The Effect of Stuffing the Patellofemoral Compartment on the Outcome of Total Knee Arthroplasty

Pierson, Jeffery L. MD; Ritter, Merrill A. MD; Keating, E. Michael MD; Faris, Philip M. MD; Meding, John B. MD; Berend, Michael E. MD; Davis, Kenneth E. MS

Journal of Bone & Joint Surgery - American Volume: October 2007 - Volume 89 - Issue 10 - p 2195–2203
doi: 10.2106/JBJS.E.01223
Scientific Articles

Background: The effect of so-called stuffing of the patellofemoral compartment at the time of total knee arthroplasty (that is, increasing the anterior patellar displacement, the anteroposterior femoral size, or the combined anteroposterior patellofemoral size) has not been well studied. The purpose of the present study was to evaluate the effect of stuffing the patellofemoral compartment on the outcome of primary total knee arthroplasty.

Methods: A retrospective review of 1100 primary total knee arthroplasties that had been performed in 1997 and 1998 was conducted. Eight hundred and thirty arthroplasties (75.5%) met the diagnostic and minimum two-year follow-up criteria for inclusion in this report. Radiographic measurements were made to determine preoperative and postoperative anterior patellar displacement, anteroposterior femoral size, combined anteroposterior patellofemoral size, anterior femoral offset, and posterior femoral offset. Regression analysis was performed to determine the effects of changes in these variables on the range of motion, the Knee Society Knee Score, the Knee Society Function Score, the Knee Society Pain Score, and the rate of lateral retinacular release.

Results: Preoperative to postoperative changes in anterior patellar displacement, anteroposterior femoral size, combined anteroposterior patellofemoral size, anterior femoral offset, and posterior femoral offset had no clinically meaningful effect on the range of motion of the knee or on any of the Knee Society scores. Increases in anterior patellar displacement were associated with a lower probability of the need for a lateral retinacular release. Increases in measured anteroposterior femoral size were associated with a higher probability of the need for lateral release. Even when combined, however, these relationships explained only 10.1% of the observed variance in the need for lateral retinacular release. Moreover, analyses indicated that patient gender, large as opposed to medium patellar size, and absolute femoral component size influenced the likelihood of lateral release more than did anterior patellar displacement and measured anteroposterior femoral size.

Conclusions: Our findings do not support the widely held belief that stuffing of the patellofemoral joint results in adverse outcomes after total knee arthroplasty. Furthermore, the need for lateral release appears to be multifactorial and likely involves a more complex set of factors. Thus, without evidence of other identifiable causes of failure, we do not recommend revision for the treatment of pain of an overstuffed knee joint.

Level of Evidence: Therapeutic Level III. See Instructions to Authors for a complete description of levels of evidence.

1 Joint Replacement Surgeons of Indiana, St. Vincent Center for Joint Replacement, 8402 Harcourt Road, Indianapolis, IN 46260. E-mail address: jlpierso@stvincent.org

2 Joint Replacement Surgeons of Indiana Research Foundation, Center for Hip and Knee Surgery; St. Francis Hospital Mooresville, 1201 Hadley Road, Mooresville, IN 46158

Article Outline

Total knee arthroplasty is an effective treatment for severe arthritis of the knee. The technical aspects of performing the arthroplasty are known to affect both the short and the long-term outcome. One of the technical details considered to be important is to reproduce the anteroposterior patellofemoral size.

As the patellar retinaculum is relatively fixed in length, some believe that increasing the anteroposterior size of the patella, the femur, or both (and thus increasing the tension on the lateral retinaculum) adversely affects the outcome of total knee arthroplasty1-6. This situation, in which the anteroposterior size of the patella or femur (or both) is increased, is referred to as stuffing of the patellofemoral joint1,2. Some studies have shown that stuffing of the joint causes reduced range of motion1,3,4,7-10. It also has been suggested that stuffing might result in pain and inferior function of the joint2,3,5,6.

Although frequently speculated upon, stuffing has been infrequently evaluated clinically. The purpose of the present study was to determine the effect of stuffing on the outcome of total knee arthroplasty.

