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Posterior Stabilized Polyethylene Inserts in Total Knee Arthroplasty: A Retrieval Study Comparing Conventional to High-Flexion Designs
The Journal of Arthroplasty 31 (2016) 495–500
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Posterior Stabilized Polyethylene Inserts in Total Knee Arthroplasty: A Retrieval Study Comparing Conventional to High-Flexion Designs Erik A. Schnaser, MD, Marcella E. Elpers, BS, Chelsea N. Koch, BS, Stephen B. Haas, MD, Geoffrey H. Westrich, MD, Timothy M. Wright, PhD Hospital for Special Surgery, New York, New York
a b s t r a c t Background: High-flex (HF) total knee arthroplasties are modified posterior-stabilized (PS) implants designed to accommodate greater flexion. Methods: We examined differences between HF and PS retrieved tibial inserts with regard to polyethylene surface damage. Twenty HF inserts from each of 3 manufacturers were matched using patient demographics with 20 PS inserts from the same manufacturers. Ranges of motion between matched patients were not different. Results: Based on subjective damage scores, no differences were detected between HF and PS groups. Differences were found, however, among manufacturers, consistent with design approaches taken for PS and HF implants. Conclusion: In our series, high flexion did not influence damage, although this was likely influenced by the fact that few HF patients in our study had larger range of motions than their PS counterparts. Article history: Received 6 April 2015 Accepted 11 September 2015 Keywords: knee arthroplasty, high flexion, polyethylene damage, posterior stabilized, retrieval analysis
© 2016 Elsevier Inc. All rights reserved.
High-flexion (HF) total knee systems have been introduced by several manufacturers to provide implant components designed to accommodate higher degrees of flexion than can be safely achieved with traditional total knee arthroplasty (TKA) implants. Design approaches vary by manufacturer and include changes to the femoral component, the tibial insert, or both. Changes to the femoral component in posterior-stabilized (PS) HF designs include increasing the size of the intercondylar cam to promote enhanced roll-back with flexion and modifying the posterior condyles to provide more contact with the tibial insert in deep flexion. Changes to the tibial insert include changing the plateau geometry, translating the PS post more posteriorly, changing the post geometry, and modifying the posterior edge of the insert to avoid impingement with the femur in deep flexion. The clinical literature supporting the use of HF PS designs over conventional PS implants is conflicting [1–6]. Most reports suggest no difference in postoperative range of motion (ROM) [3,7,8]. Some reports raised concerns over early failures [2,9], whereas others showed excellent clinical results at follow-up periods to 10 years, even in patients reaching a mean ROM of 135° [5,10,11].
One or more of the authors of this paper have disclosed potential or pertinent conflicts of interest, which may include receipt of payment, either direct or indirect, institutional support, or association with an entity in the biomedical field which may be perceived to have potential conflict of interest with this work. For full disclosure statements, refer to http://dx.doi.org/10.1016/j.arth.2015.09.011. Reprint requests: Timothy Wright, PhD, Department of Biomechanics, Hospital for Special Surgery, 535 East 70th St, New York, NY 10021. http://dx.doi.org/10.1016/j.arth.2015.09.011 0883-5403/© 2016 Elsevier Inc. All rights reserved.
Only one other retrieval analysis of HF TKA components has been reported, in which the performance of HF tibial inserts of a single manufacturer was compared with PS inserts of the same design and manufacturer [12] based on subjective grading of polyethylene surface damage. We sought to extend retrieval analysis to compare damage among PS and HF inserts from different manufacturers to explore whether the different design changes would create differences in polyethylene damage. The goals of this study were to determine if (1) damage modes differ in HF designs from those observed in traditional PS designs, (2) differences in damage exist among different HF designs, and (3) clinical ROM affects damage among different HF and PS designs. Materials and Methods We matched 20 HF polyethylene inserts from each of 3 manufacturers to 20 traditional PS inserts from the same manufacturers, all of which were collected as part of our ongoing institutional review board–approved implant retrieval program. Twenty Zimmer (Warsaw, IN) LPS inserts were matched to 20 LPS-Flex inserts; 20 Exactech (Gainesville, FL) Optetrak PS inserts were matched to 20 Optetrak Logic inserts; and 20 Smith & Nephew (Memphis, TN) Genesis II PS inserts were matched to 20 Genesis II High-Flex inserts. Matching was performed based on length of implantation (LOI; ±9 months) and patient sex and body mass index (BMI; ±5 kg/m2; Table 1). We decided not to match on revision diagnosis and ROM in an effort to keep the number of implants in each cohort as large as possible from those available from our implant retrieval system.
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Table 1 Patient Demographics (Mean [Range]) Among the HF and PS Implant Designs. Zimmer
Age (y) BMI (kg/m2) LOI (mo) Sex (% female)
Exactech
Smith & Nephew
LPS-Flex
LPS
Logic (HF)
Optetrak (PS)
Genesis II (HF)
Genesis II (PS)
62 (39-81) 33 (25-53) 22 (1-47) 65
61 (32-82) 32 (24-48) 21 (1-41) 65
63 (49-79) 30 (24-38) 17 (3-29) 60
66 (47-66) 31 (23-41) 17 (3-38) 60
68 (50-105) 29 (24-38) 17 (0-52) 55
64 (47-82) 31 (22-42) 19 (1-52) 55
P
.39 .27 .67 .57
Matching was performed based on BMI, LOI, and patient sex.
All inserts were composed of conventional, gamma-irradiated ultrahigh-molecular-weight polyethylene. Exactech components (Optetrak PS and Optetrak Logic) were direct compression molded, whereas Smith & Nephew (Genesis II PS and Genesis II HF) and Zimmer (NexGen LPS and NexGen LPS-Flex) inserts were machined from extruded stock. The clinical medical record of each patient was reviewed to gather demographic variables, the revision diagnosis, and the pre-revision ROM. Sixty-one inserts were implanted at our institution, with the remaining 59 implanted elsewhere. All 120 inserts were revised at our institution by 24 surgeons. The dominant reasons for revision were instability, stiffness, infection, and loosening (Table 2). The breakdown of revision diagnoses did not differ (P = .39) among the 6 groups (3 manufacturers × 2 design types). When multiple ROM values existed in the medical record, the maximum flexion reached by the patient was used as the best-case performance of the TKA. The location and severity of surface damage on the inserts should be a reflection of the largest ROM reached by the patient throughout the duration of implantation. Each of the implants was subjectively graded for polyethylene surface damage using a modified version of the Hood grading scale [13]. Grading was done under low power (× 10) light stereomicroscopy (WILD Type 355110, Heerbrugg, Switzerland) by 2 independent graders blinded to the clinical information. The implants were evaluated for 7 damage modes: surface deformation, embedded debris, scratching, burnishing, delamination, pitting, and abrasion. The damage from each mode was assigned a score from 0 to 3, where 0 indicated no damage, 1 indicated damage to b10% of the zone, 2 indicated damage to 10% to 50% of the zone, and 3 indicated damage to N50% of the zone. Each mode was assessed in 14 zones across the superior surface of the tibial insert (Fig. 1). Zones 0 through 7 were located on the articular surfaces of the tibial plateaus, zone 8 was located in a noncontacting area (to be used in a subsequent study involving laser scanning of the components), and zones 9 through 13 were located on the PS post. Because our main focus was to compare HF to PS designs, the posterior articular zones (3 and 7) and zone 11 depicting the posterior post were of particular interest. Because design changes were made to the anterior post (zone 9) in the Exactech and Smith & Nephew HF designs compared with their PS counterparts, we also analyzed this region separately among the 3 manufacturers. The damage mode scores from all zones were combined to produce a total damage score for each insert with a maximum possible score of 294 (maximum score of 3 × 7 modes × 14 zones).
