Orthopedics

Feature Article 

Can High-Flexion Tibial Inserts Improve Range of Motion After Posterior Cruciate-Retaining Total Knee Arthroplasty?

Bradley D. Crow, MD; Julie C. McCauley, MPHc; Kace A. Ezzet, MD

  • Orthopedics. 2010;33(9)
  • Posted September 1, 2010

Abstract

Interest in high-flexion total knee arthroplasty (TKA) prostheses designed to provide better postoperative range of motion (ROM) is widespread. We sought to determine whether changes in surface geometry of the tibial polyethylene insert could improve postoperative ROM in a consecutive series of patients undergoing TKA with retention of the posterior cruciate ligament (PCL). Two cohorts with Smith & Nephew (Memphis, Tennessee) Posterior Cruciate-Retaining Genesis II total knee prostheses were compared, 79 knees (65 patients) using standard tibial inserts and 85 knees (72 patients) using high-flexion inserts. The standard insert has a slightly raised posterior lip, whereas the high-flexion insert is recessed downward at the posterior margin to facilitate femoral rollback in flexion and eliminate impingement of the femoral component on the back of the polyethylene during rollback.

Mean ROM 1 year postoperatively was 112.0° in patients receiving the standard insert and 119.3° in patients receiving the high-flexion insert. Preoperative ROM was similar in both groups. Flexion improvement in the high-flexion group over the standard insert group was statistically significant (P<.001). Final Knee Society Scores did not differ among patients receiving the standard and high-flexion inserts.

Our study demonstrates that improved postoperative flexion can be achieved without changing surgical technique, bony cuts, or metallic prosthetic parts. This is the first report that we are aware of that documents improvement in ROM after PCL-retaining TKA through the use of high-flexion inserts.

The goal of total knee arthroplasty (TKA) is to provide pain relief and restore function to patients with arthritis. Optimum function requires that patients achieve a satisfactory range of motion (ROM) postoperatively. Early studies on prosthetic knees have demonstrated that increasing ROM after TKA has positively affected patient outcomes,1 although similar data with contemporary prostheses are lacking.

Although greater postoperative flexion is desirable, the best methods of achieving this goal remain controversial. Adequate posterior condylar offset is believed to be an important factor for maximizing ROM after TKA.2,3 Based on such observations, several investigators have tried to improve motion through femoral component design alterations aimed at optimizing the posterior condylar offset. Despite the theoretic advantages of improved condylar offset, studies evaluating so-called high-flexion femoral components have reported variable outcomes.4-10 Although some earlier nonrandomized series showed potential benefit to these designs,5,6,10 subsequent randomized, prospective studies failed to demonstrate any benefit.4,11-14 The majority of studies investigating high-flexion femoral components have restricted the analysis to PCL-substituting prostheses,4-6,10,11,14 and although benefit was found in some of the studies evaluating PCL-substituting designs,5,6,10 three studies that investigated PCL-retaining designs found no benefit to the high-flexion femoral component.12,13,15

A less studied approach to improving ROM is to alter the geometry of the tibial polyethylene insert. In a single published study, high-flexion tibial insert modifications have been found to be beneficial in a posterior-stabilized TKA design.9 Results of similar insert modifications to PCL-retaining designs have not yet been reported. Our hypothesis was that a high-flexion tibial polyethylene insert in a consecutive series of PCL-retaining primary TKAs would provide the potential benefit of increased ROM. To our knowledge, a similar study has not yet been reported.

All patients undergoing TKA at our institution are entered prospectively into a comprehensive database that tracks implant, preoperative, and postoperative data. The database is approved by an institutional review board, and all patients in the database provide consent to have their data tracked prospectively and reviewed retrospectively. This database was used for the current investigation.

