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Patella in Total Knee Arthroplasty

Barrack, Robert, L.; Burak, Corey

Clinical Orthopaedics and Related Research: August 2001 - Volume 389 - Issue - p 62-73
SECTION I SYMPOSIUM: Patella and Related Disorders
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The patella is a reliable guide to the success or failure of a total knee replacement. Patients who do not experience peripatellar symptoms or a patellar complication usually have a successful result. Conversely, peripatellar symptoms or complications usually reflect an underlying problem with surgical technique, component designs, or both. Current designs still do not replicate normal kinematics, and current instrumentation and techniques significantly alter the anatomy of the patellofemoral articulation in a substantial percentage of patients. Reproducing extensor mechanism balance and using components that provide adequate congruency and contact area through a physiologic arc of motion should lead to a successful result with minimal patellar symptoms or complications whether or not the patella is resurfaced. Attempting to achieve normal patellofemoral kinematics and minimize patellar complications has led to a better understanding of total knee arthroplasty.

From Tulane University Health Sciences Center, Department of Orthopaedic Surgery, New Orleans, LA.

Reprint requests to Robert L. Barrack, MD, Tulane University Health Sciences Center, Department of Orthopaedic Surgery, 1430 Tulane Avenue, SL32, New Orleans, LA 70112.

Many early designs of total knee arthroplasty replaced only the tibiofemoral joint and did not address the patellofemoral articulation. Symptoms referable to the patellofemoral joint often were encountered with these devices, which led to incorporation of an anterior femoral flange and resurfacing of the patella with subsequent designs such as the Total Condylar Knee (Zimmer, Warsaw, IN). 48 Although patellofemoral complications were uncommon with this implant, the arc of motion was limited to only approximately 90°. 46 Attempts to leave the patella unresurfaced with this design also led to suboptimal results. 44 Designs such as the Posterior Stabilized and Insall-Burstein (Zimmer) were developed to increase the arc of motion and thereby improve the functional results of total knee arthroplasty. Although the range of motion (ROM) improved substantially with these components, patellofemoral complications emerged as a major problem after knee replacement. 17 The popularity of uncemented knee replacements in the 1980s contributed to the high incidence of patellofemoral complications. Failure of metal-backed patellar components was the most common total knee device-related complication reported to the Food and Drug Administration during a 10-year period in one study of medical device reports. 15 Complications relating to the patella emerged as the most common cause for a total knee revision procedure. 3,5,8,10,12,17,24,39,46

The response to this major emerging problem was threefold. First, it brought into question the need to resurface the patella routinely. Because many of these complications seemed to be secondary to the patellar resurfacing procedure, it seemed reasonable that not resurfacing the patella routinely would lower the incidence of the associated complications. Second, it led to reevaluation of many total knee component design features. Some components seemed to have a much higher complication rate than others, so that some problems were perceived as design specific to some degree. Finally, and most importantly, the entire technique of knee replacement came under careful scrutiny. Inconsistency in surgical technique and instrumentation seemed to be the most common contributing factor to most patellar complications. Errors in sizing, alignment, and rotation of the tibial and femoral component eventually were appreciated as contributing factors to many patellofemoral problems. These numerous perceived problems led to many anatomic and kinematic studies aimed at gaining a better understanding of the patellofemoral articulation.

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Anatomy and Kinematics

Surprisingly high loads are transmitted across the patellofemoral articulation. During normal level gait, the joint reaction force generally measures .5 to 1 times body weight. This increases to 3 to 4 times body weight with stair climbing and can be as high as 8 times body weight during deep knee bends. 17 These high forces are distributed across a relatively small contact area in the normal patella. When a knee replacement is done, there is an additional decrease in the contact area and consequent increase in the contract stress. 36 A study by Matsuda et al 36 showed that resurfacing the patella decreased contact area to a greater degree compared with not resurfacing the patella. This finding was confirmed by Benjamin et al. 7 A subsequent study by Matsuda et al 37 used a digital electronic sensor before and after knee replacement with various component designs. They found that there was substantial variability in contact stresses between the unresurfaced patella and femoral components of various designs. In three of the designs examined there was significantly higher contact stresses than the normal knee throughout the arc of flexion. One design did not have higher contact stresses at flexion angles less than 90° but beyond 100° the contact areas became small as the patella encountered the intercondylar notch. One component maintained full contact and low compressive stresses throughout the arc of motion because of the extended patellofemoral groove. Buechel et al 14 also described low contact stress and high contact area with a mobile bearing patellar component. These findings, however, were contradicted by Matsuda et al 36 who reported that the proximal pole of the patella tented the quadriceps tendon leading to a higher contact stress.

