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Effect of Total Knee Prosthesis Design on Patellar Tracking and Need for Lateral Retinacular Release
The Journal of Arthroplasty 28 (2013) 772–777
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Effect of Total Knee Prosthesis Design on Patellar Tracking and Need for Lateral Retinacular Release Kim C. Bertin MD , Weston W.S. Lloyd MPH Hofmann Arthritis Institute, Salt Lake City, Utah
a r t i c l e
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Article history: Received 27 August 2012 Accepted 25 December 2012 Keywords: total knee arthroplasty lateral retinacular release lateral release prosthesis design
a b s t r a c t Intraoperative lateral retinacular release (LRR) during primary total knee arthroplasty (TKA) is discouraged, except when LRR is necessary to centralize patellofemoral tracking. This study compares the LRR rates in four designs of total knee implants and correlates how changes in prosthesis design affect LRR rates. 2881 primary TKAs performed by one surgeon using a single surgical technique were reviewed. After controlling for all variables, LRR rates dropped from 71.6% to 19.5% to 9.7% to 2.7% with each design change (P b .0001). Differences in varus/valgus alignment and male/female proportions were compared in each group and the differences did not correlate with LRR rates. This study concludes that changes and improvements in knee implant designs play a significant role in decreasing lateral retinacular release rates in TKA. © 2013 Elsevier Inc. All rights reserved.
Over the past 30 years, total knee arthroplasty (TKA) devices have evolved from basic, mechanical replacements to complex, anatomic reproductions [1]. Advances in knee design and surgical technique have served to improve patellofemoral tracking, lower the incidence of intraoperative lateral retinacular release (LRR), and reduce extensor mechanism complications following TKA [2–8]. The relationship between intraoperative lateral retinacular release and patellofemoral complications following TKA is contentious [9]. Various studies demonstrate that, if needed, LRR improves the tracking of the patellar component during TKA [8,10–14]. Proper patellar tracking results in a decreased occurrence of patellar subluxation and/or dislocation [5-7,15–18]. Proper patellar tracking also helps minimize prosthesis loosening, component wear, and soft tissue impingement [2,3,7,15]. However, LRR is believed to be associated with numerous complications, such as decreased patellar blood flow, compromised wound healing, patellar osteonecrosis, and anterior knee pain [5,19–25]. Studies usually show that lateral retinacular releases are also associated with an increase in patellar fracture rates [17,23,25–30]. Consensus is that the patella must track properly, and due to negative effects of LRR, the performance of intraoperative LRR has been discouraged, except in cases where it is necessary to treat significant patellar maltracking [13,26,31]. Numerous surgical techniques help minimize the need for intraoperative lateral retinacular releases. Such methods include the
Research funding was provided by Zimmer. The Conflict of Interest statement associated with this article can be found at http:// dx.doi.org/10.1016/j.arth.2012.12.012. Reprint requests: Kim C. Bertin, MD, Institution: Hofmann Arthritis Institute, 723 Mont Clair Drive, North Salt Lake, UT 84054. 0883-5403/2805-0012$36.00/0 – see front matter © 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.arth.2012.12.012
subvastus approach, external rotation of the femoral component, and tourniquet release prior to performing LRR [2,5,32–40]. In addition, the “rule of no thumb” test is recommended to quantify the possible need for lateral retinacular release [9,41,42]. While various studies have analyzed the impact of surgical techniques on patellar tracking and extensor mechanism complications, this is the first study that investigates the influence that specific changes in knee design have on LRR rates. The design features of four different knee devices are compared, and this study postulates which features are most important in reducing the incidence of intraoperative lateral retinacular releases. Materials and Methods The study population consists of a consecutive series of 2483 patients (3533 TKAs) who underwent primary total knee arthroplasty performed by a single surgeon (KCB) over a 22-year period (September, 1989–June, 2011). Surgical and clinical data were prospectively collected at the time of surgery, and then retrospectively analyzed in 2012. Informed patient consent and institutional review board approval was obtained for this cross-sectional, comparative study. Each patient received one of the four following knee systems: • • • •
Miller-Galante II Cruciate-Retaining implant (1989–1995) — “MGII” NexGen Cruciate Retaining implant (1994–2003) — “CR” NexGen CR-Flex Fixed Bearing implant (2002–present) — “Flex” Gender Solutions NexGen High-Flex implant (2006–present) — “Gender”
Each patient underwent a TKA performed with the same surgical technique: anterior referencing, measured resections, intramedullary
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773
Surgical Procedure
Table 1 Exclusion Criteria for Primary TKAs. Exclusion Criteria
N (Knees)
1. TKA without patellar replacement 2. Femoral bone prep instrumentation that used: a. Posterior referencing b. Femoral epicondylar axis to reference the tibial cut c. Minimally invasive surgery 3. Extramedullary femoral prep alignment 4. Patellar placement that was: a. Central b. Superomedial 5. TKA devices other than MGII, CR, FLEX, GENDER, such as: a. Constrained Condylar Knee device (CCK) b. Cruciate Retaining Augmentable device (CRA) c. Legacy Posterior Stabilized device (LPS) d. Other posterior-stabilizing devices Total knees excluded from the study
72 142 7 9 22 97 8 59 30 174 32 652
femoral alignment, and rotational control of the femoral component. Patellar placement and thickness data were not routinely recorded until October 2001. Therefore, after this date, inclusion criteria also consisted of medial patellar placement and patellar thickness differences of ≤ 2 mm (preoperative to postoperative). Revision and unicompartmental knee arthroplasties were excluded from the study. Additional exclusions are listed in Table 1. From the original cohort of 2483 patients (3533 TKAs) who underwent primary TKAs during the 23-year study period, 2106 patients (2881 TKAs) met the above criteria, and were included in this study. Patient demographics are summarized in Table 2.
Design Features of the Studied Implants The four knee designs studied are sequential products of one manufacturer. They incorporate, as features, evolutionary changes thought by the manufacturer to be improvements in design, particularly in relation to the morphology of the patellofemoral joint. The shape, angle, width, and depth of the anterior flange changed between designs and these modifications directly impact patellofemoral tracking [43]. The patellar component was an identical axisymmetric dome for the last three implant designs; however, the MGII was unique, having a flat periphery and a central raised eminence. Anterior–posterior sizing was similar in all designs, with size increments of 4 mm. Design dimensions are illustrated in Fig. 1 and summarized in Table 3.
