Ultracongruent Total Knee Arthroplasty

Adolph V. Lombardi Jr, MD, FACS

Noah T. Mallory, Pre-medical Year-4

INTRODUCTION

Total knee arthroplasty (TKA) is a widely successful surgical procedure to help severe cases of arthritis in the knee.¹ TKA can attribute much of its success to its continuously evolving and refining the procedure and implants, with each advancement taking surgeons one step closer to restoring the stability and function of a healthy, anatomical knee. One of the many debated aspects of primary TKA is the retention or sacrifice of the posterior cruciate ligament (PCL).²,³ Orthopedic surgeons are divided into camps over whether to choose: (1) the anatomic approach of always preserving one or both of the cruciate ligaments and using bicruciate-retaining or posterior cruciate-retaining (CR) implants, or (2) the functional approach of sacrificing and substituting for the cruciate ligaments with posterior-stabilized (PS) implants, or (3) to choose based on pathology. Although no clear answer is apparent, it is universally agreed that the best clinical results occur when the function of the PCL is retained or restored if it is resected.⁴ Specifically, the implant must permit femoral rollback, prevent the femoral condyles from subluxing anteriorly, and restore the anteroposterior stability of the knee.⁵,⁶,⁷ PS implants substitute for the PCL with a cam and post mechanism, but these bearings are not without complications, which include post dislocation and breakage,⁵,⁶,⁷,⁸ patellar clunk syndrome and patellar crepitation,⁵,⁶,⁷,⁹ intercondylar distal femoral fracture,¹⁰ increased wear around the post, as well as noise from the contact of the cam and post.⁶,⁹,¹¹ The ultracongruent (UC) bearing was designed as an alternative to the PS bearing, substituting for the PCL but without the above-noted complications.

The UC bearing, sometimes known as anterior stabilized or condylar stabilized, is a deep-dished polyethylene insert with a large anterior buildup and a more conforming articular surface to prevent the condyles from subluxing anteriorly.⁷ This large anterior buildup replaces the function of the post and thereby eliminates post-related complications. Because the UC is used with a CR femoral component, the need to resect bone from the intercondylar notch to accommodate a PS femoral is avoided and the procedure requires less time under tourniquet, which can be a factor in postoperative pain,¹²,¹³ and makes the UC a bone-conserving alternative to the PS.⁶,¹⁴,¹⁵ According to the most recent annual report of the American Joint Replacement Registry,¹⁶ PS implants continue to be the most common device used for primary TKA in the Unites States, but the use of UC bearings has increased steadily over time, from 1.1% in 2012 to 4.5% in 2018. For 2018, PS designs were utilized in 51.6% of primary TKA, followed by CR designs in 43.8%, and UC bearings, which, while starting to build momentum, are still relatively limited in use.

The UC is not solely a replacement for a PS bearing but also a noteworthy alternative to a standard CR implant as well.⁷ One of the major difficulties in putting in a CR implant is balancing the soft tissues, but this becomes considerably easier when the PCL is resected.⁵,⁷ The PCL can also rupture or become deficient in a knee with a standard CR bearing, rendering the patient unstable and in need of a revision, another complication not associated with the UC.¹⁷

INDICATIONS AND SURGICAL TECHNIQUE

PCL retention is heavily debated, and every surgeon takes a different stance regarding their preference and training; the use of the UC is no different.¹⁸ Regardless of the condition of the PCL, some surgeons will resect it while others will try to balance it.²,³,¹⁸ The view of the senior author is to spare whenever possible and put in the least amount of constraint that stabilizes the knee. Current TKA systems offer a continuum of constraint, from CR designs with variable degree of posterior slope, CR-lipped designs without posterior slope, to UC deep-dished anterior-stabilized bearings, to PS bearings, to mid-level PS bearings with a degree of varus/valgus constraint, to superstabilized constrained bearings, and ultimately to rotating hinges. The goal in both primary and revision TKA is to obtain balanced flexion/extension gaps.

