HSS Journal ®The Musculoskeletal Journal of Hospital for Special Surgery201410:9389

DOI: 10.1007/s11420-014-9389-5

The Surgeon’s Role in Relative Success of PCL-Retaining and PCL-Substituting Total Knee Arthroplasty

Merrill A. Ritter , Kenneth E. Davis1, Alex Farris1, E. Michael Keating1 and Philip M. Faris1
(1)
The Center for Hip and Knee Surgery, St. Francis Hospital, Mooresville, 1199 Hadley Road, Mooresville, IN 46158, USA
 
 
Merrill A. Ritter
Received: 10 October 2013Accepted: 26 March 2014Published online: 24 May 2014
© The Author(s) 2014

Abstract

Background

The orthopedic literature has not shown a universal and replicated difference, outside of flexion, in clinical results between posterior cruciate ligament retention and posterior cruciate ligament substitution in total knee arthroplasty.

Questions/Purposes

This study was performed to compare the restoration of flexion and knee function in a large series of cruciate-retaining and cruciate-substituting total knee arthroplasties (TKRs). In addition, we aimed to study how other variables, such as those unique to each surgeon, may have affected the results.

Patients and Methods

The current study evaluated 8,607 total knee arthroplasties in 5,594 patients performed by six surgeons, each using one of four prosthesis designs (two posterior cruciate ligament retaining, two posterior cruciate ligament substituting). Knees were compared at the level of cruciate-retaining and cruciate-substituting knees, at the level of the four prostheses, and at the level of surgeon-implant combinations. Least squared means scores were obtained through multiple linear regression, analysis of variance, and the maximum likelihood method.

Results

At the level of posterior cruciate ligament treatment, posterior cruciate ligament substitution as a whole showed 3.2° greater flexion than posterior cruciate ligament retention. At the prosthesis level, cruciate-substituting models provided greater flexion and cruciate-retaining models provided higher function scores. In the surgeon-implant combinations, surgeons provided mixed results that often did not reflect findings from other levels; one surgeon's use of a posterior cruciate ligament retaining prosthesis achieved 14.7° greater flexion than the surgeon's use of a corresponding posterior cruciate ligament substituting design.

Conclusions

Posterior cruciate ligament treatment is confounded by other variables, including the operating surgeon. The arthroplasty surgeon should choose a prosthesis based, not only on outside results, but also on personal experience and comfort.

Keywords

posterior cruciate ligament retention posterior cruciate ligament substitution total knee arthroplasty surgeon effect

Introduction

One of the most persistent issues discussed in total knee arthroplasty is the role of retention of the posterior cruciate ligament. There are two current options for contemporary TKR, retention (CR) and substitution (PS), which are based on divergent philosophies for each replacement method which cite the importance of preservation of joint structures and potential kinematic benefits in PCL retention, and relative ease of surgery and increased range of motion in PCL substitution [4, 19]. Both methods have produced excellent long-term survivorship, function, and flexion results [2, 5, 7, 8, 1214, 21], and the published literature has not found a significant, replicable and universal difference in their clinical outcomes other than increased flexion for PCL substitution [10].

For this study, the authors formed two hypotheses: (1) A prosthesis-level comparison of individual PCL-substituting and PCL-retaining implants would produce clinically significant differences in Knee Society evaluation measurements, with PCL-substituting implants producing consistently greater flexion and PCL-retaining implants producing greater function scores; (2) Observed clinical differences between PCL-substituting and PCL-retaining implants may not be due solely to the difference in treatment of the PCL, and more complex variables such as surgeon and patient selection should be examined.

At The Center for Hip and Knee Surgery, St. Francis Hospital, Mooresville, Mooresville, IN, total knee replacement using four predominant designs (2 are CR and 2 are PS) has been performed for 20 years. The authors aimed to use this large experience in an effort to provide an answer as to the difference in flexion and Knee Society Scores that can be expected between CR and PS designs. The authors also aimed to assess using statistical analysis what factors such as the operating surgeon and patient selection might contribute to differences in outcome.

Patients and Methods

From January 1, 1983, through April 1, 2011, 15,953 total knee arthroplasties were performed at the authors’ center; 14,153 of these TKAs were primary operations by one of six surgeons and using one of four prosthesis most frequently used at the center (Biomet, Warsaw, IN.; Zimmer, Warsaw, IN.). Because this was a clinical outcomes study, exclusion criteria that eliminated patients with less than 2 years of follow-up was applied, after which 8,830 total knee arthroplasties in 5,594 patients remained. Sixty-one percent of the patients were female, their mean age at time of surgery was 68.3 years (standard deviation, 8.9), their mean body mass index was 31.2 kg/m2 (SD, 5.9), and the diagnosis was osteoarthritis in 8,335 knees (96.8%), rheumatoid arthritis in 196 knees (2.3%), osteonecrosis in 61 knees (0.7%), and any other reason in 15 knees (0.2%).