Each total knee arthroplasty was performed by the senior author (KCB), and the same basic surgical technique was used in all patients. A posterior cruciate ligament preserving prosthesis was used in all TKAs. Surgeries were usually performed under spinal anesthetic and tourniquet control. An anterior midline skin incision consistently preceded a medial parapatellar arthrotomy. Intramedullary instrumentation was used for alignment of the distal femoral cut. Measured resection was used to determine the amount of distal femoral resection. Anterior referencing for size was used in all patients. If the distal femur was between sizes, the smaller femoral component size was chosen. Over the period of time covered in this paper, the shapes and sizes of the instruments changed but their purpose and application remained the same. For varus and neutral knees, proper femoral rotation was set at 3° external rotation to the posterior condylar line, or perpendicular to Whitesides line. For valgus knees, femoral rotation was set by referencing the transepicondylar axis or Whitesides line, and then positioning the prosthesis perpendicular to that reference. When establishing the mediolateral position of the femoral component, the component was lateralized, if possible, in order to improve patellar tracking. Usually the implant filled the mediolateral dimension and was centered on the femur. Tibial component rotation was determined by referencing the tibial A/P axis, defined as a line extending from the PCL to the junction of the medial and mid-thirds of the tibial tubercle and placing the implant perpendicular to that axis. The implantation of the patellar component was done to achieve optimal medialization. Using a caliper to measure preoperative and postoperative patellar thickness, appropriate care was taken to reproduce thickness within 2 mm. Cement was used for patellar fixation in all cases. After implantation of all components, tracking of patellofemoral articulation was assessed using the “rule of no thumb” test (no manual support from the surgeon's thumb to stabilize the patella in the trochlear groove) throughout the entire range of motion [42,44,45]. Any subluxation, dislocation, or visible elevation of the medial edge of the patellar component resulted in a positive test. In these cases, a lateral retinacular release was performed in order to optimize patellofemoral tracking and minimize patellar tilt. The threshold for lateral release did not change during the period being reported. Lateral retinacular releases were uniformly performed from the inside-out, approximately 1 cm from the margin of the patella. The lateral retinaculum was released in 1–2 cm sequential increments.
Table 2 Patient Demographics. MGII Patients Male Female Mean Age (SD) Male Female Operative Side Right Left Bilateral Pre-Op Alignmenta Varus Knees Valgus Knees Neutral Knees a
93 31 62 67.4 (8.8) 70.9 (5.1) 65.7 (9.7) 116 45 41 15 116 82 28 6
Percent 33.3% 66.7%
38.8% 35.3% 25.9% 70.7% 24.1% 5.2%
CR 807 278 529 69.1 (10.0) 69.5 (8.9) 68.9 (10.6) 1067 377 314 188 1067 695 299 73
Percent 34.4% 65.6%
35.3% 29.4% 35.2% 65.1% 28.0% 6.8%
Pre-operative Alignment: For this study, we used the following parameters: Varus: b 4° VALGUS alignment, or ANY VARUS alignment. Neutral: 4°–8° VALGUS alignment. Valgus: N 8° VALGUS alignment.
FLEX 603 322 281 68.0 (9.2) 66.8 (9.2) 69.5 (9.0) 760 249 275 118 760 530 168 62
Percent 53.4% 46.6%
32.8% 36.2% 31.1% 69.7% 22.1% 8.2%
GENDER 740 128 612 67.6 (8.8) 68.3 (8.7) 67.5 (8.9) 938 348 308 141 938 594 260 84
Percent 17.3% 82.7%
37.1% 32.8% 30.1% 63.3% 27.7% 9.0%
TOTAL 2106 710 1396 68.2 (9.3) 68.2 (8.8) 68.3 (9.6) 2881 1019 938 462 2881 1901 755 225
Percent 33.7% 66.3%
35.4% 32.6% 32.1% 66.0% 26.2% 7.8%
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knee alignment were then retrospectively compared and analyzed with SAS/STAT software. The intraoperative LRR rates of patients receiving each of the four knee devices were compared using the following statistical analyses. For categorical variables (i.e. prosthesis type and preoperative knee alignment), univariate analyses were conducted with chi-squared tests. Analysis of variance (ANOVA) and Tukey adjusted multi-testing was used for the comparison of averages (i.e. degrees of varus/valgus malalignment). Univariate analysis identified the factors that had an influence on the lateral retinacular release rate. Logistic regression was then used to classify these factors by odds ratios (OR) with 95% confidence intervals. Logistic regression was also used to analyze the effects of gender, knee type, malalignment, operative side, and age on the incidence of LRR. Two-sample proportions tests compared LRR rates and identified statistically significant differences. Statistical significance was defined by a P-value ≤ .05. Results Intraoperative Lateral Retinacular Release (LRR) Rates
Fig. 1. Location of implant dimensions.
Patellar tracking was reassessed between each incremental release until the patella sat properly (no patellar tilting or subluxation) in the trochlear groove throughout ROM. Any exposed bone on the lateral patellar facet was never resected. Partial releases were not distinguished from full releases for the purpose of this review. Therefore, if any portion of a release was performed, it was recorded as a patella requiring a release. Postoperatively, all patients received physical therapy, which began with full weight-bearing on the first day after surgery. Subsequent clinical evaluations were performed in a routine fashion.
Data Collection and Statistical Analysis All preoperative, intraoperative, and postoperative data were prospectively collected and stored in a research database, using PATS software (Patient Analysis and Tracking System; Axis Clinical Software). Parameters such as age, gender, operative side, and preoperative
Table 3 Implant Dimensions (in Millimeters) Based on Fig. 1.
MGII Implant — Size 5 Section A–A Section B–B Section C–C CR Implant — Size E Section A–A Section B–B Section C–C FLEX Implant — Size E Section A–A Section B–B Section C–C GENDER Implant — Size E Section A–A Section B–B Section C–C
A
B
C
D
66.86 57.92 15.48
7.47 6.22 3.70
N/Aa 2.91 2.18
7.47 6.22 5.86
66.37 55.43 38.02
7.75 6.11 3.42
2.17 4.51 1.95
7.75 6.18 4.38
64.57 54.61 36.04
7.75 6.16 4.73
2.87 4.65 3.48
7.75 6.17 5.53
59.37 49.25 33.13
7.75 5.92 2.99
2.87 4.22 2.52
7.75 6.05 3.21
a In the MGII knee, the patella support from the femoral component terminates anterior to the point of measurement; therefore, the A–A measurement is not included for this device.
From 1989 to 2011, LRR rates dropped from 71.6% (MGII devices) to 2.7% (Gender devices), demonstrating a 96% decrease in the incidence of lateral retinacular releases. With the advent of each new device, there was a statistically significant decrease in LRR rates (P b .0001). The overall LRR rate during this 22-year period was 13.5% (Table 4). A logistic regression model, controlling for age, gender, operative side, and knee alignment determined that patients receiving MGII knees were 13.2 times more likely to require a lateral retinacular release than those receiving CR implants. Patients receiving a CR implant were 1.9 times more likely to require an LRR than patients receiving Flex implants. Patients who received the Flex implant were 5.5 times more likely to require a lateral retinacular release than patients receiving Gender implants. Analysis of variance testing determined that there were no statistically significant differences in age (P = .6123) between patients in the four groups (MGII, CR, Flex, Gender). Likewise, the operative side (right, left, bilateral) was commonly distributed between the four groups with no statistically significant differences (P = .4151) (Table 2). Impact of Preoperative Varus/Valgus Alignment LRR rates of patients with valgus preoperative knee alignments were significantly higher (20.7%) than the LRR rates of patients with neutral (13.8%) or varus (10.7%) knee alignments (P b .0001). The proportions of valgus knees in each device group were similar, falling within six percentage points of each other (Table 5). Chi-squared testing showed that the only statistically significant difference in the proportions of pre-operative varus, valgus, and neutral alignments in the four groups was found when comparing the valgus proportion of the Flex group (22.1%) with the CR group (28.0%) and the Gender group (27.7%) (Table 5: Column 2). P-values were 0.004 and 0.007, respectively. The lower percentage of valgus knees in the Flex group is largely explained by demographics. There is a higher percentage of
Table 4 Lateral Retinacular Release Rates by Device.