Another study of patellar contact areas and forces after knee replacement also found that patellar replacement with an oval biconcave component significantly increased the patellar shear force from inferior to superior and an additional increase was seen with an oval domed patellar component. Once again, not resurfacing the patellar component was associated with an insignificant change in forces on the component compared with resurfacing (Fig 1). 49 McClean et al 38 reported on the effect of femoral component design on the contact area and stresses of the nonresurfaced patella after total knee replacement with numerous components. In the early degrees of flexion, the patellar prosthesis contact was near normal. Beyond 60° flexion, however, the contact areas decreased between 15% and 25%. The decrease in contact areas increased more dramatically at higher flexion angles. Two of the five femoral components tested induced 10° or greater of abnormal tilt as measured by an electrogoniometer during flexion. This showed the variability in the kinematics of the patellofemoral joint and the effect of the geometry of the femoral component that was combined with an unresurfaced patella.

Fig 1.

Fig 1.

The stresses measured in most studies of contact area have exceded the yield point of polyethylene. 36 Based on this, the clinical incidence of component failure and excessive wear would be expected to be much higher than has been observed clinically. This implies that the in vivo conditions in the laboratory are not an accurate reflection of the in vivo situation. These biomechanical studies do not take into account soft tissue remodeling and the pseudomeniscus that invariably forms after knee replacement. It is likely that some degree of load is transmitted to this soft tissue and may effectively diminish the load on the polyethylene as measured in biomechanical studies.

Kinematic studies of motion of the patellofemoral joint have consistently showed some degree of altered kinematics after knee replacement. Chew et al 16 used a three-dimensional tracking device to measure kinematics of the patellofemoral articulation of three recently designed total knee components. All three components had significant lateral tilt in the range of 5° compared with less than 1° for the normal patella (Fig 2). Stiehl et al 50 used a fluoroscopic examination of subjects with well-functioning total knee components of various designs and showed that with in vivo weightbearing, normal patellar kinematics were different after total knee replacement of all designs. They tested cruciate-retaining, posterior-stabilized, mobile-bearing unresurfaced, and mobile-bearing resurfaced total knee replacements and compared them with normal knees and with anterior cruciate deficiencies. The patellofemoral contact position was altered significantly compared with the normal for all knee component designs studied. From full extension to 90° flexion, the patellofemoral contact position moved in a superior position and the patella extended away from the tibia. Dynamic fluoroscopy showed that most subjects with cruciate-retaining total knee replacements and approximately ½ of the subjects with posterior-stabilized, fixed-bearing knee replacements experienced patellofemoral separation at full extension whereas patients with mobile-bearing knee replacements did not experience patellofemoral separation. Patellofemoral kinematics were altered significantly after total knee replacement, and there was substantial difference in the kinematics between total knee replacements of different designs.

Fig 2.

Fig 2.

Numerous anatomic studies in recent years have shown a higher degree of variability in the anatomy of the distal femur than was appreciated previously. Feinstein et al 22 examined 15 femurs from cadavers and found that the patellar groove was perpendicular to the transepicondylar axis but that there was a range of 16° in the orientation. Eckhoff et al 18 studied 85 specimens and found that the femoral sulcus was on average 2.4 mm lateral from the midpoint between the two femoral epicondyles but there was a range from 2.5 mm medial to 8.3 mm lateral. Despite these significant variations, surgeons tend to place the components in the same orientation relative to fixed landmarks, which means that the orientation of the patellofemoral joint is altered in a significant percentage of patients in the course of doing a total knee arthroplasty.

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Surgical Technique

Suboptimal technique is probably the contributing factor to the majority of patellofemoral complications and persistent patellofemoral symptoms after total knee replace-ment. 45,48 The major surgical goals in addressing the patellofemoral joint are balancing the extensor mechanism and achieving proper alignment and orientation of the components 45 This is true whether or not the patella is resurfaced. Numerous details of surgical technique help achieve this goal. At the time of arthrotomy, the capsule can be marked with transverse lines so that the capsule can be reapproximated side to side at the same level in an attempt to reestablish appropriate soft tissue balance. After arthrotomy, release of the patellofemoral ligament as described by Krackow 31 assists in retracting the patella out of the way to improve exposure and improving patellar tracking and avoiding excessive soft tissue tightness (Fig 3). It is necessary to flex the knee well beyond 90° and it also is desirable to evert the patella to assess the patellofemoral joint and to do patellar resurfacing if this is elected. If there is limited motion before knee replacement, these maneuvers may place undue tension on the patellar tendon insertion into the tibial tubercle. To avoid damaging the patellar tendon insertion, it may be necessary to do a quadriceps snip or other modified exposure. 3 It also is helpful to place a pin in the tibial tubercle to reinforce the patella tendon insertion and to direct attention to this region so that the initiation of detachment becomes apparent to the surgical team before complete avulsion occurs. Proper positioning of the femoral component substantially effects patellar component tracking. 38,40 External rotation of the femoral component relative to the posterior condyles is desirable, although this varies substantially between patients and between males and females. Relative lateralization of the femoral component also is desirable in terms of patellar tracking. It also is important to avoid component flexion or anterior placement to avoid increasing patellofemoral forces. Orientation of the tibial component also is crucial particularly avoiding relative internal rotation. Lateral placement of the tibial component also is desirable for improving patellar tracking.