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Materials and Methods

We conducted a retrospective review of 1100 consecutive primary total knee arthroplasties that had been performed in 742 patients in 1997 and 1998. Institutional review board approval was obtained from both participating centers. The exclusion of fifty-three knees (4.8%) that had been lost to follow-up, 208 knees (18.9%) that had fewer than two years of follow-up, and nine knees (0.8%) in patients with rheumatoid arthritis rather than osteoarthritis left 830 knees with a minimum two-year follow-up, which are the focus of the present report. These 830 knees represented 75.5% of all arthroplasties and 74.9% (556) of all patients included in the initial review. The mean duration of follow-up (and standard deviation) for these 348 women (62.6%) and 208 men (37.4%) was 4.4 ± 1.1 years (range, two to seven years; mode, five years). The mean age of the study patients at the time of the index arthroplasty was 68.9 ± 8.7 years) (range, thirty-five to eighty-eight years).

All procedures had been performed by one of the six surgeon authors (J.L.P., M.A.R., E.M.K., P.M.F., J.B.M., or M.E.B.). A posterior cruciate-retaining prosthesis (Anatomic Graduated Component AGC Knee; Biomet, Warsaw, Indiana) was used in 715 knees (86.1%), and a posterior cruciate-substituting prosthesis (Legacy Posterior Stabilized/LPS Knee; Zimmer, Warsaw, Indiana) was used in 115 knees (13.9%). Four of the surgeons routinely used posterior cruciate-retaining prostheses, and two routinely used posterior cruciate-substituting prostheses. No surgeon used both methods. All prostheses were cemented. The patella was resurfaced in all of the knees with an all-polyethylene component. The femoral component was rotationally aligned along the epicondylar axis.

Preoperative and postoperative radiographic measurements of anterior patellar displacement, anteroposterior femoral size, combined anteroposterior patellofemoral size, anterior femoral offset, and posterior femoral offset were made. One-sixth of all radiographic evaluations were done by each surgeon. Radiographs were randomly assigned for review, and the reviewers were blinded with regard to the clinical outcome. Measured anteroposterior femoral size was compared with the absolute anteroposterior femoral size by entering the known size of the prosthetic implants.

Radiographic techniques were standardized for each radiographic view. The radiographs were made by two radiography technicians, each of whom had twenty years of experience. The latest available postoperative radiographs were used for analysis unless they were determined to be inadequate for accurate radiographic measurements, in which case the best available postoperative radiographs were used.

The skyline view was made with the knee in 45° of flexion. Preoperative anterior patellar displacement was determined on the skyline view and was defined as the anteroposterior distance from the anterior cortex of the patella to the femoral trochlear groove (Fig. 1; Measurement A). Postoperative anterior patellar displacement was determined on the skyline view and was defined as the anteroposterior distance from the anterior margin of the resurfaced patella to the prosthetic femoral trochlear groove (Fig. 2; Measurement A').

Preoperative anteroposterior femoral size was determined on the lateral knee radiograph (made with the knee positioned in 45° of flexion on the radiography table and with a tube-to-tabletop distance of 101.6 cm) and was defined as the distance from the most anterior aspect of the distal part of the femur to the posterior cortex of the femoral condyles (Fig. 3; Measurement B). In the event that the lateral radiograph was not a true-lateral (oblique) radiograph, the posterior reference point was the midpoint between the posterior margins of the medial and lateral femoral condyles. Postoperative anteroposterior femoral size was determined on the lateral radiograph and was defined as the distance from the anterior margin of the prosthetic femur to the posterior margins of the prosthetic femoral condyles (Fig. 4; Measurement B'). In the event that the lateral radiograph was not a truelateral (oblique) radiograph, the posterior reference point was the midpoint between the posterior margins of the medial and lateral femoral condyles.

The combined anterior patellar displacement and anteroposterior femoral size was calculated by adding the values of these measurements (Measurement A + Measurement B = preoperative combined anterior patellar displacement and anteroposterior femoral size; Measurement A' + Measurement B' = postoperative combined anterior patellar displacement and anteroposterior femoral size).

Anterior femoral offset, defined as the distance between the anterior femoral cortical margin and the anterior margins of the femoral condyles, also was measured preoperatively (Fig. 3, Measurement C) and postoperatively (Fig. 4, Measurement C'). Similarly, posterior femoral offset, defined as the distance between the posterior femoral cortical margin and the posterior margins of the femoral condyles, was measured both preoperatively (Fig. 3, Measurement D) and postoperatively (Figure 4, Measurement D').