MGII (1989–1995) CR (1994–2003) FLEX (2002–2011) GENDER (2006–2011) TOTAL
N (Procedures)
LRR
LRR Rate (LRR/N)
116 1067 760 938 2881
83 208 74 25 390
71.6% 19.5% 9.7% 2.7% 13.5%
K.C. Bertin, W.W.S. Lloyd / The Journal of Arthroplasty 28 (2013) 772–777 Table 5 Percentage of Varus, Valgus, and Neutral Knees by Device With Associated LRR Rate.
MGII Varus Valgus Neutral CR Varus Valgus Neutral FLEX Varus Valgus Neutral GENDER Varus Valgus Neutral TOTAL
N (% of Group Total)
LRR
LRR Rate (LRR/N)
116 82 (70.7%) 28 (24.1%) 6 (5.2%) 1067 695 (65.1%) 299 (28.0%) 73 (6.8%) 760 530 (69.7%) 168 (22.1%) 62 (8.2%) 938 594 (63.3%) 260 (27.7%) 84 (9.0%) 2881
83 54 24 5 208 106 85 17 74 36 30 8 25 7 17 1 390
71.6% 65.9% 85.7% 83.3% 19.5% 15.3% 28.4% 23.3% 9.7% 6.8% 17.9% 12.9% 2.7% 1.2% 6.5% 1.2% 13.5%
Preoperative Alignment: For this study, we used the following parameters: Varus: b 4° VALGUS alignment, OR ANY VARUS alignment. Neutral: 4°–8° VALGUS alignment. Valgus: N 8° VALGUS alignment.
Severity of Preoperative Varus/Valgus Deformities The average preoperative alignments among the four groups differed very little, in terms of valgus severity (Table 6). In fact, there was only one statistically significant difference between the mean valgus alignments of each of the four device groups. The CR group's valgus mean of 15.15° was significantly higher than the Flex group's valgus mean of 13.36° (P = .0038). Overall, there was no distinct trend in the severity of valgus deformities that could help explain the
Table 6 Severity of Preoperative Malalignment in Relation to LRR Rates.
Implant Type
Mean Alignment of Knees Requiring LRR (in Degrees)
MGII (all knees) Varus knees Valgus knees Neutral knees CR (all knees) Varus knees Valgus knees Neutral knees FLEX (all knees) Varus knees Valgus knees Neutral knees GENDER (all knees) Varus knees Valgus knees Neutral knees TOTAL (all knees in study)
1.84° valgus 3.20° varus 15.82° valgus 5.50° valgus 2.52° valgus 3.43° varus 15.15° valgus 6.01° valgus 0.69° valgus 4.00° varus 13.36° valgus 6.48° valgus 2.38° valgus 3.23° varus 14.03° valgus 5.95° valgus 1.96° valgus
3.07° valgus 2.89° varus 16.00° valgus 5.40° valgus 5.31° valgus 3.18° varus 15.86° valgus 5.53° valgus 4.78° valgus 2.94° varus 13.53° valgus 6.75° valgus 9.52° valgus 4.29° varus 15.29° valgus 8.00° valgus 5.00° valgus
significantly lower LRR rates that coincided with the development of each new device. Impact of Male/Female Ratio Differences in Each Device Group The LRR rate of female knees (16.2%) was nearly twice the LRR rate of male knees (8.4%). When classified by device type, each LRR rate among female knees was significantly higher than its corresponding male LRR rate (P b .0001) (Table 7). 79.2% of all lateral retinacular releases in this study were performed on women (309 of 390), whereas only 20.8% were performed on men (81 of 390). 82.7% of patients receiving Gender implants were women. This proportion of female patients is significantly higher than that of the other three groups (P b .0001) (Table 2). Since the introduction of the Gender implant in May 2006, 98.5% of women undergoing TKA received this device, whereas only 37.8% of men received the Gender implant. During this period of time, male patients who did not receive the Gender device were implanted with the Flex device. Discussion
males in the Flex group (53.4%) than any other group (Table 2); and men, on average, present with fewer valgus knees. Overall, there was no discernible pattern among the valgus proportions of each device group that could explain the significantly lower LRR rates that resulted with the advent of each new device.
Mean Alignment of All Knees (in Degrees)
775
This is the first clinical study to quantify the contribution of component design in lowering lateral retinacular release rates. The question of whether advancements in knee prosthetic design significantly affect patellofemoral tracking and contribute to decreased intraoperative LRR rates is currently pertinent because patellofemoral articulation remains a problematic aspect of TKA [46– 48]. Up to 50% of the indications for revision TKA surgery are caused by complications related to this part of the knee joint [7,37,49–51]. Research suggests that patellar maltracking and intraoperative lateral retinacular releases contribute to increases in patellofemoral problems, including avascular necrosis of the patella, patella fracture, anterior knee pain, and postoperative swelling [2-9,11,15,19-30,46]. Therefore, all contributors to this problem require delineation and attention to assure success. This study compares the designs of four total knee implants (MGII Cruciate-Retaining, NexGen CR, NexGen CR-Flex and NexGen Gender Solutions High-Flex), with particular observation to how changes in design affect the need for lateral retinacular release. The changes from the MG II to the NexGen CR design include a narrower and thinner anterior flange and a more curved and anatomic trochlear groove. This resulted in lateral retinacular release rates decreasing significantly from 71.6% to 19.5% (P b .0001). The implant design changes from the NexGen CR to the CR Flex include further narrowing and thinning of the anterior flange. This resulted in a further statistically
LRR Rate
Preoperative Alignment: For this study, we used the following parameters: Varus: b4° VALGUS alignment, OR ANY VARUS alignment. Neutral: 4°–8° VALGUS alignment. Valgus: N8° VALGUS alignment.
71.6% 65.9% 85.7% 83.3% 19.5% 15.3% 28.4% 23.3% 9.7% 6.8% 17.9% 12.9% 2.7% 1.2% 6.5% 1.2% 13.5%
Table 7 Male/Female LRR Rates by Device. Female Knees MGII CR Flex Gender FEMALE TOTALa Male Knees MGII CR Flex Gender MALE TOTALa
N (Procedures)
LRR
Female LRR Rates
77 696 357 781 1911
57 167 61 24 309
74.0% 24.0% 17.1% 3.1% 16.2%
N (Procedures)
LRR
Male LRR Rates
39 371 403 157 970
26 41 13 1 81
66.7% 11.1% 3.2% 0.6% 8.4%
a The figures from Table 7 do not match the patient demographics in Table 2 because in Table 7, we are comparing the number of TKAs; whereas in Table 2, we report the number of actual patients. In other words, if a female patient underwent a bilateral TKA, the results from each knee would be included in Table 7, but the patient would only be counted once among the “male” or “female” categories of Table 2.
