Orthopedics

Feature Article 

Incidence of Patella Baja Before and After Primary Total Knee Arthroplasty Based on Body Mass Index

Elvis L. Francois, MD; Matthew P. Abdel, MD; Paul L. Sousa, MD; Danielle M. Chapman, BS; Mike J. Miller, PA-C; David F. Dalury, MD; Daniel J. Berry, MD

Abstract

In the setting of total knee arthroplasty, patella baja has been associated with decreased range of motion and increased risk of certain extensor mechanism complications. The goal of this study was to determine whether obese patients had an increased prevalence of patella baja before and/or after primary total knee arthroplasty. A multicenter retrospective review of 5089 unilateral total knee arthroplasties performed between 1998 and 2012 for osteoarthritis was conducted. Only total knee arthroplasties with cemented modular, metal-backed constructs and patellar resurfacing were included. An a priori power analysis determined that 500 patients (stratified into 5 World Health Organization body mass index groups, matched by age and sex) were needed to detect a significant Insall–Salvati ratio difference of 0.07. Patella baja was defined as an Insall–Salvati ratio of less than 0.8. Preoperative and postoperative Insall–Salvati ratios were compared between groups and analyzed using linear regression and analysis of variance. Preoperatively, there was a higher prevalence of patella baja in the higher body mass index groups (>25 kg/m2) compared with normal weight patients (10% vs 6%; P=.02). Postoperatively, there was no difference in the prevalence of patella baja between the 2 groups (5% vs 5%; P=.91). On comparison of postoperative Insall–Salvati ratio with preoperative Insall–Salvati ratio, the higher body mass index groups had a greater change in Insall–Salvati ratio (Δ 0.10 vs Δ 0.07; P=.01). This is the first study to report an increased prevalence of patella baja in obese patients and to show that the prevalence normalizes to that of a control group after total knee arthroplasty. Obese patients undergoing primary total knee arthroplasty had a higher rate of preoperative patella baja. [Orthopedics. 2019; 42(2):90–94.]

Abstract

In the setting of total knee arthroplasty, patella baja has been associated with decreased range of motion and increased risk of certain extensor mechanism complications. The goal of this study was to determine whether obese patients had an increased prevalence of patella baja before and/or after primary total knee arthroplasty. A multicenter retrospective review of 5089 unilateral total knee arthroplasties performed between 1998 and 2012 for osteoarthritis was conducted. Only total knee arthroplasties with cemented modular, metal-backed constructs and patellar resurfacing were included. An a priori power analysis determined that 500 patients (stratified into 5 World Health Organization body mass index groups, matched by age and sex) were needed to detect a significant Insall–Salvati ratio difference of 0.07. Patella baja was defined as an Insall–Salvati ratio of less than 0.8. Preoperative and postoperative Insall–Salvati ratios were compared between groups and analyzed using linear regression and analysis of variance. Preoperatively, there was a higher prevalence of patella baja in the higher body mass index groups (>25 kg/m2) compared with normal weight patients (10% vs 6%; P=.02). Postoperatively, there was no difference in the prevalence of patella baja between the 2 groups (5% vs 5%; P=.91). On comparison of postoperative Insall–Salvati ratio with preoperative Insall–Salvati ratio, the higher body mass index groups had a greater change in Insall–Salvati ratio (Δ 0.10 vs Δ 0.07; P=.01). This is the first study to report an increased prevalence of patella baja in obese patients and to show that the prevalence normalizes to that of a control group after total knee arthroplasty. Obese patients undergoing primary total knee arthroplasty had a higher rate of preoperative patella baja. [Orthopedics. 2019; 42(2):90–94.]

A rapid increase in the prevalence of primary and revision total knee arthroplasties (TKAs) has occurred in the past 2 decades.1 Furthermore, the demand for primary TKA is expected to grow to an estimated 673% by the year 2030.2 This uptrend is associated with an increased number of TKAs being performed in patients with increasing comorbidities.1 Of particular concern, obesity has been shown to be more prevalent in patients undergoing TKA.3,4 In the United States, obesity has approached epidemic proportions and continues to place a disproportionate strain on the health care system, with $79 billion in obesity-related costs in 1998 increasing to $147 billion in 2008.4,5

As the number of obese patients undergoing TKA continues to rise, the complication rate associated with this ever-changing patient base mandates reassessment. It has been reported that patella baja after TKA alters patellofemoral joint mechanics and may result in decreased range of motion, extensor lag, anterior knee pain, anterior polyethylene impingement and wear, and diminished outcomes.6 Moreover, TKAs are technically more challenging in obese patients for several reasons, and concurrent patella baja could further increase the complexity of the procedure. To the authors' knowledge, no study has investigated the incidence of patella baja in obese patients before and after primary TKA.