The CR standard bearing in one particular system has 3° of posterior slope built into it (Fig. 36D-1). This allows the surgeon to cut the tibia at approximately 5° of posterior slope. With the additional 3° built into the insert and the tibia cut to 5°, the knee will have approximately 8° of posterior slope. This allows for satisfactory retention of the PCL and minimizes the need to balance the PCL. Therefore, when the PCL is intact, the medial and lateral collateral ligaments are intact and are balanced, and the flexion and extension gaps are balanced and equal, a CR standard bearing can be used. Upon completion of preparation of the distal femur and proximal tibial and balancing of the medial and lateral collaterals, there is occasionally a scenario where the flexion gap is slightly less than the extension gap. A way to treat this is to increase the posterior slope which will increase the flexion
gap. This allows the surgeon to appropriately balance the medial and collateral ligament. The extramedullary alignment jig is reapplied to the tibia and the tibial cut is refined with increased posterior slope, which increases the flexion space. With the trial components put back in place, an anteroposterior (AP) drawer test is performed. The knee should be stable in multiple degrees of flexion and articulating at the junction of the medial and posterior third. The knee should be balanced in extension when brought to full extension. The problem of knee with a tight flexion space and with a well-balanced extension space is solved by increasing the posterior slope and the surgeon may proceed with utilizing a standard CR bearing.

FIGURE 36D-1 Three bearing options for use with a cruciate-retaining femoral component are, from left to right, a standard cruciate-retaining that incorporates 3° of built-in posterior slope to facilitate ligamentous balancing, a cruciate-retaining-lipped bearing with no posterior slope that is approximately 2 mm thicker posteriorly, and an ultracongruent deep-dished bearing with an anterior build-up of polyethylene to afford stability. (Reproduced courtesy of Joint Implant Surgeons, Inc.)

The indications for a CR-lipped bearing are an intact PCL, balanced medial and lateral collateral ligaments, and a flexion gap that is slightly greater than the extension gap. There is no posterior slope in a CR-lipped bearing (Fig. 36D-1). Therefore, the CR-lipped bearing is approximately 2 mm thicker posteriorly than the CR standard bearing with 3° of posterior slope. The CR-lipped bearing can be used to the surgeon’s advantage in a scenario where, with a 10 mm thick CR standard bearing placed, the AP drawer test reveals the knee to be loose in flexion. When the knee is brought to full extension, it is well balanced, it is stable, and it obtains full extension. The medial and lateral collateral ligaments are stable. When the knee is flexed, there is instability. The surgeon can certainly tighten the flexion gap by increasing the thickness of the bearing, going from a 10 mm to a 12 mm, but what effect will this have on extension? It should leave the knee with a slight flexion contracture. With a 12 mm bearing placed, testing in both flexion and extension reveals the knee to be springy in extension—it does not quite get full extension. When the knee is flexed it is balanced, is stable in flexion, and has a very satisfactory drawer test. This situation is where a CR-lipped bearing can provide a solution. The surgeon can place a 10 mm CR-lipped bearing, which will be approximately 2 mm thicker in flexion than the CR standard bearing that has 3° of posterior slope built in (Fig. 36D-1 ). When the CR-lipped bearing is inserted, it will tighten the flexion gap relative to the CR standard bearing and should balance our flexion/extension gap. The surgeon looks for full, stable extension with stable medial and lateral collateral ligaments. When the knee is brought to flexion, the AP drawer test will now be appropriate. The knee is balanced simply by placing a CR-lipped bearing.

FIGURE 36D-2 Three methods of balancing the posterior cruciate ligament (PCL) are, from left to right, releasing the PCL from the posterior aspect of the tibia, performing a selective recession of fibers from their insertion on the distal femur, and performing a “V” osteotomy. (Reproduced courtesy of Joint Implant Surgeons, Inc.)

When performing TKA with PCL-retaining designs, just as one balances the medial and lateral collateral ligaments, there is a need, occasionally, to balance the PCL. There are three techniques that can be utilized (Fig. 36D-2). One is releasing the PCL from the posterior aspect of
the tibia. Another is performing a femoral recession. And the final one is performing a “V” osteotomy.

