Few issues have received more attention in total knee arthroplasty (TKA) than the role of the posterior cruciate ligament. Multiple studies on longevity and functional outcome have shown benefits for posterior-stabilizing or cruciate-retaining designs, and many models currently offer either choice.1"5
Since the introduction of the posteriorstabilized total condylar knee implant in 1978,4 the benefits of posterior-stabilized versus cruciate-retaining systems have been debated. The retention of both cruciate ligaments in TKA also has been reported to yield good success.6 The debate concerning posterior cruciate ligament stabilization and retention in TKA has been reviewed by Pagnano et al,7 who outlined the controversies concerning kinematics, knee stability, gait analysis, and wear.
Stiehl et al8 raised concerns regarding knee kinematics in cruciate-retaining implants and whether femoral rollback occurs. Normal kinematics were noi reproduced in any of the 47 cruciateretaining knee implants in their study.8 Dennis et al9·10 showed anterior femoral translation (femoral rollforward) with flexion. Abnormal knee kinematics and lack of femoral rollback changes the load characteristics of knee implant surfaces, which may affect polyethylene wear.
Polyethylene failure in TKA and the factors contributing to this failure have been a focus of industry-wide research for many years. Differences in sterilization techniques and contact pressure differences relating to polyethylene failure have been described previously.11"22 The concept that abnormal knee kinematics may play a role in long-term function prompted the current study, which sought to determine whether patterns of articular contact differ between cruciate-substituting and cruciate-retaining press-fit condylar TKA Implants retrieved at revision surgery.
MATERIALS AND METHODS
From 1990 to L999, a total of 27 patients underwent revision TKA after having prior knee replacement with a press-fit condylar knee system (Johnson & Johnson, Warsaw, Ind). All surgeries were performed by one of three total joint surgeons. Demographic data, reason for revision, duration, history, clinical examination findings, and radiographie findings were retrieved through chart review.
At the time of revision surgery, all retrieved implants were analyzed. Each articular polyethylene surface was scored individually for wear by one of the authors (M.B.M.) using the technique modified by Hood et al.23 The polyethylene inserts were examined with a binocular microscope. Each implant was rated for the following seven modes of wear: burnishing, scratching, abrasion, pitting, delamination, creep, and cracking. The following ratings were assigned:
* 0=not present,
* I=visible to mild presence,
* 2=moderate damage, and
* 3=severe damage.
Tray summaries were calculated and equaled the sum of the wear scores for all seven modes. Both nonarticular (backside) and articular surfaces were measured. Additionally, patterns of markings were noted to determine whether femoral rollback or rotary motion was present.
During the assessment of backside wear, excursion of the polyethylene against the metal tray may be discernible by small scuff marks that are etched into the reverse (nonarticular) side of the tibial polyethylene. If present, these marks were measured with a caliper to gauge the length of the scuff mark and the distance from the apparent center of rotation of the rotatory movement that produced them.
Additional studies included mapping the articular surface of the polyethylene to determine the articular track the femoral component exerted on the tibial insert during weight bearing. The area of depression in the polyethylene surface was marked and outlined.
Measurements of implants initially were made using a digital camera and image computer software created by the National Institutes of Health. Digital pictures of the polyethylene were loaded into the computer and then precise measurements were calculated. Each implant was measured manually and calculations were within ,5 mm, verifying the technique.
Medial and lateral compartments were measured and recorded separately. The anteroposterior (AP) limit of each polyethylene articular surface was compared to the actual articular track in an AP direction. Next, a comparable analysis was made in a mediolateral direction.
All measurements were normalized with proportions. Also, the distance between the posterior portion of the articular track to the posterior limit of the articular surface of the tibial insert was calculated. Polyethylene not available for measurements was either sectioned for oxidation analysis or embedded during prior studies. Student /, Fisher exact, and chi-square tests were used to compare differences in the two groups using InStat Software (GraphPad, San Diego, Calif).
Of 27 patients ( 1 1 women and 1 6 men), 14 bad posterior-stabilized press-fit condylar systems before revision surgery while 13 patients had cruciate-retaining press-fit condylar knee systems. The diagnosis before primary TKA was osteoarthritis in 25 patients and rheumatoid arthritis in 2 patients.
Average patient age at the time of revision surgery was 68 years (range: 44-88 years). The interval between primary TKA and revision surgery was a mean of 3 years (range: 1.8 months to 8 years).
The reasons for revision surgery were similar in both groups, with infection (41%) and osteolysis (30%) being the most common reason overall. Group characteristics such as age, gender, duration of implant, primary disease, and reason for revision surgery were similar (Table 1).
Patients' records were reviewed for comparison of clinical examination and radiographie findings. For patients whose primary surgery was performed at the authors' institution, findings of crepitance and an effusion with varus alignment were cited most often in patients' records before their primary surgery. These findings appeared >50% of the time in both groups before primary TKA with either a posterior-stabilized or cruciate-retaining knee system.
After primary TKA, mean range of motion (ROM) with passive flexion before revision surgery was 105° (range: 60°-140°) in 10 of 14 patients with posterior-stabilized implants and 96,5° (range: 60M350} in IO of 13 patients with cruciate-retaining implants; this difference was not statistically significant. Patients had an effusion and antaigic gait more man 25% of the time, but other examination findings were not consistent. Instability was noted in only one patient with a posterior-stabilized implant.
Radiographie findings were not consistent, and no comparisons could be made. No evidence of malalignment was appreciated radiographically in either group.
