Accuracy of matching tibial slope in manual kinematically aligned total knee arthroplasty

Whisper Grayson; Ethan Teich; Nicholas M. Brown

doi:10.1016/j.jor.2025.03.033

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62 (); 152-155

doi:

10.1016/j.jor.2025.03.033

Accuracy of matching tibial slope in manual kinematically aligned total knee arthroplasty

Whisper Grayson^a,, Ethan Teich^b, Nicholas M. Brown^a

a Department of Orthopaedic Surgery & Rehabilitation, Loyola University Health System, Maywood, IL, USA

b Loyola University Stritch School of Medicine, Maywood, IL, USA

⁎Corresponding author: Whisper Grayson. whisper.grayson@luhs.org

Received: 2025-1-29, Accepted: 2025-3-14, Published: 2025-3-25

Professor P K Surendran Memorial Education Foundation

Disclaimer:
This article was originally published by Reed Elsevier India Pvt. Ltd. and was migrated to Scientific Scholar after the change of Publisher.

Abstract

Reproduction of the posterior tibial slope (PTS) during total knee arthroplasty (TKA) improves patient outcomes and minimizes complications including subsidence, polyethylene wear, and instability. The use of imaging has been previously studied as a way of measuring PTS angle and its association with surgical outcomes. There is a lack of literature on the accuracy of matching PTS during manual kinematically aligned (KA) TKA.

This is a retrospective study including 299 primary manual KA TKAs between October 2021 and August 2024 by a single surgeon. The primary outcome was comparison of pre-operative and post-operative PTS angles. Measurements were performed on lateral radiographs as the angle between the tangent line of the tibial plateau and a line perpendicular to the tibial shaft axis.

The average pre-operative tibial slope was 10.3° (std dev, 4.8) compared to an average post-operative measurement of 5.3° (std dev, 3.2) (p < 0.00001). Range of motion improved post-operatively, with average extension increasing from 4.4° to 0.9° and flexion maintained from 118.5° to 117.2°. Complications were infrequent, with one superficial infection, two prosthetic joint infections requiring revision, two additional reoperations for polyethylene exchanges, and five manipulations under anesthesia with subsequent improvement in range of motion.

In this study, we found a significant difference in pre-operative and post-operative tibial slope angle on radiographic measurements following manual KA TKA.

Keywords

Total knee arthroplasty

Kinematic alignment

Tibial slope

Range of motion

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1 Background

Kinematically aligned (KA) total knee arthroplasty (TKA) has been gaining recent popularity, with a goal of restoring pre-arthritic knee anatomy and alignment.¹^,² When compared to mechanically aligned TKA, this technique has demonstrated excellent functional and biomechanical outcomes.³^,⁴ With this approach, the implant components are aligned on three knee axes: the femur's flexion axis, the tibia's axial rotation axis, and the patella's flexion axis.² Proper implant positioning is essential for maximizing patient outcomes and limiting potential complications with goal being to reproduce a patient's pre-arthritic anatomy.⁵

One important factor in achieving optimal sagittal alignment is replication of the posterior tibial slope (PTS).⁵ The positioning goal of the tibial component is to match the unworn native tibial slope, with changes to this angle holding the potential to impact clinical and functional outcomes.²^,⁶^,⁷ Inadequate restoration of this angle increases the risk of implant loosening, instability, or poor range of motion.⁵ Previous studies have focused on using imaging modes to measure PTS and its implications on patient outcomes following TKA.⁵^,⁸^,⁹

Despite these previous studies, there remains a lack of literature evaluating the accuracy of matching tibial slope during KA TKA. In this study, we primarily aim to evaluate the accuracy of matching tibial slope during manual kinematically aligned total knee arthroplasty compared to pre-operative tibial slope as measured on pre- and post-operative lateral knee radiographs in patients ≥18-years old who underwent a primary KA TKA. Secondary aims include assessing post-operative outcomes including range of motion and complication rates.

