1 Introduction

In total knee arthroplasty (TKA), a significant amount of literature has focused on coronal plane alignment of the knee (CPAK), while there is less literature focused on sagittal plane alignment of the knee (SPAK). One component of SPAK is posterior tibial slope (PTS), which is a critical determinant of flexion mechanics, ligament balance, and long-term implant performance.¹,² Restoration of native PTS facilitates posterior femoral rollback, decreases quadriceps demand, and maximizes flexion capacity. However, excessive slope can increase cruciate ligament stress, polyethylene wear, and risk of instability.³,⁴ Clinical studies have demonstrated that deviations from the native slope may compromise range of motion and contribute to component subsidence or early failure.

Despite its importance, conventional mechanical instrumentation has yielded inconsistent restoration of sagittal alignment, resulting in wide variability in restoration of postoperative slope.⁵,⁶ Inconsistency with manual instrumentation stems from intraoperative estimation, patient and lower leg anatomy, and manual cutting guides. Early systematic review of computer-assisted navigation revealed improved alignment accuracy but did not translate into measurable clinical benefits. While this technology improved coronal plane accuracy, it did not reliably address sagittal parameters, demonstrating persistent inconsistencies in sagittal plane positioning. Robotic-assisted systems have since emerged, offering intraoperative feedback and haptic boundaries that enable greater accuracy and reproducibility in slope restoration compared to manual techniques.⁷ However, errors in intraoperative mapping with robotic surgery, especially with regards to the transmalleolar and tibial axis, can lead to underestimation of PTS in navigated cases and may alter postoperative mechanics.³,⁷,⁸,⁹

Beyond surgical technology, alignment philosophy exerts a profound influence on PTS. Mechanical alignment (MA) standardizes resections to a neutral axis, while kinematic alignment (KA) seeks to restore the patient's native prearthritic anatomy, including slope.²,¹⁰ Restricted kinematic alignment (rKA) restores each patient's native slope within predefined boundaries and frequently relies on advanced technology such as robotic assistance or patient-specific guides to achieve this precision.¹¹ The impact of hybrid approaches on PTS reproducibility with robotic assistance remains underexplored. Some studies indicate that modifications to native PTS in KA do not compromise outcomes, while others warn of risks like arthrofibrosis from excessive reduction in robotic cases.¹²

The interaction between alignment strategy and robotic assistance in achieving reproducible slope restoration remains incompletely defined. The purpose of this study was to compare the accuracy and reproducibility of PTS restoration across robotic-assisted and conventional TKA, stratified by alignment philosophy. We hypothesize that robotic-assisted TKA with restrictive KA would demonstrate lower variance in slope correction compared to conventional MA techniques, thereby enhancing patient-specific precision.

2 Methods

This retrospective cohort study was approved by exemption through WIRB-Copernicus Group, Inc. approval number 20260786 on March 2, 2026. We reviewed consecutive primary TKAs performed between January 2020 and December 2023 by four fellowship-trained, high-volume arthroplasty surgeons at a single tertiary academic medical center. Each surgeon utilized a consistent surgical technique and implant system (Persona CR knee, medial congruent bearing, Zimmer Biomet, Warsaw, IN) throughout the study period. A total of 400 consecutive cases were analyzed, with 100 procedures per surgeon to ensure balanced representation.

2.3

2.3 Surgeon 4

Gap balancing with mechanical alignment, prioritizing symmetric flexion-extension gaps after initial cuts.

Inclusion criteria included patients >18 years undergoing unilateral primary TKA for end-stage osteoarthritis or inflammatory arthritis with available preoperative and postoperative standardized lateral radiographs. Exclusion criteria included prior proximal tibial osteotomy, revision TKA, severe extra-articular deformity (>15°), or inadequate imaging quality precluding slope measurement.

Preoperative and postoperative PTS were measured on standardized weight-bearing lateral radiographs using a validated digital measurement tool, shown in Fig. 1A and B. Slope was calculated as the tibial anatomic axis, the acute angle between a line perpendicular to this axis and the tibial plateau or tibial baseplate surface postoperatively. All measurements were performed by two independent observers blinded to surgeon identity and surgical technique, with the mean value used for analysis.