In mid-2004, the senior author (K.A.E.) switched from a standard tibial insert for posterior cruciate-retaining TKAs to a high-flexion cruciate-retaining insert. Both inserts were part of the same Genesis II TKA system (Smith & Nephew, Memphis, Tennessee), which had…

Abstract

Interest in high-flexion total knee arthroplasty (TKA) prostheses designed to provide better postoperative range of motion (ROM) is widespread. We sought to determine whether changes in surface geometry of the tibial polyethylene insert could improve postoperative ROM in a consecutive series of patients undergoing TKA with retention of the posterior cruciate ligament (PCL). Two cohorts with Smith & Nephew (Memphis, Tennessee) Posterior Cruciate-Retaining Genesis II total knee prostheses were compared, 79 knees (65 patients) using standard tibial inserts and 85 knees (72 patients) using high-flexion inserts. The standard insert has a slightly raised posterior lip, whereas the high-flexion insert is recessed downward at the posterior margin to facilitate femoral rollback in flexion and eliminate impingement of the femoral component on the back of the polyethylene during rollback.

Mean ROM 1 year postoperatively was 112.0° in patients receiving the standard insert and 119.3° in patients receiving the high-flexion insert. Preoperative ROM was similar in both groups. Flexion improvement in the high-flexion group over the standard insert group was statistically significant (P<.001). Final Knee Society Scores did not differ among patients receiving the standard and high-flexion inserts.

Our study demonstrates that improved postoperative flexion can be achieved without changing surgical technique, bony cuts, or metallic prosthetic parts. This is the first report that we are aware of that documents improvement in ROM after PCL-retaining TKA through the use of high-flexion inserts.

The goal of total knee arthroplasty (TKA) is to provide pain relief and restore function to patients with arthritis. Optimum function requires that patients achieve a satisfactory range of motion (ROM) postoperatively. Early studies on prosthetic knees have demonstrated that increasing ROM after TKA has positively affected patient outcomes,1 although similar data with contemporary prostheses are lacking.

Although greater postoperative flexion is desirable, the best methods of achieving this goal remain controversial. Adequate posterior condylar offset is believed to be an important factor for maximizing ROM after TKA.2,3 Based on such observations, several investigators have tried to improve motion through femoral component design alterations aimed at optimizing the posterior condylar offset. Despite the theoretic advantages of improved condylar offset, studies evaluating so-called high-flexion femoral components have reported variable outcomes.4-10 Although some earlier nonrandomized series showed potential benefit to these designs,5,6,10 subsequent randomized, prospective studies failed to demonstrate any benefit.4,11-14 The majority of studies investigating high-flexion femoral components have restricted the analysis to PCL-substituting prostheses,4-6,10,11,14 and although benefit was found in some of the studies evaluating PCL-substituting designs,5,6,10 three studies that investigated PCL-retaining designs found no benefit to the high-flexion femoral component.12,13,15

A less studied approach to improving ROM is to alter the geometry of the tibial polyethylene insert. In a single published study, high-flexion tibial insert modifications have been found to be beneficial in a posterior-stabilized TKA design.9 Results of similar insert modifications to PCL-retaining designs have not yet been reported. Our hypothesis was that a high-flexion tibial polyethylene insert in a consecutive series of PCL-retaining primary TKAs would provide the potential benefit of increased ROM. To our knowledge, a similar study has not yet been reported.

Materials and Methods

All patients undergoing TKA at our institution are entered prospectively into a comprehensive database that tracks implant, preoperative, and postoperative data. The database is approved by an institutional review board, and all patients in the database provide consent to have their data tracked prospectively and reviewed retrospectively. This database was used for the current investigation.

In mid-2004, the senior author (K.A.E.) switched from a standard tibial insert for posterior cruciate-retaining TKAs to a high-flexion cruciate-retaining insert. Both inserts were part of the same Genesis II TKA system (Smith & Nephew, Memphis, Tennessee), which had been his standard knee prosthesis for several years. The current investigation attempts to define the benefit of the new insert by comparing the ROM of TKAs done in the 3 months prior to the adoption of the new insert to that of the TKAs done in the 3 months after the adoption. The senior author had already performed >500 TKAs with the Genesis II system prior to the study time frame and was well versed in the system.