In this series of 8,830 TKAs there were 6,515 AGC PCL-retaining knees (Biomet, Warsaw, IN) (73.8%), 376 Legacy PCL-substituting knees (Zimmer, Warsaw, IN) (4.3%), 853 Vanguard PCL-retaining knees (Biomet) (9.7%), and 1,086 Vanguard PCL-substituting knees (Biomet) (12.3%) performed by six surgeons at our center with more than 100 TKAs per year. None of these implants have undergone any significant changes in design throughout the study period; other prosthesis models have been used at the authors’ center during this period, and improvements in polyethylene formation have been introduced, but the prostheses used in this study have remained unchanged in their relevant characteristics (articulation conformity, patellar tracking, position of the cam-post mechanism, etc.).

Patient follow-up was performed in person at the authors’ clinic at 2 months, 6 months, and 1, 3, 5, 7, 10, 12, 15, 17, and 20 years after surgery (when available). Follow-up appointments included Knee Society score evaluations [9], flexion measurements using a standard goniometer, and a standardized radiograph; measurements were performed by either one of the six surgeons or an experienced physician’s assistant. After the appointment, data were entered using a standardized form into a patient database maintained at the authors’ center.

Demographic data for the patient groups for cruciate-retaining and posterior-stabilized implants are included in Table 1. The AGC prosthesis has a flat tibial surface in the anteroposterior and coronal planes, while the Legacy prosthesis and the Vanguard prostheses have a highly conforming tibial surface throughout. There were no differences in tibiofemoral articulation between the Vanguard PCL-retaining and PCL-substituting designs outside of the cam-and-post mechanism in the PCL-substituting implant.

Table 1

Demographics of patients between posterior-stabilized and cruciate-retaining prosthesis

Statistical measurement

Posterior stabilized

Cruciate retaining

n

1,093

7,514

Avg. age (SD)

66.8 (9.0)

68.4 (8.8)

% female

61.1

60.6

% diagnosis OA

98.9

96.5

Avg. BMI (SD)

33.0 (6.1)

31.0 (5.8)

Avg. pre-op flexion (SD)

102.5 (12.8)

111.6 (12.8)

% Pre-op varus < −8°

1.2%

7.4%

% pre valgus >11°

16.7%

7.7%

SD standard deviation

The authors performed a retrospective analysis of the clinical measurements found at follow-up (performed at 2 months, 6 months, and 1, 3, 5, 7, 10, 12, 15, 17, and 20 years, when available) as measured by the Knee Society clinical rating system [9]. ANOVA/multiple linear regression with the maximum likelihood method was used to find the least squares means (LSM) of each variable (Knee Society score, function score, flexion, pain score, stairs score, medial lateral stability and anterior posterior stability). Each model included for covariates preoperative alignment < −8°, preoperative valgus >11°, bmi > 41, height, age ≥ 71, gender, follow-up interval, and cruciate-retaining prosthesis compared to posterior-stabilized prosthesis or surgeon with nested prosthesis, or individual prosthesis. The nested model had 24 groups (6 surgeons, 4 implant models: Surgeon 1 × AGC, Surgeon 1 × Vanguard PS, Surgeon 2 x AGC, etc.). Four of the surgeons implanted a greater variety of TKA designs than the other two, and they were the focus of most of the present analysis. In all models, the level of significance for post hoc LSM-tested p values was set at p = 0.05.

Source of Funding

No outside source of funding was used in support of this study.

Results

Significant differences were found in flexion, function, and the stairs subscore in most comparisons with every significant difference in flexion favoring a PCL-substituting design, while significant differences in function and stairs more often favored retention over substitution (Table 2). No significant differences were found in the Knee Society knee score and the pain and walk subscores between any implant types (p > 0.0528).

Table 2

Comparison of clinical outcomes between TKA designs

Implant

Number

LSM

Effect size (SD)a

p value

Knee score

 Legacy

376

88.2

2.2 (1.1)

0.0528

 Vanguard CR

853

86.1

0.0 (1.1)

0.9839

 Vanguard PS

1,086

87.2

1.2 (1.0)

0.2299

 AGC

6,515

86.1

Base

Base

Pain subscore

 Legacy

376

47.8

0.3 (0.4)

0.4821

 Vanguard CR

853

47.4

−0.1 (0.4)

0.7595

 Vanguard PS

1,086

47.5

−0.0 (0.3)

0.9318

 AGC

6,515

47.5

Base

Base

Flexion

 Legacy

376

117.3

3.2 (0.5)

<0.0001

 Vanguard CR

853

113.7

−0.4 (0.6)

0.4857

 Vanguard PS

1,086

117.5

3.4 (0.5)

<0.0001

 AGC

6,515

114.1

Base

Base

Function score

 Legacy

376

85.1

1.4 (0.8)

0.0853

 Vanguard CR

853

85.9

2.3 (0.9)

0.0095

 Vanguard PS

1,086

83.2

−0.4 (0.7)

0.5577

 AGC

6,515

83.6

Base

Base

Stairs subscore

 Legacy

376

41.2

1.0 (0.5)

0.0273

 Vanguard CR

853

42.7

2.5 (0.5)

<0.0001

 Vanguard PS

1,086

40.0

−0.2 (0.4)

0.5588

 AGC

6,515

40.2

Base

Base

Walk subscore

 Legacy

376

45.0

0.4 (0.5)