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Fig. 1 A. Radiographs depicting the measurement of preoperative posterior slope on a lateral weight-bearing radiograph. Fig. 1B. Radiographs depicting the measurement of postoperative posterior slope of the tibial implant on a lateral weight-bearing radiograph.

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3 Results

3.1

3.1 Preoperative and postoperative posterior tibial slope

Pre- and post-operative PTS differed significantly between surgeons (p < 0.01, p < 0.003, respectively), as shown in Table 2 and Fig. 2. Surgeon 1's robotic restrictive-kinematic alignment (rKA) cohort had a mean preoperative slope of 9.16° ± 3.51, similar to Surgeon 2 (8.27° ± 3.81) and Surgeon 3 (8.32° ± 3.97), and slightly lower than Surgeon 4 (9.89° ± 3.50). Postoperatively, all surgeons achieved average slopes within a moderate range of 5-7°. Surgeon 3's mechanical measured-resection technique produced the highest final slope (6.90° ± 1.97), whereas Surgeon 1's robotic rKA and Surgeon 4's gap-balancing cohort yielded mean slopes of 6.13° ± 2.14 and 5.88° ± 2.60, respectively. Surgeon 2's mechanical cohort averaged the lowest (5.70° ± 2.61). All values fall within the 3-7° range that is often recommended to balance flexion mechanics and stability.

Table 2 Average posterior tibial slope measurements for all surgeons. Bold p-values denote statistical significance.

	Surgeon 1 (n = 100)	Surgeon 2 (n = 100)	Surgeon 3 (n = 100)	Surgeon 4 (n = 100)	p - value
Preoperative Tibial Slope	9.16° ± 3.51	8.27° ± 3.81	8.32° ± 3.97	9.89° ± 3.50	0.0112
Postoperative Tibial Slope	6.13° ± 2.14	5.70° ± 2.61	6.90° ± 1.97	5.88° ± 2.60	0.0031
Delta	−3.03° ± 3.70	−2.58° ± 4.37	−1.42° ± 4.24	−4.02° ± 4.13	0.0289

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Fig. 2 Visual representation of average measured posterior tibial slope with standard deviations for all surgeons involved in this study.

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4 Discussion

SPAK and PTS restoration remains a nuanced challenge in TKA. The native tibial plateau slopes posteriorly and varies widely among patients with studies reporting a mean value range from 7° to 14° with inter-individual (medial and lateral posterior tibial slopes) differences reaching as much as 30°.¹³,¹⁴,¹⁵,¹⁶,¹⁷ Mechanical alignment paradigms traditionally aim for a uniform tibial slope of 3-7° to avoid flexion instability, and some surgeons will alter the PTS based on the bearing surface used.¹⁸,¹⁹ This standardized target fails to consider the large variation in native anatomy and can inadvertently tighten or slacken compartments of the knee. In contrast, restricted-kinematic alignment (rKA) seeks to restore the patient's native joint lines within safe boundaries (cuts within 5-7° of mechanical axis and global alignment within ±3° of neutral) and often requires advanced technology such as robotics to execute accurately.¹¹,²⁰,²¹

Our study compared four surgeons employing different alignment philosophies (robotic rKA, mechanical measured resection, gap balancing, and mechanical alignment) and found that although all groups reduced PTS, postoperative slopes still differed significantly. Surgeon 3's manual mechanical technique yielded the most consistent final slopes, whereas the robotic rKA cohort (Surgeon 1) exhibited the greatest consistency in restoration of native PTS. These findings mirror cadaveric studies showing that robotic systems can execute planned bone resections within <1° of the target.²² These findings suggest that manual techniques can achieve precise targets in high volume surgeons, and robotic assistance enhances reproducibility of planned corrections.