His standard practice was to use cruciate-retaining designs for all primary TKAs unless the PCL was found to be absent or incompetent at the time of surgery. When the cruciate-retaining high-flexion insert for the Genesis II prosthesis became available at our institution in 2004, he adopted it as his standard of practice. During the 6 months from which the cases in this study were collected, there were no changes instituted in the surgical approach, anesthetic management, rehabilitation protocol, or postoperative care path after the adoption of the new insert. The femoral and tibial components were also unchanged. The same quadriceps-sparing minimally invasive surgical (MIS) approach was used for all knees in this study.

In 2007, we queried our database to determine whether the high-flexion insert changed ROM as intended. We evaluated a consecutive cohort of patients operated on immediately before the new insert became available and compared them to the cohort of patients operated on immediately after the change. Patients with <1 year of follow-up were excluded. All patients with a diagnosis of osteoarthritis or degenerative joint disease were included, regardless of preoperative ROM or any comorbidity. To keep the evaluation interval consistent, we chose the 1-year follow-up visit as the endpoint for evaluating ROM in all patients. This decision was made based on previous studies demonstrating that improvements in ROM reach a plateau at 1 year postoperatively.4,6

Prospective data were collected on all patients at the preoperative history and physical examination, which occurred up to 30 days preoperatively. Data collected at this visit included demographic data, ROM (extension, flexion, and total arc of motion), and Knee Society Scores (total scores and functional subset score). Range of motion and Knee Society Scores were also obtained postoperatively at each patient’s 1-year follow-up. Measurement of ROM was done in a standardized fashion for all patients. All ROM measurements were made actively with the patient supine on an examination table with the thigh flexed 90°, using a long-armed goniometer placed parallel to the thigh and tibial crest. Preoperative measurements were made by the senior author’s lower extremity reconstruction orthopedic surgery fellow (B.D.C.); the senior author collected all postoperative ROM measurements at 1-year follow-up. The technique for measuring ROM was standardized for all clinical measurements.

Anesthetic choice varied based on the preference and judgment of the anesthesiologist, but most commonly consisted of a general anesthetic, with regional anesthesia used sparingly. In all patients, a standard midline incision of approximately 9 to 12 cm was made initially, and then extended if needed for adequate exposure (final incision length range, 9-20 cm). A mini-medial parapatellar arthrotomy was performed, and the patellae were not everted. The PCL was preserved in all cases. Measured bony resections were based on intramedullary alignment of the femur and tibia. A posterior referencing jig was used to prepare the femur. Tibias were resected with a 5° posterior sloped jig. All patellae were resurfaced with an all-polyethylene button, and surgical bone cement was used on all components (Simplex P; Stryker, Mahwah, New Jersey). Each of the knees was drained with a Constavac reinfusion drain (Stryker). Surgical navigation was not used.

In the first cohort, a standard cruciate-retaining ultra high molecular weight polyethylene (UHMWPE) insert was implanted. The second cohort was implanted with a high-flexion cruciate-retaining UHMWPE insert. The only difference between the 2 cohorts was the design of the modular tibial polyethylene insert (Figures 1, 2). The standard insert has a slightly raised posterior lip, whereas the high-flexion insert is recessed downward at the posterior margin to facilitate femoral rollback in flexion and eliminate impingement of the femoral component on the back of the polyethylene during rollback. Other design modifications of the high-flexion insert include a more beveled (lower) anterior margin to minimize impingement of the patellar tendon on the front of the insert during high-flexion, a deepened PCL notch, and 1 more degree of posterior slope built onto the surface of the insert. The standard insert has 4° of posterior slope; the high-flexion insert has 5° of posterior slope. The high-flexion insert also has a more conforming anterior surface to help prevent paradoxical (anterior) motion of the femur during flexion.