Statistical Analyses Interobserver reliability was assessed using Cronbach α coefficient, which was .91, denoting excellent consistency. Total damage scores were first compared between HF and PS groups (n = 60 for each). Further analysis was conducted to compare total damage scores between the subgroups of HF and PS inserts from each manufacturer. Each group was compared as a whole based on damage mode and total damage as well as by subgroup analysis examining the specific posterior zones (zones 3 and 7 and zone 11). When data were normally distributed, a t test was conducted when comparing means of 2 groups, and an analysis of variance with a post hoc Bonferonni correction was conducted when comparing multiple groups. When the test for normality failed, a Mann-Whitney rank sum test was used when comparing 2 groups or an analysis of variance with a Tukey post hoc test when comparing multiple groups. Categorical values such as gender were compared with a χ 2 analysis. Significance was set to P b .05. Data are presented as means ± SDs. Statistics were performed with SigmaPlot 12.3 (Systat Software Inc, San Jose, CA). Results No difference (P = .25) was found between the mean overall damage scores for the 60 HF inserts (37.1 ± 17.2) compared with those of the 60 PS inserts (40.1 ± 16.3). The dominant damage modes were deformation, scratching, burnishing, and pitting (Table 3). No significant differences were found when considering damage to the tibial post or damage to the articular surfaces and their relation to indication for revision (P = .3 for both comparisons). However, when analyzing the specific damage modes, significantly more scratching (P = .005) and pitting (P = .03) were seen in the PS designs when compared with the HF designs. No significant differences were seen in the other damage modes. Most importantly, no differences were observed in the total damage score (P = .06) or in the scores for any of the individual damage modes when comparing scores of the posterior articular surfaces between the 60 HF and 60 PS inserts (zones 3 and 7). Similarly, no differences (P = .11) were seen in total damage scores on the posterior post (zone 11) between HF and PS inserts. Nonetheless, PS designs had significantly more scratching (P b .001) and pitting (P b .001) on the posterior post, whereas the HF designs had more burnishing (P = .04). On the surface
Table 2 Reasons for Revision for the 120 TKAs Included in the 3 Matched Groups Shown in Table 1. Zimmer
Instability Stiffness Infection Loosening Pain/unknown/miscellaneous Allergy Total
Exactech
Smith & Nephew
LPS Flex
LPS
Logic
PS
4 (20%) 7 (35%) 3 (15%) 3 (15%) 0 (0%) 3 (15%) 20
8 (40%) 4 (20%) 3 (15%) 5 (25%) 0 (0%) 0 (0%) 20
7 (35%) 3 (15%) 3 (15%) 2 (10%) 5 (25%) 0 (0%) 20
7 35%) 7 (35%) 1 (5%) 3 (15%) 2 (10%) 0 (0%) 20
Genesis II HF
Genesis II PS
6 (30%) 1 (5%) 5 (25%) 2 (10%) 6 (30%) 0 (0%) 20
3 (15%) 3 (15%) 8 (40%) 2 (10%) 4 (20%) 0 (0%) 20
Total
35 25 23 17 17 3 120
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Table 4 HF and PS Damage Scores (Mean [SD]) Among 20 Matched Pairs of 3 Designs of Tibial Inserts. LPS-Flex vs LPS (Zimmer)
Total damage Deformation Debris Scratching Burnishing Delamination Pitting Abrasion
LPS-Flex (n = 20)
LPS (n = 20)
50.7 (12.1) 6.1 (3.3) 0.2 (0.4) 13.4 (3.6) 19.9 (9.1) 0 (0) 11.2 (3.2) 0.1 (0.5)
43.7 (10.1) 5.6 (2.2) 0.2 (0.6) 15.7 (4.3) 7.7 (5.4) 0 (0) 13.0 (3.8) 1.4 (1.6)
P .05 .7 .5 .7 b.001 1 .1 .002
Optetrak Logic vs Optetrak (Exactech) Fig. 1. Subjective grading of polyethylene surface damage was done on separate regions of the superior surface of the tibial insert, including the tibial post.
of the anterior post (zone 9), significantly more damage was seen in the PS inserts (P = .02). The significant difference reflected greater scores for embedded debris (P = .04), scratching (P b .001), and pitting (P b .001) in the PS inserts. When analyzing the overall damage of the inserts based on design (Table 4), the Zimmer LPS-Flex inserts had the greatest amount of damage and were significantly more damaged than their LPS counterparts (P = .05). The difference was driven by the fact that the Zimmer LPS-Flex inserts displayed considerably more abrasion near the top of zones 10 and 12 reflecting the medial and lateral sides of the post than did their matched LPS counterparts. Conversely, the Exactech Optetrak Logic (HF) inserts had the lowest amount of damage among the 6 designs and were significantly less damaged Table 3 HF and PS Damage Scores (Mean [SD]) for the Cohort of 120 Retrieved Tibial Inserts. HF (n = 60) Entire articular surface (zones 0 through 13) Total damage 37.1 (17.2) Deformation 4.3 (3.6) Debris 0.2 (0.5) Scratching 11.5 (5.4) Burnishing 10.9 (11.2) Delamination 0.0 (0.0) Pitting 9.5 (5.5) Abrasion 0.7 (1.1) Posterior articular surface (zones 3 and 7) Total damage 8.0 (3.87) Deformation 0.9 (1.51) Debris 0.03 (.1) Scratching 2.5 (1.3) Burnishing 2.0 (2.5) Delamination 0.0 (0.0) Pitting 2.3 (1.4) Abrasion 0.1 (0.4) Anterior post (zone 9) Total damage 1.9 (1.6) Deformation 0.9 (0.9) Debris 0.0 (0.0) Scratching 0.2 (0.4) Burnishing 0.5 (0.7) Delamination 0.0 (0.0) Pitting 0.1 (0.3) Abrasion 0.2 (0.5) Posterior post (zone 11) Total damage 2.4 (2.5) Deformation 0.7 (0.9) Debris 0.02 (0.1) Scratching 0.3 (0.5) Burnishing 1.0 (1.3) Delamination 0.0 (0.0) Pitting 0.1 (0.3) Abrasion 0.05 (0.2) Bold p-values denote significance at b0.05.
PS (n = 60)
P
40.1 (16.3) 5.7 (4.1) 0.3 (0.8) 14.1 (6.2) 8.0 (6.5) 0.0 (0.0) 11.8 (5.9) 1.1 (2.2)
.25 .06 .73 .005 .99 1.0 .03 .57
6.5 (2.9) 0.5 (0.8) 0.03 (0.1) 2.2 (1.1) 1.3 (1.8) 0.0 (0.0) 2.2 (1.3) 0.1 (0.5)
.06 .56 1.0 .06 .30 1.0 .72 .58
3.0 (2.4) 0.9 (0.9) 0.07 (0.2) 0.7 (0.5) 0.7 (0.7) 0.0 (0.0) 0.4 (0.6) 0.1 (0.5)
.02 .98 .04 b.001 .08 1.0 b.001 .25
2.7 (1.5) 0.8 (0.7) 0.02 (0.1) 0.8 (0.5) 0.4 (0.6) 0.0 (0.0) 0.5 (0.5) 0.1 (0.4)
.11 .42 1.0 b.001 .04 1.0 b.001 .4
Total damage Deformation Debris Scratching Burnishing Delamination Pitting Abrasion
Logic (n = 20)
Optetrak (n = 20)
24.0 (8.5) 1.2 (1.4) 0.4 (0.7) 10.7 (4.4) 0.2 (0.6) 0 (0) 10.1 (4.7) 1.5 (1.2)
47.0 (10.0) 8.6 (4.3) 0.7 (1.1) 16.9 (2.9) 4.9 (3.8) 0 (0) 15.1 (4.7) 0.9 (1.1)
P b.001 b.001 .32 b.001 .002 1 0.14 b.001
Genesis II High-Flex vs Genesis II (Smith & Nephew) Smith & Nephew
High-Flex (n = 20)
Genesis II (n = 20)
Total damage Deformation Debris Scratching Burnishing Delamination Pitting Abrasion
36.6 (18.0) 5.6 (3.5) 0.1 (0.5) 10.4 (7.2) 12.8 (10.0) 0 (0) 7.2 (7.2) 0.6 (1.1)
31.6 (22.1) 2.7 (3.4) 0 (0) 9.6 (7.6) 11.5 (7.9) 0 (0) 7.0 (5.8) 1.0 (3.4)
P .19 .002 .8 .7 .8 1 .8 .5
Bold p-values denote significance at b0.05.
