Orthopedics

Feature Article 

Reproducing the Native Posterior Tibial Slope in Cruciate-Retaining Total Knee Arthroplasty: Technique and Clinical Implications

James L. Howard, MD, FRCSC; Mina W. Morcos, MD, MSc, FRCSC; Brent A. Lanting, MD, FRCSC; Lyndsay E. Somerville, PhD; James P. McAuley, MD, FRCSC

Abstract

Total knee arthroplasty (TKA) profoundly influences knee biomechanics. Using an arbitrary (often 3° to 5°) posterior tibial slope (PTS) in all cases seldom will restore native slope. This study examined whether the native PTS could be reproduced in cruciate-retaining TKA and how this would influence clinical outcome. Radiographic and clinical outcomes of 215 consecutive TKAs using the PFC sigma cruciate-retaining implant were evaluated. The tibial bone cut was planned to be made parallel to the native anatomical slope in the sagittal plane. The PTS was measured with reference to the proximal tibial medullary canal (PTS-M) and the proximal tibial anterior cortex (PTS-C) on true lateral radiographs using a picture achieving and communication system. Knee range of motion (ROM), Knee Society Score, Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC), and Short Form Health Survey (SF-12) were evaluated. Mean preoperative PTS-M was 6.9°±3.3°, and mean postoperative PTS-M was 7.0°±2.4°. Mean preoperative PTS-C was 12.2°±4.2°, and mean postoperative PTS-C was 12.6°±3.4°. Preoperative and postoperative PTS were not significantly different for both techniques (P>.05). An arbitrary 3° as an acceptable range for PTS-M was achieved in 144 knees (67%) (group 1), and 71 knees (33%) had a difference of more than 3° (group 2). Group 1 had a significantly larger gain in ROM (P=.04) as well as improved Knee Society, WOMAC, and SF-12 physical scores compared with group 2 (P<.01). The modified surgical technique reproduced the native tibial slope in cruciate-retaining TKA. Reproduction of the native PTS within 3° resulted in better clinical outcomes manifested by gain in ROM and knee functional outcome scores. [Orthopedics. 20xx; xx(x):xx–xx.]

Abstract

Total knee arthroplasty (TKA) profoundly influences knee biomechanics. Using an arbitrary (often 3° to 5°) posterior tibial slope (PTS) in all cases seldom will restore native slope. This study examined whether the native PTS could be reproduced in cruciate-retaining TKA and how this would influence clinical outcome. Radiographic and clinical outcomes of 215 consecutive TKAs using the PFC sigma cruciate-retaining implant were evaluated. The tibial bone cut was planned to be made parallel to the native anatomical slope in the sagittal plane. The PTS was measured with reference to the proximal tibial medullary canal (PTS-M) and the proximal tibial anterior cortex (PTS-C) on true lateral radiographs using a picture achieving and communication system. Knee range of motion (ROM), Knee Society Score, Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC), and Short Form Health Survey (SF-12) were evaluated. Mean preoperative PTS-M was 6.9°±3.3°, and mean postoperative PTS-M was 7.0°±2.4°. Mean preoperative PTS-C was 12.2°±4.2°, and mean postoperative PTS-C was 12.6°±3.4°. Preoperative and postoperative PTS were not significantly different for both techniques (P>.05). An arbitrary 3° as an acceptable range for PTS-M was achieved in 144 knees (67%) (group 1), and 71 knees (33%) had a difference of more than 3° (group 2). Group 1 had a significantly larger gain in ROM (P=.04) as well as improved Knee Society, WOMAC, and SF-12 physical scores compared with group 2 (P<.01). The modified surgical technique reproduced the native tibial slope in cruciate-retaining TKA. Reproduction of the native PTS within 3° resulted in better clinical outcomes manifested by gain in ROM and knee functional outcome scores. [Orthopedics. 20xx; xx(x):xx–xx.]