0.5057

 Vanguard CR

853

44.6

−0.0 (0.6)

0.9468

 Vanguard PS

1,086

44.1

−0.6 (0.5)

0.2002

 AGC

6,515

44.7

Base

Base

AP stability

 Legacy

376

10.01

0.04 (0.02)

0.0087

 Vanguard CR

853

10.05

0.08 (0.01)

<0.0001

 Vanguard PS

1,086

10.01

0.05 (0.01)

0.0010

 AGC

6,515

9.97

Base

Base

ML stability

 Legacy

376

14.98

−0.01 (0.02)

0.6040

 Vanguard CR

853

15.00

0.01 (0.02)

0.4168

 Vanguard PS

1,086

15.00

0.01 (0.01)

0.5664

 AGC

6,515

14.99

Base

Base

aEffect size compared to AGC with standard deviation in parenthesis

The authors were unable to find differences in the knee score (p = 0.1565), function score (p = 0.3112), pain subscore (p = 0.6952), stairs subscore (p = 0.1442), and walk subscore (p = 0.4583) between PCL retention and PCL substitution with the four implants included in this study. A significant difference was found in flexion, with PCL substitution providing 3.2° greater flexion than PCL retention (117.5 vs. 114.3, p < 0.0001) (Table 3).

Table 3

Overall comparison of clinical results after TKA using either PCL-retaining (CR) or PCL-substituting (PS) prostheses

Treatment

Number

LSM

Effect size (SD)a

p value

Knee score

 CR

7,368

86.3

−1.1 (0.7)

0.1565

 PS

1,462

87.3

Base

Base

Pain subscore

 CR

7,368

47.5

−0.1 (0.3)

0.6952

 PS

1,462

47.6

Base

Base

Flexion

 CR

7,368

114.3

−3.2 (0.4)

<0.0001

 PS

1,462

117.5

Base

Base

Function score

 CR

7,368

84.1

0.6 (0.6)

0.3112

 PS

1,462

83.5

Base

Base

Stairs subscore

 CR

7,368

40.6

0.5 (0.3)

0.1442

 PS

1,462

40.1

Base

Base

Walk subscore

 CR

7,368

44.7

0.3 (0.4)

0.4583

 PS

1,462

44.4

Base

Base

AP stability

 CR

7,368

9.98

−0.02 (0.01)

0.1520

 PS

1,462

10.00

Base

Base

ML stability

 CR

7,368

14.99

0.00 (0.01)

0.6525

 PS

1,462

14.99

Base

Base

aEffect size compared with PCL substitution, with standard deviation in parenthesis

Four-by-six matrices of all possible combinations of surgeon, implant, and clinical measure are shown in Tables 4, 5 and 6; further analysis is limited to surgeons 1 through 4, who implanted a greater variety of prostheses. Using a combination of LSM score differences and intra-surgeon ranking, the data indicate that surgeons displayed varying levels of success with each of the four implants examined in the study, after controlling for demographic and preoperative factors. Overall, surgeon 1 showed relatively less success with AGC, surgeon 2 showed relatively less success with Vanguard CR, surgeon 3 showed relatively less success with Vanguard PS, and surgeon 4 showed relatively less success with Vanguard PS. For example, in knee score, surgeon 1 obtained a score 2.3 points lower with his worst prosthesis (AGC) than with his best prosthesis (Vanguard PS); surgeon 2 obtained a score 8.3 points lower with his worst prosthesis (Vanguard CR) than with his best prosthesis (AGC); surgeon 3 obtained a score 14.8 points lower with his worst prosthesis (Vanguard PS) than with his best prosthesis (Legacy); and surgeon 4 obtained a score 5.9 points lower with his worst prosthesis (Vanguard PS) than with his best prosthesis (Legacy).

Table 4

Knee Society knee scores and subscores for surgeon-implant combinations

 

Surgeon

Implant

Statistical measurement

1

2

3

4

5

6

Knee score

Legacy

n

2

306

3

63

2b

0

Least squared mean

91.2

84.6

91.0

87.5

  

LSM rank w/in surgeon

2

3

1

1

  

Effect size (SD)a

5.3 (3.3)

−1.4 (1.1)

5.1 (2.8)

1.6 (2.6)

  

p value

0.1066

0.2174

0.0672

0.5445

  

VCR

n

369

216

37

146b

85b

0

Least squared mean

90.8

79.7

86.94

   

Rank w/in surgeon

3

4

2

   

Effect size (SD)

4.9 (1.4)

−6.2 (2.1)

1.0 (3.0)

   

p value

0.0005

0.0039

0.7325

   

VPS

n

319

328

28

294

117

0

Least squared mean

91.9

84.8

76.2

81.6

90.3

 

Rank w/in surgeon

1

2

4

3

  

Effect size (SD)

6.0 (1.3)

−1.1 (2.3)

−9.7 (2.9)

−4.3 (2.2)

4.4 (3.1)

 

p value

<0.0001

0.6316

0.0008

0.0573

0.1646

 