The goal of this study was to determine the preoperative prevalence of patella baja in patients undergoing primary TKA based on body mass index (BMI) classification. The secondary outcome was to determine the prevalence of patella baja after primary TKA based on BMI classification.

Materials and Methods

A multicenter retrospective review of 5089 primary cemented unilateral TKAs performed between 1998 and 2012 for osteoarthritis was conducted. Revision procedures, unicompartmental knee arthroplasties, and indications other than osteoarthritis were excluded. Moreover, to minimize bias, only cemented, modular, metal-backed constructs and patellar resurfacing were included. Both cruciate-retaining and posterior-stabilized implants were included. All cases were identified with use of institutional total joint registries, which follow patients prospectively at regular intervals from the time of index arthroplasty. Institutional review board approval was obtained at both sites prior to initiation of the study.

A Priori Power Analysis

An a priori power analysis determined that 500 patients were needed to detect a significant difference of 0.07 for Insall–Salvati ratio (ISR) at an alpha of 0.05 and a beta of 0.80, assuming a standard deviation of 0.18. Of the 5089 TKAs performed between the selected dates, a random sample of 500 TKAs (including bilateral knees) were included for review.

Patient Demographics

The mean age at index arthroplasty was 70 years (range, 48–90 years). The mean BMI was 33 kg/m2 (range, 18–57 kg/m2), and 362 patients were female (72%). Patient demographics are provided in Table 1.

Patient Demographics

Table 1:

Patient Demographics

Body mass index was calculated using weight and height information from each patient on the date of surgery. The World Health Organization's classification of BMI was used to stratify patients into 6 categories: underweight (18.50 kg/m2), normal (range, 18.50–24.99 kg/m2), over-weight (range, 25.00–29.99 kg/m2), obese class I (range, 30.00–34.99 kg/m2), obese class II (range, 35.00–39.99 kg/m2), and obese class III (≥40.00 kg/m2). Body mass index values below 14 kg/m2 or above 70 kg/m2 were presumed to be data entry errors and were discarded. Additionally, due to the low number, all underweight patients (n=13) were excluded from further analysis.

Patients from the Mayo Clinic (n=276) were matched by age and sex between the BMI categories, whereas patients from the University of Maryland, St Joseph Medical Center (n=224) were not. One hundred patients in each of the 5 BMI categories were included, yielding an overall analysis cohort totaling 500 patients. Radiographs of all 500 patients were evaluated using 1 of 2 medical image viewer programs (Mayo Clinic: QREADS, Mayo Clinic; University of Maryland, St Joseph Medical Center: Merge OrthoPACS, Watson Health Imaging Headquarters, Chicago, Illinois). Radiographic techniques were standardized across all patients and for each radiographic view. The ISR was chosen as the outcome tool because of lack of dependence on the degree of knee flexion.7 The ISR was standardized and defined as the distance between the most distal point of the patellar articular surface and insertion of the patellar tendon divided by the length of the patellar articular surface.8,9 Patella baja was defined as an ISR of less than 0.8 on true lateral radiographs at 30° of flexion (Figures 12).

Preoperative radiograph of a non-obese patient (body mass index, 24 kg/m2) with normal patellar height (A). Postoperative radiograph of the same patient (B). Abbreviation: ISR, Insall–Salvati ratio.

Figure 1:

Preoperative radiograph of a non-obese patient (body mass index, 24 kg/m2) with normal patellar height (A). Postoperative radiograph of the same patient (B). Abbreviation: ISR, Insall–Salvati ratio.

Preoperative radiograph of an obese patient (body mass index, 41 kg/m2) with patella baja (A). Postoperative radiograph of the same patient with corrected patellar height (B). Abbreviation: ISR, Insall–Salvati ratio.

Figure 2:

Preoperative radiograph of an obese patient (body mass index, 41 kg/m2) with patella baja (A). Postoperative radiograph of the same patient with corrected patellar height (B). Abbreviation: ISR, Insall–Salvati ratio.

Statistical Analysis

The data were reported using summary statistics such as mean (standard deviation) for continuous variables and count (percentage) for categorical variables. The primary outcomes were the preoperative and postoperative ISR, the change in ISR before and after primary TKA, and the proportion of patients with patella baja. Differences in ISR between BMI categories were assessed using one-way analysis of variance for continuous measures, such as the ISR, and chi-square tests for categorical measures, such as the proportion of patients with patella baja. When significant between-group differences were observed, further analysis was performed using multiple comparisons procedures, including the Ryan–Einot–Gabriel–Welsch multiple range test and the Benjamini–Hochberg procedure, to identify pairwise differences. Linear regression was used to assess the relationship between BMI (as a continuous variable) and ISR.Statistical significance was set at P≤.05. All analyses were conducted using SAS version 9.4 software (SAS Institute Inc, Cary, North Carolina) and R version 3.1.1 software (R Core Team, R Foundation for Statistical Computing, Vienna, Austria).