Cruciate-retaining total knee arthroplasty (TKA) has provided good results with more than 95% survivorship after follow-up of 10 years or longer.1–3 The posterior tibial slope angle (PTS) in posterior cruciate-retaining TKA influences the knee kinematics, knee stability, flexion gap, knee range of motion (ROM), and tension of the posterior cruciate ligament (PCL).2,4–6

A traditional cruciate-retaining surgical technique consists of selecting a tibial slope regardless of preoperative tibial anatomy, similar to a posterior-stabilized technique. This is counterintuitive and nonphysiological as several studies have reported the PTS of the normal knee ranges from 7° to 14.8°.7–12 Such nonphysiological tibial slope in the proximal tibial cut results in decreased ROM.12 Because virtually all cruciate-retaining knee systems result in tibial slope below the normal anatomical range, reproducing the patient's native slope may help to reduce tension on the PCL and facilitate knee flexion.13 Appropriate incorporation of the optimal PTS in cruciate-retaining TKA may improve the outcome after surgery.

Posterior tibial slope traditionally is measured on plain radiographs, being low cost and reproducible. The accuracy of these measurements has been questioned because of overlap of the medial and lateral tibial plateaus on the lateral radiograph. However, a recent magnetic resonance imaging (MRI) study found no difference between PTS of the medial and lateral tibial plateau.4

The primary objective of this study was to determine whether the native PTS could be surgically reproduced in cruciate-retaining TKA, which potentially could result in better knee soft tissue balance, function, and ROM postoperatively. The second objective was to determine whether reproduction of the native PTS would influence the postoperative clinical outcome.

Materials and Methods

After receiving approval from the institutional research ethics board, the authors evaluated the radiographic and clinical outcomes of a series of consecutive TKAs using the PFC Sigma cruciate-retaining total knee implant (Johnson & Johnson, Professional Inc, Raynham, Massachusetts). Between January 2007 and December 2013, a total of 599 primary TKAs were performed at London Health Science Centre, University Hospital, University of Western Ontario, London, Ontario, Canada.

A total of 215 knees with adequate preoperative and postoperative true lateral radiographs of the knee were reviewed. Indications for TKA included degenerative osteoarthritis (212 knees) and inflammatory arthritis (3 knees). Patients' mean body mass index was 34.7±19 kg/m2.

Operative Technique

The senior author (J.P.M.) performed all surgeries using PFC Sigma cruciate-retaining total knee implants. All surgeries were performed without patella resurfacing via a midline skin incision and medial parapatellar approach with conventional anterior referencing knee instruments. Intramedullary alignment guides for the femur were set at 5° of valgus. Elevation of 1 to 2 mm was accepted, confirmed by direct measurement of the resected fragment. Appropriate femoral rotation was confirmed using Whiteside's line and the transepicondylar axes. The cutting block was placed and checked to assure anatomic restoration of the flexion space, again by direct measurement of the posteromedial resection.

For the tibia, the extramedullary alignment guides were used with an alignment goal of 0° varus/valgus. The tibial bone cut was planned to be parallel to the patient's native anatomical slope in the sagittal plane. An angel wing instrument was placed on the higher tibial plateau, depending on whether the knee was varus or valgus. The slope of the cutting guide was adjusted so the cutting block was parallel to the patient's native tibial slope (Figure 1), ignoring the angle of the alignment rod that would have been used instead to set the tibial slope cut as described in the recommended instructions provided by the manufacturer's technical guide. Reproduction of the native slope was confirmed by direct examination of the resected fragment.

Intraoperative photograph showing restoration of the posterior tibial slope using the angel wing instrument as a reference guide. To aid in restoring posterior tibial slope, the angel wing was placed on the upper surface of the cutting block parallel to the patient's native tibial slope at the lateral tibial plateau.

Figure 1:

Intraoperative photograph showing restoration of the posterior tibial slope using the angel wing instrument as a reference guide. To aid in restoring posterior tibial slope, the angel wing was placed on the upper surface of the cutting block parallel to the patient's native tibial slope at the lateral tibial plateau.

Posterior Tibial Slope Measurement

The primary outcomes were the correlation between the preoperative and postoperative PTS measured on lateral radiographs. Preoperative and postoperative true lateral radiographs of the knee were obtained using a picture achieving and communication system. All of the radiographs were reviewed by 1 (J.P.M.) of the authors.

The PTS was measured with reference to the proximal tibial medullary canal (PTS-M) and the proximal tibial anterior cortex (PTS-C). The reference line of the PTS-M was defined as the line connecting the center of the proximal tibial medullary canal at 5 and 10 cm distal to the tibial plateau (Figure 2). The reference line of the PTS-C was defined as the line between 5 and 10 cm distal to the tibial plateau on the anterior proximal tibial cortex (Figure 3). The preoperative PTS was the angle between the perpendicular line of reference and the line connecting the anterior and posterior border of the lateral tibial plateau (Figure 2). The postoperative PTS was the angle between the perpendicular line of reference and the line connecting the anteroinferior and posteroinferior border of the tibial baseplate (Figure 3).