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significant decrease in lateral retinacular release rates from 19.5% to 9.7% (P b .0001). The third design evolution involved changes from the NexGen CR Flex to the NexGen CR Flex Gender. The Gender implant demonstrated the lowest intraoperative LRR rate. The Gender implant was specifically designed to more accurately match the female anatomy of the distal femur [1,52]. The implant design changes include more closely approximating the thickness of the resected anterior condyle, increasing the patellar sulcus angle of the implant by 3°, and narrowing the femoral implant mediolaterally [53–55]. These design changes provided a further statistical improvement in LRR rates from 9.7% to 2.7% (P b .0001). Theiss et al demonstrated in a clinical follow-up study that lower postoperative patellofemoral complication rates were design dependent when comparing two designs of implants [56]. In our series, the use of earlier prosthetic models was associated with a higher probability of lateral retinacular release. Our findings demonstrate that the emergence of recent, improved TKA designs has resulted in decreased LRR rates. It would be anticipated that long term follow-up studies would show lower patellofemoral complication rates associated with these decreased lateral retinacular release rates. Valgus knees and female patients had higher rates of lateral retinacular releases in this study. Even in these subgroups, this study showed significantly lower release rates with each sequential design change (P b .0001). In fact, the Gender group, having the highest proportion of female patients (82.7%) and nearly the highest percentage of valgus knees (27.7%), demonstrated the lowest LRR rate (2.7%) of the four groups. This strengthens our hypothesis that device design is the variable most responsible for reducing LRR rates during TKA procedures. Additionally, since the number of valgus knees and the severity of preoperative varus/valgus deformities was similar in each group, the sequential reductions in LRR rates can be directly attributed to changes in prosthesis design. Patellar placement data and preoperative-postoperative reconstruction thickness data were not routinely collected prior to October 2001. Consequently, these data were available only when comparing the Flex and Gender groups, but was not used as inclusion or exclusion criteria for TKAs using MGII and CR implants. It is unlikely that reconstruction thickness and patellar positioning could have contributed to the higher LRR rates of both MGII and CR groups, as the surgeon's common technique did not change when newer designs were implanted. During the 22-year study interval, the standard surgical technique was to place the patellar prosthesis medially on the patella, and to attempt to perform an anatomic reconstruction of the patella. This study has a number of strengths. The four patient populations were comparable, both preoperatively and postoperatively. The procedures were performed by the same surgeon using a single, standard technique. Each prosthesis system had nearly identical alignment guides, and the judgment regarding femoral component rotation was the same. Thus, after controlling for many important technical variables that affect the patellofemoral joint, the central remaining variable was prosthesis design. Small changes in design can also have significant effects on knee function and long-term success. Because this study compared only four implant designs, the results of this comparison cannot be extrapolated to other implants. Another weakness of this study includes the fact that since all procedures were performed by only one surgeon, our study may demonstrate a potential systemic bias, and the same results may not be applicable to other surgeons. On the other hand, examining the results of only one surgeon allows for greater consistency in analysis. In this study, the operative procedure, surgical materials, and physician skillset remained constant, providing stronger comparisons of LRR rates. The retrospective nature of the current analysis may allow for the introduction of recall bias. Attempts to reduce this bias included data retrieval and statistical analysis performed by one independent
researcher and one independent statistician, neither of whom was present during any of the TKA procedures. All research and analysis were based on datasets that were prospectively collected by the senior author/surgeon at the time of surgery. It is also noted that these patients were not followed postoperatively for possible complications related to the patella or patellofemoral joint. It is anticipated that this will be the next phase of this study. Conclusions The current study demonstrates that knee design may be the key factor in influencing lateral release rates performed during primary TKA. The results show that with a single surgeon, using a single technique performed with proper femoral rotation, anterior referencing, medial patellar placement, and after controlling for patellar thickness differences, the main differentiating variable between the four knee implants studied is prosthesis design. This study concludes that changes and improvements in knee implant design have played a significant role in decreasing lateral retinacular release rates in TKA procedures. References 1. Booth RE. The gender specific (female) knee. Orthopedics 2006;29:768. 2. Yoshii I, Whiteside LA, Anouchi YS. The effect of patellar button placement and femoral component design on patellar tracking in total knee arthroplasty. Clin Orthop Relat Res 1992;275:211. 3. Grace JN, Rand JA. Patellar instability after total knee arthroplasty. Clin Orthop Relat Res 1988;237:184. 4. Andriacchi TP, Yoder D, Conley A, et al. Patellofemoral design influences function following total knee arthroplasty. J Arthroplasty 1997;12:243. 5. Chew JT, Stewart NJ, Hanssen AD, et al. Difference in patellar tracking and knee kinematics among three different total knee designs. Clin Orthop Relat Res 1997;345:87. 6. Berger RA, Crossett LS, Jacobs JJ, et al. Malrotation causing patellofemoral complications after total knee arthroplasty. Clin Orthop Relat Res 1998;356:144. 7. Rhoads DD, Noble PC, Reuben JD, et al. The effect of femoral component position on patellar tracking after total knee arthroplasty. Clin Orthop Relat Res 1990;260:43. 8. Lachiewicz PF, Soileau ES. Patella maltracking in posterior-stabilized total knee arthroplasty. Clin Orthop Relat Res 2006;452:155. 9. Weber AB, Worland RL, Jessup DE, et al. The consequences of lateral release in total knee replacement: a review of over 1000 knees with follow up between 5 and 11 years. Knee 2003;10:187. 10. Landon GC, Galante JO, Casini J. Essay on total knee arthroplasty. Clin Orthop Relat Res 1985;192:69. 11. Kusuma SK, Puri N, Lotke PA. Lateral retinacular release during primary total knee arthroplasty: effect on outcomes and complications. J Arthroplasty 2009;24(3): 383. 12. Clayton ML, Thompson TR, Mack RP. Correction of alignment deformities during total knee arthroplasties: staged soft-tissue releases. Clin Orthop Relat Res 1986;202:117. 13. Clayton ML, Thirupathi R. Patellar complications after total condylar arthroplasty. Clin Orthop Relat Res 1982;170:152. 14. Webster DA, Murray DG. Complications of variable axis total knee arthroplasty. Clin Orthop Relat Res 1985;193:160. 15. Brick GW, Scott RD. The patellofemoral component of total knee arthroplasty. Clin Orthop Relat Res 1988;231:163. 16. Ranawat CS. The patellofemoral joint in total condylar knee arthroplasty. Pros and cons based on five- to ten-year follow-up observations. Clin Orthop Relat Res 1986;205:93. 17. Figgie HE, Goldberg VM, Figgie MP, et al. The effect of alignment of the implant on fractures of the patella after condylar total knee arthroplasty. J Bone Joint Surg Am 1989;71:1031. 18. Merkow RL, Soudry M, Insall JN. Patellar dislocation following total knee replacement. J Bone Joint Surg Am 1985;67:1321. 19. Scuderi G, Scharf SC, Meltzer LP, et al. The relationship of lateral release to patella viability in total knee arthroplasty. J Arthroplasty 1987;2:209. 20. McMahon MS, Scuderi GR, Glashow JL, et al. Scintigraphic determination of patellar viability after excision of infrapatellar fad pad and/or lateral retinacular release in total knee arthroplasty. Clin Orthop Relat Res 1990;260:10. 21. Johnson DP, Eastwood DM. Lateral patellar release in knee arthroplasty. Effect on wound healing. J Arthroplasty 1992;7:427. 22. Rand JA. The patellofemoral joint in total knee arhtroplasty. J Bone Joint Surg Am 1994;76:612. 23. Scott RD, Turoff N, Ewald FC. Stress fracture of the patella following duopatellar total knee arthroplasty with patellar resurfacing. Clin Orthop Relat Res 1982;170: 147. 24. Brick GW, Scott RD. Blood supply to the patella. Significance in total knee arthroplasty. J Arthroplasty 1989;4(Suppl):S75.