Characteristics and scores for articular and nonarticular (backside) polyethylene wear were summarized. Mean tray summaries for posterior-stabilized implants versus cruciate-retaining implants were not statistically significant (22.3 versus 18.1) There were no significant differences in individual polyethylene wear scores between the two groups. Also, for the posterior-stabilized implants, minimal tibial post wear was observed, with special attention to the anterior aspect at the femoral articulation.
Patterns of anicular contact were different between posterior-stabilized and cruciate-retaining groups. Ten posteriorstabilized polyethylene trays had rotary motion compared with 4 cruciate-retaining inserts (71% versus 31%; />=.057); this difference was not statistically significant. All 14 of the posterior-stabilized implants and 8 of the cruciate-retaining implants had evidence of femoral rollback (100% versus 62%; P=.016).
Figure 1: Articular track ratios for posterior-stabilized retrievals.
Figure 2: Articular track ratios for cruciate-retaining retrievals.
Nineteen of the 27 polyethylene tibial inserts were available for articular track mapping. Ratios and percentages for articular track and articular limit were calculated in the AP and mediolateral directions. Statistically significant differences in articular track area and position of the contact surface were observed. Of the 9 implants in the posterior-stabilized group, 8 medial and 8 lateral compartments (16 total) were analyzed (Figure 1), and of the 10 cruciate-retaining implants, 10 medial and 7 lateral compartments (17 total) were analyzed (Figure 2).
Comparison of Articular Track Area for Posterior-Stabilized and Cruciate-Retaining Implants
Differences in articular track area ratios were statistically significant between posterior-stabilized and cruciateretaining implants (93% versus 59%; /*<.001) in an AP direction. Medial compartments were 94% for posterior-stabilized and 57% for cruciate-retaining implants (/><.01), while lateral compartments were 92% for posterior-stabilized and 61% for cruciate-retaining implants (/><.01)(Table2).
Differences in mean distance from the posterior aspect of the articular track to the posterior limit of the polyethylene also were statistically significant. The mean posterior distance ratios were less for posterior-stabilized than for cruciateretaining implants (2% versus 23%; P<.001). Medial compartments were .1% for posterior-stabilized and 26% for cruciate-retaining implants (P<,001), while lateral compartments were 4% for posterior-stabilized and 19% for cruciate-retaining implants (/><.05) (Table 2).
Although linear differences posteriorly were difficult to compare because of different size tibial inserts, this corresponds to a large mean linear difference for posterior-stabilized implants versus cruciateretaining implants (.08 cm versus .97 cm). The articular track size for cruciate-retaining implants was not only much smaller than for posterior-stabilized implants, but also was situated more anterior. This may offer additional evidence of lack of femoral rollback in cruciate-retaining implants. Comparisons of measurements in a mediolateral direction for medial compartments (84% for posterior-stabilized versus 71% for cruciate-retaining implants) and lateral compartments (80% for both posterior-stabilized and cruciateretaining implants) were not significant.
Polyethylene wear is a function of stress (load per area) and the type of stresses the polyethylene is subjected to. Wear leads to paniculate material, and wear debris has been recognized as the major contributor to osteolysis and aseptic loosening, the leading cause of failed implants.24'25 Several authors agree that decreasing the area for the same load (increasing stress) will lead to earlier failure.24·25 This study found statistically significant differences in the articular track area between posterior-stabilized and cruciate-retaining knee implants.
Figure 3: Measuring the articular track in a retrieved implant.
The role and function of the posterior cruciate ligament in TKA remains controversial, and multiple studies have debated the functional results.3·5·26·27 Furthermore, debate of the posterior cruciate ligament in TKA over knee kinematics in studies performed on cadavers,4·28 gait analysis,29'30 and knee stability31"-13 continues. However, direct measures of a well -functioning TKA that help identify in vivo kinematics requires additional research. Several studies using fluoroscopy have helped determine some kinematic data of posterior-stabilized versus cruciate-retaining implants; the results suggest neither design mimics normal knee kinematics.8-10
Measuring the articular track area in the current sample population compared only the articular track contact markings of the femoral component on the tibial polyethylene. Contact patterns and markings may be present as soon as the weightbearing surface of the implant is functioning. Therefore, the duration and reasons for revision may differ for the implants, bui the contact markings are present and measurable whether implanted for only a couple of months or for 8 years.
In the authors' laboratory, measuring differences in actual polyethylene wear usually requires a minimum duration of 24 months in vivo to conclude meaningful wear data. The authors have included the wear measurements as part of the complete analysis, but conclusions on these wear differences cannot be made. The focus was the data on articular track measurements. Furthermore, excessive anterior tibial post wear was not observed, which suggests posterior contact occurred in flexion (rollback) and was not present during extension.
This study indicates there may be distinct contact patterns of the articular track in posterior-stabilized and cruciate-retaining knee implants. However, the implications of the current study should be made cautiously. If the expectation of preservation of the posterior cruciate ligament is to maintain its effect on the knee's kinematics, the current study suggests that particular goal is not being achieved. The pattern of articular contact in cruciateretaining knee implants shows little or no migration of the femoral contact surface across the tibial polyethylene plateau. In particular, most of the changes visible in the tibial surface suggest the femoral contact surface moves about on the tibial plateau very little and only rarely achieves any degree of rollback.
This observation may suggest posterior cruciate-retaining total condylar knee designs are not achieving rollback, and this observation may offer at least partial explanation for the observation of the difference between preserving and substituting designs in the degree to which they each achieve flexion motion postoperatively. Whether these observations also imply differences in the consequences to in-vivo stresses for the polyethylene will require additional analysis and is beyond the scope of this study. As more data provide additional insight into the wear mechanisms of the two designs, this observation about contact patterns may shed more light.
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Comparison of Articular Track Area for Posterior-Stabilized and Cruciate-Retaining Implants