2 Methods

2.1

2.1 Patient selection

Following Institutional Review Board (IRB) approval, a retrospective chart review of primary kinematic TKAs performed by a single arthroplasty-trained surgeon from October 2021 to August 2024 was conducted. Current Procedural Terminology (CPT) code 27447 was used for initial patient extraction, followed by chart review to ensure inclusion and exclusion criteria were met. Inclusion criteria included patients ≥18-years old who had a primary kinematic TKA (confirmed with the operative report) with pre-operative and post-operative lateral radiographs of the operative knee. Patients with less than 6-weeks follow-up or with incomplete imaging were excluded (n = 22). Following exclusions, 299 patients were included in the study.

2.2

2.2 Variables of interest

Patient demographics including age, sex, race/ethnicity, body mass index (BMI), tobacco use, hypertension, diabetes mellitus, and follow-up time were collected via chart review. Comorbidities were analyzed in the Charlson Comorbidity Index (CCI). Tibial slope was measured on both pre-operative and post-operative lateral radiographs in Picture Archiving and Communication System (PACS) software as the angle between the tangent line of the tibial plateau and a line perpendicular to the tibial shaft axis (Fig. 1). Range of motion pre-operatively and at 6-weeks post-operative were recorded. Post-operative outcomes assessed included length of hospital stay, discharge location, superficial surgical site infection (defined as an infection treated with oral antibiotics and/or superficial tissue debridement), prosthetic joint infection, reoperation, revision surgery, periprosthetic fracture, and manipulation under anesthesia (MUA).

Radiographic measurement of the tibial slope as the angle between the tangent line of the tibial plateau and a line perpendicular to the tibial shaft axis.

2.3

2.3 Operative technique

The operative technique of an unrestricted calipered kinematic total knee arthroplasty has been previously published.¹⁰^,¹¹ An attempt was made to replicate tibial slope by aligning an angel wing along the medial aspect of the plateau and setting the tibial cut guide such that the cut was parallel to this plane. All procedures were cruciate-retaining and performed with medial stabilized implants.

2.4

2.4 Statistical analysis

Means and standard deviations are provided for continuous variables while percentages and sample size provided for categorical variables. Univariate comparisons included independent t-tests for continuous variables, with an alpha level set at 0.05.

3 Results

3.1

3.1 Patients

A total of 299 patients who underwent a primary kinematic TKA from October 2021 to August 2024 were identified. A majority (62.5 %) of the patients were female, with an overall average age of 66.6 years (std dev, 8.8) at the time of the TKA. The average CCI was 2.7 (std dev, 1.2). Common comorbidities included elevated BMI, with an average BMI of 33.6 (std dev, 7.0), and hypertension (61.5 % of patients). Most of the patients had never smoked (60.5 %) and a diagnoses of diabetes mellitus was less common (22.4 %). The minimum follow-up time for the patients was six-weeks, with an average follow-up time of 9.5 months post-TKA (std dev, 7.2) (Table 1).

Table 1 Baseline demographics of study population (n = 299).

	Study Cohort n = 299
Average Age at TKA (years)	66.6 (8.8)
Average Follow-up (months)	9.5 (7.2)
Gender
Female	62.5 % (187)
Male	37.5 % (112)
Race/Ethnicity
Caucasian	63.5 % (190)
African American	12.7 % (38)
Hispanic	16.1 % (48)
Asian	1.7 % (5)
Other	6.0 % (18)
Average Charlson Comorbidity Index	2.7 (1.2)
BMI	33.6 (7.0)
Tobacco Use
Current	6.7 % (20)
Former	32.8 % (98)
Never	60.5 % (181)
Hypertension (% Yes)	61.5 % (184)
Diabetes Mellitus (% Yes)	22.4 % (67)

3.2

3.2 Outcomes following total knee arthroplasty

Pre-operative tibial slope measurements demonstrated an average slope of 10.3° (std dev, 4.8), with an average post-operative measurement of 5.4° (std dev, 3.2) (p < 0.00001). A majority (81.3 %) of the patients had a reduction in their tibial slope angle as opposed to an increase (18.7 %) and there was a mean absolute deviation of 5.9°. (Table 2). Range of motion was recorded pre-operatively and at six-weeks post-operative. The average flexion pre-operatively and at 6-weeks post-operatively were 118.5° (std dev, 10.5) and 117.2° (std dev, 12.2), respectively. Respective extension measurements were 4.4° (3.7) and 0.9° (2.9) (Table 3).