Preoperative (A) and postoperative (B) radiographs showing posterior tibial slope measurements with reference to the proximal tibial medullary canal (PTS-M; posterior slope: 90-A). The reference line of the PTS-M was defined as the line connected at the center of the proximal tibial medullary canal 5 cm and 10 cm distal to the tibial plateau.

Figure 2:

Preoperative (A) and postoperative (B) radiographs showing posterior tibial slope measurements with reference to the proximal tibial medullary canal (PTS-M; posterior slope: 90-A). The reference line of the PTS-M was defined as the line connected at the center of the proximal tibial medullary canal 5 cm and 10 cm distal to the tibial plateau.

Preoperative (A) and postoperative (B) radiographs showing posterior tibial slope measurements with reference to the proximal tibial anterior cortex (PTS-C; posterior slope: 90-B). The reference line of the PTS-C was defined as the line between 5 cm and 10 cm distal to the tibial plateau on the proximal tibial anterior cortex.

Figure 3:

Preoperative (A) and postoperative (B) radiographs showing posterior tibial slope measurements with reference to the proximal tibial anterior cortex (PTS-C; posterior slope: 90-B). The reference line of the PTS-C was defined as the line between 5 cm and 10 cm distal to the tibial plateau on the proximal tibial anterior cortex.

Patient Outcomes

The secondary outcome variables were functional improvement using the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC), Knee Society Score (KSS), Short Form Health Survey (SF-12), and knee ROM. The WOMAC, KSS, SF-12, and knee ROM were assessed preoperatively and at patients' latest follow-up. Mean follow-up was 4.4 years (range, 1.5 to 7.6 years).

Statistical Analysis

All data were collected and analyzed using Microsoft Excel Software (Microsoft Corporation, Redmond, Washington). Statistical analyses were performed using SPSS version 13 software for Windows (IBM Corporation, Armonk, New York). Means, standard deviations, and descriptive statistics were calculated for each variable. Student t test was used to compare preoperative and postoperative PTS, functional score, and knee ROM. The subgroup analysis was performed using an independent Student t test. P<.05 was considered statistically significant.

Results

The current study showed the preoperative PTS could be reproduced consistently in cruciate-retaining TKA. Mean pre-operative PTS-M was 6.9°±3.3° (range, 1.3° to 15°), and mean postoperative PTSM was 7.0°±2.4° (range, 1.4° to 10.5°). Mean preoperative PTS-C was 12.2°±4.2° (range, 4.7° to 20.9°), and mean postoperative PTS-C was 12.6°±3.4° (range, 4.2° to 19°). There was no statistically significant difference between the preoperative and postoperative PTS measurement in both techniques (P>.05) (Table 1).

Measurement Results and Functional Outcome

Table 1:

Measurement Results and Functional Outcome

Mean KSS improved significantly from 95.0±23.3 preoperatively to 171.0±34.3 at the latest follow-up (P<.05). Mean WOMAC improved from 45.6±16.7 preoperatively to 77.0±20.0 at the latest follow-up (P<.05). Mean SF-12 mental scores were 53.3±11.3 preoperatively and 52.9±10.4 postoperatively (P>.05). Mean SF-12 physical score improved significantly from 30.9±9.0 preoperatively to 41.2±11.1 postoperatively (P<.05). Mean knee ROM improved from 4.3° to 103.2° preoperatively to 0.9° to 116.3° postoperatively (P<.05) (Table 1).

With no similar study in the literature, the current authors used an arbitrary 3° as an acceptable range for PTS-M reproduction. The PTS-M was reproduced within 3° in 144 knees (67%), designated as group 1. The 71 knees with a difference greater than 3° (33%) were designated as group 2. Group 1 showed a significantly larger gain in ROM compared with group 2 (P=.04). Group 1 also had significant improvement in Knee Society, WOMAC, and SF-12 physical scores compared with group 2 (P<.01) (Table 2).

