avoiding posterior tibiofemoral im- pingement. A knee with a prosthesis cannot mimic this motion in extreme deep flexion.It is difficult for the pros- thetic designer to balance these three factors to a fine de- gree. The wide variety of knee prosthesis designs cur- rently on the market means that a consensus regarding the effects of articular geometry on longevity, stability, and kinematics has not been obtained among knee pros- thesis designers [12].

Longevity in High-flexion Knees

When a high-flexion knee is designed,it is important that the articular surface maintains an area of contact between femur and tibia to 135° or more of flexion. There are sev- eral ways to solve this problem. One is to extend the ar- ticular surface in the posterior condyles with smaller radii of curvature. Without the extension of the posterior condylar surface,the edges of the femoral posterior flange can dig into the polyethylene tibial insert in deep flexion, resulting in premature polyethylene damage. One solu- tion is to thicken the posterior femoral flange with an ad- ditional posterior condylar resection of about 2-4 mm [13]. However, reduction of bone stock due to additional bone resection may produce problems requiring revision surgery [14]. The other method is to cover the articular surfaces of the posterior femoral condyles completely with flexed posterior flanges, which fit obliquely into the cut surfaces of the posterior condyles. Because the ante- rior and posterior cut surfaces are not parallel in this de- sign, a special method of assembly would be necessary to attach the femoral component to the femur.Although ad- ditional bone resection is not necessary, retrieval of the femoral component may be difficult during revision surgery. Another way to avoid digging into the tibial in- sert with the sharp edge of the posterior flanges is to shorten and round the edges of the femoral flanges.How-

⊡ Fig. 49-3a, b. The Bisurface knee prosthesis (Type 4): (a) posterior view, (b) lateral view. Note the ball-and-socket joint in the mid-posterior portion of the femoral (composed of zirconia ceramics) and tibial component. Al- though this prosthesis is classified as a posterior-stabilized knee, the box- cut in the intercondylar part for the cam mechanism is not necessary and the bone loss for implantation is comparable to that with PCL-retaining prostheses

a b

sufficient, leading to premature polyethylene wear attrib- utable to the high forces exerted during deep squatting.

In the Bisurface knee, the major bearing surface between the femur and the tibia is the ball-and-socket joint in- stalled in the mid-posterior portion of the tibiofemoral joint in deep flexion [15]. Therefore, in this prosthesis, the area of contact at the articular surface is not disrupted in- sofar as the posterior impingement occurs in deep flex- ion.

The next point to consider regarding the longevity of the high-flexion knee is wear and fracture of the tibial post in the posterior-stabilized knee [16-19]. The posteri- or-stabilized knee has a post-cam mechanism facilitating the predictable femoral roll-back, which is advantageous to achieving high flexion. However, it has been indicated that the cam mechanism may be another source of wear debris, resulting in osteolysis [16, 17]. Furthermore, con- sidering the high load bearing on the tibial post during deep squatting [20], further attention to the strength and the design of the post is needed [19].The cam mechanism should be designed to work not only as a femoral roll- back provider and a posterior stabilizer but also as a weight-bearing articular surface, particularly during deep flexion. To prevent the fracture of the post and dis- location of the cam, the femoral cam should be designed to engage as near the base of the post as possible during deep flexion. The thickness and mechanical strength of the post should be sufficient to bear the large load during deep flexion.However,this cam design may cause the tim- ing of the cam engagement to occur later in the range of flexion,and the knee may not have posterior stability dur- ing early and mid flexion.Anterior wear of the tibial post attributable to impingement between the edge of the femoral box and the base of the tibial post at hyperexten- sion may not only be a source of polyethylene debris but may also cause post fracture. Some new prostheses ad- dressing this problem have an anterior femoral cam to

breakage of the tibial insert has not been reported so far, although the follow-up period is still short (the maxi- mum is 14 years). One concern about wear in the Bisur- face knee is the thickness of the polyethylene in the sock- et. The thickness of the thinnest tibial component avail- able is 9 mm and the polyethylene thickness at the lowest point of the socket is 4 mm. The contact area appears to be the anterior half of the socket, and the ball does not contact the lowest point of the socket during deep flexion.

The size of wear debris produced by the ball-and-socket joint may be as small as has been reported in total hip arthroplasty. This smaller size of particulate wear debris might cause periprosthetic osteolysis.

Stability in High-flexion Knees

Prostheses aiming at high flexion usually have low intrin- sic stability due to their articular geometry. The posterior flat surface of the tibia is necessary for large femoral roll- back on the tibia, for freedom of axial rotation, and to de- crease posterior bony and soft-tissue impingement. Con- tact areas between the femur and tibia during deep flex- ion should be small because the posterior femoral condyle has a small radius on the flat surface of the tibial insert.

Again,the small contact area must bear a large load trans- mitted through the knee during deep flexion. This can af- fect the durability of the tibial polyethylene insert.