Results

Preoperatively, the frequency of patella baja was higher among patients in the obese class I group compared with patients in the normal, overweight, and obese class II groups (normal=6%, over-weight=6%, obese class I=17%, obese class II=6%, and obese class III=11%; P=.02). The preoperative mean ISR was significantly higher in normal weight patients (mean=1.08) and overweight patients (mean=1.07) than in obese class I (mean=0.99), obese class II (mean=1.02) and obese class III (mean=1.00) patients (P<.001) (Table 2).

Insall–Salvati Ratio by Body Mass Index Group

Table 2:

Insall–Salvati Ratio by Body Mass Index Group

Postoperatively, no significant difference was observed in the rate of patella baja between the BMI groups (normal=5%, overweight=4%, obese class I=7%, obese class II=5%, and obese class III=5%; P=.914) (Figures 12). However, ISR was significantly associated with BMI group (P=.008). Specifically, the mean postoperative ISR was significantly higher in normal weight patients (mean=1.14) than in obese class III patients (mean=1.08) and also in overweight patients (mean=1.18) than in obese class II (mean=1.10) and obese class III (mean=1.08) patients. The mean change from preoperative ISR to postoperative ISR was significantly higher in obese class I patients than in patients in the normal weight, obese class II, and obese class III groups (0.14 vs 0.07, 0.08, and 0.07, respectively; P=.011) (Figures 12, Table 2).

Discussion

Obese patients make up a disproportionately large portion of TKA candidates.1,4 Further, obesity is associated, in a dose-response manner, with increased complications and revision procedures.10–13 Thus, continued characterization of the nuances in this patient population that may possibly affect outcomes of TKA is warranted. To the authors' knowledge, this is the first study to attempt to characterize the incidence of patella baja in obese patients undergoing TKA.

The importance of recognizing patella baja in the preoperative setting cannot be overstated. Recent reports have indicated that patella baja statistically increases the rate of patella fracture (0.9% vs 2%; P=.05).14 The authors further suggest that these results should lead to reconsidering the practice of systematic patella resurfacing in patients with preoperative patella baja. As mentioned earlier, it has also been reported that patella baja after TKA alters patellofemoral joint mechanics and may result in decreased range of motion, extensor lag, anterior knee pain, anterior polyethylene impingement and wear, and diminished outcomes.6

Among the 500 patients studied, patella baja was associated with higher BMI. Preoperatively, there was a higher rate of patella baja in the patients in the higher BMI groups (BMI >25 kg/m2) compared with normal weight patients (10% vs 6%; P=.02). As a continuous variable, the ISR was lower in patients with higher BMI compared with normal weight patients (mean, 1.02 vs 1.08; P<.01).

Postoperatively, there was no difference in the rate of patella baja in the higher BMI groups compared with normal weight patients (5% vs 5%; P=.91). However, the mean ISR was significantly lower in the higher BMI groups compared with normal weight patients (1.12 vs 1.14; P=.01). On comparison of postoperative ISR with preoperative ISR, the higher BMI groups had a greater change in ISR compared with normal weight patients (Δ 0.10 vs Δ 0.07; P=.01).

In this study, patients with a higher BMI were more likely to have patella baja preoperatively. The possible reason for patella baja in obese patients has not been elucidated in the literature. Currently, it is an empirical finding. It is most likely multifactorial, being potentially related to a host of variables, such as decreased activity and thigh-calf impingement, and potentially other epigenetic factors that may impact collagen and tenocytes. Postoperatively, the higher BMI groups showed the most significant change in patellar height. Previous studies have shown that both patella baja and postoperative change in patellar height have been associated with worse outcome.6

This study had some limitations. Foremost, the patients in this series were from institutional joint registries at 2 large orthopedic institutions in the Midwest and the Northeast. As such, the distribution of BMI seen in these patients may not be representative of patients undergoing TKA nationally. Furthermore, the authors evaluated the relationship of obesity to patella baja using preexisting World Health Organization classifications instead of pairwise comparisons between study groups or regression analyses to determine arbitrary BMI intervals. Although the analysis of specific BMI intervals may facilitate identifying statistical significance, it also greatly increases the risk of type 1 error and may offer less clinical relevance.