K.C. Bertin, W.W.S. Lloyd / The Journal of Arthroplasty 28 (2013) 772–777 25. Boyd Jr AD, Ewald FC, Thomas WH, et al. Long-term complications after total knee arthroplasty with or without resurfacing of the patella. J Bone Joint Surg Am 1993;75:674. 26. Ritter MA, Pierce MJ, Zhou H, et al. Patellar complications (total knee arthroplasty). Effect of lateral release and thickness. Clin Orthop Relat Res 1999;367:149. 27. Ritter MA, Campbell ED. Postoperative patellar complications with or without lateral release during total knee arthroplasty. Clin Orthop Relat Res 1987;219:163. 28. Ritter MA, Herbst SA, Keating EM, et al. Patellofemoral complications following total knee arthroplasty. Effect of lateral release and sacrifice of the superior lateral geniculate artery. J Arthroplasty 1996;11:368. 29. Goldberg VM, Figgie III HE, Figgie MP. Technical considerations in total knee surgery. Management of patella problems. Orthop Clin North Am 1989;20:189. 30. Tria Jr AJ, Harwood DA, Alicea JA, et al. Patellar fractures in posterior stabilized knee arthroplasties. Clin Orthop Relat Res 1994;299:131. 31. Bindelglass DF, Vince KG. Patellar tilt and subluxation following subvastus and parapatellar approach in total knee arthroplasty. Management of patella problems. J Arthroplasty 1996;11:507. 32. Fauré BT, Benjamin JB, Lindsey B, et al. Comparison of the subvastus and paramedian surgical approaches in bilateral total knee arthroplasty. J Arthroplasty 1993;8:511. 33. Hofmann AA, Plaster RL, Murdock LE. Subvastus (Southern) approach for primary total knee arthroplasty. Clin Orthop Relat Res 1991;269:70. 34. Harwin SF. Patellofemoral complications in symmetrical total knee arthroplasty. J Arthroplasty 1998;13:753. 35. Engh GA, Parks NL, Ammeen DJ. Influence of surgical approach on lateral retinacular releases in total knee arthroplasty. Clin Orthop Relat Res 1996;331:56. 36. Sodha S, Kim J, McGuire KJ, et al. Lateral retinacular release as a function of femoral component rotation in total knee arthroplasty. J Arthroplasty 2004;19:459. 37. Archibeck MJ, Camarata D, Trauger J, et al. Indications for lateral retinacular release in total knee replacement. Clin Orthop Relat Res 2003;414:157. 38. Marson BM, Tokish JT. The effect of a tourniquet on intraoperative patellofemoral tracking during total knee arthroplasty. J Arthroplasty 1999;14:197. 39. Laskin RS. Lateral release rates after total knee arthroplasty. Clin Orthop Relat Res 2001;392:88. 40. Lombardi Jr AV, Berend KR, Mallory TH, et al. The relationship of lateral release and tourniquet deflation in total knee arthroplasty. J Knee Surg 2003;16:209. 41. Clifton R, Ng CY, Nutton RW. What is the role of lateral retinacular release? J Bone Joint Surg Am 2010;1:1.
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Effect of Total Knee Prosthesis Design on Patellar Tracking and Need for Lateral Retinacular Release Kim C. Bertin MD , Weston W.S. Lloyd MPH Hofmann Arthritis Institute, Salt Lake City, Utah
a r t i c l e
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Article history: Received 27 August 2012 Accepted 25 December 2012 Keywords: total knee arthroplasty lateral retinacular release lateral release prosthesis design
a b s t r a c t Intraoperative lateral retinacular release (LRR) during primary total knee arthroplasty (TKA) is discouraged, except when LRR is necessary to centralize patellofemoral tracking. This study compares the LRR rates in four designs of total knee implants and correlates how changes in prosthesis design affect LRR rates. 2881 primary TKAs performed by one surgeon using a single surgical technique were reviewed. After controlling for all variables, LRR rates dropped from 71.6% to 19.5% to 9.7% to 2.7% with each design change (P b .0001). Differences in varus/valgus alignment and male/female proportions were compared in each group and the differences did not correlate with LRR rates. This study concludes that changes and improvements in knee implant designs play a significant role in decreasing lateral retinacular release rates in TKA. © 2013 Elsevier Inc. All rights reserved.
