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Common adjustments made in robotic total knee arthroplasty to achieve functional alignment and minimize soft tissue releases
, Brenna E. Blackburn, Christopher L. Peters, Michael J. Archibeck
⁎Corresponding author: Michael J. Archibeck. michael.archibeck@hsc.utah.edu
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Received: ,
Accepted: ,
This article was originally published by Reed Elsevier India Pvt. Ltd. and was migrated to Scientific Scholar after the change of Publisher.
Abstract
Abstract
The accuracy and precision of robotics in total knee arthroplasty (TKA) allows for intentional adjustments in bone cuts to accommodate soft tissue sleeve characteristics. This study evaluated common femoral and tibial resection adjustments in robotic-assisted TKA (rTKA) categorized by preoperative deformity, using an imageless, semi-autonomous robotic system.
In this retrospective study, 292 patients underwent rTKA between October 2019 and December 2023 at a single institution by two fellowship-trained surgeons. Functional alignment was attained by registering soft tissue gaps prior to bony resection and adjusting the bony cuts to partially or completely accommodate these gaps. Patients were categorized by preoperative Hip-Knee-Ankle (HKA) angles. Chi-square tests and ANOVA models compared adjustments across preoperative alignment groups.
The cohort included 167 (57.2 %) females, an average age 66.6 ± 9.0 years and average BMI of 30.6 ± 5.6 kg/m2. Preoperative alignment was 54.8 % varus, 22.9 % neutral and 22.3 % valgus. Common adjustments in varus and neutral knees involved the addition of varus (relative to neutral mechanical alignment) to the femoral cut (85 %) and/or tibial cut (85 %) to accommodate the often looser lateral compartment. In valgus knees, the most common adjustment included the addition of valgus to the femoral cut (63 %), but no change to the mechanically neutral tibial cut (66 %) to accommodate the often looser medial compartment. To balance the flexion gap, external rotation (relative to the posterior condylar axis) was added to all knees with a mean 5.3° (range: 1.9–10°) to accommodate the looser lateral compartment in flexion.
This study outlines the common femoral and tibial resection adjustments made from default neutral mechanical alignment based on pre-operative HKA. The range of bony resection adjustments highlights the ability of robotics to accommodate variations in ligamentous integrity and preoperative alignment findings. Further research is needed to determine if these adjustments result in improved outcomes or longevity.
Keywords
Robotic-assisted surgery
Total knee arthroplasty
Functional alignment
Femoral resection
Tibial resection
1 Introduction
In 2020, over one third of AAHKS surgeons reported using robotics in total knee arthroplasty (TKA).1 Since that time, we have seen continued growth in surgeon adoption and patient interest.2 Historical TKA dissatisfaction rates of 20 % have been recently questioned by studies reporting satisfaction in over 90 % of patients.3,4 It has been previously hypothesized that a contributing factor to dissatisfaction may be related to “unnatural alignment” and instabilities related to neutral mechanically alignment in all patients.5 In response, differing alignment philosophies have been proposed that focus, to varying degrees, on the recreation of native knee anatomy.6–8
One such philosophy, which relies on the use of technology, is functional alignment (FA). This technique relies on the measurement of soft tissue laxity and subsequent adjustments of bone resections to provide a balanced knee.9 This technique has shown to obtain better balance without soft tissue release when compared to kinematic and mechanical alignments.10–12 The rise in robotic technology allows objective measurement of gaps intraoperatively, before any cuts are made, and facilitates adjustments in the coronal and sagittal planes. Several robotic systems have demonstrated superior accuracy and precision in component placement compared to traditional methods.13–16 With experience using the functional alignment philosophy with robotics, common patterns of bone cut adjustments arise largely dependent on preoperative alignment.
The purpose of this study was to evaluate the common femoral and tibial resection adjustments required to functionally balance the knee in extension and flexion during robotic-assisted TKA (rTKA) based on preoperative alignment.