Conclusion

The authors have shown that, when compared with normal weight patients, an increasing BMI is inversely proportional to the ISR. Furthermore, patients with a higher BMI showed a greater change from preoperative ISR to postoperative ISR compared with normal weight patients. This study provides insight into knee biomechanics in obese patients and can further aid reconstructive surgeons with preoperative planning and management for this growing population.

References

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Patient Demographics

CharacteristicBody Mass Index GroupTotal (N=500)

Normal (n=100)Overweight (n=100)Obese Class I (n=100)Obese Class II (n=100)Obese Class III (n=100)
Location, No.
  Mayo5461595250276 (55.2%)
  Towson4639414850224 (44.8%)
Sex, No.
  Female78 (78.0%)69 (69.0%)68 (68.0%)72 (72.0%)75 (75.0%)362 (72.4%)
  Male22 (22.0%)31 (31.0%)32 (32.0%)28 (28.0%)25 (25.0%)138 (27.6%)
Age, y
  Mean (SD)72.7 (9.0)73.0 (7.9)70.0 (9.6)69.4 (9.7)67.1 (9.7)70.6 (9.4)
  Range50.0–90.152.0–90.250.0–90.448.0–86.751.0–84.948.0–90.4
Side, No.
  Left43 (43.0%)38 (38.0%)55 (55.0%)51 (51.0%)49 (49.0%)236 (47.2%)
  Right57 (57.0%)62 (62.0%)45 (45.0%)49 (49.0%)51 (51.0%)264 (52.8%)
Body mass index, kg/m2
  Mean (SD)23.2 (1.4)27.7 (1.6)32.2 (1.4)37.4 (1.5)46.2 (4.1)33.3 (8.3)
  Range18.9–24.9825.0–29.9930.0–34.9635.0–39.9640.0–57.918.9–57.9

Insall–Salvati Ratio by Body Mass Index Group

Insall–Salvati RatioBody Mass Index GroupTotal (N=500)P

Normal (n=100)Overweight (n=100)Obese Class I (n=100)Obese Class II (n=100)Obese Class III (n=100)
Preoperative<.001a
  Mean (SD)1.08 (0.20)1.07 (0.18)0.99 (0.18)1.02 (0.17)1.00 (0.16)1.03 (0.18)
  Range0.72–1.710.50–1.560.64–1.410.54–1.560.68–1.460.50–1.71
Preoperative.023b
  <0.86 (6.0%)6 (6.0%)17 (17.0%)6 (6.0%)11 (11.0%)46 (9.2%)
  ≥0.894 (94.0%)94 (94.0%)83 (83.0%)94 (94.0%)89 (89.0%)454 (90.8%)
Postoperative.008c
  Mean (SD)1.14 (0.22)1.18 (0.22)1.13 (0.24)1.10 (0.20)1.08 (0.18)1.12 (0.21)
  Range0.64–1.880.47–1.890.63–2.200.58–1.500.71–1.650.47–2.20
Postoperative.914
  <0.85 (5.0%)4 (4.0%)7 (7.0%)5 (5.0%)5 (5.0%)26 (5.2%)
  ≥0.895 (95.0%)96 (96.0%)93 (93.0%)95 (95.0%)95 (95.0%)474 (94.8%)
Change in Insall–Salvati ratio (postoperative–preoperative).011d
  Mean (SD)0.07 (0.16)0.11 (0.16)0.14 (0.20)0.08 (0.15)0.07 (0.14)0.09 (0.16)
  Range−0.36–0.56−0.28–0.57−0.31–1.22−0.37–0.43−0.19–0.65−0.37–1.22
Authors

The authors are from the Department of Orthopedic Surgery (ELF, MPA, PLS, DJB), Mayo Clinic, Rochester, Minnesota; and the University of Maryland, St Joseph Medical Center (DMC, MJM, DFD), Towson Orthopaedic Associates, Towson, Maryland.

Dr Francois, Dr Sousa, Ms Chapman, and Mr Miller have no relevant financial relationships to disclose. Dr Abdel is a paid consultant for Stryker. Dr Dalury is a paid consultant for DePuy and receives royalties from Johnson & Johnson. Dr Berry holds stock in Bodycad and holds a patent with DePuy.

Correspondence should be addressed to: Matthew P. Abdel, MD, Department of Orthopedic Surgery, Mayo Clinic, 200 First St SW, Rochester, MN 55905 ( abdel.matthew@mayo.edu).

Received: August 02, 2018
Accepted: February 19, 2019

10.3928/01477447-20190225-04

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