For the femoral recession technique of balancing the PCL, with the knee flexed, the surgeon performs the AP drawer test and the knee appears tight in flexion. Is it tight because of the medial collateral ligament or because of the PCL? If the medial collateral appears satisfactory but the PCL is tight, the surgeon can selectively release fibers of the PCL from their insertion on the femur using electrocautery. This will allow us to balance the PCL just like we balance the medial collateral and the lateral collateral in this TKA. As fibers are selectively removed, the surgeon gently pushes on the tibia to see if appropriate balance has been obtained. The patella is now reduced and the knee is taken through a range of motion, first checking it in flexion, checking the AP drawer test, noting that indeed there is a good AP drawer test. The knee should feel balanced. The PCL is palpated and should appear to be satisfactory and more normal. The knee is then brought to extension and the varus/valgus stability is assessed. When brought back to flexion, appropriate drawer test verifies that knee is balanced. The PCL has been partially released and obtained a stable arthroplasty obtained.

To balance the PCL with an osteotomy of the insertion of the PCL on the tibia, the knee is tested in flexion with the trial components in place. The medial collateral ligament should appear satisfactory, but the PCL is tight when palpated. Performing the AP drawer test confirms that the PCL is tight. The knee is balanced in extension, tight in flexion secondary to a tight PCL. Therefore, the trial components are removed and the proximal tibia is exposed with appropriate retractors placed. The insertion of the PCL is outlined with methylene blue marker. The surgeon then takes a quarter inch osteotome and carefully performs a semilunar osteotomy of the insertion. The osteotome is driven in approximately a centimeter to a centimeter and a half, and angled slightly posterior. The osteotomy is then completed with a broader osteotome and a mallet to displace the fragment. This will free the PCL insertion. Trial components are again placed. The stability of the knee is the assessed in flexion. By performing the osteotomy, the insertion autoregulates and the tension autoregulates. The AP drawer test will now be appropriate with an intact PCL that has been released to the appropriate tension for a good drawer test and good stability in flexion as well as extension. The knee is checked in multiple degrees of flexion as well as extension to be certain that it is balanced and stabilized, and the articulation is appropriate.

FIGURE 36D-3 Radiographic comparison of an ultracongruent implant (A: anteroposterior and B: lateral views) versus a posterior-stabilized device (C: anteroposterior and D: lateral views) for total knee arthroplasty. (Reproduced courtesy of Joint Implant Surgeons, Inc.)

The indications for an UC anterior-stabilized bearing are similar to those for a PS bearing (Fig. 36D-3). There are many surgeons who, in light of the excellent results afforded by these bearings, prefer to always use either a UC or PS insert and avoid the necessity of ligamentous balancing required for a CR standard bearing, while other surgeons, including the senior author, reserve UC and PS bearings for situations where the required ligamentous balancing for a CR standard bearing cannot be achieved.²,³ For surgeons who prefer to use selective constraint, the UC is indicated where the PCL is deficient but the medial and lateral collateral ligament are balanced and the flexion and extension gaps are equal.

The UC anterior-stabilized bearing is useful when instability in flexion is noted with the standard CR bearing; however, in extension the knee is well balanced and stable. When the knee is brought to flexion, the PCL is deficient. The surgeon can substitute for the deficient PCL in such a knee by using an UC bearing, while still using a CR femoral component and avoiding additional bony resection that would be required to switch to PS device.
An UC bearing is designed to provide stability by virtue of a long, broad anterior bumper. This enhanced polyethylene anteriorly will prevent posterior subluxation of the tibia when the knee is in flexion. When the UC trial bearing has been placed and the knee is brought to extension, it is stable in extension, both medially and laterally. In flexion the UC bearing provides resistance to posterior subluxation. The knee is balanced in flexion and extension. A high degree of flexion is obtainable with UC bearing and stability is provided by the anterior bumper. The trial bearing is exchanged for the definitive UC bearing, inserted, and assessed a final time for stability. It should be stable in both flexion and extension, with an appropriate AP drawer test, again noting resistance to posterior subluxation afforded by the anterior bumper of the UC bearing.