Table 2 Comparison of pre-operative versus post-operative tibial slope angle.

	Study Cohort n = 299	Statistical Analysis
Tibial Slope (Degrees)
Pre-operative	10.3 (4.8)	p < 0.00001^a
Post-operative	5.4 (3.2)	p < 0.00001^a
Reduction in Tibial Slope	81.3 % (243)
Increase in Tibial Slope	18.7 % (56)

a Result is significant at p < 0.05.

Table 3 Post-operative outcomes following primary kinematic total knee arthroplasty (n = 299).

	Study Cohort n = 299
Length of Stay (Days)	2.3 (2.0)
Preoperative Range of Motion
Extension	4.4 (3.7)
Flexion	118.5 (10.5)
6-week Post-operative Range of Motion
Extension	0.9 (2.9)
Flexion	117.2 (12.2)
Superficial Surgical Site Infection	0.3 % (1)
Prosthetic Joint Infection	0.7 % (2)
Reoperation	1.0 % (3)
Revision Surgery	0.7 % (2)
Periprosthetic Fracture	0.0 % (0)
Manipulation Under Anesthesia	1.7 % (5)

Post-operative complications were rare, with one superficial surgical site infection managed non-operatively and two prosthetic joint infections requiring a revision. One more reoperation was performed for a liner exchange due to laxity. Lastly, there were five manipulations under anesthesia performed for post-operative stiffness, with improvement in range of motion subsequently documented (Table 3). The change in PTS in the six patients necessitating MUA or reoperation for liner exchange is documented in Table 4.

Table 4 Change in posterior tibial slope in patients necessitating liner exchange or manipulation under anesthesia (n = 6).

	Pre-operative Tibial Slope (degrees)	Post-operative Tibial Slope (degrees)	Change in Degrees
Liner Exchange	11.03	5.27	−5.76
Manipulation Under Anesthesia	6.78	0.88	−5.90
Manipulation Under Anesthesia	6.00	8.02	+2.02
Manipulation Under Anesthesia	0.40	5.97	+5.57
Manipulation Under Anesthesia	7.71	8.59	+0.88
Manipulation Under Anesthesia	16.33	1.51	−14.82

4 Discussion

Proper alignment of implants during a total knee arthroplasty plays an essential role in determining the success of the prosthesis.⁵ Previous studies have focused on evaluating the efficacy of imaging in determining PTS, with the general population demonstrating a high variability in slope, ranging from 7° to 14.7°.¹²^,¹³ In our study, we found an average baseline PTS of 10.3° in our osteoarthritic cohort, falling within this previously accepted range. Post-operatively, however, the PTS was found to be 5.3°, indicating a low accuracy in matching tibial slope during manual KA TKA.

This study aligns with current literature by seeking to measure PTS on imaging and determine its role in determining TKA success. Previous studies have compared various imaging modes when determining PTS, including radiographs, computed tomography (CT), and magnetic resonance imaging (MRI).⁵^,⁸^,⁹ The use of lateral radiographs, as used in this study, have shown variable accuracy dependent on the quality of the radiograph and the projection of the tibial plateaus potentially influencing measurements.¹²^,¹³ While the use of CT scans allows for more accurate measurements, they are not consistently performed for patients, thus making radiographs a more accessible tool.¹⁴^,¹⁵ Nonetheless, the lack of more precise imaging presents a limitation to this study. Further hindrances for making accurate PTS measurements include the anatomical differences in the medial versus lateral tibial slopes. A study conducted by Chiu et al. found that the medial plateau sloped 3° more posteriorly compared to the lateral plateau, with a similar study by Nunley et al. finding an average medial slope of 6.8° compared to a lateral slope of 8.0° as measured on 3D CT scans.¹³^,¹⁴ Additional challenges include the difficulties of measuring a slope on a concave surface and medial side osteoarthritic wear altering the interpretation of the slope.¹⁶