AGC

n

354

851

2,887

459

0

1,964

Least squared mean

89.6

88.0

83.8

85.9

 

85

Rank w/in surgeon

4

1

3

2

  

Effect size (SD)

3.6 (1.0)

2.1 (2.4)

−2.1 (0.9)

Base

 

−0.9 (1.0)

p value

0.0004

0.3893

0.0225

Base

 

0.3607

Best vs. worst

LSM difference

2.3

8.3

14.8

5.9

  

p value

0.0336

0.0056

0.0001

0.0680

  

Pain subscore

 Legacy

n

2

306

3

63

2

0

Least squared mean

50.0

47.1

49.2

47.4

50.6

 

LSM rank w/in surgeon

1

3

3

1

1

 

Effect size (SD)a

4.0 (2.4)

1.1 (0.4)

3.2 (2.0)

1.3 (0.7)

4.6 (3.2)

 

p value

0.0907

0.0056

0.1099

0.0524

0.1465

 

 VCR

n

369

216

37

146

85b

0

Least squared mean

48.9

46.88

50.49

42.69

  

Rank w/in surgeon

3

4

1

4

  

Effect size (SD)

2.9 (0.6)

0.8 (0.6)

4.5 (2.1)

−3.3 (1.3)

  

p value

<0.0001

0.1555

0.0375

0.0093

  

 VPS

n

319

328

28

294

117

0

Least squared mean

49.6

47.6

45.87

44.39

48.39

 

Rank w/in surgeon

2

2

2

3

2

 

Effect size (SD)

3.6 (0.6)

1.6 (0.5)

−0.2 (1.7)

−1.6 (0.5)

2.4 (1.4)

 

p value

<0.0001

0.0007

0.9252

0.0010

0.0958

 

 AGC

n

354

851

2,887

459

0

1,964

Least squared mean

48.2

48.2

47.3

46.0

 

47.6

Rank w/in surgeon

4

1

4

2

  

Effect size (SD)

2.2 (0.4)

2.1 (1.0)

1.3 (0.3)

Base

 

1.6 (0.4)

p value

<0.0001

0.0247

<0.0001

Base

 

<0.0001

 Best vs. worst

LSM difference

1.8

1.3

3.2

4.7

2.2

 

p value

0.4334

0.2168

0.1317

0.0009

0.5170

 

Flexion

 Legacy

n

2

306

3

63

2

0

Least squared mean

120.9

110.4

111.0

117.6

126.73

 

LSM rank w/in surgeon

1

1

3

2

1

 

Effect size (SD)a

4.6 (3.2)

−5.9 (0.5)

−5.2 (2.6)

1.3 (0.9)

10.5 (5.5)

 

p value

0.1443

<0.0001

0.0474

0.1654

0.0554

 

 VCR

n

369

216

37

146

85b

0

Least squared mean

115.8

105.0

121.1

116.6

  

Rank w/in surgeon

3

4

1

3

  

Effect size (SD)

−0.5 (0.8)

−11.3 (0.8)

4.8 (2.9)

0.3 (1.7)

  

p value

0.5354

<0.0001

0.0942

0.8397

  

 VPS

n

319

328

28

294

117

0

Least squared mean

120.1

109.6

106.4

120.3

123.0

 

Rank w/in surgeon

2

2

4

1

2

 

Effect size (SD)

3.8 (0.9)

−6.6 (0.6)

−9.9 (2.3)

4.1 (0.7)

6.8

 

p value

<0.0001

<0.0001

<0.0001

<0.0001

0.0010

 

 AGC

n

354

851

2,887

459

0

1,964

Least squared mean

114.7

107.2

117.2

116.3

 

109.2

Rank w/in surgeon

4

3

2

4

  

Effect size (SD)

−1.6 (0.6)

−9.1 (1.3)

0.9 (0.4)

Base

 

−7.1 (0.5)

p value

0.0050

<0.0001

0.0342

Base

 

<0.0001

 Best vs. worst

LSM difference

6.2

5.4

14.7

4.0

3.7

 

p value

0.0495

<0.0001

<0.0001

<0.0001

0.5257

 

aEffect size compared to surgeon 4 × AGC, with standard deviation in parenthesis

bThese observations were not full rank in the model because of missing values such as preoperative alignments or intermediate follow-up

Table 5

Knee Society function scores and subscores for surgeon-implant combinations

 

Surgeon

Implant

Statistical measurement

1

2

3

4

5

6

Function score

 Legacy

n

2

306

3

63

2

0

Least squared mean

89.3

88.5

83.0

81.6

91.8

 

LSM rank w/in surgeon

2

1

1

2

1

 

Effect size (SD)a

7.3 (5.2)

6.5 (0.8)

1.0 (4.3)

−0.4 (1.5)

9.9 (6.9)

 

p value

0.1547

<0.0001

0.8108

0.8055

0.1513

 

 VCR

n

369

216

37

146

85b

0

Least squared mean

89.9

87.4

78.8

78.7

  

Rank w/in surgeon

1

4

2

3

  

Effect size (SD)

8.0 (1.2)

5.5 (1.3)

−3.1 (4.7)

−3.3 (2.8)