It is mandatory to balance the PCL in posterior CR TKA. This requires appropriate tibial resection with appropriate posterior slope. This also requires being facile with various techniques of PCL balance such as femoral recession or osteotomy of the insertion. Utilization of various degrees of constraint, including posterior CR standard bearings, posterior CR-lipped bearings, and UC anterior-stabilized bearings, may be required to address the full spectrum of instability and deformity encountered in primary TKA.

OUTCOMES

Extensive data and literature from randomized controlled trials, comparative cohort studies, meta-analyses, and systematic reviews support that there are excellent results with both posterior CR and PS TKA procedures.¹⁹,²⁰,²¹,²²,²³,²⁴,²⁵ However, many large registry studies report lower survival for PS TKA.²⁶,²⁷,²⁸,²⁹ In a retrospective review of the Mayo Clinic registry that included 8117 TKA, with 5389 CR and 2728 PS performed between 1988 and 1998, survival at 15 years was 90% for CR compared with 77% for PS TKA (P < .001), and remained better for CR knees after adjusting for age, sex, and preoperative diagnosis and deformity.²⁶ In an international comparative evaluation from registries in six countries of 371,527 fixed-bearing TKA implanted from 2001 to 2010 for treatment of osteoarthritis, it was found that non-PS TKA performed better with or without patellar resurfacing than PS TKA, with the effect most pronounced in the first 2 years.²⁷ In a study encompassing 63,416 TKA and 138 high-volume surgeons from the Australian National Joint Replacement Registry, researchers examined the effect of surgeon preference for PS versus minimally stabilized TKA on long-term survivorship.²⁹ At 13 years there was a 45% higher risk of revision for patients of surgeons who preferred PS TKA compared with patients of surgeons who preferred minimally stabilized TKA. A recent study from the Dutch Arthroplasty Register of 8-year revision rates for 133,841 cemented, fixed-bearing primary CR and PS TKA performed from 2007 to 2016 found that PS knees were 1.5 times (95% CI 1.4-1.6) more likely to be revised than CR knees.²⁸

Several cohort studies have examined clinical outcomes using a range of UC TKA devices by different manufacturers with varying results (Table 36D-1). An early study by Sathappan et al reported 95% survival at 10 years in 77 patients (114 TKA) with UC implants.³⁰ With mean follow-up of 8.3 years, three patients (4 knees) were revised; two as a consequence of traumatic falls and one bilateral patient for polyethylene wear treated with bearing exchanges. Likewise, a recent study by Yoon and Yang reported 100% survival at 8.1 years in 233 patients with UC TKA in which a navigation-assisted gap-balancing technique was used.¹ A smaller study by Chavoix reported 100% survival at 5.6 years in 28 patients with a mobile-bearing design UC TKA device, which was comparable to his center’s experience with a mobile-bearing PS TKA.³² In contrast, other cohort studies have reported inferior results with UC devices.⁴,¹²,³⁴ In the largest study by Marion et al, in 121 TKA patients implanted with a UC insert made of conventional gamma-irradiated polyethylene, survival was only 88% ± 17% at 9 years with alarming incidence of osteolysis with aseptic loosening.³⁴ They concluded the device was inferior to PS TKA and discontinued its use. While the authors theorize that sheer stresses associated with the deep-dished design may increase backside wear and compromise fixation, the primary failure mechanism appears to be poor polyethylene quality rather than inferiority of the UC bearing concept. In two smaller studies involving intraoperative measurements of kinematics and stability, authors reported unsatisfactory findings associated with UC inserts.⁴,¹² Massin et al conducted intraoperative testing with navigation on 10 knees with UC inserts and concluded that posterior stabilization was imperfect.¹² Likewise, Akkawi et al treated 20 patients with UC TKA assisted by navigation and followed up at 6 months.⁴ They observed that UC inserts failed to prevent anterior translation of the patella, thus causing inferior clinical scores.