Figure 1: Standard and high-flexion cruciate-retaining articular inserts
Figure 1: Standard and high-flexion cruciate-retaining articular inserts. Increased slope on anterior chamfer (A), larger PCL cutout (B), and beveled posterior lip (C).

Figure 2: Change in surface topography
Figure 2: A computer-generated model demonstrating the change in surface topography between a standard and high-flexion design: areas in green are removed to create the deep-flexion design.

The first cohort receiving the standard cruciate-retaining insert consisted of 79 TKAs in 65 patients, and the second cohort receiving the high-flexion cruciate-retaining insert consisted of 85 TKAs in 79 patients. In total, 164 consecutive primary TKAs in 144 patients used the Genesis II posterior cruciate-retaining system. Two patients were included in both cohorts because they received a standard insert in 1 knee and a high-flexion insert in the other. The demographics were similar between groups with the exception of sex and height (Table 1).

Table 1: Patient Demographics at the Time of TKA

Drains were discontinued on postoperative day 1, and parenteral antibiotics were continued for 24 hours postoperatively. A routine postoperative protocol was followed that included chemoprophylaxis for deep venous thrombosis prevention and physical therapy twice daily beginning on postoperative day 1. All patients were allowed to bear weight as tolerated, and standard ROM exercises were initiated. Continuous passive motion machines were not used.

Data were analyzed using SPSS version 13.0 (SPSS, Inc, Chicago, Illinois). Independent t tests were used to analyze differences in continuous variables, and chi-square tests were used to test differences in categorical variables between the standard and high-flexion cohorts. Stratified analyses were performed on the demographic characteristics that differed between cohorts to control for the potentially confounding effect of those variables on postoperative ROM and Knee Society Scores. All statistical tests were two-tailed, and the alpha level was set at 0.05.

Results

Mean preoperative extension, flexion, and total arc of motion were not statistically significantly different between the standard and high-flexion cohorts (Table 2). At 1 year postoperatively, however, flexion and total arc of motion were statistically significantly increased in the high-flexion cohort compared to the standard insert cohort (Table 2). Total flexion 1 year postoperatively was 119.8° in the high-flexion group, compared to 112° in the standard insert group. Total arc of motion also differed by 7° in favor of the high-flexion group (119.8° vs 112°). Neither cohort had difficulty achieving extension. The 5.4° greater increase in the total arc of motion from preoperatively to 1 year postoperatively was also significant in the high-flexion group as compared to the standard insert cohort (P=.05).

Table 2: Pre- and Postoperative Extension, Flexion, and ROM for Standard and High-Flexion Inserts

Mean preoperative Knee Society functional scores for the standard and high-flexion cohorts were 49.9 and 47.2, respectively (P=.45). Mean preoperative Knee Society total scores were 51.2 and 54.8, respectively (P=.17). The functional and total Knee Society Scores improved 1 year postoperatively for the standard cohort to 80.7 and 87.4, respectively, and to 75.6 and 86.8, respectively, in the high-flexion cohort. The difference in Knee Society scores between groups at 1 year was not statistically significant (P=.32, P=.84, respectively). Subgroup analysis by sex revealed no differences between men and women in the standard and high-flexion cohorts in any of the parameters tested.

One deep infection in the standard cohort was successfully treated with irrigation and debridement, polyethylene exchange, and intravenous antibiotics. One knee in the high-flexion cohort developed a small asymptomatic avulsion fracture of the distal pole of the patella that was treated conservatively. No other major complications occurred in either of the cohorts, and no patients required reoperation. Three knees in the standard cohort and 1 knee in the high-flexion cohort required a closed manipulation under anesthesia for stiffness. All 4 knees had improved ROM postmanipulation. No knees in either cohort were felt to be unstable at final examination. All knees had <5 mm of mediolateral opening to varus-valgus stress, and no knee had >4 mm of estimated anterior or posterior movement using the 90° drawer tests. No patients subjectively reported instability.