than their Optetrak PS counterparts. A significant reduction in deformation, scratching, and burnishing, tempered somewhat by a significant increase in abrasion, in the Optetrak Logic inserts accounted for the difference. Although the Logic inserts demonstrated significantly less total damage than both of the Zimmer insert designs (P b .05), the damage was not significantly less than either of the Smith & Nephew insert designs. Furthermore, no significant difference was found when comparing the total damage between the Smith & Nephew Genesis II High-Flex and PS inserts, although a significant increase in deformation (P b .002) was noted for the High-Flex inserts. Whether comparing HF vs PS designs within or among manufacturers, the posterior regions of the articular surfaces (zones 3 and 7) and the posterior surface of the post (zone 11) are most relevant (Fig. 2). Although contact can occur in these zones when the knee is in lesser degrees of flexion, these zones are the primary contact points between the femoral and tibial components when the knee is in high degrees of flexion. Within each of the 3 manufacturers (matched pairs of n = 20), significant differences in damage scores were found between HF and PS versions for the posterior articular surfaces (Table 5). For 2 of the designs, Zimmer and Smith & Nephew, the HF inserts showed more damage (P b .01 and P b .02, respectively) than the matched PS inserts, whereas for Exactech, the HF inserts showed significantly less damage (P b .03) than the matched PS inserts. The increased damage for the Zimmer HF was dominated by increased scratching and burnishing scores, whereas for the Smith & Nephew HF, the difference was dominated by increased deformation. However, for the posterior post, only Exactech showed a significant difference between matched HF and PS inserts, with the HF inserts showing significantly less damage (P b .001). Range-of-motion data were available for 106 (88%) of the 120 knees. No significant differences (P = .6) were detected between the HF and PS implants with regard to mean flexion. Similarly, no differences were found in mean extension (P = .7) and total combined ROM (P = .6).
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Medial Post
Posterior Post
Posterior Articular Surface
Smith & Nephew Genesis II High-Flex
Exactech Optetral Logic
Zimmer LPS-Flex
498
Fig. 2. Photographs of the post surfaces and the posterior articular surfaces for retrieved HF inserts show the polyethylene damage modes that typically dominated in these regions: deformation, scratching, burnishing, and pitting.
High-flexion designs were intended to improve performance when patients had greater than 120° of flexion, and few of the patients in our study achieved this. Twenty-three percent of our patients achieved greater than 110° of flexion, with no differences found among the 6 groups: LPS Flex (5 patients), LPS (4), Optetrak Logic (6), Optetrak PS (4), Genesis II HF (4), and Genesis II PS (4). No significant differences were seen among the ROM achieved by the patients with different implant designs (Fig. 3). However, significant ROM differences (P b .05) were seen among indications for revision, but subsequent between group comparisons showed that this finding was driven by the ROM for patients revised for stiffness being substantially lower than those revised for instability.
data that demonstrate their efficacy exist, although many of the authors of studies examining HF TKAs made the erroneous claims that efficacy was not demonstrated because the use of the HF designs did not in and of itself increase ROM (e.g., [3]). No published studies have
Discussion High-flex total knee designs were developed to better accommodate more flexion than their standard PS counterparts. However, few clinical
Table 5 HF and PS Damage Scores (Mean [SD]) for the Posterior Articular Surfaces (Zones 3 and 7) and the Posterior Post (Zone 11) Among the 3 Designs of Tibial Inserts. HF (n = 20) Posterior articular surface (zones 3 and 7) Zimmer 10.0 (3.5) Exactech 5.6 (2.1) Smith & Nephew 8.4 (4.4) Posterior post (zone 11) Zimmer 3.6 (2.8) Exactech 0.3 (0.4) Smith & Nephew 3.6 (2.0) Bold p-values denote significance at b0.05.
PS (n = 20)
P
7.0 (2.9) 6.9 (1.9) 5.8 (3.7)
.01 .03 .02
2.3 (.08) 3.3 (1.2) 2.8 (2.2)
.20 .001 .12
Fig. 3. A box and whisker plot of the ROMs achieved by the patients from whom the tibial inserts were retrieved showed no differences among the 6 designs (LPS Flex, LPS, Optetrak Logic, Optetrak PS, Genesis II PS HF, and Genesis II PS). The box shows the median (the horizontal line within the box) and the upper and lower quartiles (the top and bottom of the box). The whiskers span 1 SD above and below the mean, and the x's denote outliers beyond the range of the whiskers.
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compared HF designs from different manufacturers. However, a previous retrieval study by Paterson et al [12] compared HF to PS Genesis II knees, 1 of the 3 TKA systems in our study. Like us, they found no difference in the articular surface damage scores or damage modes between PS and HF Genesis II retrieved inserts. They also found more post damage on the HF than the PS inserts; although we found higher damage scores for the Zimmer and Smith & Nephew posterior posts, the differences did not reach significance (Table 5). The strength of our study is the large number of retrieved implants that were examined and the ability to match the implants across 3 manufacturers and among contemporary PS and HF designs from the same implant systems. Matching on crucial clinical and demographic factors strengthens the study. Although we could not match patients on revision diagnosis, we were able to match the patient's LOI, sex, and BMI. Matching increased the likelihood that the differences observed in polyethylene surface damage on the tibial inserts are related to design differences. In answering our first question—what are the differences in damage modes associated with HF designs when compared with traditional PS designs?—no differences were observed in the types of polyethylene surface damage between the HF and PS inserts. Presence and severity of individual damage modes differed (eg, PS implants had a small but significant increase in scratching and pitting), but the overall damage scores were similar. These results demonstrate that, in general, the polyethylene tibial component surfaces behave similarly between HF and PS designs. This is not an unexpected finding. For the HF designs in our study, few, if any, changes in articular geometry were incorporated in regions that would be in contact through much of the ROM (b 125°130°). Furthermore, changes were made in contact regions for higher flexion specifically intended to reduce polyethylene stresses, so little additional polyethylene damage would be expected in the posterior regions of the articular surfaces (Table 3). If these changes had been ineffective, we would have observed considerably more damage in the posterior zones (3 and 7) or damage modes such as delamination, long associated with fatigue wear caused by large cyclic polyethylene stresses [14]; instead, no delamination was observed in these regions. Our second research question was as follows: what are the differences in damage modes between the HF and traditional PS designs from different manufacturers? Unlike the aggregate analysis used to answer our first question, significant differences in surface damage scores did emerge when comparing HF and PS inserts within the 3 manufacturers' designs. The largest difference was with the Exactech Optetrak Logic HF design. The overall damage in this design was significantly lower than its PS counterpart, although these scores may be biased by the fact that the tibial inserts are compression molded from polyethylene, creating a shiny surface, potentially masking the appearance of