  

p value

<0.0001

<0.0001

0.5086

0.2422

  

 VPS

n

319

328

28

294

117

0

Least squared mean

87.5

87.8

74.7

77.0

83.9

 

Rank w/in surgeon

3

3

3

4

2

 

Effect size (SD)

5.5 (1.2)

5.9 (1.0)

−7.3 (3.7)

−4.9 (1.1)

2.0 (3.1)

 

p value

<0.0001

<0.0001

0.0489

<0.0001

0.5150

 

 AGC

n

354

851

2,887

459

0

1,964

Least squared mean

84.0

88.3

75.2

81.9

 

88.0

Rank w/in surgeon

4

2

4

1

  

Effect size (SD)

2.0 (0.9)

6.4 (2.1)

−6.8 (0.7)

Base

 

6.1 (0.8)

p value

0.0251

0.0021

<0.0001

Base

 

<0.0001

 Best vs. worst

LSM difference

5.9

1.1

7.8

4.9

7.9

 

p value

<0.0001

0.3960

0.0674

<0.0001

0.2931

 

Stairs subscore

 Legacy

n

2

306

3

63

2

0

Least squared mean

39.2

43.6

34.6

40.2

46.8

 

LSM rank w/in surgeon

3

3

2

1

1

 

Effect size (SD)a

−0.5 (2.8)

3.9 (0.5)

−5.2 (2.4)

0.5 (0.8)

7.0 (3.8)

 

p value

0.8610

<0.0001

0.0290

0.5616

0.0630

 

 VCR

n

369

216

37

146

85b

0

Least squared mean

44.3

43.6

35.2

37.2

  

Rank w/in surgeon

1

1

1

3

  

Effect size (SD)

4.6 (0.7)

3.9 (0.7)

−4.5 (2.6)

−2.5 (1.5)

  

p value

<0.0001

<0.0001

0.0806

0.0992

  

 VPS

n

319

328

28

294

117

0

Least squared mean

41.9

43.3

34.5

36.5

41.9

 

Rank w/in surgeon

2

4

3

4

2

 

Effect size (SD)

2.1 (0.7)

3.6 (0.5)

−5.3 (2.0)

−3.2 (0.6)

2.2 (1.7)

 

p value

0.0017

<0.0001

0.0095

<0.0001

0.1969

 

 AGC

n

354

851

2,887

459

0

1,964

Least squared mean

39.0

43.6

33.2

39.7

 

44.0

Rank w/in surgeon

4

2

4

2

  

Effect size (SD)

−0.8 (0.5)

3.8 (1.1)

−6.6 (0.4)

Base

 

4.3 (0.5)

p value

0.1170

0.0007

<0.0001

Base

 

<0.0001

 Best vs. worst

LSM difference

5.3

0.3

2.0

3.7

4.9

 

p value

<0.0001

0.7404

0.4129

<0.0001

0.2386

 

Walk subscore

 Legacy

n

2

306

3

63

2

0

Least squared mean

49.4

45.7

48.9

42.5

47.6

 

LSM rank w/in surgeon

1

2

1

2

1

 

Effect size (SD)a

6.4 (3.3)

2.7 (0.5)

5.9 (2.7)

−0.5 (1.0)

4.6 (4.4)

 

p value

0.0510

<0.0001

0.0326

0.6199

0.2942

 

 VCR

n

369

216

37

146

85b

0

Least squared mean

46.4

44.7

44.1

42.5

  

Rank w/in surgeon

2

4

2

3

  

Effect size (SD)

3.4 (0.8)

1.7 (0.8)

1.1 (3.0)

−0.5 (1.8)

  

p value

<0.0001

0.0416

0.7189

0.7905

  

 VPS

n

319

328

28

294

117

0

Least squared mean

45.9

45.2

39.8

41.3

45.3

 

Rank w/in surgeon

3

3

4

4

2

 

Effect size (SD)

2.9 (0.8)

2.2 (0.6)

−3.2 (2.4)

−1.7 (0.7)

2.3 (2.0)

 

p value

0.0003

0.0006

0.1699

0.0141

0.2506

 

 AGC

n

354

851

2,887

459

0

1,964

Least squared mean

45.5

46.0

42.9

43.0

 

45.0

Rank w/in surgeon

4

1

3

1

  

Effect size (SD)

2.5 (0.6)

3.0 (1.3)

−0.1 (0.4)

Base

 

2.1 (0.5)

p value

<0.0001

0.0233

0.7773

Base

 

0.0001

 Best vs. worst

LSM difference

3.9

1.3

9.1

1.7

2.3

 

p value

0.3605

0.3554

0.0109

0.0141

0.6221

 

aEffect size compared to surgeon 4 x AGC, with standard deviation in parenthesis

bThese observations were not full rank in the model because of missing values such as preoperative alignments or intermediate follow-up

Table 6

Stability measurements for surgeon-implant combinations

 

Surgeon

Implant

Statistical measurement

1

2

3

4

5

6

Anteroposterior stability

 Legacy

n

2

306

3

63

2

0

Least squared mean

10.01

10.00

10.00

10.00

9.99

 