The effect of postoperative PTS on pain relief, function, and range of motion is still presently debated. Early studies suggested that increasing PTS improves femoral rollback and quadriceps efficiency, thereby facilitating flexion.⁴,²³,²⁴,²⁵ Simulation study findings include increasing slope increases tensile loading on the PCL, may risk patellar mal-tracking, and that slopes >3° increase polyethylene liner contact pressure and ligament stress.²⁶,²⁷ Another found that a modest slope (4°) reduces wear and reduces anteroposterior translation, with further increases to 8° not affecting wear rate significantly.²⁸

Large clinical series have demonstrated that changes in PTS do not meaningfully affect postoperative function. A retrospective analysis of 793 posterior-stabilized TKAs compared knees in which the difference between native and prosthetic slope was ≤10° (n = 703) versus >10° (n = 90) and found no significant differences in International Knee Society scores or complication rates.¹⁷ Notably, they found a statistically higher postoperative flexion in the >10° group (123° vs 119°, p < 0.001) that was deemed clinically irrelevant. A cohort study found anteroposterior stability in posterior cruciate-retaining (CR) TKA was best when slopes were 4-6°, although they found no correlation between postoperative PTS and patient-reported outcomes or range of motion.²⁹ A CR-TKA series suggested that approximating the preoperative PTS or a slightly flattened (delta <0.7°) one resulted in statistically significant greater weight-bearing flexion (129° vs. 116°, p < 0.01).³⁰ These findings collectively indicate that moderate deviations in slope do not undermine clinical outcomes and patient-specific restoration may optimize stability and flexion.

Some KA advocates emphasize restoring patient's native slope within ±2°, noting that deviations tighten or slacken the PCL and alter tibial internal rotation.² Our robotic rKA cohort achieved mean postoperative slopes of approximately 6° with low variance, suggesting that robotics may be advantageous when attempting to replicate a specific slope. Our data also showed that a high volume mechanical measured-resection surgeons can achieve reproducible PTS without robotic assistance. A recent study demonstrated that simple modifications to manual instrumentation can improve slope accuracy in unicompartmental knee arthroplasty that more closely replicated the preoperative slope, reducing implantation error from −3.0° to −1.1° and increasing the proportion of knees within ±2° of target from 34% to 73%.³¹,³²,³³

Robotic systems improve accuracy and enhance reproducibility across surgeons. A recent systematic review of a robotic platform found accuracy within 0.61-1.87° for coronal and sagittal parameters noting that sagittal plane accuracy was lower than coronal yet still superior to manual techniques.³⁴ These improvements may be particularly valuable for kinematic alignment strategies, where safe zones are enforced to avoid extremes in soft-tissue tension. Findings suggest robotics aids precision in rKA but not mandatory for consistency.

Limitations to this study include the retrospective design, reliance on radiographic measurements, and lack of functional outcome data limit the generalizability of our findings. Additionally, all implants were CR implants, potentially limiting the applicability to other implant designs. Future prospective studies should correlate slope restoration with patient-specific kinematics, soft-tissue loading, and long-term implant survival.

5 Conclusion

In conclusion, the utilization of robotic technology can potentially help decrease variance in restoration of PTS. While high volume manual surgeons demonstrate similar consistency in PTS, there is a significant increase in variance of restoration of native PTS compared to robotic surgeons.

Patient guardian consent

The study did not require consent to be obtained.

Author statement

John Dundon: Conceptualization, Methodology, Supervision, Project administration, Writing – Reviewing & editing, visualization, investigation. Joshua Uffer: Data curation, Writing – Original draft preparation, Writing – Review & Editing. Noe Trevino: Data curation, Writing – Original draft preparation, Writing – Review & Editing. Skylar Lewis: Data Curation, Writing – Original draft preparation, Writing – Review & Editing. Nicholas Brown: Data curation, Writing – Original draft preparation, Writing – Review & Editing. Paul Lombardi: Writing – Reviewing & editing.

Ethical statement

The ethics committee, WIRB-Copernicus Group, Inc., approved an exemption for this retrospective original study on March 2nd, 2026, WCG 20260786. The committee's reference number is IRB00000533.

Funding

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