Postoperative data on range of motion, the Knee Society Knee Score, the Knee Society Function Score, the Knee Society Pain Score, and lateral retinacular release were retrieved electronically from our patient database, which reflects data collected by the operating surgeon at the time of office follow-up visits. The technique for measuring range of motion did not vary by surgeon or over time. Passive flexion was measured with use of a standard goniometer while the patient was supine. Because of the large number of patients, we were able to examine Knee Society Function and Pain Scores separately to identify whether differences in the Knee Society Knee Score were due to function, pain, or both. Consistent with the Knee Society clinical rating scale, pain was defined as no pain or as mild through severe pain. The indication for lateral retinacular release was the same for all surgeons. A lateral release was performed when patellar tilt or lateral patellar subluxation were present when the extensor mechanism was approximated with a towel clip or suture at the superior pole of the patella with the knee flexed to 90°.

Linear regression was used to analyze the effect of the measured radiographic variables on Knee Society Knee Scores. Log-linear regression was used to analyze the effect of the measured radiographic variables on range of motion and Knee Society Function Scores. Logistic regression was used to analyze the effect of the measured radiographic variables on Knee Society Pain Scores and the need for a lateral retinacular release. Statistical analysis was performed with use of SAS Version 8.2 statistical software (SAS Institute, Cary, North Carolina).

All regression models were analyzed for power, with the level of significance set at 0.05 and power equal to 0.80 (probability of type-II-beta error = 0.20). These models were adequately powered (n = 830) to detect any variable explaining >0.9% of the total variance in postoperative range of motion, the Knee Society Knee Score, the Knee Society Function Score, the Knee Society Pain Score, and the need for lateral retinacular release. Power analysis was performed with nQuery Advisor 5.0 (Statistical Solutions, Cork, Ireland).

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Results

Changes in anterior patellar displacement had a significant effect on the range of motion (p = 0.0079) and the Knee Society Function Score (p < 0.0001) but had no association with the Knee Society Knee Score (p = 0.1824) or the Knee Society Pain Score (p = 0.4838) (Table I). Changes in measured anteroposterior femoral size had a significant effect on range of motion (p = 0.0182) and the Knee Society Function Score (p = 0.0094) but had no correlation with the Knee Society Knee Score (p = 0.8030) or the Knee Society Pain Score (p = 0.2688). Absolute anteroposterior femoral size was less strongly associated with outcomes than was measured anteroposterior femoral size, most likely because of the tendency for any nonsystematic magnification and measurement error to average out with the use of a consistent preoperative and postoperative measure and to approach zero with the large number of study subjects. Changes in combined anterior patellar displacement and anteroposterior femoral size had no significant effect on range of motion, the Knee Society Knee Score, the Knee Society Function Score, or the Knee Society Pain Score (p = 0.8869, p = 0.3127, p = 0.3191, and p = 0.5372, respectively). Both anterior femoral offset (p = 0.0092) and posterior femoral offset (p = 0.0385) had small negative effects on function. The Knee Society Function Score decreased by 0.27 and 0.20 points for every millimeter of increase in anterior and posterior femoral offset, respectively. Posterior offset alone was related to range of motion, with a small positive correlation being observed (0.12° of flexion obtained for every millimeter of increase in posterior offset, p = 0.0492).

Simple correlations of stuffing measurements and preoperative-to-postoperative changes in outcomes were consistent with the findings of regression analysis (Table II). The data in Table II largely mirror the findings of regression analysis shown in Table I, indicating that preoperative outcome scores did not selectively favor “stuffed” or “unstuffed” knees. Discrepancies between the results in Tables I and II are accounted for by the difference between the regression and correlation procedures. The former controlled for the effects of covariates such as gender, age, body mass index, and preoperative range of motion, whereas the latter did not.

To further test and confirm the regression findings, we defined stuffing as a 15% increase in anterior patellar displacement and compared mean outcome scores among “stuffed” and “unstuffed” groups on the basis of this definition (Table III). The absence of significant differences between these groups supports the results of our regression analyses as presented in Table I. Mean outcome scores also were examined with stuffing defined as a 15% increase in combined anterior patellar displacement and anteroposterior femoral size (Table IV). Contrary to the regression findings that indicated no changes in knee and pain scores as this measure of stuffing increases, the Knee Society Knee Score (mean, 97.5 for nineteen stuffed knees as compared with 94.1 for 769 unstuffed knees; p < 0.0001) and the Knee Society Pain Score (mean, 49.7 for nineteen stuffed knees as compared with 47.5 for 769 unstuffed knees; p < 0.0001) were significantly higher for the stuffed knee group. Large differences in the number of knees in these two study groups (nineteen compared with 769) and differences in the sensitivity of descriptive and regression procedures may account for this discrepancy.