burnishing. However, this design had significant changes from the PS version in both the tibial insert design and the femoral component that were an evolution in the design rather than simply a design intended solely to accommodate HF; in that regard, the matched pairs reflect design differences not solely intended to accommodate increased flexion. Nonetheless, the tibial post in the Exactech HF design has eyelets on the posterior condyles to accommodate the femoral component in deep flexion, and the post is more posteriorly placed and designed to allow the cam to ride up the back of the post throughout increasing flexion. Also, the anterior post is rounded to be more conforming with the femoral component in hyperextension [15]. On the femoral component, the radius of the posterior condyle was increased to allow deeper flexion, the cam was extended and made more conforming with the tibial insert, and the anterior box was made more conforming to the rounded anterior tibial post. The design changes in the post-box articulation led to a significant reduction in post damage in earlier work from our group [15], consistent with the findings from our current study. The Zimmer HF femoral design changes consisted of an extension of the posterior femoral condyles to allow for increase tibial rollback,
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deepening and elongating of the anterior flange, and extension of the femoral cam to induce more rollback while resisting subluxation. Interestingly, the Smith & Nephew HF knee design differs from the PS design for the polyethylene tibial insert only; no changes were made to the femoral component. The Smith & Nephew HF insert has a deeper cutout for the patellar tendon and a chamfered anterior post, and the condyles are more conforming to the femoral component when compared with its PS counterpart. For both the Zimmer and Smith & Nephew designs, the HF inserts had more damage than the PS inserts in the posterior regions of the articular surfaces (zones 3 and 7). These findings, though statistically significant, may not be clinically significant. For example, the Zimmer HF design had higher burnishing and scratching scores, but this may not reflect more polyethylene wear. Instead, the increased score may reflect that more of the posterior zones (3 and 7) were damaged, consistent with a greater area over which contact occurs. For the Smith & Nephew HF design, the dominant difference with the PS version was deformation of the articular surface. Again, this may not relate to increased polyethylene wear, although the increased deformation may be associated with higher contact stresses. The increased deformation may reflect the fact that twice as many knees in the Smith & Nephew HF group were revised due to instability. These differences should be explored by measuring the 3-dimensional changes in the inserts in relation to pristine versions of the same inserts. Another consideration in interpreting the results from these posterior regions is the fact that contact can occur between the femoral and tibial components in these regions during activities other than HF. In such situations, the contacting portion of the femoral component will not be the posterior condyles. Instability could also play a role in the pattern of posterior region damage. For example, unstable total knees may show greater AP and coronal plane translations throughout flexion, not just at HF angles, so that differences between unstable and stiff knees could be masked within the damage observed on the retrieved inserts. Techniques such as laser scanning [16] and computed tomographic imaging [17] could provide objective measurements of the deformation experienced by the polyethylene tibial components in these regions, which in turn would provide important clues as to the source of the posterior damage. A laser scanning study of our HF and PS retrieved tibial inserts is currently underway. Our final research question dealt with whether differences existed in clinical ROM among different HF and traditional PS designs. The patients in our study had a mean ROM less than 100°, lower than most published clinical studies using these implants [4,5,7,10,18–20]. Despite studies that suggest a benefit to HF designs [1,21], we did not find differences in the pre-revision ROMs between HF and PS designs. Of course, one cannot conclude from our data that HF designs do not allow more flexion than PS designs, because we do not know the ROMs for all the still functioning implants from which our retrieved implants are but a subset. Nonetheless, our results agree with other authors who failed to show a difference in ROM between HF and PS implants [4,5,7,18,19]. Range of motion after a TKA likely depends on multiple variables, such as the patient's own inflammatory response, activity level, compliance with rehabilitation, preoperative ROM, and appropriate pain management postoperatively. Regardless, our data show that all 6 implant designs had a similar ROM and a similar number of patients who had achieved greater than 110° of flexion postoperatively. Therefore, the differences that we observed in damage scores were not due to differences in ROM. Because few patients in our study had greater than 120° of motion, we could not determine the impact of HF designs on patients who achieved HF. Our study has several limitations. First, this is a retrieval study of implants that were revised for a variety of reasons, and thus, these implants may not be representative of well-functioning implants. A strength of our study is that all 6 implant designs were matched by LOI, sex, and BMI, but numerous reasons for revision existed among the cases from which the implants were retrieved, and we could not
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match based on reason for revision and still maintain sufficient numbers to power our comparisons. Notably, higher rates on instability were found in some subgroups. Dolan et al [22] demonstrated in a regression analysis of tibial post damage that patients revised for instability or aseptic loosening had more tibial post damage than did those patient revised for infection or stiffness, so the differences in the posterior post scores in our study might have been influenced by differences in diagnosis. A second limitation is that we could not extend the matching to include pre-revision ROM. We obtained pre-revision ROM on only 88% of our retrieved implants, making matching on ROM impossible to achieve and still maintain large numbers of specimens. Our average ROM for both HF and PS knee were less than other published reports for these types of implants [4,5,7,10,18–20], and the generalizability to patients who have greater than 120° of flexion is limited. Our data were captured retrospectively, so they may be susceptible to inaccuracy in ROM capture. Dennis et al [1] published that high-flex designs may be beneficial in patients with lower preoperative ROM when compared with the PS design in primary TKA. We did not capture primary preoperative ROM in our cohort because many of the primary TKAs in our group were done at outside institutions. In conclusion, our data show no difference in the overall damage scores for HF and PS inserts and also show that implant damage is dependent on implant design characteristics. When HF and PS knees are compared, changes on both the femoral and tibial components must be considered in understanding the resulting differences in polyethylene damage. Although retrievals with greater flexion and longer implantation times should be examined as they become available, our results show little meaningful differences between HF and their PS counterparts.
Acknowledgments We acknowledge funding from the Mary and Fred Trump Institute for Implant Analysis.
References 1. Dennis DA, Heekin D, Clark CR, et al. Effect of implant design on knee flexion. J Arthroplasty 2013;28:429.