Fig 3.

Fig 3.

If a decision is made to resurface the patella, numerous surgical details are crucial to optimizing the result. It generally is desirable to maintain the preoperative patellar thickness or to reduce it by 1 or 2 mm. 26 Symmetric bone resection also is important to achieve the goal of balancing the extensor mechanism. Asymmetric resurfacing occurs in 5% to 10% of cases, even among experienced surgeons, and this has been associated with a high percentage of patients who are symptomatic. 41 A caliper should be used before and after patellar resection to ensure that the bony surface is symmetric and equal to or less than the prepatellar bone thickness. In achieving an accurate cut, it is desirable to remove synovium from the entire periphery. The use of electrocautery has been suggested to outline the edge of the medial and lateral facets. 25 In addition to providing landmarks for resection, this may provide some degree of denervation. Patellar guides can be used to ensure the proper depth of resection. Alternatively, a free hand resection can be done using the insertion of the quadriceps and patellar tendon as a reliable landmark. 34 It is advisable to use the largest component that covers the bony surface of the patella. Placement to the medial edge of the cut surface is desirable to reproduce the normal high point of the medial ridge. 25 Although superior placement has been recommended for the patellar component, 4,48 it is desirable to avoid any superior overhang to avoid tenting of the soft tissue with knee flexion. It is probably also advisable to resect any synovia superior to the patellar component to minimize the risk of a fibrous nodule forming in this area leading to patellar clunk, particularly if a posterior-stabilized component is used (Fig 4). The three-pegged cemented component has become fairly standard. Large central pegs have been associated with a high incidence of fracture (Fig 5). Numerous metal-backed components have been associated with a higher complication rate and great care must be taken in considering the use of a metal-backed component.

Fig 4.

Fig 4.

Fig 5A–C.

Fig 5A–C.

Patellar tracking should be judged with the tourniquet deflated because an inflated tourniquet has been shown to effect patellar tracking. 35 If gross maltracking occurs after patellar resurfacing, the orientation of the tibial and femoral components should be reassessed. Internal rotation of the tibial and femoral components, in particular, should be avoided. Patellar tracking should be judged before final insertion of the components so that the orientation of the components can be altered if necessary. If the components are judged to be in an optimal or acceptable position and maltracking occurs with the tourniquet deflated with the rule of no thumb, lateral release should be done. 21 It generally is considered desirable to preserve the superior lateral geniculate artery, although it has been difficult to prove that sacrificing the vessel, in itself, leads to specific complications such as avascular necrosis or fracture. Care also should be taken not to incise the vastus lateralis tendon because this may lead to quadriceps tendon rupture or weakness. If slight maltracking persists after lateral release, medial imbrication should be done to additionally improve patellar tracking. 45

The majority of patellofemoral complications can be directly or indirectly attributed to errors in surgical technique. 45,48 With proper alignment and rotation of the components and soft tissue balancing of the extensor mechanism, the incidence of complications relating to the patella should be very low. The two major areas of improvements in additionally decreasing the incidence of patellofemoral complications relate to more reproducible surgical technique and improvements in component design. Bindelglass and Dorr 10 concluded that surgical technique was of greater importance than specific design factors, specifically the presence of an asymmetric component, a raised lateral flange, or an angled trochlear groove. Harwin 24 also reported that a symmetrical dome patellar component and a symmetrical femoral component were compatible with a very low patellar femoral complication rate, in agreement with that Bindelglass and Dorr. The surgical technique probably is more important than certain design details of the femoral component in minimizing the incidence of patellofemoral complications. A persistent problem with surgical technique is the variability of component alignment that is obtained with currently available alignment guides. Eckhoff et al 19 compared four different methods of determining tibial component rotation, for instance, and found a range of 20° from 2° internal rotation to 19° external rotation. A similar study evaluated the alignment and rotation of the femoral component. Olcott and Scott 40 evaluated four commonly used methods of determining component rotation and determined that asymmetry of the flexion gap occurred in 10% to 30% of patients depending on the anatomic landmarks used to determine femoral component rotation.