An increase in anterior patellar displacement was associated with a lower probability of the need for a lateral retinacular release (odds ratio = 0.50/cm; p = 0.0022) (Table V). Furthermore, an increase in measured anteroposterior femoral size was associated with a higher probability of the need for a lateral retinacular release (odds ratio = 1.9/cm; p = 0.0010) (Table V). These relationships, even when combined, explained only 10.1% of the observed variance in the need for a lateral release.

Increases in anterior femoral offset also increased the odds of lateral release (odds ratio = 2.2/cm; p = 0.0006) (Table V). Measurement B (anteroposterior femoral size) overlaps and encompasses measurement C (anterior femoral offset). The two measures were analyzed in separate regressions to ensure that the effect of length C was uniquely represented. The results indicated that anteroposterior femoral size (odds ratio = 1.9/cm; p = 0.0010) and anterior femoral offset (odds ratio = 2.2/cm; p = 0.0006) equivalently increased the likelihood of lateral release.

Table VI demonstrates additional factors associated with the probability of lateral retinacular release. The forward/stepwise selection procedure of the logistic regression revealed that gender, large as opposed to medium patellar size, and absolute femoral component size were more important than either anterior patellar displacement or measured anteroposterior femoral size (Table VI).

There were no differences in outcome between the AGC or LPS knee systems with regard to the Knee Society Knee Score, the Knee Society Function Score, the Knee Society Pain Score, or the need for lateral release (p = 0.9949, p = 0.7898, p = 0.0851, and p = 0.0612, respectively). There was a +4.4° difference in the postoperative range of motion for the LPS prosthesis compared with the AGC prosthesis (p < 0.0001), and this effect was accounted for in the analysis. Interactions between stuffing measures that had a significant effect on range of motion and the use of either prosthesis were not significantly related to range of motion (anterior patellar displacement, p = 0.5755; anteroposterior femoral size, p = 0.0924).

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Discussion

Increasing the anteroposterior size of the patella or the distal part of the femur, or both, during total knee arthroplasty is referred to as stuffing the patellofemoral joint1,2. Some authors have observed that stuffing of the patellofemoral joint adversely affects the outcome of knee arthroplasty1-7,9-11. Shoji et al. reported that 17% of 141 knees with a ≥10% increase in anteroposterior patellar thickness after total knee arthroplasty achieved >120° of flexion postoperatively, compared with 50% of ninety knees with a <10% increase in patellar size4. Ryu et al. observed that postoperative patellar thickness was significantly greater in twenty-one knees with flexion of ≤90° after total knee arthroplasty than in twenty-nine knees with flexion of ≥120°10. Daluga et al. reported an increased need for manipulation following total knee arthroplasty in ten of ten patients with a postoperative increase of 12% in combined patellofemoral anteroposterior size3. Koh et al. compared fifty-six patients with residual patellar bone thickness of ≤12 mm with sixty-six patients with residual patellar bone thickness of >12 mm12. Postoperative differences in the Knee Society Score, Knee Society Function Score, and range of motion were not observed, but the authors noted that a higher rate of patellar complications may occur when the patellofemoral articulation is “excessively” increased. It is important to note that those studies involved relatively small numbers of patients and knees3,4,10,12.

Our study evaluated the effect of stuffing the patellofemoral compartment on the outcome of total knee arthroplasty in a large sample of 556 patients and 830 knees. For every millimeter of increase in anterior patellar displacement, we observed a corresponding 0.18° decrease in range of motion (p = 0.0079) and a 0.52-point increase in the Knee Society Function Score (p < 0.0001). It is possible that anterior displacement shifted the extensor mechanism, limiting its full excursion and adversely affecting range of motion, and improved the efficiency of the quadriceps mechanism by increasing the moment arm of the quadriceps. Clinically, however, our findings indicate that even a 1-cm change in anterior patellar displacement, which rarely (if ever) occurs, could result in only a 1.8° change in the range of motion and a 5.2-point change in the function score. For every millimeter of increase in anteroposterior femoral size, the range of motion increased by 0.14° (p = 0.0182) and the Knee Society Function Score decreased by 0.23 points (p = 0.0094). Combined anterior patellar displacement and anteroposterior femoral size had no effect on outcomes. Both anterior femoral offset and posterior femoral offset had small negative effects on function scores. Posterior offset alone was related to range of motion, with a small positive correlation being observed. None of the measures were associated with significant changes in the Knee Society Knee Score or the Knee Society Pain Score.