2. Han HS, Kang S-B, Yoon KS. High incidence of loosening of the femoral component in legacy posterior stabilised-flex total knee replacement. J Bone Joint Surg (Br) 2007; 89:1457. 3. Jain S, Pathak AC, Kanniyan K, et al. High-flexion posterior-stabilized total knee prosthesis: is it worth the hype? Knee Surg Relat Res 2013;25:100. 4. Kim Y-H, Choi Y, Kwon O-R, et al. Functional outcome and range of motion of high-flexion posterior cruciate-retaining and high-flexion posterior cruciate-substituting total knee prostheses. A prospective, randomized study. J Bone Joint Surg Am 2009;91:753. 5. Kim Y-H, Sohn K-S, Kim J-S. Range of motion of standard and high-flexion posterior stabilized total knee prostheses. A prospective, randomized study. J Bone Joint Surg Am 2005;87:1470. 6. Tanavalee A, Ngarmukos S, Tantavisut S, et al. High-flexion TKA in patients with a minimum of 120 degrees of pre-operative knee flexion: outcomes at six years of follow-up. Int Orthop, published online, Oct 24, 2010. 7. Nutton RW, van der Linden ML, Rowe PJ, et al. A prospective randomised doubleblind study of functional outcome and range of flexion following total knee replacement with the NexGen standard and high flexion components. J Bone Joint Surg (Br) 2008;90:37. 8. Seon J-K, Yim J-H, Seo H-Y, et al. No better flexion or function of high-flexion designs in Asian patients with TKA. Clin Orthop Relat Res 2012;471:1498. 9. Namba RS1, Inacio MC, Cafri G. Increased risk of revision for high flexion total knee replacement with thicker tibial liners. Bone Joint J 2014;96-B(2):217. 10. Kim Y-H, Park J-W, Kim J-S. High-flexion total knee arthroplasty: survivorship and prevalence of osteolysis. J Bone Joint Surg Am 2012;94:1378. 11. Kim TH, Lee DH, Bin SI. The NexGen LPS-flex to the knee prosthesis at a minimum of three years. J Bone Joint Surg (Br) 2008;90:1304. 12. Paterson NR, Teeter MG, MacDonald SJ, et al. The 2012 Mark Coventry award: a retrieval analysis of high flexion versus posterior-stabilized tibial inserts. Clin Orthop Relat Res 2013;471:56. 13. Hood RW, Wright TM, Burstein AH. Retrieval analysis of total knee prostheses: a method and its application to 48 total condylar prostheses. J Biomed Mater Res 1983;17:829. 14. Bartel DL, Bicknell VL, Wright TM. The effect of conformity, thickness, and material on stresses in ultra-high molecular weight components for total joint replacement. J Bone Joint Surg Am 1986;68:1041. 15. Gilbert SL, Rana AJ, Lipman JD, et al. Design changes improve contact patterns and articular surface damage in total knee arthroplasty. Knee 2014;21:1129. 16. Stoner KE, Nassif NA, Wright TM, et al. Laser scanning as a useful tool in implant retrieval analysis: a demonstration using rotating platform and fixed bearing tibial inserts. J Arthroplasty 2013;28(8 Suppl.):152. 17. Teeter MG, Naudie DD, Charron KD, et al. Three-dimensional surface deviation maps for analysis of retrieved polyethylene acetabular liners using micro-computed tomography. J Arthroplasty 2010;25:330. 18. Ng FY, Wong HL, Yau WP, et al. Comparison of range of motion after standard and high-flexion posterior stabilised total knee replacement. Int Orthop 2008;32:795. 19. Springorum HR, Maderbacher G, Craiovan B, et al. No difference between standard and high flexion cruciate retaining total knee arthroplasty: a prospective randomized trial. Knee Surg Sports Traumatol Arthrosc 2014;1–7. 20. Endres S. High-flexion versus conventional total knee arthroplasty: a 5-year study. J Orthop Surg (Hong Kong) 2011;19:226. 21. Bin SI, Nam TS. Early results of high-flex total knee arthroplasty: comparison study at 1 year after surgery. Knee Surg Sports Traumatol Arthrosc 2007;15:350. 22. Dolan MM, Kelly NH, Nguyen JT, et al. Implant design influences tibial post wear damage in posterior-stabilized knees. Clin Orthop Relat Res 2011;469:160.
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Posterior Stabilized Polyethylene Inserts in Total Knee Arthroplasty: A Retrieval Study Comparing Conventional to High-Flexion Designs Erik A. Schnaser, MD, Marcella E. Elpers, BS, Chelsea N. Koch, BS, Stephen B. Haas, MD, Geoffrey H. Westrich, MD, Timothy M. Wright, PhD Hospital for Special Surgery, New York, New York
a b s t r a c t Background: High-flex (HF) total knee arthroplasties are modified posterior-stabilized (PS) implants designed to accommodate greater flexion. Methods: We examined differences between HF and PS retrieved tibial inserts with regard to polyethylene surface damage. Twenty HF inserts from each of 3 manufacturers were matched using patient demographics with 20 PS inserts from the same manufacturers. Ranges of motion between matched patients were not different. Results: Based on subjective damage scores, no differences were detected between HF and PS groups. Differences were found, however, among manufacturers, consistent with design approaches taken for PS and HF implants. Conclusion: In our series, high flexion did not influence damage, although this was likely influenced by the fact that few HF patients in our study had larger range of motions than their PS counterparts. Article history: Received 6 April 2015 Accepted 11 September 2015 Keywords: knee arthroplasty, high flexion, polyethylene damage, posterior stabilized, retrieval analysis
© 2016 Elsevier Inc. All rights reserved.
High-flexion (HF) total knee systems have been introduced by several manufacturers to provide implant components designed to accommodate higher degrees of flexion than can be safely achieved with traditional total knee arthroplasty (TKA) implants. Design approaches vary by manufacturer and include changes to the femoral component, the tibial insert, or both. Changes to the femoral component in posterior-stabilized (PS) HF designs include increasing the size of the intercondylar cam to promote enhanced roll-back with flexion and modifying the posterior condyles to provide more contact with the tibial insert in deep flexion. Changes to the tibial insert include changing the plateau geometry, translating the PS post more posteriorly, changing the post geometry, and modifying the posterior edge of the insert to avoid impingement with the femur in deep flexion. The clinical literature supporting the use of HF PS designs over conventional PS implants is conflicting [1–6]. Most reports suggest no difference in postoperative range of motion (ROM) [3,7,8]. Some reports raised concerns over early failures [2,9], whereas others showed excellent clinical results at follow-up periods to 10 years, even in patients reaching a mean ROM of 135° [5,10,11].
One or more of the authors of this paper have disclosed potential or pertinent conflicts of interest, which may include receipt of payment, either direct or indirect, institutional support, or association with an entity in the biomedical field which may be perceived to have potential conflict of interest with this work. For full disclosure statements, refer to http://dx.doi.org/10.1016/j.arth.2015.09.011. Reprint requests: Timothy Wright, PhD, Department of Biomechanics, Hospital for Special Surgery, 535 East 70th St, New York, NY 10021. http://dx.doi.org/10.1016/j.arth.2015.09.011 0883-5403/© 2016 Elsevier Inc. All rights reserved.
Only one other retrieval analysis of HF TKA components has been reported, in which the performance of HF tibial inserts of a single manufacturer was compared with PS inserts of the same design and manufacturer [12] based on subjective grading of polyethylene surface damage. We sought to extend retrieval analysis to compare damage among PS and HF inserts from different manufacturers to explore whether the different design changes would create differences in polyethylene damage. The goals of this study were to determine if (1) damage modes differ in HF designs from those observed in traditional PS designs, (2) differences in damage exist among different HF designs, and (3) clinical ROM affects damage among different HF and PS designs. Materials and Methods We matched 20 HF polyethylene inserts from each of 3 manufacturers to 20 traditional PS inserts from the same manufacturers, all of which were collected as part of our ongoing institutional review board–approved implant retrieval program. Twenty Zimmer (Warsaw, IN) LPS inserts were matched to 20 LPS-Flex inserts; 20 Exactech (Gainesville, FL) Optetrak PS inserts were matched to 20 Optetrak Logic inserts; and 20 Smith & Nephew (Memphis, TN) Genesis II PS inserts were matched to 20 Genesis II High-Flex inserts. Matching was performed based on length of implantation (LOI; ±9 months) and patient sex and body mass index (BMI; ±5 kg/m2; Table 1). We decided not to match on revision diagnosis and ROM in an effort to keep the number of implants in each cohort as large as possible from those available from our implant retrieval system.
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496
Table 1 Patient Demographics (Mean [Range]) Among the HF and PS Implant Designs. Zimmer
Age (y) BMI (kg/m2) LOI (mo) Sex (% female)
Exactech
Smith & Nephew
LPS-Flex
LPS
Logic (HF)
Optetrak (PS)
Genesis II (HF)
Genesis II (PS)
62 (39-81) 33 (25-53) 22 (1-47) 65
61 (32-82) 32 (24-48) 21 (1-41) 65
63 (49-79) 30 (24-38) 17 (3-29) 60
66 (47-66) 31 (23-41) 17 (3-38) 60
68 (50-105) 29 (24-38) 17 (0-52) 55
64 (47-82) 31 (22-42) 19 (1-52) 55
P
.39 .27 .67 .57
Matching was performed based on BMI, LOI, and patient sex.