LSM rank w/in surgeon

2

4

2

3

2

 

Effect size (SD)a

0.01 (0.10)

0.00 (0.02)

0.00 (0.08)

0.00 (0.03)

−0.00 (0.13)

 

p value

0.9079

0.7611

0.9652

0.9843

0.9713

 

 VCR

n

369

216

37

146

85b

0

Least squared mean

10.01

10.01

9.99

10.01

  

Rank w/in surgeon

1

2

3

1

  

Effect size (SD)

0.01 (0.02)

0.01 (0.02)

−0.01 (0.09)

0.01 (0.05)

  

p value

0.5981

0.7340

0.9523

0.8844

  

 VPS

n

319

328

28

294

117

0

Least squared mean

9.98

10.00

9.98

10.00

10.00

 

Rank w/in surgeon

3

3

4

2

1

 

Effect size (SD)

−0.02 (0.02)

     

p value

0.5227

     

 AGC

n

354

851

2,887

459

0

1,964

Least squared mean

9.87

10.01

10.00

10.00

 

9.99

Rank w/in surgeon

4

1

1

4

  

Effect size (SD)

−0.13

0.01 (0.04)

0.01 (0.01)

Base

 

−0.01 (0.01)

p value

<0.0001

0.8232

0.6984

Base

 

0.5274

 Best vs. worst

LSM difference

0.14

0.01

0.02

0.01

0.01

 

p value

<0.0001

0.9190

0.7552

0.8844

0.9242

 

Mediolateral stability

 Legacy

n

2

306

3

63

2

0

Least squared mean

14.99

14.98

15.00

15.00

15.00

 

LSM rank w/in surgeon

1

4

3

2

2

 

Effect size (SD)a

−0.01 (0.10)

−0.02 (0.01)

−0.01 (0.08)

0.00 (0.03)

−0.00 (0.12)

 

p value

0.9254

0.2022

0.9394

0.9206

0.9846

 

 VCR

n

369

216

37

146

85b

0

Least squared mean

14.98

15.01

15.00

15.00

  

Rank w/in surgeon

2

1

1

4

  

Effect size (SD)

−0.02 (0.02)

0.00 (0.02)

−0.00 (0.08)

−0.00 (0.05)

  

p value

0.4388

0.8665

0.9569

0.9807

  

 VPS

n

319

328

28

294

117

0

Least squared mean

14.98

15.00

15.00

15.00

15.00

 

Rank w/in surgeon

3

2

2

1

1

 

Effect size (SD)

−0.02 (0.02)

0.00 (0.02)

−0.00 (0.07)

0.00 (0.02)

0.00 (0.05)

 

p value

0.3319

0.9035

0.9389

0.8792

0.9895

 

 AGC

n

354

851

2,887

459

0

1,964

Least squared mean

14.97

15.00

14.99

15.00

 

14.99

Rank w/in surgeon

4

3

4

3

  

Effect size (SD)

−0.03 (0.02)

−0.00 (0.04)

−0.01 (0.01)

Base

 

−0.01 (0.01)

p value

0.0471

0.9485

0.5572

Base

 

0.3472

 Best vs. worst

LSM difference

0.02

0.03

0.01

0.00

0.00

 

p value

0.7943

0.3048

0.9751

0.9352

0.9815

 

aEffect size compared to surgeon 4 x AGC, with standard deviation in parenthesis

bThese observations were not full rank in the model because of missing values such as preoperative alignments or intermediate follow-up

In terms of clinical goals, for Pain relief, significant differences were found between the best and worst Knee Society pain subscore in surgeon 4 (Legacy 47.4 vs. Vanguard CR 42.7, p = 0.0009). Best/worst differences in surgeons 1 (p = 0.4334), 2 (p = 0.2168), and 3 (p = 0.1317) were not significant. For Flexion, significant differences were found between the greatest and least flexion in surgeons 1 (Legacy 120.9 vs. AGC 114.7, p = 0.0495), 2 (Legacy 110.4 vs. Vanguard CR 105.0, p < 0.0001), 3 (Vanguard CR 121.1 vs. Vanguard PS 106.4, p < 0.0001), and 4 (Vanguard PS 120.3 vs. AGC 116.3, p < 0.0001). These differences favored PCL substitution in surgeons 1, 2, and 4 and favored PCL retention in surgeon 3. For Function, significant differences were found between the best and worst Knee Society function score in surgeons 1 (Vanguard CR 89.9 vs. AGC 84.0, p < 0.0001) and 4 (AGC 81.9 vs. Vanguard PS 77.0, p < 0.0001). The best/worst difference in surgeon 3 (Legacy 83.0 vs. AGC 74.7, p = 0.0674) was marginally significant (p < 0.10), and the best/worst difference in surgeon 2 (p = 0.3960) was not significant.