The patellar components used in the present study ranged in thickness from 7.5 to 10 mm, and our attempts to resolve even a 5° loss in range of motion due to anterior patellar displacement was only marginally successful (p = 0.0512, F-test, H0: β ≥ 5). Similarly, an effect size of ≥5 points in the Knee Society Function Score for a 1-cm change in anterior patellar displacement or anteroposterior femoral size was equally unattainable in this population (p < 0.0001, F-test, H0: β ≥ 5). Thus, the significant effects that we observed do not appear to be clinically relevant in terms of patient outcomes.

The probability of the need for lateral retinacular release decreased as the anterior patellar displacement increased and as anteroposterior femoral size increased. The latter finding is consistent with the concept that increasing anteroposterior femoral size tightens the lateral retinaculum and increases the likelihood of lateral patellar subluxation and tilt. However, increasing anterior patellar displacement theoretically produces a similar negative effect on patellar tracking, and yet we found that an increase in anterior patellar displacement was associated with a lower probability of the need for a lateral release. Moreover, the probability of needing a lateral release was more significantly related to gender, large as opposed to medium patellar size, and absolute femoral component size than it was to either anterior patellar displacement or anteroposterior femoral size. This observation suggests that the probability of requiring a lateral release most likely involves a more complex set of factors than just stuffing of the patellofemoral joint.

The present study was limited by its retrospective design, the potential for measurement errors on the radiographs, the inability to correct for patellar erosion and the effect of patellar tilt and/or subluxation on measured anterior patellar displacement, and the absence of an outcome survey specific to patellofemoral problems and function. Multiple surgeons performed the operations but, with the exception of the two different implant designs, all surgeons employed standardized procedures and approaches to patient care based on shared physician practice protocols and evidence-based standards of care. Although the present study reflects a retrospective review of historical cases and not a prospective collection of data from new cases, the outcome data were collected by the operating surgeons and are therefore subject to observer bias.

Our study evaluated the outcomes associated with a posterior cruciate-retaining prosthesis (AGC Knee) and a posterior cruciate-substituting prosthesis (LPS Knee). With the numbers available, the analysis did not indicate differences in outcomes with the exception of range of motion. Thus, we believe that our findings are relevant to both cruciate-retaining and cruciate-substituting total knee arthroplasty designs.

Our data do not support the concept that stuffing of the patellofemoral joint is associated with adverse outcomes. As discussed earlier, it is not likely that any of the small effect sizes that we observed would result in meaningful changes in flexion, function, or pain postoperatively. Although we still advocate the practice of attempting to reproduce anterior patellar displacement, anteroposterior femoral size, and the combined patellofemoral anteroposterior size during a total knee arthroplasty, our data show that precision of reproduction of these sizes did not affect the outcome of the procedure.

The findings of the present study have important clinical implications. In the evaluation of a patient who has pain or stiffness following a total knee arthroplasty, caution should be exercised in attributing these problems to stuffing of the patellofemoral joint even if radiographs show evidence of an increase in anterior patellar displacement, anteroposterior femoral size, or the combination of anterior patellar displacement and anteroposterior femoral size. Because our data do not demonstrate a relationship between stuffing and pain, we do not recommend a revision to correct stuffing of the patellofemoral joint in the absence of another identifiable cause of the pain, such as prosthetic loosening, osteolysis, infection, or instability. ▪

Disclosure: In support of their research for or preparation of this work, one or more of the authors received, in any one year, outside funding or grants in excess of $10,000 from Biomet. In addition, one or more of the authors or a member of his or her immediate family received, in any one year, payments or other benefits in excess of $10,000 or a commitment or agreement to provide such benefits from a commercial entity (Biomet and Zimmer: royalties, consulting). No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, division, center, clinical practice, or other charitable or nonprofit organization with which the authors, or a member of their immediate families, are affiliated or associated.

Investigation performed at the Center for Hip and Knee Surgery, St. Francis Hospital, Mooresville, Indiana, and the Center for Joint Replacement, St. Vincent Hospital, Indianapolis, Indiana

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