Reproduction of the PTS during TKA is essential for improving patient outcomes and minimizing complications post-operatively.⁵ Prosthesis loosening, instability, implant subsidence, and polyethylene wear have been associated with unsuitable angles.¹⁷^,¹⁸ A biomechanical study by Bai et al. found that the PTS angle impacts stability, with extreme angles leading to tibial subluxation and aseptic loosening.¹⁹ A similar biomechanical study by Hofmann et al. showed increased tibial subsidence with a tibial resection of 8° or more off the normal PTS angle.¹² Inadequate tibial slope restoration can impact the biomechanics of the joint, with an overslope causing reduced collateral ligament tension during flexion and predisposing to a tibiofemoral joint gap.²⁰ This may also lead to increased rollback and stress on the posterior edge of the polyethylene insert. Contrastingly, an underslope can lead to a tight PCL, restricting range of motion and placing increased stress on the construct leading to increased wear or loosening.²¹^,²² In our study, while there was a low overall complication rate, there was one patient who required reoperation for polyethylene exchange due to laxity and five patients who required MUA. However, the patient who required poly exchange for laxity had decreased post operative slope and of the MUA patients three had increased slope versus two with decreased slope.

In this study, attempts to match the tibial slope were made by aligning an angel wing along the medial aspect of the plateau and setting the tibial cut guide such that the cut was parallel to this plane. Other manual techniques for matching slope include using the tibial spine, where the stylus makes equal contact along the length of the anteroposterior spine if the slope is matched.²³ There is generally minimal cartilage wear on the tibial spines. Compared to manual techniques, technology-assisted TKAs determine PTS using the ankle-knee axis, connecting the center of the transmalleolar line to the proximal exit point of the tibial shaft axis. A retrospective study by Chalmers et al., however, found this transmalleolar sagittal axis to underestimate the tibial slope when compared to the more traditional intramedullary axis.²⁴

The results of this study must be considered within its limitations, including the retrospective design while inherently holds the potential for bias. Next, while all of the radiographic measurements were performed by the same person in an effort to limit potential variability, this also introduces the risk of intra-observer errors. Furthermore, as previously discussed, the use of radiographs may be less accurate than CT or MRI scans.¹⁴^,¹⁵ Lastly, all procedures were performed by a single surgeon, limiting the generalizability of this study. Finally, while attempts were made to match slope, it is still unknown how to achieve the optimal slope with a knee replacement and whether this should be based upon anatomy, balance, or some combination of these values.

To our knowledge, this paper is the largest study evaluating the accuracy of matching tibial slope during manual KA TKA. These results demonstrate a reduced post-operative angle on lateral radiographs when compared to pre-operative imaging. Further work is needed to understand the optimal method of matching tibial slope with a manual calipered kinematic knee replacement.

5 Conclusion

In this study, we found a statistically significant difference in tibial slope measurements when comparing pre-operative and post-operative lateral radiographs in patients who underwent manual KA TKA.

Funding

No funding to declare.

CRediT authorship contribution statement

Whisper Grayson: Investigation, Methodology, Writing – original draft, Writing – review & editing. Ethan Teich: Data curation, Writing – review & editing. Nicholas M. Brown: Conceptualization, Supervision, Writing – review & editing.

Consent

Institutional Review Board approval was submitted and received for this study, and the manuscript is not submitted elsewhere for publication consideration.

Ethical review committee statement

The study has been performed in accordance with the ethical standards in the 1964 Declaration of Helsinki and has been carried out in accordance with relevant regulations of the US Health Insurance Portability and Accountability Act (HIPAA).

Funding statement

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

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