In the Vanguard family of prostheses (Vanguard CR vs. Vanguard PS), in which the only difference between the two prosthesis designs are in the treatment of the PCL, the authors were unable to show a statistically significant difference between most comparisons (Table 7). Those that did show significant differences were the knee score for surgeon 3 (CR 86.9 vs. PS 76.2, p = 0.0069), flexion for all four surgeons (surgeons 1, 2, and 4 favoring PS, surgeon 3 favoring CR; surgeon 4 p = 0.0348, all others p < 0.0001), and the stairs subscore for surgeon 1 (CR 44.33 vs. PS 41.86, p = 0.0024). The differences in knee score for surgeon 2 (p = 0.0770), the pain subscore for surgeon 3 (p = 0.0873), and the function score for surgeon 1 (p = 0.0925) were marginally significant.

Table 7

Clinical results of TKA in 12 implant-surgeon combinations within the Vanguard prosthesis family

 

Surgeon

 

1

2

3

4

 

CR

PS

CR

PS

CR

PS

CR

PS

Knee score

90.8

91.9

79.7

84.8

86.9

76.2

n/a

81.6

 Difference

1.0

1.0

5.1

5.1

10.7

10.7

 

p-value

0.4793

0.4793

0.0770

0.0770

0.0069

0.0069

 

Pain subscore

48.9

49.6

46.9

47.6

50.5

45.9

42.7

44.4

 Difference

0.7

0.7

0.7

0.7

4.6

4.6

1.7

1.7

p-value

0.2787

0.2787

0.2440

0.2440

0.0873

0.0873

0.1969

0.1969

Flexion

115.8

120.1

105.0

109.6

121.1

106.4

116.6

120.3

 Difference

4.3

4.3

4.7

4.7

14.7

14.7

3.7

3.7

p-value

<0.0001

<0.0001

<0.0001

<0.0001

<0.0001

<0.0001

0.0348

0.0348

Function score

89.9

87.5

87.4

87.8

78.8

74.7

78.7

77.0

 Difference

2.5

2.5

0.4

0.4

4.2

4.2

1.6

1.6

p-value

0.0925

0.0925

0.7745

0.7745

0.4765

0.4765

0.5684

0.5684

Stairs subscore

44.3

41.9

43.6

43.3

35.2

34.5

37.2

36.5

 Difference

2.5

2.5

0.3

0.3

0.8

0.8

0.7

0.7

p-value

0.0024

0.0024

0.7404

0.7404

0.8103

0.8103

0.6669

0.6669

Walk subscore

46.4

45.9

44.7

45.2

44.1

39.8

42.5

41.3

 Difference

0.5

0.5

0.5

0.5

4.3

4.3

1.2

1.2

p-value

0.6049

0.6049

0.5456

0.5456

0.2512

0.2512

0.4997

0.4997

AP stability

10.01

9.98

10.01

10.00

9.99

9.98

10.01

10.00

 Difference

0.03

0.03

0.01

0.01

0.01

0.01

0.01

0.01

p-value

0.3344

0.3344

0.8748

0.8748

0.9194

0.9194

0.9163

0.9163

ML stability

14.98

14.98

15.01

15.00

15.00

15.00

15.00

15.00

 Difference

0.00

0.00

0.01

0.01

0.00

0.00

0.00

0.00

p-value

0.8507

0.8507

0.9444

0.9444

0.9959

0.9959

0.9352

0.9352

Discussion

PCL-retaining and PCL-substituting total knee arthroplasties have provided excellent results in long-term follow-up, both in survivorship and in clinical measurements like range of motion and function [2, 5, 7, 8, 1214, 21]. Because the debate between PCL retention and PCL substitution has lasted since at least the 1970s [11, 18, 20], and because the literature has not provided a consistent conclusion on the matter [10], it is difficult to determine which design provides consistently better outcomes. PCL substitution is often preferred in the absence of definitive differences between the two options due to the experience and attention required in PCL-retaining designs to correctly balance the PCL [4, 19]. One recent study [1, 6], however, reported significantly greater survivorship for PCL-retaining knees at 15 years.

The current investigation found results within one prosthesis generation (Vanguard) that are largely, but not completely, consistent with those published previously. Three of the four surgeons obtained greater flexion with the Vanguard PS implant than with the CR version, in agreement with the results of a Cochrane analysis [11]; surgeon 1 also obtained higher stairs subscores with Vanguard CR than with Vanguard PS. Other comparisons, however, did not conform to what has been expected of PCL-retaining and PCL-substituting systems. The Vanguard stairs scores for surgeons 2, 3 and 4 were not found to be significantly different, and surgeon 3, in even greater contrast, achieved more flexion with Vanguard CR than with Vanguard PS (121.1° vs. 106.4°).

These results suggest that a deeper examination of the influence of operating surgeon is required before a significant and independent difference in PCL results can be declared. The current study began with one hypothesis (hypothesis 1 as described in the Introduction) as an examination of each implant model’s independent influence on clinical outcome; the original goal was to conclude whether PCL substitution or PCL retention provided more favorable universal results. Because of the inconsistent conclusions from other investigators studying PCL treatment in TKA, however, the authors felt that a second hypothesis was required, thus the projection that examinations at other levels would elucidate further influences on TKA success.