Over the past 30 years, total knee arthroplasty (TKA) devices have evolved from basic, mechanical replacements to complex, anatomic reproductions [1]. Advances in knee design and surgical technique have served to improve patellofemoral tracking, lower the incidence of intraoperative lateral retinacular release (LRR), and reduce extensor mechanism complications following TKA [2–8]. The relationship between intraoperative lateral retinacular release and patellofemoral complications following TKA is contentious [9]. Various studies demonstrate that, if needed, LRR improves the tracking of the patellar component during TKA [8,10–14]. Proper patellar tracking results in a decreased occurrence of patellar subluxation and/or dislocation [5-7,15–18]. Proper patellar tracking also helps minimize prosthesis loosening, component wear, and soft tissue impingement [2,3,7,15]. However, LRR is believed to be associated with numerous complications, such as decreased patellar blood flow, compromised wound healing, patellar osteonecrosis, and anterior knee pain [5,19–25]. Studies usually show that lateral retinacular releases are also associated with an increase in patellar fracture rates [17,23,25–30]. Consensus is that the patella must track properly, and due to negative effects of LRR, the performance of intraoperative LRR has been discouraged, except in cases where it is necessary to treat significant patellar maltracking [13,26,31]. Numerous surgical techniques help minimize the need for intraoperative lateral retinacular releases. Such methods include the
Research funding was provided by Zimmer. The Conflict of Interest statement associated with this article can be found at http:// dx.doi.org/10.1016/j.arth.2012.12.012. Reprint requests: Kim C. Bertin, MD, Institution: Hofmann Arthritis Institute, 723 Mont Clair Drive, North Salt Lake, UT 84054. 0883-5403/2805-0012$36.00/0 – see front matter © 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.arth.2012.12.012
subvastus approach, external rotation of the femoral component, and tourniquet release prior to performing LRR [2,5,32–40]. In addition, the “rule of no thumb” test is recommended to quantify the possible need for lateral retinacular release [9,41,42]. While various studies have analyzed the impact of surgical techniques on patellar tracking and extensor mechanism complications, this is the first study that investigates the influence that specific changes in knee design have on LRR rates. The design features of four different knee devices are compared, and this study postulates which features are most important in reducing the incidence of intraoperative lateral retinacular releases. Materials and Methods The study population consists of a consecutive series of 2483 patients (3533 TKAs) who underwent primary total knee arthroplasty performed by a single surgeon (KCB) over a 22-year period (September, 1989–June, 2011). Surgical and clinical data were prospectively collected at the time of surgery, and then retrospectively analyzed in 2012. Informed patient consent and institutional review board approval was obtained for this cross-sectional, comparative study. Each patient received one of the four following knee systems: • • • •
Miller-Galante II Cruciate-Retaining implant (1989–1995) — “MGII” NexGen Cruciate Retaining implant (1994–2003) — “CR” NexGen CR-Flex Fixed Bearing implant (2002–present) — “Flex” Gender Solutions NexGen High-Flex implant (2006–present) — “Gender”
Each patient underwent a TKA performed with the same surgical technique: anterior referencing, measured resections, intramedullary
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773
Surgical Procedure
Table 1 Exclusion Criteria for Primary TKAs. Exclusion Criteria
N (Knees)
1. TKA without patellar replacement 2. Femoral bone prep instrumentation that used: a. Posterior referencing b. Femoral epicondylar axis to reference the tibial cut c. Minimally invasive surgery 3. Extramedullary femoral prep alignment 4. Patellar placement that was: a. Central b. Superomedial 5. TKA devices other than MGII, CR, FLEX, GENDER, such as: a. Constrained Condylar Knee device (CCK) b. Cruciate Retaining Augmentable device (CRA) c. Legacy Posterior Stabilized device (LPS) d. Other posterior-stabilizing devices Total knees excluded from the study
72 142 7 9 22 97 8 59 30 174 32 652
femoral alignment, and rotational control of the femoral component. Patellar placement and thickness data were not routinely recorded until October 2001. Therefore, after this date, inclusion criteria also consisted of medial patellar placement and patellar thickness differences of ≤ 2 mm (preoperative to postoperative). Revision and unicompartmental knee arthroplasties were excluded from the study. Additional exclusions are listed in Table 1. From the original cohort of 2483 patients (3533 TKAs) who underwent primary TKAs during the 23-year study period, 2106 patients (2881 TKAs) met the above criteria, and were included in this study. Patient demographics are summarized in Table 2.
Design Features of the Studied Implants The four knee designs studied are sequential products of one manufacturer. They incorporate, as features, evolutionary changes thought by the manufacturer to be improvements in design, particularly in relation to the morphology of the patellofemoral joint. The shape, angle, width, and depth of the anterior flange changed between designs and these modifications directly impact patellofemoral tracking [43]. The patellar component was an identical axisymmetric dome for the last three implant designs; however, the MGII was unique, having a flat periphery and a central raised eminence. Anterior–posterior sizing was similar in all designs, with size increments of 4 mm. Design dimensions are illustrated in Fig. 1 and summarized in Table 3.
Each total knee arthroplasty was performed by the senior author (KCB), and the same basic surgical technique was used in all patients. A posterior cruciate ligament preserving prosthesis was used in all TKAs. Surgeries were usually performed under spinal anesthetic and tourniquet control. An anterior midline skin incision consistently preceded a medial parapatellar arthrotomy. Intramedullary instrumentation was used for alignment of the distal femoral cut. Measured resection was used to determine the amount of distal femoral resection. Anterior referencing for size was used in all patients. If the distal femur was between sizes, the smaller femoral component size was chosen. Over the period of time covered in this paper, the shapes and sizes of the instruments changed but their purpose and application remained the same. For varus and neutral knees, proper femoral rotation was set at 3° external rotation to the posterior condylar line, or perpendicular to Whitesides line. For valgus knees, femoral rotation was set by referencing the transepicondylar axis or Whitesides line, and then positioning the prosthesis perpendicular to that reference. When establishing the mediolateral position of the femoral component, the component was lateralized, if possible, in order to improve patellar tracking. Usually the implant filled the mediolateral dimension and was centered on the femur. Tibial component rotation was determined by referencing the tibial A/P axis, defined as a line extending from the PCL to the junction of the medial and mid-thirds of the tibial tubercle and placing the implant perpendicular to that axis. The implantation of the patellar component was done to achieve optimal medialization. Using a caliper to measure preoperative and postoperative patellar thickness, appropriate care was taken to reproduce thickness within 2 mm. Cement was used for patellar fixation in all cases. After implantation of all components, tracking of patellofemoral articulation was assessed using the “rule of no thumb” test (no manual support from the surgeon's thumb to stabilize the patella in the trochlear groove) throughout the entire range of motion [42,44,45]. Any subluxation, dislocation, or visible elevation of the medial edge of the patellar component resulted in a positive test. In these cases, a lateral retinacular release was performed in order to optimize patellofemoral tracking and minimize patellar tilt. The threshold for lateral release did not change during the period being reported. Lateral retinacular releases were uniformly performed from the inside-out, approximately 1 cm from the margin of the patella. The lateral retinaculum was released in 1–2 cm sequential increments.
Table 2 Patient Demographics. MGII Patients Male Female Mean Age (SD) Male Female Operative Side Right Left Bilateral Pre-Op Alignmenta Varus Knees Valgus Knees Neutral Knees a
93 31 62 67.4 (8.8) 70.9 (5.1) 65.7 (9.7) 116 45 41 15 116 82 28 6
Percent 33.3% 66.7%
38.8% 35.3% 25.9% 70.7% 24.1% 5.2%
CR 807 278 529 69.1 (10.0) 69.5 (8.9) 68.9 (10.6) 1067 377 314 188 1067 695 299 73
Percent 34.4% 65.6%
35.3% 29.4% 35.2% 65.1% 28.0% 6.8%
Pre-operative Alignment: For this study, we used the following parameters: Varus: b 4° VALGUS alignment, or ANY VARUS alignment. Neutral: 4°–8° VALGUS alignment. Valgus: N 8° VALGUS alignment.
FLEX 603 322 281 68.0 (9.2) 66.8 (9.2) 69.5 (9.0) 760 249 275 118 760 530 168 62
Percent 53.4% 46.6%
32.8% 36.2% 31.1% 69.7% 22.1% 8.2%
GENDER 740 128 612 67.6 (8.8) 68.3 (8.7) 67.5 (8.9) 938 348 308 141 938 594 260 84
Percent 17.3% 82.7%
37.1% 32.8% 30.1% 63.3% 27.7% 9.0%
TOTAL 2106 710 1396 68.2 (9.3) 68.2 (8.8) 68.3 (9.6) 2881 1019 938 462 2881 1901 755 225
Percent 33.7% 66.3%
35.4% 32.6% 32.1% 66.0% 26.2% 7.8%
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knee alignment were then retrospectively compared and analyzed with SAS/STAT software. The intraoperative LRR rates of patients receiving each of the four knee devices were compared using the following statistical analyses. For categorical variables (i.e. prosthesis type and preoperative knee alignment), univariate analyses were conducted with chi-squared tests. Analysis of variance (ANOVA) and Tukey adjusted multi-testing was used for the comparison of averages (i.e. degrees of varus/valgus malalignment). Univariate analysis identified the factors that had an influence on the lateral retinacular release rate. Logistic regression was then used to classify these factors by odds ratios (OR) with 95% confidence intervals. Logistic regression was also used to analyze the effects of gender, knee type, malalignment, operative side, and age on the incidence of LRR. Two-sample proportions tests compared LRR rates and identified statistically significant differences. Statistical significance was defined by a P-value ≤ .05. Results Intraoperative Lateral Retinacular Release (LRR) Rates
Fig. 1. Location of implant dimensions.