Discussion

Successful TKA is predicated on the surgeon’s ability to restore function and provide pain relief to the arthritic knee. Satisfactory ROM is an important component of a successful result, although what constitutes satisfactory is likely to vary from 1 patient to another. This reality makes it difficult to determine how much ROM should be sought after TKA. Although current TKA implant designs allow the majority of patients to carry out most activities of daily living, improving ROM allows for a potentially higher degree of function. This may be important to certain patients. Studies have shown that increased ROM leads to improved knee scores, stair-climbing ability, and walking ability.1 Patients with <90° of motion tend to have lower knee scores than those with >90°.1

In this cohort of patients, the high-flexion insert, on average, allowed for 7.8° of greater flexion than the standard insert. This was achieved without changing the bony cuts or the geometry of the metallic prosthesis. Our findings were in contradistinction to the 3 other studies to evaluate high-flexion cruciate-retaining TKA designs.12,13,15 In those studies, a modified femoral component was used rather than a change in tibial insert, but the enhanced femoral component did not improve flexion. We hypothesize that the differing results between our study and those referenced above may have to do with femoral rollback. Lack of femoral rollback and the presence of paradoxical femoral movement (anterior sliding during flexion) have been associated with reduced flexion.3,16

Several design features on the insert in our study were developed to enhance femoral rollback, which is needed for optimum flexion and is oftentimes lacking in PCL-retaining TKAs. Specifically, this insert provides a conforming anterior surface to keep the femoral component from rolling anteriorly during flexion. Secondly, the back of the tibial insert was changed from a lipped design to a recessed design so that there would be no inhibition to femoral rollback by tibial insert. Thirdly, the posterior slope of the high-flex insert is increased 1° over the standard insert. Lastly, the anterior edge of the implant was chamfered aggressively to allow clearance for the patellar tendon, and the notch posteriorly was deepened and chamfered to reduce strain on the PCL. It is not possible to determine which of these design features played an important role in improving the flexion in this cohort of patients.

Conversely, the prosthesis under study by the other investigators was designed to increase flexion through better posterior condylar offset. Increased offset can only increase flexion if the femur adequately rolls back in flexion. If rollback is not achieved, the extra posterior condylar offset may not enhance the ability to flex postoperatively. It is also important to recognize that the senior author’s technique calls for sizing the femoral prosthesis through a posterior referencing jig. If a femur is between sizes, we generally do not anteriorize the cutting block, preferring to leave the posterior condylar offset and the flexion gap intact (at the expense of slight anterior femoral notching). When this technique is used, adequate posterior condylar offset should be reliably maintained without the need for a modified femoral prosthesis. In 1 of the above-referenced studies, anterior femoral referencing was used,15 and in the other 2, the referencing system was not specified.13,15 These 3 studies occurred in eastern Asia (South Korea and Japan) and may have important cultural or anatomic differences between the patients in those cohorts and ours that may play a role in obtaining flexion postoperatively. Ultimately, design features to enhance flexion may be beneficial under some clinical parameters but not others.

Our study looked at cruciate-retaining designs, but the majority of studies looking at high-flexion knee prostheses have focused on posterior-stabilized designs. Several early reports indicated that femoral components with improved posterior condylar offset and/or alterations in the posterior metallic condyles improved ROM.5,6,10 None of these studies were randomized. Subsequent randomized studies failed to show benefit from these designs.4,11-14 Although we postulated that lack of femoral rollback may be a factor in explaining the variance in outcomes associated with high-flexion cruciate-retaining knees, the majority of PCL-substituting prostheses should have adequate rollback due to the cam and post mechanism. The reasons for the variable benefit reported with the enhanced condylar offset posterior-stabilized knee designs remains unclear and may reflect differences in surgical technique and patient specific factors.

As is the case with high-flexion cruciate-retaining knees, the majority of reports on high-flexion posterior-stabilized TKA have so far focused on the femoral component. However, 2 studies have looked at tibial insert changes to enhance flexion after posterior-stabilized TKA. One study was a clinical outcome study using an insert similar to ours,7 and the other was an intraoperative navigation-assisted analysis of a high-flexion posterior-stabilized insert.17 Both studies of tibial insert changes in posterior-stabilized knees found that flexion improved, which is in agreement with our findings with cruciate-retaining knees.