Mean Change in ROM and Functional Score by Group Using an Arbitrary 3° as an Acceptable Range for PTS-M Reproduction

Table 2:

Mean Change in ROM and Functional Score by Group Using an Arbitrary 3° as an Acceptable Range for PTS-M Reproduction

Discussion

The kinematics of cruciate-retaining TKA are influenced by many factors including surgical technique, status of the PCL, implant design, and surgeon's experience. Posterior tibial slope is an important surgical factor in cruciate-retaining knee kinematics and biomechanics affecting postoperative knee ROM, tibial shearing force, and anterior translation of the knee, while also decreasing the excessive PCL load in knee flexion and enhancing the femoral rollback patterns during deep knee flexion.6,14–18

Several reports have highlighted the importance of PTS in cruciate-retaining TKA. Jojima et al19 and Whiteside et al20 reported the effectiveness of increasing PTS for improving varus and valgus as well as rotational and anteroposterior laxity in the knee that had flexion tightness. Increasing PTS can reduce the risk of flexion gap tightness in cruciate-retaining TKA, result in a wider femorotibial gap at 135° of knee flexion, and improve postoperative knee ROM.21,22 Conversely, decreasing PTS could result in improper knee kinematics and reduce ROM at a higher flexion angle.23

To the current authors' knowledge, this is the first study proposing reproduction of individual native tibial slope as a goal of cruciate-retaining TKA. A key in addressing this was to evaluate the ability to reproduce slope without the use of complex preoperative imaging or specialized instrumentation.

Several studies have reported variability in the native PTS, with a wide range of values in different populations (Table 3). In recent studies using computed tomography (CT) in 13,546 knees and 2031 knees, mean preoperative PTS was 7.2°±3.7° and 6.8°±3.3°, respectively.24,25 In the current study, the results were similar; average preoperative and postoperative PTS-M were 6.9° (range, 1.3° to 15°) and 7.0° (range, 1.4° to 10.5°), respectively. There was no difference in preoperative or postoperative PTS measurement (P>.05). The current results demonstrate the goal of surgically reproducing patients' native slope can be achieved.

Comparison of Posterior Tibial Slope in Published Studies

Table 3:

Comparison of Posterior Tibial Slope in Published Studies

The optimum cutting angle for the PTS in cruciate-retaining knees is still controversial.6,11,22,26 This brings into question what the appropriate PTS should be for a given patient. In the current study, the technique used an angel wing instrument on the lateral tibial plateau to plan a parallel cut to the patient's native anatomical slope instead of using the predetermined angle provided with the instrument used. This technique reproduces patients' pre-operative PTS, thereby providing better postoperative ROM and outcomes. Moreover, the current study confirmed that standard techniques will result in reduced posterior slope in the majority of patients. Whether PTS should be altered for patients based on their preoperative PTS or should aim for the traditional target of 3° to 6° in other designs remains controversial.

A balanced PCL in cruciate-retaining TKA is critical to postoperative ROM. If the PCL is too tight, there is a potential for limitation in knee ROM or damage to the polyethylene insert.27,28 Ritter et al29 reported one-third of cruciate-retaining TKAs required ligament balancing to obtain a smooth flexion arc, whereas Lombardi et al6 reported the PCL was released less frequently when the PTS was greater than 3°. The rate of intraoperative release of PCL in the current study was low after restoring the patient's native PTS, and the need for PCL release was found to be directly correlated with the PTS.6

As in any contemporary series of TKA, patients' functional score and knee ROM improved after surgery. The current authors cannot isolate the effect of reproduction of the tibial slope on clinical outcome without a control group of cases using a standard technique. The current results suggest increasing the PTS more than a patient's native PTS is unnecessary and restoring the native PTS will result in better soft tissue balance and normal knee kinematics.9,30 The current study showed reproduction of the native PTS within 3° resulted in better clinical outcomes manifested by gain in ROM and knee functional outcome scores. The current authors postulate this restoration of the normal native anatomy of the knee may help maximize patients' functional outcome.