All inserts were composed of conventional, gamma-irradiated ultrahigh-molecular-weight polyethylene. Exactech components (Optetrak PS and Optetrak Logic) were direct compression molded, whereas Smith & Nephew (Genesis II PS and Genesis II HF) and Zimmer (NexGen LPS and NexGen LPS-Flex) inserts were machined from extruded stock. The clinical medical record of each patient was reviewed to gather demographic variables, the revision diagnosis, and the pre-revision ROM. Sixty-one inserts were implanted at our institution, with the remaining 59 implanted elsewhere. All 120 inserts were revised at our institution by 24 surgeons. The dominant reasons for revision were instability, stiffness, infection, and loosening (Table 2). The breakdown of revision diagnoses did not differ (P = .39) among the 6 groups (3 manufacturers × 2 design types). When multiple ROM values existed in the medical record, the maximum flexion reached by the patient was used as the best-case performance of the TKA. The location and severity of surface damage on the inserts should be a reflection of the largest ROM reached by the patient throughout the duration of implantation. Each of the implants was subjectively graded for polyethylene surface damage using a modified version of the Hood grading scale [13]. Grading was done under low power (× 10) light stereomicroscopy (WILD Type 355110, Heerbrugg, Switzerland) by 2 independent graders blinded to the clinical information. The implants were evaluated for 7 damage modes: surface deformation, embedded debris, scratching, burnishing, delamination, pitting, and abrasion. The damage from each mode was assigned a score from 0 to 3, where 0 indicated no damage, 1 indicated damage to b10% of the zone, 2 indicated damage to 10% to 50% of the zone, and 3 indicated damage to N50% of the zone. Each mode was assessed in 14 zones across the superior surface of the tibial insert (Fig. 1). Zones 0 through 7 were located on the articular surfaces of the tibial plateaus, zone 8 was located in a noncontacting area (to be used in a subsequent study involving laser scanning of the components), and zones 9 through 13 were located on the PS post. Because our main focus was to compare HF to PS designs, the posterior articular zones (3 and 7) and zone 11 depicting the posterior post were of particular interest. Because design changes were made to the anterior post (zone 9) in the Exactech and Smith & Nephew HF designs compared with their PS counterparts, we also analyzed this region separately among the 3 manufacturers. The damage mode scores from all zones were combined to produce a total damage score for each insert with a maximum possible score of 294 (maximum score of 3 × 7 modes × 14 zones).
Statistical Analyses Interobserver reliability was assessed using Cronbach α coefficient, which was .91, denoting excellent consistency. Total damage scores were first compared between HF and PS groups (n = 60 for each). Further analysis was conducted to compare total damage scores between the subgroups of HF and PS inserts from each manufacturer. Each group was compared as a whole based on damage mode and total damage as well as by subgroup analysis examining the specific posterior zones (zones 3 and 7 and zone 11). When data were normally distributed, a t test was conducted when comparing means of 2 groups, and an analysis of variance with a post hoc Bonferonni correction was conducted when comparing multiple groups. When the test for normality failed, a Mann-Whitney rank sum test was used when comparing 2 groups or an analysis of variance with a Tukey post hoc test when comparing multiple groups. Categorical values such as gender were compared with a χ 2 analysis. Significance was set to P b .05. Data are presented as means ± SDs. Statistics were performed with SigmaPlot 12.3 (Systat Software Inc, San Jose, CA). Results No difference (P = .25) was found between the mean overall damage scores for the 60 HF inserts (37.1 ± 17.2) compared with those of the 60 PS inserts (40.1 ± 16.3). The dominant damage modes were deformation, scratching, burnishing, and pitting (Table 3). No significant differences were found when considering damage to the tibial post or damage to the articular surfaces and their relation to indication for revision (P = .3 for both comparisons). However, when analyzing the specific damage modes, significantly more scratching (P = .005) and pitting (P = .03) were seen in the PS designs when compared with the HF designs. No significant differences were seen in the other damage modes. Most importantly, no differences were observed in the total damage score (P = .06) or in the scores for any of the individual damage modes when comparing scores of the posterior articular surfaces between the 60 HF and 60 PS inserts (zones 3 and 7). Similarly, no differences (P = .11) were seen in total damage scores on the posterior post (zone 11) between HF and PS inserts. Nonetheless, PS designs had significantly more scratching (P b .001) and pitting (P b .001) on the posterior post, whereas the HF designs had more burnishing (P = .04). On the surface
Table 2 Reasons for Revision for the 120 TKAs Included in the 3 Matched Groups Shown in Table 1. Zimmer
Instability Stiffness Infection Loosening Pain/unknown/miscellaneous Allergy Total
Exactech
Smith & Nephew
LPS Flex
LPS
Logic
PS
4 (20%) 7 (35%) 3 (15%) 3 (15%) 0 (0%) 3 (15%) 20
8 (40%) 4 (20%) 3 (15%) 5 (25%) 0 (0%) 0 (0%) 20
7 (35%) 3 (15%) 3 (15%) 2 (10%) 5 (25%) 0 (0%) 20
7 35%) 7 (35%) 1 (5%) 3 (15%) 2 (10%) 0 (0%) 20
Genesis II HF
Genesis II PS
6 (30%) 1 (5%) 5 (25%) 2 (10%) 6 (30%) 0 (0%) 20
3 (15%) 3 (15%) 8 (40%) 2 (10%) 4 (20%) 0 (0%) 20
Total
35 25 23 17 17 3 120
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497
Table 4 HF and PS Damage Scores (Mean [SD]) Among 20 Matched Pairs of 3 Designs of Tibial Inserts. LPS-Flex vs LPS (Zimmer)
Total damage Deformation Debris Scratching Burnishing Delamination Pitting Abrasion
LPS-Flex (n = 20)
LPS (n = 20)
50.7 (12.1) 6.1 (3.3) 0.2 (0.4) 13.4 (3.6) 19.9 (9.1) 0 (0) 11.2 (3.2) 0.1 (0.5)
43.7 (10.1) 5.6 (2.2) 0.2 (0.6) 15.7 (4.3) 7.7 (5.4) 0 (0) 13.0 (3.8) 1.4 (1.6)
P .05 .7 .5 .7 b.001 1 .1 .002
Optetrak Logic vs Optetrak (Exactech) Fig. 1. Subjective grading of polyethylene surface damage was done on separate regions of the superior surface of the tibial insert, including the tibial post.
of the anterior post (zone 9), significantly more damage was seen in the PS inserts (P = .02). The significant difference reflected greater scores for embedded debris (P = .04), scratching (P b .001), and pitting (P b .001) in the PS inserts. When analyzing the overall damage of the inserts based on design (Table 4), the Zimmer LPS-Flex inserts had the greatest amount of damage and were significantly more damaged than their LPS counterparts (P = .05). The difference was driven by the fact that the Zimmer LPS-Flex inserts displayed considerably more abrasion near the top of zones 10 and 12 reflecting the medial and lateral sides of the post than did their matched LPS counterparts. Conversely, the Exactech Optetrak Logic (HF) inserts had the lowest amount of damage among the 6 designs and were significantly less damaged Table 3 HF and PS Damage Scores (Mean [SD]) for the Cohort of 120 Retrieved Tibial Inserts. HF (n = 60) Entire articular surface (zones 0 through 13) Total damage 37.1 (17.2) Deformation 4.3 (3.6) Debris 0.2 (0.5) Scratching 11.5 (5.4) Burnishing 10.9 (11.2) Delamination 0.0 (0.0) Pitting 9.5 (5.5) Abrasion 0.7 (1.1) Posterior articular surface (zones 3 and 7) Total damage 8.0 (3.87) Deformation 0.9 (1.51) Debris 0.03 (.1) Scratching 2.5 (1.3) Burnishing 2.0 (2.5) Delamination 0.0 (0.0) Pitting 2.3 (1.4) Abrasion 0.1 (0.4) Anterior post (zone 9) Total damage 1.9 (1.6) Deformation 0.9 (0.9) Debris 0.0 (0.0) Scratching 0.2 (0.4) Burnishing 0.5 (0.7) Delamination 0.0 (0.0) Pitting 0.1 (0.3) Abrasion 0.2 (0.5) Posterior post (zone 11) Total damage 2.4 (2.5) Deformation 0.7 (0.9) Debris 0.02 (0.1) Scratching 0.3 (0.5) Burnishing 1.0 (1.3) Delamination 0.0 (0.0) Pitting 0.1 (0.3) Abrasion 0.05 (0.2) Bold p-values denote significance at b0.05.