The height of the tibial post in a high-flexion knee prosthesis may be relatively low to avoid patellar-post im- pingement in the deep patellar groove, which can de- crease tension in the quadriceps and provide an increase in the range of postoperative flexion [21]. The knee pros- thesis with the low post is designed to engage the tibial post at the base of the post with the femoral cam during deep flexion.Therefore,the dislocation safety factor (DSF, the so-called jumping distance) [22] is estimated to be

49

a b

⊡ Fig. 49-4a, b. Tangential views of the patellar groove in deep flexion. (a) A prosthesis with a deep patellar groove; a high tibial post may impinge a patella during deep flexion. (b) A prosthesis with a shallow patellar groove; the quadriceps tension seems to be high during deep flexion with this design

large during deep flexion despite the low post. In early to mid flexion, however, the cam mechanism does not en- gage and does not provide posterior stability in this cam design. Correct ligament balance and alignment of the knee in the frontal, sagittal, and axial planes may be es- sential to prevent cam dislocation in such prostheses. A prosthesis with a high post and a deep patellar groove has a risk of patellar-post impingement in deep flexion be- cause the position of the tibial post relative to the femoral articular surface is anterior due to the large roll-back dur- ing deep flexion (

) [21].

A prosthesis with a high post and a shallow patellar groove cannot loosen the high tension of the contracted quadriceps during deep flexion. Therefore, a prosthesis with a shallow patellar groove is somewhat disadvanta- geous for achieving deep flexion, although the patellar- post impingement can be avoided (

). In the Bisurface knee,the posterior half of the intercondylar em- inence is replaced with the socket. Therefore, the medial- lateral intrinsic stability is as small as in the the cruciate- retention prosthesis. Correct ligament balancing and component setting are as important for postoperative knee stability as in the low-post design. The patello- femoral joint congruity between the patella and the patel- lar groove may be disrupted in full flexion because of an- terior protrusion of the femoral ball.

In general,the intrinsic stability of a high-flexion pos- terior-stabilized knee may be low compared with that of a conventional posterior-stabilized knee. Appropriate surgical technique is necessary to obtain both deep flex- ion and stability. Design features of the high-flexion knee prosthesis should be sufficiently examined preoperative- ly in each patient, with each deformity, contracture, and laxity of the knee requiring assessment.

Kinematics of High-flexion Knees

In normal knees,the knee can flex fully until the heel touch- es the buttock.In full squatting,the lateral condyle of the fe- mur rolls back extremely on the tibia, resulting in subluxa- tion. The medial condyle also rolls back greatly, but main- tains the articulation with the femur (see Fig. 49-2). This alignment of the femur and tibia in the squatting position seems to provide enough room for the soft tissue and neu- rovascular bundles posterior to the knee joint,and reduces tension in the soft tissue.No knee prosthesis can mimic this kinematic (relative position between the femur and tibia) during deep flexion.A post-cam mechanism or a ball-and- socket joint is installed in the middle of the knee in a me- dial and lateral direction. Therefore, axial rotation during deep flexion results in posterior translation of the contact point in one compartment with anterior translation of the contact point in the other (the so-called center pivot mo- tion). Because the anterior translation of the femoral

condyle on the tibia might pinch the posterior soft tissues with the posterior edge of the tibial insert during deep flex- ion, it may be difficult for the knee to rotate freely. A later- al plain X-ray of the knee with a prosthesis during full squatting demonstrates condylar liftoff, suggesting the posterior soft-tissue impingement (

). Actually, my observation shows that patients who can squat fully af- ter TKA often complain of an abnormal compressive sen- sation in the popliteal region when squatting fully and can- not maintain the position for a long time.

Consideration of the Knee with a High-flexion Prosthesis

The knee prostheses aiming at full flexion that have been presented to the market recently involve many new ideas for achieving their design objectives: namely, longevity, stability, and kinematics. If a patient with a good preop- erative range of flexion undergoes a technically good op- eration, maintains a high degree of motivation to obtain full flexion, and endures vigorous postoperative rehabil- itation, he or she will end up with a knee having almost the full range of motion and good stability postopera- tively. However, we surgeons cannot be overjoyed by such wonderful results. There are no long-term follow-up data available on patients with high-flexion knees after TKA.

It has been demonstrated that the prosthetic knee with full flexion cannot achieve normal knee kinematics, and a very high load is transmitted to the knee when a person is standing up from the squatting position [20]. I consid- er it very important to educate patients with high-flexion knees. They need to learn how to use their knee safely in their daily activities to obtain sufficient longevity. For ex- ample, standing up from the squatting position and squatting down without aids should be prohibited.When

⊡ Fig. 49-5. Lateral radiograph in full squatting position. Note the condy- lar liftoff, suggesting posterior soft-tissue impingement. This patient com- plains of abnormal compressive sensation in the popliteal region when squatting fully and cannot maintain the position for a long time

standing up from a full squatting position on the floor,the patient should keel at first, next place one leg to stand up, and then stand up using both legs (

).

It is possible for high-flexion knee prostheses to func- tion quite adequately and for patients to be completely satisfied with them. However, until now this has been a new frontier and a great challenge in the world of TKA.

Surgeons must be meticulous in their selection of pa- tients and prostheses and must perform technically good surgery. Furthermore, appropriate education and careful long-term follow-up are necessary for the patients, and doctors must report the outcomes (knee function, longevity, stability, and patient satisfaction) of high-flex- ion prostheses.

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⊡ Fig. 49-6a-d. A safe way to stand up from a full squatting position: (a) Sitting in a full squatting position on the tatami; (b) kneeling on the mat;

(c) placing one leg prior to standing up; (d) standing up using both legs a

c

b

d