Patellar tracking was reassessed between each incremental release until the patella sat properly (no patellar tilting or subluxation) in the trochlear groove throughout ROM. Any exposed bone on the lateral patellar facet was never resected. Partial releases were not distinguished from full releases for the purpose of this review. Therefore, if any portion of a release was performed, it was recorded as a patella requiring a release. Postoperatively, all patients received physical therapy, which began with full weight-bearing on the first day after surgery. Subsequent clinical evaluations were performed in a routine fashion.
Data Collection and Statistical Analysis All preoperative, intraoperative, and postoperative data were prospectively collected and stored in a research database, using PATS software (Patient Analysis and Tracking System; Axis Clinical Software). Parameters such as age, gender, operative side, and preoperative
Table 3 Implant Dimensions (in Millimeters) Based on Fig. 1.
MGII Implant — Size 5 Section A–A Section B–B Section C–C CR Implant — Size E Section A–A Section B–B Section C–C FLEX Implant — Size E Section A–A Section B–B Section C–C GENDER Implant — Size E Section A–A Section B–B Section C–C
A
B
C
D
66.86 57.92 15.48
7.47 6.22 3.70
N/Aa 2.91 2.18
7.47 6.22 5.86
66.37 55.43 38.02
7.75 6.11 3.42
2.17 4.51 1.95
7.75 6.18 4.38
64.57 54.61 36.04
7.75 6.16 4.73
2.87 4.65 3.48
7.75 6.17 5.53
59.37 49.25 33.13
7.75 5.92 2.99
2.87 4.22 2.52
7.75 6.05 3.21
a In the MGII knee, the patella support from the femoral component terminates anterior to the point of measurement; therefore, the A–A measurement is not included for this device.
From 1989 to 2011, LRR rates dropped from 71.6% (MGII devices) to 2.7% (Gender devices), demonstrating a 96% decrease in the incidence of lateral retinacular releases. With the advent of each new device, there was a statistically significant decrease in LRR rates (P b .0001). The overall LRR rate during this 22-year period was 13.5% (Table 4). A logistic regression model, controlling for age, gender, operative side, and knee alignment determined that patients receiving MGII knees were 13.2 times more likely to require a lateral retinacular release than those receiving CR implants. Patients receiving a CR implant were 1.9 times more likely to require an LRR than patients receiving Flex implants. Patients who received the Flex implant were 5.5 times more likely to require a lateral retinacular release than patients receiving Gender implants. Analysis of variance testing determined that there were no statistically significant differences in age (P = .6123) between patients in the four groups (MGII, CR, Flex, Gender). Likewise, the operative side (right, left, bilateral) was commonly distributed between the four groups with no statistically significant differences (P = .4151) (Table 2). Impact of Preoperative Varus/Valgus Alignment LRR rates of patients with valgus preoperative knee alignments were significantly higher (20.7%) than the LRR rates of patients with neutral (13.8%) or varus (10.7%) knee alignments (P b .0001). The proportions of valgus knees in each device group were similar, falling within six percentage points of each other (Table 5). Chi-squared testing showed that the only statistically significant difference in the proportions of pre-operative varus, valgus, and neutral alignments in the four groups was found when comparing the valgus proportion of the Flex group (22.1%) with the CR group (28.0%) and the Gender group (27.7%) (Table 5: Column 2). P-values were 0.004 and 0.007, respectively. The lower percentage of valgus knees in the Flex group is largely explained by demographics. There is a higher percentage of
Table 4 Lateral Retinacular Release Rates by Device.
MGII (1989–1995) CR (1994–2003) FLEX (2002–2011) GENDER (2006–2011) TOTAL
N (Procedures)
LRR
LRR Rate (LRR/N)
116 1067 760 938 2881
83 208 74 25 390
71.6% 19.5% 9.7% 2.7% 13.5%
K.C. Bertin, W.W.S. Lloyd / The Journal of Arthroplasty 28 (2013) 772–777 Table 5 Percentage of Varus, Valgus, and Neutral Knees by Device With Associated LRR Rate.
MGII Varus Valgus Neutral CR Varus Valgus Neutral FLEX Varus Valgus Neutral GENDER Varus Valgus Neutral TOTAL
N (% of Group Total)
LRR
LRR Rate (LRR/N)
116 82 (70.7%) 28 (24.1%) 6 (5.2%) 1067 695 (65.1%) 299 (28.0%) 73 (6.8%) 760 530 (69.7%) 168 (22.1%) 62 (8.2%) 938 594 (63.3%) 260 (27.7%) 84 (9.0%) 2881
83 54 24 5 208 106 85 17 74 36 30 8 25 7 17 1 390
71.6% 65.9% 85.7% 83.3% 19.5% 15.3% 28.4% 23.3% 9.7% 6.8% 17.9% 12.9% 2.7% 1.2% 6.5% 1.2% 13.5%
Preoperative Alignment: For this study, we used the following parameters: Varus: b 4° VALGUS alignment, OR ANY VARUS alignment. Neutral: 4°–8° VALGUS alignment. Valgus: N 8° VALGUS alignment.
Severity of Preoperative Varus/Valgus Deformities The average preoperative alignments among the four groups differed very little, in terms of valgus severity (Table 6). In fact, there was only one statistically significant difference between the mean valgus alignments of each of the four device groups. The CR group's valgus mean of 15.15° was significantly higher than the Flex group's valgus mean of 13.36° (P = .0038). Overall, there was no distinct trend in the severity of valgus deformities that could help explain the
Table 6 Severity of Preoperative Malalignment in Relation to LRR Rates.