Several published reports compare UC bearings with standard CR bearings in TKA (Table 36D-2). With the PCL resected, the UC high anterior buildup restores normal knee kinematics and provides comparable results to the CR.⁵,⁶,³⁶ Several studies support that functional outcomes and postoperative range of motion do not differ significantly between standard CR and UC impla nts,⁶,¹⁵,³⁹,⁴⁰ and several report that the UC was better.⁵,¹⁴,⁴³ In the earliest published report of UC bearings, Scott and Thornhill compared patients undergoing CR TKA using a single TKA system with 50 having a flatter posterior-lipped insert (CR) and 50 having a sagittally curved, more conforming insert with retention but balancing of the PCL.³⁶ Range of motion and incidence of tibial radiolucencies were similar at 1 year, and the authors concluded that UC inserts form an attractive compromise between schools of cruciate preservation and cruciate substitution, maximizing advantages while minimizing disadvantages
and preserving bone. Likewise, a recent study by Song et al comparing 38 patients with CR and 38 with UC inserts in a single TKA system found similar clinical and functional outcomes and in vivo stability between groups at 3.7 years, and concluded that UC inserts are a good option when the PCL is damaged without the need for a bony intercondylar box cut required for a PS device.⁶ Interestingly, Roh et al conducted a study using the same implant system as Song et al, but compared preservation versus sacrifice of the PCL when using UC inserts at a minimum follow-up of 2 years.³³ In 42 patients with PCL preservation versus 43 with PCL sacrifice, PCL-preserving knees had more varus rotation over 90° flexion, more anterior translation of the femur in all ranges of flexion than PCL-sacrificing knees, and more revisions—two for instability and one for PCL tightness, versus none revised in the PCL-sacrificing group. The authors concluded that preserving the PCL was not helpful for improving kinematics and clinical outcomes in UC TKA. Hofmann et al compared 100 patients with primary and revision UC TKA to a matched group of 100 patients with CR TKA.⁵ At 5 years mean follow-up, range of motion and clinical outcome scores were similar between groups, but there was a higher rate of revision for anteroposterior instability and PCL insufficiency in the CR group (5 of 100) compared with no revisions in the UC group. Similarly, Peters et al reported higher revision rates with CR versus UC TKA.⁴⁰ In their comparison of 228 patients with UC bearings and 240 with CR in a single TKA system with mean follow-up of 3.5 years, 3.1% of patients with UC TKA required revision versus 8.8% of CR TKA (P = .03), with six CR knees revised for instability versus no UC knees.

Knee Society Scores, radiographic results, frequency of nonsurgical complications, and frequency of manipulation under anesthesia were similar between groups. In a study from the current authors’ center by Berend et al, using the same TKA system as the study by Peter et al, 312 patients with UC inserts were compared to 1334 with standard CR inserts and 803 with CR-lipped inserts.¹⁴ At mean follow-up of 2.3 years, improvement in ROM was greatest in the UC group (5.9°) compared with 3.1° with standard CR and 3.0° with CR-lipped inserts, and the manipulation rate was lowest in the UC group, despite those patients having less preoperative ROM and greater tibiofemoral deformity and flexion contracture. In contrast to the findings of Hofmann et al and Peters et al in their series with longer follow-ups, no knees were revised for instability in any group. Mont et al reported on 32 knees in 29 patients with UC inserts and 139 knees in 124 patients with CR inserts in a single TKA system and observed similarly excellent clinical and functional outcomes.⁴¹ Survival was 100% in the UC group at 2.7 years and 99% in the CR group at 2.9 years with one knee revised for instability at 6 months. The senior author in that study preferentially uses CR inserts but occasionally requires the UC to achieve stability. There are concerns that the increased conformity and constraint of the UC potentially could cause increased polyethylene wear. In a recent study by Rajgopal et al with 2-year follow-up comparing knees in 105 patients treated with simultaneous bilateral TKA using UC inserts on one side and CR inserts contralaterally, they reported that knees with UC inserts had statistically better Western Ontario and McMaster Universities Osteoarthritis Index scores, Modified Knee Society Score, and ROM than knees with CR inserts.⁴³ There were no revision surgeries in either group, and gait analysis measuring foot pressures and step length showed no differences between insert groups.

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