Although the group of patients in our study that had a high-flexion insert achieved 7.8° greater flexion than those with a standard insert, we saw no corresponding improvement in Knee Society Scores. It is unclear whether the extra motion truly provided no clinical benefit to the patients or whether our scoring systems are simply not sensitive enough to detect subtle improvements in knee function (so-called ceiling effect). It is likely that at very high or very low degrees of flexion, the extra 7.8° of flexion afforded by the high-flex insert would not change a patient’s overall function. We believe, however, that in the middle ranges of postoperative motion, the extra 7.8° of flexion could be of significant benefit. The difference between 95° and 102° of flexion could translate into a much greater ability to stand from a chair unassisted or easily ascend stairs. Similarly, an improvement in flexion from 110° to 117° can improve a patient’s ability to kneel. Improving motion by 7.3° may improve the overall function in those patients who are a few degrees shy of having sufficient flexion to carry out certain concrete tasks such as kneeling, walking on steep terrain, climbing stairs, and arising from chairs without pushing up with one’s arms. Getting out of a car with limited door distance is another task where 5° to 10° of motion may make a significant difference.

One strength of this study is that all factors were held constant except for the single variable of the tibial insert. All data were collected in a prospective fashion. However, our study has some limitations. Although the data were collected prospectively, the study design included 2 consecutive cohorts rather than a randomized cohort. To minimize any error inherent with consecutive cohort series, we chose to restrict the analysis to a time frame when no other changes with respect to surgical technique or the rehabilitation protocol were implemented. Another limitation of the study was the difference in sex distribution between the standard and high-flexion cohorts. A matched-pair analysis could have mitigated this problem but would significantly reduce the number of patients in the study. However, subgroup analyses showed that sex did not act as a confounding variable; both men and women who received the high-flexion insert had a statistically significant increase in flexion and total arc of motion compared to those who received the standard insert.

Conclusion

A tibial insert designed for high flexion significantly improved flexion over a large cohort of patients presenting for primary posterior cruciate-retaining TKA. To our knowledge, this report is the first to find improved ROM in cruciate-retaining TKAs as a result of a high-flexion design modification. Flexion in the group receiving the high-flexion insert was 7.8° better than in the group that received a standard insert. No changes in surgical technique, instrumentation, or metallic prosthetic components were required. The comparatively greater motion did not come at the expense of demonstrable instability or other short-term detriment. Despite the improved flexion, Knee Society Scores were unchanged, so we are unable to state with certainty that the increased flexion truly made a positive impact on the overall function of our patients. Further functional studies are needed to determine which patients, if any, will obtain a clinically relevant functional benefit from this extra motion.

References

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  16. Stiehl JB, Komistek RD, Dennis DA, Paxson RD, Hoff WA. Fluoroscopic analysis of kinematics after posterior-cruciate-retaining knee arthroplasty. J Bone Joint Surg Br. 1995; 77(6):884-889.
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Authors

Dr Crow is from East Bay Sports Medicine Association, Concord, Ms McCauley is from Shiley Center for Orthopaedic Research & Education at Scripps Clinic, Scripps Research Services, and Dr Ezzet is from Scripps Clinic/Scripps Green Hospital, La Jolla, California.

Dr Crow and Ms McCauley have no relevant financial relationships to disclose. Dr Ezzet receives research funds from Smith & Nephew and Wright Medical Technology, Inc, and is on Smith & Nephew’s speakers bureau.

Correspondence should be addressed to: Kace A. Ezzet, MD, Department of Orthopedics, Scripps Clinic/Scripps Green Hospital, 10666 N Torrey Pines Rd, MS116, La Jolla, CA 92037 (ezzet.kace@scrippshealth.org).

doi: 10.3928/01477447-20100722-03

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