2 Material and methods
For this retrospective study, 292 patients underwent rTKA between October 29, 2019 and December 28, 2023. All procedures were completed at a single institution by one of two fellowship trained arthroplasty surgeons. The cohort comprised predominantly female patients (57.2 % female, N = 167), with an average age of 66.6 ± 9.0 years and an average BMI of 30.5 ± 5.6 kg/m2 (Table 1).
| rTKA Patients (N = 292) | |
| N (%) | |
| Sex | |
| Female | 167 (57.2) |
| Male | 125 (42.8) |
| Mean (std) | |
| Age | 66.6 (9.0) |
| BMI | 30.6 (5.6) |
Patients were categorized based on preoperative alignment using Hip-knee ankle (HKA) angles using long standing preoperative radiographs. For HKA measurements, points were placed at the center of the femoral head, the femoral intercondylar notch, the tibial interspinous groove, and the center of the tibial plafond. The femoral mechanical axis was defined by the first two points, while the tibial mechanical axis was defined by the latter two. The angle formed at the intersection of these lines represented the HKA. Varus and valgus were classified as exceeding the neutral alignment defined as −3 to +3° of valgus/varus. Preoperative alignment distribution was 54.8 % varus (n = 160), 22.9 % neutral (n = 67), and 22.3 % valgus (n = 65) (Table 1).
Exclusion criteria was established to minimize abhorrent measurements. Patients were excluded if the center of the patella grossly deviated from the center of the knee and if the fibular head lacked superimposition by at least one‐third of its width with the tibia on radiographic imaging.
2.1 Surgical technique
In 2019, our hospital acquired the ROSA® complete knee system (Zimmer, Warsaw, Indiana). All cases in this study utilized the imageless robotic platform. The robotic protocol involved registration of bony landmarks and measurements of “gaps” medially and laterally in extension and ninety degrees of flexion using distraction techniques. Using the intraoperative planning tools, the default neutral mechanical alignment was modified by adjusting femoral and/or tibial bony resections (angle and depth) to achieve a rectangular extension and flexion space of appropriate thickness. This technique has been termed functional alignment (FA) with the protocol previously explained in depth (14, 18). These surgeons generally limited adjustments of alignment within certain constraints: coronal resections boundaries ranged from 2° of valgus to 3° varus for the femur, 2° of valgus to 3.5° of varus for the tibia, and 8° of external rotation from the posterior condylar axis (ranged values depict variation between Surgeon A and Surgeon B). The default setting for all cases, prior to operative planning, were as follows:
2.1.1 Femur
distal resection depth of 10 mm.
3° of external rotation from PCA.
4° of flexion.
2.1.2 Tibia
Proximal resection of 3–6 mm
posterior slope of 5°
tibia varus/valgus of 0°
Following determination of “gaps”, adjustments were made to these settings to balance the soft tissue sleeve prior to bone resection. During surgical planning the robotic system interface allows for preset to be altered in increments of 0.5 mm or degrees. The data for planned and measured resections were recorded to the nearest 0.1 mm.
The same groups of physician assistants, anesthesiologists and nurses and the same industry representative were involved in all rTKA procedures. All procedures were performed with either a trainee (fellow/resident) or physician assistant as a surgical assist.
2.2 Data analysis
Non-continuous variables are reported as frequency statistics while descriptive statistics and all continuous variables are reported as means with standard deviations. Descriptive statistics including t-tests, Chi-squared, and fisher's exact tests were performed comparing the demographics and operative measures between the two groups. Statistical significance was set at 0.05. All statistical analysis was performed using Statistical Analysis Software (SAS) 9.4 (SAS Institute, Cary, NC).