A nested model of prosthesis within surgeon was necessary as the surgeon variable seemed to confound the authors’ preliminary results across both PCL treatment and implant generation, preventing a conclusion on the efficacy of the two PCL treatments independent of the operating surgeon. This nested model through successive layers has shown results that, in the uppermost layer of PCL retention versus PCL substitution, only showed a difference in flexion; many differences did not surface until the surgeon variable was considered.

This study follows a line of evidence gathered in previous published studies from the authors’ center. A study published in 2004 [3] cited abnormal anatomic knee alignment along with preoperative factors like morbid obesity and ligamentous imbalance as the main mechanisms of failure in AGC cruciate-retaining total knee replacement. Further studies expounded on the influence of postoperative [17] and preoperative [15] anatomic alignment on failure rates, while another [16] concluded that, even if the PCL is completely excised during TKA, the surgeon need not convert to a posterior-stabilized prosthesis if anteroposterior and coronal stability are maintained. These studies collectively argue that prosthesis selection with regard to the PCL may not affect the results of TKA as much as do other variables like anatomic alignment or patient comorbidities. More studies are needed to substantiate this argument, but the present series of published manuscripts may currently provide enough rationale to merit its application in a clinical setting.

A recent study by Abdel et al. [1] of 8,117 primary TKAs (Press-Fit Condylar, DePuy, Warsaw, IN; and Genesis I, Smith & Nephew, Memphis, TN) performed between 1988 and 1998 reported significantly greater survivorship rates for cruciate-retaining implants, with 15-year survivorship for PCL retention at 89.8% versus 76.5% for PCL substitution (p < 0.001). The difference extended to those knees with preoperative flexion contracture and angular deformity (89.8% vs. 70.5%, p = 0.04); however, only 52 PCL-substituting knees in this group were followed for 15 years. A concurrent comment [6] noted this limitation, as well as the possibility of differences in sterilization and polyethylene oxidation between the two groups caused by PCL substitution’s limited use in the early phase of the study period.

A cited strength of the above study is the use of data exclusively from surgeons performing at least 50 total knee arthroplasties per year. This technique controls for the influence of surgeons who are relatively inexperienced with TKA; it does not, however, address any preferences or familiarities that experienced orthopedic surgeons may hold toward a specific implant.

This study is a retrospective review of prospectively gathered data, so any findings must be considered in this light; however, the sample size and statistical methods counteract the weakness from which a retrospective cohort study typically suffers. Comparisons between the various designs in this study were only made after the use of generalized linear regression and the maximum likelihood method to minimize the influence of confounding variables. This statistical test was used on a large set of nearly 9,000 knees, which produced sufficient power to determine differences between most individual surgeon-prosthesis combinations. A second possible limitation arises from the prevalence of each TKA design at the center. The surgeon-authors and their colleagues implanted more PCL-retaining than PCL-substituting TKAs, and they may have used the PCL-substituting design only in patients with the worst deformities, such as extreme preoperative varus or valgus. These cases, however, did not unduly influence the results described here because the surgeons only used PCL-substituting implants in these cases to compensate for the deformity, not to specifically increase flexion in a preoperatively low-flexion patient. There was no systematic bias in the surgeons involved in this study that would result in a disproportionate amount of any preoperative patient population receiving one of the four implant designs.

Future studies, especially from large-volume centers with high statistical power, should use a nested model to examine the possible interacting variables of surgeon and implant design. Such evaluations may show that it is more important, not for the surgeon to choose which total knee design provides universally improved results, but instead for the surgeon to determine which total knee design provides consistently favorable results for the surgical technique he or she feels comfortable with in practice. In this case, the surgeon may find that different prostheses may provide different advantages in function, flexion, and pain relief, and that these advantages (and any possible disadvantages) may not extend to other surgeons at his/her practice or in the orthopedic field. The same conditions in situ (preoperative angular deformity, anteroposterior stability, etc.) may spur different surgeons to implant different arthroplasty models based on their familiarity and technique, while still obtaining good or excellent long-term clinical results.

Previous studies have asserted that PCL-substituting total knee arthroplasty provides more consistent results than PCL-retaining TKA, particularly in flexion. In light of the data from the present study and from documented success with PCL retention, however, the operating surgeon may prove to be a substantially influential variable of overall TKA success than previously thought. If this is the case, then it is the surgeon’s responsibility to establish which TKA design is most suited to his or her operative technique.

Disclosures

Conflict of Interest

Merrill A. Ritter, MD reports grants from Biomet, Inc. during the conduct of the study and grants from Exactech, Pacira and DePuy, outside the work. Philip M. Faris, MD reports grants and personal fees from Biomet, Inc. during the conduct of the study and grants from Exactech, Pacira and DePuy, outside the work. E. Michael Keating, MD reports grants from Biomet, Inc. during the conduct of the study and grants from Exactech, Pacira and DePuy, outside the work. Kenneth E. Davis, MS and Alex Farris, BA have declared that they have no conflict of interest.

Human/Animal Rights

All procedures followed were in accordance with the ethical standards of the responsible committee on human experimentation (institutional and national) and with the Helsinki Declaration of 1975, as revised in 2008 (5).

Informed Consent

Informed consent was obtained from all patients for being included in the study.

Open Access This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and the source are credited.