The current study had several limitations. First, the PTS was measured only on a conventional lateral radiograph. However, in clinical practice, PTS has been measured on plain radiographs both preoperatively and postoperatively with an acceptable level of accuracy.31–33 The authors used 2 methods of PTS measurement using different anatomical reference points to ensure consistency of reading for each knee. The more accurate measurement system such as 3D reconstruction CT for the PTS measurement is not routinely used as a preoperative or postoperative assessment tool in the authors' center or in routine clinical practice. Second, interobserver measurement was not performed. Third, PTS measurement of the medial and lateral compartments was not performed. However, the current authors reference a recent MRI study that found no difference between the PTS of the medial and lateral tibial plateau.4

The strengths are that the current study was based on a single surgeon's experience, thereby eliminating surgeon and technique as confounding factors. The study also assessed preoperative and postoperative PTS in a large cohort of patient and is one of the largest series regarding preoperative and postoperative PTS measurements in cruciate-retaining TKA.

Conclusion

The current authors believe in the concept of reproducing tibial slope for the aforementioned reasons, but at this point the findings demonstrated that their modification of standard surgical technique reliably reproduced the native tibial slope in cruciate-retaining TKA. In this study, the reproduction of patients' native PTS within 3° resulted in better clinical outcomes manifested by gain in ROM and knee functional outcome scores compared with patients who had a PTS greater than 3°.

References

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Measurement Results and Functional Outcome

MeasurementMean±SDP

PreoperativePostoperative
Posterior tibial slope
  PTS-M6.9°±3.3°7.0°±2.4°.51
  PTS-C12.2°±4.2°12.6°±3.4°.22
Functional score
  WOMAC45.6±16.777.0±20.0<.05
  KSS95.0±23.3171.0±34.3<.05
  SF-12
    Mental score53.3±11.352.9±10.4.70
    Physical score30.9±9.141.2±11.1<.05
Knee ROM
  Extension4.3°±6.1°0.9°±2.5°<.05
  Flexion103.2°±15.3°116.3°±12.2°<.05

Mean Change in ROM and Functional Score by Group Using an Arbitrary 3° as an Acceptable Range for PTS-M Reproduction

OutcomeGroup 1a (N=144)Group 2b (N=71)P
ROM
  Gain18°16.9°.04
Functional score
  Improvement in KSS83.962.3<.01
  Improvement in WOMAC44.138.1<.01

Comparison of Posterior Tibial Slope in Published Studies

StudySample SizeMeasurement TechniqueMean±SD PTS

PreoperativePostoperative
Yue et al34Male, 20CT-3D reconstruction5.3°±2.5°
Female, 166.1°±2.5°
Meric et al2413,546CT-3D reconstruction7.2°±3.7°
Kuwano et al850CT-3D reconstruction8.6°
Nunley et al252031CT-3D reconstruction6.8°±3.3°
Dejour and Bonnin31281Radiograph10°±3°
Mohanty et al32100Radiograph11.64°±4.54°
Yoga et al3591Radiograph10.1°±3.9°
Hofmann et al933Radiograph7°±3°
Yoo et al3390Radiograph10.8°±3.4°
Khattak et al3640Radiograph12.5°±3.7°
Moore and Harvey3750Radiograph14±°3.6°
Current215Radiograph (PTS-M)6.9°±3.3°7.0°±2.4°
Radiograph (PTS-C)12.2°±4.2°12.6°±3.4°
Authors

The authors are from the Bone and Joint Institute (JLH, MWM, BAL), Western University, London Health Sciences Centre (JLH, MWM, BAL, LES, JPM), and the Schulich School of Medicine and Dentistry (JLH, MWM, BAL, LES, JPM), University of Western Ontario, London, Ontario, Canada.

Dr Morcos has no relevant financial relationships to disclose. Dr Howard is a paid consultant for Smith & Nephew, DePuy, Stryker, and Intellijoint and has received research support from Smith & Nephew, DePuy, Stryker, and Zimmer. Dr Lanting is a paid consultant for Smith & Nephew, DePuy, Stryker, Integra, and Intellijoint and has received research support from Smith & Nephew, DePuy, Stryker, and Zimmer. Dr Somerville has received research support from Smith & Nephew, DePuy, Stryker, and Zimmer. Dr McAuley is a paid consultant for DePuy and has received research support from Smith & Nephew, DePuy, Stryker, and Zimmer.

Correspondence should be addressed to: Brent A. Lanting, MD, FRCSC, Bone and Joint Institute, Western University, London Health Sciences Centre, Rm B9 003, 339 Windermere Rd, London, ON N6A 5A5, Canada ( brent.lanting@lhsc.on.ca).

Received: January 29, 2018
Accepted: November 16, 2018
Posted Online: November 26, 2019

10.3928/01477447-20191122-06

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