3 Results
The femoral and tibial resections between knee alignment groups were statistically different (P < 0.001). To accommodate the often looser lateral compartment in varus knees with neutral mechanical TKA alignment, varus adjustments were made to the femoral cut in 84.9 % of varus knees and 73.1 % of neutral knees, and to the tibial cut in 84.6 % of varus knees and 68.7 % of neutral knees. The average magnitude of varus added to the femur was 0.4° for varus, 0.8° for neutral, and 0.5° for valgus knees. The average magnitude of varus added to the tibia was 1.9° for varus, 1.6° for neutral, and 1.6° for valgus knees (Fig. 1).
In valgus knees, the most common adjustment included adding valgus to the femoral cut in 63.1 % of cases, with an average magnitude of 1.4°. The tibial cut in valgus knees was left as a mechanically neutral cut (no adjustment) in 66.2 % of cases (Table 2).
| Valgus (N = 65) | Neutral (N = 67) | Varus (N = 160) | ||
| N (%) | N (%) | N (%) | P-value | |
| Femur Distal | ||||
| More valgus | 41 (63.1) | 9 (13.4) | 7 (4.4) | <0.0001 |
| No change | 3 (4.6) | 9 (13.4) | 17 (10.7) | |
| More varus | 21 (32.3) | 49 (73.1) | 135 (84.9) | |
| Tibial Proximal | ||||
| More valgus | 10 (15.4) | 3 (4.5) | 0 | <0.0001 |
| No change | 43 (66.2) | 18 (26.9) | 24 (15.4) | |
| More varus | 12 (18.5) | 46 (68.7) | 132 (84.6) | |
| Rotation from PCA (includes preset) | ||||
| More internal rotation | 0 | 0 | 0 | – |
| More external rotation | 64 (98.5) | 66 (98.5) | 160 (100) | |
To balance the flexion gap, external rotation relative to the posterior condylar axis was applied to all knees (100 %), with a mean of 5.3° relative to the PCA (range: 1.9–10°) (Table 3). No internal rotation adjustments were made. External rotation was consistent across preoperative alignment groups, with valgus knees averaging 5.1° (range: 1.9–9.0°), neutral knees averaging 5.4° (range: 1.9–10.0°), and varus knees averaging 5.3° (range: 1.9–9.5°) (Table 3).
| Valgus (N = 65) | Neutral (N = 67) | Varus (N = 160) | |
| Mean (range); median | Mean (range); median | Mean (range); median | |
| Femur Distal | |||
| More valgus | 1.4 (0.1–3.1); 1.3 | 0.9 (0.1–1.9); 0.8 | 0.6 (0.1–1.1); 0.9 |
| More varus | 0.5 (0.1–2.2); 0.2 | 0.8 (0.1–3.2); 0.3 | 0.4 (0.1–2.3); 0.3 |
| Tibial Proximal | |||
| More valgus | 1.4 (0.4–3); 1.0 | 1.0 (0.1–2.0); 1.0 | – |
| More varus | 1.6 (1.0–2.0); 2.0 | 1.6 (0.1–3.0); 0.2 | 1.9 (0.5–4.0); 2.0 |
| Rotation from PCA | |||
| More internal rotation | – | – | – |
| More external rotation | 5.1 (1.9–9.0); 5.1 | 5.4 (1.9–10.0); 5.3 | 5.3 (1.9–9.5); 5.3 |
No adjustments to the perpendicular femoral resection were needed in 10.7 % of varus, 13.4 % of neutral, and 4.6 % of valgus knees. For the tibial resections, no planned adjustments were needed in 15.4 % of varus, 26.9 % of neutral, and 66.2 % of valgus knees (Table 3).
4 Discussion
The increasing adoption of robotics, with its improved accuracy, in the performance of total knee replacement has provided additional surgeon willingness to modify alignment from mechanically neutral to reduce soft tissue releases. These resection adjustments, as influenced by measured “gaps”, seem to show consistent patterns based on preoperative deformity. To our knowledge, this is the first study to assess the quantitative resection trends of rTKA using this commercially available semi-autonomous robotic system categorized by preoperative deformity. The key findings are: 1) knees with preoperative varus deformity or neutral alignment are most commonly balanced with the addition of slight tibial varus and additional femoral external rotation, 2) knees with preoperative valgus deformity are most commonly balanced with the addition of slight femoral valgus and additional femoral external rotation.