PS (n = 60)
P
40.1 (16.3) 5.7 (4.1) 0.3 (0.8) 14.1 (6.2) 8.0 (6.5) 0.0 (0.0) 11.8 (5.9) 1.1 (2.2)
.25 .06 .73 .005 .99 1.0 .03 .57
6.5 (2.9) 0.5 (0.8) 0.03 (0.1) 2.2 (1.1) 1.3 (1.8) 0.0 (0.0) 2.2 (1.3) 0.1 (0.5)
.06 .56 1.0 .06 .30 1.0 .72 .58
3.0 (2.4) 0.9 (0.9) 0.07 (0.2) 0.7 (0.5) 0.7 (0.7) 0.0 (0.0) 0.4 (0.6) 0.1 (0.5)
.02 .98 .04 b.001 .08 1.0 b.001 .25
2.7 (1.5) 0.8 (0.7) 0.02 (0.1) 0.8 (0.5) 0.4 (0.6) 0.0 (0.0) 0.5 (0.5) 0.1 (0.4)
.11 .42 1.0 b.001 .04 1.0 b.001 .4
Total damage Deformation Debris Scratching Burnishing Delamination Pitting Abrasion
Logic (n = 20)
Optetrak (n = 20)
24.0 (8.5) 1.2 (1.4) 0.4 (0.7) 10.7 (4.4) 0.2 (0.6) 0 (0) 10.1 (4.7) 1.5 (1.2)
47.0 (10.0) 8.6 (4.3) 0.7 (1.1) 16.9 (2.9) 4.9 (3.8) 0 (0) 15.1 (4.7) 0.9 (1.1)
P b.001 b.001 .32 b.001 .002 1 0.14 b.001
Genesis II High-Flex vs Genesis II (Smith & Nephew) Smith & Nephew
High-Flex (n = 20)
Genesis II (n = 20)
Total damage Deformation Debris Scratching Burnishing Delamination Pitting Abrasion
36.6 (18.0) 5.6 (3.5) 0.1 (0.5) 10.4 (7.2) 12.8 (10.0) 0 (0) 7.2 (7.2) 0.6 (1.1)
31.6 (22.1) 2.7 (3.4) 0 (0) 9.6 (7.6) 11.5 (7.9) 0 (0) 7.0 (5.8) 1.0 (3.4)
P .19 .002 .8 .7 .8 1 .8 .5
Bold p-values denote significance at b0.05.
than their Optetrak PS counterparts. A significant reduction in deformation, scratching, and burnishing, tempered somewhat by a significant increase in abrasion, in the Optetrak Logic inserts accounted for the difference. Although the Logic inserts demonstrated significantly less total damage than both of the Zimmer insert designs (P b .05), the damage was not significantly less than either of the Smith & Nephew insert designs. Furthermore, no significant difference was found when comparing the total damage between the Smith & Nephew Genesis II High-Flex and PS inserts, although a significant increase in deformation (P b .002) was noted for the High-Flex inserts. Whether comparing HF vs PS designs within or among manufacturers, the posterior regions of the articular surfaces (zones 3 and 7) and the posterior surface of the post (zone 11) are most relevant (Fig. 2). Although contact can occur in these zones when the knee is in lesser degrees of flexion, these zones are the primary contact points between the femoral and tibial components when the knee is in high degrees of flexion. Within each of the 3 manufacturers (matched pairs of n = 20), significant differences in damage scores were found between HF and PS versions for the posterior articular surfaces (Table 5). For 2 of the designs, Zimmer and Smith & Nephew, the HF inserts showed more damage (P b .01 and P b .02, respectively) than the matched PS inserts, whereas for Exactech, the HF inserts showed significantly less damage (P b .03) than the matched PS inserts. The increased damage for the Zimmer HF was dominated by increased scratching and burnishing scores, whereas for the Smith & Nephew HF, the difference was dominated by increased deformation. However, for the posterior post, only Exactech showed a significant difference between matched HF and PS inserts, with the HF inserts showing significantly less damage (P b .001). Range-of-motion data were available for 106 (88%) of the 120 knees. No significant differences (P = .6) were detected between the HF and PS implants with regard to mean flexion. Similarly, no differences were found in mean extension (P = .7) and total combined ROM (P = .6).
E.A. Schnaser et al. / The Journal of Arthroplasty 31 (2016) 495–500
Medial Post
Posterior Post
Posterior Articular Surface
Smith & Nephew Genesis II High-Flex
Exactech Optetral Logic
Zimmer LPS-Flex
498
Fig. 2. Photographs of the post surfaces and the posterior articular surfaces for retrieved HF inserts show the polyethylene damage modes that typically dominated in these regions: deformation, scratching, burnishing, and pitting.
High-flexion designs were intended to improve performance when patients had greater than 120° of flexion, and few of the patients in our study achieved this. Twenty-three percent of our patients achieved greater than 110° of flexion, with no differences found among the 6 groups: LPS Flex (5 patients), LPS (4), Optetrak Logic (6), Optetrak PS (4), Genesis II HF (4), and Genesis II PS (4). No significant differences were seen among the ROM achieved by the patients with different implant designs (Fig. 3). However, significant ROM differences (P b .05) were seen among indications for revision, but subsequent between group comparisons showed that this finding was driven by the ROM for patients revised for stiffness being substantially lower than those revised for instability.
data that demonstrate their efficacy exist, although many of the authors of studies examining HF TKAs made the erroneous claims that efficacy was not demonstrated because the use of the HF designs did not in and of itself increase ROM (e.g., [3]). No published studies have
Discussion High-flex total knee designs were developed to better accommodate more flexion than their standard PS counterparts. However, few clinical
Table 5 HF and PS Damage Scores (Mean [SD]) for the Posterior Articular Surfaces (Zones 3 and 7) and the Posterior Post (Zone 11) Among the 3 Designs of Tibial Inserts. HF (n = 20) Posterior articular surface (zones 3 and 7) Zimmer 10.0 (3.5) Exactech 5.6 (2.1) Smith & Nephew 8.4 (4.4) Posterior post (zone 11) Zimmer 3.6 (2.8) Exactech 0.3 (0.4) Smith & Nephew 3.6 (2.0) Bold p-values denote significance at b0.05.
PS (n = 20)
P
7.0 (2.9) 6.9 (1.9) 5.8 (3.7)
.01 .03 .02
2.3 (.08) 3.3 (1.2) 2.8 (2.2)
.20 .001 .12
Fig. 3. A box and whisker plot of the ROMs achieved by the patients from whom the tibial inserts were retrieved showed no differences among the 6 designs (LPS Flex, LPS, Optetrak Logic, Optetrak PS, Genesis II PS HF, and Genesis II PS). The box shows the median (the horizontal line within the box) and the upper and lower quartiles (the top and bottom of the box). The whiskers span 1 SD above and below the mean, and the x's denote outliers beyond the range of the whiskers.