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Component Design

Early femoral component designs were characterized by a shallow flat trochlear groove, which resulted in a relatively high incidence of patellar subluxation and dislocation. A priority in early cruciate-retaining components was minimal bone resection that resulted in a shallow groove. A gait study by Andriacchi et al 2 subsequently showed that a deeper groove was associated with more normal function particularly in stair climbing. More recent designs have incorporated a separate intercondylar bone cut to allow adequate depth of resection to more closely replicate the normal anatomic trochlear groove. 42 Despite these design changes, patellofemoral kinematics still has not been reproduced completely. 16,50 The other design consideration is the distal extent of the trochlear groove. Earlier designs did not provide support for the patellar component other than two small areas of point contact beyond 90° flexion. By extending the metal surface of the trochlea farther distally, higher contact areas and lower contact stresses can be maintained beyond 90° flexion (Fig 6). This design feature has been incorporated into some recent designs. There is some evidence that this design feature particularly is advantageous when the patellar is left unresurfaced. 37

Fig 6.

Fig 6.

The issue of the design of the trochlear groove also has been addressed recently with some posterior-stabilized components. Patellar clunk is thought to be largely a design-related complication that virtually is restricted to total knee replacements done with posteriorstabilized components. 17 With these designs, there often is a sharp transition from the trochlear groove to the anterosuperior edge of the intercondylar notch of the femoral component. 46 The combination of a shallow patellar groove, a prominent intercondylar notch, and a small patellar component is thought to contribute to the development of fibrous nodule leading to patellar clunk syndrome 33 (Fig 4). Design modifications that are thought to decrease the risk of this complication include a deeper patellar groove, more distal extent of the trochlear groove, a smoother transition to the anterior edge of the intercondylar notch, and using a larger patellar component. 33 A surgical technique factor that also may diminish the incidence of the formation of fibrous nodule is excision of synovium of the undersurface of the quadriceps tendon particularly superior to the patellar component. By incorporating some of these design and technique considerations, the incidence of patellar clunk and other patellofemoral complications has been reduced dramatically. 33

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The Decision to Resurface the Patella

Whether to resurface the patella remains among the most controversial topics in total knee arthroplasty. The practice varies widely among the options of routine resurfacing, selective resurfacing, and not resurfacing. 30,43 Clinical studies reporting the results of knee replacement with and without patellar resurfacing have yielded widely variable results. Among the first widely quoted studies of total knee replacement with an unresurfaced patellar was that by Picetti et al 44 who reported a 29% incidence of anterior knee pain without patellar resurfacing. There was no control group, however, and the component used was the Total Condylar knee replacement, which is radically different than current designs. Subsequently, a study by Boyd et al 12 compared a total knee replacement with and without patellar resurfacing and reported a much higher incidence of anterior knee pain with unresurfaced patellas. However, this study included patients with inflammatory arthritis. When the analysis was restricted to patients with osteoarthritis, the complication rate was comparable between the two groups with 6% of patients who did not have resurfacing experiencing complications compared with 4% among patients who had resurfacing. Again, this was an older design component that does not meet current design standards as described previously.

The results of many studies may be, to some degree, design specific. Studies by Schroeder Boersch et al 47 using the Duracon Knee (Howmedica, Rutherford, NJ) and Kajino et al 28 using the Kajino-Shoji Knee (Biomet, Warsaw, IN) reported that their patients in whom the patella was resurfaced were able to climb stairs better than patients in whom the patella was not resurfaced. Studies by Ishinishi et al 27 with the PFC Knee (Johnson & Johnson, Raynham, MA), Braakman et al 13 with the Miller-Galante Knee (Zimmer), and Abraham et al 1 with the Variable Axis Knee (Howmedica) reported no difference in functional results between patients with and without patellar resurfacing. Some studies have compared bilateral knee replacements in which one side was resurfaced and the contralateral knee was not resurfaced. Using the LCS Knee replacement (DePuy, Inc, Warsaw, IN) Keblish et al 29 reported no difference in clinical results between the two sides. Enis et al 20 reported on patients with bilateral knee replacements with and without patellar resurfacing using the Townley Knee (DePuy) and found that the patients had a preference for the resurfaced side.

