The Journal of Arthroplasty

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1 The Journal of Arthroplasty 28 (2013) Contents lists available at SciVerse ScienceDirect The Journal of Arthroplasty journal homepage: 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 article info abstract Article history: Received 27 August 2012 Accepted 25 December 2012 Keywords: total knee arthroplasty lateral retinacular release lateral release prosthesis design 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 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 (Pb.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 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 dx.doi.org/ /j.arth Reprint requests: Kim C. Bertin, MD, Institution: Hofmann Arthritis Institute, 723 Mont Clair Drive, North Salt Lake, UT 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 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 ( ) MGII NexGen Cruciate Retaining implant ( ) 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 / $36.00/0 see front matter 2013 Elsevier Inc. All rights reserved.

2 K.C. Bertin, W.W.S. Lloyd / The Journal of Arthroplasty 28 (2013) Table 1 Exclusion Criteria for Primary TKAs. Exclusion Criteria N (Knees) 1. TKA without patellar replacement Femoral bone prep instrumentation that used: a. Posterior referencing 142 b. Femoral epicondylar axis to reference the tibial cut 7 c. Minimally invasive surgery 9 3. Extramedullary femoral prep alignment Patellar placement that was: a. Central 97 b. Superomedial 8 5. TKA devices other than MGII, CR, FLEX, GENDER, such as: a. Constrained Condylar Knee device (CCK) 59 b. Cruciate Retaining Augmentable device (CRA) 30 c. Legacy Posterior Stabilized device (LPS) 174 d. Other posterior-stabilizing devices 32 Total knees excluded from the study 652 femoral alignment, and rotational control of the femoral component. Patellar placement and thickness data were not routinely recorded until October 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. Surgical Procedure 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 Percent CR Percent FLEX Percent GENDER Percent TOTAL Percent Patients Male % % % % % Female % % % % % Mean Age (SD) 67.4 (8.8) 69.1 (10.0) 68.0 (9.2) 67.6 (8.8) 68.2 (9.3) Male 70.9 (5.1) 69.5 (8.9) 66.8 (9.2) 68.3 (8.7) 68.2 (8.8) Female 65.7 (9.7) 68.9 (10.6) 69.5 (9.0) 67.5 (8.9) 68.3 (9.6) Operative Side Right % % % % % Left % % % % % Bilateral % % % % % Pre-Op Alignment a Varus Knees % % % % % Valgus Knees % % % % % Neutral Knees 6 5.2% % % % % a 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.

3 774 K.C. Bertin, W.W.S. Lloyd / The Journal of Arthroplasty 28 (2013) 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. A B C D MGII Implant Size 5 Section A A N/A a 7.47 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 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 (Pb.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 (Pb.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 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. N (Procedures) LRR LRR Rate (LRR/N) MGII ( ) % CR ( ) % FLEX ( ) % GENDER ( ) % TOTAL %

4 K.C. Bertin, W.W.S. Lloyd / The Journal of Arthroplasty 28 (2013) Table 5 Percentage of Varus, Valgus, and Neutral Knees by Device With Associated LRR Rate. N (% of Group Total) LRR LRR Rate (LRR/N) MGII % Varus 82 (70.7%) % Valgus 28 (24.1%) % Neutral 6 (5.2%) % CR % Varus 695 (65.1%) % Valgus 299 (28.0%) % Neutral 73 (6.8%) % FLEX % Varus 530 (69.7%) % Valgus 168 (22.1%) % Neutral 62 (8.2%) % GENDER % Varus 594 (63.3%) 7 1.2% Valgus 260 (27.7%) % Neutral 84 (9.0%) 1 1.2% TOTAL % 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. 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. 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 was significantly higher than the Flex group's valgus mean of (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 All Knees (in Degrees) Mean Alignment of Knees Requiring LRR (in Degrees) LRR Rate MGII (all knees) 1.84 valgus 3.07 valgus 71.6% Varus knees 3.20 varus 2.89 varus 65.9% Valgus knees valgus valgus 85.7% Neutral knees 5.50 valgus 5.40 valgus 83.3% CR (all knees) 2.52 valgus 5.31 valgus 19.5% Varus knees 3.43 varus 3.18 varus 15.3% Valgus knees valgus valgus 28.4% Neutral knees 6.01 valgus 5.53 valgus 23.3% FLEX (all knees) 0.69 valgus 4.78 valgus 9.7% Varus knees 4.00 varus 2.94 varus 6.8% Valgus knees valgus valgus 17.9% Neutral knees 6.48 valgus 6.75 valgus 12.9% GENDER (all knees) 2.38 valgus 9.52 valgus 2.7% Varus knees 3.23 varus 4.29 varus 1.2% Valgus knees valgus valgus 6.5% Neutral knees 5.95 valgus 8.00 valgus 1.2% TOTAL (all knees in study) 1.96 valgus 5.00 valgus 13.5% 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. 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 (Pb.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 (Pb.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 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% (Pb.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 Table 7 Male/Female LRR Rates by Device. Female Knees N (Procedures) LRR Female LRR Rates MGII % CR % Flex % Gender % FEMALE TOTAL a % Male Knees N (Procedures) LRR Male LRR Rates MGII % CR % Flex % Gender % MALE TOTAL a % 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.

5 776 K.C. Bertin, W.W.S. Lloyd / The Journal of Arthroplasty 28 (2013) significant decrease in lateral retinacular release rates from 19.5% to 9.7% (Pb.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% (Pb.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 (Pb.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 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: 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: Grace JN, Rand JA. Patellar instability after total knee arthroplasty. 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