Implant Type
Mean Alignment of Knees Requiring LRR (in Degrees)
MGII (all knees) Varus knees Valgus knees Neutral knees CR (all knees) Varus knees Valgus knees Neutral knees FLEX (all knees) Varus knees Valgus knees Neutral knees GENDER (all knees) Varus knees Valgus knees Neutral knees TOTAL (all knees in study)
1.84° valgus 3.20° varus 15.82° valgus 5.50° valgus 2.52° valgus 3.43° varus 15.15° valgus 6.01° valgus 0.69° valgus 4.00° varus 13.36° valgus 6.48° valgus 2.38° valgus 3.23° varus 14.03° valgus 5.95° valgus 1.96° valgus
3.07° valgus 2.89° varus 16.00° valgus 5.40° valgus 5.31° valgus 3.18° varus 15.86° valgus 5.53° valgus 4.78° valgus 2.94° varus 13.53° valgus 6.75° valgus 9.52° valgus 4.29° varus 15.29° valgus 8.00° valgus 5.00° valgus
significantly lower LRR rates that coincided with the development of each new device. Impact of Male/Female Ratio Differences in Each Device Group The LRR rate of female knees (16.2%) was nearly twice the LRR rate of male knees (8.4%). When classified by device type, each LRR rate among female knees was significantly higher than its corresponding male LRR rate (P b .0001) (Table 7). 79.2% of all lateral retinacular releases in this study were performed on women (309 of 390), whereas only 20.8% were performed on men (81 of 390). 82.7% of patients receiving Gender implants were women. This proportion of female patients is significantly higher than that of the other three groups (P b .0001) (Table 2). Since the introduction of the Gender implant in May 2006, 98.5% of women undergoing TKA received this device, whereas only 37.8% of men received the Gender implant. During this period of time, male patients who did not receive the Gender device were implanted with the Flex device. Discussion
males in the Flex group (53.4%) than any other group (Table 2); and men, on average, present with fewer valgus knees. Overall, there was no discernible pattern among the valgus proportions of each device group that could explain the significantly lower LRR rates that resulted with the advent of each new device.
Mean Alignment of All Knees (in Degrees)
775
This is the first clinical study to quantify the contribution of component design in lowering lateral retinacular release rates. The question of whether advancements in knee prosthetic design significantly affect patellofemoral tracking and contribute to decreased intraoperative LRR rates is currently pertinent because patellofemoral articulation remains a problematic aspect of TKA [46– 48]. Up to 50% of the indications for revision TKA surgery are caused by complications related to this part of the knee joint [7,37,49–51]. Research suggests that patellar maltracking and intraoperative lateral retinacular releases contribute to increases in patellofemoral problems, including avascular necrosis of the patella, patella fracture, anterior knee pain, and postoperative swelling [2-9,11,15,19-30,46]. Therefore, all contributors to this problem require delineation and attention to assure success. This study compares the designs of four total knee implants (MGII Cruciate-Retaining, NexGen CR, NexGen CR-Flex and NexGen Gender Solutions High-Flex), with particular observation to how changes in design affect the need for lateral retinacular release. The changes from the MG II to the NexGen CR design include a narrower and thinner anterior flange and a more curved and anatomic trochlear groove. This resulted in lateral retinacular release rates decreasing significantly from 71.6% to 19.5% (P b .0001). The implant design changes from the NexGen CR to the CR Flex include further narrowing and thinning of the anterior flange. This resulted in a further statistically
LRR Rate
Preoperative Alignment: For this study, we used the following parameters: Varus: b4° VALGUS alignment, OR ANY VARUS alignment. Neutral: 4°–8° VALGUS alignment. Valgus: N8° VALGUS alignment.
71.6% 65.9% 85.7% 83.3% 19.5% 15.3% 28.4% 23.3% 9.7% 6.8% 17.9% 12.9% 2.7% 1.2% 6.5% 1.2% 13.5%
Table 7 Male/Female LRR Rates by Device. Female Knees MGII CR Flex Gender FEMALE TOTALa Male Knees MGII CR Flex Gender MALE TOTALa
N (Procedures)
LRR
Female LRR Rates
77 696 357 781 1911
57 167 61 24 309
74.0% 24.0% 17.1% 3.1% 16.2%
N (Procedures)
LRR
Male LRR Rates
39 371 403 157 970
26 41 13 1 81
66.7% 11.1% 3.2% 0.6% 8.4%
a The figures from Table 7 do not match the patient demographics in Table 2 because in Table 7, we are comparing the number of TKAs; whereas in Table 2, we report the number of actual patients. In other words, if a female patient underwent a bilateral TKA, the results from each knee would be included in Table 7, but the patient would only be counted once among the “male” or “female” categories of Table 2.
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significant decrease in lateral retinacular release rates from 19.5% to 9.7% (P b .0001). The third design evolution involved changes from the NexGen CR Flex to the NexGen CR Flex Gender. The Gender implant demonstrated the lowest intraoperative LRR rate. The Gender implant was specifically designed to more accurately match the female anatomy of the distal femur [1,52]. The implant design changes include more closely approximating the thickness of the resected anterior condyle, increasing the patellar sulcus angle of the implant by 3°, and narrowing the femoral implant mediolaterally [53–55]. These design changes provided a further statistical improvement in LRR rates from 9.7% to 2.7% (P b .0001). Theiss et al demonstrated in a clinical follow-up study that lower postoperative patellofemoral complication rates were design dependent when comparing two designs of implants [56]. In our series, the use of earlier prosthetic models was associated with a higher probability of lateral retinacular release. Our findings demonstrate that the emergence of recent, improved TKA designs has resulted in decreased LRR rates. It would be anticipated that long term follow-up studies would show lower patellofemoral complication rates associated with these decreased lateral retinacular release rates. Valgus knees and female patients had higher rates of lateral retinacular releases in this study. Even in these subgroups, this study showed significantly lower release rates with each sequential design change (P b .0001). In fact, the Gender group, having the highest proportion of female patients (82.7%) and nearly the highest percentage of valgus knees (27.7%), demonstrated the lowest LRR rate (2.7%) of the four groups. This strengthens our hypothesis that device design is the variable most responsible for reducing LRR rates during TKA procedures. Additionally, since the number of valgus knees and the severity of preoperative varus/valgus deformities was similar in each group, the sequential reductions in LRR rates can be directly attributed to changes in prosthesis design. Patellar placement data and preoperative-postoperative reconstruction thickness data were not routinely collected prior to October 2001. Consequently, these data were available only when comparing the Flex and Gender groups, but was not used as inclusion or exclusion criteria for TKAs using MGII and CR implants. It is unlikely that reconstruction thickness and patellar positioning could have contributed to the higher LRR rates of both MGII and CR groups, as the surgeon's common technique did not change when newer designs were implanted. During the 22-year study interval, the standard surgical technique was to place the patellar prosthesis medially on the patella, and to attempt to perform an anatomic reconstruction of the patella. This study has a number of strengths. The four patient populations were comparable, both preoperatively and postoperatively. The procedures were performed by the same surgeon using a single, standard technique. Each prosthesis system had nearly identical alignment guides, and the judgment regarding femoral component rotation was the same. Thus, after controlling for many important technical variables that affect the patellofemoral joint, the central remaining variable was prosthesis design. Small changes in design can also have significant effects on knee function and long-term success. Because this study compared only four implant designs, the results of this comparison cannot be extrapolated to other implants. Another weakness of this study includes the fact that since all procedures were performed by only one surgeon, our study may demonstrate a potential systemic bias, and the same results may not be applicable to other surgeons. On the other hand, examining the results of only one surgeon allows for greater consistency in analysis. In this study, the operative procedure, surgical materials, and physician skillset remained constant, providing stronger comparisons of LRR rates. The retrospective nature of the current analysis may allow for the introduction of recall bias. Attempts to reduce this bias included data retrieval and statistical analysis performed by one independent
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