There have been a limited number of prospective randomized studies addressing the decision of whether to resurface the patella. Barrack et al 5 reported the results of a prospective randomized, double-blind study using the Miller-Galante II total knee replacement (Zimmer) and found no difference in knee score or patient satisfaction; however, 10% of patients without resurfacing subsequently underwent resurfacing. 5 Among the patients who had bilateral knee replacements with one side resurfaced and the other side not resurfaced, the knee scores were equivalent and patients expressed no preference for one side over the other. A minimum 5-year followup report of this same group of patients continued to show no significant difference between the groups. The incidence of anterior knee pain had increased in both groups, but more in the patients who had resurfacing. 6 Bourne et al 11 did a similar study using the AMK total knee replacement (DePuy) and reported less pain and higher flexion in patients who did not have resurfacing. Four percent of the patients without resurfacing, however, subsequently required patellar resurfacing. Feller et al 23 reported better stair climbing ability in patients with unresurfaced knees using the PCA component (Howmedica).

Because of the highly variable reports in the literature of the results of total replacement with and without patellar resurfacing, the indications remain controversial. There is a consensus to do patellar resurfacing in cases of primary patellofemoral arthritis in older patients (Fig 7), when a severely deformed patella is present that does not track normally and in patients with inflammatory arthritis. Patellar resurfacing should be avoided when there is a small osteopenic patella present in which case resurfacing may carry with it a high risk of patellar fracture or component loosening. There is also a preference on the part of many surgeons to avoid patella resurfacing in young active patients who have normal or near normal appearing articular cartilage. Recently, computer-assisted decision analysis was applied to the issue of resurfacing of the patella in total knee arthroplasty. 51 This is a technique that is based on probability theory and Bayesian logic that uses computer software and metaanalysis of the available literature. Using this methodology, not resurfacing the patella became the procedure of choice if the probability of postoperative anterior knee pain with an unresurfaced patella decreased below 14%, if the probability of having pain with a resurfaced patella increased above 8%, or if the use of patellar implant failure decreased below 80% of the use of a perfect health state. Several studies reported an incidence of postoperative anterior knee pain below 14% with unresurfaced patellas and an incidence of anterior knee pain above 8% with resurfaced patellas. 4,11,32,33 Surgeons can use these guidelines and the results of their practice to determine whether patellar resurfacing is indicated in their patients.

Fig 7A–B.

Fig 7A–B.

Among patients with degenerative osteoarthritis there is support in the literature for resurfacing or not resurfacing the patella. 5,6,11,29 If the patella is not resurfaced, it is important to choose a femoral component that is compatible with a native patella. Design features that seem to be favorable include a deep congruent trochlear groove that extends distally to maintain contact beyond 90° flexion. Using such an implant, Kulkarni et al 32 reported excellent results with or without patellar resurfacing. The incidence of anterior knee pain was 7% (seven of 96 patients) compared with 10% in the group that did not have resurfacing (12 of 115 patients); however, only one patient in each group had pain severe enough to require medication. 32 This indicates that at least with some designs, patellar resurfacing may not be necessary. If the patella is not resurfaced, patients also should be informed that subsequent resurfacing may be necessary. If the patella is resurfaced routinely, it is incumbent on the surgeon to maintain a very low complication rate because several series without patellar resurfacing have achieved success in more than 90% of patients. 4–6 The final decision on whether to resurface the patella rests with the surgeon based on his level of training, experience, and intraoperative judgement of the status of the patellofemoral articulation. With optimal surgical technique and implant design, excellent results are obtainable in the majority of knee replacements with or without resurfacing of the patella.

In gaining an understanding of the causes of patellofemoral problems, a more clear understanding of the optimal technique and design of knee arthroplasty has emerged. It has become clear that the patella is a reflection of the overall success of the knee arthroplasty procedure. The occurrence of a patellar complication, such as maltracking or anterior knee pain usually indicates an underlying problem in surgical technique, component design, or both (Table 1). This explains why isolated patellar resurfacing is associated with a high rate of complications and persistent symptoms, 9 why resurfacing a previously unresurfaced symptomatic patella is associated with persistent or recurrent symptoms, 6,32 and why even doing patellectomy after patellar fragmentation can fail to relieve symptoms (Fig 5). Conversely, normal patellar tracking and absence of any peripatellar symptoms is a strong indication of a successful arthroplasty procedure. The patella is like the canary in the mineshaft. In the early days of coal mining in Appalachia, canaries were placed in coal mines to detect low oxygen content and the presence of methane gas. 52 A somnolent canary indicated an underlying problem in the mine just as a patellar problem often is a reflection of a suboptimal total knee replacement. In the process of understanding and minimizing patellofemoral symptoms and complications after knee arthroplasty, a better understanding of the optimal total knee technique and design has been attained.

TABLE 1

TABLE 1

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References

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Section Description

O. Sahap Atik, MD, Guest Editor

© 2001 Lippincott Williams & Wilkins, Inc.