Achieving a balanced knee is a primary goal in TKA, traditionally defined by equal medial and lateral compartment laxity through full extension and flexion.17 Clark et al. showed that utilizing FA as intended can provide a more frequently balanced TKA compared to MA or KA.10 Robotic systems, often paired with FA philosophies, allow for measurement of “gaps” and adjustment of bone resections prior to execution. Murphy et al. found MAKO femoral resections generally had more external rotation (2.3° vs. 0.1° from the posterior condylar axis) and less valgus angulation (1.6° vs. 2.7°) than OMNIbot, with MAKO tibial resections showing more varus (2.4° vs. 1.9°).18 In our study, when using a robotic system for gap balancing, we observed similar trends, although more dependent on preoperative alignment.
We found that, with the use of semi-autonomous robotics, common adjustments were preformed when categorized by preoperative deformity. We also found that the addition of femoral component external rotation was greater than prior reports across all deformity groups (across the three groups, the mean ranged from 5.1 to 5.4° of external rotation from PCA). Most modern instrumentation guides are set to 3° of PCA external rotation, as seen in our preset.19–22 Chalmers et al. showed a wide variability in rotational axis of preoperative arthritic knees. Nearly half of patients significantly deviated from a routine 3° rotation from PCA (23). Among our ROSA® patients the range of external rotation across each knee type was 1.9–10°, similar to previous studies.23 Femoral component rotations are thought to influence patellar tracking, flexion gap symmetry, and component sizing.24 Recent studies have been debating the magnitude of impact rotation has in connection with surgical outcomes with papers both supporting25–27 and questioning28,29 any effect on outcomes.
This study has several potential limitations. The study was conducted at a single institution with only two surgeons, which may introduce bias and limit the generalizability of the findings. The patient population primarily consisted of individuals with varus alignment, potentially limiting applicability to those with neutral or valgus alignments. Exclusion criteria aimed at minimizing radiographic inadequacy for HKA measurement might have excluded patients who have complex deformities, reducing the study's scope. Additionally, findings specific to the ROSA® system (Zimmer,Biomet) may not be directly applicable to other robotic systems. Despite these limitations, this study included a substantial cohort size with detailed intraoperative data collection.
5 Conclusion
We found, using a commercially available semi-autonomous robotic system with a functional alignment philosophy, that pre-execution adjustments to bone resections demonstrated common patterns when grouped by preoperative deformity. Knees with preoperative varus deformity or neutral alignment were most commonly balanced with the addition of slight tibial varus and additional femoral external rotation. Knees with preoperative valgus deformity are most commonly balanced with the addition of slight femoral valgus and additional femoral external rotation. All groups demonstrated the need for more than typically (3°) of standard external rotation to achieve a rectangular flexion space with high variability. These findings potentially underscore the value of robotic systems in accommodating variability in soft tissue laxity to achieve a balanced “functional alignment”. Given that the current study included only two surgeons, additional investigations involving a larger and more diverse group of surgeons are essential to assess the generalizability of these findings. Further research is needed to confirm if these adjustments lead to improved clinical outcomes and implant longevity.
Patient consent
Patient consent was not required as this was a retrospective exempt study.
Author contributions
DLB: Data curation; writing – original draft.
IMC: Data curation; writing – review & editing.
AS: Data curation; writing – review & editing.
BEB: Data curation; formal analysis, methodology, writing – review & editing.
CLP: Conceptualization, investigation, writing – review & editing.
MJA: Conceptualization, investigation, methodology, writing – review & editing.
Ethical approval
This study was approved as exempt by the University of Utah (IRB #71733).
The study has been conducted in accordance with the ethical principles mentioned in the Declaration of Helsinski (2013).
Funding
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
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