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compared HF designs from different manufacturers. However, a previous retrieval study by Paterson et al [12] compared HF to PS Genesis II knees, 1 of the 3 TKA systems in our study. Like us, they found no difference in the articular surface damage scores or damage modes between PS and HF Genesis II retrieved inserts. They also found more post damage on the HF than the PS inserts; although we found higher damage scores for the Zimmer and Smith & Nephew posterior posts, the differences did not reach significance (Table 5). The strength of our study is the large number of retrieved implants that were examined and the ability to match the implants across 3 manufacturers and among contemporary PS and HF designs from the same implant systems. Matching on crucial clinical and demographic factors strengthens the study. Although we could not match patients on revision diagnosis, we were able to match the patient's LOI, sex, and BMI. Matching increased the likelihood that the differences observed in polyethylene surface damage on the tibial inserts are related to design differences. In answering our first question—what are the differences in damage modes associated with HF designs when compared with traditional PS designs?—no differences were observed in the types of polyethylene surface damage between the HF and PS inserts. Presence and severity of individual damage modes differed (eg, PS implants had a small but significant increase in scratching and pitting), but the overall damage scores were similar. These results demonstrate that, in general, the polyethylene tibial component surfaces behave similarly between HF and PS designs. This is not an unexpected finding. For the HF designs in our study, few, if any, changes in articular geometry were incorporated in regions that would be in contact through much of the ROM (b 125°130°). Furthermore, changes were made in contact regions for higher flexion specifically intended to reduce polyethylene stresses, so little additional polyethylene damage would be expected in the posterior regions of the articular surfaces (Table 3). If these changes had been ineffective, we would have observed considerably more damage in the posterior zones (3 and 7) or damage modes such as delamination, long associated with fatigue wear caused by large cyclic polyethylene stresses [14]; instead, no delamination was observed in these regions. Our second research question was as follows: what are the differences in damage modes between the HF and traditional PS designs from different manufacturers? Unlike the aggregate analysis used to answer our first question, significant differences in surface damage scores did emerge when comparing HF and PS inserts within the 3 manufacturers' designs. The largest difference was with the Exactech Optetrak Logic HF design. The overall damage in this design was significantly lower than its PS counterpart, although these scores may be biased by the fact that the tibial inserts are compression molded from polyethylene, creating a shiny surface, potentially masking the appearance of burnishing. However, this design had significant changes from the PS version in both the tibial insert design and the femoral component that were an evolution in the design rather than simply a design intended solely to accommodate HF; in that regard, the matched pairs reflect design differences not solely intended to accommodate increased flexion. Nonetheless, the tibial post in the Exactech HF design has eyelets on the posterior condyles to accommodate the femoral component in deep flexion, and the post is more posteriorly placed and designed to allow the cam to ride up the back of the post throughout increasing flexion. Also, the anterior post is rounded to be more conforming with the femoral component in hyperextension [15]. On the femoral component, the radius of the posterior condyle was increased to allow deeper flexion, the cam was extended and made more conforming with the tibial insert, and the anterior box was made more conforming to the rounded anterior tibial post. The design changes in the post-box articulation led to a significant reduction in post damage in earlier work from our group [15], consistent with the findings from our current study. The Zimmer HF femoral design changes consisted of an extension of the posterior femoral condyles to allow for increase tibial rollback,
499
deepening and elongating of the anterior flange, and extension of the femoral cam to induce more rollback while resisting subluxation. Interestingly, the Smith & Nephew HF knee design differs from the PS design for the polyethylene tibial insert only; no changes were made to the femoral component. The Smith & Nephew HF insert has a deeper cutout for the patellar tendon and a chamfered anterior post, and the condyles are more conforming to the femoral component when compared with its PS counterpart. For both the Zimmer and Smith & Nephew designs, the HF inserts had more damage than the PS inserts in the posterior regions of the articular surfaces (zones 3 and 7). These findings, though statistically significant, may not be clinically significant. For example, the Zimmer HF design had higher burnishing and scratching scores, but this may not reflect more polyethylene wear. Instead, the increased score may reflect that more of the posterior zones (3 and 7) were damaged, consistent with a greater area over which contact occurs. For the Smith & Nephew HF design, the dominant difference with the PS version was deformation of the articular surface. Again, this may not relate to increased polyethylene wear, although the increased deformation may be associated with higher contact stresses. The increased deformation may reflect the fact that twice as many knees in the Smith & Nephew HF group were revised due to instability. These differences should be explored by measuring the 3-dimensional changes in the inserts in relation to pristine versions of the same inserts. Another consideration in interpreting the results from these posterior regions is the fact that contact can occur between the femoral and tibial components in these regions during activities other than HF. In such situations, the contacting portion of the femoral component will not be the posterior condyles. Instability could also play a role in the pattern of posterior region damage. For example, unstable total knees may show greater AP and coronal plane translations throughout flexion, not just at HF angles, so that differences between unstable and stiff knees could be masked within the damage observed on the retrieved inserts. Techniques such as laser scanning [16] and computed tomographic imaging [17] could provide objective measurements of the deformation experienced by the polyethylene tibial components in these regions, which in turn would provide important clues as to the source of the posterior damage. A laser scanning study of our HF and PS retrieved tibial inserts is currently underway. Our final research question dealt with whether differences existed in clinical ROM among different HF and traditional PS designs. The patients in our study had a mean ROM less than 100°, lower than most published clinical studies using these implants [4,5,7,10,18–20]. Despite studies that suggest a benefit to HF designs [1,21], we did not find differences in the pre-revision ROMs between HF and PS designs. Of course, one cannot conclude from our data that HF designs do not allow more flexion than PS designs, because we do not know the ROMs for all the still functioning implants from which our retrieved implants are but a subset. Nonetheless, our results agree with other authors who failed to show a difference in ROM between HF and PS implants [4,5,7,18,19]. Range of motion after a TKA likely depends on multiple variables, such as the patient's own inflammatory response, activity level, compliance with rehabilitation, preoperative ROM, and appropriate pain management postoperatively. Regardless, our data show that all 6 implant designs had a similar ROM and a similar number of patients who had achieved greater than 110° of flexion postoperatively. Therefore, the differences that we observed in damage scores were not due to differences in ROM. Because few patients in our study had greater than 120° of motion, we could not determine the impact of HF designs on patients who achieved HF. Our study has several limitations. First, this is a retrieval study of implants that were revised for a variety of reasons, and thus, these implants may not be representative of well-functioning implants. A strength of our study is that all 6 implant designs were matched by LOI, sex, and BMI, but numerous reasons for revision existed among the cases from which the implants were retrieved, and we could not
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match based on reason for revision and still maintain sufficient numbers to power our comparisons. Notably, higher rates on instability were found in some subgroups. Dolan et al [22] demonstrated in a regression analysis of tibial post damage that patients revised for instability or aseptic loosening had more tibial post damage than did those patient revised for infection or stiffness, so the differences in the posterior post scores in our study might have been influenced by differences in diagnosis. A second limitation is that we could not extend the matching to include pre-revision ROM. We obtained pre-revision ROM on only 88% of our retrieved implants, making matching on ROM impossible to achieve and still maintain large numbers of specimens. Our average ROM for both HF and PS knee were less than other published reports for these types of implants [4,5,7,10,18–20], and the generalizability to patients who have greater than 120° of flexion is limited. Our data were captured retrospectively, so they may be susceptible to inaccuracy in ROM capture. Dennis et al [1] published that high-flex designs may be beneficial in patients with lower preoperative ROM when compared with the PS design in primary TKA. We did not capture primary preoperative ROM in our cohort because many of the primary TKAs in our group were done at outside institutions. In conclusion, our data show no difference in the overall damage scores for HF and PS inserts and also show that implant damage is dependent on implant design characteristics. When HF and PS knees are compared, changes on both the femoral and tibial components must be considered in understanding the resulting differences in polyethylene damage. Although retrievals with greater flexion and longer implantation times should be examined as they become available, our results show little meaningful differences between HF and their PS counterparts.
Acknowledgments We acknowledge funding from the Mary and Fred Trump Institute for Implant Analysis.
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